Impact of Short-Circuit Ratio on Grid Integration of Wind Farms - a New Zealand Perspective

2nd Int'l Conference on Large-Scale Grid Integration of Renewable Energy in India| New Delhi, India | 4-6 Sep 2019 Impact of Short-circuit Ratio on Grid Integration of Wind Farms - A New Zealand Perspective

Raghavender D. Goud and Ramesh Rayudu Vikash Mantha Ciaran P. Moore School of Engineering and Electrical Design Engineer Department of Electrical and Computer Science UK Power Networks Computer Engineering Victoria University of wellington Bury St Edmunds, UK University of Canterbury Wellington, New Zealand Christchurch, New Zealand Email: [email protected]

Abstract—The high wind energy potential sites are increas- ingly remote from the load centers which create problems in integrating wind power plants into power grids. One of the major issues is the short-circuit ratio (SSR) at the point of common coupling. A lower short-circuit ratio will lead to a weaker grid which in turn creates power system stability problems. The definition of a weak grid depends on short- circuit ratio values. In New Zealand, it is often noted that the definition of a weak grid, based on SSR,is different from that of standard values (of SSR) due to several reasons. New Zealands grid is long and slender with arguably major generation at one end and load centers at the other side. Close to 80% of daily consumption of electricity is generated from renewables with 5-8% contributed by Wind generation. Wind farm locations in New Zealand share the same difficulties as shared by many nations in terms of remote locations, far away from load centers and wind variability. However, New Zealand wind farm capacity factor is close to 45% and maximum Fig. 1. Energy by technology type in NZ [2]. capacity of around 400 hours a year. Over 20 new wind farms have been consented for future with capacity close to 3GW. This work shares our experience in connecting future wind farms to New Zealand grid and their effect on the weak grid the load has grown and the power system was connected concept. We will discuss the reasons for our definition of short- using high voltage transmission system [3]. circuit ratio in New Zealand scenario and, in support, share some results of our simulations.

I.INTRODUCTION

The demand for electricity is dependent on various factors viz. GDP, energy costs, wealth and number of the popu- lation [1]. The energy consumption of New Zealand was about 38,800 gigawatt hours (GWh) in 2017 consisting of 1.72 million residential, 175,000 commercial and 123,000 industrial customers. NZ has over 219 generating stations with a mix of generation types, as shown in Fig. 1 [2]. On average, from all the generation in New Zealand, 59% of the generation is from hydro, 17% geothermal, 16% thermal, 5% wind and 3% from co-generation [2]. Wind and solar energy installations are growing rapidly in the past few years. The generation contribution of wind energy is growing at a proportion of nations electricity demand [1]. Currently, 700 MW large-scale wind energy is operational and new Fig. 2. Map of the New Zealand electricity system showing wind farms are either in consent and construction phases [2]. the main load centres and main generation sources [3]. Geographically, NZ is divided into two Islands, viz. North Island and South Island. The major load centres of the The first wind farm connected to the power grid in New country are in North Island, as shown in Fig.2 [3]. His- Zealand was in the year of 2004. Since then, over 16 wind torically, NZ’s electrical power system was distributed with farms have been developed with 490 turbines and a capacity local generation supplying the load centres until 1950s [3]. of 690MW [4]. The major power generation source was Hydro. Eventually, The interconnection of wind farms to the transmission 2nd Int'l Conference on Large-Scale Grid Integration of Renewable Energy in India| New Delhi, India | 4-6 Sep 2019 networks can cause the fluctuations in voltage and power 3) In pre-fault condition, the power dispatch at the PoC flows in the system [5]. Also, the inherent nature of the should be Maximum Real power with Zero Reactive wind power variability results in the need for reactive power power output. that affects the bus voltages and transformer taps [6]. Hence, 4) Create a solid 3-phase fault at the PoC with a fault the operators of wind farms should regulate the voltage at clearance time of 140 ms. point of connection (PoC) [7] imposed by system operators. 5) Record the V, P and Q graphs at the PoC and WF The impedance and mechanical inertia of a power system terminals. determines the strength of any grid [8]. An alternative way of 6) Repeat the study for different values of SCR. determining the strength is with short circuit ratio (SCR) [9]. III.CASE STUDY SCR does not represent the strength of entire grid but only at specific (measured) nodes of the power grid [10]. Hence, A. System model different parts of the grid may have different values of The wind farm used in this study was a type 4 model SCR [11]. from Powerfactory, as shown in Fig. 3 The base model A grid is defined as a weak grid if its SCR values is less consist of six wind turbines with a rating of 2.5 MW. Each than 3 [12] at the point of connection. The definition is sup- wind turbine has a step-up transformer of 0.69/20 kV. The ported by both IEEE [13] and NERC [9]. However, research wind turbines are connected to a wind farm substation which states there should be a distinction between a weak grid has a capacitor bank to support the reactive power. A park and grid with low SCR [10]. The SCR of a node represents controller at PoC has been used to operate the farm in the ability of a bus to withstand the voltage fluctuations in Voltage/Reactive Power/Power Factor mode, as depicted in response to a fault. Although, SCR is evaluated based on the Fig. 4 [15]. An individual controller at each Wind Turbine steady state parameters of the power system, it represents the has been to control the active/reactive power output from capacity of a bus during dynamic system disturbances [14]. the wind turbines, as shown in Fig. 5 [15]. A Master-Slave This paper presents an analysis of SCR on voltage and control approach is applied by selecting the park controller active power profiles at PoC and wind turbine terminals in Voltage control mode and the individual wind turbines in during a fault ride through scenario. The transmission system reactive power mode such that the wind farm will support model of the entire New Zealand power grid is used in the grid at PoC for any voltage variations. At the wind evaluating the short circuit levels of the system at all the farm substation, a step-up transformer of 20/110(220) kV nodes. is modelled at the PoC. Wind farm rating is scaled up to 150 MW to perform wider SCR scenarios.

