210718140 Sample Calculation for Differential Relays

Differential Relays

2.1 Differential Relay Settings Calculations MiCOM P63X Relay Type: MICOM P63X Required Data Ratings of the Power and Current Transformers MVA Voltage Ratio Rated Voltage in kV (HV Side) Rated Voltage in kV (LV Side) Vector Group CT Ratio (HV Side) CT HV Side Vector Group CT Ratio (LV Side) CT LV Side Vector Group Minimum Tap = - % Maximum Tap = +% Rated Current (HV Side) = MVA / (√3*kV) Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio Required Ratio Compensation = 1/ Current on CT secondary (HV) Rated Current (LV Side) = MVA / (√3*kV) Current on CT Secondary (LV) = Rated Current (LV Side)/CT Ratio

Required Ratio Compensation = 1/ Current on CT Secondary (LV) Relay current on LV side I2 = Current on CT secondary LV side / Ratio compensation for LV side Calculations for OLTC tap setting -% and +% Full load Current (- % or +% HV Side) = MVA / (√3*kV) Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio Relay Current for HV side I1= Current on Ct secondary (HV)/ Ratio compensation for HV side I1 is calculated for extreme transformer taps The differential current is = I1-I2 Bias current I bias = (I1+I2)/2 Differential current I dif and Ibias is calculated foe extreme Transformer taps Therefore the operating current of the relay If Ibias < 2 = Is + m1* I bias If Ibias > 2 = m2*( Ibias – 2)+ m1 * 2 Where m1 and m2 are the slope of relay char Pick up setting is chosen such that the Differential current at worst tap condition should not be more than 90% of operating current for better stability Substation Name : 220 kV GSS Debari Relay Type: MICOM P633 Ratings of the Power and Current Transformers Transformer Name: CROMPTON MVA = 100

Voltage Ratio = 220/132/11 Rated Voltage in kV (HV Side) = 220 Rated Voltage in kV (LV Side) = 132 Vector Group = YNA0d11 CT Ratio (HV Side) = 300/1 CT HV Side Vector Group = Star/Star CT Ratio (LV Side) = 500/1 CT LV Side Vector Group = Star/Star Minimum Tap = -15 Maximum Tap = +10 Rated Current (HV Side) = MVA / (√3*kV) =100*10^6/ (√3*220*10^3) =262.4319 A

Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio = 262.4319/300 =0.875 A Required Ratio Compensation = 1/ 0.875 = 1.143 A Rated Current (LV Side) = MVA / (√3*kV) =100*10^6/ (√3*132*10^3) =437.38 A

Current on CT Secondary (LV) = Rated Current (LV Side)/CT Ratio =437.38 /500 = 0.875 A Required Ratio Compensation = 1/ 0.875 =1.143 A Calculations for OLTC tap setting -10% Full load Current for 220 kV %( HV Side) Winding at -15% = MVA / (√3*0.9*kV) =100*10^6/ (√3*220*0.85*10^3) =308.743 A Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio = 308.743 /300 =1.029 A As the adopted ratio correction is 1.143 A the current to relay bias terminal = 1.029 * 1.43 =1.176 A Hence the differential current is = 1.176 – 1 = 0. 176 A Bias current = (I1+I2)/2 = (1.176+1)/2 = 1.088 A [Since the bias current is less than 1.5 A the slope will be within 30%] Therefore the operating current of the relay will be = Is + (0.3* I bias)

= 0.2 + (0.3 * 1.088) = 0.526 A Differential current at worst tap condition should not be more than 90% of operating current for better stability. Here it s less than 90% so the setting is acceptable. Calculations for OLTC tap setting +10% Full load Current for 220 kV %( HV Side) Winding at +10% = MVA / (√3*1.15*kV) =100*10^6/ (√3*220*1.10*10^3) =238.54A Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio = 238.54 /300 =0.795 A As the adopted ratio correction is 1.143 A the current to relay bias terminal = 0.795* 1.143 =0.909A Hence the differential current is = 1 - 0.909 = 0.091 A Bias current = (I1+I2)/2 = (0.909 +1)/2 = 0.955 A [Since the bias current is less than 1.5 A the slope will be within 30%] Therefore the operating current of the relay will be = Is + (0.3* I bias) = 0.2 + (0.3 * 0.93) = 0.48 A

Differential current at worst tap condition should not be more than 90% of operating current for better stability. Here it s less than 90% so the setting is acceptable.

2.2 Procedure for Relay Setting of Transformer Differential Relay MBCH Data Required MVA Rating Voltage ratio Vector group CT ratio on HV Side Winding connection of CT on HV side ICT ratio on HV Side Winding connection of ICT on HV side CT ratio on LV Side Winding connection of CT on LV side Transformer Tap Min -% OLTC Tap and max % OLTC tap

At Normal tap Rated current (HV Side) = MVA / (3*Rated voltage (HV side)) Rated current (LV Side) = MVA / (3*Rated voltage (LV side)) If CT is Star/Star Current on CT Secondary (HV) = Rated Current (HV Side) / CT Ratio (HV Side) If CT is Star/Delta the current shall be multiplied by 3.

Current on the secondary of ICT for Star/Star = Current on CT Secondary (HV Side) / ICT Ratio (HV Side) For Star/Delta the current shall be multiplied by 3. The same procedure is followed for getting the relay current on LV side also. Idiff = Relay Current = Difference between the HV and LV Current (Current at the secondary of ICT’s) To make sure that the relay does not operate when the Transformer taps are changed the currents are calculated at extreme taps. At Tap on -% Voltage corresponding to the above tap, Full load current = MVA / 3 * Voltage corresponding to –ve tap.

At Tap on +% Voltage corresponding to the above tap, Full load current = MVA / 3 * Voltage corresponding to +ve tap.

Currents at the CT Secondary and ICT Secondary are calculated by using the above procedure. For extreme +ve and extreme –ve tap. Relay current I1= the difference between the HV and LV Currents for extreme +ive tap. I2= The difference between the HV and LV Currents for extreme +ve tap. MBCH has an adjustable basic threshold setting of 10% to 50% current I selectable in 10% steps. Dual Slope – 20% Slope upto In. - 80% Slope for Current > In. Relay operating current = Pickup setting + Bias Setting * Bias current

Bias Current = (I1+I2)/2

I Operating 1A = Pickup setting + 0.20 * Bias Current less than 1A +0.8*current above 1A Bias current = (I1+I2)/2 I Difference = I1- I2 Operating current at extreme taps is calculated with the same procedure. The pick up setting is acceptable if the I operating is less than the bias current at extreme taps. Differential Relay Settings Calculations Substation Name: 220 kV GSS Debari Relay Type: MBCH Ratings of the Power and Current Transformer MVA = 50 Rated Voltage in kV (HV Side) = 220 Rated Voltage in kV (LV Side) = 132 Vector Group = YNa0d1 CT Ratio (HV Side) = 150 CT HV Side Vector Group = Star/Star CT Ratio (LV Side) = 300 CT LV Side Vector Group = Star/Star CT Ratio (HV Side) = 150 CT Ratio (LV Side) = 300 Rated Current (HV Side) = MVA / (√3*kV)

=50*10^6/ (√3*220*10^3) = 131.22 A Rated Current (LV Side) = MVA / (√3*kV) =50*10^6/ (√3*132*10^3) = 218.6 A Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio = 131.22/150 = 0.87 A Current on CT Secondary (LV) = Rated Current (LV Side)/CT Ratio =218.6/300 = 0.73 A Interposing CT (HV Side) = 0.87/0.58 Vector Group Interposing CT (HV Side) = Star/Delta Interposing CT (LV Side) = 0.73/0.58 Vector Group Interposing CT (LV Side) = Star/Delta Current on ICT Secondary (HV) = √3*(Current on CT Secondary (HV)/ Interposing CT (HV Side)) =√3*(0.87/0.58) =1.01 Current on ICT Secondary (LV) = √3*(Current on CT Secondary (LV)/ Interposing CT (LV Side)) =√3*(0.73/0.58)

