IEEE TRANSACTIONS ON SUSTAINABLE ENERGY, VOL. 4, NO. 2, APRIL 2013 451

An Index for STATCOM Placement toFacilitate Grid Integration of DER

Tareq Aziz, Member, IEEE, U. P. Mhaskar, Tapan Kumar Saha, Senior Member, IEEE, andNadarajah Mithulananthan, Senior Member, IEEE

Abstract—In recent years, the penetration level of renewable-based distributed energy resource (DER) units has increased sig-nificantly. Consequently, standards have been developed and de-ployed demanding small DER units to operate in constant powerfactor mode and large DER units in voltage control mode. This re-sults in exposing small DER units to the problem of slow voltagerecovery for contingencies like faults. Hence, this paper proposesa methodology of static and dynamic reactive power compensa-tion to avoid tripping of small DER units due to slow voltage re-covery. A new sensitivity index has been developed for the place-ment of STATic synchronous COMpensator (STATCOM) to en-sure fast voltage recovery at all the buses of interest. The casestudies involving two IEEE test systems with varying size and loadcompositions validate the proposed methodology and index.

Index Terms—Distributed energy resources (DERs), placement,static synchronous compensator (STATCOM), voltage recovery.

I. INTRODUCTION

I N recent years, several environmental and economic ben-efits along with policies have led to increased penetration

of renewable as well as nonrenewable distributed energyresources (DERs) into the electricity grid. System operatorshave addressed the issue of increased penetration by specifyinggrid integration requirements at the point of common coupling(PCC). The independent power producers are placing fast andslow acting reactive power controllers at the PCC to meet gridrequirements, which include specific voltage rules for bothsteady state as well as abnormal operation conditions. Gridoperators demand low voltage ride through (LVRT) capabilityfrom all new large-scale DERs and hence several approacheshave been adopted to improve the LVRT capability of theseDER units [1], [2]. A comprehensive literature review ofSTATic synchronous COMpensator (STATCOM) and otherFlexible AC Transmission System (FACTS) controllers alongwith reactive power generation capability of generators sug-gests that the present studies and methods are focused towardintegration of large DER units in distribution networks [3]–[6].STATCOM ensures faster operation compared to other shuntFACTS devices and hence, works as a superior device to supplyfast reactive power under postfault conditions [7].

Manuscript received April 16, 2012; revised August 05, 2012; acceptedSeptember 19, 2012. Date of publication January 09, 2013; date of currentversion March 18, 2013. This work was supported by the CSIRO IntelligentGrid Flagship Collaboration Research Fund.The authors are with the School of Information Technology and Electrical

Engineering, The University of Queensland, Qld, 4072, Australia (e-mail:taziz@itee.uq.edu.au; udaymhaskar@gmail.com; saha@itee.uq.edu.au;mithulan@itee.uq.edu.au).Digital Object Identifier 10.1109/TSTE.2012.2227517

The voltage related issues reach a new dimension when smallscale DER units in an area tend to increase. The grid codes andstandards address these issues by requesting DER units below acertain size to operate in constant power factor control mode andasks for tripping if an abnormal condition persists for more thana stipulated period [8]. This act not only affects the uptime ofsmall-scale DER units but also reduces the maximum utilizationof renewable energy resources in the system.The problems mentioned can be alleviated by choosing an

appropriate location and type of reactive power controller thatresults in a cost-effective quick recovery of voltage at all busesof interest. This paper addresses the issues mentioned above intwo steps. The first step develops a sensitivity index that utilizesthe characteristic and essential variables (current and voltage) ofvoltage source converter (VSC)-based series and shunt FACTScontrollers. The second step develops a method that utilizes thesensitivity index along with an optimization procedure and timedomain simulation to find out the best location of STATCOMthat ensures fast voltage recovery of generator buses. The paperis organized as follows.Section II gives a brief introduction to the current grid in-

terconnection requirements for the DER unit. Section III de-scribes the formulation of index and methodology proposed toimprove the voltage profile during prefault and postfault sce-narios. Section IV presents the results obtained towards the im-provement in DER unit uptime and also compares the proposedmethodology with the existing ones. Section V describes theconclusions and contributions of the paper.

