Reliable and continual performance of power transformers is the key to beneficial generation and transmission of electric power.
Generally, reasons for failure include external factors such as lightning strikes, system overload, short circuits, and internal factors such as insulation deterioration, winding failure, overheating, and the presence of oxygen, moisture, and solids in the transformer oil.
To minimize unexpected outages, periodic transformer testing and diagnostics is necessary.
Three categories can be defined for transformer testing:
- Performance acceptance test after installation and commissioning of the transformer.
- Predictive maintenance plan-based test during normal operation of the transformer to verify that electrical properties have not changed from design specifications.
- Failure test for identify breakdown cause of the transformer.
These tests are required to determine electrical, thermal, and mechanical characteristics.
A daily checklist procedure should be established to perform the visual routine test. It should contain oil temperature, winding temperature, oil level, humming (noisy operation), and oil leakage checks. An annunciation window (an indicator that announces which electrical circuit has been active) displays alarm and trip signals generated from the load.
A Buchholz relay is a safety device normally mounted at the middle of the pipe connecting the transformer tank to the conservator. It is a gas detection relay used to detect minor and major faults in the transformer. A Buchholz relay operates by detecting the volume of gas generated in the transformer tank. Gas produced by faults accumulates over time within the relay chamber. Whenever the volume of gas exceeds a certain safe level, the float moves lower, closes the contact, and generates an alarm. The fault alarm can be displayed on an annunciation window and the master trip relay will cause the circuit breaker to open.
Thermal Imaging (Thermography)
Thermal imagers capture images of infrared energy or temperature. They can detect heat patterns or temperature changes in equipment. The engineer can determine problems prior to an expensive down time by analyzing these temperature changes. Conveniently, one can measure and compare heat readings for each part of the equipment without disrupting the transformer's operation.
Prevention, diagnosis, and repair benefits can be obtained for transformers by introducing Infrared thermography into your predictive maintenance plan.
Insulation Resistance Test
Insulation ages and deteriorates because of moisture, dust, and electrostatic stress. Insulation should be monitored continually to avoid sudden failure of the equipment.
An insulation resistance test detects insulation quality within the transformer. The conductive impurities or mechanical flaws in the dielectric can be analysis based on this test. The instrument used to measure insulation resistance is known as the "megger." Normally meggers have a test voltage of 500V, 2500V, or 5000V.
Each winding should be short circuited at the bushing terminals. The resistance value should be measured between each winding and with respect to ground also. The winding should be discharged after the test is completed by connecting to the ground.
The insulation resistance value measured is usually in the order of mega-ohms. Generally the value should be greater than 1 megohm for every 1kV rating of the equipment.
Insulation resistance values decrease with increase in the temperature. Therefore the values should be normalized for a standard temperature. It is necessary to have the insulation resistance as high as possible.
Transformer Turns Ratio Test
Each winding of a transformer contains a certain number of turns of wire. The "transformer turns ratio" is the ratio of the number of turns in the high voltage winding to that in the low voltage winding. The ratio is calculated under no-load conditions.
The transformer ratio can change due to several factors like physical damage because of faults, deteriorated insulation, contamination of oil etc. If a transformer ratio changes more than 0.5 percent from the rated voltage ratio, it needs immediate attention.
The turns ratio will establish the proper relationship between the primary and secondary winding impedances. The turns ratio is the square root of the impedance ratio, i.e.
iZpri/Zsec = (Npri/Nsec)2
Zpri = Primary Impedance
Zsec = Secondary Impedance
Npri = Number of turns on the primary coil
Nsec = Number of turns on the secondary coil
Dissolved Gas Analysis (DGA)
Transformer overloading, overheating, corona, sparking, and arcing can cause thermal degradation of the oil and paper insulation within the tank. Thermal and electrical faults can accelerate the decomposition of dielectric fluid and solid insulation. Gases generated by this process include hydrogen, methane, ethane, acetylene, carbon monoxide, and carbon dioxide, all which will dissolve in the transformer oil.
The DGA test involves extracting the gases from the oil and injecting it into a gas chromatograph. Gas concentrations are detected using a flame ionization detector and a thermal conductivity detector.
Diagnostic and analysis of the specific proportions of each gas shall help to identify the fault type (thermal conditions involving the oil or the paper, partial discharge, sustained arcing, etc.).
A DGA test study can minimize damage by taking precautionary actions at an early stage.
Magnetic Balance Test
The magnetic balance test is conducted on transformers to detect inter-turn faults and magnetic imbalance. It gives an idea about the flux distribution in the core. It is only an indicative test and does not reduce the need for other tests in diagnostics.
The magnetic balance test is carried out on a three phase transformer by applying a two phase supply across the phases (i.e. one winding say U and V) with a lower than rated voltage. Other phases should be kept open circuit. The sum of voltage measured between V-W and U-W should be equal to U-V. A voltage measured in the secondary side will also be proportional to the voltage measured at the primary.
A very low voltage will induce in defected winding because it will not allow flux to pass in the magnetic path around the core. It may result in the sum of the two voltages not being equal to the applied voltage.
Tan Delta Test
Degradation of the insulation takes place because of mechanical vibration, over temperature operation, and gaseous and metallic impurities in the transformer. This may cause insulation ageing and breakdown. It is very important to study the insulation quality of the machine. The dissipation factor Tan or Power Factor Cos Ø is considered to indicate the quality of insulation. It is also known as the loss angle test or the dissipation factor test.
A clean insulation acts as a capacitor. The current should lead the voltage by 90 degrees in a pure capacitor. The pure insulation should also conduct similarly. If the insulation is deteriorated, the current will also have resistive factor. This will cause the angle of the current to be less than 90 degrees. This measured difference in the angle is described as the loss angle. The tangent of the angle (i.e. opposite/adjacent) indicates the condition of the insulation. A greater loss angle value points to a high contamination of the insulation.
Transformer Oil Break Down Test
The BDV test measures the dielectric strength of the oil using an oil tester. During this test, spherical electrodes having a 2.5 mm gap shall be gradually applied voltage until the oil loses its dielectric strength. This test should be performed for one minute, and the breakdown voltage displayed on the oil tester meter should be considered the BDV. Normally it may be 60 kV and over for one minute or as per ASTM D877-82 or IS-335.
A low value in this test indicates the presence of impurities in the oil. In this case it should be filtered to remove impurities and moisture.
Followings are other tests that can be used to detect oil based faults:
- Acidity test
- Electric strength test
- Color test
- Polychlorinated Biphenyl Analysis (PCB) test
- Fiber estimation
- Furfuraldehyde analysis test
- Metal in oil test
- Resistivity test
- Furan analysis
- Frequency Response Analysis
- Author's own experience.
- How to Test Insulation Resistance
- Pacific crest transformer resource center: http://www.pacificcresttrans.com/resource-center/21/Transformer-Faults-and-Detection.html
- Can an Open-Delta Connection Provide True Three-Phase with Two Transformers?
- Power transformer testing :http://www.newdelhilab.com/transformers/power-transformers