Lord Consulting
Power Industry Asset Management Consultants
Lord Consulting
Power Industry Asset Management Consultants
Home » Technical Papers » Experiences with continuous partial discharge monitoring
Presented by Teleconference at the 3rd AVO New Zealand International Technical Conference 15-17 October 2002, Methven, NZ
Detection of incipient failures, specifically those exhibiting partial discharge, in cables and substation plant can be of significant benefit in terms of quality of supply and customer satisfaction statistics. Targeted replacement of sections of cables or individual items of plant, rather than replacing whole cable routes or complete substations, can enable the service life of the system to be extended at least cost. When the supply company's performance is measured by a regulator in terms of 'customer minutes lost', avoidance of financial penalties can be significant.
The CableTrend® and NetworkTrend® devices, which are designed for continuous on-line monitoring of the discharge activity in cables and switchgear, are instruments derived from a programme of investigation conducted in London England over recent years.
The techniques developed within London have been adapted and refined to produce two instruments which are simple to install without the need for an outage. The PD Wizard software supplied permits easy initial evaluation of the data acquired by the instruments. CableTrend® and NetworkTrend® are now being used in differing circumstances in various parts of the world.
Almost all the medium voltage (MV) electricity distribution within the London area is by means of underground 6.6/11kV cables. The majority of this cable, totalling 10,000km, is three core belted PILCSWA construction, much of it well over forty years old and some cables are still in service after nearly ninety years. Around 98% of all supply interruptions in London are the result of cable faults. Incidents on the 6.6/11kV system are likely to affect the largest number of consumers. The underground network at higher voltages, 22-132kV, is designed such that single faults rarely result in total loss of supply. Cable incidents at these voltages are quite rare. Failures on HV switchgear are extremely rare but even one failure could result in many thousands of customers being off supply. The focus in London for asset management is targeted on the MV cable network and higher voltage switchgear.
The objective in London is to detect and locate incipient faults in cables and switchgear before electrical failure can occur. Off-line techniques such as AC or DC over voltage testing, dielectric dissipation factor (tanD) measurement and partial discharge mapping have been in use for a number of years to assess the condition of buried cables. The tanD and discharge mapping utilises a very low frequency (0.1Hz) power source to separately energise the cable. The use of this low frequency allows the large capacitance of the cable to be fed from a portable test set. However, it has been estimated that, using existing resources, it could take more than ten years to test all significant circuits just once.
On-line monitoring has demonstrated that cable conditions can deteriorate rapidly. The application of new on-line condition monitoring techniques is now well advanced in London, mainly as a tool to identify and prioritise those circuits most at risk of failure. Off-line test methods are then used to locate the fault and assess the condition of the cable.
Early work in London centred around the use of high frequency sensors coupled to digital oscilloscopes. Recognising that discharge activity can vary during a normal day, and also be considerably different from day to day, this technique was developed further by using a 32-channel multiplexer system for continuous monitoring of several circuits simultaneously.
In order to provide an instrument with wider market appeal, especially one that would be attractive to smaller utilities and industrial network operators, the task of acquiring the discharge signals and processing for simple evaluation was refined into a new 8-channel instrument known as uPD-800. Further development work, now with the close involvement of MW Test Equipment, resulted in the production of CableTrend® uPD-800. This instrument has been marketed for a little over one year as a neatly packaged product consisting of the CableTrend® instrument, one high frequency current transformer PDCT36 and accessories in a convenient robust plastic carrying case. Additional PDCT36 are supplied should the user wish to monitor several cable circuits. Easy to use software called PDWizard™, to make the task of downloading and evaluating acquired data as simple as the press of a button, is supplied with each instrument.
These monitors utilise the latest techniques available to industry, complemented by intuitive software to simplify the acquisition of partial discharge trends in the circuits targeted. These monitors can be used to establish a base-line figure for discharge activity to allow them to be prioritised for further investigation. Important circuits can be monitored continuously to detect any changes in discharge behaviour, to aid prediction of failure and prevention of unplanned outage.