II. GRID INTERCONNECTION REQUIREMENTS

Grid interconnection standards specify the required behaviorof renewable and nonrenewable independent power plants atPCC under all operating conditions. Most of the standards dealwith the following issues of power system control [8]–[10]:

A. Reactive Power Generation Capability

In order to access a maximum amount of real power andto avoid unnecessary interactions among voltage controllers,different grid codes around the world limit the usage of reac-tive power capability of DER units to a minimum level [11].For example, IEEE Standard 1547-2003 does not allow DERunits to regulate voltage at the PCC actively [8]. The FederalElectric Regulatory Commission (FERC) of the U.S. specifiespower factor at the PCC as 0.95 leading to 0.95 lagging for largewind parks as found from the system impact studies [9]. TheAustralian Energy Market Commission (AEMC) requests dis-tributed generation with a capacity of less than 30 MW to main-tain a power factor close to unity [10].

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452 IEEE TRANSACTIONS ON SUSTAINABLE ENERGY, VOL. 4, NO. 2, APRIL 2013

TABLE IDER RESPONSE TO ABNORMAL VOLTAGE [8]

B. Steady State Voltage: Continuous Operation Range

The steady state voltage level at each bus is one of the mostsalient parameters for maintaining the quality of supply. In gen-eral, it is expected that the voltage of a bus remains within therange of 10% of nominal value irrespective of the presence ofDER units in the network [10].

C. Response of DER Units to Abnormal Voltage

During abnormal system conditions, IEEE and other stan-dards demand DER units below a certain size to cease ener-gizing according to the clearing time in Table I [8].The clearing time as listed in Table I, is a maximum threshold

for DER units with a capacity of 30 kW or less. For DER unitswith a generation capacity above 30 kW, the listed clearing timeis the default value. However, this can vary with different utilitypractices.

III. SENSITIVITY INDEX AND FORMULATION OFMETHODOLOGY

Based on the literature review and analysis of grid intercon-nection requirements, a new sensitivity index and methodologyis proposed here to improve the uptime of small scale DER units.The following section presents detail derivation of the index andthe methodology.

A. Sensitivity Index for VSC Based Shunt FACTS Controller

For incorporation of VSC based FACTS controllers involtage stability studies the following issues are important:1) Formulation of Equations:

a) Choice of controllable variables (bus voltage andangle in conventional tools).

b) Choice of output variables (shunt real and reactivepower injections and/or current injection).

2) Methods of Solution:a) Stability assessment using – curve.b) Time domain simulation representing load dynamics,control strategy for excursion in voltage at buses ofinterest.

The above choices are dictated by the basic operating char-acteristics, response time, and control capability of the devices[12]. The indices and methodologies use power balance equa-tions to carry out the analysis [13], [14]. The VSC-based se-ries and shunt FACTS controllers are capable of injecting realand reactive components of voltage/current independent of aline current/bus voltage within a few milliseconds. Thus, for

Fig. 1. General FACTS device model for the study [13].

the present study (placement of VSC-based reactive power con-trollers), the power system along with VSC-based FACTS con-trollers can be presented with Fig. 1.List of Symbols:

Phase angle of voltage at bus.

Phase angle of current flowing in a branch.

I Magnitude of current flowing in a branch.

Injected shunt reactive current.

Series injected voltage in phase with line-current.

Series injected voltage in quadrature withline-current.

Shunt injected real power.

Series injected real power.

Shunt injected reactive power.

Series injected reactive power.

Magnitude of element in matrix, connectedbetween th and th bus.

Angle of element in matrix, connected betweenth and th bus.

Set of buses connected to bus where series FACTSdevices are not connected.

