IES78624B2

IES78624B2 - A wind energy system

Info

Publication number: IES78624B2
Authority: IE
Inventors: John Gillespie; John Olav Tande
Original assignee: Gineadoiri Gaoithe Teicneolaio
Priority date: 1997-10-07
Filing date: 1997-10-07
Publication date: 1998-02-25
Also published as: GB2330256A; IES970724A2; GB2330256B; GB9727394D0

Abstract

A control unit 11 senses the phase voltages for each phase of a utility grid 2, and the control unit 11 also senses the three phase sum active and reactive powers of the grid 2. A microcomputer (24) in the control unit 11 processes the measured data to determine the maximum output power Plim which wind turbine driven generators 13, connected to the grid 2, can be permitted to generate. The limit value is sent from control unit 11 to a control processor 12 which changes the operating parameters of the wind turbines 13 to limit their output power to below Plim. The wind turbines 13 may be adjustable with respect to wind direction according to the output from control processor 12. The problem of over-voltage supply on a weakly loaded grid is thereby avoided.

Description

The invention relates to a wind energy system, particularly for supply of electrical power to relatively weak grids.

Various proposals have been made for preventing over-voltage on a grid in circumstances where the wind energy system has a high output and the grid load is weak. These proposals include regulation of reactive power, management of the load, and energy storage. These proposals are applicable in some circumstances. For example, if a suitable water system is available, and the cost is not too high in relation to the electrical benefits, energy storage may be used by pumping water during peak supply periods. However, there are many situations where these proposals are just not feasible.

The invention is therefore directed towards providing a system and method for overcoming the problems of over-voltage supply to weak grids.

Statements of Invention According to the invention, there is provided a wind energy system comprising:a wind turbine comprising means for connection to a utility grid; and a controller comprising means for sensing grid voltage level and for limiting wind turbine output to maintain quality of grid supply.

In one embodiment, the controller senses active power, reactive power, and line voltage.

In one embodiment, the controller senses the input parameters at the high-voltage side of a connection transformer. -2In one embodiment, the controller comprises an analogue sub-system for receiving the sensed data, for converting to digital format, and for capturing signals for data acquisition, and a digital sub-system for performing the data processing to determine the current maximum voltage output.

Detailed Description of the Invention The invention will be more clearly understood when the following description of some embodiments thereof, given by way of example only with reference to the accompanying drawings in which:Fig. 1 is a schematic diagram illustrating a wind energy system of the invention; Fig. 2 is a diagram illustrating a voltage control unit of the system in more detail; Fig. 3 is a full diagram illustrating operation of the voltage control unit; and Figs. 4 to 7 inclusive are diagrams illustrating operation of the voltage control unit.

Referring to the drawings, and initially to Fig. 1 there is illustrated a wind energy system 1 of the invention. The system 1 is shown connected to a utility grid 2 at a 38/10 kV transformer Point of Common Coupling. The system 1 comprises a tap 10 on the grid 2 and being connected to a voltage control unit (VCU) 11. The VCU 11 is connected to a control and supervision unit 12, which is in turn connected to five 600 kW wind turbines 13 and to a meteorological station 14.

Referring also to Fig. 2, the VCU 11 is now described in more detail. The tap 10 comprises voltage and cunent transformers and is mounted at the 38 kV side of the 38/10 kV transformer.. Measurement converters 20 are connected to the tap 10 for conversion -3of the measured values to 4-20 mA DC signals. The converters 20 provide one signal for the three phase sum power, one for the three phase sum reactive power, and one for each of the three line voltages. The converters have an accuracy similar to class one or better and a response time less than 50 msec. The VCU 11 also comprises an A/D converter (ADC) 22 which samples the 4-20 mA DC signals and converts them to additional signals compatible with a microcomputer. The VCU 11 also comprises a microcomputer 23 which receives the output of the ADC 22. The microcomputer 23 is programmed for reading the digital data, storing time series of sample data, calculating the maximum allowable wind farm output power, and transmitting this limit value as a signal on an RS232 port to be read by the unit 12. The sampling rate per channel is at least two times the response frequency of the measurement converters 20.

The VCU 11 also comprises a terminal board 21 which captures the 4-20 mA signals. This allows the VCU 11 to operate both as a controller to limit over voltage output of the turbines 13, and also as a data acquisition system. This data includes the sum active power, sum reactive power, and the three line voltages, as well as the calculated maximum permitted output power. This data is stored as accessible text files, one for each month, containing time series with 10 minutes average value and 10 minutes standard deviation values, and minimum and maximum values.

