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Wednesday, September 4, 2013

Pick Up Current and Drop off current

describing of pick up current and drop off current on over current relay, lest take care on the picture bellow, and look at a problem clearly,
Picture 1, Over Current relay circuit with time delay
TC  = Tryping Coil
A    = Alrem
DC  = DC source
T    = Relay definite time
b     = Contact Relay
R    = Over Current relay
a     =  Contact Over Current Relay
Ir    = Secondary Current CT

Picture 2, Characteristic of Pick up and Drop off Current
Ip  = Pick up Current
Id  = Drop off Current
tp  = time pick up
td  = time drop off
ta  = differences between time pick up and time drop off
ts  = Setting value of Ip, value of relay should close (contact a), so circuit on definite relay closed (relay working), and Id is value when relay should stop working, after the current flow disconnected.

When ta <ts, then relay is not working
         ta > ts  then  relay is working

the value which comparison between value of drop off current and pick up current, usually notate as kd, so the value of kd can be write as equation bellow:

kd =Id/Ip

Where kd have the specified value, 0.7 to 0.9 for definite relay, and 1,0 for inverse relay.

Source: Diktat proteksi Universitas Islam Malang

Tuesday, September 3, 2013

Prinsip Kerja Electromagnetic/mekanik Relay


Today i will share one of my knowledge that I got this morning, its about electromechanic Relay. Most of you might be familiar with this kind of relay, but fsor you who do not understand at all, we can learn together.

Picture 1, Electromechanic Relay
picture 1 is principle of electromechanic relay. In the end this system should working and open the circuit breaker which attached on the line cable. How it work?

First, the current flow on the line, sensed by current transformer. Current that flow via (kumparanA), then electromagnetic field on kumparan A will pull on (contact A), so the current will flow on (kumparan B). Electromagnetic field on kumparan B, will move the circular plate which connected on contact E. when Contact E on, then DC source will applied on the circuit, and (kumparan C) will pull off the PMT (breaker on the line feeder).

Kumparan C, is for special case, when the current that sense by Current Transformer (CT), have high enough value or higher than determined setting value, kumparan C will work and turn on contact D, and drive (kumparan C) directly to pull off PMT (Breaker).

Relay which have a specific configuration that work base on the current flow value , high or low value are term to over current or under current relay. other side, Relay which have a specific configuration that work base on the voltage value, high or low value are term to over voltage or under voltage relay.

Now, you already understand the general principle of electro mechanic relay, which most trust relay in electrical power system. Thank you for your attantion.

Source: Djiteng Marsudi, Book

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