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(a) Ik (steady state short circuit current) or 30 cycle Ib (symm break current) for short time current rating of the breaker and switchboard - Thermal (b) Ib (3 or 5 (for older MV breakers) cycle symm break current) should be use for the breaker's fault current breaking capability (c) ip...

Yes, for all three phase balanced faults. Your bus admitance matrix for unbalance faults will have to be compiled from an equivalent sequence component circuit or using Modal domain to decouple the impedances.

Thanks chuky for your valuable post. Do you have a reference to that info as the Yyd transformer zero sequence impedance is different from the typical values I have used in the past. I did recieved actual values on a small Yyd Tx once from a manufacturer and the zero sequence imp was about 1.3...

You will have to ask the manufacturer for the exact impedance values. For preliminary calcs use X(0)/X(1)=1. This is normally an adequate approximation for all YNd (0.8-1.0), Dyn (0.9) and YNyn transformers. Different values should be use for Yzn (0.1-0.15) and YNyn0d (1.5-3.2) transformers. I...

I am more familiar with the Australian Standards (AS3851 - Calc 3 phase short circuit currents) but it is based on the IEC 60909. AS 3851 (and I assume IEC 60909) provides a very simple guide on how to represent a three winding transformer using three two winding transformers in either a star...

Hi David, it does look very interesting. Would you mind elaborate a bit on your research or is there an abstract available on the Web?

Hi Arango, I have never try to simulate inrush current using ATP but can give you some advise in entering saturation data into your model. As far as I know, you will not be able to use BCTRAN to simulate saturation and hysteresis. I would appreciate it if you can let us know if you do find a...

cgrondzinski, thanks for showing me the difference in resistivity and resistance. Hopefully I will not mixed it up in future. I aggree with dpc that the influence of duct size on the overall ampacity is very small and the duct size is a function of the cable diameter and not a way to increase...

Your cable rating will increase (because thermal resistivity will reduce) with the increase in duct diameter. The influence are: (a) Thermal Resistivity through cable air from cable surface to duct inside surface - mostly (but not totally) independant of duct size (b) Thermal Resistivity...

swgrmfg, you are correct - I have refered to a series and shunt gapped MOSAs as I thought they might be the worst case.

It sounds as if you are on one of the gold/platinum mines in S.A. - most of the older mines used ONAN transformers. If so, you can then possibly uprate the transformer to ONAF and get a few extra MVA. Also, the main incoming transformers normally have on-line tap changers that will easilly...

I aggree it is an interesting question. My knowledge is limited to modeling and some theory. The first MOSA that have a spakover of the gap will cause a drop in the voltage that can cause a reduction in the current of the second MOSA to below the spark-over point(this current is well within...

Hi Mineguy, the particular switchgear that we will be using specify various ceilling heights for various fault currents for Internal Arcing Current Certification - the busbars is very high in comparrison to other MV switchgear. IEC 62271-200 or IEEE 37.20.7-2001 provides more detail for IAC...

It is a ABB - UNITROL-F AVR. The purpose of the calculations is for the substation ceiling hight design. The manufacturer provide certain minimum ceiling hights for specific fault currents (busbar arcing faults) and fault duration. I want to calculate the I2t during the first 0.5 sec or 1 sec...

This might be a dump question but I get lost every time I have to do something on a generator. I must calculate the steady state fault current (energy of fault) of a bus being fed by both the utility and two small 13 MW MV generators. I have the AVR settings. It states: "Current Limiter...

MV incomers and bus ties are these days all C/B. I have only seen one installation (radially fed - 1975 design) using isolators as incomers. This design used double busbar with a isolator as tie between two busbars and was mechanically interlocked with the two incomers to prevent parr the main...

It is not that suprising in heavy industry. Your standard installation is MV circuit breakers feeding transformer sizes that varies between 1MVA and 4MVA. It is simpler to add an additional MV panel with C/B feeding the odd small transformer than install an odd Fuse Isolator switch - most...

No difference. Class X is the old BS naming and Class PX the IEC name.

I have used both EDSA and ETAP. No argument - ETAP is way ahead of EDSA. EDSA have a lot of bugs especially when new modules are released (can not trust the results of these modules). Just normal day to day use of ETAP is much easier (and more trustworthy) than EDSA. Always needs to be...

You might also need to have a quick look at the X/R of the fault since you have generators feeding onto the bus. Standard breakers are rated and type tested for a time constant of 45ms (X/R of fault < 14) - dc component of the fault.