## Short Circuit Rating

## Short Circuit Rating

(OP)

The Short Circuit MVA level of a 132/33kv substation is 20.31pu on 100MVA base. 2 x 60MVA Transformers were installed in the substation and the two packed up within a very short time. What factors do we consider in choosing a transformer for this substation please.

## RE: Short Circuit Rating

## RE: Short Circuit Rating

In terms of short circuit you need to know the rating of the 33kv breakers, their X/R ratio interrupting rating, buss bar (short circuit force withstand) and equipment down line of the 33kv as it may see an increase in current during faults. Second you need to know the source strength of the 132kv supply if not using an infinite assumption in modeling 33kv short circuit.

When simulating faults, always do L-L-L; L-L; L-L-G; and L-G.

A but more info is needed, but there are a number of ways to go about selecting transformers or another transformer.

## RE: Short Circuit Rating

## RE: Short Circuit Rating

Transformers are built in order to be able to withstand a short circuit limited only by it's own impedance for 2 seconds (IEC and ANSI)

Notice the difference betweeen total shortcircuit current on the faulted bus and shortcircuit current flowing through the transformer

## RE: Short Circuit Rating

Now in terms of secondary short circuits outside the transformer itself the current is limited well below 35ka due to the transformer's own impedance. Most power transformers can easily survive multiple short circuits for a few seconds through out their life time. In fact it is usually the mechanical stress that concerns the manufactures well more than the thermal aspect- they do take that into account very well.

## RE: Short Circuit Rating

## RE: Short Circuit Rating

## RE: Short Circuit Rating

a) Which side of the transformer (primary or secondary side) the short circuit current is going to be increased? The terminal box and the connected cables (or bus duct) of that particular side should be capable of withstanding the new short circuit current.

b) The other side would witness a marginal increase in the short circuit current.

c) The transformer through fault current withstand capability and its characteristics, remain almost un-affected. Even if the thru fault current increases marginally, (due to the increased SC level at one side), the transformer design value takes care of this. Hence no issue.

## RE: Short Circuit Rating

Let us consider a case of 100 MVA load to be catered by a station at say 132/33 kV. With an impedance of 10% , the secondary fault level in 100 MVA transformer will be 100/0.1 =1000 MVA, assuming infinite fault level at 132 kV grid. If the secondary breaker cannot handle this ,then transformer impedance has to be increased or transformer MVA has to be reduced. Number of transformers is decided as Mbrooke explained-(n-1) principle. Even with the failure of one unit, the entire load has to be handled by balance transformers. So you can have 2x 100 MVA units or 3X50 MVA units. If the 132 kV fault level is 10,000MVA (43.7 kA)then the fault MVA seen by transformer will come down to 100/(10% +1%) =909 MVA. But when the secondarys of two 100 MVA units are paralleled with common secondary breaker, then fault level will be 2000 MVA. So either impedance should be increased to 20 % or use 3x50 MVA transformers with 15 % impedance to limit fault level.

## RE: Short Circuit Rating

You have to take only one transformer into consideration and to use the actual short-circuit

impedance[about 12.5%].

## RE: Short Circuit Rating

## RE: Short Circuit Rating

## RE: Short Circuit Rating

Hi Mbrooke, i've done some research:

IEC: 60076-5 (Power transformers - Ability to withstand short circuits)

ANSI: IEEE Std C57.12.00 (IEEE Standard for Standard General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers)

IEEE Std. defines four categories of transformers based on their size:

ANSI Category I: from 5 to 500 kVA single phase or from 15 to 500 kVA three phase

ANSI Category II: from 501 to 1667 kVA single phase or from 501 to 5000 kVA three phase.

ANSI Category III: from 1688 to 10000 kVA single phase or from 5001 to 30000 kVA three phase

ANSI Category IV: above 10000 kVA single phase or above 30000 kVA three phase

For Category I t=1250/I^2

"For Category II, III, and IV units, the duration of the short-circuit current as defined in 7.1.4 is limited to 2 s, unless otherwise specified by the user. "

7.1.4.1 Category I: "The symmetrical short-circuit current shall be calculated using transformer impedance only except that the maximum symmetrical current magnitudes shall not exceed the values listed in Table 16. "

7.1.4.2 Category II: "The symmetrical short-circuit current shall be calculated using transformer impedance only"

7.1.4.3 Categories III and IV: "The symmetrical short-circuit current shall be calculated using transformer impedance plus system

impedance, as specified by the transformer user."

PS: It's not as simple as I stated in my previous comment

Hope this clarifies

Regards,

Juan

## RE: Short Circuit Rating

IEEE C37.91-2008

IEEE C57.109-1993

The buff book (IEEE 242) section 11.9.2.2.1 discusses transformer through-fault capability.

## RE: Short Circuit Rating

## RE: Short Circuit Rating

Can you elaborate on this? I admit I don't know much.

BTW Juan and Wroggnet, much thanks.

## RE: Short Circuit Rating

As the X:R ratio increases the asymmetric current will approach 2 x 2.82 of the RMS value.

The magnetic forces increase as the square of the current.

(√2 x 2)

^{2}= 8 times the magnetic forces of the forces developed by the RMS current.Bill

--------------------

"Why not the best?"

Jimmy Carter

## RE: Short Circuit Rating

The asymmetry factor varies according to X/r ratio of Transformer. It will vary 1.76 (X/r=2) 2.46(X/r=10) 2.55 (X/r=14) As per IEC 60076-5 Power Transformer- Ability to withstand Short circuit, Clause 4.2.3, if X/r >14, asymmetry factor to be taken as 2.55 up to 100 MVA 3 phase rating and 2.69 for higher ratings.

## RE: Short Circuit Rating

Bill

--------------------

"Why not the best?"

Jimmy Carter

## RE: Short Circuit Rating

î = I × k ×√2

where I=U/(Zs+Zt) the rms value as prc said, U[kV]; Zs =system impedance [ohm]=U^2/Sscsystem Zt=Ur^2/Srt*zt%/100 [ohm]

Ur[kV] it is the rated voltage of the referred winding and Srt=transformer rated power[MVA].

k=(1+(e^–(φ+π/2)R/X)sinφ) where:

e is the base of natural logarithm;

φ is the phase angle which is equal to arctan X/R, in radians.

That means for X/R=∞ k=2 and this is the theoretical limit.

Usually for II category [2.5-100 MVA] k=1.8 and for III category[above 100 MVA]=1.9

## RE: Short Circuit Rating

## RE: Short Circuit Rating

JBC

.......

"The more I read, the more I acquire, the more certain I am that I know nothing"

## RE: Short Circuit Rating