Black start of wind farm

Abstract

A method of controlling at least one converter of at least one wind turbine connectable to a wind farm grid of a wind farm during a black start or island mode of a wind farm is described, the method comprising: ramping up a converter voltage reference (Vref, 105, 77) (9) unless a power-related condition and / or a wind farm grid frequency-related condition or a converter current-related condition is violated.

Description

Technical Field

[0001] The present invention relates to a method of controlling at least one converter of at least one wind turbine connected to a wind park grid of the wind park during a black start of the wind park, to a corresponding arrangement, and further to a wind park comprising a plurality of wind turbines, wherein at least one wind turbine comprises an arrangement adapted to control or perform the control method during a black start. Background Art

[0002] EP 3 116 085 A1 discloses a method of operating a wind turbine by means of a network bridge controller with power and voltage control, the wind turbine being connected to a utility grid via an umbilical AC cable.

[0003] WO 2012 / 139667 A1 discloses a wind turbine device that, in one embodiment, has a diesel generator to initiate a black start in the event of a blackout in a power grid to which the wind turbine device is coupled. To perform a black start, the diesel generator provides a predetermined voltage to the power output of the wind turbine device.

[0004] A black start involves starting a wind farm when the wind farm is disconnected from the utility grid. Thus, during a black start, no power supplied from the utility grid is available to start one or more wind turbines of the wind farm.

[0005] Conventional methods of performing black starts involve large thermal power stations; wind farms currently do not perform this role because the power converters that form the grid interface of the wind turbines require stable voltage and frequency to operate. In the future, the availability and reliability of conventional thermal power stations will be limited, and therefore wind farms may need to have the ability to perform their own black starts.

[0006] Hence, there may be a need for a method of controlling at least one converter of at least one wind turbine connected (or connectable) to a wind park grid of a wind park during a black start, a corresponding device, and a wind park having the capability to perform a black start. Summary of the Invention

[0007] When a grid forming control algorithm is used to black start a power converter's local bus bar, its local electrical string cables, a larger network such as the entire electrical network of a wind farm, and / or a predetermined portion of a power grid that is initially in a de-energized state, embodiments of the present invention cover or acknowledge limitations on the active power, frequency, and current of the power converter in the context of a grid forming control algorithm.

[0008] According to an embodiment of the present invention, a method of controlling at least one converter of at least one wind turbine connectable (or connected) to a wind farm grid of a wind farm during a black start or island mode of a wind farm (in particular disconnected from a utility grid) is provided, the method comprising: ramping up a converter voltage reference unless a power-related condition and / or a wind farm grid frequency-related condition or a converter current-related condition is violated.

[0009] The converter can be coupled to a generator driven by a rotating shaft on which a plurality of rotor blades are mounted. The converter can, for example, include or be an AC-DC or DC-AC converter. In particular, the converter can include: a generator-side portion, in particular an AC-DC converter portion; a DC link; and a utility-side converter portion, such as a DC-AC converter portion. In particular, the utility-side converter portion can be controlled according to embodiments of the present invention, in particular without also controlling the generator-side converter portion.

[0010] During normal operation, when a wind farm or wind turbine is generating energy, the utility grid converter portion can be connected to a utility grid, for example, via a point of common coupling and via one or more transformers, which supplies power to a plurality of consumers. In particular, each converter of each wind turbine in a wind farm can be controlled according to a control method according to an embodiment of the present invention. While performing the method, the wind turbine, and in particular the entire wind farm, can be disconnected from the utility grid. Control signals can be supplied to the converter, including a converter voltage reference, a converter power reference, in particular a converter active power reference, etc. Based on the control signals to the converter, a pulse-width modulated signal can be derived, which can be supplied to the gate of a controllable switch (such as an IGBT) included in the converter.

[0011] During the black start process, the network bridge of the converter can ramp the voltage at its terminals from zero volts to the nominal voltage and thereby energize the AC electrical system external to the wind turbine generator (WTG). Depending on the configuration of the electrical network to which it is connected, the ramping up of the voltage at the converter terminals can result in an increase in the voltage in the electrical network and, therefore, in the energization of any passive loads on the external AC network. For example, passive loads can be the capacitance of cables, the inductance of transformers, and / or resistive power losses.

[0012] A power converter performing a voltage ramp must balance both the active and reactive power loads of the electrical system while adhering to finite limits on the amount of active power it can provide to the electrical network, the maximum allowable frequency deviation from a target frequency it can allow, and the current limits of the power converter.

