GB2217856A - Testing high voltage devices - Google Patents

Abstract

The condition of a device, such as a capacitive voltage divider (10), is tested whilst connected to a high voltage transmission system (12) by monitoring its insulation partial discharge activity using a capacitive probe (20) arranged in shunt with the device. Frequency selective transducers (14,24) are coupled to both the device (14) and the probe (20) for sensing high frequency discharge currents therein. The outputs of the transducers are connected to signal processing means (Fig 9 not shown) whereby signals of opposite polarity (corresponding to the directions shown in fig 2) resulting from partial discharges are extracted, using a differential technique. The probe (20) comprises a remotely activated short circuit (27) in shunt with a charging resistor (26) and is arranged on a pneumatic hoist (25). <IMAGE>

Description

" INSTRUMENT TRANSFORMER TESTING APPARATUS" This invention relates to apparatus for testing the condition of high voltage equipment. In particular, the invention is directed to apparatus for checking the insulation integrity of high voltage instrument transformers while they are in service, i.e. connected to a high voltage transmission system, by measuring their partial discharge activity. The invention also provides a high voltage capacitive probe for use with such apparatus.

BACKGROUND ART Instrument transformers, such as current transformers and voltage transformers, are commonly used in high voltage transmission systems for obtaining an accurate indication of current and voltage levels in the system. Such instrument transformers are normally located in a substation switchyard. The high voltage instrument transformers have a limited life and are liable to explode upon failure. In some cases, the porcelain insulating casings of the instrument transformers may explode with such force that they disintegrate into pieces which are propelled like shrapnel around the switchyard, thereby creating a grave risk of injury to personnel and of damage to other equipment.

There is no known satisfactory technique which provides a suitably sensitive condition diagnosis at an acceptable resources cost. Oil sampling and dissolved gas analysis is one universally accepted technique; however this method is slow, requires a scheduled outage and cannot provide adequate coverage of a system having a large number of instrument transformers with the resources normally available to a power utility or authority. The problem becomes more pressing each year as the average age of the equipment increases and the percentage of deteriorating transformers grows.

It is known that the level of partial discharge activity in the insulation of such instrument transformers increases towards the end of their useful life, and failure of an instrument transformer can often be predicted by measuring its partial discharge level.

Partial discharge monitoring is already a well established technique for testing insulation integrity in a low noise laboratory environment by measuring the intensity of high frequency (20kHz to several MHz) currents flowing in the transformer's earth connection. However, the successful application of partial discharge techniques in a substation environment has been precluded to date by a number of factors. First, a great deal of electromagnetic interference is produced by power line carrier (PLC) signals used by electricity supply authorities for communicating voice/data/protection information, and by radio transmitters, typically AM radio stations broadcasting around the frequencies of interest.Secondly, corona noise produced by high voltage busbars and other conductors in a substation closely resembles the signals produced by the actual internal partial discharges within the instrument transformer. Such corona noise is often several orders of magnitude greater than the partial discharge signals, and therefore interferes with the measurement of partial discharges. Thirdly, it is necessary to place a measuring impedance (analogous to an ammeter) in the earth lead of the instrument to be tested in order to be able to measure or observe the high frequency currents that are characteristic of partial discharges.

Furthermore, any device for testing instrument transformers while in service must comply with strict safety requirements in view of the high voltages involved.

It is an object of the present invention to overcome, or substantially ameliorate, the abovementioned problems by providing apparatus for safely testing a high voltage instrument transformer in situ, i.e. while connected to the high voltage transmission system.

It is another object of the present invention to provide a high voltage capacitive probe for use with the instrument transformer testing apparatus.

SUMMARY OF THE INVENTION In one broad form, the present invention provides apparatus for testing the insulation of a device, such as an instrument transformer or the like, when operatively connected to a high voltage transmission system, said apparatus comprising: a transportable high voltage capacitive probe for connection between said high voltage transmission system and earth, adjacent to and in parallel with said device; frequency selective sensing means for detecting high frequency currents in said device and said probe caused by partial discharges in the insulation of said device, said sensing means providing respective output electrical signals representative of the detected currents; and means for processing said signals to detect currents of opposite polarity, indicative of a partial discharge.

As explained in more detail below, partial discharges in the insulation of the device can be distinguished from external discharges by the fact that the resultant currents in the device and probe are of opposite polarity. Monitoring partial discharge activity in situ thereby enables faulty devices to be identified and removed from service before complete failure occurs.

