Because there are decreased voltages required, the no-load size is carried out on the low-voltage facet of the transformer underneath the test. Built-in cutting-edge transformers need to be shorted all through the check and condenser bushing faucets need to be earthed. Before carrying out the no-load loss test, the voltage ratio has to be checked. For oil transformers, the bushings and Buchholz relay have to be vented and the oil stage of the transformer needs to be checked. Before the proper loss measurements take vicinity the transformer needs to be excited by way of 1,1 to 1,15 instances of rated voltage. The over-excitation reduces the consequences of remanence brought about by way of direct cutting-edge excitation throughout the resistance dimension or from the Switching impulse test. The genuine loss dimension can't start until the measured figures are not steady.
Switching impulses as well as lightning impulses have standardized wave shapes. To prevent flashovers on the air-side of the transformer bushings, the polarity of the impulse voltages is generally negative. Because of their unusual aperiodic waveform transient voltages cannot be generated perfectly.
Preparation
Before conducting the impulse tests (Lightning- and Switching impulse), the transformer will be checked for the following points:
standing time
confirming measurement of voltage ratio, polarity and winding resistance
oil level and quality
vent the Buchholz relay and porcelain bushings
short-circuit and earth any built-in transformers
earth the capacitive bushing taps
check on-load or off-load tap changer positions
Test Steps
The actual impulse tests:
1. calibrating and verifying the impulse voltage waveform
2. applying the impulse voltage to the transformer under test (see test sequence)
3. verifying that the transformer under test had withstood the stresses without damage (by comparing oscillographic recordings)
The switching and lightning impulse tests are successful if the oscillograph recordings do not show any changes (close similarity in waveform; between calibrated voltage and applied test voltage). If the waveforms deviate from each other, the reasons must be determined.