=1.00 Difference between HV side and LV side currents = Current on ICT Secondary (HV) Current on ICT Secondary (LV) = 1.01-1.00 =0.01 A Calculations for OLTC tap setting -12% Full load Current for 220 kV %( HV Side) Winding at -12% = MVA / (√3*0.88*kV) =50*10^6/ (√3*220*0.88*10^3) = 149.11 A Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio = 149.11 /150 =0.99 A Current on ICT Secondary (HV) = √3*(Current on CT Secondary (HV)/ Interposing CT (HV Side)) =√3*(0.99 /(0.87/0.58)) =1.15 A Difference between HV side and LV side currents = Current on ICT Secondary (HV) - Current on ICT Secondary (LV) = 1.15 – 1.00 = 0.15 A I bias = (I1+I2)/2 = (1.00 + 1.15)/2 =1.075 A Consider the setting of Is = 0.1

The I Operating Current = 0.1 + (0.2 * 1) + (0.8*(1.075-1) = 0.1 + 0.2 + 0.8*0.075 =0. 36A At tap of -12% with full load the relay require a current of 0.36 A at Is = 0.1 Since I1-I2 = 0.15 which is less than relay operating value, relay will not operate. So a setting Is = 0.1 is acceptable. Calculations for OLTC tap setting +5% Full load Current for 220 kV %( HV Side) Winding at +5% = MVA / (√3*1.05*kV) =50*10^6/ (√3*220*1.05*10^3) =124.97 A Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio = 124.97 /150 =0.83 A Current on ICT Secondary (HV) = √3*(Current on CT Secondary (HV)/ Interposing CT (HV Side)) =√3*(0.83 /(0.87/0.58)) =0.96 A Difference between HV side and LV side currents = Current on ICT Secondary (HV) Current

on ICT Secondary = 1.00-0.96 = 0.04 A

I bias = (I1+I2)/2 = (0.96+1.00)/2

=0.98 A Consider the setting of Is = 0.1 The I Operating Current = 0.1 + (0.2 * 0.98) = 0.1 + 0.196 = 0.296 A

At tap of +5% with full load the relay require a current of 0.29 A at Is = 0.1 Since I1-I2 = 0.04 which is less than relay operating value, relay will not operate. So a setting Is = 0.1 is acceptable.

2.3 Procedure for Relay Setting of Transformer Differential Relay KBCH Data Required MVA Rating Voltage ratio Vector group HV voltage LV voltage Transformer percentage impedance: Transformer vector group: OLTC Tap: +%

OLTC Tap: -%

CT ratio and winding configuration HV side LV side At Normal tap HV Side full load current = MVA / 3*kV Current on CT Secondary Ict (HV) = Rated Current (HV Side) / CT Ratio (HV Side) N1=Required ratio compensation = 1/ Ict sec Assuming Relay current = 1A N1 is set on the relay Current on HV side = Current on CT secondary (HV)/ N1

At Normal tap low voltage side LV Side full load current = MVA / (3*kV) Current on CT Secondary Ict (LV) = Rated Current (LV Side) / CT Ratio (LV Side) N2=Required ratio compensation = 1/ Ict sec (LV) N2 is set on the relay Current on LV side = Current on CT secondary (LV)/N2 At -% Tap HV side current = MVA / (3*kV Where kV is the voltage corresponding to -% tap on HV side Current on CT Secondary Ict (HV) = Rated Current (HV Side) / CT Ratio (HV Side) With ratio compensation Current on CT Secondary =Ict sec /N1 At Tap on +% HV side current = MVA / (3*kV ) Where kV is the voltage corresponding to +% tap on HV side Current on CT Secondary Ict (HV) = Rated Current (HV Side) / CT Ratio (HV Side) With ratio compensation Current on CT Secondary =Ict sec /N1 Differential current at extreme taps At -% tap =Idiff1 =IHV-ILV At +% tap =Idiff2 =IHV-ILV Ibias = IHV+ILV /2

Operating current of Relay I Operating = Pickup setting + 0.20 * Bias Current less than 1A +0.8*current above 1A Relay Operating current current is calculated using the above equation. For extreme taps the Idiff and Ibias current are calculated., the pick up setting is chosen such that it will not operate for extreme taps Differential Relay Settings Calculations Substation Name: 220 kV GSS Nagaur Relay Type: KBCH (Areva) Ratings of the Power and Current Transformers Transformer Name: Transformer 2 MVA = 100 Voltage Ratio = 220/132 kV Rated Voltage in kV (HV Side) = 220 Rated Voltage in kV (LV Side) = 132 Vector Group = YNa0d1 CT Ratio (HV Side) = 400/1 CT HV Side Vector Group = Star/Star CT Ratio (LV Side) = 600/1 CT LV Side Vector Group = Star/Star Minimum Tap = -15 Maximum Tap = +10 Rated Current (HV Side) = MVA / (√3*kV) =100*10^6/ (√3*220*10^3)

=262.43 A Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio = 262.43/400 =0.66 A Required Ratio Compensation = 1/ 0.66 =1.52 Rated Current (LV Side) = MVA / (√3*kV) =100*10^6/ (√3*132*10^3) =437.39 A Current on CT Secondary (LV) = Rated Current (LV Side)/CT Ratio =437.39/600 = 0.73 A Required Ratio Compensation = 1/ 0.73 =1.37 A Calculations for OLTC tap setting -10% Full load Current for 400 kV %( HV Side) Winding at -15% = MVA / (√3*0.85*kV) =100*10^6/ (√3*220*0.85*10^3) =308.74 A Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio = 308.74 /400 = 0.77 A As the adopted ratio correction is 1.52 A the current to relay bias terminal

= 0.77 * 1.52 =1.18 A Hence the differential current is = 1.18 – 1 = 0.18 A Bias current = (I1+I2)/2 = (1.18+1)/2 = 1.09 A

[Since the bias current is less than 1.5 A the slope will be within 20%] Therefore the operating current of the relay will be = Is + (0.2* I bias) = 0.1 + (0.2 * 1.09) = 0.37 A Differential current at worst tap condition should not be more than 90% of operating current for better stability. Here it is less than 90% so the setting is acceptable. Calculations for OLTC tap setting +10% Full load Current for 220 kV %( HV Side) Winding at +10% = MVA / (√3*1.1*kV) =100*10^6/ (√3*220*1.1*10^3) = 237.26 A

Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio = 238.57 /400 = 0.60 A

As the adopted ratio correction is 1.52 A the current to relay bias terminal = 0.60 * 1.52 = 0.82 A Hence the differential current is = 1 – 0.82 = 0.18 A Bias current = (I1+I2)/2 = (0.82 +1)/2 = 0.91 A

[Since the bias current is less than 1.5 A the slope will be within 20%] Therefore the operating current of the relay will be = Is + (0.2* I bias) = 0.1 + (0.2 * 0.91) = 0.28 A Differential current at worst tap condition should not be more than 90% of operating current for better stability. Here it is less than 90% so the setting is acceptable.

