GB2357642A - Trip circuit fault protection apparatus - Google Patents

Abstract

A trip circuit protection apparatus for use in combination with a trip circuit comprising a circuit breaker coil 5 connected in series in a supply line 8 from a supply voltage 7, comprises a sensing means 12 adapted to produce an output signal indicative of the condition of the supply to the coil 5, a fault detection means 13, adapted to monitor the output of the sensing means 12 and a switching device 11 in series in the supply line 8 between the supply 7 and the coil 5 which during normal condition of the trip circuit is closed. The fault detection means 13 is adapted to operate the switching device 11 to isolate the coil 5 from the supply 7 if the condition of the line meets predetermined criteria indicative of a fault in the trip circuit, for example, excessive current level or current pulse duration.

Description

2357642 A FAULT PROTECTION APPARATUS This invention relates to

improvements in fault protection apparatus and in particular to a trip circuit protection apparatus for protecting a circuit 5 breaker included in a fault protection scheme.

An example of a typical prior art fault protection apparatus is shown in figure 1 of the accompanying drawings. It is known to provide a fault protection apparatus for high voltage electrical lines 2, such as those found in a domestic power transmission network, by locating a circuit breaker 3 in series with a load 1 on the line 2 to be protected. In the event of a fault on the line 2, such as an excessive current or short circuit, the circuit breaker 3 is tripped, forcing open a normally closed contact 4 which isolates the load 2 from its supply 6. Many such devices may be located at various locations along the line in order to allow sections of the line to be isolated.

Operation of the circuit breaker 3 is achieved using a circuit breaker control circuit. This is often referred to as a trip circuit. This typically 210 comprises a normally open relay contact 9 which is connected in a line 8 in series with an operating coil 5 of the circuit breaker 3 and an independent voltage supply 7. The relay is normally open to isolate the coil 5 from the supply but is adapted to move to a closed position if a fault occurs on the main line 2 to be protected. This in turn connects the circuit breaker coil to the independent supply 7 and energisation of the coil 5 opens the circuit breaker contact, i.e. trips the circuit breaker.

For high voltage protection, large circuit breakers are needed to provide the force necessary to break the flow of current along the line to the load.

In many applications the operating coil of the circuit breaker may require 2 a current pulse in excess of 50 amps for a short period of as low as 100 milliseconds or less in order to trip the breaker. In one well known example, up to seven pulses in rapid sequence (less than 100 millisecond) are required to trip the circuit breaker.

Faults can arise in the separate supply for the circuit breaker coil. For example, where the circuit breakers are mounted on poles in a distribution network this supply is typically a self contained battery of high current rating. To protect the coil from battery faults or other problems with the trip circuit, a fuse F is typically provided in series between the battery and the coil. Excess current will cause the fuse to blow, protecting the coil. Another common fault with fuses occurs if the circuit breaker is tripped several times in close succession. On each trip the fuse will warm up. This could cause the fuse to blow in error during a subsequent operation.

Whilst the crude protection provided by the fuse can prevent some instances of damage to the circuit breaker coil it is difficult to provide a fuse that will not operate during a normal operation of the circuit and yet still protect if the operating pulse is too long or at too high a current. Most devices have a long time/current tolerance band and may allow short duration pulses to pass even if they are able to damage the coil or continuous current to flow of a small value which would eventually damage the coil after a long period of time.

An object of the present invention is to overcome at least partially some of the disadvantages of the prior art.

According to a first aspect, the invention provides a trip circuit protection apparatus for use in combination with a trip circuit comprising at least a 3 circuit breaker coil connected in series in a supply line from a supply voltage, the trip circuit protection apparatus comprising:

a sensing means adapted to produce an output signal indicative of the condition of the supply line to the coil; a fault detection means adapted to monitor the output of the sensing means, a switching means provided in series in the supply line between the supply and the coil which during a normal condition of the trip circuit is closed, and in which the fault detection means is adapted to operate the switching device to isolate the coil from the supply in the event that the condition of the line meets a predetermined criteria indicative of a fault in the trip circuit.

