WO2016033769A1 - Method and system for coordinating control of wind farm during disconnection to utility grid - Google Patents

Method and system for coordinating control of wind farm during disconnection to utility grid Download PDF

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Publication number
WO2016033769A1
WO2016033769A1 PCT/CN2014/085909 CN2014085909W WO2016033769A1 WO 2016033769 A1 WO2016033769 A1 WO 2016033769A1 CN 2014085909 W CN2014085909 W CN 2014085909W WO 2016033769 A1 WO2016033769 A1 WO 2016033769A1
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WO
WIPO (PCT)
Prior art keywords
wind
wind turbines
cable
wind turbine
turbines
Prior art date
Application number
PCT/CN2014/085909
Other languages
French (fr)
Inventor
Yao Chen
Xing Huang
Original Assignee
Abb Technology Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Abb Technology Ltd filed Critical Abb Technology Ltd
Priority to PCT/CN2014/085909 priority Critical patent/WO2016033769A1/en
Priority to CN201480081567.4A priority patent/CN106662073B/en
Publication of WO2016033769A1 publication Critical patent/WO2016033769A1/en

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    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F03—MACHINES OR ENGINES FOR LIQUIDS; WIND, SPRING, OR WEIGHT MOTORS; PRODUCING MECHANICAL POWER OR A REACTIVE PROPULSIVE THRUST, NOT OTHERWISE PROVIDED FOR
    • F03D—WIND MOTORS
    • F03D7/00—Controlling wind motors 
    • F03D7/02—Controlling wind motors  the wind motors having rotation axis substantially parallel to the air flow entering the rotor
    • F03D7/028—Controlling wind motors  the wind motors having rotation axis substantially parallel to the air flow entering the rotor controlling wind motor output power
    • F03D7/0284—Controlling wind motors  the wind motors having rotation axis substantially parallel to the air flow entering the rotor controlling wind motor output power in relation to the state of the electric grid
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F03—MACHINES OR ENGINES FOR LIQUIDS; WIND, SPRING, OR WEIGHT MOTORS; PRODUCING MECHANICAL POWER OR A REACTIVE PROPULSIVE THRUST, NOT OTHERWISE PROVIDED FOR
    • F03D—WIND MOTORS
    • F03D7/00—Controlling wind motors 
    • F03D7/02—Controlling wind motors  the wind motors having rotation axis substantially parallel to the air flow entering the rotor
    • F03D7/04—Automatic control; Regulation
    • F03D7/042—Automatic control; Regulation by means of an electrical or electronic controller
    • F03D7/048—Automatic control; Regulation by means of an electrical or electronic controller controlling wind farms
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F03—MACHINES OR ENGINES FOR LIQUIDS; WIND, SPRING, OR WEIGHT MOTORS; PRODUCING MECHANICAL POWER OR A REACTIVE PROPULSIVE THRUST, NOT OTHERWISE PROVIDED FOR
    • F03D—WIND MOTORS
    • F03D9/00—Adaptations of wind motors for special use; Combinations of wind motors with apparatus driven thereby; Wind motors specially adapted for installation in particular locations
    • F03D9/20—Wind motors characterised by the driven apparatus
    • F03D9/25—Wind motors characterised by the driven apparatus the apparatus being an electrical generator
    • F03D9/255—Wind motors characterised by the driven apparatus the apparatus being an electrical generator connected to electrical distribution networks; Arrangements therefor
    • F03D9/257—Wind motors characterised by the driven apparatus the apparatus being an electrical generator connected to electrical distribution networks; Arrangements therefor the wind motor being part of a wind farm
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/18—Arrangements for adjusting, eliminating or compensating reactive power in networks
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H02J3/381—Dispersed generators
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F05—INDEXING SCHEMES RELATING TO ENGINES OR PUMPS IN VARIOUS SUBCLASSES OF CLASSES F01-F04
    • F05B—INDEXING SCHEME RELATING TO WIND, SPRING, WEIGHT, INERTIA OR LIKE MOTORS, TO MACHINES OR ENGINES FOR LIQUIDS COVERED BY SUBCLASSES F03B, F03D AND F03G
    • F05B2240/00—Components
    • F05B2240/90—Mounting on supporting structures or systems
    • F05B2240/95—Mounting on supporting structures or systems offshore
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F05—INDEXING SCHEMES RELATING TO ENGINES OR PUMPS IN VARIOUS SUBCLASSES OF CLASSES F01-F04
    • F05B—INDEXING SCHEME RELATING TO WIND, SPRING, WEIGHT, INERTIA OR LIKE MOTORS, TO MACHINES OR ENGINES FOR LIQUIDS COVERED BY SUBCLASSES F03B, F03D AND F03G
    • F05B2240/00—Components
    • F05B2240/90—Mounting on supporting structures or systems
    • F05B2240/96—Mounting on supporting structures or systems as part of a wind turbine farm
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F05—INDEXING SCHEMES RELATING TO ENGINES OR PUMPS IN VARIOUS SUBCLASSES OF CLASSES F01-F04
    • F05B—INDEXING SCHEME RELATING TO WIND, SPRING, WEIGHT, INERTIA OR LIKE MOTORS, TO MACHINES OR ENGINES FOR LIQUIDS COVERED BY SUBCLASSES F03B, F03D AND F03G
    • F05B2270/00—Control
    • F05B2270/10—Purpose of the control system
    • F05B2270/107—Purpose of the control system to cope with emergencies
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J2300/00—Systems for supplying or distributing electric power characterised by decentralized, dispersed, or local generation
    • H02J2300/10—The dispersed energy generation being of fossil origin, e.g. diesel generators
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J2300/00—Systems for supplying or distributing electric power characterised by decentralized, dispersed, or local generation
    • H02J2300/20—The dispersed energy generation being of renewable origin
    • H02J2300/28—The renewable source being wind energy
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H02J3/388—Islanding, i.e. disconnection of local power supply from the network
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/70—Wind energy
    • Y02E10/72—Wind turbines with rotation axis in wind direction
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/70—Wind energy
    • Y02E10/727—Offshore wind turbines
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/70—Wind energy
    • Y02E10/76—Power conversion electric or electronic aspects
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
    • Y02E40/30—Reactive power compensation

Definitions

  • the invention relates to a method and system for coordinating control of the wind turbines of a wind farm during its disconnection to a utility grid.
  • an independent emergency source of electrical power is to be provided, which should be able to take over the supply of the emergency consumers, e. g. lighting and important communications and signalling systems, etc. , when the main power supply fails; and the supply time period should meet the requirement from e. g. several hours to several days for different loads.
  • Offshore wind farm is one typical offshore application which is expected to grow in a steady pace and the installed power is expected to increase.
  • the electrical system design should also comply with the requirements for emergency auxiliary load supply, including navigation lights,sensor and communication apparatus, ventilation and heating equipment etc. , when the whole wind farm is disconnected from the onshore grid, e. g. in the circumstance of export cable failure.
  • patent EP 2 503 146 describes method and its arrangement for operating an electric energy production facility (wind farm) during a fault in a utility grid to which energy is to be delivered.
  • the energy production facility includes at least one wind turbine.
  • the method includes disconnecting the utility grid from the wind turbine.
  • the method further includes connecting an external generator via a cable to an auxiliary equipment of the wind turbine, to deliver active power to the auxiliary equipment.
  • the cable is connected to a converter of the wind turbine where the converter is capable of supporting reactive power. Since the generated reactive power can be absorbed by the converter connected to the cable by appropriately switching one or more power transistors, so that the absorption of generated reactive power by the external generator can be reduced, thus reduction in size and costs of the external generator can be enabled, and hence the safe operation of the external generator can be enabled.
  • the converter is adapted to consume reactive power generated by application of a voltage to the cable, so that the reactive power is consumed more easily and in a more cost-effective manner using the converter instead of using the external generator or installing other compensation equipment such as shunt reactors.