II.METHODOLOGY B. Short circuit ratio calculations A three-phase short circuit analysis was performed on The calculation of short-circuit ratio requires short-circuit the NZ transmission network model to identify the short- levels at the nodes of the power system. The minimum three- circuit capacities. The short circuit capacities varied from phase short-circuit levels are calculated according to IEC 222 MVA to 3376 MVA across the country. However, only 60909 at all the nodes of the NZ power network using Sys- three cases with least short circuit capacity from each North tem Operator’s model in DiGSilent PowerFactory. The single and South Island are considered for this analysis. The wind line diagram consists of the complete transmission network farm name plate rating of 15 MW to 150 MW (Capacities of North and South Island along with the interconnecting currently under operation in NZ) were connected to calculate underwater HVDC link. the SCR. SCRs evaluated using eq(1), are summarised in Based on the short-circuit levels observed, the point of Tables-I to VI. S connections for wind farm interconnection were selected SCR = cc (1) for this study. The lower and higher short-circuit capacity Swf nodes were selected to compare the system performance for where Scc is the short circuit level at the bus without different short circuit ratios. The short circuit ratios were connecting the wind farm and Swf is the wind farm name evaluated based on the short circuit levels and the wind farm plate rating in MW [7]. name plate ratings. At PoC, an inﬁnite grid with the selected short circuit TABLE I: Short Circuit Ratios for Westport node of South capacity is modelled to connect the wind farm. The wind Island. farm with different name plate ratings were modelled and Wind farm S.No. SC (MVA) SCR connected at the PoC to create a low and high SCR scenarios. rating (MW) 1 222.68 10 22.27 This completes the modelling of case to be studied in 2 222.68 15 14.85 DigSilent Powerfactory. The load data included in the study 3 222.68 30 7.42 was for 13 December 2018. 4 222.68 45 4.95 The fault ride through (FRT) studies were carried as 5 222.68 60 3.71 6 222.68 75 2.97 follows: 7 222.68 90 2.47 1) The FRT study shall be carried out by keeping the 8 222.68 105 2.12 9 222.68 120 1.86 park controller in V control mode. 10 222.68 135 1.65 2) The OLTC of the WF Transformer (if any) shall be 11 222.68 150 1.48 locked at a nominal position. MV 3.1 MV 5.1 MV 1.2 MV LV1.2 LV5.1 LV3.1

Trf 1.2 WT Trf Type 2.8MVA 20kV/0.. 2.78 MVA 2.78 Trf .. Trf .. 2.50MW 1.2 WTG WT .. WT .. WTG WTG 5.. WTG 3.. 2.50 .. 2.78 .. 2.50 .. 2.78 .. NA2XS(F)2Y NA2XS(F)2Y .. 1 Cable5.1 1.0 km 0 0 NA2XS(F)2Y 1x150RM 12/20kV NA2XS(F)2Y it NA2XS(F)2Y .. NA2XS(F)2Y MV 5.2 MV Cable3.1 1.0 km Cable1.2 0.8km MV 1.1 MV MV 3.2 LV5.2 LV1.1

Trf ..

