22-06-2011, 03:50 PM
An Improved Nonlinear STATCOM Control for Electric Arc Furnace Voltage Flicker Mitigation
Abstract
Electric arc furnaces (EAFs) are prevalent in the steel industry to melt iron and scrap steel. EAFs frequently cause large amplitude fluctuations of active and reactive power and are the source of significant power-quality (PQ) disturbances. Static synchronous compensators (STATCOMs) provide a power-electronicbased means of embedded control for reactive power support and PQ improvement. This paper introduces a new nonlinear control for the STATCOM that provides significant reduction in EAF-induced aperiodic oscillations on the power system. This method is compared with traditional PI controls and has shown to have improved performance. Index Terms—Arc furnace flicker, nonlinear control, static synchronous compensator (STATCOM).
I. INTRODUCTION
ELECTRIC arc furnaces (EAFs) comprise a major portion of industrial loading on the bulk power system. EAF flicker is induced by low-frequency modulation (generally between 5–35 Hz) of the voltage at the point of common coupling (PCC) with the system. This fluctuation in load leads to fast nonperiodic voltage variations with appreciable voltage distortion. Customers who share the distribution feeder with these nonlinear loads frequently experience significant voltage variations that produce disturbances in their equipment operation. Typically, a staticVAr compensator (SVC) or static synchronous compensator (STATCOM) is added to compensate for the reactive power fluctuation [1]–[3]. Analyses of EAF loads indicated that a variation in active power is nearly as great as the variation in reactive power and is a significant contributor to voltage flicker [4]. Therefore, it is necessary to develop controls that can impact active and reactive power flows to mitigate electric arc furnace disturbances. The SVC cannot react rapidly enough to counteract the rapidly varying flicker; therefore, the STATCOM is an attractive solution [5]. Recent work has investigated the use of multilevel-converter- based STATCOMs for arc furnace flicker mitigation [6], [7]. Multilevel converters are attractive due to the reduction in harmonics and smaller-sized components. In this paper, an 11-level cascaded multilevel STATCOM with PWM control is introduced to compensate for a nonlinear load that emulates an EAF. Most STATCOM control proposed for EAF flicker mitigation has focused on the use of PI or PID linear control to provide the reactive and active power compensation through current control [3]–[7]. Linear control often provides adequate control, but can suffer from degradation in performance if the operating conditions change or if multiple modes of oscillation are present. One recent STATCOM control development was reported in [8]. In this approach, an energy-based control law is designed to provide stability whereas an adaptive mechanism is used to improve the robustness to parametric uncertainties. In this paper, the authors coordinated the generator excitation and the STATCOM for improved performance. Although the authors used a simplified model of the STATCOM, they were able to acheive an effective control law that provided significant oscillation damping. The primary drawback with the proposed approach is that in some applications, such as EAFs, the STATCOM may be located significantly distant from the generator so that coordinated generator/STATCOM control may not be realistic. Another similar nonlinear STATCOM control developed specifically for fast load regulation, such as electric arc furnace applications, is presented in [9]. In this paper, the authors propose a nonlinear controller that is robust in the face of system variations. The authors design a nonlinear control strategy that achieves asymptotic regulation of the voltage magnitude while compensating for uncertainties in the load conductance. While the goal of this control is different than that of the STATCOM for EAF flicker mitigation, this paper provides several salient approaches that will be exploited. In particular, the authors proposed a coordinate transformation that allows for the development of a stable control strategy utilizing a novel Lyapunov function. While the proposed control is significantly different, there are still several conceptual similarities between these two approaches. For this reason, we propose a new nonlinear controller that provides improved performance for flicker mitigation and power-quality (PQ) improvement for EAF applications.
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