APPLICATIONS

MANUAL

AM8:1992

PRIVATE AND STANDBY GENERATION OF ELECTRICITY

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PRIVATE AND STANDBY GENERATION OF ELECTRICITYAM8:1992

The Chartered Institution of Building Services Engineers Delta House, 222 Balham High Road, LondonSW129BS

The rights of publication or of translation of this publication may be reproduced,

are reserved. No part stored in a retrieval

system or transmitted in any form or by any means without the prior permission of the institution.

G 1992 THECHARTERED INSTITUTION OF BUILDINGSERVICES ENGINEERS LONDON ISBN0900953551

Printedin Great Britainby UnwinBrothers Ltd,The GreshomPress, ld Woking,Surrey,GU22 9LH O

ForewordThkApplicatiotu Manual k intended to give guidance on the design processes and principles which should be followed to provide private or standby generation of electricity. Details of appropriate commissioning and maintenance procedures are ako irmhtded. The intention of this document is to provide engineers with guidance that reflects good current UK practice but it cannot be regarded as a panacea. In view of the changing situation with regard to the former public supply authorities, it is expected that it will become necessary to review and/or amplifi some of the information contained herein. The CIBSE gratefully acknowledges the contributions made by many members and co-opted non-members, particularly the late John Vollborth, in the preparation of this document. R C Gmnings Task Group Chairman

Private and Standby Generation of ElectricityTask GroupR C Cunnings (Chairman) R M Bennett P W Edrnunds F Barrington R B Tremlett B RT Waddell R A Wheadon A R Wilkes B R Walker

Publications SecretaryK J Butcher

EditorR E Yarham

The Institution gratefully acknowledges the help of the following organisations in the preparation of this document: Alcad Ltd, Andrew Wilkes Management, Association of British Generating Set Manufacturers, Aukett Ltd, Building Design Partnership, Chloride Ltd, Donald Smith Seymour and Rooley, Oscar Faber and PartnersPLC, Haden Young Ltd, Hoare Lea and Partners, James H Pull Partnership, Petbow Ltd, Rex Justham and Associates, Tremlett Consulting Electrical Engineers, Sound Research Laboratories Ltd. Cover photographs: The Institution gratefully acknowledges Puma Power Plant, Ash, Kent, for permission to reproduce photographs of private and standby generator installations.

ContentsPage

1 1.11.2 1.3

Use and applications

1 1 1

Reasons for private and standby generationUses of private and standby generation Types of prime mover Power output Generator size related to load

4 6 6 8

1.4 1.5 1.6 2 2.1 2.2 3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 4 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.? 4.10 5 5.1 5,2 5.3 6 6.1 6.2 6.3 6.4 6.5 6.6 7

Choice of high or low voltage systems

Mode of opemtionIndividual generator systems systems

1010 10

Parallel operating

generator

Specification requirementsPrime movers Starting methods Batteries Battery chargers Alternators Control Operator controls, instrumentation and alarms

11 1112 12 13 14 16 18

Installation requirementsSpace requirements Fuel systems Exhaust systems Air requirements Noise and noise attenuation Flexible connections Earthing Electrical distribution system Power factor correction of generator Fire prevention, loads and room layout

2020 21 23 24 24 27 28 29 31 31 33 33 33 35 37 37 instructions 37 37 37

detection and extinguishing

Testing and commissioningIntroduction Works testing Site testing and commissioning

Operation and maintenanceUser training Operating and maintenance

Special tools Spare parts Maintenance contract

37 38 40

Periodic maintenance

References

Appendices AlAll Al .2 A2 A3 A3.1 A3.2 A3.3 A3.4 A3.5

Availability of wind powerGeneral Bibliography

42 42 42

Acoustics-commonly

used technical terms and units

43 45 45 46 46 46 46

Operation with loads corrected for power factorAlternator field excitation Limitation due to field heating Maximum power output

Effect of sudden loss of load Parallel operation with authority supply

Private and standby generation of electricity

1

Use and applications

the former, private generation is employed to meet the base load. Typically, base load generating plant is used to meet all the electrical demands of a particular site continuously. Therefore, some inherent spare plant capacity must be incorporated to allow for breakdowns, routine maintenance and occasional peak loads. In some special circumstances, generating plant is used for peak-lopping to reduce the maximum demand on a site. The primary source of supply maybe either privately generated or provided by the supply authority but, in both cases, sets are operated only when required to meet peak loads. Uninterruptible power supplies (uPs) and no-break sets provide continuity of supply for loads which require an unbroken power supply at all times. A UPSmaintains continuous power by using the energy stored in integral batteries when the normal supply fluctuates or fails. The capacity of the batteries is chosen to suit the time for which interruptions to the normal supply may be tolerated. This time is often referred to as the battery autonomy time. No-break sets employ a motor/alternator powered by the normal supply coupled, via a clutch, to an engine on the same shaft, The rotating mass provides an energy source to cover supply fluctuations and, in the case of failure of the supply, to start the engine. For the temporary provision mobile sets are available.1.2,1 Standby generation

1.1

Reasons for private and standby generation

There are clear reasons for considering the provision of private or standby generation of electricity: standby generation is required if the particular application demands an alternative source of supply during any failure or disruption of the public supply private generation provides a total independence from the public supply. Computers, communications and similar equipment can require high-grade no-break power supplies, whereas heating and air conditioning or similar equipment will accept relatively coarse supplies and also a break before switching to a standby supply. Unless specifications dictate a high-grade type of standby with very fine voltage and frequency limit control, it is usual to employ standard-build, competitively priced, but nevertheless reliable, types of generation equipment. Private generation may be viable in the situation of a known local load and, if the finance is available, in providing multiple sets with inherent standby. This is preferred in order to allow for breakdowns and essential maintenance. In this situation it may be feasible to use the rejected heat by means of a combined heat and power system (CHP). This may lower the effective unit cost to an attractive level as long as the additional expenditure on plant and equipment has been included. The justification for standby generation may be based upon one or more of the following reasons: safety: where loss of supply could be a danger to life or

of electricity

generation,

The range of standby generating plant extends from simple sets with manual controls to complex systems with automatic operation in parallel with other sets and/or with the mains supply. Specialist systems are available that allow no-break operation from failure of the primary supply to provision of the standby supply. As part of the design process for standby generating plant, it is necessary to select those loads which should receive standby power. This will enable a proper judgement to be made of the type and size of load to be supported by the proposed set. Normally, the rating of the set should be larger than the connected load for a number of reasons. These are discussed insection 1.5. The following checklist will assist in selecting the loads which need to be supported. These loads should be assessed bearing in mind each of the following questions. 1

health; this could be a critical industrial process, pumping applications, ventilation or life safety systems.

security: for essential communications, operational requirements

defence or

economy: to maintain commercial communications for airlines~shops, hotels, offices and warehouses etc.

1.2

Uses of private and standby generation

Generating sets may be used to provi& the primary source of electricity at a particular location as an alternative to that provided by the supply authority (i.e. private generation) or, alternatively, as a standby supply in the event of failure of the primary source (i.e. standby generation). Ln the case of

CIBSE APPLICATIONS MANUAL Is the supply required to: (a) allow safe evacuation? (~) enable restricted operation of a service? (c) maintain continued operation of a service? (d) enable continued occupation of the premises? The following checklist is not exhaustive and does not imply an order of priority. .

security of supply: need for electrical supply to be independent of industrial action and/or failure of public supply voltage and/or frequency deviations: need to maintain supply voltage and/or frequency within strict limits to avoid damage or malfunction of equipment such as computers; supply authority unable to maintain supply voltage and/or frequency within acceptable tolerances idle standby plant: where a high proportion of standby power is required it is usually uneconomic to have this plant idle for most of its life waste heat recoverv: where there is a large electrical demand and a good-load factor, together with a use for waste heat, a combined heat and power system (cHP) may be advantageous.

Fire fighting lifls; fire service pumps; smoke extraction plant; pressurisation fans for smoke lobbies/staircases; generator services (including pumps and lighting) air conditioning for personnel; air conditioning for equipment; heating for personnel; heating for protection of equipment from frost or condensation damage mechanical ventilation for personnel; mechanical ventilation for equipment; toilet ventilation; kitchen ventilation; restaurant ventilation voice communications equipment; data communications equipment; mainframe computers and peripherals; mainframe computer remote terminals; stand-alone and personal computers; control systems for plant; security system internal lighting (with and without daylight); external lighting; passenger lifts; goods lifts; photocopying machines; facsimile machines; cash registers; vending machines; cash dispensers sewage pumps; drainage/sump pumps; water heaters; process equipment; kitchen equipment (including refrigerators and freezers); trafllc lights; trafiic barriers uninterruptible power supplies; halon extraction plant.

