EE 4223/5223 - B.A. Mork HMWK “CAP” __________________________ Capacitor Bank Protection Basics Complete by Tue Apr 12, 2005 A 230-kV 80-MVAR capacitor bank is connected in grounded wye. Differential-voltage protection is to be done by comparing L-N bus voltages to L-N voltages across the capacitors closest to the neutral. Sketch out the capacitor bank connections, including the VTs. Refer to the NEMA-standard capacitor sizes on the attached sheets when sizing the capacitor bank. The individual capacitors can withstand a continuous 10% 60-Hz overvoltage without damage. Decide on a series-parallel configuration for each phase and then answer the following: a) How many series groups of capacitors are required on each phase? How many capacitors are paralleled in each series group? How many total capacitors are needed for the entire three-phase bank? (Feel free to make use of the capbank.xls spreadsheet that is available on the course web page). b) What is the required voltage ratio of the VTs? Can you specify exact ratios or must you use standardly- available CCVT/VT ratios? For normal operation, what voltage will be measured by the VTs on each phase? c) If one capacitor in the series group closest to the neutral fails and is cleared by its fuse, what voltage will be measured by the VT on that phase? How many capacitors in a group could fail before exceeding the rated voltage of the remaining capacitors? If the 3 VT secondary outputs are summed together to measure voltage imbalance, what will the unbalance voltage be? (This calculation is not done by the spread sheet). d) If one capacitor in one of the series groups not monitored by the VTs fails, what voltage will be measured by the VT on that phase? If the 3 VT secondary outputs are summed together to measure voltage imbalance, what will the unbalance voltage be? e) What is the ratio of the 2 VT secondary voltages for normal operation of the cap bank? Recommend Voltage differential settings. As backup, a CT is used to monitor the neutral current. Recommend a CT ratio and type- 51 overcurrent relay pickup setting. In both cases, recommend alarm thresholds and trip thresholds for the relays. f) Make a recommendation on whether it’s good to have fewer or more capacitors in parallel in each of the series groups. Factors to include are economics, reliability of the capacitor bank and the system it’s connected to, and maintenance.

Protection of capacitors

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Electric Power Protection of capacitors is explained via examples and numerical explanation

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EE 4223/5223 - B.A. Mork HMWK “CAP” __________________________Capacitor Bank Protection Basics Complete by Tue Apr 12, 2005

A 230-kV 80-MVAR capacitor bank is connected in grounded wye. Differential-voltage protection is to be done bycomparing L-N bus voltages to L-N voltages across the capacitors closest to the neutral. Sketch out the capacitor bankconnections, including the VTs. Refer to the NEMA-standard capacitor sizes on the attached sheets when sizing thecapacitor bank. The individual capacitors can withstand a continuous 10% 60-Hz overvoltage without damage. Decideon a series-parallel configuration for each phase and then answer the following:

a) How many series groups of capacitors are required on each phase? How many capacitors are paralleled ineach series group? How many total capacitors are needed for the entire three-phase bank? (Feel free to makeuse of the capbank.xls spreadsheet that is available on the course web page).

b) What is the required voltage ratio of the VTs? Can you specify exact ratios or must you use standardly-available CCVT/VT ratios? For normal operation, what voltage will be measured by the VTs on each phase?

c) If one capacitor in the series group closest to the neutral fails and is cleared by its fuse, what voltage will bemeasured by the VT on that phase? How many capacitors in a group could fail before exceeding the ratedvoltage of the remaining capacitors? If the 3 VT secondary outputs are summed together to measure voltageimbalance, what will the unbalance voltage be? (This calculation is not done by the spread sheet).

d) If one capacitor in one of the series groups not monitored by the VTs fails, what voltage will be measured bythe VT on that phase? If the 3 VT secondary outputs are summed together to measure voltage imbalance, whatwill the unbalance voltage be?

e) What is the ratio of the 2 VT secondary voltages for normal operation of the cap bank? Recommend Voltagedifferential settings. As backup, a CT is used to monitor the neutral current. Recommend a CT ratio and type-51 overcurrent relay pickup setting. In both cases, recommend alarm thresholds and trip thresholds for therelays.

f) Make a recommendation on whether it’s good to have fewer or more capacitors in parallel in each of the seriesgroups. Factors to include are economics, reliability of the capacitor bank and the system it’s connected to,and maintenance.