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IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 24, NO. 4, DECEMBER 2009 983

Power Engineering Letters

Nighttime Application of PV Solar Farm as STATCOM to Regulate Grid Voltage

Rajiv K. Varma, Vinod Khadkikar, and Ravi Seethapathy

AbstractThis letter presents a novel concept of utilizing pho-tovoltaic (PV) solar farm (SF) as a flexible ac transmission systemscontrollerstatic synchronous compensator, to regulate the pointof common coupling voltage during nighttime when the SF is notproducing any activepower. This concept, although general, is pre-sented for the scenario of a distribution feeder, which has both PVsolar and wind farms connected to it. The proposed control will en-able increased connections of renewable energy sources in the grid.A MATLAB/Simulink-based simulation study is presented undervariable wind power generation and fault condition to validate theproposed concept.

Index TermsDistributed generation, flexible ac transmissionsystems (FACTS), solar energy, static synchronous compensator(STATCOM), voltage regulation, wind energy.

I. INTRODUCTION

Utilities are presently facing a major challenge of grid-

integrating an increasing number of renewable-energy-based

distributed generators (DGs) while ensuring stability, voltage

regulation, and power quality [1]. During the nighttime, feeder

loads are usually much lower compared to daytime, while the

wind farms (WFs) produce more power due to increased wind

speeds. This potentially causes reverse power to flow from thepoint of common coupling (PCC) toward the main grid result-

ing in feeder voltages to rise above allowable limits, typically

5% [1][3]. To allow further DG connections, utilities need to

install expensive voltage regulating devices (e.g., static var com-

pensator (SVC), static synchronous compensator (STATCOM),

voltage regulators, etc.) [1], [4]. Voltage-source inverters are es-

sential components of PV solar farms (SFs), which provide solar

power conversion during daytime (normal operation). However,

PV SFs are practically inactive during nighttime and do not pro-

duce any real power output. The proposed novel concept is to

use the existing SF inverter as a STATCOM during nighttime

to regulate voltage variations at the PCC due to increased andintermittent WF power and/or by load variations.

Manuscript received June 5, 2009. Current version published November 20,2009. This work wassupported byOntario Centres of Excellence (OCE),Centrefor Energy. Paper no. PESL-00043-2009.

R. K. Varma and V. Khadkikar are with the Department of Electrical andComputer Engineering, University of Western Ontario, London, ON N6A 5B9,Canada (e-mail: rkvarma@uwo.ca; vkhadkik@uwo.ca).

R. Seethapathyis with Hydro OneNetworks, Inc., Toronto, ON M5G, Canada(e-mail: ravi.seethapathy@hydroone.com).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TEC.2009.2031814

Fig. 1. Single-line diagram of the distributed generation study system.

Fig. 2. SF as STATCOMcontroller diagram. (a) Synchronization. (b) PCCvoltage regulation loop. (c) DC bus voltage regulation loop.

II. DG SYSTEM MODELING

Fig. 1 shows the single-line diagram of the study system.

A WF modeled as a fully controlled converterinverter-based

permanent-magnet synchronous generator and a PV SF mod-

eled as a voltage-source inverter with a dc-bus capacitor, are

integrated in a 12.7-kV distribution network.

The load is considered to be of passive RL type.

III. SF INVERTER CONTROL

Fig. 2 shows the block diagram of the control scheme used to

achieve the proposed concept. Thecontroller is composed of two

proportionalintegral (PI) based voltage-regulation loops. One

loop regulates the PCC voltage, while the other maintains the

dc-bus voltage across SF inverter capacitor at a constant level.

The PCC voltage is regulated by providing leading or lagging

reactive power during bus voltage drop and rise, respectively.

A phase-locked loop (PLL) based control approach is used to

maintain synchronization [5] with PCC voltage. A hysteresis

current controller is utilized to perform switching of inverter

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984 IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 24, NO. 4, DECEMBER 2009

Fig. 3. SF as STATCOMPCC voltage regulation.

switches. To facilitate the reactive power exchange, the dc-side

capacitor of SF is controlled in self-supporting mode, and thus,

eliminates the need of an external dc source (such as battery).

IV. SIMULATION STUDY

MATLAB/Simulink simulation results for the study system

during steady-state and transient conditions are depicted inFig. 3. All parameters are in per unit (pu) with base values

of 10 MVA and 12.7 kV. Three scenarios of WF generation are

considered: 1) generation less than load; 2) generation approxi-

mately 2.5 times load; and 3) generation much higher than the

load ( 5 times the load). The different time instants of interest

are: t1 at which WF (PW F = 0.77 pu) is connected to network;

t2 when load is changed from 0.8+j0.08 to 0.3+j0.08 pu; and

t3 when WF power generation increases to 1.46 pu. Initially,

SF is not controlled as STATCOM. Fig. 3(a) depicts the volt-

age profile of PCC voltage VPC C without any compensation. A

voltage drop during 11.25 s due to heavy loading and voltage

rise during 1.251.75 s due to the reverse power flow, are ob-

served. With increased reverse power flow, PCC voltage risesup to 8.4%, exceeding the utility specification of5%.

The SF inverter is now operated as STATCOM. The regu-

lated PCC voltage VPC C

is shown in Fig. 3(a). The voltage

variation is only 3% (between t1 and t3 ), and is within utility

norm. Even during significant amount of reverse power flow

(after t3 ), SF maintains the PCC voltage at 1.0412 pu. The

distribution transformer secondary side voltage Vse c also does

not vary more than 1%. The pu active and reactive powers

of the load (PL , QL ), the WF (PW F ,QW F ) and SF (PSF ,QSF )

are illustrated in Fig. 3(b). The reactive power exchanged be-

tween SF and the distribution network (leading/lagging), and the

self-supporting dc-bus voltage Vdc profile [see Fig. 3(b)] vali-

Fig. 4. SF as STATCOMtransient performance during 3LG fault.

dates the proposed concept of utilization of SF as STATCOM

to regulate the PCC voltage. The performance of system under

a three-line to ground (3LG) fault condition (for six cycles) is

shown in Fig. 4(a) and (b). It is evident that the SF as STATCOM

helps achieve better fault recovery.

V. CONCLUSION

A novel concept of optimal utilization of a PV SF (virtu-ally inactive during nighttime in terms of active power genera-

tion) as a STATCOM has been proposed and validated through

MATLAB/Simulink simulations. The SF inverter is used to reg-

ulate the distribution voltage at PCC within utility specified

limits even during wide variations in WF output and loads. The

proposed strategy of PV SF control will facilitate integration

of more wind plants in the system without needing additional

voltage-regulating devices. This novel strategy implies operat-

ing PV solar plant as a generator during the day [providing

megawatts (MW)] and ancillary services provider at night [pro-

viding megavoltamperes (Mvars)]. It is worth noting that grid

codes typically do not allow DGs to manage the PCC voltages,this is considered to be the task of utilities. Therefore, in effect,

the PV solar plant may become a utility tool at night (control

is passed on to the utility supervisory control and data acqui-

sition (SCADA) operator). This may pose interesting questions

on ownership/partnership/lease options, license to operate, etc.,

and possible code changes by the regulator. These aspects are

outside the scope of the letter. The technical aspects, however,

warrant a look at such mixed usages.

ACKNOWLEDGMENT

The authors gratefully acknowledge Dr. C. Schauder for pro-

viding his insights in this research.

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