Methods of Operating Doubly-Fed Induction Generator Systems

Abstract

A doubly-fed induction generator (DFIG) system (100) is described. The DFIG system (100) includes an induction electric machine (102) including a stator having a stator winding and a rotor having a rotor winding. The stator winding is electrically connected to at least one output terminal (108) and the rotor winding is electrically connected to the at least one output terminal (108) by means of a power converter. The power converter includes a first active rectifier/inverter (130a) with alternating current AC terminals electrically connected to the rotor winding, and direct current DC terminals, and a second active rectifier/inverter (136a) with DC terminals electrically connected to the DC terminals of the first active rectifier/inverter by a DC link (138a), and AC terminals electrically connected to the at least one output terminal (108). A controller is adapted to control the first active rectifier/inverter (130a) so that the frequency of the AC current at its AC terminals is substantially constant during at least one of a “line charging mode” and an “islanded mode”.

Claims

1. A method of operating a doubly-fed induction generator DFIG system comprising an induction electric machine including a stator having a stator winding and a rotor having a rotor winding, wherein the stator winding is electrically connected to at least one output terminal of the DFIG system and the rotor winding is electrically connected to the at least one output terminal by means of a power converter that includes: a first active rectifier/inverter with alternating current AC terminals electrically connected to the rotor winding, and direct current DC terminals; and a second active rectifier/inverter with DC terminals electrically connected to the DC terminals of the first active rectifier/inverter by a DC link, and AC terminals electrically connected to the at least one output terminal; wherein the method comprises the step of controlling the first active rectifier/inverter so that the frequency of the AC current at its AC terminals is kept substantially constant during at least one of a line charging mode and an islanded mode.

2. A method according to claim 1, wherein the frequency of the AC current at the AC terminals of the second active rectifier/inverter varies during the at least one of the line charging mode and the islanded mode.

3. A method according to claim 1, wherein the step of controlling the first active rectifier/inverter further comprises using a rotor angle as a transformation angle, e.g., to convert between a three-phase reference frame and a rotating reference frame.

4. A method according to claim 3, wherein the step of controlling the first active rectifier/inverter further comprises vector control, e.g., two-axis vector control, wherein the step of controlling the first active rectifier/inverter further comprises generating drive pulses for controlling semiconductor switches of the first active rectifier/inverter using output signals from at least one axis controller, wherein each axis controller receives an input signal derived from a difference between a respective axis current reference and a respective axis measured rotor current.

5. A method according to claim 4, wherein the input signal to each axis controller is further derived from a respective axis measured stator current in the rotating reference frame, and optionally from a measured stator voltage in the three-phase reference frame.

6. A method according to claim 5 wherein each respective axis measured stator current is modified by a respective controller or gain function.

7. A method according to claim 5, wherein the respective axis measured stator current is provide to a transfer function to derive a setpoint that aligns the respective axis measured rotor current with the respective axis current reference to maintain alignment of the rotating reference frame with the stator voltage.

8. A method according to claim 3, wherein the rotor angle is derived from a substantially constant rotor frequency reference that is indicative of a desired rotor frequency.

9. A method according to claim 3, wherein the rotor angle is derived from an algorithm that uses a measured value of the rotor shaft speed.

10. A method according to claim 9, wherein the rotor angle is derived using a controller, e.g., a proportional-integral controller, that receives an input signal that is derived from the difference between a rotor shaft speed reference and the measured value of the rotor shaft speed.

11. A method according to claim 10, wherein the speed reference is provided by a speed regulator that regulates the rotor shaft speed of the DFIG.

12. A method according to claim 10, wherein the output of the controller is a dynamic rotor angle that is added to a base rotor angle that is derived from a stator angle and a mechanical angle.

13. A method according to claim 1, wherein the step of controlling the first active rectifier/inverter further comprises using an algorithm to select the frequency of the AC current at its rotor terminals so that the induction electric machine behaves like a synchronous electric machine.

14. A doubly-fed induction generator DFIG system comprising: an induction electric machine including a stator having a stator winding and a rotor having a rotor winding, wherein the stator winding is electrically connected to at least one output terminal of the DFIG system and the rotor winding is electrically connected to the at least one output terminal by means of a power converter that includes: a first active rectifier/inverter with alternating current AC terminals electrically connected to the rotor winding, and direct current DC terminals; and a second active rectifier/inverter with DC terminals electrically connected to the DC terminals of the first active rectifier/inverter by a DC link, and AC terminals electrically connected to the at least one output terminal; and a controller adapted to control the first active rectifier/inverter so that the frequency of the AC current at its AC terminals is kept substantially constant during at least one of a line charging mode and an islanded mode.

