Author Topic: Implementation of Cascade Multilevel Inverter in Distribution Systems as Power L  (Read 2529 times)

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Author : Rajasekhar.G.G, N.Sambasiva Rao, T.Vijay Muni
International Journal of Scientific & Engineering Research Volume 2, Issue 10, October-2011
ISSN 2229-5518
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Abstract— This paper deals with the implementation of cascade multilevel inverter-based STATCOM, which employs H-bridge inverter. The STATCOM system is modeled using the d-q transform, which calculates the instantaneous reactive power. In this paper, a power line conditioner using a cascade multilevel inverter is presented for voltage regulation, harmonic filtering and reactive power compensation (var). 11 level STATCOM is selected as demonstration. The cascade multilevel converter consists of five single-phase full bridges in which each bridge has its own DC source. This new inverter can: 1) It can eliminate transformers of multilevel inverters used in conventional static var compensators; 2) make possible to direct connect to power distribution system in parallel and series without any transformer; 3) generate almost sinusoidal voltage. This paper focuses on feasibility and control schemes of the cascade inverter foe voltage regulation and harmonic filtering in distribution systems. The results are analyzed and discussed.

Index Terms— Active power filter, STATCOM, cascade multilevel inverter, power line conditioner, 

1   INTRODUCTION                                                                     
IN  large power network, the active control of reactive power is indispensable to stabilize the power systems and to maintain the supply voltage. A static synchronous compen-sator (STATCOM) using the voltage source inverters (VSIs) have been widely accepted as the next generation of the reac-tive power controllers of power system. Recently, power quality and custom power have been hot topics because of widespread use of nonlinear electronic equipment and the power quality requirements of sensitive loads. To provide high power quality at the point of common coupling (PCC) of distribution system, line conditioning, including voltage regulation, reactive power compensator, harmonic compensator is an indispensably necessary remedy.

Traditionally, a multipulse inverter consisting of several voltage source inverters connected together through zig-zag arrangement transformers is used for var compensation. These transformers are: 1) most expensive equipment in the system; 2) produce about 50% of the total losses of the sys-tem; 3) causes difficulties in control due to dc magnetizing and surge over voltage problems resulting from saturation of the transformers.

A cascade multilevel inverter have been proposed for static var compensation and generation applications, The new cascaded inverter eliminates the bulk of transformers required by static var compensators (SVC’s) that employ the multipulse inverter and that can respond much faster. This inverter generates almost sinusoidal staircase voltage with only one time switching per line cycle.
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2 SYSTEM CONFIGURATION OF POWER LINE CONDITIONER
The proposed power line conditioner is using the cascaded multilevel inverter is presented for voltage regulation, reactive power (var) compensation and harmonic filtering of a power distribution system in this paper.  A STATCOM is connected to the power network at PCC, where the voltage-quality problem is concern. All required voltages and currents are measured and fed into the controller to be compared with the commands. The controller then performs feedback control and outputs a set of switching signals to drive the main semiconductor switches of the power converter accordingly. The single line diagram of the STATCOM system is illustrated in Figure 2-1. In general, VSC is represented by an ideal voltage source associated with internal loss connected to the AC power via coupling reactors

Figure 2-1 Single line diagram of VSC based STATCOM

A power line conditioner can be connected in series with the source before the PCC as shown in Figure 2-2 or connected in parallel with the system at the PCC.
The series power line conditioner can be control to provide pure, constant sine-wave voltage to the loads that are sensitive to voltage fluctuations, sags, swings and harmonics
The parallel power line conditioner is to compensate the reactive power and harmonics.
This paper focuses on this power line conditioner and re-veals a control method for dc voltage balancing of cascade inverters

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