Implementation of the Low-Voltage Ride-Through Curve After Considering Offshore Wind Farms Integrated Into the Isolated Taiwan Power System

energies

Article Implementation of the Low-Voltage Ride-Through Curve after Considering Offshore Wind Farms Integrated into the Isolated Taiwan Power System

Shiue-Der Lu 1 , Meng-Hui Wang 1,* and Chung-Ying Tai 2

1 Department of Electrical Engineering, National Chin-Yi University of Technology, Taichung 41170, Taiwan; [email protected] 2 Department of Electrical Engineering National Taiwan University of Science and Technology, Taipei 10607, Taiwan; [email protected] * Correspondence: [email protected]; Tel.: +886-4-23924505 (ext. 7233)

Received: 3 March 2019; Accepted: 27 March 2019; Published: 1 April 2019

Abstract: In response to the power impact effect resulting from merging large-scale offshore wind farms (OWFs) into the Taiwan Power (Taipower) Company (TPC) system in the future, this study aims to discuss the situation where the offshore wind power is merged into the power grids of the Changbin and Changlin areas, and study a Low-Voltage Ride-Through (LVRT) curve ﬁt for the Taiwan power grid through varying fault scenarios and fault times to reduce the effect of the tripping of OWFs on the TPC system. The Power System Simulator for Engineering (PSS/E) program was used to analyze the Taipower off-peak system in 2018. The proposed LVRT curve is compared to the current LVRT curve of Taipower. The research ﬁndings show that if the offshore wind turbine (OWT) set uses the proposed LVRT curve, when a fault occurs, the wind turbines can be prevented from becoming disconnected from the power grid, and the voltage sag amplitude of the connection point during the fault and the disturbances after the fault is cleared are relatively small. In addition, according to the transient stability analysis results, the system can return to stability after fault clearance, thereby meeting the Taipower transmission system planning criteria and technical key points of renewable energy power generation system parallel connection technique.

Keywords: large-scale offshore wind farms (OWFs); Taiwan Power Company (TPC); low-voltage ride-through; power system simulator for engineering; renewable energy

1. Introduction With the rise of environmental considerations and the growth of renewable technology, various countries’ energy strategies have proposed increasing the ratio of renewable energy in succession to decrease their dependence on thermal power plants. As wind power technology has gradually developed and the generating efﬁciency thereof is currently greater than that of other renewable energy sources, it has become one of the most important parts of the green energy mixes developed by various countries in recent years. According to the annual research report of GWEC, the installed global wind power generation capacity was 539 GW up to 2017, and the newly installed wind power capacity in 2017 was as high as 52 GW [1]. The Taiwanese government has proposed a four-year (2017–2020) promotion project for wind power generation to increase Taiwan’s accumulated OWF capacity to 520 MW by 2020. Taiwan’s plan is to promote the development of large-scale OWFs through economies of scale, eventually expanding its offshore wind power capacity to 3000 MW [2]. Therefore, comprehensive standard operating procedures (SOPs) on grid connection, research on the quality of power grids, and response strategies

