Automatic Underfrequency Load Shedding in ESCOM

LOAD SHEDDING SCHEMES

AUTOMATIC UNDERFREQUENCY LOAD SHEDDING IN ESCOM C. P. Levy and C. A. Haupt Operations Department, Electricity Supply Commission, Republic of South Africa

Abstract. Low frequency conditions can occur on power systems as a result of the loss of large amounts of generation or when sections with insufficient generation are severed from the main system. Protection against these conditions is reqUired. This paper describes the approach adopted by the Electricity Supply Commission of South Africa. A four stage load shedding scheme supplemented by a fifth emergency generator islanding scheme has been implemented. Some of the more important practical aspects are also included in the text. Keywords. Power management; load regulation; relay control; frequency control; load shedding. INTRODUCTION

to cater for frequency excursions of different magnitudes and durations. The absolute minimum tolerable frequency on a system is normally determined by boiler auxiliaries. The minimum allowable system frequency for normal operation is, in the case of ESCOM, fixed by the Electricity Act at a value of 48,75 Hz (50 Hz - 2,5 %).

In order to appreciate the different aspects of the underfrequency protection scheme in use on the ESCOM system, it is neceisary to give a brief description of the power system. The power system comprises 147 turbogenerators, including gas-turbine and hydro-sets, providing a generation sendout capacity of 15 063 MW. The largest generator is rated at.600 MW. Import from a hydro power station (Cahora Bassa) in a neighbouring state amounts to a maximum of 1820 MW. This import is transmitted via a i 533 kV direct current link and is supplied to the ESCOM system at Apollo Converter station. Apollo and the majority of power stations are located in one small area of the country, as can be seen in figure 1. From figure 1 it should be noted that a number of areas have relatively weak connections to the main system.

The amount of load to be shed depends mainly on the relative magnitude of the generation loss, the amount of spinning reserve carried on the system at the time of generation loss and the load characteristics. If an underfrequency load shedding scheme is properly designed to cope with varying frequency excursions, an exact knowledge of the system's frequency response characteristics is not essential. It is preferable to shed load in small blocks at a number of frequency levels and with varying time delays to ensure that just the right amount of load is shed. This has the disadvantage that supply has to be restored at a large number of points which could cause unnecessary delays. More frequency relaying is also required. Load should, as far as possible, be shed at manned stations or stations with supervisory control facilities.

In 1979 the maximum system demand was For this demand, the 12 855 ~l. maximum import at Apollo is approximately 14 % of the total generation of the system. GENERAL PHILOSOPHY OF UNDERFREQUENCY LOAD SHEDDING

When determining which load is to be shed, the nature of the load is taken into account. For example, pumped storage load is disconnected very rapidly while mining loads, where people underground rely on supply for ventilation, and continuous process industries

In order to protect a power system against low frequency conditions, an underfrequency load shedding scheme of some form is reqUired. Such a scheme has to be versatile in order 3

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P. Levy and C. A. Haupt

are shed at a later stage or possibly not shed at all. The physical layout of a system governs, to a large extent where load can be shed. THE ESCOM UNDERFREQUENCY LOAD SHEDDING SCHEME AND PROGRAM FOR CORRECTIVE ACTION IN THE EVENT OF LOW FREQUENCY CONDITIONS Because the amount of generation that can be lost on the ESCOM system varies from approximately 20 MW to 1800 MW (between 0,15 %and 12,5 %of the installed capacity) it should be appreciated that widely varying frequency deviations can occur upon loss of generation. A comprehensive underfrequency protective system has thus been devised. This system has been broken down into an automatic and manual scheme, the most important section being the automatic scheme. Normal Load Shedding - Automatic Involuntary Scheme The underfrequency load shedding scheme adopted by ESCOM has been designed to deal with both slow and fast decays of frequency. The scheme incorporates time delayed shedding at four different frequency levels. The total load to be shed at any one frequency level is therefore shed over a period of a few seconds. The first frequency level at which shedding takes place is 48,80 Hz. This level has been selected so as to be just above the statutory minimum frequency of 48,75 Hz. The other three stages are 48,50 HZ, 48,20 Hz and 47,9 Hz respectively. Time delays at the various frequency levels vary from 0,5 to 5 seconds. The scheme is diagramatically shown in figure 2. At each frequency level approximately 10 % of system load is shed. Loads are shed in the reverse order of their importance, i.e. the more important loads are shed at the lower frequency levels. (Because of this, it is reqUired that all less important loads shOuld be shed before the more important loads are shed). In order to achieve this, all the stage 1 loads should be shed before the stage 2 loads, or, all the stage 1 and 2 loads should be shed before stage 3 loads.

