PMSG-Based Black-Start Technology and Its Field Tests

energies

Article PMSG-Based Black-Start Technology and Its Field Tests

Min-gang Tan 1,* , Yi Tang 2 and Chaohai Zhang 1

1 College of Automation Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 211106, China; [email protected] 2 School of Electrical Engineering, Southeast University, Nanjing 210096, China; [email protected] * Correspondence: [email protected]

Received: 9 April 2019; Accepted: 31 May 2019; Published: 4 June 2019

Abstract: It is of great importance for power grids to have black-start capability for rapid recovery, and there is great theoretical significance and practical application value in studying how to use wind farms as the black-start power supply source for power grids with large-scale renewable energy generation. In this paper, a black-start scheme using a permanent-magnet synchronous generator (PMSG)-based wind farm as black-start power supply source is formulated. First, a diesel generator is used as an external supporting power supply for the self-start of a wind power unit (WPU). Then, after all the planned WPUs operate normally, the wind farm with the diesel generator and static var generator (SVG) is used to black start the simulated auxiliary load of a thermal power plant. A field test of the proposed black-start scheme is carried out on an actual wind farm in Jiangsu Province (China). The results of the field test show that wind farms can act as a black-start power supply source for the grid after appropriate technological transformation.

Keywords: wind power; diesel generator; self-start; simulated auxiliary load; black start; ﬁeld test

1. Introduction With the rapid development of UHV AC and DC transmission, large-scale grid-connected renewable energy generation and communication technologies, the possibility of power grid blackouts is increasing due to uncertainty of renewable energy generation, cascading failures and cyber-attacks [1,2]. It is of great significance to develop reasonable black-start schemes for power grids with large-scale renewable energy generation. Benefiting from the advantages such as faster start-up and lower energy requirements, hydroelectric and combustion gas turbines are often the primary choices of traditional black-start power supply sources. At the same time, hydroelectric power availability is limited by geographical conditions and seasonal factors, and the operation and maintenance costs of the combustion gas turbines are extremely high. Therefore, there is an urgent requirement to find new forms of energy to support the black start of power grids [3–5]. China has built large numbers of wind farms along the northwestern, northeastern, northern and eastern coasts. The penetration rate of wind power is increasing year by year. The capacity of wind power generation in China reached 188,390 MW in 2017. More importantly, the distribution of wind power is extremely broad, and covers 31 provinces in China. The spatial distribution, seasonal difference and potential black-start supporting ability makes it an effective supplement for hydroelectric power sources [5–8]. Scholars from all over the world have conducted many research studies on this topic [9]. A new wind turbine structure that can be applied for black starts has been proposed by a Chinese professor from the Huazhong University for Technology and Science [2]. Aktarujjaman et al., from Australia,

Energies 2019, 12, 2144; doi:10.3390/en12112144 www.mdpi.com/journal/energies Energies 2019, 12, 2144 2 of 18 have proposed a new method and its control system, which can be used to black start a double-fed induction generator (DFIG) [6]. The North China Electric Power Research Institute attempted to launch a black-start field test using the National Scenery Storage Demonstration Power Station [10]. In general, the current achievements in this field include: (1) the self-start ability of wind turbine based on external power supply [10–12]; (2) the decision making and effect evaluation of black-start scheme for wind farms [13–17]; (3) the voltage and frequency issues that occurred in wind farms during black start process [18–20]. Studies have shown that the following problems should be considered when using wind farms as a black start power supply source: (1) the contradiction between the fluctuating nature of wind power and the high stability requirement of black-start power source [21,22]; (2) the rationality of external power supply configuration [23,24]; (3) the influence on wind turbine body and its isolation system caused by the severe attack of grid-connected power components during black start process [25,26]; (4) the black-start transformation requirements for wind turbine structure, control strategy and wind farm system [2,10,27,28]. There is much research in the field of black-start practice involving wind farms. Sun built a wind farm model and simulated the three stages of a black start of the power grid [2]. The results showed that wind farm participation in a black start is feasible. In order to increase load and minimize interruption time, Bizon studied the possibility of insertion of wind power plants in the hydroelectric plants corridors with black-start capacity [7]; El-Zonkoly proposed a firefly algorithm to find the optimal final sequence of non-black start units restoration, transmission paths and load pick-up sequence [29]; Golshani developed a novel offline restoration planning tool for harnessing wind energy to enhance the resilience of power grid [1]. Based on the National Wind Storage Demonstration Power Station, a wind farm was used as a black-start power supply, and the reasons for the failure of the self-start of the wind turbine in this test were analyzed [10]. A black start test was carried out successfully by the State Grid Jibei Electric Power Co., Ltd. at the above power station [30]. Through a simulation analysis of the wind farm and thermal power plant data under an actual substation, it has been used as a good reference for the black-start application of wind farms [8]. A black start test using wind power generation and diesel generator was completed in Jiangsu Province of China [31]. Based on the current theoretical and practical research status of wind farms as black-start power sources at home and abroad [29,32–35], there is no technical plan that involves a field test of wind farms as a black start power source in an actual power grid. To verify the black-start capability of the wind farm and promote its engineering applications, this paper proposes a black-start scheme based on a PMSG-based wind farm as power source: first, the diesel generator is used as an external supporting power supply for the self-start of a WPU, and after all planned WPUs are running normally, the wind farm with the diesel generator and the SVG is used to black start the simulated auxiliary load in a thermal power plant. Then, the field test of the scheme is realized on an actual wind farm in Jiangsu Province in China, and the detailed test process is analyzed.

2. Black-Start Scheme

2.1. Situation on Site Figure1 shows the topology of a wind farm in Jiangsu. The installed capacity of the wind farm is 100 MW, and there are three collection lines equipped with 50 PMSG-based WPUs with a rated capacity of 2 MW. In addition, there is an SVG with a rated capacity of 0.5 MVar. The electrical distance from a near thermal power plant to the wind farm is 30 kilometers. The installed capacity of the thermal power plant is 9 MW, and the maximum auxiliary power of the single unit in the thermal plant is 300 kW. EnergiesEnergiesEnergies 20172019 2017, ,10,12 10, ,1797, 2144 1797 33 3of ofof 19 1819

