Short circuit on LV Switchgear fed from two transformers in paralell. Cases a.b.c

[mcm00048]

Hello All,

I have to calculate the Short circuit rating of a LV Switchgear. ( See pdf attached). The LV Switchgear is fed from two 800kVA transformers running in paralell and each transformer is working at 50% Load. I have made a quick calculation of SC current but I need to know what case would be appropiate for sizing:

a. Short circuit current provided by two 800kVA trafo at 100% Load ( It will never happen, since the transformers are sized to provide N+1 redundancy)
b. Short circuit current provided by two 800kVA trafo at 50% Load
c. Short circuit current provided by one 800kVA trafo at 100 % load

For the calculation of one transformer 800kVA full load, I get 19,27kA. If I have two transformers 2x800kVA @50% Load, this value would be 2xTimes of SC provided from two 400kVA transformers? See table of cases:

Thank you so much in advance

 

[wbd]

The loading of the transformers has nothing to do with the short circuit current that the transformer can deliver. Under a short circuit, the transformers will deliver what ever the source (the grid they are tied to) can deliver.

Theoretically, the maximum would be using an infinite bus and this would be ~38.5 kA as noted in your chart. This is what you need to use for sizing the equipment.

 

[jghrist]

As wbd noted, the fault contribution from the transformers does not depend on the load. It depends on the rated capacity and impedance. If the load is direct connected motors, they will contribute to the fault based on how many are connected to the bus and running, their ratings, and the impedance of feeders. You have not considered the impedance of the system on the source side of the transformers.

 

[cuky2000]

Since the transformers are sized to provide N+1, it is suggested to consider a tie-breaker in the LV switchgear with the possible benefits:
a) Reduce the SC duty in 50% (~20 kA instead 40 kA)
b) Increase the reliability since LV system could remain in service in the event of a SC.
See the suggested option below.

 

[crshears]

The suggestion by cuky2000 can be very cost-effective, and if an outage is required to one of the transformers, from an operational standpoint [ at least within my utility ] it is considered an acceptable practice to close the bus tie breaker, adjust the taps on the retiring transformer so as to reduce its output of reactive power to zero while raising those on the companion to maintain normal LV bus voltage, then unload the retiring transformer via its secondary breaker before removing it from potential and isolating for work protection.

The risk of briefly paralleling the two sources for switching purposes has been deemed acceptable, as the probability of a fault occurring in this brief state is not considered so likely as to warrant a complete interruption of half of the supplied loads. There will of course be customers or entities who think the risk of paralleling is too great and insist on a break-before-make approach to switching, and that is of course their right; there can be a cost to this, however, as one simple-cycle CT generating station I know of learned to their chagrin, as they never anticipated how often the IESO would ask them to switch their station service supplies from one 230 kV circuit to the other...it happened so often that they finally chose to permanently supply their station service from a much less reliable rural low-tension feeder rather than violate their perceived risk of swapping station service supplies on a make-before-break basis.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[7anoter4]

According to IEC 60909-0 standard [see 3.3.3 chapter]:
A network transformer is a transformer connecting two or more networks at different voltages. For two-winding transformers with and without on-load tap-changer, an impedance correction factor KT is to be introduced in addition to the impedance evaluated according to equations (7)[ZT=RT+jXT ZT=ukr/100*UrT^2/SrT]
If the long-term operating conditions of network transformers before the short circuit are known for sure, then the following equation (12b):
KT=Un/Ub*cmax/(1+xT*(IbT/IrT)*sin(φbT))
cmax is the voltage factor from table 1, related to the nominal voltage of the network connected with low-voltage side of the network transformer;
Ub is the highest operating voltage before short circuit;
IbT is the highest operating current before short circuit (this depends on network configuration and relevant reliability philosophy);
Un is the rated voltage
φbT is the angle of power factor before short circuit.
However, the current influence is weak so I do not agree with the results from the
above table[mainly the short-circuit current for one half of rated [IbT].

 

[ScottyUK]

Hi crshears,

In the UK short-term paralleling does occur in some plants, but those doing so are in breach of the UK's Electricity at Work Regulations which require that no equipment is operated beyond its maximum capability. No allowances for short periods only, no allowances for risk assessments. My own opinion is that if something is going to go wrong, it's more likely to do so when breakers are changing states or when loads are suddenly applied or removed to equipment - anything which disturbs the existing steady-state. Clearly I'm not going to write a switching instruction which requires breaking the law - you'd have to be a special kind of stupid to do that - but from a more selfish point of view almost all our gear requires local manual operation, and I'd rather not have one of us stood in front of it when it comes apart.

 

[unclebob]

To the OP,

it's very odd that your 2-ended switchgear has no tie breaker in it, or you just didn't draw it (and the main breakers too).
In what country are you?

 

[crshears]

Hi Scotty, I'm thankful that there is at least a little wiggle room allowed in my neck of the woods when it comes to make-before-break switching, as our SAIDI and SAIFI numbers would get real nasty real quick otherwise...

That being said, the point in your last sentence is well taken, and for what it's worth we're not cavalier about the risks, either; in those instances where such switching is taking place the area in proximity to the switchgear being operated is confirmed clear of all personnel prior to, and this and numerous other local/manual operations of switchgear are prohibited for the very types of reasons you describe.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]