HV industrial network design

8/7/2019 HV industrial network design

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n 169

HV industrialnetwork design

Georges THOMASSET

Graduated as an engineer from theIEG (Grenoble Electrical EngineeringInstitute) in 1971.Since then, he has been designingcomplex industrial networks in theframework of Merlin Gerincompany's Technical Division.

After having headed the "MediumVoltage public distribution andhydroelectric installations"engineering department, he hasbeen in charge of the engineeringsection of the Contractingdepartment's industrial unit since1984.

E/CT169 first issued october 1994


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Cahier Technique Merlin Gerin n 169 / p.2

lexicon

flicker: periodic fluctuations the lightoutput of lamps.

HTA and HTB: categories of mediumvoltage defined by a French decreedated 14 November 1988.

Voltage levels are classified by differentdecrees, standards and other particularspecifications such as those of utilities.AC voltages greater than 1000 V aredefined by:s the French decree of 14 November1988 which defines two categories ofvoltage:HTA = 1 kV < U 50 kV,HTB = U > 50 kV.s CENELEC (European committee forelectrotechnical standardisation), in acircular dated 27 July 1992, specifies:MV = 1 kV < U 35 kV,HV = U > 35 kV.

s the IEC publication sets forth thehighest voltage ranges for equipment:

protection system used, the choice ofdevices and device settings.

network structure: overall networkarrangement, often represented in asingle-line diagram, which indicates the

relative arrangement (interconnections,separation of circuits, etc.) of thedifferent sources and loads.

plant: grouping (in a location) ofseveral energy consumers.

private network: electrical networkwhich supplies power to one or moreuser sites (plants) which generally havethe same owner.

utility supply: electrical network whichbelongs to the national or local energydistributor and serves severalindependent consumers.

sensitive loads: loads for which noabsence of power supply is tolerated.

range A = 1 kV < U < 52 kV,range B = 52 kV U < 300 kV,range C = U 300 kV.A revision is pending, which will includeonly two ranges:range I = 1 kV < U 245 kV,range II = U 245 kV.s the French national utilities EDF nowuses the classification given in thedecree cited above.

network dynamic stability: thecapacity of a network, which includesseveral synchronous machines, toreturn to normal operation after asudden disturbance that has caused atemporary change (i.e. short-circuit) orpermanent change (line opening) in thenetwork configuration.

network protection plan: overall

organization of electricalprotection gear, including: the

appendices

appendix 1: extension of an hypotheses p. 19

approximate calculation of a p. 19

current limiting reactor

appendix 2: computerized means calculation software programs p. 20

expert system for evaluating p. 20

electrical network design quality

appendix 3: general principle of compensation p. 20

appendix 4: choice of the earthing system for a HV industrial network p. 21appendix 5: network voltage drops p. 22

appendix 6: stages in industrial network design p. 22

appendix 7: bibliography p. 24

existing industrial network

used for network analyses


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Faced with increasingly fiercecompetition, industrialists must employhighly rigorous management and theirproduction facilities must have a high

level of availability.Electrical networks supply the energyrequired to run the production facilities.The provision of a continuous powersupply to loads is strived for from thestart of the network design, in particularduring preliminary choices in the single-line diagram.Reductions in electrical installation andoperating costs, together with reliablefailure-free operation, are vitalconditions for profitability. Thistechnical and economic optimizationcalls for detailed and global preliminary

analyses, which include:s specific needs and constraintsrelated to the type of industry,s integration of the limits andconstraints of the public distributionnetwork,s standards and local practices,s particularities of the operatingpersonnel, facilities manager andmaintenance personnel.The scope of this study is limited to theanalyses involved in the design of HighVoltage ("HTA" and "HTB") high powerindustrial installations which have the

following main characteristics:s installed capacity in the 10 MVArange,s autonomous electrical energyproduction (when applicable),s power supplied by a nationaltransmission or distribution network( 20 kV),s private Medium Voltage distribution.

HV industrial network design

contents

1. needs and main constraints to be met needs to be met p. 4

main constraints p. 5

2. main rules of industrial network load survey, load and diversity p. 7

design factors

choice of voltages p. 7reactive power compensation p. 7

backup and replacement sources p. 7

autonomous electrical energy p. 7production

division of sources p. 7

overall electrical diagram p. 8

3. validation and technical and choice of the earthing system p. 9

feeder definition p. 9

insulation coordination analysis p. 9

protection system definition p. 9

short-circuit currents calculation p. 10

calculation of voltage fluctuations p. 11under normal and disturbedoperating conditions

choice of motor starting method p. 11

network dynamic stability p. 12

synthesis p. 13

standard network structures p. 14

concrete example of a structure p. 16

choice of equipment p. 16

optimal operation p. 16

5. conclusion p. 18

appendices see page on left

economic optimization

4. choice of optimal network structureand operation


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sinitial investment cost,soperating and maintenance costs,scost of production losses associatedwith the network design and protection

plan (protection system used, choice of

devices and settings).

Optimization of electrical energy

When a plant includes electrical energy

generators, it is necessary to manage

the energy supplied by the utility and

the energy produced locally in the best

manner possible.A control and monitoring system makes

it possible to optimize the cost of plant

power consumption in accordance with:

sthe contract with the utility (billingrates according to the time, day and

season);

sthe availability of the plantgenerators;

sindustrial process requirements.Network changes and future

extensions

When designing an industrial network,

it is of primary importance to make acareful assessment of the future

development of the plants, especially

when extensions are foreseeable.

Changes that are liable or due to be

made in the future should be taken intoaccount:sin sizing the main power supplycomponents (cables, transformers,switching devices),sin designing the distribution diagram,sin calculating the areas to be setaside for electrical rooms.This forward planning will result in

increasingly flexible energymanagement.