WT .. WindSubstation/Busbar Farm Distribution Distribution NetworkSubstation/Busbar PoC WTG 5.. WTG Trf 1.1 2.50 .. 2.50 .. 2.78 WT Trf Type 2.8MVA 20kV/0.. 2.78MVA 2.50 MW 2.50 1.1 WTG LV3.2 0 1 Trf .. WT .. NA2XS(F)2Y .. NA2XS(F)2Y WTG 3.. WTG LVCap 2.50 2.50 .. 2.78 .. Cable5.2 1.0 km 0 NA2XS(F)2Y NA2XS(F)2Y .. Cable1.1_a 1.0km NA2XS(F)2Y.. MV 5.3MV Cable3.2 1.0km

Trf Capacitor Bank CapacitorBank Trf Type 20MVA 20kV/0.4kV Bay 4 MV 3.3MV LV 5.3 0 2

Trf .. WT .. WTG5.. 2.50.. 2.78.. Cable 1.1 LV 3.3

NA2XS(F)2Y 1x150RM 12/20kV it Bay 3 Bay 2 1.0 km WF_GT 110kVGrid 0 90MVA_110/20kV Bay1 Bay1 Bay3

Trf ..

WT .. 0 WTG3.. 2.50.. 2.78.. NA2XS(F)2Y NA2XS(F)2Y .. Cable5.3 1.0km 0 PoC/PCC NA2XS(F)2Y 1x150RM 12/20kV NA2XS(F)2Y it NA2XS(F)2Y.. a a a Bay 8 Bay7 Bay6 Bay5 Cable3.3 1.0km Cable2.1 3.0 km MV 5.4 MV MV 3.4 MV 2.1 MV LV5.4 LV 2.1

Trf .. WT .. WTG 5.. WTG LV3.4

2.50 .. 2.78 .. Trf 2.1 WT Trf Type 2.8MVA 20kV/0.. 2.78MVA 2.50 MW 2.1 WTG 0 ParkControl System (dynamic)

Trf .. ControllerPark (steady-state)

WT .. 1 WTG 3.. WTG WindFarm Power at PoC 2.50.. 2.78.. NA2XS(F)2Y.. Cable5.4 1.0km 0 NA2XS(F)2Y 1x150RM 12/20kV NA2XS(F)2Y it NA2XS(F)2Y .. NA2XS(F)2Y MV 5.5 Cable3.4 1.0 km Cable2.2 1.1km MV 3.5 MV 2.2 MV LV5.5 LV2.2

Trf .. WT .. WTG 5.. WTG LV3.5

2.50 .. 2.50 .. 2.78 Trf 2.2 WT Trf Type 2.8MVA 20kV/0.. 2.78MVA 2.50 MW 2.50 2.2 WTG 0 Trf ..

WT .. 1 WTG 3.. WTG 2.50 .. 2.50 .. 2.78 NA2XS(F)2Y.. Cable5.5 0 1.0 km NA2XS(F)2Y.. NA2XS(F)2Y 1x150RM 12/20kV NA2XS(F)2Y it i.3 Wind 3. Fig. Cable3_ 1.0km NA2XS(F)2Y .. NA2XS(F)2Y Cable3.5 1.0 km NA2XS(F)2Y .. NA2XS(F)2Y Cable2.3 1.1km MV 5.6MV Cable4_ 1.0km MV 3.6 MV 2.3 MV NA2XS(F)2Y NA2XS(F)2Y .. Cable5_ LV 5.6 LV 2.3 1.0 km LV 3.6

Trf .. Trf 2.3 WT .. WT Trf Type 2.8MVA 20kV/0.. 2.78 MVA .. NA2XS(F)2Y 2.50MW 2.3 WTG WTG5..

2.50.. 2.78.. Trf 3.6 Cable6_

WT Trf Type 2.8MVA 20kV/0.69kV 1.0km 2.78MVA 2.50MW WTG 3.6 1 0 0 NA2XS(F)2Y 12/20kV NA2XS(F)2Y it1x150RM ammdli Powerfactory. in model farm Cable2.4 1.5km MV 2.4 LV2.4

Trf 2.4 WT Trf Type 2.8MVA 20kV/0.. 2.78MVA 2.50 MW 2.50 2.4 WTG 1 MV 6.1MV MV 4.1 MV LV 6.1 LV 4.1

Trf .. WT .. WTG 6.. WTG

2.50.. 2.78.. Trf .. WT .. WTG 4.. WTG 2.50.. 2.78.. 0 0 NA2XS(F)2Y.. NA2XS(F)2Y .. NA2XS(F)2Y Cable6.1 1.0km Cable4.1 1.0km MV 6.2MV MV 4.2 MV LV6.2 LV 4.2

Trf .. WT .. WTG WTG 6..

2.50 .. 2.78 .. Trf .. WT .. WTG 4.. WTG 2.50.. 2.78.. 0 0 NA2XS(F)2Y .. NA2XS(F)2Y Cable6.2 1.0km NA2XS(F)2Y NA2XS(F)2Y .. Cable4.2 1.0km MV 6.3 MV MV 4.3 LV6.3

Trf .. WT .. WTG6.. LV4.3 2.50 .. 2.50 .. 2.78 0 Trf .. WT .. WTG 4.. WTG 2.50 .. 2.50 .. 2.78 0 NA2XS(F)2Y .. NA2XS(F)2Y Cable6.3 .. NA2XS(F)2Y 1.0 km Cable4.3 1.0km MV 6.4MV MV 4.4 MV LV 6.4 LV4.4

Trf .. WT .. WTG WTG 6..

