GB2029616A - Electrical load monitoring arrangement - Google Patents

Abstract

The red signal lamps 1,2 and 3 for a vehicle approach of a controlled pedestrian road crossing are connected in parallel via individual coupling transformers 4, 5 and 6 across a mains supply source 7. A current transformer 10 couples a monitoring arrangement 12 to this circuit. Because of a high loss reactance characteristic of the coupling transformers, the load current waveform of the source 7 has an initial portion of each half-cycle, prior to magnetic saturation of the coupling transformers, which is of low magnitude when the circuit is on no-load, but this portion has a much higher magnitude when any of the lamps is drawing current. This magnitude can be detected using a sampling technique, a peak magnitude sensing technique or a pulse width modulation technique. <IMAGE>

Description

SPECIFICATION Electrical load monitoring arrangement This invention relates to an electrical load monitoring arrangement of a kind suitable for detecting the failure of all of a plurality of electrical loads connected in parallel, the electrical loads being connected for energisation via respective transformers from an a.c. supply source.

An electrical load monitoring arrangement of the above kind has a particular but non-exclusive application for monitoring the performance of signal lamps of certain road traffic signal installations. For example, in the United Kingdom it has been proposed for a controlled pedestrian road crossing, known as a "Pelican" crossing, that the performance of the red signal lamps shown to traffic for both vehicle approaches should be monitored, so that a controller for the crossing can be switched-off by a monitoring arrangement if the latter detects that all the red signal lamps (usually 2 or 3) for either vehicle approach have failed, thereby avoiding a dangerous situation.The dangerous situation that would occur is the giving of a pedestrian "cross now" signal, which may be an audible signal (essentially to assist blind persons) as well as a green visual signal, which is presented to pedestrians during a green pedestrian phase and which should be accompanied by a red (stop) traffic phase on both vehicle approaches; but would in fact not be so accompanied on one or both vehicle approaches if there were total red lamp failure in respect of the or each vehicle approach.

Once the controller for the crossing has been switched-off, the crossing reverts to an uncontrolled pedestrian crossing.

One suggested method of monitoring signal lamps of a controlled pedestrian crossing is to detect the correct light level of each lamp being monitored.

This method of monitoring would involve incorporating a photo-electric sensor, such as a cadmium sulphide cell, in the lamp reflector, so that an arrangement for carrying it out would be expensive to design and install since the sensor would have to be shielded against ambient light and modification to the reflector would be necessargyto mount the sensor. There would also be the expense of provid ing electrical connections between the reflector mounted sensors and an associated electric circuit which controls the switching-off of the controller for the crossing.

Therefore, in view of the cost of design and installation, it has now been proposed in the road traffic signal art that any arrangement for monitoring the performance of the signal lamps of a controlled pedestrian road crossing should be installed with the controller for the crossing. However, this immediate ly imposes the restriction that any monitoring has, in practice, to be in respect of the voltage and current for energising the lamps. Where the voltage rating for the signal lamps is the same as the voltage of an a.c. supply source for the lamps, the signal lamps can be connected in parallel directly across the a.c.

supply source, and a simple monitoring arrange ment can be formed by a current sensor connected in series with the a.c. supply source and a voltage sensor connected across the a.c. supply source, together with means for detecting from these two sensors the condition of a.c. supply voltage present but no a.c. supply current. When this condition obtains, none of the signal lamps connected in parallel across the a.c. supply source is drawing current, so that all these lamps (or their energising circuits) have failed and the monitoring arrangement will switch-off the controller.

However, practical considerations in the design of the controlled pedestrian road crossing, known as the "Pelican" crossing as aforesaid, preclude the use of such a simple monitoring arrangement. More specifically, a type of signal lamp now in common use in such crossings is a tungsten halogen lamp having a 12 volt rating, so that step-down voltage transformation is necessary in order to energise such a signal lamp from an a.c. (mains) supply source of, say, 240 volts. Furthermore, in order in the interests of economy to reduce to a minimum the length of the relatively large diameter cable which is required for energizing such a signal lamp (i.e.

compared with a lamp having a 240 volt rating and the same watt rating) an individual step-down transformer is provided for each signal lamp and is mounted adjacent thereto in the relevant signal post head of the crossing.

The design of this step-down transformer is such that it draws a significant no-load current and that it introduces considerable distortion into the current waveform from the a.c. supply source. As regards the no-load current, this can be so large that the total average no-load current of three such transformers connected in parallel can be commensurate with the average energising current drawn by one signal lamp. As regards the distortion introduced by the transformer, this is due to the transformer being designed as a high reactance transformer so that the loss reactance due to it is made high deliberately.

This has the effect of reducing the surge current through the signal lamp when it is switched-on, thereby prolonging the life of the lamp.

