ON-SITE TESTING OF TRANSFORMERS - SAVE COSTS AND TIME Dr. Mario Jochim, Bernhard Nick - HIGHVOLT Prueftechnik Dresden GmbH Marie-Curie-Straße 10, 01139 Dresden, German, jochim@highvolt.de Abstract Our modern life strongly depends on the availability of electrical power at any time. Therefore the reliability of electrical power systems is of outmost importance. Even if the overall performance of well-maintained transmission and distribution systems can be described as good, the breakdown of key network components is dangerous, costly, and sometimes long-lasting and may harm the environment. Common reasons for system outages caused by equipment breakdowns are problems in the electrical insulation of network components like transformers, cables or switchgear. Test procedures and results in the factory and on-site need to be according to the relevant standards (IEC 60076, GOST 1516.3, IEEE Std C57.12.00). While commissioning tests of newly installed equipment are common practice, field testing of aged assets with high voltage becomes increasingly popular, too. Examples are induced or applied voltage tests on transformers showing suspicious and increasing rates of fault gases during routine tests or after having been repaired on-site. Such onsite tests are reliable procedures to guarantee quality and status of installed transformers and to reduce time and cost significantly compared to factory testing. KEYWORDS On-site and factory testing, transformer testing, state-of-the-art technology Introduction After being produced, new transformers are tested in the routine test field according to standards IEEE C57.12.90 or IEC 60076-3. Routine test protocols are generated and the transformers are shipped to the place of installation. The shipment as well as the installation activities can put a tremendous strain on the transformer. Therefore, customers are always interested in the internal conditions of the transformer. So, prior to energizing, it is best to perform as much tests as possible on site and even more after repair service in the field or at a shop. During the tests, the transformers are applied to different voltages, currents, load conditions and especially different frequencies like higher than 80% of power frequency for the applied potential test or higher than two times of power frequency for the induced voltage test. Also with regard to the life of the transformer, diagnostic testing needs to be performed to verify the condition of the insulation coordination. As a standard solution, transformer manufacturers have been using motor-generator sets with two or more machines on the main shaft for different tests. However this solution is not practical for on-site testing, due to the cost-intensive service and the limited range of the output frequency of motor-generator sets as well as the wear of rotating machine parts, heavy weight and size. Design of the mobile transformer test system The mobile test system installed in a 40 ft container includes all components for induced voltage, no-load loss and load loss test. The container is divided into three compartments; converter/transformer room, control room and filter room (see Figure 1). The converter/transformer room is situated on the rear side of the container and houses the static frequency converter, power sine wave filter and the step-up transformer. As alternative the system can also be installed in three 20ft containers (see Figure 2). This makes it easier to transport the test system into substations in narrow city centres or mountain areas. For power supply, an input power supply of 750 kva from a stiff grid or 1.2 MVA from a diesel generator are necessary.
The major components are provided with sufficient cooling by supply air and exhaust air openings integrated in the side walls. The system is designed and optimized for on-site testing and mobility in compliance with relevant legal road criteria. The main data of the test system is shown in Table 1. It can also be used in the test shop as substitute of an M/G set if this is not powerful enough or needs to be repaired. Table 1 Main data of the transformer test system Type of test system WV 620-1000/80 Active power continuous 620 kw Apparent power Test voltage continuous short time (15 min ON / 120 min OFF / 3 x per day) 1000 kva 1500 kva 8.9 80.2 kv (11 steps) Max. rating of transformer to be tested (induced) Max. rating of transformer to be tested (no-load) Mass of the test system (container, trailer, truck) 500 MVA 350 MVA (THD < 5%) 500 MVA (THD > 5%) < 40 t Figure 1 Design of the mobile transformer test system for induced voltage and no-load losses installed in a 40ft container
Figure 2 Design of the mobile transformer test system for induced voltage and no-load losses installed in three 20ft containers Most common tests on site Induced voltage tests To conduct the test, an alternating voltage shall be applied to the terminals of one winding of the transformer. A sinusoidal alternating voltage shall be applied with a frequency sufficiently above the rated frequency of the transformer to avoid excessive magnetizing currents during the tests and to limit the flux density in the core. Basically, the induced voltage test should be started with a test frequency in the range of 120 to 150 Hz and a low test voltage. The required testing power depends on the test frequency which will become increasingly capacitive with higher frequencies. Especially when testing large power transformers up to 1200 MVA, the required testing power can be reduced by shifting the test frequency and searching for the self compensation point. By using the self compensation the required test power will be at a minimum and only the active power needs to be provided by the test system. As a requirement for testing, a low PD noise level < 100 pc is requested. This requirement can be easily fulfilled by state of the art static frequency converters due to measures in the control of the inverter components, an optimized grounding system and well adapted filter components 2, 4. Even lower PD levels of 30 50 pc (broad band measurement) can be achieved by comprehensive and customized use of the above mentioned measures. Figure 3 shows an example of the typical PD noise level of the transformer test system.
