**3.3. False trip due to tap changer**

224 MATLAB – A Fundamental Tool for Scientific Computing and Engineering Applications – Volume 1

Generally, three main difficulties handicap the conventional differential protection. They induce the differential relay to release a false trip signal without the existing of any fault. These complications must be overcome in order to make the differential relay working

This phenomenon, the transient magnetizing inrush or the exciting current, occurs in the primary side of the transformer whenever the transformer is switched on (energized) and the instantaneous value of the voltage is not at 90�. At this time, the first peak of the flux wave is higher than the peak of the flux at the steady state condition. This current appears as an internal fault, and it is sensed as a differential current by the differential relay. The value of the first peak of the magnetizing current may be as high as several times the peak of the full load current. The magnitude and duration of the magnetizing inrush current is

The instantaneous value of the voltage waveform at the moment of closing CB,

The effect of the inrush current on the differential relay is false tripping the transformer without of any existing type of faults. From the principle of operation of the differential relay, the relay compares the currents coming from both sides of the power transformer as explained above. However, the inrush current is flowing only in the primary side of the power transformer. So that, the differential current will have a significant value due to the existence of current in only one side. Therefore, the relay has to be designed to recognize

The performance of the differential relays depends on the accuracy of the CTs in reproducing their primary currents in their secondary side. In many cases, the primary

**3. Differential protection difficulties** 

CTs Mismatch and saturation,

**3.1. Magnetizing inrush current** 

Magnetizing inrush current during initial energization,

influenced by many factors, some of these factors are [2], [6], [7];

The type of the iron laminations used in the transformer core,

The maximum flux-carrying capability of the iron core laminations,

that this current is a normal phenomenon and to not trip due to this current.

The value of the residual (remnant) magnetizing flux,

The saturation flux density of the transformer core,

The sign of the residual magnetizing flux,

 The total impedance of the supply circuit, The physical size of the transformer,

**3.2. False trip due to C.T characteristics** 

The input supply voltage level,

Transformation ratio changes due to Tap changer.

properly [2], [3]:

On-Load Tap-Changer (OLTC) is installed on the power transformer to control automatically the transformer output voltage. This device is required wherever there are heavy fluctuations in the power system voltage. The transformation ratio of the CTs can be matched with only one point of the tap-changing range. Therefore, if the OLTC is changed, unbalance current flows in the differential relay operating coil. This action causes CTs mismatches. This current will be considered as a fault current which makes the relay to release a trip signal [11], [12].