In order to provide a monitoring arrangement which can detect the failure of all of a plurality of red traffic signal lamps which are connected in parallel for energisation via respective step-down transformers from an a.c. supply source, an adaptation of the simple monitoring arrangement mentioned previously has been proposed. In this adaptation, it is not the absence of a.c. supply current (with a.c.

supply voltage present) which is detected, because the no-load currents of the transformers are always present in the absence of signal lamp energisation.

Nor is it a finite average magnitude of the a.c. supply current which is detected, because a low magnitude of the average a.c. supply current may be due to either the total of the no-load currents or to a combination of no-load currents and a single lamp being energised, and these currents may be indistinguishable from each other. Instead, in this adaptation of the simple monitoring arrangement, the relative phase between the current waveform and the voltage waveform of the a.c. supply source is used as a criterion for determining when all of the signal lamps have failed. The phase difference between these current and voltage waveforms varies in a range between 900 to 0" from a no-load condition to a load condition.Because of the use of high reactance transformers, because none, one, two, or more signal lamps may be energised, and because of other contributing factors such as cable resistance, the actual variation in the phase difference is over a relatively small range. Nevertheless, the variation in phase difference which has been obtained between the condition when all of the signal lamps connected in parallel have failed and the condition when at least one of them is energised has been thought to be sufficient to provide a reliable monitoring arrangement, but this is open to doubt.

It is an object of the present invention to provide an electrical load monitoring arrangement of the kind referred to which utilises what is thought to be a more reliable criterion of the a.c. supply source current waveform for determining the failure of all of the plurality of electrical loads, for example, traffic signal lamps.

According to the invention an electrical load monitoring arrangement of the kind referred to is characterised in that it comprises means for sensing the magnitude of the a.c. supply current waveform during a period of a recurrent cycle thereof prior to magnetic saturation of the coupling transformers by the a.c. supply current in that cycle, and means for determining in accordance with the sensed magnitude whether or not to produce an output signal signifying the failure of all of the electrical loads.

An arrangement according to the invention has the advantage that prior to the magnetic saturation of the coupling transformers, that is, before the transformer core has been driven to the knee of its B-H curve, the no-load currents of the coupling transformers is sufficiently small to be distinguishable from the load current due to a single load being energised. Furthermore, the no-load currents are relatively constant prior to the magnetic saturation.

In carrying out the invention, the a.c. supply current waveform may be full-wave rectified prior to the magnitude sensing being effected. This gives the advantage that magnitude sensing can be effected twice per cycle of the a.c. supply current waveform, thereby doubling the sensing resolution.

In one particular embodiment of the invention, the magnitude sensing of the a.c. supply current waveform is effected by a sampling circuit which is operable to sample the current magnitude for a predetermined duration within said period, commencing at a predetermined time after the beginning ofthe period. The sampled current magnitude is then compared with a reference magnitude to determine whether or not the output signal is to be produced.

In another particular embodiment of the invention, the peak magnitude of the a.c supply current waveform is sensed following the magnetic saturation of the transformer cores. The ratio of or the difference between this peak current magnitude and the current magnitude which is sensed prior to the magnetic saturation is then compared with a reference magnitude to determine whether or not the output signal is to be produced.

In a further particular embodiment of the invention, the magnitude sensing of the a.c. supply current waveform is effected by a pulse width modulation circuit which is operable to produce a pulse having a width proportional to the duration, including said period, that the magnitude in each cycle of the a.c. supply current waveform exceeds a first reference magnitude. The pulse thus produced is then integrated to produce a resultant signal which is compared with a second reference magnitude to determine whether or not the output signal is to be produced.

Each of these particular embodiments of the invention gives the advantage that it relies on the relatively constant no-load currents which exist prior to the magnetic saturation of the transformer cores.

In order that the invention may be more fully understood reference will now be made by way of example to the accompanying drawings of which Figure 1 shows a simple circuit of a plurality of electrical loads connected in parallel across an a.c.

supply source; Figures 2, 3 and 4 show diagrammatically respective electrical load monitoring arrangements according to the invention; Figure 5 shows a circuit for the monitoring arrangement shown in Figure 4; and Figure 6 shows explanatory waveform diagrams.

Referring to the drawings, the simple circuit shown in Figure 1 comprises three lamps 1,2 and 3 which are connected in parallel via respective coupling transformers 4, 5, and 6 for energisation from an a.c. supply source 7. The a.c. supply source 7 is assumed to be an a.c. mains supply having the usual line and neutral sides Land N. A switch 8 and the primary winding 9 of a current transformer 10 are connected in series in the line side L of the mains supply. As discussed in some detail earlier in this specification, the lamps 1, 2 and 3 are assumed to be the red signal lamps shown to traffic for one vehicle approach of a controlled pedestrian road crossing.