1.2.2.2 Combined heat and power (ctIP) systems With abase load generation system the cost of electricity can be somewhat higher than that of a grid supply. However, the extra thermal energy recoverable from the plant can result in large energy and running cost savings related to electricity obtained fkom the grid. The amount of usefitl energy that can be obtained from fhel used on a CHPsystem depends largely on the following, and varies for every project. Power-to-heat/absorption cooling demand ratio type of generator prime mover used i.e. reciprocating engine or turbine suitable local application for the recovered heat stilcient electrical demand to enable plant to operate at its most efficient plant loading. Table 1 shows the approximate thermal efilciencies which may be obtained from a range of generator sets where conditions for heat recovery and the use of the electricity generated and heat recovered are the optimum. From gas turbines the heat recovered is entirely from the exhaust at temperamres around 500 C. From diesel engines the exhaust heat recovered can beat temperatures around 500C, but the heat recovered from water and lubricating oil jackets is at a temperature of only about 80C and that from the intercooler at about 40 C. The temperature of the heat

.

1.2.2

Base load generation

7.2.2.1 Conditions for base load generation

Base load generation would be considered for one or more of the following reasons: Availability of supply: public supply unavailable locally and too costly to have installedgeneration plant

o atoutgoingterminals ndflanges a of Table 1 Grosspercentage f energyrecoverable

Enginesize(MVT)

Grosspercentage f energyrecoverable o fhm stated sywtn (%)Electrical Exhaust Water Lubriea- Intercwler jacket~ Ting oil jacket

Reeipro- Turbine Recipro- Turbine caring eating

0.5 1.0 2.0 4.0 20.0

37 38 40 40

17 20 24 28 36

19 19 19 19

58 56 54 52

10 10 10 10

3 4 4 4

7 8 8 s

46

~ Reciprocating engines only

2

PRIVATE ANDSTANDBY ENERATION G OFELECTRICITY from exhausts is suitable for steam production, absorption chillers and hot water systems whilst the temperature of the heat from intercoolers, lubricating oil and water jackets is more suitable for general air heating applications. The percentage recoverable energy from gas and dual fiel engines is similar to that from diesels but the temperature of the recovered heat will vary depending on the type of fuel used. Steam turbine generators are often used for large generation schemes and here the recoverable heat can be made available for district heating schemes and the like at an economical energy cost. 1.2.2.3 Factors influencing provision of base loadgeneration The

G59(1J,HSE Guidance Note PM 53(2)and supply authority requirements. Short-duration operation in parallel with the supply, allowing a no-break changeover for test purposes, can usually be negotiated with the supply authority. Speeial tariffs are usual in these cases and need careful study to determine the optimum economy ofa service/standby capacity ratio. With the grid supply mnning permanently in parallel with private generation the security of the customers supply is increased considerably and this could perhaps reduce the number of staff required to operate the system, thus reducing costs. Whether the grid supply is used as a standby or is run in parallel with private generation, there is usually an appreciable charge levied by the supply authorities for the use, or possible use, of a large capacity of their supply at some time during their tariff period. 1.2.2.5 Exporting electricity Where capacity sometimes exceeds demand, consideration should be given to exporting surplus electricity either to the grid or direct to other consumers. Similar considerations to those given in section 2.2 for parallel operation are necessary, but added protective devices will be required and different tariff arrangements will apply. Where electricity is exported to other consumers, security of supply is essential and every aspect of both the engineering and economics of such arrangements must be considered very earefidly.1.2.3 Peak-lopping sets

following factors should be considered when planning a base load genemtion scheme: high capital cost of equipment; however, if energy costs continue to rise at a much greater rate than equipment costs the payback period reduces accordingly; this is especially true for cHP systems additional plant space required for generator sets, heat recovery boilers, switchgear, controls, mechanical services, fuel tanks etc. noise can usually be reduced to acceptable levels but some additional costs must be considered air pollution from generator exhausts: this can be overcome by discharging exhausts above roof level but, if there are taller buildings adjacent, some form of fume cleaning equipment may be required incurring additional costs high standards of maintenance are essential, adding to the running costs although most systems can run automatically, competent staff will be required to monitor the operation of the system; this usually involves additional running costs over those for a grid-fed system base load generation sets should be run at or near to full load capa~ity for maximum efficiency; diesel engines should always be run at SO%load or greater to minimise problems of carbonisation; gas reciprocating engines are erratic and difficult to control when lightly loaded.

.

Peak-lopping generating plant is often an economical proposition in situations where an abnormally high load occurs at certain known times of the day, or for specific plantlprocess fi.tnctions. Such plant may also be designed to act as standby in the event of mains failure. With supply authority tariffs commonly based on the maximum demand, the charge can be contained by using private peak-lopping generating sets to share this abnormal load. It is usual for standby duty generating plant to be interlocked with the normal mains supply to prevent parallel operation. For peak-lopping applications, however, it is common to have parallel-running and in such cases the control unit must accommodate this facility, in addition to those for a standard standby set. When peak-lopping sets are used in parallel operation with a mains system, their capacity must satis~ continuous duty conditions. In addition, the agreement of the supply authority must be obtained and its requirements observed. Such discussions should take place at an early stage of the design consideration. Peak-lopping generating plant can also be used to run essential plant that cannot tolerate power supply interruptions during certain operations. In these situations the mains supply to such essential plant is commonly retained 3

1.2.2.4 Connections with supply authority With most base load systems it is generally advisable to have grid connections brought into a building ready for connection in the event of either generation failure or industrial action concerning generator fuel supplies or spares, for instance. Usually, supply authorities prefer private generation to run separately from the grid supply rather than run in parallel. However, many cases exist of parallel installations run with the consent of the supply authority and such situations are becoming more widespread with the changing attitudes following privatisation. Careful consideration must be given to the electrical system fault levels and the necessary protective and interlocking devices. Reference should be made to Electricity Council Engineenng Recommendation

CIBSE APPLICATIONS MANUAL as standby, a charge being levied by the supply authority depending upon the capacity of the mains supply being required to remain available. In addition to the above general considerations, it is important that the use ofpeak-lopping sets is not chosen without awareness of other potential problem areas. Such areas include: tariff penal~ies if the generating set fails and supplies must be obtained solely from the supply authority the protection requirements imposed by the supply authority, including earthing and synchronisation the need for competent staff to operate and/or monitor the generating plant for optimum performance . possibility of higher fault levels.Uninterruptible power supplies

control and protection equipment; this must be separately considered. Larger mobile sets, such as trailer-mounted units, can commonly be obtained with ratings up to 1 MW. Exceptionally high output sets, normally of the gas turbine type, are available with ratings up to 3 MW. These very large sets are often made into mobile units by mounting them on to a separate towing trailer for transportation purposes and are commonly employed on construction sites. Trailers for use on the public highway must comply with the relevant legislation. Starting of mobile sets is usually by hand cranking for ~he smaller units and electric hand start for the larger trailer mounted sets. Large mobile sets are not always assembled with the control and protection equipment fitted as an integral part of the unit. With such separate items, particular care is needed when resiting the mobile set to ensure proper safety requirements are met. Since mobile sets are used in non-permanent locations, often on open sites, it is important to consider noise emission from the unit. Some environments will require more stringent control of noise than others. Refer to section 4.5 for further guidance. See section 4.7 for earthing arrangements for mobile sets.

1.2.4

Rotary or static equipment is available to isolate the critical load from flucmations and failures of the elecuicity supply. The electrical starting and running characteristics of this equipment can be onerous when supplied by generating sets. Particular care is required when specifying the duty for these sets. 1.2.!5No-break generating sets

No-break sets generally comprise a large flywheel rotating with the elecrnc generator and a clutch isolating the engine. The flywheel is kept rotating by an electric motor when the mains supply is available. On failure of mains supply the engine is automatically started by engagement of the clutch. Only a small transient reduction to frequency results due to the large inertia of the flywheel. Alternatively, an electromagnetic clutch and patented winding/control arrangement can be utilised to maintain frequency during engine starting. It is a more expensive system than a linked UPSand generator arrangement.1.2.6 Mobile generating sets

1.31.3.1

Types of prime moverGeneraI

The prime mover is the principal source of power or the initial source of motive power which drives the alternator shaft. Prime movers for private and standby generation include:

reciprocating internal combustion engines gas turbines wind generators steam turbines water turbines reciprocating steam engines water wheels.