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Figure 1. Topological structure diagram of the wind farm. FigureFigure 1. 1. Topological Topological structure structure diagram diagram of of the the wind wind farm. farm. Figure2 shows the electrical topology of the PMSG-based WPU in the wind farm. The cut-in and Figure 2 shows the electrical topology of the PMSG-based WPU in the wind farm. The cut-in and cut-outFigure wind 2 speedsshows the are electrical 2.5 m/s and topology 18 m/s, of respectively. the PMSG-based The cabin WPU load in the of wind the WPU farm. is The powered cut-in by and a cut-out wind speeds are 2.5 m/s and 18 m/s, respectively. The cabin load of the WPU is powered by 400cut-out V power wind supply, speeds andare 2.5 the m/s maximum and 18 instantaneousm/s, respectively. power The of cabin a single load WPU of the load WPU is approximatelyis powered by a 400 V power supply, and the maximum instantaneous power of a single WPU load is approximately 50a 400 kVA. V power The PMSG supply, is connected and the maximum to the box instantaneous transformer throughpower of a a back-to-back single WPU load full-power is approximately converter 50 kVA. The PMSG is connected to the box transformer through a back-to-back full-power converter for50 kVA. power The output. PMSG is connected to the box transformer through a back-to-back full-power converter forfor power power output. output. FullFull Power Power Converter Converter PMSGPMSG

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AccordingAccordingAccording to toto the thethe investigation investigationinvestigation of ofof the thethe site site topology, topology, geographical geographicalgeographical conditions conditionsconditions and andand communication communicationcommunication network,network,network, the thethe three three three WPUs WPUs connected connected toto to collectorcollector collector line line line 3, 3, Y1,3, Y1, Y1, Y3 Y3 Y3 and and and Y5, Y5, Y5, were were were selected selected selected for for testing,for testing, testing, and and and the thetotalthe total total length length length of theof of the powerthe power power line line line was was was 8.5 8.5 kilometers. 8.5 kilometers. kilometers. 2.2. Black-Start System 2.2.2.2. Black-Start Black-Start System System For black start testing, an external power supply, some balanced loads and a black start target are ForFor black black start start testing, testing, an an external external power power supply, supply, some some balanced balanced loads loads and and a ablack black start start target target needed. It is necessary to carry out the corresponding transformations and increase the measuring areare needed. needed. It It is is necessary necessary to to carry carry out out the the corresponding corresponding transformations transformations and and increase increase the the measuring measuring equipment. The ﬁeld test system diagram for the black start is shown in Figure3. equipment.equipment. The The field field test test system system diagram diagram fo for rthe the black black start start is is shown shown in in Figure Figure 3. 3. The equipment used in the black-start system is listed in Table1. The diesel generator, shown as TheThe equipment equipment used used in in the the black-start black-start system system is is li listedsted in in Table Table 1. 1. The The diesel diesel generator, generator, shown shown as as Gd, is used as an external power supply. The minimum power output of a WPU is 300 kW, which Gd,Gd, is is used used as as an an external external power power supply. supply. The The minimum minimum power power output output of of a aWPU WPU is is 300 300 kW, kW, which which cannot be completely consumed by the equivalent load of the wind turbine, shown as M2, in Figure3, cannotcannot be be completely completely consumed consumed by by the the equivalent equivalent lo loadad of of the the wind wind turbine, turbine, shown shown as as M2, M2, in in Figure Figure and the remainder needs to be consumed by the balanced load shown as Lb, which is an electronic load 3,3, and and the the remainder remainder needs needs to to be be consumed consumed by by the the balanced balanced load load shown shown as as Lb, Lb, which which is is an an electronic electronic composed of resistors, inductors and capacitors. However, the Lb should be powered by Gd before loadload composed composed of of resist resistors,ors, inductors inductors and and capacitors. capacitors. However, However, the the Lb Lb should should be be powered powered by by Gd Gd the WPU operates normally. Considering the active and reactive power characteristics of the WPU, beforebefore the the WPU WPU operates operates normally. normally. Considering Considering the the active active and and reactive reactive power power characteristics characteristics of of the the WPU,WPU, this this paper paper selects selects the the capacity capacity of of the the diesel diesel generator generator as as 400 400 kVA, kVA, of of which which 100 100 kVA kVA is is reserved reserved

Energies 2019, 12, 2144 4 of 18 Energies 2017, 10, 1797 4 of 19 forthis safety. paper For selects the same the capacity reason, ofthe the power diesel of generatorthe balanced as 400load kVA, is 1000 of whichkVA. The 100 maximum kVA is reserved auxiliary for powersafety. Forof a the single same thermal reason, thepower power plant of theis 300 balanced kW. Taking load is 1000the kVA.impactful The maximum power into auxiliary account, power the instantaneousof a single thermal power power of the plant simulated is 300 kW.auxiliary Taking load the of impactful the thermal power power into plant, account, shown the instantaneousas La, is 3000 kVA.power of the simulated auxiliary load of the thermal power plant, shown as La, is 3000 kVA.

#3 Feeder Recorder 1 CV1 QF103 G1

T101 T102 QF106 QF102 QF104 M1 Y1 T100 QF101 Gd Recorder 4

Recorder 2 CV2 QF203 T201 G2 QF206 QF202

T202 QF204 Y3 M2

T301 Recorder 3

QF306 QF302 Lb Y5

FigureFigure 3. 3. FieldField test test system system diagram diagram for for black black start. start. Table 1. Equipment list for the black-start test. Table 1. Equipment list for the black-start test. Symbol Equipment Type Symbol Equipment Type Gd diesel generator 400 kW GdLa auxiliary machine diesel generator in thermal power 690 V400/3000 kW kVA Lb balanced load 690 V/1000 kVA La auxiliary machine in thermal power 690 V/3000 kVA G1, G2 PMSG GW115/2000 kW M1,Lb M2 cabin balanced equivalent load load 69040–60 V/1000 kVA kVA CV1, CV2 full-power converter GW115/2000 kW G1, G2 PMSG GW115/2000 kW T100 step-up transformer 500 kVA T101,M1, T201, M2 T301 box-type cabin equivalent transformer load 40–602200 kVAkVA T102, T202 cabin load transformer 90 kVA CV1, CV2 full-power converter GW115/2000 kW QF101 circuit breaker for Gd 400 V–2000 A QF102,T100 QF202 circuit step-up breaker transformer for WPU 690 500 V–2000 kVA A QF103, QF203 circuit breaker for full-power converter 690 V–2000 A T101, T201, T301 box-type transformer 2200 kVA QF104, QF204 circuit breaker for cabin equivalent load 400 V–100 A T102,QF302 T202 circuit cabin load breaker transformer for load 400 90 V–1500 kVA A QF106, QF206, QF306 circuit breaker for box-type transformer 35000 V–100 A QF101 circuit breaker for Gd 400 V–2000 A QF102, QF202 circuit breaker for WPU 690 V–2000 A From Figures1–3, the adjustments are as follows: (1) on the low voltage busbar of the box-type transformer, shownQF103, asQF203 T101, Gd is circuit connected breaker through for full-power a step-up converter transformer, 690 shown V–2000 asA T100; (2) the removal of allQF104, equipment QF204 on the low-voltage circuit breaker busbar for cabin of equivalent the box-type load transformer 400 V–100 that A belongs to Y5, where La and LbQF302 are connected in parallel; circuit (3) thebreaker oscilloscopes for load are installed 400 at V–1500 the outlet A of the grid side of Y1 and Y3, and at the access point of La, Lb, and Gd. QF106, QF206, QF306 circuit breaker for box-type transformer 35000 V–100 A

