An improved control strategy for a hybrid series compensator

Karthik, Kannan (2001) An improved control strategy for a hybrid series compensator. Masters thesis, Memorial University of Newfoundland.

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Abstract

This thesis presents an improved control strategy to ameliorate the load voltage and supply current profiles in a power system. A synchronous d-q-0 based control scheme is developed for a hybrid series compensator topology to cancel the zero, negative and harmonic components from the load voltage and also suppress the supply current harmonics. -- The proposed d-q-0 based control scheme combines the decoupling of the current and voltage variables, transformation and filtering to extract the harmonic voltages and currents. In the harmonic current extraction procedure, the zero sequence components (including the fundamental frequency) are removed before applying the required transformations to filter out the harmonics from the supply current. This is done to ensure that the zero sequence components do not produce alternating components in the synchronous frame, which lead to errors in the filtered output. This control approach was developed as an improvement over the instantaneous active and reactive power (IRP) theory based scheme, which assumes a balanced and sinusoidal supply voltage. It is shown that the IRP theory based harmonic extraction procedure does not provide an accurate estimate of the supply current harmonics in the presence of imbalance, voltage distortion and zero sequence components. -- The feed-forward control approach based on the synchronous transformations in the d-q-0 frame is susceptible to gain variations which affect both system stability and the compensation error. Hence, a simplified single phase harmonic equivalent model is developed to carry out stability and distortion analysis of the hybrid series compensator topology. The effect of the controller gain parameters Ky and Rh on the stability limits is investigated with some root-locus plots. The influence of the controller gain parameters on the distortion levels in the load voltage and supply current is examined through a distortion analysis. From the analysis it is shown that limited controller gain margin is one of the drawbacks of the hybrid single-series compensator topology. Moreover, stability constraints on the values of the control parameters limit the range of compensation for wide variations in the supply voltage and load harmonics. -- A two-compensator topology is proposed to reduce the interaction between the voltage and current signals by allowing independent control of the two voltage components, thereby increasing the stability limits. A much larger controller gain margin (hence larger compensation range) and suppression of voltage drop across the passive shunt filter are shown to be some of the advantages of the proposed topology. -- A simulation model of the overall system with the proposed two-compensator topology is constructed using the SIMULINK toolboxes in MATLAB. The simulation results are used to demonstrate the effectiveness of the proposed d-q-0 based control scheme and the two-compensator topology when applied to load voltage compensation and harmonic isolation of non-linear loads.

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