Mobile generating sets are built on their own chassis and are useful where the sets could be required at various locations, either within buildings or outside, on sites remote from a suitable mains supply and for breakdowns, maintenance, or construction purposes, The type of mounting assembly will depend on the anticipated extent of mobilit y and location. Any of the following maybe suitable, depending on the application: hand-portable units hand trolley units trailer-mounted skid-base units lorry or low-loader transportable units. The hand-portable range of mobile sets usually have ratings of less than 10 kW and, particularly the smaller sets, normally employ single cylinder petrol engines rather than diesel. Such small units are often provided with little, if any, 4 units

The selection of the type of prime mover and the fuel to be used depend on the following:

capital and running costs reliabilityy, ease of maintenance and availability of parts and service storage and availability of fuel size of generating set and associated plant performance (including control, governing, load acceptance and fuel consumption) environmental visual impact). aspects (e.g. fumes, noise, vibration,

PRIVATE ANDSTANDBYGENEMTtON OF ELECTRICITY 1.3.2Reciprocating internal combustion engines

Reciprocating internal combustion engines are available for use with various fitels: e.g. petrol, diesel and gas. The most commonly used for standby generation is the diesel engine. Gas engines are normally used for base load plant. 1.3.2.1 Diesel engines Where diesel engines are employed as prime movers it is manufacturers normal practice to use engines with turbo-chargers and charge-air coolers to obtain maximum power output from the engines at minimum capital cost. Due to the infrequent running of standby sets, their running hours are usually low and, provided they are checked regularly in accordance with good maintenance procedures, the actual maintenance requirements will be low. This means that units running at 1500 rev/rein are quite satisfactory and commonly used and, for smaller sets, even higher speeds are acceptable. The starting time for diesel generators of up to about 1 MW is approximately 10 seconds from commencement of cranking to rated speed. Typically, a further 5 seconds should be allowed for full load acceptance. It is not normal for a 100% step load to be applied to the generator. The acceptable maximum load step depends upon the type of engine and should be agreed with the manufictttrer. 1.3.3 Gas turbines

The sizes of wind power generation systems which are physically possible, range from an electrical output of a few watts to ihe multi-megawatt generators used for peakloppil,g and supplying large distribution systems. Table 2 gives rotor diameters for given output at various wind speeds. The basis of this table is given in Appendix A 1. Figure 1 shows the approximate wind speeds likely to occur in the UK.

1.3.4.2 Siting of wind generators The output power is proportional to the cube of the windspeed, therefore a much greater output is available for a small increase in windspeed. Thus the siting of the wind generator is very important, both in terms of location and height above ground. Consideration must also be given to its proximity to obstructions and the size of the generating machine if mounted on the same axis as the rotor.Table 2 Rotor diameterfor given outputatvarious wind speeds (2-bladed rotor) Rotordiameterm) forgivenwindapeed ( (m/s) Estimated output power 10 12 14 (kW) 4 68 0.5 5.9 S.3 18.5 26.2 37.0 S8.6 3.2 4.5 10.1 14.3 20.2 31.9 2.1 2.9 6.5 9.3 13 sHYects on apparatus:

-1%

~

apparenteed-5% \

-15%

\

-20%4G

%. ~b-25%

I

r04 *@e q~ \

severe flick an fluorescent lightinglimit for correct motor storting

-30% - .

likelihood \, \\ aperatlan

\

..

of ot

,hips

undervaltage

Figure3

Effect of voltage dIp on

electrical pparatus a

It should be noted that high voltage cables, connections and switchgear are more expensive than the low voltage equivalents. It maybe necessary to consult a specialist to advise on the options available for a particular situation. 1.6.2Operational and other implications

standby system that is independent system.

of

the high voltage

The safety factors to be considered with high voltage systems are more stringent than those with low voltage systems. Only persons authorised to operate high voltage systems can be allowed to do so in the interests of safety. All electrical systems should be isolated before work starts. Work on high voltage equipment should not be carried out until a permit has been issued to state that the equipment is isolated, locked off and earthed. The area of safe working must be indicated physically by barriers and notices, together with a description and sketch on the permit. It is possible to use outside organisations to provide the service of operating and maintaining the private 11 kV system. The supply authority is one of these, subjeet to its agreement and charges. It has to be considered if this is an acceptable situation or whether directly-employed specialist staff are necessary. These costs need to be taken into account when studying the implications of high voltage operations. There maybe instances where it would be more attractive to use multiple low voltage sets distributed around a site rather than use central high voltage generators. This will provide a

The additional insulation and high fault withstand levels of high voltage generators make them more expensive to build than low voltage sets. For sets above about 1.5 MVA, the effect of the very high load current on the size, weight and cost of the low voltage alternator tends to make high voltage sets more viable. High voltage sets are usually made to order and as a result could have a longer delivery period than standard ex-stock units. If the unit suffers a major failure, the repair work may be specialised and need non-standard parts. This could mean that the set may be out of service for a considerable time. The circuit protection and switchgear differs to that for low voltage operation. Load current tends to be low whereas fault energies are high. The value of the equipment is also high. As a result an enhanced quality and degree of protection above that for a low voltage system is desirable. The fault level will increase as the rating of the set increases to a point where the method of earthing the high voltage system neutral needs to be reviewed. Methods of limiting earth fault current are to introduce an earthing transformer, reactor or resistor. The advice of a specialist should be sought for a particular application which, for example, could involve parallel operation of sets or parallel operation with the mains supply,

9

CIBSEAPPLICATIONS MANUAL

2

Mode of operation

operated regularly on load, the likelihood of ftiiure is very low.

Generator systems may be provided for standby or base load applications. The modes of operation for the alternative arrangements are outlined below. In a standby application, the set maybe an individual unit to replace all or part of the normal supply when that supply is not available. Alternatively, more than one set may be needed to support the site under these conditions. In some applications it maybe necessary to provide a spare set to the standby generating plant to ensure the availability of this supply. In base load operation, the option of obtaining a standby supply from the supply authority may not exist or may not be economically viable. In this case there would be no alternative supply and a spare generator would usually be provided. This may comprise a second machine identical to the primary generator or, in a multi-set installation, one set additional to those required to meet the site load. Provision of an additional set allows one set to be out of action for maintenance while the remaining sets are available for service. This applies to both base load and multi-set installations where continuous operation is essential and a standby supply must always be available.

2.2

Parallel operating generator systems

These may be installed for one or more of the following reasons: to match the load efllciently to improve overall reliability . to allow one set to be off-line for maintenance to increase flexibility. The installation of multiple sets will affect the operating costs of the generating plant. These costs include those for fiel, lubricating oils, ~ters and maintenance personnel. Fuel costs will vary according to the load and the operating eiliciency of the plant. It is advisable to optimise the number of sets supporting the load at any one time in order to minimise fuel costs. Unnecessary starting and running at low loads can result in inefficient use of fuel and increased wear. Automatic control systems are available which provide continuous load optimisation while the plant is running. As the load is reduced, sets are disconnected progressively and shut down. Such control systems usually enable the operating parameters to be adjusted to avoid unnecessary operations and to maintain a minimum percentage of spinning reserve (i.e. the spare generating capacity of sets mnning on load). This allows for starting of larger loads and for fault clearing.

2.1

Individual generator systems

An individual set is frequently adequate as standby to the normal supply. If the set is properly maintained and

10

PRWATE AND STANDBYGENERATIONOF ELECTRICITY

3

Specification requirements

in water-cooled engines in cold climates and for sand filters in areas prone to sand storms. In addition, for ambient temperatures at or below freezing it would be normal practice to provide electric heater facilities in the engine block to avoid water freezing and to assist starting.

3.1

Prime movers

The various types of prime mover have been discussed in section 1.3. The following deals principal y with reciprocating internal combustion engines. 3.1.1

Protectionf primemovers o

Any potential hazards to maintenance personnel associated with the prime mover should be indicated in the manufacturers health and safety policy documentation. This should also detail the measures to be taken under emergency situations. In particular, guarding in accordance with 13.S 5304{9) should be provided to all exposed moving parts. Such guarding should be removable to facilitate maintenance and danger notices should be provided. As an aid towards preventing undue wear and tear on the prime mover components, an hours-run meter should be incorporated either on the prime mover or within the generator set control panel. This will enable planned preventive maintenance to be carried out at the appropriate intervals.3.1.2 Environmental

Safeguards are required to warn of malfunction of prime movers and to initiate automatic safety measures to protect both the prime mover and operatives near the machinery. The main risks of failure are due to the following: low oil pressure high oil temperature high coolant temperature low coolant level prolonged overload

rating factors

engine overspeed.

