Bus Reactor BHEL

International Journal of EmergingElectric Power Systems

Volume2, Issue1 2005 Article 1038

Utilization of Controlled Shunt Reactor in a400KV Interconnected Network

VENKATA NATARAJA JITHIN SUNDAR SISHTLA∗ Reshmi M†

∗”Corporate R&D, BHARAT HEAVY ELECTRICALS LIMITED”, [email protected]†[email protected]

Copyright c©2005 by the authors. All rights reserved. No part of this publication may be re-produced, stored in a retrieval system, or transmitted, in any form or by any means, electronic,mechanical, photocopying, recording, or otherwise, without the prior written permission of thepublisher, bepress, which has been given certain exclusive rights by the author.International Jour-nal of Emerging Electric Power Systems is produced by The Berkeley Electronic Press (bepress).http://www.bepress.com/ijeeps

Utilization of Controlled Shunt Reactor in a400KV Interconnected Network∗

VENKATA NATARAJA JITHIN SUNDAR SISHTLA and Reshmi M

Abstract

Fixed shunt reactors are traditionally used in long distance EHV and UHV lines to reduceFerranti over voltages and over voltages that arise due to load rejection. A new equipment calledControlled shunt reactor (CSR) has been developed [3] and [4]. Detailed load flow and voltagestability analysis on a typical 400KV interconnected network prove that replacement of fixed shuntreactors with CSR is beneficial. This study report is an attempt to justify and quantify the economicand technical benefits of CSR in a 400KV system

KEYWORDS: Compensation, economics, EHV transmission lines, impedance transformers, in-terconnected power systems, load flow analysis, reactive power, thyristors, voltage collapse, powersystem dynamic stability

∗The authors express their sincere thanks to Mr. R.K Bhattachaya GM (EM) and Mr.M.P SoniAGM (TPS) of Corp. R&D, BHEL for encouraging us to carrying out this work. The authorsthank the management of Corp. R&D, BHEL. for giving permission to publish this paper. Theauthors also thank Dr. Federico Milano, University of Waterloo for his timely clarifications for thetechnical queries.

1.0 Introduction In India 400 kV transmission was introduced in 1971-72. The first two lines

were Obra-Sultanpur and Dehar-Panipat in Punjab. These long lines were provided with Fixed Shunt Reactors (FSR) to control over voltages during line energization and light load conditions that prevailed for longer duration during the initial period. By 2001, the 400kV network has grown over 42000-ckt km. More than ¾ th of the lines are compensated with FSRs to 60% of their charging capacity. The total shunt reactor capacity connected in the grid was around 10,500 MVAR. The grid was catering to a load of 65,000MW and 39,000 MVAR of reactive power. Permanent connection of 10,500 MVAR shunt reactor capacity was taking the total MVAR load close to 50,000 MVAR. Today, the above figures could have reached higher levels. This additional burden of reactive load under peak load conditions results in reduced voltage profile and additional losses due to increased currents in the net works. Higher losses and reduced voltage profile also indicate higher reactive power requirement from generators forcing them to operate nearer to their reactive power limits. In order to meet this higher reactive power demand, the generator excitation voltages are kept higher increasing the possibility of higher dynamic over voltage under sudden load rejection. The possibility of a voltage collapse also increases in the grid due to sudden loss of a line with permanently connected reactors in the grid.

Recent blackouts reported in the WREB have drawn the attention of the power system community in the country. One important inference from these events was that the grid was short of reactive power support and with increased load conditions have led to considerable low voltages. The low voltage trips have virtually split up the grid into two parts. One with more generation and light loads, and another with weak generation and heavy loads. The subsequent trips occurred due to low/over voltages and resulted in total system blackout. Prior to the blackout the grid was experiencing poor voltages due to increased agriculture loads. Voltages prevailing before the blackout in some parts of the western grid are shown below. Station Voltage(kV) Station Voltage(kV) Indore 321 Itarsi 371 Nagda 325 Bina 367 Satpura 367 Bhopal 355 Dhule 380 Parli 379 Asoj 370 Kalwa 395

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SISHTLA and M: Utilization of Controlled Shunt Reactor in a 400KV Interconnected