Set of branches connected to bus where seriesFACTS devices are connected.

In the presence of both series and shunt FACTS devices asshown in Fig. 1, the real and reactive power injected by seriesFACTS device can be given as

(1)

(2)

Voltage in quadrature with injected line current by seriesFACTS device can be expressed as

(3)

AZIZ et al.: INDEX FOR STATCOM PLACEMENT TO FACILITATE GRID INTEGRATION OF DER 453

Similarly for shunt FACTS device connected at bus , in-jected real and reactive power can be given as

(4)

(5)

Reactive current injected at bus can be expressed as

(6)

For sensitivity analysis, linearizing above equations aroundthe operating point yields

(7)

Here, elements of matrix are the sensitivity factors, whichare the partial derivatives of (4), (6), (1), and (3). Consideringthe power system with only shunt FACTS devices, (7) can besimplified as

(8)

With zero real power injection from the shunt FACTS device,simplification of (8) results in the expression of sensitivity indexas in

(9)

where elements of various submatrices in (9) are as follows:

(10)

(11)

(12)

(13)

Fig. 2. Flowchart for determination of reactive power controller location.

Replacing above submatrices in (9) results in (14), whichshows the sensitivity index at bus

(14)

As reactive current injection by shunt FACTS controlleris independent of node voltage to which it is connected ( asin Fig. 1), is proposed as the sensitivity index to findout the best location for STATCOM, to alleviate slow voltagerecovery problems in a DER integrated distribution system.As STATCOM can absorb as well as inject , the value of

can be both positive and negative. A positive valueof the index indicates the requirement of supplying , whereasa negative value stands for the necessity of inductive currentat the connection bus of STATCOM. Therefore, a large valueof for a particular bus in system indicates highvoltage sensitivity with injection/absorption of reactive currentfrom/by STATCOM. Some preliminary results have been

published in [15].

B. Methodology for Placement of STATCOM

The key steps developed to secure a voltage profilethroughout a distribution system for all operating conditions(steady state and postfault) are depicted in Fig. 2. Inclusion ofDER unit results in reduction of real power intake from theutility grid. Steady state voltage requirement (as in Table I) atthe load end as well as the DER connection point is maintainedby following an optimal capacitor placement algorithm. VARoptimization and planning is associated with a nondifferentiableobjective function, which leads to a heuristic algorithm for thesolution. In the present work, Tabu search-based optimization[16], [17] has been chosen to obtain optimal location and sizesof fixed capacitor banks.With the application of various faults such as single line to

ground faults and three-phase faults in the system, time domainsimulations can be carried out to check the dynamic voltagerestoring capability of the generator and load bus. As a three-phase fault is the most severe contingency, the present studyconcentrates on voltage recovery with a three-phase fault nearto the generator bus, which makes the proposed methodologyapplicable for all other faults.

454 IEEE TRANSACTIONS ON SUSTAINABLE ENERGY, VOL. 4, NO. 2, APRIL 2013

Placement of STATCOM is considered only when the staticcompensator (capacitor bank) fails to maintain the requiredvoltage profile under postfault conditions.For the placement of STATCOM, the proposed sensitivity

index along with its direction is used to find outthe single best location. The bus having the highest negative

indicates the necessity of inductive current injectionby STATCOM at that bus. The optimal capacitor on that bus isreplaced by STATCOM of the same reactive power set pointand voltage recovery time is obtained with the help of time do-main simulation for the generator buses. If the first STATCOMfails to support recovery within an allowed time frame, withthe first STATCOM in place the bus with the second mostnegative sensitivity index is searched with the helpof (14). This leads to the placement of a second STATCOM.The procedure is repeated until the voltage excursion of allbuses concerned fall within the boundary specified by thestandard/grid code. The proposed index utilizes aninherent feature of STATCOM, which allows reactive currentinjection even at diminishing terminal voltage [18]. Index

, calculated with frequent snapshots, allows capturingfast dynamics [19]. Performance of in choosingthe best place for STATCOM is compared with an existingsensitivity index [20], [21]. Effectiveness of the index

and the methodology based on this index has beenverified through time domain simulations with two differenttest systems as presented in Section IV.

