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Enhancing the Utilization of the Cape’s 765kV Cape Corridor by Series Compensation
Nhlanhla Mbuli
Department of Electrical and Electronic Engineering, University of Johannesburg, South Africa,
mbulin@eskom.co.za
JHC Pretorius Department of Electrical and Electronic Engineering,
University of Johannesburg, South Africa, jhcpretorius@uj.ac.za
Mondli Mkhize Eskom Research Center
Rosherville, South Africa, MkhizeMO@eskom.co.za
Silence Sithole
Department of Electrical and Electronic Engineering, University of Johannesburg, South Africa,
sitholfs@eskom.co.za,
Abstract—To enhance the voltage stability limit of the Western Cape network, in the 1980’s Eskom introduced 765kV technology to build additional lines into the Cape. This was because introducing more 400kV lines seemed not to yield substantial, additional transfer, and, because of its transfer potential compared to 400kV, 765kV was considered a better option for future expansions. The challenge here was that the then existing 400kV system was very well-developed, with a number of lines built in parallel. As a result, when the 765kV lines were introduced, their utilization was far less than adequate. This paper reports the results of a study that evaluated the possibility of series compensating the lines in the 765k Cape corridor. The areas dealt with here are the impact of series compensation on (1) loading of the 400kV and 765kV lines, (2) the system active power losses, and (3) voltage stability limit of the system.
I. INTRODUCTION
The geographic layout of the Eskom’s Cape network is presented in Figure 1 below. Referring to this diagram, Grootvlei substation is shown in the northern-most part of the system. Around this location are major coal reserves Hence, a big generation pool of coal-fired power stations was built in here.
Also shown in the figure, is the geographic layout of the transmission Western Cape network, located on the South Western side of Hydra substation. The distance from the location of the generation pool on the northern part of the figure to the Western Cape network is in the order of 1500km. Although there are some power stations located in the Western Cape, they cannot supply all the demand requirements, and
power has to be imported from the north to supply the entire load.
The large distance mentioned above, make it quite challenging to move power from the generation pool to the Western Cape, with voltage stability a limiting factor to transfer capability.
Initially, the supplies to the Cape were achieved by commissioning 400kV lines. As load grew, and more lines were required, it was found that not acceptable improvements in the transfer capability (i.e., voltage stability improvements) could be realized by adding more 400kV lines. A decision was taken in to build and operate additional lines at 765kV. As is known, the transmission capability of a transmission line increases as a square of the voltage chosen [1,2].
The utilization of the 765kV lines has had positive impact on the transfer capability into the Western Cape. However, close scrutiny of their utilization, as determined by their loading, suggests that this is poor. Under high loading conditions, they reach only about 20% of their thermal capacity. With normal, steady state rating, normally called rate A, of these lines at 4800 MVA, this represents a huge underutilization of infrastructure.
The underutilization can be ascribed to the strength of the 400kV corridor of lines in relation to the 765kV lines. Since the 400kV corridor is well-developed, and therefore its impedance is comparatively lower, substantial power still prefers to flow in its path, and, despite the fact that 765kV line has a higher transfer capability, lesser than desired power is directed by the system into this path, especially in the absence of any mechanism of directing power into a preferred path.
978-1-4577-0547-2/12/$31.00 ©2012 IEEE
In this paper, the authors evaluate the possibility of introducing series compensation on the lines in the 765kV corridor. It is expected that by doing so, the impedance of the 765kV corridor will be reduced in relation to that of the 400kV corridor, and, in this way, the 765kV corridor will pick up higher loading. The benefits of this should be increased loading of the 765kV corridor, reduction in active power system losses, and improvement in voltage stability limit of the network supplying the Western Cape network.
In Section II of this paper, a brief, theoretical discussion of the impact of series compensations on transfer capability of a system is discussed. Thereafter, in Section III, the methodology followed in assessing the impact of series compensation of 765kV on the performance of the Cape network is discussed. The results of the study are presented in Section IV and conclusions are summarized in Section V.