Energies 2019, 12, 1258; doi:10.3390/en12071258 www.mdpi.com/journal/energies Energies 2019, 12, 1258 2 of 12 for the plan are required to guarantee the successful integration of wind farms with established power grids and to reduce the impact on power grids caused by the integration of renewable energy. SOPs for integrating renewable energy with power grids have been established in European and North American countries, where wind power is flourishing. In particular, Low-Voltage Ride-Through has become a key topic for wind power research. Regarding power system dispatching, when wind power capacity reaches a certain high proportion of the total generating capacity of an entire power grid system, power generation instability can affect the operation of the entire grid system. In particular, if accidents or disturbances occur in the mains supply, causing a voltage sag at the point of common coupling (PCC) in the wind farms, then curtailment from the PCC may cause an instantaneous, massive drop of power in the system. Consequently, the power system stability can be upset, leading to a sudden imbalance between power demand and power generation in the power supply system, a massive variation in the mains supply frequency, and even the collapse of the entire system [3,4]. Therefore, several studies have proposed to mitigate the power quality issues due to high penetration of renewable energy sources in electric grid systems [5–8]. In [5], two control techniques were used to different reactive power values of loads and varying scenarios of distributed generation unit positions in order to control the angle between the current and voltage for improving the voltage profile and reducing the power losses. In [6], a useful emergency energy system was designed for Latakia city to ensure that the system would be steady and able to support most of the requirements of the city. In [7], an advanced control strategy combined with time-varying gains of two control loops was presented to enhance frequency regulation in an actual power system. In [8], a stable inertial control program was presented by using spatially dependent gains to enhance the frequency nadir and confirming stable operation of wind turbines. In addition, LVRT issues have played an increasingly important role in many countries developing renewable energy in recent years. Therefore, an appropriate LVRT curve must be implemented to guarantee that wind power generators remain operational without trips for a specified period of time, even when faults occur in the power system. According to the key point, as determined by Taipower Company, the wind power equipment are merged into a voltage system (greater than 25 kV and less than 345 kV) with LVRT capability, thus, when the power system fails, inducing a PCC voltage sag, the wind power generation equipment must be able to run continuously [9]. Only the reverse power transmission and voltage regulation factor were updated and LVRT was not discussed while the new edition of the key points was published in 2016. The formulation of the curve refers to the first LVRT grid connection requirement of the world, as presented by TenneT transmission system operator (TSO) GmbH (formerly known as E.ON Netz GmbH) in 2003 [10–15]. However, the power transmission guidelines of Germany proposed a newer version of LVRT according to the features of wind turbines as well as a power grid, and put it in the grid connection specifications. Therefore, Taiwan’s power experts and scholars have suggested revising and enhancing the LVRT curve and the fault ride-through clauses of wind turbines in the existing renewable energy grid connection technology code of Taipower, in order to require large-scale renewable energy power plants to enhance equipment performance to improve the steady running ability. In terms of the present studies considering LVRT after the wind turbines are added to the system, since TenneT proposed the first LVRT curve, some countries with developed wind power have presented LVRTs meeting their power grid characteristics in succession. Reference [14] describes the LVRT requirements of Canada, the United Kingdom, and Denmark. Reference [15] is the LVRT formulated by the state grid of the United Kingdom, which is applied to power systems with a voltage class higher than 200 kV; regardless of whether the fault is balanced or unbalanced, as long as the fault duration does not exceed 0.14 seconds, even if the voltage drops to 0 % of voltage before the fault occurrence, not all wind turbines can be disconnected from the system. The wind turbines can be disconnected from the power system when the fault duration exceeds 0.14 seconds, and the fault severity is under the curve. References [16,17] indicate that the wind turbine working voltage classes in Denmark are divided into below 100 kV and above 100 kV. When the system has a serious Energies 2019, , x FOR PEER REVIEW 3 of 14 two-phase short-circuit fault continuing 0.1 second, and a short-circuit fault continuing 0.1 second within 0.3–0.5 seconds, meaning more than two two-phase short-circuit faults within two minutes, the wind turbines need not be linked to the power system. The Canadian LVRT, developed by the power companies of Alberta and Quebec, is described in [18]. The LVRT proposed by Quebec is also referred to as the Hydro–Quebec curve, and it is Energies 2019, 12, 1258 3 of 12 applicable for three-phase balance or imbalance faults at PCC regardless of the locations of the fault points in the system. In [19], the DIgSILENT software was used to create a grid system coupled withfault, permanent including a three-phasemagnet generator short-circuit (PMG) fault continuingwind turbine 0.1 second, systems a two-phase and doubly-fed short-circuit induction fault generatorscontinuing 0.1(DFIGs) second, to and simulate a short-circuit the transient fault continuing responses 0.1 and second low-voltage within 0.3–0.5 capability seconds, of meaning these two typesmore thanof generators two two-phase when short-circuit encountering faults faults within in two the minutes, system. the In wind [20], turbines a dynamic need notsimulation be linked was conductedto the power on system. the OWFs of various collector system topologies during system faults, and the fault enduranceThe Canadian of the wind LVRT, farms developed and individual by the power wind companies turbines of on Alberta the LVRT and Quebec, capability is described curve was compared.in [18]. The In LVRT[21], approaches proposed by to Quebec improve is alsoLVRT referred capability, to as such the Hydro–Quebec as increases in curve, rotor andimpedance, it is choppers,applicable and for three-phasearc suppression balance circ oruits, imbalance were employed faults at PCC to regardlessexplore their of the effects locations on wind of the turbines. fault Thepoints LVRT in the characteristics system. In [ 19of], wind the DIgSILENT turbines were software examined was used in to[22], create which a grid focused system on coupled the direct currentwith permanent capacitor magnet voltage generator of the (PMG)transducer wind in turbine a DFIG; systems the andtime doubly-fed from the inductionfault occurrence generators to the (DFIGs) to simulate the transient responses and low-voltage capability of these two types of generators point when the system voltage exceeded its limiting value was identified to define the LVRT when encountering faults in the system. In [20], a dynamic simulation was conducted on the OWFs of applicable for the DFIG. To sum up, there are no studies regarding the formulation and revision of various collector system topologies during system faults, and the fault endurance of the wind farms Taiwan’s power grid LVRT curve. Therefore, this paper aims at the present Taipower system, and and individual wind turbines on the LVRT capability curve was compared. In [21], approaches to studies a LVRT curve fit for wind turbines according to different fault scenarios and fault times improve LVRT capability, such as increases in rotor impedance, choppers, and arc suppression circuits, after a 520 MW offshore wind generating set is merged into Taipower. As Taiwan is an isolated were employed to explore their effects on wind turbines. The LVRT characteristics of wind turbines islandwere examinedpower system, in [22], which focusedis special, on theand direct considering current capacitorthe present voltage green of theenergy transducer development in a planning,DFIG; the wind time from power the objectives, fault occurrence and toenhancement the point when of the systemgrid conne voltagection exceeded level, the its limitingsingle unit installedvalue was capacity identified and to definethe overall the LVRT wind applicable farm capacity for the DFIG.are increased, To sum up, the there main are failure no studies type is complex,regarding and the formulationpower grid andaccident revision cases of Taiwan’s have exc powereeded grid the LVRTexisting curve. LVRT, Therefore, the operation this paper and aims safety ofat the the power present grid Taipower system system, with wind and studies power a are LVRT influenced, curve fit forthus, wind it is turbines necessary according to discuss to different and revise thefault present scenarios LVRT and curve. fault times after a 520 MW offshore wind generating set is merged into Taipower. As Taiwan is an isolated island power system, which is special, and considering the present green 2.energy Introduction development to Simulation planning, windof Taipower power objectives, and Grid and Connection enhancement Systems of the grid connection level, the single unit installed capacity and the overall wind farm capacity are increased, the main failure typeThe is complex, OWFs and poweronshore grid substations accident cases in the have coastal exceeded area the of existing Changbin LVRT, are the discussed operation and and the actualsafety ofoff-peak the power system grid systemof Taiwan’s with wind power power grid are in influenced, 2018 is analyzed. thus, it is necessaryBased on tothe discuss 36 potential and offshorerevise the sites present and LVRTthe existing curve. sea area opened in the offshore wind power generation planning site application operation key points, the total development potential is estimated at about 23 GW [23], 252. sites Introduction are concentrated to Simulation in Yun-Chang of Taipower Rise and area, Grid as Connection shown in SystemsFigure 1, the offshore blocks in the area areThe relatively OWFs and concentrated, onshore substations and the in scale the coastalof each area block of Changbinexceeds 200 are MW. discussed and the actual off-peak system of Taiwan’s power grid in 2018 is analyzed. Based on the 36 potential offshore sites 2.1.and Introduction the existing to sea Changlin area opened Areas inand the Power offshore Grid windin Changbin power generation planning site application operationThe Lutung, key points, Luhsi, the totalHsienhsi, development Changbin, potential and is Tacheng estimated substations at about 23 GWin [the23], 25Changlin sites are and Changbinconcentrated areas in Yun-Changare chosen Riseas the area, OWFs’ as shown landing in Figure connection1, the offshore points, blocks as shown in the in area Figure are relatively 1. concentrated, and the scale of each block exceeds 200 MW.

