SYSTEM AND METHOD FOR MONITORING PARTIAL DISCHARGE IN ELECTRICAL COMPONENTS
The present invention relates to the field of monitoring electrical components, and more particularly to monitoring partial discharge events in electrical components .
Gas insulated substations (GIS) are vital items of plant in the electrical power systems of developed and developing countries. GIS are utilised in many electricity transmission networks because of their very compact size. In a GIS, the conductors and circuit devices, such as instrument transformers and switchgear, are housed within pressure vessels containing an atmosphere of sulphur hexafluoride at elevated pressure. The pressure vessels are typically made of aluminium and electrically are held at earth potential. In a 3 -phase network, the conductors of the individual phases are separately encased in pressure vessels and are mechanically mounted therein via insulators. The pressure vessels typically comprise interconnected tubular members which may have side wall inspection ports .
Fault conditions which arise in the GIS lead to lengthy disconnection periods due to the mechanical complexity of the GIS, and there is therefore a requirement to provide diagnostic measurements in service use of the GIS in order to predict the possibility of a fault condition and to enable corrective action at a planned and convenient time. These potential faults almost always show partial electrical discharge activity before breakdown occurs, and this discharge activity can be sensed from the consequential ultra-high frequency (TJHF) resonant modes established in the pressure vessels. Accordingly, the UHF modes can be sensed by couplers built into the pressure vessels at the inspection ports, combined with the use of a spectrum analyser connected to the coupler output and human interpretation of the analyser result.
The electricity supply network in developed countries is divided broadly into transmission and distribution systems. The most economic design of an overhead line (or underground cable) to carry a certain load is to use its optimum voltage/current ratio. In other words, if a line needs to carry more power, then both its rated voltage and rated current need to be increased. Transmission systems are the backbone of the network and often operate at 400kV and either 275kV or 230kV. Below this, the distribution systems operate typically at 132kV, 66kV, 33kV and llkV. The interchange between these voltage levels occurs at substations that are increasingly of the GIS type. All power transmitted by a 400kV substation passes through every other substation along the chain, and since at each interchange the current and voltage both decreases, the number of
substations at lower voltages increases very approximately as the square of the voltage ratio. For example, we would expect to need 36 GIS at 66kV for every one at 400kV. In practice, as it is the number of switchgear bays that increases, there may be as many as 96 GIS at 66kV for two at 400kV. Also in practice the 66kV GIS are physically as large the 400kV GIS.
Electricity consumers such as semiconductor manufacturers who are supplied directly at 66kV distribution voltages need a very stable supply of electricity because even minor dips in voltage can cause their plant to malfunction.
Typically electricity suppliers monitor the transmission voltage GIS. However, any plant breakdown, even at distribution voltage GIS, causes a dip in the power in the supply voltage over much of the network. Furthermore, distribution voltage transformers have been known to explode, so catching fire and causing extensive damage to the substation.
UHF monitoring can be used wherever the defect is enclosed in a metallic housing, because it is this enclosed space that resonates and gives the UHF signal picked up by the couplers. In this respect, a distribution voltage network comprising 66kV GIS is no different from any other, except that the total number of couplers needed for all the substations would be large. Furthermore, other low voltage components which are large in number in distribution networks, such as 33kV cable terminations, are enclosed in aluminium compartments that
would support UHF resonances which are therefore suitable for monitoring by such diagnostic systems.
In the prior art, systems are known which are suitable for application to monitoring of transmission voltage (-IS •
US Patent No 5396180 to the University of Strathclyde describes a diagnostic measuring system for GIS, comprising means for automatically monitoring a plurality of UHF couplers, and communicating data representing the detection signals to an analysing means which carries out the appropriate analysis and characterisation of the signal .
US Patent No 5982181 to Hitachi Limited describes an insulation device diagnosing system which-prepares detection data from the partial discharge signal, such that periodic elements are given to a plurality of specific frequencies of the partial discharge signal. The deterioration lifetime and the kind and extent of abnormality of the insulated device are judged from the pattern and intensity of the spectral distribution.
US Patent No 6239723 to Siemens AG describes an encapsulated installation at high or medium voltage, including a plurality of sensors located within the interior of an encapsulation. The surface acoustic wave (SAW) sensors are monitored by wireless communication from outside the encapsulated installation. The sensors can be interrogated selectively.