2.4 Procedure for Relay Setting of Transformer Differential Relay DTH 31/32 Data Required MVA Rating Voltage ratio Vector group CT ratio on HV Side Winding connection of CT on HV side ICT ratio on HV Side Winding connection of ICT on HV side CT ratio on LV Side Winding connection of CT on LV side Transformer Tap Min -% or OLTC Tap max +% Rated current (HV Side) = MVA / (3*Rated voltage (HV side)) Rated current (LV Side) = MVA / (3*Rated voltage (LV side)) If CT is Star/Star Current on CT Secondary (HV) = Rated Current (HV Side) / CT Ratio (HV Side) If CT is Star/Delta the current shall be multiplied by 3. Current on the secondary of ICT for Star/Star = Current on CT Secondary (HV Side) / ICT Ratio (HV Side)

For Star/Delta the current shall be multiplied by 3. The same procedure is followed for getting the relay current on LV side also. Idiff = Relay Current = Difference between the HV and LV Current (Current at the secondary of ICT’s) To make sure that the relay does not operate when the Transformer taps are changed the currents are calculated at extreme taps. At Tap on -% Voltage corresponding to the above tap, Full load current = MVA / 3 * Voltage corresponding to –ve tap.

At Tap on +% Voltage corresponding to the above tap, Full load current = MVA / 3 * Voltage corresponding to +ve tap.

Currents at the CT Secondary and ICT Secondary are calculated by using the above procedure. For extreme +ive and extreme –ive tap.

Relay current I1= The difference between the HV and LV Currents for extreme +ive tap. I2= The difference between the HV and LV Currents for extreme +ive tap.

DTH 31 relay has a fixed pick up setting and variable single slope bias setting.

For setting the Bias Setting The operating current of DTH 31 is given by the equation

Relay operating current = Pickup setting + Bias Setting * Bias current The pickup setting in DTH 31 = 0.15 (constant) Bias Current = (I1+I2)/2 Operating Current at normal tap with Bias setting Is = 0.15(or) 0.3 I Operating 1A = 0.15 + 0.15 * Bias Current Bias current = (I1+I2)/2 I Difference 1A = I1- I2 Operating current at extreme taps is calculated with the same procedure. Operating current at extreme taps is calculated with the same procedure. Operating Current at extreme Minimum tap I Operating 2A. I Difference 2A = I1 – I2.

Operating Current at extreme Maximum tap I Operating 3A. I difference 3A = I1 – I2. In each of the above cases IOperating current >IDifference + Tolerance If the above is true a setting of 0.15 for bias is selected. In case the above is not true. Similar calculation is carried out for the next Bias Setting. Bhinmal Differential Relay Settings Calculations Relay Type: DTH31 Station Name : 220 kV GSS Modak Ratings of the Power and Current Transformers

Transformer Name: NGEF MVA = 100 Voltage Ratio = 220/132 kV Rated Voltage in kV (HV Side) = 220 kV Rated Voltage in kV (LV Side) = 132 KV Vector Group = YNyn0d1 CT Ratio (HV Side) = 300/1 CT HV Side Vector Group = Star/Star CT Ratio (LV Side) = 600/1 CT LV Side Vector Group = Star/Star ICT Ratio (HV Side) = 7.3/5 ICT Ratio (LV Side) = 3.6/2.89 Rated Current (HV Side) = MVA / (√3*kV) =100*10^6/ (√3*220*10^3) =262.4319 A Rated Current (LV Side) = MVA / (√3*kV) =100*10^6/ (√3*132*10^3) =437.39 A Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio = 262.4319/300 =0.874 A Current on CT Secondary (LV) = Rated Current (LV Side)/CT Ratio

=437.39/600 = 0.728 A

Interposing CT (HV Side) = = 7.3/5 Vector Group Interposing CT (HV Side) = Star/Delta Interposing CT (LV Side) = 3.65/2.89 Vector Group Interposing CT (LV Side) = Star/Delta Current on ICT Secondary (HV) = √3*(Current on CT Secondary (HV)/ Interposing CT (HV Side)) =√3*(0.656/1.128) =1.03 A

Current on ICT Secondary (LV) = √3*(Current on CT Secondary (LV)/ Interposing CT (LV Side)) =√3*(0.728/1.261) = 0.99 A

Difference between HV side and LV side currents = Current on ICT Secondary (HV) – Current on ICT Secondary (LV) = 1.03 – .99 =0.04

Calculations for OLTC tap setting -15% Full load Current for 220 kV %( HV Side) Winding at -10% = MVA / (√3*0.85*kV) =100*10^6/ (√3*220*0.85*10^3) =308.743 A Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio = 308.74 /300 =1.029 A Current on ICT Secondary (HV) = √3*(Current on CT Secondary (HV)/ Interposing CT (HV Side)) =√3*1.029=1.22 A Current on ICT Secondary (LV) = 1.00 A Difference between HV side and LV side currents = Current on ICT Secondary (HV) Current on ICT Secondary (LV) = 1.22- 1.00 = 0.22 A Ibias = ( I1 + I2 ) /2 = ( 1.22+ 1 ) /2 = 1.11 A Operating Current = Bias setting + Slope 1 * I bias = 0.15 +0.15*1.11 = 0.3165 A As Id= 0.22, Operating current = 0.31, Hence stable

Calculations for OLTC tap setting +10% Full load Current for 220 kV %( HV Side) Winding at +15% = MVA / (√3*1.10*kV) =100*10^6/ (√3*220*1.10*10^3) = 238.57A Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio = 228.20 /300 = 0.795 A Current on ICT Secondary (HV) = √3*(Current on CT Secondary (HV)/ Interposing CT (HV Side)) =0.9434 Current on ICT Secondary (LV) = 1.00 A Difference between HV side and LV side currents = Current on ICT Secondary (HV) Current on ICT Secondary (LV) =1- 0.9434 = 0.056 A At Tap of -10 % I bias = (I1+I2)/2 = (1+0.9434)/2 A =0 .97 A Consider a bias setting of Is = 0.30 The I Operating Current = 0.15 + Is * I bias = 0.15 + 0.15* 0.97 =0.29 A At tap of -10% with full load the relay require a current 0.47 A at Is = 0.30.

Since I1-I2 = 0.056 which is less than the operating value, relay will not operate. So a setting Is = 0.30 is acceptable.

2.5 Differential Relay Settings Calculations RET 670 Differential Relay Settings Calculation Procedure Relay Type: RET 670 Required Data Ratings of the Power and Current Transformers MVA Voltage Ratio Rated Voltage in kV (HV Side) Rated Voltage in kV (LV Side) Vector Group CT Ratio (HV Side) CT HV Side Vector Group CT Ratio (LV Side) CT LV Side Vector Group CT resistance value (Rct) Lead resistance value (Rl) Minimum Tap = - % Maximum Tap = +% Rated Current (HV Side) = MVA / (√3*kV) Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio

Required Ratio Compensation = 1/ Current on CT secondary (HV) Rated Current (LV Side) = MVA / (√3*kV) Current on CT Secondary (LV) = Rated Current (LV Side)/CT Ratio Required Ratio Compensation = 1/ Current on CT Secondary (LV) Relay current on LV side I2 = Current on CT secondary LV side / Ratio compensation for LV side Calculations for OLTC tap setting -% and +% Full load Current (- % or +% HV Side) = MVA / (√3*kV) Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio Relay Current for HV side I1= Current on Ct secondary (HV)/ Ratio compensation for HV side I1 is calculated for extreme transformer taps The differential current is = I1-I2 Bias current I bias = (I1+I2)/2 Differential current I dif and Ibias is calculated foe extreme Transformer taps Therefore the operating current of the relay = Is + m1* I bias for current less than (4.Iref) + m2* Ibias for current greater than (4 Iref) Where m1 and m2 are the slope of relay char Pick up setting is chosen such that the Differential current at worst tap condition should not be more than 90% of operating current for better stability..

