GB2158253A - Transformer tap changing switch wear monitor - Google Patents

Abstract

Apparatus for monitoring the wear occurring at arcing contacts of a transformer tap changing switch is embodied in microprocessor form and reads the load current at each tap change by means of transformer 29 from which it determines the arcing current at the circuit breaking contacts 17, 15. The wear is determined as a contact wear figure (CWF) related to the current by either calculation or reference to a look-up table, both being empirically derived from wear measurements on contacts at different arcing currents. The wear figures are accumulated for the movable contacts and for fixed contacts in relation to their tap positions. The number of circuit breaking operations of each arcing contact may also be accumulated, again with reference to tap positions, and the total number of operations may be accumulated to monitor overall mechanical wear, e.g. on main current carrying contacts. The tap position, current and/or accumulated data may be displayed for all or any selected tap position or printed or supplied to, or interrogated by, a remote circuit. <IMAGE>

Description

SPECIFICATION Transformer tap changing switch wear monitor This invention relates to transformer on-load tap changing switches and in particular to the monitoring of contact wear therein.

On-load tap changing switches are normally provided with a plurality of fixed contacts connected to appropriate tapping points of the transformer and a movable contact arranged to connect a selected one of the fixed contacts to a transformer output. The fixed and movable main current carrying contacts, are also provided with arcing contacts which function to perform breaking of current carrying circuits, and suffer any consequential arcing, to protect the contact making surfaces of the main current carrying contacts.

Periodically the arcing contacts have to be replaced before they are rendered ineffective and expose the main current carrying contacts to damage.

The frequency with which the arcing contacts require replacement is subject to a number of variables.

The arcing contacts wear as a function of the number of circuit breaks they handle, the current carried at the break that is, the arcing current, and duration of the arc, the wear being principally erosiqn by the arc.

The relationship between arcing contact wear and the current broken is complex and relationships normally considered are empiri cally obtained. For instance, it has been found that the contact erosion increases with current at a greater than linear rate, of the order of the power 1.5, and also is dependent upon the duration of an arcing current. Depending upon which point in the current cycle the circuit is broken an arc may be developed extending from anywhere between 0 and 10 milliseconds.

Manufacturers of on-load tap changing switches usually define and determine the wear on arcing contacts in terms of the number of milligrams of material eroded by an arc passing a predetermined current in accordance with l.E.C. 214. The wear function of their arcing contacts is often expressed as a percentage of permissible wear by a curve relating contact life/contact wear to the number of operations at a given average current or by a curve relating the contact life/contact wear to different average currents for a given number of operations.

Based upon experience of a particular installation as to its average current load a suitable estimate of current and number of operations may be made. It is found that the majority of tap changes take place between a relatively small number of tap positions leading to accelerated wear on the arcing contacts associated therewith and little or no wear on those associated with the other tap positions.

Accordingly, to mitigate any danger of complete wear on any arcing contact it has been normal practice to inspect all arcing contacts at conservatively frequent intervals, including those which have an intermediate degree of wear. Alternatively, to provide some degree of control, the number of tap changing operations have been counted and the total used as a muitiplying factor on the assumed average wear to indicate when a maximum wear may have occurred.

Such a maintenance policy, while cheaper to implement than the consequences of a break-down, is costly and wasteful of resources.

It is an object of the present invention to provide a method of monitoring transformer tap changing switch arcing contact wear and wear monitoring means which enables more cost-effective operation and maintenance.

According to one aspect of the present invention a method of monitoring arcing contact wear in a transformer on-load tap changing switch, comprises determining for each change of tap position the contacts at which a current-carrying circuit breaking operation occurs, determining the current flowing through the fixed and movable arcing contacts when the circuit is broken, determining as a wear figure the degree of wear caused to each arcing contact as a function of the determined current, and accumulating for each movable contact and for the fixed arcing contact at each tap position, the arcing contact wear figures.

The determination of the contact wear figures may be achieved by reference to a iook-up table containing a set of wear figure values related to arcing current through the contact, or may be achieved by calculation using relationships derived empirically from wear measured on an arcing contact at different arcing currents.

