GB2420456A

GB2420456A - Generator control having grid imbalance detector

Info

Publication number: GB2420456A
Application number: GB0425662A
Authority: GB
Inventors: Phillip Cartwright; Lie Xu
Original assignee: Areva T&D UK Ltd
Current assignee: UK Grid Solutions Ltd
Priority date: 2004-11-23
Filing date: 2004-11-23
Publication date: 2006-05-24
Also published as: GB0425662D0

Abstract

A controller 101 for one or more generators, for example doubly fed induction generators (DFIG) attached to a wind-turbine 100, where each generator has voltage source converters 110, 112 connected on its rotor side for controlling the power at its output side. An imbalance detector 116 detects any imbalance in the grid served by the generators and the voltage source converters are controlled accordingly. Processing means 118 are provided for calculating the current orders for both positive and negative sequences according to a required active power output and these are communicated to the controllers of the voltage source converters. A plurality of DFIGs (e.g. a wind farm) may be provided (100a-g, Figure 2) having a system controller (200, figure 2).

Description

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A METIOD OF CONTROLLING OF A WIND-TURBINE

This invention relates to a method of controlling of a wind-turbine. In particular, hut not exclusively, the invention may relate to a windturbine incorporating a Doubly Fed Induction Generator (DFIG).

Due to concerns about emissions from burning fossil fuels and the depletion of fossil fuel resources, renewable energy systems are a topic of great interest and investment. In particular, wind farms are now considered a viable energy source and their utility and efficiency has been the subject of much recent research and development.

Most early wind farms used asynchronous generators with fixed speed and stall control of the blades. The early turbines used stall control because pitch control wasn't available (see Hansen M 0 L, Aerodynamics of Wind Turbines", James & James, 2000, ISBN 1-902916-06-9). Such wind farms are directly coupled to a power grid, with the connection being present when the wind speed (and therefore the electrical power produced) is sufficient to supply energy to the grid. When the power falls below a set amount (which may be zero as, if left connected, the wind-turbine will operate as a motor), they are de-coupled from the grid.

The performance of fixed speed wind-turbines depends on the characteristics of the mechanical system and they often provide variable output power which can lead to instability in the grid voltage.

In order to overcome the problems associated with fixed speed systems, new wind farms tend to use variable speed turbines that utilise an active voltage source. There are two types of generators used in variable speed generation: Fully Fed Synchronous Generators (FFSGs) and Doubly Fed Induction Generators (DFIGs). S t

* I * 1 q, *.; : : * * I * * * * * . *:. . *, DFIGs (sometimes called cascade machines) are so called as an electrical voltage is applied to both the stator and the rotor.

As the speed of the turbine rotor fluctuates, the frequency of the S alternating current flowing from the generator rotor also varies. A power converter, positioned between the generator rotor and the stator/grid, transforms the variable-frequency alternating current to direct current, and then converts it back to alternating current having a constant frequency.

The active and reactive power output of the generator stalor is controlled by the rotor side converter. Thus, taking into account the amplification factor of the inverse slip ratio of the DFJG, controlling the active and reactive power via the rotor side allows for a converter rating typically 25% of the machine rating rather than a potential 100% rating if control is realised using the grid side converter DFJGs are conventionally controlled using vector control based on d-q decoupling. The Control system is usually defined in the synchronous d-q reference frame (d and q are two arbitrary axes, one real, one imaginary.

The synchronous reference frame is stationary with respect to the magnetic

field in the generator stator.

For an ideal balanced three-phase ac system, the three-phase voltages have equal amplitude and are phase-shifted by 120 with respect to each other.

However, in practice, a three-phase ac system is usually unbalanced, i.e., either the voltage amplitudes of the three phases are different or the phase shift angles between each phase aren't 120 .

At present, steady state imbalances in an ac system are corrected by using single-phase transformers and associated voltage control relays (see Basic Active Network Management by Peter Thomas and Eric Paalman of Scottish Power). Alternative methods of resolving steady state or transient ac imbalances may be achieved by installation of an appropriately rated . ; .: , , te : : . * * *,, :. l*l *,a voltage sourced converter based Static Synchronous Compensator (STATCOM). An alternative method of resolving steady state ac imbalances may he achieved by installing a Static Var compensator or passive inductive and/or capacitive shunt elements using taps for off line adjustments.

There may he connection requirements for any power source contributing to a grid. The ability to meet connection requirements as well as the ability to accurately control voltage may have commercial benefits to a power source operator.

According to the invention, there is provided a system controller for one or more generators, each generator having voltage source converters connected on its rotor side for controlling the power at its stalor side, the system controller comprising an imbalance detector for detecting an imbalance in a grid served by the generators; processing means for calculating current orders for both positive and negative sequences according to a required active power output and negative sequence voltage; and communication means for transmitting a signal indicative of the current orders to master controllers for the voltage source converters at each generator.