Network upgrading

Electrical energy consumptionincreases as extensions are made tomeet the needs of new types ofmanufacturing and ever more powerfulmachines. This makes it mandatory toupgrade and/or restructure the network.

Greater care must be taken in networkupgrading analyses than in analyses ofnew installations since additionalconstraints are involved, i.e.:

1. needs and main constraints to be met

sovercurrent (short-circuits andoverloads),

sovervoltage.The solutions that are used must

ensure at the least the following:

squick fault clearance and continuedpower supply to the fault-free sections

of the network (discrimination),

ssupply of information on the type ofinitial fault, for quick servicing.

Continuous power supply to loads

Continuous power supply to the loadsis necessary for the following reasons:

ssafety of people, e.g. lighting;ssustained production performance,e.g. glass wire drawing;

sproductivity;soperating convenience, e.g.simplified machine or workshop restart

procedure.

Loads are divided into three groups

according to their operating

requirements:

s

"normal" loads,

s"essential" loads,s"sensitive" loads for which noabsence of power supply is tolerated.

Network operating ease

In order to carry out their tasks safely

and reliably, network facilities operators

need the following:

sa network that is easy to operate inorder to act correctly in the event of a

problem or a manoeuvre;

ssufficiently sized switchgear andequipment, which require littlemaintenance and are easy to repair

(maintainability);sefficient means of control andmonitoring which facilitate remotecontrol of the network by real timecentralization in a single location of allthe information relating to the state ofthe "electrical process", under normaland disturbed operating conditions.

Minimum electrical installation cost

The minimum electrical installation costdoes not necessarily mean theminimum initial cost, but the sum ofthree costs:

Industrial electrical networks must

supply power to all the plant loads, at

optimal investment, operating and loss

of production costs, taking into account:

sthe needs to be met:sssafety of people,sssafety of property,sscontinuous power supply,ssnetwork operating ease,ssminimum installation cost,ssoptimization of electrical energy(cost / quality),

ssnetwork changes and futureextensions,

ssnetwork upgrading;sconstraints linked to:ssthe industrial process,ssthe electrical process,ssthe utility,ssthe climate and geography of the site,ssstandards, regulations and localpractices.

It is clear that not all the needs can be

optimally met, which means that the

network designer must endeavour to

find the best compromise.

needs to be met

Safety of people

Even if not all countries have standards

and rules, certain obvious principles

must be adhered to:

sprohibiting access to energized parts(protection against direct contact),

sa system to protect against the rise inpotential of metal structures (protectionagainst indirect contact),

sprohibiting on-load line disconnectingswitch operations,sprohibiting earthing of live conductorssquick fault clearance.Safety of property

Electrical installations should not besubmitted to stress that they are notable to withstand. The choice ofmaterials and equipment is therefore ofprime importance. Two electricalphenomena are to be considered inorder to avoid fire and to limitdestructive effects:


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s insufficient electrodynamic anddielectric strength of existing devices orequipment;

s capacity to supply power to large

loads (starting current, dynamicstability, etc.);

s area and height of existing electricalrooms that cannot be changed;

s imposed geographical location ofequipment and loads.

Example of upgrading with the additionof a new transformer: the installation ofa three-phase reactor between the oldinstallation and the transformer makesit possible to limit the increase in short-circuit current and therefore to continueusing the existing equipment (see

appendix 1).Quality of analyses

The design analysis and detailedanalysis must be carried outmethodically and precisely. ISO 9001certification entails setting upprocedures which describe theessential steps involved in design anddetailed analyses in the view ofmastering these analyses and meetingthe clients' needs in the best waypossible.

Merlin Gerin's Contracting Departmentwas granted ISO 9001 certification

in 1992.

main constraints

Constraints linked to the industrialprocess

Apart from the need for a high level ofcontinuous power supply in certainindustrial processes, those veryprocesses also impose constraints:s power supply and, in particular,starting of very large motors whichdrive crushers, grinders, fans, pumpsand conveyor belts (a major cause ofvoltage drops during starting);s power supply to arc furnaces, the arcoften being unstable, which causesbrief but repeated unbalanced voltagedrops (resulting in flicker), and cancreate harmonics;s power supply to high capacityelectronic devices (rectifiers, thyristors,etc.) which create major voltage wavedeformations (harmonics) in thenetwork and lower the power factor;this is the case for potlines, DC electricfurnaces and variable speed drives.

In addition, some industrial processes

pollute their environment. They produce

substances (dust, gas) that are

sometimes corrosive and can jam

mechanisms or degrade the

performance of electrical devices

(e.g. dielectric strength) or even cause

explosions in the presence of electric

arcs.

Constraints linked to the electrical

process

During the analysis, it is necessary to

take into consideration various

"electrotechnical" conditions that all

electrical networks must fulfil, in

particular:

s limitation of short-circuit currents and

their durations starting or restarting of large motors

without excessive voltage drops,

s stability of alternators after a

disturbance has occurred.

Constraints imposed by the utility

electrical power distribution network

s short-circuit capacity

The short-circuit capacity of the

upstream network supplying the private

network is a decisive element when

choosing:

ss the structure of the privatedistribution network,ss the maximum load,ss particular loads that are sensitive tovoltage drops.

s utility earthing system

The same type of earthing system, as

used by the utility, is often used for the

private network but it is sometimes not

compatible with particular loads. Insuch cases, it is necessary to create a

separate system with:

ss a type of electrical protection forphase-to-earth faults,

ssa special network operating method(fault tracking and/or clearing by

operating personnel in the case of

isolated neutral systems).

s momentary dips or substantialtransient single-phase or multiphase

voltage drops.