  • Patent EP 2 236 821 describes a method and a system for island operation of at least two wind turbines associated with a wind farm, wherein said wind farm is configured for providing power generated by wind turbines in said wind farm to a main grid and wherein the method comprises: detecting at least two or more deactivated wind turbines in said wind farm, said deactivated wind turbines being disconnected from said main grid;configuring at least one islanded local grid for electrically connecting said two or more deactivated wind turbines; activating at least one of said deactivated wind turbine using a black start operation; and, connecting said at least one activated wind turbine and at least one of said deactivated wind turbines to said local grid, said activated wind turbine acting as a power supply for said at least one deactivated wind turbine connected to said local grid.
  • patent EP 2 236 821 does not discuss the scenario where generators of a multiple of wind turbines act as auxiliary equipment power supply, and alternatively does an external generator as well, which thus induces a technical problem of over supply or insufficient supply with respect to the work load.
  • each of the wind turbines includes auxiliary equipment, a transformer unit, a converter and a generator, the transformer unit has a first winding circuit electrically connectable to input of the auxiliary equipment, a second winding circuit electrically connectable to the converter via a third switch, a third winding circuit electrically connectable to the respective one of at least one cable via the respective second switch, and the converter is electrically connectable to the generator via a fourth switch; and the control system,
  • Figure 1 schematically illustrates a wind farm according to an embodiment of present invention
  • Figure 2 is a flow chart illustrating the hybrid APS solution for the wind farm according to figure 1;
  • Figure 3 shows a flowchart on the system controller obtaining the frequency and voltage references f * /V * for the diesel generator according to an embodiment of present invention
  • Figure 4 shows block diagram for diesel generator’ slocal control according to an embodiment of present invention.
  • FIG. 1 schematically illustrates a wind farm according to an embodiment of present invention.
  • the wind farm 1 includes a plurality of wind turbines 10-15, 20-25, 30-35, a substation 4, at least one cable 50, 51, 52 and a control system 6.
  • Each of the cables 50, 51, 52 is electrically connectable to a utility grid via a first switch 7, for example a circuit breaker.
  • the first switch 7 can be opened where a fault in the utility grid occurred.
  • the wind turbines are arranged in three groups, where group G1 consists of the wind turbines 10-15, group G2 consists of the wind turbines, 20-25, and group G3 consists of the wind turbines 30-35.
  • Each of the wind turbines 10-15 of group G1 is electrically connectable to the cable 50 via respective one of second switches 800-805
  • each of the wind turbines 20-25 of group G2 is electrically connectable to the cable 51 via respective one of second switches 810-815
  • each of the wind turbines 30-35 of group G3 is electrically connectable to the cable 52 via respective one of second switches 820-825.
  • the skilled person shall understand that the number of cable can be equal or above one, and the wind turbines can be arranged in one or more group to deliver energy to the one or more cables.
  • the wind farm 1 can include a bus bar 8 arranged between the first switch 7 and the respective cable 50, 51, 52 and electrically connecting the first switch 7 and the respective cable 50, 51, 52.
  • a bus bar 8 arranged between the utility grid and the first switch 7, in the substation 4 one or more transformers may be arranged to transform a voltage supplied by the wind turbines to a higher voltage, and the first switch 7 can be arranged in the substation 4 as well.
  • Each of wind turbines 10-15, 20-25, 30-35 includes a generator 100-150, 200-250, 300-350, a converter 101-151, 201-251, 301-351, auxiliary equipment 102-152, 202-252, 302-352,and a transformer unit 103-153, 203-253, 303-353.
  • the transformer unit103-153, 203-253,303-353 has a first winding circuit electrically connectable to input of the auxiliary equipment 102-152, 202-252, 302-352, a second winding circuit electrically connectable to the converter 101-151, 201-251, 301-351 via a third switch 104-154, 204-254, 304-354, a third winding circuit electrically connectable to the respective one of at least one cable via the respective second switch 800-805, 810-815, 820-825, and the converter is electrically connectable to the generator via a fourth switch 105-155, 205-255, 305-355.
  • the respective generator 100-150, 200-250, 300-350 can supply energy to the respective converter 101-151, 201-251, 301-351.
  • Each converter 101-151, 201-251, 301-351 converts a generally variable frequency energy stream delivered from the generator 100-150, 200-250, 300-350 to a fixed frequency energy stream supplied to the respective one of the cable 50, 51, 52.
  • the fixed frequency energy stream supplied to the respective one of the cable 50, 51, 52 is supplied via the respective third switch 104-154, 204-254, 304-354, the respective transformer unit 103-153, 203-253, 303-353 and the respective second switch 800-805, 810-815, 820-825.
  • the wind farm 1 can also include an external generator 9 located inside the substation 4,which can be electrically connectable to the cable 50, 51, 52 via a fifth switch 2 and the respective bus bar 8.
  • the external generator can be for example a diesel generator.
  • the control system 6 can control to switch the on/off state of the second switch 800-805,810-815, 820-825, the third switch 104-154, 204-254, 304-354, the fourth switch 105-155,205-255, 305-355, and the fifth switch 2.
  • the control system 6 can send command to the switches so that they act switch accordingly, either from on to off or vice versa.
  • the second switch, the third switch, the fourth switch and the fifth switch can be for example circuit breaker.
  • energy can be delivered from the respective one of the wind turbines 10-15, 20-25, 30-35 to the utility grid via the respective closed second switch 800-805, 810-815, 820-825, the respective closed third switch 104-154, 204-254, 304-354, the respective closed fourth switch 105-155, 205-255, 305-355, and the respective one of the cables 50, 51, 52, and the closed first switch 7.
  • the cables 50, 51, 52 comprise distributed capacitance which may result in generation of reactive power.
  • the effect of the distributed capacitance is denoted by capacitor C besides the respective cable.
  • the first switch 7 can be opened such that the wind turbines 10-15, 20-25, 30-35 are electrically disconnected to the utility grid.
  • the circuit breaker automatically opens to protect an electrical circuit from damage caused by overload or short circuit.
  • all of the second switches 800-805,810-815, 820-825connecting the wind turbines 10-15, 20-25, 30-35 to the cables 50, 51, 52 will be opened as well. So can be the third switch 104-154, 204-254, 304-354 and the fourth switch 105-155, 205-255, 305-355.
  • either the diesel generator9or both the diesel generator 9 and thewind turbine 10-15, 20-25, 30-35 can be started up to supply the auxiliary loads of the wind turbines 10-15, 20-25, 30-35 in the wind farm 1 during islanding operation caused by utility grid failure.
  • the selection of the power source to the auxiliary equipment of the wind turbines can be controlled by the control system 6 commanding different on/off status of the second switch 800-805, 810-815, 820-825, the third switch 104-154, 204-254,304-354, the fourth switch 105-155, 205-255, 305-355, and the fifth switch 2.
  • the control system 6 selects the respective diesel generator 9 as the power source to the auxiliary equipment of the wind turbines. As soon as the diesel generator 9 is in operation, the control system 6 can send command to close the fifth switch2 between the diesel generator 9 and the respective cable 50, 51, 52. After that, the control system 6 controls the second switch 800-805, 810-815,820-825 to be closed and active power generated by the diesel generator 9 flows through the cable 50, 51, 52. The active power flowing through the cable 50, 51, 52 is supplied to the auxiliary equipment 102-152, 202-252, 302-352 of the respective wind turbines 10-15,20-25, 30-35.
  • the control system 6 can also select one of the deactivated wind turbines 10-15, 20-25, 30-35 and instructs the selected wind turbine to initiate a start-up procedure, for example wind turbine 22 is selected.
  • start-up procedures are well known in the art.
  • the control system 6 connects the wind turbine 22 to the cable 51 by closing the second switch812 and the third switch 224 and the fourth switch 225of the wind turbine 22. Further, the power generation of wind turbine 22 is gradually increased until stable operation is achieved.
