Method of starting a wind park

Abstract

Provided is a method for starting a wind park including plural wind turbines connectable in a collector system connectable to a utility grid, the method including: starting at least one first wind turbine, each being equipped with an utility grid independent energy supply and a grid forming function, to produce electrical energy from wind energy, thereby utilizing the respective grid independent energy supply for starting; performing the grid forming function by the first wind turbine to achieve a reference voltage in the collector system; starting at least one second wind turbine and/or at least one third wind turbine to produce energy by conversion of wind energy, thereby utilizing energy provided in the collector system for starting.

Claims

1. A method of starting a wind park comprising a plurality of wind turbines connectable in a collector system connectable to a utility grid, the method comprising: starting at least one first wind turbine, each being equipped with a utility grid independent energy supply and a grid forming function, to produce electrical energy from wind energy, utilizing the respective grid independent energy supply for starting; performing the grid forming function by the at least one first wind turbine to achieve a reference voltage in the collector system; starting at least one second wind turbine and/or at least one third wind turbine to produce energy by conversion of wind energy, utilizing energy provided in the collector system for starting, wherein performing the grid forming function by the first wind turbine includes: ramping up a converter output voltage of a converter of the first wind turbine in a controlled fashion until a current limit and/or power limit of the converter is reached, wherein ramping up the converter output voltage of the converter of the first wind turbine is performed such as to balance the system load by: monitoring an actual frequency of the collector system voltage; deriving a limited reference voltage based on a frequency difference between the actual frequency and a nominal frequency of the collector system voltage; and controlling the converter of the first wind turbine based on the limited reference voltage.

2. The method according to claim 1, wherein the at least one second wind turbine is equipped with a grid forming function, the method further comprising after starting the at least one second wind turbine: performing the grid forming function by the second wind turbine to achieve the reference voltage in the collector system.

3. The method according to claim 1, further comprising: connecting the first wind turbine to the collector system after having started the first wind turbine; connecting the second wind turbine to the collector system before starting the second wind turbine; connecting the third wind turbine to the collector system before starting the third wind turbine; automatically synchronizing the electrical output of the second wind turbine and/or the third wind turbine with that of the first wind turbine, wherein each of the third wind turbine performs a current control at a converter output terminal of the third wind turbine.

4. The method according to claim 1, wherein at least one of the second wind turbine and the third wind turbine is started, if a collector system voltage is between predefined voltage limits.

5. The method according to claim 1, wherein deriving the limited reference voltage comprises: deriving a maximum, a minimum and an offset of the reference voltage based on a measured collector system voltage and the frequency difference; deriving the limited reference voltage based on the maximum, the minimum and the offset of the reference voltage and the reference voltage.

6. The method according to claim 1, wherein at least one of the reference voltage and the limited reference voltage is lower than a nominal voltage of the collector system until the power produced by conversion of wind of collector system connected wind turbines essentially matches at least one of active and reactive power demand of the collector system and wherein at least one of the reference voltage and the limited reference voltage is greater than an actual voltage of the collector system.

7. The method according to claim 1, wherein performing the grid forming function by the first wind turbine supports active and/or reactive power requirements of the collector system including the wind turbines connected thereto.

8. The method according to claim 1, wherein a power reference of each of the at least one first and the at least one second wind turbine is derived, based on a preliminary power reference and a power reference offset derived based on the frequency difference, and based on a set fraction of maximum available power of the respective first or second wind turbine.

9. The method according to claim 1, wherein a wind turbine power reference of each of the first and second wind turbines is limited by a power limit derived from a park master maximum power reference and a power offset derived, in dependence of the frequency difference.

10. The method according to claim 1, further comprising: starting at least one other first wind turbine to produce electrical energy from wind energy before starting any second wind turbine, utilizing the respective grid independent energy supply for starting.

11. The method according to claim 1, further comprising: running synchronisation control regarding adjusting magnitude and phase of the collector system voltage to respective values of the utility grid; and connecting the collector system to the utility grid, if a magnitude and/or a phase of the collector system voltage essentially match those of the utility grid in a predefined margin.