15. A local AC power network comprising a DFIG system according to claim 14 and an AC external circuit electrically connected to the at least one output terminal of the DFIG system and electrically connectable to a remote AC power network by means of a remote circuit breaker, the AC external circuit including a local circuit breaker and an AC power line that is electrically connected between the local circuit breaker and the remote circuit breaker.

Description

DRAWINGS

[0079] FIG. 1 is a schematic diagram of a basic DFIG system;

[0080] FIG. 2 is a schematic diagram of part of the basic DFIG system of FIG. 1 showing a controller for the machine-side converter;

[0081] FIG. 3 is a schematic diagram of a possible practical implementation of a DFIG system according to the present invention;

[0082] FIG. 4 is a flow diagram of an operating sequence for the DFIG system of FIG. 3;

[0083] FIG. 5 is a flow diagram of a blackstart step of the operating sequence;

[0084] FIG. 6 shows characteristic values of the blackstart step;

[0085] FIG. 7 is a flow diagram of a line charging step of the operating sequence;

[0086] FIG. 8 shows characteristic values of the line charging step;

[0087] FIG. 9 shows characteristic values of a network connection step of the operating sequence;

[0088] FIG. 10 is a schematic diagram of part of the basic DFIG system of FIG. 1 showing a first controller for the machine-side converter according to the present invention;

[0089] FIG. 11A is a schematic diagram of a first rotor angle generator;

[0090] FIG. 11B is a schematic diagram of a second rotor angle generator;

[0091] FIG. 12 is a schematic diagram of a second controller for the machine-side converter according to the present invention; and

[0092] FIG. 13 is a schematic diagram of a third controller for the machine-side converter according to the present invention.

[0093] FIG. 3 shows a possible practical implementation of a DFIG system 100 specifically adapted for hydro power generation. The DFIG system 100 includes a DFIG 102 whose rotor shaft is mechanically connected to a turbine assembly 104 by means of a drive train 106. The turbine assembly 104 includes a plurality of blades that may be rotated by water flow for hydro power generation. The rotation speed of the turbine assembly 104 is regulated by a turbine regulator (not shown) which controls the rotation speed by opening and closing wicket gates (not shown) that control the flow of water to the turbine assembly.

[0094] The stator winding of the DFIG 102 is electrically connected to output terminals 108 by a three-phase AC circuit 110. The AC circuit 110 includes a circuit breaker 112 and a phase reversal switch 114.

[0095] The output terminals 108 of the DFIG system 100 are electrically connected to a remote three-phase AC power network or utility grid 128 (hereinafter “remote power network”) by means of an external three-phase AC circuit 116. The AC circuit 116 includes a first switchyard 118 to which the output terminals 108 are electrically connected by a step-up transformer 120 and a local (or HV) circuit breaker 122. Additional DFIG systems may be electrically connected to the first switchyard 118 as shown.

[0096] The first switchyard 118 is electrically connected to a second switchyard 124 by an AC power line 126. Additional AC power lines may be electrically connected to the second switchyard 124 as shown.

[0097] The AC circuit 116 forms part of a local three-phase AC power network (hereinafter “local power network”) that is electrically connected to the remote power network 128 through the second switchyard 124, and in particular by means of a remote circuit breaker 168—see below. In this arrangement, the local power network would extend as far as the second switchyard 124 and possibly to further parts of the remote power network as long as there is no other significant power source with a higher rated power and/or inertia. Any additional DFIG systems electrically connected to the first switchyard 118 may be operated in a “slave mode” where they follow the operation of the DFIG system 100 shown in FIG. 3. Alternatively, any additional DFIG systems may be operated with a known control scheme such as the control scheme described with reference to FIG. 2, for example. As such, any additional DFIG systems may be considered to be local power sources that do not have a higher rated power or inertia.

[0098] The second switchyard 124 is electrically connected to the remote power network 128 operating at a fixed grid frequency, e.g., 50 or 60 Hz, when the remote circuit breaker 168 is closed.

[0099] The rotor winding of the DFIG 102 is electrically connected to three machine-side converters 130a-130c arranged in parallel by a three-phase AC circuit 132. It will be readily understood that the number of machine-side converters is not limited to three and will depend on the overall system requirements. A crowbar 134 is electrically connected to the AC circuit 132. A grid-side converter 136a-136c is electrically connected to each machine-side converter 130a-130c by a DC link 138a-138c with one or more capacitors. Each DC link 138a-138c includes a DC chopper 140a-140c.

[0100] Each grid-side converter 136a-136c is electrically connected to the output terminals 108 by a three-phase AC circuit 142a-142c that includes a transformer 144a-144c. The AC circuits 142a-142c are electrically connected to a circuit breaker 146 that is electrically connected in turn to the output terminals 108 by a three-phase AC circuit 148.

