The nonlinear internal control of STATCOM: theory and application

Abstract

In this paper, a two-level control structure of Static Synchronous Var Compensator (STATCOM), which consists of internal and external control system, is described. The control targets of two levels and relations between them are also stated. The suggested configuration of control systems can benefit the systemic design for STATCOM controller. H_∞ control approach, which considers imprecise parameters and external disturbances, is applied to design the internal controller of STATCOM. With the proposed internal control, the model of STATCOM can be simplified to be a first-order inertia block, which can significantly simplify the design of system control. After that, some improvements and optimizations based on the dynamical analysis of STATCOM have been developed to achieve better dynamical performance and higher tracking accuracy. Simulations are carried out to verify the performance of the proposed controller comparing with that of the conventional PI controller. The results show that the proposed controller has faster dynamic response, higher accuracy of tracking the reference, and stronger robustness to parameters variation and external disturbances.

Introduction

Flexible AC transmission systems (FACTS) devices, which are based on voltage-source or current-source inverters, such as Static Synchronous Var Compensator (STATCOM), Static Synchronous Series Compensator (SSSC), and Unified Power Flow Controller (UPFC), have been used to flexible power flow control, secure loading and damping of power system oscillations [1], [2]. As one kind of the typical FACTS devices, STATCOM is playing more and more important roles in reactive power provision and voltage support because of its attractive steady state performance and operating characteristics, which has been well studied in the past years. Moreover, lots of work about its dynamic characteristics and applications to improve the transient stability of power systems also have been done [3], [4]. As we know, proper control strategies corresponding to the control objectives are necessary in order to achieve full utilization of STATCOM. So far, various control approaches have been applied to STATCOM dynamical control and achieve many good results [5], [6], [7], [8], [9]. Nevertheless, most of these literatures have not clearly documented the holistic configuration of control system of STATCOM, which is obstruction of the systemic designing STATCOM controllers in industrial practice. Usually, when one designs STATCOM controllers, two usual ways are utilized as follows:

• Adopting a detail model of STATCOM without distinguishing the internal and external control. The design process is obviously extremely complex and difficult to handle since the controller must consider the electromechanical dynamics of power systems and internal electromagnetic transience of STATCOM itself, which is much faster than the former one, at the same time. Furthermore, in the design process, the model of STATCOM involves the electromagnetic transience caused by inductances and capacitors, while the model of power systems does not consider such transience but only the sine steady state of voltages and currents. It is difficult to explain why different ways are adopted to handle the dynamics from the viewpoint of physical reality and mathematical reasoning.

• Adopting a simplified model without distinguishing the internal and external control. Such way significantly simplifies the design process because simpler model is adopted. Whereas, there are two potential problems: (a) What is the theoretical foundation of the simplification? (b) How to deal with the internal electromagnetic transience of STATCOM? In most literatures, these two problems are not documented.

Basically, the difficult is caused by different time scale of external electromechanical dynamics and internal electromagnetic transience. For the sake of overcoming this difficult, one natural way is to decouple the control system into two levels: internal control and external control (or system control), which is shown in Fig. 1. Each of them has its specific control targets:

• The internal controller: operates the inverter power switches to generate a fundamental output voltage waveform with the demanded magnitude and phase angle in synchronism with the AC system. Besides, the internal control, the voltage on the DC capacitor should be constant in a PWM based STATCOM.

• The external controller: figures out reference inputs of the internal controller according to system dynamics, or rather, ‘tells’ the internal controller that how much reactive current the STATCOM should generate or absorb in order to meet the requirement of systems.

The inverter of static generator comprises a large number of GTO or IGBT. The gating pulses of these semiconductor power switches are generated by the internal inverter control in response to the demand of reference signals. From the viewpoint of injecting current, a STATCOM can be regarded as a controller to generate or absorb a demanded reactive current from the connected system. The reference signals of STATCOM (also the internal controller) are provided by the external controller, which determine the operating modes of STATCOM in response to the demand of the AC system. Besides, the internal control should also meet the requirements of secure operation of STATCOM itself. So another target of internal control is to keep the voltage on the DC capacitor to be constant in a PWM based STATCOM. Additionally, the real current exchange between the system and STATCOM should be limited too. There are also many other tasks that the internal control executes, such as to keep the maximum voltage and current within a safe limit. However, they are not the most important problems from the viewpoint of control systems. It is more suitable to discuss such issues in the design of protection systems. So we will not discuss these problems in the subsequent parts.

In industrial practice, the internal controller is an integral part of the inverter. Then, if the internal control strategy is well designed, the whole STATCOM system with its internal controller can be simplified to be a first-order inertia block when designing the external controller (system controller). Then the control variable of the external controller is changed to be the reactive current, which is also the reference signal of internal control. It is determined by the functional operations of STATCOM, the operator instructions and system variables. Therefore, the design of external controller can evade the complexity of internal dynamics of STATCOM itself, and the design of system control can emphasis on the dynamics between the AC system and STATCOM.

Fig. 1 shows the configuration of the two-level control system of STATCOM. Because of the similar structures of FACTS devices based on controllable inverter bridge, this control system configuration can be extended to most FACTS devices based on voltage source inverter (VSI).

Based on the two-level configuration of STATCOM control system, H_∞ control approach, considering imprecise parameters and external disturbances [10], [11] is applied to design the internal controller of STATCOM. With this controller, STATCOM can be simplified to be a reactive current source with a first-order inertia block during the design of external controller. Based on the dynamical analysis of STATCOM, some improvements and optimizations have been developed to achieve a better dynamical performance and tracking accuracy. Since the emphases of this paper focuses on the internal control, the design of external controller will not be discussed here. Simulations are carried out to test the performance of this controller. The results show that the proposed controller can track the output reference with fast dynamic response. The robustness to parameters' variation and disturbances also has been verified. The comparison of performances between the proposed controller and conventional PI controller shows the advantage of the former one.