12. The method according to claim 1, wherein the respective grid independent energy supply of each first wind turbine is disconnected from the collector system and connectable to the collector system, wherein at least one of the grid independent energy supply of any of the at least one first wind turbine comprises at least one of: a Diesel and/or hydrogen powered generator; a solar cell system; an electric energy storage, in particular battery, accumulator, capacitor bank.

13. A wind park comprising a plurality of wind turbines connectable in a collector system connectable to a utility grid, the wind park being adapted to perform the method according to claim 1.

Description

BRIEF DESCRIPTION

(1) Some of the embodiments will be described in detail, with reference to the following figures, wherein like designations denote like members, wherein:

(2) FIG. 1 schematically illustrates a wind park according to an embodiment of the present invention;

(3) FIG. 2 schematically illustrates a (e.g. first) wind turbine of a wind park according to an embodiment of the present invention;

(4) FIG. 3 schematically illustrates a method scheme performed in the context of a grid forming function performed by one or more wind turbines of the wind park illustrated in FIG. 1;

(5) FIG. 4 schematically illustrates a method schemes performed by one or more wind turbines of the wind park illustrated in FIG. 1; and

(6) FIG. 5 schematically shows the additional power commanded by the governor function could also be added to the limiting function of the wind turbine's main power control.

DETAILED DESCRIPTION

(7) The illustration in the drawings is in schematic form. It is noted that in different figures, similar or identical elements are provided with the same or similar reference signs or with reference signs, which are different from the corresponding reference signs only within the last letter.

(8) The wind park 100 schematically illustrated in FIG. 1 comprises at least one first wind turbine 101a, 101b, 101c each of which is provided with an utility grid independent energy supply 103a, 103b, 103c, respectively, which may be utilized for starting the first wind turbine 101a, 101b, 101c, respectively, in a situation where a collector system 106 comprising cables 105 (and potentially other components like capacitor banks, transformer, filters and the like) which are all connected to a point of common coupling 107 is disconnected from a utility grid 109 by opening a wind park breaker 111. Note that the wind park breaker in this context defines the boundary between the islanded wind farm system and the utility grid, which for example includes both the wind farm side and the grid side of an export line as well as an extended wind farm network including a portion of the nearby utility grid. In this situation, the wind park 100 is disconnected from the utility grid 109 and is thereby also referred to as an islanded wind park.

(9) The wind park 100 further comprises a second group of wind turbines comprising the second wind turbines 113a, 113b which are, as in the first wind turbines 101a, 101b, 101c equipped to perform a grid forming function (this functionality being indicated with a grey colour of the wind turbines).

(10) The wind park 100 further comprises third wind turbines 115a, 115b, 115c, 115d of a third group of wind turbines which neither have a utility grid independent power supply nor are capable of performing a grid forming function.

(11) Via a park transformer 117, the point of common coupling 107 may be connected, when the wind park breaker 111 is closed, to the utility grid 109. For controlling functions or the operation of the wind turbine, also a park level controller 119 is provided which may receive an external input 121 (such as regarding a demanded voltage, frequency, active power, reactive power) and which may output control signals 123 to all wind turbines of the wind park 100 in order to control the operation, in particular to supply coordination signals, for example release signals, regarding power limit, regarding voltage reference and the like. The park level controller 119 may also be capable of actuating the wind park breaker 111 for controlled closing and opening the breaker. Furthermore, via sensors 125, 127, electrical properties for example between the point of common coupling 107 and the wind park transformer 117 are sensed or measured and supplied via measurement signals or sensor signals 129 to the park level controller 119. The measurement signals 129 may for example comprise measurement values of the frequency and/or the voltage at the point of common coupling 107 corresponding also to a voltage of the collector system collectively referred to with reference sign 106.