Energies 2017, 10, 1797 5 of 19

From Figure 1 to Figure 3, the adjustments are as follows: (1) on the low voltage busbar of the box-type transformer, shown as T101, Gd is connected through a step-up transformer, shown as T100; (2) the removal of all equipment on the low-voltage busbar of the box-type transformer that belongs Energies 2019, 12, 2144 5 of 18 to Y5, where La and Lb are connected in parallel; (3) the oscilloscopes are installed at the outlet of the grid side of Y1 and Y3, and at the access point of La, Lb, and Gd. In order order to to synchronize synchronize the the steps, steps, a synchronous a synchronous control control device for device black for start black (SCDBS) start is (SCDBS) developed, is developand theed relationship, and the relationship of the logic of is shownthe logic in is Figure shown4. in The Figure equipment 4. The inequipment the green in block the green belongs block to belongsthe wind to farm, the windthe SCADA farm, the is used SCADA to monitor is used the to parameters monitor the of parameterstransformers, of circuit transformers, breakers, circuit loads breakers,and SVG. loads Then, and the SVG. above Then, parameters the above are par sentameters to SCDBS, are sent where to theSCDBS, monitoring where the and monitoring control system and controlof WPUs system is. The of equipmentWPUs is. The of theequipment thermal of power the thermal plant is power shown plant in the is light-yellow shown in the block, light- andyellow the block,SCADA and here the plays SCADA the samehere plays role as the it doessame inrole the as wind it does farm. in the The wind equipment farm. The in theequipment red block in is the added red blockthrough is added an abovementioned through an abovementioned transformation for transformation the purpose of for changing the purpose the normal of changing grid structure the normal to a gridblack structure start system. to a black start system.

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SCDBS is thethe centralcentral controllercontroller of of the the system, system, which which a blacka black start start strategy strategy is embeddedis embedded in. Afterin. After the theoperating operating information information of theof the whole whole system system is is collected, collected, a a series series of of blackblack startstart operationsoperations will be automatically executed executed according according to to the the above above strategy. strategy. The implementationThe implementation of the deviceof the and device strategy and strategyare described are described in detail in below. detail below. The hardware framework of SCDBS is presented in Figure5. There are four types of interfaces for interacting with external devices and users. Part I is a data acquisition and processing module, which adopts FPGA to control the AD converter and transforms analog quantities into digital quantities. The real time value of voltage, current frequency, phase diﬀerence, active and reactive power can be measured. Part II is designed to communicate with the external devices and users. There are three kinds of communication ports, six Ethernet ports with TCP/IP and PC 104 protocol, three RS-485 port and two RS-232 port with Modbus RTU. The Ethernet ports are used for the communication between SCDBS and SCADAs, monitoring and controlling systems in wind farms and thermal power plant. The remaining Ethernet, RS-485 and RS-232 ports are reserved for external power supply, balanced loads and improved control-nodes. In addition, there is a human machine interaction, the running state, the key parameters and the interface of control instruction are displayed on an LCD screen. The black start operations can be manually performed by user. Part III is a digit input and output module, which monitors the states of circuit breakers and operates them according to the instructions. Part IV is the central processing unit, where a black start strategy is implemented, and a GPS pulse timing is used to synchronize with the system clock. All the above information will be sent to the central processing unit, and then the black start strategy determines the next execution based on the current steps and the obtained information. Figure6 shows the control strategy of SCDBS, it is a general strategy for any step (such as step Figure 5. The hardware framework of SCDBS. N) in the black start process. After the program starts, all the running parameters will be initialized, N is the current step number, Nmax is the last step number of the test. Then, the program performs the corresponding judgment according to the value of N, if the current condition meets the requirements for performing step N, the operation will be executed; and a delay is executed if some parameters or Energies 2017, 10, 1797 5 of 19

Monitoring and #1 WPU #3 WPU #5 WPU Wind Farm Control System { of WPUs Other WPUs

Synchronous Thermal Wind Farm Thermal Power Control Device Power SCADA Plant SCADA for Black Start Plant { { { Circuit Breakers Circuit Circuit Breakers Circuit Balanced Loads External Power Power External Control-Nodes Induced Draft Transformers Transformers Power Plant Key Electric Wind Farm Wind Parameters Generators Generator Improved Improved Static Var Supply Loads Loads Fan Energies 2019, 12, 2144 6 of 18

states beyond the range set at step N. After the delay, the step N is performed if the condition meets the expected one, otherwise the programFigure 4. willThe logic go backrelationship to initialization of SCDBS. and display the corresponding N fault code. FollowingSCDBS is the this central process, controller the of program the system, is wh ﬁnallyich a executedblack start strategy to step is embeddedmax, all prompt in. After message will be displayedthe operating on information the LCD screen of the whole in the system whole is process. collected, In a series conjunction of black withstart operations Figure7, thewill judgmentbe criteriaautomatically in every main executed step will according be introduced to the above in detail. strategy. The implementation of the device and strategy are described in detail below.

Energies 2017, 10, 1797 6 of 19

The hardware framework of SCDBS is presented in Figure 5. There are four types of interfaces for interacting with external devices and users. Part I is a data acquisition and processing module, which adopts FPGA to control the AD converter and transforms analog quantities into digital quantities. The real time value of voltage, current frequency, phase difference, active and reactive power can be measured. Part II is designed to communicate with the external devices and users. There are three kinds of communication ports, six Ethernet ports with TCP/IP and PC 104 protocol, three RS-485 port and two RS-232 port with Modbus RTU. The Ethernet ports are used for the communication between SCDBS and SCADAs, monitoring and controlling systems in wind farms and thermal power plant. The remaining Ethernet, RS-485 and RS-232 ports are reserved for external power supply, balanced loads and improved control-nodes. In addition, there is a human machine interaction, the running state, the key parameters and the interface of control instruction are displayed on an LCD screen. The black start operations can be manually performed by user. Part III is a digit input and output module, which monitors the states of circuit breakers and operates them according to the instructions. Part IV is the central processing unit, where a black start strategy is implemented, and a GPS pulse timing is used to synchronize with the system clock. All the above information will be sentFigure to theFigure central 5. 5.The The processing hardware hardware unit, framework and then of of SCDBS.the SCDBS. black start strategy determines the next execution based on the current steps and the obtained information.

