Unsynchronized Phasor-Based Protection Method for Single ... - MDPI

energies Article

Unsynchronized Phasor-Based Protection Method for Single Line-to-Ground Faults in an Ungrounded Offshore Wind Farm with Fully-Rated Converters-Based Wind Turbines Liuming Jing, Dae-Hee Son, Sang-Hee Kang and Soon-Ryul Nam * Department of Electrical Engineering, Myongji University, Yongin 449-728, Korea; [email protected] (L.J.); [email protected] (D.-H.S.); [email protected] (S.-H.K.) * Correspondence: [email protected]; Tel.: +82-31-330-6361 Academic Editor: Ying-Yi Hong Received: 12 December 2016; Accepted: 10 April 2017; Published: 13 April 2017

Abstract: This paper proposes a protection method for single line-to-ground (SLG) faults in an ungrounded offshore wind farm with fully-rated converter-based wind turbines. The proposed method uses the unsynchronized current phasors measured by unit protections installed at the connection point of the fully-rated converter (FRC)-based wind turbines (WTs). Each unit protection collects the unsynchronized current phasors from two adjacent nodes and synchronizes them by aligning the positive-sequence current to the same phase angle. The faulted section is identified by comparing the phase angles of the synchronized zero-sequence currents from adjacent nodes. Simulations of an ungrounded offshore wind farm with relay models were carried out using power system computer-aided design (PSCAD)/ electromagnetic transients including direct current (EMTDC). Keywords: fully-rated converter-based wind turbines; offshore wind farm; single line-to-ground fault; unit protection

1. Introduction There have been rapid developments and significant momentum in the use of offshore wind farms in the recent years, because wind power provides a source of clean and renewable energy [1–3]. Voltage source converter (VSC)-based high voltage direct current (HVDC) systems have increased in popularity because VSC-HVDC systems are economical for long-distance bulk power transmission. Unlike high-voltage alternating-current (HVAC) systems, the VSC-HVDC transmission system capacity is not limited by the capacitive charging current. Moreover, the offshore wind farm can be decoupled from the onshore grid in this system, and the active and reactive power can be controlled independently at both ends [4–6]. Fully-rated converter (FRC)-based wind turbines (WTs) with a permanent magnet synchronous generator (PMSG) have a wide speed control range, good reactive power support, and fault-ride-through capability [7], and offshore wind farms incorporating FRC-WTs and VSC-HVDC connections are likely to play an important role in the future of wind energy utilization [8–10]. The use of offshore wind farms has resulted in some technical challenges, including fault detection, isolation, and restoration (FDIR) methods for offshore wind farms. In the offshore collector grid, fault currents are mainly determined by FRC-WTs and HVDC controls in the case of a fault [11]. Offshore wind farms with a VSC-HVDC connection will have different fault responses compared to conventional alternating current (AC) systems. The fault current capability of an FRC-WT is usually limited to its rated current or slightly above its rated current, and is determined by the short-time current-carrying Energies 2017, 10, 526; doi:10.3390/en10040526

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capacity of the semiconductor switches. A low fault current level will be insufficient to activate conventional over-current protection schemes in the inverter-dominated AC-networks of offshore wind farms [12–14]. Here, we consider an ungrounded configuration for detecting single-line-to-ground (SLG) faults in offshore wind farms. The fault ride-through capability of FRC-WTs in an offshore wind farm will be considered for an offshore wind farm protection scheme. Faults in an offshore wind farm should be detected and isolated sufficiently quickly that the FRC-WTs can operate stably through the fault [15]. SLG faults in ungrounded offshore wind farms do not lead to large currents, however, during an SLG fault, an ungrounded offshore wind farm is subject to a high and destructive transient overvoltage, which is potentially hazardous to equipment [16]. Thus, locating and isolating the SLG fault is important in an ungrounded offshore wind farm. There are many feeder protection methods for offshore wind farms with FRC-WTs, such as overcurrent protection, methods based on directional power and peak current, or impedance protection. The author of [17] presented a method based on the coordination of over-current relays. However, offline analysis is required to obtain approximate values for the fault current levels under different operating conditions. If the system topology is changed, the fault current levels should be recalculated. The authors of [18] proposed a protection method based on peak current and directional power; the protection system can detect the faulted feeder and faulted feeder section. However, this method can only respond to a three-phase fault. The authors of [19] proposed an offshore wind farm feeder protection strategy based on impedance. The impedance protection system was designed using two zones, encircling the feeder impedance and the load area. It has the advantage of selectively having high sensitivity to fault currents and low sensitivity to load currents. However, the impedance characteristic for both zones should be of a quadrilateral shape, with some time delay. This paper analyzes the behavior of an ungrounded offshore wind farm in the case of an SLG fault at different locations to evaluate the feasibility of effective fault detection strategies in the grid. Various protection methods relying on communication have been described in the literature [20,21]. In conventional ungrounded distribution feeders, there is only one source to supply fault currents. Several techniques have been proposed for locating SLG faults in conventional ungrounded distribution feeders, such as signal injection [22], traveling-wave [23], and phasor-based [24] methods. However, in ungrounded offshore wind farms, there are two sources: HVDC and FRC-WTs. Therefore, it is difficult to use conventional methods for locating SLG faults in ungrounded offshore wind farms. The proposed method can locate SLG faults in ungrounded offshore wind farms without the installation of expensive synchronization devices. First, a collector bus protection detects the fault status and then a unit protection detects the fault direction. Finally, the fault section is identified by coordination of unit protection and collector bus protection. The proposed method, therefore, requires communication only when there is an event in the system, and time synchronization is not required. The remainder of the paper is organized as follows. Section 2 gives an overview of the major protection issues in offshore wind farms with FRC-WTs, Section 3 describes the proposed protection method for an offshore wind farm with FRC-WTs, Section 4 presents simulation results, and conclusions are provided in Section 5. 2. Fault Characteristics in an Offshore Wind Farm with FRC-WTs Figure 1 shows the system being studied. It consists of ten FRC-WTs equally distributed and separated into two feeders. The offshore wind farm is interconnected to a VSC-HVDC transmission system via a Y/∆ offshore converter transformer. The offshore converter transformer has a nominal voltage ratio of 33/150 kV. The 150-kV terminals of the converter transformer serve the high voltage (HV) system, and this HV system enables the suitable operation of the VSC-HVDC. The 33-kV terminals of the converter transformer, consisting of delta connection winding, serve as the collector bus, and the offshore wind farm is, therefore, an ungrounded system [25–27]. Two medium-voltage (MV) feeders are connected to the collector bus, with five switchboards (SWBs) on each feeder and three circuit breakers (CBs) installed at each SWB. The FRC-WT is connected

