The analysis of an unsymmetrical fault by symmetrical components method can be conveniently done by drawing sequence networks. A Sequence Networks Unsymmetrical Faults of a particular sequence current in a given power system is the path for the flow of that sequence current in the system. It is composed of impedances offered to that sequence current in the system. Since there are three sequence currents (viz. positive sequence current, negative sequence current and zero sequence current), there will be three sequence networks for a given power system, namely ;
- Positive sequence network
- Negative sequence network
- Zero sequence network
1. Positive sequence network: The positive sequence network for a given power system shows all the paths for the flow of positive sequence currents in the system. It is represented by one-line diagram and is composed of impedances offered to the positive sequence currents. While drawing the positive sequence network of a given power system, the following points may be kept in view:
- Each generator in the system is represented by the generated voltage in series with appropriate reactance and resistance.
- Current limiting impedances between the generator’s neutral and ground pass no positive sequence current and hence are not included in the positive sequence network.
- All resistances and magnetising currents for each transformer are neglected as a matter of
- For transmission lines, the shunt capacitances and resistances are generally neglected.
- Motor loads are included in the network as generated e.m.f. in series with appropriate
2. Negative sequence network: The negative sequence network for a given power system shows all the paths for the flow of negative sequence currents in the system. It is also represented by one line diagram and is composed of impedances offered to the negative sequence currents. The negative sequence network can be readily obtained from positive sequence network with the following modifications :
- Omit the e.m.fs. of 3-phase generators and motors in the positive sequence network. It is because these devices have only positive sequence-generated voltages.
- Change, if necessary, the impedances that represent rotating machinery in the positive sequence network. It is because negative sequence impedance of rotating machinety is generally different from that of positive sequence impedance.
- Current limiting impedances between generator’s neutral and ground pass no negative sequence current and hence are not included in the negative Sequence Networks Unsymmetrical Faults.
- For static devices such as transmission lines and transformers, the negative sequence impedances have the same value as the corresponding positive sequence impedances.
3. Zero sequence network: The zero sequence network for a given power system shows all the paths for the flow of zero sequence currents. The zero sequence network of a system depends upon the nature of connections of the 3-phase windings of the components in the system. The following points may be noted about zero Sequence Networks Unsymmetrical Faults:
- The zero sequence currents will flow only if there is a return path e. path from neutral to ground or to another neutral point in the circuit.
- In the case of a system with no return path for zero sequence currents, these currents cannot exist.
Reference Bus for Sequence Networks:
While drawing the sequence networks, it is necessary to specify the reference potential 11).1:t. which all sequence voltage drops are to be taken. For this purpose, the reader may keep in mind the following points :
- For positive or negative Sequence Networks Unsymmetrical Faults, the neutral of the generator is taken as the reference bus. This is logical because positive or negative sequence components represent balanced sets and hence all the neutral points must be at the same potential for either positive or negative sequence currents.
- For zero sequence network, the reference bus is the ground at the generator.