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FigureFigure 6. The6. The control control strategy strategy of ofSCDBS. SCDBS.

Energies 2017, 10, 1797 7 of 19

Figure 6 shows the control strategy of SCDBS, it is a general strategy for any step (such as step N) in the black start process. After the program starts, all the running parameters will be initialized, N is the current step number, Nmax is the last step number of the test. Then, the program performs the corresponding judgment according to the value of N, if the current condition meets the requirements for performing step N, the operation will be executed; and a delay is executed if some parameters or states beyond the range set at step N. After the delay, the step N is performed if the condition meets the expected one, otherwise the program will go back to initialization and display the corresponding fault code. Following this process, the program is finally executed to step Nmax, all prompt message Energieswill be2019 displayed, 12, 2144 on the LCD screen in the whole process. In conjunction with Figure 7, the judgment7 of 18 criteria in every main step will be introduced in detail.

FigureFigure 7. (a 7.) equipment(a) equipment installation installation and system and system debugging, debugging, (b) charging (b) charging for the forprimary the primary equipment, equipment, (c) powering(c) powering Lb, (d) Lb,self-tarting (d) self-tarting Y1, (e) self-tarting Y1, (e) self-tarting Y3, (f) black Y3, (startingf) black La starting La.

2.3.2.3. Black-Start Black-Start Scheme Scheme The proposed black-start scheme consists of eight steps in three stages. (1) For the purpose of a systematic experimental topology and the follow-up data analysis, transformation stage of black-start system is necessary: (A) the topology of power grid should be adjusted, the equipment including generators and loads should be installed, and the measuring instrument needs to be debugged; (2) Since the wind power is acted as a power source in the black start, the self-starting stage of wind farm is essential: (B) the charging of the primary equipment is the first step for the self-start of the WPU because of the existence of preheating, dehumidification and mechanical transmission; (C) then, Lb is energized to avoid the reverse power flowing to Gd; (D) Y1 is Started after the isolated system is running normally, (E) Y3 is started at the same condition with Y1, the both self-start progress are recorded; (F) for the sake of stability, the capacity of the system is increased; (3) due to the complicated Energies 2019, 12, 2144 8 of 18 process, black starting the target load is the last but most important stage: (G) under almost consistent initial conditions, the simulated auxiliary load with different power value is black started in three trials respectively, (H) for the experiment, the system should be split after La is black started: (A) Equipment installation and system debugging In this step, the operation mode adjustment for the black-start system in the wind farm is carried out: the collector line 3 is in cold stand-by, the high voltage side of all WPUs are disconnected from collector line 3 and the low voltage circuit breakers are in the open state. After connecting all the devices listed in Table1, the monitoring power supply, the voltage source and the current source are turned on to ensure the normal operation of the monitoring equipment. The system topology after the completion of debugging is shown in Figure7a. (B) Charging for the primary equipment Before the diesel starts, all the circuit breakers QF101, QF102, QF106, QF206, QF202 and QF306 must be closed. To avoid the circuit breaker tripping or system collapse caused by the magnetizing inrush current of a transformer or charging line, Gd is stepped up from zero in this test. In the process of Gd’s terminal voltage rising from 0 to the rated voltage, the voltage of the transformer and the line also reach their respective rated values. As shown in Figure7b, while the line is red, it is indicating that the line is energized. (C) Powering Lb If the system frequency, the voltage of the transformer and the line reach their respective rated values at this step, circuit breaker QF302 is closed, and then the active power of Lb is gradually increased to 400 kW from 0 at a rate of 10 kW/2 s. At this point, a balancing system with Gd, the transmission line, the transformers and Lb is formed, which is presented in Figure7c. (D) Starting Y1 Similar to the previous step, if the frequency and voltage of each node of the system are operating in the allowable range set at this step, the wind condition is acceptable, and there is no fault reported by the SCADA in wind farm. Circuit breaker QF104 for M1 is closed, and then M1 is energized. Y1 is ready to provide electricity while the “START” command has arrived. Then a start command from SCDBS is sent to Y1, which begins startup and runs at maximum output power at 300 kW. A new balancing system containing one WPU is formed, as shown in Figure7d. (E) Starting Y3 On the basis of normal operation of the above system, the active power of Lb is increased to 800 kW in the same way as step (C). If all parameters such as the system voltage and frequency, the output power of Y1, the temperatures of the equipment in the whole system are normal, Y3 is started in the same way in step (D). A new balancing system involving two WPUs is formed, as presented in Figure7e. (F) Increasing the capacity of the system The whole capacity of the isolated system in step (E) is approximately 800 kW, and Gd runs in power output mode. For the purpose of providing sufficient voltage and frequency support by Gd during the black-start process, the output power of Y1 is adjusted to 300 kW, and the output power of Y3 should be adjusted manually to satisfy that the output power of Gd is close to 0. The above process is executed by SCDBS and monitoring and control system in wind farm. (G) Black starting La The power of La is 0 at a relative time of zero, and the new output limit of the Y1 unit is increased to the rated value of the simulated load in the thermal power plant based on the original operating conditions. When t = 10 s, La is energized, and Lb is gradually removed according to the power Energies 2019, 12, 2144 9 of 18 shortage. If the system voltage or frequency is high, the inductance or resistance of Lb will be increased; and the inductance or resistance of Lb will be reduced if there is a low system voltage or frequency. The voltage, frequency and power of Gd, Y1, Y3, La and Lb are recorded, the system topology is shown in Figure7f. (H) Splitting the system After La is black started, Y1 is stopped first, then the power of La is decreased slowly to 0. In the same fashion, Y3 is stopped and Lb is removed. Finally, Gd is shut down, the system topology is set to the normal operation mode.

3. Analysis of the Test Results Based on the data from the WPU operation log and the recorder, the Lb access, self-start of WPU Y1 and Y3, the black start of the La, and the splitting process are analyzed during the ﬁeld test.

3.1. Lb Access Lb is used to consume the power of the isolated system. Its switching purpose is to maintain the system stability for voltage and frequency and to prevent the reverse power operation of Gd. Before starting WPU Y1 and Y3, it is necessary to create a load in advance, and Lb is divided into two parts considering the capacity of Gd and fuel consumption. The processes of increasing the Lb from 0 to 400 kW is set before self-starting any WPU, and the processes of increasing the Lb from 400 kW to 800 kW is set after self-starting Y1. The rate of increase during the Lb power increase step is 10 kW every two seconds. The main parameters when Lb is connecting are listed in Table2. The actual measured power of Lb after the two inputs are 340 kW and 984 kW, which are lower or higher than the expected value. As shown in Table2, both voltage fluctuation and frequency fluctuation are much severer than the first time.

