Coordination control of statcom and ultc of power transformers

STATCOM provides the opportunity to improve power quality and reliability due to its fast response, and has the functional capability to handle dynamic conditions, such as transient stability and power oscillation damping in addition to providing voltage regulation. When both STATCOM and under-load tap changers (ULTCs) are used to control system voltage, the STATCOM reacts to the voltage deviation faster than the ULTC. If the STATCOM output reaches the maximum capacity limit, it loses active control and behaves similar to a shunt reactor/capacitor bank. Keeping reactive power reserve in an STATCOM during steady-state operation is always needed to provide reactive power requirements during emergencies. This paper presents a new control strategy to limit the steady-state reactive-power output of the STATCOM to a desired value during the steady-state voltage range based on a variable ULTC reference voltage control scheme. When STATCOM settles to a new operating point following a disturbance, the reference voltage of ULTC will be effectively changed based on the STATCOM output; thereby activates slow voltage regulators to return back and resetting the STATCOM to be within the steady-state margin.