The problem with applying the known systems for diagnostic measurements of GIS to the large number of substations in a distribution network is the large amount of data generated by the detection system. This data is typically transmitted across a communication network, analysed centrally, and the results need expert interpretation.
Figure 1 shows a monitoring system for a gas insulated substation (GIS) according to US Patent No 5396180, generally depicted at 10. The system includes a plurality of couplers 11 for detecting UHF signals from inside combustion chambers of the GIS. The couplers are connected to units 21, which contain circuits for signal amplification, level detection and timing of the pulse. The units also digitise the outputs of the couplers 11 and transfer the data to a host computer 25 via a loop network 13.
Host computer 25 is used to store the coupler data for analysis. The prior art system as described is essentially a two-stage process. Unit 21 transmits characterised data via network loop 13, and host computer performs the analysis necessary to determine whether a partial discharge event has occurred. The partial discharge information can then be transmitted to an engineer at a remote location 27 via any suitable data transmission means. Here, the data will often be analysed further by the engineer or by an expert system in order to gain additional information on the nature and location of the defect.
The system will generally comprise twenty to thirty three-phase sets of couplers and therefore twenty to thirty units 21, each transmitting information on the characterised signal into the network 13. Within a GIS, this represents a large amount of data transmitted across the network. Inside the noisy environment of a GIS, these data are prone to interference, and are therefore typically transmitted by an optical fibre network.
The system described in US 5396180 is not suitable for implementation across a distribution network due to the high expense involved in installing such a system. In addition, the amount of data generated would be difficult to handle.
It would be advantageous to provide a system suitable for monitoring a large number of GIS and other components at distribution voltages which avoided the generation of large amounts of data and also provided status reporting in a manner easily understood. Furthermore, it would be advantageous to provide a system simple enough to be installed and maintained by local agents.
It is an object of the present invention to provide a monitoring system which can monitor a large number of GIS or other components and transmit a small amount of data about the status of the GIS or other components through the communication network.
According to a first aspect of the present invention, there is provided an integrated processing unit for use in an electrical component monitoring system, the integrated processing unit comprising:
receiving means for receiving a signal from a UHF detector; analysing means for analysing the received signal and determining if the received signal corresponds to a partial discharge event; means for generating a notification signal in the event that the received signal corresponds to a partial discharge event .
The integrated processing unit may comprise means for transmitting the notification signal to a local control unit .
Preferably, the analysing means is adapted to distinguish a received signal corresponding to a partial discharge event from a received interference signal.
Preferably, the analysing means is a CPU running a software module, the software module analysing one or parameters of the received signal, the parameters selected from the group consisting of: signal count, signal spacing, statistical pulse, and pattern deviation.
More preferably, the software module further comprises a noise suppression algorithm.
The means for transmitting a notification signal may be a switch operable to act on a pair of wires connected to the local control unit.
Preferably, the means for transmitting a notification signal is a two-way data link.
More preferably, the integrated processing unit further comprises a housing, wherein the UHF detector is a coupler partially enclosed by said housing.
Preferably, the integrated processing unit comprises an input means adapted to receive an input signal from an auxiliary UHF detector.
The integrated processing unit further may comprise a visible display to indicate the presence of a partial discharge event.
Preferably, the integrated processing unit is adapted to transmit a signature of the received signal to a local control unit when a partial discharge event is identified.
According to a second aspect of the invention, there is provided a system for monitoring electrical components, said system comprising a plurality of integrated processing units according to the any preceding Claim.
The system may further comprise a plurality of UHF detectors for detecting UHF radiation and providing detection signals to the integrated processing units.
Preferably, one UHF detector is provided for each integrated processing unit.
Preferably, the system further comprises a local control unit configured to receive notification signals from a plurality of integrated processing units.
The local control unit may comprise a memory device storing data representative of known fault conditions, and a comparator for comparing a signature signal from an integrated processing unit with known fault conditions .
Preferably, the system further comprises a control centre, remote from the integrated processing units and local control unit, connected to the local control unit by a communications link and adapted to receive a second notification signal from a local control unit.
