Testing of Transformers is used to find the circuit parameters which are heavily dependent upon from the analysis of the equivalent circuit one can determine the electrical parameters. But if the temperature rise of the transformer is required, then the test method is the most dependable one. There are several tests that can be done on the transformer; however a few common ones are discussed here.
The performance parameters of interest can be obtained by solving that circuit for any load conditions. The equivalent circuit parameters are available to the designer of the transformers from the various expressions that he uses for designing the transformers.
1. Winding resistance test
Resistance measurement of the windings by applying a small d.c voltage to the winding and measuring the current through the same. The ratio gives the winding resistance, more commonly feasible with high voltage windings. For low voltage windings a resistance-bridge method can be used. From the d.c resistance one can get the a.c. resistance by applying skin effect corrections.
2. Polarity Test
Polarity test is used for identifying the primary and secondary phasor polarities. Both a.c. and d.c methods can be used for detecting the polarities of the induced emf. The transformer is connected to a low voltage a.c. source with the connections made as shown in the fig
A supply voltage Vs is applied to the primary and the readings of the voltmeters V1 , V2 and V3 are noted. V1 : V2 gives the turns ratio. If V3 reads V1 – V2 then assumed dot locations are correct (for the connection shown). The beginning and end of the primary and secondary may then be marked by A1 – A2 and a1 – a2 respectively.
If the voltage rises from A1 – A2 in the primary, at any instant it does so from a1 – a2 in the secondary. When the switch S is closed if the secondary voltage shows a positive reading, with a moving coil meter, the assumed polarity is correct. If the meter kicks back the assumed polarity is wrong.
3. Open Circuit Test
In the open-circuit test, the secondary winding of the transformer is open-circuited and the nominal value of the input voltage is applied to the primary winding, and the input current and power are measured. Where V, A, W are the voltmeter, ammeter, and wattmeter respectively.
Let these meters read V1 , Io and Wo respectively. The no-load current at rated voltage is less than 1 percent of nominal current and hence the loss and drop that take place in primary impedance r1 + jxl1 due to the no-load current Io is negligible.
The active component Ic of the no-load current Io represents the core losses and reactive current Im is the current needed for the magnetization. Thus the wattmeter reading are
Open Circuit Characteristics
This graph is obtained by noting the current drawn by the transformer at different applied voltage, keeping the secondary open-circuited. The usual operating point selected for operation lies at some standard voltage around the knee point of the characteristic. After this value is chosen as the nominal value the parameters are calculated as mentioned above.
Sometimes the primary voltage required may be in kilo-Volts and it may not be feasible to apply nominal voltage to primary from the point of safety to personnel and equipment. If the secondary voltage is low, one can perform the test with a low voltage(LV) side energized keeping the high voltage (HV) side open-circuited. In this case the parameters that are obtained are in terms of LV. These have to be referred to the HV side if we need the equivalent circuit referred to the HV side.
4. Short Circuit Test
The short circuit test of the transformer is to determine the series branch parameters of the equivalent circuit. In this test primary applied voltage, the current and power input are measured keeping the secondary terminals short-circuited. Let these values be Vsc , Isc and Wsc respectively.
The supply voltage required to circulate rated current through the transformer is usually very small and is of the order of a few percent of the nominal voltage. The excitation current which is only 1 percent or less even at rated voltage becomes negligibly small during this test and hence is neglected.
The shunt branch is thus assumed to be absent. Also I1 = I2’ as Io ≃ 0. Therefore Wsc is the sum of the copper losses in primary and secondary put together. The reactive power consumed is absorbed by the leakage reactance of the two windings.
Wscis the sum of the copper losses in primary and secondary put together. The reactive power consumed is absorbed by the leakage reactance of the two windings.
However if the exact equivalent circuit is needed then either r1 or r2’ is determined from the resistance measurement and the other separated from the total. As for the separation of xl1 and xl2’ is concerned, they are assumed to be equal. This is a fairly valid assumption for many types of transformer windings as the leakage flux paths are through air and are similar.
5. Load Test or Back to Back or Phantom loading Test
Load Test used to determine the total loss that takes place, when the transformer is loaded.This test is used mainly to determine
- Rated load of the machine and the temperature rise
- Voltage regulation and efficiency of the transformer
Rated load is determined by loading the transformer on a continuous basis and observing the steady-state temperature rise. The losses that are generated inside the transformer on load appear as heat. This heats the transformer and the temperature of the transformer increases. The insulation of the transformer is the one to get affected by this rise in the temperature. Both paper and oil which are used for insulation in the transformer start get- ting degenerated and get decomposed.
If the flashpoint of the oil is reached the transformer goes up in flames. Hence to have a reasonable life expectancy the loading of the transformer must be limited to that value which gives the maximum temperature rise tolerated by the insulation.
Equivalent loss methods of loading and Phantom loading are commonly used in the case of transformers. The load is applied and held constant until the temperature rise of the transformer reaches a steady value. If the final steady temperature rise is lower than the maximum permissible value, then the load can be increased else it is decreased.
In Phantom loading method two identical transformers are needed. The windings are connected back to back as shown in Fig. Suitable voltage is injected into the loop formed by the two secondaries such that full load current passes through them. An equivalent current then passes through the primary also.
The voltage source V1 supplies the magnetizing current and core losses for the two transformers. The second source supplies the load component of the current and losses due to the same. There is no power wasted in a load and hence the name Phantom or virtual loading. The power absorbed by the second transformer which acts as a load is pushed back into the mains. The two sources put together to meet the core and copper losses of the two transformers. The transformers work with full flux drawing full load currents and hence are closest to the actual loading condition with a physical load.