Load management for power-system emergencies

W.R. Lachs

Indexing terms: Load and voltage regulation, Power system protection, Batteries and cells

Abstract: A new approach to load management isoutlined, having as a major objective, provisionfor the emergency operation of a power system.This load-management proposal would not incon-venience consumers, yet would be accessible tooperators at any time. In an emergency, thisimmediate load control would greatly enhance thepotential for safeguarding the system. This load-management approach extends to individual con-sumers within distribution networks. Batteryinstallations at selected consumers would back-upportions of the load under local microprocessorcontrol. This control could be provided by a cur-rently available credit and load-management unit(CALMU), which, with its remote interrogationfacility, is already cost competitive with conven-tional metering. The proposal has been conceivedas an important component of a system-protection arrangement which would be capableof countering calamitous disturbances on majorpower systems. It has also the potential to sub-stantially improve distribution-network security.The stored energy of the battery installationscould extend the use of this proposal for normaloperations. Not only would it reduce the need forpeaking generators and improve the system capac-ity factor, but it would facilitate the introductionof renewable and alternative generation. Only thenormal operational benefits would allow tariffincentives encouraging consumers to meet thecapital costs of battery installations.

With this policy, the proposal would allow elec-tric utilities, with a modest capital outlay, to sig-nificantly raise security levels of their total powergrid.

1 Introduction

Existing load-management measures have been devel-oped to only meet limited numbers of objectives [4]. Onthe other hand, to meet all operational requirements,controllable loads should have the following features:

(a) could be switched at any time(b) would cause no consumer inconvenience(c) would constitute a significant portion of the system

load(d) would be widely dispersed to handle all possible

contingencies

Paper 5523C (P9, Pl l) , first received 1st December 1986 and in revisedform 21st April 1987

The author resides at 7 Garnet Avenue, Lilyfield, New South Wales,Australia 2040

(e) would allow the switching off, or switching on, ofsubstantial load blocks

(/) could be integrated with emergency system-controlarrangements.

This paper presents a proposal which has all these andthe additional feature of energy storage. As well as beingvaluable for system emergency control, it can have appli-cation to a wide range of other operational situations.

1.1 System emergency operationThe power industry faces a major challenge in devisingmeasures for the automatic control of emergency condi-tions following calamitous system disturbances. Methodsfor automatically controlling these system emergencieshave been proposed in two papers [2, 3], in which com-binations of measures are required to handle each of thedifferent causes of breakdown. A common ingredient ofmany of these is the requirement of flexible load-sheddingmeasures, namely:

(i) against a system reactive-power deficit [5, 6](ii) against severe transmission-line overloads [7](iii) against loss of generators.

With its readily controllable loads, the load-managementproposal would be of great value in the development ofthis system protection.

2 Load control in emergencies

During most power-system emergencies, load sheddingprovides the most powerful measure for safeguarding thepower network's integrity. At present, load sheddingcauses inconvenience to consumers, particularly if theyhave had no prior warning. This causes load shedding tobe used as a measure of last resort.

With its infrequent use, and the limited number of pre-selected load blocks, there is a lack of flexibility in theutilisation of load shedding. Thus it cannot always beused in ways to best control the range of disturbanceswhich do beset a power system. This deficiency would becircumvented by the potential flexibility of this load-management proposal.

2.1 Eliminating consumer inconvenienceDire power-system emergencies, even though they rarelyoccur, are always unexpected. Whenever consumers areinterrupted without warning and suffer inconvenience,their ill will is directed against the utility. Yet for effectiveemergency control, it is often necessary to shed con-sumers without warning. In fact, a delay in load sheddingmay have serious system repercussions. If consumers hada back-up for a portion of their demand this dilemmacould be resolved.

Present battery developments allow almostmaintenance-free installations. Such an installation pro-vided with a rectifier and inverter, could alternatively be

IEE PROCEEDINGS, Vol. 134, Pt. C, No. 5, SEPTEMBER 1987 337

charged from the grid, or provide AC power to an associ-ated consumer. With suitable controls, that portion of thedemand supplied by the battery installation could be dis-connected from the grid without inconvenience to theconsumer.

2.2 Tariff incentivesFor a domestic consumer, such an installation should, inthe forseeable future, incur the same order of cost as anelectric off-peak hot-water system. With existing tariffincentives many domestic consumers have installed thesehot-water systems. A similar tariff incentive may wellencourage consumers to meet the capital costs of batteryinstallations. Electricite de France provides a year roundtariff only 14% greater than that of off-peak waterheating to any consumer prepared to accept, at 24 hoursnotice, restriction for 22 days per year. Obviously a lowertariff would apply for an instantly controllable load.

