Draft - to be published and presented with "2012 IEEE International Symposium on Electrical Insulation", 10-13 June 2012, San Juan, Puerto Rico On-site Partial Discharge Diagnosis

Detlev W. Gross, Markus Soeller Power Diagnostix Systems GmbH Aachen, Germany

Abstract—In the lab, partial discharge diagnosis has widely Thus, acceptance testing as commonly applied in the lab, replaced the traditional RIV measurements. Additionally, partial will be increasingly used on-site to extend the service life of discharge acceptance levels are being reduced due to the old , to validate the success of on-site repair, and increasing use of composite material and a growing awareness of to ensure successful commissioning of new units. partial discharge phenomenon and their consequences. Adequate filtering of the supply voltage for induced voltage testing and the Likewise, applying on-line monitoring of various operation use of sensitive acoustic measurement has greatly improved the parameters including partial discharge can assist to extend the detection and location of partial discharge activity in power lifetime of service aged substation equipment. transformers. II. PARTIAL DISCHARGE TESTING An increasing population of service aged substation equipment having reached their projected service life demands on-site repair Detecting high frequency signals using narrow-band and, hence, on-site testing to factory standards. receivers based on heterodyne principles has been used already very early in the history of insulation systems [1]. Using inverter based three-phase mobile test sets allows on-site With early meter-type instruments diagnosis was mostly application of tests previously limited to a mere test room limited to the observation of magnitude and inception vs. environment. Besides the unmatched portability of inverter- extinction voltage, while later oscilloscope-base instruments based sources, especially the less critical generation of reactive added the phase position of the discharge activity. An in-depth power simplifies on-site testing, if compared with motor- understanding of the gas discharge physics and the statistics of generator sets. Additionally, the lack of high short circuit partial discharge was supported with the introduction of currents limits the damage in case of breakdown. However, instruments using the phase resolved partial discharge (PRPD) adequately removing the switching noise spectrum becomes a pattern, or ϕ-q-n pattern [3, 4, 5]. demanding task in order to reach the required sensitivity of the partial discharge measurements.

Keywords: partial discharge; on-site; diagnosis; inverter-based; acoustic location

I. INTRODUCTION The deregulation and privatization of the energy sector, which started in the 1970ies in many areas of the world, has had a further impact on transformers and their life. Before deregulation with often government-owned or government controlled utilities, availability was the most prominent design and sourcing criterion. A side effect of this policy was “over- engineering”. Deregulation shifted the emphasis to profitability. As a consequence, re-investment into the grid dropped significantly. However, in light of the typical service life of a large power transformer of about 40 years, the consequences of this change took decades to materialize, while profits went up immediately. Figure 1. ϕ-q-n pattern of multiple cavities (voids) Nowadays, we do have in many parts of the world service- aged populations of sub-station equipment, of which a large As an example, fig. 1 shows such a ϕ-q-n pattern of several portion has reached or already exceeded its projected service voids in epoxy resin. Here, each individual sine-shaped trace life. In Europe, for instance the majority of the 400kV- belongs to an individual gas inclusion. Moreover, the well- transmission-system was commissioned in the 1970ies and distributed pattern is caused by a low availability of the starting 1980ies. Moreover, the ongoing change from fossil fuels to for the discharge avalanche, as it is typical for bubbles renewable sources further increase the required transmission in polymeric material such as fresh epoxy resin [6]. capacity of a service-aged grid. Fig. 2, instead, shows the activity of several gas inclusions With a better understanding of the deterioration processes with the casein glue of barriers and spacers on top of the static of materials used in power transformer, partial discharge shield of large distribution transformer coils. Here, increasing measurements gained a more prominent role in the acceptance the field strength due to customer demands reached the testing of large power transformers. The relevant standards for limitations of the factory's production methods. transformer testing, such as the IEEE C57.113 [9] have shifted the emphasis to partial discharge detection with the more recent revisions. A partial discharge acceptance level of 500pC was commonly used. However, the increasing use of composite materials and their defect mechanism led to a reduction of the partial discharge acceptance test levels during the past decade.

III. MOBILE TEST SET Generally, on-site partial discharge testing of large transformers is a demanding task. In order to allow tests at elevated voltage, as it is part of the standard short duration or the long duration test [10], the transformer needs to be energized at a higher frequency. Heavy motor-generator-sets are commonly used in the test room of a transformer factory. A motor-generator-set, the step-up transformer and the control circuits cover several freight container loaded up to their permissible weight, when testing transformers of 500MVA or more.

