WO2007072007A1

WO2007072007A1 - Electrically resistive power dissipation apparatus

Info

Publication number: WO2007072007A1
Application number: PCT/GB2006/004837
Authority: WO
Inventors: Andrew Causebrook
Original assignee: Newcastle University of Upon Tyne
Current assignee: Newcastle University of Upon Tyne
Priority date: 2005-12-22
Filing date: 2006-12-21
Publication date: 2007-06-28
Anticipated expiration: 2008-06-22
Also published as: GB0526133D0

Abstract

A dynamic braking apparatus (6) for temporarily increasing electrical power dissipation in a power circuit connected to an electrical generator (4) is disclosed. The dynamic braking apparatus (6) includes a resistor connectable by means of a controller in series with conductors (16a, 16b) of the power circuit by closing a switch in response to detection of a fault on the power circuit.

Description

^'j ELECTRICALLY RESISTIVE POWER DISSIPATION APPARATUS

The present invention relates to apparatus for temporarily increasing the electrical power resistively dissipated in power circuits connected to electrical generators and relates particularly, but not exclusively, to such apparatus for dynamically braking wind powered electrical generators.

Wind powered electrical generators are known which contribute power to the electrical grid system. Such generators suffer from the drawback that if a fault such as a short circuit occurs in a power circuit connected to the generator, the power drawn from the generator can decrease significantly. Damage to the generator could therefore result from the generator being driven too rapidly.

In order to avoid such damage, it is known to provide generators either an overspeed protection device on the mechanical drive train of the generator, or a voltage or current protection relay on the electrical circuit.

In order to maximise stability of the grid system to which the generator is connected, it is desirable to prevent tripping of any generating units on the system during a specified range of grid faults, and to enable a generator to restore its pre-fault output power to the grid as soon as possible after clearance of the fault. A number of techniques are known which attempt to achieve this. High-speed mechanical actuators change the pitch of turbine blades or the setting of inlet valves to reduce the power input to the generator drive train, which consequently limits rotor speed of the generator. However, this technique suffers from the drawback that restoration of power export after clearance of a fault is relatively slow. Fast acting reactive power sources can be used to raise the voltage at the generator terminals and thus increase the power transfer capability of the generator during and after the grid fault, but application of this method is limited because of the very large quantity of reactive power often required.

US Patent 5,198,745 discloses an arrangement in which electrical power is dissipated in dynamic braking resistors which are selectively connectable between phase conductors connected to the generator and earth. On detection of a fault on the power circuit, braking resistors are selectively connected between the phase conductors and earth, and the dissipation of electrical power in the braking resistors can limit the speed at which the generator rotor is driven during a power circuit disturbance. The braking resistor can also be controlled to damp the rotor speed oscillations after the disturbance. These effects may avoid the necessity of tripping of the generator and may improve the stability of the electrical grid to which the generator is connected.

This arrangement can contribute to the stability of traditional grids comprising synchronous generators because the synchronous generators have an independent source of excitation that can provide significant support to the voltage across the parallel braking resistors during and after the disturbance. This in turn means that the power dissipation, proportional to the square of residual voltage across the resistors, can be sustained at a higher level during fault. However, this arrangement does not contribute significantly to generator power take off when used with asynchronous machines that generally demand reactive power and thereby depress voltage during and after the disturbance .

Preferred embodiments of the present invention seek to overcome or at least alleviate at least some of the above disadvantages of the prior art.

According to an aspect of the present invention, there is provided an apparatus for temporarily increasing electrical power dissipation in at least one power circuit connected to an electrical generator, the apparatus comprising :

at least one resistive component adapted to be connected in series with at least one phase conductor of a power circuit; and

switching means for connecting the or each said resistive component in series with said phase conductor in response to detection of a fault on a said power circuit, to increase the electrical power dissipation in the power circuit .

By connecting at least one said resistive component in series with a phase conductor of a power circuit, this provides the advantage that voltage at the terminals of the generator is supported in most cases by transferring all the generated power across the series resistor. This in turn means that electrical torque, proportional to the square of voltage at the generator terminals, is supported and power take off, the product of electrical torque and rotor speed, is increased while the resistor is inserted. This in turn means that the acceleration of the generator rotor during the fault is reduced and the stabilising (decelerating) torque during the recovery period is increased. The advantage is also provided that reduction of torque transients on the generator drive train and the peak currents in the generator windings can be achieved by inserting the resistive component. These transient torque and current reductions will be achieved on fault clearance only for dynamic braking resistors inserted by mechanical bypass switches. However, reductions will be achieved during fault initiation and clearance if the said resistors are inserted within a few milliseconds using transistors or active thyristors. Transient torque reductions are beneficial to the mechanical drive trains of all generator types. Reduced peak currents are particularly beneficial to certain types of wind turbine generators systems using power electronic switches in the rotor circuit (doubly-fed induction generators) .

