So, why the open circuit tests and short circuit tests are preferred over the direct loading method? In acutal practice, the load test, polarity test, winding resistance test is sufficient for smaller rated transformers. But additionally the short circuit and open circuit tests are necessary for high voltage power transformers in orderto design those transformers for most profitable operation. When the short circuit (phase of ground or phase to phase) occurs in those large rated transformers, huge amount of current is subjected to flow through the transformers windings. This huge current will cause a winding damage if the winding resistances and reactances are not designed accurately to handle the massive heat dissipitation. If the short circuit test is conducted with the grid power and acutal load in secondary, there will effective power disipation, which is nothing but a wastage. Therefore, short circuit test provides the virtual environment of the short circuit condition, without any risk or power wastage.
Open Circuit Test of Transformer
The open circuit test of transformer is also called as no load test of transformer. This test gives the shunt branch parameters of a transformer. A voltmeter, an ammeter and a wattmeter is required as the measuring devices to perform the test. Open circuit refers the situation of a circuit where the two terminals are kept open, that is no physical wired connection will be there. So there will be an infinite resistance in between the two open circuited terminals. In transformer open circuit test, the high voltage side (HV side may be the primary or the secondary) is kept at open circuited condition. In some other words, there will be no load in the high voltage side of the transformer. Sometime a additional voltmeter is connected with the high voltage side (as we know that the internal resistance of a voltmeter is so high, that it may considered as infinite). The voltmeter of HV side shows the open circuit voltage on HV side of the transformer. See the following connection diagram of open circuit test.
The ammeter is connected in series with the low voltage side and the voltmeter is connected in parallel with the low voltage side. The wattmeter is also connected acoordingly with the low voltage side. Then a variable AC voltage is applied to the low voltage side of the transformer with the help of an AC variac. The supply frequency will be the rated one and by maintaining this constant frequency, the applied voltage is increased slowly by moving the variac jocky. After sometime, the applied voltage reached to the rated LV side rated voltage. So, the voltmeter which is connected in parallel gives the reading of the rated primary voltage. As there is no distribution loads are connected with the seconday, so very small current flows through the LV side, which is termed as no load current and abbreviated as INL.
Calculation For Open Circuit Test
As the ammeter is connected in series with the LV side, therefore it gives the reading of the no load current of LV side. The resistance of the winding is constant, so the copper loss that is the I2 loss is depended upon the current flowing through the winding. So, there will be no copper loss in the HV side as no curent is there due to the open circuited terminals. But a small no load current will flow through the LV side, so a negligable copper loss will occur in the LV side of the transformer. But the iron loss is depended upon the flux generated. As rated voltage in rated frequency is applied into the LV terminals, so the normal core loss will occur in the LV side. Therefore, the wattmeter measures only the iron core loss of transformer. Suppose, the applied rated voltage in LV side is V1 and the wattmeter reading is Pw.
So, Pw = V1INLcosφNL
Therefore, the no load power factor is cosφNL = Pw / V1INL.
Short Circuit Test of Transformer
The short circuit test is also termed as full load test. This test gives the full load circuit parameters and series branch parameters of the transformer. The required measuring instruments are volmeter, ammeter and wattmeter. The term short circuit means providing a very low resistance path in between two circuit terminals. So, as the name suggests, in short circuit test of transformer, any of the side must be kept in short circuited condition to perform the test. Generally, the low voltage side is short circuited to avoid the risk and power wastages associated with high voltage power. A thick conductor is used to provide the short circuit path, or sometime an ammeter is connected additionally (as ammeter provides the least resistance). The voltmeter is connected in parallel with the HV side and the ammeter is connected in series with the HV side. And the wattmeter is connected accordingly with the HV side.
The high voltage side of the transformer is fed from a variable AC voltage source. The applied voltage must be very smaller as compared to the rated voltage of the HV side. The applied voltage is increased very slowly until the full load primary current flows (short circuit current ISC) through the winding. As soon as the current reached into the full load value, the secondary current will also reached into the full load value. Then the meter readings are noted. The voltmeter shows the value of voltage at which the full load current starts flowing through both of the LV and HV windings. The ammeter shows the full load primary current of the transformer. The windings are subjected to full load current, so the copper loss that is the I2R loss is occured into both windings. But the applied voltage is the HV terminals is very small as compared to the rated voltage of the transformer, and in this way the generated flux φ is quite low. And there is no LV voltage due to the short circuited LV terminals. Therefore the core loss, with is depended upon the generated flux is very small and can be neglected. So, the wattmeter gives the reading of the copper loss of the transformer.
Calculation For Short Circuit Test
As there is no voltage in LV side, so the total applied voltage is used to supply the whole transformer impedance with respect to the primary side. The applied voltage V1 = ISCZe1.
Therefore, the equivalent impedance referred to primary, Ze1 = primary voltage / ISC
And the wattmeter reading PSC = I2Re1
So, the equivalent resistance referred to primary is Re1 = PSC / I2
And full load power factor cos φSC = Re1 / Z e1
The equivalent resistance of the other side is Re1 / a2 and the equivalent reactance of the other side is Ze1 / a2, where a is the transformation ratio of the transformer. In this way, all equivalent resistances and impedances are derived from the short circuit test which are further used to develop the equivalent circuit of the transformer.
Why Short Circuit Test is done on the HV side?
There are some specific reasons for which the short circuit measurements are done on the HV side, by shorting the LV side. They are:
- The transformer KVA is constant for the both LV and HV side. In short circuit test we have to measure the series parameter, so we need the rated current flow in the winding. As the total KVA is constant in both side, so in LV side the rated current will more (Low Voltage so High current) as compared to the HV side. So, in HV side, it is more easy to get the accurate measurements with lesser rated current. That is the main reason for which the LV side is shorted and the test is done on the HV side.
- The appled voltage on the HV side is very less as compared to the rated HV side voltage. Therefore better accuracy is acheived, when the voltmeter shows the reading of the HV voltage.
- Short circuit of HV side is not very easy, as this winding carries a huge amount of rated voltage. If the winding is shorted, due to the low resistance path the current flow will be very high which is dangerous for the copper winding. But when the LV side is shorted, the increase of the current flow is bounded in a safe limit, due to the reduced voltage AC supply on HV side. So the short circuit test is usually done on the HV side by shorting the LV side.