[0101] A pre-charge circuit/auxiliary grid 150 is electrically connected to the AC circuit 148 and includes a circuit breaker 152. The pre-charge circuit/auxiliary grid 150 is electrically connected to each DC link 138a-138c by a contactor 154a-154c, a pre-charge transformer 156a-156c and a rectifier 158a-158c. The pre-charge circuit/auxiliary grid 150 includes a transformer 160 and is electrically connected to an electric machine 162 whose rotor shaft is driven by a prime mover, for example a diesel engine 164.

[0102] Other electrical loads may be electrically connected to the pre-charge circuit/auxiliary grid 150 as shown.

[0103] All circuit breakers (CBs), contactors etc. are controlled to open and close by a controller (not shown).

[0104] From standstill (step 0) the DFIG system 100 may be operated in a sequence of steps shown in FIG. 4. The operation sequence will also transition the local power network through various operating modes, and in particular a line-charging mode, an islanded mode, and eventually a grid-connected mode.

[0105] Pre-Charge Blackstart Step (Step 1)

[0106] Summary: The pre-charge circuit/auxiliary grid 150 receives power from the electric machine 162 and the DC links 138a-138c are charged using the pre-charge circuit/auxiliary grid to an initial DC link voltage.

[0107] Blackstart Step (Step 2)

[0108] Summary: The excitation of the DFIG 102 is ramped up, until the grid-side converters 136a-136c may be started. After starting the grid-side converters 136a-136c, the DC link voltage is increased from the initial DC link voltage.

[0109] The detailed sequence for the blackstart step is shown in FIG. 5. Characteristic values are shown in FIG. 6. In particular, FIG. 6 shows: [0110] the load on the diesel engine 164 that drives the electric machine 162 supplying power to the pre-charge circuit/auxiliary grid 150 (the “diesel load”), [0111] the load on the turbine assembly 104 (the “turbine load”), [0112] the DC link voltage, and [0113] the stator voltage.

[0114] Switch state during the blackstart step: [0115] circuit breaker 112 is closed, [0116] phase reversal disconnector 114 is connected to “turbine” (T), [0117] circuit breaker 146 is closed, [0118] local circuit breaker 122 is open, [0119] pre-charge contactors 154a-154c are closed, and [0120] circuit breaker 152 is open.

[0121] Control state during the blackstart step: [0122] turbine regulator controls the opening/closing of the wicket gates to control rotor shaft speed, [0123] grid-side converters 136a-136c control DC link voltage, and [0124] machine-side converters 130a-130c control rotor current amplitude and frequency.

[0125] After the DC links 138a-138c have been pre-charged to the initial DC link voltage in step 1, the blackstart step may be started.

[0126] The turbine regulator may bring the rotor shaft speed of the DFIG 102 to close to the synchronous speed. For example, the rotation speed of the turbine assembly 104, and hence the rotation speed of the rotor shaft, may be controlled by operating the wicket gates to control the flow of water to the turbine assembly 104.

[0127] The speed setpoint will preferably lead to low slip to minimise the rotor active power.

[0128] The machine-side converters 130a-130c are started (step 2b) and inject a magnetisation current into the DFIG 102 to excite it. As a result of the excitation, the stator voltage will increase. Any active current flow on the stator, will apply a torque to the rotor shaft.

[0129] During step 2c, the magnetisation current reference is increased by a ramp. When the stator voltage reaches a pre-defined level (e.g., 20% to 32% of the rated stator voltage), the grid-side converters 136a-136c are started (step 2d). The grid-side converter controls the DC link voltage.

[0130] The DC link voltage reference is increased by a ramp (step 2e). With increasing DC link voltage from the initial DC link voltage, the active power flow through the pre-charge rectifiers 158a-158c will cease, and instead the active power will start to flow from the stator via the transformers 144a-144c into the DC link.

[0131] As it is unloaded, the pre-charge circuit/auxiliary grid 150 may be disconnected from the DC links 138a-138c by opening the pre-charge contactors 154a-154c (step 2f). However, it may be required that the pre-charge contactors 154a-154c remain closed to supply power to the DC links 138a-138c in case of a transient during the following steps, which leads to a decrease in DC link voltage.

[0132] With fully ramped up DC link voltage, optimization of the overall DFIG system 100 (e.g., power converter and/or turbine assembly operation) may be carried out (step 2g).

[0133] The DFIG system 100 now operates in isolated step (step 3)—see below.

Isolated Step (Step 3)

[0134] Summary: The DFIG system 100 is operated in steady state where the rotor shaft speed is controlled by the turbine regulator at de facto no load condition. The machine-side converters 130a-130c are excited by the DFIG 102 to a level in the range of 20% to 32% of rated stator voltage. The grid-side converters 136a-136c control the DC link voltage and cover losses within the electrical system.