  • the control system 6 connects theauxiliary equipment 202 of the deactivated wind turbine 20 to the respective cable 51 by closing the second switch 810of the deactivated wind turbine 20;in a similar way, the control system 6 connects the auxiliary equipment of the other deactivated wind turbines to the local grid, which is powered by the diesel generator 9 and the operational wind turbine 22.
  • the above two operations of supplying auxiliary load by the diesel generator 9 and by the activated wind turbine can be coordinated by the control system 6 and thus the load for auxiliary equipment of the wind turbines is cooperatively supported by the external generator and the selected wind turbine.
  • the hybrid auxiliary power supply solution is described by embodiment hereafter.
  • the hybrid Auxiliary Power Supply (APS) system includes both diesel generator 9 and distributed wind turbine generators 10-15, 20-25, 30-35, the control system 6 for receiving measurements and sending control references. It should be noticed that the hybrid APS system should be applicable to different type of transmission technology for offshore wind farm, e. g. high voltage AC transmission, high voltage DC transmission, etc. Below are listed different connection modes of the wind turbine generators.
  • Mode l only auxiliary loads are connected to the collection grid here as the cable 50, 51,52, which only play as a load in the islanding system.
  • the control system 6 controls the respective second switch 800-805, 810-815, 820-825 to be closed, thus the active power that flows on the respective cable 50, 51, 52 is supplied to the respective auxiliary equipment 102-152, 202-252, 302-352 via the respective transformer unit 103-153, 203-253, 303-353.
  • Mode 2 both wind converter and the auxiliary loads are connected, which play as a reactive power source in the islanding system.
  • the wind turbine 10 is selected to operate in mode 2 by closing the second switch 800 and the third switch 104.
  • the active power generated by the diesel generator 9 and/or the active power generated by another wind turbine, for example wind turbine22 flows through the cable 50, 51, 52, thereby generating reactive power.
  • the active power flowing through the cable is supplied to the auxiliary equipment 102being electrically connected to the cable 50.
  • the reactive power,or at least a portion of the reactive power generated due to the cable 50, 51, 52 is fed to the converter 101of the wind turbine 10 (in Mode 2) by the closedsecond switch 800 and the third switch 104.
  • the converter 101 consumes the reactive power generated due to flow of energy through the cable 50, 51, 52.
  • the diesel generator may be designed for moderate nominal capability. In particular, the diesel generatordoes not need to support reactive power.
  • Mode 3 wind turbine generator, wind converter and auxiliary loads are connected to the collection grid, which further play as active/reactive power source in the islanding system.
  • the wind turbine 20 is selected to operate in Mode 3 by closing the second switch 810, the third switch 204, and the forth switch 205.
  • the active power generated by the diesel generator 9 and/or the active power generated by the wind turbine20 flows through the cable 50, 51, 52,thereby generating reactive power.
  • the active power flowing through the cable is supplied to the auxiliary equipment 202 of the wind turbine 20.
  • the reactive power, or at least a portion of the reactive power generated due to the cable 50, 51, 52 is fed to the converter 201of the wind turbine 20 (in Mode 3) by the closedsecond switch 810 and the third switch 204.
  • the converter 201 consumes the reactive power generated due to flow of energy through the cable 50, 51, 52.
  • the diesel generator may be designed for moderate nominal capability. In particular, the diesel generator does not need to support reactive power. Plus, the diesel generator can be operated with less fuel consumption,because it supplies less energy to the auxiliary load of the wind farm.
  • At least one of the wind turbine generator 100-150, 200-250, 300-350 can be selected to take care of large part of active power and wind turbine converter 101-151, 201-251, 301-351 can be selected to supply the reactive power output; while diesel generator 9 only needs to contribute small part of active/reactive power to provide a power source to wind turbine generators; and
  • wind turbine converterl01-151, 201-251, 301-351 can be selected to take care of large part of reactive power output; while diesel generator 9 need to not only to provide a power source to wind turbine converters,but also take care of large part of active power.
  • the input factors include acceptable frequency/voltage variation range, wind speed, etc.
  • the possible outputs include power/voltage references for diesel generator, active/reactive power commands for group of wind turbine generators,etc. ;
  • the wind turbine generators should be selective switched on, to take care of the active/reactive power commands allocated by the first tier coordination and meanwhile achieve optimal efficiency within wind turbine generators and the collection grid;
  • the input factors include generator type, location, wind turbine generator condition, etc.
  • the possible outputs include on/off commands of the corresponding switches and active/reactive power commands for multiple wind turbine generators, etc.
  • Table I shows the input/output definition of the control system 6 of hybrid AP S solution according to figure 1.
  • Figure 2 is a flow chart illustrating the hybrid APS solution for the wind farm according to figure 1.
  • the control system 6 can run the calculation method according to figure 2. It shows that when the system power supply is not available and the hybrid APS solution is enabled, the system controller 6 will start to calculate the P/Q (P: active power, Q: reactive power) references for diesel generator 9 and the wind turbine 10-15, 20-25, 30-35, and further allocate the P/Q references to individual wind turbine 10-15, 20-25, 30-35 accordingly.
  • P/Q P: active power
  • Q reactive power
  • Figure 3 shows a flowchart on how the system controller 6 obtain the frequency and voltage references f * /V * for the diesel generator.
  • the limits of active/reactive power output of the diesel generator [P DG_min , P DG_max ] and [Q DG_min , Q DG_max ] will be either defined by the wind farm operator directly, or determined based on the acceptable frequency/voltage range of island system [f min , f max ] and [V min , V max ] , diesel generator safety operation range, fuel storage conditions, etc.
  • the measured active/reactive power output from the diesel generator P DG /Q DG will be compared with the limits respectively.
  • the frequency and voltage references f * /V * will be adjusted accordingly ⁇ For example, if the measured active power output P DG is higher than the limit P DG_max , the frequency reference f * will be reduced so as to reduce P DG ; if the measured reactive power Q DG is higher than the limit Q DG_max , the voltage reference V * will be reduced so as to reduce Q DG .
  • the diesel generator will do local control as illustrated in Figure 4, where P DG will be regulated through the close-loop governor control, Q DG will be regulated through close-loop excitation voltage (indicated by V f in figure 4) control.
  • the incremental active power reference for group of wind turbinesfor time instant t ⁇ P wind (t) can be calculatedaccording to the active power output from diesel generator P DG (t) , and the limits of active power output of diesel generator [P DG_min , P DG_max ] , as shown in equation (1) , which means that, if the active power output of diesel generator exceeds the limit, the incremental active power reference for group of wind turbines will be calculated to compensate the differences, otherwise the incremental active power will kept unchanged.
  • the total active power reference for group of wind turbines for time instant t, P wind (t) can be calculated by adding up the incremental active power ⁇ P wind (t) and the total output active power for time instant t-1,P wind (t-1) , and the limits of the available active power from wind [P wind_min (t) , P wind_max (t) ]which can be calculated in a real-time manner by using wind speed, the design data and the conditions of the wind turbines, as shown in equation (2) .
  • the incremental reactive power reference for group of wind turbinesfor time instant t ⁇ Q wind (t) can be calculatedaccording to the reactive power output from diesel generator Q DG (t) , and the limits of reactive power output of diesel generator [Q DG_min ,Q DG_max ] , as shown in equation (3) , which means that, ifthe reactive power output of diesel generator exceeds the limit, the incremental reactive power reference for group of wind turbines will be calculated to compensate the differences, otherwise the incremental reactive power will kept unchanged. .
  • the total reactive power reference for group of wind turbines for time instant t Q wind (t) can be calculated by adding up the incremental reactive power ⁇ Q wind (t) and the total output reactive power for time instant t-1 Q wind (t-1) , and the limits of the available reactive power from wind converter [Q wind_min (t) , Q wind_max (t) ] which can be calculated by the design data and the conditions of the wind turbine generators, as shown in equation (4) .
  • the following four equations illustrate one way of calculating ⁇ P wind (t) , P wind (t) , ⁇ Q wind (t)and Q wind (t) .
