Effective Design of STATCOM Considering Fundamental Frequency Current, Active Harmonic Filtering and Zero Sequence Current

Abstract

The main objective of this thesis was to investigate the effect of parallel reactive power compensation (RPC) and active harmonic filtering (AHF) operation on a STATCOM design, in terms of needed number of submodules (SMs), DC link voltage capacity, MV busbar voltage, zero sequence current demand, transformer and coupling inductor reactance. To achieve this objective, two design scenarios were carried out. In the first scenario, fundamental reactive current of studied STATCOM was prioritized over its current for active harmonic voltage filtering. In the second scenario, studied STATCOM was required to produce the nominal fundamental reactive power and perform active harmonic voltage filtering simultaneously.

The problem was studied in PSCAD based simulation environment. In all simulations, q-component current was supplied manually to enable the RPC operation of studied STATCOM. To enable AHF operation, harmonic current control mode was used, and the reference value of the desired harmonic filtering current was supplied accordingly. However, before proceeding with any simulation, first, the system limitations based on the studied STATCOM technology were studied and adequate majors were placed inside the simulation mode accordingly. Thereafter, simulations providing information on the basic STATCOM design operating in RPC mode only (and without AHF functionality) were carried out so that it can be compared later with the aforementioned scenarios of parallel RPC and AHF operations.

In the first design scenario, it was found out that additional AHF operation affects the STAT-COM design in three ways. First was the magnitude of AHF current where an increment in the needed number of SMs w.r.t basic design was noticed with increasing magnitude of AHF current. The second was the phase angle references of AHF current where if phase angle references of AHF current are chosen such that peaks of produced voltage source converter’s (VSC’s) fundamental and harmonic voltages are aligned then the amount of needed SMs to produce the same VSC voltage was increased. But, if phase angle references of AHF current are such that the peaks of VSC voltages are opposite to each other, then fewer SMs are required to produce the same VSC voltage. The third effect on STATCOM design was based on the harmonic order of AHF current produced. It was noticed that when harmonic order of AHF current was high, then the amount of needed SMs to produce the same magnitude of AHF current was increased.

In the second design scenario, it was found that maximum fundamental reactive current and maximum filtering current cannot be achieved at the same time with a geometrical summation principle of these currents, but possible with an arithmetical summation principle with a trade-off between optimum utilisation of current capacity and extra hardware cost. Hence, an optimum design to achieve the maximum of RPC and AHF current simultaneously exists between economical (based on the geometrical summation principle) and conservative (based on the arithmetical summation principle) design, but rather close to the economical one. In last, it was also noticed that the maximum demand of zero sequence current occurred when STATCOM was producing fundamental reactive current and negative sequence AHF current simultaneously in the maximum capacitive operation point, with an unbalanced network. And, peaks of positive and negative sequence network voltage and peaks of produced VSC voltages (fundamental and harmonic) were aligned.