# directional and non directional relay

Discussion in 'Homework Help' started by samina, Oct 21, 2010.

1. ### saminaMember

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what are the differences between directional and non directional relay. I want to get information about relay. can anyone provide me any site address for this?

2. ### crutschowWell-Known MemberMost Helpful Member

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Never heard of them. What do you mean by directional and non-directional?

3. ### dknguyenWell-Known Member

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NEver heard of them either but Google showed this:
http://www.electro-tech-online.com/custompdfs/2010/10/art03.pdf

It sounds like directional relays are just relays that only turn on when the voltage/current are the right polarity and magnitude, whereas a regular relays just turns on if magnitude is large enough.

Maybe these were used before semiconductors were widespread? You'd just stick a diode in series with the coil of a regular relay nowadays.

But they seem pretty complicated compared to normal relays. The directional DC relays sound like they use magnetic elements (either field windings or permanent magnet) on the switch (rather than just steel that always gets attracted to any polarity magnetic field) so that only the right coil polarity move the switch on the correct direction to activate the relay. And then it talks about relays that use multi-tap transformers for their coils which allows you to make relays that activate on current imbalances between multiple wires, or relays that activate based on AC phase-shifts and current polarity.

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5. ### Gary BNew Member

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Directional and nondirectional could mean DC verses AC. In that case the AC relay has a copper slug around its core that acts like a shorted winding. This allows it to stay energized while the power reverses.

The other possible meaning is that you are talking about a Polar Relay. It has a permanent magnet and multiple windings. It is energized by a specific amount of current in one direction only and remains energized until the current drops below another specific value. For instance, it might require 35ma to pull in but will not release until the current drops below 18ma. The pull-in and drop currents are adjustable. Telephone MF receivers and central office trunks used then in cross-bar offices.

Last edited: Oct 22, 2010
6. ### GauthamvgNew Member

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Guys Directional and non directional relays are over voltage protection devices. They are used to protect the power system under faulty condition.
The directional relays are the one which operates under the direction of flow of fault current. See the following quote
i found this in my "Power system switch gears and protection" text book.

7. ### saminaMember

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thank you all for your help.Gauthamvg may I know which book you do follow?
Thanks all

8. ### GauthamvgNew Member

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Its provided by our college dude. Actually its not a text book. its study material for us. I have that paper in this semester. But if i found the correct source means i'll let you know.

9. ### RODALCOWell-Known Member

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Gauthamvg is correct.

These relays are used in substations on transformer circuit brakers.

In case a line fault occurs and a transformer trips out in a two transformer 33kV / 11kV substation for example, and with the 11kV bus coupler closed.

The second transformer will backfeed the first transformer via the low voltage 11kV winding and keeps the fault going.

At e certain set reversed current sensed by the relay , usually 10% the braker will trip and take the transformer off line.

The older style relays CDD do this my means of a ferraris disc and the new numerical relays can be programmed for the purpose.

I will take one or two photo's today during my travels.