The imbalance indicator may include a power quality analyser for determining whether the grid contains a negative sequence component.

According to the invention, there is also provided control circuitry for voltage source converters connected on a rotor side of, and for controlling the power at a stalor side of, a generator. the control circuitry comprising communication means for receiving a signal indicative of positive and negative sequence current orders from a system controller; * * * * * **.

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* *5S S * * I * * I S * S S I S.. * *IS 115 5 * means for receiving signals indicative of the output current and voltage of the generator; and a master controller for determining the required modifications to the voltage source controllers.

The control circuitry may include a current controller responsive to signals from the master controller to control at least one of the voltage SOUFCC controllers.

According to the invention, there is also provided a method of controlling one or more generators serving a grid, each generator having voltage source converters connected on its rotor side for controlling the power at its stator side, the method comprising a. detecting an imbalance in the grid; b. generating positive and negative sequence current orders in response to the imbalance; c. feeding the orders to a current controller serving at least one of the voltage source regulators of each generator; and d. regulating by current controller(s) the positive and negative currents according to their respective orders.

The method may include, between steps a and b, the step of deciding whether to contribute towards balancing the system or to export as much active power as possible.

According to the invention, there is also provided a method of operating a system controller for one or more generators, each generator having voltage source converters connected on its rotor side for controlling the power at its stalor side, the method comprising detecting an imbalance in a grid served by the generators; calculating current orders for both positive and negative sequences according to a required active power output and negative sequence voltage; and * . . . S *IS * S S *5 85 S S S S * S S * I SI * se. . S * * * * S S S I S S.. * us *SS * * transmitting a signal indicative of the current orders to master controllers for the voltage source converters at each generator.

The step of detecting an imbalance may include the step of determining whether the grid contains a negative sequence component.

According to the invention, there is also provided a method of operating a master controller for voltage source converters connected on a rotor side of, and for controlling the power at a stator side of, a generator, the method comprising receiving a signal indicative of a current order from a system controller; receiving signals indicative of the output current and voltage of the generator; and determining the required modifications to the voltage source controllers.

According to the invention, there is also provided a controller, the controller being operable to control one or more wind-powered generators, the controller comprising a power quality analyser operable to obtain data by sampling a power supply supplied by at least one wind-powered generator controlled by the controller and analyse the data in order to determine whether the power supply has an imbalance between the phases and the controller being further operable such that, if it is determined that the power supply is unbalanced, it calculates an output to be made to a generator being controlled thereby in order to cancel the imbalance on the power supply and subsequently operable to make the output to the generator.

A state of imbalance is detected by the power analyser using the measured three-phase voltage.

* S I S * SIS * S S IS IS S S S S * * 5 0 S SI * *SS * S I I S * S S * * S S SSS S III *SS S S This has the advantage that a wind- powered generator controlled by such a controller would be able to make a positive contribution to both the steady state power quality and to the dynamic system recovery of the grid by correcting imbalances. This in turn increases the attractiveness of wind farm technology to grid operators. Further, the controller could protect the generator from over- speeding during a temporary AC system imbalance caused by single-phase fault by controlling the generator such that maximum power output from the generator is achieved.

The controller is capable of generating a positive sequence current order.

Further, the controller is capable of generating a negative sequence current order. The controller may further he arranged to transmit the generated sequence current order (whether positive or negative sequence) to at least one generator controlled thereby. The skilled person will appreciate that a negative sequence will only be required when the power supply is in an unbalanced state and thus, when the power supply is balanced only a positive sequence current order is required whilst when the power supply is unbalanced both a positive and a negative sequence may be required.

According to the invention, there is also provided a method of determining an unbalanced state in a power supply supplied to one or more windpowered Doubly Fed induction Generators (DFIG), the method comprising: 1. monitoring the power supply with a controller and calculating whether the power supply is in an unbalanced state; 2. if it is determined that the supply is in an unbalanced state calculating how to control the DFIG in order to cancel the imbalance, and outputting the calculated control to the DFIG in order to cancel the unbalance monitored in the power supply.

According to the invention, there is also provided a system comprising a controller according to the invention, a Doubly Fed Induction Generator * * S S * *t* * S S IS 55 5 5 S S S S * S S 55 * SIS S S S S * * S S * * S S I** * *IS III * S (DFIG) powered by a wind-turbine and a power supply fed by the DFTG, the controller being arranged to monitor the power supply to determine whether it is balanced or not and ii it is determined that the power supply is not balanced to cause the DFIG to operate such that the imbalance is cancelled.

The invention is applicable to any wound rotor induction machine or generator with a voltage source converter connected on the rotor side for controlling the power at the stator side.

