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.

This discussion is locked.

You cannot post a reply to this discussion. If you have a question start a new discussion

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.

Parents

0 John Peckham over 4 years ago

Roger

Following on from Mike.

The max load current for your 1.5MVA Tx will be around 2165A. Assuming a 5% percentage impedance (check the data plate on the TX.) PFC = 2165 x 100/5 = 43.3kVA. That assumes an infinite bus on the HV side which it wont be so a bit less than 43.3kVA.

Then the fault current will be further reduced by the current limiting effect of the circuit protection devices downstream of the Tx. The calculation assumes a of negligible impedance fault which it will not be. So lots of assumptions.

The cables away from the Tx. will further limit the fault current. If you had stated the length of the cables I could have put it in to my Amtec but I would also have needed the TX percentage impedance.

The maximum fault current, assuming an infinite bus an no contribution from stored energy in the installation, with the cables cold for a symmetrical fault will be the open circuit phase voltage (usually 250V if tapped for 433V) divided by the sum of the impedance of one phase winding and one line conductor with the neutral playing no part in the fault current.

As for measurement of very low impedances and high fault current that is beyond the capability of hand held installation testers. They may have a resolution of 0.01 ohms but resolution is not accuracy. As Mike says lead resistance of ordinary testers comes in to play. Also the actual test current with installation testers on "high current" is around 5A. For the real deal you will need something like the Megger MIMS 1000 wich delivers up to 1000A of fault current with 4 wire Kelvin leads. I have used one once at the terminals of a transformer all kitted up with flame retardant overalls and full face protection and pressing the test button was a bum clenching moment.
Cancel
Vote Up 0 Vote Down

Cancel

Reply

0 John Peckham over 4 years ago

Roger

Following on from Mike.

The max load current for your 1.5MVA Tx will be around 2165A. Assuming a 5% percentage impedance (check the data plate on the TX.) PFC = 2165 x 100/5 = 43.3kVA. That assumes an infinite bus on the HV side which it wont be so a bit less than 43.3kVA.

Then the fault current will be further reduced by the current limiting effect of the circuit protection devices downstream of the Tx. The calculation assumes a of negligible impedance fault which it will not be. So lots of assumptions.

The cables away from the Tx. will further limit the fault current. If you had stated the length of the cables I could have put it in to my Amtec but I would also have needed the TX percentage impedance.

The maximum fault current, assuming an infinite bus an no contribution from stored energy in the installation, with the cables cold for a symmetrical fault will be the open circuit phase voltage (usually 250V if tapped for 433V) divided by the sum of the impedance of one phase winding and one line conductor with the neutral playing no part in the fault current.

As for measurement of very low impedances and high fault current that is beyond the capability of hand held installation testers. They may have a resolution of 0.01 ohms but resolution is not accuracy. As Mike says lead resistance of ordinary testers comes in to play. Also the actual test current with installation testers on "high current" is around 5A. For the real deal you will need something like the Megger MIMS 1000 wich delivers up to 1000A of fault current with 4 wire Kelvin leads. I have used one once at the terminals of a transformer all kitted up with flame retardant overalls and full face protection and pressing the test button was a bum clenching moment.
Cancel
Vote Up 0 Vote Down

Cancel

Children

No Data

Ipf Measurement on a 3-phase system

Community Rules & Guidelines