1. Introduction

Voltage source converter (VSC) is widely used in industrial, commercial and renewable power generation applications. Out of many possible configurations, the three-phase and three-wire is widely used. During the last decade, the penetration of renewable energy sources has increased around the world. This is due to the increased concern worldwide about the carbon emission [1, 2]. The utilisation of energy from uncertain and variable sources has become possible with the use of these converters only.

For the purpose of this study, the analysis of VSCs is carried out assuming the ideal operating conditions of grid. But VSCs are never operating under such

conditions in practical. In a relatively weak system, VSCs are subject to various power quality disturbances, such as unbalance voltage, voltage swell and swag, notches, etc. [3]. The occurrence of such disturbances causes various problems like ripple in torque of generator and motor, increased losses, abnormal tripping of protective devices, malfunction of sophisticated control system, reduction in the expected lifetime of equipment, etc. [4].

calculated by the PI controller, which considers the DC voltage and desired active power through the VSC to the grid. The instantaneous reactive power is calculated by the separate loop, which may consider either the desired power factor or the reference voltage. The dq frame is synchronised with the positive sequence fundamental voltage of the grid at PCC with the help of PLL. It converts three phase voltages Va, Vb and Vc into Vd+ and Vq+, which are converted in the alpha-beta reference frame voltages Vα and Vβ. Then current reference is obtained by the α-β to abc transformation. These reference currents are compared with actual current, and modulating signals md and mq are generated. These signals are finally transformed

Harmonic Resonance Analysis for Wind Integrated Power System and Optimized Filter Design

Both switching frequency and grid voltage distortion can cause poor power quality. A filter design is a subject that requires trade-off between filter performance and the control bandwidth. Filters are required to meet power quality stan-

Inverter for grid interfacing will need to incorporate interface filters to attenuate

This method was proposed in [7]. The system impedance is partitioned into source and grid impedance. The source impedance is either represented by Thevenin's equivalent circuit or Norton equivalent circuit (Figures 2 and 3). Thevenin's equivalent circuit consists of ideal voltage source in series with the series impedance (Zs), whereas the load impedance is modelled by series impedance (Zl). Since the converter circuit is non-linear, it is represented by the small signal circuit. This linear representation of circuit is valid only for the small perturbation of signal. With this assumption, the current (I) flowing from source to load is given by

I sðÞ¼ Vsð Þ<sup>s</sup>

ZlðÞþs Zsð Þs

(1)

into ma, m<sup>b</sup> and mc to generate pulse width modulated (PWM) signal.

dard, avoid parallel resonance and improve power quality.

the injection of current harmonics.

DOI: http://dx.doi.org/10.5772/intechopen.89167

Figure 2.

Figure 3.

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Thevenin's equivalent circuit.

Norton's equivalent circuit.

3. Impedance-based stability analysis

There are two types of harmonics generated by VSCs. One is characteristics harmonics, which are related with the switching operations of the IGBTs inside the VSC. And second is non-characteristics harmonics. The voltage ripple on DC side of VSC generates harmonics on its AC side current. According to [5], the noncharacteristics harmonics are generated by the unbalanced voltage in the AC side. However the quantification of magnitude of such harmonics is not simple and cannot be done with deterministic method. The non-characteristics harmonics are considered as the steady-state low-frequency components which would not appear if the grid voltage is balanced. The unbalance grid voltage has fundamental frequency negative sequence component and third-order positive sequence component.

Though it is possible to eliminate zero sequence third harmonic component using transformer of proper vector group, the non-characteristics third-order positive sequence harmonics cannot eliminate transformers with delta-connected winding.

In [6], the author has proposed DC voltage control to eliminate DC oscillating voltage when AC side is unbalanced. To achieve this, VSC has to operate with constant AC power control. However, the effect of control on AC current is not discussed. It is important to analyse the distorted and unbalanced AC side current, when such control is implemented. In this case, the currents of AC side of converter contain non-characteristics low-frequency component such as fundamental negative sequence and third harmonic components of positive and negative sequence.