The effect of temperature on the performance and output of diesel engine prime movers can vary significantly between one type of engine and another. Specific reference should be made to manufacturers data relating to applicable de-rating factors for a given prime mover. When diesel engine prime movers are operated at high altitudes, the combustion air is of low density and the reduced amount of oxygen available will result in reduced power output. At very high altitudes there is always some derating necessary but the point at which this begins will vary with different engines. Therefore, reference to manufacturers data is essential Some de-rating for atmospheric humidity is generally necessary for naturally aspirated engines. With turbocharged and turbocharged/intercooled engines this is not necessary, but other factors, such as water temperature, will need to be considered.3.1.3 Diesel engine speeds

Engine status and warning indication should be incorporated in the prime mover/generating set control panel to indicate when any of these criteria are approaching their safe limits. Under such situations the plant should be automatically shut down. An audio-visual alarm should also be activated as a warning to quickly bring attention to the malfunction, the sounder being capable of being muted, leaving a visual warning until cancelled. Automatic fuel isolation should be provided to protect the prime mover against abnormal conditions if they reach critical values whilst the machine is running. A warning that the system has operated should appear at a manned station. For frequency control, the running speed of the prime mover is maintained within pre-set limits by a governor which may be mechanical, hydraulic or electronic (see also section 3.3). It is normal practice to provide a safety device to prevent engine overspeed which could cause damage. The prime mover starting equipment should incorporate fail-to-start protection, where automatic mode of operation is arranged, such that the starter motor for the engine is automatically disconnected if the engine fails to start after a preset number of attempts or within a reasonable time (see also section 3.2). In addition to protecting the alternator and prime mover, this also avoids undue discharge of the batteries and should initiate audible and visuaJ warnings. Engine vibrations should be effectively damped by suitable resilient mountings on the prime mover and generator set main and sub-frame assemblies. Refer to section 4.5. Prime movers may need protection against the climatic conditions that are likely to prevail under both normal and abnormal situations. Examples are the need for anti-freeze

Engine speeds are normally bands:

divided into the following

high speed: 1500 rev/rein (typical) medium speed: 600-1000 rev/rein slow speed: below 600 rev/rein.

For base load operation medium or slow speed units are normally used. However, slow and medium speed sets are more costly, heavier and appreciably larger than fast running types. Also, with slow running sets there is more difficulty in controlling frequency within fine limits, due to the cyclic variation of rotational speeds of engines. Electronic governors and the use of high inertia flywheels will help to control the frequency to finer limits but can be costly. These considerations normally lead to high speed engines being chosen for standby purposes. 11

CIBSEAPPLICATIONS MANUAL

3.1.4

Reciprocating engines and turbine drives

3.2.4

Hand start

Reciprocating engines can be obtained with ratings of up to about 2 MW using high speed engines (i.e. 1500 revlmin) and up to about 13 MW using medium speed engines (i.e. 600 to 1000 rev/rein). Turbine drives are not generally available below 500 kW but above this rating they can meet power requirements up to any capacity. The choice between reciprocating engines or turbines for base load generation plant should be made after considering the information given on prime movers in this section and seetion 1.3.

Hand start is defined as being able to start an engine by means of a starting handle or rope. Normally, hand started engines are equipped with cylinder decompressors to allow the crankshaft to be turned more easily. This only applies to relatively small single or twin cylinder engines, of less than 20 kW shaft power, due to the amount of physical effort required. 3.2.5Starting aids

3.2

Starting methods

3.2.1

Electric start

If an engine is to operate in relatively low ambient temperatures, e.g. less than 2 C for more than 12 hours, it maybe necessary to provide some form of an aid to start. The type of aid will depend to a large extent on the minimum temperature specified. The following are typical:

Most generators used for standby purposes employ electric starting methods. The engine crankshaft is rotated by an electric motor engaging onto a toothed wheel attached to the flywheel. Motor engagement can be of the inertia type or pre-engaged by means of a solenoid. The latter is preferred because less damage is caused to the toothed wheel on engagement. In the case of a gas turbine engine the motor rotates the compressor via a suitable gear train and free wheel clutch. On some small engines the voltage of the starter motor could be 12 V DCbut normally 24 V DCis used.

coolant heaters of the elecrnc type can be fitted into the jacket of a water cooled engine; these should be controlled by a suitable thermostat to maintain the jacket at about 40C when the ambient temperature is near freezing timed lubricating oil priming system to assist quick starting of large sets maybe appropriate glow-plug aids can be fitted to air cooled engines; these require approximately two minutes to heat the combustion space ether injection can be used for manual or automatic start; this involves injecting ether into the air intake manifold during cranking; recommended operation at or below -10C air intake manifold heaters can be used to preheat the air before cranking commences thereby assisting fuel combustion; as with the glow plug system, this can take at least two minutes to provide sufficient air temperature rise methods of preheating the fuel and/or the use of additives should be corisidered if there is a danger of sustained low outside temperatures.

3.2.2

Air start

The engine crankshaft is rotated either by injecting high pressure air into the cylinders by means of a sequenced distributor to suit the number of cylinders and the disposition of the pistons, or by an air motor similar to the electric start motor. Starting by means of an air motor is similar to that described in section 3.2.1 except that the starter motor is driven by air rather than electricity. Air is normally supplied at about 700 kPa (7 bar) and a storage cylinder ensures adequate capacity to provide at least three starting attempts. The compressor can be either electric motor- or diesel engine-driven with both being provided where mutual back-up is required. If air start is the only form of starting, then consideration must be given to a back-up air supply in the event of tiilure (e.g. by diesel motor compressor). 3.2.3Hydraulic start

3.3

Batteries

It is essential that the batteries are carefully selected as battery problems are the most common causes of generators failing to start. Rechargeable storage batteries are used to provide the power for electric starter motors.The capacity of the battery will depend on the type of battery to be used and also on the current required to crank the engine when cold. In addition, the battery should have sufficient capacity to provide six consecutive starting attempts when at 00 C, without recharge. For locations other than the UK a temperature above or below 0 C may have to be specified according to the local environmental conditions. Batteries should always be placed as near to the starter motor as possible to prevent excessive voltage drops due to long cables. To prevent damage by vibration, high performance Plant& batteries should be installed on a stand conveniently near to the engine and not on the engine bedplate.

This method is often used for applications in hazardous areas, the pumps being located outside the hazardous area. It is also used as an alternative emergency starting system. The engine crankshaft is rotated by a hydraulic motor which is fed from a hydraulic accumulator. The accumulator is primed to about 3 MPa (30 bar) from an independent hydraulic system using a hand pump. The priming of the accumulator can take anything from five minutes to one hour depending on the size of engine. 12

PIUVATE AND STANDBY GENERATIONOF ELECTRICITY

It is imporant to ensure that the battery voltage drop during starting, particularly with repeated attempts to start, does not cause maloperation of the DCcontrols. Battery capacity is specified by the discharge current over a stated time period (usually 3, 5, 10 or 20 hours) to a particular end voltage at the battery or cell terminals, at a stated ambient temperature. For example, 80 ampere hours at the 3 hour rate means that, at an ambient temperature of 20C, the battery can supply 26.7A for 3 hours at not less than 1.80 V per cell after 3 hours discharge. Nickel cadmium battery capacity is normally specified at the 5 hour rate of discharge to 1.00 V at 20 * 5C. The types of battery normally provided for motor starting duties are lead acid or nickel cadmium. There are many variations of these basic types, the most important of which are &scribed below.3.3.1 High performance Plant6 balteries

If automotive type batteries are used in conjunction with a trickle charger their life can be reduced to as little as 8 months. With a constant potential charger of suitable quality this effect is not so marked but the low charging rate may not be stilcient to bring the battery back to t%llcharge, Lead-acid batteries using thicker pasted plates in flooded versions are also available in automotive and stationary designs with life expectancies of between 4 and 10 years when used with constant potential chargers. A better option is to use a charger specifically designed for use with automotive batteries, having a taper characteristic chosen to suit the battery capacity and desired recharge time, This type of charger monitors the charge condition of the battery to which it is (permanently) connected and automatiea.liy provides the float or boost charge required. 3.3.5Sealed rechargeable batteries

These batteries (manufactured to BS 6290[10): Part 2) employ thick pure lead positive plates and pasted grid negative plates in a transparent container. They have a life expectancy of 20 years or more when used for stationary applications with a constant potential charger, They are not suitable for mobile applications and should not be subjected to vibration.3.3.2

These batteries are not, in fact, completely sealed but have safety valves to vent any excess pressure and maintain a constant internal pressure slightly above atmospheric. Automotive sealed rechargeable batteries will have approximately the same life expectancy as the flooded type batteries described in section 3.3.4. Valve regulated, gas recombination batteries (manufactured to BS 6290(10):Ilrti 4) in stationary applications have a life expectancy of about 10 years when used in conjunction with a constant potential charger. These batteries contain a gelled electrolyte and are covered by 3S 6745(13): art 1. P

Free electrolyte nickel cadmium batteries

These batteries (manufactured to BS 626@lJ) are normally used for engine starting and other standby duties. They can operate for long periods with minimal maintenance and have a life expectancy in excess of 25 years. Most nickel cadmium batteries accept rapid recharge and have excellent resistance to electrical or mechanical abuse. They operate over a wide range of temperatures. While the initial cost is higher than that for lead acid automotive batteries, this may be offset by the expected longer life.3.3.3 Pocket plate nickel cadmium batteries