Hence, by providing a trip circuit protection circuit according to the first aspect of the invention the supply line from the supply to the coil is broken automatically by the switching means in the event of a fault condition.

The sensing means may detect the magnitude of the current in the line as an indicator of line condition. This magnitude may be an instantaneous magnitude value. This provides for a fast switching response.

In a notable arrangement a fault condition may be indicated if the current in the line exceeds a predetermined maximum value. During normal operation the current would lie below this value. In practice, for a trip circuit in which the coil is driven by pulses the value would be chosen to 4 exceed the maximum expected value of current magnitude during a pulse in normal trip circuit operation.

Alternatively, we may mean that the predetermined criteria is met if the current in the line exceeds a predetermined value for a predetermined time duration. Again, this is especially suitable for use in which the trip circuit normally provides short duration pulses to trip the circuit breaker. In that case, this can indicate the presence of a current which exceeds the maximum normal expected pulse length for current in the line. This would typically arise if a short circuit occurs.

The sensing means may comprise, in combination, a resistive element placed in series between the coil and the supply voltage and a voltage sensing means adapted to sense the voltage dropped across the resistance element. In a purely resistive element this voltage drop will be proportional to the current flow. The output of the voltage sensing means may therefore represent the output of the sensing means which is fed to the fault detection means. Thus, the current sensing means may be adapted to indirectly sense the current in the line by measuring the voltage drop across the component. A voltage drop across another part of the circuit may be measured instead, such as the voltage drop across the terminals of a battery source for the trip circuit.

The fault detection means may comprise at least a first comparator circuit.

A first input to the first comparator circuit may be dependent upon the output of the sensing means, whilst a second input of the comparator may be connected to a reference signal. As is well known to the skilled man, the output of the comparator will be a function of the difference between the signals on the two inputs.

For example, where the sensing means produces an output signal dependent on the current in the trip circuit line the second input may be connected to a reference voltage which is greater than the maximum expected output voltage from the sensing means. If the sensed voltage (and hence current) exceeds the reference voltage, the output of the comparator will switch states to indicate a fault condition in the trip circuit.

The output of the comparator may directly feed an input to the switching means. For instance, the switching means may comprise a coil-operated relay. The output from the comparator may be applied across the relay coil so that when the comparator output changes state (sensed input voltage exceeds the reference input voltage) the relay is energised. Of course, the relay coil may be normally energised and instead is de- energised when the output of the comparator changes state indicative of a fault. Other switching devices are also envisaged within the scope of the invention such as semi-conductor switching devices. By providing either a metallic relay contact or a semiconductor device, the switch does not erode over time as the contact is normally closed. This improves on thermal fuses which are thermally degraded over time due to the passage of regular current pulses.

In an alternative, or in addition, a second comparator may be provided. Again, a first input to the second comparator may be dependent upon the output of the sensing means. The second input may also be connected to a reference signal. This reference will differ from that to the first comparator an should be chosen such that the output of the comparator changes state for substantially the duration of a pulse in the trip circuit. To do this it should be between the value of the signal applied to the first input for both the pulse and non-pulse conditions of the trip circuit.

6 The output of the second comparator may be adapted to trigger a timer or counter circuit, the output of the timer circuit being adapted to indicate a fault if the input voltage applied to the comparator exceeds a reference value for a duration in excess of an expected maximum time. This can therefore be used to indicate a fault where the current value is within accepted levels but the duration of any signal on the line is excessive.

The timer may be reset after each trip of the circuit breaker. The protection circuit thus performs identically for each trip operation unlike a warming fuse.

The counter or timer circuit may comprise an RC circuit which provides a time constant for the timer.

is The trip circuit protection apparatus for the trip circuit coil may be connected in series with an independent power supply, such as a battery. Alternatively, the circuit may be self powered and draw power from the line containing the coil or the line protected by the circuit breaker.

