Page 276 - Practical Power System and Protective Relays Commissioning

270 Practical Power System and Protective Relays Commissioning

Abnormal operating conditions, such as over voltages, over excitation, or
overloading;
Uncleared external faults;
Transformer connections.

Group 1 0 degrees Phase displacement Y y0 , D d0 , Z d0
Group 2 180 degrees Phase displacement Y d6 , D d6 , D z6
Group 3 30 degrees Lag phase displacement Y d1 , D y1 , Y z1
Group 4 30 degrees Lead phase displacement Y d11 , D y11 , Y z11

Differential protection is not enough to protect the transformer against
earth faults, especially in solid-earthed systems, and there is a need to have
separate earth fault protection.

18.12.1.1 Transformer Inrush Current
When the transformer is first energized a transient current follows, which
may reach instantaneous peaks of 8 30 times the transformer full load
current.
Factors affecting the duration and magnitude of inrush current are as
follows:

Size of the transformer bank;
Size of the power system;
Resistance in the power system from the source to the transformer bank;
Residual flux level;
Type of iron used for the core and its saturation level.
There are three conditions which can produce a magnetizing inrush
effect:

1. First energization;
2. Voltage recovery following external fault clearance;
3. Inrush current due to a parallel transformer being energized.
Inrush current can be detected by the second harmonic contents, which
are high in inrush current and low in fault current so we can detect the
second harmonic in inrush current and block the operation of differential
protection at the starting of transformer energization.
Another method used to detect the inrush current is the zero detection in
current waveform as the inrush current has a significant period in each cycle
where the current is substantially zero, but the fault current passes through
the zero point very quickly.