3.4

Battery chargers

Battery manufacturers provide specific instructions for bench charging or commissioning new batteries. Both lead-acid and nickel cadmium batteries gradually dkcharge when not in use (i.e. open-circuit) and therefore must be provided with some means of recharging. All batteries produce explosive gases when on charge. On trickle charge or float charge the amount of gas is very small but if the batteries are boost charged at an elevated voltage adequate ventilation should be provided. The explosive gases are often heid within the separators and not freed until some time after the last charge period. This is particularly important as batteries approach the end of their life as they are then more likely to produce gases at even the lower voltage levels. Appendix A of BS 6133[14)provides guidance on ventilation requirements. There are two main methods of recharging described in the foiiowing sections.3.4.1 Engine-driven

These batteries are used for general standby duties and display the same characteristics as free electrolyte types (see section 3.3.2) but are essentially maintenance-free and can operate for up to 20 years without topping up. 3.3.4Lead+cid automotive starter batteries

These batteries (manufactured to BS 3911(1ZJ) employ nominal 2 V ceils in 3-, 6- or 12-cell monoblock containers, giving nominal 6,12 and 24 V units. Each cell contains thin positive and negative lead alloy grids, pasted with lead compounds and with dilute sulphuric acid as the electrolyte. The cells are capable of supplying high currents for short periods. They have a limited life, however, of about 4 to 5 years when operating from an open circuit condition, with freshening charges being given from the generator output when the generator is started. Therefore batteries of this type are usually sized to give the required current output when the battery has been discharged from 100%o 75% to state of charge.

and these are

alternator or dynamo

A voltage regulator, suitable for the battery being recharged, must be incorporated. Recharging is only available when the engine is running but provides rapid recharge. If the battery becomes discharged, it will need to be charged with a portable charger or be removed for bench charging. 13

CIBSEAPPLICATIONS MANUAL 3.4.2 Mains operated batte~ chargers

3.5.2.1 Salient poleRotating

These chargers are essential for all applications where the battery may remain idle for some time. It is common practice to use constant potential chargers set to 2.20 to 2.25 V per cell for Plant&and industrial leadacid batteries. This provides a charge current of approximately 0.5 to 1.0 mA for each ampere-hour of rating. Mains operated chargers should be capable of supplying a current of approximately 7V0of the nominal 10-hour capacity, together with a boost circuit to raise the voltage to 2.5 V per cell. Sealed lead-acid batteries (to ll!$ 6290: Part 4(~0J) require a constant potential charger set to a nominal 2.27 V per cell and capable of supplying a current ofapproximately 10Aof the nominal 3-hour capacity. A boost circuit is not recommended for sealed leadacid recombination batteries. It is recommended that automotive batteries are float charged with a purpose-designed charger, see section 3.3.4. The charging rates and float voltages vary for different types of cell and reference must be made to manufacturers literature when commissioning a battery/charger system. For nickel cadmium batteries, it is usual to set constant potential chargers to 1.40/1.45 V per cell. Chargers should be capable of supp~ ying current usually 20A or 10% of the nominal capacity of the battery but currents of 1007ocan be used if rapid recharge is needed.3.4.3 Alarms and indicators

main field comprising 2,4,6,8, 10 or 12 poles depending on the speed of the prime mover for synchronous operation. Excitation is provided by a separate exciter mounted on the same shaft or by other methods as described below. The excitation for the main field is supplied via a commutator and slip ring assembly. 3.5.2.2 Brushless salient pole This is the type of generator most commonly adopted for standby and private generation applications. It is similar to the conventional salient pale machine with the exception of the excitation arrangement. The output from the exciter is fed to a rotating rectifier assembly, the output of which is connected directly to the rotating field. This arrangement eliminates the need for brush gear. 3.5.2.3 Stofic excitation Similar to the conventional salient pole machine except that the excitation requirements are provided totally by external means. A slip ring assembly is used to supply the main rotating field. 3.5.2,4 Self-regulatingsalient pole

Possibly the simplest form Of ACgenerator in use. The excitation system is static, self-exciting having open-loop control supplying the rotating armature via slip rings. 3.5.2.5 Distributed fiekl Usually associated with large and high voltage ACgenerators or where, for manufacturing difficulties, salient pole is not possible. The main field windings are drop-in coils in a slotted cylindrical rotor. 3.5.3Asynchronous generators

It is recommended that an ammeter and voltmeter are included in the battery charger circuit. Alarms can be provided for the following conditions: battery charger failure low voltage high voltage mains failure.

3.5

Alternators

Asynchronous generators are, essentially, induction motors driven by a prime mover. They can operate independently, or in parallel with other machines or the mains supply, with simpler control than synchronous machines. However, they are suitable only where frequency stability is not critical. There are two basic types of asynchronous generators, compensated and uncompensated, and the choice will depend upan the application. Compensated asynchronous generators are self-exciting and therefore can operate independently of the mains. With uncompensated machines, the magnetizing cument is taken from the mains and therefore they have limited uses in standby applications. However, no special synchronizing control is required to parallel with the mains. 3.5.4Protection

3.5.1

British and international standards

The relevant British Standards applicable to rotating electrical machines areBS 499Y3Jand BS 500(%4). These standards are broadly in line with International Electrotechnical Commission standards L!3C34(15)and IEC 72(WZ 3.5.2

Synchronous generators

Synchronous generators may be either single or three phase and can be operated independently or in parallel. The various types of synchronous generators are described in the following sections. 14

Potential faults on generator systems include starter insulation faults, overload, overvoltage, unbalanced loading, rotor faults, loss of excitation or synchronism, overspeed, and prime mover failure. Protection against some, or all of

PRtVATE AND STANDBYGENERATIONOF ELECTRICITY

these faults shouM be provided, the extent of which will depend upon the size or application of the generator. Alternators should be equipped with electrical protection to guard against excessive overload, overcurrent and earth leakage currents. This protection is often in the form of suitable inverse definite minimum time lag (IDMTL] relaysIoeated in the associated essential supplies circuit breaker. Earth fault or leakage protection is of the unrestricted or restricted type, depending upon the overall distribution system protection design, using separate IDMTL relays and associated current transformers. Earth fault protection is of particular importance for portable or semi-portable sets. Unrestricted earth fault protection is normally applicable only to generating sets up to 300 kW rating, Restricted earth

circuit when the temperature setting is exceeded. However, difficulties in the accuracy and reliability of such devices, coupled with their extra cost, suggest that such facilities are not normally justified. Overspeed protection is provided normally by means of an electronic overspeed protection circuit built into the generator control panel. Certain types of alternator are also susceptible to damage due to operation below normal speed for an extended period and hence such equipment should be protected against underspeed. All alternators used with engine driven sets are required, as a standard provision, to withstand a 209ooverspeed above nominal value without damage. Physical protection to alternator windings is provided in the form of impregnation and external treatment of the windings with suitable varnish to guard against moisture and other contaminating substances. This is of particular importance for tropical climates. Other physical protection measures include anti-vibration mountings for the alternator assembly as part of the overall generating plant support I%tme. Radio interference suppressors should be provided in the automatic voltage regulator (AVR) circuit to give suppression within the limits given in BS80@8J.3.5.5 De-rating faetora

fault protection will detect most winding earth faults and is best tiorded by using core-balance earth leakage protection. p Further information on typicalrotection arrangements is given in thel%orecriw RelaysApplication Guide(]7),published by GEC Alsthom Instruments Ltd. Earthing and bonding arrangements are described in section 4.7. Reverse power relay protection should be provided for sets intended for operation in parallel or where regenerative loads may be encountered. On large installations with sets running in parallel, it is normal to incorporate circulating current protection to detect winding faults. It is preferable on parallel set installations to have a field failure relay, rather than an under-voltage relay, to trip the alternator and engine in the event of an excitation fault such as a diode failure. The generator should be sized to contain the transient voltage dip to acceptable limits upon starting the largest motor. Care is required to prevent simultaneous starting ofa large number of motors under standby generation conditions. Ideally, the motors having the largest starting current should be started first. With small machines the output voltage may collapse under a fault condition whilst the machine continues to run, In this situation an under-voltage relay device should be provided to trip the alternator circuit breaker after a suitable time interval and to shut off the fuel supply to the prime mover. Excitation systems can be provided with field forcing/ current maintenance to maintain the short circuit levels at up to four times the full load current for 5 to 10 seconds. This ensures the operation of protective devices. See also section 4.8 on distribution system protection. Alternators should be continuously maximum rated to BS

