1. Analysis of the Amplitude and Frequencies of the Voltage Magnification Transients in Distribution Networks due to Capacitor Switching
Mohamed Saied
Electrical Engineering Department
College of Engineering and Petroleum
Kuwait University

2. Contents _________________________
1. Introduction
2. Objectives
3. Method of Analysis
4. Model Validation
5. Results of Parameter Studies
6. Conclusions

3. Introduction-1 _________________________
The increased use of capacitors in order to:
i) improve the network voltage profile,
ii) reduce the network losses,
iii) allow a more efficient utilization of the
network components’ transmission
capabilities
iv) the increased use of load types, such as
ASD , which are sensitive to abnormalities
in their voltage waveform

4. Introduction-2 _________________________
2. Of special interest are those stresses appearing at the terminals of a low voltage customer following the switching of a utility capacitor at a higher voltage level, and the possibility of their magnification.

5. Introduction-3 _________________________

6. Introduction-4 __________________________ 4
Introduction-4 __________________________ 4. Usually, these situations are analyzed applying either time domain techniques, frequency domain methods, or using sophisticated software packages such as EMTP.

7. Objectives _________________________
To provide the distribution engineer with a simple procedure for predicting the amplitudes and frequencies of the transients
2. To identify, in advance, the general trends of the impact of the parameters, and possible critical combinations of the network parameters, that can lead to dangerously high network stresses.

8. Method of Analysis-1 ____________________________
1.First, we apply circuit techniques to find the pre-switching voltage distribution at the network buses, which represent the initial conditions for the follow-up transient analysis using Laplace transforms.

9. Method of Analysis-2 _______________________________________
2. A derivation of analytical expressions for the expected maximum magnitudes of the total voltage, as well as their frequencies .
3. Finally, a relation will be found to identify the critical reactive power levels of both the customer and the switched utility capacitor that can lead to excessively high values of the switching transient voltage.

10. Initial Conditions
With the suffix a denoting pre-switching values:

11. Fig. 2: The Laplace domain equivalent circuit used to find the customer post-switching voltage and its frequencies. ___________________________________

12. Post-switching voltage in Laplace domain, at the customer

13. Finding the relevant frequencies : ____________________________

14. The amplitude of the transient component of the voltage at the customer (bus 1) ___________________________________

15. Model Validation
To validate the suggested procedure, a case study based on the power network described in [4] is analyzed.:
Supply Voltage=10kV, 60-Hz,=200 MVA,
= 1.5 MVA, Tranf.impedance(pu)=0.06,
Rating of the utility switched capacitor
=3 MVAR ,
Rating of the already connected capacitor at the customer=300kVAR

16. Fig. 3:The pu voltage at the customer bus, versus the customer reactive power compensation, for different values of as a parameter. Thicker curves correspond to higher .

17. Fig. 4:The pu voltage at the customer bus, versus the customer reactive power compensation for 3 different transformer sizes as parameter. Thicker curves correspond to larger sizes . ___________________________ .

18. Fig. 5: The expected frequencies, as multiples of the supply frequency, versus the customer reactive power compensation level . ____________________________

19. Fig.6: The maximum customer transient voltage as a function of both the customer compensation level and the utility compensation for the base case. _______________________________________________ )
(x-axis:
and y-axis:

20. Fig.7:Effect of the supply short circuit level (200MVA for left plot, and 50 MVA for the right )on the per unit customer post-switching transient overvoltage . Both diagrams are for the same switched utility capacitor size=3MVAR. ______________________________________________________________

21. Fig.8: Effect of the transformer per unit reactance on the maximum expected switching overvoltage across the customer. Thicker curves correspond to larger Transf.impedance(pu). ___________________________

22. CONCLUSIONS-1
1. The voltage magnification at the customer following switching of distant utility capacitors is discussed. Model has been suggested, giving closed-form expressions for the maximum voltage and its frequencies .
2. Suggested model validated for cases using solutions available by applying EMTP.

23. CONCLUSIONS-2
3. For the case study, one of the frequencies in the transient varies between 5 and 8 times the power frequency, while the other is very sensitive to changes in the load compensation. The second frequency reaches values as high as 30 times the power frequency at small customer compensation levels.

24. CONCLUSIONS-3
4. There are combinations of the values of both the customer and utility compensation levels that can lead to extreme amplitudes of the transient voltage across the customer which can attain values close to 2.5 per unit.

25. CONCLUSIONS-4
The pre-switching customer voltage is more sensitive to changes in the load reactive power in weaker networks.
6. The maximum switching transient, is almost independent on the network S.C. level, will occur approximately at same load reactive power. Beyond this load MVAR, the switching will initiate higher voltages in the weaker networks.

26. CONCLUSIONS-5
7. The switching transients become more severe and occur at smaller customer compensation levels, as the transformer reactance increases

27. Thank You!