WO2002018964A1

WO2002018964A1 - Partial discharge injector

Info

Publication number: WO2002018964A1
Application number: PCT/AU2001/001040
Authority: WO
Inventors: Joseph Unsworth; David Tallis
Original assignee: Siemens Limited
Priority date: 2000-08-31
Filing date: 2001-08-22
Publication date: 2002-03-07
Also published as: AUPQ985000A0

Abstract

A testing device (10) for safely testing the operation and calibration of partial discharge measuring equipment or partial discharge monitoring devices adapted to monitor high voltage electrical transformers and other electrical equipment. The testing device (10) comprises a chamber (11) having a flange (12) that is mountable to the wall of the transformer. Mounted in the chamber (11) is a spark plug (19) that is used to inject electrical discharges into the chamber (11). The electrical discharges injected into the transformer can be detected by partial discharge monitors and so provides a mechanism for testing the operation of such devices.

Description

"Partial discharge injector"

Field of the Invention

The present invention relates to a safe testing device for high voltage electrical equipment and in particular to a device for delivering full and partial electrical discharges at will into the electrical equipment to allow testing and calibration of partial discharge detectors and partial discharge measuring instruments used for monitoring the equipment. Background of the Invention

High voltage generator and transmission transformers form an integral part of any electrical power generation, distribution and transmission system. Other transformers, such as rectifier transformers are also used in industrial processes, such as smelting and electro-deposition processes. Also, current transformers (CTs) are used for protection and metering of electricity distribution systems. The most important part of the insulation for oil filled transformers comprises paper which is wound around the copper windings. There are spacers, connections, washers, seals, lead through plates, taps and bushings, which are also part of the insulation system within the transformer. In order to enhance the insulation and stability, the paper is permeated with a dielectric, typically mineral oil or silicone oil, which fills the transformer. This insulating oil also serves as a coolant, distributing heat by convection or forced flow, and also quenches discharges. Other types of transformers include solid insulation filled transformers, which use solid polymeric dielectrics such as epoxy thermoset, which is vacuum back-filled into the transformer, and gas-filled transformers. Gas-filled transformers, for example those used in underground mines, are usually filled with argon or sulfur hexafluoride for safety. There are also some low voltage air filled transformers.

The operating lifetime of a high voltage transformer can be greater than 35 years. The lifetime depends on the loading, design and quality of manufacture, and materials and maintenance routines. During its lifetime, the transformer insulation can degrade, the rate of degradation being dependent upon the workload and the internal operating environment of the transformer, such as temperature, moisture content, pH and the like. Any degradation of the insulation, such as electronic and ionic plasma erosion of solid insulation surrounding an air bubble occluded due to faulty manufacture, can result in increasing levels of partial discharge within the transformer. Occurrence of partial discharges in mineral oil also leads to evolution of gases such as hydrogen and acetylene within the transformer. Such increased partial discharge leads to further degradation of the insulation which in turn leads to increasing levels of partial discharge. Continued degradation of the insulation can result in severe discharges, short-circuit faults or a catastrophic failure due to an explosion of the gases, for example, hydrogen, acetylene and ethylene, produced as chemical by-products of the degradation process. Such failure can result in reduction or loss of supply (outage) to the power system, incur considerable expense for the replacement or repair of the transformer and also present a serious risk to nearby personnel and the environment.

Partial discharge in transformers can also occur due to faulty manufacture and/or mechanical or electrical fatigue. For example, the movement of loose components, and creep and stress relaxation of metallic components, such as fastenings, or foreign metallic bodies within the transformer, provide an opportunity for discharges to occur even when there has been no or little degradation of the insulation.

Partial discharge in transformers can also arise due to windings becoming loose within the transformer. Wear and tear suffered by the tap connectors and backlash in the tap changer can also cause partial discharges and arcing. Faults in the bushings can also result in partial discharges.

It is known that a partial discharge can produce electrical signals at different locations within a large transformer including a discharge current in neutral caused by imbalance, a displacement current through the capacitive tapping of a bushing, a radiated radio frequency (RF) pulse and a radiated ultrasonic pulse.