Yun-Chang Rise Area Hsiensi Changbin

Lutung Luhsi Changbin Area Offshore Wind Farms 161/23 kV Distribution Changlin substation 345/161 kV Extra high voltage substation Tacheng Changlin Area

FigureFigure 1. 1. GraphGraph showing showing the location ofof substationssubstations along along the th Changline Changlin and and Changbin Changbin areas. areas. Energies 2019, 12, 1258 4 of 12

2.1. Introduction to Changlin Areas and Power Grid in Changbin The Lutung, Luhsi, Hsienhsi, Changbin, and Tacheng substations in the Changlin and Changbin Energies 2019, , x FOR PEER REVIEW 4 of 14 areas are chosen as the OWFs’ landing connection points, as shown in Figure1. In theIn case the of case wind of turbines wind turbines merged merged into various into substations,various substations, transient transient stability, stability, voltage ﬂuctuation,voltage and three-phasefluctuation, short-circuitandEnergies three-phase 2019, , x FOR fault PEER short-circuit REVIEW are carried fault out. are Finally, carried aout. LVRT Finally, curve a suitableLVRT4 of 14 curve for suitable merging for wind turbinesmerging into the wind Taiwan turbinesIn the power case into of system windthe turbinesTaiwan is formulatedmerged power into va rioussy basedstem substations, is on formulated thetransient simulation stability, based voltage results. on the simulation Theresults. wind turbinesfluctuation, adopted and three-phase in this short-circuit study fault were are carried GEWT out. 4.0Finally, MW a LVRT full curve scale suitable converter for fed induction merging wind turbines into the Taiwan power system is formulated based on the simulation generators,The which wind are turbinesresults. manufactured adopted in by this the study General were ElectricGEWT 4.0 Company MW full (Bostonscale converter City, USA) fed induction and feature generators, which Theare wind manufactured turbines adopted in bythis studythe wereGene GEWTral 4.0 Electric MW full scale Company converter fed (Boston induction City, USA) and a maximal output ofgenerators, 4 MW which each are (Figuremanufactured2). by Thesethe General turbines, Electric Company which (Boston are City, variable-speed USA) and models and feature a maximal output of 4 MW each (Figure 2). These turbines, which are variable-speed models feature a wide rangefeature of rotor a maximal speeds, output of were4 MW each connected (Figure 2). These to turbines, transformers which are variable-speed through models converters before being and feature a wideand feature range a wide of rotorrange of speeds, rotor speeds, were were conneconnectedcted to transformers to transformers through converters through before converters before coupled with the powerbeing coupled grids; with these the power converters grids; these converters can be can used be used for for reactive reactive power power regulation regulation [24]. being coupled [24].with the power grids; these converters can be used for reactive power regulation [24]. AC/DC DC/AC Gear Box AC/DC DC/AC Grid Gear Box Transformer Direct Drive Grid Synchronous Generator

Figure 2. DiagramFigure 2. Diagram of a full of a full scale scale conver converter-fedter-fed inductioninduction generator. generator. Transformer 2.2. Inside Grid Connection2.2. Inside Grid Configuration Connection Configuration of OWFs of OWFs in in Chang-Yuen Chang-Yuen Rise Area Rise Direct Area Drive Blocks of large-scale wind farms were defined for the insideSynchronous framework of Generatorthe simulated Blocks of large-scaleoffshore area. wind Each farms of the five were blocks definedfeatured 26 wind for turbines the inside with a wind framework power capacityof of 104 the simulated offshore MW, givingFigure a total 2. Diagram capacity of of520 a MW. full Each scale gene converrator windter-fed turbine inductio had its voltagen generator. raised from area. Each of the five0.69 blocks kV to 33 featuredkV through an 26 internal wind boost turbines transformer.with Each block a wind was coupled power with a capacity main bus of 104 MW, giving and had its voltage increased from 33 kV to 161 kV through an offshore substation. The power was a total2.2. capacity Inside Grid of 520 Connectionthen MW. transmitted Each Configuration to the generator land substations of windOWFs through turbine in161-kV Chang-Yuen submarine had cables. its Rise voltage Area raised from 0.69 kV to 33 kV through an internal boost transformer. Each block wasOffshore Onshore coupled with a main bus and had its voltage Changpin Blocks of large-scale wind farmsarea were defined for the inside framework of the simulated increased from 33 kV to 161 kV through an52MW offshore substation. The power was then transmitted to the Changbin 161 kV XLPE offshore area. Each of the five blocks featuredwindfarms 26 windSubmarine cable turbines with a wind power capacity of 104 52MW land substations through 161-kV submarine cables. Changbin MW, giving a total capacity of 520 MW. Each generatorH(2301) wind turbine had its voltage raised from

As shown in Figure3, four of the offshore52MW blocks were connected to the Changbin H (2301), 0.69 kV to 33 kV through an internal boostHsienhsi transformer.161 kV XLPE Each block was coupled with a main bus windfarms Submarine cable 52MW Hsienhsiand Hhad (5801), its voltage Lutung increased H (5801), from and33 kV Luhsi to 161 H kV (5811) throughHsienhsi substations, an offshore and substation. the remaining The power block was was H(5801) connectedthen transmitted to the Tacheng to the H land (5841) substations substation. through 161-kV submarine cables. 52MW Lutung 161 kV XLPE windfarms Submarine cable 52MW

Lutung H(5809) Offshore Onshore

52MW Changpin Luhsi 161 kV XLPE windfarms Submarine cable area 52MW Luhsi 52MW H(5811) ChanglinChangbin 161 kV XLPE area windfarms Submarine cable 52MW 52MWTacheng 161 kV XLPE windfarms Submarine cable 52MW Changbin Tacheng H(2301) H(5841) Offshore wind Offshore farms capacity substations 52MW Hsienhsi 161 kV XLPE Figure 3. Graph of connectingwindfarms the offshore wind farm blocSubmarineks into cable the land distribution substations. 52MW

Hsienhsi H(5801)

52MW Lutung 161 kV XLPE windfarms Submarine cable 52MW

Lutung H(5809)

52MW Luhsi 161 kV XLPE windfarms Submarine cable 52MW Luhsi H(5811)

