Objective: The combination of increasing electric power demand, restriction on transmission system expansion, and flexible utilization patterns for transmission network, make the development and application of power electronics-based controller for power system a high priority. Generally, Flexible AC Transmission System (FACTS) devices refer to the use of power electronics to enhance the power system performance. Voltage-Sourced Converter (VSC) based Static Synchronous Compensators (STATCOMs) are used for voltage regulation in transmission and distribution systems. Unlike PWM-controlled STATCOMs, Angle-controlled STATCOMs are switched at line frequency to limit the system losses. In recent years, angle-controlled STATCOMs have been deployed by utilities for the purpose of transmission system voltage regulation, voltage stability improvement and increasing operational functionality.
Summary: VSC consists of four three level Neutral Point Clamped (NPC) three phase inverters. Their square wave outputs are combined electromagnetically to generate a 48-pulse output voltage waveform and their DC sides are connected to a common DC-bus. The output voltages of the three level NPC poles are combined through auxiliary and shunt transformers. The primary winding of the auxiliary transformer is doubly fed open wye winding with 11.9 kV rated voltage across the winding. The secondary winding is doubly fed open zigzag winding with the same voltage rating as the primary winding i.e. 11.9 kV. The voltage across the secondary leads the primary by 30 degree. The shunt transformer aids in voltage waveform construction and serves to couple the synthesized voltage to the 345 kV transmission system. The shunt transformer has standard delta connected primary windings rated 345 kV, and open delta secondary windings rated 21.4 kV across the winding. The AC-system zero-sequence current, in case of ground faults in the AC-system, is filtered out by the primary winding delta connection. Therefore, the AC-system zero-sequence current does not flow on the STATCOM tie line. The zero angle is set to 3.75 degree to eliminate the 23rd and 25th harmonics of the NPC configuration.
Impact: CSC project is currently up and running in upstate New York. Having the actual implementation of the system in the real time based simulation allows to do any type of study faster than with offline simulations programs in case of any abnormal condition. A complex network such as CSC system is simulated using a typical timestep of 25-50 μs. Besides, switching functions can be simulated in the range of 1-4 μs.
Semhi Isik, North Carolina State University