NGT Sizing Calculation

ABBCOASTAL ENERGEN PRIVATE LIMITED

2 X 600 MW

THERMAL POWER PLANT

Doc No: 1013-3VYN294303-01

Rev No:00

= 20000 V

a = 0.227 mF

b = 0.25 mF

c = 108.8 pF/m/ph

= 84 m

= 0.009139 mF

d = 66.1 pF/m/ph

= 27 m

= 0.001785 mF

e = 43.94 pF/m/ph

= 53 m

= 0.002329 mF

f = 14700 pF/m/ph

= 0.0147 mF

g = 3000 pF/m/ph

= 0.003 mF

h = 3000 pF/m/ph

= 0.003 mF

i = 3280 pF/m/ph

= 0.0033 mF

Vp = 20000 V

Vs = 240 V

N = 83.333

= 11547 V

Considering 30% of force field condition Normal phase to neutral voltage =1.3X Vp1 Vff = 15011

= 0.514233 mF

XC = 1/2pifC ohms/ph

XC = 6190.00 ohms/ph

Ic = 2.425 Amps

It = 7.275 Amps

P = 145.50 kVA

= 13.86 kVA

= 15 kVA

XCt = 2063.33 ohm

Rsec = 0.297 ohm

= 0.297 ohm

Resistance value required at secondary side, Rsec = Req/N^2

Selected secondary Voltage of the Grounding Transformer

II. CALCULATION OF NGT PARAMETER :

(From UT manufacturer Data)

(From UT manufacturer Data)

(From ST manufacturer Data)

Selected primary Voltage of the Grounding Transformer

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

Selected Neutral Grounding Transformer rating

III. CALCULATION OF NGR PARAMETER :

Zero-Sequence Capacitive charging current/phase = Vff x Xc

Total system Capacitive charging current, 3 * Ic

Total length of IBPD

Total Busduct capacitance/phase

Hence, turns ratio

Normal Phase to Neutral Voltage = VP1 = VL-L / Sqrt(3)

Equivalent UT-1A capacitance / phase

Equivalent ST-1 capacitance / phase

Equivalent GT capacitance / phase

Busduct capacitance Delta run (in ref. to standard Manufacturer's Data)


SIZING CALCULATION OF GENERATOR NEUTRAL GROUNDING TRANSFORMER (NGTR)

I. EQUIPMENT DATA :

Generator Voltage, VL-L

Generator stator winding capacitance / phase

Capacitor considered per phase =

Busduct capacitance main run (in ref. to standard Manufacturer's Data)

( in ref. to 600MW HPE Generator data)

(in ref. to standard manufacturer data)


(From GT manufacturer Data)

Total zero sequence capacitance C = (a)+(b)+(c)+(d)+(e)+(f)+(g)+(h)+(i)

Power rating of the grounding transformer = (VpXIt)/1000

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

Busduct capacitance Tap off run (in ref. to standard Manufacturer's Data)



Equivalent UT-1B capacitance / phase

Zero-sequence Capacitive reactance to ground

Residual Capacitive reactance Xc/3

Selected value of neutral grounding resistor

IV. CALCULATION OF NGR CURRENT RATING :

of 2

ABBCOASTAL ENERGEN PRIVATE LIMITED

2 X 600 MW

THERMAL POWER PLANT

Doc No: 1013-3VYN294303-01

Rev No:00

Rp = 2063.33 ohms

If = Vff/ Rp Amps

If = 5.60 Amps

Isec = 466 Amps

A : 13.86 kVA

: 15 kVA

: 20000/240 Volts

: 10 sec

: 157.5 kVA

: 175 kVA

B : 0.297 Ohms

: 0.223 Ohms

: 0.149 Ohms

: 0.074 Ohms

: 500 Amps

: 10 sec

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

Ratio

Resistance Value of Grounding Resistor at 75% Tap


Short Time Rating of Trafo = Overloading factor x Continuous Rating

Required rating of the Neutral Grounding Transformer (NGT)

Selected rating of the Neutral Grounding Transformer (NGT) continious

Resistance Value of Grounding Resistor (NGR)


Current Rating

Duty

Duty

Selected rating of the Neutral Grounding Transformer (NGT) for short time rating of

10sec

Single line to ground fault current (NGT's self impedance is not considered)

Current in secondary of Neutral Grounding Transformer

V. SUMMARY :

Selected Grounding Resistor value reflected in the primary side (Rsec/N^2)

Fault current through generator neutral under single line to ground fault condition

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NGT Sizing Calculation

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