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

yep. reduced neutral when we expect most loads either to be delta or well balanced star configurations.

but 40m of 300mm core is going to be about 1.5 milliohms, so on 230V over 100kA - and that is just one core. Basically the resistance of the cable is not really doing much to reduce the fault level - it is all in the TX, and perhaps, if the 11kV lines are very long, a bit on that side.

we can estimate the fault level another way - if it is a 5% regulation Tx then we can look at one phase only -

a 1.5MVA TX, would give 1/3 of its MVAs on each phase. Let us pretend we had a 500kVA single phase Tx - full load would be 2000A amps between friends (250V * 2000 amps = 500kVA, so 8% off maybe)

If the regulation was for 5% droop at full load, then the for 100% droop, we'd be looking at 20 tims this current, or about 40kA (single phase) This is a long way from the single phase measured value of 8kA - indeed if the 8kA is true, then at full load (~2kA) you will drop a quarter of the volts- if this was true, something would be cooking, so I hope the measured PSSC is in quite a bit in error.

To be honest to measure PSSC of this magnitude is not easy, - do you know how it was done ? As I alluded above, anything looking like a normal meter lead (about 10-20milliohms/ metre of test lead) will dominate.
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0 mapj1 over 4 years ago

yep. reduced neutral when we expect most loads either to be delta or well balanced star configurations.

but 40m of 300mm core is going to be about 1.5 milliohms, so on 230V over 100kA - and that is just one core. Basically the resistance of the cable is not really doing much to reduce the fault level - it is all in the TX, and perhaps, if the 11kV lines are very long, a bit on that side.

we can estimate the fault level another way - if it is a 5% regulation Tx then we can look at one phase only -

a 1.5MVA TX, would give 1/3 of its MVAs on each phase. Let us pretend we had a 500kVA single phase Tx - full load would be 2000A amps between friends (250V * 2000 amps = 500kVA, so 8% off maybe)

If the regulation was for 5% droop at full load, then the for 100% droop, we'd be looking at 20 tims this current, or about 40kA (single phase) This is a long way from the single phase measured value of 8kA - indeed if the 8kA is true, then at full load (~2kA) you will drop a quarter of the volts- if this was true, something would be cooking, so I hope the measured PSSC is in quite a bit in error.

To be honest to measure PSSC of this magnitude is not easy, - do you know how it was done ? As I alluded above, anything looking like a normal meter lead (about 10-20milliohms/ metre of test lead) will dominate.
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Ipf Measurement on a 3-phase system

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