(12) The wind park 100 is capable of performing a method of starting the wind park according to an embodiment of the present invention. Thereby, first at least one first wind turbine 101a, 101b, 101c is started to produce electrical energy from the wind energy (whenever sufficient wind is present), thereby utilizing the respective grid independent energy supply 103a, 103b, 103c, respectively, for starting. Thereupon, after having reached a state where power production can commence, the respective first wind turbine 101a, 101b, 101c may be connected (via not explicitly illustrated wind turbine breakers) to the collector system 106. After the first wind turbine (e.g. 101a, 101b and/or 101c) has been connected to the collector system, the respective first wind turbine(s) may perform a grid forming function for achieving desired electrical properties within the collector system 106 and thus also at the point of common coupling 107.

(13) After the collector system 106 is sufficiently energized (for example has reached a desired collector system voltage, such as a reference voltage in the collector system), at least one of the second and/or third wind turbines 115a, 115b, 115c, 115d may be started thereby utilizing energy from the collector system 106. Thereby, not explicitly depicted wind turbine breakers may be closed and connect the respective third wind turbines to the collector system 106.

(14) Before or concurrently with starting and/or connecting one or more of the third wind turbines 115a, 115b, 115c, 115d to the collector system, one or more of the second wind turbines 113a, 113b may be started and may be connected to the collector system. Thereby, also the second wind turbines 113a, 113b may perform the grid forming function to further stabilize the collector system electrical properties, in particular the voltage and/or frequency to desired values.

(15) One or more of the first wind turbines may start independently from each other, thus providing a distributed black start. Once the collector grid's electrical properties are within desired values, the second wind turbines may start independently from each other and the from the first wind turbines. The third wind turbine(s) may, once released for operation, start up independently, but may be unreleased if the number of first and second wind turbines reduce below the level required to support the number of online third turbines. Furthermore, frequency to power offset droop (or similar) may be utilized within the grid forming functions of the first and/or the second wind turbine to balance the power produced by the wind turbines with the power required by the islanded collector system.

(16) In particular, the first wind turbines and/or the second wind turbines may be equipped with the respective grid forming function which may be configured to allow ramping up the voltage at respective output terminals of the respective converters, until they reach their local current or power limit. In particular, (additional) black start equipped wind turbines (for example the first wind turbines of the first group having an individual power supply) may autonomously synchronize and connect to the partially energized collector system (AC system) and may share the total real and reactive power load of the islanded collector system (e.g. medium voltage system). The grid forming function of the first and/or the second wind turbines may support the start and/or operation of the third group of wind turbines (not being equipped with the grid forming function). Thereby, the third wind turbine may run for example a net side current control.

(17) FIG. 2 schematically illustrates a wind turbine 101 of the first group of wind turbines as illustrated in FIG. 1. The first wind turbine 101 comprises a rotation shaft 131 at which plural rotor blades 133 are connected. Upon impact of wind, the rotation shaft 131 rotates and, since being connected to a rotor of a generator 135, thereby provides electric energy. The electric AC energy is supplied to a converter 137 comprising a generator side portion 139 (for example a AC-DC converter), a DC link 141 (for example comprising one or more capacitors) and a utility grid side portion 143 (for example a DC-AC converter). At the output terminal 145 of the converter 137, an active and reactive power (having in steady state operation a frequency equal to the grid frequency) is output which is further supplied to a wind turbine transformer 147 which transforms the output voltage to a medium voltage. Between the converter and the wind turbine transformer a pulse width modulation filter 149 and/or an inductance 151 may for example be utilized.

(18) A wind turbine controller 155 may receive measurement signals such as a measurement signal 157 regarding the output voltage at or close to the output terminal 145 of the converter 137. The wind turbine controller 155 may further receive the control signals 123 from the park level controller 119 as is illustrated in FIG. 1. The wind turbine controller 155 may supply control signals 159 to the converter 137 for example controlling the rotational speed of the rotor 131 and also in particular in order to perform a grid forming function as implemented in a module 161.

