VOLTAGE SUPPORT TRANSFORMER A NOVEL DEVICE FOR IMPROVED POWER QUALITY

This paper describes an innovative device Voltage Support Transformer (VST) for improved power quality during power system transients. The VST applications that reduce voltage sags caused by faults in power systems and reduce overvoltages caused by shunt capacitor bank switching are presented. Voltage sags (dips) are caused by short circuits on either the utility transmission or distribution system. Duration of voltage sags depend on the fault types. Whenever a fault occurs in the power system that causes voltage sag at the fault location it will also cause voltage sag at loads that are connected to the nearby feeders. Unfortunately, even if an industrial facility with a sensitive manufacturing process is not connected on the faulted circuit, voltage sags may affect the manufacturing process. When installed at the utility substations that provide power for an industrial facility, the VST will support the system voltage for the whole facility. Shunt capacitor bank switching may cause high overvoltages that can endanger normal operation of sensitive load and, in some cases, cause high overvoltages that may damage power transformer insulation. The VST may effectively reduce the capacitor bank switching overvoltages. The VST is a specially designed single-phase transformer that mutually couples the same phases of two or more lines and is connected in series with the loads. It uses a change in current from one line in which a fault condition or other power perturbation exists to create a voltage change in one or more of the other lines of the network connected to the same bus, to maintain or support the voltage in those lines at or near pre-perturbation levels.