WO2009083446A2

WO2009083446A2 - Apparatus and method for controlling reactive power from clusters of wind turbines connected to a utility grid

Info

Publication number: WO2009083446A2
Application number: PCT/EP2008/067695
Authority: WO
Inventors: Allan Holm JØRGENSEN; Lars Helle; Leonard Schaier
Original assignee: Vestas Wind Systems A/S
Priority date: 2007-12-28
Filing date: 2008-12-17
Publication date: 2009-07-09
Also published as: WO2009083446A3

Abstract

The present invention relates to a method for operating a wind farm comprising a plurality of wind turbine generators having substantially the same real output power ratings supplying power to a grid. The method comprises the steps of selecting at least one turbine in a first group of wind turbines to support reactive power needs of the farm or grid by supplying or absorbing reactive power when needed; selecting at least one turbine in a second group of wind turbines to support reactive power needs of the farm or grid by absorbing reactive power when needed; and commanding at least one wind turbine in the first or second group to support reactive power needs of the farm or grid.

Description

APPARATUS AND METHOD FOR CONTROLLING REACTIVE POWER FROM CLUSTERS OF WIND TURBINES CONNECTED TO A UTILITY GRID

FIELD OF THE INVENTION

The present invention relates to controlling the reactive power from clusters of wind turbines to control voltage on an electric power grid, and, more particularly, to an apparatus and method for controlling the reactive power contributions of at least two clusters of individual turbines, with at least one cluster adapted to provide positive reactive power and at least one cluster adapted to provide negative reactive power.

BACKGROUND OF THE INVENTION

A wind turbine is an energy converting device that converts kinetic energy in the wind into electrical energy for use by customers connected to a utility power grid. This type of energy conversion typically involves using wind to turn turbine blades that, in turn rotate the rotor of an AC electrical generator either directly or through a gearbox.

The electrical power available from a wind driven generator and supplied to a utility grid is a function of the power available from the wind, its speed, losses in the grid and the characteristics of the distribution system and loads connected thereto. Because the wind speed and load fluctuates, voltage levels applied to the grid may vary. Likewise since most electric power transmission components have a significant reactive component; voltages in the grid are also a function of the reactive characteristics of loads and components connected to the grid.

To prevent damage to equipment, grid voltage must be held within certain tolerances. One method of supporting grid voltage control is the use of suppliers or absorbers of variable amounts of reactive power to compensate for voltage changes due to the reactive nature of the grid. When overhead lines are primarily inductive, for example, a passive device such as an inductor absorbs reactive power and tends to lower grid voltages while a capacitor supplies reactive power which tends to raise grid voltages.

A synchronous generator with wound rotor or a doubly-fed induction generator (DFIG) can also be used as a supplier or absorber of reactive power and therefore contribute to voltage control. The state of the machine depends on whether the level of rotor current provided is greater or less than that needed to provide sufficient flux to generate rated output voltage. When excess current is applied to the rotor of either machine it is considered to be overexcited. In this state more flux than necessary is generated by rotor current, the machine supplies or generates reactive power from the stator and, with regard to reactive power, acts like a capacitor. By convention, this reactive power from the generator is considered to be positive (flowing from the generator) and typically labeled "+Q".

If on the other hand the machine receives too little rotor current it is considered to be under excited. In this state the machine absorbs reactive power into the stator to help supply flux not provided by its rotor. By convention, reactive power absorbed into the stator is considered to be negative (flowing into the generator) and typically labeled "-Q".

A machine's rating, and hence its frame size, is determined by its ability to simultaneously deliver real and reactive power. An ability to deliver real and positive reactive power is dependent on rotor current. Rotor heating is directly proportional to the square of total rotor current (I²R) and is therefore proportional to the vector sum of direct and quadrature components of the rotor current. The direct component of rotor current is responsible for generating flux while the quadrature component is responsible for producing torque and power. Rotor heating is also due to rotor core losses from excitation flux. Any increases in rotor current to increase reactive power generation therefore increases rotor heating by the aforementioned I²R losses and also rotor core losses. Power output limits of a prior art doubly fed induction generator due to rotor heating are shown in Fig. 1 by line 112 Likewise while absorption of reactive power does not increase rotor heating above what would be experienced by delivery of only real power, the increase in flux due to absorption of reactive power does cause excess heating of end sections of a machines stator. Power output limits of the prior art doubly fed induction generator due to Stator end heating are shown in Fig. 1 by line 114.