The presence of such phenomena can

cause disturbances, or even production

shutdowns and machine destruction.

These phenomena may result in:

ss operating errors and/or the loss ofdata of the industrial process computer

system (central computers, PLCs), andmanagement control system as well asscientific calculation errors.ss mechanical destruction of motors,motor coupling and/or the drivenmachines. This occurs duringmomentary dips (re-energizing ofmotors that are still in motion), sincecoupling may take place with phaseopposition between the mains voltageand the residual voltage generated bythe motor, this resultat in motorcurrents being very high , i.e. 4 to5 times the rated current, and causeexcessive electrodynamic stress.

s overvoltage of external origin, inparticular lightning strokes (refer to"Cahier Technique" n168).

s value and quality of the supplyvoltage:ss the value of the supply voltagedictates to a certain extent theorganization of private networkvoltages. If the power supply is in HighVoltage, it may be of benefit to use thatvoltage level for the plant's maindistribution system;ss the quality of the supply voltage.Various voltage fluctuations may botheror even prevent production equipmentoperation. The chart in figure 1 showssuch faults, the causes and

consequences, as well as the mainremedies.

Remarks:In terms of supply voltage quality,frequency deviations must bewithin 2% and harmonic voltagesmust be less than 3%.

A major European national utility hasinstalled more than 2000 recordingdevices on the power supply system formajor industrial sites in order to assessthe quality of the electrical power beingsupplied.The devices measure:s rms voltage and current;s active and reactive power,s short and long power outages,s voltage dips,s harmonic voltages and currents,s voltage unbalance.They also detect 175 and 188 Hzremote control signals.

Climatic and geographicalconstraintsIn order to determine the bestspecifications for equipment anddevices according to the types of


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voltage fluctuations a few % 5 to 25% 5 to 25% 100%

duration 1/100 to 1 s 0.5 to 20 s 20 s to 1 hour 0.1 to 0.3 s or 10 to 30 s

periodicity YES NO NO NO

causes presence of electric s starting of large motors absence of voltage fast and/or slow HV linearc furnace s single- or multiphase regulation in primary reclosing system

fault substations

consequences variations in incandescent s risk of network instability s motor overload momentary suspension oflighting (Flicker) s high risk of network power supply to all loads

instability

main remedies use static var s change the starting equip the transformers use shunt circuit breakerscompensation equipment method with on-load tap changers

s increase protectionplan rapidity

fig. 1: voltage fluctuations, causes ,consequences and main remedies

installations, it is necessary to know thefollowing:s average and maximum dailytemperatures,s relative humidity rate at maximumtemperature,s maximum wind speed,s presence of frost, ice, sand-bearingwinds,

s environment (corrosive atmosphere

or risk of explosion),

s altitude,

s frequency of lightning strokes in the

region,

s difficulties in gaining access to the

site (for the transport of materials and

also for maintenance).

Compliance with standards and localpracticesIn this regard, it is especially importantto be familiar with:s national and/or internationalswitchgear and installation standards,sregulations and rules which applyspecifically to the industrial complex,s local practices.


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network. The division of sources (seefig. 3) is used to isolate the fluctuatingloads and it offers two additionaladvantages as well:

s improved discrimination between theprotection devices, thereby increasingcontinuous power supply to the otherloads;s adaptation of the earthing system tosuit the loads.

overall electrical diagramThe network designer uses the variouselements described earlier in thischapter to establish a preliminarystructure, which he then fine-tunes inaccordance with the constraints of theindustrial site to obtain the "overall

single-line diagram" of the plant'selectrical distribution system. This is thestarting point for technical andeconomic network optimization.The next chapter presents the differentchoices that are available and thecalculations that are required to find theoptimal solution.

fig. 3: the division of sources is a means of separating the "fluctuating" loads from the other

loads.

M M M

normal

loads

"sensitive"

loads

"fluctuating" loads

lighting

computers

high power

motors

variable

speed motor


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3. validation and technical and economic optimization

choice of the earthingsystemStandards and regulations make

protection against direct and indirect

contact compulsory for all electrical

installations. In general, protection

functions automatically interrupt the

power supply (upon the first or second

phase-to-earth fault according to the

earthing system used). There are also

special protection devices designed for

specific situations. High Voltagenetwork earthing systems may be

chosen according to the criteria given inappendix 4.

However, it is very often of benefit to

use different earthing systems in the

same industrial network, each of which

provides a predominant advantage.

feeder definitionFeeders account for a large portion of

electrical installation investment. It is

therefore important, for safety and costreasons, to:

s choose the best type of equipment(cables),

s make the most accurate calculations

of the minimum cross section, whiletaking into account short-circuit and

starting currents, voltage drops, losses,

etc.

insulation coordinationanalysisThe coordination of insulation provides

the best technical and commercial

compromise in protecting people and

equipment against the overvoltage that

may occur in electrical installations,

whether such overvoltage originates

from the network or from lightning.

Three types of overvoltage may cause

flashovers and hence insulation faults,

with or without destruction of the

equipment:

s power frequency overvoltage (50 to

500 Hz),

s switching surges,

s atmospheric overvoltage (lightning

strokes).

Insulation coordination contributes to

obtaining greater electrical power

availability. Correct insulation

coordination entails:

s knowing the level of overvoltage that

is liable to occur in the network;

s specifying the desired degree of

performance or, more explicitly, the

acceptable insulation failure rate;

s installing protection devices that aresuited to both the network components

(insulation level) and the types of

overvoltage;

s choosing the various network

components based on their level of

overvoltage withstand, which must

meet the constraints described above.