2.3 Load ControlFor utilities to gain the greatest benefit of these interrupt-ible loads, particularly during system emergencies, theymust be integrated into a system-wide control arrange-ment. At each consumer's installation the followingcontrol features would be valuable:

(a) separate metering of a number of subcircuits(b) separate switching control of each subcircuit(c) fast switching — in a matter of cycles(d) quick response to external signals(e) identification of signals for each of the differing

system states, e.g. normal, alert and emergency(/) transfer of one subcircuit (interruptible load) from

the grid to the battery inverter supply(g) switching in rectifier(h) selecting a rate of battery charging(i) monitoring the level of charge of the batteries.

All these control functions at each individual consumer'sinstallation can be met by a presently developed micro-processor — the credit and load management unit(CALMU) [1]. With their remote interrogation facility,these units are cost competitive with existing metering.Besides being thief proof these microprocessor units canbe interrogated by the consumer and can be made tocontrol the switching in of appliances, a feature thatwould be invaluable for time of use tariffs.

It has been proposed that CALMU units be arrangedin an hierarchical control array extending from distribu-tion, through the subtransmission circuits, to regionalcontrol centres. For system emergency control, communi-cations would need to be extended to the major controlcentres.

2.4 Method of utilisationThe main emphasis of this paper is the use of these con-trollable loads for the main system needs. But as theywould be predominantly located within the distributionnetworks, there would be the scope for also raisingsecurity levels in these networks. Nonetheless Table 1only lists their applications for main system require-ments.

For greater operational flexibility, interruptible loadsshould be divided into three parts:

(i) the first for peak lopping(ii) the second to replace spinning reserve generators(iii) the last to provide for system emergency oper-

ation.

The possible benefits are enumerated below.

Table 1: Operational modes for controllable load

System state

Normalmoderate loadpeak loadlight load

Emergencyafter load shedexcessgeneration

Supply

consumersubcircuits

from gridfrom batteryfrom grid

from batteryfrom grid

of controllable

batteryinstallation

disconnecteddisconnectedfrom gridtrickle charge

disconnectedfrom gridhigh charge

load

combinedloads

normalnilhigh

nilvery high

3.1 Demand benefits

3.1.1 Normal operation :(i) The need for operating out of priority generators

would be reduced by selectivity switching off interrupt-ible consumers at the build up, and during daily peakperiods.

(ii) The need for spinning reserve generators, to coverthe loss of the largest unit or a heavily loaded transmis-sion link, could be removed.

(iii) Peak lopping loads could be selected to reduce theflows on heavily loaded circuits, so gaining a substantialloss reduction.

3.1.2 Abnormal operation:(iv) Controllable loads not switched for peak lopping

could cover the loss of any circuit — be it transmission,subtransmission or distribution.

(v) During emergencies it may be difficult to maintainvoltage levels at vulnerable load centres — those suppliedthrough heavily loaded lines. Selective switching of thethird quantum of loads could correct this situation. Toillustrate — a 20% reduction on a transmission linecarrying 2.5 SIL, as well as improving the receivingvoltage, would reduce series reactive losses by 75%.

3.2 Energy benefits

3.2.1 Normal operation:(i) The amount of demand reduction at peak load is

also a function of the energy storage of the interruptibleloads that are switched. Fig. 1 shows how a fixed quan-

100

0 1 2 3 4 5 6 7 8 9 10energy, %> peak month average weekday total

Fig. 1 Load duration curve for peak load region(i) extreme peak day(ii) peak day(iii) peak month average weekday

338 IEE PROCEEDINGS, Vol. 134, Pt. C, No. 5, SEPTEMBER 1987

tity of stored energy can produce different demandreductions on different days. Thus interruptible loadswith 2% energy storage could reduce demand 9%, 14%or 23% on different days.

(ii) System capacity factor would be improved bybattery charging at light-load periods.

322 Abnormal operation:(iii) On loss of generators, interruptible loads could be

switched off until additional units could pick up the load.(iv) For short-term periods of energy shortfall, inter-

ruptible load switching would buy time until neighbour-ing utilities could raise their exports.

3.3 Planning flexibility(i) Unexpected factors, such as the delay in commis-

sioning a new power station, or a sudden growth spurt,can alter planning assumptions. The energy storage ofthis proposal could help to tide over the difficult period.