  • control system 6 After the control system 6 calculates the total active and reactive power references for the wind turbine 10-15, 20-25, 30-35, it will further execute allocationlogic to determine the connection mode and the control commands for individual wind turbine 10-15, 20-25, 30-35.
  • Wind turbine connection mode determination should consider following criteria:
  • each of the wind turbine groups Gl, G2 and G3 working as connection Mode 2.
  • Each of the wind turbines 10-15 of group G1 is electrically connectable to the cable 50 via respective one of second switches 800 -805
  • each of the wind turbines 20-25 of group G2 is electrically connectable to the cable 51 via respective one of second switches 810-815
  • each of the wind turbines 30-35 of group G3 is electrically connectable to the cable 52 via respective one of second switches 820-825.
  • the location of the selected wind turbine should be as close to the substation as possible, i.
  • the selected wind turbine has the shortest electrical distance to the substation 4 out of the wind turbine group; secondly, the selected wind turbine has the capability of reactive power control; and thirdly, the selected wind turbine can operate normally, which means no failure occurs inside wind turbine which will make it unable to work.
  • the reason for selecting wind turbine this way is to ensure the most effective cable charging power compensation. According the typical power flow distribution of offshore wind farms, the closer to the substation, the higher the cable charging power will be. This way of selection can ensure that the largest portion of charging power can be absorbed locally, thus ensure the highest effectiveness.
  • control system 6 can select the wind turbines 10, 20, 30 who are electrically close to the substation 4 and accordingly close the respective second switch 800, 810, 820 and the respective third switch 104, 204, 304. Consequently, the active power that flows on the respective cable 50,51, 52 can be supplied to the respective auxiliary equipment 102, 202, 302 of the selected wind turbines via the respective transformer unit 103, 203, 303.
  • the selection of the wind turbines can vary from selection of some of the wind turbines 10, 20,30, for example wind turbines 10, 30 and accordingly close the respective second switch 800, 820 and the respective third switch 104, 304, to selection of the wind turbines 10, 20,30 and wind turbine in the remaining which is as close as possible to the substation 4, for example the wind turbines 10, 20, 30 together with wind turbines 11, 31 and accordingly close the respective second switch 800, 810, 820, 801, 821 and the respective third switch 104, 204, 304, 114, 314.
  • the control system 6 can choose the wind turbine 20 out of the selected wind turbines 10, 20, 30 and accordingly close the fourth switch 205. Since the number of the Mode 3 wind turbine generator is depending on the required total active power P wind (t) ,and also the redundancy requirement for system frequency reliability consideration, the choice of the wind turbines can vary from choice one of the selected wind turbines 10, 20,30 to full choice of them. The skilled person shall understand the wind turbine for supplying active power can also be chosen from those except for the selected one. For example, the control system 6 chooses the wind turbine 21 by closing the second switch 811, the third switch 214 and the fourth switch 215.
  • control system 6 selects the wind turbines 10, 20, 30, chooses wind turbine 20 from the selected and closes the respective second switch 800, 810, 820 and the respective third switch 104, 204, 304 and the fourth switch 205, consequently the control system 6 controls the respective second switch 801-805, 811-815, 821-825 to be closed, thus the active power that flows on the respective cable 50, 51, 52 can be supplied to the respective auxiliary equipment 112-152, 212-252, 312-352 via the respective transformer unit 113-153, 213-253, 313-353.
  • the control system 6 can control to distribute the total reactive power Q wind (t) among all wind turbinesworking as Mode 2 and Mode 3 equally. For example, if wind turbine 10, 20,30 are selected to work as Mode 2 and Mode 3, the reactive power control command for each wind turbine is equal to Q wind (t) /3. Another way is to allocate the reactive power control command proportionally to their available capacity of reactive power, to avoid overloading of wind turbine, as illustrated in equation (5) , where Q wind1 , Q wind2 , Q wind3 represent the reactive power control command for wind turbine 10, 20, 30; Q ava1 , Q ava2 ,Q ava3 represent the available reactive power capacity of wind turbine 10, 20, 30.
  • control system 6 can control to distribute the total active power P wind (t) among all wind turbinesworking as Mode 3 equally. For example, if wind turbine 10, 20 are selected to work as Mode 3, the active power control command for each wind turbine is equal to P wind (t) /2. Another way is to allocate the active power control command proportionally to their available capacity of active power, to avoid overloading of wind turbine, as illustrated in equation (6) , where P wind1 , P wind2 represent the active power control command for wind turbine 10, 20; P ava1 , P ava2 represent the available active power capacity of wind turbine 10, 20.
  • connection mode and the control commands After the connection mode and the control commands are determined, it is up to the local controller of wind turbine generator to follow and keep tracking of the references.

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Abstract

A method and a system for coordinating control of wind turbines of a wind farm during a fault in a utility grid to which energy is to be delivered via at least one cable and via a substation of the wind farm are provided. The method comprises: disconnecting the utility grid from the wind turbines; electrically connecting auxiliary equipment of at least one group of the wind turbines via respective one of at least one cable; selecting at least one wind turbine electrically close to the substation of the wind farm in the respective one of the at least one group of wind turbines; and absorbing reactive power generated on the at least one cable by converter of the selected at least one wind turbine. Said method and system ensure the most effective cable charging power compensation.

Description

METHOD AND SYSTEM FOR COORDINATING CONTROL OF WIND FARM DURING DISCONNECTION TO UTILITY GRID Technical Field
The invention relates to a method and system for coordinating control of the wind turbines of a wind farm during its disconnection to a utility grid.
Background Art
It has been well defined that all offshore installations/units， an independent emergency source of electrical power is to be provided， which should be able to take over the supply of the emergency consumers， e. g. lighting and important communications and signalling systems， etc. ， when the main power supply fails； and the supply time period should meet the requirement from e. g. several hours to several days for different loads. Offshore wind farm is one typical offshore application which is expected to grow in a steady pace and the installed power is expected to increase. The electrical system design should also comply with the requirements for emergency auxiliary load supply， including navigation lights，sensor and communication apparatus， ventilation and heating equipment etc. ， when the whole wind farm is disconnected from the onshore grid， e. g. in the circumstance of export cable failure.
The concept of using diesel generator for supplying the auxiliary loads has been disclosed in patent EP 1 752 659. However， in this circumstance， the charging power (capacitive reactive power) from the cable array needs to be absorbed by the diesel generator， which will lead to the oversizing problem of the diesel generator. In addressing the technical problem， patent EP 2 503 146 describes method and its arrangement for operating an electric energy production facility (wind farm) during a fault in a utility grid to which energy is to be delivered. The energy production facility includes at least one wind turbine.The method includes disconnecting the utility grid from the wind turbine. The method further includes connecting an external generator via a cable to an auxiliary equipment of the wind turbine， to deliver active power to the auxiliary equipment. The cable is connected to a converter of the wind turbine where the converter is capable of supporting reactive power. Since the generated reactive power can be absorbed by the converter connected to the cable by appropriately switching one or more power transistors， so that the absorption of generated reactive power by the external generator can be reduced， thus reduction in size and costs of the external generator can be enabled， and hence the safe operation of the external generator can be enabled. The converter is adapted to consume reactive power generated by application of a voltage to the cable， so that the reactive power is consumed more easily and in a more cost-effective manner using the converter instead of using the external generator or installing other compensation equipment such as shunt reactors. However， it has not been addressed for the scenario where the wind farm consists of a multiple of wind turbines and any of their converters is able to absorb the reactive power generated on the cable， the effect of the reactive power compensation varies according to various selection of the wind turbine converter in terms of their electrical distance to the substation. Selection of the reactive power absorbing converter among those of the multiple of wind turbines without considering their relative effectiveness in compensating the cable generated reactive power， as disclosed by patent EP 2 503 146，may cause problem of less effective cable charging reactive power compensation.