According to another aspect of the present invention transformer on-load tap changing switch arcing contact wear monitoring means comprises tap position indicating means operable to determine the tap position at which a current carrying circuit is broken, current measuring means operable to determine current carried by movable and fixed arcing contacts at the tap position at the time the circuit is broken, processing means responsive to a change in tap position to determine for the arcing contacts as a function of the determined arcing contact current a contact wear figure, representing a proportion of the total permissible wear thereof, and accumulator means operable to (accumulate and) store the cumulative wear figures associated with the movable arcing contacts and the arcing contacts in relation to their tap positions.

An embodiment of the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 (a) is a schematic electromechanical representation of a transformer on-load tap changing selector switch showing the disposition and relationship between arcing contacts and existing tap-position indicating means, Figure 1 (b) is a block circuit diagram of contact wear monitoring means according to the present invention showing its relationship with the switch of Figure 1 and derivation of inputs therefrom, and Figure 2 is a flow chart of the functional steps taken by the monitoring means to monitor the arcing contact wear and to monitor the number of circuit breaks for each contact.

Referring to Figure 1 a transformer on-load tap-changing switch 10 comprises a plurality of fixed tap contacts 11 arrayed in an arcuate manner for contact by a movable main contact 12 carried in an arm 1 3 rotatable by a drive shaft 14, which shaft is driven by a mechanism (not shown) such as a stored spring energy device to move the arm stepwise between tap positions. The constructions applied to such tap changing switches are well known and do not require, nor will be, described in detail.In accordance with common practice damage to the main current carrying contacts due to arcing by breaking contact on-load is avoided by providing special 'fixed' arcing contacts 1 5 for each fixed contact to engage with two or more corresponding 'movable' arcing contacts 16, 1 7 carried by the movable arm 1 3 and associated with the movable contact 1 2. The movable arcing contacts are connected to the transformer output, that is, main contact 12, either directly or by way of a current limiting resistor.

The movable arcing contacts 16, 1 7 move relative to the arm 1 2 during the tap changing operation and function by providing a current path between the tapping points and the output whilst the main current carrying contact is between adjacent taps. For example, if the arm 1 3 is moved anticlockwise in the Figure, the movable contact 1 7 engages the fixed arcing contact at the old tap position until the movable arcing contact 1 6 and/or main contact 1 2 engages the adjacent fixed contact, a final part of the tap changing motion being breaking of contact between the trailing arcing contact 1 7 and the fixed arcing contact 1 5.

It is such a current-breaking action which leads to an arc developing between the contacs, through which an arcing current flows, before the circuit is finally broken. This arc vapourises a small quantity of contact material and constitutes the wear on the fixed arcing contacts and the movable arcing contact or contacts.

The tap changing switch is operated under the control of control means 1 8 which monitors transformer loading and regulation and determines when, and by how many tap positions, the movable contact should be moved.

To this end the switch is provided with a tap position indicator (TPI) 19, usually in the form of a potentiometer, which produces an analog signal voltage on lines 20 for the control means.

The tap changing switch thus far described is conventional. In accordance with the present invention wear of the fixed arcing contacts is monitored by wear monitoring means indicated generally at 21. This is a microprocessor based device and as such may be located somewhat remotely from the tap changing switch housing, for example, with the control means 18.

The wear monitoring means comprises a microprocessor consisting of a conventional CPU 22, ROM or PROM 23, in which the operating program and any system constants are stored, and RAM 24 which forms a store both for the CPU operation and for data monitored by the system. The RAM 24 is of a non-volatile construction to ensure retention of data in the event of a power interruption. The component parts are interconnected by address and data busses 25, 26.

Data is input by way of analog to digital converters (ADC) 27, 28 from the tap position indicator 1 9 and from a current transformer 29 which monitors the load current flowing from the power transformer by way of the tap changing switch.

The ADC's 27 and 28 provide signals to the busses 25, 26 as required by the CPU and may be individual units, as shown, or a single device in which the input signals are multiplexed if desired. The analog signals applied thereto are fed by way of signal conditioning means, (not shown) which convert the signal levels to usable form and remove potentially disruptive transients.

The microprocessor provides the function of measuring means by which the measured current values are used to determine the actual value of arcing current which is processed, also by the microprocessor, to derive contact wear figures.

Output means, so called to distinguish it from the signal input ADC's described above, comprises microprocessor input/output (I/O) means 30 by which data from the monitor is fed to display means 31, where it is decoded and displayed alphanumerically (or graphically), and to an output interface unit 32, to which a printer 33 or a remote or portable storage means (not shown) may be coupled.

