RESIDUAL LIFE ASSESSMENT OF GENERATOR TRANSFORMERS IN OLD HYDRO POWER PLANTS

Transcription

1 RESIDUAL LIFE ASSESSMENT OF GENERATOR TRANSFORMERS IN OLD HYDRO POWER PLANTS Authored by: Sanjay Srivastava, Chief Engineer (HE&RM), Rakesh Kumar, Director (HE&RM), R.K. Jayaswal, Dy. Director (HE&RM) & Amit Roy Singal, Asst. Director (HE&RM). Presented by: Rakesh Kumar, Director (HE&RM), CEA

2 INTRODUCTION Renovation, Modernization, Uprating and Life Extension (RMU&LE) has been recognized worldwide as a well proven cost effective technique for improving the performance, efficiency and reliability of old Hydro Power Plants (HPPs). Generator Transformer is one of the most important and costly component of power generation next only to the generator. Any discussion on the topic of Life Extension of the generating station will not be complete without discussing the ways and means of Life Extension of the Generator Transformers.

3 AGEING OF GENERATOR TRANSFORMERS Ageing of solid insulation materials like Paper, Press Board,Wood based laminates etc. Ageing and degradation of Insulating Oil. Wear and tear of moving parts like Tap Changing Mechanism, Pressure Relief Device, Monitoring equipments etc. Corrosion of metallic parts exposed to atmosphere. In addition to the above, accelerated ageing taking place in the transformer on account of the following abnormal operating conditions: Overloading of Transformers. Operation under overvoltage conditions. Impact of lightning/ switching surges from the system. Operation under short circuit conditions.

4 TESTS ON GENERATOR TRANSFORMERS Dissolved Gas Analysis (DGA) of transformer oil. Monitoring of water content in transformer oil. Monitoring of Capacitance and Tan-Delta values of Condenser Bushing. Partial Discharge (PD) measurement. Winding Resistance Measurement Percentage Impedance Measurement Insulation Resistance Test Furan Analysis Magnetizing Current Test Sweep Frequency Response Analysis (SFRA) DIRANA Test Harmonics Measurement

5 DISSOLVED GAS ANALYSIS (DGA) Number of gases are produced as by-products of the ageing taking place in the insulating materials 1. Hydrogen 5. Acetylene 2. Methane 6. Carbon Dioxide 3. Ethane 7. Carbon Monoxide 4. Ethylene 8. Oxygen

6 MECHANISM OF GAS GENERATION IN TRANSFORMERS Partial Discharge in Oil Large amount of H 2 and little CH 4 Arcing in Oil (Power Discharge) Large amount of H 2 and C 2 H 2 Oil Overheating CH 4, C 2 H 6, C 2 H 4 are formed in increasing order of severity Traces of C 2 H 2 in case of very high temperatures Cellulose Overheating by Electric Fault Large quantities of CO & CO 2

7 DISSOLVED GAS ANALYSIS (DGA) Method recommended by CIGRE Task force Gas Concentration Ratios Ratio Value Indication C 2 H 2 /C 2 H 6 H 2 / CH 4 CO 2 / CO C 2 H 2 / H 2 >1 >10 >10 <3 >2 (with C 2 H 2 >30) Discharge Partial discharge Cellulose overheating Cellulose degradation by electric fault Fault gases diffusing into main tank from OLTC

8 DISSOLVED GAS ANALYSIS (DGA) Method recommended by IEC (1999) Concentration ratio between gases Case Characteristic fault C 2 H 2 /C 2 H 4 CH 4 /H 2 C 2 H 4 /C 2 H 6 CO 2 /CO C 2 H 2 /H 2 O 2 /N 2 PD Partial discharges (*) < 0.1 < D1 Low energy discharges > > D2 High energy discharges > T1 Thermal fault (<300 o C) (*) (*) < T2 Thermal fault (300,700 o C) <0.1 >1 1, T3 Thermal fault (>700 o C) <0.2 >1 > Cellulose (paper) degradation by electric fault Fault gases diffusing into main tank from OLTC Excessive consumption of oxygen as a result ofoil oxidation and/or paper ageing < >2to <0.3 (*) Not significant Note: In some countries, the ratio CH 4 /H 2 is used instead of C 2 H 4 /C 2 H 6.

9 WATER CONTENT MEASUREMENT Hydrogen generated during ageing combines with the free oxygen present in the oil and forms water. A part of this is absorbed by the cellulose insulation and the balance remains as moisture in oil. As paper is highly hygroscopic in nature, when transformer is filled with oil, it absorbs the moisture from oil, which affects its insulation properties as well as reduces its life System Voltage of Transformer kv Water content ppm, max. Above Up to and including

10 TAN DELTA AND CAPACITANCE ON WINDINGS AND BUSHINGS This test is used to assess the condition of the oil and cellulose in terms of moisture, insulation, etc. The advantage of this method is that it identifies the winding (HV or LV) that has a possible problem. Dissipation factor of 0.5% is considered high whereas value of 1% is considered hazardous for operation of transformer and may require its replacement if completed its normal operating life. Capacitance value of bushing in service need to be compared with factory test result value for deciding the extent of degradation.

11 WINDING RESISTANCE MEASUREMENT Transformer winding resistance measurement is carried out to calculate the I 2 R losses and to calculate winding temperature at the end of a temperature rise test. Winding resistance result should not vary by 5% with factory test results or duplicate transformer result.