Table 2. The list of parameters for balance load Lb connecting.

Sequence Parameter Value Parameter Value Parameter Value

Umax 707.5 V Umin 669.65 V Voltage fluctuation 2.95% Part 1 f max 50.71 Hz f min 49.24 Hz Frequency fluctuation 1.52% Qmax 8.6 kVar Q 16.6 kVar min − Umax 718.2 V Umin 629.22 V Voltage fluctuation 8.81% Part 2 f max 52.01 Hz f min 48.391 Hz Frequency fluctuation 4.03% Qmax 22.3 kVar Q 28.1 kVar min −

As shown in Figure8, Lb is gradually increased from 400 to 800 kW before Y3 is self-started. There is an obvious voltage and frequency fluctuation in the system while Lb is increasing and the fluctuation intervals of frequency, voltage, active and reactive power are all larger before and after Lb is increased. Lb consists of resistance, inductance and capacitor, and Ohm’s law is followed by the algorithm of power control system of load bank. The values of Lb are set based on the initial voltage, frequency and the target power. In the process of Lb is increased, the voltage and frequency are also fluctuating. In other words, the impedance of Lb is constant; the voltage and frequency are time variable; the power of Lb will be changed based on Ohm’s law. In the first processes of increasing Lb, Gd only supplies power to Lb, which is small at that time. But in the second processes, Gd not only supplies power to Lb, which is much bigger than before, but also provides voltage and frequency support for Y1. More seriously, Y1 is set to limited power operation mode, and its output power is fluctuating when the wind speed is low, the load increment is undertaken by Gd. That’s to say, when the power consumption of Lb is set to 400 kW and 800 kW respectively, the output power of Gd may be both large than 400 kW. As a result, the stability of the voltage and frequency exceeds the upper limit of Gd, which may be the cause of system fluctuation. Energies 2019, 12, 2144 10 of 18

Energies 2017, 10, 1797 10 of 19 The power shortage is obviously bigger the second time, so the system fluctuation is significantly severer than it is at the first time.

52 700

U/V f/Hz 50 650

48 600 0 100 200 300 400 500 0 100 200 300 400 500 t/s t/s (a) (b) 1200 50

1000 P/kW 800 Q/kVar 0

600

400 -50 0 100 200 300 400 500 0 100 200 300 400 500 t/s t/s (c) (d) Figure 8. (a) system frequency, (b) system voltage, (c) active power, (d) reactive power. Figure 8. (a) system frequency, (b) system voltage, (c) active power, (d) reactive power The above two problems are caused by a shortage of power supply in the system that provides As shown in Figure 8, Lb is gradually increased from 400 to 800 kW before Y3 is self-started. the support of power, voltage and frequency. There are three solutions to the problem: the most direct There is an obvious voltage and frequency fluctuation in the system while Lb is increasing and the and effective way is to configure an external power supply with a larger rated capacity; the most fluctuation intervals of frequency, voltage, active and reactive power are all larger before and after economical and practical way is to choose a wind farm with a larger annual average wind speed as a Lb is increased. black start power plant to increase the output power and stability of the wind turbine; and the best Lb consists of resistance, inductance and capacitor, and Ohm's law is followed by the algorithm way is to develop a wind turbine with the ability to maintain the stability of voltage and frequency. of power control system of load bank. The values of Lb are set based on the initial voltage, frequency At the same time, the maximum value of the fluctuation is also related to the input load power. and the target power. In the process of Lb is increased, the voltage and frequency are also fluctuating. To improve the stability of the system, the single load with a large power should be energized in the In other words, the impedance of Lb is constant; the voltage and frequency are time variable; the initial stage, and then the loads with small power can be connected. power of Lb will be changed based on Ohm’s law. 3.2.In WPU the Self-Startingfirst processes of increasing Lb, Gd only supplies power to Lb, which is small at that time. But in the second processes, Gd not only supplies power to Lb, which is much bigger than before, but also providesAt this stage, voltage the and failure frequency of the WPU support to start for Y1 and. More to suddenly seriously, stop Y1 running is set to after limited a successful power operationstartup both mode occurred., and its Shown output in power Table 3is is fluctuating the main parameter when the listwind for speed six WPU is low, startup the tests.load increment No. 1 and isNo. undertaken 2 are the self-startingby Gd. That’s process to say, of w Y1,hen No. the 3power to No. consumption 6 are the self-starting of Lb is set process to 400 of kW Y3. and After 800 taking kW respectively,11 seconds,83 the seconds output andpower 32 seconds,of Gd may respectively, be both large the self-startingthan 400 kW processes. As a result, at No. the 1,stability No. 2 and of the No. voltage6 are successful. and frequency At a windexceeds speed the greaterupper limit than of 2.5 Gd m/, s,which Y3 is may started be fourthe cause times of and system failed fluctuation. for the first Tthreehe power times, shortage and a grid is obvious side voltagely bigger abnormality the second fault time, is reported. so the system fluctuation is significantly severer than it is at the first time. The above two problems are caused by a shortage of power supply in the system that provides the support of power, voltage and frequency. There are three solutions to the problem: the most direct and effective way is to configure an external power supply with a larger rated capacity; the most economical and practical way is to choose a wind farm with a larger annual average wind speed as a black start power plant to increase the output power and stability of the wind turbine; and the best way is to develop a wind turbine with the ability to maintain the stability of voltage and frequency.

Energies 2019, 12, 2144 11 of 18

Table 3. The list of parameters of wind turbine generator for self-starting process.

Max Fluctuation No. Wind Speed Parameter Max Min Result Ratio(%) U (V) 715.98 689.66 3.80 Last 11 s 1 5.85 m/s I (A) 272.41 // successful f (Hz) 50.60 49.30 1.41(0.71 Hz) U (V) 714.49 652.1 5.60 Last 83 s 2 2.55 m/s I (A) 223.83 // successful f (Hz) 51.13 48.94 2.25(1.1 Hz) U (V) 823.47 568.91 19.34 3 4.26 m/s I (A) 418.65 // Failed f (Hz) 52.39 47.29 5.42(2.7 Hz) U (V) 822.17 565.32 19.16 4 5.91 m/s I (A) 427.09 // Failed f (Hz) 52.739 46.41 7.18(3.6 Hz) U (V) 818.99 562.64 18.70 5 4.53 m/s I (A) 384.13 // Failed f (Hz) 52.35 47.72 4.70(2.4 Hz) U (V) 695.72 640.05 7.24 Last 32 s, 6 3.65 m/s I (A) 231.81 // successful f (Hz) 51.12 49.00 2.24(1.1 Hz)

