Transformer Overflux Protection is provided to protect the Transformer core from overfluxing. A Transformer is designed to operate at a particular flux level. In case the flux in the core of Transformer exceeds a certain level, the core loss increases which may lead to overheating of components which in turn may result into internal fault. Therefore, overflux protection is provided.
A transformer is designed to operate at or below a maximum magnetic flux density in the transformer core. Above this design limit the eddy currents in the core and nearby conductive components cause overheating which within a very short time may cause severe damage. The magnetic flux in the core is proportional to the voltage applied to the winding divided by the impedance of the winding. The flux in the core increases with either increasing voltage or decreasing frequency. During start-up or shutdown of generator-connected transformers, or following a load rejection, the transformer may experience an excessive ratio of Volts to Hertz (V/f), that is, become overexcited. When a transformer core is overexcited, the core is operating in a non-linear magnetic region, and creates harmonic components in the exciting current. A significant amount of current at the 5th harmonic is characteristic of overexcitation.
Assuming Number of turns constant, Flux is directly proportional to V/f. Here V is supply voltage and f is frequency of supply.
In case of any Transformer, signal for supply voltage V is taken from PT. Let us assume that Transformer Primary is connected with 220 kV. Thus normal voltage of primary of Transformer will be 220 kV at a frequency of 50 Hz. Also assume that the PT ratio is 220 kV/110 V.
V/f ratio = 110/50 = 2.2
Thus at a V/f ratio of 2.2 the Transformer will operate satisfactorily. So the question arises which V/f ratio may cause the overfluxing. For answering this we need to have a look at the Hysteresis curve of the core material and from the curve we can judge at which flux level Transformer can be subject for a particular time safely.
Normally the setting of overfluxing is kept 110% of nominal value or 1.1 pu. This means at a flux level of 1.1×2.2 = 2.42 the Transformer will operate safely but above 2.42 the Transformer core will be subjected to overflux.
Does this mean that at a V/f ratio of 2.5 Transformer shall be tripped instantaneously? No it doesn’t mean so. Because Transformer core may tolerate such an overflux for some short time duration and hence instantaneous tripping is not required. Therefore, wise decision is to give an INVERSE characteristics to the tripping which mean more the ratio of V/f less will be time of tripping.
Now we consider two cases:
Case1: Transformer Primary voltage rises to 247 kV while frequency is 50.1 Hz
As primary of Transformer rises to 247 kV at a frequency f = 50.1 Hz
The PT secondary Voltage = 247×110/220 = 123.5 V
Hence, V / f = 123.5/50.1 = 2.465
Thus the Relay will pick-up and as the characteristics is inverse, the relay will trip after some time because we have kept the setting 2.42. If the Primary Voltage is maintaining at 247 kV , then we can do nothing and the Relay will definitely trip.
Case2: Transformer is provide with Tap Changer
Suppose the Transformer is provided with Tap Changer. As the Transformer is provided with Tap Changer in the primary side, we can increase the Tap position from the nominal value which will result in increase in the value of N1 (Primary number of turns) and hence,
But this is not going to help us as we have taken the voltage signal from the PT which is connected to the Primary side i.e. and primary side voltage is maintained at 247 kV, hence V/f will be same.
Thus we observe that, even though we have Tap Changer, in the present scenario we can do nothing to prevent tripping of Transformer on overfluxing though the Transformer is not actually in overflux condition (as we have increased the number of turns in the primary side.)
Therefore, to take advantage of Tap Changer, we can make a provision of taking voltage signal from the secondary side PT of Transformer Relay. In such case, if the primary turn of Transformer is increased then its reflection on secondary side PT will be observed proportionally and tripping on Overflux protection can be prevented.
In case of no load operation of Transformer, we can give voltage signal to the Relay from the Primary side PT.
In this way the purpose will be served without compromising the overflux protection. Thus we see, how important is tap changer in preventing tripping of Transformer from Overfluxing.