Figure 3 Basic PD noise level of the transformer test system No load loss tests The no-load loss test needs to be performed to check the transformer after a repair service or after maintenance. One main requirement for this test is the waveform distortion which is described in IEEE C57.12.90 where a distortion of 5% is acceptable 1. This requirement can be fulfilled due to the excellent dynamic behaviour of the static frequency converter which allows the application of enhanced control features. So, the systematic suppression of the voltage harmonics can be enabled and the waveform distortion can be significantly reduced. An example of the non-linear test current and the perfect sine wave test voltage is shown in Figure 4 3. The needed power rating of the static frequency converter required is only twice the needed test power. In comparison, the M/G set has a power rating of 5 10 times higher to conduct the no-load loss test on the same tested transformer depending on nonlinear impedances of the iron core of the tested transformer and the additional impedances of the step-up transformer.
Figure 4 Non-linear current and sine wave test voltage during no-load loss test of a 120 MVA transformer Parallel operation Due to the modular design of the frequency converter system, up to 3 test systems can be paralleled and routine tests can be performed on transformers with a rated power of up to 1500 MVA. Figure 5 Test of a 1.1GVA generator transformer at nuclear power station in Grundremmingen by SMIT Transformers and RWE
Power analysis Being equipped with a complete set of current and voltage instrument transformers, the mobile transformer test system can measure all relevant values during the no-load losses test. Gathered data can be displayed and analyzed with the integrated power analyzer equipment (see Fig. 6). Together with the remote software package the test procedure can be performed and recorded automatically. Figure 6 Control room with integrated power analyzer equipment and computer aided control software System for applied test The purpose of the applied voltage test is to verify the integrity of the main insulation of the transformer. Since the transformer to be tested can be seen as a primarily capacitive load, a resonant circuit with variable inductance or variable frequency can be used. The resonant tuning is realized via a static power inverter and control system located in front of the exciter transformer and the fixed inductance, (see Figure 7). The result is an on-site applied voltage test system which is smaller, lighter, more compact and transportable. Moreover, an automation of test procedures via control software can easily be realized, e.g. test voltage, test time and the search of the resonance point. International standards such as IEC 60076-3 allow test frequencies at 80% of the power frequency for the applied test. The IEEE C57.152 Field Test Guide, now being revised, will consider an applied test frequency consistent with the IEC Standard. A test voltage level up to 360 kv with one single reactor and up to 720 kv with two reactors in series can be generated. The main components can be stored and transported on a 40 ft trailer.
Figure 7 Resonant test system 360kV/5A for power transformers Maintenance and Service Transformer test systems based on static frequency converter technology have no rotating parts in the main power path. The power flow is completely controlled by the switching operation of the IGBT s. So, the need for maintenance is very limited (once a year) and depends on the environmental conditions [4]. M/G-sets are based on the utilization of rotating parts and encounter all the mechanical problems of rotating machines with increasing lifetime as shown in Figure 8. The maintenance for static frequency converter based systems is usually restricted to the replacement of components of the cooling system (e.g. filter pads of fans). In comparison the life cycle costs of M/G-sets are much higher. In case of a defect, a defective power converter module can be replaced by two persons within 30 minutes. Therefore, the overall downtime can be kept very short. If an M/G-set fails, the entire alternator or motor has to be disassembled. The masses to be handled are often in the range of several tons. Consequently, the downtime and the cost inccurred will be much longer and higher. Figure 8 Comparison of failure rates between electronic and mechanical devices
Conclusion Static frequency converters are used as state of the art technology to generate an AC voltage with continuously adjustable frequency. Induced voltage tests with PD measurements and no-load loss testing on power transformers is being performed today with static frequency converter technology. Today s mobile transformer test systems require minimum test power, have no moving parts, and are lightweight and easy to transport. The mobile test system can also be used in the factory as substitute of existing M/G sets. Loss measurements and heat run tests can be performed with addition of compensation capacitor banks. All requirements of the relevant standards can be fulfilled. Power rating can be increased by the application of the modular principle of parallel connections of several systems. A comparison between motor-generator sets and static frequency converters results in the clear conclusion that the future of transformer testing will be dominated by static frequency converters. REFERENCES 1. IEEE C57.12.90 Standard Test Code for liquid-immersed Distribution, Power, and Regulating Transformers, 2006 2. IEC 60076-3 Power transformers Part 3: Insulation levels, dielectrics test and external clearances, 2000 3. Y. Huang, A. Thiede, A. Winter: A New Generation of On-Site Test Systems, CPRI India 2009 4. A. Winter, A. Thiede: A New Generation of On-Site Test Systems for Power Transformers, ISEI Vancouver, 2009 5. D. Bouley, J.-F. Christin: Comparison of Static and Rotary UPS, APC, White Paper 92, Rev. 2, Schneider Electric