To satisfy this reqUirement a special feature termed "upward shedding" was introduced. This is reqUired for rapid frequency decays into stages 2, 3 or 4. For a very fast frequency decay down to, say, stage 2, the timers of both stages would start almost simultaneously. Because a dip down to stage 2 would only be caused by a large loss of generation, a large amount of load should be shed. The tripping signal that trips the stage 2 loads after the shortest stage 2 time delay is used to trip the entire or selected loads of stage 1. Similarly the shortest time delayed stage 3 trip signal trips all stage 1 and stage 2 loads (or sometimes selected loads of these sYages). A number of the larger municipal consumers with local generation running in parallel with ESCOM have load shedding SChemes. The frequency levels of the various stages sometimes differ slightly from those used by ESCOM. Islanding of Regions with Local Generation The concept of islanding regions has been introduced to cater for the loss of connection between the main system and a region with local generation. The connection may consist of one, two or three lines running in parallel. The basis of islanding is explained by means of figure 3. If the breaker at A were to open, the local generation plus all the load in the area would be islanded from the major system. As local generation will not be sufficient to supply the entire load, the frequency in the island will begin to drop rapidly. If no action is then taken, the local generation will collapse. To prevent this from happening, underfrequency relays can be used to open breaker B at a pre-defined level. The local generation will then be left feeding a load of matching size. Two islands will thus be formed, namely that of the small local generation with its matching load, and that of the bulk load which has no generation (known as the black island). If, for instance, the tripping of the breaker at A were caused by a line fault, the breaker would automatically reclose after 4 seconds, restoring supply to the bulk load. The local generation and matching load would be unaffected

Automatic underfrequency load shedding

5

by the interruption. When supply to the black island is restored, the local generation is re synchronised to the main system either automatically or manually.

islanded. Also, it must be ensured that the voltage supply for the underfrequency relay comes from the bar on which the islanding is to take place.

To facilitate rapid restoration of supply to the black island, it is required that no underfrequency load shedding take place in the black island. The only shedding would be the "trimming" of load that remained connected to the local generation.

To cater for further drops in frequency in the newly formed island, two load shedding steps are included. The frequency levels used are 47,0 Hz and 46,7 Hz. If the frequency continues to drop after the load has been shed, then the generators are islanded with their auxiliaries at a frequency level of 46,5 Hz.

Underfrequency islanding is applied in three areas on the ESCOM system, i.e. on the single line supply system to the East London and Port Elizabeth system, on the three line supply system to the Western Cape and on the three line supply system to the Natal Region (see figure 1). Restoration of supply to the bulk load in the black island presents no problem provided the major part of the system is healthy and capable of picking up the entire bulk load. Islanding of Generators to Preserve start-up Power Following Major System Disruptions. Following a major system disruption in 1975, various projects were initiated to help prevent major disturbances and to facilitate system restoration when disturbances do occur. As the majority of the larger generating sets are powered by pulverised fuel boilers, no start-up power is available at any of these power stations to start the boilers from cold. To ensure that start-up power will be available under emergency conditions, a scheme has been devised whereby generators are islanded with matching load or on their auxiliaries at some major power stations. At a frequency level of 47,2 Hz at least one generator per station is islanded onto matching load. This is done by clearing one section of busbar at the power stations of all infeeds and outfeeds except for the generator and the matching load. For this scheme to be successful, special attention must be paid to the linking arrangements at the power stations to ensure that the load is always linked onto the same bar as the generator with which it is to be