(19) For starting up the first wind turbine 101 which belongs to the first group of wind turbines, the first wind turbine 101 comprises a grid independent power supply 163 which may be for example configured as an electrical storage or a Diesel generator or a battery or the like. Before the wind turbine 101 is started, a breaker 165 which, when closed, connects the utility grid independent power supply 163 to essentially the output terminal of the converter 137, is opened. The grid independent power supply 163 is however connected to auxiliary and operational equipment of the wind turbine 101. In particular, the power supply 163 may be connected or connectable for energy supply to pumps or fans 167, may be connected to a pitching system 169, may be connected to a yawing system 171 and may be connected to measurement equipment 173 and may further be connected to a DC link pre-charging equipment 175 which may all or at least in part need electrical energy in order to support starting the wind turbine 101.

(20) Not only one but two or even more of the first wind turbines of the first group of wind turbines may be started and may energize the collector system after a low wind period, rather than consider each wind turbine as an individual system. Rather than having stored energy and a suitably sized diesel generator for energizing the first cluster (e.g. string or group of strings) of wind turbines, X first wind turbine may be equipped with a small amount of storage that allows them to supply the auxiliary loads (for example 167, 169, 171, 173, 175) for a given number of hours and pre-charge the main converter DC link 141 when the start command (for example via signal 123 or via a locally set command) is given.

(21) It should be noted that there is no direct electrical connection between the medium voltage collector system 106 and the individual grid independent energy supply 163, other than optional “recharge” state which may be achieved by closing the breaker 165.

(22) The first turbines may comprise X wind turbines which may energize the islanded medium voltage collector system 106.

(23) When there is insufficient wind to cover the auxiliary load and losses in the system, the wind farm system may be de-energized leaving the black start capable first wind turbines in hibernation mode waiting for sufficient wind to start up. In addition, the energy storage required to ride through period of low/no wind can be minimized if a group of wind turbines act in a coordinated (or combined) manner (which may also be referred to as distributed black start). A group of M turbines with M>4 is considered. If these wind turbines are oriented such that they are all pointing in different directions, and that these turbines are all full pitched, then when the wind returns, one of these four wind turbines may start to rotate. If these four wind turbines all contain sufficient energy storage to maintain their internal environmental control, then as the wind starts, one of these wind turbines may be enabled and start to energize the collector system (AC system) as described above. This one first wind turbine may now have sufficient generation capacity to provide for its own pitch, yaw and environmental control and internal systems. Thus, at this stage, the auxiliary systems 167, 169, 171, 173, 175 may be supplied with energy from the output terminal 145 of the converter 137.

(24) As the voltage rises on the collector system, one of the other fully pitched turbines may start, initially by yawing into the wind than to start power production, as this will increase the collector system voltage. Each of these “selected wind turbines” will need to have the ability to start its pitch and yaw controls at reduced AC grid voltage. Once these black start capable wind turbines (for example the first group of wind turbines each being equipped with the grid forming algorithm) are all enabled, it can be calculated that the AC voltage on the islanded system will be sufficiently high such that remaining wind turbines may start using energy from the collector system. That may mean that the AC voltage at the collector system may be sufficient for the remaining Y and Z (e.g. second and/or third) wind turbines (where the third group are not provided with a grid forming algorithm) to start. If the low/no wind period is greater than the period at which dry is needed, then these wind turbines will need to go through the dry out procedure, that might be applied, if the turbine is left de-energized, such that moisture can gather inside the components. For electrical components moisture can cause flash-over when energized which can damage the component. To avoid this, the components are heated to allow any moisture to evaporate before starting.

(25) For offshore wind farms it is understood that there is rarely a no wind condition which persists for >3 days.

(26) Additionally, there could be a low rated, lower voltage cable between wind turbines to power up the auxiliary system without needing to energize the main transformers.