However because the main function of the wind turbine is to extract real power from the wind and power is established by the quadrature component of rotor current plus a small direct current to establish flux, there is a lower limit on rotor heating and therefore on rotor and frame size.

Generally speaking a utility grid requires positive reactive power more often and in greater quantities than negative reactive power. This results from the tendency of transmission systems and loads to be absorbers of reactive power thus causing grid voltage drops that can only be compensated for by applying positive reactive power.

Advantageously, because a large portion of the reactive power to be supplied is based on slowly varying grid conditions such as changes in desired power factor, a large portion of the reactive power generation task, normally provided by generators, can be provided cost effectively by relay switched capacitors and in some cases by FACTS or STATCOM type devices. The benefit to using capacitors or other FACTS or STATCOM type devices is that more generators with smaller frame sizes, with their attendant cost savings, can be used. Of course a disadvantage of relay switched capacitors is speed of response but this disadvantage is not shared with in FACTS or STATCOM devices.

DESCRIPTION OF THE INVENTION

According to the present invention reactive power is generated or absorbed by at least two groups of wind turbines configured with doubly fed induction generators. Each of these groups of wind turbines are herein termed a cluster. One cluster comprises wind turbines that both generate reactive power (+Q) by operating in an overexcited mode and, if desired, also absorb reactive power in an underexcited mode while the second group is adapted to only absorb reactive power (-Q).

According to one preferred embodiment of the present invention the first cluster may both generate and absorb reactive power because once a rotor is sized for reactive power generation, there may be little or no benefit to restricting its ability to absorb reactive power.

The wind turbines configured for supplying reactive power have generators with rotors sized for the extra heat when operating in an overexcited mode while generators in wind turbines assigned to absorb reactive power do not require the larger rotors and may utilize a smaller frame size and are therefore smaller and lighter.

In the preferred embodiment additional reactive power generation capability may also be provided by FACTS or STATCOM type devices located at a farm level or switched capacitors at a wind turbine or farm level.

Thus, by use of two clusters; one for generating Q (and if necessary for absorbing Q) and the other for absorbing Q a more efficient system for addressing reactive power requirements, particularly in instances of out of the ordinary conditions, is provided. Although not intended as limiting, an exemplary embodiment of the present invention would include a wind farm with 100 wind turbines with 75 turbines assigned to the cluster that generates Q and 25 in the cluster that absorbs Q.

In a preferred embodiment each wind turbine includes a Reactive Power Selector that selects an appropriate reactive power command for each turbine. That is, if grid conditions demand or suggest that a farm generate additional reactive power, then the command for additional reactive power is sent to all turbines, but only those having their Reactive Power Selector set to accept a command for generation of reactive power will direct the reactive power command to a Power Controller in the appropriate wind turbine. In an alternate embodiment a park controller maintains a cluster map that details the addresses of those turbines that are designated to generate reactive power and those that are designated for absorption of reactive power. When reactive power is needed by the grid, the park controller uses the map to identify those turbines that are to receive a command to either supply or absorb reactive power to fulfill the need and addresses the grid requirements by selectively directing the reactive power requirements to the appropriate wind turbines.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring to the figures in which like numbers refer to like elements:

Fig. 1 is graphical representation of real and reactive power capabilities of a prior art doubly-fed induction generator.

Fig. 2 is an illustration of a typical wind turbine assembly

Fig. 3 is a schematic illustration of a wind park having two clusters of multiple wind turbines.

Fig. 4 gives a block diagram that shows the major control systems of the wind turbine of the present invention.

Fig. 5 is a graphical representation of a transfer function that relates a wind farm reactive power output to voltages measured at the point of common connection for the farm and grid to which it provides power.

Figs. 5a and 5b represent transfer functions of individual turbines of the present invention.

While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. It should be understood, however, that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE DRAWINGS

As graphically illustrated in Fig. 2, a Wind Turbine 10 is supported on a Tower 20 and a Wind Turbine Nacelle 30 is positioned on top of Tower 20.

As is typical, Wind Turbine Rotor 23 has three Wind Turbine Blades 25 connected to Hub 24 through Pitch Mechanisms 26. Each Pitch Mechanism 26 includes a blade bearing and pitch actuating means which allows the blade to pitch. The pitch process is controlled by a pitch controller. Details of the blade bearings, pitch actuating means and pitch controller are well known in the art and not shown.

