IEEE PES PowerAfrica 2012 - Conference and Exhibition Johannesburg, South Africa, 9-13 July 2012

Modelling of Large-Scale Grid-Connected Photovoltaic Systems: Static Grid Support by Reactive Power Control

Mitra Mirhosseini, Vassilios G. Agelidis, Jayashri Ravishankar School of Electrical Engineering and Telecommunications

The University of New South Wales, Sydney, Australia Email: m.mirhosseini@student.unsw.edu.au

Abstract - Modelling of large-scale grid-connected pllOtovoltaic systems (GCPSs) is needed for different time frameworks to assess different aspects of botlt component and system performance. Tlte paper presents extended models of large-scale GCPSs witlt DigSILENT PowerFactory for static grid support using reactive power control. Grid codes are taken into account to develop tlte proposed models wlticlt are able to control tlte system's reactive power (Q) support under fIXed Q, fixed power factor (cosqJ), cosqJ dependent of active power (cosqJ(P)) and droop control Q(U). Selected results of a simulation block for a 10 MVA of PV generation demonstrates tlte effectiveness of tlte modified models.

I. INTRODUCTION

The ever increasing size of grid-connected photovoltaic (PV) systems (GCPSs) will have a profound impact on the power system and how the GCPS and the power system interact with one another [1]. As a result, both component and system studies of large-scale GCPSs are becoming more crucial. This is due to the fact that the generated power of the PV systems can be highly variable due to weather conditions impacting on the power system performance. Therefore, CGPSs bring about new integration issues to the utility and PV plant developers, especially when the size of the GCPS reaches hundreds of MWs to be connected in a single location both geographically and electrically.

The integration issues of large-scale GCPSs can be categorized into static and dynamic. Research has been widely reported on dynamic features of GCPSs using computer modelling. For example in [2] an electromechanical transient model of large-scale GCPS has been developed by the FASTEST (Fast Analysis of STability using the Extended equal area criterion and Simulation Technologies) software. The research in [3] has proposed a coordinated control system to enhance the low voltage ride through (L VRT) capability of a 5 MW PV plant under various fault conditions using the PSCAD/EMTDC software. In [4] the impact of short circuit power at the point of common coupling (PCC) as well as droop value of the inverter controller on the L VRT capability is investigated with DigSILENT software [5].

From the static point of view, several areas of research have been addressed. In [6] the impact of large-scale PV penetration on the static voltage stability of power system has been investigated with MATLAB software. Another study in [7] proposed an overall maximum power point tracking (MPPT) strategy for a multi-peak problem in the PV array.

978-1-4673-2550-9112/$31.00 2012 IEEE

In [8], a capability chart has been defined in the P-Q plane using MA TLAB, to find all possible pairs of active and reactive power (P, Q) that can be selected as the inputs of the inverter control system of the PV generator. From the grid support perspective, it has been demonstrated in [9] that converting the function of the PV inverter from unity power factor to voltage regulation control reduces the operation of voltage regulators and capacitor banks embedded in the power system. In [10] the impact of a large GCPS on the voltage and the reactive power characteristics at the PCC has been investigated through a P-V curve method using DigSILENT software. The focus has been on the control mode of the PV system i.e. current-source or voltage-source mode. It should be noted that in the above mentioned references, there is a lack of research on the various methods of supporting the grid under static conditions with the reactive power, Q without using external reactive power compensation devices.

The DigSILENT PowerFactory 14.1 software [5] has a template model of a GCPS. However, as it will be demonstrated later in this paper, the current model requires modification to address reactive power support under static conditions to meet medium voltage (MV) grid code requirements.

The objective of this paper is to report a 10 MV A GCPS using DigSILENT PowerFactory 14.1 software. This paper will fill the gap in the static reactive power support, by developing a control system and applying it to the existing PV inverter configuration to operate under four different methods of reactive power support as required by the grid codes.

The remainder of the paper is organized as follows. The grid codes (GCs) impacting the design and operation of large GCPSs [11, 12] are presented in Section II. The existing model of a GCPS in DigSILENT is introduced in detail in Section III. Then a 10MW GCPS is designed and reported in Section IV. The IOMW GCPS model is evaluated and extended to deal with the requirements of the GCs under static conditions in Section V. Finally, Section VI summarizes the conclusions.

II. GRID CODES

Electricity utilities must fulfill certain national and/or international obligations in supplying electric power to

customers. These obligations are known as GCs and are set by transmission system operators (TSOs).