The trip circuit protection apparatus may be adapted to open its switching means substantially instantaneously to detecting a fault condition on the line.

It is envisaged that a switching time of 1 millisecond or less can be achieved whilst at the same time providing a protection envelope of the order of 1 Amp, or perhaps less than 1 Amp.

It is also envisaged that the fault detection circuit may include additional comparator circuits which can be tailored to the expected operating 7 conditions of the trip circuit. Of course, other microprocessor based systems may be used in place of the comparators whilst fulfilling a similar function.

In one refinement, the fault detection circuit may be adapted to respond t varying cut-off times for different expected pulse in a train of pulses, i.e. allow the first pulse to be linger than the second without opening the switch, or of a different peak current.

In accordance with a second aspect, the invention provides a trip circuit for a load to be protected, in which the trip circuit is protected by a trip circuit protection apparatus in accordance with the first aspect of the invention.

The trip circuit may include a trip contact which is normally open in use to isolate the coil from the supply voltage but is adapted to be closed in the event of a fault in the line be protected to operate the breaker coil.

The circuit breaker may comprise an auto-recloser type circuit breaker. 20 The switching means of the trip circuit protection apparatus may be located in the trip circuit between the trip contact and supply voltage. Thus, in the event of a fault in the trip circuit the switching means isolates both the coil and the trip contact from the supply. Alternatively, 25 it may be located between the coil and the trip contact.

Similarly, where the sensing means comprise a current sensing means this may be provided between the supply voltage and the trip contact.

8 There will now be described by way of example only, one embodiment of the present invention with reference to the accompanying drawings of which:

Figure 1 is an illustration of a prior art fault protection apparatus;

Figure 2 is an illustration of a trip circuit protection apparatus in accordance with the invention; Figure 3 illustrates the fault detection circuit which forms a part of the apparatus of Figure 2; and Figure 4 is a block diagram illustrating the operation of the fault detection circuit.

As shown in Figure 2, a load 1 on a line 2, such as a high voltage power transmission line is protected by a circuit breaker 3. The circuit breaker 3 includes a contact 4 which is normally closed and a coil 5. The contact 4 is connected in series into the line 2. In normal operating conditions the breaker contact 4 connects the load 1 to a supply 6 which feeds the line 2. An energising current, typically comprising one or more short duration pulses can be applied to the circuit breaker coil 3 to open the contact 4 and isolate the load 1.

In order to energise the circuit breaker coil it is connected to an independent power supply 7 through a control line 8. A contact 9 of a trip circuit relay 10 is provided in series in the line 8. This is normally open to present current flow through the circuit breaker coil 5. To trip the circuit breaker the trip contact 9 may be closed in a known manner when a fault is detected on the main supply line 2 to the load 1.

9 In the event of a fault at the battery 7, or perhaps a fault with the trip contact 9, an excessive current may be fed to the breaker coil 5 which could damage the coil. This may also occur if the contact 9 stays closed far too long, which can cause a current to flow in the coil for long enough to damage the coil.

To protect the circuit breaker coil 5 a trip circuit protection apparatus is provided. This includes a protection switch 11 in series with the battery 7 in the trip circuit line 8 which is normally closed. A sensing device 12 is also provided which monitors the flow of current along the line 8 and a fault detection circuit 13, shown generally as a block, opens the switch 11 when an abnormal current flow is detected by the sensing device.

In the example shown in Figure 2 of the accompanying drawings the sensing device 12 comprises a resistive load which is connected in series in the trip circuit line 8. As current flows a volts drop is produced across the resistance element. This voltage dropped across the resistance load according to the relationship V = IR. A measurement taken across the resistance load can therefore act as an input to the fault detection circuit.

The switch 11 comprises a normally closed contact of a coil operated relay. When the fault detection circuit detects a fault on the trip circuit line 8 it applies a current to the energising coil of the relay which opens the switch 11. The relay can be latched so as to stay open until it is reset by an operator, or alternatively the output from the fault detection circuit may be similarly latched.

