Voltage Dip Effect

[tharabsd]

Dear All,

Right now, I am in the middle of studying the new electrical source connection to our plant. We plan to change from self generation to purchase electricity from power company. One of consideration is effect of voltage dip occur to our plant operation.

First consideration.
I believe that if there is a voltage dip occur in transmission, motor ampere will be increased. Is there any formula to calculate how much the ampere increase when there si a voltage dip?

Second consideration.
Still connecting with first consideration. How to calculate the transformer sizing to occupy the ampre increase when there is a restarting after voltage dip occur?

If there are any article or book explain about the effect of voltage dip, sharing is very welcome.

Thank you very much

 

[davidbeach]

Consider the motor to be a constant kW load. Transformers shouldn't be a problem unless they are at the ragged edge of being dangerously overloaded with normal voltage.

 

[RRaghunath]

tharabsd,
Main challenge you face when the power is taken from grid is momentary voltage dip during faults in the utility OHL system and dropping-off of contactors in the motor power / control circuits. If the utility connection is HV, the problem may not be that big.

Further, when you are considering restarting, if the restart is after a momentary voltage dip, then the motor speed would still be above 90% and the reacceleration may not be an issue.

If it is after a couple of seconds, it is preferable to trip the motors and start sequentially to control the current and the associated voltage dip at the motor switchboards.

 

[Bronzeado]

Tharabsd,

Davidbeach and Raghun have givem good advice.

Also, after momentary voltage dips, transformer may saturate, draining high currents similar to inrush currrents.

So, from the point of view of supply systems, you may have both transformer inrush and motor re-aceleration currents.

Regards,

Herivelto Bronzeado

 

[tharabsd]

Thank you very much for the sharing.

Like Raghun mention, the challenge is momentary power dip in plant site. For low voltage voltage, I assume if power drop more than 50%, magnet in contactor can not hold anymore so motor will be stopped.
For HV motor, we use mechanically latch VCB so connection will remain to electrical line when there is a power dip.

Raghun also mention that, if the power dip not so long, motor speed will be still above 90% so the acceleration is not an issue. My question is, what about the limitation of power dip duration so the accelration of motor when restarting is not an issue?
I am very aware about this because we have 5500KW Slip ring motor connected in the line....


Thank you very much

 

[RRaghunath]

Typically 2sec for HV motors and can be upto 5sec for LV motors.

 

[slavag]

Hi.
As Raghun saied.
Voltage dip is usually up to 250ms.
Regards.
Slava