MV Circuit Breaker Close and Trip DC Circuits

[tmaly1]

Dear Folks,

We are about to replace an old oil MV Circuit Breaker with new MV Vacuum one.

The old breaker trip circuit coil is rated 12.7 amps @ 48VDC, close coil and charge motor was 240 VAC.

The new breaker trip circuit coil is rated 17 amps @ 48VDC, The close coil (17 amps) and charge motor (10 amps) are also rated 48VDC not 240 VAC as the old one.

The DC Battery Bank consists of 37 NI-CD cells for a total output voltage of 48VDC, the charger/converter output is 12 amps DC.

Do you think we need to replace/upgrade the exisitng DC Battery Bank/Charger to handle the new configuration (all 48VDC operations)?! what size is adequate in your opinion?

I appreciate your feedback and help on this.

Best regards all,

 

[dpc]

You need to know the amp-hour rating of the existing batteries.

Also, you'll have to make some assumptions about # of breaker operations in sequence and your other dc loads.

But the difference in the trip coil current is not an issue since this current exists only for a cycle or so.

If this is the only breaker and only dc load, you're probably OK.

 

[rbulsara]

I would not charge the motor from a station battery. Consider providing a 240vac to 48Vdc power supply the motor charging.

This may allow you to use the existing batteries for the protection devices. I doubt if the existing batteries can handle 10A motor charge. Also chargin motor is a non-critical load, and should not be put on a critical power supply (batteries).

 

[dpc]

rbulsara,

Breaker spring-charging motors are powered by station batteries all the time - the vast majority of the time in my experience.

 

[ScottyUK]

UK practice frequently uses DC spring charging motors.

I agree with dpc's comments: you need to determine how many breaker operations you need to support from the battery. Also consider: what is the minimum supply voltage that the protection relays can accept; how many simultaneous breaker operations could there potentially be; are the conductors between the battery and switchgear adequately sized from a volt-drop perspective?

In many ways a higher voltage station battery brings benefits, unless you are talking about a small installation where there are few breakers and distances are small. 110V or 240V is a common choice for larger installations.


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I don't suffer from insanity. I enjoy it...

 

[nightfox1925]

You need to re-evaluate your DC loading or re-visit your DC load profile to determine whether the existing battery ampere-hour and the battery charger ratings are adequate. Upgrading your batteries to higher AH capacity or going for higher DC voltage (125VDC) would result in replacement of your battery chargers as well. You may have to re-check your cables for DC voltage drops and DC short circuits.

You first determine all your relays power consumptions as continuous loads, determine the nature of breaker operations as close/open breaker momentary loads and lastly, consider all breakers trip at the same time (interrupting) momentary loads. Determine the backup time and create your DC load profile. From here check your equivalent AH requirement and compare with the existing Ah capacity of your battery and battery charger rating.

You may use the IEEE 1115 battery sizing guidelines using a special worksheet. I agree with dpc, most of the spring charge mechanism of modern breakers nowadays use DC voltage. Ensure that you consider the battery kT or Rt Factors in your calculations (from battery performance curves).

 

[RajT]

In the worse case it appears that you may have to upgrade your station batt arrangement ie bigger Ah capacity.
I would suggest not to rush into changing the present arrangement.

With the new CB wired up, carry out checks with various cycles/O-C-O etc and evaluate the result. You will then be in a better poistion to assess the way forward.

 

[tmaly1]

Thanks folks,

Your input has been extremely helpful and I appreciate the time you took to answer and help me with my question.

I'm going to contact ALCAD since they are the battery manufacturer, also, I'm going to check IEEE 1115 as 0210972 suggested.

Since dpc and Rajt stressed that I need to know the AH capacity for the battery, I'm wondering what is the significance of the AH for the operation?

Thanks all and best regards

 

[ScottyUK]

A larger AH battery has two significant properties:

1) More total stored energy, hence more operations of the breaker.

2) Ability to deliver more peak current to the load without the battery voltage collapsing or damaging the cells. This is a rule of thumb for batteries of the same basic design - there are batteries which are designed specifically for very high discharge rates which have can deliver peak currents far greater than their size would suggest.


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I don't suffer from insanity. I enjoy it...

 

[jghrist]

You can get free battery sizing software from Alcad. See

http://www.alcad.com/application_battsize.htm

 

[rbulsara]

dpc:

I agree with you as long as the battery was sized for that.

I presumed (alwyas dangerous) that in this case, because the brekaer charging motor was 240V, the batteries were not sized for it. Unless, it is imparartive that the breaker charging has to be from the station batteries, it is OK to use a power supply.

On the other hand the battery setup itself may be due a replacement than its a different story.