Settings of Transformer HV Side Overcurrent and Earthfault Relay

As mentioned previously the setting methodology is common whether transformers operating separately or in parallel.

PSM for overcurrent relay is set such that transformer full load capacity can be used when transformer operating on extreme tap. Transformers having on load tap changing facility generally having taps to adjust LV voltage to rated value either for ± 10% or for +5% to -15% variations in HV voltage. Accordingly maximum HV current while transformer operating on extreme tap would be 

Where 0.0125 (or 1.25%) is the factor corresponding to voltage change for 1 tap position.

HV side Overcurrent PSM can be decided by calculating HV side maximum current using above formulae or by observing name plate details of the transformer.

In case of transformer having alarm and load trimming scheme it is necessary to select HV overcurrent PSM corresponding to HV current of

Here 10% higher value is selected considering overload alarm setting at 100% and load trimming setting at 105% of transformer full load current. Alarm and load trimming scheme needs to be provided on LV side of the relay.

(NOTE: For under-loaded transformer this minute calculations may not necessary one may select PSM corresponding to Transformer HV full load current) 

Above said facts are explained here by an example. Let us consider a 132/33 kV 50 MVA transformer fully loaded with HV CTR as 400/1 Amp. Let transformer OLTC normal tap is 5. Total number of taps 17. Voltage variation per tap as 1.25%. Let this transformer is provided with overload alarm and load trimming scheme on LV side. Decide HV overcurrent PSM.

Transformer LV full load current = 17.5*50 = 875 Amp.

LV side alarm setting = 875 A

LV side load trimming scheme setting = 1.05*875 = 919 Amp. (5% higher than alarm setting to allow for manual action to reduce the load)

Let at such situation transformer is operating on extreme tap. Hence its HV side current at the time of operation of LV side load trimming scheme would be

= 919 / Transformation Ratio at extreme tap

= 919 / (4*0.85) = 270 Amp.

Hence to avoid racing between HV side overcurrent relay and LV side load trimming scheme due to inaccuracies of CT ratio and relay measurement it is necessary to adopt HV side PSM as 110% of calculated HV side full load current while transformer operating on extreme tap. Hence-

Transformer HV full load at rated voltage = 50000/(1.732*132) = 218 Amp.

Maximum HV current while transformer operating on extreme tap would be 

Thus HV side PSM would be 283/400 = 0.7075 ≈ 70%

For earth fault relay selected PSM shall be 20% of transformer full load current. It shall be in steps of 15%, 20% and 30%. Thus for above transformer earthfault PSM would be

0.2*218/400 = 0.109 thus select earth fault PSM as 15%.

Selection of such low PSM for earthfault may pose some constraint for time discrimination between LV side and HV side relay operation but that shall be accepted considering importance of isolating the transformer from supply in case of any abnormality. However it is the general practice to set TMS for E/F below 0.25 which shall also be followed by selecting proper PSM for E/F relay.

TMS for HV side overcurrent relay shall be such that HV side relay operational time would be 500 ms for LV side bus fault (See NOTE). This will allow transformer LV side relay to operate first opening transformer LV side breaker only and clearing LV bus fault. This avoids unnecessary switching operation of the transformer and thereby reduces supply restoration time. Obviously for calculations of overcurrent TMS consider 3-Ph fault on transformer LV bus while for calculating earthfault TMS consider 1-Ph fault on transformer LV bus.

(For details of calculating TMS see section Overcurrent Relay Settings Calculations for EHV Lines)

Most of the overcurrent and earthfault protection relays are provided with facility to select definite time operation for set value of current (Generally called as High Set element). Proper selection of this current value and time of operation for the HV relay definitely strengthens the protection. Thus for High Set element current setting shall be equal to 70% of HV relay current for fault on transformer HV terminal (which is same as transformer HV bus fault current). The time of operation for HV High Set element shall be 50 ms.  

(NOTE: Time discrimination between HV and LV side overcurrent relay at the fault level lower than considered for TMS calculations will always more. See HV and LV relay operating characteristics plotted as in next section)