The fault detection circuit 13 is shown schematically in Figure 3 of the accompanying drawings, and the operation of the circuit of Figure 3 is illustrated by way of a block diagram in Figure 4.

The circuit comprises two comparators 20,30. Each comparator 20,30 has a positive and a negative input 21,22,31,32, and produces a respective output signal 23,33 according to the sum of its two inputs. In practice, each comparator has an infinite gain and their output will swing between a minimum level when the value applied to the positive terminal 21 or 31 is 10 less than that applied to the corresponding negative terminal 22 or 32, and a maximum level when the value applied to the positive terminal exceeds that applied to the negative input terminal. These levels depend on the comparator type and supply voltages used. 15 The positive terminal of each comparator 20,30 is fed with an input voltage determined by the voltage drop, W developed across the resistive load in the trip circuit line 8 to the coil 5. The negative terminal of each comparator 20,30 is connected to a respective reference voltage, Vrefl and Vref2. In the example, this reference voltage is derived by 20 providing a resistive divider circuit connected between the supply rails for the fault detection circuit. Each reference voltage in the example is different as will become apparent. In normal operation, with no faults in the trip circuit line to the breaker 25 coil, the current flowing in the line to the coil will be zero or thereabouts because the trip contact is open. If a fault is detected on the main line the trip contact is momentarily closed to send an energising pulse to the coil to trip the circuit breaker. In some arrangements, a train of similar pulses may be needed. A fault in the supply to the trip coil will be 30 indicated if either the magnitude of the current rises too high in the line, or if the duration of a current "pulse" exceeds an expected maximum. The first comparator 20 is connected to a circuit which enables a fault in which the current is too high to be detected. The second comparator is connected to a circuit which enables a fault in which the 5 current pulse is too long to be detected.

The operation of the first comparator is best understood with reference to figure 4 of the accompanying drawings.

The negative input to comparator 20 is connected to a reference voltage Vrefl which is slightly higher than the expected peak voltage (dependent upon peak current by the relationship V = IR) in the line. In normal operation the voltage dropped across the resistor R in the line is first measured 200. This is fed 201 to the positive input of the first comparator which compares 202 the sensed voltage to the reference. Normally, the output of the first comparator 20 will remain at its normal state, i.e. low, when a pulse is applied to trigger the breaker coil. However, if the current rises to an excessive level, the output of the first comparator 20 will change state, i.e. go high. This provides an input 203 to a latch 40 which in turn energises the relay that opens the switch 11. Thus, the coil 5 is isolated from its supply battery 7.

The second comparator 30 also has a reference voltage Vref2 applied to its negative terminal 32. However, this is selected to be at a level which is lower than the peak voltage value yet higher than any possible noise on the line to the coil 5. The output of the second comparator 30 will therefore swing high for the duration of a pulse, or whenever current flows in the line. This output is fed to a counter 50. The function of the counter 50 can be seen with reference to Figure 4.

12 In operation, the voltage Vd is measured across the resistor R in a first step 100 and subsequently fed 210 to the positive input to the second comparator 30. Whenever current flows in the trip circuit the output 34 of the second comparator will change states 220, and the counter is started 230. The counter continues to increment 240 until the output of the second comparator 30 returns to its normal state when the counter is held 250 at its last value. The count value is then compared 260 to a reference value. The pulse duration is then compared 260 to a reference value. If the counter value is above the reference (indicating that the pulse on the line exceeds its maximum permitted length), the counter value will exceed the reference count value. This event then triggers a latch 60 to energise the protection relay. If the count does not exceed the maximum, the counter 50 is reset 270 and the latch is not energised.

Of course, it will be appreciated that the embodiment illustrated in the drawings is provided by way of example only and is not limiting. Many alterations are envisaged. For example, only one of the two comparator circuits may be provided in some instances. Also, the detection circuit may be adapted to detect a fault by processing the current measurement signal in other ways. For example, the inputs to the comparator(s) may be current fed rather than voltage fed as descried in the specific embodiment.

