3
Control of hydro-electric plant E.G. Davidson Indexing terms: Natural resources, Control systems, Power systems and plant Abstract: The paper briefly details the evolution of control systems as applied to hydro-electric plant in the North of Scotland Hydro-Electric Board area over the past 20 years. Automatic control is described stressing the basic simplicity of hydro-electric plant. Remote control is treated in some depth with particular emphasis being placed on features peculiar to hydro stations. These include water level monitoring, dam gate controls and the various means of achieving communication channels. It also explains how display techniques have changed due mainly to the increasing use of VDUs. The paper also looks ahead to the types of systems which will be developed and implemented in the next 10 years. It pays particular attention to the greatly improved presen- tation methods that are becoming increasingly available to the operators in the control centres. 1 Background The geographical location with stations sharing the catch- ment area of a single river and its tributaries, and the sim- plicity of hydro-electric plant lends itself to remote control from a group control centre. Remote control of hydro-electric plant, although involv- ing basically the same problems as the remote control of any other part of the electricity supply network, has certain requirements which are not found in the control substations. In the control of a substation almost all the devices which are controlled ranging from the circuit breakers to the auto-reclose equipment to protection switching, are two-state digital devices (namely open/close, off/on etc.). The exception to this is transformer tap changers which are multi-position digital devices. The hydro-electric generator is an analog device whose output of both real and reactive power is continuously variable. For a long time this was one of the major prob- lems associated with the remote control of hydro-electric generators. Up to about 1960, virtually all hydro stations were either manned or controlled by direct wire from an adjac- ent manned station or control centre. Some of these sta- tions where a full shift rota was not employed, had telemetering systems which sent back alarm indication and metering information to the Control Centre. Four main problems prevented the implementation of successful remote control. First, equipment at this time was electromechanical (using relays and uniselectors) and the reliability was gen- erally poor, especially when subjected to the high uti- lisation required for the control of hydro generating plant compared with the control of substations. In a substation there may, on average, be one controlled circuit breaker operation per week. In a hydro-electric generating station on the other hand, there could be as many as 20 starts and stops per day plus a much greater number of load and voltage adjustments. Secondly, the systems that were available operated slowly with quite considerable delays between sending an instruction and it being executed at the outstation. This made load and voltage adjustment almost impossible, as these signals had to be sent as a series of pulses with each pulse being of a fixed time duration (generally of the order of two seconds). Thirdly, the techniques used for transmitting meter Paper 4398C, (P10, Pll), first received 2nd October 1984 and in revised form 3rd December 1985 The author is with Central Technical Services, North of Scotland Hydro-Electric Board, Burghmuir, Perth PHI 1QE, United Kingdom readings were unsatisfactory. Each telemetered quantity was represented by a pulse rate, with the quantity being measured and the displayed reading being proportional to the pulse rate. Apart from the poor accuracy, the main problem with this type of system is the inherent time delay, with readings generally having time constants in the range 30-90 seconds. Stations that were controlled using electromechanical systems had to use what were non-standard techniques to overcome the latter two problems. Load and voltage con- trols used separate signalling channels after the particular function that was to be controlled had been selected in the normal way, and high speed telemetering with each meter way having a separate 50 baud signalling channel was employed. The number of facilities handled by these systems was extremely limited (see Fig. 1). 1960 1973 1982 Controls Indications Alarms Meter readings Integrated readings Check addresses 6 4 4 2 12 15 32 7 12 23 53 8 2 2 Fig. 1 Remote control facilities: single generator station The advent of the transistor used as a digital switch completely changed the situation. Equipment could now be continuously scanning, controls could be made to operate as if they were direct wire and meter readings could be transmitted accurately and quickly. The fourth problem was the fact that these stations gen- erally had no automatic control equipment (i.e. to run-up and synchronise or shutdown the plant). An automatic run-up and shutdown sequence is a prerequisite for remote control. 2 Automatic control The simplicity of hydro plant gives rise to an automatic sequence that is straightforward and which can be solved using a variety of methods. The automatic sequence for the starting operation comprises the preparation stage, actual running-up and synchronising. The control system now generally employed is the sequential/parallel system with as many operations as pos- sible being initiated in parallel in each stage. In a typical sequence for the control of a hydro-electric generator there are very few auxiliaries to be controlled; in fact, the largest motor in most stations is the crane motor. Early automatic control sequences were electro- mechanical (i.e. relay logic) and did no more than run-up or shutdown the machine after being activated by the IEE PROCEEDINGS, Vol. 133, Pt. C, No. 3, APRIL 1986 145