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Two medium-voltage (MV) feeders are connected to the collector bus, with five switchboards (SWBs) on each feeder and three circuit breakers (CBs) installed at each SWB. The FRC-WT is Energies 2017, 10, 526 3 of 15 connected to the SWB via a step-up transformer. The FRC-WT side of the step-up transformer is a wye connection winding and the collector bus side is a delta connection winding. There are five MW FRC-WTs feedertransformer. and, therefore, total capacity of one feeder is 25 MW.isTwo feeder strings to the SWB on viaeach a step-up The the FRC-WT side of the step-up transformer a wye connection are connected the MV bus collector andconnection the rated capacity the offshore farm is, therefore, winding and thetocollector side isbus, a delta winding.ofThere are fivewind MW FRC-WTs on each 50 MW. The limited number FRC-WTs provides acceptable computation times inconnected simulations and feeder and, therefore, the total of capacity of one feeder is 25 MW. Two feeder strings are to the can collector provide bus, someand insight into capacity the behavior ungrounded offshore wind farms with FRC-WTs MV the rated of the of offshore wind farm is, therefore, 50 MW. The limited under fault. As shown in Figure 1, a feeder fault occurred at position 1 and aand FRC-WT branchsome fault number of FRC-WTs provides acceptable computation times in simulations can provide occurred at position 2. insight into the behavior of ungrounded offshore wind farms with FRC-WTs under fault. As shown in collector bus, a ground potential1 transformer (GPT) is used tooccurred measure at theposition zero-sequence FigureAt1,the a feeder fault occurred at position and a FRC-WT branch fault 2. voltage and create an artificial ground, because delta-connected systems have no wye point At the collector bus, a ground potential transformer (GPT) is used to measure the zero-sequence available forcreate connection to ground. The wye-connected primary windings ofno thewye GPT are grounded voltage and an artificial ground, because delta-connected systems have point available solidly with atocurrent-limiting resistor (CLR)primary connected across of the delta of the solidly tertiary for connection ground. The wye-connected windings thebroken GPT are grounded R windings. The CLR connected at the tertiary can be transformed into the equivalent grounding with a current-limiting clresistor (CLR) connected across the broken delta of the tertiary windings. The CLR Rcl connected at the tertiary can be transformed into the equivalent grounding resistance RCL RCL connected resistance at the primary: connected at the primary: n22 R RCL =n ⋅ Rclcl (1) RCL = 3 · 3 (1) 3 3 wheren nis is GPT turns ratio.Because Becausethe theCLR CLRhas hasthe theeffect effectofofproviding providingvery veryhigh-resistance high-resistance where thethe GPT turns ratio. groundingon onungrounded ungroundedsystems systemswith withaafew fewtens tensof ofkilo-ohms, kilo-ohms,ititintroduces introducesfew fewside-effects. side-effects.SLG SLG grounding faults produce produce zero-sequence zero-sequence currents currents that that have haveextremely extremelysmall smallmagnitudes magnitudes compared compared to tophase phase faults currents.Because Because ititisisalmost almostimpossible impossibleto tocalculate calculatezero-sequence zero-sequencecurrents currentsfrom fromphase phasecurrents, currents,aa currents. zero-sequencecurrent currenttransformer transformer (ZCT) is used. ZCT a ratio of mA/1.5 200 mA/1.5 which zero-sequence (ZCT) is used. TheThe ZCT hadhad a ratio of 200 mA,mA, which is theis the typical in South Korea. typical ratioratio usedused in South Korea. EachSWB SWB current transformer (CMs). Each CM can measure the Each hashas two two current transformer modulesmodules (CMs). Each CM can measure the zero-sequence zero-sequence current and positive-sequence current through the SWB. current and positive-sequence current flow through theflow SWB.

Figure 1. Offshore wind farm with fully-rated converter-based wind turbines. GPT: ground potential Figure 1. Offshore wind farm with fully-rated converter-based wind turbines. GPT: ground potential transformer; CM: current transformer module; SWB: switchboards; VSC-HVDC: voltage source transformer; CM: current transformer module; SWB: switchboards; VSC-HVDC: voltage source converter based high-voltage direct current transmission. converter based high-voltage direct current transmission.

Figure 2 shows the sequence networks and their interconnections for the SLG fault at position Figure shows1the sequence networks and their interconnections for the SLG fault at position 1 1 shown in 2Figure using the following notation [28,29]: shown in Figure 1 using the following notation [28,29]: (m~ n) : line impedance from the m th node to the n th node, Z Lk (m∼n) ZLk : line impedance from the mth node to the nth node, Z ( m ~ n ) : line-to-ground capacitive impedance from the m th node to the n th node, (m∼n) Ck ZCk : line-to-ground capacitive impedance from the mth node to the nth node, Z ( m ~ n ) : FRC-WT source impedance including the step-up transformer, (m∼n) Dk ZDk : FRC-WT source impedance including the step-up transformer, Z Sk : HVDC source impedance including the offshore converter transformer, ZSk : HVDC source impedance including the offshore converter transformer, (m)

Ik

: current from the mth node to the next node radially,

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I k( m) : current from the m th node to the next node radially, IFk : fault I :current, fault current, (m)

Fk

(m) :Vvoltage at theatmth : voltage thenode, m th node, k (m∼n) ~ n) EWT E :( mFRC-WT source from from the mth to the nth node, : FRC-WT source thenode m th node to the n th node, WT ES : HVDC source, ES : HVDC source, R F : fault resistance. RF : fault resistance.

Vk

The term k = 0, 1 and 2 denotes the zero, positive, and negative sequence, respectively. respectively. I F 0 = I F1 = I F 2

( F ~5) Z L0

(2) I0

(6~10) Z L0

(1~ F ) Z L0

( F ~5) ZC 0

(1~ F ) ZC 0

( F ~5) IC 0

(1~ F ) IC 0

(1) I1

(2) I1

ZS 0

( F ~5) Z L1

(1) I0

(6~10) ZC 0 (6~10) IC 0

(1~ F ) Z L1

(6~10) Z L1

RF

( F ~5) IC1

(1~ F ) EWT

(1~ F ) IC1

ES

(1~ F ) Z L2

( F ~5) Z L2

(6~10) ZC1 (6~10) EWT

(6~10) IC1

(6~10) Z L2

(1~ F ) IC 2

(6~10) Z D2

(1~ F ) ZC 2

ZS 2

( F ~5) IC 2

(1~ F ) Z D2

( F ~5) Z D2

( F ~5) ZC 2

(6~10) Z D1

(1~ F ) ZC1

Z S1

( F ~5) EWT

(1~ F ) Z D1

( F ~5) Z D1

( F ~5) ZC1

(6~10) ZC 2 (6~10) IC 2

Figure 2. Interconnection of the sequence networks for the single line-to-ground (SLG) fault in the Figure 2. Interconnection of the sequence networks for the single line-to-ground (SLG) fault in the system shown in Figure 1. system shown in Figure 1.