[0013] To comply with these constraints, the network bridge controller of the power converter (e.g., executing the control method) may include features that allow (ideally) autonomous control of voltage increases while taking into account all three sets of constraints. That is, any WTG participating in the incentive can support the load of the external AC electrical system within its capabilities without knowing the status of the remaining WTGs in the wind farm; and any WTG's local decision to increase the voltage level must (or may) not cause the remaining participating WTGs to violate their constraints on power, frequency, and current.

[0014] Similarly, when the converter has increased the voltage to the nominal level, it must continue to ensure that the frequency, active power and current limits are observed. When the converter is in this state, it is described as operating in island mode; this indicates that the external AC electrical network supported by the converter(s) is not connected to the larger grid, and the converter must therefore take the necessary actions to independently support the grid and balance the active and reactive loads.

[0015] Embodiments of the present invention disclose design details that allow features to comply with various limitations of the converter(s) during a black start voltage ramp or when operating in island mode. Thus, embodiments of the control method can be applied during a black start or during island mode where the entire wind farm is disconnected from the utility grid.

[0016] The wind farm grid may be an electrical network via which a plurality of wind turbines are connected to one another, in particular at their power output terminals. Each individual wind turbine may be disconnected from or connected to the wind farm grid by means of a corresponding circuit breaker or switch. Similarly, the wind farm grid may be disconnected from or connected to the utility grid by means of a corresponding circuit breaker or switch. In particular, when executing the control method, initially, when the wind farm grid voltage is not equal to (e.g., lower than) the utility grid voltage, the wind farm grid may be disconnected from the utility grid. When the wind farm voltage is substantially equal to (or greater than) the utility grid voltage, the corresponding wind farm may be connected to the utility grid.

[0017] When performing a black start, the wind turbines involved will be connected to the wind park grid and work together to increase the wind park grid voltage to a target voltage that may be close to or greater than the utility grid voltage.

[0018] The ramp-up of the converter voltage reference may also be constrained so that the converter voltage reference does not exceed the target wind farm grid voltage. When power-related conditions and / or wind farm grid frequency-related conditions or converter current-related conditions are observed, the converter may be protected from damage and, further, the desired wind farm grid frequency may be achieved.

[0019] Embodiments of the present invention may require wind turbines to limit their active power, frequency and / or current during the excitation of the wind farm grid (black start) during a black start. Thus, a black start can be performed in a more reliable manner and in particular in a fast manner without the risk of damaging components of the wind turbine, in particular the converter.

[0020] Power-related conditions may include consideration of the converter power reference and available power. Wind farm grid frequency-related conditions may consider wind farm grid upper and lower frequency limits as well as the actual wind farm grid frequency. Converter current-related conditions may adhere to or consider converter current limits compared to the actual converter current.

[0021] According to an embodiment of the present invention, ramping up the converter voltage reference comprises: measuring the wind farm grid voltage; supplying the sum of the wind farm grid voltage and a positive increment to the converter as the converter voltage reference; increasing the converter power reference when the wind farm grid frequency falls below a target wind farm grid frequency (e.g., due to an imbalance between the converter power reference and the active power load); and continuing to increase the converter voltage reference to the target value of the wind farm grid voltage unless: the converter power reference is greater than the available converter (active) power and / or the converter current is greater than the converter current limit.

[0022] The converter voltage reference may be ramped up at a predetermined rate. The ramping may be performed in steps such that a positive increment is (repeatedly) added to a previous converter voltage reference. The wind farm grid voltage may relate to the voltage of a wind farm grid, i.e., an electrical network connecting a plurality of wind turbines of a wind farm. The positive increment may be defined by a desired ramp-up rate of the converter voltage reference. After the converter voltage reference is supplied to the converter, the converter may control its respective controllable switch so as to actually output the desired modulation voltage at its terminals. At this stage, the converter of the respective wind turbine may already be connected to the wind farm grid.

[0023] The wind farm also comprises energy consumers, such as other wind turbines that have not yet been started or other auxiliary equipment that requires electrical energy to operate.For a stable black start there must be an approximate balance of the power produced and the power consumed.

[0024] The nominal wind farm grid frequency may be a frequency that is ultimately desired to be achieved before connecting the entire wind farm to the utility grid. The nominal wind farm grid frequency may be substantially equal to the nominal utility grid frequency. If the converter power reference is not greater than the available converter power, or when the converter current is not greater than the converter current limit, the converter voltage reference may continue to be increased.

