Research On High Voltage Large Capacity Alternate Active Power Filters

In this dissertation, three novel active power filters are proposed for 10kV~35kV power distribution systems application, which are the combination of traditional transformer theory and modern power electronic technology. The disadvantages of the conventional active power filters for high voltage gird are overcome in the proposed schemes, thus the security and reliability, the dynamic responses and the ability of harmonic suppression and reactive power compensation are satisfactory. The correctness, the validity and the practicality of the three schemes are demonstrated by the theory deduction, the simulation analysis and the experimental results, which establishes definite base for the practical applications of high voltage large capacity alternate active power filters.To manufacture a high-power active filtering device, a novel transformer structure that contains one primary and multiple secondary compensation windings is presented. The proposed linear transformer is applied to a series hybrid active power filter that is based on the fundamental magnetic flux compensation and thus a 100kVA prototype is successfully developed. Firstly, the series transformer's primary fundamental current is detected. Then, the reference signals of fundamental compensation currents are obtained according to the fundamental magnetic flux compensation condition. Fundamental compensation currents that produced by power electronic converters are injected into the multiple compensation windings. In this condition, the series transformer can really exhibit very low primary leakage impedance to fundamental and very high primary self-impedance to harmonics and thus acts as a"harmonic isolator". The topology consisting of three independent single- phase transformers and single-phase converters is adopted in three-phase active power filters in order to deal with unbalanced harmonic-producing loads. Since the converters are the same, modular design scheme can be adopted. The proposed active power filter together with the TCRs can be used to realize the combination of harmonic suppression and reactive power compensation.A novel shunt active power filter is proposed, which is based on the harmonic impedance control of a linear parallel transformer. Firstly, the primary harmonic current of the parallel transformer is detected. Then, the reference signal of harmonic compensation current can be obtained according to the harmonic compensation condition. Harmonic compensation current that is produced by power electronic converter is injected into the secondary compensation winding. In this condition, the parallel transformer can exhibit nearly zero impedance to harmonic current and very high primary self-impedance to fundamental current and thus lead harmonic current in power systems to flow into the transformer branch. With a step-down transformer, the power semiconductor switches of the active power filter are always worked in relatively low voltage, which ensure the security and reliability of the active filtering device. To greatly increase the capacity of the proposed active power filter, a novel parallel transformer structure with multiple secondary compensation windings should be adopted.A novel shunt hybrid active power filter is proposed, which is based on the harmonic impedance control and fundamental impedance adjust of a linear parallel transformer. Firstly, the parallel transformer's primary harmonic current and primary fundamental current are separated. Then, the reference signal of compensation current is calculated according to the fixed harmonic compensation coefficient and the variational fundamental compensation coefficient. Compensation current that is produced by power electronic converter is injected into the secondary compensation winding. In this condition, the parallel transformer can exhibit nearly zero impedance to harmonic current and continuous adjustable reactance to fundamental current. Thus, the proposed shunt hybrid active power filter can not only lead harmonic current in power systems to flow into the transformer branch but also compensate reactive power of power systems together with passive power filter. In the same way, a novel parallel transformer structure with multiple secondary compensation windings should be adopted to greatly increase the capacity of the proposed active power filter.Three reference current detecting methods are presented for single-phase active power filter, i.e. fundamental current detecting method based on parallel resonance of passive LC filter, fundamental current detecting method based on switched-capacitor filter (SCF), harmonic current detecting method based on d-q transformation and its improved circuits. A reference current off-line detecting method is also presented for the two shunt active power filters proposed in the dissertation, which has ideal dynamic response since detecting delay does not exist. With strategy of each phase independent detecting, these methods can also be implemented in three-phase active power filter.Unified accurate mathematic model of the three active power filters presented in the dissertation is established, transfer function is deduced, sufficient and necessary condition of system steady is also put forward. Two current control schemes are discussed, i.e. current nonlinear control method based on instantaneous compare and current linear control method based on triangle compare.Finally, assistant circuits and control systems of the prototypes are discussed, which mainly contains five aspects, i.e. operation principle of IGBT grid driver and introduction of three drivers implemented in the prototypes, typical IGBT cushion circuits for voltage source converters, protection methods of over-current over-voltage and over-temperature, design principles of transformers, inductances and control systems.