The frequency selective sensing means suitably comprise a matched pair of resonant or tuned transducers. A clip-on transducer is preferably used for the instrument transformer to eliminate the need to break into the earth lead or DLA link of the transformer. (Direct connection to the earth lead would require high voltage switching and, in any case, is undesirable for reasons of safety, both to plant and personnel). In a preferred embodiment of the invention, the resonant transducer is a current-to-voltage transducer consisting of a wound ferrite toroid with specific electrical characteristics at the transducer's resonant frequency, and is mechanically constructed so as to render negligible the effects of splitting it into two halves to enable it to be clipped around an earth link or DLA link of the device under test.A toroidal transducer is also placed around the earth lead of the probe prior to connection to the HV system, and is therefore not required to be of split construction. The permeability and losses of the toroid material, and the number of secondary turns, are chosen so as to produce a decaying sinusoidal output voltage in response to an input current impulse.

As mentioned above, one of the problems associated with the application of partial discharge techniques in a substation environment is the continuous interference from PLC and radio stations which is generally much greater than the pulse discharge levels in the instrument transformer, particularly at an early stage of deterioration. To maximise the signal to noise ratio therefore, two techniques are preferably used. First, a resonant or tuned current-tovoltage transducer is used, the resonance lying in a relatively quiet part of the frequency spectrum. Secondly, the resulting signals are heavily filtered by the processing means to further attenuate interfering signals.

Furthermore, the problem of corona interference or other external noise is eliminated, or at least substantially diminished, by connecting the high voltage capacitive probe in parallel with the transformer under test, and substracting from each other the respective signals detected in the transformer under test and the capacitive probe. Any residual interference from PLC, radio beacons etc. that has not been eliminated by filtering is virtually eliminated by this technique.

The high frequency discharge currents resulting from an internal discharge in the transformer flow in opposite directions in the earth leads of the transformer and the capacitive probe. However, for external discharges (e.g.corona discharges), the currents in the respective earth leads flow in the same sense. By comparing the polarities of the detected currents, namely by subtracting the detected currents from each other, partial discharges within the transformer can be separated from other externally produced interference. As the probe is able to be brought physically close to the instrument transformer under test, there is no appreciable phase difference between the two detected currents.

The high voltage capacitor probe used in the transformer testing apparatus comprises a vertically extendible member to enable the probe to be elevated into contact with the high voltage transmission line, and retracted into a compact form for ease of transportation.

Preferably, the extendible member is a telescoping hoist which is pneumatically operated. In this manner, the hoist can be operated from a manual pump or source of compressed air via a plastic tube, and the operator is therefore both spacially and electrically isolated from the hoist.

The probe is suitably mounted on a wheeled platform so as to be easily manoeuvrable around a high voltage switchyard. By using a telescoping hoist, the probe can be of slender design thereby enabling it to be used in a cramped substation environment while maintaining safe clearances from live contacts.

The probe is also provided with a charging/discharging resistor that limits inrush currents during make/break operations. The resistor protects not only the long term integrity of the capacitor itself but also the instrumentation of the test apparatus which can be easily damaged by sharp transients.

The resistor must of course be short circuited for partial discharge testing. The probe is therefore provided with a remotely switchable short circuit in parallel with the resistor. In a preferred embodiment of the invention, the switchable short circuit is formed by a tape consisting of a length of metal braid connected to a length of insulating tape, and wound between two spools between the top and bottom of the resistor respectively. At least one of the spools is driven by a motor which is remotely controlled by the operator e.g. by infra-red signals. The tape is in contact with terminals on either side of the resistor. To short circuit the resistor, the tape is wound until the metal braid portion makes contact with both terminals. The switching can therefore be carried out quickly and efficiently from a safe distance, and compactly i.e. without encroaching on adjacent HV plant.

Preferably, the processing means includes means for adjusting the relative levels of the signals to compensate for differences in various devices under test, and also comparator means for comparing the difference between the transducer output signals with a predetermined threshold value and producing a valid output if the said threshold is exceeded, thereby avoiding false readings resulting from low level spurious readings.

In order that the invention may be more fully understood and put into practice, a preferred embodiment thereof will now be described with reference to the accompanying drawings.

DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic diagram of apparatus for testing an instrument transformer connected to a high voltage transmission system according to a preferred embodiment of the invention; Fig. 2 is a schematic diagram illustrating the direction of current flow for internal discharges; Fig. 3 is a schematic diagram illustrating the direction of current flow for external discharges and EMI; Fig. 4 is an elevational view of the probe used in the apparatus of Fig. 1; Fig. 5 is a side elevational view of the switchable resistor of the probe of Fig. 4; Fig. 6 is a schematic sectional view of the switchable short circuit in shunt with the resistor of Fig.