Several European countries are obliged to satisfy interconnection requirements for the large-scale GCPS based on power system operation and stability. Germany and Spain are the leaders in utilizing PV systems with respect to both the installation and integration technologies. Therefore, they have more comprehensive GCs in this field. The studies in this paper will be done according to German GCs [11, 12] for connecting PV systems to the MV grid. The size of PV system to be connected to MV grid depends on the respective grid conditions and is determined by the calculation of TSO. Possible PV sizes are in the range of 500kW to 100MW [12]. These GCs do not have a major impact on the design of the PV inverter. However, new control algorithms must be developed to satisfy them.

A. Reactive Power Control for Static Grid Support

In MV grids, the grid support under static conditions implies the voltage stability of the system under normal operation. This is done based on the system requirement or on demand by the TSO to keep the voltage variation within acceptable limits. Besides that, the PV plant should be capable of providing reactive power at any operating point to keep the power factor within the boundary of 0.95 underexcited to 0.95 overexcited at the PCc. The setpoint for reactive power control can be adjusted by the TSO via each of the four methods of supporting reactive power in MV systems [12]: fixed Q, fixed cos

L. Solar 11 E _ , PhotovoItaic

Radiation JI Model

Uarray

larra), DC busbar and

Capacitor Model

idre/ _.

I TemperatoreJF 2

, iqre/

, Vdcref t +--+---------------- ,

1::::::=====:jJ

I Power

l\'leasurement I ft----P iS-' -.J I AC Voltage 1 1 lIac

ControUer

I Slow Frequency I I Fmeas I 1_ red ,

II I Active Power Reduction t======= l\'leasurement _

'========l.J

I Si17re/" COSre/ r 2 I

I Phase l\feasurement

Fig. 2. Control frame of PV system in DigSILENT

Static generator

IV. CASE STUDY

The system considered for the study is a 1 OMV A GCPS as seen in Fig. 3 which consists of 10 parallel IMVA PV generators (with transformer embedded inside the inverter) connected to a 20 kV MV grid through an AC line. The system's specifications are provided in Table I.

to be strong grid and its SCP is calculated to be 1 OMV A x 30 = 300MVA. According to [14], the short circuit ratio value below 20 is considered as a "weak grid" connection. The details of selected PV module and PV inverter can be found in [15] and [16], respectively.

External Grid

PCC

LO

MVO

LI L2 LlO

BI B2 BIO

PVI PV2 PVIO

Fig. 3. The configuration of a 10 MVA GCPS

To connect this 1 OMV A PV system to a stable external grid, the short circuit power (SCP) of the external grid is selected to be 30 times the power of PV plant i.e. the short circuit ratio equals to 30. In this case the external grid is said

Table I. The case study datasheet

PV Module Characteristics in STC PV Inverter Characteristics

Optimum Operating 35.2 V Max. DC power 1133 kW Voltage U''''P1') (at cosqJ=l)

Optimum Operating 7.95 A Max. DC input 1000 V Current (l",vo) voltage Open Circuit

44.8 V Max. DC input

2484 A Voltage (Vv,) current Short Circuit 8.33 A Min. DC input 450 V Current (1,,) voltage Temperature -0.0033fOC Rated AC power 1l00kVA Coefficient of Va, (at STC) Temperature 0.00055/oC Nominal AC 20 kV Coefficient of 1" Voltage

Number of Parallel 175

Max. output 31.8 A Modules current

N umber of Series 19 AC power 50 Hz Modules frequency Line Characteristics

Rated Voltage External Grid Characteristics (LO-L 10) 20 kV

Rated Current Min. Short 0.117 kA Circuit Power 300 MVA (LI-LIO) (Skillin) Min. Short

Rated Current (LO) 0.456 kA Circuit 8.66 kA Current(lk",;,,)

Based on the PV module parameters in Table I, the maximum active power is calculated as 0.93 MW for each PV generator at STC, using

P max = (35.2V X 19serieJ X (7.95A X 175parn,Ie,) (1) =

O.93MW

V. SIMULATION AND RESULTS

In this section, results are initially reported from the tests of GCPS under environmental conditions. Then, the results under static conditions using all four methods of reactive power support, namely, fixed Q, fixed cos 0.1. More information on dynamic grid support is given in [12].

The output of droop control block iq( is limited by maximum and minimum reactive current i e i and I'

. . . q-max q-min,

respectIvely. Then iq( enters the current limiter block along with

.calculated id and duac to compute reference values idrej

and Iqref' These values are used by the PV inverter to adjust the active and reactive power, respectively. The outputs of the current limiter are restricted by maximum allowed

absolute current (maxAbsCur) and maximum absolute reactive current in normal operation (maxlq).

