Harmonics Mitigation using Shunt Active Power Filter Based on Modified Synchronous Reference Frame Theory
The dependency on electrical energy is being increased rapidly with the advancement of technology. The use of smart appliances has also increased to save the electrical energy. The power electronic converters and non-linear loads are also the sources of harmonics and reactive power which, greatly reduces the power quality. Therefore, proper methods are required for mitigation of current harmonics which will enhance the reliability of grid-connected systems.
Many techniques have been used by researchers for mitigation of harmonics but, the most common method is the active power ltering due to its excellent performance in reducing harmonic distortion. This research is focused on shunt active power lter used for mitigation of current harmonics. The SAPF is consisting of two main parts which are reference current generation and injection of current at the point of common coupling. Many control strategies have been developed for reference current generation like instantaneous reactive power theory, synchronous reference frame theory and discrete fourier transform. Similarly many techniques are being used for current injection but hysteresis current control is most widely used. These techniques have been hardly evaluated under adverse grid conditions. The aim of this thesis is to develop a control strategy that works under all types of grid disturbances.
Synchronization of active power lter with the grid voltage is very important under the eects of frequency drift, phase angle variation and presence of dc oset. Existing phase locked loop techniques like synchronous reference frame PLL, moving average lter PLL and cascaded delayed signal cancellation PLL have been investigated for estimation of frequency and phase under adverse grid conditions. Cascaded delayed signal cancellation technique is shown to have best performance in terms of dynamic response and ltering capability. This identied technique is being applied to synchronize the active power lter with the grid using synchronous reference frame theory.
This thesis contributes by developing a modied reference current generation technique that employs cascaded delayed cancelation PLL instead of the conventional synchronous reference frame PLL to estimate frequency and phase of the grid voltage. Furthermore, sliding mode control strategy has been developed to inject the reference current accurately at the point of common coupling. The proposed strategy is tested under various grid disturbances and results are compared with the instantaneous reactive power theory which does not require frequency and phase information. Matlab/Simulink environment is used to evaluate the performance of shunt active power lter. The performance of the conventional control strategy of shunt active power lter is being investigated under normal grid conditions and found satisfactory but under distorted grid conditions such as presence of frequency drift, phase angle variation and dc oset, it is not able to reduce harmonics distortion within limits of IEEE-519 standard. On the other hand a control strategy is proposed that is able to show excellent performance under both normal and adverse grid conditions. The total harmonic distortion of the compensated source current in each test case of grid disturbances does not exceed the standard limit. The proposed topology enhances the harmonic compensation capability and robustness compared with the conventional APF topologies in grid-connected systems.