Sequence Impedances

• The are the impedances offered by the circuit elements to positive, negative and zero sequence currents
• Positive sequence impedance The impedance offered by an element when only positive sequence current is flowing through it
• Negative sequence impedance:The impedance offered by an element when only negative sequence current is flowing through it
• Zero sequence impedance:The impedance offered by an element when only zero sequence current is flowing through it

Sequence Networks

• The single phase equivalent circuit ofapower system network formed using any particular sequence impedance is known as sequence networks
• A diagram formed using positive sequence impedances is called positive sequence network

Eg:Reactance diagram

• A diagram formed using negative sequence impedances is called negative sequence network
• A diagram formed using zero sequence impedances is called zero sequence network

Sequence Networks of generators

• Under balanced conditions, In will be zero
• Hence, Zn can be omitted in positive and negative sequence networks
• Generated emf's are of positive sequence only
• Positive sequence network consists of emf in series with positive sequence impedance
• Negative & zero sequence networks includes their respective sequence impedances only

Sequence Networks of generators

• In zero sequence network, all the currents$I_{a 0}, I_{b 0} \& I_{c 0}$ flows through Z,
• Current flowing through $$ Z_{n}=I_{a 0}+I_{b 0}+I_{c 0}=3 I_{a 0} $$
• Let, Zog-Zero sequence impedance per phase of the generator
• Zo -Total zero sequence network per phase of zero sequence network

• Zero sequence voltage drop from point 'a' to ground is $$ -3 I_{a 0} Z_{n}-I_{a 0} Z_{0 g}=-I_{a 0}\left(3 Z_{n}+Z_{0 g}\right) $$

• Total impedance in the zero sequence network is $$ 3 Z_{n}+Z_{0 g} $$

Sequence Networks of Transmission Lines

• Due to the symmetry of transposed transmission lines, sequence impedances are independent of phase sequence. :. Z1=Z2
• Zero sequence currents hasareturn path through the overhead ground wires. Positive and negative sequence currents have no return path
• Hence, magnetic field due to zero sequence current is different from that due to positive and negative sequence currents

• Zero sequence reactance is2-3.5 times the positive sequence reactance

• $ \text { i.e, } \begin{aligned} Z_{1} &=Z_{2} \\ Z_{0} &=2-3.5 \text { times } Z_{1} \end{aligned} $

• Sequence impedances are represented as series impedances in their respective sequence network