There is a continuous voltage higher than 818 V in the processes of No. 3 to No. 5. When the power factor of Lb is adjusted to 0.8, and the initial voltage on the grid side is reduced to approximately 670 V, Y3 self-started when the wind speed was 3.65 m/s. The waveforms of frequency, voltage, active and reactive power are shown in Figure9, and the fluctuation range of the main electrical parameters during the self-starting process is shown for No. 6 in Table3. In all successful trials, the maximum frequency and voltage fluctuation value are 7.24% and 2.25%. Benefitting from consulting the WPU operating manual, it is learned that the grid-connected protection of the WPU will be triggered when the grid side voltage continues to be higher than 110% of the rated value for 2 s, the grid side voltage continues to be higher than 115% of the rated value for 0.5 s, or the grid side frequency fluctuation continue to be greater than 105% for 2 s. After a comprehensive analysis of the WPU operating manual, the recorded data and the operating log of the WPUs, the reasons why the WPU failed to self-start or stop running are found. The first three Y3 startup attempts failed due to the internal reported failure. Y1 is turned off after it is started in grid-connected mode since the wind speed is less than the cut-in requirement for an extended period. The failure of No.3 and No.5 are caused by high voltage, test No. 4 failed for both high voltage and low frequency. The high voltage recorded during the Y3 startup process is caused by the capacitive effect of the 8.5 km feeder for the wind farm. From the three successful tests, it can be found that within the cut-in and cut-out wind speed range, the higher the wind speed, the faster the WPU start-up process. Besides, the start-up time has a nonlinear relationship with the wind speed; and the startup process of the WPU is affected by the fixed value of the grid-connected protection. By summarizing and analyzing the startup process of the above six tests, the following solutions can be used to speed up the self-start process or reduce the possibility of failure. Firstly, it is essential to choose a wind farm with a large and stable annual average wind speed when selecting a black start power plant. Secondly, the self-start process may be affected by the WPU protection settings, so it should be calculated precisely and adjusted as needed before the actual black start. Thirdly, in the case of a small system capacity, a long transmission line will increase the stability of the system voltage but affect the startup process of the WPU, and its negative impact can be reduced by connecting an SVG to the isolated system. Energies 2017, 10, 1797 12 of 19 is started in grid-connected mode since the wind speed is less than the cut-in requirement for an extended period. The failure of No.3 and No.5 are caused by high voltage, test No. 4 failed for both high voltage and low frequency. The high voltage recorded during the Y3 startup process is caused by the capacitive effect of the 8.5 km feeder for the wind farm. From the three successful tests, it can be found that within the cut-in and cut-out wind speed range, the higher the wind speed, the faster the WPU start-up process. Besides, the start-up time has a nonlinear relationship with the wind speed; and the startup process of the WPU is affected by the Energies 2019, 12, 2144 12 of 18 fixed value of the grid-connected protection.

51 700

50.5 z

V 680

H /

50 U f

49.5 660

49 640 50 100 150 50 100 150 t/s t/s (c) (b)

200

W V

k 0

/ /

P 100 Q

-50 0

50 100 150 50 100 150 t/s t/s (c) (d) Figure 9. (a) system frequency, (b) system voltage, (c) active power, (d) reactive power. Figure 9. (a) system frequency, (b) system voltage, (c) active power, (d) reactive power. 3.3. Black Starting La By summarizing and analyzing the startup process of the above six tests, the following solutions After the normal operation of the WPUs, three on-site black-start simulation tests have been can be used to speed up the self-start process or reduce the possibility of failure. Firstly, it is essential carried out, and the power used in each experiment are 100, 200 and 300 kW, respectively. Before to choose a wind farm with a large and stable annual average wind speed when selecting a black start each La input, the output power of Y1 is adjusted so that the output active power value of Gd is power plant. Secondly, the self-start process may be affected by the WPU protection settings, so it approximately 200 kW. During the black-start process, the power is regulated by Y1, Y3 is always should be calculated precisely and adjusted as needed before the actual black start. Thirdly, in the caserunning of a in small 500 kW system limited capacity, power a mode. long transmission line will increase the stability of the system Table4 shows the main electrical parameters of the system when 100, 200 and 300 kW simulated voltage but affect the startup process of the WPU, and its negative impact can be reduced by connectingauxiliary loads an SVG are black-startedto the isolated under system. the same initial conditions. To meet the power demand of La quickly, the preadjusted power value of Y1 is greater than the value of La to be started. 3.3. Black Starting La Table 4. The list of parameters for starting simulation auxiliary load. After the normal operation of the WPUs, three on-site black-start simulation tests have been carried out, and the Sequencepower used in each experiment Parameters are 100, 200 and Value300 kW, respectively. Before each La input, the output power of Y1 Poweris adjusted of La (kW) so that the 100 output 200active power 300 value of Gd is approximately 200Initial kW. state During before the blackInitial-start voltage process, (V) the power 690.0 is regulated 710.0 710.0by Y1, Y3 is always black start Preadjusted power (kW) 200 300 400 running in 500 kW limited power mode. Preadjusted time (s) 5.0 5.0 4.5 Max power of G (kW) 280 250 230 d Min power of G (kW) 67 20 5 d − − − Main parameters of Max Voltage (V) 710.4 710.2 710.7 the black start Min voltage (V) 620.2 629.1 650.2 process Max frequency (Hz) 52.06 51.43 51.09 Min frequency (Hz) 48.31 48.57 49.15 Startup duration (s) 1.5 2.2 3.2 Energies 2019, 12, 2144 13 of 18

The process for the WPU output power occurs after the action of the mechanical parts such as yaw and pitch, usually takes a few seconds. Therefore, it is necessary to adjust the power change value according to the actual response speed of the wind turbine before black starting the thermal power plant simulation auxiliary machine. On the basis of checking the power change response of the wind turbine and the self-starting of the wind farm, the coordinated control process of the wind farm participating in the black start is as follows: (1) the program running in the control device for the wind farm black start is initialized; (2) an output power adjustment value to the WPU monitoring system is sent by the abovementioned device; (3) the wind farm black-start device starts counting down after receiving the correct information from the wind turbine monitoring feedback; (4) a load input command is sent to the monitoring system for the auxiliary machine in the thermal power plant by the above device after receiving the response of the wind turbine and the countdown is over. As shown in Figure 10, the initial voltage of the system is 690 V, the power limit of Y1 has been changed for 5 seconds before putting La into the power system, and the black-start process is performed onEnergies an auxiliary 2017, 10, 1797 load of 100 kW. The black start duration is about 2.3 seconds, the output power change14 of 19 of Y1 takes 7.5 seconds, and the changed power is about 250 kW.