The I/O means 30 also is arranged to provide signals to an alarm unit 34, the initiation of an alarm condition by data satisfying a preset threshold condition causing data associated with that condition to be displayed.

The I/O means 30 is connected to receive control inputs from display mode selection means 35, the control inputs being either by discrete function switches or a coded sequence from a key-pad in known manner.

The control inputs may determine amongst other things what information is required to be displayed, to operate the printer and reset the alarm.

The functioning of the monitoring apparatus is best described in relation to a tap change operation and with respect to the processor operating sequence as depicted by the flow diagrams of Figure 2.

Initially however consideration is given to the criteria used to determine arcing contact wear.

As discussed above, arcing contact wear occurs when a current carrying connection is broken between moving arcing contact 1 7 and a fixed arcing contact 15, the degree of wear at each circuit break being a function of the current flowing at the time of the break or the arcing current flowing subsequent to the break.

The wear at each circuit break may, for example, be considered as a percentage of allowable wear of the contact but to preserve the generality of measurement it is considered herein as a contact wear figure (CWF) and an accumulation of such figures indicating the total wear that has taken place on a particular fixed arcing contact or on the movable arcing contacts. Such accumulations may be considered as the cummulative fixed and movable contact wear figures (CFCWF) and (CMCWF) respectively.

It is desirable that if the CFCWF or CMCWF exceeds a safe value at any time an alarm condition is initiated and an alarm actuated in order to enable action to be taken to replace the worn contact.

Although by monitoring the degree of wear occuring on each arcing contact each time it is involved in a tap changing operation it is unnecessary to count the number of such operations to monitor wear of the arcing contacts it is still desirable for complete wear monitoring of the tap changing switch to do so.

The arcing contacts, as described, function to prevent any erosive wear of the main current carrying contacts due to arcing but they are still subject to mechanical wear by continual engagement and rubbing motion. To this end it is desirable to keep account of the number of contact breaking operations (CBO) undertaken so that the state of wear of the main contacts can be inferred. For the moving contact the total number of operations is important whilst for the fixed contacts the numbers of operations are related to the tap position. Such main contact wear is relatively slow and may be considered by normally checking the cumulative number of tap changing operations only at times when arcing contact replacement is needed or may be linked to an alarm system which is actuated when any cumulative total number of tap changing operations exceeds a desirable figure.

The processor controlled circuit of the monitor means reads the input from the tap position indicator continually and responds to a change in tap position to execute the monitoring sequence.

Referring now to Figure 2 the sequence of monitoring operations as performed by the circuit of Figure 1 (b) will be explained.

Starting at 0 the processor continually samples, or reads, the present tap position (TP) from the tap position indicator (TPI) to derive a NEW TP. The tap position resulting from a previous operation OLD TP is read from store and compared with NEW TP. If the comparison is true the TPI has not moved and the sampling cycle is repeated starting at O again. If the TPI has moved then the comparison becomes unsuccessful snd the sampling loop exits at A.

Upon detection of a change in tap position the program jumps to B. The load current passing through the transformer, and thus the switch, represents the arcing current flowing between the fixed and movable contacts which have just broken but also may be increased or decreased by circulating currents flowing in a bridging resistor between the movable contact and another tap position. The relationship between arcing current and that flowing through the transformer output where such multiple paths exist and the arcing current value is readily determinable by the microprocessor. The CPU employs the determined arcing current to calculate the contact wear figures CWF for that current breaking operation.

The tap position OLD TP is read from store and used to address a stored value of cumulative fixed contact wear figure (CFCWF) associated with the tap position, causing it in turn to be read from the store.

A value of cumulative movable contact wear figure (CMCWF) is read from the store.

Assuming that the fixed and movable arcing contacts are subject to the same wear and have the same wear figures, the cumulative wear figures CFCWF and CMCWF are each increased by the value of CWF.

The new cumulative totals are compared with threshold wear figures stored in PROM 23, which determine when wear has reached a dangerous level. If the comparison is positive and indicates excessive contact wear the alarm procedure is initiated. This procedure results in an alarm signal and/or the display of the appropriate tap position on the display 31 over-riding any information at the time displayed. The CMCWF and the CFCWF, are then returned to the appropriate addresses in the store. The program is shown at a position C on the flow diagram from whence it jumps to the point D to continue.

