UNSYMMETRICAL FAULT CALCULATIONS – PART – 34 – SEQUENCE NETWORK OF LOADED THREE-PHASE GENERATOR SEQUENCE NETWORKS

A single-phase equivalent circuit of power system consisting of impedances to the current of any one sequence only is called sequence network. [OR] An equivalent network for the balanced power system under an imagined operating condition such that only one sequence component of voltages and currents is present in the system.

Sequence network is composed of impedances offered to that sequence current in the system. Three sequence networks for a given power system namely

(i) Positive sequence network (ZO) – The positive sequence network for a given power system shows all the paths for the flow of positive sequence currents in the system.

(ii) Negative sequence network (Z1) – The negative sequence network for a given power system shows all the paths for the flow of negative sequence currents in the system

(iii) Zero sequence network (Z2) - The zero sequence network for a given power system shows all the paths for the flow of zero sequence currents in the system.

Each sequence network is replaced by a Thevenin’s equivalent circuit between two points i.e. each sequence network can be reduced to a single voltage and single impedance.

One point is the fault point ‘F’ and the other point is the zero potential of reference bus N.

Va1 = Vf - Z1 IR1, Va2 = -Z2 Ia2 and Va0 = -Z0 Ia0

Ia is the current flowing from the system into the fault, its components Ia1, Ia2 and Ia0.

Z1, Z2 and Z0 are the total impedance of the positive, negative and zero sequence networks up to the fault point.

For a fault on the unloaded generator with excitation voltage Eg the following will be the sequence voltage drops.

Va1 = Eg - Z1 IR1, Va2 = -Z2 Ia2 and Va0 = -Z0 Ia0