[0025] The target value for the wind farm grid voltage can also be referred to as the nominal wind farm grid voltage, i.e., the desired voltage of the wind farm grid. This approach allows the wind farm grid voltage to be increased continuously or in steps while maintaining power balance and complying with converter current limits. Furthermore, by maintaining the wind farm grid frequency close to the wind farm grid frequency limit, the frequency can be constrained from deviating from the target frequency by more than a maximum deviation.

[0026] According to an embodiment of the invention, the available converter power is based on the available power of a generator driven by a rotating shaft to which the plurality of rotor blades are connected. The available converter power may for example be equal to the available power of the generator reduced by losses.

[0027] According to an embodiment of the invention, the method further comprises: stopping further increasing the converter voltage reference if the converter power reference is greater than the available converter (eg active) power; and maintaining the wind farm grid frequency at the wind farm grid frequency limit using trim control.

[0028] This feature can ensure that the wind farm grid frequency does not deviate too much from the target wind farm grid frequency.

[0029] According to an embodiment of the invention, using smooth control comprises changing, in particular reducing, the converter voltage reference in dependence on a frequency difference between the wind park grid frequency and a wind park grid frequency limit in order to prevent a further decrease in the wind park grid frequency, in particular in order to keep the wind park grid frequency close to the wind park grid frequency limit.

[0030] Therefore, instead of further increasing the converter voltage reference, the converter voltage reference is changed depending on the frequency difference between the wind farm grid frequency and the wind farm grid frequency limit. As a result, the wind farm grid frequency can be kept within a predictable range.

[0031] According to an embodiment of the present invention, the method further includes: if the converter current is greater than the converter current limit, stopping further increasing the converter voltage reference; and using reactive current limit control to maintain the converter current at the converter current limit. By applying reactive current limit control, the converter can be protected from damage.

[0032] According to an embodiment of the present invention, using reactive current limit control includes varying, in particular reducing, a converter modulation voltage limit depending on the difference between the reactive converter current limit and the reactive converter current, in particular to keep the converter current at the converter current limit. Thus, a reliable process for keeping the converter current at the converter current limit can be provided. The converter modulation voltage is the voltage generated at the terminals of the converter by its switching action, and the converter modulation voltage limit is a limitation of this voltage to prevent the converter current from exceeding the current limit.

[0033] According to an embodiment of the present invention, the reactive current limit control selects whether to limit the converter modulation voltage to an upper limit or a lower limit depending on whether the reactive converter current is inductive or capacitive.

[0034] Depending on whether the converter is sourcing capacitive reactive current or sinking inductive reactive current, respectively, either the upper or lower modulation voltage limit may be used to limit the converter current.

[0035] According to an embodiment of the present invention, the wind farm frequency limit is given as the difference between the nominal frequency and the maximum frequency deviation. Thus, it can be ensured that the wind farm grid frequency does not change unpredictably.

[0036] According to an embodiment of the present invention, a method of controlling a plurality of converters of a plurality of wind turbines connected to a wind farm grid of a wind farm during a black start (e.g., disconnection from a utility grid) of a wind farm is provided, the method comprising: independently performing a method according to one of the aforementioned embodiments for each of the plurality of converters.

[0037] Thus, the converters can be controlled independently of each other according to the control method as described above. Thus, in particular, synchronization of the control converters may not be required. Thus, a coordination feature for coordinating the control of different converters may not be necessary, thereby simplifying the method.

[0038] According to an embodiment of the present invention, a device for controlling at least one converter of at least one wind turbine connectable to a wind farm grid of a wind farm during a black start or island mode of a wind farm (e.g. disconnected from a utility grid) is provided, the device being adapted to control or perform a method according to one of the preceding embodiments.

[0039] It should be understood that the features disclosed, described, explained or provided, alone or in any combination, for the method of controlling at least one converter of at least one wind turbine connected to a wind farm grid of a wind farm during a black start or island mode may also be applied, alone or in any combination, to the apparatus for controlling at least one converter of at least one wind turbine connected to a wind farm grid of a wind farm during a black start or island mode according to embodiments of the present invention, and vice versa.

[0040] According to an embodiment, the arrangement may comprise an available real power clamp adapted to ensure that the converter power reference is constrained by the available power.

[0041] Available active power clamps can include, for example, a frequency-to-reference power droop, which outputs a change in reference power based on the deviation of the wind farm grid frequency from a target frequency. The actual power output by the converter can be measured as power feedback. This power feedback can be filtered and added to the reference power as the output of the frequency-to-reference power droop.

