GB2382734A

GB2382734A - Combined parking and emergency electro-dynamic brake for a wind turbine with permanent magnet generator

Info

Publication number: GB2382734A
Application number: GB0128808A
Authority: GB
Inventors: Michael Andrew Wastling; David Irving; Robert James Cann; John Charles Balson
Original assignee: Individual
Current assignee: Individual
Priority date: 2001-12-01
Filing date: 2001-12-01
Publication date: 2003-06-04
Also published as: GB0128808D0

Abstract

A wind turbine fitted with a permanent magnet generator uses a combination of resistive and inductive loads to provide both high speed emergency braking and parking brake functions without applying excessive electrical torque loads to the turbine. Curve 3 shows the generator torque for a combined resistive and inductive load and enables the generator to perform adequately as a brake. The inductive load can be a coil of copper wire with a ferrous core.

Description

Combined Parking and Emergency Electrodynamic Brake for a Wind Turbine with a Permanent Magnet Generator It is common for wind turbines to employ a permanent magnet generator, for the purpose of generating electricity. A separate requirement of the design of any wind turbine is to provide a means of bringing the wind turbine to rest during normal operation and in the event a fault condition. This patent relates to the use of a permanent magnet generator to provide these braking functions.

It is possible to arrange for a generator to act as a brake by simply applying a resistive load to the electrical output from the generator. If the resistance is correctly chosen then the generator will provide more torque than is provided by the wind and the wind turbine will slow down. Unfortunately, as the wind turbine reduces its speed, the generator voltage reduces and the electrical torque reduces. Potentially, this reduction may be more than the reduction in torque provided by the wind and the net result is that the wind turbine may merely slow down but not stop.

In order to ensure that the wind turbine does come to a virtual standstill, it would be necessary to connect a very low resistive load to the output of the generator, potentially even a short circuit connection. This is likely to be effective, but would suffer the disadvantage that a very high electrical torque would be imposed on the wind turbine if the short circuit were applied when the machine was operating at full speed.

Therefore, there is no single resistive load that is suitable for achieving a suitable torque for slowing machine down when operating at normal speeds, and for reducing the rotational speed to a virtual standstill. This patent relates to a solution to this problem, based on the use of an inductive load.

The patent relates to the use of an inductive load connected to the output of the generator. An inductive load provides a reactance that is dependent on the frequency of the output from the generator, and therefore is dependent on the rotational speed of the wind turbine. By obtaining the right combination of resistance and inductance, it is possible to achieve an electrodynamic brake that will slow the wind turbine down from full speed to a virtual standstill, without introducing excessive loads. It can therefore replace a conventional mechanical brake.

The invention will now be described with reference to Figure 1 in which generator rotational speed is plotted against torque.

Referring to Figure 1, the solid line represents the maximum aerodynamic torque that a wind turbine is capable of producing at the design wind speed as a function of rotational speed. In order to slow the wind turbine down, the generator must provide torque above this curve. The upper dotted curve (1)

represents the generator torque for a short circuit. It shows that in this state, the generator is able to apply a very high torque at high rotational speeds.

Potentially this may damage the machine and is therefore unsatisfactory. By comparing curve (1) with the solid curve, representing the maximum aerodynamic torque, it is clear that with a short circuit applied, the generator is capable of slowing the wind turbine to a very low rotational speed. Therefore, a short circuit is valuable to prevent the wind turbine from starting if it is Initially stationary, but it is not suitable as a brake to slow it down from high rotational speed.

Curve (2) shows the generator torque for a resistive load that is selected to achieve a braking torque of 1.4 times the aerodynamic torque at full rotational speed. Curve (2) is higher than the solid curve only above about 60% of the maximum rotational speed. Therefore, in this case, the wind turbine may not slow down below this speed. Therefore a purely resistive load IS also unsatisfactory in providing a brake.

Curve (3) shows the generator torque for a combined inductive and resistive load that has the same maximum braking torque as curve (2). The curve is always above the curve representing the aerodynamic torque, right down to very low rotational speeds. Therefore, this combined resistive and inductive load is enabling the generator to perform adequately as a brake in a manner that replaces a conventional mechanical brake.

The inductive load is trivial to construct and can consist of a coil of copper wire, possibly with a ferrous core. The length of the copper and the crosssectional area can be varied to control the resistance. The number of turns and the coil size and core details control the inductance. The total mass of copper has to be adequate to absorb the energy that is dissipated by the resistive load during braking, without becoming too hot. Three such coils would be required for a standard 3-phase generator.

Claims

CLAIMS (1) The use of a combined inductive and resistive load to enable a permanent magnet generator to perform as a brake.

(2) The use of simple coils of copper wire in combination with a ferrous core to achieve the required induction to provide the load described in Claim 1.