Changlin area 52MW Tacheng 161 kV XLPE windfarms Submarine cable 52MW

Tacheng H(5841) Offshore wind Offshore farms capacity substations

FigureFigure 3. Graph 3. Graph of connecting of connecting the the offshore offshore wind wind farm farm blocksblocks into the the land land distribution distribution substations. substations. Energies 2019, 12, 1258 5 of 12

3. LVRT Curve Program of Meeting OWFs into Taipower System In this study, the PSS/E power analysis program (Siemens, Berlin, Germany) was employed for our LVRT simulation, including power flow analysis, transient stability analysis, fault analysis, etc. In addition, the actual system parameters in Taiwan and the off-peak system in the coastal area of Changbin in 2018 were utilized. In order to obtain practical analysis results, a series of conditions need to be considered. The system is free of new (expanded) transmission lines, N-1 and N-2 planning criteria are used, the systems under different fault scenarios and fault times are compared, the system power flow is simulated, and the minimum voltage, transient stability, and three-phase short-circuit fault of wind turbines during the fault are analyzed. Finally, a wind turbine LVRT standard is formulated according to the simulation results to avoid the trip of OWTs influencing the Taipower system. According to the requirements of technical key points, when the voltage at the OWT connection point is 15% of the rated voltage, the OWTs shall be capable of keeping the grid connected for at least 0.5 seconds, and the wind power generation equipment shall be grid connected for at least 3 seconds when the voltage is recovered to 90% of the rated voltage [9]. According to the simulation results in Table1, when the onshore bus fails, the terminal voltage of OWTs is close to 0 p.u., thus, in terms of the proposed LVRT curve, the OWTs shall be able to connect the grid within a time range.

Table 1. The OWTs Minimum Voltage during a fault.

Fault Location Tripping of Transmission Line Minimum Voltage of the OWTs Changbin Changbin H (2301)-Tsaokang #A (7710) Changbin Windfarms/0.032 p.u. Hsienhsi Hsienhsi H (5801)-Changbin H (2301) Hsienhsi Windfarms/0.032 p.u. Lutung Lutung H (5809)-Luhsi H (5811) Lutung Windfarms/0.032 p.u Luhsi Luhsi H (5811)-Lutung H (5809) Luhsi Windfarms/0.032 p.u Tacheng Tacheng H (2321)-Changlin H (2321) Tacheng Windfarms/0.031 p.u.

The longer the fault clearing time, the longer the stable voltage recovery time. The voltage may even break down if the fault clearing time is too long, and cannot recover to a steady state. Therefore, it is necessary to consider whether the voltage at the OWT connection point can recover its stability during a fault when formulating the LVRT curve. The simulated fault time is increased to 0.625 seconds in this paper, the voltage variation at the wind turbine end is observed, and the current LVRT, as speciﬁed by Taipower, is followed after 0.625 seconds. The A→B→F→I in Figure4 is the Taipower speciﬁed wind turbine LVRT; A→C→D→E→G→H→I is the proposed LVRT in this paper. The simulation process is described as follows:

A→C: When a severe three-phase short circuit happens in the system, the voltage falls to 0 p.u. C→D: When a fault occurs at one of the spots in the system, during the fault clearance time, the voltage of the wind turbines at PCC recovers to 90% of the rated voltage in 3 seconds; no turbines can be decoupled from the grids during the 3 seconds of voltage recovery regardless of the magnitudes of their voltage sags. E→G: The minimum voltages of the turbines when disturbances occurred at various fault durations were simulated. G→H→I: The TPC LVRT curve was retained.

The voltage will drop to 0 (A→C) when the three-phase short-circuit fault occurs in the power system, and the G→H→I maintains the original LVRT curve established by Taipower Company. Therefore, the subsequent simulation analysis lays stress on how to make the value of point D and the values of point E to point G by simulation analysis (Figure4). Energies 2019, , x FOR PEER REVIEW 6 of 14

EnergiesTherefore,2019, the12, 1258 subsequent simulation analysis lays stress on how to make the value of point D6 ofand 12 the values of point E to point G by simulation analysis (Figure 4). Voltage(Per Unit)

A I 0.90

H E F B TPC LVRT Curve (Taipower-specified) 0.15 G C Proposed LVRT Curve 0 0.5 3 Time (Seconds) D

Figure 4. The proposed LVRT curve versus TPC LVRT curve. Figure 4. The proposed LVRT curve versus TPC LVRT curve. 3.1. Establishment of the Value of Point D—Fult Cearing Tme of Ofshore Wnd Frms at Dfferent Fult Pints 3.1. EstablishmentThis subsection of the simulates Value of aPoint three-phase D—Fult short-circuitCearing Tme of fault Ofshore in the Wnd 161 Frms kV and at Dfferent 345 kV Fult buses Pints of an adjacentThis offshore subsection wind simulates farm with a three-phase different line short-circ trip accidents,uit fault in orderin the to161 determine kV and 345 whether kV buses the OWTs of an willadjacent recover offshore to 90% wind within farm 3 seconds with different after the line fault. tr Theip accidents, trip line follows in order the to TPC determine transmission whether system the planningOWTs will criteria, recover the extra-highto 90% within voltage 3 cableseconds uses afte N-2r criteriathe fault. in theThe 345 trip kV transmissionline follows line,the thus,TPC thetransmission non-fault system end is clearedplanning by crit foureria, cycles the whenextra-high a three-phase voltage cable short-circuit uses N-2 fault criteria occurs in the in the345 line, kV andtransmission the critical line, clearing thus, the time non-fault of fault end is cleare higherd thanby four 4.5 cycles cycles. when A non-extra a three-phase high voltage short-circuit cable usesfault N-1occurs criteria in the at theline, extra-high and the voltagecritical substationclearing time (E/S). of Infault terms end of is the higher 161 kV than transmission 4.5 cycles. line, A ifnon-extra the renewable high voltage energy cable grid-connected uses N-1 criteria installed at th capacitye extra-high exceeds voltage 420MW, substation the N-1 (E/S). criteria In areterms used of (athe three-phase 161 kV transmission short-circuit line, fault), if the the renewable non-fault end energy is cleared grid-connected by seven cycles,installed and capacity the fault exceeds critical clearing420MW, timethe N-1 is higher criteria than are 12 used cycles (a three-phase [25]. short-circuit fault), the non-fault end is cleared by sevenTable cycles,2 lists and the the 19 fault fault critical scenarios, clearing 10 time of which is higher occurred than 12 in thecycles 161-kV [25]. circuit and nine in the 345-kVTable circuit. 2 lists the 19 fault scenarios, 10 of which occurred in the 161-kV circuit and nine in the 345-kV circuit. Table 2. Codes of the fault scenarios.

