PEDS 2007 A Modular Structured Multilevel Inverter Active Power Filter with Unified Constant-Frequency Integration Control for Nonlinear AC Loads P. Y. Lim School of Engineering and Information Technology Universiti Malaysia Sabah Sabah, Malaysia lpy@ums.edu.my Abstract- Active power filter (APF) has a vital role for compensating the reactive power and harmonics from a group of nonlinear loads connected at the common point of coupling to ensure that the resulting total current drawn from the main incoming supply is sinusoidal. This paper is to present the performance of a Modular Structured Multilevel Inverter (MSMI) APF with Unified Constant Frequency Integration (UCI) control technique for compensating the distorted current caused by various nonlinear ac loads. MSMI is well known for its capability in reducing the voltage and current stresses on the semiconductors switches and providing the convenience for future expansion in order to achieve higher power ratings for the APF system. Whereas, the UCI control technique is adopted due to its constant switching frequency operation and simple analog circuitry. To demonstrate the capability of the single phase MSMI APF with UCI control technique for compensation of harmonics caused by various AC nonlinear loads, simulation results are presented and discussed. Index Terms--Active power filter, current harmonics compensation, modular structured multilevel inverter, unified constant frequency integration control, nonlinear loads. I. INTRODUCTION Active power filter (APF) has been widely used for harmonics and reactive power compensation and it now becomes a mature technology for improving the power quality. Recently, semiconductor constraints and the voltage stress on the semiconductor switches have drawn the attraction of the researchers. In order to achieve higher voltage level of the filtering operation, inverter configuration such as Hybrid Asymmetric Multilevel Inverter [1] was proposed. The performance of the APF inverter depends very much on the control scheme. Many technical papers related with active power filters and their control methods were presented. Some researchers spent their effort in developing the APF system with Unified Constant-Frequency Integration (UCI) controller [2][3] recently. APF controlled by UCI Controller has the attractive advantages comparing to the other control method such as the PQ Theory [4], Linear Current Control, Digital Deadbeat Control and Hysterisis Control [5]. Those proposed control approaches need to sense the input voltage, load current and then calculate the harmonic reactive N. A. Azli Department of Energy Conversion Faculty of Electrical Engineering Universiti Teknologi Malaysia Johor, Malaysia naziha@ieee.org component in the load in order to generate the reference current for controlling the inverter to produce a current that is equal to the amplitude and opposite in direction of the reactive current of nonlinear loads. Those control methods require fast and real-time calculation which require several high precision analogue multipliers or high speed DSP chip with fast A/D converter that yields high cost, complexity and low stability. [3] As for the UCI controller, it is based on the theory of one cycle control [6], which employs an integrator with reset port as its core component to control the duty ratio of the APF inverter switches. This method ensures that the current drawn from the AC main supply will be sinusoidal waveform with different non-linear AC loads connected to the supply. No calculation of reference current for the controller is required which greatly simplifies the control circuit. This paper is to realize the implementation of UCI Controller to Modular Structured Multilevel Inverter (MSMI) APF which will generate harmonics current that cancels the harmonics current from a nonlinear load in order to form better sinusoidal incoming supply. MSMI has an advantage of reducing the voltage stress of switching devices. Furthermore, the modularized circuit layout of MSMI allows expansion of the APF structure to be done easily. [7] This will be convenient if expansion of APF is required to meet higher power level demand, such as in power transmission line. The performance of a single phase simulation of MSMI APF which is connected to different AC nonlinear loads will be presented for the purpose of demonstration. II. OPERATION OF THE MSMI WITH UCI CONTROLLER MSMI is an inverter structure, which has cascaded inverters with Separate DC Sources (SDCs). This topology allows expansion of the number of levels, which provides flexibility in increasing of higher voltage level. This can be done easily as the inverter has modularized circuit layout. Furthermore, the cascaded inverter structure help to reduce voltage stress on switches, as lower voltage will be imposed by the DC side capacitor voltage to the switches or in other words, switches only have to bear on smaller value of voltage. 1-4244-0645-5/07/$20.00©2007 IEEE 244 Authorized licensed use limited to: IEEE Xplore. Downloaded on January 6, 2009 at 19:34 from IEEE Xplore. Restrictions apply.