2.50.. 2.78.. Trf .. WT .. WTG WTG 4.. 2.50.. 2.78.. 0 0 NA2XS(F)2Y .. NA2XS(F)2Y NA2XS(F)2Y .. NA2XS(F)2Y Cable4.4 Cable6.4 1.0km 1.0km MV 6.5 MV 4.5 LV4.5 LV6.5

Trf .. WT .. WTG 4.. WTG

Trf .. .. 2.50 .. 2.78 WT .. WTG 6.. WTG 2.50 .. 2.78 .. 0 0 NA2XS(F)2Y NA2XS(F)2Y .. Cable4.5 NA2XS(F)2Y .. NA2XS(F)2Y 1.0km Cable 6.5 1.0km MV 6.6 MV 4.6 MV LV 4.6 LV 6.6

Trf .. Trf .. WT .. WT .. WTG6.. WTG4.. 2.50.. 2.78.. 2.50.. 2.78.. 0 0

DIgSILENT 2nd Int'l Conference on Large-Scale Grid Integration of Renewable Energy in India| New Delhi, India | 4-6 Sep 2019 Sep 4-6 | India Delhi, New India| in Energy Renewable of Integration Grid Large-Scale on Conference Int'l 2nd AL I:SotCrutRto o aaund fSouth of node Oamaru for Ratios Island. Circuit Short III: TABLE South of node Reefton for Ratios Island. Circuit Short II: TABLE 7 6 5 4 3 2 1 0 WTG FRC Frame incl Frame Current Ctrl: FRC WTG Controller:Park ControllerPark u q p i.4 idfr akcnrle [15]. controller park farm Wind 4. Fig. v_wind S.NO. S.No. i.5 idtriecnrle [15]. controller turbine Wind 5. Fig. Overfrequ. Power Reduc.. Voltage Measurement 1.1 Current Measurement 1.1 Overfrequency Power R.. 2 1 0 PQ Measurement LV1.1 Park Control System (d.. System Control Park 1/(1+sT) 1/(1+sT) 1/(1+sT) Vac Measurement Turbine Controller Turbine PQ Measurement Iac Measurement Tu Tq Tp q_sign_cmd cosphi_cmd 10 Park Controller Park mode_cmd p_lim_cmd 10 Plim_park Turbine 1.1 Fmeas q_cmd PLL 1.1 6 5 4 3 2 1 9 8 7 9 8 7 6 5 4 3 2 1 PLL 2nd Int'l Conference on Large-Scale Grid Integration of Renewable Energy in India| New Delhi, India | 4-6Sep2019 New India Delhi, India| Renewable Energy Large-Scale GridIntegrationof in Int'l Conferenceon 2nd u_meas q_meas p_meas 2 1 0 1 0 2 1 0 1 0 1 0 C(MVA) SC C(MVA) SC 1 0 321.23 321.23 321.23 321.23 321.23 321.23 321.23 321.23 321.23 321.23 rdfeuiq_ref pred_frequ 275.17 275.17 275.17 275.17 275.17 275.17 275.17 275.17 275.17 275.17 Protection 1.1 Mode,u_min,Tunblock,array_cosp.. 6 5 4 3 2 1 0 1 0 Qref Pref Qin Protection Pin u Power Reduction Power Mode Selector cosref sinref ir ii 6 5 4 3 2 1 0 PQ Controller 1.1 Controller PQ ui PQ Controller ur aig(MW) rating aig(MW) rating idfarm Wind idfarm Wind 1 0 delta_q delta_p id_ref q_min 0 Current Controller 1.1 7 6 5 4 3 2 1 0 Current Controller Current [Kp+Ki/s] [Kp+Ki/s] Kqp,Kqi Kpp,Kpi 150 135 120 105 150 135 120 105 90 75 60 45 30 15 90 75 60 45 30 15 q_max 1 _i_nenp_lim p_lim_intern q_ref_intern 1 0 u1r_in u1i_in 10.71 21.42 18.34 SCR SCR 2.14 2.38 2.68 3.06 3.57 4.28 5.35 7.14 1.83 2.04 2.29 2.62 3.06 3.67 4.59 6.11 9.17 1 0 gradlim gradlim Generator WTG 1.1 dq_max dp_max q_ref 1 0

