Modeling, control and simulation of a chain link STATCOM in EMTP-RV☆
Introduction
An advanced static VAR compensator (ASVC), using a voltage–source-converter (VSC), popularly known as a STATic synchronous COMpensator (STATCOM), has been acknowledged as the next-generation reactive power controller in power systems. This controller’s capability is analogous to that of a rotating synchronous condenser and it is used in a similar way to provide dynamic voltage compensation, increased transient stability and damping for the power transmission system [1], [2], [3]. Several STATCOMs, based on gate turn offs (GTOs) and a special zigzag transformer, have been put into operation [4], [5]. These STATCOMs have advantages over conventional SVCs of lower harmonic generation, improved performance and smaller reactor size. However, zigzag transformers used in these STATCOMs are bulky, expensive and as yet unreliable.
An alternative method to eliminate the zigzag transformer and to increase the rating with a corresponding reduction in harmonics is to use a multilevel converter [6], which produces a ‘multi-stepped’ output voltage waveform. A relatively new multilevel converter application has been developed by connecting a number of GTO-based VSCs in series to form a chain [1], [7], [8]. This chain link STATCOM (CLS) has advantages compared to more conventional versions of the STATCOM, i.e. good harmonic performance, lower losses, reduced number of diodes–capacitors and lower cost. Moreover, the packaging and physical layout is much easier due to a modular structure. As the CLS is switched at low frequency, the GTO is selected as a generic switch in the CLS. However, other types of semiconductor switches (IGBT, IGCT, etc.) can also be used for this application. The first CLS application is in service in the National Grid Company, East Claydon substation, UK, since mid-1990s [8].
A 3-phase CLS having 3-links per phase is presented in this paper. Only three links are used here, since the model exemplifies an adequate degree of complexity without incurring the excessive computational burden of simulating a larger (and maybe more realistic) number of links. The proposed CLS, with its controller, is modeled using dq-transformation and is connected at the end of a radial transmission line, and across the load to support the voltage at the point of common coupling (PCC). This CLS system is simulated with the digital simulation software package EMTP-RV for performance investigation. Results of the simulation tests in steady- and transient-states are presented.
Section snippets
Basic circuit arrangement and principle of operation
A CLS comprises of a number of GTO converter ‘links’ connected in series on their ac side to form a separate ‘chain’ per phase (Fig. 1a). Each ‘link’ is a 1-phase, 3-level, full bridge VSC comprised of four GTO-diode pairs as switches and an independent self-controlled dc capacitor, Cdc for energy storage (Fig. 2a). In this arrangement, the dc capacitors are floating with the voltage, Vdc on each of them. Consequently, no real power needs to be supplied, other than the losses that are
CLS power circuit, system modeling and control strategies
A 3-phase CLS having 3-links per phase is modeled in EMTP-RV to investigate its behavior under different power system operating conditions. The model of the CLS is comprised of three fundamental blocks: a switch, a VSC link and an SPWM circuit. Approximate mathematical models of the CLS system are obtained and the control strategies are also explained in this section.
Simulation results
The CLS system and the controllers explained in the preceding sections are simulated with EMTP-RV using a 10 μs time-step; a small time-step is needed for an accurate simulation. Although not reported here, simulations at 1 μs time-step were also made to verify the validity of the results obtained. The performance results are presented next.
Conclusions
A 3-phase CLS having 3-links per phase is presented in this paper. The SPWM technique to drive the switches of the CLS is used such that each switch turns ON/OFF once per cycle of the fundamental frequency to reduce the converter losses by producing a 3-level output voltage waveform per link and to synthesize a multilevel CLS output voltage in a close approximation to a sinusoidal waveform. Approximate static and dynamic mathematical models and the analysis of the CLS system are presented using
Acknowledgment
The authors acknowledge funding support from the Natural Sciences and Engineering Research Council of Canada.
Nikunj M. Shah received MA.Sc. degree in Electrical & Computer Engineering from Concordia University, Montreal, Canada in 2007. He worked as an Electrical Engineer at Bombardier Transportation (erstwhile ABB Inc.) in Baroda, India from 1998 to 2001. Presently, he is a Sr. Product Engineer at Siemens Energy & Automation in Alpharetta, GA, USA. His research interests are in modeling and simulation of high power converters and power systems.
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Nikunj M. Shah received MA.Sc. degree in Electrical & Computer Engineering from Concordia University, Montreal, Canada in 2007. He worked as an Electrical Engineer at Bombardier Transportation (erstwhile ABB Inc.) in Baroda, India from 1998 to 2001. Presently, he is a Sr. Product Engineer at Siemens Energy & Automation in Alpharetta, GA, USA. His research interests are in modeling and simulation of high power converters and power systems.
Vijay K. Sood obtained a Ph.D. degree from University of Bradford, England in 1977. From 1976 to 2007, he was a Researcher at IREQ (Hydro-Québec) in Montreal. He is an Adjunct Professor at Concordia University, Montreal where he teaches post-graduate courses in Power Electronics, HVDC transmission and FACTS. Presently, he is a professor at the University of Ontario Institute of Technology (UOIT) in Oshawa, Ontario (Canada). He is a Fellow of the Engineering Institute of Canada (1999) and Fellow of IEEE (2006). He was the Managing Editor of the IEEE Canadian Review magazine from 1996 to 2006 and was the Secretary of IEEE Canada (2005–2006). He is author of the textbook “HVDC and FACTS Controllers—Applications of Static Converters in Power Systems”, published by Kluwer Academic Publishers in 2004.
Venkat Ramachandran is a Professor, Dept. of Electrical and Computer Engineering, Concordia University, Montreal, Canada. He got the Ph.D. degree from Indian Institute of Science, Bangalore, India. He is a Fellow of IEEE and has won several teaching excellence awards. He is also the recipient of Outstanding Engineering Educator Award from IEEE, Canada. His main interests are in Multidimensional Systems and 2D Signal Processing.