"Cahier Technique" n 151 describes

voltage disturbances, ways of limiting

them and the measures set forth in the

standards to ensure dependable,

optimized electrical energy distribution,

thanks to insulation coordination.

protection system

definitionWhen a fault appears in an electrical

network, it may be detectedsimultaneously by several protection

devices situated in different areas ofthe network. The aim of selective

tripping is to isolate as quickly aspossible the section of the network that

is affected by the fault, and only that

section, leaving all the fault-free

sections of the network energized. Thenetwork designer must first of all selecta protection system that will enable him

to specify the most suitable protection

devices. He must also create a relay

tripping plan, which consists ofdetermining the appropriate current andtime delay settings to be used to obtain

satisfactory tripping discrimination.

Different discrimination techniques are

currently implemented, which use or

combine various data and variablessuch as current, time, digital

information, geographicalarrangements, etc.

Current discrimination

Used (in Low Voltage) when short-circuit current decreases rapidly as thedistance increases between the source

and the short-circuit point being

examined, or between the supply and

load sides of a transformer.

Time discrimination

Used in Low Voltage and very often in

High Voltage as well. Protection device(circuit breaker) tripping time delays areincreased as the devices becomecloser to the source. In order to

eliminate the effects of transient

currents , the minimum time delay at

the furthest point from the source(directly upstream from the load) is

0.2 s or even 0.1 s if the current settingis high, with a maximum time delay

of 1 s at the starting point of the privatenetwork.

These time delays may be definite time

or short-circuit current dependent.

Discrimination by logical data

transmission, or logical

discrimination

This type of discrimination consists ofthe transmission of a "blocking signal"

of a limited duration, by the firstprotection unit situated directly

upstream of the fault, and which is

supposed to open the circuit, to the

other protection units situated furtherupstream.

This technique, which has been

developed and patented by Merlin

Gerin, is described in detail in "CahierTechnique" n 2.

The tripping time delay is short and

definite, wherever the fault may be

situated in the network. This helpsincrease to the dynamic stability of the

network and minimizes the damagingeffects of faults and thermal stress.

Discrimination using directional or

differential protection

This type of discrimination provides

specific protection of a portion orparticular element of the network.


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Examples: transformers, parallel

cables, loop systems, etc.

Discrimination using distance

protectionThis type consists of splitting the

network into zones. The protectionunits locate the zone area in which the

fault is situated by calculating the circuit

impedance.

This technique is seldom used except

when the private HV network is very

widespread.

An example of a protection system that

uses several discrimination techniques

is given in figure 4.

N.B. The criteria used for selecting

protection systems, relays andprotection settings are not discussed in

this section. A number of other

publications deal with those matters,

e.g. "Cahiers Techniques" n 2 and

158.

short-circuit current

calculationIn order to find the best technical and

economic solution, it is necessary to

know the different short-circuit values

so as to determine:

s making and breaking capacityaccording to the maximum peak and

rms short-circuit current;

s equipment and switchgear resistanceto electrodynamic stress, according to

the maximum peak short-circuit current;

s protection tripping settings in

discrimination analyses according to

the maximum and minimum rms short-

circuit current.

"Cahier Technique" n 158, after

reviewing the physical phenomena to

be taken into consideration, reviews the

calculation methods set forth in thestandards.

When rotating machines (alternators or

motors) are included, the form of short-

circuit current can be broken down into

three portions:

s subtransient,

s transient,

s steady state.

The subtransient and transient stages

are linked to the extinction of the flux

0.5 s

0.5 s

0.2 s

0.2 s

0.2 s

pilot wire

pilot wire

discrimination by data

transmission

time discrimination

current discrimination

discrimination by data

transmission

fig. 4: protection system using several discrimination techniques: current, time and logic.

built up in the synchronous and

asynchronous rotating machines.In both of these stages, it is necessaryto deal with the aspect of an

asymmetrical component, also referredto as a DC component, the damping ofwhich depends on the R/X ratio of theupstream network and the timing of the


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fault with respect to the phase of the

voltage.

In addition, it is necessary to take intoaccount motor contribution to short-

circuit current. When a three-phase

short-circuit occurs, asynchronous

machines are no longer supplied with

power by the network, but the magnetic

flux of the machines cannot suddenly

disappear.

The extinction of this flux creates a

subtransient and then transient current

which intensify the short-circuit current

in the network.

The total short-circuit current is the

vectorial sum of two short-circuit

currents: source short-circuit current

and machine short-circuit current.

calculation of voltagefluctuations under normaland disturbed operatingconditions

Normal operating conditions

The calculation of voltage fluctuations

under normal operating conditions is a

design task which serves to verify thevoltages throughout the network.If the voltages are too low, the network

designer should verify whether:s the active and reactive power flowsare normal,s the feeders are correctly sized,s the transformer power ratings aresufficient,s the reactive power compensationscheme is appropriate,s the correct network structure is beingused.

Disturbed operating conditions

It is necessary to calculate voltagefluctuations under disturbed operating

conditions in order to verify whether thefollowing phenomena will result inexcessive voltage drops or rises:s starting of large motors (see fig. 5),

s downgraded network operation (e.g.2 transformers operating instead of the3 intended for normal operation),s no-load network operation, with orwithout reactive power compensation.Appendix 5 gives the mathematicalexpression and vector diagram of a

network voltage drop.

choice of motor starting

methodThe starting method used (delta-star,

autotransformer, resistors or statorreactors, etc.) must obviously provide

sufficient accelerating torque

(accelerating torque is generally greater

than 0.15 times the rated torque), and,

in addition, must only cause acceptable

voltage drops (< 15%).

Reminder: the equation governing the

interaction of the motor with the driven

mechanism is:

T'm - Tr= Jd

dt

T'm = motor torque when energized atactual supply voltage (Ur),

Tr = braking torque of the drivenmachine,J = inertia of all the driven masses,dw/dt = angular acceleration,

T'm = TmUr

Un

2

Tm = motor torque when energized atrated voltage (Un).Ta = T'm - TrTa = accelerating torque.