Patent EP 2 236 821 describes a method and a system for island operation of at least two wind turbines associated with a wind farm， wherein said wind farm is configured for providing power generated by wind turbines in said wind farm to a main grid and wherein the method comprises： detecting at least two or more deactivated wind turbines in said wind farm， said deactivated wind turbines being disconnected from said main grid；configuring at least one islanded local grid for electrically connecting said two or more deactivated wind turbines； activating at least one of said deactivated wind turbine using a black start operation； and， connecting said at least one activated wind turbine and at least one of said deactivated wind turbines to said local grid， said activated wind turbine acting as a power supply for said at least one deactivated wind turbine connected to said local grid. However， patent EP 2 236 821 does not discuss the scenario where generators of a multiple of wind turbines act as auxiliary equipment power supply， and alternatively does an external generator as well， which thus induces a technical problem of over supply or insufficient supply with respect to the work load.
Brief Summary of the Invention
It is therefore an objective of the invention to provide a method for coordinating control of wind turbines of a wind farm during a fault in a utility grid to which energy is to be delivered via at least one cable and via a substation of the wind farm， wherein the wind turbines are arranged in at least one group， comprising： disconnecting the utility grid from the wind turbines； electrically connecting auxiliary equipment of at least one group of the wind turbines via respective one of at least one cable； selecting at least one wind turbine electrically close to a substation of the wind farm in the respective one of the at least one group of wind turbines； and absorbing reactive power generated on the at least one cable by converter of thc selected at least one wind turbine.
According to another aspect of present invention， it provides coordination system for controlling wind turbines of a wind farm during a fault in a utility grid to which energy is to be delivered via at least one cable and via a substation of the wind farm， comprising： a first switch， being arranged between the utility grid and each of the wind turbines and being adapted for being opened where a fault in the utility grid occurred； at least one second switch， being arranged between the respective one of the wind turbines of the respective one of the at least one wind turbine group and the respective one of the at least one cable； and a control system； wherein： each of the wind turbines includes auxiliary equipment， a transformer unit， a converter and a generator， the transformer unit has a first winding circuit electrically connectable to input of the auxiliary equipment， a second winding circuit electrically connectable to the converter via a third switch， a third winding circuit electrically connectable to the respective one of at least one cable via the respective second switch， and the converter is electrically connectable to the generator via a fourth switch； and the control system， being adapted for following an opening of the first switch，selecting the wind turbine electrically close to the substation in the respective one of the at least one group of wind turbines and closing the third switch of the selected wind turbine so that the converter of the selected wind turbine absorbs reactive power generated on the at least one cable.
By having the method and system of present invention， the most effective cable charging power compensation is ensured.
Brief Description of the Drawings
The subject matter of the invention will be explained in more detail in the following text with reference to preferred exemplary embodiments which are illustrated in the drawings，in which：
Figure 1 schematically illustrates a wind farm according to an embodiment of present invention；
Figure 2 is a flow chart illustrating the hybrid APS solution for the wind farm according to figure 1；
Figure 3 shows a flowchart on the system controller obtaining the frequency and voltage references f*/V*for the diesel generator according to an embodiment of present invention；and
Figure 4 shows block diagram for diesel generator’ slocal control according to an embodiment of present invention.
The reference symbols used in the drawings， and their meanings， are listed in summary form in the list of reference symbols. In principle， identical parts are provided with the same reference symbols in the figures.
Preferred Embodiments of the Invention
Figure 1 schematically illustrates a wind farm according to an embodiment of present invention. As shown in figure 1， the wind farm 1 includes a plurality of wind turbines 10-15， 20-25， 30-35， a substation 4， at least one  cable  50， 51， 52 and a control system 6. Each of the  cables  50， 51， 52 is electrically connectable to a utility grid via a first switch 7， for example a circuit breaker. The first switch 7 can be opened where a fault in the utility grid occurred. The wind turbines are arranged in three groups， where group G1 consists of the wind turbines 10-15， group G2 consists of the wind turbines， 20-25， and group G3 consists of the wind turbines 30-35.
Each of the wind turbines 10-15 of group G1 is electrically connectable to the cable 50 via respective one of second switches 800-805， each of the wind turbines 20-25 of group G2 is electrically connectable to the cable 51 via respective one of second switches 810-815，and each of the wind turbines 30-35 of group G3 is electrically connectable to the cable 52 via respective one of second switches 820-825. The skilled person shall understand that the number of cable can be equal or above one， and the wind turbines can be arranged in one or more group to deliver energy to the one or more cables. Where there is more than one  cable  50， 51， 52， the wind farm 1 can include a bus bar 8 arranged between the first switch 7 and the  respective cable  50， 51， 52 and electrically connecting the first switch 7 and the  respective cable  50， 51， 52. Between the utility grid and the first switch 7， in the substation 4 one or more transformers may be arranged to transform a voltage supplied by the wind turbines to a higher voltage， and the first switch 7 can be arranged in the substation 4 as well.
Each of wind turbines 10-15， 20-25， 30-35 includes a generator 100-150， 200-250， 300-350，a converter 101-151， 201-251， 301-351， auxiliary equipment 102-152， 202-252， 302-352，and a transformer unit 103-153， 203-253， 303-353. The transformer unit103-153， 203-253，303-353 has a first winding circuit electrically connectable to input of the auxiliary equipment 102-152， 202-252， 302-352， a second winding circuit electrically connectable to the converter 101-151， 201-251， 301-351 via a third switch 104-154， 204-254， 304-354， a third winding circuit electrically connectable to the respective one of at least one cable via  the respective second switch 800-805， 810-815， 820-825， and the converter is electrically connectable to the generator via a fourth switch 105-155， 205-255， 305-355. The respective generator 100-150， 200-250， 300-350 can supply energy to the respective converter 101-151， 201-251， 301-351. Each converter 101-151， 201-251， 301-351 converts a generally variable frequency energy stream delivered from the generator 100-150， 200-250， 300-350 to a fixed frequency energy stream supplied to the respective one of the  cable  50， 51， 52. The fixed frequency energy stream supplied to the respective one of the  cable  50， 51， 52 is supplied via the respective third switch 104-154， 204-254， 304-354， the respective transformer unit 103-153， 203-253， 303-353 and the respective second switch 800-805， 810-815， 820-825.
The wind farm 1 can also include an external generator 9 located inside the substation 4，which can be electrically connectable to the  cable  50， 51， 52 via a fifth switch 2 and the respective bus bar 8. The external generator can be for example a diesel generator.
The control system 6 can control to switch the on/off state of the second switch 800-805，810-815， 820-825， the third switch 104-154， 204-254， 304-354， the fourth switch 105-155，205-255， 305-355， and the fifth switch 2. For example， the control system 6 can send command to the switches so that they act switch accordingly， either from on to off or vice versa. The second switch， the third switch， the fourth switch and the fifth switchcan be for example circuit breaker.
During normal operation， energy can be delivered from the respective one of the wind turbines 10-15， 20-25， 30-35 to the utility grid via the respective closed second switch 800-805， 810-815， 820-825， the respective closed third switch 104-154， 204-254， 304-354， the respective closed fourth switch 105-155， 205-255， 305-355， and the respective one of the  cables  50， 51， 52， and the closed first switch 7.
The  cables  50， 51， 52 comprise distributed capacitance which may result in generation of reactive power. The effect of the distributed capacitance is denoted by capacitor C besides the respective cable.
During a fault condition in the utility grid， the first switch 7 can be opened such that the wind turbines 10-15， 20-25， 30-35 are electrically disconnected to the utility grid. For example， the circuit breaker automatically opens to protect an electrical circuit from damage caused by overload or short circuit. Further， all of the second switches 800-805，810-815， 820-825connecting the wind turbines 10-15， 20-25， 30-35 to the  cables  50， 51， 52 will be opened as well. So can be the third switch 104-154， 204-254， 304-354 and the fourth switch 105-155， 205-255， 305-355.