Embodiments of the present invention are now described with reference to the accompanying Figures of which: Figure 1 shows a schematic representation of a Doubly Fed Induction Generator (DFIG); Figure 2 shows a system controller arranged to control a wind farm comprising a plurality of DFTG wind-turbines according to a first embodiment of the present invention; Figure 3 shows a flow chart outlining the actions of the system controller of Figure 2; Figure 4 shows a flow chart outlining the actions of a master controller of the DFIG of Figure 1; and Figure 5 is a flowchart representing the actions taken by a power quality analyser of the system controller to determine whether or not an unbalanced condition exists.

The block diagram of Figure 1 shows the components and the layout of a Doubly Fed Induction Generator (DFIG) 100 together with associated control circuitry 101. The DFIG 100 comprises rotor blades 102 * * I I I Ill * I S IS IS S * S S I S S * I S IS * I.. * S * S S * S * I S S S ISS S 55* II. S * connected through a gearbox to the rotor 106. The rotor 106 rotates within a stator 108. The stator 1 08 is connected to a power output 11 6.

The DFIG 1 00 further comprises a first voltage source controller 11 0 and a second voltage source controller 11 2.

The first voltage source controller 11 0 is driven via a Pulse Width Modulator (PWM) which is in turn driven by a current controller which is controlled by a master controller 114. The master controller 114 is arranged to control the output of the DFIG 1 00 through control of the two voltage source controllers 110,112. The master controller 114 comprises an unbalance controller 116 and a power and voltage controller 118. The unbalance controller 11 6 and the power and voltage controller 11 8 have input thereto the current Is and voltage Vs of the output of the DFJG 1 00 which are then processed in order to determine the current references of the positive and negative sequence components. Thus, it will be appreciated that the current is controlled in order to try and cancel the imbalance.

A direct voltage controller 120 drives the second voltage source controller 112 via a PWM.

In use ol the DFIG 100, the power and voltage controller 118 controls the voltage source controller 110 (through the current controller and the PWM) in order to control the rotor appropriately to produce the desired output from the DFIG 100. 11 an imbalance occurs on a grid Vn to which the DFIG 100 is connected, this is detected and the unbalance controller 116 causes the first voltage source controller 110 to he driven appropriately to try and cancel the unbalance in Vn.

Figure 2 shows a system controller 200 for a wind farm 202 comprising a plurality of DFIGs lOOa-g and associated control circuitry (not shown).

The system controller 200 is connected to each DFJG I 00ag and its control circuitry and is capable of sending signals to and receiving signals * S S S * III * S S St *1 * S S S S S I S S *5 * uS S S S * S * I S S * S 5 *5* I 5*5 5.5 * S from the master controllers I 14a-g (not shown) of each DFIG lOOa-g.

The system controller 200 is also connected to a power supply grid 204 and includes a power quality analyser (essentially a voltmeter) which monitors the three-phase voltage of the power supply grid 204 for a state s of imbalance.

Figure 3 shows a flow chart outlining the actions performed by the system controller 200 in use. In step 300, the system controller 200 determines from the parameters that it is monitoring that there is an imbalance in the grid 204.

Figure 5 shows the steps taken by the power quality analyser of the system controller 200 to determine whether or not an unbalanced state exists.

The three-phase voltages are measured (step 500) and any negative sequence component extracted (step 502). The analyser determines whether a negative sequence component exists (step 504): if one exists, the analyser indicates a state of imbalance (step 506); if none exists, the analyser indicates that the system is balanced (step 508).

Referring again to Figure 3, the quality of fault is then determined, i.e. if it is a steady state fault, such as may he caused by a mechanical fault in the grid (such as a short to earth or the like) or a transient fault (such as may be caused by lightning strike or the like) (step 302).

In order to rehalance the grid, current references for both positive and negative sequences are then calculated according to the required active power output and negative sequence voltage (step 304)., This is achieved by controlling the reactive power. The required modifications to the two voltage source controllers 110,112 is subsequently determined (step 306).

A signal containing the information to generate these currents is generated (step 308) and this signal is sent to the control means 11 4a-g of the wind- * . S S S 555 * S S *5 *5 5 * S I S * S I S S ** * *5* S S S S S * I S S S S S S **V *SS S * turbines (step 310). All the turbines are sent the same signal, hut the extent to which each turbine contributes depends on its operating condition.

Figure 4 shows a flow chart outlining the response of the control means 114 of any DF!G 100 on receipt of the signal generated by the system controller 200. The signal is received in step 400. The sequence current order is regenerated from the signal in step 402 and the two voltage source controllers 110,112 are controlled by the master controller 114 in order to drive the stator of the DFIG 100 accordingly (step 404).

