GB2070356A - Apparatus for recording a disturbance on an alternating electrical supply - Google Patents

Abstract

An apparatus for recording a disturbance on an alternating electrical supply utilizes a recording apparatus (14), means (12a) for receiving an alternating electrical signal to be monitored and for coupling the signal to a record input (14a) of the recording apparatus through an analogue delay means (16), and means (12) arranged to detect a disturbance on the electrical signal and to provide a control signal to cause the recorder to record the signal applied to its record input, the analogue delay means being such that the recorder records the signal to be monitored immediately before, during and immediately after the occurrence of a disturbance on the alternating electrical signal. The delay means may be a charge-coupled or bucket-brigade device supplied with sampled values of the signal. <IMAGE>

Description

SPECIFICATION Apparatus for recording or indicating a disturbance on an alternating electrical supply This invention relates generally to an apparatus for recording signals and in particular to an apparatus for recording or indicating a disturbance on an alternating electrical supply.

There is often a requirement for recording a disturbance which occurs in an alternating current supply, such as a mains, or line frequency supply.

One substantial user of such apparatus is the electric utility industry which uses such apparatus to monitor both the generation and distribution of the mains supply. Such apparatus also finds application with users of equipment who need to know whether there has been a mains supply disturbance which might have affected operation of the equipment so that they can take some appropriate action to correct or repeat the operation of the equipment. Some typical uses of such apparatus are in monitoring the a.c.

supply to, inter alia, computers, control systems, tele-communications and medical equipment, longterm destructive testing equipment and the like.

One way to monitor such an a.c. supply would be to continuously record the line currents and/or voltages against time on, for example, a chart recorder, but this clearly would be wasteful in the use of recording medium and in time because of the need for the recorded information to be checked to ascertain whether a fault or disturbance had occurred.

One known apparatus embodies a magnetic drum memory in which signals are fed to recording heads in contact with a ferrous oxide coated drum which is continuously rotating. If a fault occurs, a recording paper transport is started, and signals from reproduce heads feed moving coil galvanometers. The signals on the drum are erased before they reach the record heads. This system is limited in bandwidth, noisy (electrical noise) and expensive.

Another known method uses a digital shift register to achieve a delay in the disturbance signal. Briefly, the apparatus operates as follows: 1. Signals are conditioned by appropriate signal conditioning modules. Discriminators in these modules are set so that in the event of a fault or disturbance the recorder is switched on via the control logic. The signals and switching command can be supplied by the customer if required.

2. The signals are connected via low pass filters to a multiplexer which samples each input signal in turn.

3. The output of the multiplexer is fed into a sample and hold unit, which retains its momentary value and then via an analogue to digital converter into an 8 bit data word.

4. The output of the A to D converter is then fed into a digital shift register which is clocked at the same rate as the sampling frequency thus moving information through the register.

5. The delay introduced is dependent on the capacity of the shift register and of the frequency of the sampling clock.

6. The output of the shift register is then fed into a digital to analogue converter.

7. The multiplexed signals are decoded in synchronism with the input multiplexer.

8. The decoded signals are then fed through a low pass filter to recompose the original signal.

The typical bandwidth of this system for one channel continuously sampled is 2 kilohertz.

When more channels are required, the bandwidth is reduced in proportion to the number of channels, e.g. a four channel system will have an ideal bandwidth of 500 hertz each. The input and output filters have to be changed to accommodate the new bandwidths. Because of the complexity of the system, it is relatively expensive. In addition, in order to increase the bandwidth of the system, a much larger shift register would be necessary. Also, since there is no channel to channel isolation afterthe input filter and before the output filter, common mode problems are difficult to reject.

Accordingly, it is desirable to provide an apparatus which will record disturbance on an alternating electrical signal, where the apparatus is less expensive and less complex than previously known systems.

The present invention is directed toward an apparatus for indicating or recording disturbances on an alternating current supply.

The invention provides an apparatus for recording a disturbance on an alternating electrical signal, comprising: means for detecting a disturbance on the alternating electrical signal, operative to provide a control signal in response to the occurrence of the disturbance; a first filter for filtering the alternating electrical signal, operative to provide a filtered electrical signal; analogue delay means for delaying the filtered electrical signal, operative to provide a delayed signal; a second filter operative to filter the delayed signal and thereby provide an output signal; and recorder means operative to record the output signal for a predetermined period upon the occurrence of the control signal.