(A similar circuit comprising three further red signal lamps would be provided for the other vehicle approach of the crossing). On the assumption that the current transformer 10 introduces negligible distortion between its primary and secondary currents, a current I which flows in the secondary winding 11 of the current transformer 10, when the circuit is energised by closure of the switch 8, has substantially the same waveform shape as that of the a.c. supply current waveform. This current I is represented by the waveform (a) in Figure 6. If none of the lamps 1,2 and 3 is energised (e.g. due to lamp failure), then the current I is representative only of the no-load currents of the three coupling transformers 4, 5 and 6. On the basis of the discussion given earlier in the specification regarding use of high reactance transformers for the coupling transformers, waveform (a) in Figure 6, shows that during the period that1 in each half-cycle, the current I has only a relatively low substantially constant magnitude i1 (or-il). However, at the time tri, the transformer cores of the coupling transformers 4, 5 and 6 become magnetised beyond the "knee" of their B-H curve, so that as a result during the following period t1-tO their no-load currents change sharply to a peak magnitude i3 (or -i3) because of the decreased inductance of the primary windings of these coupling transformers.The dotted line Le in waveform (a) symbolises that when one (or more) of the lamps 1, 2 and 3 is energised and thus drawing load current, the current I changes substantially during each period tO-tl to an approximate magnitude i2 (or -i2). A monitoring arrangement 12 is connected to the secondary winding 11 of the coupling transformer 10 to monitor the operation of the lamps 1,2 and 3, using the criterion of the magnitude of the current I during each period that1 to determine whether or not all three lamps have failed.

As mentioned previously, this criterion of the magnitude of the current I during each period tO-tl can be utilised in different ways, and the monitoring arrangements shown in Figures 2,3 and 4 exemplify three different ways, respectively.

In the monitoring arrangement shown in Figure 2, the secondary winding 11 of the current transformer 10 is connected to a full-wave rectifier circuit 13 the output waveform I' from which is thus as represented by waveform (b) in Figure 6. This output waveform I' is applied to a sampling circuit 14 which is operable in each half-cycle of the waveform to produce a signal sample Ss representing the magnitude of the waveform I' during a recurrent sampling period ts. This sampling period ts occurs within the period tO-tl, commencing after an initial delay period td, as shown in waveform (b). The period td is determined by a delay circuit 15 which is rendered operable at the beginning of each half-cycle of the waveform I' by a zero-crossing detector circuit 16 which detects each time the magnitude of the waveform I is zero.The signal sample Ss produced by the sampling circuit 14 is applied to a comparator circuit 17, to which is also applied a reference signal Sr. If the signal sample Ss has a magnitude less than that of the reference signal Sr (i.e. a magnitude less than it, then the failure of all the lamps 1,2 and 3 has been detected and the comparator circuit 17 produces an output signal So signifying this failure. This output signal So operates a relay or other device 18 which then, in the case of a controlled pedestrian crossing, would switch-off the controller for the crossing.

In the monitoring arrangement shown in Figure 3, the secondary winding 11 of the current transformer 10 is connected to a full-wave rectifier circuit 19 the output waveform I" from which is similar to the output waveform I' produced in the monitoring arrangement of Figure 2. The output waveform 1", which is represented by waveform (c) in Figure 6, is applied to a minimum magnitude detector circuit 20 and to a maximum magnitude detector circuit 21.

Two delay circuits 22 and 23, which are rendered operative atthe beginning of each half-cycle of the waveform I" by a zero-crossing detector 24, cause the circuits 20 and 21 to detect after respective delay periods td and td' the prevailing (low) and (high) magnitude il and ih of the waveform 1", These detected magnitudes are applied to a logic circuit 25 which may function as a divider circuit to determine the ratio between the detected magnitudes or as a subtract circuit to determine the difference between the detected magnitudes.The magnitude of the resultant signal Sds is compared in a comparator circuit 26 with the magnitude of a reference signal Sr' to cause the production of an output signal So' for operation of a relay or other device 27 in the same circumstances as explained for the monitoring arrangement of Figure 2.

In the monitoring arrangement shown in Figure 4, the secondary winding 11 of the current transformer 10 is also connected to a full-wave rectifier circuit 28 the output waveform I"' from which is represented bythewaveform (d) in Figure 6, this waveform being similar to the waveforms (b) and (c). This output waveform I"' is applied to a threshold detector circuit 29 to which is also applied a first reference signal 1 Sr which has a magnitude just greater than il (see waveform (d) in Figure 6).During the time that the magnitude ofthewaveform i"' exceeds the magnitude of the first reference signal 1Sr, an output signal Sp from the threshold detector circuit 29 has a constant magnitude of one value (e.g. high), whereas during the time that the magnitude of the waveform I"' does not exceed the magnitude of the first reference signal 1 Sr, the output signal Sp has a constant magnitude of another (low) value. As a consequence, the output signal Sp is, in effect, a pulse width modulated signal having the form represented by waveform (e) or waveform (f) of Figure 6 according as the shape of the waveform I"' is due to total lamp failure or due to at least one lamp being still energised.