Because the magnitude of the zero-sequence impedance of an ungrounded system is very large, Because the magnitude of the zero-sequence impedance of an ungrounded is very when large, the negative-sequence impedance can be ignored without significant loss system of accuracy the negative-sequence impedance can be ignored without significant loss of accuracy when considering considering SLG faults. This leads to the simplified zero-sequence network and the corresponding SLG faults. This leads toin theFigure simplified phasor diagram shown 3. zero-sequence network and the corresponding phasor diagram shown in Figure 3. The best solution for operating a wind farm is to keep a constant power factor close to 1. The best solution operating a windatfarm is to keep a constant factor closeangle to 1. Assuming Assuming that a windforfarm is operated a constant power factorpower of 1, the phase difference (1)

that a wind farm is operated at a constant power factor theare phase angle between V1 between and andof V1,1(2) equal indifference the positive-sequence 0  . Since V1(1) I1(1) becomes V1(1) (1) (1) (2) ◦ and I1 becomes 0 . Since V1 and V1 are equal in the positive-sequence network shown in Figure 3, the network shown in Figure 3, the phase angle difference between I1(1) and I1(2) is almost 0  (.1In (1) (2) ) ◦ the phase angle difference between I1 (1) and I1 is almost(2) 0 . In the zero-sequence network, I0 lags (1) (2)  zero-sequence network, by 90 due to capacitive I 0 (2)lags V◦0 by 90 , and I 0 leads V0 (1) (2) V0 by 90◦ , and I0 leads V Since the phase angle difference 0 by 90 due to capacitive reactance. (2) 1) (2) (1) 1) 1) reactance. (Since the(2)phase angle difference between V0(1) and (V is 0  , I 0(2) leads V0(1) (by between V0 and V0 is 0◦ , I0 leads V0 by 90◦ . In addition, V0 0 is equal to the inverse of V1 .

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90  . In addition, V0(1) is equal to the inverse of V1(1) . Therefore, I 0(1) leads I1(1) by 90  ,

5 of 15 5 of 15 and I 0(2)

(1)  . Consequently, (1) (2) (2) lags I1(2) by while zero-sequence on the left sidethe of zero-sequence the fault leads Therefore, I0 90 leads I1 by 90◦ , and I0 the lags I1 by 90◦ . current Consequently, while ◦  current on thesequence left side of the fault sequencecurrent currenton bythe 90 right , the zero-sequence current the positive current byleads thepositive zero-sequence side of the fault lags 90 ,the positive sequence current by 90◦ . on the right side of the fault lags the the positive sequence current by 90 .

Figure 3. Simplified zero-sequence network and corresponding phasor diagram. Figure 3. Simplified zero-sequence network and corresponding phasor diagram.

3. Proposed Protection Method for SLG Faults in an Offshore Wind Farm 3. Proposed Protection Method for SLG Faults in an Offshore Wind Farm Similar to the microgrid protection method in [30], the proposed protection method utilizes Similar to the microgrid protection method in [30], the proposed protection method utilizes both both unit and collector bus protection, as shown in Figure 4. The main difference between the unit and collector bus protection, as shown in Figure 4. The main difference between the microgrid Energies 2017,and 10, 526 6 of of 15 microgrid proposed protection methods is the identification of sections in fault. In the case and proposed protection methods is the identification of sections in fault. In the case of the microgrid the microgrid protection method, a lateral branch is required for each node, and three ZCTs are protection method, a lateral branch is required for each node, and three ZCTs are used to determine protections and bus Synchronization is achieved phase compensation on used to determine theprotection. flow direction of zero sequence currents.using For the proposed protectionbased method, the flow direction of zero sequence currents. For the proposed protection method, no lateral branch is thelateral characteristics the ungrounded offshore farm. Therefore, the additional GPS-receiving no branch isof required and only two ZCTs wind are used to determine flow direction of zero required and only two ZCTs are used to determine the flow direction of zero sequence currents. equipment is not required. sequence currents. Figure 4a presents the proposed FRC-WT unit protection, which consists of an intelligent electronic device (IED) situated locally (at the SWB) that is able to detect the fault and send a required action at the hardware level. Three wye-connected CBs are installed in the FRC-WT connection point and controlled by the IED. The breakers can be used to isolate the fault section. The IED can obtain the required information from the two current transformers and two ZCTs installed at the left and right sides of the SWB. The three-phase currents from two adjacent measurement points are exchanged using the communication channel. Each unit protection collects the unsynchronized current phasors from two adjacent nodes and synchronizes the current phasors by aligning the positive-sequence current from the adjacent nodes to the same phase, and then the phases of the zero-sequence currents from the two adjacent measurement points are compared. (a) (b) As shown in Figure 4b, the collector bus protection system can obtain the required information Figure 4. Fully-rated converter (FRC)-based wind turbines (WTs) unit protection and collector bus fromFigure the GPT and the two ZCTs installed at the end of the(WTs) feeder. The two breakers installed 4. Fully-rated converter (FRC)-based wind turbines unit protection and collector busat the protection: (a) unit protection; (b) collector bus protection. IED: intelligent electronic device. end of each feeder areprotection; controlled(b)by the collector bus protection and can be useddevice. to isolate the fault protection: (a) unit collector bus protection. IED: intelligent electronic section. Time synchronization is not required, and ground potential transformers (GPTs) are also Figure 5atshows communication between current modules and IEDs. not required every the SWB. It is difficult tochannels install GPT at every nodetransformer in an offshore wind farm. Figure 4a presents the proposed FRC-WT unit protection, which consists of an intelligent electronic IEDsInare usually equipped withphasors communication the International conventional methods, all need to befunctionality, synchronized such using as a global positioning device (IED) situated locally (at the SWB) that is able to detect the fault and send a required action Electrotechnical Commission (IEC) 61850.from Eacheach IEDnode will with exchange the three-phase current system (GPS) because they are collected a different time reference. In and the at the hardware level. Three wye-connected CBs are installed in the FRC-WT connection point and zero-sequence current signals with its upstream and downstream IED. The IED will synchronize the proposed method, only positive-sequence phasors need to be synchronized between unit current by aligning the positive-sequence current angle. The zero-sequence current angle is then compared and the faulted feeder section can be located by the coordination of IEDs. If fault F1 occurs, the IED at SWB3 will detect that the zero-sequence current leads the positive sequence at CM2, and the IED at SWB2 will detect that the zero-sequence current lags the positive sequence at CM1. This identifies an SLG fault in the cable between CM1 and CM2. If fault F2 occurs, the IED at