Following that， generally speaking， either the diesel generator9or both the diesel generator 9 and thewind turbine 10-15， 20-25， 30-35 can be started up to supply the auxiliary loads of the wind turbines 10-15， 20-25， 30-35 in the wind farm 1 during islanding operation caused by utility grid failure. The selection of the power source to the auxiliary equipment of the wind turbines can be controlled by the control system 6 commanding different on/off status of the second switch 800-805， 810-815， 820-825， the third switch 104-154， 204-254，304-354， the fourth switch 105-155， 205-255， 305-355， and the fifth switch 2.
For example， after formation of the islanded wind farm 1， the control system 6 selects the respective diesel generator 9 as the power source to the auxiliary equipment of the wind turbines. As soon as the diesel generator 9 is in operation， the control system 6 can send command to close the fifth switch2 between the diesel generator 9 and the  respective cable  50， 51， 52. After that， the control system 6 controls the second switch 800-805， 810-815，820-825 to be closed and active power generated by the diesel generator 9 flows through the  cable  50， 51， 52. The active power flowing through the  cable  50， 51， 52 is supplied to  the auxiliary equipment 102-152， 202-252， 302-352 of the respective wind turbines 10-15，20-25， 30-35.
As an alternative， after the diesel generator is connected to the  respective cable  50， 51， 52，the control system 6 can also select one of the deactivated wind turbines 10-15， 20-25， 30-35 and instructs the selected wind turbine to initiate a start-up procedure， for example wind turbine 22 is selected. Such start-up procedures are well known in the art. The control system 6 connects the wind turbine 22 to the cable 51 by closing the second switch812 and the third switch 224 and the fourth switch 225of the wind turbine 22. Further， the power generation of wind turbine 22 is gradually increased until stable operation is achieved. The control system 6 connects theauxiliary equipment 202 of the deactivated wind turbine 20 to the respective cable 51 by closing the second switch 810of the deactivated wind turbine 20；in a similar way， the control system 6 connects the auxiliary equipment of the other deactivated wind turbines to the local grid， which is powered by the diesel generator 9 and the operational wind turbine 22.
The above two operations of supplying auxiliary load by the diesel generator 9 and by the activated wind turbine can be coordinated by the control system 6 and thus the load for auxiliary equipment of the wind turbines is cooperatively supported by the external generator and the selected wind turbine. The hybrid auxiliary power supply solution is described by embodiment hereafter.
A) Control system architecture of hybrid APS solution
As shown in figure 1， the hybrid Auxiliary Power Supply (APS) system includes both diesel generator 9 and distributed wind turbine generators 10-15， 20-25， 30-35， the control system 6 for receiving measurements and sending control references. It should be noticed that the hybrid APS system should be applicable to different type of transmission technology for offshore wind farm， e. g. high voltage AC transmission， high voltage DC transmission， etc. Below are listed different connection modes of the wind turbine generators.
Mode l： only auxiliary loads are connected to the collection grid here as the  cable  50， 51，52， which only play as a load in the islanding system. For example， the control system 6 controls the respective second switch 800-805， 810-815， 820-825 to be closed， thus the active power that flows on the  respective cable  50， 51， 52 is supplied to the respective auxiliary equipment 102-152， 202-252， 302-352 via the respective transformer unit 103-153， 203-253， 303-353.
Mode 2： both wind converter and the auxiliary loads are connected， which play as a reactive power source in the islanding system. For example， the wind turbine 10 is selected to operate in mode 2 by closing the second switch 800 and the third switch 104. The active power generated by the diesel generator 9 and/or the active power generated by another wind turbine， for example wind turbine22， flows through the  cable  50， 51， 52， thereby generating reactive power. The active power flowing through the cable is supplied to the auxiliary equipment 102being electrically connected to the cable 50. The reactive power，or at least a portion of the reactive power generated due to the  cable  50， 51， 52 is fed to the converter 101of the wind turbine 10 (in Mode 2) by the closedsecond switch 800 and the third switch 104. Thereby， the converter 101 consumes the reactive power generated due to flow of energy through the  cable  50， 51， 52. Thereby， the diesel generator may be designed for moderate nominal capability. In particular， the diesel generatordoes not need to support reactive power.
Mode 3： wind turbine generator， wind converter and auxiliary loads are connected to the collection grid， which further play as active/reactive power source in the islanding system.For example， the wind turbine 20 is selected to operate in Mode 3 by closing the second  switch 810， the third switch 204， and the forth switch 205. The active power generated by the diesel generator 9 and/or the active power generated by the wind turbine20 (or together with another wind turbine， for example wind turbine 11) flows through the  cable  50， 51， 52，thereby generating reactive power. The active power flowing through the cable is supplied to the auxiliary equipment 202 of the wind turbine 20. The reactive power， or at least a portion of the reactive power generated due to the  cable  50， 51， 52 is fed to the converter 201of the wind turbine 20 (in Mode 3) by the closedsecond switch 810 and the third switch 204. Thereby， the converter 201 consumes the reactive power generated due to flow of energy through the  cable  50， 51， 52. Thereby， the diesel generator may be designed for moderate nominal capability. In particular， the diesel generator does not need to support reactive power. Plus， the diesel generator can be operated with less fuel consumption，because it supplies less energy to the auxiliary load of the wind farm.
B) Coordinated control method for hybrid APS solution
Considering the wind condition， two scenarios need to be considered in the control solution of hybrid APS solution：
When the utility grid fails and wind is blowing， at least one of the wind turbine generator 100-150， 200-250， 300-350 can be selected to take care of large part of active power and wind turbine converter 101-151， 201-251， 301-351 can be selected to supply the reactive power output； while diesel generator 9 only needs to contribute small part of active/reactive power to provide a power source to wind turbine generators； and
When the utility grid fails and wind is not blowing， wind turbine converterl01-151， 201-251， 301-351 can be selected to take care of large part of reactive power output； while diesel generator 9 need to not only to provide a power source to wind turbine converters，but also take care of large part of active power.
To realize these functionalities， two-tier coordinated control is needed with objective and required signals are elaborated as follows：
First tier coordination between diesel generator9 and wind turbine 10-15， 20-25， 30-35： of which the objective is to balance the active and reactive power while still maintaining the minimum fuel consumption； the input factors include acceptable frequency/voltage variation range， wind speed， etc. ， the possible outputs include power/voltage references for diesel generator， active/reactive power commands for group of wind turbine generators，etc. ；
Secondly tier coordination among multiple wind turbinesl0-15， 20-25， 30-35： Since the total capacity of the wind turbine generators of an offshore wind farm is far larger than the auxiliary loads， the wind turbine generators should be selective switched on， to take care of the active/reactive power commands allocated by the first tier coordination and meanwhile achieve optimal efficiency within wind turbine generators and the collection grid； the input factors include generator type， location， wind turbine generator condition， etc. ， the possible outputs include on/off commands of the corresponding switches and active/reactive power commands for multiple wind turbine generators， etc.
Table I shows the input/output definition of the control system 6 of hybrid AP S solution according to figure 1.
Figure PCTCN2014085909-appb-000001
Table I
Figure 2 is a flow chart illustrating the hybrid APS solution for the wind farm according to figure 1. The control system 6 can run the calculation method according to figure 2. It shows that when the system power supply is not available and the hybrid APS solution is enabled， the system controller 6 will start to calculate the P/Q (P： active power， Q： reactive power) references for diesel generator 9 and the wind turbine 10-15， 20-25， 30-35， and further allocate the P/Q references to individual wind turbine 10-15， 20-25， 30-35 accordingly.
Three steps of the figure 2， diesel generatorcontrol reference calculation， wind turbine control reference calculation， wind turbine connection mode determination and control reference allocation are explained in detail hereafter.