(ii) Transmission elements that would be overloadedduring short peak periods could be relieved by selectiveswitching of interruptible loads. This would allow savingsby deferring the need for their augmentation.

3.3.1 Future technological developments:(iii) Energy storage facilities are necessary for the

development of renewable and alternative generatingsources as their availability does not match systemheavy-demand periods. It may even be possible toconnect the smaller generators into the distribution net-works, electrically closer to the battery installations.

(iv) Electrical car developments may provide batterybanks suitably sized for this proposal. As well as provid-ing the energy storage, the batteries could be rechargedfrom the proposed installation.

4 System emergencies

Existing load-management schemes are often limitedduring power-system emergencies. Off-peak water heaterscan only be switched off at light-load periods and airconditioners during hot weather. The immediate controlof loads in proximity to a disturbance, on the other hand,provides the most effective recourse during an emergency.

The proposal provides for interruptible loads locatedmainly within distribution networks. This would broadenstrategic load shedding, which can select the best loca-tions and the correct quantity of loads for the most effec-tive control of calamitous system disturbances [5, 6]. Byextending the hierarchical control structure from themajor transmission substation, as proposed in Reference3, to distribution levels, further benefits would be pos-sible :

(a) this proposal would allow loads to be shed withoutcausing customer inconvenience

(b) a more precise shedding could match each dis-turbance, both in load quantity and location

(c) a better reliability within distribution networkscould be achieved

(d) operators could switch additional loading by con-necting battery chargers.

Table 2 shows how this can be applied to system emer-gency control.

4.1 Enhancement of operational securityThe availability of widely scattered controllable loadswould have a great benefit in improving power-system

Table 2: Load-management measures for system emergencycontrol

Cause of system emergency Load-control measures

System reactive-power deficitSevere transmission-line overloadSystem reactive-power surplus

System generation deficitSystem generation surplusTransient stability

Steady-state stability

strategic load shedding [5, 6]fast strategic load shedding [7]

connect battery chargers atpoints of high voltage rises [5]shed loads near lost generatorsswitch in battery chargerscontrolled switching in and outof loads to best damp transientswings [8]measures as for system reactive-power mismatches

operational security. This can be seen by consideringthree different emergency situations.

4 2 System reactive -po wer deficitStrategic load shedding has been demonstrated as themost effective control measure [5, 6].

4.2.1 Existing situation: With its inconvenience to con-sumers, operators would tend to limit its use and delaythe application of load shedding during the pre-restorative state.

4.2.2 With proposal: The elimination of consumerinconvenience would remove restraint in the quantityand timing of load shedding, both for automatic or oper-ator application. This, more quickly, would providebetter power-system resilience to any further shocks, aswell as speeding up the restoration process.

4.3 System generator surplus

4.3.1 Existing situation: With loss of load centres atlight-load periods, the unco-ordinated actions of turboge-nerator governors are the only constraint to system overfrequency. In actual incidents there have been over-frequencies causing some units to trip, followed byislanding and breakdown of much of the grid.

4.3.2 With proposal: The immediate switching in ofbattery chargers at load centres near to those lost, wouldsupplement governor actions and so avoid dangerousoverfrequency excursions.

4.4 System generator deficit

4.4.1 Existing situation: Existing underfrequency pro-tections, although effective in most cases of generatorloss, have been found inadequate in some instances. Spe-cifically when heavy loading on transmission lines followsthe severe loss of generation at a major load centre.Underfrequency would cause load shedding throughoutall sectors of the grid to match the lost generation, butexcessive line loading would cause voltage instability atthe vulnerable load centre.

4.4.2 With proposal: The proliferation of interruptibleloads would allow the development of an improved 'stra-tegic underfrequency load shedding'. This would utiliseboth voltage and frequency changes to select loads,mostly near to the lost generators, for shedding.

5 Implementation of proposal

Economic considerations will influence the implementa-tion of this load-management proposal. For a lower

IEE PROCEEDINGS, Vol. 134, Pt. C, No. 5, SEPTEMBER 1987 339

inaugural cost, industrial and commercial consumersshould be chosen, as a greater magnitude of controllableload would become available with a lesser number ofconsumers. In fact, a number of industries can presentlyreap benefits solely by reducing demand charges, evenafter an outlay for battery installations [9]. However, thiswould not allow the utility any direct control over asegment of the consumer's load, nor access to the energystorage of the battery installation. This difficulty could beovercome by an interruptible tariff which could alsoprove attractive to a greater range of consumers.

