Key Technology Research To Improve The Operational Performance Of The Cascade Static Synchronous Compensator

Static Synchronous Compensator (STATCOM) is a new generation of dynamic reactive power compensation equipment, following the synchronous condenser, a mechanical switch capacitor switching and Static Var Compensator (SVC). It has all the advantages of fast reactive power regulation speed, wide range of adjustment, strong ability to regulate reactive power in undervoltage conditions, low harmonic content, small footprint, access the system without resonance. It can play an important role in terms of maintaining or controlling node voltage, improving the transmission capacity, damping power oscillation, static and transient stability, and improving power quality. In the field of medium voltage, the cascade structure STATCOM development is most rapid.At present, cascade STATCOM theoretical research and engineering practice have been carried out both at home and abroad, and mathematical modeling, parametric design calculation methods and decoupling control algorithm are mastered. But in cascade STATCOM engineering design and application, there are still a number of key technical issues affecting the cascade STATCOM running performance. In this paper, theoretical analysis and laboratory validation have been carried out in three key technical issues of the cascade STATCOM easy off-grid when the grid undervoltage and containing negative sequence, low harmonic compensation accuracy, and the control and protection system experimental verification technology.This paper points out that the DC voltage balance control is a key technology to inhibit the cascade STATCOM off-grid. The global DC voltage balance control strategy based on fundamental positive sequence current control and the DC voltage balance control strategy among phases based on fundamental negative sequence current control are proposed considering grid voltage negative sequence component influence in view of detailed mathematical models after analyzing the active power exchange during the cascade STATCOM running process. On this basis, the idea and implementation of double-sequence current decoupling control method are given.This paper analyzes the frequency characteristics of the current control loop among double-sequence current decoupling control method, then gives harmonic current compensation algorithm based on the Fast Fourier Transform (FFT).This algorithm is applied in10kV/3.5MVA STATCOM industrial prototype, and the problem of low accuracy of the harmonic current compensation is solved by eliminating the affects of the controller delay via harmonic amplitude and phase correction independently.This paper studies the closed-loop test methods of STATCOM control and protection system using the real-time digital simulation (RTDS) platform. The comparison result between the RTDS test results and the offline digital simulation results shows that the RTDS test results is closer to the industrial prototype test results in terms of the steady-state error and dynamic performance of the controller.On this basis, this paper builds the RTDS test platform for directly connected cascade STATCOM control and protection system, and carries on the experimental verification of double-sequence current decoupling control method to solve the problem of cascade STATCOM poor run stability using in the wind farm. The experiment result shows that the control strategy can effectively solve the problem of the cascade STATCOM easy off-grid when the grid under voltage and containing negative sequence, and ensure that the device is stable and reliable.