Sample Calculation for ABB RET 670 A) Biased Differential Substation Name : 220 kV GSS Rajwest Relay Type: ABB RET 670 Ratings of the Power and Current Transformers

Transformer Name: TELK MVA = 315 Voltage Ratio = 400/220/33 Rated Voltage in kV (HV Side) = 400 Rated Voltage in kV (LV Side) = 220 Vector Group = YNA0d11 CT Ratio (HV Side) = 1000/1 CT HV Side Vector Group = Star/Star CT Ratio (LV Side) = 1000/1 CT LV Side Vector Group = Star/Star Minimum Tap = -10 Maximum Tap = +15 Rated Current (HV Side) = MVA / (√3*kV) =315*10^6/ (√3*400*10^3) =454.66 A

Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio = 454.66/1000 =0.454 A Required Ratio Compensation = 1/ 0.454 = 2.199 A Rated Current (LV Side) = MVA / (√3*kV)

=315*10^6/ (√3*220*10^3) =826.66 A Current on CT Secondary (LV) = Rated Current (LV Side)/CT Ratio =826.66/1000 = 0.826 A Required Ratio Compensation = 1/ 0.826 =1.209 A Compensated current on LV side=√3*0.826/1.209

Calculations for OLTC tap setting -10% Full load Current for 220 kV %( HV Side) Winding at -15% = MVA / (√3*0.85*kV) =315*10^6/ (√3*400*0.85*10^3) =534.89 A Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio = 534.89 /1000 =0.534 A Inter posing CT (HV) winding = √3*Current on CT sec (HV)/Ratio correction factor =√3*0.534/2.199 =0.421 A Hence the differential current is = ABS (0.421– 1.183) = 0. 762 A Bias current = (I1+I2)/2

= (0.421+1.183)/2 = 0.802 A [Since the bias current is less than 1.5 A the slope will be within 40%] Therefore the operating current of the relay will be = Is + (1.5* M1)+M2*Ibias = 0.1 + (0.4 * 1.5)*0.8*0.697 = 1.25 A Differential current at worst tap condition should not be more than 90% of operating current for better stability. Here it s less than 90% so the setting is acceptable.

Calculations for OLTC tap setting +10% Full load Current for 220 kV %( HV Side) Winding at +10% = MVA / (√3*1.1*kV) =315*10^6/ (√3*400*1.1*10^3) =413.33 A Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio = 413.33 /1000 =0.413 A Interposing HV winding current=√3*0.413/2.199 =0.325 I bias = (interposing CT HV current side+ Interposed LV side CT current side)/2 = (0.325+1.183)/2 = 0.754 A Hence the differential current is = 1.183-0.325 = 0.858 A

[Since the bias current is less than 1.5 A the slope will be within 30%] Therefore the operating current of the relay will be = Is + (m1+m2)* I bias = 0.1 + (0.4 +0.8) * 0.754 = 1.01 A The operating value is larger than the differential value hence the system is Stable for the MAX tap condition.

B) High Impedance Differential Substation Name : 220 kV GSS Rajwest Relay Type: ABB RET 670 Ratings of the Power and Current Transformers Transformer Name: TELK MVA = 315 Voltage Ratio = 400/220/33 Rated Voltage in kV (HV Side) = 400 Rated Voltage in kV (LV Side) = 220 Vector Group = YNA0d11 % Impedance of the transformer=0.1198 CT Ratio (HV Side) = 1000/1 CT HV Side Vector Group = Star/Star CT Ratio (LV Side) = 1000/1 CT LV Side Vector Group = Star/Star CT resistance (Rct) =4Ω Lead Resistance value=1.5 Ω

Rated Current (HV Side) = MVA / (√3*kV) =315*10^6/ (√3*400*10^3) =454.66 A Maximum fault current contributed =Rated current /Percentage Impedance

=454.66/0.1198 =3795.15 A Maximum fault current contributed secondary=3795.15/1000 = 3.795 A Operating Voltage =3.795*(4+2*1.5) =26.57V

Rstab

= 250Ω

Stabilizing resistance value (Rstab) to be set as per the operating voltage range given in the technical reference manual table provided by the manufacturer.

2.6 Procedure for Relay setting of Transformer Differential Relay ABB RADSB Substation

: 220kV GSS Dausa

Tranformer Name : CGL Relay Type: RADSB [ABB] Data Required MVA Rating Voltage ratio Vector group CT ratio on HV Side Winding connection of CT on HV side ICT ratio on HV Side Winding connection of ICT on HV side CT ratio on LV Side Winding connection of CT on LV side Transformer Tap Min -% OLTC Tap and max % OLTC tap Calculation Rated Current (HV Side) = MVA / (√3*kV) Rated Current (LV Side) = MVA / (√3*kV) Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio

Current on CT Secondary (LV) = Rated Current (LV Side)/CT Ratio Interposing CT (HV Side) = Current on CT Secondary (HV) / ICT ratio on HV side Interposing CT (LV Side) = Current on CT Secondary (LV) /0.578 Current on ICT Secondary (HV) = √3*(Current on CT Secondary (HV)/ Interposing CT (HV Side)) Current on ICT Secondary (LV) = √3*(Current on CT Secondary (LV)/ Interposing CT (LV Side)) Difference between HV side and LV side currents = Current on ICT Secondary (HV) – Current on ICT Secondary (LV) Calculations for OLTC tap setting -% Voltage corresponding to the above tap, Full load current = MVA / 3 * Voltage corresponding to –ve tap. Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio Current on ICT Secondary (HV) in A = √3*(Current on CT Secondary (HV)/ Interposing CT (HVSide)) Difference between HV side and LV side currents = Current on ICT Secondary (HV) Current on ICT Secondary (LV)

Calculations for OLTC tap setting +% Voltage corresponding to the above tap, Full load current = MVA / 3 * Voltage corresponding to +ve tap.

Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio Current on ICT Secondary (HV) in A = √3*(Current on CT Secondary (HV)/ Interposing CT (HVSide)) Difference between HV side and LV side currents = Current on ICT Secondary (HV) Current on ICT Secondary (LV) Relay setting range available : Operating value can be Settable 20, 25, 35, or 50% of In Id = (I1-I2) Ibias = ( I1+I2)/2

Operating current = 0.25+ Slope1 setting * I bias Id should be less than I operating current The setting shall be selected in such a way that the relay shall not operate for the extreme tap positions Differential Relay Settings Calculations Relay Type: RADSB [ABB] Substation

: 220kV GSS Dausa

Tranformer Name : CGL Ratings of the Power and Current Transformers Transformer Name: CGL MVA = 100 Voltage Ratio = 220/132kV Rated Voltage in kV (HV Side) = 220 Rated Voltage in kV (LV Side) = 132 Vector Group = YnYa0 CT Ratio (HV Side) = 300/5

CT HV Side Vector Group = Star/Star CT Ratio (LV Side) = 600/5 CT LV Side Vector Group = Star/Star ICT Ratio (HV Side) = 4.37/2.88 ICT Ratio (LV Side) = 3.64/2.88 Rated Current (HV Side) = MVA / (√3*kV) =100*10^6/ (√3*220*10^3) =262.4319 A Rated Current (LV Side) = MVA / (√3*kV) =100*10^6/ (√3*132*10^3) = 437.39A Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio = 262.4319/(300/5) = 4.37 A Current on CT Secondary (LV) = Rated Current (LV Side)/CT Ratio =437.39/(600/5) = 3.64 A Interposing CT (HV Side) = 4.37/2.88 = 1.517 Interposing CT (LV Side) = 3.64/2.88 =1.263