The same data processing ideas, but covering a completely different frequency spectrum for the acquired discharge signals, has been incorporated into the NetworkTrend® instrument. This instrument recognises the need to monitor items of fixed plant, such as switchgear, busbars and cable end boxes to detect increasing discharge activity in solid insulation which may result in catastrophic electrical failure. NetworkTrend® is also an 8-channel device but uses magnetically attached capacitive sensors to capture the discharge pulses. The download and evaluation software is the same and automatically recognises the type of instrument being downloaded.
If this paper appears to be less than 'impartial' in focussing further discussion on two specific condition monitoring instruments, the authors justify such specific intention by the need to talk about their own actual experiences of on-line discharge monitoring.
Incipient faults in the solid insulation of cables and switchgear manifest themselves as small voids. Under electrical stress from the system operating voltage, minute discharges occur across these voids. Because the discharges are within the voids and do not bridge the whole insulation between high voltage and earth electrodes, they are known as 'partial' discharge. Over time, discharges within these voids will grow under electrical stress until they form a discharge path which develops into a breakdown channel. At this point the discharge ceases to be partial and full electrical breakdown occurs.
Detection and identification of the pattern of pulses coming from partial discharge in insulation has been an established technique for assessment of the quality of the insulation in power apparatus for over fifty years. The pulses generated by different types of insulation failure produce differing patterns when displayed on the sinewave of the applied voltage. Pulses from partial discharge appear at specific points on the sinewave; pulses occurring all around the sinewave usually indicate background noise. The different failure modes all have well-known recognisable characteristics.
Most discharge detection measurements have been carried out under laboratory conditions but, more recently, techniques have been developed to measure partial discharge to laboratory accuracy under field conditions. However, these measurements usually require the plant to be isolated from the system, which requires an outage, and provision of a separate test voltage source to energise the test object. The test and measurement equipment is bulky and expensive and in order to perform the measurements it is necessary to undertake invasive activity on the plant. In addition, as soon as the plant is de-energised from system voltage, the physical nature of the insulation will undergo small changes as a result of temperature changes. When re-energised with the test voltage, discharge activity may have altered or even been extinguished. While on the system, the plant can experiences phase-earth voltages equal to line voltage (3.Uo) during system disturbances. Some asset owners are reluctant to allow applied voltage testing as high as 1.7Uo on aged plant so, under the artificial conditions of an off-line outage test, the test voltage may be below the discharge inception voltage. The off-line test may fail to find any partial discharge but, when the plant is back on the system, the discharge may re-strike. Similarly, as discharge activity can vary with loading and temperature as a result of minute pressure changes in the voids, a single off-line discharge test may give an incomplete impression of the state of the insulation under system conditions.
The ideal is to be able to monitor the condition of the insulation system continuously under system operating conditions. While this is now achievable using on-line versions of laboratory-style discharge detectors, often needing discharge measurement experts or consultants to operate them, such detailed analysis is seldom necessary. Relatively simple, low cost, trending instruments can usually provide sufficient information for the network maintenance manager to make an informed decision about the general condition of a major asset. CableTrend® and NetworkTrend®, as their names suggest, are instruments designed to monitor continuously trends in the development of potentially damaging discharge in cables and static plant, providing information for prioritising items needing closer investigation and warning of developing faults. Graphical presentation of trend data through PD Wizard™ simplifies analysis.
The prime requirements for any continuous monitoring system, apart from all the obvious considerations like ensuring that the correct parameters are being measured in a safe and accurate manner, must be:
Meeting all these considerations inevitably requires a trade-off between cost, simplicity and accuracy. The best continuous monitors strike a good compromise across all factors.
In the case of discharge monitoring, to ensure that the correct signals are detected in the first place, the design and selection of sensor has to be appropriate to both the magnitude and frequency spectrum of the discharge signals of interest. The sensors must be easy to install and the processing of the acquired data must be presented as clear information.
Discharges in the solid insulation of power cables cause small current pulses to flow to earth through the cable screen and earth bonds. It has been found that discharge current pulses produced in lengths of paper cable, typically 200-2000m, can be acquired with a ferrite cored current transformer with 3dB response of 25kHz-5MHz. This is the bandwidth chosen for the PDCT 36 supplied with the CableTrend®.
