GB2159010A

GB2159010A - Protecting alternators in electricity supply systems

Info

Publication number: GB2159010A
Application number: GB08510695A
Authority: GB
Inventors: John William Warin
Original assignee: Northern Engineering Industries PLC
Current assignee: Rolls Royce Power Engineering PLC
Priority date: 1984-05-19
Filing date: 1985-04-26
Publication date: 1985-11-20
Also published as: GB8510696D0; GB2159963B; GB2159010B; GB8412856D0; GB2159963A; GB8510695D0

Abstract

In alternator systems connected to a grid, it is necessary to detect disconnection of the grid supply therefrom and, in that event, to isolate the alternator system from the grid. The frequency of the electricity in the grid changes slowly and, as the alternator system is in synchronism with the grid supply, changes in the alternator systems frequency are dictated by the grid supply. In the event of a disconnection, however, the changes of frequency in the alternator system occur relatively rapidly. Consequently, the alternator system is operated such that it is isolated from the grid if a current signal of a repeatedly updated signal indicative of rate of change of frequency of the electricity generated by the alternator is outside a predetermined acceptance band. During each positive half cycle of one phase of the alternator supply a counter 30, counted pulses from a 2 MHz oscillator 28, counts up to 2<14> then sets a bistable 34 ON which locks counter 30 into a reset state and releases a counter 32, which counts up to 2<14> during each negative half cycle then resets bistable 34 to OFF. Thus ON and OFF state of the bistable 34 corresponds to the alternator half cycles but are delayed by a certain amount, whereby transients are not counted by a counter 12. A counter 16 counts pulses from an 18kHz oscillator 14. For every 10 cycles counted by counter 12 a store 18 is indexed, whereupon counter 16 is stopped, the oldest count in store 18 is discarded, the remaining counts shifted, and the current figure in counter 16 is introduced into store 18; the counter 16 is then restarted. The counts in store 18 are processed in circuit 20 to give a signal indicative of the rate of change of frequency of the alternator. <IMAGE>

Description

SPECIFICATION Electricity Supply Systems The invention relates to electricity supply systems.

Alternators, driven by a steam turbine, a gas turbine or a diesel engine and located on an industrial site for example, are often used to supply electricity in parallel with a relatively large electricity supply system such as the public supply system (usually termed "the grid") to reduce the amount of power taken from the grid. Under such circumstances, it is essential to detect a disconnection of the grid supply from the alternator system to avoid damage to the alternator and its drive equipment. The damage can arise from the out-of-phase synchronisation of the alternator system and the grid which is likely to occur upon indiscriminate reconnection of the grid supply to the alternator system by the manual or automatic operation of the circuit breakers in the grid.

In alternator systems in which the alternator is relatively small and does not normally export power to the grid (i.e. it only reduces the amount of power imported from the grid), the disconnection of the grid from the alternator system can often be detected by detecting the export of power to the grid. Such detection can be used to isolate the alternator system from the grid before indiscriminate reconnection of the grid supply to the alternator system can occur.

However, the detection of the disconnection of the grid supply from the alternator system in systems in which the alternator is of a size that it normally exports powers to the grid has become a more frequent requirement.

The frequency of the electricity in the grid slowly changes with time owing to changes in load on the grid. When an alternator system is connected in parallel with a grid, because the alternator system is in synchronism with the grid supply, the changes in frequency in the alternator system are dictated by the grid supply and are relatively slow. However, if the alternator system is not connected to the grid supply, i.e. the grid supply has been disconnected therefrom, such changes of frequency occur relatively rapidly because changes in load affect the smaller alternator system to a greater extent.

According to one aspect of the invention, a method of operating an alternator system which is connected in parallel with a grid comprises repeatedly updating a signal indicative of rate of change of frequency of the electricity generated by the alternator of the system, determining whether the current signal is within a predetermined acceptance band and, if the current signal is outside said band, isolating the alternator system from the grid.

Preferably, the signal is repeatedly updated by repeatedly counting a number of cycles of a phase of the electricity generated by the alternator equal to a predetermined number of cycles to determine periods during which high frequency time pulses are counted, storing at least three successive counts of time pulses, discarding the oldest count as the newest count is stored and obtaining the signal by subtracting the differences between the oldest stored count and the median stored count and the median stored count and the newest stored count.