In the part of the program beginning at D the (cumulative) number of fixed contact breaking operations (CFCBO) associated with the tap position OLD TP and addressed thereby, is read from the store as is the cumulative number of movable contact breaking operations (CMCBO), this being independent of the value of OLD TP.

CFCBO and CMCBO are both incremented by one and the new values returned to the store, this portion of program ending at E.

If desired, before the new cumulative values are returned to the store they may be compared with appropriate threshold values from PROM 23, in analogous manner to the cumulative wear figures, in order to initiate an alarm sequence if the number of tap changing operations to which any contact has been subjected is excessive.

The program jumps from point E to point F at which the newly detected value of tap position NEW TP replaces OLD TP in the store. The monitoring sequence ends at G and the program then jumps to step 0 to continue sampling the TPI for a change.

It will be appreciated that at any time the cumulative values for tap break operation and contact wear figure the movable contacts and for any tap position contact can be read from the store and displayed in relation to that tap position. Depending on the form of display a single tap position may be entered and the relevant data displayed or all of the data may be displayed simultaneously in relation to all tap positions. Similarly, a print out of the stored data in relation to the tap positions may also readily be produced or the data read out electronically for passage to a further store or processing apparatus. Such an electronic read out may be initiated by appropriate manual manipulation of the display mode selection means 35 or by an interrogation signal received from the remote apparatus.

Means by which data is selected for display, displayed, printed or transferred to another store is well known in the art and requires no further explanation.

It will be appreciated that the contact wear figure CWF can only be calculated approximately based upon the empirical data available for specific currents by extrapolation thereof.

Whilst such a calculation does not provide an accurate wear figure for each contact breaking operation, particularly as the duration of arcing current at each circuit break is not known, over tens of hundreds of such operations employment of an average duration time and integration of the cumulative wear figure smooths out the irregularities caused by such approximation.

As an alternative to employing approximate calculations it may be preferred to store in the RAM 24 a look-up table relating current to arcing contact wear figure and based also upon empirical predetermination from actual measurements of contact wear at different current values. Such contact wear figures are again based upon an average arcing current duration but the accuracy of cumulative wear figure determination increases with the number of values accumulated.

The added complexity of measuring the load current and then deducing the value of arcing current is justified by the ability then to locate the current transformer 29 at a readily accessible point outside of the tap changing switch housing. It will be appreciated that the transformer may, of course, be located within the housing adjacent the movable arcing contact to measure the arcing current directly.

It will be recalled that although a count is kept of the number of contact breaking operations this is not of direct relevance to the monitoring of arcing contact wear and if such wear only is required to be monitored the monitoring program then jumping straight from C to F.

The above description has described the manner in which wear figures and circuit breaking operations are counted for the fixed contacts by reference to the tap position, and for the movable contact by a simple summation, in order to illustrate the monitoring principles employed.

In doing so the description is limited to consideration only of a single movable arcing contact. It will be appreciated that in general tap-changing switch constructions, such as shown in Figure 1(a), two movable arcing contacts share the circuit breaking for tap changes in either directional sense.

It will be appreciated that the monitoring program is readily adapted for two or more movable contacts by having an appropriate number of stores or storage areas associated therewith the one whose contents are read and modified by the operation being determined by the directional sense of the TPI change, that is, either up or down in level.

With a single movable arcing contact the CMCBO is in fact equal to the total number of tap changing switch operations by which the mechanical wear on the switch in general, and main contacts in particular can be judged. If more than one movable contact is employed the total number of switch operations may be stored separately, or derived as required, from their cumulative sum.

It will be appreciated that by correllating contact breaking operations with the magnitude of load current for each arcing contact the wear on the many contacts within the switch can be monitored more accurately enabling routine maintenance and planned contact replacement to be carried out more effectively. Furthermore, in a particular installation some contacts may be brought into circuit very infrequently and use of the monitoring means of the present invention would enable such contacts to avoid automatic replacement with an attendent saving in running costs.

Claims (25)

1. A method of monitoring arcing contact wear in a transformer on-load tap changing switch, comprising determining for each change of tap position the contacts at which a current-carrying circuit breaking operation occurs, determining the current flowing through the fixed and movable arcing contacts when the circuit is broken, determining as a wear figure the degree of wear caused to each arcing contact as a function of the determined current, and accumulating for each movable contact and for the fixed arcing contact each at tap position, the arcing contact wear figures.