[0042] According to an embodiment of the invention, the apparatus further comprises a smoothing controller adapted to control the wind park grid frequency to a wind park grid frequency limit by changing the converter reference voltage.

[0043] The smoothing controller may include a difference element that derives a frequency error as a difference between the actual frequency of the wind farm grid and a frequency limit and supplies the frequency error to a controller, such as a PI controller.The PI controller may output a change in a reference voltage.

[0044] According to an embodiment of the invention, the apparatus further comprises a reactive current limit controller adapted to control the converter reactive current to a converter current limit by adjusting the converter modulation voltage limit.

[0045] Furthermore, a wind park is provided, comprising a plurality of wind turbines connected to a wind park grid, wherein at least one wind turbine comprises a device according to one of the preceding embodiments.

[0046] The aspects defined above and further aspects of the invention are apparent from the examples of embodiment to be described hereinafter and will be explained with reference to the examples of embodiment.The invention will be described in more detail hereinafter with reference to examples of embodiment but to which the invention is not limited. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Embodiments of the present invention will now be described with reference to the accompanying drawings. The present invention is not limited to the embodiments described or illustrated.

[0048] Figure 1 Schematically illustrates a method of controlling at least one converter according to an embodiment of the present invention;

[0049] Figure 2 schematically illustrates a portion of an apparatus for controlling at least one converter, the portion illustrating droop control between a converter power reference limited by an available real power clamp and a converter frequency;

[0050] Figure 3 illustrates the evolution of different electrical parameters as obtained in an embodiment of the invention;

[0051] Figure 4Schematically illustrates parts of an apparatus for controlling at least one converter according to an embodiment of the invention, comprising a smoothing controller;

[0052] Figure 5 illustrates the evolution of electrical parameters as obtained in an embodiment of the present invention; and

[0053] Figure 6 Parts of an apparatus for controlling at least one converter according to an embodiment of the invention are illustrated, with an emphasis on a reactive current limiting controller. DETAILED DESCRIPTION

[0054] Figure 1 The method 1 schematically illustrated in FIG1 for controlling at least one converter of at least one wind turbine connectable to a wind farm grid of a wind farm during a black start or islanding mode starts with method step 3, in which a converter black start is initiated. Therefore, a precondition is that the wind turbine rotor of the wind turbine is rotating and the generator is controlling the DC link voltage.

[0055] Method 1 illustrates in particular how the utility grid (wind farm grid) side portion of the converter is controlled. At method step 5, the current wind farm grid voltage is measured. At method step 7, a voltage reference ramp is initialized to start from the measured wind farm grid voltage. In method step 9, a loop is entered in which the converter voltage reference is ramped up at a specified rate. As indicated in box 11, the active and reactive power loads of the converter increase proportionally with the increase in the converter voltage reference. As indicated in box 13, the wind farm grid frequency also changes proportionally with the controller power error Perr=Powerref-Pactiveload. In method step 15, according to the frequency to power reference droop, the converter power reference increases as the frequency drops, as indicated by the reference Figure 2 As explained in further detail.

[0056] In decision block 17, a check is made to see if the converter power reference is greater than the available power. If this is not the case, it branches to branch 19, returning to the ramp-up loop step 9. Therefore, the converter voltage reference is increased, and elements 11, 13, and 15 are executed again. At some point, the converter power reference will be greater than the available power, and it branches from decision element 17 to the "yes" branch 21.

[0057] Branch 21 leads to method step 23 in which the voltage ramp is frozen. In the next method step 25, the frequency is kept at the wind farm grid frequency limit using smooth control. After method step 25, it loops back to decision element 17 via branch 27.

[0058] After the converter voltage reference has been increased in method step 9 and after the active and reactive power load of the converter has increased proportionally to the voltage, according to block 29 the converter current is also increased proportionally to the active and reactive load.

[0059] In decision element 31, a check is made as to whether the converter current is greater than the converter current limit. If this is not the case, it branches into branch 33, leading to a further decision element 35. In decision element 35, a check is made as to whether the converter current has just exceeded this limit. If this is not the case, it branches into the "No" branch 37, returning to the ramp-up loop 9.

[0060] If, on the other hand, the converter current has just exceeded the limit, it branches into branch 39 and thus returns to method step 5 , in which the current wind farm grid voltage is measured.

