Research On Control Strategies For Shunt Active Power Filter

With the continuous proliferation of non linear loads, harmonic pollution is becoming a serious problem and harmonic distortion has been considered as one of the major problems that degrade power quality. Besides conventional solutions such as passive power filters, the use of active power filters to improve the power quality has attracted much attention. Owing to its convenience in installation and maintenance, shunt active power filter has become the mainstream of practical applications. As a dynamic compensating device, the control strategies for active power filter have a vital influence on its performance. Therefore, the researches on the control strategies for shunt active power filter are deeply significative.The control strategies for active power filter include reference current detecting approach and compensating current tracking control approach. The p-q method based on instantaneous power theory applies only to the sinusoidal and balanced voltage conditions. And then the d-q method based on synchronous coordinate transformation can apply to the distorted voltage condition, while its detecting results have small errors in the case of voltage asymmetry. This thesis adopts a novel reference current detecting approach by combining instantaneous power theory with two kinds of fundamental positive sequence detecting methods, which are selective band pass filter and generalized integrator based positive sequence calculator. Compared with the existent p-q method and d-q method, the detecting approach adopted in this thesis does not need phase locked loop(PLL) and has a quicker dynamic response, furthermore, which can still get accurate detecting results under non-ideal voltage conditions.Two compensating current tracking control approaches for active power filter are studied in this thesis, that is voltage space vector pulse width modulation control method and voltage space vector modulation based double hysteresis current control method. This thesis adopts a novel SVPWM algorithm implementation structure which is simpler and more compact than that of the traditional SVPWM algorithm which suffers from the shortcomings of complex implementation structure and large amount of computation. In addition, this thesis improves the existent voltage space vector modulation based double hysteresis current control method and then obtains a novel control method which control the main currents of the active power filter system directly without detecting the harmonic and reactive power currents. It can still achieve a good control performance when the supply voltage is non-ideal. In order to realize digitalization of active power filter control system, this thesis uses VHDL and DSP Builder to design two digital control systems for active power filter, and verifies the correctness and effectiveness of the designed control system with the hardware in the loop simulation.