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Page 1: Control of hydro-electric plant

Control of hydro-electric plantE.G. Davidson

Indexing terms: Natural resources, Control systems, Power systems and plant

Abstract: The paper briefly details the evolution of control systems as applied to hydro-electric plant in theNorth of Scotland Hydro-Electric Board area over the past 20 years. Automatic control is described stressingthe basic simplicity of hydro-electric plant. Remote control is treated in some depth with particular emphasisbeing placed on features peculiar to hydro stations. These include water level monitoring, dam gate controls andthe various means of achieving communication channels. It also explains how display techniques have changeddue mainly to the increasing use of VDUs. The paper also looks ahead to the types of systems which will bedeveloped and implemented in the next 10 years. It pays particular attention to the greatly improved presen-tation methods that are becoming increasingly available to the operators in the control centres.

1 Background

The geographical location with stations sharing the catch-ment area of a single river and its tributaries, and the sim-plicity of hydro-electric plant lends itself to remote controlfrom a group control centre.

Remote control of hydro-electric plant, although involv-ing basically the same problems as the remote control ofany other part of the electricity supply network, hascertain requirements which are not found in the controlsubstations.

In the control of a substation almost all the deviceswhich are controlled ranging from the circuit breakers tothe auto-reclose equipment to protection switching, aretwo-state digital devices (namely open/close, off/on etc.).The exception to this is transformer tap changers whichare multi-position digital devices.

The hydro-electric generator is an analog device whoseoutput of both real and reactive power is continuouslyvariable. For a long time this was one of the major prob-lems associated with the remote control of hydro-electricgenerators.

Up to about 1960, virtually all hydro stations wereeither manned or controlled by direct wire from an adjac-ent manned station or control centre. Some of these sta-tions where a full shift rota was not employed, hadtelemetering systems which sent back alarm indication andmetering information to the Control Centre.

Four main problems prevented the implementation ofsuccessful remote control.

First, equipment at this time was electromechanical(using relays and uniselectors) and the reliability was gen-erally poor, especially when subjected to the high uti-lisation required for the control of hydro generating plantcompared with the control of substations. In a substationthere may, on average, be one controlled circuit breakeroperation per week. In a hydro-electric generating stationon the other hand, there could be as many as 20 starts andstops per day plus a much greater number of load andvoltage adjustments.

Secondly, the systems that were available operatedslowly with quite considerable delays between sending aninstruction and it being executed at the outstation. Thismade load and voltage adjustment almost impossible, asthese signals had to be sent as a series of pulses with eachpulse being of a fixed time duration (generally of the orderof two seconds).

Thirdly, the techniques used for transmitting meter

Paper 4398C, (P10, Pl l ) , first received 2nd October 1984 and in revised form 3rdDecember 1985

The author is with Central Technical Services, North of Scotland Hydro-ElectricBoard, Burghmuir, Perth PHI 1QE, United Kingdom

readings were unsatisfactory. Each telemetered quantitywas represented by a pulse rate, with the quantity beingmeasured and the displayed reading being proportional tothe pulse rate. Apart from the poor accuracy, the mainproblem with this type of system is the inherent time delay,with readings generally having time constants in the range30-90 seconds.

Stations that were controlled using electromechanicalsystems had to use what were non-standard techniques toovercome the latter two problems. Load and voltage con-trols used separate signalling channels after the particularfunction that was to be controlled had been selected in thenormal way, and high speed telemetering with each meterway having a separate 50 baud signalling channel wasemployed. The number of facilities handled by thesesystems was extremely limited (see Fig. 1).

1960 1973 1982

ControlsIndicationsAlarmsMeter readingsIntegrated readingsCheck addresses

6442

——

1215327

——

122353822

Fig. 1 Remote control facilities: single generator station

The advent of the transistor used as a digital switchcompletely changed the situation. Equipment could nowbe continuously scanning, controls could be made tooperate as if they were direct wire and meter readingscould be transmitted accurately and quickly.

The fourth problem was the fact that these stations gen-erally had no automatic control equipment (i.e. to run-upand synchronise or shutdown the plant). An automaticrun-up and shutdown sequence is a prerequisite for remotecontrol.

2 Automatic control

The simplicity of hydro plant gives rise to an automaticsequence that is straightforward and which can be solvedusing a variety of methods. The automatic sequence for thestarting operation comprises the preparation stage, actualrunning-up and synchronising.

