VK+Formula Calculation +Sample

Customer : Project : ABB Ref.no. : Vk Value calculation for Transformer Diff. Protection ( Relay used : CAG34) System Voltage in kV CT Primary CT Secondary ( 1 or 5A) Trasnformer rating in kVA Rct (CT Sec.) RL (Lead Resistance)

11 1200 1 500 8 100

Formula : Vk = 40 I (Rct+2RL)

Vk

Vk value required is Magnetising current, Ie < 30mA at VK / 2

394.40 414

Volts

Customer : Project : ABB Ref.no. : Vk Value calculation for Transformer REF Protection ( Relay used : CAG14) System Voltage in kV CT Primary CT Secondary ( 1 or 5A) Trasnformer rating in kVA Percentage Impedence ( Z %) Rct (CT Sec.) RL (Lead Resistance)

6.6 250 1 2250 6.5 2 5

Full load current = Transformer Rating / ( V3 / System Voltage)

196.83

Fault current = Full load current / Percentage Impedence Fault Current reflected to CT Sec. = (Fault current / CT Primary) x CT Secondary Formula : Vk = 2 If (Rct+2RL)

3028.15

If

12.11

Vk

50.70

Vk value required is Magnetising current, Ie < 30mA at VK / 2

56

Volts

Customer : Project : ABB Ref.no. : Vk Value calculation for Motor Diff. Protection ( Relay used : CAG34) System Voltage in kV CT Primary CT Secondary ( 1 or 5A) Motor rating Power factor Percentage Impedence ( Z %) Rct (CT Sec.) RL (Lead Resistance)

6.6 350 1 2555 0.8 8 2 5

Full load current = Transformer Rating / ( V3 / System Voltage)

279.39

Fault current = Full load current / Percentage Impedence Fault Current reflected to CT Sec. = (Fault current / CT Primary) x CT Secondary Formula : Vk = 2 If (Rct+2RL)

KV A A kW Cos % Ohm Mtrs.

3492.36

If

Vk

Vk value required is Magnetising current, Ie < 30mA at VK / 2

9.98

41.77 46

Volts

Customer : Project : ABB Ref.no. : Vk Value calculation for Stabilising Resistors & Metrocils ( Relay used : CAG34) System Voltage in kV CT Primary CT Secondary ( 1 or 5A) Motor rating Power factor Percentage Impedence ( Z %) Rct (CT Sec.) RL (Lead Resistance) Relay rated current (1 or 5A) Assuming relay setting (10-40%) Relay burden at setting current Relay setting in Amp (Is)

6.6 250 1 1805 0.8 8 2 5 1 0.1 1 0.1

Full load current = Transformer Rating / ( V3 / System Voltage)

KV A A kW Cos % Ohm Mtrs. A VA A 197.38

Fault current = Full load current / Percentage Impedence

2467.21

Fault Current reflected to CT Sec. = (Fault current / CT Primary) x CT Secondary

If

Formula : Vk = 2 If (Rct+2RL)

Vk

Vk value required is

9.87

41.31 45

Volts

Magnetising current, Ie < 30mA at VK / 2 Voltage across relay Formula : Vr = If (Rct+2RL)

Vr

20.66

Stabilising Resistor Formula : Rs = {Vr-Is - VA Burden of Relay / (Is) Sq.}

Rs

106.55

Vk

2038.45

V Peak

812.30

Vf = Maximum volts that would produce if CT did not sature under internal faults. Formula : Vf = If {Rs + VA Burden of Relay / (Is) Sq.} V Peak = Peak Volts developed by CT under internal faults Formula : V Peak = 2 V2 Sqrt ( Vk ( Vf -Vk ) )

Since Peak Voltage is less than 3kV Metrocil is not required.