1200 kW= 5 nI

4.5 kV4.75 kV Un = 5 kVUn = 5 kV

plant network

motor during

starting phase

57 kVUn = 60 kV

Un = 60 kV

30kmline

I

StartI

StartI

StartI

Start

fig. 5: example of the determination of voltage dips during motor starting for the installation of a

1200 kW motor supplied by a 30 km long, 60 kV line. It should be noted that separation of the

"plant network" power supply and the motor power supply have made it possible to have a

voltage drop of only 250 V in the "plant network".

xx kV = voltage during motor starting


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The curve showing Ta as a function ofspeed is given in figure 6 below.

The chart in figure 7 shows the mostfrequently used starting methods (forfurther details, please refer to "CahierTechnique" n 165).

network dynamic stabilityUnder non-disturbed operatingconditions, all the rotating machines(motors and alternators) included in theinstallation form a stable system withthe utility network.This equilibrium may be affected by aproblem in one of the networks (utilityor private), such as: major loadfluctuation, change in the number of

transformers, lines or supply sources,multiphase fault, etc.This results in either short-livedinstability, in the case of a well-designed network, or a loss of stabilityif the disturbing phenomenon is veryserious or if the network has a lowrecovery capacity (e.g. short-circuitcapacity too low).

Asynchronous motor performance inthe presence of a three-phase fault

Here, as an example, is the analysis ofasynchronous motor performance inthe presence of a three-phase fault(see fig. 8).After the fault has been cleared, twosituations may occur:

s the motor torques are greater than

the braking torques, in which case themotors can reaccelerate and return to

their stable state;

s the motor torques are less than thebraking torques, in which case the

motors continue to slow down, drawinglarge currents which are detected by

the motor and/or network protectiondevices, which trip the related circuit

breakers.

Motor reacceleration, and hencenetwork dynamic stability, are favouredby:s a suitable load shedding plan(tripping of normal, non-essentialmotors in the event of serious faults);

s a powerful network (correctlyregulated alternators and low voltagedrop);

s fast-acting protection systems whichreduce the duration of motor slowdown(e.g. logical discrimination);s a suitable network structure with:

ss separation and regrouping inseparate circuits of normal, non-essential loads and essential andsensitive loads, which facilitates loadshedding,ss minimal impedance connections foressential and sensitive loads so as to

limit voltage drops.The duration of the return to normalmotor speed depends on:

s motor accelerating torque, and hence

voltage drop,

s rotating machine inertia.

Calculation software may be used tosimulate the dynamic performance of

electrical networks, making it easier to

fig. 7: the most commonly used starting methods

T'm = motor torque when energized at theactual supply voltage (Ur),Tr = braking torque of the driven machineTm = motor torque when energized at therated voltage (Un)Ta = accelerating torque

fig. 6: diagram of different motor torquesaccording to speed, with a voltage drop ofabout 15% (hence T'm = 0.7 Tm) andbraking torque TR = 0 during starting.It should be noted that this voltage dropwould not allow the motor to be started witha Tr > 0.6 Tn when started, since Tr > T'm.

0

TTn

torque

1

0

speed ( )

Tm

T'm

Tr

TnTa

0.6

fig. 8: performance of asynchronous motors

in the presence of a three-phase fault

three-phase fault

voltage drop

increase

in current

consumed

by motors

decrease inmotor torque

(proportional tovoltage

squared)

slowing down

of all motors

application application starting advantagesneeds characteristics method drawbacks

continuous or virtually machines that require direct simplicity, reducedcontinuous process strong starting torque investment.starts 1 per day upon starting:

sstrong torque,frequent motors with low inrush direct shigh inrush current,starts > 1 per day current or low power sstrong mechanical stress.

pumps, fans, machines that start with stator- reactor reduction of torque andcompressors, weak torque inrush current upon startingfrequent starts (adjustment possible).

optimization of when starting current stator - optimization of torquestarting needs to be reduced autotransformer (reduced) and inrush currentcharacteristics while maintaining the upon starting (adjustment

torque needed for possible).starting

optimization of strong the most difficult starts rotor low inrush current andtorque starting strong starting torque.characteristics


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make the correct choices, in particular,when establishing the procedures to beused to separate power supply sources(creation of "isolated systems"), non-

essential load shedding plans andprotection systems (logicaldiscrimination for very short trippingtimes). All of these factors contribute tomaintaining the dynamic stability of thenetwork when disturbances occur.

To summarize:

If the disturbance is minor (two-phase

short-circuit far from the load), stability

is restored by speed and voltage

regulators. When there is a high risk of

instability, it is necessary to include a

protection device that will clear the fault

within a very short time (0.2 to 0.3 s)

and/or a device to divide the network so

as to sustain it (load shedding) and

avoid the risk of a complete installation

shutdown (see fig. 9).

synthesisAll of the information presented in this

chapter is summarized in a logic

diagram in appendix 6.

M1 M2 M3 M4 M5

shed circuit

G(rad/s)

300

250

0.15 1 2 3 t (s)

alternator

motor M1, with load shedding

motor M1, without load shedding

clearing of the short-circuit

G

fig. 9: presentation of the dynamic performance of a network with and without load shedding


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Different network structures are

possible, the most common of which

are described in this chapter together

with the main areas in which they are

used.

The choice of a network structure,

which is always a decisive factor in

terms of energy availability, is often a

difficult one to make.

The most rational method consists ofmaking a quick comparison of the

unavailability of voltage at a particular

point in the network for differentstructures and using a very interesting

expert system (see appendix 2).

standard network

structures

Open or closed loop, also called

"primary loop system"

(see fig. 10)

Recommended for very widespreadnetworks, with major future extensions.