Referring to Fig. 3, a number of Wind Turbines 10 and 12 are located in a Wind Park 100 having a Park Substation 110. Park Substation 110 includes Static Var Compensator 115 for providing reactive power to support grid voltage control and power transfer. Wind Turbines 10 are adapted to generate reactive power while Wind Turbines 12 are adapted to absorb reactive power.

As discussed further, Wind Turbines 10 receive a reactive power reference derived from the transfer function of Fig. 5a while Wind Turbines 12 receive a reactive power reference derived from Fig 5b. In view of the fact that Generators 245 (shown in Fig. 4) in Wind Turbines 12 are only to be absorbers of reactive power, they may be built using a smaller frame size because, as absorbers of reactive power, their rotor heating is minimized.

Fig. 5 represents a reactive power output of Wind Farm 100 which is the combination of reactive power output from Wind Turbines 10 and 12 when no other major generators or absorbers of reactive power are active at Wind Farm 100. Fig. 5 represents a condition where Wind Farm 100 is operating at unity power factor (no net flow of reactive power) in Region II of Fig 5. In a preferred embodiment, the curves of Fig 5 are stored in Flash memory in Reactive Power Selectors 280 and are changeable under control of Park Controller 110 if it is desired to alter the slope of lines 205 and or 206 or change the limits of Region II of Fig. 5.

Substation 110 also includes Point of Common Connection (PCC) Voltage Sensor 125 and Reactive Power Controller 117. Reactive Power Controller 117 operates in a commanded reactive power mode whereby reactive power output is responsive to PCC Voltage Sensor 125 and grid voltage conditions around nominal voltage as well as to reactive power (or power factor) commands from a system operator (not shown).

Referring to Fig. 4, Wind Turbine Rotor 23 (in Fig. 2) mechanically drives Doubly-Fed Induction Generator (DFIG) 245, through a gearbox (not shown). The electrical outputs of Generator 245 are provided to a utility grid via substation 110, from Stator 245a of Generator 245 at all power-producing generator speeds, and also from Rotor 245b via Rotor Side Inverter 225, DC Link 230 and Grid Side Inverter 235 for generator speeds above synchronous speed. An illustrative synchronous speed is 1800 rpm for a grid frequency of 60 hertz. Output from Stator 245a and Grid Side Inverter 235 of the converter are combined down stream of Transformer 240 shown in Fig 4.

A Power Reference Set Point based on wind speed is fed to Error Detector 205 where its is compared to the combined real current output from the Stator 245a and Grid Side Inverter 250 as measured in Power Sensor 275. Error output from Error Detector 205 is fed to the Rotor Inverter Controller 220 which is based on a Scalar Control Algorithm and is processed to provide variable frequency gating signals for Pulse Width Modulator (PWM) 224 semiconductors in Rotor Side

Inverter 220. The processing includes generating the variable frequency gating signals so that the frequency of output signal of the Rotor Inverter Controller 220 is equal to the difference between the electrical equivalent of the rotor speed and the grid frequency. Inputs to the Rotor Inverter Controller 220 include output of Rotor Position Sensor 205 and a sample of wind turbine output voltage. Although the preferred embodiment utilizes Scalar Control of rotor signals Field Oriented Control of the rotor signals may be used in an alternate embodiment. In yet another alternate embodiment Resonance Controllers may be uses for control of rotor and inverter currents.

The output of Rotor Inverter Controller 220 feeds Generator Rotor 245b of Generator 245 where it controls output current of Stator 245a.

As shown in Fig. 4, Rotor Inverter Controller 220 receives two inputs. A first input is from Error Detector 210 that represents a difference between magnitude of real power that is based on the available power in the wind and the real output power. The second input is received from Error Detector 215 and represents the difference between a Reactive Power Command, which is derived from a Wind Turbine output voltage sensor or the voltage at the PCC (depending on the embodiment) and a reactive power feedback signal from Power Sensor 267. The Reactive Power Command is derived from a voltage measurement using the transfer functions of Fig. 5a and 5b stored in Reactive Power Selector 280 that gives a desired reactive power for a given sensed voltage. The transfer function located in Reactive Power Selector 280 located in wind turbines 10 is based on the curve of Fig 5a while the transfer function of Reactive Power Selector 280 for Wind Turbines 12 are shown in Fig 5b. In a preferred embodiment adaptive look up tables are provided so that the slopes of the reactive power curves in Fig. 5a and 5b and or the voltage at the boundaries of Region II can be changed when reactive power requirements as a function of voltage are changed.

