Equivalent UT-1A capacitance / phase (From UT manufacturer Data)
g
= =
3000 pF/m/ph 0.003 mF
Equivalent UT-1B capacitance / phase (From UT manufacturer Data)
h
= =
3000 pF/m/ph 0.003 mF
Equivalent ST-1 capacitance / phase (From ST manufacturer Data)
i
= =
3280 pF/m/ph 0.0033 mF
Selected primary Voltage of the Grounding Transformer
Vp
=
20000 V
Selected secondary Voltage of the Grounding Transformer Hence, turns ratio
Vs N
= =
240 V 83.333
=
11547 V
=
15011
=
0.514233 mF
II. CALCULATION OF NGT PARAMETER : Normal Phase to Neutral Voltage = VP1 = VL-L / Sqrt(3) Considering 30% of force field condition Normal phase to neutral voltage =1.3X Vp1
Vff
Total zero sequence capacitance C = (a)+(b)+(c)+(d)+(e)+(f)+(g)+(h)+(i) Zero-sequence Capacitive reactance to ground
XC XC
= =
1/2pifC ohms/ph 6190.00 ohms/ph
Zero-Sequence Capacitive charging current/phase = Vff x Xc
Ic
=
2.425 Amps
Total system Capacitive charging current, 3 * Ic
It
=
7.275 Amps
Power rating of the grounding transformer = (VpXIt)/1000
P
=
145.50 kVA
Power rating of the grounding transformer with overloading factor of 2.6 in ref. to table1 of IEEE C62.92.02-1989 for 10sec duty
=
13.86 kVA
Selected Neutral Grounding Transformer rating
=
15 kVA
=
2063.33 ohm
III. CALCULATION OF NGR PARAMETER : Residual Capacitive reactance Xc/3
XCt
A per IEEE C62.92.02 - 1989, The grounding resistor is sized so that the ratio between Capacitive Reactance (XCt) and effective resistance (Req) between the generator neutral and ground is 1. Hence, Req = XCt Resistance value required at secondary side, Rsec = Req/N^2
Rsec
Selected value of neutral grounding resistor IV. CALCULATION OF NGR CURRENT RATING :
Page 1 of 2
=
0.297 ohm
=
0.297 ohm
COASTAL ENERGEN PRIVATE LIMITED 2 X 600 MW THERMAL POWER PLANT
ABB
Selected Grounding Resistor value reflected in the primary side (Rsec/N^2)
Doc No: 1013-3VYN294303-01 Rev No:00
Rp
=
2063.33 ohms
Fault current through generator neutral under single line to ground fault condition
If
=
Vff/ Rp Amps
Single line to ground fault current (NGT's self impedance is not considered)
If
=
5.60 Amps
Isec
=
466 Amps
Required rating of the Neutral Grounding Transformer (NGT)
: : : : :
Current in secondary of Neutral Grounding Transformer V. SUMMARY : A
Selected rating of the Neutral Grounding Transformer (NGT) continious Ratio Duty Short Time Rating of Trafo = Overloading factor x Continuous Rating Selected rating of the Neutral Grounding Transformer (NGT) for short time rating of 10sec B
Resistance Value of Grounding Resistor (NGR) Resistance Value of Grounding Resistor at 75% Tap Resistance Value of Grounding Resistor at 50% Tap Resistance Value of Grounding Resistor at 25% Tap Current Rating Duty
Note: As per TCE/Energn contract the ground fault limiting current for generator neutral is 1A. As per system capacitance considered (As per above calculation), The capacitive current value is 7.275Amps. But according to the design philosophy the resistive component of the ground fault current should be greater than or equal to capacitive component, to minimize the voltage surges. Hence 1A is not a realistic figure as a limiting current for generator nuetral