The control centre may be provided with a display for indicating the status of the local control unit .
Preferably, the system is adapted to monitor Gas Insulated Substations (GIS) , each GIS having a local control unit and a plurality of integrated processing units.
According to a third aspect of the invention there is provided a method for monitoring electrical components, the method comprising the steps of: i) detecting a UHF signal using a UHF coupler; ii) analysing said signal in order to distinguish a detected signal corresponding to a partial discharge event from a detected interference signal; iii) generating a notification signal indicative of a partial discharge event; iv) transmitting the notification signal to a location remote from the UHF coupler.
Preferably, steps (ii) and (iii) are carried out by a single integrated processing unit.
Step (iv) may comprise the sub-steps of (a) transmitting a first notification signal to a local control unit, local to the UHF coupler, and (b) transmitting a second notification signal to the location remote from the UHF coupler.
The method may comprise the step of transmitting a signature of a received signal to the local control unit .
Preferably, the method comprises the additional step of comparing the signature of the received signal with data corresponding to known fault conditions, thereby determining the most likely cause of the partial discharge event .
In order to provide a better understanding of the present invention, an example will now be described by way of example only and with reference to the accompanying Figures, in which:
Figure 1 illustrates a block diagram of the system according to the prior art;
Figure 2 illustrates a block diagram of the system according to an embodiment of the invention;
Figure 3 illustrates an integrated processing unit according to an embodiment of the invention.
Figure 4 shows a monitoring system according to an embodiment of the invention.
Figure 5 illustrates an example of a simple front end display in the control centre;
Figure 6 illustrates the circuit diagram of an integrated processing unit according to an embodiment of the invention;
Figure 7 illustrates the circuit diagram of an integrated processing unit according to an alternative embodiment of the invention;
Figure 2 shows a schematic diagram of a monitoring system according to an embodiment of the invention. The system includes the plurality of couplers 11, connected to integrated processing units 20. Each integrated processing unit (IPU) is connected to a databus 12 via a hub 14. Hub 14 receives input signals from several IPUs 20, via links 15, and is connected by the databus 12 to a local control unit 40. The local control unit 40 is in turn connected to control centre 27. Databus 12 is for example a multicore cable. Hub 14 is not an essential part of the system, but provides a convenient cabling solution and allows, each IPU to be addressed individually by the local control unit 40.
Couplers 11 detect UHF signals, and transmit the detection signal to the integrated processing units 20. The detection signals are received by processing unit 20, which contains analysing means 21, 22 for determining whether the detected signal corresponds to a partial discharge event or is due to an external source of interference.
Analysing means contains circuitry 21 for digitising the signal, as well as a microprocessor 22 which analyses the signal in order to determine whether a real partial discharge has occurred-. The microprocessor runs a software module that includes algorithms to analyse the parameters of the received signal . The parameters analysed are selected from the signal count, the signal spacing, and the statistical pulse or pattern deviation. The software module also runs a noise rejection algorithm.
The IPU may then transmit a simple "yes" or "no" message to the local control unit 40, depending on whether or not the partial discharge (PD) event has occurred. The local control unit 40 is linked to a control centre 27 where an engineer may monitor its activity.
A key advantage of the system is the reduction in the amount of data transmitted through the system. By providing an integrated unit that characterises and analyses the detection pulse in si tu, the transmission of large amounts of data associated with the detection pulse is avoided.
There will now be described an example embodiment of a practical implementation of the monitoring system, with reference to Figures 3 to 6. Figure 3 shows an IPU 20 contained in a housing 30. The housing is also provided with an LED display 31, reset module 36, power input 32 and coupler 11. The housing 30 is mounted on chambers of the GIS 38, as shown in Figure 4. A connection 35 functions as a control link from the local control unit 40 to the IPU.
The housing 30 is mounted such that the coupler is on dielectric apertures in the chambers such as the exposed edges of the cast resin support barriers or glass observation window .
When a defect is present, the dielectric apertures in the GIS chambers pass UHF signals that are picked up by the couplers. The signal from the coupler is analysed within the processing unit 20, and in the presence of a defect a notification signal is transmitted via switch 34. In this embodiment, each IPU is associated with one UHF coupler 11.
