According to a first aspect of the invention, there is provided a generator which includes a shaft; an electromagnetic bearing for rotatably supporting the shaft; an auxiliary bearing for supporting the shaft in the event of failure of the electromagnetic bearing; a sensor for sensing failure of the electromagnetic bearing; and an electrically operated brake for slowing rotation of the shaft in the event of failure of the electromagnetic bearing.

The electrically operated brake may include a resistive braking load which includes a switching device to connect an output of the generator to the resistive braking load and to disconnect the generator from its operative load when failure of the electromagnetic bearing is sensed.

The generator may further include a braking resistor control system for controlling braking of the shaft.

The generator may further include an exciter, an excitation transformer and an excitation power converter.

As indicated above, the generator may be part of a nuclear power plant. In particular, the power plant may be modular in nature and of the pebble bed type. With such a power plant, the generator may be located inside a pressure vessel which contains helium.

According to a second aspect of the invention, there is provided a method of protecting an electricity generator which includes the steps of sensing failure of an electromagnetic bearing of the generator; supporting a shaft of the generator by means of an auxiliary bearing in the event of failure of the electromagnetic bearing; and slowing the shaft by means by of an electrically operated brake in the event of failure of the electromagnetic bearing.

The method may include the step of disconnecting an output of the generator from an operative load in the event of failure of the electromagnetic bearing.

The invention is now described, by way of an example with reference to the accompanying drawings, in which: Figure 1 shows a conceptual layout of a pebble bed modular reactor (PBMR) generator, illustrating the method of protecting the generator according to

the invention: and Figure 2 shows a typical connection diagram of an electric brake arrangement for the PBMR generator of Figure 1.

Referring now to Figure 1, a PBMR power turbine and generator 10 is shown schematically to include a helium filled pressure vessel 12 which encloses an electricity generator 14. The generator 14 includes a rotor 16 mounted on a shaft 18 which is supported by electromagnetic bearings 20. Auxiliary bearings 22 are provided to support the shaft during a plant shutdown or in the event of the electromagnetic bearings failing, as will be described hereunder. Failure of the electromagnetic bearings is detected by a sensor 64. The generator 14 further includes a stator 24 and a stator frame 26. Coolers 28 are provided to cool the generator 14.

A rotating exciter 30 is mounted to one end of the shaft 18 as shown and exciter entries and terminals 32 are connected to the exciter 30. A power turbine 34 is connected to the opposite end of the shaft 18.

The vessel 12 includes a helium supply port 36 and water cooling supply ports 38 for connection to the coolers 28.

The generator 14 is connected to a generator busbar 40 via terminals 42. The terminals 42 are enclose in a terminal enclosure 44.

The terminals 42 are also connected via a control busbar 46 to an

excitation transformer 48. Excitation power is thus derived from the generator via the excitation transformer 48 and fed to the exciter 30 via excitation power converters 50. A braking resistor control system 52 is connected to the sensor 64 via entries and terminals 55 for the electromagnetic bearings 20 and the sensor 64.

An automatic voltage regulator 53 is provided to control the voltage at the exciter 30 during a braking cycle as will be described hereunder.

The terminals 42 are furthermore connected via a control busbar 54 to electric brake transformers 56. Power thus flows from the generator 14 to a braking resistor switching device 58 and a braking resistor bank 59 via the transformers 56. The braking resistor control system 52 controls the braking resistor switching device 58.

Referring now to Figure 2, a connection diagram 60 shows schematically a typical connection of an electric brake for the generator of Figure 1. It will be appreciated that the diagram shows only a connection to one of the phases of a three phase generator. In a three phase generator, the connections to all three phases are the same as shown in the diagram.

In this drawing, the lines terminating in filled (solid) arrows represent power flow and the lines terminating in unfilled arrows represent control links.

Referring now to both drawings, the generator 14 is shown to include the sensor 64 which monitors and senses failure of the electromagnetic bearings

20. The generator 14 is shown to include the shaft 18 and the rotating exciter 30.

The braking resistor control system 52 is provided to control the rotation speed of the shaft 18 in the event of a failure of the electromagnetic bearings shown as 20 in Figure 1. In such an event, the control system 52 opens a circuit breaker 72, thereby disconnecting the generator output from an operative load comprising a generator transformer 73 and a high voltage electrical network 74. The generator output is now redirected to a braking resistor bank 78 via an electric braking resistor transformer 80 and a braking resistor switching device 82. In this configuration, a resistive load is placed on the generator 14 to control the shaft speed.

As the shaft speed is reduced, the frequency at the output terminals 42 drops accordingly. The control system 52 ensures that the capability of the generator 14 and the exciter 30 are not exceeded by maintaining a ratio of generator voltage to frequency constant throughout the braking cycle. This is accomplished by providing a generator voltage and frequency monitor 84 which is connected to an automatic voltage regulator 86. The voltage at the exciter is thus controlled and adjusted via an excitation converter 88 which is powered from the generator output terminals 42 via an excitation transformer 90.

In use, when the electromagnetic bearings 20 fail, the shaft 18 drops onto the auxiliary bearings (22 in Figure 1) and the shaft braking cycle as described above commences. The shaft speed is reduced rapidly during the braking cycle, thus ensuring reduced wear on the auxiliary bearings. The auxiliary bearings are not

oil lubricated due to the fact that the bearings are located within the helium pressure vessel 12 and oil lubricants have the effect of contaminating the helium within the pressure vessel 12.