The PDCT36 is an encapsulated split-core device designed to be clipped around a suitable earth bond. It can be opened in 'scissors' form with one side hinged or, for ease of access in difficult locations, its two halves can be completely separated to position around an earth bar. Connection to the CableTrend® is by means of a 3m long coaxial cable with BNC connector.
Partial discharge in insulation inside metalclad static power plant such as switchgear, cable end boxes and busbar chambers, causes small current pulses to flow in the outer skin of the apparatus. These high frequency pulses produce small transient voltages which flow to earth across the skin of the plant. Applying a suitably designed capacitive sensor to the metal skin of the plant allows these transient earth voltages to be coupled into the measurement circuit.
The capacitive coupler type MCC100 is an encapsulated unit incorporating strong magnets to attach the sensor to the steelwork of metalclad equipment. A BNC socket is moulded into the sensor and provided with a 5m coaxial cable for connection to the NetworkTrend®.
Although their outward appearance is similar, the internal electronics of CableTrend® and NetworkTrend® are quite different because of the different operating bandwidth and number of pulse count threshold levels.
The CableTrend® uPD-800 and NetworkTrend® uPD-801 have been designed to withstand normal use in a substation environment. They are packaged in robust plastic housings and designed for permanent fixing to a rigid structure. These instruments can also be supplied mounted in an enclosure meeting IP65 standard for protection against moisture and dust ingress when used in aggressive or tropical environments. Although intended primarily for continuous monitoring application in a specific location, both instruments can be used for short-term surveying of discharge activity on different items of plant.
The instruments are powered from normal mains supply through a plug-top AC adaptor which supplies the 9V required. This adaptor isolates the instrument from the mains but still provides a supply phase reference. The adaptor can be sourced readily in all countries to allow the instruments to be used on any mains voltage, 50 or 60Hz, and with any style of socket outlet.
Basic control functions are readily accessible at the instrument through a membrane keypad which forms the front panel. A large LCD display shows instrument status and instructions through a hierarchical menu. Selection of the number of active monitoring channels (1-8), threshold levels, date/time setting and memory clearing are available through the keypad. A dry contact NO/NC relay operates when any preset threshold is exceeded on any channel and can be used to control a local or remote annunciator.
Acquired data is stored in non-volatile memory inside the instrument, up to 124 days of data for each channel. Front end data processing, compression and achiving is all handled inside the instrument. Stored data my be downloaded at any time over a simple RS-232 serial connection to a DB-9 socket on the side of the instrument case using any common terminal emulation program. This data can then be manipulated by the user through spreadsheets.
The simplest way of communicating with CableTrend® and NetworkTrend® is by means of software called PD Wizard™ which is supplied with each instrument. This can be installed quickly on a PC or laptop computer from the CD-ROM provided. With this software, data can be downloaded from any instrument at the click of a single button and is then stored in a standard database format. Downloaded data is appended to any previously stored data for that instrument in the database. Provided downloads are done at least every 120 days, no data is ever lost. This software also permits certain information to be uploaded to the instruments.
Communication with CableTrend® or NetworkTrend® can be by direct RS-232 link at the instrument using the supplied cable. Alternatively, using a telephone landline with modem, or via GSM modem, data can be download remotely, again just at the click of a button.
One of the major benefits of PD Wizard™ is the ease with which acquired data can be viewed and evaluated graphically on screen without any further processing by the operator. Any period from just one single day up to many hundreds of days can be selected for viewing. This can best be demonstrated by looking at some real examples of data acquired under field conditions.
CableTrend® units have been evaluated by several utilities and industrial operators in the UK and other countries. Some are in use for continuous monitoring of circuits. Many are being used as 'survey' tools and moved from one location to another every few weeks.
Being a slightly newer concept, NetworkTrend® has seen less field use to date but it is being evaluated and has demonstrated its ability to respond to typical service discharge conditions.
A early system delivered by MW Test Equipment was used for on-line discharge monitoring on three circuits of single core 132kV XLPE cables. The discharge signals were acquired by a standard design split-core HFCT clipped around the main earth bonds at the cable sealing ends at one end of each cable. Long coaxial cables, up to 100m, were required to connect the CTs back to the multiplexer in the substation control room. Remote communication with the PC in the substation was via GSM modem. Using this discharge monitoring technique, no PD activity was found from the polymeric cable itself but some activity appeared to come from accessories. One joint was removed from service following subsequent off-line testing.