Current on ICT Secondary (HV) = (Current on CT Secondary (HV)*Interposing CT (HV Side)) =4.37/(4.37 /2.88) = 2.88 A

Current on ICT Secondary (LV) = √3*(Current on CT Secondary (LV)/ Interposing CT (LV Side)) =3.64/(3.64/1.263) =2.88 A Difference between HV side and LV side currents = Current on ICT Secondary (HV) – Current on ICT Secondary (LV) = 4.9891 – 4.9916 = 0.0019606708 A Calculations for OLTC tap setting -15% Full load Current for 220 kV %( HV Side) Winding at -15% = MVA / (√3*0.85*kV) =100*10^6/ (√3*220*0.85*10^3) =308.74 A Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio = 308.74/(300/5) =5.15 A Current on ICT Secondary (HV) = (Current on CT Secondary (HV)/ Interposing CT (HV side ) =5.15/(4.862 /1.517)

=3.39 A Current on ICT Secondary (LV) =(Current on CT Secondary (HV)/ Interposing CT (HV side) = 2.88 A

Difference between HV side and LV side currents = Current on ICT Secondary (HV) Current on ICT Secondary (LV) = 3.39-2.88 = 0.51 A I bias = ( I1+I2)/2 = (3.39+2.88)/2 = 3.34 Operating current = 0.25+0.2*3.34 = 0.88 A Id = 0.51 is less than I operating = 0.88, hence stable Calculations for OLTC tap setting +10% Full load Current for 220 kV %( HV Side) Winding at +10% = MVA / (√3*1.1*kV) =100*10^6/ (√3*220*1.1*10^3) = 238.57 A Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio = 238.57 /(300/5) =3.98 A Current on ICT Secondary (HV) = √3*(Current on CT Secondary (HV)/ Interposing CT (HV Side)) =3.98/(3.954 / 1.517) =2.62 A

Current on ICT Secondary (LV) = 2.88 Difference between HV side and LV side currents = Current on ICT Secondary (HV) Current on ICT Secondary (LV) = 2.88-2.62 = 0.26 I bias =( I1+I2) /2 = ( 2.88+2.62) /2 = 2.75 A I operating = 0.25 + 0.2* 2.75 = 0.8 A Id= 0.26 is less than I operating = 0.8 A, hence stable

2.7 Procedure for Relay Setting of Transformer Differential Relay GE T60 Data Required MVA Rating Voltage ratio Vector group HV voltage LV voltage Transformer percentage impedance: Transformer vector group: OLTC Tap: +%

OLTC Tap: -%

CT ratio and winding configuration HV side LV side At Normal tap HV Side full load current = MVA / 3*kV Current on CT Secondary Ict (HV) = Rated Current (HV Side) / CT Ratio (HV Side) N1=Required ratio compensation = 1/ Ict sec Assuming Relay current = 1A N1 is set on the relay Current on HV side = Current on CT secondary (HV)/ N1

At Normal tap low voltage side LV Side full load current = MVA / (3*kV) Current on CT Secondary Ict (LV) = Rated Current (LV Side) / CT Ratio (LV Side) N2=Required ratio compensation = 1/ Ict sec (LV) N2 is set on the relay Current onLV side = Current on CT secondary (LV)/N2 At -% Tap HV side current = MVA / (3*kV Where kV is the voltage corresponding to -% tap on HV side Current on CT Secondary = Rated Current (HV Side) / CT Ratio (HV Side) With ratio compensation Current on CT Secondary =Ratio of compensation * CT secondary current At Tap on +% HV side current = MVA / (3*kV ) Where kV is the voltage corresponding to +% tap on HV side Current on CT Secondary Ict (HV) = Rated Current (HV Side) / CT Ratio (HV Side) With ratio compensation Current on CT Secondary =Ratio of compensation * CT secondary current Differential current at extreme taps At -% tap =Idiff1 =IHV-ILV At +% tap =Idiff2 =IHV-ILV Ibias = IHV+ILV

Operating current of Relay I Operating = Pickup setting + 0.3 * Bias Current for Bias current less than Break Point I Operating = Pickup setting + 0.3 * Bias Current + 0.8 (Bias current – Break point) for Bias current greater than Break point.

Relay Operating current current is calculated using the above equation. For extreme taps the Idiff and Ibias current are calculated., the pick up setting is chosen such that it will not operate for extreme taps.

Differential Relay Settings Calculations Substation Name: 220 kV GSS Kushkhera Relay Type: GE T60 Ratings of the Power and Current Transformers

Differential Current Setting = 0.25 A Break Point = 2 Slope1 = 0.3 Slope 2= 0.8 Transformer Name: IMP MVA = 100 Voltage Ratio = 220/132 kV Rated Voltage in kV (HV Side) = 220 Rated Voltage in kV (LV Side) = 132 Vector Group = YNa0d1 CT Ratio (HV Side) = 400/1 CT HV Side Vector Group = Star/Star CT Ratio (LV Side) = 500/1 CT LV Side Vector Group = Star/Star Minimum Tap = -15 Maximum Tap = +10 Rated Current (HV Side) = MVA / (√3*kV) =100*10^6/ (√3*220*10^3) =262.43 A Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio = 262.43/400 =0.66 A

Required Ratio Compensation = 1/ 0.66 =1.52

Rated Current (LV Side) = MVA / (√3*kV) =100*10^6/ (√3*132*10^3) =437.39 A

Current on CT Secondary (LV) = Rated Current (LV Side)/CT Ratio =437.39/500 = 0.874 A Required Ratio Compensation = 1/ 0.874 =1.143 A

Calculations for OLTC at Nominal tap setting Full load Current for 220 kV %( HV Side) Winding = MVA / (√3*0.9*kV) =100*10^6/ (√3*220*10^3) = 262.43 A Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio = 262.43 /400 = 0.656 A As the adopted ratio correction is 1.52 A the current to relay bias terminal = 0.656 * 1.52

Compensated current internally = 0.9972 A Hence the differential current is = 0.9972 – 1 = 0.0027A Bias current = Max (I1, I2) = (0.9972, 1) =1A

[Since the bias current is less than Break point (2) the slope will be within 30%] Therefore the operating current of the relay will be = Is + (0.3* I bias) = 0.25 + (0.3* 0.9972) = 0.549 A Differential current at worst tap condition should not be more than 90% of operating current for better stability. Here it is less than 90% so the setting is acceptable.

Calculations for OLTC tap setting -15% Full load Current for 400 kV %( HV Side) Winding at -15% = MVA / (√3*0.85*kV) =100*10^6/ (√3*220*0.85*10^3) = 308.74 A Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio = 308.74 /400 = 0.772 A As the adopted compensation is 1.33 A the current to relay bias terminal = 0.772 * 1.33

Compensated current internally =1.029 A Hence the differential current is = 1.029 – 0.87 = 0.15 A Bias current = Maximum of (I1, I2) = Maximum of (1.029, 0.87) = 1.029 A

[Since the bias current is less than Break point (2) the slope will be within 25%] Therefore the operating current of the relay will be = Is + (0.2* I bias) = 0.25 + (0.3 *1.029) = 0.558 A Differential current at worst tap condition should not be more than 90% of operating current for better stability. Here it is less than 90% so the setting is acceptable. Calculations for OLTC tap setting +10% Full load Current for 220 kV %( HV Side) Winding at +10% = MVA / (√3*1.1*kV) =100*10^6/ (√3*220*1.1*10^3) = 238.57A

Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio = 238.57 /400 = 0.5964 A As the adopted ratio correction is 1.52 A the current to relay bias terminal = 0.596 * 1.52

Compensated current internally = 0.905 A Hence the differential current is = 1-0.905 = 0.095 A Bias current = Maximum of (I1, I2) = Maximum of (0.905, 1) =1A

[Since the bias current is less than Break point (2) the slope will be within 30%] Therefore the operating current of the relay will be = Is + (0.3* I bias) = 0.25 + (0.3 * 1) = 0.55 A Differential current at worst tap condition should not be more than 90% of operating current for better stability. Here it is less than 90% so the setting is acceptable.

