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FACTS compensation modelling for optimal power transfer

by A Ndimurwimo, R T Harris, A G Roberts, and W Phipps, Nelson Mandela Metropolitan University, Port Elizabeth Eskoms Eastern Cape Albany-Wesley 66/22 kV transmission system has been experiencing a large voltage drop caused by its length. The voltage drop is an obvious indication of the imbalance between the generated and consumed reactive power in the transmission system. Utilities take different measures to raise the nominal voltage, but although this satisfies the consumers, it is done at the expense of other parameters in the system. If careful planning is not done, transmission compensation can result in the inability to transfer power at the designed capacity. The installation of a compensator must take into account its quality, rating and location and must target a specific constraint in the transmission system. Reactive var compensation, generated by capacitors, was identified as one method suitable to improve the power transfer capacity by overcoming a number of transmission constraints on the transmission line. Normally the voltage at consumer terminals should be within defined limits ,based on the nominal voltage. The limits vary between different classes of services. The desirable voltage range under normal operating conditions is usually set as the nominal voltage 5 to 10 %. The main cause of voltage instability is the inability of the power system to meet the demand for reactive power. Hence the loadability of a bus in a system depends on the reactive power support that the bus can receive from the system [1]. Eskom uses shunt capacitor banks to support the voltage within limits [2]. These capacitor banks are switched on in discrete steps and the resultant reactive power cannot be accurately controlled. Moreover, as the voltage limit is reached before the designed thermal limit, the system cannot deliver the designed full load power and ultimately cannot meet the consumers' power demand [Fig. 1, Table 1] The purpose of this paper is to:

observed at power factors, 0,65, 0,8 and 0,95 lagging, characterizing the three scenarios based on:

Nominal voltage and load angle Transformer and line loading

Modelling and simulations Scenario 1: Uncompensated transmission model Different simulations were carried out at full load. The uncompensated, large and small scale compensated models are shown in Figs. 2, 3 and 4. The layout in Figs. 3 and 4 also show the loadflow result box report. The results at the nodes (busbars) indicate the line to line nominal and per unit voltage as well as load angle in degrees. The branch elements, namely lines and transformers, show the active and reactive power and then p e r c e n t a g e l o a d i n g. T h e l o a d i n g i s expressed in terms of the percentage of

the designed full load current. The loads indicate the active and reactive power. Any transmission system has a number of variables such as: reactive and real power, bus voltage magnitude and load angle. Thermal capacity of a line sets a limit to the maximum apparent power (MVA) transfer [3]. Normally the thermal ratings for transmission lines are expressed in terms of current flows. The optimum transmission capacity is where the voltage and thermal limit are attained at the same loading. By convention, the voltage angle at the slack bus is the reference. Therefore as a result the following will happen:

A more positive voltage angle corresponds to an injection of power into the system. A more negative voltage angle corresponds to a consumption of real power.

The stability limit (where more power

Evaluate the reliability of different FACTS technologies and develop a suitable specification for FACTS, for this situation Identify a suitable compensation technology for use on the electrical distribution network

The DigSilent program was applied to get the load flow solutions for three transmission model scenarios: uncompensated, small scale compensated and large scale compensated. Different trends were

Fig. 1: Uncompensated transmission model.

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Substation Committees Wesley Wesley Peddie Table 1: Peak demand. Wesley Hamburg Wesley Peddie Feeder 2006 peak demand (MVA) 1,8 2,7 3,1 6,5 Till June 2007 peak demand (MVA) 1,1 2,4 2,8 6,9

receiving end terminal voltage to enhance the power transmission capability of the system. The SVC is a reactive power generator whose output is varied so as to maintain or control specific parameters (e.g., voltage, frequency) of the electric power system A compensator, when connected in a transmission system, can supply reactive power, capacitive or inductive and this can be varied to control the voltage at given terminals [3]. The compensator can be shunt or series. The shunt compensator is known as a controlled reactive current source, which is connected in parallel with the transmission line to control voltage [4]. The series compensator is a reciprocal of the shunt compensator as it is functionally a controlled voltage source, connected in series with the transmission line to control current [5]. The most economical means available for reactive power supply for voltage control is a fixed or a mechanically switched shunt capacitor. Scenario 2: Large scale compensated model Fig. 2 is the large scale compensation model at 0,95 power factor lagging when the system was loaded to full load. For radial transmission, the best location of the compensator is at the load end [3]. A 20 Mvar capacitor was connected at the Fishriver busbar to improve the voltage of the transmission system. While there was a significant voltage improvement at the receiving end of the line, other design parameters were observed to go beyond their limits, particularly the loading of the lines and transformers. If the load current is not kept within the transmission system operating limits, current flow may cause a thermal loading problem that can in turn become another constraint in the power transmission. Scenario 3: Small scale compensation model Fig. 3 is the layout of the small scale compensation model at 0,95 power factor lagging. The small scale compensation idea was implemented using a number of smaller rated capacitors that aimed at mitigating a number of constraints encountered in the large scale compensated model such as; poor voltage, line and transformer overloading a t s a m e t i m e, a n d h e n c e a c h i e v e an optimised compensation. A shunt compensator between 2 to 4 Mvar was modelled at Committees 11 kV and Committees 22 kV busbars separately. Other aspects that were considered in this model were the location and technology type of the shunt and series compensators. The addition of a series capacitor

Fig. 2: Large scale compensated model.