Figure 1: 2013 Cape Corridor Network
II. IMPACT OF SERIES COMPENSATION ON POWER
TRANSFER
The transfer of active power along a transmission line can be expressed by the following equation:
δsinline
RSR X
VVP = (1)
where
RP is the active power flow in the line
SV is the sending end voltage
RV is the receiving end voltage
lineX is the reactance of the transmission line
δsin is the angle difference between the sending and the receiving ends
The introduction of some series capacitance, with reactance of CX , on the line has the impact of reducing the overall
reactance of the line, leading to a new power transfer equation of the form:
δsinCline
RSR XX
VVP−
= (2)
Now, in the situation under discussion, series compensation of the 765kV line is expected to have a direct influence on how power is split between the 400kV corridor of lines and the 765kV corridor. By having the 765kV lines series compensated, it is expected that their loading will increase substantially, whereas that on the 400kV system will drop.
The overall benefits on the overall system performance are expected to be improved utilization of the existing 765kV corridor, reduction in active power losses of the system, and enhancement of voltage stability of the Cape system.
III. STUDY METHODOLOGY
The following methodology was followed in assessing the impact of series compensation on the performance of the Cape corridor.
A. Scenarios Evaluated
The 2013 network was set-up and analyzed to obtain a base a performance of the expected network without series compensation. Thereafter, a scenario (called Scenario A) was constructed by series compensation some of the 765kV lines. The affected lines are Perseus Hydra, Perseus Gamma, Hydra Gamma, Gamma Kappa 765kV line and Omega Kappa 76kV line. The degree of series compensation assumed in the study was 50%.
B. Loadflows Studies Done
For base case and Scenario A, loadflow simulations were done for peak loadflow condition. The aim was to assess the impact of series compensation on the power flow in the 400kV and 765kV corridors.
The loadflow studies were carried out only for system healthy conditions using Power System Simulator for Engineering (PSS/E) [3] software.
Criteria for power system loadflow analysis during healthy conditions is that voltages must remain in the range of 0.95 and 1.05 pu (must not exceed 1.045 for 765kV) for, whereas equipment loading must not exceed rate A of equipment.
C. Active Power Sytsem Losses
Losses were calculated for both scenarios to assess if restructuring of the power flows would have any impact on the total active power losses of the system.
D. Voltage Stability Calculations
Voltage stability limits were also calculated for the two scenarios to assess the transfer capability of the two scenarios in relation to each other. The power transfer at the nose point of voltage stability curve was used. Voltage stability limits. Voltage stability limits were calculated using VSTAB [4].
IV. RESULTS
The result of the analyses are presented and briefly discussed in the sub-sections below.
A. Loadflow Analysis
This inserting series capacitors on the 765kV lines leads to a substantially increased loading on the 765kV lines and a drop in the loading of the 400kV lines as shown in Figure 2. Similar information is shown in Figure 3, where series compensation leads to the loading of the 765kV from about 24% to 38% of line’s MVA rating. This is very close to doubling the utilization of the line.
Figure 2: Powerflow in MVA rating in various corridors of
the Cape network
Figure 3: Powerflow in % of MVA rating
B. Active Power System Losses
The introduction of series compensation leads to a reduction of active power losses of the system by 75 MW, as shown in Figure 4. The net present value of such a saving in losses could amount to a value of R 1.5 billion considering the
assumptions [5] used in Eskom to calculate the benefits or cost of losses.
Figure 4: Change in active losses of the system due to series
compensation
C. Voltage Stability Limits
Once series compensation is introduced, the restructured Western Cape network ends up with a higher voltage stability limit compared to the initial network. Refer to Figure 5.
Figure 5: Voltage stability limits on implementing series
compensation
V. CONCLUSIONS
The study has shown that by using series capacitors in higher voltage HVAC lines it is possible to substantially improve the loading, and thereby utilization of higher voltage lines.
Further to the increase in higher voltage HVAC lines, a substantial reduction in system active power losses can be realized.
Once the power flows are reconfigured, the resulting system realizes higher voltage stability, meaning an increase in power transfer capacity.
REFERENCES
[1] Glover, J.D., Sarma, M.; Power System Analysis and Design;
2nd Edition; PWS Publishing Company; 1993.
[2] van Cutsem, T., Vournas, C.; Voltage Stability of Electric Power
System; Springer; 2008. [3] PSS/E: Power System Simulator for Engineering; Online
Documentation; Version 32; June 2009. [4] VSTAB: Voltage Stability Analysis Program; Powertech Labs;
Version 4.1, December 1995.
[5] Corporate Finance; Economic Evaluation Parameters 2006-2010;
April 2006.
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