[0061] If decision element 31 determines that the converter current is greater than the converter current limit, then a "yes" branch 41 leads to method step 43, in which the voltage ramp is frozen. As a next method step 45, reactive current limit control is utilized to maintain the converter current at the converter current limit. After method step 45, the method loops back to decision element 31 via branch 47.

[0062] After the converter voltage reference has been increased in method step 9, a check is performed in decision element 48 to determine whether the target wind farm grid voltage has been reached. If this is the case, the black start process is completed, as indicated in method step 49, and island mode is entered. Alternatively, the entire wind farm can be connected to the utility grid. If decision element 48 determines that the target wind farm grid voltage has not yet been reached, it loops back to ramp-up loop method step 9.

[0063] Figure 1 The diagram illustrated in illustrates a sequence of method steps for each individual wind turbine converter. The startup of the converter in each wind turbine is not intended to be coordinated, but may depend on, for example, the wind speed. In particular, each wind turbine may increase its voltage until a frequency or current limit is reached, or indeed a target voltage level for the wind farm grid is reached. When a converter reaches any of these limits, the controller will appear in an appropriate loop until a decision can be made to exit the loop because the converter is no longer in limit (see loops 27, 47). The mechanisms by which a converter may transition from "in limit" to "out of limit" may be: 1) if another wind turbine in the wind farm is connected and starts supplying part of the total load; or 2) if the load (active or reactive) drops for some reason; or 3) for example an increase in wind speed.

[0064] It is possible that a second or more wind turbines can be connected before the first turbine reaches either limit. In this case, all connected wind turbines can ramp up the voltage together, cycling the loops 27, 45, and 19 until they hit the current or power limit or the target voltage. The target voltage may not always be equal to the nominal voltage of the wind farm grid.

[0065] Figure 2 A portion 50 of an apparatus for controlling at least one converter (active power clamp) according to an embodiment of the present invention is schematically illustrated. A grid forming control block 51 receives a converter power reference 53 and a power feedback 55. The grid forming control 51 outputs a frequency 57 (ω). Frequency 57 is supplied to a frequency to power reference droop module 59, which outputs a deviation 61 of the power reference. The deviation 61 of the power reference can be added to the power feedback 55 using an adding element 63, which can be filtered using a filter 65 or not. The addition of the power feedback is optional and may not always be necessary. However, it can result in fewer changes in the converter frequency, thereby providing a cleaner behavior.

[0066] The sum obtained by summing element 63 yields the final power reference 53, which the grid forming control also acts to control. Final power reference 53 is the sum of output 61 and the output of filter 65, which has been limited by limiter 69 and is supplied to grid forming controller 51. Limiter 67 receives available power 69 and limits output 54 of summing element 63 to available power 69, thereby yielding final power reference 53.

[0067] Limiting the active power and frequency can be achieved using the available power clamp described below:

[0068] As the converter increases the voltage at its terminals, it begins to feed active power into the electrical network to satisfy the passive loads. The design of the grid forming controller algorithm uses the error between the power feedback and the power reference to drive a frequency change and subsequent voltage vector angle shift to control the power output by the converter. As the active load on the converter increases, the droop between the frequency error and the power reference can be used to control the frequency. This droop operates by observing the frequency change caused by the increase in active load and then increasing the converter's power reference proportionally to balance the power feedback. It therefore acts to stabilize the frequency (see Figure 2 ), and provides the operating point for the converter. The gain of this droop is chosen so that for a given frequency change, a specific change in the power reference applies.

[0069] The operator of a power converter or electrical network may place limits on the maximum frequency deviation from the nominal value (usually specified by international standards). Furthermore, the amount of active power that a converter can supply to the electrical network is also limited by the amount of power available from its prime mover (e.g. wind). The droop between the frequency and the power reference alone cannot prevent violations of the available power limit or the frequency limit, and therefore this must be supplemented by two additional control features:

[0070] Available Active Power Clamp - When the available power limit is reached (defined by prevailing wind conditions, less losses), increases in the controller power reference are clamped at the limit. This has the effect of disengaging the droop between frequency and power reference, thus allowing the frequency to increase or decrease in proportion to the load until it hits its own limit.

