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Fault Level by MVA

Russell Fox / February 22, 2017

There are two methods of fault level analysis that I am aware of, firstly the 'per unit' (p.u.) base method, the second being the ‘MVA’ method.  Essentially, the 'per unit' method considers elements as p.u.’s of a base network.  The MVA method considers all components as loads within the system, contributing or absorbing.

I find the MVA method slightly easier to get to grips with and this [age lists some of the essential elements in compiling the analysis.


Components

Utility base:             MVA(c) = MVA(ut)

Transformer:            MVA(c) = MVA²(tx) / Z(tx)%

Cable:                     MVA(c) = V² / Ω   x10-6

Motor (Reactors):     MVA(c) = MVA(mt)/ K%

 
Motor loads may be lumped together with a percentage contribution factor applied:
For motors <37kW . . . K = 25%
For motors >37kW . . . K = 17%

Where:
MVA(c)        = Component contribution in MVA
MVA(ut)    = Utility network contributing fault level in MVA
MVA(tx)    = Transformer rating in MVA
MVA(mt)   = Motor operational load in MVA
V             = The voltage at that particular point in the system
Z(tx)%     = Transformer impedance as a percentage
K%          = Motor contributing factor


Summating components:

Summate the contributing loads from source(s) to the fault location.
Where components in parallel are added

Where components in series are added inversely. i.e. the inverse of the sum of inversed components . . . as for resistance in parallel


Results

To determine the resulting fault levels, divide the resulting MVA component by the system voltage.


Ready Reckoner:

In determining the anticipated fault level at a supply intake from a substation transformer, for a simple arrangement of transformer connected to a utility network, given typically expected equipment ratings, the following are expected at the LV connection of 400V, 50Hz:

Earth Fault Current (kA)
Transformer

System



Infinity500250125
kVA
Z(tx)%
kA
kA
kA
kA
100
4.00%
3.63.63.63.5
225
4.00%
8.18.07.97.8
300
4.00%
10.8
10.7
10.5
10.2
500
4.50%
16.0
15.7
15.4
14.7
750
5.00%
21.7
21.0
20.4
19.3
1,000
5.00%
28.9
27.8
26.7
24.9
1,250
5.00%
36.1
34.4
32.8
30.1
1,500
5.50%
39.4
37.3
35.5
32.3
2,000
5.50%
52.5
48.9
45.8
40.7
Three Phase Fault Current (kA)
Transformer

System



Infinity500250125
kVA
Z(tx)%




100
4.00%
6.36.26.26.1
225
4.00%
14.113.913.813.5
300
4.00%
18.8
18.518.217.7
500
4.50%
27.8
27.226.625.5
750
5.00%
37.5
36.435.433.5
1,000
5.00%
50.0
48.146.343.1
1,250
5.00%
62.5
59.556.852.1
1,500
5.50%
68.2
64.7
61.5
56.0
2,000
5.50%
90.9
84.7
79.4
70.4
Notes:
*The above values are ‘Rule of thumb’ values, no allowance has been made for cable connection impedances. Exacting calulations are recommended for all actual installations.
** Transformer percentage impedance are approximate to manufacturer's data. Use particular specification for actual installations.


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Canberra,
Australia.
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