Published by The Berkeley Electronic Press, 2005

The recent three blackouts on 6/10/03, 5/11/03 and 7/11/03 were due to low voltage problem prevailing in the region. The poor voltage profile continues to prevail causing serious insecurity in the grid. The capacitors already installed are quite inadequate and that the presently available capacitors have not come to the rescue of improving the voltages to the desired level. Large quantum of MVARs is drawn from the grid causing large depression in the voltages, which is not at all conducive to healthy grid operation. This situation clearly indicates lack of proper reactive power support, which can be improved considerably by providing the same. Another important observation is that the low voltage situation occurred while lines were carrying power much lower than the natural capacity. The immediate remedial suggestion could be to install capacitor banks or static var compensation. Implementation of this suggestion calls for detailed system analysis and large investments, which may take considerable amount of time and effort. A significant issue, which has come for discussion during these disturbances, is the role of shunt reactors in the 400kV system. It is noteworthy that almost all the shunt reactors connected in the lines and buses were in operation during the moments of crisis. Definitely, existence of these reactors must have played a detrimental role leading to deterioration of system voltages. There is a suggestion that shunt reactors are to be switched off during such crises. But by permanently removing the shunt reactors, over voltages may develop during light load conditions leading to grid disturbance. 2.0 Role of Shunt Reactors and Proposed Application Criteria

During the early 70’s when 400kV transmission was introduced, there were some trials to use breaker controlled shunt reactors. Due to the inherent problems involved in reactor switching and also due to lack of suitable breakers it was decided to use permanently connected shunt reactors for line compensation. In the initial days of 400kV grid, as the lines were lightly loaded, the 60% permanent compensation of line charging capacity was not imposing any limitations.

The 400kV network has become the main constituent in the national grid today. The dependency on 400kV grid for transmission of power to load centers from the remote generating centers has increased. In this context, maintenance of healthy voltages, maximum power transfer capacity, increased voltage stability margin and reduced line losses in the 400kV grid assumes maximum importance. With the backdrop of recent black outs it is

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International Journal of Emerging Electric Power Systems, Vol. 2 [2005], Iss. 1, Art. 1038

http://www.bepress.com/ijeeps/vol2/iss1/art1038

necessary to redefine the guidelines for the use of shunt reactors for line and bus applications.

1.A shunt reactor connected at either ends of the line (line reactor) should not remain connected on permanent basis. The line reactor should be in circuit during line energization, during light load conditions and when the line voltage is above the permissible limit. 2.The line reactor should get disconnected as the loading of the line increases or when the steady state line voltage decreases below the permissible limit. 3.The line reactor should be able to protect the line and associated equipment from possible over voltages that may occur due to sudden load throw off conditions. 4.A line reactor should facilitate successful auto reclosure during single line to ground fault conditions. Under the present conditions of the Indian grid, Controlled Shunt Reactor (CSR) will be an ideal choice of equipment. CSR will provide the necessary reactive power support to the lines during line energization and during sudden load rejection. During full load and reduced voltage conditions CSR cuts itself out of circuit. In simple words CSR is a switchable reactive load which will come into the circuit only when it is required thus eliminating the unnecessary permanent burden on the grid. 3.0 Principle and Implementation

The principle behind the CSR is to control the reactive current of the shunt

reactor by using a thyristor valve, which can provide the necessary speed of switching and control by means of firing angle control. It is not practical to use a thyristor valve at transmission voltage levels and the concept of a reactor transformer with primary winding connected to high voltage side and a secondary (control) winding suitable for connection of a thyristor valve forms the basis for CSR. In other words, CSR is a high impedance transformer controlled by an anti parallel pair of thyristors (valve) on the secondary side at a suitable voltage level. The impedance of CSR can be controlled by varying the firing angle of the pair through a Controller.

CSR can be controlled to vary reactive power consumption anywhere between zero to its full capacity by varying the firing angle of the thyristors.

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A smooth control of reactive power consumption is thus achieved. This operating mode is referred to as “Continuous” mode. The CSR can be mad “On/Off” type when thyristors operate at only two firing angles corresponding to full and zero conduction. The general arrangement of CSR scheme for a 400 kV transmission system is as shown in Fig.1. The reactor transformer primary is connected to high voltage through an isolator and the secondary or control winding is connected across the thyristor valves. The controller which is a logical and programmable device, generates firing pulses based on the input signals. The bypass circuit comprising a breaker and a choke is provided to bypass the thyristor circuit under various conditions. When this circuit is closed, CSR acts as a fixed reactor irrespective of the thyristor firing status. The bypass circuit ensures the availability of the reactor in case of thyristor valve breakdown or during its maintenance. The presence of a small choke allows smooth online changeover of current from bypass circuit to thyristor valves. Isolators in the bypass and thyristor circuits are provided to take care of maintenance requirements.