[0071] Voltage Ramp Freeze and Smoothing Controller - This controller detects when the frequency reaches a limit and then freezes the ramp of the converter voltage and applies a proportional-integral type control to the converter voltage to control the frequency to the limit. By freezing the voltage ramp, the controller prevents further increases in the active load (assuming that the active power loads of the system are proportional to the grid voltage, i.e. they are passive resistive loads). This slows the increase in frequency deviation, allowing the smoothing controller to assume control of the converter voltage to regulate the active power load placed on the converter and bring the frequency back to the limit by changing the balance between the power reference and the feedback (see Figure 3 ).

[0072] An alternative implementation of the droop between frequency and power reference is to sum the measured power feedback (unfiltered or filtered) with the output of the droop and then clamp it by the available power (see Figure 2 This additional link allows the converter frequency to remain at or near nominal until the available power limit is reached. At this point, the frequency will deviate until it hits the voltage ramp freeze and smoothing controller thresholds described above.

[0073] To allow the electrical network voltage to continue ramping up to nominal levels without violating the power and frequency limits of any one converter, additional black-start-capable converters connected in parallel are required to share the load. When multiple grid-forming controlled converters are connected in parallel, the frequency of the electrical network is a common variable that allows for sharing of active power loads. When the target voltage of the electrical network has been reached, the voltage ramp freeze and smoothing controllers can no longer operate, and frequency limits may be imposed solely on the availability of sufficient power from all connected converters to supply the load.

[0074] Figure 3 Several graphs are shown for converter voltage (ordinate 71), load power (ordinate 73), and frequency (ordinate 75), with abscissa 70 indicating time. According to curve 77, the black start voltage is ramped up at a constant rate. According to curve 79, the load power also increases. Simultaneously, the frequency according to curve 81 decreases below target frequency 83 and encounters frequency limit 85 at time 87. At this instance, the ramp-up of the converter reference voltage is stopped in order to keep frequency 81 close to frequency limit 85. 72 indicates the voltage target value.

[0075] Figure 4 Part of an apparatus for controlling at least one converter is schematically illustrated, with the focus on smoothing controller 100. Frequency limit 85 and converter frequency 91 are subtracted from one another, wherein if frequency 91 is less than zero, the frequency limit is added and frequency 91 is subtracted. If frequency 91 is greater than zero, the frequency limit is subtracted and frequency 91 is added. The result of difference element 93 is denoted by reference symbol 95 and supplied to controller 97, which derives therefrom a change in reference voltage (ΔVref), which is denoted by reference symbol 99.

[0076] Figure 5 The diagram shows a graph of electrical parameters as implemented in an embodiment of the present invention. Thus, the converter voltage (ordinate 101) and the reactive current (ordinate 103) are indicated as a function of time (abscissa 103). According to curve 105, the converter voltage rises according to the black start voltage ramp.

[0077] In the reactive current diagram, a first current limit 107 and a second current limit 109 are indicated. Depending on whether the load on the wind farm grid is inductive or capacitive, the reactive current will be positive or negative, respectively. When the load is inductive and the reactive current is positive ( Figure 5 Limitations 107 and 109 may apply when (not shown in FIG, but it may be the case according to another embodiment) the ?nctions 107 and 109 may apply.

[0078] When the load is capacitive and the reactive current is negative (see Figure 5 When the current curve 112 is used, limits 119 and 121 may be applied.

[0079] Thus, for the case of a positive reactive current, there may be a first and a second current limit defined. Furthermore, for the case of a negative reactive current, there may be a further first and a further second current limit defined.

[0080] Curve 112 indicates the reactive current of the first converter. Curve 114 indicates the reactive current of the second converter. At time 111, the first converter encounters its first converter current limit 119. As a result, further increases in the converter voltage reference are frozen or stopped during time interval 113. At time 115, the second converter is connected to the wind farm grid and continues to ramp up the voltage. During time interval 117, reactive current control is activated to continue limiting the current 112 of the first converter. Thus, a first current limit 119 associated with the first converter and a second current limit 121 also associated with the first converter are indicated. According to reactive current control, the reactive current 112 of the first converter is controlled to approach the second current limit 121.

[0081] Figure 6 A schematic diagram illustrates a portion of a controller for at least one converter, with a focus on reactive current limit control 120. A difference element 123 is used to calculate the difference between a voltage reference 125 and a voltage feedback signal 127. This difference is supplied to a PI controller 129, which outputs a converter modulation voltage demand 131. Converter output voltage 131 is subject to a limiter 133, which receives upper and lower limits 136 from another PI controller 135. The difference between a converter current limit 137 and a feedback converter current 139 is supplied to another PI controller 135, with a difference 141 calculated by a difference element 143. If reactive current feedback 139 is greater than zero, difference element 143 calculates the difference between signals 139 and 137. If the reactive feedback current is less than zero, difference element 143 calculates the difference between signals 137 and 139. If the feedback reactive converter current is greater than zero, an upper limit is applied to limiter 133. If the reactive feedback converter current is less than zero, the lower limit of the limiter 133 is applied.