Figure 2. ϕ-q-n pattern of gas inclusions trapped in casein glue In order to overcome the operational and logistic limitations of such a conventional solution, an inverter-based three-phase Finally, fig. 3 shows a pattern that is caused by a source was developed [11]. The unit is built into a modified delamination in transformer pressboard. Here, the Lichtenberg 40ft high-cube container (Fig.4) and stays within the load and figure of the surface discharge does cause the steep increase of size limitations of a conventional road-worthy trailer truck. the partial discharge magnitude vs. phase.

Figure 4. Mobile transformer test set built into a 40ft container

The mobile test set requires a 400V three-phase supply feeding three individual 450kVA inverter units covering an output frequency range of 20-200Hz. The 2MVA step-up Figure 3. ϕ-q-n pattern of pressboard delamination transformer consists of three single-phase transformers in a Power transformer acceptance testing initially focussed on common tank. This allows running the unit in single-phase "radio interference voltage" (RIV). The original intention of mode at full power on all three inverters. Both ends of the HV this test, however, was to avoid hampering AM radio reception winding as well as two taps are accessible via four bushings in due to partial discharge activity. Therefore, narrow-band line for each of the single-phase units (fig. 5). Additionally, circuits and a weighting circuit were used with the RIV meters each low voltage coil has a tap as well. Thus, by selecting the [2]. However, the used bandwidth does not allow to process LV and by applying jumpers equipped with multi-contact high-repetition partial discharge, while the weighting circuit connectors, a large variation of output voltages ranging from confuses the detection of low-repetition discharge with high 8.5kV to 90kV full-scale can be chosen by interconnecting the magnitudes. different taps in star or delta configuration (Table 1) to have a close match to the load requirements. Figure 6. Noise pattern due to (unfiltered) inverter switching action

Finally, the mobile test set comes with a 500kV reactor for resonant applied voltage testing. The reactor sits on a frame that can be moved out of the container to provide the required spacing (fig. 7). The reactor has an inductance of 400H, which together with the coupling of 2nF results in a resonant frequency of about 178Hz. With the minimum frequency of the inverter of 15Hz, a load capacitance of up to 200nF can be Figure 5. Step-up transformer with taps and HV filters covered for applied voltage testing. However, with increasing capacitance, the current limitation of the reactor limits the Running the inverters with 120° phase shift offers three- maximum voltage. Given the current limit of 4A, the 500kV phase induced voltage testing, while 0° phase shift allows can be reached up to about 25nF, whereas the resonance single phase induced voltage testing at full power. Although frequency is close to 50Hz, then. intended mainly for transformer testing, this also offers testing a cable of 5µF at 36kV and 50Hz, for instance.

TABLE I. MOBILE TEST SET, 2MVA STEP-UP TRANSFORMER

Three-Phase Output, Voltage and Current Configuration LV Input 1 LV Input 2 HV Output 1, Delta 11.8kV 97.9A 8.5kV 135.2A

