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  • Research ArticleTransient Stability Enhancement of Multimachine Power SystemUsing Robust and Novel Controller Based CSC-STATCOM

    Sandeep Gupta1 and Ramesh Kumar Tripathi2

    1Department of Electrical Engineering, RIET, Jaipur 302026, India2Department of Electrical Engineering, MNNIT, Allahabad 211004, India

    Correspondence should be addressed to Sandeep Gupta; jecsandeep@gmail.com

    Received 3 August 2014; Revised 22 December 2014; Accepted 23 December 2014

    Academic Editor: Francesco Profumo

    Copyright 2015 S. Gupta and R. K. Tripathi. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

    A current source converter (CSC) based static synchronous compensator (STATCOM) is a shunt flexible AC transmission system(FACTS) device, which has a vital role as a stability support for small and large transient instability in an interconnected powernetwork. This paper investigates the impact of a novel and robust pole-shifting controller for CSC-STATCOM to improve thetransient stability of the multimachine power system. The proposed algorithm utilizes CSC based STATCOM to supply reactivepower to the test system to maintain the transient stability in the event of severe contingency. Firstly, modeling and pole-shiftingcontroller design for CSC based STATCOM are stated. After that, we show the impact of the proposedmethod in the multimachinepower system with different disturbances. Here, applicability of the proposed scheme is demonstrated through simulation inMATLAB and the simulation results show an improvement in the transient stability of multimachine power system with CSC-STATCOM. Also clearly shown, the robustness and effectiveness of CSC-STATCOM are better rather than other shunt FACTSdevices (SVC and VSC-STATCOM) by comparing the results in this paper.

    1. Introduction

    The continuous developments of electrical loads due to themodification of the society structure result in todays trans-mission structure to be faced close to their stability restric-tions. So the renovation of urban and rural power networkis more and more necessary. Due to governmental, financial,and green climate reasons, it is not always possible toconstruct new transmission lines to relieve the power systemstability problem at the existing overloaded transmissionlines. As a result, the utility industry is facing the challengeof efficient utilization of the existing AC transmission linesin power system networks. So transient stability, voltageregulation, damping oscillations, and so forth are the mostimportant operating issues that electrical engineers are facingduring power transfer at high levels.

    In above power quality problems, transient stability is oneof the most important key factors during power transfer athigh levels. According to the literature, transient stability of a

    power system is its ability tomaintain synchronous operationof the machines when subjected to a large disturbance [1].While the generator excitation system with PSS (power sys-tem stabilizer) can maintain excitation control and stabilityit is not adequate to sustain the stability of power system forlarge faults or overloading occurs near to generator terminals[2].

    So many researchers worked on this problem in findingthe solution for a long time.These solutions are such as usingwide-area measurement signals [3], phasor measurementunit [4], and flexible AC transmission system. In thesesolutions, one of the powerful methods for enhancing thetransient stability is to use flexible AC transmission system(FACTS) devices [58]. Even though the prime objective ofshunt FACTS devices (SVC, STATCOM) is to maintain busvoltage by absorbing (or injecting) reactive power, they arealso competent of improving the system stability by diminish-ing (or increasing) the capability of power transfer when themachine angle decreases (increases), which is accomplished

    Hindawi Publishing CorporationAdvances in Power ElectronicsVolume 2015, Article ID 626731, 12 pageshttp://dx.doi.org/10.1155/2015/626731

  • 2 Advances in Power Electronics

    by operating the shunt FACTS devices in inductive (capaci-tive) mode.

    In many research papers [2, 911], the different typesof these devices with different control techniques are usedfor improving transient stability. In between these FACTSdevices, the STATCOM is valuable for enhancement powersystem dynamic stability and frequency stabilization due tothe more rapid output response, lower harmonics, superiorcontrol stability and small size, and so forth [7, 12]. By theirinverter configuration, basic type of STATCOM topologycan be realized by either a current-source converter (CSC)or a voltage-source converter (VSC) [1317]. But recentresearch confirms several merits of CSC based STATCOMover VSC based STATCOM [18, 19]. These advantages arehigh converter reliability, quick starting, and inherent short-circuit protection, and the output current of the converteris directly controlled and in low switching frequency thisreduces the filtering requirements comparedwith the case of aVSC.Therefore CSC based STATCOM is very useful in powersystems rather than VSC based STATCOM in many cases.