Open loop operation is advisable.

Double radial feeder, also called

"primary selective system" (manualor automatic)

(see fig. 11)

Recommended for very widespread

networks with limited future extensions

and which require a high level of

continuous power supply.

Radial feeder, also called "single

power supply"(see fig. 12 on the opposite page)

Recommended when continuous power

supply requirements are limited. It is

often used for cement plant networks.

Dual power supply

(see fig. 13 on the opposite page)

Recommended when a high level of

continuous power supply is required or

when the operating and maintenance

teams are small.

It is very often used in the steel and

petrochemical industries.

4. choice of optimal network structure andoperation

fig. 10: diagram of a primary loop.

fig. 11: diagram of a primary selective network.

HV

LVLVLV

HV

LV

LV

LV


15/24


G

fig. 15: diagram of a network with local generation.

mainsource

replacementsource

essential

loads

(network with backup)

non-essential

loads

G

fig. 16: diagram of a network with a replacement source and load shedding.

With replacement source and loadshedding(see fig. 16)This is the typical case for an industrial

network in which a very high level ofcontinuous power supply is requiredusing a single power supply source,i.e. the utility.

LV

HV

subdistribution

board

LV

HV

fig. 12: diagram of a single power supply

network.

HV

HV

LV

fig. 13: diagram of a dual power supply

network.

Dual busbar(see fig. 14)Recommended when a very high levelof continuous power supply is required

or when there are very strong load

fluctuations. The loads may be

distributed between the two busbars

without any break in the power supply.

With energy generating sets(see fig. 15)

This is the simplest structure, and is

very often used.

fig. 14: diagram of a dual busbar network.

feeders

incomers

busbar B

busbar A


16/24


concrete example of astructureThe diagram in figure 17 was designed

and developed for a multi-metal mine inMorocco. It includes different networkstructures that were described earlier.The power supply to the variousworkshops is provided by a loop, aduplicate feeder, or a main source andreplacement source.

choice of equipmentWhatever the structure that is used, theequipment must comply with:s standards in effect;s network characteristics:

ss rated voltage and current,ss short-circuit current (making andbreaking capacity, electrodynamic andthermal withstand);s the required functions (fault breaking,breaking in normal operation, frequencycontrol, circuit isolation, etc );s continuous power supplyrequirements (fixed, disconnectable,withdrawable switchgear);s operating and maintenance staffqualifications (interlocks, varyingdegrees of automatic control, breakingtechnique requiring maintenance or

maintenance-free);s requirements related to maintenanceand possible extensions (extra spacefor future use, modular system, etc.).

N.B. At this stage of the analysis, allthe network characteristics result fromcalculations that are often carried outusing scientific calculation software.

optimal operationOptimal electrical distribution systemoperation means finding:s the best level of continuous power

supply,s minimum energy consumption cost,s optimization of the operating andmaintenance means which contribute tocorrect network operation, both steadystate and transient, and in the presenceof a fault.The solution lies in the implementationof an electrical Energy ManagementSystem for the entire network.

The Energy Management Systems thatare currently offered by manufacturers,such as Merlin Gerin, make full use ofmicroprocessor performance. These

fig. 17: structure of the electrical network of a Moroccan mine (Merlin Gerin).

G

crushingpit head

63 kV

10 MVA2 MVA10 MVA

replacement source

5 kVLV LV

20 kV

level - 300

pit n 1

5 kV

treatment grinders

level - 500

pit n 2


17/24


components, integrated in local andremote management centres, and inthe protection and control gear installedat the actual energy consumption sites,

are at the origin of the concept of"decentralized intelligence". The term"decentralized intelligence" means thatthe control centres and devicesautonomously carry out theirassignments at their levels (without anyhuman intervention) and do not callupon the "upper" level except whenfailures occur. The remote monitoringand control system continuouslyinforms the network manager or user ofchanges.This explains the importance of clearlyspecifying the network architecture.

Description of an Energy

Management System

(see fig. 18)

Energy Management Systems are set

up in four levels:

s level 0: sensors (position, electrical

variables, etc.) and actuators (trip units,

coils, etc.);

s level 1: protection and control units,

e.g. HV cubicle;

s level 2: local control, e.g. a HV/LV

substation of a plant or LV switchboard

in a workshop:

s level 3: remote control of an entire

private network.

All this equipment, particularly levels 1

to 3, is linked by digital communication

buses (networks via which theinformation is conveyed).

Assignments carried out by EnergyManagement Systems

s managing energy supply andconsumption according to:ss subscribed power,ss utility billing rates,ss availability of the private generatingstation,ss industrial process requirements.s maintaining continuous power supplythrough:ss quick, discriminating protection (e.g.logical discrimination system),ss automatic power supply sourcechangeover,

fig. 18: example of Energy Management System architecture.

level 3

remote control of the entire

private electrical network

(remote control and

monitoring system and

data loggers)

level 2

local management of

"plant-substation" networks

(network control centre)

level 1

protection and control units(HV cubicles, workshop main

LV switchboards)

level 0

sensors and actuators

(trip units, limit switches,

instrument transformers, etc.)

control-monitoring station

n1

control-monitoring station

n2

feeder 2: trigged, ground faulter 2: Overload 150A, Load shedding

I 1 = 136 A

Welding works branch: intensity 32A

I 1 = 96 ADef A1


18/24


ss efficient load shedding/reconnection,the parameters of which can be set viathe man/machine interface with theestablishment of load shedding criteria

(load shedding/reconnection plan);ss sequential workshop restarting,ss adjustment of voltage, power factor,etc.ss maintaining power supply toessential loads during outages of theutilily supply the local alternators.

s enabling man/machine dialogue:ss real time display of network andequipment status via mimic diagrams(single-line diagrams, detail diagrams,curves, etc.);

ss remote control of switching devices;ss

data and measurement logging,ss chronological recording of faults andalarms (10 ms),ss filing of events,ss metering, statistics,ss archiving.