(27) Multi-Sequence Energization

(28) A single black start capable wind turbine (for example a wind turbine of the first group of wind turbines) may not have sufficient power available or reactive current capability to supply the (real power) loads on the islanded system or the reactive power requirements of the islanded system. Furthermore, embodiments of the present invention allow that multiple (first or second or third) wind turbines connect at different time intervals or time points so that a tight time coordination between the wind turbines may not be necessary. In particular, the energization of the collector system may be performed in a staged approach where it is the combined effort of all participating wind turbines that energizes the collector system, i.e. to keep the collector system at operating points where the number of connected wind turbines can support the complete electrical system both in terms of active and/or reactive power. On two levels this may be done as: having a sequential build-up of the electrical system, i.e. where string cables are connected one by one and the wind turbines on each string are started before the next string is connected. This may reduce the total load for the black start capable wind turbines. operating below nominal voltage until such time that sufficient wind turbines are connected to the collector system (for example are online) to cover the full active and reactive power load at nominal voltage.

(29) The first black start wind turbine (a first wind turbine or a wind turbine of the first group of wind turbines) which is connected will ramp its pulse width modulation voltage magnitude from zero with a fixed ramp rate and until a threshold is reached. This threshold could be a preselected value or could be driven by the absolute value of the reactive power and/or absolute value of active power and/or a set current limit (e.g. total current) and/or a percentage of a set current limit. Thus, the first wind turbine may ramp up the collector system voltage to a limit value. In alternative embodiments, the voltage may be ramped until the power converter reaches its reactive power capacity and/or power capacity based on the prevailing wind conditions or a given percentage thereof. This may ensure that the wind turbine (in particular first wind turbine) will never exceed its physical restriction in terms of current rating etc. However, the real and/or reactive power requirements of the islanded collector system may and probably will exceed the capabilities of a single (first) wind turbine. The following and subsequent wind turbines may synchronize to the electrical system (which is now defined by the wind turbine(s) already connected) both in terms of measured voltage magnitude and (phase) angle. The ramping may be implemented as a slew-rate limit, a saturation that is gradually released or similar, as is schematically illustrated in FIG. 3.

(30) When the following wind turbines connect, they will start from the measured voltage magnitude of the collector system, initially have zero reactive power flow and hence they will start to ramp up the voltage until they reach their capability limit or set threshold as described for the first wind turbine. The ability to control and synchronize to a less than nominal AC voltage supply is an enabling feature of the converter control. As the wind turbines are trying to increase the collector system voltage, the ramped voltage magnitude may or should always be larger than the measured voltage, i.e. V.sub.reference,max≥V.sub.measured, and if a wind turbine were to have a voltage reference that was lower than the measured voltage, this wind turbine would in fact be pulling the voltage down. So, when a wind turbine is ramping up its voltage reference, it needs to make sure that the ramped value is larger than or equal to the measured voltage. When this other first wind turbine connects, it may contribute power and reactive power to the islanded collector system and thus may reduce the load on the previously connected first wind turbine, which may allow both wind turbines to increase the AC voltage of the collector system. It is this “sharing” of the total islanded load (both real and reactive powers) which may permit the energization of the collector system.

(31) When the wind farm is at the intermediate stage of being energized to partial voltage level, it may be necessary to monitor the capabilities of the connected wind turbines (which may be changing due to wind conditions or the like), such that the level of energization is always at a level that matches the capabilities of the connected wind turbines. Thus, it may be ensured that the real and reactive power required by the islanded medium voltage collector system is equal to the real and reactive power produced by the wind turbines connected to the islanded collector system. It may be necessary to reduce the voltage level if for example a wind turbine disconnects during the ramping up. If a wind turbine disconnects, then the power available from the wind turbines will reduce. Reducing the voltage on the medium voltage collector system may reduce real and reactive power loads.