Fig. 3: Small scale compensated model.

flow would cause excessive separation of voltage angle) also tends to be a concern, only for long transmission lines in this case. Fig. 1 shows the topology of the transmission system in its physical and electrical design. It was simulated as an uncompensated transmission model. Observations were made of the voltage, load angle, transformer and line loading responses. Table 1 is the actual recorded peak demand for the period 2006 2007, as compared to the line design power

demand: Wesley = 5 MVA, Peddie = 10 MVA and Committees 7,5 MVA. Under the uncompensated transmission model the system could not deliver the designed full load power as it was confirmed that the peak demand was less than the system was designed to deliver. The uncompensated transmission model highlighted the need for direct voltage support as at full load the receiving end voltage was below the accepted standard, after maintaining a sufficient energize - September 2010 - Page 33

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Fig. 4: Transformer loading.

Fig. 5: Line loading.

necessitated the creation of another busbar. Apart from the C-shunt capacitors used at Committees 11 kV and Committees 22 kV busbars, other types or technologies of shunt capacitors considered were; RLC and RL. Results and analysis A number of criteria on the performance of the lines, transformers and busbars under different conditions had to be met using various capacitor technologies and ratings. Essential selection criteria considered were:

Wesley transformer (WESTR) was loaded beyond its rated value. The uncompensated transmission caused the WESTR loading to rise to one and half times its full load rating. The large scale compensation scenario caused t h e We s l e y t r a n s f o r m e r ( W E S T R ) a n d Fishriver transformer (FISHTR) loading to rise as following: WESTR to about 367%, FISHTR; 184%. The small scale compensation, showed that the WESTR (Wesley Transformer) loading was reduced to around 107% while the FISHTR (Fishriver Transformer) went down to 50%, this is less than half its large scale compensation loading. The large scale compensation analysis refers to the 20 Mvar capacitor connected at the transmission receiving end i.e. Fishriver busbar. Other simulations done in the large scale compensation entailed the same capacitor (20 Mvar) connected individually at busbars linked to small rated transformers like; Committees 11 kV (COM11TR) and Committees 22 kV (COM22TR). Each transformer experienced an unprecedented overloading, the highest overloading happened at Committess 11 kV substation where the transformer loading rose to more than 900%. This called for a smaller capacitor to mitigate this overloading problem and hence a small scale compensation. The next step was to introduce a series capacitor into the system in order to change the inherent impedance characteristics of the mentioned transmission system. Despite the series compensation, o v e r l o a d i n g a t We s l e y t r a n s f o r m e r remained a critical problem. In order to solve this problem, other capacitor types or technologies, namely RLC and RL, were energize - September 2010 - Page 34

explored at both Fishriver and Peddie substations for optimal power transfer. Fig. 5 is the line loading comparison for lines: Albany-committees (ALB-COM); Breakfastvlei-Peddie (BRK-PED) and PeddieWesley (PED-WES) . In all three scenarios, the loadings in ALBCOM and BRK-PED lines are within limits i.e. less than 100%. Moreover the line loading in the uncompensated transmission remains constant. In large scale compensation however, the PED-WES line displays an opposite trend with results in an increase in loading of more than 180%, while with small scale compensation the loading was around 90%. The reason PeddieWesley line (PED-WES) was greatly affected is because of the large reactive power generated into the transmission system by the compensator at the receiving end. Fig. 6 is a comparison of load angle (voltage angle) response. The uncompensated transmission load angle response remained almost constant around 0. The small scale compensation load angle response was more positive at the Fishriver LV, Wesley LV and HV busbars attaining the highest positive angles. The opposite response happened with the large scale compensation whereby the same busbars, i.e. Fishriver LV,Wesley LV and HV attained the highest negative angles. This illustrates the different effects of large scale compensation, as opposed to small scale compensation in terms of absorbing or generating reactive power. Fig. 7 is a comparison of the voltage response. The uncompensated transmission system profile showed a generally poor v o l t a g e r e g u l a t i o n. T h e l a r g e s c a l e compensation model raised the voltage regulation and the terminal voltage at the

Avoiding the overloading of transformers and lines hence loading must be less than 100% when maximum load is transmitted. Rated voltage to customers at all times must be within 5% of the nominal voltage The recommended model must be characterised by a positive voltage angle.

