Testing the transformers in your factory is essential to keeping your production running safe and efficiently. The following are 12 standard tests to check your transformers:
1. No-Load Losses
This test involves using two voltmeters to read the no-load losses at specific excitation voltages and frequencies. By connecting two voltmeters in parallel, one commonly being an average-responding RMS calibrated voltmeter and the other one being a true RMS-responding voltmeter. When these are connected, you can adjust the specific values that the average-responding voltmeter is reading and correct the no-load losses to a sine-wave. A sine-wave is most commonly used, but the waveform can be adjusted to match the need of the transformer.
2. No-Load Excitation Current
In this test, the winding that excites the transformer is measured for current while all other windings are left in an open circuit. This uses an RMS meter which reads the square root sum of the squares of the harmonic currents. We use this since the no-load excitation current is not sinusoidal, but rather has odd harmonics.
3. Load Losses and Impendance Voltage
For this test, the transformer must already be in a specific state before the load losses and impedance voltage are measured. The insulating liquid must be stabilized at a certain temperature and the difference in temperatures between the top and bottom oil must be less than 5 degrees Celsius. There are two methods for this test to measure the load losses and impedance: wattmeter-voltmeter-ammeter method and impedance bridge method.
4. Dielectric Tests
Applied-voltage tests and induced-voltage are used in these tests. Applied-voltage tests use high voltage through the winding around the entire transformer, but only testing one winding at a time while the rest are grounded. Typically a 60 Hz voltage is increased gradually over 15 second and then held there for 40 seconds before reducing it back down to zero over 5 seconds. Induced-voltage tests use a high voltage across one winding at a time while the rest are in open circuit. This tests the quality of the turn-to-turn insulation. To keep core saturation from happening at the higher excitation voltage, you increase the frequency of the induced-voltage test, usually to around 120 Hz. This is applied for 7200 cycles or 60 seconds, whichever takes a smaller amount of time.
5. Switching Impulse Test
By switching impulse waves between each high-voltage line terminal and ground, you can test the voltage of the transformer. The test involves one reduced voltage wave, around 50-70% of specified test level, then two full-voltage waves, either positive or negative polarity waves may be used. Throughout the test, a voltage oscillogram is taken for each applied wave. If there is no collapse in voltage, the test is successful.
6. Lighting Impulse Test
This test is done by using one reduced full wave, two chopped waves, then two full waves through the system. Tap connections are made using the smallest number of effective turns in the winding under tests and regulating transformers are set at the maximum buck position. The voltage is monitored with oscillograms of each wave. Then to analyze the graphs, you look for differences in the shapes of the reduced full wave and then the two full waves, which show turn-to-turn insulation failure.
7. Partial Discharge Test
Radio-frequency noise is detected during this test to find if the transformer is generating partial discharges within the voids in the insulation. An applied voltage increases gradually until the point when discharge begins to occur, then the voltage is decreased until the discharge stops. This final voltage needs to be less than the operating voltage of the transformer for it to be running properly.
8. Insulation Power Factor
This is the ratio of dissipated power to apparent power throughout the insulation, which is measured in watts while inputting a sinusoidal voltage. The test results are referenced to 20 degrees Celsius and are temperature corrected.
9. Insulation Resistance
A high voltage DC voltage is applied to one winding at a time meanwhile the other windings are grounded. This test measures the leakage current and then the insulation resistance is calculated using Ohm’s law.
10. Noise Measurement
This test takes place while the transformer is still energized at a rated voltage and with the cooling equipment running. The shape of the space in which the transformer is will also affect the test (an anechoic chamber is best), otherwise there cannot be any acoustic reflecting surface within 3m of the microphone. The microphones are 1 meter intervals around the perimeter of the transformer and at least four microphones for small transformers. The sound power levels are measures at specific band frequencies and converted into decibels.
11. Temperature Rise (Heat Run)
This test requires energizing the transformer to create core losses. Simultaneously, the windings of the transformer are attached to a loading transformer which is circulating rated currents through the windings. A large power transformer will consume up to 1 megawatt of total losses so the heat run test is expensive to perform.
12. Short-Circuit Test
This final test is usually used for sample transformers to confirm the design of a core and coil assembly. This low voltage impulse (LVI) current waveform is tested on the transformer before and after the applications of the short circuit test. Oscillograms of the voltage monitor the currents are checked for major changes in waveshape that could indicate damage to the windings.
So what tests and check ups can you do to ensure your transformers are running well and efficiently? See what PanelShop.com can offer you.
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