power converter

Abstract: Because of grid faults and generator faults disturbance will be created in the supply voltage .In case of unbalanced voltage source, the converters suffer from the problems of power oscillations and over current problems. These problems can be solved by controlling the positive and negative sequence currents. In this paper, a new control strategy is established by utilising the zero sequence components to enhance the power control ability. The paper explains how to use the zero sequence current controls and circuit topologies and concludes with operation with better performance in the delivered power and the load current when the supply voltage unbalanced.

Index: dc-ac converters, zero sequence converter, unbalanced supply voltage

 I.Introduction

Power Electronics plays a vital role in the power control and conditioning. In so many applications of power electronics like solar energy , wind generation, in speed control of dc motor  and renewable energy sources. The power control and conditioning can be acheieved by power electronic components i.e., converters[1], [2].  A conventional dc–ac voltage source converter is shown in fig which is used to convert the dc voltage to three phase AC sources generating units and depending upon the loads and applications[3]–[5].  

Since  the power electronics are very important and essential in the energy conversion technology , the failures and faults may introduce the serious problems in  converters .There is a need to increase the reliability of power electronic converters in many applications to withstand failures and faults [6]–[13]. A good example can be seen in the wind power application, where both the total installed capacity and individual capacity of the power conversion system are relatively

high. The sudden disconnection of the power converter may cause significant impacts on the grid stability and also on the high cost for maintenance/repair [1].

            Because of disturbances and faults the ac source from  the converters  becomes distorted, These unbalanced ac voltage is a big challenge to control the converter operation [2],[14] .Existing control methods  which can regulate both the positive and negative sequence are introduced to  handle these disturbances[2], [16]–[21].The performance by these methods are up to some extent only in distorted power  oscillations and distorted load current oscillations and considering cost of the power converter design. 

This paper presents the improvement in power control limits of a three phase dc-ac converter system for unbalanced ac source. A new approach has been implemented to resolve the issues and enhance the power control capability.

This new approach is utilising the zero sequence components in the converters under other applications like where the unbalanced ac source is to be presented; in that way basic principle and feasibility are mainly focussed.

II. LIMITATIONS OF A CONVERTER SYSTEM

To analyse the performance of the power electronic converter , a unbalanced ac source is introduced firstly   .the distorted three phase ac voltage is shown in fig.3 ,the phasor diagram ,assumed that the phase B fault occurred with voltage dip on the phase A of the ac source.

According to [2] and [19], any distorted three-phase voltage can be expressed by the sum of components in the positive sequence, negative sequence, and zero sequence. For simplicity of analysis, only the components with the fundamental frequency are considered in this paper, however, it is also possible to extend

the analysis to higher order harmonics. The distorted three-phase ac source voltage in Fig. 3 can be represented by

where V +, V −, and V0 are the voltage amplitude in the positive, negative, and zero sequence, respectively. And ϕ+, ϕ−, and ϕ0 represent the initial phase angles in the positive sequence, negative sequence, and zero sequence, respectively. The predefined voltage dip as indicated in

 Fig. 3 should contain voltage components in all the three sequences [2], [11].

Because there are only three wires and a common neutral point in the windings of the ac source, the currents flowing in the three phases do not contain zero-sequence components. As a result, the three-phase load current controlled by the converter can be written as

With the voltage of the ac source in (1) and the current controlled by the converter in (2), the instantaneous real power p and the imaginary power q in αβ coordinate, as well as the real power p0 in the zero coordinate can be calculated .