DIgSILENT DIgSILENT h hr-ici ee tec oei xd ti observed is it ﬁxed, As is MW. node 15 of each steps at in level output short-circuit farm the wind the increasing by cleared is fault and of ms. s A 2 140 [15]. at after created tool is fault Powerfactory solid DIgSILENT three-phase different in create simulations The performed to network. were varied system power gener- was the power in output scenarios wind farm SCR the wind varying The by ation. levels SCR different for North of node Gisborne for Island. Ratios Circuit Short VI: TABLE of node Park National for Island. Ratios North Circuit Short V: TABLE North of node Kaitaia for Ratios Island. Circuit Short IV: TABLE ubn ee r nlsddrn n fe laigtefault the values. clearing SCR after selected and during for wind farm analysed at Wind are and level PoC The the turbine ms. at 140 Power) Active of the short and (Voltage time three-phase study, responses clearance solid this fault a with In as at circuit scenarios. modelled disturbance is SCR sudden disturbance different a sudden under on PoC farm Wind the of performance the studies stability Transient C. h C ausaecluae tsxndso h network the of nodes six at calculated are values SCR The stability network the test to performed were Simulations h i fti rnin tblt tde st analyse to is studies stability transient this of aim The S.No. S.No. S.No. V R IV. 10 10 10 9 8 7 6 5 4 3 2 1 9 8 7 6 5 4 3 2 1 9 8 7 6 5 4 3 2 1 C(MVA) SC (MVA) SC C(MVA) SC SLSAND ESULTS 297.11 297.11 297.11 297.11 297.11 297.11 297.11 297.11 297.11 297.11 238.57 238.57 238.57 238.57 238.57 238.57 238.57 238.57 238.57 238.57 367.15 367.15 367.15 367.15 367.15 367.15 367.15 367.15 367.15 367.15 aig(MW) rating (MW) rating aig(MW) rating idfarm Wind farm Wind idfarm Wind D ISCUSSION 150 135 120 105 150 135 120 105 150 135 120 105 90 75 60 45 30 15 90 75 60 45 30 15 90 75 60 45 30 15 19.81 15.90 12.24 24.48 SCR SCR SCR 1.98 2.20 2.48 2.83 3.30 3.96 4.95 6.60 9.90 1.59 1.77 1.99 2.27 2.65 3.18 3.98 5.30 7.95 2.45 2.72 3.06 3.50 4.08 4.90 6.12 8.16 2nd Int'l Conference on Large-Scale Grid Integration of Renewable Energy in India| New Delhi, India | 4-6 Sep 2019 that the SCR will decrease with increase in the wind farm 1.60 output, as illustrated in Fig. 6. The nodes with low short- [p.u.] circuit capacity will see low SCR values for the increased 1.20 wind farm output.

0.80

SC Level - 222 MVA

SC Level - 275 MVA

SC Level - 321 MVA

0.40 SC Level - 238 MVA

SC Level - 297 MVA

SC Level - 367 MVA 20

0.00

15 SCR -0.40 -0.1000 1.9200 3.9400 5.9600 7.9800 [s] 10.000 Distribution Network Substation\Busbar PoC: Voltage, Magnitude

Plot PoC(1) Date: 23/07/2019 Annex: /2 10 Fig. 8. Voltage proﬁle at PoC for SCR=1.65.

1.60

[p.u.]

1.20

0 20 40 60 80 100 120 140 160

W ind Farm Output (MW )

0.80 Fig. 6. Variation of SCR for short-circuit capacity and wind farm output. 0.40 Stability of the power system for selected nodes is tested for different SCR values. Fig. 7 shows the voltage profiles 0.00 at the PoC for SCR of 1.48. It is observed that the voltage profile resumes to pre-fault condition after 1029 ms of -0.40 -0.1000 1.9200 3.9400 5.9600 7.9800 [s] 10.000 Distribution Network Substation\Busbar PoC: Voltage, Magnitude the fault clearance. Also, during the restoration phase, the Plot PoC(1) Date: 23/07/2019 Annex: /2 voltage overshoot of 41.75%. Fig. 9. Voltage profile at PoC for SCR=1.86.

1.60 1.20

[p.u.] [p.u.]

1.20 0.90

0.80 0.60

0.40 0.30

0.00 0.00

-0.40 -0.1000 1.9200 3.9400 5.9600 7.9800 [s] 10.000 -0.30 Distribution Network Substation\Busbar PoC: Voltage, Magnitude -0.1000 1.9200 3.9400 5.9600 7.9800 [s] 10.000 Distribution Network Substation\Busbar PoC: Voltage, Magnitude Plot PoC(1) Date: 23/07/2019 Plot PoC(1) Date: 23/07/2019 Annex: /2 Fig. 7. Voltage proﬁle at PoC for SCR=1.48. Fig. 10. Voltage proﬁle at PoC for SCR=2.12. Annex: /2