Fault Scenario Fault LocationTable 2. Codes of the fault scenarios. Tripping of Transmission Line Fault 1 Luhsi H (5811) Luhsi H(5811)-Lutung H (5809)1st line Fault ScenarioFault 2 FaultChangbin Location H (2301) Changbin Tripping H(2301)-Tsaokang of Transmission #A (7710) Line 1st line FaultFault 1 3 Luhsi Hsienhsi H (5811) H (5801) Hsienhsi Luhsi H(5801)-ChangbinH(5811)-Lutung H H (5809)1st (2301) 1st line FaultFault 2 4 Changbin Lutung H H (2301) (5809) Changbin Lutung H(2301)-Tsaokang H(5809)-Luhsi (5811) #A H (7710) 1st line 1st line Fault 5 Hanpao H (5817) Hanpao H(5817)-Tsaokang #B (7711) 2nd line 161 kV Fault 3 Hsienhsi H (5801) Hsienhsi H(5801)-Changbin H (2301) 1st line Fault 6 Chuanhsiang H (2351) Chuanhsiang H(2351)-Changhsin #A (7750) Rth line FaultFault 4 7 Lutung Fuhsiang H H(5809) (5855) Fuhsiang Lutung H(5855)-Changhua H(5809)-Luhsi (5811) H (3551) H 1st 1st line FaultFault 5 8 Hanpao Tsaokang H H(5819)(5817) Tsaokang Hanpao H(5817)-Tsaokang H(5819)-Tsaokang #B #B (7711) (7711) 2nd 2nd line line 161 kV FaultFault 6 9 Chuanhsiang Tacheng H H (5841) (2351) Chuanhsian Tachengg H H(2351)-Changhsin (5841)-Changlin H (2321) #A (7750) 1st line Rth line FaultFault 7 10 Fuhsiang Changhsin H H(5855) (5851) Changhsin Fuhsiang H H(5855)-Changhua (5851)-Changhsin #A H (7750) (3551) Rth 1st line line FaultFault 8 11 Tsaokang Houli EH(5819) (2130) Tsaokang Houli E (2130)-Omei H(5819)-Tsaokang E (2000) Wth#B (7711) & Rth 2nd line line Fault 12 Chunghuopei E (530) Chunghuopei E (530)-Chungkang E (2150) Nth & Sth line Fault 9 Tacheng H (5841) Tacheng H (5841)-Changlin H (2321) 1st line Fault 13 Changbin E (2300) Changbin E (2300)-Chuanhsiang E (2350) 1st line FaultFault 10 14 Changhsin Changlin EH (2320)(5851) Changhsin Changlin H E (5851)-Changhsin (2320)-Nantou E (2400) #A (7750) Nth line Rth line 345 kV FaultFault 11 15 Chuanhsiang Houli E (2130) E (2350) Chuanhsiang Houli E (2130)-Omei E (2350)-Chunghuonan E (2000) Wth E (540) & Rth 1st line FaultFault 12 16 Chunghuopei Chunghuonan E E(530) (540) Chunghuopei Chunghuonan E (530)-Chungkang E (540)-Changbin E E (2150) (2300) 1stNth line & Sth line FaultFault 13 17 Changbin Chungkang E (2300) E (2150) Changbin Chungkang E E(2300)-Chuanhsiang (2150)-Cchungko E (2170) E (2350) Nth line1st line Fault 18 Nantou E (2400) Nantou E (2400)-Chuanhsiang E (2350) 1st line 345 kV Fault 14 Changlin E (2320) Changlin E (2320)-Nantou E (2400) Nth line Fault 19 Chiamin E (2500) Chiamin E (2500)-Chungliaopei E (2480) 1st & 2nd line Fault 15 Chuanhsiang E (2350) Chuanhsiang E (2350)-Chunghuonan E (540) 1st line Fault 16 Chunghuonan E (540) Chunghuonan E (540)-Changbin E (2300) 1st line Tables3Fault and 174 show Chungkang when the bus E (2150) in the Taipower Chungkang grid fails, E (2150)-Cchungko and whether the E OWT(2170) Nth connection line points will recover to 90% of rated voltage in three seconds after the fault occurs [26]. According to the Energies 2019, 12, 1258 7 of 12 results, when an accident occurs in an adjacent offshore wind farm, the OWT connection points will recover to 90% of rated voltage in 3 seconds at accident time points 0.1 second, 0.14 second, and 0.15 second. Therefore, no matter how much the voltage dips at the OWT connection point, the OWTs shall be able to remain grid-connected for at least 0.15 seconds when an accident occurs, this is, point D in Figure4 is 0.15.

Table 3. The fault clearance time (0.1, 0.14, and 0.15 seconds) inﬂuences on wind farms (Lutung, Luhsi, Hsienhsi, Changbin, and Tacheng) [26].

Fault Clearing Time Fault Clearing Time Fault Scenario Fault Scenario (seconds) (seconds) (161 kV) (345kV) 0.1 0.14 0.15 0.1 0.14 0.15 Fault 1 O O Fault 11 O O O Fault 2 O O Fault 12 O O O Fault 3 O O Fault 13 O O O Fault 4 O O Fault 14 O O O Fault 5 O O Fault 15 O O O Fault 6 O O Fault 16 O O O Fault 7 O O Fault 17 O O O Fault 8 O O Fault 18 O O O Fault 9 O O Fault 19 O O O Fault 10 O O Notes: O: The wind turbines voltage at the PCC recovered to 90% of the rated voltage in 3 seconds after the fault happened; X: The wind turbines voltage at the PCC unrecovered to 90% of the rated voltage in 3 seconds after the fault happened.

Table 4. The fault clearance time (0.2, 0.5, and 0.625 seconds) effects on wind farms (Lutung, Luhsi, Hsienhsi, Changbin, and Tacheng) [26].