The power, now with an amended profile, can be output to the network in step 406 and can contribute to rebalancing the network.

Claims

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I. A system controller for one or more generators. each generator having voltage source converters connected Ofl its rotor side for controlling the power at its stalor side, the system controller comprising an imbalance detector for detecting an imbalance in a grid served by the generators; processing means for calculating current orders for both positive and negative sequences according to a required active power output and negative sequence voltage; and communication means for transmitting a signal indicative of the current orders to master controllers for the voltage source converters at each generator.
2. The system controller of Claim 1 wherein the imbalance indicator includes a power quality analyser for determining whether the grid contains a negative sequence component.
3. Control circuitry for voltage source converters connected on a rotor side of, and for controlling the power at a stator side of, a generator, the control circuitry comprising communication means for receiving a signal indicative of positive and negative sequence current orders from a system controller; means for receiving signals indicative of the output current and voltage of the generator; and a master controller for determining the required modifications to the voltage source controllers.
4. The control circuitry of Claim 3 including a current controller responsive to signals from the master controller to control at least one of the voltage source controllers.

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5. A method of controlling one or more generators serving a grid, each generator having voltage source converters connected on its rotor side for controlling the power at its stator side. the method comprising a. detecting an imbalance in the grid; b. generating positive and negative sequence current orders in response to the imbalance; c. feeding the orders to a current controller serving at least one of the voltage source regulators oleach generator; and d. regulating by current controller(s) the positive and negative currents according to their respective orders.
6. The method of Claim 5 wherein, between steps a and b, the method includes the step of deciding whether to contribute towards balancing the system or to export as much active power as possible.
7. A method of operating a system controller for one or more generators, each generator having voltage source converters connected on its rotor side for controlling the power at its stator side, the method comprising detecting an imbalance in a grid served by the generators; calculating current orders for both positive and negative sequences according to a required active power output and negative sequence voltage; and transmitting a signal indicative of' the current orders to master controllers for the voltage source converters at each generator.
8. The method of Claim 7 wherein the step ol detecting an imbalance includes the step of determining whether the grid contains a negative sequence component.
9. A method of operating a master controller for voltage source converters connected Ofl a rotor side of. and for controlling the power at a stator side of, a generator. the method comprising * , * * * .*S t V V V VS S V S S S I V * V * a.. * * S I S * S S * I * S I.. * aSS S55 * S receiving a signal indicative of a current order from a system controller; receiving signals indicative of the output current and voltage of the generator: and S determining the required modifications to the voltage source controllers.
10. A controller, the controller being operable to control one or more wind-powered generators, the controller comprising a power quality analyser operable to obtain data by sampling a power supply supplied by at least one wind-powered generator controlled by the controller and analyse the data in order to determine whether the power supply has an imbalance between the phases and the controller being further operable such that, if it is determined that the power supply is unbalanced, it is calculates an output to be made to a generator being controlled thereby in order to cancel the imbalance on the power supply and subsequently operable to make the output to the generator.
11. The controller of Claim 1 0 wherein the controller generates a positive sequence current order.
12. The controller of Claim 10 or 11 wherein the controller generates a negative sequence current order.
13. The controller of any of Claims 10 to 12 wherein the controller is further operable to transmit the generated sequence current order (whether positive or negative sequence) to at least one generator controlled thereby.
14. A method of determining an unbalanced state in a power supply supplied to one or more wind-powered Doubly Fed Induction Generators (DFIG), the method comprising: monitoring the power supply with a controller and calculating whether the power supply is in an unbalanced state; * , I I I III * I I II SI I I 4

I I I I S I I IS

* IS* * I S S * * I S I S * S S.. S *** I,. * * 2. if it is determined that the supply is in an unbalanced state, calculating how to control the DFTG in order to cancel the imbalance, and outputting the calculated control to the DFIG in order to cancel the unbalance monitored in the power supply.
15. A system comprising a controller according to Claim 10, a Doubly Fed Induction Generator (DFIG) powered by a wind-turbine and a power supply fed by the DFIG, the controller being operable to monitor the power supply to determine whether it is balanced or not and, if it is determined that the power supply is not balanced, to cause the DFTG to operate such that the imbalance is cancelled.
16. A system substantially as herein before described with reference to, is and/or as illustrated in, any one or more of Figures 1 and 2 of the accompanying drawings.
17. A system controller substantially as herein before described with reference to, and/or as illustrated in, Figure 2 of the accompanying drawings.
18. Control circuitry substantially as herein helore described with reference to, and/or as illustrated in, Figure 1 of the accompanying drawings.

1 9. A method substantially as herein before described with reference to, and/or as illustrated in, any one or more of Figures 3 to 5 of the accompanying drawings.