Sliding Mode Control And Digital System Design Of STATCOM

Voltage-sources-converter-based Flexible AC Transmission System(FACTS)plays an important role in modern high voltage transmission system.Static synchronous compensator (STATCOM)is classified as second generation FACTS.The fundamental principle of STATCOM is the generation of a controllable ac voltage source by voltage source inverters connected to adc capacitor.In this way,STATCOM can provide smooth and rapid reactive power compensation to power system.It has been reported that STATCOM can offer a number of performance advantages for reactive power control applications over the conventional static VAR compensator(SVC)because of its greater reactive current output capability at depressed voltage,faster response,better control stability,lower harmonics,and smaller size,etc.Up to now,a great many successful applications of STATCOM equipment for voltage control and transient stability improvement have been reported in the literature.Conventionally,the STATCOM regulator is designed from a small-signal model at a given operating point.But the traditional controller suffers from the inadequacies of providing suitable control and transient stability enhancement over a wide range of power system operation conditions.The power system is a typical nonlinear system.Thus,it is reasonable to design the controller for STATCOM using nonlinear control theory.After having investigated the present research situation of STATCOM nonlinear controller,this dissertation studies on a series of effective nonlinear coordinated regulators for STATCOM inner-loop and outer-loop controllers by using fuzzy control(FC),sliding mode control(SMC), direct feedback linearization(DFL),etc.Besides,a DSP-based lab prototype model is built to validate the concepts of three-phase distribution level small capacity STATCOM and the control scheme proposed in this dissertation.The main content of this dissertation falls into the following seven parts:1.The system structure and operation principle of STATCOM are thoroughly investigated.Using in-out modeling and energy equations,a time domain and a steady state mathematical model of a STATCOM are developed.Based on the mathematical model,the fundamental running characteristic of STATCOM main circuit is analyzed.Aim at large capacity compensation situation,the design method of STATCOM multiple main circuit configuration and its control parameters are discussed.The results of simulative experiment are given to verify the feasibility of the main circuit scheme.2.Two-level structure control model of power system STATCOM controller is proposed. Firstly,two basic reactive current inner-loop control laws are derived.Based on Matlab/Simulink tools,a STATCOM device level control system is modeled.Then the digital simulation is carried out,the results illustrate that the model is correct and these control laws are effectiveness.Finally,concerning power system stability control,the outer-loop control system structure and voltage type control characteristics of STATCOM are comprehensively analyzed.These conclusions are necessary to the future study in the following sections.3.The stable region of one-machine infinite-bus power system with STATCOM is studied.According to the power system mathematic model,a damping-reflected energy function is derived with first integral.Based on the energy function,the system stable region is estimated,and some simulation results are given.The simulation results show the changing tendency of the system stable region is equivalent to reactive current capacity of STATCOM, which indicates STATCOM is beneficial for the system stable operation and efficiently expand the power system asymptotic stable region.4.A sliding mode controller(SMC)for STATCOM reactive current inner-loop control system is designed.Firstly,the reactive current control model is built up based on STATCOM device level nonlinear.mathematical model.The model is transformed into a linear system using direct feedback linearization(DFL)technique.Then a pole-placement method is employed to select a suitable sliding mode hyper plane,from which the variable structure control scheme considering disturbance is constructed.The chattering in the control signal is suppressed by replacing the sign function with a smoothing function in the control function. Finally,the control logic is evaluated in Matlab/Simulink,and the simulation results are compared with the traditional PID control design.It is shown that the proposed approach is superior to the traditional PID method in response performance,has stronger robustness against external disturbance.5.According to the nonlinear model of the power system including STATCOM,the paper presents the design of a nonlinear fuzzy sliding mode controller(FSMC)for STATCOM to enhance the transient stability performance of power system.For the three-order nonlinear system model,it is be transformed into a linear system by use of the DFL technique.Based on the linear sliding mode hyper plane designed by means of pole-placement method,a Mamdani-type fuzzy sliding mode control scheme is carried out. The parameters of the fuzzy controller can be varied widely by a suitable choice of membership functions and parameters in the rule base.In order to eliminate the control dead-zone of FSMC signal and improve voltage regulation accuracy,a proportional-integral (PI)regulator is implemented into the control system.The simulation results show the effectiveness and robustness of the proposed approach over a wide range of operating condition changes.6.A coordinated nonlinear controller of STATCOM and generator excitation is designed according to the DFL technique and the T-S type FSMC theory.The proposed coordinated controller can improve both the power angle stability of generator and the voltage behavior at the STATCOM location.The characteristics of the gain variation are parameterized and governed by the rule proportionality.The controller can provide suitable voltage control and transient stability enhancement over a wide rang of power system operating conditions.It is designed based on local measurements and possess good robustness.The simulating studies indicate the proposed coordinated controller can improve the accuracy of voltage control and strengthen the stability of power system.7.The algorithm of harmonic and reactive current detection by ip-iq method based on instantaneous reactive power theory is analyzed.Voltage Source Inverter(VSI)main circuit of small capacity STATCOM is introduced and its operational principles are deduced.The main circuit parameters design method and the current-track-method's SVPWM control algorithm are presented.In order to meet the real time requirement of STATCOM with direct current control strategy to reference current acquisition,a hard circuit based on Digital Signal Processor(DSP)TMS320F2812 and Complex Programmable Logic Device(CPLD) EPM7128SLC is designed.The key hardware and software of system are presented.The algorithm simulation on three-phase ac circuit is done.Experimental results confirm the validity and feasibility of the proposed method.