IV. RESULTS AND ANALYSIS

A. Test Distribution System

To test the effectiveness of the proposed index and themethodology, two distribution test systems with differentconfigurations and load compositions are considered in thiswork. All studies reported are carried out using DIgSILENTPowerFactory 14.0 [22]. The first test distribution system asin Fig. 3, consists of 16 buses. Total load in the system is28.7 MW and 9.48 MVAr, respectively. This system is themodified form of the one used in [23] and has been treated asa commercial feeder throughout the study. The second systemwith 21.76 MW and 9 MVAr of real and reactive power load, asshown in Fig. 4, is obtained from [24]. This system is treated asan industrial feeder for the present study. Short circuit capacity(SCC) has been taken as 100 and 300 MVA for the 16 and43 bus systems, respectively, and the grid ratio is takenas 4. Low values of both—short circuit capacity and ratioimplies that the grid connections are weak and hence, they areunable to supply a large amount of reactive power under peakdemand or in contingency situations [25].This study considers practical load compositions as they have

a profound impact on the bus voltages of a system. The break-down typically used by the utilities for commercial and indus-trial feeders are shown in Table II [26]. Here, motors with apower rating greater than 100 hp are treated as large motors,which are principal loads in an industrial feeder. This break-down of load is utilized in the study for static as well as fordynamic load modeling of both feeders.

Fig. 3. Single line diagram: 16 bus test system.

Fig. 4. Single line diagram: 43 bus test system.

TABLE IITYPICAL LOAD COMPOSITION [26]

TABLE IIIPEAK LOAD DATA [27]

For each type of feeder, the customer load has its typical loadcomposition as well as daily load curve (DLC) set [27]. Alongwith peak demand, values found from DLC sets arepresented in Table III.

B. DER Units and Their Capacity

As present grid standards do not allow small-scale generatorsto contribute reactive power, DER units in this study are con-ventional synchronous generators with unity power factor oper-ation [10]. DER connection details for both systems are givenin Table IV at base load condition. The location of DER unitsin the 16 bus system are chosen randomly at bus 3 and bus 6,whereas for the 43 bus system, locations are maintained as givenby the system data.

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TABLE IVDER UNITS: CAPACITY AND LOCATION

TABLE VOPTIMAL CAPACITOR SOLUTION FOR 16 BUS SYSTEM

TABLE VIOPTIMAL CAPACITOR SOLUTION FOR 43 BUS SYSTEM

C. Steady State Voltage With Optimal Capacitor andSensitivity Index Values

In most practices, steady state voltage requirementthroughout the network is ensured by following an optimalcapacitor placement algorithm. As described in Section III-B,the “Tabu search” technique is used to find out the feasiblelocations of fixed capacitors in the system at peak load con-dition followed from Table III. The maximum number ofiterations is set as stop criterion for this algorithm [28]. Table Vsummarizes results for optimal capacitor places for the 16 bussystem, whereas Table VI shows the places for the 43 bus testsystem to ensure steady state voltage within the limits statedin Section II-B. For the 16 bus test system, placing capacitorsof 10.2 MVAr helps in maintaining the voltage profile withinthe limits and also reduces grid loss significantly from 1.20to 0.90 MW under peak load condition. In the 43 bus system,inclusion of 11.20 MVAr of fixed capacitor bank improvesthe voltage profile. However, grid loss is found to reducemarginally from 0.54 to 0.45 MW.

D. Voltage Recovery and STATCOM Placement

According to [8], the voltage at the DER terminal must comeback to 88% of its normal operating voltage within 2 s after initi-ation of an abnormal condition. STATCOM has been introducedto provide dynamic reactive power support in the test system

Fig. 5. Voltage at bus 3 with three-phase fault for peak load (16 bus system).

