It was observed that, couple of our MV motors 4.16kV are experiencing earth leakage readings on their protection relay with 0.8amps and when tested IR, on motor and cables were satisfactory. Even the TanDelta results were found within the acceptable value. Now, Motors are running with same fixed earth leakage value. We have downloaded our Bus condition through PQM to analyze the quality of electricity.
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Observing earth leakage 0.8amp on our MV motors on same Bus
- Thread starter SSK18
- Start date
All windings have a small leakage current to ground. 0.8 A seems fine.
Muthu
Muthu
electricpete
Electrical
I agree it's not necessarily a problem.
Ac ground current is not monitored for motor condition monitoring, but for protective relaying (The odds of a sustained steady state ground "fault" of a few amps or less seems pretty darned low, even if we didn't have those megger results). You'd probably get better answers in the power engineering forum from people who set those type of relays.
There is of course a capacitive current to ground from each phase, but due to physical symmetry of the three phases within the motor, the sum of capacitive currents from the three phases to ground tends to cancel unless there is a zero sequence component of voltage unbalance on the system.
Along with any actual unbalance in capacitive ground currents from the three phases, the ground relay setting has to accommodate expected errors associated with the measurement. Residual CT connection adds error if CT characteristics/burden are different. Core balance CT can add error due to conductor positioning.
fwiw at our plant I believe our 4kv motor 51G relays are set near 0.4A (that's the long-time asymptote) referred to the primary of the core balance CT. Our 4kv system is resistance grounded thru 240 ohm resistor, which I guess limits ground fault current to somewhere around 10A. I'd think the setting has to be low enough to trip on ground fault and also low enough to coordinate tripping before upstream ground fault devices, but higher than what the relays is expected to measure during non-fault conditions. We've had that 0.4A setting for decades, and we've had no spurious 51G trips attributable to the setting to my knowledge.
Ac ground current is not monitored for motor condition monitoring, but for protective relaying (The odds of a sustained steady state ground "fault" of a few amps or less seems pretty darned low, even if we didn't have those megger results). You'd probably get better answers in the power engineering forum from people who set those type of relays.
There is of course a capacitive current to ground from each phase, but due to physical symmetry of the three phases within the motor, the sum of capacitive currents from the three phases to ground tends to cancel unless there is a zero sequence component of voltage unbalance on the system.
Along with any actual unbalance in capacitive ground currents from the three phases, the ground relay setting has to accommodate expected errors associated with the measurement. Residual CT connection adds error if CT characteristics/burden are different. Core balance CT can add error due to conductor positioning.
fwiw at our plant I believe our 4kv motor 51G relays are set near 0.4A (that's the long-time asymptote) referred to the primary of the core balance CT. Our 4kv system is resistance grounded thru 240 ohm resistor, which I guess limits ground fault current to somewhere around 10A. I'd think the setting has to be low enough to trip on ground fault and also low enough to coordinate tripping before upstream ground fault devices, but higher than what the relays is expected to measure during non-fault conditions. We've had that 0.4A setting for decades, and we've had no spurious 51G trips attributable to the setting to my knowledge.
- Moderator
- #4
If that current is real current it represents over 3 kW losses which would be serious.
3 kW of losses will often progress to a complete failure.
If it is apparent current due to winding capacitance to ground, it will not be generating heat and will not be serious.
--------------------
Ohm's law
Not just a good idea;
It's the LAW!
3 kW of losses will often progress to a complete failure.
If it is apparent current due to winding capacitance to ground, it will not be generating heat and will not be serious.
--------------------
Ohm's law
Not just a good idea;
It's the LAW!
"...If that current is real current it represents over 3 kW losses which would be serious. 3 kW of losses will often progress to a complete failure. If it is apparent current due to winding capacitance to ground, it will not be generating heat and will not be serious".
Please advise how to measure and differentiate whether it is a "real current" or "apparent current" Apparently the motor is working fine with 0.8A.
Che Kuan Yau (Singapore)
Please advise how to measure and differentiate whether it is a "real current" or "apparent current" Apparently the motor is working fine with 0.8A.
Che Kuan Yau (Singapore)
- Moderator
- #6
I would go with a dual trace scope.
One trace would be a signal derived from the leakage current from one phase.
The other trace would be from the phase voltage.
If the current is capacitive it will be offset 90 degrees on the scope.
Measuring the ground current may be a challenge and may have to be done indirectly.
The best indication that it is capacitive current to ground is that it is not getting worse and that capacitive current to ground is common in motors.
--------------------
Ohm's law
Not just a good idea;
It's the LAW!
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