Fault Clearing Time (seconds) Fault Changbin, Hsienhsi, Lutung and Tacheng Windfarm Scenario Luhsi Windfarms 0.2 0.5 0.625 0.2 0.5 0.625 Fault 1 O O O O O O Fault 2 O O O O O O Fault 3 O O O O O O Fault 4 O O O O O O Fault 5 O O O O O O Fault 6 O O O O O O Fault 7 O O O O O O Fault 8 O O O O O O Fault 9 O O O O O O Fault 10 O O O O O O Fault 11 O X X O X X Fault 12 X X X X X X Fault 13 O X X O X X Fault 14 O O O O O O Fault 15 X X X X X X Fault 16 X X X X X X Fault 17 O X X O X X Fault 18 O X X O X X Fault 19 O O X O X X Notes: O: The wind turbines voltage at the PCC recovered to 90% of the rated voltage in 3 seconds after the fault happened; X: The wind turbines voltage at the PCC unrecovered to 90% of the rated voltage in 3 seconds after the fault happened. Energies 2019, , x FOR PEER REVIEW 8 of 13

Notes: O: The wind turbines voltage at the PCC recovered to 90% of the rated voltage in 3 seconds after the fault happened; X: The wind turbines voltage at the PCC unrecovered to 90% of the rated Energiesvoltage2019, 12 in, 1258 3 seconds after the fault happened. 8 of 12

3.2. Establishment of the Values of Point E to Point G—Aalysis of the Vltage of Ofshore Wnd Farms under 3.2.Dfferent Establishment Fult Cearing of the Tmes Values of Point E to Point G—Aalysis of the Vltage of Ofshore Wnd Farms under Dfferent Fult Cearing Tmes Subsection 3.1 has defined point D as 0.15 seconds, thus, the fault time simulation shall be greaterSection than 3.1 or has equal deﬁned to point 0.15 Dseconds. as 0.15 seconds, According thus, to the [2 fault5], when time simulation the fault is shall the be most greater serious than orthree equal-phase to 0.15 short seconds.-circuit According fault, the tominimum [25], when voltage the fault at the is the OWT most connection serious three-phase point is obtained, short-circuit i.e., fault,the valley the minimum of voltage voltage at the at OWT the OWT connection connection point point at faultis obtained, times of i.e., 0.15 the second,valley of 0.2 voltage second, at the 0.5 OWTsecond, connection and 0.625 point second at fault occur times when of 0.15the second, three-phase 0.2 second, short-circuit 0.5 second, fault andhappens 0.625 in second an adjacent occur whenoffshore the wind three-phase farm bus. short-circuit Figures 5 illustrates fault happens the wind in an turbines adjacent minimum offshore wind PCC farmvoltages bus. in Figure the TPC5 illustratessystem for the varying wind turbinesfault durations minimum and PCC scenarios voltages [26] in. Table the TPC 5 shows system the for minimum varying faultvoltage durations of each andwin scenariosd farm at [ 26 PCC.]. Table The5 shows minimum the minimum voltage of voltage the OWTs of each at windPCC was farm 0.091 at PCC. p.u. The when minimum a fault voltagehappened of theon OWTsthe shore at PCCfor offshore was 0.091 wind p.u. farms. when The a fault extent happened of each voltage on the shoresag was for especially offshore wind large farms.when Thethe fault extent happened of each voltage due to sag the was OWFs especially were near large to when the onshore the fault PCC happened voltag duees. Therefore, to the OWFs the wereminimum near to voltage the onshore must PCCbe 0.09 voltages. p.u. at Therefore, E, and the the length minimum of time voltage must mustbe 0.625 be 0.09 seconds p.u. atfrom E, and E to the G lengthto prevent of time the must offshore be 0.625 turbines seconds disconnection from E to G with to prevent the power the offshore grid without turbines impacting disconnection the stability with theof powerthe power grid withoutsystem. impactingAs shown the in stability Figure 4 of, the the pointspower system. A→C→ AsD→ shownE→G→ inH Figure→I were4, the linked points to → → → → → → AconstituteC D theE proposedG H ILVRT were linkedcurve in to this constitute study (Figure the proposed 6). LVRT curve in this study (Figure6).

(a) (b)

(c)

FigureFigure 5. 5.Minimum Minimum voltage voltage of of OWTs: OWTs (a:) ( 0.2a) 0.2 seconds seconds at the at faultthe fault time time point; point (b) 0.5; (b seconds) 0.5 seconds at the at fault the timefault point; time point; (c) 0.625 (c) seconds0.625 seconds at the faultat the time fault point time [point26]. [26].

TableTable 5. 5.Minimum Minimum voltagevoltage valuevalue ofof offshoreoffshore wind wind farm farm connection connection point point under under different different fault fault times. times. Offshore Wind Farms Fault Location Offshore Wind Farm Voltage (p.u.) Offshore Wind Farms Fault Location Offshore Wind Farm Voltage (p.u.) Changbin Changbin H 0.091 ChangbinHsienhsi Changbin Hsienhsi H H 0.0910.091 Lutung Lutung H 0.091 Luhsi Luhsi H 0.091 Tacheng Tacheng H 0.091 Energies 2019, , x FOR PEER REVIEW 9 of 13

Hsienhsi Hsienhsi H 0.091

Lutung Lutung H 0.091

Luhsi Luhsi H 0.091

Tacheng Tacheng H 0.091

Figure 6 depicts the proposed LVRT curve, which is detailed as follows:  The turbine should proceed to operate for a minimum of 0.15 seconds when the PCC voltage of the wind turbines sags to 0 p.u.  The turbine should proceed to operate for a minimum of 0.475 seconds when the PCC voltage of the wind turbines sags to 0.09 p.u.  When the PCC voltage of the wind turbine exceeds the proposed LVRT curve (shown as a red Energies 2019, 12, 1258 9 of 12 dotted line) during the voltage sag, the turbine remains in operation.

