A 35-kV system voltage sag improvement

Electric utilities began to operate distribution systems at 35 kV about 20 years ago. Experience soon revealed that some unique conditions existed as the distribution voltage was increased to this level. Customers served by a substation feeder complained of blinking lights and equipment problems when a fault occurred on the adjacent feeder of the same substation. A comparison can be made between the voltage sag that occurs on a 12-kV and 35-kV system using system impedance and symmetrical component calculations. Most of the 35-kV voltage drop, during a fault, occurs in the substation power transformer. The low voltage appears at the substation low side bus. This paper includes field test data that verify the symmetrical component calculations for existing system conditions. It explains how lower impedance power transformers and special considerations for 35-kV system design will make improvements in voltage sag of up to 20%.