Fig. 8 depicts the voltage profile at the PoC for a short 1.20 circuit ratio of 1.65. The voltage over-shoot reduces to 38.9% [p.u.] with a restoration of time of 516 ms after the fault clearance. Figs. 9 to 14 illustrate the voltage profile for SCR values 0.90 from 1.86 to 4.95. The voltage overshoot ranges from 34.7% to 15.44% and restoration times are between 420 ms and 0.60 230 ms. Voltage profiles for SCR values beyond 5 are not demon- 0.30 strated in this paper, as the percentage over-shoot is within 10% and restoration times in 200 ms after the fault clear- ances. 0.00 Fig. 15 shows the effect of reactive power support at the

-0.30 PoC. It is observed that with the reactive power using the -0.1000 1.9200 3.9400 5.9600 7.9800 [s] 10.000 Distribution Network Substation\Busbar PoC: Voltage, Magnitude Plot PoC(1) Date: 23/07/2019 capacitor bank at the PoC improves system stability. The Annex: /2 restoration time of 1679 ms is noted without the reactive Fig. 11. Voltage proﬁle at PoC for SCR=2.47. 2nd Int'l Conference on Large-Scale Grid Integration of Renewable Energy in India| New Delhi, India | 4-6 Sep 2019

1.60

1.6

W ithout Reactive Power Support

[p.u.] Reactive Power Support at the PoC

1.4

1.20

1.2

1.0 0.80

0.8

0.40

0.6 Voltage(p.u)

0.4

0.00

0.2

0.0 -0.40 -0.1000 1.9200 3.9400 5.9600 7.9800 [s] 10.000 Distribution Network Substation\Busbar PoC: Voltage, Magnitude

Plot PoC(1) Date: 23/07/2019

Annex: /2 -0.2

Fig. 12. Voltage proﬁle at PoC for SCR=2.97. 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8

T ime (S) 1.60 Fig. 15. Effect of reactive power support on the voltage [p.u.] proﬁle at PoC for SCR=1.48.

1.20

1.60

[p.u.] 0.80

1.20

0.40

0.80

0.00

0.40

-0.40 -0.1000 1.9200 3.9400 5.9600 7.9800 [s] 10.000 Distribution Network Substation\Busbar PoC: Voltage, Magnitude Plot PoC(1) Date: 23/07/2019 0.00 Fig. 13. Voltage proﬁle at PoC for SCR=3.71. Annex: /2

-0.40 -0.1000 1.9200 3.9400 5.9600 7.9800 [s] 10.000 1.60 Distribution Network Substation\Busbar PoC: Voltage, Magnitude

Plot PoC(1) Date: 27/07/2019 Annex: /2 [p.u.] Fig. 16. Voltage proﬁle at PoC for SCR=2.45.

1.20

(i.e. SCR=1.48), there is an overshoot of 44.63% and the 0.80 settling time is around 1800 ms. For high SCR condition (i.e. SCR=22.27), there is an overshoot of 9.47% and the 0.40 setting time is around 480 ms. The large spikes can drive the wind turbines into over-voltage ride through (OVRT) mode.

0.00 The measured voltage disturbances can also be fatal for the power electronic components in the WTG converters.

-0.40 -0.1000 1.9200 3.9400 5.9600 7.9800 [s] 10.000 1.35 Distribution Network Substation\Busbar PoC: Voltage, Magnitude

Plot PoC(1) Date: 23/07/2019 Annex: /2 Fig. 14. Voltage proﬁle at PoC for SCR=4.95. [p.u.]

1.10 power support as compared to 1029 ms with the reactive 0.85 power support. Voltage proﬁle at the PoC for a SCR value of 2.45 at a different node and short circuit capacity is depicted in 0.60 Fig. 16. It is important to note here that although the SCR is same but with different short circuit capacity, the voltage 0.35 overshoot is same as that of SCR of 2.47. Hence, it is observed that SCR plays a predominant role in the stability 0.10 -0.1000 1.9200 3.9400 5.9600 7.9800 [s] 10.000 LV 1.2: Voltage, Magnitude of the system. Plot PoC(6) Date: 23/07/2019

Figs. 17 to 24 show the voltage profiles at one of the wind Fig. 17. Voltage profile at WTG for SCR=1.48. Annex: /7 turbines low-voltage terminal. It can be observed that the voltage profile resumes to pre-fault condition after clearance Figs. 25 to 27 illustrate the active power response at the of the fault with an increased voltage overshoot and settling PoC for SCRs of 1.48, 1.65 and 14.85. During pre-fault time at PoC, for all SCR values. For low SCR conditions condition, at PoC, the wind farm was injecting 145.34 MW 2nd Int'l Conference on Large-Scale Grid Integration of Renewable Energy in India| New Delhi, India | 4-6 Sep 2019

1.35 1.35

[p.u.] [p.u.]

