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Adaptive Filter Implementation for Dstatcom

N.Narasimhulu, K.Swathi

Non sinusoidal current in a distribution system is mainly due to nonlinear characteristics of equipment such as adjustable speed drives, switch-mode power supplies, rectifiers, and others such type of loads. In most cases, nonlinear loads are represented either as a harmonic current source or a harmonic voltage source in practical applications and responsible for creating power-quality problems. Mitigation of power-quality problems can be achieved by using passive or active filters; however, due to certain advantages of active filter (AF) with digital control, it is used for improving power quality. An improved version of a shunt-connected AF used in the distribution system is known as a distribution static compensator (DSTATCOM). It is used for compensation of current-related power-quality problems such as reactive-power compensation, harmonics elimination, and load balancing in power factor correction (PFC) mode and zero voltage regulation (ZVR) mode. Improved responses and capacity for transient overload even at a reduced voltage level are the major advantages of DSTATCOM. Various international standards such as IEEE and IEC have reported the guidelines of harmonics limit at the point of common coupling (PCC). Effective utilization of a converter used as a DSTATCOM depends upon the control algorithm used for extraction of reference currents and switching schemes. For extracting reference signals, many time-domain control algorithms are available, which are based on phase-dictated sinusoid-tracking parameter extraction of non-stationary sinusoidal, amplitude phase-locked loop (PLL), etc; unit template-based control algorithm without sensing load currents; one-cycle control, which has excellent harmonics suppression, simple circuitry, robust performance, and low cost linear feedback control and signal-processing algorithm for selective harmonic identification based on heterodyning, moving-average finite-impulse response filters, and PLL using feed forward based control. These algorithms are based on basic arithmetic operation of mathematical function, transform, tuning of internal constants, clock, an integrator, flip-flops, comparators and logic circuits, etc. The performance of these classical control algorithms depends upon selection and tuning of internal parameters, circuit components, and their formulations. An AF is implemented in three-phase distorted voltage ac mains for reactive-power compensation, harmonics elimination, and load balancing with a self-supporting dc link in PFC and ZVR modes of DSTATCOM. This proposed control algorithm is also modified for DSTATCOM operation in ZVR mode. The main features of this control approach are high convergence speed and robustness with respect to input frequency and internal parameter variations and less sensitivity to voltage pollutions when it is used as a reactive-power component of current extraction. Design of control algorithm only needs multiplications, integral, gain, and subtraction blocks. Thus, the structure of this control algorithm is based on basic arithmetic operation; hence, its implementation is simple, and it does not require any extra synchronization circuit. Selected values of integral constants are not affecting the performance of the filter within a certain range. The simulations were performed in the environment of MATLAB/SIMULINK.