Graphs of the load flow are indicated in Fig. 4, 5, 6 and 7. The analysis compares the voltage and loading trends of the transmission system at 0,95 power factor at full load for the uncompensated, large scale and small scale compensation scenarios. The trend at 0,95 power factor was found to be consistently typical of the results obtained at 0,65 and 0,8 power factors. Large scale compensation was analysed when the compensator was connected at the Fishriver substation. Fig. 4 is a comparison of transformer loadings at 0,95 power factor lagging under all three scenarios: Uncompensated, small scale and large scale compensation. In uncompensated and large scale compensated transmission scenarios, the

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Fig. 6: Load angle response.

Fig. 7: Voltage response.

transmission's receiving end (Fishriver) to 0,95 pu. The small scale compensation improved the overall transmission system voltage profile to around 0.95 pu. The large scale compensation was implemented with the shunt C-type capacitors. The series capacitor improved the voltage at Peddie HV busbar. Line losses comparison Table 2 shows the line loss comparison under the three scenarios at 0,95 power factor. The uncompensated model shows the total loss of 4469 kW: The highest loss happened in the Albany-Committees (ALBCOM) line at 1574,44 kW, while the lowest loss happened in the Peddie-Wesley (PEDWES) line at 615 kW. Though the ALB-COM line is longer than PED-WES line, their loadings were almost equal, and the difference in losses is attributed to the amount and location of generated and absorbed reactive power In the large scale compensated transmission model, The PED-WES line loss rose to 3884,92 kW, more than six times (600%) of the uncompensated losses,

while the increase in the losses on ALBCOM line was 16%. The small scale compensated transmission model was characterized by the lowest total line losses as well as the lowest individual line losses of the three scenarios with PED-WES and ALB-COM lines losses reduced from the uncompensated model by 23% and 47% respectively. Conclusion The transformers and lines loading in the uncompensated and large scale compensated models explains why the recorded peak demand as seen in Table 1 is less than the designed capacity. The small scale compensated transmission model has enabled the transfer of the full load designed power. The optimised capacitor specifications for the small scale compensated model are: 11 kV, 2,25 Mvar shunt capacitor bank for Committees 11 substation, 22 kV, 2,25 Mvar shunt capacitor bank for Committees 22 substation, 66 kV, 5 , 5 M v a r s h u n t r e a c t o r f o r Pe d d i e

Substation, 66 kV, 415, 220A, series capacitor bank. The capacitor ratings and location of capacitors used in the compensation models established the difference between large and small scale compensation, then the results obtained after simulations produced distinct trends in the transformer loading, line loading, voltage (load) angle and voltage response that characterised the large and small scale compensation. The large scale compensation managed to raise the receiving end voltage at the expense of other transmission line parameters, while the small scale compensation displayed a reliable voltage supply, safe transformer and line loading and overall reduced line losses. References
[1] S Yome, Arthit and Mithulananthan (2004 April), Comparison of shunt capacitor, SVC and STATCOM in static voltage stability margin enhancement, International Journal of Electrical Engineering of Education. Retrieved February 11, 2005 from Find articles database. [2] C S l a b b e r t ( 2 0 0 3 ) T h r e e p h a s e l o a d compensator, Report no RES/RR/03/21253. Unpublished Eskom: Resources and Strategy Research Division, (p. 3). [3] N G Hingoran and L Gyugyi (2000). Understanding FACTS, Concepts and Technology of flexible AC Transmission systems ,1st edition, IEEE Press book, New York, (pp: 2,268 ,chapter 5) [4] N P Vores and P et al Gareth (2004), Optimal power flow as a tool for fault level constrained network capacity analysis www.era.lb.ed.ac.uk/ bitstream/184/634/1EEE TPWB Retrieved on 05/04/2006. [5] J A Momoh, (2001). Electrical power system applications of optimisation, 1st Edition, Marcel Dekker, Inc 2001, New York,(chapter 6).

Line name

Small scale compensated (kW) 1213,94 752,22 1050,18 325,33 3342

Large scale compensated (kW) 1825,91 1349,01 1883,35 3884,92 8943,19

Uncompensated (kW) 1574,44 950,96 1327,64 615,97 4469

Albany-committees Committees-Breakfastvlei Brakfastvlei-Peddie Peddie-Wesley Total

Table 2: Line losses.

Contact Alexis Ndimurwimo, Nelson Mandela Metro University, Tel 041 504-3332, alexis@nmmu.ac.za

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