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protections and bus protection. Synchronization is achieved using phase compensation based on controlled by the IED. can be used towind isolate the fault section.additional The IED can obtain the the characteristics of The the breakers ungrounded offshore farm. Therefore, GPS-receiving required information from the two current transformers and two ZCTs installed at the left and right equipment is not required. sides of the SWB. The three-phase currents from two adjacent measurement points are exchanged using the communication channel. Each unit protection collects the unsynchronized current phasors from two adjacent nodes and synchronizes the current phasors by aligning the positive-sequence current from the adjacent nodes to the same phase, and then the phases of the zero-sequence currents from the two adjacent measurement points are compared. As shown in Figure 4b, the collector bus protection system can obtain the required information from the GPT and the two ZCTs installed at the end of the feeder. The two breakers installed at the end of each feeder are controlled by the collector bus protection and can be used to isolate the fault section. Time synchronization is not required, and ground potential transformers (GPTs) are also not required at every SWB. It is difficult to install GPT at every node in an offshore wind farm. (a) (b) In conventional methods, all phasors need to be synchronized using a global positioning system (GPS)Figure because they are collected each node a different time reference. the proposed 4. Fully-rated converter from (FRC)-based windwith turbines (WTs) unit protection andIn collector bus method, only positive-sequence phasors need to be synchronized between unit protections and bus protection: (a) unit protection; (b) collector bus protection. IED: intelligent electronic device. protection. Synchronization is achieved using phase compensation based on the characteristics of the ungrounded farm. Therefore, additional GPS-receiving equipment is not required. Figure 5offshore shows wind the communication channels between current transformer modules and IEDs. Figure 5 shows the communication channels between current transformer modules and IEDs. IEDs are usually equipped with communication functionality, such as the International IEDs are usually equipped with communication functionality, as the International Electrotechnical Electrotechnical Commission (IEC) 61850. Each IED will such exchange the three-phase current and Commission (IEC) 61850. Each IED will exchange the three-phase current and zero-sequence current zero-sequence current signals with its upstream and downstream IED. The IED will synchronize the signals its upstream downstream IED. The IED willThe synchronize the current byangle aligning the currentwith by aligning the and positive-sequence current angle. zero-sequence current is then positive-sequence current angle. Thesection zero-sequence currentby angle then compared and the compared and the faulted feeder can be located the iscoordination of IEDs. If faulted fault F1 feeder section can be located by the coordination of IEDs. If fault F1 occurs, the IED at SWB3 willat occurs, the IED at SWB3 will detect that the zero-sequence current leads the positive sequence detect current leads sequence at CM2, and IED at sequence SWB2 willat CM2, that and the the zero-sequence IED at SWB2 will detect thatthe thepositive zero-sequence current lags thethe positive detect zero-sequence positive sequence at CM1. identifies an SLG CM1. that Thisthe identifies an SLGcurrent fault inlags the the cable between CM1 and CM2.This If fault F2 occurs, thefault IEDinat the cable between CM1 and CM2. If fault F2 occurs, the IED at SWB3 will detect that the zero-sequence SWB3 will detect that the zero-sequence current leads the positive sequence at CM3, and will also current leadsthe thezero-sequence positive sequence at CM3, will alsosequence detect that zero-sequence current lagsfault the detect that current lags and the positive atthe CM2. This identifies an SLG positive sequence at CM2. This identifies an SLG fault in the FRC-WT branch of SWB3. In the case in the FRC-WT branch of SWB3. In the case of a feeder fault, two SWBs near the fault position can of a feeder fault,section, two SWBs fault position canbranch locate fault, the fault in the case of an locate the fault and near in thethe case of an FRC-WT twosection, currentand transformer modules FRC-WT tworight current transformer modules installed theposition left and by right sides of the installed branch on the fault, left and sides of the SWB can indicate the on fault comparing the SWB can indicate the fault position by comparing the sequence angle difference within the SWB. sequence angle difference within the SWB.

Figure5.5.Offshore Offshorewind windfarm farmprotection protectionconfiguration. configuration. Figure

If an SLG fault occurs in the offshore wind farm, the IED installed at the 33-kV collector bus will If an SLG fault occurs in the offshore wind farm, the IED installed at the 33-kV collector bus detect that the zero-sequence voltage magnitude exceeds the pre-set value, which enables detection will detect that the zero-sequence voltage magnitude exceeds the pre-set value, which enables of the fault status. The collector bus protection will detect the faulted feeder from the angle detection of the fault status. The collector bus protection will detect the faulted feeder from the difference between the zero-sequence voltage and the zero-sequence current. In the faulted feeder, angle difference between the zero-sequence voltage and the zero-sequence current. In the faulted the zero-sequence current will lag the zero-sequence voltage by 60 to 90 , whereas in a healthy feeder, the zero-sequence current will lag the zero-sequence voltage by 60◦ to 90◦ , whereas in a healthy [30].The Thecollector collectorbus bus feeder,the thezero zerosequence sequencecurrent current will will lead lead the the zero-sequence zero-sequence voltage voltage by 90◦ [30]. feeder, by 90 protection method is shown in Figure 6. protection method is shown in Figure 6.

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Figure Figure 6. 6. Block Block diagram diagram for for collector collector bus bus protection. protection. Figure 6. Block diagram for collector bus protection.

After After the the faulted faulted feeder feeder is is detected, detected, the the fault fault status status signal signal can can be be sent sent to to IEDs IEDs in in the the faulted faulted After the faulted feeder ischannel, detected,and the IEDs fault installed status signal canfaulted be sentfeeder to IEDs in process the faulted feeder by in will the feeder by the the communication communication channel, and IEDs installed in the the faulted feeder will process the feeder by the communication channel, and IEDs installed in the faulted feeder will process positive-sequence and the zero-sequence current signals. Each unit protection collects the positive-sequence and the zero-sequence current signals. Each unit protection collects the the positive-sequence the zero-sequence signals. Each unit collects the unsynchronized current phasors from in SWB, side of SWB, unsynchronized currentand phasors from two two CMs CMscurrent in the the local local SWB, the the left left sideprotection of the the upstream upstream SWB, unsynchronized current phasors from two CMs in the local SWB, the left side of the upstream SWB, and and the the right right side side of of the the downstream downstream SWB, SWB, and and the the IED IED can can synchronize synchronize these these by by aligning aligning the the and the right side of the downstream SWB, and the IED can synchronize these by aligning the adjacent adjacent positive-sequence current to the same phase angle. The faulted section is identified adjacent positive-sequence current to the same phase angle. The faulted section is identified by by positive-sequence current to theof angle. The faulted section is identified by comparing comparing angles the synchronized zero-sequence currents. The unit comparing the the phase phase angles ofsame the phase synchronized zero-sequence currents. The FRC-WT FRC-WT unit the phase angles of is the synchronized zero-sequence currents. The unit protection method is protection method in 7. diagram of unit Iabc_n_L denotes protection method is shown shown in Figure Figure 7. In In the the block block diagram ofFRC-WT unit protection, protection, Iabc_n_L denotes shown in Figurecurrent 7. In theat diagram unit protection, Iabc_n_L denotes three-phase current the the side nth SWB, denotes the three-phase current at the three-phase three-phase current atblock the left left side of ofofthe the nth SWB, Iabc_n_R Iabc_n_R denotes thethe three-phase current at at the left side of the nth SWB, Iabc_n_R denotes the three-phase current at the right side of the nth the the right right side side of of the the nth nth SWB, SWB, and and section section [(n [(n −− 1)~n] 1)~n] denotes denotes the the section section between between SWB SWB (n (n −− 1) 1) and and SWB, and section [(n − 1)~n] denotes the section between SWB (n − 1) and SWB (n). SWB SWB (n). (n).

