GB2276022A - Frequency stabilised A.C.power supply system - Google Patents

Abstract

The frequency of the power supply from an a.c. induction generator 5 or 7 is controlled by monitoring the output frequency and comparing it at 9 with a reference. The comparator 9 generates an a.c. error signal having a frequency equal to the difference between the desired and actual frequencies. The error signal is fed to the wound rotor of the controlled generator to correct its output frequency. The output voltage may also be stabilised by adjusting the magnitude of the error signal. The generator may form part of a propulsion and auxiliary power system of a ship. Prime movers D1, D2 etc drive propulsion generators G1, G2 as well as auxiliary generators 5,7. The control system protects the supply from variation due to load changes in the propulsion system. <IMAGE>

Description

Power Supply Systems This invention relates to power supply systems, and particularly to a system for supplying electrical power to busbars from generators connected in tandem. The system is particularly, but not solely, applicable to the supply of electrical power in ships, and more especially in ships wherein the propellors are driven by electric motors.

The use of electric propulsion of ships is now quite common, because the speed of the propulsion electric motors can be readily varied over a wide range, whereas the speed of diesel engines otherwise used for ships' propulsion can be varied over only a relatively small range. This easy variation in propulsion motor speed enables ships to enter and leave port at low speed, and to cruise at any desired low, intermediate or high speed. It also enables warships to cruise at low speed (and hence with relatively low energy consumption) but to accelerate readily to full speed when necessary.

The propulsion motors are driven from an ac supply which is provided by generators driven by diesel engines, gas turbines or other prime movers.

The prime movers occupy a large amount of space, so it has been proposed to use a single prime mover to drive both a main generator for feeding a high-power busbar which supplies the propulsion motors and an auxiliary generator for feeding a low-power busbar for supplying auxiliary equipment such as lighting and also for supplying computer and other equipment which requires a supply which is both voltage and frequency stable.

This can give rise to problems, because any fault or appreciable change in load which occurs on the high-power busbar will affect the speed of the prime mover and so will affect the frequency of the supply on the low-power busbar.

It is an object of the present invention to provide an improved power supply in which such changes in supply frequency prevented or at least alleviated.

According to the invention there is provided an ac power supply comprising a prime mover coupled to drive the shaft of a wound-rotor induction motor which thereby acts as a generator for providing an ac output; and means to monitor the frequency of the ac output and to feed to the rotor of'the motor a frequency error signal for controlling the freqeuncy of the ac output in dependence upon any difference between the ac output frequency and a reference frequency.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawing, in which Figure 1 is a block schematic diagram of a conventional power supply system, and Figure 2 is a block schematic diagram of the system of Figure 1, modified in accordance with the invention.

Referring to Figure 1, a conventional power supply for a ship comprises a high-power busbar 1 which is supplied by an ac generator G1 which is driven by a diesel engine, a gas turbine or other prime mover D1. A low-power busbar 3 is supplied by an ac generator G5, which is also driven by the prime mover D1. Duplicate generators G2 and G6, driven by a prime mover D2, are connected in parallel with the generators G1 and G5, respectively. Other prime mover/generator sets such as D3,G3; D4,G4; D5,G7 and D6,G8 may be connected to the busbars as and when required to satisfy the power requirements at the busbars.

It will be seen that any fault or change in load at the busbar 1 will affect the speed of the prime movers D1 and D2 and so will affect the frequency of the supply on the busbar 3 as explained above. Furthermore, if either the generator G1 or the generator 62 trips, i.e. goes off load, the corresponding prime mover will speed up, so that the frequency at the busbar 3 will increase until such time as the governor on that prime mover can restore its speed to normal. Clearly, any such change in frequency on the busbar 3 will be detrimental to the operation of the equipment connected thereto.

Referring to Figure 2, in accordance with the invention the conventional synchronous generators G5 and G6 are replaced by wound-rotor induction motors 5 and 7, the shafts of which are driven by the prime movers D1 and D2, respectively. A frequency monitoring circuit 9 is connected to the busbar 3, and provides a frequency error signal on lines 11, 13 in dependence upon any frequency difference occurring between the factual supply frequency on the busbar 3 and a reference frequency. The error signal is a dc signal for zero frequency error, becoming an ac signal of frequency equal to the error frequency where there is an error.

The error signal is fed to the rotor windings of the motor 5 and 7 so that the frequency of the output produced thereby is corrected. This correction will happen very rapidly, so there is no wait while the governors of the prime movers respond.

The error signal generator may comprise thyristors, gate turn-off devices or a rotary machine.

Voltage stabilisation of the auxiliary supply is effected by adjusting the magnitude of the signal fed to the rotors of the motors 5 and 7.

The coupling of the two generators (e.g. G1 and G5) to a single prime mover (e.g. D1) enables regenerative power from the high-power busbar 1, generated, for example, when the propulsion motors are used to slow the ship, to reduce the load on the prime mover D1 or D2, so that the generator 61 or 62 in effect drives the motor 5 or 7, respectively. In effect, therefore, the regenerative power feeds the low-power busbar 3.

Although the invention is described above in relation to ships' supply systems, the use of a wound-rotor induction motor for providing frequency correction may also be applied to other supply systems.

Claims (7)

CLAIMS:

1. An a.c. power supply system comprising a prime mover coupied to drive the shaft of a wound-rotor induction motor which thereby acts as a generator for providing an a.c. output; and means to monitor the frequency of the a.c. output and to feed to the rotor of the motor a frequency error signal for controlling the frequency of the ac output in dependence upon any difference between the a.c. output frequency and a reference frequency.

2. An a.c. power supply system as claimed in Claim 1 in which the monitoring means comprises a rotary dynamo-electric machine.

3. An a.c. power supply system as claimed in Claim 1 in which the monitoring means comprises a solid state switch device.

4. An a.c. power supply system as claimed in Claim 3 in which the solid state switch device is a thyristor.

5. An a.c. power supply system as claimed in any preceding claim in which, in operation, voltage stabilization of the generator is carried out by adjusting the magnitude of the current fed to the rotor of the generator.

6. An a.c. power supply system as claimed in any preceding claim in which the output of the generator is fed to a low power bus line and the prime mover for the generator also drives a further, principal, generator to supply power to a high power busline.

7. An a.c. power supply system substantially a as herein before described with reference to Figure 2 of the accompanying drawings.