The magnitude of partial discharge within a transformer provides one means of determining the integrity of the transformer's insulation. For example, a detected partial discharge having a magnitude of 50pC would normally be ignored at normal voltage operations, a reading of 500pC would be viewed with some concern, whilst a reading of 5000pC would be considered potentially dangerous. Just as important is the frequency of occurrence or activity of the discharges. For example, 200pC to 500pC occurring frequently can do more damage than lOOOpC occurring infrequently. Power authorities typically test transformers by sampling the dielectric oil within the transformer about once a year to determine and analyse the oil's dissolved gas concentration (DGA) and dielectric loss angle (DLA). Dissolved Gas Analysis (DGA) has the disadvantage that monitoring is typically only periodic and can only give a long term trend. Using this method, deterioration of the insulation may not be detected and transformers have failed catastrophically even when this infrequent DGA sampling has been carried out. Since it is known that partial discharges of high magnitude develop shortly before a major failure, continuous monitoring of electric equipment while it is kept on-line to provide early warning is very desirable. Partial discharge can also be measured using instruments such as Robinson, Siemens Radio, Haefly or Tettex partial discharge detectors, which detect high frequency electrical (radio) signals only, by coupling to the lower part of the bushing on the transformer or to the windings using capacitor dividers and a toroid system. These instruments are normally used in a test bay during high voltage proving tests for a new or re-wound transformer. These measurements can, however, normally not be undertaken in a substation location due to the high level of electrical interference. Taking reliable readings with these instruments also requires considerable skill. One device for detecting the occurrence of partial discharge events in a transformer is described in International Application No PCT/AU94/00263 (WO 94/28566). This device comprised an ultrasonic transducer and a radio frequency antenna that were mounted in the transformer wall and adapted respectively to detect the ultrasonic and radio frequency waves generated by a partial discharge. If a radio frequency signal was detected within a pre-set time period before detection of an ultrasonic signal, a partial discharge was assumed to have occurred. While able to detect such signals, one problem with the device described in WO 94/28566 was that electrical noise within the transformer would generate randomly occurring radio signals that lead to the triggering of false alarms on occurrences of partial discharge. Shutting down a transformer based on a false alarm is clearly undesirable and costly. An improved device for the detection of partial discharge is described in International Patent Application No PCT /AU00/01028 entitled "Partial discharge monitoring system for transformers" which was developed by one of the co-inventors of the present application. This improved device uses signal processing techniques to discriminate detection of signals indicative of partial discharge from other signals generated due to the noisy electromagnetic environment normally present in on-line high voltage electrical equipment, such as transformers.

Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.

Summary of the Invention

Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

While devices are available that can detect occurrences of partial discharge, there remains a requirement to check the initial performance and calibrate the devices on installation to ensure that the device will detect partial discharges of desired magnitudes. It is also desirable to be able to provide injected partial discharges at will. Given that in some instances the monitoring means may be installed for many years, a means of periodically and simply testing the monitoring means to ensure that it is operating with a desired level of performance is also desirable. The present invention is directed to a device for testing the performance of partial discharge measuring instruments or monitors that are used for high voltage electrical equipment, such as high voltage transformers and the like.

According to a first aspect, the present invention is a testing device for testing the operation and calibration of partial discharge measuring equipment or partial discharge monitoring devices adapted to monitor high voltage electrical equipment, the testing device comprising a chamber mountable to the high voltage electrical equipment and an electrical discharge injector means mountable at least partially within the chamber and adapted to inject electrical discharges into the chamber. This device can be fitted on to oil sampling valves or outlet or inlet valves of transformers.

The testing and calibration device is preferably adapted to safely inject full or partial electrical discharges into high voltage electrical equipment, such as high voltage transformers. On discharging, the testing device generates radio frequency pulses and ultrasonic pulses which are injected into the electrical equipment. These pulses can be detected and analysed by partial discharge monitoring devices.

This partial discharge injector can be used during proving tests in a test bay to check the operation of partial discharge measuring instruments, such as Robinson, Siemens Radio, Haefly and Tettex partial discharge detectors. However, the partial discharge measuring instruments such as Robinson, Siemens Radio, Haefly and Tettex can only detect the radio frequency pulses. One example of a partial discharge detecting and monitoring device that can detect both ultrasonic and radio frequency pulses generated by a partial discharge is described in International Patent Application No PCT/AU00/01028 entitled "Partial discharge monitoring system for transformers". Once such a detecting and monitoring device is mounted to an item of high voltage electrical equipment, such as a transformer, its performance is preferably calibrated by firstly mounting a testing device according to the present invention and injecting electrical discharges into the equipment. Once calibrated and the performance of the monitoring system is checked, the testing device can be removed from the equipment. The testing device can also be used to demonstrate the operation of the monitoring and detecting means. Further, the device can be installed on a transformer to allow testing of a partial discharge monitoring means that has been installed in the transformer for some time.

