Active Power Compensation of Voltage Source Converters with Energy Storage Capacitors
2006 IEEE PES Power Systems Conference and Exposition
Electrical Machines and Power Electronics, School of Electrical Engineering, Royal Institute of Technology (KTH). This project is financed from the Competence Center in Electric Power Engineering (EKC 2 ) at KTH and is one of the ongoing projects within the research program of Controllable Power Systems at EKC 2 . The main contributions of this work can be summarized as following: • A novel flux modulation scheme combined with a deadbeat current control strategy has been developed for two level
... loped for two level Voltage Source Converters (VSC). By this control system, both the positive and negative sequence components of the converter current and bus voltage are controllable . • Effective active power compensation schemes have been proposed for improvement of the power quality at the Point of Common Coupling (PCC) in power systems and for performance enhancement of certain phase sensitive applications . • The impact of energy storage on the performance of a certain system under fault conditions has been investigated . • The possible use of a Static synchronous Compensator (StatCom) with energy storage to improve the power quality at the PCC of a system with cyclic loads has been studied (Chapter 6). i ii Abstract This project deals with voltage source converters with energy storage capability. The main objective is to study the possible benefits of energy storage to a power system with a VSC as the interface between them. First of all, a converter control system is proposed for a two level VSC. In the conventional converter control, the control system usually takes the voltage measured at the point where the converter is connected and calculates the reference voltage for the converter; with a modulation system the converter then produces the required 'average voltage'. In this project, a novel flux modulation scheme, combined with the deadbeat current control strategy, is proposed. The current controller is capable of controlling both positive and negative sequence current components. With flux modulation, the control system measures the bus flux and commands the converter to generate the required flux. Based on the proposed control strategies, several application studies have been carried out. The first application study investigates the effect of energy storage on the power quality at the point of common coupling when a system is subject to load disturbances. The voltage at PCC in a weak network is very sensitive to load changes. A sudden change in active load will cause both a phase jump and a magnitude fluctuation in the bus voltage, whereas reactive load changes mainly affect the voltage magnitude. With the addition of energy storage to a StatCom, it is possible to compensate for the active power change as well as providing reactive power support. In this thesis, some effective active power compensation schemes are proposed. Simulations and experiments have been performed to verify the compensation schemes. The iii results show that a StatCom with energy storage can significantly reduce phase jumps and magnitude deviations of the bus voltage. The impact of the energy storage on the performance of weak systems under fault conditions has been investigated. The investigation was done by studying an example system. The system model was established based on a real system, in which some induction motors driving pumps along a pipeline are fed from a radial transmission line. Studies show that for a weak system with induction motor loads, a StatCom with certain energy storage capacity will effectively improve the system recovery after faults. Although this incurs extra cost for the increasing dc voltage rating and size of the dc side capacitor, the overall rating of the converter can be reduced by utilization of the proposed active power compensation scheme. The last case study investigates the possible use of a StatCom with energy storage to improve the power quality at the point of common coupling where a cyclic load is connected. Studies show that by providing both fast reactive and fast active power support to the network, not only the voltage magnitude can be well controlled, but also the voltage phase jump can be reduced significantly.