Alstom IDMT relay range 3

[tonynccg]

We have a switchgear protected by 800/5A PCT with IDMT O/L relay, how can i setting the IDMT if there is a load of 900A for 50sec. by setting the IDMT relay?

Also what is the meaning of the following for IDMT relay
1. Starting current
2. Closing current
3. Resetting current.

 

[RalphChristie]

Are you refering to the CDG-relay (electromechanical) from Alstom?

1. Starting current:
Current flowing when the disc starts to move (103% to 110% of current setting)
2. Closing current
Current flowing that will operate the contacts for a trip (130% of current setting)
3. Resetting current
Current up to which the disc will totally reset (90% of current setting)

This means for a setting of 5A (5A-relay):
If a current between 5.15A to 5.5A enters the relay, the disc will start to turn, but not necessary operate (trip contacts will not operate).
A current of 6.5A will definitely operate the relay (trip contacts will operate).
For a current of 4.5A the disc will totally reset.(trip contacts will not operate)

Your first question is a bit difficult to answer - I do not know your setup. You'll have to do a fault study and have to discriminate with feeders upstream and downstream. This relays is not for overload, it is for overcurrent (earth or phase faults)

For a setting:
To stay above 900A you can set the plug-setting on 6.25A.

110% of 900A is 990A
setting = 990/800*5
= 6.1875A

It is a 5A relay (CT is 800/5) and the settings on a 5A relay is 2.5A; 3.75A; 5A; 6.25A; 7.5A; 8.75A and 10A.
The next setting above 6.1875A is 6.25A.

With your time-multiplier setting you have to discriminate between upstream and downstream relays. A recomended time is 0.4seconds between electromechanical relays. (The time setting is not expressed in seconds, it is just a multiplier)

You can also visit this sites:

http://www.ee.polyu.edu.hk/staff/eekkli/BEngOpt/Lect/Ch3_OC2003.pdf

http://www.geindustrial.com/pm/notes/artsci

http://www.areva-td.com/servlet/Con...48589697&lid=en&tab=Chapters&id=1056536208254

Ralph

 

[CJFlatters]

Hi

Ralph is correct, but if you need a specific answer to a specific question, it would help if you could confirm the CT ratio and relay tye/s used.


If the relay is an

_______________________________________
Colin J Flatters BSc(Hons) IEng MIEE MIIE
Electrical Engineer / Project Manager
Email - cflatters@colin7.demon.co.uk.

 

[CJFlatters]

With respect to the grading margin Ralph mentions, 0.4 sec's is a fairly common and practical value which is valid for the majority of applications.

However above relay operating times of 0.6 sec's, or where you are struggling to get adequate grading between successive devices, you may find that you need to look at the margin more carefully.

The formula for calculating the required grading margin (and a far better explanation than miy own) is detailed in the GEC PRAG manual and may prove useful.

The grading margin takes into account relay operating errors, CT ratio errors, circuit breaker operating times, relay overshoot times and a safety margin.

From memory, the formula is as follows;

t' = ((2Er% + Ect%)/100)x t + t_Oshoot + t_cb + t_safety

t = relay operting time of downstream device
Er% = Relay error (7.5% for CDG's)
Ect% = 10%
t_oshoot = typically 50msec for CDG
t_cb = Circuit breaker total interrupting time, dependant upon switchgear
t_safety = typically 100mSec for CDG type

For OCB's, (T_cb approx 100mSec) substituting values yields

t'= 0.25t + 0.05 + 0.10 + 0.10
= 0.25t + 0.25

Thus

for t = 0.1 secs, t' = 0.275
for t = 0.5 secs, t' = 0.375
for t = 0.8 secs, t' = 0.45
for t = 1.0 secs, t' = 0.50

Hope this helps


_______________________________________
Regards -

Colin J Flatters
Consulting Engineer & Project Manager