The transformer draws a large current at the time of switching on of the transformer at no load. The large current drawn by the transformer is known as inrush current or magnetizing inrush current, The magnitude of the inrush current may be 8 to 10 times or even larger than this. The magnitude and duration of inrush current depends on the instant of the AC waveform at which the transformer is switched on. The waveform of the transformer inrush current is as given below.
The secondary current is zero and the primary current is high at the time of charging of the transformer. This difference in current between primary and secondary may lead to operation of differential protection relay. Thus, it is not possible to charge the transformer. If the protection relay is bypassed the differential protection of the transformer will get defeated. Bypassing the differential protection relay is not the solution.
If the relay senses the current of 100 Hz frequency and if the current is more than the differential current setting of the relay, the relay will observe that the transformer is taking the inrush magnetizing current and the relay blocks its main tripping mechanism. Therefore, the 2nd order harmonic current needs to be blocked in the differential relay because 2nd order harmonic current will operate under no fault within the transformer.
Thus, the harmonic current blocking is an important feature of the differential protection relay. In the harmonic restraint differential protection scheme, the harmonic restraint differential relay has the 2nd,3rd,5th order harmonic blocking circuit and the differential protection circuits.
If the inrush current contains predominant 2nd order harmonic current,it is symptom of transformer energization and differential protection is supposed to be blocked during normal transformer energization. If the ratio of the second harmonic current to the fundamental is greater than 2nd order harmonic current set point, the relay blocks the differential operation because relay sees this condition as transformer energisation.
The magnitude of 2nd order harmonic current in inrush current depends on transformer material, construction and remeanace. Therefore, the setting of the 2nd order harmonic blocking current can not be calculated in a straightforward way. The relay measures the ratio of 2nd order harmonic current and the fundamental current and the setting of the 2nd order harmonic blocking current can be done accordingly.
The setting of the 2nd order harmonic restraining is generally kept between 20-25 % of the differential relay setting. If the current is below the differential relay setting, the relay does not trip because the relay will observe it as a transformer charging phenomenon. If the harmonic restraining current is more than the differential relay setting, the relay will trip with differential protection fault.
During over fluxing of the transformer, the transformer current has predominant 5th order harmonic current. The over fluxing happens when there is sudden rise in voltage or sudden drop in the frequency. The measurement of 5th order harmonic current is very important for over excitation monitoring of the transformer. The magnitude of the 5th order harmonic current under over excitation depends on the magnetization curve of the transformer.
The differential protection relay will trip in case of the over fluxing. If the over fluxing happens for short period of time, the differential relay should not trip. Therefore, to prevent the operation of differential protection relay in case of the over fluxing, 5th order harmonic restraining is provided.The 5th order harmonic current varies as the voltage applied to transformer increase from its rated voltage.
If the magnitude of the 5th order harmonic current is more than the the setting of the differential protection relay setting the relay will trip. If the magnitude of the 5th order harmonic current is less than the the setting of the differential protection relay setting the relay will not trip. The setting of the 5th order harmonic restraining is generally kept between 20- 25 % of the differential relay setting.
The operation of transformer is not possible without having the harmonic restraint in differential protection.
The basic circuit of harmonic restraint relay is as given below.
“If the transformer has been out of use for some time (i.e. after storage) its remanence may be very small, causing the 2nd harmonic blocking not to operate at the first energizing attempt. Therefore, the setting could be lowered to 10% for the first energizing attempt.”