The great preponderance of consumer interruptionsresult from incidents affecting distribution networks.Controllable domestic loads would make the most sig-nificant impact on the improvement of distributionnetwork reliability. Therefore, as a longer term objective,utilities should aim to include domestic consumers. Thiswould in turn allow a greater dispersion of controllableloads, a necessary objective to be able to handle the com-plete gamut of system disturbances.

5.1 Quan tity of con troltable loadsThis proposal has a broader purpose than just reducingdemand for short-term emergencies and the extra dimen-sion is provided by the energy-storage feature. Unfor-tunately there is no simple relationship between thedemand reduction and the amount of back-up energystorage. Fig. 1 illustrates the considerable variation of theamounts of energy storage, on different days, to effect thesame magnitude of peak load reduction. This analysissuggests an energy storage of 3-4% of daily usage couldbe justified to back-up controllable loads utilised forpeak lopping. This would allow demand reductions of upto 27% of the daily peak.

The second tranche of controllable loads would elimi-nate the need for spinning reserve generation. Thedemand component would correspond to the output ofthe largest generator, and the energy component wouldbe influenced by the time taken to run up replacementgeneration, again a very variable quantity.

For emergency operation, the emphasis would be on awide scatter of a significant amount of controllable loads.If effective in eliminating serious interruptions, not muchenergy back-up would be necessary, but the associatedbattery installations would incorporate a substantialenergy storage.

These considerations have provided an estimatedobjective of 50% daily peak demand and 10% energystorage for the ultimate development of this proposal.

52 Policy requirementsIn the evolution of such a proposal, utilities would needto make a number of policy decisions which can bedivided into short term and longer term for ease of dis-cussion.

52.1 Short term:(i) The potential benefits justify an interruptible tariff

to encourage the participation of consumers.(ii) Resources would need to be allocated to the devel-

opment of battery installations.(iii) Communication networks would need to be

extended to allow control of interruptible loads fromcontrol centres.

522 Longer term:(i) Facilities, such as CALMU, should be provided to

all consumers.

(ii) Special tariff incentives should be provided fordomestic consumers.

(iii) Efforts should be initiated for reduced batteryinstallation costs with quantity production runs.

(iv) There would be need to develop a higher-gradecommunication networks capable of functioning with anautomatic system protection.

6 Conclusion

By eliminating consumer inconvenience, the load-management proposal allows complete operational flex-ibility. The additional facility of allowing controllableloads to be switched in or out, makes it particularly valu-able for system emergency operation.

With a widespread acceptance, controllable loadswithin distribution networks would be scattered through-out the power grid. This would provide the followingrange of benefits for the utility:

(a) It would provide system operators a powerful oper-ational aid in their control of the grid.

(b) It would have application within transmission, sub-transmission or distribution networks.

(c) It would be valuable for normal, abnormal orsystem emergency situations.

(d) It would raise the levels of power-system reliabilityand security.

(e) It would allow a delay in augmenting heavilyloaded elements, so defraying costs.

(/) It would facilitate the introduction of renewableand alternative generation.

(g) It could take advantage of new battery technologyassociated with electric vehicles.

The proposal could initially be developed with industrialand commercial consumers so allowing a substantialamount of controllable load with a lesser number ofinstallations. An interruptible tariff would encourage par-ticipation of consumers as well as gaining utility access tothe controllable loads. Battery installations by selectedindustrial consumers have been shown to be cost effective[9], but nonetheless a special tariff would be justified toallow utility load control as well as access to the batteryenergy storage.

With the establishment of a control structure ema-nating from control centres, the inclusion of domesticconsumers would become more attractive. As well asincreasing the amount and dispersion of controllableloads, it would allow a significant improvement of dis-tribution network reliability. In addition, even if therewere an interruption, consumers with a battery install-ation would be spared inconvenience.

The energy component significantly enhances the ver-satility of this load-management proposal. The fact thatloads could be switched on or off at any time or at anylocation could provide a boon to power-system oper-ation. By providing special control facilities for this load-management proposal, it could provide a completely newdimension for system emergency control.

