05-05-2011, 12:18 PM
Abstract
This paper presents a new unified power-quality conditioningsystem (MC-UPQC), capable of simultaneous compensationfor voltage and current in multibus/multifeeder systems. Inthis configuration, one shunt voltage-source converter (shunt VSC)and two or more series VSCs exist. The system can be applied to adjacentfeeders to compensate for supply-voltage and load currentimperfections on the main feeder and full compensation of supplyvoltageimperfections on the other feeders. In the proposed configuration,all converters are connected back to back on the dc sideand share a common dc-link capacitor. Therefore, power can betransferred from one feeder to adjacent feeders to compensate forsag/swell and interruption. The performance of the MC-UPQC aswell as the adopted control algorithm is illustrated by simulation.The results obtained in PSCAD/EMTDC on a two-bus/two-feedersystem show the effectiveness of the proposed configuration.
Index Terms—Power quality (PQ), PSCAD/EMTDC, unifiedpower-quality conditioner (UPQC), voltage-source converter(VSC).
I. INTRODUCTION
WITH increasing applications of nonlinear and electronicallyswitched devices in distribution systems andindustries, power-quality (PQ) problems, such as harmonics,flicker, and imbalance have become serious concerns. In addition,lightning strikes on transmission lines, switching ofcapacitor banks, and various network faults can also cause PQproblems, such as transients, voltage sag/swell, and interruption.On the other hand, an increase of sensitive loads involvingdigital electronics and complex process controllers requires apure sinusoidal supply voltage for proper load operation [1].In order to meet PQ standard limits, it may be necessary toinclude some sort of compensation. Modern solutions can befound in the form of active rectification or active filtering [2].A shunt active power filter is suitable for the suppression ofnegative load influence on the supply network, but if there aresupply voltage imperfections, a series active power filter may beneeded to provide full compensation [3].In recent years, solutions based on flexible ac transmissionsystems (FACTS) have appeared. The application of FACTSconcepts in distribution systems has resulted in a new generationof compensating devices. A unified power-quality conditioner(UPQC) [4] is the extension of the unified power-flow controller (UPFC) [5] concept at the distribution level. It consistsof combined series and shunt converters for simultaneouscompensation of voltage and current imperfections in a supplyfeeder [6]–[8].Recently, multiconverter FACTS devices, such as an interlinepower-flow controller (IPFC) [9] and the generalized unifiedpower-flow controller (GUPFC) [10] are introduced. The aimof these devices is to control the power flow of multilines or asubnetwork rather than control the power flow of a single lineby, for instance, a UPFC.When the power flows of two lines starting in one substationneed to be controlled, an interline power flow controller (IPFC)can be used. An IPFC consists of two series VSCs whose dccapacitors are coupled. This allows active power to circulatebetween the VSCs. With this configuration, two lines can becontrolled simultaneously to optimize the network utilization.The GUPFC combines three or more shunt and series converters.It extends the concept of voltage and power-flow controlbeyond what is achievable with the known two-converterUPFC. The simplest GUPFC consists of three converters—oneconnected in shunt and the other two in series with two transmissionlines in a substation. The basic GUPFC can control totalfive power system quantities, such as a bus voltage and independentactive and reactive power flows of two lines. The concept ofGUPFC can be extended for more lines if necessary. The devicemay be installed in some central substations to manage powerflows of multilines or a group of lines and provide voltage supportas well. By using GUPFC devices, the transfer capability oftransmission lines can be increased significantly. Furthermore,by using the multiline-management capability of the GUPFC,active power flow on lines cannot only be increased, but also bedecreased with respect to operating and market transaction requirements.In general, the GUPFC can be used to increase thetransfer capability and relieve congestions in a flexible way.This concept can be extended to design multiconverter configurationsfor PQ improvement in adjacent feeders. For example,the interline unified power-quality conditioner (IUPQC),which is the extension of the IPFC concept at the distributionlevel, has been proposed in [11]. The IUPQC consists of one seriesand one shunt converter. It is connected between two feedersto regulate the bus voltage of one of the feeders, while regulatingthe voltage across a sensitive load in the other feeder. In thisconfiguration, the voltage regulation in one of the feeders is performedby the shunt-VSC. However, since the source impedanceis very low, a high amount of current would be needed to boostthe bus voltage in case of a voltage sag/swell which is not feasible.It also has low dynamic performance because the dc-linkcapacitor voltage is not regulated.
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