Fig. 6. Voltage at bus 6 with three-phase fault for peak load (16 bus system).

under such conditions. Instead of going for additional locations,the best possible bus for placing STATCOM has been decidedwithin the optimal buses, which were chosen by the optimal ca-pacitor placement algorithm. Sensitivity indices and

are calculated for optimal capacitor buses and listedin Tables V and VI.In order to check the voltage profile under abnormal condi-

tions, both systems are subjected to a three-phase fault (with afault reactance of 0.05 ) near the generator bus and the faultis cleared after 10 cycles. The following test cases are investi-gated to find out a possible solution for supporting voltage re-quirements mentioned in Table I:Case 1) without capacitors;Case 2) with capacitors at optimal locations;Case 3) replacing capacitor with STATCOM at a bus with the

highest ;Case 4) replacing capacitor with STATCOM at a bus with the

highest (for both inductive, i.e., negativeand capacitive, i.e., positive values).

During the simulation studies, STATCOM is equipped witha PI-based voltage source controller [29]. The static rating ofSTATCOM for each system is chosen based on the optimal ca-pacitor size found from Tables V and VI. For example, in the43 bus test system, STATCOM with 1.1-MVAr reactive powerset point is chosen based on the findings from Table VI.Figs. 5–7 show the time domain simulation for the 16 bus

system at peak load condition (as in Table III) with a three-phasefault at bus 5. Figs. 5 and 6 show excursions in voltage at DER

456 IEEE TRANSACTIONS ON SUSTAINABLE ENERGY, VOL. 4, NO. 2, APRIL 2013

Fig. 7. Voltage at bus 5 with three-phase fault for peak load (16 bus system).

TABLE VIIVOLTAGE RECOVERY TIME FOR 16 BUS SYSTEM (PEAK LOAD)

bus 3 and 6, respectively. Load voltage at bus 5 is shown inFig. 7.As shown in Table V, the most negative value of is

found at bus 7, whereas the most positive value is found at bus3. has the highest value of 0.0069 Vp.u./MVAr at bus7. A STATCOM is placed at bus 7 and bus 3 alternately to findthe effect of its placement on voltage recovery time and alsoto investigate the effectiveness of sensitivity indicesand , respectively.Without capacitors, the system failsto reach the required voltage due to the three-phase fault. Forgenerator bus 3, capacitors at optimal places with optimal sizeshelp to recover the bus voltage within 1.15 s. STATCOM atbus 7 reduces the restoring time significantly to only 0.26 s. Itis interesting to note, though bus 3 is a generator bus, placingSTATCOM on bus 3 results in a longer recovery time (1.26 s)than that on bus 7. Table VII summarizes the voltage recoverytime for both generator buses of the 16 bus system.In order to check the effectiveness of the proposed placement

of STATCOM with an unbalanced fault, another case study iscarried out with a single phase to ground fault on phase “A.”The fault is featured with the same fault reactance of 0.05and a clearing time of 10 cycles. The voltage plots at bus 6, forphases “A,” “B,” and “C” are shown, respectively, in Figs. 8,9, and 10. The simulation plots depict that in each phase thevoltage recovery is faster compared to every other case withthe three-phase fault and prove that a single line to ground faultdoes not have a severe impact on recovery. Therefore, for plan-ning purposes, the rest of the simulations are carried out withonly a three-phase fault. As simulation results for the 16 bussystem under peak load condition do not reflect any problem ofslow voltage recovery of generator buses, the base case is notconsidered.Time domain simulations for the 43 bus system are plotted

with two scenarios—base load and peak load. Simulation is

Fig. 8. Voltage of phase “A” at bus 6 with single phase to ground fault (16 bussystem).

Fig. 9. Voltage of phase “B” at bus 6 with single phase to ground fault (16 bussystem).