[0082] Limiting the reactive power / total output current can be performed as described below:

[0083] As the converter ramps up the voltage, it will also begin to exchange reactive power with the electrical network to satisfy the reactive load of its components. The reactive load placed on the converter is also a function of the voltage in the electrical network and will therefore increase proportionally as the converter voltage ramps up. The limiting factor within the converter on the amount of reactive load it can supply is the current limit, which accounts for the conduction of both active and reactive current components.

[0084] In order to ensure that the reactive load placed on the converter does not violate the converter current limit, a calculation is performed given the maximum capacity available for reactive current flow (Equation (1)).

[0085]

[0086] This calculation prioritizes active current, and so once the active current is subtracted from the total current limit, the remaining current capacity is allocated to the reactive component.Once the reactive current limit is known, it is used to drive the reactive current limit feature.

[0087] The reactive current limiting feature can be operated by controlling the voltage drop across the converter filter reactance. When performing a black start, there are actually two levels of current limiting that may exist:

[0088] 1) The level at which the black start voltage ramp is frozen.

[0089] 2) The actual "hard" reactive current limit of the converter.

[0090] When the current has risen to a first limit (e.g., Figure 5 119 or 107 in ), the ramp up of the converter voltage is frozen, thereby allowing the reactive current load on the converter and the electrical network voltage to stabilize. This limit is chosen to be within the hard current limit of the converter so that there is some slack for the converter to balance the reactive load and thus stabilize the voltage. Once the voltage is frozen after hitting this limit, continuation of the black start voltage ramp up towards its target can only be achieved by adding one or more parallel, black start capable converters with which the first converter can share the load (see Figure 5 ).

[0091] Once the additional converter has been connected, it can begin to ramp up the electrical system voltage from the level where the first converter has reached its current limit. When this ramp up begins, it will cause the current of the first converter to increase beyond its first current limit until it reaches its second "hard" current limit (e.g., Figure 5 109 or 121), at this time, the reactive current limit controller can be activated.

[0092] The reactive current limit controller can override the voltage controller of the grid forming algorithm and uses proportional-integral (PI) action to control the reactive current to the limit value by adjusting the converter output voltage (see Figure 6 The reactive current controller can choose to limit the converter voltage to an upper or lower limit depending on the direction of current flow (inductive or capacitive).

[0093] As the second converter continues to ramp up the electrical system voltage, the reactive current controller of the first converter can force its output voltage to follow the ramp so that its current remains at the limit. As the voltage ramp continues, the second converter will supply any changes in the reactive current load on the electrical network until it reaches its first current limit and freezes the voltage ramp, or the target voltage for the ramp is reached.

[0094] Once the black start voltage ramp has increased the electrical system voltage to a target level (typically the nominal voltage, or at least 90% of the nominal voltage), the first current limit may become ineffective because the voltage controller maintains the voltage equal to the target. However, when the hard current limit is reached, the reactive current limit controller may continue to function regardless of the converter voltage.

[0095] Features and advantages according to embodiments of the present invention may include the following or may provide the following advantages:

[0096] Regarding active power and frequency limitations:

[0097] 1) Limitation of the converter power reference at the level of the motive force available power, which may then force the frequency to its limit and may freeze the black start voltage ramp, as per 2).

[0098] As an advantage, the converter controller will respect the limitations of the prime mover (wind) available power while performing a black start voltage ramp or operating in island mode.

[0099] 2) When the frequency limit is reached, the black start voltage ramp is frozen and a smooth controller is used to vary the electrical system voltage to modulate the load placed on the converter, allowing the frequency to track the limit.

[0100] As an advantage, the converter controller can be relied upon to automatically increase the electrical system voltage until a prescribed frequency limit is reached, and can then be relied upon to prevent the frequency from exceeding that limit.

[0101] 3) The addition of power feedback (unfiltered or filtered) to the output of the frequency to power droop to keep the frequency close to the nominal value until the available power limit is reached may have advantages because no large frequency deviations will be experienced during the black start voltage ramp unless the available power limit is reached, giving the process a cleaner action.

