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IRON CORE EARTHINGH
- Thread starter Elecme
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electricuwe
Electrical
Connection of the core of a power transformer serves two purposes:
Without this connection the potential of the core could be anywhere, making it impossible to design the insulation system
In case of some kind of insulation failures in the transformer there will be short circuit from the winding to the core. With an earthed core this will trip the transformers protection.
Without this connection the potential of the core could be anywhere, making it impossible to design the insulation system
In case of some kind of insulation failures in the transformer there will be short circuit from the winding to the core. With an earthed core this will trip the transformers protection.
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if you do not earth the core of the transformer, then it will float at a potential determined by the capacitive coupling of the transformer system. Under normal conditions, this probably won't be as big an issue with balanced loads and voltages, and a nice sinusoidal system frequency. Troubles could come when dealing with lightning strikes, switching transients or faults, where you have a number of higher frequencies, and more often than not unbalance. The insulation of the windings is designed to the known voltages that appear between the winding and the core (which is earthed).
Capacitive coupling is also the reason that you should always tie down unused windings in a transformer when doing an applied (hi-pot) test. You don't know the voltage that the other winding will float up to when applying volts to the winding under test. If the coupling is such, you may raise the winding not under test up to greater than its insulation level, and damage the insulation.
It is handy to have the core earth connection brought out through a bushing and earthed external to the tank with an earth cable, which allows the core insulation resistance can be measured. We have had situations where the earth connection was high resistance, and the core insulation was discharging, producing gas. It happens due to the core not being tied hard down to earth, and then the potential arcing over in the oil. With an externally connected earth, we could isolate it fairly easily, otherwise you have to do a lot more testing to try to determine the location of the discharge, whether its in the winding or not etc. Just make sure your technicians reconnect the earth after testing, otherwise you'll a step change in your gas results next time you analyse your oil sample.
Capacitive coupling is also the reason that you should always tie down unused windings in a transformer when doing an applied (hi-pot) test. You don't know the voltage that the other winding will float up to when applying volts to the winding under test. If the coupling is such, you may raise the winding not under test up to greater than its insulation level, and damage the insulation.
It is handy to have the core earth connection brought out through a bushing and earthed external to the tank with an earth cable, which allows the core insulation resistance can be measured. We have had situations where the earth connection was high resistance, and the core insulation was discharging, producing gas. It happens due to the core not being tied hard down to earth, and then the potential arcing over in the oil. With an externally connected earth, we could isolate it fairly easily, otherwise you have to do a lot more testing to try to determine the location of the discharge, whether its in the winding or not etc. Just make sure your technicians reconnect the earth after testing, otherwise you'll a step change in your gas results next time you analyse your oil sample.
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