1.10 1.10

0.85 0.85

0.60 0.60

0.35 0.35

0.10 0.10 -0.1000 1.9200 3.9400 5.9600 7.9800 [s] 10.000 -0.1000 1.9200 3.9400 5.9600 7.9800 [s] 10.000 LV 1.2: Voltage, Magnitude LV 1.2: Voltage, Magnitude Plot PoC(2) Date: 23/07/2019 Plot PoC(6) Date: 23/07/2019 Annex: /3 Fig. 18. Voltage proﬁle at WTG for SCR=1.65. Annex: /7 Fig. 22. Voltage proﬁle at WTG for SCR=2.97.

1.35 1.35

[p.u.] [p.u.]

1.10 1.10

0.85 0.85

0.60 0.60

0.35 0.35

0.10 0.10 -0.1000 1.9200 3.9400 5.9600 7.9800 [s] 10.000 -0.1000 1.9200 3.9400 5.9600 7.9800 [s] 10.000 LV 1.2: Voltage, Magnitude LV 1.2: Voltage, Magnitude Plot PoC(2) Date: 23/07/2019 Plot PoC(6) Date: 23/07/2019 Annex: /3 Fig. 19. Voltage proﬁle at WTG for SCR=1.86. Annex: /7 Fig. 23. Voltage proﬁle at WTG for SCR=3.71.

1.35 1.35

[p.u.] [p.u.]

1.10 1.10

0.85 0.85

0.60 0.60

0.35 0.35

0.10 0.10 -0.1000 1.9200 3.9400 5.9600 7.9800 [s] 10.000 -0.1000 1.9200 3.9400 5.9600 7.9800 [s] 10.000 LV 1.2: Voltage, Magnitude LV 1.2: Voltage, Magnitude Plot PoC(2) Date: 23/07/2019 Plot PoC(6) Date: 23/07/2019 Annex: /3 Fig. 20. Voltage proﬁle at WTG for SCR=2.12. Annex: /7 Fig. 24. Voltage proﬁle at WTG for SCR=4.95.

1.35 of active power and zero reactive power with SCR of 1.48. It [p.u.] seen that the wind farms are marginally stable and the active 1.10 power reduces to zero during the disturbance for all SCR values. It is also observed that the active power recovered

0.85 to its pre-fault level once the fault is cleared for all SCR values. For low SCR condition (i.e. SCR=1.48), the recovery time is around 2200 ms, and for high SCR condition (i.e. 0.60 SCR=14.85), it is around 420 ms. The percentage overshoot of voltage proﬁles at PoC and 0.35 WTG terminal are depicted in Fig. 28. It is observed that the WTG terminals experiences higher voltage dynamics

0.10 -0.1000 1.9200 3.9400 5.9600 7.9800 [s] 10.000 compared to the PoC. Also, an increase in SCR reduces LV 1.2: Voltage, Magnitude Plot PoC(6) Date: 23/07/2019 Annex: /7 the voltage overshoots and are within 20% (for SCR values Fig. 21. Voltage proﬁle at WTG for SCR=2.47. above 5). 2nd Int'l Conference on Large-Scale Grid Integration of Renewable Energy in India| New Delhi, India | 4-6 Sep 2019

PoC

W T G T erminal

160

140

120

100

60 VoltageOvershoot (%)

40 ActivePower (MW)

0 0

0 5 10 15 20 25

-20

SCR

2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6

T ime (S) Fig. 28. Voltage overshoot for different SCR values. Fig. 25. Active power proﬁle at PoC for SCR=1.48. Fig. 29 shows the restoration time for voltage at PoC and WTG low-voltage terminal. The settling time decreases with increase in SCR values at both PoC and WTG terminal, however, the restoration times at WTG terminal are greater

140 compared to PoC.

120

PoC

100

2000 W T G T erminal

1500

40 ActivePower (MW)

20 1000

0 RestorationTime (ms)

500

-20

2.0 2.2 2.4 2.6

T ime (S)

Fig. 26. Active power proﬁle at PoC for SCR=1.65. 0

0 5 10 15 20 25

SCR Fig. 29. Voltage restoration time for different SCR values.

16 V. DISCUSSION

14 The short circuit ratios are lower for South Island of New 12 Zealand compared to North Island mainly due to longer

10 transmission lines connecting the generation to the load

8 centres. For this study, we have chosen two locations of interest (Reefton and Oamaru) in South Island and three 6 locations (Kaitaia, National Park and Gisborne) in North 4 island using a speciﬁc criteria in conﬁdence. ActivePower (MW) 2 All nodes exhibit SCR of 5 or less beyond 60 MW of wind

0 farm output rating; a medium size farm for New Zealand

-2 installations. The voltage overshoot analysis suggests over 15% below SCR of 4 and increases sharply below SCR of -4

