UNSYMMETRICAL FAULT CALCULATIONS – PART – 35 – SEQUENCE NETWORK OF TRANSFORMER

The presence of a fault at any point in a system means generally that the system is put into an unbalanced state of operation.

In unsymmetrical fault calculations, each piece of equipment will have three value of impedance offered by each sequence current.

The zero sequence network is a single-phase component and its existence is dependent upon a closed path that must be completed through reference ground.

1. For positive or negative sequence networks, the neutral of the generator is taken as the reference bus. The reason is the positive and negative sequence components represent balanced sets and hence all the neutral point must be at the same potential for positive or negative sequence currents.

2. If there is no ground return, zero sequence currents cannot flow and in the corresponding zero-sequence networks are considered an open circuit.

3. In star connected transformer if the neutral is grounded zero-sequence current can flow in the winding.

4. If the star point is ungrounded zero sequence currents cannot flow in the winding.

5. In a delta connected, there is no neutral, hence zero-sequence currents cannot flow in the line because there is no return path. However, zero-sequence currents can circulate in the legs of the delta connected winding.

6. Due to the presence of zero sequence voltages the zero-sequence currents flowing around a delta winding.

7. When the magnetizing current in a transformer is neglected, the primary ampere-turns balance the secondary ampere-turns and current can flow in the primary only if the there is a current in secondary.

8. If the neutral point of the star connection is grounded through an impedance Zn and impedance 3Zn appears in series with Z0 in the zero-sequence network.

9. The zero sequence impedance depends upon earth connection. If there is a through the circuit for earth current, zero-sequence impedance is equal to positive sequence impedance otherwise it will be infinite.

10. Positive sequence impedance = Negative sequence impedance 

zero sequence impedance = Positive sequence impedance if there is a circuit for earth current (infinite, if there is no circuit for earth current)