Low-pass + duo, Filter

Un

maxAbsCur, maxlq

Current Limiter

Fig. 6. Dynamic reactive power support integrated with the existing PV model in DigSILENT

These principles of the dynamic controller can be extended to a static control system that can operate in parallel with the dynamic controller to support the grid with reactive power under static conditions (Iduacl < 0.1). Such a static controller is a proportional-integral (PI) controller and is shown in Fig. 7. The desired reactive power Qrej(which is set by the TSO) is compared with the actual reactive power (Qm) measured by the voltage measurement device at the output of PY generator and the mismatch value passes through the PI controller. The output of the PI controller (Qnorm) then divided by Uac to obtain respective reactive current (iqnorlll) which goes to the current limiter. In the current limiter a switch is introduced and embedded to select the suitable iq (iqj or iqnorm) depending on the level of voltage drop. In addition, Qm passes through a low-pass filter to mitigate its high frequency components.

_P_in_-+t1 QLimits = = - - - - - i I I

Low-pass Filter

+ PI Controller

Qnorm

Fig. 7. Proposed control system to support reactive power control in static conditions

Furthermore, to implement the cos

cos

mode and the droop value is set to 2 (200%) and a voltage drop less than 0.1 Un is applied to the MVO bus. In the voltage drop duration this controller should inject reactive

1:::; :tC:T== =0800 _ 4 150s 0950 0999 0 40 =--=-==':--=----:----+--:.:.- ------ ----=--=-- ::r ?_----___ m ___ m __ m_mn - 0.80L---------------------- 0.0000 2.0000 4.0000 6.0000 8.0000 [s] 10.01 --_. PVI: Power Factor -- PQ Measurement_BI: Active Power in p.ll. ---- Controller: Minimum Reactive Power Limit in p.lI. ---- Controller: Maximum Reactive Power Limit in p.n. -- PQ Measurement_BI: Reactive Power in p.ll.

Fig. 13. Performance of proposed cosq>(P) method

Fig. 14 shows the results after activating the droop mode of the existing model with droop equal to 200%. For better illustration of the results in the voltage drop, solar radiation and temperature values are set constant throughout the simulation. It is obvious that there is no reactive power injection during the voltage drop.

1.60

1.201-__________ ___ _

0.80 6.372 s 0.921 p.ll.

0.40

0.00 --------------------.------------,---------

-0.40'-------'-------'------'------'-------"-0.0000 2.0000 4.0000 6.0000 8.0000 [s] 10.0 -- Cub_l\Voltage Measurement_PCC: Output Voltage. Absolute in p.ll. -0-0

- PVI: Droop in p.u. (base: 400.00 %) PV 1 : Power Factor -- PQ Measurement_BI: Reactive Power in p.ll.

Fig. 14. Performance of the droop control in existing PV model of DigSILENT

Therefore, a method which is the same as droop control in the dynamic grid support in DigSILENT is proposed. However, the static controller is triggered when the voltage drop is less than 10%. The Q-U relationship is shown in (2).

(2)

where uac is the voltage at the output of PV generator and duac is the voltage deviation from the Un. The capability of the developed model is shown in Fig. 15. For better comparison with the existing model, the droop value in the proposed method is set to 2 (200%).

Based on the results in Fig. 15, there is voltage improvement in the PCC bus by the injection of reactive power. Nevertheless, the power factor drops a bit due to the rise in reactive power, but it is still within the GC limits.

1.60

1.20f-__________ ___ _ 0.80

0.40

0.00 _____________ .J

5.482 s 0.935 p.u.

t------

-0.401------'---------'----------' 0.0000 2.0000 4.0000 6.0000 8.0000 [s] 10.00 -- Voltage Measurement-PCC: Output Voltage, Absolute in p.ll. -- Controller: Droop for static Q support in p.ll. (base: 4.00 p.ll.) --- Controller: Minimum Reactive Power Limit in p.u. --- Controller: Maximum Reactive Power Limit in p.u.

PV 1: Power Factor - - PQ Measurement Reactive Power in p.u.

Fig. 15. Performance of modified static droop control

VI. CONCLUSION

In this paper a GCPS system is modeled with the software DigSILENT PowerFactory and assessed for supporting the grid under static conditions with different reactive power control methods. The existing model has been modified to address fixed Q, fixed cos

controller under fault conditions

iq-max Maximum acceptable reactive current

iq-nun Minimum acceptable reactive current

iqnorm Generated reactive current by the proposed PI controller under static conditions

iqre[ Reference value for the reactive current

lsc Short circuit current of PV module

maxAbsCur maximum allowed absolute current

maxlq maximum absolute reactive current in normal operation

P, Pin Generated active power by PV generator

Pmax Maximum generated active power by PV generator

Pn Nominal power of PV inverter

Q, Qm Generated reactive power by PV system

Qnorm Generated reactive power by the proposed PI controller under static conditions

Qmax Maximum acceptable reactive power

Qmin Minimum acceptable reactive power

Qre[ The desired reactive power to be injected into the grid under static conditions

Uac Voltage at the output of PV system

Un Nominal voltage of PV generator

Vmpp Voltage value at maximum power point of PV module

Voc Open circuit voltage of PV module

REFERENCES

[1] F. Katiraei and 1. R. Aguero, "Solar PV Integration Challenges," IEEE Power and Energy Magazine vol. 9, pp. 62-71,2011.

