Zero Sequence Network of Three Winding Transformer

In every transmission network it is very common to have 220/132kV Inter Connecting Transformers (ICT). Such large capacity transformers generally having three windings. The third winding called as tertiary winding and connected in delta having rated voltage either 11kV or 33kV. This winding works as third harmonic suppressor or seldom used for supplying sub-station auxiliaries. This ICT is modeled as three terminal network as shown below. 

For any ICT tertiary winding is having lowest voltage hence thus it is called as LV winding and nomenclature of ICT winding is as- Primary Winding – HV, Secondary Winding – IV and Tertiary Winding – LV. Though the ICT is represented by three terminal network with impedances as Z_P, Z_S, and Z_T they are not directly available on ICT nameplate. Instead whatever given is

Z_ps – Leakage impedance measured from primary with secondary short-circuited and tertiary open

Z_pt – Leakage impedance measured from primary with tertiary short-circuited and secondary open

Z_st – Leakage impedance measured from secondary with tertiary short-circuited and primary open

And using following formulae we can have Z_P, Z_S, and Z_T

Note: ICT are auto transformers and if neutral is solidly grounded (without intentional insertion of earthig resistance) its modeling is same as three winding transformer. Generally ICT neutral is grounded solidly.