Step： Diesel Generatorcontrol reference calculation
Figure 3 shows a flowchart on how the system controller 6 obtain the frequency and voltage references f*/V*for the diesel generator. Firstly， the limits of active/reactive power output of the diesel generator [PDG_min， PDG_max] and [QDG_min， QDG_max] will be either defined by the wind farm operator directly， or determined based on the acceptable frequency/voltage range of island system [fmin， fmax] and [Vmin， Vmax] ， diesel generator safety operation range， fuel storage conditions， etc. Secondly， the measured active/reactive power output from the diesel generator PDG/QDG will be compared with the limits respectively. If the differences between the measured values and the given limits exceed certain threshold，the frequency and voltage references f*/V*will be adjusted accordingly。For example， if the measured active power output PDG is higher than the limit PDG_max， the frequency reference f*will be reduced so as to reduce PDG； if the measured reactive power QDG is higher than the limit QDG_max， the voltage reference V*will be reduced so as to reduce QDG.
With the f*/V*references， the diesel generator will do local control as illustrated in Figure 4， where PDG will be regulated through the close-loop governor control， QDG will be regulated through close-loop excitation voltage (indicated by Vf in figure 4) control.
Step： wind turbine control reference calculation
The incremental active power reference for group of wind turbinesfor time instant t ΔPwind (t) can be calculatedaccording to the active power output from diesel generator PDG (t) ， and the limits of active power output of diesel generator [PDG_min， PDG_max] ， as shown in equation (1) ， which means that， if the active power output of diesel generator exceeds the limit， the incremental active power reference for group of wind turbines will be calculated to compensate the differences， otherwise the incremental active power will kept unchanged.
After obtaining the incremental active power reference， the total active power reference for group of wind turbines for time instant t， Pwind (t) can be calculated by adding up the incremental active power ΔPwind (t) and the total output active power for time instant t-1，Pwind (t-1) ， and the limits of the available active power from wind [Pwind_min (t) ， Pwind_max (t) ]which can be calculated in a real-time manner by using wind speed， the design data and the conditions of the wind turbines， as shown in equation (2) .
Similarly， the incremental reactive power reference for group of wind turbinesfor time instant tΔQwind (t) can be calculatedaccording to the reactive power output from diesel generator QDG (t) ， and the limits of reactive power output of diesel generator [QDG_min，QDG_max] ， as shown in equation (3) ， which means that， ifthe reactive power output of diesel generator exceeds the limit， the incremental reactive power reference for group of wind turbines will be calculated to compensate the differences， otherwise the incremental reactive power will kept unchanged. . After obtaining the incremental reactive power reference， the total reactive power reference for group of wind turbines for time instant t Qwind (t) can be calculated by adding up the incremental reactive power ΔQwind (t) and the total output reactive power for time instant t-1 Qwind (t-1) ， and the limits of the available reactive power from wind converter [Qwind_min (t) ， Qwind_max (t) ] which can be calculated by the design data and the conditions of the wind turbine generators， as shown in equation (4) .The following four equations illustrate one way of calculating ΔPwind (t) ， Pwind (t) ， ΔQwind (t)and Qwind (t) .
Figure PCTCN2014085909-appb-000002
Figure PCTCN2014085909-appb-000003
Figure PCTCN2014085909-appb-000004
Figure PCTCN2014085909-appb-000005
Step： wind turbine connection mode determination and control reference allocation
After the control system 6 calculates the total active and reactive power references for the wind turbine 10-15， 20-25， 30-35， it will further execute allocationlogic to determine the connection mode and the control commands for individual wind turbine 10-15， 20-25， 30-35.
Wind turbine connection mode determination should consider following criteria：
Select at least one from the each of the wind turbine groups Gl， G2 and G3 working as connection Mode 2. Each of the wind turbines 10-15 of group G1 is electrically connectable to the cable 50 via respective one of second switches 800 -805， each of the wind turbines 20-25 of group G2 is electrically connectable to the cable 51 via respective one of second switches 810-815， and each of the wind turbines 30-35 of group G3 is electrically connectable to the cable 52 via respective one of second switches 820-825. The location of the selected wind turbine should be as close to the substation as possible， i. e.firstly， the selected wind turbine has the shortest electrical distance to the substation 4 out of the wind turbine group； secondly， the selected wind turbine has the capability of reactive power control； and thirdly， the selected wind turbine can operate normally， which means no failure occurs inside wind turbine which will make it unable to work. The reason for selecting wind turbine this way is to ensure the most effective cable charging power compensation. According the typical power flow distribution of offshore wind farms， the closer to the substation， the higher the cable charging power will be. This way of selection can ensure that the largest portion of charging power can be absorbed locally， thus ensure the highest effectiveness. Furthermore， comparing with the method that use every possible wind turbine for cable charging power compensation， this way of selection actually aggregate the reactive power demand to a limited number of wind turbines， and thus ensure a more reasonable operating point of wind turbine converters. For example， the control system 6 can select the  wind turbines  10， 20， 30 who are electrically close to the substation 4 and accordingly close the respective  second switch  800， 810， 820 and the respective third switch 104， 204， 304. Consequently， the active power that flows on the  respective cable  50，51， 52 can be supplied to the respective  auxiliary equipment  102， 202， 302 of the selected wind turbines via the  respective transformer unit  103， 203， 303. Since the number of the Mode 2 wind turbine generator is depending on the required total reactive power Qwind (t) ，and also the redundancy requirement for system voltage reliability consideration， the selection of the wind turbines can vary from selection of some of the  wind turbines  10， 20，30， for  example wind turbines  10， 30 and accordingly close the respective second switch 800， 820 and the respective third switch 104， 304， to selection of the  wind turbines  10， 20，30 and wind turbine in the remaining which is as close as possible to the substation 4， for example the  wind turbines  10， 20， 30 together with  wind turbines  11， 31 and accordingly close the respective  second switch  800， 810， 820， 801， 821 and the respective third switch 104， 204， 304， 114， 314.
Choose at least one available wind turbine out of the selected ones working as connection Mode 2， which to be operated as Mode 3. By doing this way， firstly the capacity of wind turbine converters for both active power and reactive power output can be fully utilized；secondly， the total number of wind turbines activated during the islanding operation of the  offshore wind farm can be limited to reduce the control complexity of the offshore wind farm. For example， the control system 6 can choose the wind turbine 20 out of the selected  wind turbines  10， 20， 30 and accordingly close the fourth switch 205. Since the number of the Mode 3 wind turbine generator is depending on the required total active power Pwind (t) ，and also the redundancy requirement for system frequency reliability consideration， the choice of the wind turbines can vary from choice one of the selected  wind turbines  10， 20，30 to full choice of them. The skilled person shall understand the wind turbine for supplying active power can also be chosen from those except for the selected one. For example， the control system 6 chooses the wind turbine 21 by closing the second switch 811， the third switch 214 and the fourth switch 215.
Otherwind turbinesexcept for the selected and chosen ones are working as Mode 1. For example， the control system 6 selects the  wind turbines  10， 20， 30， chooses wind turbine 20 from the selected and closes the respective  second switch  800， 810， 820 and the respective third switch 104， 204， 304 and the fourth switch 205， consequently the control system 6 controls the respective second switch 801-805， 811-815， 821-825 to be closed， thus the active power that flows on the  respective cable  50， 51， 52 can be supplied to the respective auxiliary equipment 112-152， 212-252， 312-352 via the respective transformer unit 113-153， 213-253， 313-353.
P/Q control commands determination for wind turbine generator working as Mode 2 and Mode 3 should consider following criteria：
The control system 6 can control to distribute the total reactive power Qwind (t) among all wind turbinesworking as Mode 2 and Mode 3 equally. For example， if  wind turbine  10， 20，30 are selected to work as Mode 2 and Mode 3， the reactive power control command for each wind turbine is equal to Qwind (t) /3. Another way is to allocate the reactive power control command proportionally to their available capacity of reactive power， to avoid overloading of wind turbine， as illustrated in equation (5) ， where Qwind1， Qwind2， Qwind3 represent the reactive power control command for  wind turbine  10， 20， 30； Qava1， Qava2，Qava3 represent the available reactive power capacity of  wind turbine  10， 20， 30.