5; Fig. 7 is an equivalent circuit diagram of an internal partial discharge showing current flows; Fig. 8 is an equivalent circuit diagram of external corona discharge showing current flows; and Fig. 9 is a schematic block diagram of the processing circuit of Fig. 1.

DESCRIPTION OF PREFERRED EMBODIMENT The apparatus of the preferred embodiment-of this invention, illustrated schematically in Fig. 1, is designed to enable a skilled operator to test a high voltage instrument transformer 10 while in service in a minimum of time without requiring high voltage system switching. The transformer 10 is tested by monitoring its insulation partial discharge activity, using a capacitive probe 20 connected to the live transmission line in parallel with the instrument transformer 10 under test. As shown in Fig. 2, when an internal discharge A occurs within the instrument transformer 10 connected to a high voltage transmission line 12, a pulse of high frequency current is generated.The high frequency current flows from the instrument transformer 10 through the interconnecting portion of the transmission line 12 and then to earth via the high voltage capacitive probe 20 connected to the transmission line 12. In contrast, external sources of noise, such as corona discharges B occurring on the transmission line 12, cause high frequency current pulses to flow to earth through a shunt connection of the instrument transformer 10 and probe 20, as illustrated in Fig. 3. Thus, the flow or polarity of the currents in the instrument transformer and probe are the same for external discharges, but opposite for internal discharges. This characteristic is utilised by the present invention to distinguish partial discharges within the instrument transformer from external discharges occurring in the high voltage system.

The probe 20 provides a high voltage capacitive link between the transmission line 12 and earth. Transducers 14, 24 are respectively coupled to the instrument transformer 10 and probe 20 in order to detect the high frequency currents flowing in the respective devices as a result of a discharge. The detected currents are then compared in signal processing circuitry 30 to ascertain whether a partial discharge has occurred. The level of partial discharges can be used to ascertain the condition of the insulation of the transformer 10 under test.

The transducers 14, 24 are preferably resonant current-to-voltage wound ferrite toroid transducers which are designed to have a resonance lying in a relatively quite part of the frequency spectrum to thereby maximize signal-to-noise ratio and overcome interference from PLC and radio stations.

The transducers 14, 24 are tuned by tuning elements 41 (Fig.

9). The instrument transformer's toroid is constructed in two parts which enables the transducer 14 to be clipped onto the instrument transformer 10 by placing it around the earth lead or DLA link thereof as shown in Fig. 1. Hence, there is no need to actually connect directly to the earth lead and no high voltage switching is required. The permeability and losses of the material of the toroid transducers, and the number of secondary turns thereon, are chosen so as to produce a decaying sinusoidal output voltage V0a, V0b in response to current impulses in the transformer 10 and capacitive probe 20, respectively. The outputs of the respective transducers 14, 24 are connected to signal processing circuitry 30 (to be described in more detail below).

The high voltage capacitive probe 20 is shown in more detail in Figs. 4, 5 and 6. The probe is mounted on, but insulated from, a wheeled platform 19 to enable it to be easily manoeuvred into close proximity to the instrument transformer 10 under test. (The probe is placed in close proximity to the transformer in order to render negligible any phase difference in the discharge currents passing through the instrument and probe). Jacking screws 22 can be provided on the platform for added stability and to assist in aligning the probe with the line 12. The probe 20 comprises a vertically extendible member to enable the probe to be elevated into contact with the high voltage transmission line 12. In the preferred embodiment, the extendible -member is a pneumatically operated telescoping hoist 25.The use of a telescoping hoist allows the probe to be constructed in a slender configuration thereby enabling the probe to be used in a cramped substation environment while maintaining a safe clearance from high voltage contacts. The hoist is designed to suit operating conditions. For example, in high lift or windy conditions, a larger or heavier hoist is preferably used. Counterbalance weights can be provided to optimise the hoist extension characteristic, i.e. to ensure a constant rate of extension through the extension stages.

The hoist 25 is operated from a remotely situated pump or compressed air supply, via an interconnecting tube.