The tertiary winding is in delta, which takes care of the triplen harmonics.

For other harmonics, which are generated at various firing angles during continuous control of reactive power, harmonic filters can be connected in the tertiary winding. The MVAR requirement of these filters is very small and is to the tune of 5% of the CSR capacity.

Fig1. 400kV CSR schematic diagram

Thyristors

400kV line

Reactor transformer

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4.0 Site Trials and Performance of CSR The 420 kV, 50 MVAR CSR, the first of its kind in India, has been commissioned in September 2001 at 400 kV Itarsi substation of PGCIL, located in the state of Madhya Pradesh. The CSR was installed in the place of an existing fixed shunt reactor of the same capacity. The location is the Itarsi end of a 238 km line from Jabalpur to Itarsi. It is primarily placed as a line reactor but can also be used as a bus reactor when the line is disconnected. Itarsi site was chosen as it had parallel redundant circuits to allow installation in an existing system without effecting the operation. A suitable layout was prepared to fit in all the components of CSR in the existing area available for the conventional shunt reactor. Thyristor valves were housed in a small room. The existing protection scheme of the shunt reactor was retained and additional protection relays were provided for the new equipment. Controller can be chosen to operate in “ON/OFF” or “CONTINUOUS” mode through a selector switch. In case of ON/OFF mode the controller decides the firing mode of thyristors on the basis of bus voltage. The CSR is made to conduct if the bus voltage goes above set level and stops conducting if the bus voltage goes below the lower voltage level. These levels can be modified by the operator “on line”. The response time of the controller is less than 10 ms. In case of CONTINUOUS mode the controller measures the line load current, calculates the required reactive compensating current and generates the appropriate firing pulses.

A dedicated transient fault recorder is also installed with the CSR to monitor all the parameters related to the scheme and to capture its response during various system conditions. Following were some important observations during the installation and testing of the CSR at Itarsi:

a) The line energization with CSR instead of a fixed reactor does not have any problems. With by pass scheme, the presence of CSR is ensured right from the first cycle. b) The change-over from bypass circuit to thyristor circuit and vice versa is smooth and bumpless.

c) When CSR is not drawing any reactive power the line voltage improves by 1kV at Itarsi bus and the power transfer in the corresponding line increases by 20 MW. The change may look small but it may be noted that the CSR is

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installed at only one end of the line and that Itarsi bus has a very high short circuit level, where only one reactor is converted into CSR out of a total of eleven.

d) CSR has operated in AUTO mode with ON/OFF control successfully responding to the voltage variations.

e) A single line to ground fault (SLG) was simulated by momentarily removing feedback of one of the phase voltages to the controller. The CSR came into full conduction within the planned response time.

f) Continuous control mode was also tested with manually changing the firing angle from full conduction to zero conduction. The reactive power at each of the firing angles was noted. It was found to be varying smoothly with the firing angle from full MVAR to zero.

g) CSR is functioning without any operational problems. It does not require any operator intervention during normal operation and comes in and goes out of circuit at the set limits automatically.

CSR provides reactive power support depending on the bus/line voltage levels. At Itarsi an upper voltage limit (UVL) of 415 kV and a lower voltage limit (LVL) of 400kV were set. Depending on the voltage condition CSR will be operating. The ON and OFF times are continuously logged. Fig.2 depicts the operation pattern of CSR in the year 2002-03. The graph shows percentage time for which CSR has been providing the 50 MVAR reactive power support based on the prevailing voltage condition. On the contrary, a fixed shunt reactor will be continuously in circuit resulting in drop in voltage as well as higher equipment and line losses. CSR was in circuit for an average period of 10.85% of time during a year.