Figure Figure 7. 7. Block Block diagram diagram for for unit unit protection. protection. Figure 7. Block diagram for unit protection.

The The faulted faulted feeder feeder and and faulted faulted section section can can then then be be located located by by the the coordination coordination of of IEDs. IEDs. The The The faulted feeder and faulted section can then be located by the coordination of IEDs. The proposed proposed fault fault detection detection and and location location scheme scheme is is shown shown in in Figure Figure 8. 8. proposed fault detection and location scheme is shown in Figure 8.

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Figure 8. Flowchart of the proposed fault detection and location scheme.

Figure 8. Flowchart of the proposed fault detection and location scheme.

With regard to the fault section isolation method, two scenarios were explored with different

Figure 8. Flowchart of the proposed fault detection and location scheme. fault positions: fault section and FRC-WT branch fault intwo an offshore wind farm. The proposed unit With regard tofeeder the fault isolation method, scenarios were explored with different protection will disconnect the smallest possible portion of the collector feeder in the case of a feeder fault positions: feeder fault and FRC-WT branch fault in an offshore wind farm. The proposed unit With regard to the fault section isolation method, two scenarios were explored with different SLG fault. The CB will open if the tripping condition is reached. As shownfeeder in Figure 9, incase the case protection will disconnect the and smallest possible collector in the ofunit a of feeder fault positions: feeder fault FRC-WT branchportion fault inof anthe offshore wind farm. The proposed the feeder fault F1, the unit protection installed at SWB3 and the unit protection installed at SWB 2 SLG fault. The CB will open if the tripping condition is reached. As shown in Figure 9, in the case protection will disconnect the smallest possible portion of the collector feeder in the case of a feeder of detect the fault within the cable between the SWB2 and SWB3, and CB3.2 and CB2.1 will open to the feeder faultThe F1,CB thewill unit protection installed at SWB3 and the protection installed at of SWB 2 SLG fault. open if the tripping condition is reached. As unit shown in Figure 9, in the case isolate the fault section. If there is a tie-breaker and cable between feeder 1 and feeder 2, then SWB3, F1,the thecable unit protection installed SWB3 and and the unit protection installed at SWB 2 detectthe thefeeder fault fault within between the SWB2atand SWB3, CB3.2 and CB2.1 will open to isolate SWB4, and SWB5 can be transferred to feeder 2 and resupplied via SWB10. The proposed protection detect the fault within the cable between the SWB2 and SWB3, and CB3.2 and CB2.1 will open to the fault section. If selective there is aoperation tie-breaker andThe cable between feeder andafeeder 2, then SWB3, SWB4, method allows of CBs. unit protection will 1 send trip signal to the central isolatecan the fault section. If there is a tie-breaker and cablevia between feeder 1proposed and feederprotection 2, then SWB3, and SWB5 transferred 2 and branch resupplied SWB10. method breaker of be SWB in the casetooffeeder an FRC-WT fault F2. Once the The FRC-WT fault is detected, the SWB4, and SWB5 can be transferred to feeder 2 and resupplied via SWB10. The proposed protection allows selective operation of from CBs.the The unit protection send trip to signal tothe thefault central breaker FRC-WT should be cut off offshore wind farm.will CB8.3 willaopen isolate section, method allows selective operation of CBs. The unit protection will send a trip signal to the central while other parts offshorebranch wind farm work. Once theis fault is cleared, the of SWB in the case of of anthe FRC-WT faultwill F2. continue Once thetoFRC-WT fault detected, the FRC-WT breaker of SWB in the case of an FRC-WT branch fault F2. Once the FRC-WT fault is detected, the FRC-WT can be reconnected to the offshore wind farm. should be cut off from the offshore wind farm. CB8.3 will open to isolate the fault section, while FRC-WT should be cut off from the offshore wind farm. CB8.3 will open to isolate the fault section,other partswhile of theother offshore will wind continue work. Oncetothe faultOnce is cleared, can be partswind of thefarm offshore farmto will continue work. the faultthe is FRC-WT cleared, the reconnected the wind FRC-WTto can beoffshore reconnected to farm. the offshore wind farm.

Figure 9. Feeder fault and FRC-WT branch fault in the offshore wind farm.

Because the proposed method relies on communication channels, communication failure Figure 9. Feeder fault and FRC-WT branch fault in the offshore wind farm. managementFigure is critical for system protection, and backup protection shouldwind be considered to cope 9. Feeder fault and FRC-WT branch fault in the offshore farm. Because the proposed method relies on communication channels, communication failure management critical for method system protection, backup protection should becommunication considered to cope Because the isproposed relies onand communication channels, failure management is critical for system protection, and backup protection should be considered to cope

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withcommunication communicationfailure. failure.Collector Collectorbus busprotection protectionwill willbe beoperated operatedafter afteraagiven giventime, time,as asaabackup backup with inthe thecase case of communication failure. this method, the protection system willoperation maintain in of communication failure. UsingUsing this method, the protection system will maintain operation of an offshore farm with FRC-WTs. of an offshore wind farm wind with FRC-WTs. 4.4.Case CaseStudy Study To Toinvestigate investigatethe theeffectiveness effectivenessof ofthe theprotection protectionmethod, method,an anoffshore offshorewind windfarm farmwas wassimulated simulated using usingPower Powersystem systemcomputer-aided computer-aideddesign design(PSCAD)/ (PSCAD)/ electromagnetic electromagnetic transients transients including including direct direct current (EMTDC), as shown in Figure 10. current (EMTDC), as shown in Figure 10.