Preferably, means is provided for sampling each signal at a predetermined frequency, and means is provided for isolating the sampling signal for any one signal channel from each of the other signal channels to be monitored. The recorder may have a plurality of record inputs.

The invention is illustrated by way of example in the accompanying drawing, the single figure of which is a block diagram of one embodiment of apparatus in accordance with the invention.

Referring to the drawing in Figure 1, there is shown an apparatus 10 for recording a disturbance on an electrical input signal. The apparatus 10 includes a fault detection circuit 12 which is also referred to as a fault sensor. Circuits such as discriminators are well known in the art and may be utilized for the fault detection circuit 12. The circuit 12 is arranged to detect a disturbance on an alternating electrical signal applied to an input terminal 12a and to provide a control signal at terminal 12b. This control signal is applied to an input terminal 13a to thereby trigger into operation a recorder 14. The recorder 14 may be a multi-channel light-beam recorder of suitable frequency response such as a BHL 5000 MEMOGRAPH which is manufactured by Bell & Howell Limited, Basingstoke, England. Other suitable recording devices may also be used.

The circuit 12 may be arranged to provide the control signal for a predetermined period to thereby maintain the recorder 14 in a recording state.

Alternatively, the circuit 12 may be arranged to provide a short duration signal, such as an impulse, as the control signal. In this case, the recorder 14 would be such that upon an impulse signal being applied to the input terminal 13a, the recorder 14 would record for the predetermined period.

The fault detection circuit 12 also provides a signal at a terminal 12c. This signal corresponds to the alternating signal to be monitored, and is suitably conditioned by the fault detection circuit 12 for processing by remainder of the apparatus 10. The signal at terminal 12c is coupled to a low-pass linear phase filter 15 which is preset to have a specific bandwidth. In the preferred ebodiment of the invention, the filter 15 is a 6-pole linear phase filter having a bandwidth of d.c. to 1500 Hertz. The main purpose of the filter 15 is to eliminate "aliasing" errors which are inherent in any sampling system.

The output of the filter 15 is fed to an analogue delay line 16 which is sampled periodically by a square wave transistor logic (TTL) clock pulse generator 18. The output of the analogue delay line 16 is fed to a 6-pole low-pass filter 20 which has a bandwidth of d.c. to 1000 Hertz. The filter 20 operates to filter out the high frequency components added by the sampling operation to thereby recompose the electrical signal. The output of the filter 20 is then amplified by a driver amplifier 24. This output signal is then coupled from the output of the amplifer24to an input 14a of the recorder 14.

A RETICON SAD-4096, which is a general purpose n-channel bucket brigade audio delay line used in audio systems for reverberation effects, sound effects, variable speech control and the like, may be utilized as the analogue delay line 16. Other suitable delay lines could be used, for example, a chargecoupled device (CCD) or any device which will provide a true analogue delay, rather than a digital shift register which first requires an input analogue to digital converter and an output digital to analogue converter.

Additional signals may be monitored and recorded with the use of duplicate circuitry (not shown) consisting of the fault sensor 12, the low pass filters 15 and 20, the analog delay line 16 and the amplifier 24. The output of the amplifier 24 of each duplicate circuit may be fed into any of the unused inputs 14b, 14c, 14d, 14e, or 14g of the recorder 14 as desired. The control signal of each duplicate circuit may be fed from the terminal 1 2b of the duplicate circuit to the additional input terminals used to trigger the recorder 14. Only two additional input terminals 136 and 13c are shown in the drawing.

In order to permit synchronization of each chan nel, the square wave generator 18 has an additional output which is fed to an opto-coupler 22. The opto-coupler 22 provides isolation between the square wave generator 18 and an analogue delay line in a duplicate circuit. Additional opto-couplers may be utilized to provide a clock signal from the square wave generator 18 to additional analogue delay lines used in duplicate circuits.