[0135] Line Charging Step (Step 4)

[0136] Summary: The AC power line 126, which was previously not energized, is electrically connected to the DFIG 102 through the step-up transformer 120 by closing the local circuit breaker 122. The power converter is operated to increase the line voltage and to transition the local power network into a line charging mode of operation.

[0137] The detailed sequence for the line charging step is shown in FIG. 7. Characteristic values are shown in FIG. 8. In particular, FIG. 8 shows: [0138] the line voltage, [0139] the load on the diesel engine 164 that drives the electric machine 162 supplying power to the pre-charge circuit/auxiliary grid 150 (the “diesel load”), [0140] the load on the turbine assembly 104 (the “turbine load”), [0141] the DC link voltage, and [0142] the stator voltage.

[0143] Switch state during the line charging step: [0144] circuit breaker 112 is closed, [0145] phase reversal disconnector 114 is connected to “turbine” (T), [0146] circuit breaker 146 is closed, [0147] local circuit breaker 122 is open and will be closed during line charging (circuit breakers 166A, 166B that connect the AC power line 126 to the first and second switchyards 118, 124 are closed), [0148] pre-charge contactors 154a-154c are open or closed, and [0149] circuit breaker 152 is open.

[0150] Control state during the line charging step: [0151] turbine regulator controls the opening/closing of the wicket gates to control rotor shaft speed, [0152] grid-side converters 136a-136c control DC link voltage, and [0153] machine-side converters 130a-130c control rotor current amplitude and frequency. The first, second or third control schemes described below may be used to control the machine-side converters 130a-130c during the line charging step and while the local power network is operated in the line charging mode. The second control scheme may be preferred.

[0154] The speed setpoint should consider the operational characteristic of the machine-side converters 130a-130c with preference to medium output voltage and of the turbine assembly 104, which is preferably run at low speed.

[0155] The line charging step is initiated (e.g., by a control signal) and preparation is made to close the local circuit breaker 122 (step 4a). The circuit breakers 166A, 166B that connect the AC power line 126 to the first and second switchyards 118, 124 are closed. If all necessary internal conditions are fulfilled, the controller (not shown) closes the local circuit breaker 122 to connect the DFIG system to the first switchyard 118 (step 4b) and transition the local power network to a line charging mode.

[0156] With the closing of the local circuit breaker 122, the AC power line 126 will be electrically connected to the DFIG system 100 and may be energised. Due to the AC power line's capacity against ground, an inrush current is expected which imposes a transient in the DFIG 102 and the power converter. The stator voltage is expected to partially decrease.

[0157] After receiving the closed feedback from the local circuit breaker 122, and after the transient due to line connection is over, the power converter starts to increase the stator voltage and at the same time the AC power line voltage (step 4c).

[0158] With increasing voltage, the losses of the DFIG system will increase and are covered by the turbine assembly 104. For example, the wicket gates opening will be adjusted.

[0159] Islanded Step (Step 5)

[0160] Summary: The AC power line 126 is charged to a voltage of >90% of rated voltage.

[0161] Passive loads may be electrically connected and active power may be consumed up to a pre-defined level. The local power network is operated in an islanded mode. During the islanded mode there is no other significant power source with a higher rated power and/or inertia connected to the local power network. The DFIG system 100 therefore regulates the voltage and frequency of the local power network. Any additional DFIG systems electrically connected to the first switchyard 118 may support the DFIG system 100 and may be operated in a “slave mode”, i.e., where the additional DFIG system follow the operation of the DFIG system and may consequently be considered to be an extension of the DFIG system.

[0162] The first, second or third control schemes described below may be used to control the machine-side converters 130a-130c during the islanded step and while the local power network is operated in the islanded mode. The third control scheme may be preferred.

[0163] Network Connection Step (Step 6)

[0164] Summary: The power converter and the DFIG control prepares the settings for connection of the islanded local power network to the remote power network 128 through the second switchyard 124. This transitions the local power network from the islanded mode to a grid-connected mode (or normal mode).

[0165] Characteristic values for the network connection step are shown in FIG. 9. In particular, FIG. 9 shows: [0166] the line voltage, [0167] the line frequency, [0168] the opening/closing of the wicket gates that control flow of water to the turbine assembly 104 (the “hydraulic power”), [0169] the turbine speed, and [0170] the stator voltage.

[0171] Switch state during the network connection step: [0172] circuit breaker 112 is closed, [0173] phase reversal disconnector 114 is connected to “turbine” (T), [0174] circuit breaker 146 is closed, [0175] local circuit breaker 122 is closed (circuit breakers 166A, 166B that connect the AC power line 126 to the first and second switchyards 118, 124 are closed), [0176] pre-charge contactors 154a-154c are open, [0177] circuit breaker 152 is open, [0178] remote circuit breaker 168 is open and will be closed during the network connection step.

