Hello All, While checking an EIC and comparing design values vs measured values, a few fundamental questions have been puzzling me and I would be grateful of any advice or comments. The installation is an embedded LV generator connected to the site 11kV ring via a 1.5MVA transformer. It’s a TN-S system with the N-E link at the transformer. The cables from the generator to the breaker panel are L=3x300, N=2x300, PE=1x240. The breaker panel is considered the point of connection and where Ipf and Ze were measured; Ipf=8.58kA and Ze=0.02Ohms. The calculated 3-phase symmetrical fault at the breaker panel (not including the generator contribution) was ~34kA (assuming a 250MVA fault level at 11kV) i.e. significantly higher that the measured Ipf. This lead me think that the actual fault level at 11kV must be much lower than 250MVA. On reflection, I’m thinking that the Ipf measurement is however a worst case measurement, as the meter only measures the impedance on the LV side of the transformer and the downstream cables i.e. assumes an ‘infinite source’ on the 11kV side, so the measured Ipf should be much higher than 8.58kA? Maybe the Ipf needs to be multiplied by 2, as the measurement was with a 1-phase meter? The On-Site Guide states that ‘For three-phase supplies, the maximum possible fault level will be approximately twice the single-phase to neutral value.’ Thinking about this multiply by 2 (a round-up of 1.732), while this may be an acceptable approximation for domestic installations, I don’t think it is for an installation like this. Simply doubling the measured L-N value assumes that L and N impedances are the same (they are not) and doesn’t allow for the additional transformer winding impedance for a phase-phase fault. My understanding is also that a 3-phase (symmetrical) fault is effectively a single-phase calculation, so doubt the accuracy of this x2 factor in this case. Could the discrepancies be due to meter inaccuracies at these low impedance readings e.g. a Ze measurement of 0.01Ohms vs 0.02Ohms has a significant impact on Ipf. Should the contractor be using a more specialist meter? Thanks.

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Ipf Measurement on a 3-phase system

Hello All,
While checking an EIC and comparing design values vs measured values, a few fundamental questions have been puzzling me and I would be grateful of any advice or comments.
The installation is an embedded LV generator connected to the site 11kV ring via a 1.5MVA transformer. It’s a TN-S system with the N-E link at the transformer. The cables from the generator to the breaker panel are L=3x300, N=2x300, PE=1x240. The breaker panel is considered the point of connection and where Ipf and Ze were measured; Ipf=8.58kA and Ze=0.02Ohms.

The calculated 3-phase symmetrical fault at the breaker panel (not including the generator contribution) was ~34kA (assuming a 250MVA fault level at 11kV) i.e. significantly higher that the measured Ipf. This lead me think that the actual fault level at 11kV must be much lower than 250MVA. On reflection, I’m thinking that the Ipf measurement is however a worst case measurement, as the meter only measures the impedance on the LV side of the transformer and the downstream cables i.e. assumes an ‘infinite source’ on the 11kV side, so the measured Ipf should be much higher than 8.58kA?

Maybe the Ipf needs to be multiplied by 2, as the measurement was with a 1-phase meter? The On-Site Guide states that ‘For three-phase supplies, the maximum possible fault level will be approximately twice the single-phase to neutral value.’ Thinking about this multiply by 2 (a round-up of 1.732), while this may be an acceptable approximation for domestic installations, I don’t think it is for an installation like this. Simply doubling the measured L-N value assumes that L and N impedances are the same (they are not) and doesn’t allow for the additional transformer winding impedance for a phase-phase fault. My understanding is also that a 3-phase (symmetrical) fault is effectively a single-phase calculation, so doubt the accuracy of this x2 factor in this case.

Could the discrepancies be due to meter inaccuracies at these low impedance readings e.g. a Ze measurement of 0.01Ohms vs 0.02Ohms has a significant impact on Ipf. Should the contractor be using a more specialist meter?

Thanks.

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0 mapj1 over 4 years ago

Well the answer could be 'all of the above' though that is not much help.

I will try and simplify the question, to make sure I have it right and am not making some silly assumptions

First assumption.

You are interested in the fault levels on the LV side of the transformer, measured when the genset is not running in parallel with the supply - so the feed is from the 11kV side, and its 250MVA fault level is attenuated via the regulation (effective series inductance and resistance ) of the transformer perhaps 2% or 5% or whatever of full load.

Is the 250MVA a DNO figure or a guessed PIDOOMA figure ? Even if the 11kV line is very 'stiff' (i.e. totally non droopy) the effect of the TX will be to limit the fault level on the load side to 20-40 times the transformer kVA rating - some 50 - 75MVA might be more credible on the LV side.

You have measured one phase L-N fault loop at the panel end of the cables - but the genset and its cables are out of the equation then. How much cable is there between the panel and the transformer secondary?

IF the cables dominate, then maybe as you have reduced neutral, the doubling rule will slightly under estimate the 3 phases bolted fault current. but your cable sizes are not that different.

Yes - these sort of impedances are at the end of the meter where the fruit machine nature of the least significant digits on the meter start to matter and so does how you bolt the meter to the point of measurement - conventional probes and leads are no longer to be trusted, (On the test house kit a 4 wire connection is often used so the voltage reading part is not upset by the voltage drop in the current drawing part - the probe 'leads' may actually be coaxial cables, with the current down the braid, and the voltage reading up the middle. I have yet to see this on a field portable instrument, but it could be done.)
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0 mapj1 over 4 years ago

Well the answer could be 'all of the above' though that is not much help.

I will try and simplify the question, to make sure I have it right and am not making some silly assumptions

First assumption.

You are interested in the fault levels on the LV side of the transformer, measured when the genset is not running in parallel with the supply - so the feed is from the 11kV side, and its 250MVA fault level is attenuated via the regulation (effective series inductance and resistance ) of the transformer perhaps 2% or 5% or whatever of full load.

Is the 250MVA a DNO figure or a guessed PIDOOMA figure ? Even if the 11kV line is very 'stiff' (i.e. totally non droopy) the effect of the TX will be to limit the fault level on the load side to 20-40 times the transformer kVA rating - some 50 - 75MVA might be more credible on the LV side.

You have measured one phase L-N fault loop at the panel end of the cables - but the genset and its cables are out of the equation then. How much cable is there between the panel and the transformer secondary?

IF the cables dominate, then maybe as you have reduced neutral, the doubling rule will slightly under estimate the 3 phases bolted fault current. but your cable sizes are not that different.

Yes - these sort of impedances are at the end of the meter where the fruit machine nature of the least significant digits on the meter start to matter and so does how you bolt the meter to the point of measurement - conventional probes and leads are no longer to be trusted, (On the test house kit a 4 wire connection is often used so the voltage reading part is not upset by the voltage drop in the current drawing part - the probe 'leads' may actually be coaxial cables, with the current down the braid, and the voltage reading up the middle. I have yet to see this on a field portable instrument, but it could be done.)
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Ipf Measurement on a 3-phase system

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