Figure PCTCN2014085909-appb-000006
Similarly， the control system 6 can control to distribute the total active power Pwind (t) among all wind turbinesworking as Mode 3 equally. For example， if  wind turbine  10， 20 are selected to work as Mode 3， the active power control command for each wind turbine is equal to Pwind (t) /2. Another way is to allocate the active power control command proportionally to their available capacity of active power， to avoid overloading of wind turbine， as illustrated in equation (6) ， where Pwind1， Pwind2 represent the active power control command for  wind turbine  10， 20； Pava1， Pava2 represent the available active power capacity of  wind turbine  10， 20.
Figure PCTCN2014085909-appb-000007
After the connection mode and the control commands are determined， it is up to the local controller of wind turbine generator to follow and keep tracking of the references.
Though the present invention has been described on the basis of some preferredembodiments， those skilled in the art should appreciate that those embodiments should by noway limit the scope of the present invention. Without departing from the spirit and conceptof the present invention， any variations and modifications to the embodiments should bewithin the apprehension of those with ordinary knowledge and skills in the art，and thereforefall in the scope of the present invention which is defined by the accompanied claims.

Claims (22)

  1. A method for coordinating control of wind turbinesof a wind farm duringa fault in a utility grid to which energy is to be deliveredvia at least one cable and via a substation of the wind farm， wherein the wind turbines are arranged in at least one group，comprising：
    disconnecting the utility grid from the wind turbines；
    electrically connecting auxiliary equipment of at least one group of the wind turbines viarespective one ofat least one cable；
    selecting at least one wind turbine electrically close to a substationof the wind farm in the respective one of the at least one group of wind turbines； and
    absorbing reactive power generated on the at least one cable by converter of the selected at least one wind turbine.
  2. The method for coordinating control of wind turbines in a wind farm according to claim 1， comprising：
    choosing at least one of the wind turbines to supply active power to the auxiliary equipment of the at least one group of wind turbines by its generator via the corresponding one of the at least one cable.
  3. The method for coordinating control of wind turbines in a wind farm according to claim 2， wherein：
    the at least one wind turbine is chosen from the selected wind turbines.
  4. The method for coordinating control of wind turbines in a wind farm according to claim 1 or 2 or 3， wherein：
    a plurality of the at least one cable being electrically connected by a bus bar；
    the auxiliary equipment of a plurality of the at least one group of the wind turbines being electrically connected via the respective one of the plurality of cables；
    distributing the absorption of the reactive power generated on the plurality of cables among the converters of the selected wind turbines in the at least one wind turbine.
  5. The method for coordinating control of wind turbines in a wind farm according to claim 1 or 2 or 3， wherein：
    a plurality of the at least one cable being electrically connected by a bus bar；
    the auxiliary equipment of a plurality of the at least one group of the wind turbines being electrically connected via the respective one of the plurality of cables；
    choosing a plurality of the at least one of the selected wind turbines to supply the active power to the auxiliary equipment of a plurality of the at least one group of wind turbines by their generators via the plurality of cables and the bus bar.
  6. The method for coordinating control of wind turbines in a wind farm according to claim 4， wherein：
    the absorption of the reactive power being equally distributed among the converters of the selected wind turbines.
  7. The method for coordinating control of wind turbines in a wind farm according to claim 4， wherein：
    the absorption of the reactive power being distributed among the converters of the selected wind turbines in proportion to reactive power capacities of their converters.
  8. The method for coordinating control of wind turbines in a wind farm according to claim 5， wherein：
    the chosen wind turbines supplying equal amount of the active power.
  9. The method for coordinating control of wind turbines in a wind farm according to claim 5， wherein：
    the chosen wind turbines supplying amount of the active power in proportion to active power capacities of their generators.
  10. The method for coordinating control of wind turbines in a wind farm according to claim 1 or 2 or 3， further comprising：
    connecting an external generator via the at least one cable to the auxiliary equipment of the at least one group of wind turbines；
    wherein：
    the absorption of reactive power generated is distributed in the converter of the selected at least one wind turbine and the external generator.
  11. The method for coordinating control of wind turbines in a wind farm according to claim 1 or 2 or 3， further comprising：
    connecting an external generator via the at least one cable to the auxiliary equipment of the at least one group of wind turbines to supply active power；
    wherein：
    the supply of active power generated is distributed in the generator of the chosen wind turbine and the external generator.
  12. A coordination system for controlling wind turbines of a wind farm during a fault in a utility grid to which energy is to be delivered via at least one cable and via a substation of the wind farm， comprising：
    a first switch， being arranged between the utility grid and each ofihe wind turbines and being adapted for being opened where a fault in the utility grid occurred；
    at least one second switch， being arranged between the respective one of the wind turbines of the respective one of the at least one wind turbine group and the respective one ofthe at least one cable； and
    a control system；
    wherein：
    each of the wind turbines includes auxiliary equipment， a transformer unit， a converter and a generator， the transformer unit has a first winding circuit electrically connectable to input of the auxiliary equipment， a second winding circuit electrically connectable to the converter via a third switch， a third winding circuit electrically connectable to the respective one of at least one cable via the respective second switch， and the converter is electrically connectable to the generator via a fourth switch； and
    thecontrol system， being adapted forfollowing an opening of the first switch， selecting the wind turbine electrically close to thesubstationin the respective one of the at least one group of wind turbines and closing the third switch of the selected wind turbine so that the converter of the selected wind turbine absorbs reactive power generated on the at least one cable.
  13. The coordination system according to claim 12， wherein：
    the control system is further adapted for choosing at least one of the wind turbines to supply active power to the auxiliary equipment of the at least one group of wind turbines by its generator via the corresponding one of the at least one cable and closing  the third and the fourth switches of the chosen wind turbine so that the generator of the chosen wind turbine supplies active power to the respective one of the at least one cable.
  14. The coordination system according to claim 13， wherein：
    the at least one wind turbine is chosen from the selected wind turbines.
  15. The coordination system according to claim 12， or 13 or 14， further comprising：
    a bus bar， being arranged to be connected with a plurality of the at least one cable；
    wherein：
    the control system is further adapted for distributing the absorption of the reactive power generated on the at least one cable among the converters of the selected wind turbines.
  16. The coordination system according to claim 12， or 13 or 14， further comprising：
    a bus bar， being arranged to be connected with a plurality of the at least one cable；
    wherein：
    the auxiliary equipment of the wind turbines are electrically connected to the respective one of the at least one cable via the respective closed second switch；
    the control system is further adapted for choosing at least one of the selected wind turbines to supply the active power to the auxiliary equipment of the wind turbines by their generators.
  17. The coordination system according to claim 15， wherein：
    the absorption of the reactive power being equally distributed among the converters of the selected wind turbines.
  18. The coordination system according to claim 15， wherein：
    the absorption of the reactive power being distributed among the converters of the selected wind turbines in proportion to reactive power capacities of their converters.
  19. The coordination system according to claim 16， wherein：
    the chosen wind turbines supplying equal amount of the active power.
  20. The coordination system according to claim 16， wherein：
    the chosen wind turbines supplying amount of the active power in proportion to active power capacities of their generators.
  21. The coordination system according to claim 12 or 13 or 14， further comprising：
    an external generator， being arranged to be electrically connected to the auxiliary equipment of the at least one group of wind turbinesvia the at least one cable；
    wherein：
    the absorption of reactive power generated is distributed in the converter of the selected at least one wind turbine and the external generator.
  22. The coordination system according to claim 12 or 13 or 14， further comprising：
    an external generator， being arranged to be electrically connected to the auxiliary equipment of the at least one group of wind turbines via the at least one cable；
    wherein：
    the supply of active power generated is distributed in the generator of the chosen wind turbine and the external generator.
PCT/CN2014/085909 2014-09-04 2014-09-04 Method and system for coordinating control of wind farm during disconnection to utility grid WO2016033769A1 (en)

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