FigureFigure 6.6. TheThe proposedproposed LVRTLVRT curve curve (with (with values values of of each each point) point) versus versus TPC TPC LVRT LVRT curve. curve. Figure6 depicts the proposed LVRT curve, which is detailed as follows: 4. Fault Analysis of the OWTs under Different LVRT Curves TheThis turbine section should compares proceed the effects to operate of the for proposed a minimum LVRT of 0.15curve seconds and the when TPC theLVRT PCC curve voltage on the of grid the performance, wind turbines namely sags to the 0 p.u. transient stability analysis, when three-phase short circuit faults occurredThe turbine in the 161 should kV proceedand 345 tokV operate buses; forthis a minimumsection then of 0.475reveals seconds the advantage when the of PCC the voltage proposed of LVRTthe curve wind over turbines the TPC sags LVRT to 0.09 curve p.u. . This section analyzes the effects of the two aforementioned specificationsWhen the of PCC LVRT voltage on the of theTaipower wind turbine system exceeds at different the proposed fault points LVRT and curve fault (shown clearance as atimes red whendotted three line)-phase during short thecircuit voltage faults sag, occurred the turbine with remains subsequent in operation. circuit trips. This system involved the OWTs with a total power generation capacity of 520 MW coupled with the Lutung, Luhsi, 4. Fault Analysis of the OWTs under Different LVRT Curves Hsienhsi, Changbin, and Tacheng distribution substations. The Changbin H (2301) substation was designatedThis section as the compares fault point, the effects and the of theRth proposed loop transmission LVRT curve line and be thetween TPC Changbin LVRT curve Hon (2301) the grid and performance,Lutung H (5809) namely was the determined transient stabilityas the tripping analysis, circuit when in three-phase investigating short the circuit real power faults occurredand PCC involtage the 161 changes kV and 345of the kV turbines buses; this under section the then two reveals specifications the advantage of LVRT of duringthe proposed and after LVRT the curve fault overoccurrence. the TPC The LVRT time curve. points This were section defi analyzesned as follows: the effects at 1 ofseconds the two of aforementioned the fault occurrence; specifications the circuit of LVRTtripped on the at the Taipower seventh system cycle at (approx. different 1.1166 faultpoints seconds); and fault the fault clearance clearance times at when the three-phase twelfth cycle short (1.2 circuitseconds). faults occurred with subsequent circuit trips. This system involved the OWTs with a total power generation capacity of 520 MW coupled with the Lutung, Luhsi, Hsienhsi, Changbin, and Tacheng distribution substations. The Changbin H (2301) substation was designated as the fault point, and the Rth loop transmission line between Changbin H (2301) and Lutung H (5809) was determined as the tripping circuit in investigating the real power and PCC voltage changes of the turbines under the two specifications of LVRT during and after the fault occurrence. The time points were defined as follows: at 1 seconds of the fault occurrence; the circuit tripped at the seventh cycle (approx. 1.1166 seconds); the fault clearance at the twelfth cycle (1.2 seconds). According to Figure7, when the fault occurred in the Changbin H substation, if the TPC LVRT

curve was implemented in the OWTs, then all the offshore turbines in the Changbin block tripped at 0.08 seconds, as shown in Figure7a. This was because the turbines were close in distance to the fault point, leading their PCC voltage to fall below 0.15 p.u. when the fault occurred. Conversely, when the proposed LVRT curve was adopted, none of the turbines tripped, as shown in Figure7b. According to Figure8, when faults occurred, the voltage dips in the Lutung H (Figure8c) and Luhsi H (Figure8d) substations were more severe if the TPC LVRT curve was adopted than if the proposed LVRT curve was adopted because of the circuit trips in the OWTs; moreover, the times for voltage recovery to 0.9 p.u. after fault clearance were longer if the TPC LVRT curve was adopted, and the disturbance to the system was higher. Energies 2019, , x FOR PEER REVIEW 11 of 14

Wind Farms Restored Stable

Wind Farms Tripped

Changbin Windfarm Changbin Windfarm Hsienhsi Windfarm Hsienhsi Windfarm Lutung Windfarm Lutung Windfarm Energies 2019, 12, 1258 Luhsi Windfarm Luhsi Windfarm 10 of 12 Tacheng Windfarm Tacheng Windfarm 9 Energies 201 , , x FOR PEER REVIEW(a) (b) 11 of 14

Figure 7. When Changbin H occurs fault: (a) Real power of the turbine under the TPC LVRT curve during fault occurrence at the Changbin H substation; (b) Real power of the turbine under the proposed LVRT curve during fault occurrence at the Changbin H substation.Wind Farms Restored Stable

According to Figure 7, when the fault occurred in the Changbin H substation, if the TPC LVRT Wind Farms Tripped curve was implemented in the OWTs, then all the offshore turbines in the Changbin block tripped at 0.08 seconds, as shown in Figure 7a. This was because the turbines were close in distance to the fault point, leading their PCC voltage to fall below 0.15 p.u. when the fault occurred. Conversely, when Changbin Windfarm Changbin Windfarm Hsienhsi Windfarm Hsienhsi Windfarm the proposed LVRTLutung Windfarm curve was adopted, none of the turbines Lutungtri Windfarmpped, as shown in Figure 7b. Luhsi Windfarm Luhsi Windfarm According to FigureTacheng Windfarm 8, when faults occurred, the vo ltage dips in theTacheng Lutung Windfarm H (Figure 8c) and Luhsi (a) (b)

H (Figure 8d) substations were more severe if the TPC LVRT curve was adopted than if the proposed LVRT curve was adopted because of the circuit trips in the OWTs; moreover, the times for FigureFigure 7. When 7. When Changbin Changbin H occursH occurs fault: fault: (a(a)) RealReal power of of the the turbine turbine under under the theTPC TPC LVRT LVRT curve curve voltage recovery to 0.9 p.u. after fault clearance were longer if the TPC LVRT curve was adopted, duringduring fault occurrencefault occurrence at the at Changbin the Changbin H substation; H substation; (b) Real (b) powerReal power of the of turbine the turbine under under the proposed the and the disturbance to the system was higher. LVRTproposed curve during LVRT faultcurve occurrence during fault at occu therrence Changbin at the HChangbin substation. H substation.