Fig.2 Operation pattern of CSR in the year 2002-03

C SR O PE R ATIO N 2002-03

010203040

March,0

2

May, 0

2

July,

02

Sep,02

Nov,02

Jan,0

3

M O N T H & YEAR

IN C

IRC

UIT

% O

PER

ATI

NG

TI

ME

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5.0 Economic Benefits

Utilization of CSR in place of a fixed shunt reactor will lead to economic benefits in addition to quality improvement in voltage profile. The benefits may vary quantitatively depending on the location and grid conditions. In Indian conditions, where the loads are on the rise and the reactive power compensation is almost nil, the utilization of CSR will have a considerable impact. Installation of a 50 MVAR CSR at a loaded line end will amount to the releasing of a 50 MVAR capacitive reactive power to the system.

5.1 Savings from the equipment self losses

A permanently connected fixed reactor will result in losses on a continuous basis. Even though CSR can have self-losses of four times to that of a fixed reactor, it will not be in circuit continuously. Under present conditions, where the 400 kV lines are carrying power from generating to distant load centers, lightly loaded or no load condition is very rare. From the recorded data at Itarsi it was observed that CSR on an average was in circuit for less than 3 hours in a day and the self-losses of both fixed shunt reactor and CSR are almost the same. However, in view of the high self-losses, it is necessary to consider fully loaded lines to derive maximum benefit from the point of view of equipment losses. A decision regarding CSR installation shall be on the basis of savings in line losses (4.2) and revenue from increased power transfer capacity (4.3), which are more prominent.

5.2 Savings in Line Losses

As CSR cuts out of circuit, there will be reduction in the reactive component of the current in the line. This in turn reduces the magnitude of RMS current of the line. As the line losses vary in proportion to the square of the current, the reduction will be substantial on an annual basis. The magnitude of savings will also increase proportionally with the line length. Another important factor will be the operating power factor. At power factors poorer than 0.90, the effect on the savings in line losses will be quite high. For example, a line of length 300 km delivering a load of 400MVA at a power factor of 0.9 will see a saving of 4.25 million units annually if one 50 MVAR shunt reactor is replaced with a CSR. This line when delivering the same 400MVA at a better power factor of say 0.95 will see a saving of 2.84 million units annually if one 50 MVAR shunt reactor is replaced with a CSR.

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5.3 Revenue due to Additional Power Flow When the lines are fully loaded the effect of removing a fixed shunt reactor will facilitate the line to carry additional power. When CSR comes out of circuit, the line will be able to carry additional power under peak demand. This will also be more pronounced when operating power factors are below 0.9. Removal of 50 MVAR reactive load will increase the power transfer capacity by 20 MW and will be more at poorer power factors. This benefit can be quantified into substantial earnings by awarding free wheeling charges for the additional power transferred.

The technical superiority of CSR is well proven with the satisfactory operation at Itarsi. While considering it in place of a normal shunt reactor, it is natural to compare CSR with the former. CSR deployment benefits the grid immensely but the additional investment in comparison with a normal shunt reactor generates a debate over the techno-economic viability of CSR. To quantify the economic benefits offered by CSR it is necessary to consider a 400 kV interconnected system and to study the exact nature of power flows. A detailed simulation study on a typical 400KV interconnected system was carried out using MATLAB/PSAT package. The load flow analysis and continuation power flow using perpendicular intersection technique is done to identify the voltage collapse point and the maximum loadability on the system. As the load flow is a steady state analysis, the system with CSR can be simplified and studied as a system with shunt reactor and without shunt reactor.

6.0 A Case Study

A 10 bus 400kV interconnected network is considered. The line diagram and system data is given below. The generation, loading, bus voltages and line flows are in line with the conditions of 400kV Indian grid. The existing method of compensation up to 60% of the line charging capacity is followed to determine the number and rating of shunt reactors. No additional compensation devices like shunt capacitors or SVC are employed.

The 400kV transmission line parameters:

Resistance, ohm/km = 0.0277. Inductance, H/km = 1.219e-3. Capacitance, F/km = 1.08e-8.

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Fig. 3. 10 bus 400kV Interconnected System

A detailed load flow analysis is done on the above system. As load flow study is steady state analysis, CSR is represented as a fixed shunt reactor.

Two cases considered for simulation:

• System with shunt reactors. • System with out shunt reactors.

However, CSR remains in the system to compensate during line energization and light load scenario and gets disconnected when the system is heavily loaded based on the line voltage. CSR can come into full conduction within 10ms in case of sudden load throw off conditions.