In a preferred embodiment, the chamber has a flange at one end that is adapted to abut or mate with a flange on the high voltage electrical equipment. The flange on the high voltage electrical equipment preferably surrounds a port used to fill the high voltage electrical equipment with oil or to drain it therefrom. The port of the high voltage electrical equipment preferably incorporates an inlet or outlet valve to allow filling or drainage of oil to or from the high voltage electrical equipment. The chamber preferably comprises an oil filled chamber that can be filled with oil once mounted to the port of the high voltage electrical equipment. The oil storage chamber preferably has a first substantially cylindrical portion extending outwardly from the flange at said one end. The chamber also preferably has a second substantially cylindrical portion extending inwardly from an end distal said one end. The first and second cylindrical portions preferably do not share a common longitudinal axis. In a preferred embodiment, when the chamber is mounted to high voltage electrical equipment, a lower portion of both the first and second substantially cylindrical portions are longitudinally aligned. This alignment results in the respective upper portions of the first and second substantially cylindrical portions being offset with the upper portion of the second substantially cylindrical portion being higher than the upper portion of the first substantially cylindrical portion. An intermediate region is preferably provided between the respective upper portions of the two substantially cylindrical portions. The interior of the second cylindrical portion immediately beneath its upper portion can serve as a gas entrapment area in the chamber for any evolved gases generated by the electrical discharge injector when in operation. This is particularly advantageous as it serves to prevent any evolved gases migrating into the high voltage electrical equipment which is undesirable.

A bleed valve is preferably disposed in the upper region of the second substantially cylindrical portion. The bleed valve can be used to allow air to escape the chamber when the chamber is being filled with oil. The bleed valve can also be used to allow escape of the evolved gases from the gas entrapment area at intervals during operation of the partial discharge injector. A drain valve is preferably disposed in the lower region of the second substantially cylindrical portion to allow drainage of oil from the chamber. While in use, the drain valve would normally remain closed. When it is desired to remove the testing device from the high voltage electrical equipment, the oil sampling valve or inlet or outlet valve in the port of the electrical equipment would firstly be closed before opening the drain valve to drain the oil from the chamber.

The electrical discharge injector means can comprise a spark plug, such as a spark plug used in an automotive engine. The spark plug is preferably mountable in a wall of the chamber. The spark plug can be powered by a 12V induction (ignition) coil. The induction coil can be powered by a 240V/12V (rms) transformer. The timing of the spark generated by the spark plug can be controlled using a timer circuit, such as a 555 timer set up as an astable multivibrator. The time intervals between the high voltage pulses (about 15kV) from the induction coil are controlled using a resistor/capacitor combination in the 555 circuit. Typical values used are

2.1MΩ for the resistance and 470 microfarads for the capacitance giving a time interval of approximately 1 second.

While the spark plug can inject electrical discharges into the chamber, it can be modified to only inject partial discharges. The insertion of insulating material between the gap of the spark plug can be used to reduce the magnitude of the full discharges to partial discharges.

Brief Description of the Drawings

By way of example only, a preferred embodiment of the invention is now described with reference to the accompanying drawings, in which: Fig. 1 is a view of a testing device according to one embodiment of the present invention.

Preferred Mode of Carrying out the Invention

A testing device or injector head according to the present invention for high voltage electrical equipment, such as a transformer, is generally depicted as 10 in Fig 1. The head 10 includes a steel chamber 11 having a flange 12.

The flange 12 is adapted to abut or mate with a corresponding flange surrounding a port on the transformer and be bolted thereto. The chamber has a first substantially cylindrical portion 13 and a second substantially cylindrical portion 14 that has a diameter greater than that of first portion 13. The longitudinal axis of the second portion 14 is offset from that of the first portion 13, with the two portions 13,14 being separated by an intermediate region 15.

The head 10 is adapted to be mounted to the transformer in the orientation depicted in Fig. 1. In this orientation, the upper edge 16 slopes upwardly from the upper edge of the first portion 13 to the upper edge of the second portion 14.