There may also be some additional signal conditioning circuitry provided between the output of the sensing means and the input(s) to the comparator(s) Providing the protection system of the present invention is advantageous over the prior art in that it is possible to increase the reaction time of the system to faults though the selection of suitable componentry when compared to a fuse protection system. It can also be easily adjusted to have varying degrees of fault sensitivity by altering the values of the 13 reference voltage applied to the comparators to provide a higher or lower permitted margin of error in the energising pulses.

Claims (18)

14 CLAIMS

1. A trip circuit protection apparatus for use in combination with a trip circuit comprising at least a circuit breaker coil connected in series in a supply line from a supply voltage, the trip circuit protection apparatus comprising: a sensing means adapted to produce an output signal indicative of the condition of the supply line to the coil; a fault detection means adapted to monitor the output of the sensing means, a switching means provided in series in the supply line between the supply and the coil which during a normal condition of the trip circuit is closed, and in which the fault detection means is adapted to operate the switching device to isolate the coil from the supply in the event that the condition of the line meets a predetermined criteria indicative of a fault in the trip circuit.

2. A trip circuit protection apparatus according to claim 1, in which the sensing means detects the magnitude of the current in the line as an indicator of line condition.

3. A trip circuit protection apparatus according to claim 2, in which a fault condition is indicated if the current in the line exceeds a predetermined maximum value.

4. A trip circuit protection apparatus according to claim 2 or claim 3, in which the predetermined criteria is met if the current in the line exceeds a predetermined value for a predetermined time duration.

5. A trip circuit protection apparatus according to any preceding claim, in which the sensing means comprises, in combination, a resistive element placed in series between the coil and the supply voltage and a voltage sensing means adapted to sense the voltage dropping across the 5 resistance element.

6. A trip circuit protection apparatus according to any preceding claim, in which the fault detection means comprises at least a first comparator circuit.

7. A trip circuit protection apparatus according to claim 6, in which a first input to the first comparator circuit is dependent upon the output of the sensing means, whilst a second input of the comparator is connected to a reference signal, the output of the comparator being a function of the 15 difference between the signals on the two inputs.

8. A trip circuit protection apparatus according to claim 7 as dependent from claim 5, in which second input of the comparator is connected to a reference voltage which is greater than the maximum expected output voltage from the sensing means.

9. A trip circuit protection apparatus according to claim 6,7 or 8, in which the output of the comparator is fed to an input to the switching means.

10. A trip circuit protection apparatus according to any preceding claim, in which the switching means comprises a coil-operated relay.

16

11. A trip circuit protection apparatus according to claim 7, in which the value of the reference signal applied to the first input lies between the valve for or both the pulse and non-pulse conditions of the trip circuit.

12. A trip circuit protection apparatus according to claim 11, in which the output of the comparator is adapted to trigger a timer or counter circuit, the output of the timer circuit being adapted to indicate a fault if the input voltage applied to the comparator exceeds the reference value for a duration in excess of an expected maximum time.

13. A trip circuit protection apparatus according to any preceding claim which is self powered and draws power from the line containing the coil or the line protected by the circuit breaker.

14. A trip circuit protection apparatus substantially as described herein with reference to and as illustrated in the accompanying drawings.

15. A trip circuit for a load to be protected, in which the trip circuit is protected by a trip circuit protection apparatus in accordance with the first 20 aspect of the invention.

16. The trip circuit of claim 15 which includes a trip contact which is normally open in use to isolate the coil from the supply voltage but is adapted to be closed in the event of a fault in the line be protected to 25 operate the breaker coil.

17. The trip circuit of claim 15 or claim 16, in which the switching means of the trip circuit protection apparatus is located in the trip circuit between the trip contact and supply voltage.

17

18. A trip circuit protected by a trip circuit protection circuit substantially as described herein with reference to and as illustrated in the accompanying drawings.