    Presently the most used techniques for controller designof FACTS devices are the Proportional Integration (PI), PIDcontroller [20], pole-shifting controller, and linear quadraticregulator (LQR) [21]. But LQR and pole-shifting algorithmsgive quicker response in comparison to PI and PID algorithm[22]. LQR controller gain () can be calculated by solvingthe Riccati equation and is also dependent on the twocost functions (, ). So Riccati equation solvers have somelimitations, which relate to the input arguments. But poleshifting method does not face this type of any problem. Sopole shifting method gives a better and robust performancein comparison to other methods.

    The main contribution of this paper is the application ofproposed pole-shifting controller based CSC-STATCOM forimprovement of power system stability in terms of transientstability by injecting (or absorbing) reactive power. In thispaper, the proposed scheme is used in the multimachinepower transmission system with dynamic loads under agrievous disturbance condition (three-phase fault or heavyloading) to enhancement of power system transient stabilitystudies and to observe the impact of the CSC based STAT-COM on electromechanical oscillations and transmissioncapacity. Furthermore, the obtained outcomes from theproposed algorithm based CSC-STATCOM are compared tothe obtained outcomes from the other shunt FACTS devices(SVCandVSC-STATCOM)which are used in previousworks[23, 24].

    The rest of the paper is prepared as follows. Section 2discusses the circuit modeling and proposed pole-shiftingcontroller designing for CSC based STATCOM. A two-areapower system is described with a CSC-STATCOM devicein Section 3. Simulation results of the test system with andwithout CSC based STATCOM for severe contingency areshown in Section 4, to improve the transient stability of themultimachine power system. Comparison among differentshunt FACTS devices (SVC, VSC-STATCOM, and CSC-STATCOM) is also described in Section 4. Finally, Section 5concludes this paper.

    Idc

    iR

    iS

    iT

    R L

    C

    Ldc , Rdc

    CR

    CS

    CT

    S1 S3

    S2 S4

    S5

    S6

    R

    S

    T

    iSR

    iSS

    iST

    iCR

    MV Bus

    Figure 1: The representation of CSC based STATCOM. , , :

    line current; V, V, V: voltages across the filter capacitors; V

    , V,

    V: line voltages; dc: dc-side current; dc: converter switching and

    conduction losses; dc: smoothing inductor; : filter capacitance; :inductance of the line reactor; : resistance of the line reactor.

    2. Mathematical Modeling of Pole-ShiftingController Based CSC-STATCOM

    2.1. CSC Based STATCOM Model. In this section, to verifythe response of the STATCOM on dynamic performance,the mathematical modeling and control strategy of a CSCbased STATCOM are needed to be presented. So in thedesigning of controller for CSC based STATCOM, the statespace equations from the CSC-STATCOM circuit must beintroduced. To minimize the complexity of mathematicalcalculation, the theory of transformation of currentshas been applied in this circuit, which makes the and components independent parameters. Figure 1 shows thecircuit diagram of a typical CSC based STATCOM.

    The basic mathematical equations of the CSC basedSTATCOM have been derived in the literature [19]. There-fore, only brief details of the primary equations for CSC-STATCOMare given here for the readers convenience. Basedon the equivalent circuit of CSC-STATCOM as shown inFigure 1, the differential equations for the system can beachieved, which are derived in the abc frame and thentransformed into the synchronous frame using trans-formation method [25]:

    dc =

    dcdc

    dc 3

    2dc

    3

    2dc, (1)

    =

    + 1

    +1

    , (2)

    =

    +

    1

    , (3)

    =

    1

    + +

    1

    dc, (4)

    =

    1

    +

    1

    dc. (5)

    In above differential equations and

    are the two

    input variables. Two output variables are dc and . Here, is the rotation frequency of the system and this is equal to the

  • Advances in Power Electronics 3

    nominal frequency of the system voltage. and are the -

    axis and -axis components of the line current. and

    are -axis and -axis components of the system modulatingsignal, respectively.

    and

    are direct and quadrature axis

    of system voltage. Here is taken as a zero.

    and

    are

    the -axis and -axis components of the voltage across filterca

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