7 References

1 WOLFF, R.F.: 'Two-way system reaches all customers', Electr.World, January 1982, pp. 102-105

2 LACHS, W.R.: 'Countering calamitous system disturbances'. IEEConf. Publ. 225, 1982, pp. 79-83

3 LACHS, W.R.: 'Resolving the impasse in emergency system oper-ation', IEE Proc. C, Gen. Trans. & Distrib., 1987, 134, (5), pp.331-336

340 IEE PROCEEDINGS, Vol. 134, Pt. C, No. 5, SEPTEMBER 1987

4 JOHNSON, W.A. et al.: 'Load management — how will operatorswant to use it', IEEE Trans., PAS-102, pp. 1811-1817

5 LACHS, W.R.: 'Insecure system reactive power balance: analysisand countermeasures', IEEE Trans., PAS-104, pp. 2413-2419

6 LACHS, W.R.: 'Dynamic study of an extreme system reactive powerdeficit', IEEE Trans., 1985, PAS-104, pp. 2420-2427

7 LACHS, W.R.: Transmission line overloads — real time control',IEE Proc. C, Gen. Trans. & Distrib., 1987,134, (5), pp. 342-347

8 LACHS, W.R.: 'A new transient stability control'. IEEE WinterPower Meeting, New Orleans, 1987, paper 87WM 061-5

9 'Economic analysis of specific customer-side-of-the meter applica-tions for battery energy storage'. EPRI Report EM-3535, May 1985

10 PRINCE, W.: 'Distributed intelligence load control: yes or no',IEEE Trans., 1984, PAS-103, pp. 1599-1604

8 Appendix

8.1 Application of battery installationsAlthough industrial and commercial consumer batteryinstallations are more attractive for inaugurating thisproposal, it is more readily explained by considering adomestic installation.

8.1.1 Domestic installation: To limit its cost, only aportion of the demand would be supplied from thebattery installation. This would include lighting, tele-vision, refrigerator and limited cooking and heatingfacilities. To meet this demand for a maximum of twohours, 5kWh of storage is envisaged. Say a 20-12 V70 AH installation to allow substantial battery life.

A number of subcircuits at the domestic installationwould include one (or two) that could be suppliedthrough the battery installation, as well as one for batterycharging, and others for noninterruptible demand. Themetering and control of all subcircuits would be central-ised through the CALMU [1], which could, oncommand, transfer the interruptible circuits from grid, tobattery inverter supply.

The major portion of the domestic demand, hot water,air conditioning, washing machines, heating and cookingwould be connected to circuits supplied at conventionaltariffs. Unless restrictions to interruptible tariff areapplied, consumers may be tempted to connect too muchdemand to these circuits. If not controlled, this couldcurtail battery life. The CALMU could monitor loadingand perhaps provide a warning to consumers to transferexcess demand off the interruptible circuits.

8.1.2 Battery-installation operation: An example isprovided of possible criteria to be adopted with thesebattery installations.

(i) Interruptible tariff: The control of each consumer'sbattery installation would be at the complete discretionof the utility, to meet its operational requirements. Thiswould be a condition of granting the interruptible tariff.

(ii) Battery charge level: Normally each battery wouldbe at above 50% and no more than 90% of its fullycharged capacity. Only under exceptional conditionswould battery capacity reduce to 25%. Although batterycharging would usually occur during load troughs, whencapacity falls below 50%, the earliest opportunity shouldbe taken to recharge, even at a heavy load period.

(iii) Rates of charging: The rectifiers at each batteryinstallation should be capable of two rates of charging.The normal charge rate would be designed for optimumbattery life. Under short, operationally difficult periods,there would be a high rate of charging. Specifically atlight-load periods when dangerous overfrequency excur-sions could be averted after a sudden loss of load. Thisbenefit would need to be weighed against the cost ofreduced battery life.

(iv) Rectifier I inverter: Solid-state technology andquantity production should allow a more reasonable costof rectifier/inverter units for the battery installations. Thiscould be dependent on utility support for the load-management proposal.

8.2 Operational applicationsThe main purpose of this load-management proposal isits operational applications. In developing a controlstructure for its application, consideration must be givento dealing with the following operational situations [10]and also on the balance that must be struck betweenoperator and automatic control.

(i) prevent off-peak cycling of base load plants(ii) add load to minimum generation periods(iii) diminish or eliminate spinning reserve generation(iv) minimise high-cost generation(v) reduce daily peak loads(vi) diminish rapid load changes on generators(vii) integrate with load frequency control(viii) emergency assistance(ix) load shedding(x) island stabilisation(xi) disconnect/reconnect arrangements(xii) reduce cold-load pick-up.

The successful development of a control structure willsignificantly influence the benefits that could be gainedby this load management.

IEE PROCEEDINGS, Vol. 134, Pt. C, No. 5, SEPTEMBER 1987 341