CableTrend® was installed for a temporary survey on three cable circuits from a small 11kV substation. Each circuit was three core PILCSWA with an available earth bond around which to clip the PDCT36 just below the cable gland entry to the switchgear. NetworkTrend® has now been used on 11kV and 33kV switchgear and cable boxes.
16-channel discharge monitor on 132kV XLPE cables
NetworkTrend couplers on 33kV cable end boxes
Typical 'temporary' CableTrend installation on an 11kV PILCSWA cable
CableTrend® and NetworkTrend® can be left permanently installed to acquire discharge data from the cables or static plant respectively. All the acquired data for pulse counts and phase is pre-processed inside the instruments and held in non-volatile memory. Pulse data is processed according to a special algorithm to produce a Virtual Destructive Energy Analogy (VDEA) based on counts per second and pulse size. This VDEA algorithm filters a huge amount of data into a simple figure which will always behave in a stable and rational way to indicate worsening or easing of discharge activity in the insulation. Up to 124 days of data is stored for all eight of the instrument channels. Thereafter, initial data is overwritten.
The LCD display on the front panel of the instruments shows the status of the instrument and gives some information such as pulse counts versus pulse size sequentially around all active channels. By using the four soft keys on the front panel it is possible to access internal menus allowing parameters to be changed, channels to be enabled/disabled, memory to be cleared and the internal clock to be re-set. Some history data can be retrieved for local viewing on this screen. Data compression and archiving is performed automatically each day just after midnight. The keypad and display provide sufficient access to the instrument to enable it to be set to work, and to check that valid data is being acquired, without recourse to a computer.
An alarm level can be set for different discharge thresholds on each channel. This alarm operates a relay to trigger a remote annuciator or send a signal to the substation data system.
The basic method for communicating with CableTrend® and NetworkTrend® is via a PC or laptop computer. A DB-9 socket for RS-232 connection is provided on the side of the instrument case. A special comms cable (supplied) is used to connect the instrument to the PC. In the simplest case, data may be downloaded as ASCII text through any terminal emulation program (eg Hyperterminal) according to standard report formats detailed in the instrument operator manual. This data may then be manipulated by the user through standard spreadsheet software. This requires the user to have a facility with designing spreadsheets to produce information in graphical form and can be time consuming.
By far the simplest method is to use the proprietary PD Wizard™ software supplied with each instrument. This opens to a clear main screen which allows the user to download data from either a CableTrend® or NetworkTrend® at the click of a single software button. The downloaded data is added to the database in the software if a record already exists for the instrument being downloaded, or a new database entry is created automatically. The software recognises the type of instrument connected. Provided each instrument is downloaded within 124 days of the previous download, a database record of unlimited date range is maintained. When downloading, it is possible to specify the date range on interest
Having downloaded the required data, all the information to make a judgement about the discharge activity recorded is instantly available through standard on-screen graphs. Counts per second for different millivolt threshold levels, VDEA, polar plot and inverse exponential pulse height distribution can be displayed for as little as one single day or any other chosen period of days or weeks.
The following illustrations show typical screens for data downloading and for modifying the instrument settings.
PD Wizard main screen - data download
PD Wizard screen for updating database and instrument settings
The graphs on the following pages show typical acquired data displayed ready to assist the user to make informed decisions about the severity and type of discharge.
This graph, downloaded from PD Wizard, shows actual discharge counts activity from an 11kV paper cable in a substation. Note that there are fairly high numbers of pulses at threshold levels up to the 20-32mV band and even significant pulse counts 32-52mV.
The polar diagram relating to the same data series indicates quite clearly that the discharge pulses fall in two diametrically opposed quadrants on the voltage cycle. This is confirmation of discharge activity over this date period.
Compare this with another downloaded counts graph which appears to show similar discharge activity but with much greater numbers of counts:
The polar diagram for this period shows that the counts activity occurred all around the voltage cycle, clearly indicating 'noise' rather than discharge.