ZCT

ZCT

CB_3_2

CB_3_1

A V

A V

Out

ABC

ZCT

In

Out

ZCT

CB_2_2 A V

out

CB_2_1 ABC

ABC CB_1_2

I0

CB_1_1 ABC

Out CB_0_1

In

ABC

ZCT

Sh C13 Sh 0.001 [ohm]

Sh C14 Sh

0.001 [ohm]

ABC

I0_feeder1

I0_2_2

In

Sh C12 Sh

SLG fault

Sh C11 Sh 0.001 [ohm]

Out

CB_1_3

ABCIn

I0_3_1

I0_3_2 OutABC

out CB_2_3

In

I0

0.001 [ohm]

A V

I0

0.001 [ohm]

CB_4_1

Sh C15 Sh

0.001 [ohm]

Sh Ctie1 Sh

ABC CB_4_2

I0

PMSG

PMSG

out I0

CB_3_3

CB_5_1 ABC

PMSG I0_4_1

CB_4_3

ABC CB_5_2

out

0.001 [ohm]

ABC

PMSG

out CB_5_3

PMSG

CB_Bus

100 [MVA] 33 [kV] / 150 [kV] #2

RRL

out

CB_7_3

CB_6_3

ABC CB_9_2

CB_9_1

ABC

ABC

CB_8_2

CB_8_1

ABC

ABC

CB_7_2

CB_7_1 ABC

CB_6_2 ABC

GPT

I0_feeder2

out

CB_10_1 ABC

V0

PMSG

PMSG

out CB_8_3

ABC CB_10_2

PMSG

out CB_9_3

ABC

PMSG

out CB_10_3

PMSG

V_PCC

CB_Tie

#1

I0

CB_6_1 ABC

ABC

In

Out

CB_0_2

ZCT

0.001 [ohm]

Sh C21 Sh 0.001 [ohm]

Sh C22 Sh 0.001 [ohm]

Sh C23 Sh 0.001 [ohm]

Sh C24 Sh 0.001 [ohm]

Sh C25 Sh 0.001 [ohm]

0.001 [ohm]

Sh Ctie1 Sh

Figure 10. Power system computer-aided design (PSCAD)/electromagnetic transients including Figure 10. Power system computer-aided design (PSCAD)/electromagnetic transients including direct direct current (EMTDC) model of the offshore wind farm. current (EMTDC) model of the offshore wind farm.

Individual wind turbine generators and cables between wind turbines were modeled, and the Individual wind turbine generators and cables wind turbines were modeled, and the onshore grid and VSC-HVDC was represented by abetween simple Thevenin’s equivalent. The 150-kV bus onshore grid and VSC-HVDC was represented by a simple Thevenin’s equivalent. The 150-kV supplied the Y/∆ offshore converter transformer, which had a nominal voltage ratio of 33/150bus kV. supplied the Y/∆ offshore converter transformer, which had a nominal voltage ratio of 33/150 kV. Two Two feeders were connected to the 33-kV terminals of the converter transformer, with five 5-MW feeders were to the 33-kV terminals of thetwo converter transformer, 5-MW FRC-WTs onconnected each feeder. The distance between adjacent FRC-WTs with was five 1 km, and FRC-WTs GPT was on each feeder. The distance between two adjacent FRC-WTs was 1 km, and GPT was modeled in modeled in the collector bus. The current transformer modules were modeled in SWB2, SWB3, the collector bus. The current SWB4, and the collector bus. transformer modules were modeled in SWB2, SWB3, SWB4, and the collector bus. Table 1 lists the parameters for the offshore wind farm. The distance between two adjacent Table 1 lists the parameters for the offshore wind farm. The distance between feeders was 1.5 km, and 33-kV cross-linked polyethylene (XLPE) submarine cables two wereadjacent used to feeders was 1.5 km, and 33-kV cross-linked polyethylene (XLPE) submarine cables were used to connect the FRC-WTs. The submarine cables between each wind turbine were connected on the connect the FRC-WTs. The submarine cables between each wind turbine were connected on the bottom of each wind turbine where the sheaths of the cables were grounded [31]. Tables 2 and 3 list bottom of parameters each wind turbine the sheaths of the cables grounded [31]. [32] Tables 2 and 3 the cable [6] andwhere the fully-rated converter wind were turbine parameters used in the list the cable parameters simulations, respectively.[6] and the fully-rated converter wind turbine parameters [32] used in the simulations, respectively. Table 1. System configuration. XLPE: cross-linked polyethylene. Table 1. System configuration. XLPE: cross-linked polyethylene. System Components Parameters Offshore converter transformer System Components potential transformer Offshore Grounding converter transformer Grounding potential transformer Zero-sequence current transformer Zero-sequence Current-limiting current transformer resistor Current-limiting resistor Rated capacity of an FRC-WT Rated capacity of an FRC-WT Low impedance fault resistance Low impedance fault resistance High impedance fault resistance High impedance fault resistance Cable Cable

150 kV/33 kVParameters ( Y / Δ) (33/ 3 kV)/(190/3 150√ kV/33 V) kV (Y/∆) (33/ 3mA kV)/(190/3 V) 200 mA/1.5 200 mA/1.5 mA 8 Ω 5 MW 8 Ω 5 MW 0.02 Ω 0.02 Ω 100 Ω 100 Ω 2 cable cable 240 mm 240XLPE mm2Three-core XLPE Three-core

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Table 2. Cable parameters. DC: direct current.

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Table 2. Cable parameters. DC: direct current. System Components Parameters System Components Parameters Cross-section 240 mm2 2 Cross-section 240 mm Rated voltage 33 kV Rated voltage 33 kV Current rating 410 A rating A DCCurrent resistance 0.0754410 Ω/km Ω /km DC resistance 0.0754 Inductance 0.33 mH/km Capacitance 0.23 Inductance 0.33µF/km mH/km 0.23 μF /km Capacitance Table 3. Fully-rated converter wind turbine parameters. Table 3. Fully-rated converter wind turbine parameters. System Components Parameters System Components Parameters Power Power 5 MW 5 MW Rotational speed 12.1 rpm Rotational speed 12.1 rpm Rotor blade diameter 126 m Rotor blade diameter 126 m Rated wind speed 12 m/s Rated wind speed 12 m/s

As shown wind farm was assumed to be From the As shown in in Figure Figure11, 11,initially initiallythe theoffshore offshore wind farm was assumed to de-energized. be de-energized. From start of the simulation to a time of 2 s, the wind speed was kept constant at 7 m/s, which is the cut-in the start of the simulation to a time of 2 s, the wind speed was kept constant at 7 m/s, which is the wind the for FRC-WT with PMSG in this paper. 2 s, the speed ramped cut-inspeed wind for speed the FRC-WT with studied PMSG studied in this At paper. At wind 2 s, the windwas speed was up from 7 m/s, and the ramping of wind speed lead to ramping of the active power output. At 7 s, ramped up from 7 m/s, and the ramping of wind speed lead to ramping of the active power output. the7wind 12 m/s and wasand kept constant, which iswhich the rated wind speed forspeed the FRC-WT At s, thespeed windreached speed reached 12 m/s was kept constant, is the rated wind for the with PMSG. activeThe power output of output the offshore farmwind also ramped upramped to the maximum FRC-WT withThe PMSG. active power of thewind offshore farm also up to the power levelpower [33–37]. maximum level [33–37].