(32) FIG. 3 schematically illustrates a method scheme 177 which may for example be implemented in the grid forming function module 161 of the wind turbine controller 155. A voltage ramp management module 179 receives V.sub.measured being the voltage at the wind turbine low voltage bus bar, for example the converter output terminal 145 as illustrated in FIG. 2. The voltage measurement may give a good indication of what the medium voltage at the collector system 106 may be as the low voltage and the medium voltage sides are electrically connected via the wind turbine transformer 147, as illustrated in FIG. 2. The voltage ramp management module 179 further receives the frequency error Δf representing a difference between the actual frequency in the collector system and a nominal frequency of the collector system voltage. The frequency error Δf may be an output of a grid forming converter control as is illustrated in FIG. 4. The signal 183 (released) may be a status flag (binary 1 or 0) indicating whether the converter is released for operation. The signal 185 (in limit) is a status flag which may indicate if the net side converter is in current limit or in power limit (when the power being exported by the net side converter to the AC system is equal to the power being extracted from the available wind). From signals 183, 185 the voltage ramp signal 131 is derived (being a flag whether to ramp up the voltage or not) and is received by the management module.

(33) V.sub.reference,limited 189 is the net side converter voltage reference. V.sub.reference,offset output by the voltage ramp management module 179 is an offset applied to the V.sub.reference to control the output voltage to balance real or reactive power when in the limit condition. V.sub.reference,min, V.sub.reference,max are output by the voltage ramp management module 179 and provided to a limiting element 187 which limits V.sub.reference. The voltage ramp management 179 outputs V.sub.reference,offset which is added to the output of the limiting element 187 resulting in the value V.sub.reference,limited, also referred to as limited reference voltage 189. Thereby, a means to modify the reference voltage to the net side converter is provided which could for example be used for the initial voltage ramp up.

(34) According to an embodiment, a droop between frequency error and a reference offset of the oscillator system voltage can be applied which may activate if the wind turbines cannot sustain the frequency or if there is a building power error (for example via cumulative sum). Another implementation could also be made like simply ramping down V.sub.reference,max until the frequency or power error has stabilized. A PI-controller or similar could also be used. The converter may at this point need to ensure that it is obeying the power reference from the turbine controller as the rotor would otherwise start to decelerate. An appropriate hysteresis band on frequency or power error may need to be put in place if ramps are used to avoid toggling between ramping up and down.

(35) When the voltage magnitude is within the normal operating range, and the absolute reactive/active/apparent power and/or the reactive/active/total current is less than a set limit, the wind turbines may transfer into normal voltage control. At this point, the energy storages may be restored to their reference levels, for example by recharging them from the low voltage bus bar by for example closing the breaker 165 illustrated in FIG. 2.

(36) Load Balancing

(37) When operating the wind turbines on a wind farm island it is necessary to supply the active and reactive load on the island which may include constant or continuous adjustment of active and reactive power output as the load may change. In particular, the total load and the collector system, i.e. both active and reactive power, may be unknown. Due to the autonomous behaviour of the wind turbines it may also be unknown for the individual wind turbine how many other wind turbines may help share the imbalance in load.

(38) Therefore, the first wind turbine (i.e. a wind turbine of the first group) and the wind turbines of the second group may run the grid forming algorithm on the network bridge converter that allows them to come online in an uncoordinated fashion, contribute to the system voltage and resist any changes to frequency and voltage. Such a grid forming algorithm may also be a virtual synchronous machine (VSM) type control or the power control as described in EP 3 116 085 A1. This may avoid the need to assign a master wind turbine that is supplying a reference voltage and which requires an additional layer of coordination because the grid forming wind turbines, by virtue of the network bridge converter control, may automatically synchronize to each other in a similar fashion as a synchronous machine and may share the task of balancing the system. That is, the wind turbines may have the ability to act as a back electromotive force (EMF) very similar to a synchronous machine.

(39) The total active load of the islanded system may be unknown and may probably be time-varying and the wind turbines therefore need to adjust their power production to match this load. In particular, if the power produced by the already collector system connected wind turbines exceeds the load on the collector system, then the frequency will rise, and the opposite will happen if the load exceeds the power production, i.e. the frequency will decrease. Thus, according to embodiments of the present invention, the frequency output from the grid forming control (for example frequency error M in FIG. 4 or 5) is used via a droop gain to modify the power reference via a ΔPref term as in FIG. 3. Thereby, the frequency and power balance of the system may be controlled.