The graph below shows the VDEA curve for the earlier example of the 11kV paper cable. The VDEA is steady at a reasonably high value ~40dB. This could indicate discharge activity that is fairly severe but that is not worsening with time. That it is actual discharge is confirmed by the polar diagram.
Compare this with another situation where the VDEA graph appears to show increasing discharge activity. In fact, from the polar diagram, this can be seen just to be noise caused by very high and varying levels of 5-8mV pulses.
Much work has been undertaken to develop continuous methods of monitoring partial discharge continuously on-line to assist asset owners and maintenance managers in assessing the condition of major plant items. On-line discharge monitoring of switchgear has been used for several years and instruments exist to give some estimate of the likely location of any discharge. Because of the dynamic nature of discharges in insulation, which vary according to changes in temperature, pressure, humidity, electrical stress (and sometimes, it seems, even with the colour of the engineer's hair!), it is extremely difficult to make precise objective measurements of magnitude and location. Huge sums of money and many tens of thousands of man hours have been expended on trying to understand the nature of partial discharge, with university departments devoting their whole life to the subject.
There are a number of complex and very sophisticated instruments on the market with which discharge can be monitored, measured and pinpointed on-line. Such instruments are also very expensive and generally cannot be justified on medium voltage apparatus. The instruments discussed here are an attempt to provide the utilities and industrial operators of MV systems with simple low-cost tools that will enable them to monitor developing trends in discharge activity over time. This may assist with making those difficult decisions about when to repair or replace major assets.
Development in the technology for continuous monitoring and diagnosis of partial discharge from cables and static plant is most likely to focus on on-line pinpoint location of the discharge source by utilising advances in time domain reflectometry and application of commercial GPS. So far no system has been developed that will give an indication of likely time to failure of a cable, but with continuing research into mechanisms of failure and improved methods for analysing trend data, this may well be achieved in the future.
C M Walton ©IEE 1998 "Networks for tomorrow - urban and city centre underground distribution for the 21st century", IEE Switchgear Conference.
R R MacKinlay & C M Walton "Some advances in PD monitoring for high voltage cables", IEE Cable Life Conference, Capenhurst UK, 21 April 1998.
Cliff Walton, "Incipient failure detection and location for underground cables", CIRED Nice 1999.
Colin Bower, "On-line diagnostic trials on London Electricity's 11kV cable system", IEE Cable Asset Management Conference, Birmingham UK, 3 February 2000.
Cliff Walton, "Regulatory and business drivers for reducing underground cable failures", ERA High Voltage Plant Life Extension Conference, Belgium, 23-24 November 2000.
Michael Webb, "Some instruments for plant monitoring and assessment", ERA High Voltage Plant Life Extension Conference, Belgium, 23-24 November 2000.
Mark Lomax, "Cable Asset Management" course at ERA Technology, Leatherhead UK, 19-20 June 2002.
Michael Webb, "Plant Monitoring Techniques", EA Technology course 'Management of Substation Assets', Capenhurst UK, 23-24 September 2002.
After working for several years with Foster Transformers Ltd, where he became Manager of the Test Equipment Division, Michael Webb joined ERA Technology as Client Liaison Manager. He then worked with Kilovolt Systems Ltd and Tettex Instruments of Switzerland before setting up MW Test Equipment in 1981. A joint venture company, Baur Test Equipment Ltd was established with Baur of Austria in 1989. These companies form the core of The Faraday House Group Ltd today.
Michael's main professional interests are insulating fluids, high voltage testing and insulation diagnostics. He is particularly keen on the application of continuous monitoring to these areas. He is a Chartered Engineer with a Diploma in Electrical Engineering, a Member of the Institution of Electrical Engineers, an Associate IEEE and an individual member of Cigre.
Mark Lomax joined the Faraday House Group in 1997 after being closely involved with partial discharge testing and detection for thirteen years with Robinson Instruments. Mark's background in electronics and measurements, coupled with his experience of high voltage discharge detection, make him the lead person in the company for all discharge monitoring activities.
Mark's other major time involvement is with condition assessment and dissolved fault gas monitoring equipment for large power transformers.