Normally an alternator is rated with an efilciency factor at full load output and assumed power factor of 0.8 lagging. The efficiency of any given machine varies between low load and full load, and also with the power factor. Generally, the larger the rating of the alternator, the more efilcient it is, e.g. a machine of 500 kVA should have an efilciency in the order of 90%. Loads containing elements which are sensitive to voltage variations may dictate that there should be some restriction on the transient voltage limits of the machine. These limits must be maintained at the alternator terminals under all conditions of load variation. Selection of the alternator, therefore, may be determined more by this factor than by thermal considerations. This factor could result in a prime mover matched to an oversize alternator having a thermal rating well in excess of that which would be required for the plant rating associated with the engine power. The output of the alternator in any given application maybe limited by either its winding operating temperature, or by the transient performance when load changes are imposed upon it. The thermal performance will usually decide the rating of the alternator for prevailing ambient temperature and altitude conditions. Temperature rises for continuously rated machines must not exceed the values permitted in BS 4999(3)for the particular class of insulation at a maximum ambient temperature of 40C at an altitude not exceeding 1000 m. If cooling conditions are more difficult to accomplish, then the machine must be de-rated accordingly. For given ambient conditions the alternator will usually have a continuous rating equal to or greater than the continuous rating of the prime mover. 15

499!X3) BS 500d4)and, to comply with BS 5514(5), and must

be able to supply a 10% overload for one hour in any twelve hour period of continuous running at rated load without injurious overheating of the generator windings. This overload capability, however, should not be employed too frequently otherwise the life of the winding insulation may be shortened due to the associated higher winding temperatures. Overheating of the alternator stator windings maybe prevented by the use of temperature detectors (e.g. thermistors) built into the windings which isolate the

CIBSEAPPLICATIONS MANUAL

As a rule of thumb, a de-rating factor of 1% per C above an ambient temperature of 40 C should be applied. Detailed information on operating conditions is given in BS 499$31: Part 102. However, accurate de-rating factors should always be confirmed by the manufacturer.

3.6.2.1 Manua/ synchronisationIt is necessary to have a means of adjusting the engine speed and the generator voltage for each generating set. The engine governors must be compatible and have similar characteristics of load versus speed, i.e. droop. The speed droop must be the same and at least equal to 4 to 5% of the nominal speed in order to achieve good engine load sharing.

3.6

Control

3.6.1 Start-up controlThe choice of start-up control method depends on the application and the requirements of the system. There are three types of start-up control: manual: cranking of the engine is initiated manual operation of a switch or pushbutton by the

The generators must be provided with quadrature droop equipment in order to achieve reactive load sharing, i.e. equal power factors. The process of synchronizing would be carried out manually by adjustment of the frequency and voltage of the incoming machine. The following control and instrumentation should be considered for manual synchronizing and parallel operation:

automatic: cranking of the engine starts on receipt of an independent signal multiple starting attempt: normally at least three cranking attempts are made should the engine fail to start. On some engines pre-lubrication is necessary before cranking can commence. This includes ensuring adequate lubrication of the main bearings. Where automatic start-up is used, caution notices should be displayed prominently to warn personnel. Automatic mains failure control systems are designed to start the generating set in the event of a complete mains fkilure or a deviation outside acceptable limits. The system is similarly designed to stop the set and restore the mains supply to the load upon restoration of the mains supply. Reference should be made to EZecmcity ouncil Enginemng C Recommen&tions G5W) and HSE Guidance Note PM53(2). Means must be provided using contactom or circuit breakers to allow changeover from mains to standby generator. Interlocking by mechanical and/or electrical means must be provided to ensure that both switches cannot be closed at the same time, Full details must be agreed with the supply authority. The rating of each switch must correspond to the maximum current to be carried when on mains or standby supply. These may not necessarily be equal. Changeover is initiated by a relay or solid state device which monitors mains voltage. This can be single or three phase depending on the nature and type of loads. When loss or partial loss of voltage is detected, a signal is provided to start the generator set. If required, this signal can be delayed to prevent starting if the loss of voltage is only of a transient nature. Likewise, on restoration of a healthy mains, the stopping of the generator set can be delayed to allow the mains to stabilise or for other operational reasons. 3.6.2Synchronisation control for parallel operation

output circuit breaker, contactor or switch with short circuit protection and synchronizing check relays voltage adjustment provision frequency adjustment provision voltmeter indication of incoming and bus supplies frequency indication of incoming and bus supplies synchronizing lamps or synchroscope to indicate phase coincidence and frequency difference current indication for each phase kVAr meter power fhctor meter kW load indication for each set reverse power protection.Automatic synchronisation

3.6.2.2

If a completely automatic system to synchronise two or more generating sets is necessary, then equipment is needed to perform the functions of frequency and voltage adjustment. In addition, if the system demands precise frequency control, i.e. isochronous operation, then some form of load control is necessary. The following control and instrumentation is considered essential for automatic synchronizing and parallel operation:

automatic synchroniser automatic load sharer for each set output circuit breaker, contactor or switch with short circuit protection and synchronizing check relays reverse power relay voltage adjusting device frequency adjusting device automatic governor. is also recom-

For parallel operation, in which a single load is supplied by more than one generating set, it is necessary to synchronise the sets by adjustment of the frequency and voltage. This may be achieved either manually or automatically, as follows. 16

The following additional instrumentation mended: current in~lcation for each phase kW load indication for each set

voltmeter indication of incoming and bus supplies

PRIVATE AND STANDBY Generation

OF ELECTRICITY

frequency indication of incoming and bus supplies synchronizing lamps or synchroscope to indicate phase and frequency difference.

Table6 Requirements ofgoverning systernd$ 19) Parameter Requirement ofstatedBritishStandard for given classof systemBS5514 Bs 649t

Al Class3.6.3

ClassA2 1.5%1.0% 8.0%

class A2 1.0% lSF%4.5%

Frequeney control

The frequency of the generator is dependent upon the speed of rotation and the number of poles, and is given by:J -

Steadystate speed -up to 25%power -over 25%power Speed dropnoloadto fullload Transient speedchange rated load off reeoverytimeTransient peedchange s

1.0?70

0.8%5.0%

%#

(3)

10.0%8sN/A N/A

15.0% 15sN/A N/A

15.0% 15s4.0% 5s

where~ is the output frequent y (Hz), lVPis the number of pairs of poles and n is the speed of rotation (rev/rein). Table 5 gives the combinations of speed and number of pairs of poles for a generator output of 50 Hz. Table 5 Numberofpairs ofpoles and speed for 50Hz output Pairsspeed

25%load onrecuvery time

s Transient peedchange(nonturbocharged engine) rated load onrecovery time

10.O%8s

15.0% 15

N/A N/A

t BS 649@) swithdrawn i andsupersededyZM55241s). b However,vatues are quotedfrom BS 649becauseexisting installationsmay have been designed tooperate totime requirements.

ofpoles 1 2 3 4 5 6

(rev/rein) 3000 1500 1000 750 600 500

3.6.3.2 /iydraulic governors

These are an improvement over the simple mechanical type. Hydraulic pressure is used to provide a servo action to act upon the engine fuel injection system. In a sophisticated form they can provide true isochronous speed control. A reasonable range of speed adjustment is available. Hydraulic governors are also particularly suitable for use in hot climates. 3.6.3.3Electronic governors

Since fkequency control is dependent upon the shafi speed of the prime mover, a speed governor must be fitted to the prime mover. The choice of governor type will depend on the frequency control limits required to meet the load, application, type of prime mover selected and whether single or parallel generator applications apply. The three types of governor are described below. The requirements of governors are laid down in BS S514(5) and BS 649(19) (withdrawn). Table 6 summarises these requirements. Class Al (as defined byBS 649) and Class AO(as defined by BS 5514)aregenerally for special purpose applications such as radar, radio, computer supplies and heavy motor starting duties. Parameters for these applications must be determined by the designer and the manufacturer. BS S514 recognises turbocharged engines as a special case. The percentage of rated load that can be applied in one step is dependent upon the brake mean effective pressure (BMEP)of the engine.

Electronic governors consist of a speed sensor, control unit and electro-hydraulic actuator. The speed sensor produces a high frequency signal proportional to the speed of rotation of the engine. This is compared with a set frequency signal in the control unit and any error is amplified and fed back to position the actuator which, in turn, acts upon the engine fuel injeetion system. A closed-loop servo system ensures stable control, This type of governor will provide the very best performance possible from any prime mover. 3.6.4Voltage control

The method of voltage control will depend upon the voltage regulation limits required to meet the particular load application. Regulation standards are laid down by BS 499!X3): run 140.

3.6.3.1 Mechanical

governors

The types of voltage control are described below. The control of the output voltage of an ACgenerator depends on the type of alternator and its excitation system, see section 3.5. 3.6.4.1 Self-regulating compounded This is the simplest form of voltage control and normally comprises a linear choke, current transformer and rectifier. It provides open-loop control with excellent overload capability, However, voltage regulation is dependent upon a17

The simplest form of mechanical governor consists of a spring-loaded rotating bob-weight assembly which acts directly on the engine fuel injection system. An inherent speed droop of 4-5% from no load to full load is typical. There is usually only limited provision for speed adjustment.