HV Output 1, Star 20.4kV 56.5A 14.8kV 78.1A

HV Output 2, Delta 26.1kV 44.3A 18.9kV 61.2A

HV Output 2, Star 45.1kV 25.6A 32.7kV 35.3A

HV Output 3, Delta 52.0kV 22.2A 37.7kV 30.7A

HV Output 3, Star 90.1kV 12.8A 65.2kV 17.7A

The inverters offer full four-quadrant operation and, hence, can supply reactive power up to their output current limit. Figure 7. 500kV reactor for applied voltage testing moved out Additionally, the mobile test set comes with switchable inductive (3 x 180kVA) and capacitive (3 x 603kVA) compensation to minimize the inverter current. IV. ON-SITE TESTING Generally, of course, the inverters produce switching noise, On-site acceptance testing of transformers including partial which strongly hamper partial discharge measurements, if not discharge tests is either triggered by abnormal behavior of the sufficiently filtered. Depending on the load situation, the transformer in service detected by on-line dissolved gas inverters produce various impulse noise patterns. Fig. 6 gives analysis (DGA) or in more extreme cases by tripping a an idea of such noise pattern. One set of filters is fitted on the Buchholz relay. Moreover, such on-site acceptance testing LV side between inverter and transformer. Another set of filters greatly simplifies assessing the current status in preparation for is placed on the high voltage side (fig. 5), which additionally an on-site repair or validation of the results after such an on-site carry the fiber-optically-isolated load current measurement. repair [11, 14, 16, 17]. Of course, the transformer needs to be disconnected from The IEC60270-2000 [7] limits the frequency range to an the grid and prepared for the test. Setting up a mobile test set as upper corner frequency of 400kHz, whereas the highest such requires few hours, only. Typically, a rented 400V diesel permissible lower corner frequency is kept to 100kHz. Often, generator is used to supply the mobile test set. Hence, this frequency range is (partly) occupied by noise signals, when decommissioning, measurement, and commissioning the doing field tests. Thus, a tunable detection circuit is very transformer again can be done within two days, if needed. helpful to optimize the signal-to-noise ratio (SNR). Currently, However, subsequent attempts to acoustically locate a found an addendum to the IEC60270-2000 is under preparation. Re- partial discharge source may be more time consuming. defining frequency ranges is part of the discussion. Besides the three-phase induced voltage test at elevated Generally, the lower cut-off frequency shall exclude frequency, the unit support as well single-phase induced and residual noise signals from the three-phase-supply for induced applied voltage tests. The available 1.3MVA active power does voltage testing regardless, whether it is IGBT switching noise not pose any limitation for no-load tests on even very large from an inverter-based source or thyristor noise of the transmission transformers. However, accurately measuring the excitation system of a conventional motor-generator set. The load losses is limited by the available output current and by the upper corner frequency shall not be chosen too high to still limited capacitive compensation. cover larger parts of the winding. However, for proper pulse processing a bandwidth of several hundred kHz is mandatory As an example, table 2 shows the power requirements for [8]. two different transformers tested. In both cases a diesel generator well below the maximum power was used. Given a well de-noised source and the comments above, on-site partial discharge testing of large power transformers offers similar sensitivity levels as found in an average, non- TABLE II. MOBILE TEST SET, POWER REQUIREMENTS optimized transformer test room. Power Requirements Transformer I Transformer II VI. ACOUSTIC LOCATION Nominal Voltage 112/22kV 235/20kV Besides electromagnetic signals and light emission, partial Nominal Power 63MVA 660MVA discharge cause also acoustical signals. Hence, its acoustic emissions and the respective travel time in different materials Apparent Power 38kVA (@1.0UN) 240kVA (@1.5UN) can be used to locate partial discharge. Power (Diesel Gen.) 24kW (@1.0UN) 190kW (@1.5UN) However, the classical triangulation approach with three sensors on three faces of a cubic tank fails in most cases of real-life transformers, as two essential conditions of this V. PARTIAL DISCHARGE TESTING method aren’t met. Firstly, the internal medium of the tank must be homogeneous and, secondly, the tank wall must have As in the lab, typically, the test tap or potential tap of the the properties of a thin membrane [15, 17]. bushings is used for coupling to the partial discharge signal. Only for smaller LV or tertiary bushings coupling Instead, a transformer is filled with different materials are used instead. Quadrupoles and preamplifiers (fig. 8) are having different density and, hence, causing different travel fitted to make the signals available to an eight-channel partial speeds of the sound wave. Thus, for a partial discharge source discharge detector. Besides covering the frequency range as deeply buried within the insulation system, the different defined with the IEC60270, this unit offers additionally a wider possible travel paths, their delays and attenuation strongly frequency range using a tunable heterodyne circuit covering hamper the location of the source. Additionally, the tank wall, frequencies up to 10MHz [12, 13]. instead of being a thin membrane, has its own transmission properties and does add lateral transmission within the steel as an additional path. Thus, understanding the transmission paths and their properties of a remote partial discharge source in a winding is as complex as trying to visually locate a light source within a complex structure of glass of different refraction indices, for example. Typically, before attempting an acoustic location, an in- depth partial discharge diagnosis is performed in order to understand the rough location of the source in terms of dominant phase, phase-to-phase, or phase-to-ground discharge, for instance. Based on these results, the suspected area of the transformer is scanned placing several acoustic sensors on the tank wall (fig. 9) to find acoustic signals correlated to the Figure 8. Quadrupoles and preamplifiers fitted to the test tap electrical partial discharge signal. The ideal situation is having a clear oil path. Here, the typical transmission speed of about 1.4m/ms can be used for calculating the distance. Of course, knowing the internal structure of the transformer under test and the impact of different materials is essential and detailed drawings should be at hand. Practical experience of acoustic location of about one hundred large power transformers led to a comparably simple method. After having found a correlating acoustical signal, three sensors are placed in a row, whereas the positions are optimized in order to have the center sensor showing the shortest distance. Placing the sensors in a row reduces the triangulation to a two-dimensional, i.e. “flat problem”. Having started with a horizontal line, for instance, the sensors are then placed in a vertical line at the horizontal position found in the first step. This process is assisted by a software tool to create Figure 9. Acoustic sensors fitted to the tank wall using magnets then the three-dimensional position results. Fig. 11 shows the oscilloscope screen of this software, while the location screen Therefore, an oscilloscope or equivalent equipment is is found with fig. 12. triggered to the dominant electrical signal, while displaying the averaged acoustic signals of the piezo sensors placed. Fig. 10 shows such typical signals with different travel time for the sensor positions.