2.8 Procedure for Relay Setting of Transformer Differential Relay CSC326 Data Required MVA Rating Voltage ratio Vector group HV voltage LV voltage Transformer percentage impedance: Transformer vector group: OLTC Tap: +%

OLTC Tap: -%

CT ratio and winding configuration HV side LV side At Normal tap HV Side full load current = MVA / 3*kV Current on CT Secondary Ict (HV) = Rated Current (HV Side) / CT Ratio (HV Side) N1=Required ratio compensation = 1/ Ict sec Assuming Relay current = 1A N1 is set on the relay Current on HV side = Current on CT secondary (HV)/ N1

At Normal tap low voltage side LV Side full load current = MVA / (3*kV) Current on CT Secondary Ict (LV) = Rated Current (LV Side) / CT Ratio (LV Side) N2=Required ratio compensation = 1/ Ict sec (LV) N2 is set on the relay Current onLV side = Current on CT secondary (LV)/N2 At -% Tap HV side current = MVA / (3*kV Where kV is the voltage corresponding to -% tap on HV side Current on CT Secondary = Rated Current (HV Side) / CT Ratio (HV Side) With ratio compensation Current on CT Secondary =Ratio of compensation * CT secondary current At Tap on +% HV side current = MVA / (3*kV ) Where kV is the voltage corresponding to +% tap on HV side Current on CT Secondary Ict (HV) = Rated Current (HV Side) / CT Ratio (HV Side) With ratio compensation Current on CT Secondary =Ratio of compensation * CT secondary current Differential current at extreme taps At -% tap =Idiff1 =IHV-ILV At +% tap =Idiff2 =IHV-ILV Ibias = IHV+ILV

Operating current of Relay I Operating = Pickup setting + 0 * Bias Current if less than 0.656 I Operating = Pickup setting + (0.3 * Bias Current -0.656) if greater than 0.656 I Operating = Pickup setting + 0.7 * (Bias Current-4) + 0.3*4

Relay Operating current current is calculated using the above equation. For extreme taps the Idiff and Ibias current are calculated., the pick up setting is chosen such that it will not operate for extreme taps.

Differential protection characteristics for transformers:

Differential Relay Settings Calculations Substation Name: 220 kV GSS Bharathpur Relay Type: CSC326 Ratings of the Power and Current Transformers Differential Current Setting = 0.2 A Transformer Name: CGL

MVA = 100 Voltage Ratio = 220/132 kV Rated Voltage in kV (HV Side) = 220 Rated Voltage in kV (LV Side) = 132 Vector Group = YNa0d1 CT Ratio (HV Side) = 400/1 CT HV Side Vector Group = Star/Star CT Ratio (LV Side) = 500/1 CT LV Side Vector Group = Star/Star Minimum Tap = -15 Maximum Tap = +10 Rated Current (HV Side) = MVA / (√3*kV) =100*10^6/ (√3*220*10^3) =262.43 A Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio = 262.43/400 =0.66 A Required Ratio Compensation = 1/ 0.66 =1.52 Rated Current (LV Side) = MVA / (√3*kV) =100*10^6/ (√3*132*10^3) =437.39 A

Current on CT Secondary (LV) = Rated Current (LV Side)/CT Ratio =437.39/500 = 0.8747 A Required Ratio Compensation = 1/0.8747 = 1.143 A Calculations for OLTC Nominal tap setting: Full load Current for 220 kV %( HV Side) Winding at

= MVA / (√3*kV) =100*10^6/ (√3*220*10^3*) = 262.43 A

Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio = 262.43 /400 = 0.656 A As the adopted ratio correction is 1.52 A the current to relay bias terminal = 0.656 * 1.52 = 0.997 A Compensated current internally on HV =0.997 A Rated Current (LV Side) = MVA / (√3*kV) =100*10^6/ (√3*132*10^3) =437.39 A

Current on CT Secondary (LV) = Rated Current (LV Side)/CT Ratio =437.39/500 = 0.8747 A Required Ratio Compensation = 1/0.8747 = 1.143 A

Compensated current on LV = 0.8747 * 1.143 = 0.9993 Hence the differential current is = (0.9973 – 0.9993 ) = 0.002 A Bias current = (I1+I2)/2 = (0.656+0.8747)/2 = 0.765 A

[Since the bias current is less than 4 A and greater than 0.656 the slope will be within 30%] Therefore the operating current of the relay will be = Is + (0.3* ( I bias-0.656) = 0.2 + (0.3 *0.109) = 0.232 A Differential current at worst tap condition should not be more than 90% of operating current for better stability. Here it is less than 90% so the setting is acceptable.

Calculations for OLTC tap setting -15% Full load Current for 400 kV %( HV Side) Winding at -15% = MVA / (√3*0.85*kV) =100*10^6/ (√3*220*0.85*10^3) = 308.74 A Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio = 308.74 /400 = 0.772 A As the adopted ratio correction is 1.52 A the current to relay bias terminal = 0.772 * 1.52 = 1.18 A Compensated current internally on HV =1.18 A Hence the differential current is = (1.18– 1.0 ) = 0.18 A Bias current = (I1+I2)/2 = (1.18+1)/2 = 1.09 A

[Since the bias current is less than 4 A and greater than 0.656 the slope will be within 30%] Therefore the operating current of the relay will be = Is + (0.3* ( I bias-0.656) = 0.2 + (0.3 *0.434) = 0.33 A Differential current at worst tap condition should not be more than 90% of operating current for better stability. Here it is less than 90% so the setting is acceptable.

Calculations for OLTC tap setting +10% Full load Current for 220 kV %( HV Side) Winding at +10% = MVA / (√3*1.1*kV) =100*10^6/ (√3*220*1.1*10^3) = 238.5 A

Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio = 238.574 /400 = 0.596 A

As the adopted ratio correction is 1 A the current to relay bias terminal = 0.596 * 1.52 Compensated current internally = 0.905 A Differential current = I1- I2 = 1-0.905 = 0.095 A Bias current = (I1+I2)/2 = (0.905 +1)/2 = 0.952 A [Since the bias current is less than 4 A and greater than 0.656 the slope will be within 30%] Therefore the operating current of the relay will be = Is + (0.3* ( I bias-0.656) = 0.2 + (0.3*(0.952- 0.656)) = 0.288 A

Differential current at worst tap condition should not be more than 90% of operating current for better stability. Here it is less than 90% so the setting is acceptable

2.9 Procedure for Relay Setting of Transformer Differential Relay DUOBIAS Data Required MVA Rating Voltage ratio Vector group HV voltage LV voltage Transformer percentage impedance: Transformer vector group: OLTC Tap: +%

OLTC Tap: -%

CT ratio and winding configuration HV side LV side At Normal tap HV Side full load current = MVA / 3*kV Current on CT Secondary Ict (HV) = Rated Current (HV Side) / CT Ratio (HV Side) N1=Required ratio compensation = 1/ Ict sec Assuming Relay current = 1A N1 is set on the relay Current on HV side = Current on CT secondary (HV)/ N1

