33/0.433kV, Dyn1 Distribution trafo with HV fuses

[RRaghunath]

This configuration is fairly common in distribution systems.

But, what would happen to the motor loads on LV side, when only one fuse blows (single phasing on HV side).

It appears that there is no more a rotating flux in the motor. Will the motor stall. It also seems that the open-delta / residual voltage will be zero on LV side. Then, how can the condition be detected (presuming that the LV incomer has a VT)?

Appreciate any experiences / references. Thanks.

 

[waross]

Several things happen.
This assumes that there is no protective relaying and the single phase condition persists.
Once a motor is rotating the magnetic field does not have to rotate to keep the motor spinning. Consider single phase motors with a starting switch.
One phase must supply all the energy to drive the load and the current will increase.
The motors will act as rotary convertors and supply energy to any non motor loads connected to the phase that has lost primary power. This will further increase the motor currents.
Small motors will shortly trip out on overload.
Many refrigeration motors are equipped with protection modules which will trip on phase loss, voltage unbalance or overcurrent. In the case of phase loss, they will trip off instantly.
A large lightly loaded motor may continue to run on single phase and also supply small loads on the lost phase, but most large motors have voltage unbalance and phase loss protection.
It is possible for a loaded, poorly protected motor to continue running on single phase. It will draw excess current, and finally burn up.
respectfully