[0179] Control state during the network connection step: [0180] turbine regulator controls the opening/closing of the wicket gates to control rotor shaft speed, [0181] grid-side converters 136a-136c control DC link voltage, and [0182] machine-side converters 130a-130c control rotor current amplitude and frequency. The first, second or third control schemes described below may be used to control the machine-side converters 130a-130c during the network connection step as the local power network is transitioned from the islanded mode to the grid-connected mode. The third control scheme may be preferred.

[0183] During the network connection step, it is necessary to connect the local power network to the remote power network 128.

[0184] It is assumed that: [0185] the remote circuit breaker 168 which connects the remote power network 128 to the second switchyard 124 is physically remote to the DFIG system, [0186] there is no instant signalling to the controller for the power converter, [0187] the difference between the frequency of the DFIG system (i.e., the “stator frequency”) and the frequency of the remote power network 128 (the “grid frequency”) is very small, and in particular is small enough for a remote synchrocheck to allow the electrical connection to be made by closing the remote circuit breaker 168, and [0188] the installed power in the remote power network 128 is higher than the installed power in the islanded local power network.

[0189] This means that the instant of connection by the remote synchrocheck is not known exactly and that, after connection, the stator frequency will be forced to follow the grid frequency (i.e., the frequency of the remote power network).

[0190] The turbine regulator (not shown) may control the rotation speed of the turbine assembly 104 (e.g., by operating the wicket gates) to a speed which is pre-defined for islanded operation. This rotation speed should take into consideration the operational characteristic of the power converter with preference to medium output voltage and of the turbine assembly, which is preferably run at low speed.

[0191] When it is intended to connect the AC power line 126 to the interconnected remote power network 126, a signal may be sent to the power converter controller and the turbine regulator.

[0192] The controller and turbine regulator will apply limits to the active and reactive power, as well as to the wicket gate opening. Transients will be used to detect the remote connection and the DFIG system 100 switches to normal operation (step 7). The local power network is transitioned to a grid-connected mode.

[0193] The difference between the stator frequency ω.sub.s and the grid frequency ω.sub.g on network connection will result in a change in the rotor shaft speed of the DFIG 102. If the grid frequency is higher than the stator frequency, the rotor shaft speed will increase and the turbine regulator may adjust the wicket gates to reduce rotor shaft speed accordingly. If the grid frequency is lower than the stator frequency, the rotor shaft speed will decrease and the regulator may adjust the wicket gates to increase rotor shaft speed accordingly.

[0194] Normal Operation Step (Step 7)

[0195] Summary: The DFIG system 100 is electrically connected to the remote power network 128 and operates within normal parameters for hydro power generation.

[0196] The local power network operates in a grid-connected mode (or normal mode).

[0197] The first, second or third control schemes described below may be used to control the machine-side converters 130a-130c for a short time after the remote circuit breaker 168 is closed and the local power network is transitioned to the grid-connected mode. The third control scheme may be preferred. After a short time (e.g., less than 15 min for a manual transition or less than about 100 ms for an automatic transition) the machine-side converters 130a-130c may be controlled by a known control scheme as described with reference to FIG. 2, for example.

[0198] Houseload (Step 8)

[0199] Summary: The DFIG system 100 is operated in steady state where the rotor shaft speed is controlled by the turbine regulator at de facto no load condition. The machine-side converters 130a-130c are excited by the DFIG 102 to a level in the range of about 90% of rated stator voltage. The grid-side converters 136a-136c control the DC link voltage and covers losses within the electrical system. The high stator voltage allows connection of the auxiliary grid 150 to the stator.

[0200] Synchronisation HV Side (Step 9)

[0201] Summary: The DFIG system 100, which was previously operated in the houseload step, is synchronised to the remote power network grid 128.

[0202] FIG. 10 shows a first controller 36B according to the present invention. The controller 36B is similar to the controller 36A shown in FIG. 2 and like parts have been given the same reference sign. The controller 36B may be used to control the machine-side converters 130a-130c of the DFIG system 100 shown in FIG. 3.

[0203] To improve clarity, only one of the machine-side converters is shown in FIGS. 10, 12 and 13. The grid-side converters, the remote switchyard, the remote power network and other non-essential components have also been omitted.

[0204] The first controller 36B controls the machine-side converter 130a according to a first control scheme.

[0205] The first controller 36B includes a pulse pattern generator 38 for generating drive pulses for controlling the semiconductor switches of the machine-side converter 130a to turn on and off. The drive pulses are generated using output signals from a direct axis (or “d-axis”) current controller 40 and a quadrature axis (or “q-axis”) current controller 42.