The output signal Sp from the circuit 29 is applied to an integrating circuit 30 which is responsive thereto to produce an output signal Si having a magnitude which is proportional to the pulse width of the output signal Sp. A comparator circuit 31 compares the magnitude of the output signal Si with the magnitude of a second reference signal 2Sr, to produce an output signal So" for operation of a relay or other device 32 in the same circumstances as for the monitoring arrangements of Figures 2 and 3.

In the circuit shown in Figure 5 of the monitoring arrangement shown in Figure 4, the individual components which form the different circuits of the arrangement have been grouped in broken line blocks, so far as is practicable. Thus, the full-wave rectifier 28 is formed by the rectifier bridge comprised by four rectifiers 33-36; the threshold detector circuit 29 is comprised by a transistor 37 and associated components (including an outputtransistor 38); the integrating circuit 30 is formed by a capacitor 39 and resistors 40 and 41, and the comparator circuit 31 is formed by an operational amplifier 42 and associated components. In the threshold detector circuit 29, a variable resistor 43 connected in the emitter circuit of the transistor 37 provides the first reference signal 1 Sr. A variable resistor 44 in one of the input circuits of the amplifier 42 provides the second reference signal 2Sr in the comparator circuit 31.

It is evident that the other two monitoring arrangements shown in Figures 2 and 3 can be implemented in similarfashion using discrete components to form their various circuits.

Claims (12)

CLAIMS:

1. An electrical load monitoring arrangement suitable for detecting the failure of all of a plurality of electrical loads connected in parallel, the electrical loads being connected for energisation via respective transformers from an a.c. supply source, which arrangement is characterised in that it comprises means for sensing the magnitude of the a.c. supply current waveform during a period of a recurrent cycle thereof prior to magnetic saturation of the coupling transformers by the a.c. supply current in that cycle, and means for determining in accordance with the sensed magnitude whether or not to produce an output signal signifying the failure of all of the electrical loads.

2. An electrical load monitoring arrangement as claimed in Claim 1, characterised in that it includes meansforfull-wave rectifying the a.c. supply current waveform prior to the magnitude sensing thereof being effected.

3. An electrical load monitoring arrangement as claimed in Claim 1 or Claim 2, characterised in that it includes a sampling circuit which is operable to sample the current magnitude for a predetermined duration within said period, commencing at a predetermined time after the beginning of the period, together with means for comparing the sampled current magnitude with a reference magnitude to determine whether or not the output signal is to be produced.

4. An electrical load monitoring arrangement as claimed in Claim 1 or Claim 2, characterised in that it includes means for sensing the peak magnitude of the a.c. supply current waveform following the magnetic saturation of the transformer cores, means for determining a resultant signal which is representative of the ratio of or the difference between said peak magnitude and the current magnitude which is sensed prior to the magnetic saturation, and means for comparing the resulting signal with a reference magnitude to determine whether or not the output signal is to be produced.

5. An electrical load monitoring arrangement as claimed in Claim 1 or Claim 2, characterised in that it includes, a pulse width modulation circuitwhich is operable to produce a pulse having a width proportional to a duration, including said period, that the magnitude in each cycle of the a.c. supply current waveform exceeds a first reference magnitude, an integrating circuit which is operable to integrate said pulse to produce a resultant signal, and means for comparing said resultant signal with a second reference magnitude to determine whether or not the output signal is to be produced.

6. An electrical load monitoring arrangement as claimed in any preceding Claim, in combination with said plurality of electrical loads connected in parallel.

7. A combination as claimed in CbiXm 6, wherein said electrial loads are traffic signal lamp-

8. An electrical load monitoring arrangement, substantially as hereinbefore described with reference to Figures 2 and 6 of the accompanying drawings.

9. An electrical load monitoring arrangement, substantially as hereinbefore described with reference to Figures 3 and 6 of the accompanying drawings.

10. An electrical load monitoring arrangement, substantially as hereinbefore described with reference to Figures 4 and 6 of the accompanying drawings.

11. An electrical load monitoring arrangement as claimed in Claim 10, having a circuit substantially as hereinbefore described with reference to Figure 5 of the accompanying drawings.

12. In combination, a plurality of electrical loads connected in parallel across an a.c. supply source, substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings, and an electrical load monitoring arrangement as claimed in Claim 8, Claim 9, Claim 10, or Claim 11.