GB2130808A

GB2130808A - Power generating machine

Info

Publication number: GB2130808A
Application number: GB08315184A
Authority: GB
Inventors: Hilton Law
Original assignee: English Electric Co Ltd
Current assignee: English Electric Co Ltd
Priority date: 1979-08-10
Filing date: 1983-06-02
Publication date: 1984-06-06
Also published as: GB8315184D0; GB2130808B

Abstract

An alternating current electrical power generating system incorporating a turbine, an electrical output generator driven indirectly by the turbine, and means for compensating for variations in the turbine speed so as to provide an output current of substantially constant frequency from the generator stator. The system comprising a generator 5 and an induction machine 6 mounted on a shaft S driven by a variable speed turbine T. The stator of the machine 6 is supplied with alternating current having a desired frequency FB. An output frequency is taken from the rotor windings of the machine 6 and fed via a static invertor I to a synchronous or induction motor 7 driving an output induction machine 8. The stator of the generator 5 is connected via a slip ring SR8 to the rotor winding of the machine 8. The arrangement between the rotor current frequency and the rotor speed of the machine 8 being such as to provide an output of constant frequency. <IMAGE>

Description

SPECIFICATION Power generating equipment This invention relates to electrical power generating equipment.

On some types of power generating equipment the rotational speed of an alternating current generator may vary widely and in a random manner. Such applications include the harnessing of wave, wind and tidal power. In order to feed this power into the National Grid or to parallel a number of generators onto a common bus, the random frequency output from these generators must be converted into a common standard, that is 50 Hz in this country.

It has been proposed to utilise two dynamoelectric machines having stator and rotor windings wound as for a wound-rotor induction motor, with their rotors mechanically coupled together so as to be simultaneously rotated by the same prime mover, the rotor windings of the two machines being interconnected, a constant frequency excitation source being applied to the stator winding of one machine, and the stator winding of the other machine providing the output from the assembly.

Such an arrangement is illustrated diagrammatically in Figure 1, in which 1 and 2 represent two induction machines mounted on a common shaft S driven by a variable speed turbine T. The rotor windings of the two machines are interconnected by conducting links as at L. A control frequency Fc is applied to the stator winding of the machine 2, and in use an output frequency FB will be generated by the stator winding of the machine 1. Now if N is the shaft speed and FR the rotor current frequency.

FR=FBN and F,=F,--N.

Therefore F,=F,--2N.

It will accordingly be seen that in order to obtain a constant output frequency FB the excitation frequency cannot be constant.

An object of the invention is to provide a power generating system which can be arranged to provide a constant output frequency.

According to one aspect of the invention an alternating current electrical power generating system incorporates a turbine, an electrical output generator, means for varying the frequency of the rotor current of the generator in response to changes in turbine speed, and an electric motor coupled to the rotor of the generator and supplied with current at a frequency which also varies in response to changes in the turbine speed so as to control the speed of the generator rotor, the relationship between the rotor current frequency and the rotor speed being such as to provide an output current of substantially constant frequency from the generator stator.

The electric motor is preferably provided by an induction motor, and the arrangement then conveniently includes a static inverter for modifying the frequency of the current supplied to the motor, the inverter being controlled to change the frequency byoan amount equivalent to motor slip.

One embodiment of the invention is illustrated in Figure 2. In this embodiment the rotors of a main generator 5 and a smaller induction machine 6 are mounted on a common shaft S driven by a variable speed turbine T. The stator of the excitation machine 6 is supplied with alternating current having a desired frequency FB. An output current of frequency FBN is taken from the rotor winding of the excitation machine 6 via a system of slip-rings SR6, and is fed to a static inverter I, which amplifies the power level and feeds it to the stator winding of a synchronous or induction motor 7 having its rotor mounted on the same shaft S2 as an output induction machine 8. The stator of the main generator 5 is connected via a slip-ring system SR8 to the rotor winding of the output machine 8.Now in use the motor 7, as it is fed by an alternating current of frequency FBN will tend to rotate at the same speed as the rotor of the output machine 8.

Now it is known that for any 2-pole induction machine Stator power Rotor power Shaft power Stator frequency Rotor frequency Shaft speed Thus if the shaft torque is zero there is no power transfer between the rotor and the stator.

To induce the motor 7 to induce torque it is necessary to change the phase of the supply frequency to the motor if a synchronous machine is used, or to change the frequency by an amount equivalent to the motor slip if an induction motor is used. This is conveniently achieved by suitable control of the static inverter by any convenient form of control shown at C.