720 52

51 700

/ U f 50 680 49

48 660 0 10 20 30 0 10 20 30 t/s t/s (c) (b) 600 300

200 500 adjustment

k /

/ process of P P Y1 100 400

0 300 0 10 20 30 0 10 20 30 t/s t/s (c) (d) Figure 10. ((a) system frequency, ((b)) systemsystem voltage,voltage, ((cc)) activeactive powerpower ofof Gd,Gd, ((dd)) activeactive powerpower ofof Y1.Y1.

AtAs theshown beginning in Figure of black10, the start, initial the voltage voltage, of frequency the system and is output690 V, the power power of Ld limit have of experiencedY1 has been achanged short-term for fluctuation. 5 seconds beforeIt is worth putting noting La that into the the amplitude power system, of the grid and side the voltage black-start is less process than the is protectionperformed value on an of auxiliary 621 V,and load it lasts of 100 for approximatelykW. The black two start cycles. duration Gd hasis about a reverse 2.3 powerseconds of, 5the kilowatts output whichpower lastchange about of 5Y1 milliseconds, takes 7.5 seconds the maximum, and the andchange minimumd power frequency is about 25 in0 the kW. black-start process are 52.1 HzAt the and beginning 48.4 Hz, respectively. of black start, The the output voltage, power frequency of Y1 and lag behindoutput power the power of Ld consumption have experienced of La, thea short power-term consumption fluctuation. of It Lais worth is almost noting supplied that the by amplitude Gd. of the grid side voltage is less than the protectionBecause value of the of extremely 621 V, and transient it lasts fluctuation for approximately time, neither two Y1 cycles. nor Gd Gd are has off -grid.a reverse And power the reverse of 5 powerkilowatts of Gdwhich can last be reducedabout 5 milliseconds by adjusting, t thehe maximum output power and andminimum presetting frequenc timey of in Y1. the Toblack prevent-start process are 52.1Hz and 48.4Hz, respectively. The output power of Y1 lag behind the power consumption of La, the power consumption of La is almost supplied by Gd. Because of the extremely transient fluctuation time, neither Y1 nor Gd are off-grid. And the reverse power of Gd can be reduced by adjusting the output power and presetting time of Y1. To prevent the falling voltage of the system from exceeding the protection value during the black-start process, the initial voltage can be preset higher. And the longer preset time of Y1 can make a contribution to the power supply of Gd during La is black started.

Energies 2019, 12, 2144 14 of 18 the falling voltage of the system from exceeding the protection value during the black-start process, the initial voltage can be preset higher. And the longer preset time of Y1 can make a contribution to the power supply of Gd during La is black started. Before starting the second test, the initial voltage is raised to 710 V, the presetting time of Y1 has been changed to 8 s. As shown in Figure 11, it is the black-start process when La is 200 kW. The voltage andEnergies frequency 2017, 10, 1797 ﬂuctuations have improved compared to the ﬁrst attempt. The black start duration15 of is19 about 3.4 s, the output power change of Y1 takes 7.5 seconds, and the changed power is about 380 kW.

51.5 720

51 700 50.5

680

/ H

/ 50 U f 660 49.5 640 49 620 48.5 0 10 20 30 0 10 20 30 t/s t/s (c) (b) 700 300 600 adjustment

200 process of

k /

/ 500 Y1 P 100 P 400 0 300 0 10 20 30 0 10 20 30 t/s t/s (c) (d) Figure 11. (a) system frequency, ((bb)) systemsystem voltage,voltage, ((cc)) activeactive powerpower ofof Gd,Gd, ((dd)) activeactive powerpower ofof Y1.Y1.

TheBefore output starting power the ﬂuctuationssecond test, the of Gd initial have voltage also decreased, is raised to but 710 it V experiences, the presetting two time short of periods Y1 has ofbeen reverse changed power; to 8 also, seconds because. As ofshown its short in Figure holding 11 time,, it is therethe black is no-start oﬀ-network process when occurs. La The is 200 inverse kW. powerThe voltage value and of Gd frequency is related fluctuations to the pre-adjustment have improved power compared value and to preset the first time attempt. for Y1, andThe theblack longer start theduration preset is time about and 3.4 the seco largernds the, the pre-adjustment output power power, change the of greater Y1 takes the possibility7.5 seconds of, theand appearance the change ofd reversepower is power. about Since380 kW the. power of La is increased, the maximum value of the reverse power is smaller than theThe previousoutput power one. fluctuations of Gd have also decreased, but it experiences two short periods of reverseUnder power; a premise also, that because the rated of its capacity short holding of Gd, the time, wind there farm is andno off its-network wind speed occurs. conditions The inverse have beenpower determined, value of Gd the is solutionrelated to can the be pre studied-adjustment as a multi-objective power value optimization, and preset time as shownfor Y1, inand Equation the longer (1): the preset time and the larger the pre-adjustment power, the greater the possibility of the appearance  min g(t , P , P ) of reverse power. Since the power of La is increased,preset the maximumchange La value of the reverse power is  tpreset A,Pchange B,PLa C,  ∈ ∈ ∈ smaller than the previous one. s.t. ∆ fsystem a  ≤ Under a premise that the rated capacity of Gd, the wind farm and its wind speed conditions(1)  ∆Usystem b  |≤ have been determined, the solution canPreverse be studiedc as a multi-objective optimization, as shown in  ≤ Equation (1):  E d Gd ≤

 ming ( tpreset , P change , P La ) tpreset A,,, P change  B P La  C

s. t . | fsystem | a   ||Ubsystem (1)  Pc  reverse  Ed  Gd

where tpreset, Pchange, PLa are the preset time of Y1, the output power change of Y1, rated power of La respectively, and △fsystem, △Usystem are the maximum fluctuation of system frequency and voltage, respectively, Preverse, EGd are the maximum reverse power and energy consumption of Gd, respectively.