The control system now generally employed is thesequential/parallel system with as many operations as pos-sible being initiated in parallel in each stage. In a typicalsequence for the control of a hydro-electric generator thereare very few auxiliaries to be controlled; in fact, the largestmotor in most stations is the crane motor.

Early automatic control sequences were electro-mechanical (i.e. relay logic) and did no more than run-upor shutdown the machine after being activated by the

IEE PROCEEDINGS, Vol. 133, Pt. C, No. 3, APRIL 1986 145

Page 2: Control of hydro-electric plant

appropriate control signal. In these early schemes theoperator had no idea on a failure-to-start whether heshould try again or immediately call for assistance. Thisshortcoming was partly solved in later schemes by includ-ing a sequence monitor. This was usually of an extremelysimple nature giving an indication as each step in thesequence was completed.

The logical follow on from the electromechanicalrun-up sequence was the solid state system using eitherdiscreet components or integrated circuits. This type ofsystem was not used greatly as there were no obvious sig-nificant advantages and it generally required a relay (orfilter or driver) on each input and output.

Modern auto run-up systems make use of program-mable logic controllers (PLC) which are microprocessorbased. The microprocessor based system is much moreflexible and is able to use the same hardware for differentapplications. It can also include complex run-up monitor-ing with time tagging included.

3 Remote control

In 1968 the North of Scotland Hydro-Electric Board(NSHEB) had only eight remotely controlled hydro sta-tions and of these eight 75% used systems which, by thestandards of today, were inadequate; the remainder usedsystems that were totally inadequate. Now all the majorhydro stations and a lot of the minor ones are either con-trolled from a manned point or remotely controlled, themajority using a standardised system developed by theBoard's staff. The Board now has 37 stations remotelycontrolled (31 of them using the Board's own system)ranging from a 2 MW conventional hydro station to a300 MW pumped storage station. Typical facilities for astation with remote control equipment installed in themid-seventies are given in Fig. 1.

The decision to design its own telecontrol system wastaken by the NSHEB in the late sixties. At that time theBoard had telecontrol equipment installed or on orderfrom three different manufacturers and using three differ-ent types of technology. This caused great training andfamiliarisation problems for the maintenance staff and thisfactor, coupled with difficulties in purchasing suitablesystems to cover the wide variety of stations, convinced theBoard to go ahead with the development of its SLC256system. This system was designed to cater for all the appli-cations that were foreseen at the time.

The SLC256 system has been, and in fact still is, reli-able, secure in operation, with a high availability. Themaintenance staff have found it easy to maintain andmodify and the ease of use by the control operators has ledto very few 'operator errors'.

Like most remote control equipments in service it usesan address-reply cyclic scanning type of system. Alarmsand most indications are transmitted as single bits of infor-mation. Meter readings use transducers to convert theprimary quantities to a standard 0-10 mA DC input. Thisinput is converted to digital form, usually of eight bits, bythe ADC (analog to digital converter) and transmitted inthis way. The resolution that can be attained is dependenton the number of bits and the type of coding arrangement.For example, eight bits in pure binary form gives aresolution of 1 in 256, whereas eight bits in BCD (binarycoded decimal) gives a resolution of 1 in 100. The accuracydepends on a number of factors, the number of bits usedbeing only one of them.

The electrical quantities that are monitored for eachmachine normally consist of megawatts, megavars and

146

stator and rotor currents. This allows a machine to be runin the event of a failure of one reading.

Apart from the electrical quantities (i.e. megawatts,megavars etc.) the main metered quantities are waterlevels. In stations which are part of, or close to, theirreservoir, the output from the transducer can be feddirectly into the telecontrol outstation. Most of the trans-ducers used are immersed in a still well (in the water) andmeasure pressure using either the strain gauge principle ora bellows arrangement which alters the air gap in a trans-former.

Where the water level to be monitored is remote fromthe telecontrol system the problem is more difficult andsome form of isolated (or barriered) system has to be usedto protect both the transmitting equipment and the tele-control outstation from damage by lightning. One type ofsystem is the frequency system shown in Fig. 2.

telecontroloutstation

lightningprotector

barriertransformer

transducer

Fig. 2 Water level transmission equipment

In a number of stations the control of flood gates in thedams is an important item, and they are at present con-trolled manually from the control centre. The controlincorporates circuitry which only allows them to open insteps normally of the order of 150-300 mm but they canbe stopped at any intermediate position.