As the tube is made of insulating material, typically plastic, the probe will be electrically isolated from its operator. The operator is further protected by the hoist being earthed by earth lead 13. For safety, an earth interlock is provided on the air pump to prevent extension, but not contraction, of the hoist. In addition, a safety interlock is preferably provided on the hoist to protect against overshoot, for example in the event of a jammed air valve. This safety interlock is designed to cut the supply of air abruptly when activated. Further safety features can include an earth stick for discharging the probe after use, a continuity checker for ensuring that the earth connections are maintained, and a "test in progress" beacon to warn others of the testing operation.

A protective device 28 is connected between the base of the capacitive probe and the earthed metal of the hoist to ensure that the output of the probe cannot rise above a safe level under abnormal conditions.

A contact terminal 29 is provided at the uppermost point of the probe 20. The contact terminal 29 is preferably in the form of an aluminium cylinder having a roughened contact plate at the top thereof which, because of its oxide breaking characteristics, provides a better electrical connection to the line 12. The width of the cylindrical contact terminal 29 is made wide enough to allow some tolerance in the positioning of the probe below the line 12.

That is, even if the probe is not exactly directly below the line, good contact will still be made with the line. Safety guides 31, preferably insulated, can be provided at the ends of the cylindrical terminal 29 to prevent the line 20 slipping off the terminal 29 during testing, e.g. as a result of wind forces.

The probe 20 further comprises a charging/discharging resistor 26 located between its top contact terminal 29 and the main capacitive body thereof.

The resistor 26 limits inrush currents during make and break operations with the line 12. The resistor 26 also protects the long term integrity of the capacitive portion of the probe and prevents sharp transients from reaching the signal processing circuitry 30. However, during partial discharge testing, the resistor 26 must be short circuited as it would otherwise severely attenuate the signal being detected. A remotely activated short circuit switch 27 is therefore provided in parallel with the resistor 26. The short circuit shunt comprises a pair of spools 21, 23 on which a tape 27A, 27B is wound. Part of the tape is an electrically conductive metal braid 27B while the other part is an insulating tape 27A. At least one of the spools (23) is wound by a motor 17 which is remotely controlled, for example, by a hand-held infra-red or UHF controller 15.A control circuit 16 includes suitable receiver circuitry for the controller 15 as well as control circuitry for operating the motor 17 in accordance with the received signals, preferably in a fail safe manner. The motor 17, spool 23 and circuitry 16 can be housed within the cylindrical casing of contact terminal 29 as shown in Fig. 6. The other spool (21) is a spring loaded reel and also serves as a terminal connection to the bottom of resistor 26.

The tape 27A, 27B is in contact with terminal 18 which is connected to the contact terminal 29. When the metal braid 27B is wound into contact with both terminals 18, 21, the resistor 26 is short circuited. The short circuit shunt is removed by rewinding the spools 21, 23 so that the insulating portion of the tape 27A is inserted between the terminals 18, 21. The length of the resistor 26 can be minimized while still maintaining adequate clearance between terminals. The use of the remotely activated short circuit shunt enables the operator to keep at a safe distance from the probe 20. Corona shields (not shown) are preferably provided on probe 20.

As can be seen from Fig. 1, the capacitor bank comprised by the probe 20 also acts as one section of a capacitor divider circuit to obtain a voltage reference signal 40 and allow phase information of the pulses to be determined. The voltage reference signal 40 is fed to the signal processing circuitry 30 together with the ' output voltages of transducers 14, 24.

Referring now to Figs. 7 to 9, it will be seen that, assuming the absence of radio interference, the tuned transducers 14, 24 produce voltage waveforms V0a, V0b in response to (a) internal discharge currents (Fig. 7), and (b) external discharge currents (Fig.8) detected in the instrument transformer 10 and capacitive probe 20, respectively. If residual radio noise is present, it will be superimposed on V0a and V0b equally and will be substantially eliminated in the difference circuit 44. (The transducers 14, 24 are preferably matched before use, and to this end, may incorporate in-built calibration or matching circuits).

The output voltage signals from the tuned transducers 14, 24 are buffered by respective buffers 42 and filtered through respective band pass filters 43 in the signal processing circuitry 30. The two signals are then matched using gain control 48 and the difference between the two signals is then amplified by differential amplifier 44 before being passed to threshold comparator 45, controlled by level control 49. The output of comparator 45 is fed to a pulse conditioner 46.

