Impact of energy storage on system performance under fault conditions

This work investigates the impact of the energy stored in the dc side capacitor of a converter on the system performance under fault conditions. The system under investigation consists of two interconnected sources feeding a radial string of heavily loaded transmission lines. Along the lines six large induction motors are connected together with eight other loads and some capacitor banks supplying the reactive power consumption in the motors and lines. Since the system is vulnerable to faults close to the sources, additional reactive power support is necessary to improve the fault ride-through capability of the system and the performance of the motors. During a fault, the motor terminal voltage is very low. The unbalance between the electrical torque and the mechanical torque decelerates the motor. After the fault-clearance, the motor starts to accelerate if the electrical torque generated is larger than the mechanical torque. In order to produce the required torque, the motor consumes much more reactive power than under normal condition. If the reactive power support is not sufficient, the motor will keep decelerating until it stops. A StatCom can be installed to provide fast and dynamic reactive power support and voltage control to the system. In this paper, a control scheme for a StatCom with energy storage capability is proposed. The system performance, especially the motor speeds, will be analyzed through simulations carried out in PSCAD. It is shown in the paper that the scheme with active power compensation effectively helps the system to recover from faults. Although this incurs extra cost for the increased dc voltage of the converter and the dc side capacitor, the overall rating of the converter can be reduced by utilization of the proposed scheme.