Selection Of The New 4 MW S1 GTG Generator Neutral Earthing & Sizing Of Its Neutral Earthing Transformer & Its Secondary Loaded Earthing Resistor 1
Basis Of Design In order to protect stator core from damage during internal stator earth fault, it is necessary to limit stator earth fault to as low a value that is practicable. Generators/Motors connected to the 11 kV system, through 33/11.5 kV transformer unit, can be treated as an isolated system which is not influenced by the earthing requirements of the distribution system The industry wide practice for such generators/motors is to use a 11 kV neutral earthing transformer suitably rated with secondary winding designed for 240 V/ 110 V or so and loaded with a resistor of a value which when referred to the primary will pass a low earth fault current of say 5 to 10 A. The Neutral earthing resistor connected shall be of low ohmic value of rugged construction while still presenting a high equivalent value in the generator 11 kV neutral circuit The Neutral resistor shall be incorporated to prevent the production of high transient over voltages (TOVs) in the event of an arcing earth fault which it does by discharging the bound charge in the circuit capacitance.
To prevent transient over voltages, it is necessary to ensure the equivalent resistance in the stator circuit should not exceed the impedance at system frequency of the total summated capacitances of the three phases of the generator/motor circuit. In this respect the resistive component of stator EF current should not be less than (not < than) the residual capacitance current ( 3.Ico ) in the generator/motor 11 kV circuit upto tranformer. For Selecting Earthing Transformer, it is necessary to ensure it never becomes saturated otherwise a very undesirable condition of ferro-resonance may occur. In this respect it is usual to select transformer having a primary winding with knee-point voltage (Vk) = to 1.3 x generator/motor rated line voltage of 11000 V (11 kV)
2.0 Sizing Calculation 2.1 Selection Of EF Current Limit 2.1.1 Generator Data Generator Rating = Rated Gen kV Maximum Overvoltage 2.1.2 Transformer Data Ratio 2.1.3 Total Capcitance Per Phase Generator Generator Connections Generator-Transformer 11 kV 11 kV Cable 100 m Surge Suppressor Capicatance
2.1.4 TOTAL RESIDUAL CAPACITIVE IMPEDANCE (10^6 ) / (3.w.C)
2222.99 Ohm
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Selection Of The New 4 MW S1 GTG Generator Neutral Earthing & Sizing Of Its Neutral Earthing Transformer & Its Secondary Loaded Earthing Resistor
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Selection Of The New 4 MW S1 GTG Generator Neutral Earthing & Sizing Of Its Neutral Earthing Transformer & Its Secondary Loaded Earthing Resistor 2.2 Neutral Earthing Resistor The Effective Resistance Should Be Made Equal To Residual Capacitive Impedance (2222.99 Ohm calculated above) To Ensure TOV Is Controlled Then Resistive Component Of EF
3.39
A
(11000/Sqrt(3)/ 2222.99
2.86
The Actual Fault Current Will Contain Equal Resistive & Capacitive Components & The Net EF Current Will Be Sqrt (Ir^2+Ic^2) 4.43 A SQRT(2.856^2+2.856^2) 2.3 Neutral Earthing Distribution Transformer (Neutral ET or N ET) & NER Sizing 2.3.1 N ET Transfo Primary Vk (Knee Point Voltage) 15.000 kV ( > 1.3*11 kV) Applied kV On N ET During EF 6.351 kV (11 / sqrt(3) ) Applied kV On N ET During EF With Field Forcing
8.256
kV
Increase in Neutral EF Current
3.714
A
Maximum N ET Loading Under Above Condition
55.709 kVA
(15*3.714)
Considering 30 sec duty, the N ET Can Have 6 Times Short Term 30 Sec Overload Withstand With kVA Considered Based On Maximun EF Amp & Knee Point kV
9.285
(55.709/6)
( 8.26/6.35)*2.86 A
kVA
10.000 2.3.2 N ET Secondary Rating Consider Secondary Knee Point Of Maximum Secondary Current
1.3 x
kVA
240.000 V 232.12 A
30 sec rated
(3.714*15000/240)
2.3.3 N ET Secondary Loading Resistor Equivalent Neutral Primary Resistor
2222.99 Ohm
(= EF Capacitance Impedance Calculated In 2.14 Clause Above)
Actual Secondary Loading Resisor Based on 15 kV / 250 V N ET
0.57
Ohm
At 240 V Side
N ET Secondary Amp
41.67
A
(10000/240)
Type N ET Cu Loss Type N ET Resistance Due Cu Loss
310 0.18
Watt Ohm
Typical For Such Rating Watt / I ^2 = 310/(40)^2
Therefore N ET Secondary Loading Resistor Rated N ET Secondary Resistor Ampere For 30 Seconds
0.39 Ohm (0.569085-0.17856) 232.12 A (For 30 Sec) ,(1.3*2.857*11000/240)
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Selection Of The New 4 MW S1 GTG Generator Neutral Earthing & Sizing Of Its Neutral Earthing Transformer & Its Secondary Loaded Earthing Resistor 2.3.4 Neutral Earthing Transformer Reactance Typical % X Of N ET Typical X pu Of N ET N ET Rating (Base kVA) N ET Rating (Base MVA) Base kV X Base N ET X in Ohm N ET R in Ohm AS Calculated Above X/R Ratio Of N ET
4 0.04 10 0.01 0.25 6.25 0.25 0.18 1.4
% PU kVA MVA kV Ohm ( kV^2/MVA) Ohm X pu x X Base Ohm (This Should Not Exceed 2)
3 Summary Of Requirements N ET Rating 10 kVA 30 sec rating N ET Ratio 15000V/240V N ET Secondary Loaded Resistor 0.39 Ohm N ET Secondary Loaded Resistor 30 Second Current Rating 232.12 A For 30 Second N ET X/R Ratio 1.4 (Not > 2.)
