First question which strikes in our mind is that why do we need to do DCRM even though we already had Contact Resistance Measurement,CRM? Seems good question and this question will take you to the depth.
So, first of all we need to know about the contacts of Circuit Breaker. Basically, a CB has two types of Contact.
- Main Contact– Main contact of CB are designed to carry current when the CB is closed but it is not designed to take arcing stress during Closing / Opening operation of CB. Therefore, obviously Main Contact will make after the making of Arcing contact during Closing operation and it will separate before the separation of Arcing contact during Opening operation.
- Arcing Contact– Arcing contact is provided in CB to take all the arcing stress.
From the picture, it is clear that how arcing contact and Main Contact work. When we measure the CRM, we do it after closing the CB so the measured resistance is basically the Net Resistance of Main Contact and Arcing Contact. If the Resistance of Main Contact is R and that of arcing contact is r the both of them are parallel when Breaker is closed. So measured resistance during CRM = R*r/(R+r)
Next, If I ask you how will you determine whether the Arcing Contact assembly is properly working or malfunctioning then will you answer by doing CRM? Obviously your answer will be a big NOOO………..
How will we confirm then? It’s by conducting DCRM.
Why dynamic? As because the arcing contact travels while Opening / Closing operation to fully take closed position / open position. Therefore the word Dynamic came in picture. Therefore as you can guess, DCRM measures the contact Resistance during dynamic condition. Basically it is the measurement of Arcing contact resistance when contact travels.
How do we conduct this test and what are the results?
This test is conducted by DCRM kits available in market of many makes like SCOPE, DOBLE etc. What we do, we extend the 220 V DC supply to the kit and form the kit 220 V DC is extended to the nearest point of Trip Coil-1 & TC-2 and Closing Coil. By this manner we take the control to kit for closing / opening of Breaker. For taking the Breaker Open / Close feedback to the kit, we connect the kit to the NO contact of Auxiliary Switch of CB. Now, we need to get the travel of contacts, for that we connect kit to the closing / opening lever via a Transducer. By this manner, we are now able to control the CB, Status of CB and contact travel in kit.
DCRM kit now injects 100 A / 25 A current through the contacts and plot a curve of Current through the contact during dynamic condition, Travel of contact and Dynamic Resistance with respect to Time. Straight forward, a fingerprint of CB we get.
How does the fingerprint looks and how to interpret?
This a typical fingerprint of healthy CB. How do we analyse this signature of CB?
See the graph, there are three quantities namely Resistance (Dynamic), Current and Travel of Contact. Lets start when the CB is open, obviously Resistance will be high as shown by horizontal red line. But as soon as we give close command to the CB, first Main Contact makes and resistance decreases to point P1. After that Arcing Contact start closing, resistance varies from P1 to P2 and finally when it is fully closed the resistance is constant P2 to P3. At P3, CB is fully close. Now we will see what happens to the Current.
Initially current is zero as CB is open. As soon as we give close command to the CB,at P1 current increases and at P2 it will be highest as CB is fully close here. Current will remain at its maximum up to P3.
At P3 we give opening command to the CB by the Kit, so Main Contact will open at P3 and Resistance will increase slightly to P4, and then Arcing Contact start opening and at P4 it fully opens and hence Resistance take the maximum value. Likewise current also, decreases to its original value i.e zero.
Now we focus on Travel of contact. Leaving this portion for you to think but in case you need help write in comment box.
Now, see the dynamic period is from P2 to P3 as in this period Arcing contact is travelling to get at fully close position. Therefore, during the dynamic period the Resistance curve is almost flat which signifies constant resistance during dynamic period which in turn meand Arching contact and main contact assembly are properly working and contacts are in good condition. Now take a look at this Signature of CB.
See the dynamic resistance(Green curve) during Close / Open period is not flat rather it is fluctuating which means contact condition is not good or assembly is malfunctioning.
That is all which I can convey. If you have more information share it by writing in comment box, I will be very happy. Thank you!!!
Nice
You are a genius.
It’s your love and support!
nice work sir!!
The above information is very muc informative and useful. I want to understand whether this test needs to be conducted on all 3 poles of CB simultaneously or on single pole individually, especially for HV CBs like 400 kV, 220 kV as we do in case of open / close time measurement to know the pole discrepancy.
If you have three channel DCRM kit, you can conduct DCRM test on all three poles simultaneously. In case you have single channel DCRM Kit then you need to carry out this test on all three poles individually. You must also compare the test results with that of factory test report to ensure the healthiness of breaker. This is applicable for EHV Breaker.
Thanks for your quick response. The query was related to site tests after installation of new 400 kV cbs and conducting pre-commissioning tests. We have single channel Scope make CB analyser.
As I understood, DCRM will provide data on contact conditions and installation accuracy as well and in our case it will serve as reference for future assessment of contact condition.
Hence single pole DCRM shall also suffice.
I think 400 kV Breaker are not ganged operated. Individual pole have dedicated operating mechanism. Therefore, DCRM test of individual pole is necessary.
Really very detailed explaination sir!
I want to share what i have seen practically while performing DCRM on 400kv CB; the test was done on 3 poles simultaneously so as to have similar timings of operation, maximum differing 2-3ms.
Dear ,
This is quite helpfull, but during closing of the CB arching contacts closes fisrt than main contact. So point P1 is the point where arching contacts closes . In this article you had explained that main contact is closed first and point P1 is main contact closed. So please correct if I am wrong.
Regards
Yes, you are correct. If you see the graph, it is shown that P1 is the point of closure of Arching Contact. During closure of CB, arching contact is first closed while during opening, main contact first open.
Good explanation sir…Thank you…Also please explain about healthy travel and unhealthy travel ..Thanks in advance
Thank you very much. Very clear explanation.
You are welcome Suresh! One request, pls share the post as it will help us.
I think one more thing should included and elaborated in how much milliseconds it works. These is most essential part of it.
The test result should always be compared with the factory test report. This is the reason, the test result part have not been discussed. Above all, DCRM test is different from Breaker Timing Test. The nature of curve obtained from the test should be in agreement with the factory test result.
Thanks for clearing the concept.
Sir,
I have one dout if CRM is good bt noise coming at starting stage of DCRM at P2 point so how we will conclude that contacts are ok r not ok
Just compare the result with factory test report. If there is mismatch, ask manufacturer.I think, this is the best way to conclude.
Sir-
I agree with both what Bhupendra said on Feb. 18, 2019 and your reply. Arcing contact closes 1st as the graph shows, but your write up below the graph says different! – Please read and correct. Thanks for the article.
How much rated speed,plot length and close open timings normally used while conducting DCRM test
Sir, please tell me how the wave travels and what’s the use of transducer
Sir, please explain me about the travel also.
What do you want to know about travel?
Sir, I have cleared the concept of DCRM.I have doubt in how i can perform POW power on wave test for my numerical relay test? I don’t have any kit. But testing on Typhoon 602.would you please send the steps and whole procedure to carry out this function testing without kit
Thanks for good explanation