A control structure for PWM-controlled static synchronous compensators under unbalanced conditions and grid faults

Highlights

• The VSC-based STATCOM are sensitive to the unbalanced grid.

• Keep the VSC-based STATCOM online under the unbalanced condition.

• Limiting the negative sequence current.

• No need to redesign/overdesign of the converter power components.

• Performance verified by simulation and Hardware-In-the-Loop test.

Abstract

Grid connected Voltage Source Converters (VSCs) are the heart of many applications with power quality concerns due to their reactive power controllability. Among the widely used grid-connected applications of the VSCs, the Static Synchronous Compensators (STATCOMs) are commonly used for compensating the voltage quality problems that come from voltage sag and swell. In spite of superior feature of fast voltage regulation and reactive power support functionality, VSC-based STATCOMs have the major drawback of being sensitive to the grid disturbances, especially the unbalanced condition and faults. Moreover, when the STATCOMs are used in the Distributed Generation (DG) applications or reactive power support of the sensitive industrial load, the unbalanced condition becomes even more intolerable. Protection system usually trips due to over current or highly distorted current caused by negative sequence current flow under unbalanced conditions and system faults.

This paper propose an alternative control structure to keep the VSC-based STATCOM online during the unbalanced condition and system faults by limiting the negative sequence current. This eliminates the need to redesign/overdesign of the STACOM power components and over rating of the semiconductor switches to operate under fault current. Converter MVA rating reduction will decrease the cost significantly. Proposed controller performance has been verified by simulation and Hardware-In-the-Loop test.

Introduction

Significant voltage drops are usually observed at industrial load buses when there is a power fault at some point in the supplier utility or a sudden change in the utility load. This happens particularly if the industrial load is supplied through a long distribution feeder. Reactive power compensation is conventionally used to improve the load voltage profile and enhance the power factor. Voltage Source Converter (VSC) based STATCOMs controlled by the PWM techniques (Fig. 1), are commonly used for the purpose of voltage regulation and power factor correction particularly for the sensitive industrial loads. The VSC based STATCOM are also widely used in the DG systems like photovoltaic, and wind generation in the distribution level [1]. A major drawback of using the VSC-based STATCOM is however, their sensitivity to the voltage disturbances especially unbalanced condition and system faults. To protect the semiconductor switches from the huge fault current flow, the STATCOM is tripped under utility system fault or severe unbalanced conditions when its reactive power support functionality isy needed. Moreover most of the faults are unbalanced and results in voltage dips, which produce current harmonics and unbalance that also cause the STATCOM current protection to trip [1]. Therefore from the protection philosophy point of view, the VSC-based STATCOM with conventional current controller, either should be tripped or its power components should be overdesigned to tolerate the huge negative sequence current under unbalanced conditions and grid fault.

Basically the inverter power components and switches must be designed for the peak continuous operating current and for the peak continuous operating voltage. Generally there is a designed margin beyond this point to accommodate some percentage overload as well as specified abnormal operating condition particularly unbalanced condition and grid faults. The MVA rating of the equipment and hence the cost is derived from the product of peak voltage and current (considering the fault and unbalanced condition), regardless of whether they occur at the same time or not [2]. Hence the appropriate controller which is capable of limiting the fault current and consequently decrease the converter design margin will significantly reduce the converter equipment rating and cost.

It is highly beneficial to keep the STATCOM available during the power system faults when its reactive power support functionality may be needed the most. On the other hand, if the protection system trips the converter, it takes several fraction of an hour, depending on the size of the converter, to discharge the DC-link and check the healthiness of the entire system [3]. Hence, several practical methods have been proposed and implemented to attenuate the effect of unbalanced line voltage on the converter performance under system faults.

As the amount of the wind power around the world increases, system operators have been forced to tighten their grid connection rules-known as grid codes-in order to limit the effect of the wind powers on the network quality and stability. To achieve compliance with grid codes, some sort of the reactive power support like VSC-based STATCOM with appropriate fault ride through capability is required [4]. Therefore revisiting the VSC dynamics under power system disturbances seems to be an imperative factor in wind power expansion. Whereas the basic operating of the VSC-based STATCOM is well established in the literature, the fault performance has received relatively lower attention. This paper proposes an alternative current control structure to improve the PWM VSC-based STATCOM performance under severe unbalanced conditions and system faults. The main improvement is to decrease the negative sequence current under fault conditions. Limiting the negative sequence current will reduce the MVA rating and cost of the converter power equipment and switches.

In the proposed control structure, the conventional current controller in synchronous frame is in charge of controlling the VSC positive sequence output voltage. This is done by calculation of modulation index (M), and desired output voltage angle ($\alpha )$ based on calculated values of reference d and q component of the output voltage in synchronous frame. A new part is added to the conventional vector current controller in synchronous frame that controls the VSC output negative sequence voltage. The controllable negative sequence voltage attenuates the effect of the line negative sequence voltage due to the unbalanced condition and reduces the negative sequence current seen by the STATCOM. The output of this part of the controller is an oscillatory angle (2nd harmonic oscillation) with appropriate amplitude (M_ac) and phase (${\alpha }_{\mathit{ac}})$ as shown in Fig. 2. The output of the negative sequence voltage controller will be added to the α.

This paper is organized as follows. Section ‘Backgrounds on control the VSC under unbalanced conditions’ reviews the research and advances in the control of VSC under unbalanced conditions and grid faults. Section ‘Analysis of VSC under unbalanced operating conditions’ analyses the behavior of the VSC under fault conditions and obtains the required equations to develop the proposed controller. Section ‘Voltage spectral content’ presents the VSC output voltage harmonic content when 2nd harmonic oscillations are added to the DC-link voltage. In section ‘Proposed control structure development’, the proposed control structure and its implementation are presented. Proposed controller stability will also be discussed in this part of the paper. PSCAD/EMTDC simulation results and Hardwar-In-the-Loop testing results present in sections ‘PSCAD simulation results and Hardware-In-the-Loop-test’. And finally, section ‘Conclusion’ draws the conclusion of the paper.