A pulse indicating a valid reading will be generated at the output of the pulse conditioner 46 only if the V0a and V0b signals are 1800 out of phase, i.e. if there has been a partial discharge inside the instrument transformer 10 (Fig. 7). No pulse will be generated at the output of 46 if the signals V0a and V0b are in phase as a result of external discharge (see Fig. 8). The threshold comparator 45 is provided so that a pulse reading is obtained at the output of the pulse conditioner 46 only if the difference signal detected by transducers 14, 24 is greater than a predetermined level, so that only internal discharges above a (variable) specified level are displayed. This allows a simple evaluation of the level of internal discharge to be made.

The output of the pulse conditioner 46 is added to the voltage reference signal 40 by a summation circuit 50 so that the output can be displayed as a sinewave having the detected pulses superimposed thereon in correct phase relationship.

The signal processing circuit 30 can further include a pulse counter for counting the detected pulses, as well as a data logging facility to record the discharge activity.

The signal processing circuit 30 further includes a number of oscilloscope outputs 47 so that an oscilloscope can simply be plugged in for diagnostics and monitoring of circuit operation.

The foregoing describes only one embodiment of the invention, and modifications which are obvious to those skilled in the art may be made thereto without departing from the scope of the invention. For example, a microprocessor may be included in the signal processing circuit 30 to count the internal discharges and provide an output indicative of the condition of the device under test. Although the apparatus has been described with particular reference to its use in testing instrument transformers, it can be suitably modified to test other high voltage equipment such as transformer bushings, or to carry out the on-line measurement of equipment dielectric loss angles (DLA) using the probe capacitor as a reference.

Claims (16)

cLAtMS:-

1. Apparatus for testing the insulation of a device, such as an instrument transformer or the like, while the device is operatively connected to a high voltage transmission system, said apparatus comprising: a transportable high voltage capacitive probe for connection between said high voltage transmission system and earth, adjacent to and in parallel with said device; frequency selective sensing means for detecting high frequency currents in said device and said probe caused by partial discharges in the insulation of said device, said sensing means providing respective output electrical signals representative of the detected currents; and means for processing said signals to detect currents of opposite polarity, indicative of a partial discharge.

2. Apparatus as claimed in claim 1, wherein said frequency selective sensing means comprises resonant currentto-voltage transdurcers for each of said device and said probe, at least the device transducer being in the form of a clip-on toroidal transducer suitable for coupling to the earth lead or DLA link of said device.

3. Apparatus as claimed in claim 1 or 2, wherein said probe comprises a vertically extendible member, and a contact terminal at the top of said probe whereby said contact terminal can be elevated into contact with a line of said transmission system.

4. Apparatus as claimed in claim 3, wherein said vertically extendible member is a hoist of slender configuration and pneumatically operable from a remote location via a connecting air line.

5. Apparatus as claimed in claim 3 or 4, wherein said probe further comprises a capacitive body, and a resistor connected in series between said contact terminal and said capacitive body.

6. Apparatus as claimed in claim 5 wherein said probe further comprises a remotely activated short circuit switch in parallel with said resistor.

7. Apparatus as claimed in claim 6, wherein said short circuit switch comprises a tape mounted on a pair of reels in parallel with said resistor and consisting of conductive and insulative lengths in series, wherein at least one of said reels is remotely controllable to selectively wind said conductive length of tape into shorting contact across said resistor.

8. Apparatus as claimed in any preceding claim wherein said probe is mounted on a movable platform.

9. Apparatus as claimed in any preceding claim, wherein said processing means comprises means for frequency filtering output signals from said sensing means.

10. Apparatus as claimed in any preceding claim wherein said processing means comprises comparator means for comparing the difference between the output signals representing respective detected currents with a predetermined variable threshold value and producing an output if said threshold is exceeded.

11. A transportable high voltage capacitive probe suitable for connection between a high voltage transmission system and earth, said probe comprising a contact terminal at the top of said probe; a vertically extendable member whereby said terminal can be elevated into contact with a line of said high voltage transmission system; a capacitive body; and a resistor connected in series between said terminal and said capacitive body.

12. A probe as claimed in claim 11 further comprising a remotely activated short circuit switch in parallel with said resistor.

13. A probe as claimed in claim 11 wherein said vertically extendible member is a hoist pneumatically operable from a remote location via a connecting air line.

14. Apparatus for testing the insulation of an instrument transformer when operatively connected to a high voltage transmission system, said apparatus being substantially as hereinbefore described with reference to the accompanying drawings.

15. - A transportable high voltage capacitive probe suitable for connection between a high voltage transmission system and earth, said probe being substantially as described herein with reference to Figs.. 4 to 6 of the accompanying drawings.

16. Any novel feature or combination of features described herein.