Diff Bias Settings Calculation

Detailed Calculations of Differential Relay for meeting Operational Requirement and preventing Unwanted tripping on the account of Harmonics. Subject: Investigation of recent trippings of power transformer on Harmonics in differential relay for taking corrective action to prevent further trippings. Calculation: Differential Relay setting calculations of the Power transformer based on Typical Power Transformer data as mentioned below: Data: 1.

Transformer:20MVA Voltage rating: 66/11 KV Taps: +5% to -15% Connection: Dyn11 % Impedance: 14%

2.

Current Transformers: HV side: 200/1 LV side: 1600/5 Primary: Delta connected Secondary: Star connected Ratio error of all CTs: +/-3%

3.

Relay: Biased differential relay Rating: 1 A /5 A Initial setting: Bias setting: Instantaneous High set unit: 2nd Harmonic Restraints: Operation is prevented when 2nd harmonic contents in the differential circuits exceeds 10% ( which is a best possible setting available in relay) 5th Harmonic Restraints: This is provided to avoid possible maloperation under over excited condition.

Primary Current HV rated current

= 20 MVA (66 * 1.732) = 175 Amp.

LV rated current

= 20MVA / (11* 1.732) = 1050 Amp

Percentage bias of the relay has to be selected such that the relay remains stable for outzone three phase short circuit fault on 11KV side of the transformer.

While selecting this bias setting, following points needs to be considered 1. Tap-changing operation of the transformer 2.

Possible CT saturation

3.

Mismatching of CT saturation characteristic

4.

CT ratio error.

Assuming Transformer operation at the highest tap on HV side, the fault current on primary and secondary side for the three phase fault in Out-zone can be calculated on the basis of % impedance of the transformer. Assuming % impedance of transformer=14% S.C fault current on 11 KV side Is= 1050/0.14=7498.21A S.C fault current on 66 KV side taking -15% tap into consideration LV Voltage-11000 Volts HV Voltage-56100 Volts Ip= 7498.275*11000/56100=1470.25A Secondary equivalent on 11KV side Is1= 7498.275*5/1600=23.43A Secondary equivalent on 66 KV side Ip1= 1470.25*1/200=7.35A After delta formation Secondary equivalent on 11KV side Is2=23.43*1.732=40.58A Correction factor for HV/LV CT Ratio-(1200/.578)/ (1600/5) =6.48 Corrected Is2 = 40.58/6.48=6.26 Amps. Differential Current=Ip1-Is2=1.09A Bias Current=(Is2+Ip1)/2=6.80A % Bias= Differential Current/ Bias Current=0.16107 Taking 3% CT Error into consideration Is1=23.43*0.97=22.72 A Ip1=7.35*1.03=7.57A Is2=6.26*.97=6.072 A Ip1-Is2=1.498 A (Is2+Ip1)/2=6.82A % Bias= Differential Current/ Bias Current=0.22

Initially based on our operational inputs and Specifications of the transformers and CT’s Bias settings/First Slope of Differential Characteristics was kept as 20% Which has proven insufficient and needs correction as mentioned above. Considering safety margin 30% setting will suffice the Purpose.

Conclusion: In NDPL generally power transformer are charged on full load during supply failure from DTL. Recently we have started operating most of the transformer in auto mode keeping their tap changer in operation. A combined effect of higher tap position & higher harmonics level due to Charging current of 33/11KV cables, starting current mismatch (differential current of Tx) is reaching on higher side.( more than 20% and causing differential tripping) Recalculation of protection settings were done to meet our new operational requirements and 30% bias settings will appropriately prevent harmonics trippings. 30% bias setting is already been implemented in all Honeywell stations and will be incorporated in ABB and Alstom stations.