All this data is used, in particular, toorganize preventive maintenance.

s drawing up "User Guide" procedureswhich, for example:ss prohibit the starting of particularmotors according to the poweravailable from the generating station,the time, or the degree of priority of themotors,ss prohibiting the use of particular HighVoltage switchboard power supplyconfigurations (paralling of supplies),ss proposing the most suitable backuparrangement for serious faults in a mainfeeder or a generator,ss proposing operating instructions andmaintenance operations (electrical,mechanical, etc.).

Advantages of an EnergyManagement System

The development of level 1 digitalprotection and control systems and therapid rise in the performance/cost ratio

of level 2 hardware and softwareprovide industrialists with technical andeconomic benefits, in particular:

s increased operating dependability;

s a broader range of accessiblefunctions, especially data logging,preventive maintenance and remotecontrol,

s easier commissioning and more

efficient operation.

The richness of the functions that areoffered by these systems, new

possibilities for self-testing or even self-

diagnosis and monitoring, as well as

the user-friendliness of the userdialogue interfaces, naturally make the

facilities manager's role more effective

and interesting. The facilities manageris better able to assess the operation ofhis network and to optimize, apart fromremote monitoring and control,maintenance and renewal of hiselectrical equipment.

investment, operating costs andproduction losses.

The best operating conditions provide a

level of continuous power supply to

loads which is compatible with

installation requirements, in the aim of

obtaining maximum productivity and

maximum safety of people andproperty.

The new generations of electricalswitchgear and equipment aredesigned to communicate, via digitalcommunication buses, with one ormore control centres. And it is thecombination of both networks, theenergy network and the informationnetwork, at an acceptable investmentcost, which provides optimal fulfilmentof users' needs.

Well-mastered electrical networkdesign makes it possible to ensureoptimal operation under normal and

disturbed network operating conditions.

The best cost does not necessarilymean the minimum initial investment,

but rather the design of an electrical

network which proves to be the most

economical from the viewpoint of initial

5. conclusion


19/24


The extension of an existing industrialnetwork by adding a transformer thatcan be connected parallel to theexisting transformer has the drawbackof increasing short-circuit currentstrength and also of making itnecessary to increase:s the breaking and making capacity ofthe existing devices,sthe old installation's resistance toelectrodynamic stress.

The installation of a three-phasereactor between the old and newfacilities eliminates these difficulties(see fig. 19).

hypothesess short-circuit current of the existinginstallation: 17 kA (Isc1),s short-circuit current in the existingbusbar must be limited to 21 kA (Isc2),XTr = 0.63 .

approximate calculation ofa current limiting reactor(resistors neglected)

The current flowing through the reactorshould be equal, in the firstapproximation, to:

fig. 19: extension of an existing industrial network by addition of an extra transformer.

existing installation new installation

new

20 MVA

transformer

Usc = 12%

36 MVA

Ucc = 12%

17 kA

Isc 1 X = 0.81

4 kA21 kA

limiting

reactor

10 kV 10 kV

appendix 1: extension of an existingindustrial network

21 - 17 = 4 kA = IscL

scL = current limited by the reactor

=V

X

X = total reactance (20 MVAtransformer and limiting reactor)

X = VIscL

= 10,000

3 x 4,000= 1.44

X = Xreact + XTrXTr = 0.63X react = 1.44 - 0.63X react = 0.81


20/24


Here is a list of the main softwareprograms used in Merlin Gerin'sdifferent departments that areresponsible for analyzing and/ordesigning electrical networks.

calculation softwareprogramss load flow study,s short-circuit currents,s voltage drops,

s network dynamic stability,

s harmonic currents and voltages,s lightning and switching voltagesurges,s transformer and capacitor switching,s unavailability of electrical powersupply.

expert system forevaluating electricalnetwork design qualityAn expert system called ADELIA has

been developed and is used by

Merlin Gerin. It is used to make a quick

comparisons of the unavailability of

voltage at a particular point in the

network using distribution schemes.

It offers the advantage of requiring

fewer calculations than the Markov

graph method, and also of providing

both qualitative information (graph of

combinations of events which lead to

system failures), and quantitative

results (network unavailability

calculations).

The principle of compensation usingcapacitors may be illustrated by the twofigures opposite.

s figure 20 shows the vectorialcomposition of the different currentsand for a given active current, thereduction in the total current in theconductors.

Ia = active current consumedIt1 = total current prior to compensationIr1 = reactive current supplied via thetransformer prior to compensation

It2 = total current after compensation

Irc = reactive current supplied by the

capacitorIr2 = reactive current supplied by thetransformer after compensation

(Ir2 = Ir1 - Irc)

s figure 21 illustrates the localexchange of reactive power whichtakes place between the load and thecapacitor. The total current supplied bythe network It2 is reduced and the

appendix 2: computerized means used fornetwork analyses

appendix 3: general principle ofcompensation

fig. 20: phasor diagram of the different

currents and the effect of compensation.

fig. 21: diagram depicting the energy

exchange in a consumer circuit and the

advantage of compensation.

installlation output is thereby improvedsince losses due to the Joule effect areproportional to the square of thecurrent.