[2] F. Li, W. Li, F. Xue, Y. Fang, T. Shi, and L. Zhu, "Modeling and simulation of large-scale grid-connected photovoltaic system," in International Conference on Power System Technology (POWERCON), 2010, pp. 1-6.

[3] G. M. S. Islam, A. AI-Durra, S. M. Muyeen, and J. Tamura, "Low voltage ride through capability enhancement of grid connected large scale photovoltaic system," in IECON 2011 - 37th Annual Conference on IEEE Industrial Electronics Society, 2011, pp. 884-889.

[4]

[5]

[6]

A. Marinopoulos, F. Papandrea, M. Reza, S. Norrga, F. Spertino, and R. Napoli, "Grid integration aspects of large solar PV installations: L VRT capability and reactive power/voltage support requirements," in IEEE Trondheim PowerTech, 2011, pp. 1-8. DigSILENT Power Factory 14.1. [Online] accessed May 2012. Available: http://www.digsilent.de/index.php/productspowerfactory.html. R. Shah, N. Mithulananthan, R. C. Bansal, K. Y. Lee, and A. Lomi, "Power system voltage stability as affected by large-scale PV

penetration," in International Conference on Electrical Engineering and Informatics (lCEEI), 2011, pp. 1-6.

[7] L. Lei, Z. Zhao, W. Xu, and J. Zhu, "Modeling and analysis of MWlevel grid-connected PV plant," in IECON 2011 - 37th Annual Conference on IEEE Industrial Electronics Society, 2011, pp. 890-895.

[8] F. Delfino, R. Procopio, M. Rossi, and G. Ronda, "Integration of largesize photovoltaic systems into the distribution grids: a p-q chart approach to assess reactive support capability," lET Renewable Power Generation, vol. 4, pp. 329-340, 2010.

[9] R. A. Walling and K. Clark, "Grid support functions implemented in utility-scale PV systems," in IEEE PES Transmission and Distribution Conference and Exposition 2010, pp. 1-5.

[10] X. Xu, Y. Huang, G. He, H. Zhao, and W. Wang, "Modeling of large grid-integrated PV station and analysis its impact on grid voltage," in International Conference on Sustainable Power Generation and Supply, 2009, pp. 1-6.

[11] Bundesverband der Energie- und Wasserwirtschaft e.V.(BDEW), "Erzeugungsanlagen am Mittelspannungsnetz (Richtlinie fur Anschluss

und Parallelbetrieb von Erzeugungsanlagen am Mittelspannungsnetz)," German Grid Codes, Berlin, 2008.

[12] E. Troester, "New German grid codes for connecting PV systems to the Medium Voltage Power Grid," presented at the 2nd International Conference on Concentrating Photo voltaic Power plant, Darmstadt, Germany, 2009.

[13] M. Kizilcay, K. Teichmann, A. Agdemir, M. Losing, and C. Neumann, "Blackstart of a 380-kV Transmission System with Unloaded

Transformers after a System Collapse," presented at the 17th Power Systems Computation Conference (PSCC), Stockholm, Sweden, 2011.

[14] M. F. Farias, P. E. Battaiotto, and M. G. Cendoya, "Wind Farm to Weak-Grid Connection using UPQC custom power device," in IEEE International Conference on Industrial Technology (ICIT) 2010, pp. 1745-1750.

[15] SUNTECH. 280 Watt RELIATHON Solar Module [Online] accessed May 2012. Available: http://am.suntechpower.com/images/stories/pdf/datasheets/20 IIISTP275 _ VRM _ UL.pdf

[16] SMA Solar Technology. Sunny CENTRAL 1000 MVA [Online] accessed May 2012. Available: http://www.sma.de/en/products/centralinverters/sunny-central-800mv-I 1-1000mv-II-1250mv-ll.htrnl

[17] S. Chowdhury, S. P. Chowdhury, G. A. Taylor, and Y. H. Song, "Mathematical modelling and performance evaluation of a stand-alone

polycrystalline PV plant with MPPT facility," in IEEE Power and Energy Society General Meeting - Conversion and Delivery of Electrical Energy in the 21st Century, 2008, pp. 1-7.