[0102] Regarding the limitation of reactive power / total output current:

[0103] 4) Freeze the black start voltage ramp to comply with the current limit while allowing some current headroom to balance the reactive current load.

[0104] 5) Use a reactive current limit controller to ensure that the converter's current output does not exceed a hard limit by ensuring that the converter output voltage tracks the electrical system voltage within a sufficient margin.

[0105] The power converters of the wind turbines can perform a distributed black start of all or part of the wind farm electrical system using one or more WTGs for energization, and then maintain voltage and frequency stability while complying with available power, frequency, and current limits. In addition, these embodiments can allow the wind farm to energize a wider portion of the national grid and thus facilitate black starting of the entire grid after a power outage.

[0106] It should be noted that the term "comprising" does not exclude other elements or steps, and "a" or "an" does not exclude a plurality. Moreover, elements described in conjunction with different embodiments may be combined. It should also be noted that reference signs in the claims should not be construed as limiting the scope of the claims.

Claims

1. A method of controlling a grid-side converter part of at least one converter of at least one wind turbine, the at least one converter of the at least one wind turbine being connected to a generator of the wind turbine via a generator-side converter part, the grid-side converter part being connectable to a wind farm grid of the wind farm during a black start of the wind farm, the method comprising: ramping up a converter voltage reference defining a desired output voltage of the utility-side converter part to the wind farm grid unless a power-related condition, a wind farm grid frequency-related condition, or a converter current-related condition is violated, the power-related condition including a limit on the amount of active power that the converter can provide, the wind farm grid frequency-related condition including a maximum allowable frequency deviation from a target wind farm grid frequency, and the converter current-related condition including a converter current limit; Further including: Measure wind farm grid voltage; Wherein ramping up the converter voltage reference comprises: The sum of the wind farm grid voltage and the positive increment is supplied to the converter as a converter voltage reference; When the wind farm grid frequency falls below the target wind farm grid frequency, the converter power reference is increased; Continue increasing the converter voltage reference to the target value of the wind farm grid voltage unless: The converter power reference is greater than the available converter power and / or The absolute value of the converter current is greater than the converter current limit. 2 . The method of claim 1 , wherein the available converter power is based on the available power of a generator driven by a rotating shaft to which a plurality of rotor blades are connected.

3. The method according to claim 1 , further comprising: If the converter power reference is greater than the available converter power: interrupting further increasing said converter voltage reference; The wind farm grid frequency is maintained at the wind farm grid frequency limit using smooth control.

4. The method of claim 3, wherein using smooth control comprises: The converter voltage reference is changed, reduced, depending on the frequency difference between the park grid frequency and the park grid frequency limit in order to prevent the park grid frequency from decreasing further in order to keep the park grid frequency (ω) close to the park grid frequency limit.

5. The method according to claim 1, further comprising: If the absolute value of the converter current is greater than the converter current limit: interrupting further increasing said converter voltage reference; Reactive current limit control is used to keep the converter current at the converter current limit.

6. The method of claim 5, wherein using reactive current limit control comprises: The converter modulation voltage limit is changed depending on the difference between the reactive converter current limit and the reactive converter current in order to keep the converter current at the converter current limit.

7. The method of claim 5, wherein the reactive current limit control selects whether to limit the converter modulation voltage to an upper limit or a lower limit depending on whether the reactive converter current is inductive or capacitive.

8. The method according to claim 3, wherein the wind farm grid frequency limit is given as a difference between a target frequency and a maximum frequency deviation.

9. A method of controlling a plurality of converters of a plurality of wind turbines connectable to a wind farm grid of a wind farm during a black start of a wind farm, the method comprising: The method according to claim 1 is performed independently for each of the plurality of converters.

10. An apparatus for controlling at least one converter of at least one wind turbine connectable to a wind park grid of a wind park during a black start of a wind park, the apparatus comprising a controller part adapted to control or perform the method according to claim 2.

11. The apparatus according to claim 10, comprising: An available real power clamp is adapted to ensure that the converter power reference is constrained by the available power.

12. The apparatus according to claim 10, comprising: A smoothing controller is adapted to control the wind park grid frequency to a wind park grid frequency limit by varying the converter reference voltage.

13. The apparatus according to claim 10, comprising: A reactive current limit controller is adapted to control the converter reactive current to a converter current limit by adjusting the converter modulation voltage.

14. A wind farm comprising a plurality of wind turbines connected to a wind farm grid, wherein at least one wind turbine comprises the apparatus according to claim 10.

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