6.1 Improved Voltage Profiles at Load Buses

The following table gives the bus voltages of the system with and without shunt reactors

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TABLE I VOLTAGE PROFILES AT DIFFERENT LOAD BUSES

Load Bus

Voltage(kV)With all the reactors in

circuit

Voltage(kV)Wit

h all the reactors removed

Voltage(kV) With reactors removed at

strategic locations

Bus C 405.85 409.00 406.00 Bus E 352.80 376.70 361.00 Bus F 361.62 378.00 367.90 Bus G 336.00 359.00 344.40 Bus H 347.76 365.40 355.70 Bus I 375.00 386.00 377.60

In the above case at Bus G for example, the voltages have improved from 336kV to 344.4kV which is about 8.4kV increased voltage, with and without reactors at strategic locations respectively.

6.2 Increased Loadability of Lines

PV curves of different load buses are obtained using continuation power flow method for the voltage stability analysis. These curves are used to determine the maximum loading point (MLP), the maximum power transfer among system areas, the voltage stability indices, and to compare planning strategies.

Fig. 4. PV curves at all load Buses with all the reactors in the circuit

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Fig. 5. PV curves at all load Buses with all the reactors removed.

Fig. 6. PV curves at all load Buses with reactors removed at strategic locations.

In figure 4, PV curves at all the load buses are generated with all the reactors remaining permanently connected in the circuit. It is evident that the maximum loading of the system in this case is 1.087 pu. Removal of reactors at some strategic locations increases maximum loading to 1.142 pu as shown in figure 5. The maximum loading point further improves to 1.2537 pu with all the permanently connected reactors removed as shown in figure 6. However, keeping in view the dynamic voltages that may arise due to sudden load throw-off or line outages, utilization of CSR is recommended as a replacement for permanently connected shunt reactors.

L O A D V O L T A G E S V, pu

POWER AT LOAD BUSES, P p u

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6.3 Improved Voltage Stability Index

The voltage stability index determines how far is the bus from its voltage stability limit. The voltage stability index is the ratio of the differential change in active load for the whole system to the differential change in the voltage, -dPTOTAL/dVj. This index will be high when the weakest bus is far from instability but zero when the weakest bus experiences voltage collapse. (Negative sign is used so that the index will be positive before the critical point is encountered and negative afterwards.)

The voltage stability index of two weakest load buses G and H under different conditions is tabulated below .The weakest bus is the one that is nearest to experiencing voltage collapse.

TABLE II

VOLTAGE STABILITY INDICES OF LOAD BUSES G AND H

Load Buses

Voltage stability

index with all the

reactors in circuit

Voltage stability

index with all the

reactors removed

Voltage

stability index with reactors removed at

strategic locations

Bus G 15.8 16.8 16.2 Bus H 18.6 19.6 19.0

The voltage stability indices of Bus G and Bus H have increased with the removal of reactors which indicate that the bus has moved away from the voltage instability or voltage collapse.

The Fig.7. below shows the PV curves under different conditions at Bus G. The curves clearly illustrate the improvement in the voltage stability margin.

6.4 Reduced Transmission Losses

In the above study, transmission loss analysis is done which shows that the transmission losses of the system with all the reactors remaining permanently connected is reduced from 0.0937 pu (93.7MW) to 0.08277pu (82.77MW) with all the reactors removed from the system. Similarly, the transmission losses are reduced from 0.0937pu (93.7MW) to 0.08933 pu (89.33 MW) removing 4

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reactors at strategic locations. The reduction in losses with removal of reactors benefits the system economically.

Fig. 7. PV curves at load Bus G under different conditions

6.5 Economic Benefits of CSR

The above analysis and estimation of reduced losses prove that the utilization of CSR is very much beneficial to the power system. The additional investment on CSR compared to a fixed reactor can be justified by the revenue accrued in terms of savings in line losses alone. From the loss estimates derived from the load flow, a simple pay back calculation for the additional investment on CSR is given below. 6.5.1 Replacing all the 17 reactors in the system with CSR

Transmission losses : With shunt reactors (CSR ON) = 93.7MW. Without reactors (CSR OFF) = 82.77MW. Savings in losses : Losses reduced by 10.93MW. (Assuming CSR to be OFF on an average for 20 hrs per day.) Savings per annum = Reduced loss x No. of hours x 365 x Rs/ kWH

= (10.93MW x 20 x 365) kWH x Rs.3.0 / kWH = 79789000 kWH x Rs.3.0 /kWH

= Rs. 23.936 Crores With an average cost of CSR* Rs.4.0 Crores, The total investment amounts to Rs.68 crores.