Figure 10. Acoustic travel time with respect to the electrical PD signal

Given the complexity of a transformer’s insulation system, the key strategy, when optimizing the sensor positions, is to Figure 12. Viewing the resulting position based on the triangulation have a signal path as simple as possible. The two-dimensional system is intentionally over determined in order to address the effects of signals traveling in the tank wall. The transmission in steel is about four times faster than in transformer oil and, hence, can produce confusing results in case of a lateral location of the discharge source. Especially with mounting turrets of high voltage bushings, the high speed in steel can produce misleading results, when signals via the epoxy resin of the bushing and the steel of the turret appear earlier at the sensor than the signals of the direct oil path. With a clear oil path, the acoustic signal has a sharp inception and placing a cursor either manually or automatically on the graph is easy. In such cases an accuracy of about +/-2cm can be achieved. If several independent transmission paths contribute to the signal placing the cursor and, hence, determining the distance becomes more and more difficult. However, even here in most cases a location precision of +/- 20cm is possible. Often, the precision can then be improved with the analysis of the partial discharge pattern, its properties, and the cross-coupling matrix with respect to phases and coils. Figure 11. Analyzing the vertical and horizontal results Based on this analysis, areas are identified, where the observed partial discharge activity is physically possible and compared with location result of poorer precision.

VII. MONITORING Generally, partial discharge monitoring suffers from electromagnetic noise interference as found in a substation environment. Thus, the local noise situation needs to be understood before setting up a monitoring system.

Figure 13. Bushing coupling unit and adapter connected to a test tap

The smoothest application can be expected, if the Figure 14. Fitting an UHF sensor to a transformer's oil drain valve transformer is connected to cables and/or gas insulated (GIS). Here, almost lab-type frequencies in the range of the IEC60270 can be used due to the low interference level. Partial discharge coupling is made using the measurement taps of the bushings. Care must be taken to have this connection made durable in order to maintain a safe operation of the bushing. Fig. 13 shows such coupling to a bushing tap – the measurement impedance that separates the power frequency synchronization and the high frequency signal follows a mere mechanical adapter with surge protection. Alternatively, higher frequencies can be covered with built- in UHF antennas mounted on a spare flange or fitted to a drain valve of the transformer under test (fig. 14). Sadly, often transformer design does not provide such UHF access via the drain valve. In more than 50% of the cases the drain valve does not offer a fully open cross-section, continues internally with an elbow, faces a 45° stiffener plate, or runs via pipe directly to the conservator. Fig. 15 shows an example of such a partial discharge monitoring instrument (upper left corner) as part of an overall transformer monitoring system. Here, the instrument talks to the local monitoring system covering temperatures, voltages, and other parameters, which then reports to the SCADA system. Alternatively, the PD monitoring device itself is already equipped with a TCP/IP interface acting according to the IEC61850. Besides the values for partial discharge monitoring, which are already included with the relevant section of this standard, further parameters can be added into the model description, including the ϕ-q-n pattern or more in-depth Figure 15. PD acquisition system as part of an overall monitoring system trending information, for instance. [4] Heitz, C. 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