A typical application of a seven channel system would be the monitoring of a three-phase supply, including the neutral line. The input channels 14(a) through 14(g) shown in the drawing could be utilized' in this application. In order to provide even further isolation between the duplicate circuits d.c. to d.c.

converters could be utilized to supply power to each duplicate circuit.

In further detail, the delay TD of the analogue delay line 16, whether it be a bucket brigade device or a charge-coupled device, is a function of the sampling frequency FC according to the relation: To = (number of samples/FC) The previously referenced BHL 5000 recorder utilizes a tungsten iodine lamp in conjunction with moving photosensitive paper in order to record data or signals. Using the characteristics of the BHL 5000 recorder for purposes of example, the delay TD required to record a monitored electrical signal during, before and after the occurrence of a disturbance is a function of: (a) The start-uptime of the tungsten iodine lamp-approximately 150 milliseconds.

(b) The acceleration of the paper transport to the approximate speed - 150 milliseconds.

(c) The duration of recording (300 milliseconds in this example) before the disturbance (fault).

Since (a) and (b) run concurrently, the total delay required is approximately 450 milliseconds. By way of example, the delay line previously referenced, the RETICON SAD-4096 will accommodate 2048 samples.

By rewriting the above relation as: FC = (number of samples TD), it is apparent that for a delay of 450 milliseconds with a 2048 sample delay device, the sampling frequency, FC = (2048 0.450 seconds) 455.1 hertz.

By utilizing a sampling frequency of 4000 hertz, the delay TD = 512 milliseconds, which therefore results in approximately 362 milliseconds of recording prior to the occurrence of the disturbance.

Although delayed by 512 milliseconds, the electrical signals which pass through the analogue line 16 retain their analogue value. The output of the analogue delay line 16 has the waveform of a switched staircase. This switched staircase waveform is recomposed by the 6-pole low-pass linear phase filter 20 and coupled to recorder 14 via a driver amplifier 24. As previously described, the period during which the recorder 14 records, following the occurrence of a disturbance, may be preselected.

In order to provide information as to the sequence, time and day of any disturbances which are recorded by the apparatus 10, a timing unit (not shown) may be connected to an unused input of the recorder 14 such that at each time the recorder 14 operates, one of the output channels of the recorder 14 will record the desired information. Timing units are well known in the art and any suitable unit may be utilized.

It is to be understood that the subject invention has been described by reference to specific embodiments and that many additions and modifications thereto will be apparent to those skilled in the art.

Accordingly, the foregoing description is not to be construed in a limiting sense.

Claims (10)

1. Apparatus for recording a disturbance on an alternating electrical signal, comprising: means for detecting a disturbance on the alternating electrical signal, operative to provide a control signal in response to the occurrence of the disturbance; a first filterforfiltering the alternating electrical signal, operative to provide a filtered electrical signal; analogue delay means for delaying the filtered electrical signal, operative to provide a delayed signal; a second filter operative to filter the delayed signal and thereby provide an output signal; and recorder means operative to record the output signal for a predetermined period upon the occurrence of the control signal.

2. An apparatus according to Claim 1, wherein said means for detecting a disturbance on the alternating electrical signal is operative to provide, as said control signal, a trigger signal having a predetermined duration, and said recorder means is operative to record the output signal in response to the existence of the trigger signal.

3. An apparatus according to Claim 1 or 2, further comprising a timing means having its output connected to the analogue delay means, operative to control the delay time of the analogue delay means.

4. An apparatus according to Claim 3, wherein the timing means further comprises a square wave generator.

5. An apparatus according to any one of Claims 1-4, wherein the first filter is a low pass filter.

6. An apparatus according to any one of Claims 1-5, wherein the analogue delay means further comprises a bucket brigade delay line.

7. An apparatus according to any one of Claims 1-5, wherein the analogue delay means further comprises a charge coupled device.

8. An apparatus according to any one of Claims 1-7, wherein the second filter is a low pass filter.

9. An apparatus according to Claim 5 or Claim 8 as appended to Claim 3, wherein the or each low pass filter has a cut-off frequency of less than one half of the frequency of the timing means.

10. An apparatus for recording disturbance on an alternating electrical signal, substantially as described herein with reference to the accompanying drawing.