12 PERCENTAGE IMPEDANCE MEASUREMENT The percentage impedance of a transformer is the volt drop on full load due to the winding resistance and leakage reactance expressed as a percentage of the rated voltage. Test results must verify the nameplate rating. Percentage error should be within 3%.

13 INSULATION RESISTANCE TEST Insulation resistance tests are made to determine insulation resistance between individual windings to ground or between individual windings. It shows the condition of insulation.

14 POLARIZATION INDEX TEST It is a ratio of the Insulation Resistance Measured for 10 minutes to the insulation resistance value measured after 1 minute. PI is a good indication of winding contamination, moisture ingress (leakage currents), and/or bulk insulation damage (conduction currents). Acceptable PI for Power Transformers is greater than 1.5. For Distribution Transformer, it should be at least 1.3.

15 DEGREE OF POLYMERISATION (DP) Remnant life of a transformer in service depends mainly on the degree of degradation of the insulating materials DP is the average molecule size and closely relates to the mechanical properties of paper Early and precise diagnosis of degradation not possible through DGA. IEEE C57.91 loading guide mentions that one should be able to operate a transformer well dried, oxygen free, and with 110 deg C hotspot for 135,000 hours until 25% of retained tensile strength (75% loss), or 150,000 hours until a DP of 200 is reached (80% loss of tensile strength).

16 LIFE ASSESSMENT BY DP METHOD

17 Limitations DEGREE OF POLYMERISATION (DP) Test is invasive in nature. Sample can be taken only after taking the equipment out of service. Taking paper sample is a tedious job. Collected sample may not be representative of entire insulation.

18 FURAN ANALYSIS The concentration of furans in the mineral oil of a power transformer is a good indicator of the condition of the cellulose materials such as paper, pressboard and cotton tapes used for insulation. Not an oil quality test. Historically used as a major input for RLA of transformers. Furan analysis is used as a supplement along with DGA & DP tests Limiting value of total furanic content is Good Health : <3mg/Kg Satisfactory health : 3-5 mg/kg Deteriorated health : >5mg/Kg

19 MAGNETIZING CURRENT TEST Magnetizing current test of Transformer is performed to locate defects in the magnetic core structure, shifting of windings, failure in turn-to-turn insulation or problem in tap changers. Useful in distinguishing between severe and less severe cases. An agreement to within 25% of the measured exciting current with the previous test is usually considered satisfactory. If the measured exciting current value is 50% higher than the value measured during pre-commissioning checks, then the winding needs further analysis.

20 SWEEP FREQUENCY RESPONSE ANALYSIS (SFRA) This test passes a range of frequencies (between 10 Hz to 2 MHz) through the transformer and then calculates the transfer function. From these responses, the mechanical condition can be assessed. Reliable and sensitive method to evaluate the mechanical and electrical integrity of transformers active part. FRA is also carried out periodically for detection of deformation of smaller magnitude, when transformer experiences several short circuit forces. Helps in monitoring health and condition assessment of transformers.

21 DIELECTRIC RESPONSE ANALYSER (DIRANA) TEST DIRANA determines the water content in paper without the need of oil sampling. It combines Frequency Domain Spectroscopy (FDS) and Polarization Depolarization Current (PDC+) The power dissipation factor of the insulation is measured over a wide frequency ranging from the µhz to the khz region. The resulting curve contains information about the insulation condition. The very low frequencies contain information on moisture in the solid insulation, while the position of the slope in the mid-range frequencies indicates the conductivity of the liquid insulation. DIRANA is the only device which compensates the influence of ageing byproducts. Otherwise, this would lead to an overestimation of the water content in aged transformers.

22 HARMONICS MEASUREMENT The harmful effects of harmonic voltages and currents on transformer performance often go unnoticed until an actual failure occurs. Application of non-sinusoidal excitation voltages to transformers increase the iron losses in the magnetic core of the transformer. A more serious effect of harmonic loads served by transformers is due to an increase in eddy current losses. The increase in transformer eddy current loss due to harmonics has a significant effect on the operating temperature of the transformer.

23 Insulation section TYPICAL CASE STUDY Test Results RLA Study on 5.888MVA, 6.6/66kV Generator Transformer in about 40 Year Old Hydro Power Plant Insulation Resistance 60sec (G ) Polarization Index Tan (%) Moisture content (%) HV Winding versus LV Winding connected to Grounded Tank HV Winding versus LV Winding (Ungrounded) LV Winding versus HV Winding connected to Grounded Tank Oil Test DGA Furan Analysis Parameters within permissible limits High concentration of ethylene 4.8 mg/kg

24 OBSERVATIONS ON TEST RESULTS Insulation Resistance shows poor condition of insulation. PI values less than 1 are questionable. Limit of Tan Delta for aged transformer is 1%. Tan Delta values for this transformer are very poor. Results largely exceed normal limits as per service life and indicate over ageing of transformers. Moisture content shows that the transformer is in wet condition. The oil test results are normal and the oil parameters are within permissible limits as per IS 1866:2000. The DGA results exhibits high concentration of ethylene gas indicating possible overheating or thermal fault in the transformer. Furan analysis shows presence of high quantity (4.8 mg/kg) of furanic compounds. This shows poor health of paper insulation.

25 Thank You