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Interest and depreciation on the investment @15% amounts to Rs.10.20 Crores. Net earnings per annum : 23.936 – 10.20 = 13.736 Crores Payback period = 68/13.736 = 4.95 years.

(* - cost of a 50MVAR CSR with thyristor ON-OFF control)

6.5.2 Replacing only 4 reactors at strategic locations with CSR

Transmission losses : With shunt reactors (CSR ON) = 93.7 MW. With out reactors (CSR OFF) = 89.33 MW. Savings in losses : Losses reduced by 4.37 MW. (Assuming CSR to be OFF on an average for 20 hrs per day ) Savings per annum = Reduced loss x No. of hours x 365 x Rs/ kWH = (4.37MW x 20 x 365) kWH x Rs.3.0 / kWH

= 31901000 kWH. x Rs.3.0 / kWH = Rs. 9.570 Crores

With an average cost of CSR Rs.4.0 Crores, The total investment amounts to Rs.16 crores. Interest and depreciation on the investment @15% amounts to Rs.2.4 Crores. Net earnings per annum : 9.57 – 2.4 = 7.17 Crores Payback period = 16/7.17 = 2.23 years.

It may be noted that the above payback calculation is done without taking into calculation the benefit of additional power transfer and savings in equipment self losses. It is also note worthy that investment on CSR will be generating revenue in terms of savings in losses whereas investment on a permanently connected shunt reactor is a liability for ever.

7.0 Conclusions

Utilization of CSR in place of fixed reactor enhances the voltage stability margin, loadability, voltage conditions and decreases transmission losses. From the pay back calculations it can be concluded that the application of CSR must be given a serious consideration for not only the new installations but also for replacing the existing permanently connected shunt reactors. Although, the calculation justifies replacement of all the shunt reactors with CSRs, replacement of the same at least at some strategically identified locations is to be immediately taken up to improve the existing situation in the 400kV network. Future SVC requirements do not obviate the decision to install CSR.

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Authors

S. V .N ..Jithin Sundar is a Deputy General Manager with Corporate Research & Development Divn Bharat Heavy Electricals Limited, Vikas nagar, Hyderabad, India (e-mail: [email protected]). M. Reshmi. is a post graduate student of power engineering, JNTU presently under training at Corp.R&D, Bharat Heavy Electricals Limited, Vikas nagar, Hyderabad, India (e-mail: [email protected]). References [1] V. Ajjarapu, C. Christy, "The Continuation Power Flow: A tool for Steady State voltage stability," IEEE Trans. Power Systems, vol. 7, No. 1, pp. 416-423, February 1992. [2] G. N. Alexandro, V. P. Lunin, Y. G. Selesney, L. N. Shifrin, S. V. N. Jithin Sundar, S. C. Bhageria, C. D. Khoday , Amitabh Singhal, “Fast-Acting Controlled Shunt Reactor.” Vii SEPOPE, MAY, 2003, Curitiba, Brazil. [3] S. V. N. Jithin Sundar, S. C. Bhageria, C. D. Khoday , Amitabh Singhal , A. K. Tripathy, G. N. Alexandrov, M. M. Goswami, I. S. Jha, Subir Sen, V. K. Prasher “ Controlled Shunt Reactor-A member of FACTS family” Eleventh National Power system Conference.(NPSC-2000), Bangalore. India. [4] S. V. N. Jithin Sundar, S. C. Bhageria, C. D. Khoday, Dr. M. Arunachalam, Amitabh Singhal, M. I. Khan, A. R. C. Rao, J. S. Kuntia, G. N. Alexandrov, M. M. Goswami, M. Arunachalam, “Design, Testing and Commissioning of First 420kV, 50 MVAR Controlled Shunt Reactor in India,” 14-120,CIGRE 2002, Paris. [5] D. A. Alves, L. C. P. Da Silva, C. A. Castro,V. F. Da Costa, “Study of Alternative Schemes for the Parameterization step of the Continuation Power Flow Method Based on Physical Parameters-part I:Mathematical Modelling,” Electric Power Components and Systems, vol. 31, no. 12, pp. 1151-1166, December 2003. [6] “Report on Grid disturbances in Western region” WRLDC, December 2003.

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