[0206] The rotor current I.sub.r may be measured using suitable current transducers or other measuring devices and is converted from the three-phase reference frame to the dq-reference frame based on a transformation angle γ.sub.r. The dq-reference frame is a rotating reference frame, typically rotating at the stator frequency of the DFIG 102. In the dq-reference frame, the measured value of the rotor current has a d-axis component (or “d-axis rotor current I.sub.dr”) and a q-axis component (or “q-axis rotor current I.sub.qr”).

[0207] The d-axis current controller 40 receives an input signal ΔI.sub.dr derived from a difference between a d-axis rotor current reference I.sub.dr* and the d-axis rotor current I.sub.dr. The q-axis current controller 42 receives an input signal ΔI.sub.qr derived from a difference between a q-axis rotor current reference I.sub.qr* and the q-axis rotor current I.sub.qr. The d-axis rotor current reference I.sub.dr* may be provided by an active power, torque or speed controller, for example, and may be indicative of a desired active power, torque or speed for the DFIG 102. The d-axis current controller 40 uses the input signal ΔI.sub.dr to derive a d-axis rotor voltage V.sub.dr to control the semiconductor switches of the machine-side converter 103a to achieve the desired active power, torque or speed that corresponds to the d-axis rotor current reference I.sub.dr*. The q-axis rotor current reference I.sub.qr* may be provided by a reactive power, voltage or power factor controller, for example, and may be indicative of a desired reactive power, voltage or power factor for the DFIG 102. The q-axis current controller 42 uses the input signal ΔI.sub.qr to derive a q-axis rotor voltage V.sub.qr control the semiconductor switches of the machine-side converter 130a to achieve the desired reactive power, voltage or power factor that corresponds to the q-axis rotor current reference I.sub.qr*.

[0208] The d-axis current controller 40 and the q-axis current controller 42 may be proportional-integral (PT) controllers, for example.

[0209] The d-axis and q-axis rotor voltages V.sub.dr, V.sub.qr derived by the d-axis and q-axis current controllers 40, 42 are converted from the dq-reference frame to the three-phase reference frame based on the transformation angle γ.sub.r and provided as an input to the pulse pattern generator 38.

[0210] The transformation angle γ.sub.r used to convert between the three-phase and dq-reference frames is a rotor angle and is provided by a rotor angle generator 44. Unlike the controller 36A shown in FIG. 2, the rotor angle γ.sub.r is not derived from the mechanical angle and the stator angle. Instead, as shown in FIG. 11A, the rotor angle generator 44 may derive the rotor angle γ.sub.r by integrating a substantially constant (or pre-set) rotor frequency reference ω.sub.r* that is indicative of the desired rotor frequency to be maintained at the AC terminals of the machine-side converter 130a as represented below:

[00002]${\gamma }_r=\mathrm{INT}\left\{{\omega }_r*\right\}\mathrm{dt}$

[0211] Alternatively, as shown in FIG. 11B, the rotor angle generator 44 may derive the rotor frequency reference ω.sub.r* using a look-up table with reference to one or more system parameters such a stator power, grid power, rotor shaft speed etc., and represented in FIG. 11B by “X”. The rotor angle generator 44 may then derive the rotor angle γ.sub.r by integrating the rotor frequency reference ω.sub.r* as represented above.

[0212] The first controller 36B uses the rotor angle γ.sub.r generated by the rotor angle generator 44 to control the semiconductor switches of the machine-side converter 130a to achieve and maintain the desired rotor frequency during the appropriate operating steps of the DFIG system 100.

[0213] FIG. 12 shows a second controller 36C according to the present invention. The second controller 36C is similar to the first controller 36B shown in FIG. 10 and like parts have been given the same reference sign.

[0214] The second controller 36B controls the machine-side converter 130a according to a second control scheme.

[0215] The d-axis current corresponds to the active current and the q-axis current corresponds to the reactive current only if the rotating reference frame is correctly aligned with the stator voltage. When using a substantially constant rotor frequency to derive a rotor angle as the transformation angle to convert between the three-phase and dq-reference frames, this alignment might be lost in the case of loading the DFIG 2 with active power.

[0216] It is known that in the case of correct alignment, the stator current and rotor current have the following relationship:

[00003]$I_{dr}=-\frac{L_s}{L_h}{I}_{ds}+\frac{L_s}{L_h{R}_{Fe}}{V}_{ds}-\frac{R_s}{L_h{\omega }_s}{I}_{qs}$$I_{qr}=-\frac{L_s}{L_h}{I}_{qs}-\frac{V_{ds}}{L_h{\omega }_s}+\frac{R_s}{L_h{\omega }_s}{I}_{ds}$

[0217] where:

[0218] I.sub.dr is the d-axis rotor current,

[0219] I.sub.qr is the q-axis rotor current,

[0220] I.sub.ds is the d-axis stator current,

[0221] I.sub.qs is the q-axis stator current,

[0222] L.sub.s is the stator inductance,

[0223] L.sub.b is the main inductance,

[0224] R.sub.Fe is the iron resistance,

[0225] V.sub.ds is the d-axis stator voltage,

[0226] R.sub.s is the stator resistance, and

[0227] ω.sub.s is the stator angular frequency.