Fig. 10. Voltage of phase “C” at bus 6 with single phase to ground fault (16 bussystem).

carried out for all four cases mentioned earlier. Figs. 11 and 12show the simulation results for generator bus voltages with athree-phase fault at bus 31 at base load condition. Sensitivityindices, and (capacitive), both have theirhighest value at bus 21. (inductive) is found only atbus 39 with a value of 0.25 (Vp.u./Ip.u.) as shown in Table VI.Table VIII summarizes the voltage recovery time at baseloading, which shows that other than STATCOM at bus 39,no controller arrangement can support the grid requirement atbus 4 as they have recovery times greater than 2 s. For bus 50,recovery time is found to be less than 2 s in all arrangementsconsidered.

AZIZ et al.: INDEX FOR STATCOM PLACEMENT TO FACILITATE GRID INTEGRATION OF DER 457

Fig. 11. Voltage at bus 4 with three-phase fault for base load (43 bus system).

Fig. 12. Voltage at bus 50 with three-phase fault for base load (43 bus system).

TABLE VIIIVOLTAGE RECOVERY TIME FOR 43 BUS SYSTEM (BASE LOAD)

Figs. 13 and 14 show the simulation results with a three-phasefault at bus 31 under peak load condition. Table IX summarizesthe voltage recovery times at peak load.It shows that the placement of STATCOM at bus 39 results

in reduced voltage recovery time supporting grid requirement,whereas the other arrangements fail to meet the standard (re-covery within 2 s as in Table I) and demand tripping of the DERunit at bus 4. Analyzing the voltage recovery time from all threeTables VII–IX, it can be concluded that STATCOM should beplaced at a bus with the highest inductive to supportfast voltage recovery requirement of DER units in the system.Table X summarizes and compares the performance of pro-

posed methodology with existing methodologies in recoveringvoltage at PCC under peak load condition.As mentioned in Table X, in [1] and [3], STATCOM has been

placed at the PCC along with the DER unit to ensure fast voltagerecovery and the ratings of STATCOM units have been chosenas 30%–100% of the rating of corresponding DER units. Hence,in a system with a large number of DER units, that approach

Fig. 13. Voltage at bus 4 with three-phase fault for peak load (43 bus system).

Fig. 14. Voltage at bus 50 with three-phase fault for peak load (43 bus system).

TABLE IXVOLTAGE RECOVERY TIME FOR 43 BUS SYSTEM (PEAK LOAD)

will lead to placement of an equally large number of STAT-COMs at points of common coupling to support fast voltage re-covery. However, adopting the proposed methodology, a singleSTATCOM on the bus with the highest inductivevalue is found to ensure fast voltage recovery for both systems.The other methodology, utilizing as an index to placeSTATCOM fails to meet the grid standard of recovery within 2 sfor the 43 bus test system.For a small radial system like the 16 bus system, it has

been found that STATCOM is not even necessary to supportvoltage recovery, though wrong placement of STATCOM leadsto longer recovery time. However, for the 43 bus system withlarge mesh configuration, STATCOM is required at both baseload and peak load conditions to support fast voltage recovery.Simulation results presented in this section are followed by twoadditional case studies—1) requirement of multiple STATCOMunits and 2) validity of the methodology with renewable gen-eration units. The first case study utilizes the 16 bus system asthe test distribution system, whereas the 43 bus test system isutilized for the second set of studies. With changed operationscenarios, these studies strengthen the above findings.

458 IEEE TRANSACTIONS ON SUSTAINABLE ENERGY, VOL. 4, NO. 2, APRIL 2013

TABLE XPERFORMANCE COMPARISON OF STATCOM PLACEMENT METHODS

Fig. 15. Voltage at bus 6 with variation of motor load (16 bus system).