CIBSEAPPLICATIONS MANUAL

good engine governor to maintain speed and even then the over the load range of regulation will not be better than t 3+0 the generator. Voltage is normally pm-set with very limited provision for adjustment. 3.6.4.2 Variable voliage compounded

automatically controls the phased connection of the load groups in accordance with a pre-determined schedule of priorities (the schedule may differ according to the number of generating sets available, which incoming supplies have failed and other operational details) monitors and controls the reconnection of loads to the mains when a stable supply has been restored (see below).

This type of voltage control is an addition to the selfregulating type which has a diverter type closed-loop regulator in place of the diverter resistor. This retains the advantages of the self-regulating generator but, in addition, gives voItage regulation better than f 2% at any Ioad and reasonable provision for adjustment of output voltage. 3.6.4.3 Automatic voltage regulator(AVR)

This last function may not be appropriate in some applications. For example, certain loads may need to be maintained on the generator supply for a period beyond mains resumption availability to avoid supply failure risk upon changeover to mains supply. In such cases, manual reconnection of loads is preferred. The software for load sequencing control systems is likely to be complex and care must be taken to avoid unduly complicated, and therefore potentially unreliable, control systems on emergency standby facilities. It is often preferable to ensure that such a control system acts as a stand alone system which cannot adversely affect the manual operation of the generating plant should a control system failure arise. If the building has a building management system (BMS) then it is normal, though not essential, for the automatic load sequencing control system to be interfaced with theBMS.

This is the most common type used with brushless type generators. It consists of electronic control applied to the excitation system of the generator. Voltage regulation better than * 2% over the load range and at least A 10XOoltage v range adjustment can be achieved. It is usual to specify either Grade 1 or 2 voltage regulation (as defined in BS 4999(3): l%zrt140) depending upon the permitted variation for both steady load and sudden load application situations. Grade 2 regulation is normally recommended, giving control to+ 2.5V0of rated voltage for balanced steady loads between no load and full load and at any power factor between unity and 0.8 lagging. Upon sudden application of a step load of 60Afull load, Grade 2 regulation will restrict the initial voltage dip to within 15% of rated value, recovering to within 3Aof rated voltage within 0.5, 1.0 or 1.5 seconds, as specified. Grade 1 regulation gives control to &5% of rated voltage for the same range of balanced steady loads, but at rated power factor. With a step load application of 35% full load the initial voltage dip will be restricted to 15Aof rated value, recovering to within 670of rated voltage within 0.5,1.0 or 1.5 seconds, as specified. Refer also to section 3.5.4 on protection. 3.6.4.4 Static excitation In this type of control the complete excitation system is provided in one unit and the voltage control is an inherent fimction. It is used for special applications only. 3.6.5Load control

Guidance on the suitability of load sequencing control systems must be sought from the generator manufacturer and, if different, the control system supplier during design of the system to explore fully the operational implications.

3.7

Operator controls, instrumentation and alarms

3.7.1 Operator controlsThe operator controls required for a generating installation may include the following:

set

stop: disconnects load and stops set at once; button will need to be reset, e.g. turn to release manual run: set will start, pick up load and run until offis selected automatic standby: set will start and pick up load on receipt of a remote signal, usually mains fail duty select: for multi-set installations, the order of loading and unloading sets may be seleeted mains Ml simulate: proves automatic standby operation by simulating a mains failure return to mains: overrides a previous signal to run on load; will not do so unless mains is healthy; set may continue to run off-load under automatic control alarm accept: acknowledges an alarm, leaving indication without audible alarm alarm reset: allows reset of alarm indication if cause of

When a standby generation system is arranged to accept and/or reject loads in accordance with a graded priority load sequence, rather than simple changeover between essential and non-essential loads, it maybe advantageous to utilise automatic load sequence control. Such controllers comprise a microprocessor-based system which performs the following fimctions: monitors mains supplies monitors which standby generators have started up as a result of a ftilure of the mains supply monitors the status of the generator start-up sequence to identify when loads can be applied 18 control

PRIVATE AND STANDBY GENERATIONOF ELECTRICITY alarm

is not present

fuel level: indicates fuel level of local (day) and/or bulktank(s) tachometer: engine shafi speed of rotation battery charger current: current flow to recharge and

. . .

lamp test: proves indicator lamps are healthy voltage adjust: allows fine tuning of control voltage frequency frequency adjust: allows fine tuning of speed, hence

maintain starter battery. 3.7.3Alarms and indicatorsencountered are the

single/parallel select: selects control circuits for parallel operation, namely voltage drop and load share voltmeter select: selects particular line-to-line or phaseto-neutral voltage or off ammeter select: selects particular phase current or off generated kW select: selects total multi-set installation

The alarms and indicators commonly following:

kW or one set of a

ready to start: set will start on receipt of a mains fail signal manual selected: set will start from a hand control automatic selected: set will start on receipt of a mains fail if read~ common alarm: remote single alarm lamp indicates when one or more alarms are activated standby on-load: load is supported b y the set mains on-load: load is supported by the mains

circuit breaker control: enables local open and close for testing, manual operation or manual synchronizing synchroscope onioffi enables manual synchronizing between set and another supply test/manual: engine starts and runs off-load power factor meter select: may select particular phases fid meter off.

which

3.7.2

Instrumentation

mains available: indicates mains has returned following a failure; operator may choose to transfer load to mains and stop set mains failed: mains not available; generator set start-up routine is activated if system is automatic battery charge failure: starting battery not receiving charge failure: set has failed to start or failed while running high engine temperature: indicates over-temperature of engine coolant water; will result in shutdown if safe limit is exceeded for a defined period low engine oil pressure: indicates inadequate pressure; will result in immediate shutdown oil

The instruments usually provided include the following: . . alarm: draws operators attention to condition its permitted tolerance outside

.

voltmeter: indicates phase and/or line voltage, both onand off-load ammeter: indicates phase current frequency voltage meter: indicates frequency of alternating

power factor meter: monitors power factor of the load kVAr meter: indicates reactive power flow hours run meter: provides record of running hours to assist planning of maintenance periods water temperature: engine indicates temperature of coolant in

earth fault: current flow to earth detected; will cause generator output circuit breaker to trip fuel level monitor: indicates high or low fuel level in local (day) or bulk tanks engine overspeed: shuts down set if normal speed is exceeded heater on: indicates operation achieve rapid warm-up. of coolant heater to

oil pressure: indicates engine oil pressure exhaust temperature {turbines only): over-temperature

will cause shutdown; full load aff&ted by temperatureof combustion air

19

CIBSEAPPLICATIONS MANUAL

4

Installation

requirements

drawings should be obtained before the design and layout are finalised. Table 8 Minimumclear room dimensions for single generator set with skidmountedcontrolpanelanddaily service fueltank output (kVA) Length Width Height

4.1

Space requirements and room layout

(mm) 4000 4350 6150 6150 6800 7100 7500 8250 9000 9000 9750 12000

(mm) 2500 2500 3500 3500 4500 4500 4500 5000 5000 5000 6000 6500

(mm) 2500 2500 3000 3500 4000 4000 4000 4500 4500 4500 4500 5000

The space required to house a generator will depend on the rating of the generator set required. For a given speed and generator set loading, the dimensions and weight vary slightly depending upon the manufacturer. The weight can sometimes be the overriding consideration when selecting a generator set, particularly if it is to be located on the roof of a building. Table 7 gives the dimensions and weights of typical generator sets. These are based on a four pole set with an automatic mains failure facility and a diesel prime mover operating at 1500 rev/rein. Allowance has been made for a local daily service fuel tank and nominal sound attenuation. This information should be used for initial design only. Detailed information appropriate to the actual machine chosen should be obtained from the manufacturer.