Voluntary Underfreguency Load Shedding Involuntary load shedding has the disadvantage that all supply to consumers is interrupted. To overcome this disadvantage to some extent, a voluntary automatic load shedding scheme has been established. A number of large consumers have agreed to shed non-essential loads at frequency levels above those at which involuntary shedding takes place i.e. above the stage 1 frequency level of 48,8 Hz. Three frequency levels have been chosen. These are 49,2 Hz, 49,1 Hz and 49,0 Hz. The total load to be shed will be spread over the three levels. This has been done mainly to ensure that only the required amount of load is shed for a slow frequency decline situation. Also, load restoration will take place in small steps, which will prevent the frequency from being depressed again. At present all load is shed instantaneously. Time delays are not considered to be feasible because of the small steps between each frequency level. All load that is to be shed at 49,2 Hz should be shed before any of the 49,1 Hz load is shed. To achieve this, and to allow the system to possibly recover sufficiently so that the next frequency level of 49,1 Hz is not reached, the load at 49,2 Hz must be tripped as fast as possible. Considering a fast rate of decline of frequency of 0,2 HZ/Sec, there will only be 0,5 sec. between each level, which may not give the system sufficient time to recover before the next frequency level is reached. This is not considered to be a major disadvantage as the three levels of frequency are primarily

6

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P. Levy and C. A. Haupt

designed to deal with slow frequency declines. The rate at which generation can be increased upon sudden declines in frequency, depends on the amount of spinning reserve on the system at the time. If the rate of decline of frequency is of the order of 0,2 Hz/sec, when the frequency reaches 49,2 Hz, then it is very unlikely that all the load shed at the three levels will be sufficient to stop the decline completely. However, if the rate of decline of frequency has slowed to less than, say, 0,1 Hz/sec., there will be at least a 1 sec. delay between the levels which could be sufficient to allow the system to recover before all load is shed, dependent on the spinning reserve on the system during the frequency decline. Following load shedding, load is restored in reverse order to that of tripping once the frequency has recovered to 49,5 Hz. This is done in three steps for every frequency level. If the alarm remains cancelled after a section of load has been switched in, the next section is switched in. This procedure will continue until all load has been reconnected. If the alarm were to come on at any stage, then all restoration of load is ceased until the alarm recancels. The total amount of load shed voluntarily is approximately 5 % of the total system load. The main drawback of a voluntary scheme as described is that the electricity supply authority has no control over the scheme. Consumers may discontinue their participation at any time without the knowledge of the supply authority. Manual Load Reduction Program When it is known beforehand that a shortage of generating capacity will exist a manual load reduction plan is put into operation. Methods and points at which load will be disconnected or reduced have been determined. Upon request from National Control, Regional Control Centres carry out the manual load reduction plan. Manual Pre-Authorised Increase In Generation at Power Stations

An underfrequency alarm is initiated at power stations at a frequency level of 49,4 Hz. On receipt of the alarm, the operators will immediately adjust the required control systems so as to make available the maximum reserve capacity. OPERATIONAL AND PRACTICAL ASPECTS Standard underfrequency load shedding panels have been designed for use in the underfrequency load shedding scheme. The most common underfrequency relay in use is the Brown Boveri FCXl03 relay. This relay operates down to a level of about 60 % of nominal voltage, which is used to advant~ge in the black island. It has been found that, under severe overload conditions in the regional islands, the voltage at the islanding point drops below this minimum relay operating level. At the islanding points the Brown Boveri FCX103b is now being installed, supplemented by an undervoltage relay. The FCXl03b r~lay can operate down to a level of around 20 % of nominal system voltage. Where the load to be shed at any relaying point exceeds 100 MW, the decision to shed load is on the basis of a two out of three scheme i.e. three underfrequency relays are prOVided and at least two out of the three relays are reqUired to operate before any load is shed. This provides security against false shedding and certainty of shedding correctly. Where the load to be shed is less than 100 MW, but is still considered very important, a two out of tvlO relEiy scheme is used. Small loads are normally shed by means of a single underfrequer.cy relay scheme. The load shedding scheme has been designed so that all the feeders or transformers at any station can be selected to any of the load shedding frequencies or time delays. This allows rotation of loads which are to be shed and also allows for any alterations to be made to accommodate future system loading conditions and changes. Some problems were experienced with load shedding in the regional islands. When the interconnection between the main system and region is lost, islanding of the local generation with matching load should take place, but no shedding should take place in the "black island". The normal load shedding time delays in the black island were too short and resulted in

