NGR Sizing Calculation

NGR Sizing Calculation Transformer Rating

7500

kVA

Primary Voltage

11000

VAC

Secondary Voltage

6900

VAC

% Impedence

8

%

Rated current at 6600 VAC Amps

627.55

Amps

Isc * Amps

6275.55

Amps

Fault kVA

75

MVA

System Charging Current (Typically 1 A per 1 MVA for Medium Voltage Systems) (7500 x 1) / 1000 =

7.5

Amps

The let‐through current of the NGR should be at least 10 times higher than the System charging current So NGR's Required minimum let‐through Current = (7.5 A x 10) =

75

Amps

From the above it is evident, that the minimum rating of NGR should be 75A, considering the stability during the system charging. However, the NGR is selected for limiting the fault current to 5 % of the Symmetrical fault current which works out to be 314 A. The NGR is rated for 10 seconds, as the protective devices will clear the ground the fault in less than 1s, depending on the type & magnitude of the earth fault. NGR Current Rating Value of Fault Current limited as a % of Isc NGR Voltage Rating (6900/√3 ) = Value of Resistance (V/I)

315 5.02 3983.72 12.65

Amps % VAC Ω

Cable selection for NGR As the NGR is rated for 10 Seconds , the cables need to be sized for a maximum of 10 seconds. S = (A /K ) x √3 Where S = Cross sectional area of the conductor in sq.mm A = Withstand capacity of the cable in kilo Amperes 0.315 K = Factor for Copper XLPE Cables = 0.143 t = Fault clearing time in Seconds. = 10

kA (as per cable manufacturer.)

S = A x √ t K S = 6.97 Hence S =

6.97

Sqmm

However, as a conservative measure, One run of 6.6 kV, 3c x 70 Sqmm, copper, XLPE cables is selected, with all the 3 cores of the cable connected to NGR and transformer

627.573