Hindawi Publishing Corporation VLSI Design Volume 2010, Article ID 512312, 14 pages doi:10.1155/2010/512312 Research Article FPGA-Based Software Implementation of Series Harmonic Compensation for Single Phase Inverters K. Selvajyothi 1 and P. A. Janakiraman 2 1 Department of Electrical Engineering, Indian Institute of Information Technology Design & Manufacturing Kancheepuram, IIT Madras campus, Chennai-600036, India 2 Department of Electrical Engineering, Indian Institute of Technology Madras, Chennai-600036, India Correspondence should be addressed to K. Selvajyothi, ksjyothi@iiitdm.ac.in Received 10 June 2009; Accepted 20 October 2009 Academic Editor: Gregory D. Peterson Copyright © 2010 K. Selvajyothi and P. A. Janakiraman. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. This paper presents a single chip FPGA (Altera Cyclone II) controlled single phase inverter, programmed for the reduction of harmonics in the output voltage. Separate composite digital observers have been designed for extracting the fundamental and harmonic components of the voltage and the highly distorted current signals, particularly when the inverter supplies nonlinear loads. These observers have been embedded into the FPGA along with the controllers and I/O interfaces. The multiple observers yield very pure in-phase and quadrature voltage signals for use in the outer loop and similar signals for stabilizing the inner current loop. The Inverter could be modeled as a feed back control system with the fundamental component of the voltage as the desired output while the voltage harmonics take the role of noise creeping into the output. To obtain a very low total harmonic distortion in the voltage waveform, the well-known control strategy of using a very large feed back around the noise signal has been employed. 1. Introduction Stand-alone inverters are commonly used in the case of power failure, to deliver power for critical loads, which demand purely sinusoidal voltage at the specified magnitude, frequency, and low total harmonic distortion (THDv). The THDv in industry should not exceed 5% as per the guidelines given in the IEEE Standard 519-1992. Fixed passive filters may not perform well, particularly when the operating frequency drifts far away from the set resonance frequency. Alternatively, active filters can be employed. Many control methods have been proposed basically for obtaining pure sinusoidal output with good voltage regulation and fast dynamic response [1–9]. Sinusoidal pulse width modulation (SPWM) schemes for stand-alone inverters have been shown to perform well with linear loads [10]. However, with nonlinear loads the SPWM scheme does not guarantee low distortion in the output voltage. The availability of low cost microprocessors has led to discrete-time methods, such as repetitive control [1, 2], sliding mode control [3], and deadbeat control [4, 5] to improve the performance. To get zero steady-state error in the output voltage and fast response virtual inductor, capacitor and a resistor were used in [6], while internal model control scheme (IMC) was employed in [7]. The control methods presented in [8, 9] employ two- feedback control loops. The inner loop is used for current control and the outer loop is used for voltage control. Many of these methods have not specifically considered the reduction in distortion due to nonlinear loads. The emergence of FPGAs has drawn much attention due to their shorter design cycle, lower cost, and higher density. The simplicity and programmability of FPGAs make them a most favorable choice for prototyping digital systems. When comparing the dynamic performance and control capabilities in PWM-controlled Power converters FPGA- based digital techniques are better than DSPs [11]. Basically a Luenberger observer (simple observer) can be used for obtaining the filtered fundamental component from the periodic output voltage and current waveforms, which leads to the indirect estimation of the total harmonics in the output voltage due to the nonlinear loads [12]. The Inverter can be modeled as a feed back control system with