At Normal tap low voltage side LV Side full load current = MVA / (3*kV) Current on CT Secondary Ict (LV) = Rated Current (LV Side) / CT Ratio (LV Side) N2=Required ratio compensation = 1/ Ict sec (LV) N2 is set on the relay Current onLV side = Current on CT secondary (LV)/N2 At -% Tap HV side current = MVA / (3*kV Where kV is the voltage corresponding to -% tap on HV side Current on CT Secondary = Rated Current (HV Side) / CT Ratio (HV Side) With ratio compensation Current on CT Secondary =Ratio of compensation * CT secondary current At Tap on +% HV side current = MVA / (3*kV ) Where kV is the voltage corresponding to +% tap on HV side Current on CT Secondary Ict (HV) = Rated Current (HV Side) / CT Ratio (HV Side) With ratio compensation Current on CT Secondary =Ratio of compensation * CT secondary current Differential current at extreme taps At -% tap =Idiff1 =IHV-ILV At +% tap =Idiff2 =IHV-ILV Ibias = IHV+ILV

Operating current of Relay I Operating = Pickup setting + 0 * Bias Current if less than 0.656 I Operating = Pickup setting + (0.3 * Bias Current -0.656) if greater than 0.656 I Operating = Pickup setting + 0.7 * (Bias Current-4) + 0.3*4

Relay Operating current current is calculated using the above equation. For extreme taps the Idiff and Ibias current are calculated., the pick up setting is chosen such that it will not operate for extreme taps.

Differential protection characteristics for transformers:

Differential Relay Settings Calculations Substation Name: 220 kV GSS Neemrana Relay Type: DUOBIAS Ratings of the Power and Current Transformers Differential Current Setting = 0.25 A Transformer Name: CGL MVA = 100 Voltage Ratio = 220/132 kV Rated Voltage in kV (HV Side) = 220 Rated Voltage in kV (LV Side) = 132 Vector Group = YNa0d1 CT Ratio (HV Side) = 400/1 CT HV Side Vector Group = Star/Star CT Ratio (LV Side) = 500/1 CT LV Side Vector Group = Star/Star Minimum Tap = -15 Maximum Tap = +10 Rated Current (HV Side) = MVA / (√3*kV) =100*10^6/ (√3*220*10^3) =262.43 A

Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio = 262.43/400 =0.66 A Required Ratio Compensation = 1/ 0.66 =1.52 Rated Current (LV Side) = MVA / (√3*kV) =100*10^6/ (√3*132*10^3) =437.39 A Current on CT Secondary (LV) = Rated Current (LV Side)/CT Ratio =437.39/500 = 0.8747 A Required Ratio Compensation = 1/0.8747 = 1.143 A Calculations for OLTC Nominal tap setting: Full load Current for 220 kV ( HV Side) Winding

= MVA / (√3*kV) =100*10^6/ (√3*220*10^3*) = 262.43 A

Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio = 262.43 /400 = 0.656 A As the adopted ratio correction is 1.52 A the current to relay bias terminal = 0.656 * 1.52* √3

= 1.727 A Compensated current internally on HV = 1.727 A Rated Current (LV Side) = MVA / (√3*kV) =100*10^6/ (√3*132*10^3) =437.39 A

Current on CT Secondary (LV) = Rated Current (LV Side)/CT Ratio =437.39/500 = 0.8747 A Required Ratio Compensation = 1/0.8747 = 1.143 A Compensated current on LV = 0.8747 * 1.143 =1 Hence the differential current is = (1 – 1 ) =0A Bias current = (I1+I2)/2 = (1+1)/2 =1A [Since the bias current is less than 4 A and greater than 0.656 the slope will be within 30%] Therefore the operating current of the relay will be = Is + (0.3* ( I bias) = 0.2 + (0.3*1) = 0.5A

Calculations for OLTC tap setting -15% Full load Current for 400 kV %( HV Side) Winding at -15% = MVA / (√3*0.85*kV) =100*10^6/ (√3*220*0.85*10^3) = 308.743 A Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio = 308.743 /400 = 0.771 A As the adopted ratio correction is 1.52 A the current to relay bias terminal = 0.771 * 1.52 = 0.888 A Compensated current internally on HV =0.888 A Rated Current (LV Side) = MVA / (√3*kV) =100*10^6/ (√3*132*10^3) =437.39 A Current on CT Secondary (LV) = Rated Current (LV Side)/CT Ratio =437.39/500 = 0.8747 A Required Ratio Compensation = 1/0.8747 = 1.143 A Compensated current on LV = 0.874* 1.143

= 0.998 A Hence the differential current is = (1.0-0.888) = 0.12 A Bias current = (I1+I2)/2 = (0.888+1)/2 = 0.944 A [Since the bias current is less than 4 A and greater than 0.656 the slope will be within 30%] Therefore the operating current of the relay will be = Is + (0.3* ( I bias-0.656) = 0.2 + (0.3 *0.28) = 0.284 A Id= 0.12 is less than I operating current = 0.284 A, hence Stable Differential current at worst tap condition should not be more than 90% of operating current for better stability. Here it is less than 90% so the setting is acceptable. Calculations for OLTC tap setting +10% Full load Current for 220 kV %( HV Side) Winding at +10% = MVA / (√3*1.1*kV) =100*10^6/ (√3*220*1.1*10^3) = 238.5 A Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio = 238.5 /400 = 0.56 A As the adopted ratio correction is 1 A the current to relay bias terminal = 0.56 * 1.52 Compensated current internally = 0.8512 A

Compensated current internally on LV = 0.851*1.143 =1A Hence the differential current is = 1-0.8512 = 0.1488 A Bias current = (I1+I2)/2 = (1 +0.1488)/2 = 0.574 A [Since the bias current is less than 0.656 the slope will be zero. Therefore the operating current of the relay will be = Is + (0.3* ( I bias-0.656) = 0.2 + (0.3*(0.574)) = 0.2A Differential current at worst tap condition should not be more than 90% of operating current for better stability. Here it is less than 90% so the setting is acceptable.

2.10 Procedure for Relay Setting of Transformer Differential Relay 7UT61X/7UT51X Data Required MVA Rating Voltage ratio Vector group HV voltage LV voltage Transformer percentage impedance: Transformer vector group: OLTC Tap: +%

OLTC Tap: -%

CT ratio and winding configuration HV side LV side At Normal tap HV Side full load current = MVA / 3*kV Current on CT Secondary Ict (HV) = Rated Current (HV Side) / CT Ratio (HV Side) N1=Required ratio compensation = 1/ Ict sec Assuming Relay current = 1A N1 is set on the relay

Current on HV side = Current on CT secondary (HV)/ N1

At Normal tap low voltage side LV Side full load current = MVA / (3*kV) Current on CT Secondary Ict (LV) = Rated Current (LV Side) / CT Ratio (LV Side) N2=Required ratio compensation = 1/ Ict sec (LV) N2 is set on the relay Current onLV side = Current on CT secondary (LV)/N2 At -% Tap HV side current = MVA / (3*kV Where kV is the voltage corresponding to -% tap on HV side Current on CT Secondary = Rated Current (HV Side) / CT Ratio (HV Side) With ratio compensation Current on CT Secondary =Ratio of compensation * CT secondary current At Tap on +% HV side current = MVA / (3*kV ) Where kV is the voltage corresponding to +% tap on HV side Current on CT Secondary Ict (HV) = Rated Current (HV Side) / CT Ratio (HV Side) With ratio compensation Current on CT Secondary =Ratio of compensation * CT secondary current Differential current at extreme taps At -% tap =Idiff1 =IHV-ILV

At +% tap =Idiff2 =IHV-ILV Ibias = IHV+ILV Operating current of Relay I Operating = Pickup setting + 0.25 * Bias Current if less than 2.5A I Operating = Pickup setting + 0.25 * Bias Current + 0.5 ( Bias current above 2.5 A )

