Research On A Series Hybrid Active Power Filter Based On Fundamental Magnetic Flux Compensation

Power quality problems such as voltage waveform distortion, voltage fluctuation, flicker and three phase unbalance become increasingly serious as the capacity of impactive and nonlinear load in electric power system continuously increases. Whereas some precise instrument and equipments used in modern production have much higher request to power quality. The author make a few researches on active power filter using for suppressing the harmonic of power systems and adjustable reactor, which have many functions, by combining the transformer with modern power electronics. At the same time the author analyzes some FACTS equipments containing transformer. The main research results are as follows: 1) The current (or voltage) of electric power system is analyzed. The amplitude calculating formulas of the fundamental and harmonics are achieved. The amplitude calculating formulas and the calculating results of one of Matlab's functions-FFT are compared according to Euler formula and the relation of the two is achieved. The validity of these analyses is proved by practical applications. Harmonic sources are classified into two kinds: current type harmonic source and voltage type harmonic source and their physical essential is analyzed in terms of simulation. An integrated parameter design of series hybrid active power filtering system containing passive power filters is introduced. 2) A novel principle of adjustable reactor based on magnetic flux controllable is introduced. The secondary winding of a transformer is inversely in phase injected a current producing by a voltage source inverter, which has the same frequency as the current of the primary winding. According to the transformer's voltage equations, the main magnetic flux of the transformer will vary continuously along with the continuous change of the injected current's amplitude; therefore, the equivalent impedance of primary winding of the transformer will change continuously. The stability condition when the amplitude and phase of the injected current from the secondary winding varies is addressed in detail. According to the stability condition, the adjustable reactor can run normally. When the two special states are applied in the active power filter, we can achieve a novel series hybrid active power filter with the fundamental magnetic flux compensation (FMFC) of a series transformer. In the active power filter, the fundamental current component of power utility is detected and tracked to produce a fundamental compensation current, which is injected into the secondary winding. When the injected fundamental current and the fundamental current component of the power utility satisfy the fundamental magnetic flux compensation condition, the series transformer can really exhibit very low impedance to fundamental and naturally magnetizing impedance to harmonics. As a result, the series hybrid active power filter with FMFC really acts as "harmonic isolator". In the active power filter, the fundamental current is detected and followed to realize the FMFC, the system bandwidth requirement is much lower and the control circuit is simple; the filtering system is more stable and more easily realized. The criterion in which the high order or high-pass filter can be omitted is addressed. The topology structure and characteristics when the series hybrid active power filter with FMFC is used in three-phase system are analyzed. 3) In order to design a high-power series active filter, a novel transformer structure with multiple secondary windings is proposed. The novel transformer structure contains one primary and multiple secondary windings, which are connected with a PWM inverter using for producing multiple fundamental compensating currents. When the overall multiple fundamental compensating currents and the fundamental current component of the electric utility in each phase satisfy the FMFC condition, the series transformer can really exhibit very low impedance to fundamental and magnetizing impedance to harmonics. The fundamental compensating current of secondary winding is provided together by multiple inverters after the transformer with multiple secondary windings is used. And then the switching devices of PWM voltage source inverter are available in the case of large capacity. The influence of the inconsistent mutual impedance and the inverter parameters is analyzed. Experimental results of active filtering system with 80kVA three-phase harmonic resource verified the validity of the novel principle of series hybrid active power filter with FMFC and the transformer structure for large capacity. 4) The timely detecting approach of the fundamental or harmonic current signal, which is specially researched in the paper, is a key technique in active power filter. The influence of the PLL influence to harmonic detection in ip-iq operation method based on the instantaneous reactive power theory is originally investigated. The conclusion is the harmonic can be detected accurately without the PLL when only harmonic instead of harmonic and reactive power is detected when the line frequency hardly change. What is more, two kinds of improved methods based on the instantaneous reactive power theory are investigated and the consistency in physical essence of both methods is proved. Furthermore, two kinds of fundamental detection approaches are introduced from the modern filter method in the paper. One is the fundamental detection method of the power systems in terms of LC parallel resonance. The other one is a simple fundamental currentdetecting approach by using Switched-capacitor filter (SCF). The principle, detection precision, dynamic response and the circuit realization of the two novel approaches are also addressed and verified in the paper. 5) The most often-used current control includes three strategies: hysteresis current control, carried modulation current control and periodical sampling method. The principle is analyzed in the light of one-phase and three-phase and simulations are made in order to verify those analysis. Their individual characteristic and suitable scope of the four strategies are explained. The parameter choosing criterion of hysteresis current control and the features of triangle comparison current control are addressed in the paper. 6)Three kinds of protecting strategies used in the series hybrid active power filter with FMFC are investigated: the secondary windings of series transformer are short-circuited, voltage protection by ZnO2 arrester and specially designing the series transformer in term of magnetic saturation characteristics. The main circuit diagram and its electrical control circuit of the series hybrid active power filter with FMFC are given. The magnetizing impedance choosing criterion of the series transformer and condition on how to use the magnetic saturation principle are explained. The dynamic switching process of the system, active part and passive part is explained in detail. Finally, static response question and the other key questions in the practical application are analyzed in the paper. 7) There are two kinds of realizing methods in the secondary winding when a transformer is used in many known FACTS equipments: injecting a current source and energizing a voltage source. The Generalized principle and relations of two kinds of realizing methods are originally addressed. The principle of a few FACTS equipments is explained in term of the generalized principle. These FACTS equipments include three kinds of realizing methods of injecting a current source and four kinds of realizing methods of energizing a voltage source.