According to Figure 7, when the fault occurred in the Changbin H substation, if the TPC LVRT curveProposed was LVRT implemented Curve in the OWTs, then all the offshoreProposed turbines LVRT Curve in the Changbin block tripped at 0.08 seconds,TPC as LVRT shown Curve in Figure 7a. This was because the turbinesTPC were LVRT Curve close in distance to the fault point, leading their PCC voltage to fall below 0.15 p.u. when the fault occurred. Conversely, when the proposed LVRT curve was adopted, none of the turbines tripped, as shown in Figure 7b. According to Figure 8, when faults occurred, the voltage dips in the Lutung H (Figure 8c) and Luhsi H (Figure 8d) substations were more severe if the TPC LVRT curve was adopted than if the proposed LVRT curve was adopted because of the circuit trips in the OWTs; moreover, the times for voltage recovery to 0.9 p.u. after fault clearance were longer if the TPC LVRT curve was adopted, [Changbin [Hsienhsi and the disturbance[Changbin to the system was higher. [Hsienhsi (a) (b)

Proposed LVRT Curve Proposed LVRT Curve Proposed LVRT Curve Proposed LVRT Curve

TPC LVRT Curve TPC LVRT Curve TPC LVRT Curve TPC LVRT Curve

[Changbin [Hsienhsi [Changbin [Hsienhsi [Lutung [Luhsi [Lutung [Luhsi ( a) ( b) (c) Energies 2019, , x FOR PEER REVIEW(d) 12 of 14 Proposed LVRT Curve Proposed LVRT Curve

TPC LVRT Curve TPC LVRT Curve Proposed LVRT Curve

TPC LVRT Curve

[Lutung [Luhsi [Lutung [Luhsi

[Tacheng (c) [Tacheng (d)

(e)

Figure 8. When Changbin H occurs fault: PCC voltage changes in the turbine under the TPC curve Figure 8. When Changbin H occurs fault: PCC voltage changes in the turbine under the TPC curve and the proposed LVRT curve:and (a) Changbinthe proposed H Voltage;LVRT curve: (b) Hsienhsi (a) Changbin H Voltage; H Voltage; (c) Lutung (b) Hsienhsi H Voltage; H Voltage; (c) Lutung H (d) Luhsi H Voltage; (e) TachengVoltage; H Voltage. (d) Luhsi H Voltage; (e) Tacheng H Voltage.

5. Conclusions This paper merges the target of 520 MW offshore wind power, as planned by Taiwan for 2020, into the Taipower Changbin district (Lutung, Luhsi, Hsienhsi, and Changbin distribution substations) and Changbin district (Tacheng distribution substation), based on the content specified of the TPC transmission system planning criteria and technical key points, as combined with the actual Taipower system, and varying fault scenarios and fault times are considered in the analysis, in order to formulate a new LVRT. This LVRT curve was analyzed and compared with the current TPC LVRT curve regarding effects on the transient stability in the system. According to the results, applying the proposed LVRT curve in the wind turbines avoided the disconnection of the turbines and the power system, reduced the PCC voltage dip during fault occurrences, lessened the disturbances caused by faults, shortened the time of voltage recovery to 0.9 p.u., and enhanced the operational stability between the turbines and the grids. This study may act as a reference for power companies for their LVRT curve revision in the future.

Author Contributions: S.D.L conceived of the presented idea, designed the simulation cases and wrote this article; M.-H.W. supervised the findings of this work; C.-Y.T performed the simulations; all authors provided critical feedback and helped shape the research, analysis and manuscript.

Acknowledgments: The authors gratefully acknowledge the financial support of the National Chin-Yi University of Technology, R.O.C. through its grants NCUT 19–R–CE–017 and also express their gratitude to the Power Dispatching Department of Taiwan Power Company for providing much precious advice and help, so as to this research could be completed successfully. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Global Wind Energy Council (GWEC). Global Wind Statistics 2017. Available online: https://www.google.com.hk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=2ahUKEwi7qqTm66PhA hWMGaYKHdZEBiMQFjAAegQIABAB&url=https%3A%2F%2Fgwec.net%2Fwp-content%2Fuploads%2 Fvip%2FGWEC_PRstats2017_EN-003_FINAL.pdf&usg=AOvVaw3kG04-vWc1O7qaCoVeVH5C (accessed on 14 February 2018). 2. Executive Yuan, "four-year plan for promoting wind power generation—clean energy is emerging," 2018. 3. Dobson, I.; Chiang, H.D.; Thorp J.S.; Fekih-Ahmed, L. A model of voltage collapse in electric power systems. In the Proceedings of the 27th IEEE Conference, Austin, TX, USA, 7–9 December, 1988, doi:10.1109/CDC.1988.194705 4. Park, C.H.; Jang, G. Voltage quality assessment considering low voltage ride-through requirement for wind turbines. IET Gener. Transm. Distrib. 2016, 10, 4205-4214. 5. Fandi, G.; Ahmad, I.; Igbinovia, F.O.; Muller, Z.; Tlusty, J.; Krepl, V. Voltage Regulation and Power Loss

Energies 2019, 12, 1258 11 of 12

5. Conclusions This paper merges the target of 520 MW offshore wind power, as planned by Taiwan for 2020, into the Taipower Changbin district (Lutung, Luhsi, Hsienhsi, and Changbin distribution substations) and Changbin district (Tacheng distribution substation), based on the content speciﬁed of the TPC transmission system planning criteria and technical key points, as combined with the actual Taipower system, and varying fault scenarios and fault times are considered in the analysis, in order to formulate a new LVRT. This LVRT curve was analyzed and compared with the current TPC LVRT curve regarding effects on the transient stability in the system. According to the results, applying the proposed LVRT curve in the wind turbines avoided the disconnection of the turbines and the power system, reduced the PCC voltage dip during fault occurrences, lessened the disturbances caused by faults, shortened the time of voltage recovery to 0.9 p.u., and enhanced the operational stability between the turbines and the grids. This study may act as a reference for power companies for their LVRT curve revision in the future.

Author Contributions: S.D.L conceived of the presented idea, designed the simulation cases and wrote this article; M.-H.W. supervised the findings of this work; C.-Y.T performed the simulations; all authors provided critical feedback and helped shape the research, analysis and manuscript. Acknowledgments: The authors gratefully acknowledge the financial support of the National Chin-Yi University of Technology, R.O.C. through its grants NCUT 19–R–CE–017 and also express their gratitude to the Power Dispatching Department of Taiwan Power Company for providing much precious advice and help, so as to this research could be completed successfully. Conflicts of Interest: The authors declare no conflict of interest.

References

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