E. Case Study: Influencing Factors and Requirement ofMultiple STATCOM Units

Multiple STATCOMs may be required in a system; this de-pends on a number of issues such as slow voltage recovery, sizeand strength of distribution network, strength of grid connec-tion, number of DER units, and loading condition. The key fac-tors behind slow voltage recovery have been discussed in detailin [30].As both commercial and industrial feeders are dominated by

induction motors, the sharing of small and large motor loadsplays a significant role in the voltage recovery of DER units.Increasing the share of motor load, which is predominantly air-conditioner load in a commercial feeder, results in a greater con-sumption of reactive power under a fault condition. Simulationresults in Fig. 15 show that the recovery situation gets worsewith the inclusion of 80% motor load in the 16 bus system andthe voltage at bus 6 fails to recover to its prefault value. Ac-cording to Table I, this ultimately demands tripping of the DERunit at bus 6.Network configuration, ratio of interconnections, line

impedance, and SCC of distribution-feeder connection are someof the influential factors, which determine the size and strengthof any distribution system. In a practical scenario, ratiois fixed for a particular distribution system. However, with aplan of expansion, the geographical area along with line lengthof the distribution system may change over a time period. An-other reason behind network expansion could be the remote lo-

Fig. 16. Voltage at bus 6 with variation of line impedance (16 bus system).

Fig. 17. Bus voltage with single STATCOM (16 bus system).

cation of renewable DER units. This act eventually changes theeffective line impedance throughwhich the buses are connected.Fig. 16 shows how the voltage recovery process of bus 6 varieswith the change in line impedance of the 16 bus system. With a100% increase in line impedance of each connection, voltage atbus 6 fails to recover and hence, DER1 demands to be trippedafter 2 s of the fault.Hence, among the factors mentioned above the two most

likely incidences that can take place over time include networkexpansion and addition of new DER units. These changesultimately modify a DER integrated distribution network andthe necessity of multiple STATCOM units may arise to supportthe changed scenario. Instead of going for a new network, the16 bus test system as in Fig. 3 is modified and taken as anexample for the present case study. The changes include in-creasing line impedance of each connection to twice its originalvalue and connection of a new 5 MVA DER unit at bus 16. Thenew DER unit at bus 16 is operated at unity p.f. Figs. 17 and 18show the voltage excursions at generator buses 6, 3, and 16 ofthe changed 16 bus system. Fig. 17 shows the voltage recoveryperformance of each generator bus with the first STATCOMin place (bus 7 as explained in Section IV-D). It can be seenthat, though the voltage at buses 6 and 3 can recover within2 s, the voltage at bus 16 with a new DER unit takes 3.23 s toreach 0.88 p.u. of postfault value. Hence, single STATCOMfails to support dynamic VAR for the new DER unit at bus 16and demands tripping.According to the methodology described in Section III-B,

a revised calculation of sensitivity index establishes bus 15as the next most sensitive node for the placement of a second

AZIZ et al.: INDEX FOR STATCOM PLACEMENT TO FACILITATE GRID INTEGRATION OF DER 459

Fig. 18. Bus voltage with multiple STATCOM (16 bus system).

Fig. 19. Voltage at bus 4 with DFIG under peak load condition (43 bus system).

STATCOM. Fig. 18 shows that voltages at all three DER busesrecover within the allowed time frame of Table I. Therefore,in a wide and weak distribution system with a large number ofDER units, placement of multiple STATCOM units could beunavoidable. Nevertheless, results in this section prove that theproposed sensitivity-based approach would effectively reducethe total number of STATCOM units in such situations.