[0228] This alignment may be corrected on the basis of the relationship between the stator current I.sub.s and the rotor current I.sub.r.

[0229] As shown in FIG. 12, the second controller 36C includes a stator angle generator 46 that derives a stator angle γ.sub.s from a measured value of the stator voltage V.sub.s using a PLL, for example. The stator current I.sub.s may be measured using suitable current transducers or other measuring devices and is converted from the three-phase reference frame to the dq-reference frame based on the stator angle γ.sub.s. In the dq-reference frame, the measured value of the stator current has a d-axis component (or “d-axis stator current I.sub.ds”) and a q-axis component (or “q-axis stator current I.sub.qs”). The d-axis stator current I.sub.ds is provided to a gain function 48 and then to the summing node 52 that provides the input signal ΔI.sub.dr to the d-axis current controller 40. The q-axis stator current I.sub.qs is provided to a gain function 50 and then to the summing node 54 that provides the input signal ΔI.sub.qr to the q-axis current controller 42. The gain function 50 also receives the measured stator voltage V.sub.s.

[0230] In case the measured d-axis and q-axis rotor currents I.sub.dr and I.sub.qr do not fulfil the above relationship with the respective d-axis and q-axis stator currents I.sub.ds and I.sub.qs, the gain functions, 48, 50 are implemented to correct for the d-axis and q-axis currents, respectively. The gain functions 48, 50 may be implemented as a constant gain value, a first order transfer function such as a low pass function, or a PID function, for example.

[0231] In one particular implementation, the gain function 48 may be represented by:

[00004]$-\frac{L_s}{L_h}$

and the gain function 50 may be zero.

[0232] The second controller 36C may align the rotor d-axis with the stator d-axis. Conventionally, d-axis control is associated with active power and the q-axis control is associated with reactive power. The stator voltage measurement is needed to distinguish between active and reactive power current components.

[0233] FIG. 13 shows a third controller 60 according to the present invention.

[0234] The third controller 60 controls the machine-side converter 130a according to a third control scheme.

[0235] The controller 60 includes a pulse pattern generator 62 for generating drive pulses for controlling the semiconductor switches of the machine-side converter 130a to turn on and off.

[0236] A turbine regulator 64 includes a grid frequency controller 66 that derives power reference P.sub.hyd* by comparing a measured grid frequency ω.sub.g (e.g., the frequency of the local power network when operated in an islanded mode before it is electrically connected to the remote power network or immediately after a connection has been made and the local power network is operated in a grid-connected mode) with a grid frequency reference ω.sub.g*. The power reference P.sub.hyd* is provided to a power controller 68 which may adjust the flow of water to the turbine assembly 4 by controlling the wicket gates. The power controller 68 may also use the power reference P.sub.hyd* as the pointer to a look-up table to derive an optimum mechanical speed reference ω.sub.m*.

[0237] The rotor current I.sub.r may be measured using suitable current transducers or other measuring devices and is converted from the three-phase reference frame to an absolute value of the rotor current I.sub.absr. In particular, the rotor current I.sub.r may be converted using the following equations:

[00005]$I_{\alpha r}=I_{ar}$$I_{\beta r}=\frac{\left(I_{br}-I_{cr}\right)}{\sqrt{3}}$$I_{absr}=\sqrt{I_{\alpha r}^2+I_{\beta r}^2}$

where I.sub.ar, I.sub.br and I.sub.cr are the rotor currents for phase “a”, “b” and “c”, respectively.

[0238] Similarly, the stator voltage V.sub.s may be measured using suitable voltage sensors or other measuring devices and is converted from the three-phase reference frame to an absolute value of the stator voltage V.sub.abss. In particular, the stator voltage V.sub.s may be converted using the following equations:

[00006]$V_{\alpha s}=V_{as}$$V_{\beta s}=\frac{\left(V_{\mathrm{bs}}-V_{cs}\right)}{\sqrt{3}}$$V_{abss}=\sqrt{V_{\alpha s}^2+V_{\beta s}^2}$

where V.sub.as, V.sub.bs and V.sub.cs are the stator voltages for phase “a”, “b” and “c”, respectively.

[0239] The absolute value of the rotor current I.sub.absr is provided to a summing node 70 where it is subtracted from a rotor current reference I.sub.absr*.