(a)

(b)

Figure farm: (a) Figure 11. 11. Simulation Simulation results results of of the the offshore offshore wind wind farm: (a) wind wind speed speed (b) (b) active active power power output. output.

An SLG fault between the cable, the sheath, and the ground was simulated on the 33-kV An SLG fault the cable, the sheath, andΩ.the ground simulated on the 1, 33-kV collector collector feeder 1, between with a fault impedance of 0.01 The fault was occurred at position as shown in feeder 1, a fault impedance 0.01 Ω.were Thepre-conditioned fault occurred atusing position 1, as shownButterworth in Figure 1. Figure 1. with The voltage and currentofsignals a second-order The voltage and current signals were pre-conditioned using a second-order Butterworth low-pass low-pass filter with a cutoff frequency of 360 Hz to prevent aliasing errors. The sampling frequency filterset with cutoff of 360 to prevent errors. sampling frequency was(DFT) set to was to a3840 Hzfrequency (64 samples per Hz cycle in 60-Hzaliasing systems) and aThe discrete Fourier transform 3840 Hz (64 samples per cycle in 60-Hz systems) and a discrete Fourier transform (DFT) was used to was used to estimate the phasors. estimate the phasors. As shown in Figure 12, the zero-sequence current at feeder 1 lagged the zero-sequence voltage shown bus. in Figure 12, the zero-sequence current at feeder 1tolagged the60° zero-sequence voltage at theAs collector The zero-sequence angle difference dropped between and 90° in feeder 1. ◦ and 90◦ in feeder 1. at the collector bus. The zero-sequence angle difference dropped to between 60 This angle difference indicated that there was an SLG fault at feeder 1. In the case of an SLG fault, the This angle difference there wasthe an phase SLG fault at feeder In the case of an SLG fault, the faulted phase may beindicated detected that readily using voltage at the1.collector bus. faulted phase may be detected readily using the phase voltage at the collector bus.

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(a) (a)

(b) (b)

Figure 12. The zero-sequence voltage and current at the 33 kV side of the converter transformer in Figure 12. The zero-sequence voltage and current at the 33 kV side of the converter transformer in the Figure zero-sequence voltage and current the 33 kV (b) sidephase of therelation. converter transformer in the case12. of aThe feeder fault: (a) zero-sequence voltage at and current; case of a feeder fault: (a) zero-sequence voltage and current; (b) phase relation. the case of a feeder fault: (a) zero-sequence voltage and current; (b) phase relation.

As shown in Figure 13, the unsynchronized phasors from the left side of SWB2 and the right As shown in 13, phasors from the left and right AsSWB3 shownwere in Figure Figure 13, the the unsynchronized unsynchronized phasors from left side side of of SWB2 SWB2 and the the right side of approximately synchronized by aligning the the positive-sequence current phasors. side of SWB3 were approximately synchronized by aligning the positive-sequence current phasors. side positive-sequence of SWB3 were approximately by aligning positive-sequence current phasors. The current and synchronized the zero-sequence currentthe at the left side of SWB2 and the right The current and the zero-sequence current at the leftleft sideside of SWB2 and and theleads right side The positive-sequence positive-sequence current and the zero-sequence current at the of SWB2 the right side of SWB3 indicated that an SLG fault occurred. The zero-sequence current the of SWB3 indicated that an SLG fault occurred. The zero-sequence current leads the positive-sequence side of SWB3 indicated that an SLG fault occurred. The zero-sequence current leads the positive-sequence current at the right side of SWB3, indicating that an SLG fault occurred at the right current at the right side of SWB3, indicating that an SLG fault occurred at the right side of the positive-sequence current at the right of SWB3, indicating thatzero-sequence an SLG fault occurred at the right side of the measurement point. The side figure also shows that the current lagged the measurement point.current The figure also shows that the shows zero-sequence current lagged positive-sequence side of the measurement point. The figure also that the zero-sequence current lagged the positive-sequence at the left side of SWB2, indicating that an SLG faultthe occurred at the left current at the left side of SWB2, indicating that an SLG fault occurred at the left side of SWB2. The fault positive-sequence at thecan, left therefore, side of SWB2, indicating an SLGresults fault occurred at there the left side of SWB2. The current fault section be located. The that simulation show that is section be located. The simulation results show that there anresults SLG fault between SWB2 sideSLG of can, SWB2. The fault section therefore, be located. The simulation show that there is an faulttherefore, between SWB2 andcan, SWB3, and the faulted section can beisidentified by comparing the and SWB3, and the faulted section can be identified by comparing the angle difference between the an SLG fault between SWB2 and SWB3, and the faulted section can be identified by comparing the angle difference between the positive-sequence and zero-sequence current phasors. positive-sequence and zero-sequence current phasors. angle difference between the positive-sequence and zero-sequence current phasors.

(a) (a)

(b) (b)

Figure 13. The positive-sequence and zero-sequence current at switchboard 2 (SWB2) and Figure 13. 13.The The positive-sequence and zero-sequence atfault: switchboard 2 (SWB2) switchboard 3 (SWB3) in the caseand of azero-sequence feeder A phase-sheath-ground positive-sequence and Figure positive-sequence currentcurrent at switchboard 2(a) (SWB2) and switchboard 3 current; (SWB3) the case of a feeder A phase-sheath-ground fault: (a) positive-sequence and zero-sequence phase 3switchboard (SWB3) in the case of in a(b) feeder Arelation. phase-sheath-ground fault: (a) positive-sequence and zero-sequence zero-sequence current; (b) phase relation. current; (b) phase relation.

As shown in Figure 14, the unsynchronized phasors from the left side of SWB3 and the right AsSWB4 shownwere in Figure 14, the unsynchronized phasors from left side of SWB3 and the right side of approximately synchronized by aligning the the positive-sequence current phasors. As shown in Figure 14, the unsynchronized phasors from the left side of SWB3 and the right side zero-sequence of SWB4 were current approximately by aligning positive-sequence current The led the synchronized positive-sequence current the at the right sides of SWB4 andphasors. SWB3. side of SWB4 were approximately synchronized by aligning the positive-sequence current phasors. The zero-sequence current positive-sequence the right of SWB4 and SWB3. This indicates that an SLG led faultthe occurred at the rightcurrent side of at SWB3. The sides two zero-sequence angles The zero-sequence current led the positive-sequence current at the right sides of SWB4 and SWB3. This indicates that an SLG fault occurred at the right side of SWB3. The two zero-sequence were in the same direction. The simulation results show that there was no SLG fault between angles SWB3 This indicates that an SLG fault occurred at the right side of SWB3. The two zero-sequence angles were wereSWB4. in the Similar same direction. Thefound simulation show there no SLG between SWB3 and trends were at theresults right side of that SWB2 andwas the left side fault of SWB1. in the same direction. The simulation results show that there was no SLG fault between SWB3 and and SWB4. Similar trends were found at the right side of SWB2 and the left side of SWB1. SWB4. Similar trends were found at the right side of SWB2 and the left side of SWB1.