It1

Irc

It2

Ir1

Ir2

Ia1 2

before

compensation

after

compensation

increased

available

power

reactive

power

output by the

transformer

reactive

power

supplied by

the capacitor

active

power


21/24


The choice of the earthing system for aHigh Voltage industrial networkinvolves the following criteria:s general policy,s legislation in effect,s constraints related to the network,s constraints related to networkoperation,

s constraints related to the type ofloads,s etc.Five diagrams may be considered:s directly earthed neutral,s reactance- earthed neutral,s resonant- earthed neutral,s resistance- earthed neutral,

s isolated neutral.

Each of them offers advantages and

drawbacks with which the network

designer should be familiar before

making his final choice. They are listed

in the chart below (see fig. 22).

appendix 4: choice of the earthing systemfor a HV industrial network

directly earthed neutral advantages drawbacks in practicediagram

directly earthed neutral sfacilitates earth fault causes high earth fault currents not useddetection and protection (dangerous for personnel and riskdiscrimination, of major equipment damage)slimits overvoltage

reactance-earthed limits earth fault currents srequires more complex protection applicable without any particularthan direct earthing, precautions only if the limitingsmay cause severe overvoltage, impedance is low with respect todepending on installation configurations the circuit's earth fault resistance

resonance-earthed is conducive to self-extinction requires complex protection ssometimes used in Eastern(Petersen coil) of earth fault current (directional devices that are difficult countries

to implement) snot used in France

resistance-earthed slimits earth fault currents, the most beneficial for industrialsfacilitates earth fault current distribution: it combines all thedetection and protection advantages

discrimination,slimits overvoltage

isolated from earth limits earth fault currents srisk of overvoltage no tripping when the first earth faultsrequires the use of over-insulated occurs entails:equipment (line voltage between phase s being granted specialand earth when a zero impedance dispensation (French law)fault occurs), s that the capacitance betweens overvoltage protection advisable, active network conductors ands insulation monitoring obligatory earth do not cause earth fault(French law), current that is dangerous fors complex discrimination between personnel and machines.earth fault protection devices

fig. 22: advantages and drawbacks of the different earthing diagrams used for a HV industrial network.


22/24


fig. 23: phasor diagram of network voltage drop

Voltage drops in a network may becalculated using the followingmathematical expression:V = R I cos + X I sin The electrical diagram and vectordiagram which correspond to thisequation are given in figure 23.

V

F

ECD

B

A

I

Va

Vd

XI

RI

RL

load

VaVd

source

I

cable

AD = RI cos

DE = BC sin + CF sin

DE = BF sin = XI sin

This logic diagram comprises twoloops:s the first one, the analysis and

selection loop, starts at "Needs andconstraints to be met" and leads to the

appendix 6: stages in industrial networkdesign

organization of a network structure inan "Overall single-line diagram";s the second is aimed at optimization

of the structure.

analysis and selection loopleading to the organization ofa structure

structure optimization loop

appendix 5: network voltage drops(mathematical expression and vector diagram)


23/24


Needs andconstraints to

be met

NO

Major needsmet and/orconstraints

mastered

load survey$ active$ reactivebased on$ load factors$ diversity factors

Choice of the motor starting methodThis depends on:$ permissible network voltage drop$ braking torques and motors$ inertia of rotating machines

N.B. The maximum voltage drop and starting timevalues currently encountered are:$ voltage drops = 10 to 15% of rated voltage$ starting times: pumps = 0.5 to 2 s grinders = 5 to 10 s conveyor belts = 5 to 30 s fans = 10 to 200 s

Choice of voltagesAccording to:$ the function to be performed: transmission distribution consumption$ power available and powerto be supplied$the distance between sourcesand loads$ local practices and habits

Reactive energy compensationUsing the following main criteriafor choosing the type ofcompensation:$ feeder length and crosssection$ desired increase in availableactive power

Backup sources$ standards$ laws

Replacement sources$ maintaining of production facilitiesoperation$ power supply convenience

Autonomous electrical powerproductionIf the industrialist:$ has available vapour produced

by the process$ has fuel available at a low cost$ is unable to obtain a strong orreliable utility supply

Division of sourcesTo delimit areas disturbed by:$ starting of large motors$ power electronic systems$ etc.

Calculation of voltage fluctuations under normaland disturbed operating conditions

Short-circuit current calculation

Protection system definition

Feeder definitionMain criteria to be used:$ steady state current (or equivalent)$ installation method$ exposure to sun$ ground temperature$ air temperature$ ground conductivity$ permissible voltage drop under normal and disturbedoperating conditions$ short-circuit current withstand$

cable screen withstand to single-phase and/ormultiphase faults$ contact voltage$ etc.

Insulation coordination analysis

Choice of earthing systemsamong the following possible diagrams:$ isolated neutral$ directly earthed neutral$ neutral earthed via a reactor or Petersen extinction coil$ neutral earthed via a resistor

YES

Overallsingle-line

diagram


24/24

Merlin Gerin Cahiers Techniques

s Protection of electrical distributionnetworks by the logic selectivity systemCahier Technique n 2F. SAUTRIAU

s Mise la terre du neutre dans unrseau industriel haute tensionCahier Technique n 62F. SAUTRIAU

s Gestion de l'nergie dans lesprocessus industriels

Cahier Technique n 133C.G. POUZOLS

s Surtensions et coordination del'isolement,Cahier Technique n 151D. FULCHRION

s Calcul des courants de court-circuit,Cahier Technique n 158R. CALVAS, B. DE METZ NOBLAT,A. DUCLUZAUX and G. THOMASSET

s Lightning and HV electricalinstallations,Cahier Technique n 168B. DE METZ NOBLAT

Miscellaneous publications

s Overvoltage when clearing short-circuits from networks with a reactance-earthed neutral system.Bulletin de la socit Franaise desElectriciens, series 8, Volume 1, n 4(April 1960)LE VERRE

appendix 7: bibliography

Documents

HV industrial network design