(40) The controller portion 177 illustrated in FIG. 3 may represent a voltage management controlling module which may modify the effective voltage reference within the converter in response to a frequency error. This controller (for example controller 177 illustrated in FIG. 3) may be implemented in any suitable controller structure, proportional only, proportional differential, lead/lag, etc. The controller may effectively act like a governor on a conventional synchronous machine.

(41) As all the Y load balancing wind turbines are subject to their local wind conditions at any point in time, each wind turbine may only have a power demand (for example Preftotal in FIG. 4) up to its available power (which may be determined by the prevailing wind conditions). For any additional reductions in frequency, the wind turbine outputting its available power may be required to push the additional demand for power to other wind turbines having room for up-regulation, which may mean that when any one wind turbine cannot produce more power it will not increase its power demand above what is available from the prevailing wind (P.sub.max(availablepower)) in FIG. 4 which is implemented therein via a clamp 191. The governor function 193 in FIG. 4 has an output which must be subject to a time-varying upper limit as given from the available power of the wind turbine. If the additional power commanded (ΔPref in FIG. 4) by the governor is added after the main wind turbine power control, the governor function must limit its output to be no more than the wind turbine's available power (or an estimate thereof), which is what the clamp block 191 in FIG. 4 does.

(42) In particular, the control scheme 190 illustrated in FIG. 4 illustrates a turbine controller 192 and a converter controller 194. The turbine controller 192 comprises a power control module 195 which generates a reference power Pref and sends it to a module 197 which evaluates various internal and external power limits including a centrally calculated maximum power P.sub.max, which is received from a park control. The turbine controller 192 outputs a reference power P.sub.ref as well as a maximum available power P.sub.max(availablepower) which is supplied to the converter controller 194. The available maximum power P.sub.max(availablepower) is supplied to the clamp (or limiting element) 191 which outputs ΔP.sub.ref which is the offsets of the reference power which is added to the reference power P.sub.ref provided from the turbine control. The value P.sub.refTotal is supplied to the grid forming converter control 161 which outputs the frequency error Δf which is supplied to the island mode control, for example droop gain or governor 193.

(43) Alternatively, as shown in FIG. 5, the additional power commanded by the governor function could also be added to the limiting function of the wind turbine's main power control (i.e. the turbine control block 201 in FIG. 5). In this case, the governor response (for example by governor or island mode controller 203) may only be active when the available power is above the limit applied to the wind turbine's power reference.

(44) For example, whilst the functionality shown in FIGS. 4 and 5 is the same or similar, it does not necessarily need to be implemented in the converter control.

(45) The system may remain energized as long as the combined available power of all the wind turbines is at least that of the total active power demand. The frequency error that is used for the described load balancing control can clearly come from a number of sources, for example the converters internal control of frequency, the converters measurement of frequency, the turbine controls measurement of frequency or an external measurement unit.

(46) In particular, the controller 200 illustrated in FIG. 5 comprises a turbine controller 201 and a grid forming converter control 205. Based on a frequency error Δf, the governor 203 (island mode control for example droop gain) outputs an offset 204 of a power which is added to the maximum power P.sub.max provided from the park control. The sum is provided to a limiting module 207 which receives a reference power P.sub.ref from a power controller 209. The module 207 outputs a reference power P.sub.ref(turbine control) which is provided to the grid forming converter control 205 which outputs the frequency error Δf.

(47) The use of a grid forming algorithm together with a frequency to power offset droop (assuming a simple proportional governor function) to balance the power production of the wind turbine to the islanded load, enables multiple parallel units to share the total load. The frequency to power offset control effectively act like a governor on a conventional synchronous machine. This controller may be implemented in any suitable controller structure, proportional only, proportional differential, lead/lag, etc. and may be supplemented by filters to achieve the desired control response.

