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Posted: 2018-03-13 08:02 AM
I have some questions about some PM8000 devices that have captured some waveforms.
First PM8000 supplies feeds a cabinet with isolation transformers (hospital operation rooms).
The PM8000 captured some some strange waveforms. It's like the neutral point has suddenly shifted...
Customer had no complaints of interruptions or distortions from the users... There a several PM8000 devices on site and only on this device we captured this kind of distortions.... Is there anybody who has seen similar captures or could explain what happened? Could this be a problem on the PM8000 meter?
Second capture is about power interruptions. As the PM8000 Auxiliary supply is not on UPS power. When we lose power, I guess the PM8000 can make a last capture with the energy stored in the internal capacity of the PM. But when the power comes back it's not possible because the meter is off and it takes some time to boot... However, we captured these waveforms... I would to see a capture where voltage and current drops but this is what we see... Can someone explain what we see here?
Reports can be downloaded from box: https://schneider-electric.box.com/s/dhgl28nuey0lsberp3y828xg3t08nisz
Maybe it would be interesting to make a place on the exchange to share some waveforms with information about incidents. There is not so much information available about these things...
The first waveform appears as if the power source has a resistance grounded neutral or an ungrounded neutral. A ground fault on one of the phases will not cause high current flow but it will raise the neutral voltage value to be approximately the Line to Neutral voltage above 0 volts. If the ground fault is momentary the neutral voltage will slowly decay to be at 0 volts again.
The first two look like there is negative DC bias into power line I saw something similar quite often in the 115 kV line when there was lightning strike near the line.
The PM8000 is in a TNS electrical network and powers some isolation transformers IT . They are also equiped with some insulation monitoring devices from Bender.
After properly checking the information L4 replied saying that, this is expected behavior of an isolated IT system during (and recovering from) a single line fault.
The second waveform, I could reproduce in our lab and is due to voltage interruption on the aux and voltage inputs. But i have no explanation what this just shows... Customer will put the auxilary power on UPS.
This will typically happen if one loses the input supply and one has motors running as loads. In this case the supply voltage will go to zero and what you are seeing is the back-emf of the motors as they run down and demagnetize. They regenerate into the supply and dissipate all electrical energy stored within them.
That is generally why one should wait until all transients have died down before switching back in as one can create large motor transient current and torque if one switches the mains back in and out of sync with the back-emf's.
This is quite normal and common and depends on the type of downstream or upstream fault which is normally due to an unsymmetrical type fault. It depends on the point at which the fault occurs as well as the nature of the unsymmetrical fault.