Mounted in the upper surface of the second portion 14 is a bleed valve

17. Mounted in the lower surface of the second portion 14 is a drainage valve

18. A spark plug 19 is mounted in an end wall 21 of the chamber 11. The spark plug 19 is positioned such that when it generates a spark, the spark occurs within the chamber 11. In the depicted embodiment, the spark plug is powered by a 12V induction (ignition) coil. The induction coil is supplied by a 240V/12V (rms) transformer. The timing of generation of sparks by the spark plug 19 is controlled using a 555 timer set up as an astable multivibrator. The time intervals between the high voltage pulses (about 15kV) from the induction coil are controlled using a resistor/capacitor combination. The time interval will typically be about 1 second.

In the depicted embodiment, the spark plug 19 is adapted to output electrical discharges into the chamber 11. By putting an electrically insulating material between the gap of the spark plug 19, the magnitude of the discharges can be reduced to partial discharges.

To mount the head 10 to a transformer, the oil sampling or outlet or inlet valve in the port of the transformer must firstly be closed. The blanking cap covering the port can then be removed before the flange 12 is mated with and bolted to the corresponding flange on the transformer. A suitable gasket, such as a rubberised cork ring, will normally be positioned between the respective flanges.

After ensuring both valves 17,18 are closed, the outlet and inlet valve of the transformer can be opened to allow mineral oil to flow into the chamber 11. As the chamber 11 fills, the bleed valve 17 can be briefly opened to bleed off any air entrapped above the oil.

The spark plug 19 can then be attached to its power source and electrical discharges can then be injected into the chamber 11. If necessary, the bleed valve 17 can be opened at regular intervals to allow bleeding off of any gases generated due to the discharges that are trapped in the upper region of the chamber 11 provided by the second cylindrical portion 14.

The monitoring system mounted to the transformer can be tested or calibrated to ensure that it is detecting the occurrences of electrical discharge within the transformer. When this is complete, the head 10 can be turned off and removed from the transformer.

Prior to its removal, the oil sampling, or outlet or inlet valve on the transformer would be closed before the drainage valve 18 was opened to allow drainage of the oil from the chamber 11 into a bucket or the like. Once drained, the head 10 can be removed before the blanking plate is re-fitted to the transformer. It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims

CLAIMS:

1. A testing device for safely testing the operation and calibration of partial discharge measuring equipment or partial discharge monitoring devices adapted to monitor high voltage electrical equipment, the testing device comprising a chamber mountable to the high voltage electrical equipment and an electrical discharge injector means mountable at least partially within the chamber and adapted to inject electrical discharges into the chamber.

2. A testing device of claim 1 wherein the device is mountable to an oil sampling valve of a transformer.

3. A testing device of claim 1 wherein the device is mountable to an outlet or inlet valve of a transformer.

4. A testing device of claim 1 wherein the electrical discharge injector means injects partial electrical discharges into the high voltage electrical equipment.

5. A testing device of claim 1 wherein the chamber has a flange at one ^'end adapted to abut with a flange surrounding a port on the high voltage electrical equipment.

6. A testing device of claim 5 wherein the chamber is fillable with oil once mounted to the port of the high voltage electrical equipment.

7. A testing device of claim 6 wherein the chamber has a first substantially cylindrical portion extending outwardly from the flange at said one end, and a second substantially cylindrical portion having a diameter greater than that of the first portion extending inwardly from an end distal said one end.

8. A testing device of claim 7 wherein the first and second cylindrical portions do not share a common longitudinal axis.

9. A testing device of claim 8 wherein an intermediate region extends between the first and second substantially cylindrical portions.

10. A testing device of claim 9 wherein the interior of the second cylindrical portion defines a gas entrapment area for any evolved gases generated by the electrical discharge injector means when in operation.

11. A testing device of claim 10 wherein a bleed valve is disposed in an upper region of the second substantially cylindrical portion.

12. A testing device of claim 10 wherein a drain valve is disposed in a lower region of the second substantially cylindrical portion to allow drainage of oil from the chamber.

13. A testing device of claim 1 wherein the electrical discharge injector means comprises a spark plug mountable in a wall of the chamber.

14. A method of testing the operation and calibration of partial discharge measuring equipment or partial discharge monitoring devices adapted to monitor high voltage electrical equipment, comprising the steps of mounting a chamber to the electrical equipment, injecting an electrical discharge into the chamber, and detecting the discharge.