F. Case Study: Wind- and Solar-Based Generation andVoltage Recovery

The synchronous generator model considered so far can beused to represent DER units such as combined heat and power(CHP) plant, geothermal, solar thermal, biomass, and smallhydro types of distributed generation. Over the last few years,there has been an increased interest in wind- and solar-basedDER units. Hence, two additional case studies are carriedout on the 43 bus system to validate the methodology withthese types of DER units. At first, the synchronous generatorat bus 4 is replaced by seven doubly-fed induction generator(DFIG)-based wind turbines each with a rating of 2 MVA, tofind out the impact of the proposed placement of STATCOMunder fault condition. DFIG machine parameters are takenfrom [31]. Fig. 19 shows the voltage recovery pattern with athree-phase fault at bus 31 under peak load condition. Simu-lation results show that a STATCOM at bus 39 results in theshortest voltage recovery time of 1.64 s.Similar results are found by replacing the synchronous gen-

erator at bus 4 with 14 MVA of photovoltaic (PV) plant [32] asshown in Fig. 20. In both cases, generation units are being oper-ated at unity power factor. Hence, simulation results in Figs. 19

Fig. 20. Voltage at bus 4 with PV plant under peak load condition (43 bussystem).

and 20 confirm that this methodology can be effectively used torecover voltage within a standard time frame with wind as wellas solar-based DER units.

V. CONCLUSION

In this paper, a new sensitivity index for placing STATCOMhas been developed. Amethodology based on the index has beenproposed for a distribution system with dispersed DER units tomaintain voltage at all buses under both steady state and ab-normal conditions. An analytical expression of the new index

has been developed, which requires only a nominalload flow solution. Case studies have been carried out with var-ious types of DER units, including wind- and solar-based gener-ation to validate the proposed methodology. Simulation resultsprove that the presence of STATCOM at a bus with the highestnegative value ensures a fast voltage recovery at thePCC as well as load buses. As a result, small-scale DER unitsremain connected to the grid under abnormal conditions im-proving their uptime. Network parameters and loadmodels havebeen identified as the influential factors that may lead to place-ment of multiple STATCOM units. In a system with a numberof DER units, this approach minimizes the number of expen-sive STATCOM units to keep DER units interconnected. Futurework will focus on the optimal mixture of static and dynamiccompensation and sizing of STATCOM for achieving voltagecontrol with improvement in DER uptime.

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Tareq Aziz (S’09–M’12) received the B.Sc. (Engg.)and M.Sc. (Engg.) degrees in electrical and elec-tronic engineering, both from Bangladesh Universityof Engineering and Technology (BUET), in 2002and 2005, respectively. Currently he is workingtoward the Ph.D. degree at the School of ITEE, TheUniversity of Queensland, Australia.His research interests include renewable energy in-

tegration in power systems, power system stability,and signal processing.

U. P. Mhaskar received the Ph.D. degree in 2003 from IIT, Mumbai, India.He has worked as a postdoc fellow at The University of Queensland. His

research areas include control system design, power electronics and drives, re-newable energy, and HIL simulation studies.

Tapan Kumar Saha (M’93–SM’97) receivedthe B.Sc.Eng. degree in 1982 from BangladeshUniversity of Engineering and Technology, Dhaka,Bangladesh, the M.Tech. degree in 1985 from theIndian Institute of Technology, New Delhi, India,and the Ph.D. degree in 1994 from the University ofQueensland, Brisbane, Australia.He is currently a Professor of electrical engi-

neering with the School of Information Technologyand Electrical Engineering, The University ofQueensland, Australia. He is a Fellow of the In-

stitution of Engineers, Australia. His research interests include conditionmonitoring of electrical plants, power systems, and power quality.

Nadarajah Mithulananthan (M’02–SM’10) re-ceived the B.Sc. (Eng.) degree from the Universityof Peradeniya, Sri Lanka, in May 1993, the M.Eng.degree from the Asian Institute of Technology,Bangkok, Thailand, in August 1997, and the Ph.D.degree from the University of Waterloo, Canada inelectrical and computer engineering in 2002.He is currently a senior lecture at the University

of Queensland (UQ), Brisbane, Australia. Prior tojoining UQ he was associate Professor at AsianInstitute of Technology, Bangkok, Thailand. His

research interests are integration of renewable energy in power systems andpower system stability and dynamics.