An Improved Phase-shifted Carrier Pwm for Modular Multilevel Converters

The modular multilevel converter is improved topology for high-voltage, high-power applications since the cascaded sub modules can generate high-voltage waveforms with excellent harmonic performance at low switching frequencies. Many publications have been presented on the modulation and control of this converter type, some of which are based on phase-shifted carrier modulation. This paper presents an analysis of how the switching frequency affects the capacitor voltages, circulating current and alternating voltage at phase-shifted carrier modulation. It is found that integer multiples of the fundamental frequency should be avoided as they can cause the capacitor voltages to diverge. Suitable switching frequencies are then defined for which the arm and line quantities will be periodic and symmetric in the upper and lower arms. The theoretical results are then validated by the simulations result in matlab/simulink. INTRODUCTION In recent day, the demand for electricity have been increasing the demand of power generation and transmission .The modular multilevel converter (MMC) is have more demand because of it great merit when compared with conventional twolevel or three-level voltage source converters. The merit like,reduced EMI noise, lower losses, scalability, less semiconductor device stress and easy assembling etc,. which make MMC as emerging topology for high-voltage high-power applications, particularly in the high-voltage direct current transmission (HVDC) sector. There are two major tasks associated along with MMC and several control solutions based on feedback control or the sorting algorithm have been proposed and reported they are capacitor voltage balancing and circulating current suppression . Pulse width modulation (PWM) techniques have been developed which is associated with the MMC. And for round method, was adopted by the nearest level control (NLC).This method is mainly used with combination with MMC for submodules (SMs). A voltage balancing method was discussed by the phase disposition (PD) level shifted PWM strategy. But it as not suitable for MMC .So we are using another PWM technique called phase-shifted carrier (PSC). which is the most commonly used method along the cascaded H-bridge converters (CHB) . The PSC modulation is also attractive to MMC as it has some distinctive features: 1) The semiconductor stress and the power handled by each SM are evenly distributed. Hence, the capacitor voltage balancing control can be easily achieved. 2) The output voltage has a high resulting switching frequency and a low total harmonic distortion(THD). 3) Consistent with the structure of MMC, each triangular carrier associated to a particular SM presents the nature of modularity and scalability. Because of it merits, the PSC scheme of CHB has been directly interacted along with MMC and also by many researchers , the carriers for SMs in the same phase are arranged with an equal phase shifts in angle. Moreover ,the significant of this two topology are differences. During the case of the CHB, we required transferring active power for large number of isolated dc sources, and there are fed from phase-shifting isolation transformers and multiphase rectifiers, which are very expensive and bulky . In contrast, MMC eliminates the bulky phase-shifting transformers and because of its additional dc terminal are formed by the upper and lower arms, which allows bi-directional power flow between ac and dc sides. when we applying PSC modulation to MMC, carriers for SMs in the lower arm and carriers for SMs in the upper arm need to be considered separately, with an interleaved displacement angle. This displacement angle are affect the high-frequency interactions between the upper and lower arms, and further determines the harmonic features of MMC. So for few studies have analyzed the principle of PSC modulation for MMC and it remains unclear how the displacement angle would influence the performance of MMC. Therefore , the aim of this paper to provide a fully mathematical analysis of the PSC modulation implemented in MMC, to investigate the impact of the displacement angle on the voltage and current harmonics, and to find out an optimal displacement angle which can minimize these harmonics. BASIC OPERATING PRINCIPLES A.STRUCTURE OF MMC The circuit configuration of a three-phase MMC as shown in Fig. 1. Each phase of MMC INTERNATIONAL JOURNAL OF PROFESSIONAL ENGINEERING STUDIES Volume VIII /Issue 3 / MAR 2017 IJPRES consists of two arms, the upper and the lower, and there are connected through buffer inductors. Fig 1 Circuit configuration of the MMC Each arm is formed by a series connection of N nominally identical half-bridge SMs and each SM contains a dc capacitor and two insulated gate bipolar transistors (IGBTs). The single coupled inductor is preferred in this paper which is smaller size and lighter weight than the total of the two separate inductors [5]. B. Mathematical Model of MMC As shown in Fig. 2, the equivalent circuit diagram of one phase of MMC is used for analysis. Fig. 2. Equivalent circuit of one phase of the MMC uoj is the output voltage of phase j (j ∈ {a,b,c }), ioj is the phase current, and E is the dc-link voltage. uuj,iuj and uwj,iwj represent the voltages and currents of the upper arm and the lower arm, respectively. The following equations can be obtained by Kirchhoff’s voltage law: R i + L di dt + M di dt = E 2 − u − u (1) R i + L di dt + M di dt = E 2 − u + u (2) u = L di dt + M di dt + L di dt + M di dt (3) where uLj is the voltage across the coupled inductors, Mu is the mutual inductance, Rup,Rlw and Lup,Llw are the resistances and self-inductances, respectively. Which is assumed that the coupling coefficient of two windings equals 1 (i.e., Lup = Llw =Mu= L0) and the resistances of the inductors are neglected for simplicity. Hence, the expressions (1)– (3) can be derived as