International Journal of Electrical Power & Energy Systems
Interactions and multivariable design of STATCOM AC and DC voltage control
Introduction
The static synchronous compensator (STATCOM) is based on the principle that a voltage source inverter generates a controllable AC voltage source behind a transformer leakage reactance so that the voltage difference across the reactance produces active and reactive power exchange between the STATCOM and the transmission network. The STATCOM is one of the new generation flexible AC transmission systems (FACTS) devices with promising applications in future. The first full-scale STATCOM has been in operation since 1995, to be followed by installation of more STATCOM in practical power systems [1], [2], [3], [4], [5], [6], [7], [8], [9], [10].
The basic function of a STATCOM installed in a power system is for the line voltage control, which is implemented by an AC voltage controller in the STATCOM by regulating the reactive power exchange between the STATCOM and the power system. A second controller installed in the STATCOM is the DC voltage controller, which regulates the DC voltage across the DC capacitor of the STATCOM. It is the conventionally suggested arrangement of STATCOM control that two individual controllers are assigned two separate control functions. In Ref. [11], the issue of the STATCOM DC voltage control is addressed comprehensively for a pulse width modulation (PWM)-based STATCOM, providing an effective scheme for the control system design of STATCOM. STATCOM AC voltage control is fully investigated based on a large example power system in Ref. [12], presenting insight into the operation of STATCOM AC voltage control in large power systems.
However, from the point of view of the control system design, the arrangement of two controllers for two control functions of STATCOM is inherently defective. The multiple controls of the STATCOM, in fact, form a multi-input multi-output (MIMO) system. The assignment and design of a single controller for each control assumes the decomposition among the multiple control channels. If there exist interactions among control channels, the control performance and the closed-loop system stability may not be ensured by the satisfactory design of individual controllers [13]. In this paper, indeed, a case study of design failure of conventional STATCOM control is demonstrated. The STATCOM AC and DC voltage controllers were designed separately in a sequence, which resulted in satisfactory control performance and closed-loop system stability. However, when they were in joint operation as they should be, control performance decreased and even instability occurred. This study case strongly indicates the existence of negative interaction between the STATCOM AC and DC voltage control. However, this case study only demonstrates the possibility that the existence of the interactions may threaten the normal operation of STATCOM control system. It does not suggest that the failure of two single-input single-output (SISO) control for STATCOM AC and DC voltage regulation always exists. In fact, the scheme suggested in Ref. [11] can provide decomposed active and reactive power control of STATCOM. This means STATCOM AC and DC voltage regulation can be implemented independently.
In this paper, it is suggested that a single multivariable controller is assigned the dual AC and DC voltage control functions of the STATCOM so as to overcome the negative interactions between the STATCOM controls. The multivariable controller can be designed by treating the STATCOM in the power system as a MIMO system, resulting in a decomposed STATCOM AC and DC voltage control. In this paper, an example of successful design is presented where a multivariable PI controller replaces the conventional multiple controllers of the STATCOM to perform AC and DC voltage control.
Section snippets
A study case of negative interactions between STATCOM AC and DC voltage control
Fig. 1 shows a single-machine infinite-bus power system installed with a STATCOM which consists of a step-down transformer (SDT) with a leakage reactance XSDT, a three-phase GTO based voltage source converter (VSC) and a DC capacitor. The VSC generates a controllable AC voltage source behind the leakage reactance. The voltage difference between the STATCOM bus AC voltage, vL(t), and v0(t) produces active and reactive power exchange between the STATCOM and the power system,
Multivariable design of the STATCOM control
The linearized model of the power system installed with the STATCOM iswhich can beas derived in Ref. [15]. Hence this is a typical MIMO control system with two control inputs and two feedback outputs. According to
Conclusions
The major contributions of this paper are
- 1.
It is reported for the first time that if the STATCOM AC and DC voltage control is performed by two individual controllers which are designed separately, the negative interactions between STATCOM AC and DC voltage controllers could result in poor control performance and even closed-loop system instability when they are in joint operation.
- 2.
It is suggested that two control functions of the STATCOM AC and DC voltage regulation can be performed by a single
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