Backup Relay Settings Calculation Methodology for ICTs

Use of directional relays

Tripping of ICTs changes power flows for either side of network drastically and thereby may lead tripping of other network elements too. To avoid such wide spread network disturbance it is utmost necessary that HV backup relays shall respond only for fault on IV bus and vice a versa.  Thus ICTs HV and IV backup relays are directional.

As discussed previously about MTA for directional relays of EHV lines same values shall be adopted for MTA for directional relays for ICTs. That means for overcurrent relay MTA shall be +45⁰ (45 degree leading) and for earthfault relay it shall be -45⁰ (45 degree lagging).

PSM Setting IV Side

Generally power flow through ICTs is from HV side to IV side and as directional relays are provided for ICTs in grid; IV side overcurrent relay will not respond for overload condition on ICTs; still IV side overcurrent PSM shall be according to ICT full load. This is only to ensure its non-operation on load whenever there is accidental change in power flow direction due to grid disturbance.

PSM Setting HV Side

ICTs tap winding is designed on IV side. Hence its HV side current for maximum capacity utilization is constant irrespective of tap position. Hence its HV over current PSM can be decided from ICTs rated current only irrespective of tap position.

As HV side current does not depends upon tap position; Alarm and Load trimming scheme shall be implemented using HV side CTs of ICTs. Hence while calculating HV side relay PSM; alarm and load trimming scheme provided for ICT needs to be considered.

For E/F relay PSM shall be selected as 20% or 30% depending upon expected earth fault current.

TMS setting-Time gradation

HV side backup relay of ICTs Protects the ICT against IV side bus faults or un-cleared fault on IV side EHV feeders. These EHV feeders are having distance protection as its main protection. We have adopted Zone-II time delay for distance relays as 350 ms. Hence if TMS of the backup relays is such that the HV side relay operation time shall be 500 ms for fault on IV bus it will provide sufficient time grading between EHV feeder Z-II and ICT HV side relay operation.

By the same logic IV side backup relay time of operation shall also be 500 ms for considering fault on HV bus.

The setting methodology can be best understood with the help of example. 

Use of Definite Time Delay ( Highset ) Setting

We may use the vast magnitude difference between fault current measured by backup relay of ICT during through fault and fault in differential zone to set definite time delay (Highset) operating characteristics of the ICT backup relay.

While using this characteristics for protecting ICT against through fault its setting shall be selected corresponding to 70% of fault current contributed by ICT for bus fault with time delay setting of 500 ms. Typically this setting is called as HS1. Obviously this delay is for proper time discrimination from EHV lines fault clearing time. This figure shows example for deciding backup relay setting for IV side relay.

For fault in IV side differential zone; current setting for Highset shall be corresponding to 70% of IV bus fault current excluding the fault current contributed by the ICT for which Highset setting being decided. Typically this setting is called as HS2. For this Highset element time delay setting of 50-100 ms shall be selected. This time delay shall be there for avoiding mal-operation due to incorrect direction decision during fault transients.