UNDERVOLTAGE RELAY 1

[Elecme]

Why sometimes it is preferred to use instantaneous undervoltage relay with a time delay one in bus protection?

Elecme

 

[alehman]

Your question is not clear. Please provide more information on about the purpose of the undervoltage relay.

It is common to use time delay on undervoltage relays to prevent undesirable operation on short term events.

 

[jbartos]

Suggestion: Often, motor inrush currents would cause nuisance trips without some suitable timing of the undervoltage relay 27.

 

[busbar]

Elecme, is the intent of your question to compare[ul][li] high-speed undervoltage relays[/li] [li]definite-time undervoltage relays[/li] [li] undervoltage relays with separate time-delay relays[/li] [li] undervoltage relays with inverse time-voltage characteristics?[/li] [/ul]

 

[Elecme]

The following statement is written in IEEE - buff book Std 242 -1986:4.5 (1) Bus Undervolatge Protection


"A time delay relay is normally used to enable it to ride through momentary dips, thus preventing nuisance operation. An instantaneous undervoltage relay with its contact connected in series with the time-delay undervoltage relay contacts may also be used to provide a fast reset time, thus preventing the inertia (overtravel) of the time delay rely from tripping the circuit"

Can someone elaborate on this.

Busbar,
Can you mention some applications in which the 4
chatrctresitics of the under voltage realy applied.

Elecme

 

[busbar]

There are several application papers and instruction books for Basler undervotage relays at:

http://www.basler.com/php/download_page.php?section=protect#BE127

 

[peterb]

Elecme, I believe that the question revolves around the overtravel inherent in an induction disk relay. If you are using an induction disk undervoltage relay, where the time delay is provided by the movement of the disk, there is a degree of overtravel due to disk inertia when the initiating condition (undervoltage) is removed. This overtravel would tend towards timer inaccuracy. Also, resetting the disk takes a variable time depending on how far the disk has travelled - this provides an integrating action for sequential events within the reset time.
The situation is not the same when you use an instantaneous relay with on/off action and a definite delay timer, as there is no overtravel and instantaneous reset of the timer.
Of course, this only applies with electromechanical relays.