Discuss the sequence network for transmission lines.
1 Answer

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


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