In a preferred embodiment the sensed voltage input to Voltage to Reactive Power Converter 280 is measured at the PCC by voltage sensor 125 in

Substation 110 and fed, either through SCADA or, for increased speed, via dedicated links to each turbine in both clusters 104 and 108 of Fig. 3. In an alternate embodiment, where a wind turbine output is representative of a voltage at the PCC, the voltage is measured at the output of each Wind Turbine 10 and 12. The output of the Reactive Power Selector's 280 of turbines 10 and 12 are fed to Rotor Inverter Controller's 220.

When Generator 245 is rotating above synchronous speed, variable frequency power is available from Rotor 245b of Generator 245, rectified in Rotor Side Inverter 220 and fed to capacitors in DC Link 230 where ripple is reduced and energy is made available for short term transients. The DC voltage at DC Link 230 is measured across the capacitors in the DC Link 230 and is the input voltage to Grid Side Inverter 235. The voltage at the input to Grid Side Inverter 235 is fed to Grid Side Inverter 235 where it is converted to AC at the grid frequency.

There are two feedback loops that control the output of Grid Side Inverter 235 through Grid Side Inverter Controller 286. In an outer loop, a sample of voltage at DC Link 230 is compared to a DC link voltage reference in a first error detector 262. Since the objective of the outer loop is to control the DC link voltage, the output of first Error Detector 262 represents a desired real component of Grid Side Inverter 235 output current. The output from Error Detector 262 is fed to PI controller 240 and then to Error Detector 277 where it is compared to a sample of a real component of Grid Side Inverter 235 from Current Sensor 255. This error signal is processed by Inverter Current Processor 282 of Grid Side Controller 286 where it is combined with any reactive power error components (discussed below) and the result fed to PWM 284. The output of PWM 284 then controls semiconductor switches in Grid Side Inverter 235 to control output from Grid Side Inverter 235.

Any reactive component of Grid Side Inverter output current is controlled to a zero value by feeding back a sample of any reactive current output. This reactive current component is combined with the real current component described above in Inverter Current Processor 288 to form a desired Grid Side Inverter 235 output current waveform having a real part that is a substantially scaled version of the output of Error Detector 277 and a reactive component having a target value of zero. The output of Inverter Current Controller 282 feeds the desired current waveform to PWM 284 where, as discussed above, timing signals needed to generate a desired output current are created and fed to Grid Side Inverter 235.

In an alternate embodiment one or more turbines in cluster 104 may be separately called upon or otherwise triggered to generate reactive power.

Likewise one or more turbines in cluster 108 may be separately called upon or otherwise triggered to absorb reactive power. In each case reactive power generated (or absorbed) may be up to a limit value for each turbine or some percentage of the limit.

Although the invention has been described by reference to certain embodiments and prior art wind turbines, the invention is not limited to the embodiments described. Modifications and variations will occur to those skilled in the art in light of the teachings. The scope of the invention is defined with reference to the following claims.

Claims

1. A method for operating a wind farm comprising a plurality of wind turbine generators having substantially the same real output power ratings supplying power to a grid, the method comprising the steps of:

- selecting at least one turbine in a first group of wind turbines to support reactive power needs of the farm or grid by supplying or absorbing reactive power when needed;

- selecting at least one turbine in a second group of wind turbines to support reactive power needs of the farm or grid by absorbing reactive power when needed; and

- commanding at least one wind turbine in the first or second group to support reactive power needs of the farm or grid.

2. A method according to claim 1, wherein said wind turbine generators comprise doubly fed induction generators.

3. A method according to claim 1 or 2, wherein the selected turbine in the first group of wind turbines is adapted to support reactive power needs of the farm or grid by supplying reactive power when needed.

4. A method according to claim 1 or 2, wherein the selected turbine in the first group of wind turbines is adapted to support reactive power needs of the farm or grid by absorbing reactive power when needed.

5. A wind farm for providing wind generated electric power to a grid, said wind farm comprising;

- a least one group of wind turbines adapted to provide real power and generate or absorb reactive power to support the farm or grid; and - a second group of wind turbines adapted to provide real power and only absorb reactive power to support the farm or grid.

6. A wind farm according to claim 5, wherein said wind turbines comprise doubly fed induction generators.

7. A wind farm according to claim 5 or 6, wherein the first group of wind turbines is adapted to provide real power and support reactive power needs of the farm or grid by supplying reactive power.

8. A wind farm according to claim 5 or 6, wherein the first group of wind turbines is adapted to provide real power and support reactive power needs of the farm or grid by absorbing reactive power.