Up to the mid 1970s in the control rooms, all the con-trols indications, alarms and metering for the remotely

Fig. 3 Fort Augustus control room

IEE PROCEEDINGS, Vol. 133, Pt. C, No. 3, APRIL 1986

Page 3: Control of hydro-electric plant

controlled stations were presented on a control desk and amimic diagram. Conventional display techniques wereused and a typical control room showing the desk anddiagram is shown in Fig. 3. Even where set point loadcontrol was used it was operated from the control desk.

Over the last ten years there has been a gradual butcontinual move to use VDUs and computer based displayand control equipment. At present there are a number ofhybrid systems using a conventional control panel withsomewhat reduced facilities but using the VDU to handleall the alarms. The use of this type of system has allowedan increase in the numbers of facilities that can be handled(see Fig. 1). It has also, in some cases, had the effect ofsimplifying the outstation equipment by allowing alarmssuch as 'head loss' to be computed by the control centrecomputer instead of using hardware at the outstation. Atypical VDU display is shown in Fig. 4.

oriented or with full graphic capabilities, will be employedand the control desk and mimic diagram as it exists todaywill eventually disappear. It will be replaced by a muchsimpler mimic diagram giving an overview of the systemand the control functions will be carried out from thespecial function keyboards associated with each operatingposition. Display formats will have to be carefully con-sidered and facilities such as megawatts will be displayedin some form of analog-digital form in order that the oper-ator can quickly glean the same amount of informationthat is presently available to him from an analog meter.

Computer processing of functions comes under threeheadings: real time, extended real time and offline func-tions. The number of real time functions (i.e. megawatts,water levels, alarms, indications etc.) available to the oper-ator will not vary greatly, but there will be a great increasein the provision of extended real time functions. These will

Generation

CashlieLubreochLochayLochayLochaySt. FillansDalchonzieLednockFinlarigTotal

G1G1G1G2G3G1G1G1G1

closedclosedclosedclosedopenclosedclosedclosedclosed

9.12.3

19.120.10.0

20.33.01.0

21.796.6

MWMWMWMWMWMWMWMWMWMW

-0.2

-1.6+0.5

+0.0

+0.3

MVAr

MVArMVAr

MVAr

MVAr

1345.01099.8

954.0

1142.0315.7

1390.41614.0

ftftft

ftftftft

Fig. 4 Typical VDU display

The computer based VDU display used as an additionto the conventional control desk and mimic diagram, hasthree main advantages. First, it can give an alarm whenany analog quantity goes over or under a preset limit. Thislimit can be easily changed by the operator to takeaccount of changing operational conditions. Secondly, itcan inhibit alarms when they are either operating erron-eously or when testing is being carried out. Thirdly, it cangive a log of all events, thus freeing the operator from thistime consuming task which is often required when he hasmore important duties to attend to.

As many hydro stations are situated in very remotelocations, this often creates a communication problem. Itis difficult to maintain a high level of reliability in tele-phone lines, when they are available, and when the circuitsare overhead they are extremely prone to damage bylightning. Any type of radio is often difficult, if not impos-sible, to install and maintain, and the only option is powerline carrier, which tends to be a robust flexible communi-cations medium. Although it is susceptible to interruptionduring work on the power line, this is not usually aproblem in hydro station applications, as planned outagesof the lines will probably coincide with station main-tenance. For this application power line carrier is used inthis country at 11, 33 and 132 kV with both phase-to-phase and phase-to-earth coupling being employed.

4 Future trends

The future is going to see a considerable increase in thenumbers and the importance of computer based VDU dis-plays in the control centres. Colour VDUs, either symbol

include water optimisation, flood control, post faultanalysis and analysis of failures to start.

Functions will be distributed with some intelligent pro-cessing being carried out at the outstations. The use of setpoint controllers either as part of electronic governors orwith the existing governors, will allow much more rapidand efficient loading of the machines. A typical powerstation installation is shown in Fig. 5.

floodcontrol

autorun-up

set pointcontroller

alarmsystem

eventlogger

telecontrol

Fig. 5 Distributed control system

The new telecontrol outstation will use more efficientcodes with data reduction techniques being employed toenable the most important information to be transmittedmore quickly. Time tagged information will also be trans-mitted from the outstation but with a lower priority.

One result of these extra aids and facilities will be theability to control more stations and generators from acontrol centre, enabling more efficient use of staff andmore secure and economic operation of the stations.

IEE PROCEEDINGS, Vol. 133, Pt. C, No. 3, APRIL 1986 147