At least one said resistive component may comprise at least one resistor.

In a preferred embodiment of the invention, at least one said resistive component comprises a plurality of resistors, and said switching means is adapted to vary the number of said resistors through which current flows.

This provides the advantage of enabling the overall resistance of the resistive component to be adjusted.

In one preferred embodiment of the invention, a plurality of first said resistors are connected in parallel, and said switching means includes a respective switch connected in series with at least one said first resistor .

In another preferred embodiment of the invention, a plurality of second said resistors are connected in series, and said switching means is adapted to enable current to by pass at least one said second resistor.

The switching means may be adapted to receive pulsed signals to control connection of at least one said resistive element to at least one said phase conductor.

This provides the advantage of enabling the effective resistance of the resistive element to be adjusted by adjusting the length and/or timing of the pulses.

The switching means may include at least one transistor .

The switching means may include at least one thyristor .

The apparatus may further comprise control means for generating a control signal for controlling said switching means in response to detection of a fault on at least one said power circuit.

According to another aspect of the present invention, there is provided a method of controlling electrical power dissipation in a power circuit connected to an electrical generator, the method comprising:- detecting a fault in at least one power circuit connected to an electrical generator; and

connecting at least one resistive component in series with at least one phase conductor of a said power circuit in response to detection of said fault.

Preferred embodiments of the invention will now be described, by way of example only and not in any limitative sense, with reference to the accompanying drawings in which : -

Figure 1 is a schematic representation of an electricity generating system incorporating a dynamic braking resistor apparatus embodying the present invention;

Figure 2A is a more detailed representation of a first embodiment of the dynamic braking apparatus of Figure 1;

Figure 2B is a schematic representation, corresponding to Figure 2A, of a second embodiment of the dynamic braking apparatus of Figure 1;

Figure 2C is a schematic representation, corresponding to Figure 2A, of a third embodiment of the dynamic braking apparatus of Figure 1; and

Figure 2D is a schematic representation, corresponding to Figure 2A, of a fourth embodiment of the dynamic braking apparatus of Figure 1.

Referring to Figure 1, an electricity generating apparatus 2 includes a wind powered generator 4 in which a rotor (not shown) is caused by wind to rotate relative to a stator in order to generate electrical power, as will be familiar to persons skilled in the art. A dynamic braking apparatus 6 embodying the present invention is connected between terminals 8 of the generator 4 and a step up transformer 10.

As shown in greater detail in Figure 2A, the dynamic braking apparatus 6 includes a resistor 12 and a switch 14 for selectively connecting the resistor 12 in series with a power circuit including phase conductors lβa lβb. The switch 14 is closed in normal operation of the generator 4. The switch 14 may be a mechanical contactor or circuit breaker, or a power transistor (for example an IGBT or MOSFET) for providing high frequency switching for insertion within less than 1 millisecond and low distortion control of the effective resistance. Alternatively, line- commutator thyristors for providing cycle by cycle switching and relatively low on state losses may be used. Active thyristors provide sub-millisecond insertion and relatively low on state losses, providing peak current limitation and some resistance control. The switching may also be controlled so that the length/timing of pulses controls the effective resistance of the braking resistor 12.

A controller 18 detects a fault on the power circuit by a suitable method as will be familiar to persons skilled in the art, and sends a control signal to open the switch 14 to connect the resistor 12 in series with the phase conductors 16a lβb. As a result, all of the available current in the phase conductors lβa lβb flows through the resistor 12, and power is dissipated in the resistor 12 in a manner related to the square of the current flowing.

Referring back to Figure 1, the generating apparatus 2 has tee-off connection 20 and a site busbar 22, both of which are connected to other generators (not shown) . A site substation 24 includes a second dynamic braking apparatus 26, similar in construction to the first dynamic braking apparatus 6 but adapted to operate at higher voltages, connected to the power circuit. A step-up transformer 28 is connected to the dynamic braking apparatus 26 and a tee- off connection 30 connects the power circuit to other generation sites (not shown) . A further dynamic braking apparatus 32 is connected between the tee-off connection 30 and a grid substation 34. As will be appreciated by persons skilled in the art, there will typically be multiple tee- offs on a wind farm equipped with the generator, of which the one shown is just typical.

