SFRA Ability to Find out Fault Inside The Winding: A Practical Case Study

Sweep Frequency Response Analysis (SFRA) has become very popular now – a days due to its ability to provide much more information regarding mechanical as well as electrical condition of power transformers. SFRA has ability to provide information about inner winding which cannot be visualized in internal inspection of power transformer. SFRA is comparative method and provides better information when evaluation of Transformer condition is done by comparing actual set of SFRA results with reference results. This field experience consists 220/66KV 160MVA, power transformer tripped in differential relay. Mechanical defects ostensibly occur due to a short circuit current flowing through power transformer in case of terminal faults i.e. fault near to power transformer. To know the mechanical integrity of power transformer SFRA test is carried out and is supported by other tests i.e. DC resistance, Magnetic balance, Winding resistance, capacitance and tan delta etc. The test results reveals that SFRA is capable of providing more information compared to other tests. Keyword Sweep Frequency Response Analysis, transformer, differential relay, terminal fault, mechanical integrity, Winding Deformation, capacitance and tan delta.


III. SFRA
The first technical paper published on Frequency Response Analysis was by Mr. E.P. Dick and Mr. C.C. Erven in 1978 [1]. Sweep frequency response analysis has been developed to detect winding movement and deformation in power transformers. High voltage power transformer can be represented as a complex electric system consisting of inductances, capacitances and resistances. The complex structure of power transformers can be represented with two port network as shown in figure. The two port network, excited by the voltage signal, produces an electrical response which is dependent on the frequency of the input signal, and the value of output voltage depend on impedance of particular winding that have withstood test at particular frequency. Output voltage Vout and Input Voltage Vin are compared in the frequency domain, the gain in dB as under 20 log 10 …………………………………………… (1) The values of RLC elements depend on the geometry and material used for each part of the transformer. Any change in geometry or change in material reflect significant change in the response. SFRA analysis involve comparison of two set of test which is compared with base result and if not available then compare with sister unit, and if sister unit test also not available then comparison is done with phase itself. If base available, then sound judgment can be taken with help of SFRA test.
VI. THROUGH FAULT In power system due to overhead construction of bus bars and lines, chances of through fault is much more, and if fault occurred near to transformers known as a terminal fault, fault current fed by the transformer is very high. Through fault passed through transformer for longer time as compared to transformer fault, as back up protection take longer time. Higher magnitude fault for more time create mechanical stresses inside the transformer and transformer windings may shift and any minor movement of active part may lead to the transformer failure.

V. CASE STUDY
At one of the 200kv sub-station, 2 nos. 160MVA power transformer were in service. During its service and normal functioning on date 16.06.2019 at 6.13Hrs, bus fault took place. It was due to heavy wind which caused conducting material fall on 66kv bus. Both transformers tripped with LV side, however transformer no. 1 tripped with differential relay and buchholz relay. Said transformer kept out of service for further investigation. Disturbance Recorder (DR) extract from both numerical relay, differential as well as back up. From the disturbance recorder it was noticed that transformer has cleared the fault successfully at 6.13 as shown in fig. Transformer tripped with backup relay and LV breaker trip, hence fault current stopped and transformer charged from HV side. After 14 second fault took place again inside the transformer and transformer tripped with differential relay. Transformer HV side breaker kept off with differential relay and magnitude of differential fault current was very low i.e. 450 Amp in R phase and differential fault cleared within 75 milliseconds including all i.e. fault detecting and fault cleared and breaker off as shown figure .

A. Capacitance and Tan delta test
To know the condition of insulation of power transformer, capacitance and tan delta test was carried out. This test provide information regarding watt loss through insulation if watt loss more than previous results than it means insulation going to aging faster. Any change in capacitance value indicate change in mechanical integrity of winding or change in dielectric constant of insulation material used.

Figure. 5. Tan delta of insulation
It is one of the most common test performed on power transformer to know the healthiness of insulation system in our case tan delta value found within limit and value of capacitance also in permissible limit as shown in table. It indicate that there is no major change in insulation system, change in capacitance and tan delta noticed in LV winding, however it is in permissible limit. Insulation resistance taken at one minute and at 10 minute with applied 5kv DC through insulation tester known as IR tester and Polarization Index PI value was derived. Result found normal and no any significant change perceived. Insulation resistance and Polarization Index before and after tripping shown in Table II.

1) HV magnetic balance test
Three phase power supply applied to HV winding and measure the magnetizing current. This test indicates about magnetic circuit created by core. Any shorted turn may result into high magnetizing current in particular phase. In our case all results found normal as shown in table. 2) LV magnetic balance test LV magnetic balance test with application of three phase voltage LV side and note down LV magnetizing current at all taps. As our transformer in doubt having two winding hence both HV as well as LV magnetizing current required to be checked and nothing found abnormal in LV magnetizing current as shown in table IV.

5) VOLTAGE RATIO TEST
Voltage ratio or turn ratio test provide conformation about proper function of power transformer at for which it design actually. Voltage ratio measured with applied primary side three phase AC supply voltage and measured the LV side voltage at each tap. Where in turns ratio applied voltage kept constant primary side and measured the voltage at secondary to avoid any change in AC supply in measurement. In our case turn ratio test carried out with kit and all results found within permissible limit, as shown in below table. Sweep frequency response analysis carried out after differential trip and compare with base signature available and all test results found normal. However, in LV and HV R phase in winding area deformation notice and it indicated that there is shift in LV winding and same reflected in HV windings. Distortions in LV winding shown in below figure blue trace is before fault and consider as base and red trace after fault and in mid frequency area clear deviation noticed.

H. Internal Inspection
After the study of all test results with analysis of SFRA, it was concluded that LV winding was deformed which cannot be visualized in internal inspection. However, to find out any other damaged caused due to deformation of LV winding, internal inspection was carried out. In internal inspection, at bottom part of R phase winding carbon particles were noticed as shown in figure 8.

VI. CONCLUSION.
This is a case where in we can state that in the event of bus fault, due to metallic object fall in bus heavy through fault current fed by the transformer. After successfully clear the fault by transformer, transformer cannot with stand more voltage stress and trip with differential relay and buchholz relay. Fault created in transformer is not major, hence detection is not easy. SFRA test in such situation provide more information about mechanical as well as electrical integrity of transformer windings. There is no any major fault inside the transformer only displacement in HV and LV winding of R ph. it is difficult to know about geometric changes would have occurred in the transformer, can be ensured by carrying out SFRA test post failure it can be matched with base SFRA test. The same is done in this case, in winding area creation of new resonance point and elimination of old resonance point indicate displacement in winding. While in internal inspection nothing found abnormal, as fault in winding and winding covered with hard insulation one cannot find anything, however SFRA clearly indicate movement in winding.