Frequency Response Analysis (FRA) test

1. Frequency Response Analysis (FRA) test Transformer Services Dept./TCB/BHEL

2. Frequency response analysis (FRA) test is now considered as one of the most reliable condition monitoring tools for detection of winding movement in transformers and reactors. In addition, it also provides useful information on possible electrical failures during service life of the equipment. FRA patterns are unique to a particular winding configuration and its insulation structure and would change either in case of any mechanical movement of the winding due to short-circuit forces or dielectric failures. The FRA patterns or graphics recorded either during final testing at works or during commissioning of the equipment at site, serves as ‘fingerprints’ for comparison with FRA patterns recorded on the same equipment subsequently at periodic intervals, e.g., during annual maintenance or planned shutdowns etc. Transformer Test group has satisfactorily established FRA test method as a condition monitoring tool to assess the condition of the transformer windings before and after short-circuit test and dielectric tests. FRA data has been collected on more than 35 transformers and reactors to serve as reference ‘fingerprints’ for comparison with FRA test data gathered at site on the same or similar transformers and reactors during service. We are now fully geared up to provide services to various customers for FRA measurements at site during commissioning of transformers and reactors and during planned annual shutdowns as a condition monitoring tool. This would help our customers to properly assess the condition of the equipment in service and take a logical decision on following possibilities. Transformer Services Dept./TCB/BHEL

3. The transformer is healthy in all respects and can be safely put back into service. • Possible minor winding movement is indicated by FRA results but the transformer is suitable for re-energization with close condition monitoring. • The transformer can be put back into service after physical inspection, minor repairs and satisfactory processing. • The transformer is damaged beyond repairs and can not be put back into service. • The above objective evaluation based on the FRA results will avoid forced shutdowns and ultimate damage to the equipment. • Features of FRA test system • FRA provides a reliable and sensitive means of detecting winding movement and other faults which after affect transformer inductances and capacitances • The test is easy to perform and the results are repeatable • FRA patterns called “fingerprints” are unique of a winding configuration • FRA measurement at site will help to make a logical decision on continual use of equipment in service Transformer Services Dept./TCB/BHEL

4. 1.Purpose Frequency response analysis (FRA) test is conducted on transformers & reactors to determine the frequency response of windings. The reference frequency responses obtained during laboratory testing serve as ‘fingerprints’ to monitor the condition of the transformer or reactor during service. 2.Reference and applicable standards The test is normally conducted before and after short-circuit test to assess the mechanical condition of the transformer during short-circuit test. In addition, the test is conducted as per customer specifications during final testing of transformer / reactor to develop reference ‘fingerprints’ data. 3.Principle The frequency response of an electrical winding is obtained by application of sweep frequency (sinusoidal). The winding will have a characteristic frequency response for the applied signal at different frequencies. The response is uniquely determined by the winding arrangement involved and any winding movement or other fault will modify the frequency response due to changes in inductances and capacitances. The sweep frequency voltage is applied through network analysers. The frequency response of the winding is determined between the frequency range of 10 Hz to 2 MHz. Transformer Services Dept./TCB/BHEL

5. 4.Procedures • 4.1 Winding connections • The FRA test is performed on one winding of the electrical equipment at a time. The transformer / reactor shall be electrically isolated from any other electrical connections or systems, including earth connections during FRA test. The two end terminals of each winding shall be made available for measuring the frequency response across the winding. # For star connected winding, the response shall be measured across the terminal & neutral. # For delta connected winding, the response shall be measured across two line terminals & in case of open-delta, across individual winding. # For auto connected winding, the response of series & common windings shall be measured separately. For a transformer, it is normal practice to earth one end of every winding that is not being tested, leave the other open end. Alternatively, all other windings may be left unconnected from each other and from earth. In every case, the termination of each winding for each test should be recorded. Transformer Services Dept./TCB/BHEL

6. 4.2 Test connections • The network analyser is connected directly across the winding under test. The sweep frequency sinusoidal signal of approximately 2 V(rms) at different frequency ranges is applied at one end of the winding and both source signal & transferred signal at the other end of the winding are measured. The frequency response of the winding is determined by plotting the ratio of the output from the winding to the input at following frequency ranges. • * 10 Hz to 2 KHz • * 100 Hz to 20 KHz • * 1 KHz to 200 KHz • * 10 KHz to 2 MHz • Alternatively frequency ranges specified by the customer can be selected. The amplitude of the applied signal is 10 dB. The test is conducted at maximum, mean and minimum taps, in case of winding having tappings. • While making measurements at mean tap, care taken to move the tap charger from higher voltage taps, for proper comparison of FRA results of different phases of same transformer or different transformer. Transformer Services Dept./TCB/BHEL

7. 4.3 Test records • Frequency response of each winding at different tap positions and frequency ranges are recorded in separate files. The test records consist of following. • Transformer / reactor details • Winding under test • Phase / winding connections • Tap position • Test arrangements • Date and time • Earthing arrangements • 5. Test results • The FRA results is analysed for • Changes in response of the winding • Difference between the FRA records of different phases of the same transformer. • FRA test is primarily a condition assessment test and can be used in conjunction with other diagnostic tests for detailed analysis and interpretation of the transformer. • 6. Test criteria The FRA test results obtained during final testing of transformer / reactor at work shall be used as reference ‘fingerprints’ for condition monitoring during service. Transformer Services Dept./TCB/BHEL

8. FRA PLOT OF 200 MVA GENERATOR TRANSFORMER Transformer Services Dept./TCB/BHEL

9. FRA PLOT OF 315 MVA AUTO TRANSFORMER FRA PLOT OF 250 MVA GENERATOR TRANSFORMER Transformer Services Dept./TCB/BHEL

10. FRA PLOT OF 80 MVA STATION TRANSFORMER FRA PLOT OF 50 MVAR SHUNT REACTOR Transformer Services Dept./TCB/BHEL

11. FRA PLOT OF 315 MVA HVDC FRA PLOTS OF 200 MVA TRANSFORMERS AT DIFFERENT TAPPINGS Transformer Services Dept./TCB/BHEL

12. FRA PLOTS OF THREE IDENTICAL 200 MVA TRANSFORMERS Transformer Services Dept./TCB/BHEL