Referring now to Fig. 3, the manner in which the VCU 11 operates is now described. Essentially, the VCU 11 reads the measured values of active power, reactive power, and line voltages and calculates the maximum permitted wind farm output, Plim. The following two tables set out the input parameters and the symbols used. -4Table 1 Description of input parameters for VCU.

Parameter Description Default value Pset_0 default setting for Plim 2400 kW Pset_min minimum value for Plim OkW Psetmax maximum value for Plim 3000 kW Pstep discretion step for Plim 10 kW Tread “loop-time” for reading measurement signals 0.025 sec Tscreen “loop-time” for writing to screen 1 sec T_RS232 “loop-time” for writing to RS232 10 min Tfile “loop-time” for writing to file 10 min Cos_phi wind farm power factor 0.95 R short-circuit resistance seen from wind farm 21.6 ohm X short-circuit reactance from wind farm 26.8 ohm Ulim maximum permitted voltage level 41.0 kV' Table 2 Description of symbols.

Symbol Description Plim maximum permitted output power from the wind farm U12 measured line voltage at 38 kV side of the transformer at the wind farm U23 measured line voltage at 38 kV side of the transformer at the wind farm U31 measured line voltage at 38 kV side of the transformer at the wind farm P measured sum active power at 38 kV side of the transformer at the wind farm positive for production Q measured sum reactive power at 38 kV side of the transformer at the wind farm, positive for production UO calculated voltage in case of zero output form the wind farm u maximum of U12, U23 and U31 N integer number -5The algorithm flow is indicated by the numeral 30 in Fig. 3 and it includes the start-up step 31 and the step 32 of reading the input parameters. In step 33 line voltages, sum active, and sum reactive power are read and the maximum permitted output is calculated according to the algorithms of block 34. In step 35 this data is written to a screen according to algorithms of block 36. In step 37 the VCU prepares the data for transmission to the microcomputer as indicated by the algorithms in the box 38. In step 39 the VCU writes the data to a file, as indicated in the block 40. In step 41 the algorithm is terminated and variable Plim is set to its default value in step 42.

In operation, the system 1 operates to provide power to the grid 2. The highest voltage level is at the Point of Common Coupling (PCC) of the system i.e. at the 38/10 kV transformer. The graph of Fig. 4 illustrates how the voltage level depends on the consumer load level at the grid. If the load is less than 40% of the maximum level, the voltage level at the PCC may become critically high i.e. above 1.08 pu. It is at this point the VCU must limit the output. The graph of Fig. 5 illustrates how the VCU operates. It limits the maximum output power from the system by reducing the voltage output from 1.08 pu to 1.04 pu with decreasing grid load.

Referring now to Figs. 6 and 7, voltage and power outputs with respect to time are given. The system has a maximum output of 5 MW and is connected to the grid with an estimated short circuit impedance of R = 2.5 Ohm and X = 2.5 Ohm. The graphs of Figs. 6 and 7 were derived using simulation with a hypothetical voltage limit of 11.25 kV. As is clear from these diagrams, the VCU limits the output power to a little over 5 MW during hours with high load and down to one MW during hours with low load.

It will be appreciated that the invention allows connection of a wind energy system which has a potentially high output to a grid which is regarded as being weak. This is achieved in a very simple an inexpensive manner. It is envisaged that the cost of lost power output will be relatively small. -6The invention is not limited to the embodiments described, but may be varied in construction and detail.

Svan.spec.Oct6

Claims

1. A wind energy system comprising:a wind turbine comprising means for connection to a utility grid; and a controller comprising means for sensing grid voltage level and for limiting wind turbine output to maintain quality of grid supply.

2. A system as claimed in claim 1, wherein the controller senses active power, reactive power, and line voltage.

3. A system as claimed in claims 1 or 2, wherein the controller senses the input parameters at the high voltage side of a connection transformer.

4. A system as claimed in any preceding claim, wherein the controller comprises an analogue sub-system for receiving the sensed data, for converting to digital format, and for capturing signals for data acquisition, and a digital sub-system for performing data processing to determine the current maximum voltage output.

5. A wind energy system substantially as described with reference to and as illustrated in the accompanying drawings. 1 /4 t ·=>