Referring to Figure 2B, a dynamic braking apparatus

106 of a second embodiment is shown. A soft-starter 130 for grid connection of an induction generator is provided, as will be familiar to persons skilled in the art. The soft- starter 130 is bypassed by a mechanical switch 114a under normal operation. The braking resistor 112 is connected in parallel with the soft-starter 130 and bypass switch 114a and controlled by adapting the control of the soft-starter 130 according to the requirements of the dynamic braking system. An additional switch 114b is required in series with the braking resistor 112 to prevent interference of the resistor 112 with the normal function of the soft- starter 130 during connection. The soft-starter 130 typically comprises a back-to-back (anti-parallel) pair of thyristors switched by pulses timed to reduce the effective voltage applied to the terminals of an induction generator. This allows the connecting voltage to be progressively ramped up to full voltage, thereby reducing the high inrush of current associated with connecting generators of this type. The soft-starter 130, as applied to control the effective braking resistance, operates in a similar manner by controlling the effective voltage across the resistor 112 and therefore the power dissipated in it during the grid fault.

A dynamic braking apparatus 206 of a third embodiment is shown in Figure 2C. Either or both of two parallel connected resistors 212 can be connected in series with the conductors 16a, 16b of the power circuit by means of separate switches 214a, 214b and controller 218 to adjust the effective resistance experienced by current flowing through the phase conductors 16a, 16b.

A dynamic braking apparatus 306 of a fourth embodiment is shown in Figure 2D, in which a pair of series connected resistors 312 can be selectively connected to the conductors 16a, 16b of the power circuit by closing one or both of switches 314a, 314b operated by controller 318.

The operation of the generating apparatus 2 shown in Figures 1 and 2A will now be described

During normal operation of the generating apparatus, switch 14 is closed and most of the available current by passes the resistor 12 because of the much lower resistance of the bypass switch path 14. On detection of a fault such as a short-circuit on the power circuit connected to the generator, the controller 18 opens the switch 14 to connect the resistor 12 in series with the conductors 16a, lβb of the power circuit. As a result, all of the current passing through the phase conductors 16a, 16b of the power circuit now passes through the resistor 12 and is available to cause electrical power dissipation, which varies with the square of the current passing through the phase conductors 16a, 16b.

The dissipation of electrical power in the dynamic braking resistor 12 limits the maximum rotational speed of the generator 4 and therefore prevents it from tripping and allows it to recover to a stable condition after fault clearance. After the fault has cleared, the controller 18 closes the switch 14 again after an appropriate delay. The controller 18 uses voltage and/or current measurements together with the status of other interactive devices (not' shown) such as reactive power compensation devices to determine the timing of control signals sent to the switch 14.

It will be appreciated by person skilled in the art that the above embodiment has been described by way of example only and not in any limitative sense, and that various alterations and modifications are possible without departure from the scope of the invention as defined by the appended claims. For example, although the embodiments described above with reference to the drawings show a single phase, it will be appreciated by persons skilled in the art that any number of phases may be used, although three phases will usually be present.

Claims

1. An apparatus for temporarily increasing electrical power dissipation in at least one power circuit connected to an electrical generator, the apparatus comprising:

at least one resistive component adapted to be connected in series with at least one phase conductor of a power circuit/ and

switching means for connecting the or each said resistive component in series with said phase conductor in response to detection of a fault on a said power circuit, to increase the electrical power dissipation in the power circuit.

2. An apparatus according to claim 1, wherein at least one said resistive component comprises at least one resistor.

3. An apparatus according to claim 1 or 2, wherein at least one said resistive component comprises a plurality of resistors, and said switching means is adapted to vary the number of said resistors through which current flows.

4. An apparatus according to claim 3, wherein a plurality of first said resistors are connected in parallel, and said switching means includes a respective switch connected in series with at least one said first resistor.

5. An apparatus according to claim 3 or 4, wherein a plurality of second said resistors are connected in series, and said switching means is adapted to enable current to by pass at least one said second resistor.

6. An apparatus according to any one of the preceding claims, wherein the switching means is adapted to receive pulsed signals to control connection of at least one said resistive element to at least one said phase conductor.

7. An apparatus according to any one of the preceding claims, wherein the switching means includes at least one transistor .

8. An apparatus according to any one of the preceding claims, wherein the switching means includes at least one pair of thyristors.

9. An apparatus according to any one of the preceding claims, further comprising control means for generating a control signal for controlling said switching means in response to detection of a fault on at least one said power circuit .

10. A method of controlling electrical power dissipation in a power circuit connected to an electrical generator, the method comprising :-

detecting a fault in at least one power circuit connected to an electrical generator; and