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(a) (b) (a) (b) Figure (a) and (b) Figure 14. 14. The The positive-sequence positive-sequence and zero-sequence zero-sequencecurrent currentat at SWB3 SWB3 and and SWB4 SWB4 in in the the case case of of an an A A

Figure 14. The positive-sequence and zero-sequence current at SWB3 and SWB4 in therelation. case of an A phase-sheath-ground fault: and current; (b) phase phase phase-sheath-ground fault: (a) (a) positive-sequence positive-sequence and zero-sequence zero-sequence current; (b) relation. Figure 14. The positive-sequence and zero-sequence at SWB3 and SWB4 in the case of an A phase-sheath-ground fault: (a) positive-sequence and current zero-sequence current; (b) phase relation. phase-sheath-ground fault: (a) positive-sequence and zero-sequence current; (b) phase relation.

The A phase-sheath fault was simulated with a fault resistance of 0.02 Ω at the same position. The fault was with aa fault resistance of 0.02 Ω position. The A A phase-sheath phase-sheath fault was simulated simulated faultindicate resistance 0.02proposed Ω at at the the same same position. The simulation results are shown in Figurewith 15, and thatof the method can The A phase-sheath faultshown was simulated with aand faultindicate resistance of the 0.02method Ω at thecan same position. The results areare shown in Figure 15, and indicate that the proposed successfully The simulation simulation in fault. Figure 15, that proposed method can successfully detectresults a single phase-sheath The simulation results arephase-sheath shown in Figure detect a single phase-sheath fault. successfully detect a single fault. 15, and indicate that the proposed method can successfully detect a single phase-sheath fault.

(a) (b) (a) (b) Figure 15. The positive-sequence (a) and zero-sequence current at SWB2 and SWB3 (b) in the case of an A Figure 15. 15. The The positive-sequence and zero-sequence zero-sequence current at SWB3 in the case case of of an an A A phase-sheath fault: (a) positive-sequence and zero-sequence (b)and phase relation. Figure positive-sequence and currentcurrent; at SWB2 SWB2 and SWB3 in the Figure 15. Thefault: positive-sequence and zero-sequence currentcurrent; at SWB2(b) and SWB3 in the case of an A phase-sheath (a) positive-sequence and zero-sequence phase relation. phase-sheath fault: (a) positive-sequence and zero-sequence current; (b) phase relation. phase-sheath fault: (a) positive-sequence and zero-sequence current; (b) phase relation.

The fault location is shown in Figure 16. The fault distance varied from 0.2 km to 0.8 km from The fault4 location is shown inresults Figurefor 16.faults The fault distancepositions. varied from 0.2table, km toZero-Positive 0.8 km from SWB3. Table lists the simulation in different In the The fault location is shown in Figure 16. The fault distance varied from 0.2 km to 0.8 km fault location shown Figure 16. km from from SWB3. Table 4 lists the simulation results for faults in different positions. In the table, Zero-Positive denotes the angle difference between the zero-sequence current and the positive-sequence current. SWB3. Table 44 lists the results for positions. In listsdifference the simulation simulation results for faults faults in in different different In the the table, table, Zero-Positive Zero-Positive denotes the angle between the zero-sequence current positions. and the positive-sequence current. denotes denotes the the angle angle difference difference between between the the zero-sequence zero-sequencecurrent currentand andthe thepositive-sequence positive-sequencecurrent. current.

Figure 16. Offshore wind farm feeder fault. Figure 16. Offshore wind farm feeder fault. Figure 16. Offshore wind farm feeder fault. Figure 16. Offshore wind farm feeder fault.

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Table 4. Simulation results for faults in different positions.

Fault Type

Fault Distance

Angle Difference between the Zero-Sequence Current and the Positive-Sequence Current Right Side of SWB 3

Left Side of SWB 2

Low impedance A phase-sheath-ground fault (0.02 Ω)

0.2 km 0.4 km 0.6 km 0.8 km

84.52◦

84.22◦ 84.29◦ 84.03◦

−93.92◦ −93.90◦ −93.91◦ −94.00◦

High impedance A phase-sheath-ground fault (100 Ω)

0.2 km 0.4 km 0.6 km 0.8 km

84.31◦ 83.99◦ 84.26◦ 83.96◦

−94.08◦ −93.86◦ −94.16◦ −93.89◦

Low impedance A phase-sheath fault (0.02 Ω)

0.2 km 0.4 km 0.6 km 0.8 km

84.14◦ 84.19◦ 84.26◦ 84.06◦

−93.82◦ −93.99◦ −94.06◦ −94.07◦

5. Conclusions This paper proposes a protection method for SLG faults in ungrounded offshore wind farms incorporating FRC-WTs. The proposed method is based on the angle difference between the zero-sequence current and the positive-sequence current, using unsynchronized current phasors measured by unit protections. Each unit protection collects the unsynchronized current phasors from two adjacent nodes and synchronizes them by aligning the positive-sequence currents to the same phase angle. The faulted section is identified by comparing the phase angles of the synchronized zero-sequence currents in adjacent nodes. Once the faulted section is identified, circuit breakers are tripped to isolate the faulted section. This method could be used even if the number of wind turbines is high. The proposed method differs from conventional protection methods because time synchronization is not necessary between different unit protections, and there is, therefore, no requirement for GPS to be installed at every node. Simulations of an ungrounded offshore wind farm with relay models were carried out using PSCAD/EMTDC. The simulation results show that the proposed method can detect an SLG fault and locate the fault section. Although this paper focused on ungrounded offshore wind farms incorporating FRC-WTs, the proposed method could also be applied to ungrounded DFIG (doubly fed induction generator)-WT based offshore wind farms. In the future, protection methods for line-to-line, line-to-line-to-ground, and three-phase faults will be developed, and an offshore wind farm with FRC-WTs will be implemented in a real-time digital simulator to evaluate the entire protection method. Acknowledgments: This research was supported in part by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. NRF-2013R1A1A2062924). This research was also supported in part by the Human Resources Program in Energy Technology of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), and granted financial resources from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20154030200770). Author Contributions: Liuming Jing prepared the manuscript and completed the simulations. Soon-Ryul Nam supervised the study and coordinated the main theme of this paper. Dae-Hee Son developed the model of offshore wind farm in the study. Sang-Hee Kang discussed the results and implications, and commented on the manuscript. All of the authors read and approved the final manuscript. Conflicts of Interest: The authors declare no conflict of interest.

References 1.

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