Draft - to be published and presented with "2012 IEEE International Symposium on Electrical Insulation", 10-13 June 2012, San Juan, Puerto Rico On-site Transformer 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 transformers, 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 high voltage 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 electron 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 capacitor 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.