Now an induction machine can be represented or replaced by a differential gear box, generator and motor as illustrated in Figure 3a and 3b.

Thus in the system described above an induction machine can be replaced by such a motor, generator, gear-box arrangement. For example the embodiment illustrated could be modified as shown in Figure 4. In this arrangement the rotor of the machine 6 is driven by the shaft S of the turbine T which shaft also provides one input to a differential gear-box G, N being the variable rotor shaft speed.

An output of frequency FBN is taken via the slip-ring system SR6 from the rotor winding of the machine 6 and fed to the static inverter I, the output from which is fed to the stator of an induction or synchronous motor 7, the shaft S2 of which provides a second input to the differential gear-box G. This shaft will have a speed equivalent to FBN, and the differential gear-box is arranged to drive its output shaft S3, and hence the rotor of the main generator 5 a speed giving an output frequency from the stator of the machine equal to the input frequency FB.

The embodiment shown in Figure 4 can be modified to the form illustrated in Figure 5, the same reference being used to denote the same elements in each of the figures. The latter arrangement may be particularly advantageous in view of difficulties which can be experienced with double fed induction machines, and because the output generator, that is the machine 1, is rotated at constant speed. It may be preferable for the motor 7 to be a synchronous motor rather than an induction motor as the static inverter I need not then be forced commutated, although the control arrangements may need to be more complex.

Claims

1. An alternating current power generating system incorporating a turbine, an electrical output generator, means for varying the frequency of the rotor current of the generator in response to changes in turbine speed, and an electric motor coupled to the rotor of the generator and supplied with current at a frequency which also varies in response to changes in the turbine speed so as to control the speed of the generator rotor, the relationship between the rotor current frequency and the rotor speed being such as to provide an output current of substantially constant frequency from the generator stator.

2. An alternating current power generating system according to Claim 1 wherein said electric motor is an induction motor.

3. An alternating current power generating system according to Claim 2 wherein an inverter serves to change the frequency of the current supplied to the induction motor by an amount equivalent to motor slip.

4. An alternating current power generating system according to Claim 2 wherein the induction motor is replaced by a differential gear-box, generator and motor, one input of which gear-box is driven at a speed determined by the turbine, another input of which is varied in response to changes in turbine speed, and the output of which is coupled to the rotor of the generator, the relationship between the inputs being such as to provide an output current of said substantially constant frequency from the generator stator.

5. An alternating current power generating system according to Claim 1 wherein said electric motor is a synchronous machine and the inverter serves to change the phase of the supply frequency to the motor.

6. An alternating current power generating system substantially as shown in and as hereinbefore described with reference to Figure 2 of the accompanying drawings.

6. An alternating current power generating system according to Claim 2, in which the turbine is connected to the rotor of a further generator having its stator winding connected via a system of sliprings to the rotor winding of the output generator, and in which the rotor of the further generator is coupled to the rotor of an dynamo-electric machine having stator and rotor windings wound as for a wound rotor induction motor, the output from the rotor of this machine being fed via a system of sliprings and a static inverter to the stator winding of the induction motor, the stator of said dynamoelectric machine being supplied with current at a predetermined frequency, and the inverter being controlled in a manner such that the output current from the generator is at said predetermined frequency.

7. An alternating current power generating system substantially as shown in and as hereinbefore described with reference to any one of Figures 2 to 5 of the accompanying drawings.

Superseded claims 1-7 New or amended claims:-

1. An alternating current power generating system incorporating a turbine, an electrical output generator, means for varying the frequency of an input current to the generator rotor in response to changes in turbine speed, a dynamo-electric machine having its stator and rotor windings wound as for a wound rotor induction motor, coupled to the turbine rotor and having its stator supplied with an input current at a predetermined frequency, the rotor of the machine providing an output current having a frequency which differs from said predetermined frequency by an amount determined by the turbine speed, an electric motor coupled to the output generator and supplied with current derived from the output of the dynamo-electric machine so as to cause itto drive the generator rotor at a speed related to the frequency of the input current supplied to the generator as to provide an output current from the generator at said predetermined frequency.

4. An alternating current power generating system according to Claim 1 wherein said electric motor is a synchronous machine and the inverter serves to change the phase of the supply frequency to the motor.

5. An alternating current power generating system according to Claim 2, in which the turbine is connected to the rotor of a main generator having its stator winding connected via a system of sliprings to the rotor winding of the output generator, the stator of said dynamo-electric machine being supplied with current at said predetermined frequency, and the output from the rotor of the machine being fed via a system of slip-rings and a static inverter to the stator winding of the induction motor.