2. A method as claimed in claim 1 in which the current flowing through the arcing contacts at which the circuit is broken is determined as a function of the transformer load current modified in accordance with the current flow along multiple paths, shared by the arcing current path, in the switch.

3. A method as claimed in claim 1 or claim 2 in which the determination of the arcing contact wear figure is achieved by reference to a look-up table containing a set of wear figure values related to arcing current through the contact.

4. A method as claimed in claim 1 or claim 2 in which each arcing contact wear figure is determine by calculation using relationships derived empirically from wear measured on an arcing contact at different arcing currents.

5. A method as claimed in any one of claims 1 to 4 including accumulating for each arcing contact the number of current-carrying circuit-breaking operations to which it is subjected

6. A method as claimed in any one of claims 1 to 5 inCluding accumulating the number of circuit breaking operations which the tap changing switch undergoes.

7. A method as claimed in any one of claims 1 to 6 including giving an alarm indication if the cumulative wear figure for any arcing contact exceeds a predetermined threshold value.

8. A method as claimed in any one of the preceding claims in which the monitoring of data associated with each tap changing operation is initiated by the change in tap position.

9. A method of monitoring arcing contact wear in a transformer on-load tap changing switch substantially as herein described with reference to the accompanying drawings.

10. Transformer on-load tap-changing switch arcing contact wear monitoring means comprising tap position indicating means operable to determine the tap position at which a currentcarrying circuit is broken, current measuring means operable to determine current carried by movable and fixed arcing contacts at the tap position at the time the circuit is broken, processing means responsive to a change in tap position to determine for the arcing contacts as a function of the determined arcing contact current a contact wear figure, representing a proportion of the total permissible wear thereof, and accumulator means operable to accumulate the cumulative wear figures associated with the movable arcing contacts and with the fixed arcing contacts in relation to their tap positions.

11. Monitoring means as claimed in claim 10 in which the measuring means is operable to measure the transformer load current flowing through the tap changer and operable to determine from the distribution of current paths within the switch the proportion of this current flowing by way of the circuit breaking arcing contacts.

1 2. Monitoring means as claimed in claim 10 or claim 11 in which the processing means includes a look-up table having stored therein a set of arcing contact wear figures related to arcing current values.

1 3. Monitoring means as claimed in claim 10 or claim 11 in which the processing means is operable to calculate the contact wear figures for the arcing contacts at which the circuit is broken from the determined current in accordance with the empirical relationship between arcing current and contact wear.

14. Monitoring means as claimed in any one of claims 10 to 1 3 in which the accumulator means is operable to accumulate for each tap position the number of circuit breaking operations undergone by the arcing contacts at that position.

1 5. Monitoring means as claimed in any one of claims 10 to 14 in which the accumulator means is operable to accumulate the number of circuit breaking operations which the tap changing switch undergoes.

1 6. Monitoring means as claimed in any one of claims 10 to 1 5 including alarm means responsive to a cumulative wear figure associated with any arcing contact exceeding a predetermined wear figure threshold level to actuate an alarm.

1 7. Monitoring means as claimed in any one of claims 10 to 1 6 including output means operable to provide a visual indication of the current tap position and/or the data determined at that tap position and/or the cumulative data associated with that tap position.

1 8. Monitoring means as claimed in claim 1 7 in which the output means comprises a visual display device operable to display the data temporarily.

1 9. Monitoring means as claimed in claim 1 8 in which the output means comprises display mode selection means responsive to selection of a tap position identity to display accumulated data associated with the selected tap position.

20. Monitoring means as claimed in any one of claims 1 7 to 1 9 in which the output means comprises a printer operable to print at least some of the cumulative data associated with at least some of the tap positions.

21. Monitoring means as claimed in any one of claims 10 to 20 in which the processing means comprises a microprocessor having stored therein the program sequence required to perform the monitoring operator and contact wear determination.

22. Monitoring means as claimed in claim 21 in which the current measuring means and accumulator means are both embodied in the microprocessor.

23. Transformer on-load tap changing switch arcing contact wear monitoring means substantially as herein described with reference to, and as shown in, the accompanying drawings.

24. A transformer on-load tap changer including arcing contact wear monitoring means as claimed in any one of claims 10 to 23.

25. A transformer on-load tap changer switch substantially as herein described with reference to, and as shown in, Figure 1 of the accompanying drawings.