According to another aspect of the invention, an alternator system which is connected in parallel with a grid comprises apparatus for repeatedly updating a signal indicative of rate of change of frequency of the electricity generated by the alternator of the system and for determining whether the current signal is within a predetermined acceptance band and means operable to isolate the alternator system from the grid if the current signal is outside said band.

Preferabiy, said apparatus comprises a first generator for generating cycles directly proportional to the cycles of a phase of the electricity generated by the alternator, a first counter for counting the cycles generated by the first generator, a second generator for generating high frequency time pulses, a second counter for counting the time pulses, a count store for storing at least three successive counts of time pulses, the store being indexable by the first counter when a predetermined number of cycles has been counted to discard the oldest stored count and to store the count in the second counter and means for subtracting the differences between the oldest stored count and the median stored count and the median stored count and the newest stored count to obtain a current signal indicative of rate of change of frequency of the electricity generated by the alternator and for determining whether the current signal is within a predetermined acceptance band.

Apparatus will now be described to illustrate the invention by way of example only with reference to the accompanying drawing which is a schematic block diagram of apparatus in accordance with the invention.

The apparatus has a generator 10 for generating cycles directly proportional to the voltage cycles of a phase of the electricity supplied by an alternator of an alternator system connected in parallel with a grid; a counter 12 for counting the cycles generated by a generator 10; a generator in the form of a high frequency oscillator 14 for generating time pulses; a counter 16 for counting the time pulses produced by the oscillator 14; a count store 18; a processor means 20; and a relay 22.

The generator 10 has a comparator 24 for determining when the voltage half cycles are positive and negative. The comparator 24 operates a solid-state switch 26 controlling the output of a 2 MHz oscillator 28. The output of the oscillator 28 passes to a counter 30 during the positive half cycles and to a counter 32 during the negative half cycles.

The counters 30 and 32 are each arranged to count up to 214 during the respective half cycles.

When the counter 30 reaches a count of 214, it sets a bistable switch 34 to the "on" state which, in turn, locks a counter 30 into a reset (count=O) condition and removes a reset signal from counter 32. As soon as the voltage cycle becomes negative counter 32 begins to count and when it reaches a count of 214 it resets the bistable switch to the "off" state which, in turn, locks the counter into a reset condition and removes the reset signal from counter 30.

The effect is to produce "on" and "off" states in synchronism with the voltage half cycles but delayed by 2'3 microseconds. However, transitory excursions of the wavefrom, caused by mains borne spikes for example, do not get counted as a cycle but merely introduce a timing error equal to the time for which the excursion causes the voltage to be of wrong polarity. The delayed synchronisation of the generated cycles with the voltage cycles is reestablished in the next half cycle orfull cycle assuming a further excursion does not occur.

The generated cycles are counted by the counter 12 which is a programmable downcounter. The counter 12 can be programmed to index the count store 18 at a suitable number of cycles, say every 10 cycles.

The oscillator 14 produces high frequency time pulses, for example 18000 Hz and the counter 16 counts the pulses continuously. When the counter capacity is reached, the counter returns to zero and continues counting upwardly again.

When the store 18, which holds eleven successive counts, is indexed by the counter 12, the counter 16 is stopped, the oldest count in the store is discarded, the remaining ten counts are shifted through the store 18 and the figure in the counter 16 is introduced into the store 18. The counter 16 then starts counting again.

The processor means 20 is arranged to ignore the highest and lowest significant figures of the counts and to subtract the differences between the oldest and the median and the median and the newest counts to obtain a current signal indicative of rate of change of frequency. If the result of the differences between the counts would be negative, the processor means takes a unit from the next higher significant figure and adds itto the lower number before performing the subtraction so that a positive result is obtained. The processor means 20 then compares the current signal with a predetermined acceptance band, say +12, and, if the current signal is outside that band (which indicates that the alternator has been disconnected from the grid), operates the relay 22 which, in turn, operates circuit breakers to isolate the alternator from the grid.

The process is repeated each time the store 18 is indexed by the counter 12 so that the signal is repeatedly updated.