(48) Park Level Coordination

(49) The park level coordination may allow for a higher level of automation but is not essential in order to perform the core function described above. The black start process may be initiated on request from the park level control or an external entity communicated to the wind turbines via the park level control. The park level controller may handle the high level coordination of releasing the operation of wind turbines with the appropriate capabilities at the appropriate time during the black start procedure. When the black start capable wind turbines have energized the collector system to within normal operation ranges, the park level control may release the remaining wind turbines in the wind farm that may run an entirely different network bridge control algorithm, for example a traditional DQ-axis current control.

(50) As the total active load and the number of connected wind turbines at any given time are unknown when parameterizing the controllers, it may be useful to have a master controller that adjusts the power dispatch of the wind turbines to have them reduce the frequency error. The auxiliary equipment in the wind farm may have frequency ranges in which they can operate and if the governor droops (assuming a droop gain based governor) are to be parameterized for the worst case power imbalance and the minimum number of connected wind turbines, a high gain would be required in order to stay within the set frequency range. Letting the park controller set the wind farm dispatch (effectively an estimate of the present load) based on a measurement of the frequency, may allow for lower governor gains to be used and reduces the dependency of parameterization on site specific conditions. Thereby, wind farm dispatch may refer to the power limit (for example shown as P.sub.max(parkcontrol) in FIG. 4) sent to the wind turbines from the park level control. In the case the internal black start was performed in order to contribute to the re-energization of the main power system, the park breaker needs to be closed when the internal black start is complete and the entity managing the complete system restoration is ready to bring the wind farm onto the utility system.

(51) If the transmission system is already energized by some other generator, the voltage phasor on the wind farm side of the park breaker must first be aligned sufficiently close to the phasor on the grid side such that the disturbance from the breaker closure is kept sufficiently small.

(52) A synchronization control may be run on the park controller to adjust voltage magnitude and phase to match those on the grid side of the breaker. When the errors are within the limits of the synchro-check relay, the park breaker will close at will and the wind farm be connected to the main power system. The voltage magnitude error may be limited through control of the wind turbine's terminal voltage and the angle error by running the wind farm network with a small (for example 100 mHz) frequency offset.

(53) An advantage of embodiments of the present invention may come from exploiting the distributed nature of wind turbines. Each single wind turbine would likely not be able to handle the whole task of black starting the system, but any K number of them would. The starting method described above retains a large degree of autonomy for the individual wind turbines in that: there is no time critical control running on a central piece of equipment to coordinate the black start, and the wind turbines equipped with an energy buffer for black start are free to start up at any time after they have been released for black start, and will do so if there is sufficient wind, and hence provide the losses for the auxiliary systems (and if battery storage used for the Auxiliary supply, this could be recharged also, hence prolonging the period that the battery storage could maintain the Aux supplies) when energized and running the wind farm island, all turbines with the grid forming control software equally share the task of balancing the active and reactive power. At this point, wind turbines without these capabilities can also be connected and started up if their power production is needed in order to supply the active loads; in this context they could be considered as base load wind turbines. There is no “master” wind turbine that the others synchronize against and the wind turbines are thus free to connect and disconnect from islanded grid as dictated by the local wind conditions (of course provided that there is enough production between the wind turbines to keep the wind farm energized).

(54) Embodiments of the present invention open up the possibility that a group of wind turbines within a windfarm, can start to energise a windfarm and then allow other wind turbines to connect and contribute to the overall real and reactive power load. Furthermore, it may be possible that additional wind turbines within the windfarm do not need to operate with the grid forming control algorithm used in the black start wind turbines, they could be standard current controlled wind turbines. This makes this solution potentially retro fitable to existing windfarms.

(55) Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention.

(56) For the sake of clarity, it is to be understood that the use of “a” or “an” throughout this application does not exclude a plurality, and “comprising” does not exclude other steps or elements. The mention of a “unit” or a “module” does not preclude the use of more than one unit or module.