1.9 2.0 2.1 2.2 2.3 2.4 2.5 2.6 3. The overshoots are worse at WTG terminal level where T ime (S) the levels are around 20% even at SCR of 10. Similar Fig. 27. Active power proﬁle at PoC for SCR=14.85. conclusions can be made for the restoration times where they are very high below SCR of 3 (the weak grid deﬁned value). From the analysis, it is evident that many nodes of 2nd Int'l Conference on Large-Scale Grid Integration of Renewable Energy in India| New Delhi, India | 4-6 Sep 2019

NZ transmission network need a detailed design study of [11] Y. Zhou, D. D. Nguyen, P. C. Kjr, and S. Saylors, “Connecting wind WTG farm installations beyond 60 MW. power plant with weak grid - challenges and solutions,” in 2013 IEEE Power Energy Society General Meeting, July 2013, pp. 1–7. As seen from above, nodes with low SCRs will have large [12] J. Liu, W. Yao, J. Wen, J. Fang, L. Jiang, H. He, and S. Cheng, impact on the voltage response i.e. large initial spikes and “Impact of power grid strength and PLL parameters on stability of slower stabilisation. It can also cause the Wind turbines to grid-connected DFIG wind farm,” IEEE Transactions on Sustainable Energy, pp. 1–1, 2019. operate in OVRT mode. Grids with low SCRs will also have [13] D. Yang, X. Wang, F. Liu, K. Xin, Y. Liu, and F. Blaabjerg, “Adaptive larger impact on the active power recovery response with reactive power control of pv power plants for improved power transfer higher times which in-turn affecting stability of the system. capability under ultra-weak grid conditions,” in 2017 IEEE Power Energy Society General Meeting, July 2017, pp. 1–5. The analysis shows that disturbances at the WTGs are [14] S. Huang, J. Schmall, J. Conto, J. Adams, Y. Zhang, and C. Carter, higher compared to PoC which can potentially affect the “Voltage control challenges on weak grids with high penetration of low-voltage side and power electronic components in the wind generation: Ercot experience,” in 2012 IEEE Power and Energy Society General Meeting, July 2012, pp. 1–7. WTG. This can also be signiﬁcant for NZ scenario due to [15] Wind farm model. [Online]. Available: https://www.digsilent.de/en/ the large HVDC network connecting North and South Island powerfactory.html networks.

VI.CONCLUSION The effect of SCR on voltage and active power proﬁles are analysed when integrating the wind energy to New Zealand’s transmission grid. With a maximum capacity of 150 MW (NZ’s largest wind farm), it is observed that the NZ grid is not entirely a weak grid with some strong nodes but there are several nodes with SCR values less than 3. From the results we can also conclude that the system performance needs attention below the SCR values of 5. The low SCR at a bus represents its inability to maintain the voltage proﬁle during the power system disturbances. Hence, the evaluation of SCR and its affect on the wind farm interconnection at individual nodes need to be analysed. We also conclude that many nodes of NZ transmission network need a detailed design study of WTG farm installations beyond 60 MW.

ACKNOWLEDGMENT The authors gratefully acknowledge the support of the Victoria University of Wellington for this work through the VUW Research trust. The authors also would like to thank Dr. Nyuk-Min Vong of Transpower NZ Limited for initial discussions on the project and Electricity Authority for the system model and load data.

REFERENCES [1] Te Mauri Hiko Monitoring Report. [Online]. Available: https: //www.transpower.co.nz/resources [2] Electricity in New Zealand. [Online]. Available: https://www.ea.govt. nz/about-us/media-and-publications/electricity-new-zealand/ [3] Case study on the development of the wind industry in new zealand. [Online]. Available: http://www.windenergy.org.nz/store/doc/ Case-Study-on-the-Development-of-the-wind-industry-in-New-Zealand. pdf [4] Wind farms operating and under construction. [Online]. Available: http://www.windenergy.org.nz/operating-&-under-construction [5] S. S. Baghsorkhi and I. A. Hiskens, “Analysis tools for assessing the impact of wind power on weak grids,” in 2012 IEEE International Systems Conference SysCon 2012, March 2012, pp. 1–8. [6] ——, “Impact of wind power variability on sub-transmission networks,” in 2012 IEEE Power and Energy Society General Meeting, July 2012, pp. 1–7. [7] J. W. Feltes and B. S. Fernandes, “Wind turbine generator dynamic performance with weak transmission grids,” in 2012 IEEE Power and Energy Society General Meeting, July 2012, pp. 1–7. [8] B. B. Chakrabarti and R. Rayudu, “Inertial support performance of grid connected DFIG,” in 2013 IEEE Innovative Smart Grid Technologies-Asia (ISGT Asia), Nov 2013, pp. 1–6. [9] Short-circuit modelling and system strength. [Online]. Avail- able: https://www.nerc.com/pa/RAPA/ra/ReliabilityAssessmentsDL/ Short Circuit whitepaper Final 1 26 18.pdf [10] A. Golieva, “Low short-circuit ratio connection of wind power plants,” Master’s thesis, NTNU, 2015.