Under voltage protection

[PAP]

Hi all
I have a question about the under voltage protection for the main incoming of industrial plant. Its main incoming has only one feeder @ 22kV and connect to step down transformer (to 400V). Some problems were come from under voltage relay and undervoltage release coil in the main circuit breaker (@400V) .So I want to remove this function from the system, are there any concerns about this function? I don't understand that why under voltage relay must be used in this simple scheme.
For the motors ,under voltage protection was cover by overload already,right?

Thanks all for your sharing

PAP

 

[jbartos]

Suggestion: The undervoltage protection protects the power distribution load equipment against continuing voltage sags that are detrimental to motors, ballasts, etc.
Therefore, it is good to have this function, ANSI Device No. 27. For various protective functions of 27, see
Reference:
IEEE Std 242-2001 Buff Book includes:
4.4.3 Undervoltage relays (Device 27) on page 80

 

[bigamp]

PAP,

Undervoltage protection is a good thing to have but it should not be instantaneous; it needs to be time delayed to allow time graded overcurrent protection to operate. You will get a dip in voltage whenever you get a short circuit fault and the extent of the dip will depend on the location and nature of the fault. The fault could be within the plant on a branch circuit in which case you need to allow time for the branch circuit protection to operate. The fault could be within the network on another 22kV feeder and you need to allow time for the network protection to operate.

I would expect the undervoltage trip coil in the 400V circuit breaker to be powered from a separate source of voltage and should be nothing other than a failsafe tripping mechanism operated by other protection or shutdown circuits? Where is it powered from in your application?

Motor overloads will not really provide direct motor undervoltage protection. A reduction in voltage will cause an increase in current but it would have to be a fair decrease in voltage before there was enough of an increase in current for the overload to operate anytime soon. The best bet would be for a fault to collapse the voltage to the extent that contactors drop off (assuming circuit powered contactors) and hence the motors trip.

You can get undervoltage protection from high spec motor protection relays but these are typically used for large MV motors.

 

[jbartos]

Suggestion: The higher upstream undervoltage relay 27 is, the wider the undervoltage protective coverage in the power distribution is. Please, notice the application diversity of 27, in general:
a) Bus undervoltage protection. The undervoltage relay may either alarm or trip voltage-sensitive loads, such as induction motors, whenever the line voltage drops
below the calibrated setting. A time-delay relay is normally used to enable it to ride through momentary sags and thus prevent nuisance operation. For electromechanical
relays, to prevent the inertia (or overtravel) of the time-delay relay from tripping the circuit, an instantaneous undervoltage relay with its contacts connected in series with the time undervoltage relay contacts may also be used to provide a fast reset time.
b) Source transfer scheme. The undervoltage relay is used to initiate the transfer and, when desired, retransfer of a load from its normal source to a standby or emergency
power source. Due to the possibility of a motor load, this relay has a time delay in order to preclude out-of-synchronism closures.
c) Permissive functions. An instantaneous undervoltage relay is used as a permissive device to initiate or block certain action when the voltage falls below the dropout
setting.
d) Backup functions. A time-undervoltage relay may be used as a backup device following the failure of other devices to operate properly. For example, a long time-delay
relay may be used to trip an isolated generator and its auxiliaries if the primary protective devices fail to do so.
e) Timing applications. A time-undervoltage relay can be used to insert a specified amount of time delay in an operating sequence. Certain protective functions, such as
a negative-sequence overvoltage relay, may require a time delay to prevent nuisance tripping.

Clearly, more details would be needed to assess the applied 27 relay in the posted application, and its potential removal.

 

[buzzp]

The result of an undervoltage on a running motor is increased current draw. The overload relay will satisfactorily protect your motor. However, if the voltage is low before it starts, it may have trouble getting up to speed causing an overload condition as well but why cause damage by starting with a low voltage to begin with.
I would definately not disable this function unless you have a back up low voltage monitor. There are many other pieces of equipment that could be damaged from operating at a reduced voltage.

 

[RajT]

On a simple dist system such as yours, it is most unusual to have a UV release on the 400V incomer.
Some UV release coils do have a fixed (slugged)delay of 1 or 3 secs. It appears that the coil in youer case may not have this delay.
If the 400V CB is causing nuisense trips, then I would suggest the following.

For a period ( determined by you) fit a trace recorder on the 22kV bus to moniter the system voltage.
In the event of the 400V incomer tripping, check to see if there was any corresponding voltg dip on te 22kV bus.

You will then know the true nature of the tripping problem and could remove the UV coil altogether to avoid further trips.

On the other hand, you could fit a 27 device (with a setting of say Un<80%; t=100msec) on the 22kV feeder end of the 400V CB. This will provide a more robust protection from severe voltage dips, if there are any.