The number of cycles counted by the counter 12 and the predetermined acceptance band can be set to obtain any desired sensitivity. Clearly, the smaller the acceptance band is (to increase sensitivity), the more likelihood there is of the apparatus spuriously operating. That problem can be reduced if the processor means 20 is modified such that two consecutive signals have to be outside the acceptance band before the means 20 will operate the relay 22.

The number of counts held in the store 18 can be as low as three. If an even number of counts are used, the median count is taken to be the two adjacent middle counts, respectively. For example, if eight counts were stored, the processor would subtract the differences between counts 1 and 4 and counts 5 and 8.

In isolating an alternator from the grid, a relatively rapid change of frequency is detected and used to initiate that isolation. In most situations, the loss of the grid supply will give rise to the relatively rapid change of frequency. However, in certain circumstances, for example if the alternator continues to export power to a load on the local part of the grid following loss of grid supply, the rate of change of frequency may not be rapid enough to cause isolation of the alternator system from the grid by the apparatus described above before manual or automatic re-connection of the grid supply occurs. In that instance, additional or alternative protection such as that described in our co-pending Application No.8510697 (Applicants' reference AP-AL 1344) can be used.

Attention is drawn to our co-pending Application No.8510696 (Applicants' reference AP-AL 1343B) which claims a method of and apparatus for repeatedly updating a signal indicative of rate of change of frequency in an electricity supply system.

Claims

1. A method of operating an alternator system which is connected in parallel with a grid comprising repeatedly updating a signal indicative of rate of change of frequency of the electricity generated by the alternator of the system, determining whether the current signal is within a predetermined acceptance band and, if the current signal is outside said band, isolating the alternator system from the grid.

2. A method according to claim 1, in which the signal is repeatedly updated by repeatedly counting a number of cycles of a phase of the electricity generated by the alternator equal to a predetermined number of cycles to determine periods during which high frequency time pulses are counted, storing at least three successive counts of time pulses, discarding the oldest count as the newest count is stored and obtaining the signal by subtracting the differences between the oldest stored count and the median stored count and the median stored count and the newest stored count.

3. A method according to claim 1 or claim 2, in which the isolation step is initiated only if the signal immediately preceding the current signal was outside said band also.

4. An alternator system which is connected in parallel with a grid comprising apparatus for repeatedly updating a signal indicative of rate of change of frequency of the electricity generated by the alternator of the system and for determining whether the current signal is within a predetermined acceptance band and means operable to isolate the alternator system from the grid if the current signal is outside said band.

5. An alternator system according to claim 4, in which said apparatus comprises a first generator for generating cycles directly proportional to the cycles of a phase of the electricity generated by the alternator, a first counter for counting the cycles generated by the first generator, a second generator for generating high frequency time pulses, a count store for storing at least three successive counts of time pulses, the store being indexable by the first counter when a predetermined number of cycles has been counted to discard the oldest stored count and to store the count in the second counter and means for subtracting the differences between the oldest stored count and the median stored count and the median stored count and the newest stored count to obtain a current signal indicative of rate of change of frequency of the electricity generated by the alternator and for determining whether the current signal is within a predetermined acceptance band.

6. An alternator system according to claim 5, in which the first generator comprises a comparator for determining when the supply half cycles are positive and negative, a first switch controlled by the comparator, a high frequency oscillator, first and second counters and a second bistable switch, the output from the oscillator being passed by the first switch to the first and second counters, respectively, when the comparator detects positive and negative half cycles, respectively, the output from the first counter, when the first counter reaches a predetermined count in its respective positive half cycle, setting the bistable switch to the "on" state which in turn, locks the first counter in a reset condition and removes a reset signal from the second counter and the output from the second counter, when the second counter reaches a predetermined count in its respective negative half cycle, setting the bistable switch to the "off" state which, in turn, locks the second counter in a reset condition and removes a reset signal from the first counter.

7. An alternator system according to any one of claims 4 to 6, in which said means operates to isolate the alternator system only if the signal immediately preceding the current signal is outside said band also.

8. A method according to claim 1 substantially as hereinbefore described with reference to the accompanying drawings.

9. An alternator system according to claim 4 substantially as hereinbefore described with reference to the accompanying drawing.