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Motor protection
As the Settings of Differential Protection are smaller or too smaller than the "Magnetic, or Short-time Protection", we can use smaller section of cables, because smaller section means bigger cable's impedance and smaller Earth Fault Current that should be enough to trip the Differential Protection, but personally and for many reasons, I don't prefer to use a Differential Protection with motors.
The initial remark is : in principal "1x35mm2" for phases is not enough to have the necessary value of VD accordingly to the VD's percentages defined by "IEC…", where my calculation is based on :
- MV: 20kV, 500MVA, R/Z equal to "10%"
- Tr. : 400kVA, 20/0.4kV, 4%, 5000W
- Earthing System: TN-S
- Cable between Tr. and the MDB is : L: 10m, Ladders, θ: +30°C, SC, PVC, Cu, Flat-Touching.
- The DOL Motor's starting Equipment "Motor Circuit Breaker + Contactor" are installed in MDB.
- Cable between MDB and the Motor is : L: 400m, Perforated Cable Tray, θ: +30°C, SC, PVC, Cu, Flat-Touching.
- Motor: 30kW, PF: 0.85, Efficiency: 0,90, DOL, Start-up current= 7 x In
- The Thermal Relay installed in the Motor Circuit Breaker is used for Over-load.
- The Short-time Protection ( Magnetic Protection ) installed in CB is used for "Short Circuit, & Earth Fault", that means without Differential Protection.
- The VD percentages should be according to "IEC" especially during Start-up.
Now to maintain the VD percentage during start-up less than "15%", there's another solution even without using a Differential Protection, in which we can use the following cables :
- The cable's section of Motor's Phases: 2x35mm2
- The cable's section of Motor's PE: 1x50mm2
- The Earth Fault current at the end of cable is : 720A, that means the Short-time setting should be "max 540A" or less, but that's a critical value as the start-up motor current is "around 400A"
- The VD during normal running from Tr. till the Motor is: 3.17%
- The VD during start-up from Tr. till the Motor is: 13.50%
The best solution is to find the suitable Motor Circuit Breaker.
The initial remark is : in principal "1x35mm2" for phases is not enough to have the necessary value of VD accordingly to the VD's percentages defined by "IEC…", where my calculation is based on :
- MV: 20kV, 500MVA, R/Z equal to "10%"
- Tr. : 400kVA, 20/0.4kV, 4%, 5000W
- Earthing System: TN-S
- Cable between Tr. and the MDB is : L: 10m, Ladders, θ: +30°C, SC, PVC, Cu, Flat-Touching.
- The DOL Motor's starting Equipment "Motor Circuit Breaker + Contactor" are installed in MDB.
- Cable between MDB and the Motor is : L: 400m, Perforated Cable Tray, θ: +30°C, SC, PVC, Cu, Flat-Touching.
- Motor: 30kW, PF: 0.85, Efficiency: 0,90, DOL, Start-up current= 7 x In
- The Thermal Relay installed in the Motor Circuit Breaker is used for Over-load.
- The Short-time Protection ( Magnetic Protection ) installed in CB is used for "Short Circuit, & Earth Fault", that means without Differential Protection.
- The VD percentages should be according to "IEC" especially during Start-up.
Now to maintain the VD percentage during start-up less than "15%", there's another solution even without using a Differential Protection, in which we can use the following cables :
- The cable's section of Motor's Phases: 2x35mm2
- The cable's section of Motor's PE: 1x50mm2
- The Earth Fault current at the end of cable is : 720A, that means the Short-time setting should be "max 540A" or less, but that's a critical value as the start-up motor current is "around 400A"
- The VD during normal running from Tr. till the Motor is: 3.17%
- The VD during start-up from Tr. till the Motor is: 13.50%
The best solution is to find the suitable Motor Circuit Breaker.
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