Energies 2019, 12, 2144 15 of 18

where tpreset, Pchange, PLa are the preset time of Y1, the output power change of Y1, rated power of La respectively, and ∆fsystem, ∆Usystem are the maximum fluctuation of system frequency and voltage, respectively, Preverse, EGd are the maximum reverse power and energy consumption of Gd, respectively. A, B, C are the set interval of tpreset, Pchange, PLa, respectively. a, b are the upper limit of absolute value of ∆fsystem and ∆Usystem, c, d are the upper limit of Preverse, EGd, respectively. The functions g(tpreset, Pchange, PLa) can be built based on the need for black start. It should be noted that g(tpreset, Pchange, PLa) is closely correlated to the system topology and its parameters. The initial voltage of the system is also set to 710 V. Figure 12 shows the black-start process when LaEnergies is 300 2017 kW., 10, To 1797 reduce the reverse power of Gd, La is black started after the power limit of16 Y1 of is 19 changed over 4.5 s. At the moment of the La black starts, the voltage fluctuation of the system is large andA, B the, C outputare the powerset interval of Gd of experiences tpreset, Pchange, a P shortLa, respectively period of oscillation.. a, b are the The upper black limit start of durationabsolute isvalue about of 4△ s,fsystem the outputand △ powerUsystem, c change, d are the of Y1upper takes limit 9.2 of s, andPreverse the, EGd changed, respectively power. The is about function 460s kW. g(tpreset There, Pchange is also, PLa) acan slight be Gdbuilt reverse based poweron the ofneed about for approximately black start. It should5 kW. Asbe noted the power that ofg(t Y1preset increases,, Pchange, PLa the) isoutput closely − powercorrelated fluctuation to the system of Gdtends topology to be and stable. its parameters.

51.5 720 51 700

50.5 680

/ H

/ 660

50 U f 49.5 640 49 620 48.5 600

t/s t/s (c) (b) 800 300 700

200 W

W 600 adjustment

/ P P process of Y1 100 500 400 0 300 0 10 20 30 0 10 20 30 t/s t/s

(c) (d) FigureFigure 12. 12.( (aa)) systemsystem frequency,frequency, ( b(b)) system system voltage, voltage, ( c()c) active active of of Gd, Gd, ( d(d)) active active power power of of Y1. Y1.

SummarizingThe initial voltage the above of the threesystem black-start is also set to processes, 710 V. Figure the following 12 shows the conclusions black-start can process be drawn: when (1)La the is 300 output kW. of To the reduce WPUs the relatively reverse lagspower the of rapid Gd, powerLa is black demand started of the after black the start.power The limit power of Y1 for is thechanged black startover is4.5 mainly seconds. provided At the moment by the diesel of the generator. La black Instart others, the words, voltage the fluctuation external power of the supply system providesis large and both the voltage output and power frequency of Gd experiences references for a short the system period and of oscillation. provides the The main black power start supportduration foris about the system 4 seconds black, the start. output (2) During power the change black of start, Y1 thetakes magnitude 9.2 seconds, of the and reverse the change powerd ofpower the external is about power460 kW supply. Ther ise is related also a to slight factors Gd such reverse as its power initial of output about power, approximately the power –5 adjustment kW. As the value power and of the Y1 presetincreases, time the of WPUs,output andpower the fluctuation black start time.of Gd (3)tends The to smaller be stable. the output power adjustment value of WPU,Summarizing the shorter the the response above three time. black The- slowerstart processes, the black-start the following process, conclusions the smaller can the be voltage drawn: and (1) frequencythe output impact of the ofWPUs the system. relatively lags the rapid power demand of the black start. The power for the black start is mainly provided by the diesel generator. In other words, the external power supply provides both voltage and frequency references for the system and provides the main power support for the system black start. (2) During the black start, the magnitude of the reverse power of the external power supply is related to factors such as its initial output power, the power adjustment value and the preset time of WPUs, and the black start time. (3) The smaller the output power adjustment value of WPU, the shorter the response time. The slower the black-start process, the smaller the voltage and frequency impact of the system.

Energies 2019, 12, 2144 16 of 18

3.4. Splitting Process To verify the support of the external power supply to the system, the output power of Y1 and Y3 is adjusted so that the output power of Gd is close to 0 after the black-start process is completed, and then Gd is shut down. After querying the operation log of the WPUs, it is known that the system voltage is pulled low when the Gd is shut down, and a network side failure is reported by the control systems of Y1 and Y3 and then the WPUs are stopped. In response to the above problems, the following three aspects are some effective solutions. Firstly, a wind turbine with the ability to maintain the stability of voltage and frequency, which produces power according to the demand of the system, is the most ideal solution. Secondly, there are trade-offs when making a black start plan, and the weight coefficients of the multi-objective optimization should be considered cautiously. Thirdly, when the auxiliary power of the black-start thermal power plant is large, it is recommended to use a variable frequency start or soft start to reduce the impact on the system; and, when the power of the thermal power plant is large, multiple WPUs can be used to participate in the black start to speed up the black start process and reduce the power shortage. Through many investigations, it is known that most of the operating WPUs do not have the ability to stabilize voltage and frequency. Therefore, in order to promote the application of wind farms to participate in a black start, a new technology such as a virtual synchronous machine can be used to study the WPU body.

4. Conclusions In this paper, a black-start scheme in which the PMSG-based wind farm is taken as black-start power supply source is formulated: First, diesel generator is used as external supporting power supply for a WPU self-start. After all planned WPUs operate normally, the wind farm with the diesel generator and SVG is used to black start the simulated auxiliary load of thermal power plant. A ﬁeld test of the proposed scheme is carried out on an actual wind farm in Jiangsu Province in China. Through the analysis of the test results, the following conclusions can be drawn:

(1) A wind farm conﬁgured with an external power supply, balanced load and coordinated controls has black-start capability. (2) The self-starting and black starting are susceptible to wind speed and system disturbances. The slower the black-start process, the smaller the voltage and frequency impact of the system. (3) The external power supply provides voltage and frequency support and main power for the black start of the power grid. (4) The output of the WPUs relatively lags the rapid power demand of the black start. The power of the black start is mainly provided by the external power supply. (5) The magnitude of the reverse power of the external power supply is related to factors such as its initial output power, the power adjustment value and the preset time of WPUs.

5. Future Studies (1) Poor voltage and frequency stability, high real-time requirements and quick system status changes are observed during the wind farms’ participation in the black-start process, so coordinated control strategies and devices to synchronize the steps and control the transients need to be further studied. (2) When the auxiliary power of the black-start thermal power plant is large, the soft start technology and frequency conversion devices can be used as an eﬀective means to improve the stability of the black-start system and to minimize the conﬁguration capacity of the external power supply. (3) It is known that most of operating WPUs do not have the ability to stabilize voltage and frequency. Therefore, in order to promote the application of wind farms to participate in a black start, a new technology such as a virtual synchronous machine could be used to study the WPU body. Energies 2019, 12, 2144 17 of 18

Author Contributions: Y.T. and M.T. conceived and designed the study; M.T. and C.Z. analyzed the data, performed the experiments and wrote the paper; C.Z. provide some actual wind power parameters and checked the results of this work. All the authors read and approved the final manuscript. Funding: This work is supported by the National Natural Science Foundation of China (NSFC) (Grant No. 51877037). Conflicts of Interest: The authors declare no conflict of interest.

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