[0240] The rotor current reference I.sub.absr* is derived from a stator voltage amplitude controller 72. The stator voltage amplitude controller 72 subtracts the absolute value of the stator voltage V.sub.abss from a stator voltage reference V.sub.abss* using a summing node 74. The difference between the stator voltage reference V.sub.abss* and the absolute value of the stator voltage V.sub.abss is provided to a controller 76 which derives the rotor current reference I.sub.absr*. The controller 76 controls the stator voltage amplitude and may be a PI controller or another suitable controller.

[0241] The difference between the rotor current reference I.sub.absr* and the absolute value of the rotor current Iabsr (i.e., ΔI.sub.absr output by the summing node 70) is provided to a q-axis current controller 78 which derives a q-axis rotor voltage V.sub.qr. The q-axis current controller 78 controls the rotor voltage amplitude and may be a PI controller or another suitable controller. In this arrangement, a d-axis rotor voltage V.sub.dr is zero.

[0242] The controller 60 includes a rotor angle generator 80 which derives a rotor angle which is used as a transformation angle γ.sub.r to convert the q-axis rotor voltage V.sub.qr into the three-phase reference frame for use by the pulse pattern generator 62.

[0243] In the rotor angle generator 80, a measured value of the mechanical shaft speed ω is subtracted from the optimum mechanical speed reference ω.sub.m* provided by the turbine regulator 64 in a summing node 86. (In an alternative arrangement, the optimum mechanical speed reference ω.sub.m* may simply be a constant value as opposed to being provided by the turbine speed controller. In this case, there would be no need for the turbine speed controller to derive the mechanical speed reference.) The difference between the speed reference ω.sub.m* and the measured shaft speed ω.sub.m (i.e., Δω) is provided to a speed controller 84 which provides an output to a summing node 90. The speed controller 84 may be a PI controller or other suitable controller. The output of the speed controller 84 represents a dynamic component of the rotor angle.

[0244] An initial stator frequency reference ω.sub.s0* is integrated by integrator 86 to derive a stator angle γ.sub.s. The mechanical angle γ.sub.m may be derived from a speed encoder fitted to the rotor shaft and is subtracted from the stator angle γ.sub.s in summing node 88 to derive a base component of the rotor angle. A summing node 90 sums the dynamic and base components of the rotor angle (i.e., γ.sub.r,dynamic and γ.sub.r,base) to derive the total rotor angle γ.sub.r. The rotor angle γ.sub.r generated by the rotor angle generator 80 is used by the controller 60 to control the semiconductor switches of the machine-side converter 103a to maintain a substantially constant rotor frequency.

[0245] The controller 60 aims to apply the same control scheme for connection to different types of AC power network or utility grid. During a line charging mode of the local power network, the DFIG 102 is effectively creating the islanded network and impregnating the voltage and frequency to the local power network and eventually to other loads that are electrically connected to it. These loads may be passive loads, active loads, or additional generators in the case of a small islanded grid that would typically have small grid inertia (i.e., reaction of grid frequency to active power changes). The dynamic of the remote power network 128 to which the DFIG system 100 is eventually connected is unknown and a robust control structure is needed. At the end of the line charging, the local power network will be electrically connected to the unknown remote power network 128 with an unknown inertia by closing the remote circuit breaker. The control scheme should remain stable during the connection to the remote power network.

[0246] The rotor angle generator 80 is initialised by the initial stator frequency reference ω.sub.s0*. If the speed reference ω.sub.m* is the same as the measured speed ω.sub.m, the initial stator frequency reference ω.sub.s0* is integrated to generate the stator angle γ.sub.s. The integrator 86 may be initialised by an initialising angle. A deviation between the measured speed ω.sub.m and the speed reference ω.sub.m* will lead to a change in the stator frequency. This deviation will be detected and corrected by the turbine regulator 64 in terms of a change in the measured grid frequency ω.sub.g, which is the same as the stator frequency. The speed controller 84 will correct for any deviation in the shaft speed. Such speed deviations may happen when a new participant has been electrically connected to the local power network, for example, after the local power network has been electrically connected to the remote power network that has a strong dynamic influence.

[0247] Whenever there is a transient in the power network that imposes a higher (or lower) torque on the DFIG 102, the shaft speed will change from the previously steady state. This will cause a reaction in the rotor angle generator 80 which may regulate the stator frequency to a higher or lower value. The turbine regulator 64 will detect the deviation in the stator frequency (which corresponds to the measured grid frequency ω.sub.g) and will use the generated power reference P.sub.hyd* to adjust the flow of water to the turbine assembly 104 by controlling the wicket gates to compensate for the higher or lower torque. The rotor frequency also remains substantially constant. The DFIG 102 therefore shows a similar frequency behaviour to a synchronous machine.