The notification signal is provided by the internal switch 34 which operates on a pair of wires connected to local control unit. Thus the output signal from the unit is a simple one, merely showing that the internal switch 34 is open or closed.
LED display 31 is also provided within the housing. The LED display shows the status of the internal switch and hence indicates whether a fault condition is present within the GIS. Resetting module 36 is also provided for resetting the unit to the "no-fault" condition.
The notification signal from the individual integrated processing units is transmitted via cable 12 to a local control unit 40 as shown in Figure 4. The local control unit incorporates the power supply, which is typically AC at a low voltage. The power supply cable would typically be a screened multi-core cable.
In addition, the local control unit 40 incorporates means for determining the status of the switches within the IPUs. The local control unit is optionally provided with one or more indicating devices to show information on the current status of the internal switches within the IPUs . For example, the level control unit may display whether the internal switch in any IPU is open or closed, or the local control unit may indicate the status of a particular IPU. Alternatively, the local control unit may indicate which IPUs have internal switches in the closed position.
Further functional circuitry is also optionally provided within the local control unit. For example, means for automatically testing the IPUs at preset intervals, or resetting the IPUs to the no-fault condition are included. Such control signals are provided via control link 35.
In an alternative embodiment, the monitoring system of the invention utilises internal couplers provided within the chambers of the GIS. In this case, the couplers are connected to the IPUs via coaxial cables.
The information on the status of the internal switches in the IPUs is transmitted from the local control unit to a remote control centre by SCADA. This control centre may monitor a large number of GIS systems and contains a PC and a monitor. Software running on the PC gives an alarm when an internal switch within an IPU is closed. The particular GIS containing that IPU is identified, and an operator in the control centre can instruct an engineer to attend to the GIS with the fault condition. In a more
sophisticated embodiment, the PC can optionally provide the operator, via the local control unit, with additional information such as the particular IPU with the fault condition. The alarm within the control centre is cancelled when the internal switch of the IPU is reset, by a command from the local control unit to the reset module running on the IPU. All of the data received in the remote control centre from the various local control units is recorded.
Figure 5 shows a simple form of display 50 provided by the software running on the PC in the control centre. The left-hand side contains a list 51 of the GIS monitored by the system. Each GIS has an array 52 of indicators 53 to display the status of the GIS over a recent period of time. An indicator of one colour (say green) indicates that no fault conditions were detected within the GIS on that particular day. An indicator of a second colour (say red) indicates to the operator that a fault condition existed in that particular GIS on a particular day. This display allows the history of the GIS to be instantly displayed in a clear and concise manner. The display is for example a Microsoft ® Windows ® based software application.
In a more sophisticated embodiment, the operator has the option to call up further information on the fault from the local control units. For example, data on the time of the fault, or the approximate location of the fault may be displayed.
In addition to the reset function implemented by a specific command to the reset module 36 from the local
control unit 40, the system can be configured so that the switches are reset to the no-fault condition at a set time. This reset function may be configured within the processor inside the housing, or alternatively can be implemented in the local control unit . The automatic reset prevents a temporary fault from being displayed on consecutive days. That is, a persistent fault can be distinguished from a short-lived one.
Optionally, when the display indicates a fault to the operator, the IPU reset function can be activated from the control centre, so that the display is set back to the green no fault condition. If the discharge is still active, the display will return to red, and the operator can instruct an engineer to attend the site. This mechanism provides a means for checking the operation of the system. Such an embodiment requires a more sophisticated communications link between the remote control centre and the local control units at the GIS, such as an RS232 connection.
When the engineer arrives at the GIS he or she must determine which coupler has detected a fault. This information can be provided by an operator who has accessed the data from local control unit 40 in advance. Alternatively, the engineer may gain the information from the local control unit himself. When the active coupler is located, the engineer connects a portable UHF monitor to the coupler. The portable device is provided with a mobile telephone capable of relaying data back to the control centre for more detailed analysis.
Figure 6 shows the circuitry of the integrated processing unit in more detail .
The UHF coupler 11 transmits the detection signal to a filter 61, which is typically a band pass filter chosen to transmit only the UHF signals of interest. The output of the filter 61 is delivered to amplifier 62. Power supply from the local control unit is shown by 32.