Automatic under frequency load shedding

unwanted shedding because the voltage in the island was kept high enough by the motor load. The settings were adjusted and since then the scheme has functioned very well. Another problem that occurred was the failure to island owing to the voltage in the severed network dropping rapidly to a level lower than the minimum operating level of the frequency relay. This was only experienced once when a very small amount of generation was present in the islanded region. Undervoltage relays were then installed at the islanding points to supplement the underfrequency scheme and the FCXl03b relays are presently being installed. On one occasion the frequency in an islanded region remained suspended at a level lower than 48,0 Hz. (The lower shedding stages in the islanded region were at that stage lower than the present level). In order to prevent frequency suspension at a level lower than the stage 1 level after all stage 1 loads have already been shed, the concept of downward shedding has been designed. After a suitable time delay selected loads of the lower shedding stages are shed in an attempt to allow the frequency to recover to a value above the stage 1 level. Islanding in networks with local generation has been very successful. Over a period of three years during which teething problems with protection were experienced and operator errors and system faults occurred, more than 30 supply interruptions to the most southern part of the system necessitated load shedding. On only one occasion was the load shedding unsatisfactory because the frequency in the islanded region remained suspended at about 47,8 Hz for some minutes when the interconnector providing the bulk power to the area tripped. As a result of faults caused by bird droppings on the single 400 kV line to the East London and Port Elizabeth area, unsuccessful single phase tripping resulted in various interruptions to the area. In virtually all the cases supply to the dark island was resumed when the lines reclosed on three phases following the unsuccessful single phase auto-reclosures. Resynchronising of the island with local generation normally takes place within three minutes.

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No practical experience has been gained with the islanding of large generators. The scheme has not yet been commissioned at all the stations where it is planned to install the scheme. A possible major limiting factor to the success of this scheme will be the ability of the boilers to cope with the sudden variation in load. CONCLUSION Underfrequency load shedding has been of tremendous assistance in keeping the system viable under overload or undergeneration conditions. This not only applies to the system as a whole, but also to any part of it which may become disconnected from the main system but which still has substantial generating capacity capable of maintaining supply to its network when the excess load on it has been shed on underfrequency. Such sections can then readily be resynchronised to the main system and supplies restored as soon as generation becomes available. The effectiveness of underfrequency load shedding in saving a part of the ESCOM system from total collapse under conditions of severe generation deficiency was demonstrated after a major system disturbance in 1975. Approximately 2800 MW of generation was lost when four power stations were severed from the main system. The total system load at the time was 7300 MW. Sufficient load was shed on the underfrequency protection scheme to leave the remaining power stations intact at near normal frequency. The voluntary load shedding scheme has been of great value in arresting frequency declines, particularly when very large amounts of generation have been lost. As a result of the voluntary scheme, involuntary shedding has only taken place once in the last two years. This is significant in view of the fact that generation losses of between 1000 MW and 1800 MW have occurred fairly often. These losses occurred mainly during the commissioning of the final stage of the d.c. import into the ESCOM system. On a system the size of the ESCOM system, large frequency deviations are more likely to occur than on the extremely large interconnected systems elsewhere in the world. As a result

C. P. Levy and C. A. Haupt

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of this ESCOM has had to devise a very comprehensive underfrequency protection program, and experience has proved that their approach has been the correct one. ACKNOWLEDGEMENTS The authors wish to thank ESCOM Management for their permission to publish this paper. REFERENCES Cardwell, G.R., and R.R. Slatem (1976). The Escom Underfrequency Load Shedding Scheme and Philosophy. Escom internal report Narayan, V., F. Ritter and H. Ungrad (1978). Frequency Relays for Load Shedding. Brown Boveri Review No. 6, pp 413-415 Slatem, R.R., and A.L. Taylor (1975). Some Protection, Underfrequency Load Shedding and Testing Aspects of the Escom Network. Electricity in South Africa - Cigre South Africa, Section T7. Stevenson, J., and O.C. S,ymons (1975). Automatic Low-frequency Load Shedding in the C.E.G.B. S,ystem. lEE Conference Publication 125, Deve!~pments in Power system Protection, pp. 34-41. Stutterheim, F.W. (1977). Comments on Keeping Power Stations Viable During Major S,ystem Disturbances and Increasing System Security. Escomointernal report.

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