30 50 100 150 250 300 400 500 700 1000 1500 2000

The following checklist identifies the main considerations when planning the room layout and generator set configuration:

Table 7

Typical dimensions for 415 V diesel

number of sets required dimensions of each set daily service fuel tank; skid-mounted or remote control panel; skid-mounted or remote access requirements for maintenance access requirements for means of escape location of emergency stops isolation of electrical supplies and fuel acoustic treatment of generator and/or room plant layout to suit exhaust, ventilation and attenuator requirements plant layout to suit structural requirements limitations or special

stmrnator set with automatic mains failure facility output (kVA) 30 50 100 150 250 300 400 500 700 1000 1500 2000 Length (mm) 2650 2900 4100 4100 4550 4750 5000 5500 6000 6000 6500 8000 Width(mm)

Height (mm) 1500 1500 1900 20Q0 2500 2500 2500 2500 3000 3000 3000 3500

Weight (k) 1300 1700 3250 3800 5000 8500 9000 9500 12000 13000 19000 26000

1100 1100 1400 1400 1750 1900 1900 20WI 2000 2000 3000 3500

The arrangement of the generator and the ancillary services (i.e. oil tanks controls panels etc.) will be determined by access for maintenance and for safety requirements. The final layout arrangement will also be influenced by the site restrictions, acoustic requirements, and weight distribution requirements. Table 8 indicates the minimum room dimensions required to accommodate a single generator set with skid-mounted control panel and daily service fuel tank.Clear space should be allowed above the generator to accommodate the exhaust pipes and to permit removal of the pistons during maintenance (cylinder head arrangements w@. These requirements vary with engine type but at least 1 m should be allowed for sets up to 700 kVA and 1.5 m for

radiator/heat exchanger/cooKng tower; local or remote any special local authority requirements any height restrictions requirements to meet local planning

cabling requirements; high or low level type and voltage of batteries required attention to engine starting currents) type of battery charger storage for tools/manuals provision of workbench and maintenance areas lighting/emergency lighting requirements fire regulations (with particular storage and enclosures) reference to fuel oil (with special

larger sets. It is common for larger sets (e.g. 1 MVA and above) to require separate rooms for the accommodation of control panels and switchgear. The space requirement will depend upon the generator set configuration and the integration with distribution switchgear, The manufacturers certified20

location and number of doors (doors should open outwards and two means of escape from the generator room should be provided)

PRIVATE AND STANDBYGENERATIONOF ELECTRICITY

oil/drip tray provision bulk oil storage and fuel transfer system low voltage switchboard and synchronizing equipment earthing protection floor loading space requirements additions. for future plant replacement and

4.2. ?.4 kdustriul fuel oils Industrial fuel oils from kerosene to heavy fuel oil Class G have flash points above 37.8 C and are therefore not subjeet to the provisions of the Petroieum ~Consoli&twn)Acr 1928. All fuel storage and handling systems should be in accordance with i9S 799(27), 286$2s) and BS 541tX29~. BS Fire insurance companies and the Fire Ofilces Committee (now incorporated into the Loss Prevention Council) issue recommendations on fuel storage and handling. In particular, see Recommendationsfm Oil Fired InsraUutions(30}, published by the Loss Prevention Council. Some local authorities also issue recommendations on fuet storage and handling. 4.2.1.5 Diesel fuel oil

4.24.2.1

Fuel systemsTypes of fuel

The properties Guide(zO). 4.2.1.7

of various fuels are given in the C113SEoil to

Natural gas

Natural gas is supplied by British Gas PLC at low pressure by means of pressure-reducing equipment to a statutory level of 150 mm water gauge, It sometimes has to be boosted to a higher pressure for generators. Provision for fuel storage is not required. Normally the supply main is brought to an agreed point, where a meter is installed; it is the responsibility of the consumer to transport the gas from the meter to the point of use. British Gas safety requirements must be met in all cases. Guidance documents are available from British Gas area offices; in particular, see British Ga Co& of Practice IM1 fi21). In instances where an interruptible gas supply is to be provided suitable alternative fuel storage should normally be incorporated. 4.2.1.2Liquefied petroleum gas (LPG)

Diesel engines for generator sets usually run on Class A fuel BS 286~28J.It is sometimes advantageous to use a fuel oil common to other systems such as heating boilers. To optimise the fuel storage requirements in these circumstances a fuel oil other than Class A may be specified. However, care should be taken to ensure compatibility with the generator diesel engine. The ignition properties of fuel oils are defined by the Cetane Index. Typical values are as follows: marine engines: Cetane 40 industrial engines and generating set prime movers: Cetane 45 engines for road vehicles: Cetane 50. Diesel fuel oil tends to wax at low temperatures and hence, where temperatures of below 5 C can be expected, precautions such as additives and electrical trace heatingneed to be considered.

4.2.2

Storage tanks

The safety requirements for the storage of no, both in bulk and in cylinders, in factories and other premises to which the Factories Act(22) applies, are covered by the Highly Flamntalh Liquids and Liquejied Petroleum Gases Regulations 197f123J. ompliance with these regulations also satisfies the C requirements of the Health and Safety Executive code of practice, 2%.s Storage OfLPGat Ft3cedInstaUations(24j. For bulk storage installations the pressure vessels should be installed away from buildings and the line of adjoining property. The speeific requirements regarding location and spacing of storage vessels are given in the Health and Safety Executive code of practice and in a code of practice published by the Liquefied Petroleum Gas Industry Technical Association, Installation and maintenance of bulk .LPG storageat consumers premi.ws~2s~. 4.2.1.3Motor spirit

Most generator set installations, particularly large installations and those used for base load applications, include a bulk fuel oil storage tank feeding a daily service tank above, or adjacent to, each generator set. Bulk and daily fuel oil storage tanks should be manufactured using good quality mild steel plate in accordance with BS 799: Part 5(27).Bulk tanks are commonly cylindrical and may be installed either horizontally or vertically. When installed horizontally, they should be mounted with a slight fhll and be fitted with a cock at the lowest point for draining sludge and/or water. The fuel oil supply draw-off should beat least 80 mm above the sludge cock and preferably at the opposite end of the tank. Filters should befitted to both the intake and draw-off pipelines associated with the bulk storage tank. Either local or remote indication of the bulk tank contents should be provided. The daily service tank should be as close as possible to the diesel engine and ideally immediately above the engine fuel inlet. The fuel outlet inside the tank should be raised above 21

(petro/)

These fuels have flash points below 22.8C and fall within the provisions of the Petrwkum (Consolidation)Act 192ti2@.

CIBSEAPPLICATIONS MANUAL

the bottom of the tank in order to avoid sediment andlor water entering the engine. A vent is also necessary. Means of returning any leaked or overflow fuel to the bulk storage tank should be provided on pumped fuel transfer systems. This prevents pressurisation of the daily service tank. A bund wall may also be required, see also section 4.2. The quantity of fuel oil to be stored, either by bulk/daily service tanks or solely by dail y service tanks, depends on:

Fill point and Bulk guard unit

Ground

floor level

II I~

period of useSolenoid valve ~ II

fuel consumption of the engine(s)

;*Wf$~

availabilityy of fuel.Daily _ --C7= @@ W

j!,

Spill wall oil proof render to 11 O% capacity of tank

The proportional division between bulk and daily (local) service tank fuel storage may be affected by the local regulations, see also section 4.2.3. While manufacturers will provide specific consumption figures for their particular diesel engine, a rough estimate of the storage requirement may be based on a rate of 0.34 litres of fuel per kWh at fill load. This should be checked using the manufacturers data as soon as details of the engine are known. It is usually advantageous to select a tank of a size that is commercially available. Table 9 gives typical dimensions for cylindrical tanks for a range of capacities.

&m ;$

ij

Float witch

~, DOWservicetank should testedto ?% be pressure ++*Y withstond the standing &$:*%*,$ :y** g head of bel in case the solenoid valve fails to

Figure 4 Gravity-feed fuel transfer system (from ABGSM Technical Msmormrdum TM3(311, reproducedby permission)

authority. Some local authorities also require an automatic warning system to indicate that the tank is full during refueling.4.2.4 Fuel transfer methods

Table 9 Typical dimensionsfor cylindrical tanks for a range of capacities

Capacity (lime) 5000 10000 25000 50000

Diameter (m) 1.50 2.00 2.50 2.75

Length (m) 2.80 3.00 5.00 9.00

A simple but uncommon method of transferring fuel oil from the bulk storage tank (or fill point) to the daily service tank is by gravity feed. The bulk storage tank is located at alevel higher than the daily service tank in order to provide the gravity head of fuel. The fhel supply to the daily service tank is either: (a) automatically controlled by means of a float switch in the daily service tank and an electrically actuated solenoid valve in the supply pipeline, see Figure 4, or

The period of use for determining the fuel oil storage capacity can vary greatly between different standby generation users. It may range from three or four hours to two weeks or more, particularly for military and other vital establishments. A period of 24 to 48 hours is typical for most standby purposes, provided fuel deliveries can be assured. It is important to establish the period of use with the client at an early stage during design. Base load generation will require considerably longer periods. 4.2.3Statutory requirements

(b) manually controlled by a hand operated gate valve and suitable level indicator in the daily service tank, an overflow pipe returning excess fuel to the bulk storage tank.

If the bulk storage tank is sited either some distance from or below the daily service tank, a remote fuel transfer method is normally provided, see Figure 5. The pump can be either filly automatic or semi-automatic. Fully automatic systems rely on a float switch in the daily service tank io initiate the pump operation with further float switches to provide low and high level alarms. Semiautomatic systems comprise a manual push button start and automatic stop by means of