A combination of the signal detector 63 and the signal peak detector 64 provide information on the peak of the pulse, the width of the pulse and the point-on-wave time of events with respect to a 50 Hz reference signal.
This information is digitised by the ADC 65 within micro- controller 60. The digitised data are then accessed by processor 66 for analysis. The processor runs a software module including a series of algorithms for analysing various parameters of the signal, including the signal count, the signal spacing, and the statistical pulse or pattern deviation. The software module also runs a noise rejection algorithm. Additional protection circuitry may be provided to avoid damage created by large discharge spikes. The software module may be updated via input 68.
The above-described embodiment allows transmission of a simple notification signal to a local control unit 40 via output 69. A more sophisticated embodiment includes the capability for additional, more detailed characterisation of the detected signal. In this example, if a partial discharge event is identified, a sample or signature of the signal will be transmitted to the local control unit 40 for further analysis.
Different types of partial discharge event produce different UHF signals with different pulse signatures. The local control unit comprises memory, storing data representative of known fault conditions. A comparator compares the signature signal with known fault conditions, and is able to determine the most likely cause of the detected signal .
For this example, the data transfer system between the IPUs and the local control unit must be capable of allowing additional information to be transmitted to the local control unit. Optionally, some or all of this information can be transmitted to the control centre. For example, if required, the characterising parameters of the pulse signature may be transmitted across a network, or information on the type of discharge event that has occurred can be provided to an operator at the control centre. However, in the preferred embodiment, a simple notification that a fault has been identified at a particular GIS is transmitted to the remote control centre via a SCADA communications link.
An alternative circuit diagram is shown in Figure 7. In this embodiment, the tuned filter 71 is provided so that measurements can be made in a part of the spectrum that is free from external interference. The signals are then amplified by amplification circuits 72. The remainder of the circuitry works in the same way as the embodiment of Figure 6, other than the transmission of the warning signal. In this embodiment a "Bluetooth" ® transceiver 73 is provided within the integrated processing unit for communicating the warning signal. The local control unit
is also provided with Bluetooth ® transceiver. By providing the Bluetooth ® transceivers, wireless communication between the local control unit and the IPU is possible. In addition, wireless communication is possible between the local control unit and the portable device carried by an investigating engineer, or between the IPU and such a mobile device directly.
The local control unit can also be used to update the IPUs with new software for analysing of detected pulses.
In practice, an engineer dispatched to the relevant site is able to obtain extra information on the discharge event by accessing the Bluetooth ® transceiver with a portable unit. This information can be obtained from the local control unit or directly from the IPUs Bluetooth ® transceiver itself. This would avoid the need for physically connecting the coupler or local control unit to the portable device during detailed fault investigation.
It is evident that many modifications could be made to the described system within the scope of the invention. Such modifications include changes to the data transmission systems, the detector circuitry, the display systems and the power supply, and would be obvious to one skilled in the art.
The present invention offers numerous advantages over the prior art systems as follows:
By providing an integrated unit for identifying a partial discharge event by analysing the detected signal, the
physical structure of the system is simplified. The one- piece unit is cheaply produced, easily distributed and readily installed in the GIS. Significantly, the units may be retro-fitted to GIS.
By analysing the signal in situ it is possible to reduce the amount of data that is transmitted through the network as a matter of course, thereby enabling larger numbers of GIS to be monitored. The invention is therefore particularly suitable for monitoring GIS and other plant equipment at distribution levels.
The systems described provide an operator with a lucid display of the statuses of a large number of GIS, complete with defect history. No expert analysis is necessary at the control centre, and therefore the operator need not be skilled in interpreting partial discharge information.
In the non- fault condition, the display provides the user with a degree of assurance by virtue of the positive indication of the status of a GIS when no fault is present.
Indicators on the local control units and the IPUs themselves indicate the presence of a defect to an investigating engineer, and direct the engineer to the approximate location of the fault.
More sophisticated embodiments allow additional information on the type of partial discharge event to be transmitted on an ad hoc basis.
Although the foregoing description relates to GIS, the claimed invention in its various aspects is suitable for monitoring other electrical systems, including power and instrument transformers, bushings, cable terminations and switch gear.
Further modifications may be made within the scope of the invention herein intended.