For advance test of potential transformer, Risatti makes a full range of machines for electrical transformers, single and three-phase, toroidal transformers, in their many forms, both in terms of power and voltage and production volumes.
In fact, the equipment ranges from simple tests for small transformers on single stations or alternating double ones, to complex highly automated tests with the application of robots and laser marking systems for high production volumes.
The case of tests on high power or high voltage transformers is different where, given the low number of pieces produced, the tests are mainly individual and have very distinctive features.
Advance testing
Polarity Test of Potential Transformer
This test is used in determine the direction of current flow in primary and secondary winding of potential transformer.
Apply sine waveform of maximum amplitude from signal generator at the primary winding of transformer and observe the waveform of voltage at primary side and voltage waveform at secondary side using Two channel CRO.
If waveform of both channels are same then polarity is correct otherwise polarity will be opposite.
Excitation Test of Potential Transformer
This Test can be performed on without connected Load. If a transformer ratio is 220:5V Apply 50 V at primary side of transformer and note down the value of current flowing through primary while secondary winding is open circuit.
Increase the primary voltage from variac in step of 50V and note down the value of Primary current.
As relationship between primary voltage and primary current is linear then it shows that transformer core is not saturated. As relationship between voltage and current at primary side is going to non-linearity then transformer core shall saturated.
Insulation Resistance Test of Potential Transformer
- To check the insulation resistance between primary winding and secondary winding, connect one terminal of Meter with primary winding one terminal and second terminal of Meter with secondary winding one terminal.
- checking the insulation resistance between primary winding and core of transformer,connect one terminal of Meter with Primary winding one Terminal and second terminal of Meter with core of Transformer.
- To check the insulation resistance between secondary winding and core of transformer,connect one terminal of Meter with secondary winding one Terminal and second terminal of Meter with core of Transformer.
Ratio-Verification Test of Potential Transformer
PT ratio is described as the ratio of primary voltage input to secondary voltage output at full load. For example, a PT with a ratio of 220:5V will produce 5V of secondary voltage when 220 voltages applied at the primary terminal.
If the primary voltage changes, the secondary voltage output will change accordingly. For example, if 100 voltages is applied at Primary terminal instead of 220 voltage, the output voltage shall be 2.27.
The relationship between voltage ratio and turn ratio is shown in this formula.
Short circuit Test of Potential Transformer:
The Short circuit Test is used to determine the copper losses of Potential transformer.
The LV side of transformer is short circuited and wattmeter (W), voltmere (V) and ammeter (A) are connected on the HV side of the transformer.
Voltage is applied to the HV side and increased from the zero until the ammeter reading equals the rated current. All the readings are taken at this rated current.
The ammeter reading gives primary equivalent of full load current (Isc).
The voltage applied for full load current is very small as compared to rated voltage. Hence, core loss due to small applied voltage can be neglected.
Thus, the wattmeter reading can be taken as copper loss in the transformer.