Relay Operating current is calculated using the above equation. For extreme taps the Idiff and Ibias current are calculated., the pick up setting is chosen such that it will not operate for extreme taps Differential Relay Settings Calculations Substation Name: 220 kV GSS Nagaur Relay Type: SIPROTECH 7UT61X Ratings of the Power and Current Transformers Differential Current Setting = 0.2 A Transformer Name: BHEL -II MVA = 100 Voltage Ratio = 220/132 kV Rated Voltage in kV (HV Side) = 220 Rated Voltage in kV (LV Side) = 132 Vector Group = YNa0d1 CT Ratio (HV Side) = 400/1 CT HV Side Vector Group = Star/Star CT Ratio (LV Side) = 500/1

CT LV Side Vector Group = Star/Star Minimum Tap = -15 Maximum Tap = +10 Rated Current (HV Side) = MVA / (√3*kV) =100*10^6/ (√3*220*10^3) =262.43 A Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio = 262.43/400 =0.66 A Required Ratio Compensation = 1/ 0.66 =1.52

Rated Current (LV Side) = MVA / (√3*kV) =100*10^6/ (√3*132*10^3) =437.39 A

Current on CT Secondary (LV) = Rated Current (LV Side)/CT Ratio =437.39/500 = 0.73 A Required Ratio Compensation = 1/ 0.73 =1.14 A Calculations for OLTC tap setting -10%

Full load Current for 400 kV %( HV Side) Winding at -15% = MVA / (√3*0.9*kV) =100*10^6/ (√3*220*0.85*10^3) = 308.74 A Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio = 308.74 /400 = 0.77 A As the adopted ratio correction is 1.52 A the current to relay bias terminal = 0.77 * 1.52 Compensated current internally =1.18 A Hence the differential current is = 1.18 – 1 = 0.18 A Bias current = (I1+I2) = (1.18 +1) = 2.18 A [Since the bias current is less than 2.5 A the slope will be within 25%] Therefore the operating current of the relay will be = Is + (0.5* I bias) = 0.2 + (0.25 *2.18) = 0.74 A Differential current at w tap condition should not be more than 90% of operating current for better stability. Here it is less than 90% so the setting is acceptable. Calculations for OLTC tap setting +10% Full load Current for 220 kV %( HV Side) Winding at +10% = MVA / (√3*1.1*kV) =100*10^6/ (√3*220*1.1*10^3)

= 238.5 A Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio = 238.5 /400 = 0.60 A As the adopted ratio correction is 1.52 A the current to relay bias terminal = 0.60 * 1.14 Compensated current internally = 0.68 A Hence the differential current is = 1 – 0.68 = 0.32 A Bias current = (I1+I2) = (0.68 +1) = 1.68 A

[Since the bias current is less than 2.5 A the slope will be within 25%] Therefore the operating current of the relay will be = Is + (0.25* I bias) = 0.2 + (0.25 * 1.68) = 0.62 A Differential current at worst tap condition should not be more than 90% of operating current for better stability. Here it is less than 90% so the setting is acceptable.

2.11 Differential Relay Settings Calculations GE SR745 Relay Type: GE SR745 Required Data Ratings of the Power and Current Transformers MVA Voltage Ratio Rated Voltage in kV (HV Side) Rated Voltage in kV (LV Side) Vector Group CT Ratio (HV Side) CT HV Side Vector Group CT Ratio (LV Side) CT LV Side Vector Group Minimum Tap = - % Maximum Tap = +% Break point Rated Current (HV Side) = MVA / (√3*kV) Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio Required Ratio Compensation = 1/ Current on CT secondary (HV) Rated Current (LV Side) = MVA / (√3*kV)

Current on CT Secondary (LV) = Rated Current (LV Side)/CT Ratio Required Ratio Compensation = 1/ Current on CT Secondary (LV) Relay current on LV side I2 = Current on CT secondary LV side * Ratio compensation for LV side Calculations for OLTC tap setting -% and +% Full load Current (- % or +% HV Side) = MVA / (√3*kV) Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio Relay Current for HV side I1= Current on Ct secondary (HV)* Ratio compensation for HV side I1 is calculated for extreme transformer taps The differential current is = I1-I2 Bias current I bias = Maximum of(I1,I2) Differential current I dif andIbias is calculated foe extreme Transformer taps Therefore the operating current of the relay = Is + Slope1* I bias for I bias current less than Break point And Is + Slope1*Break point + Slope2* (IbiasBreak point) for I biascurrent greater than Break point.

Pick up setting is chosen such that the Differential current at worst tap condition should not be more than 90% of operating current for better stability.

Sample Calculation of GE SR 745 Substation Name : 220 kV GSS Khinwasar Relay Type: GE SR745 Ratings of the Power and Current Transformers Transformer Name: EMCO MVA = 100 Voltage Ratio = 220/132 Rated Voltage in kV (HV Side) = 220 Rated Voltage in kV (LV Side) = 132 Vector Group = YNA0d1 CT Ratio (HV Side) = 400/1 CT HV Side Vector Group = Star/Star CT Ratio (LV Side) = 500/1 CT LV Side Vector Group = Star/Star Minimum Tap = -15 Maximum Tap = +10 Break point=2 Is=0.2, m1=25%, m2=80% Rated Current (HV Side) = MVA / (√3*kV) =100*10^6/ (√3*220*10^3)

=262.4319 A Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio = 262.4319/400 =0.65607 A Required Ratio Compensation = 1/ 0.65607 = 1.5242 A Rated Current (LV Side) = MVA / (√3*kV) =100*10^6/ (√3*132*10^3) =437.38 A Current on CT Secondary (LV) = Rated Current (LV Side)/CT Ratio =437.38 /500 = 0.875 A Required Ratio Compensation = 1/ 0.875 =1.143 A As the adopted ratio correction is 1.143 A the current to relay bias terminal = 0.875 * 1.143 I1=1 A Calculations for OLTC tap setting -15% Full load Current for 220 kV %( HV Side) Winding at -15% = MVA / (√3*0.85*kV) =100*10^6/ (√3*220*0.85*10^3) =308.743 A Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio

= 308.743 /400 =0.7719 A As the adopted ratio correction is 1.5242 A the current to relay bias terminal = 0.7719 * 1.5242 I2=1.176 A Hence the differential current is = (I1-I2) =1.176 – 1 = 0. 176 A Bias currentIbias = Max(I1,I2) = (1.176,1) = 1.176 A [Since the bias current is less than Break point the slope will be within 25%] Therefore the operating current of the relay will be = Is + (0.25* I bias) = 0.2 + (0.25 * 1.176) = 0.494 A Differential current at worst tap condition should not be more than 90% of operating current for better stability. Here it s less than 90% so the setting is acceptable.

Calculations for OLTC tap setting +10% Full load Current for 220 kV %( HV Side) Winding at +10% = MVA / (√3*1.15*kV) =100*10^6/ (√3*220*1.1*10^3) =238.574A Current on CT Secondary (HV) = Rated Current (HV Side)/CT Ratio = 238.574 /400

=0.5964 A As the adopted ratio correction is 1.5242 A the current to relay bias terminal = 0.5964* 1.5242 I2=0.909 A Hence the differential current is= (I1-I2) = 1 - 0.909 = 0.091 A Bias current Ibias = Max(I1,I2) = (1, 0.909) =1A [Since the bias current is less than Break point the slope will be within 25%] Therefore the operating current of the relay will be = Is + (0.25* I bias) = 0.2 + (0.25 * 1) = 0.45 A Differential current at worst tap condition should not be more than 90% of operating current for better stability. Here it s less than 90% so the setting is acceptable.