Molded/ HV Circuit Breaker O/C operation

[sobeys81]

Hi All,
I read quite a bit of posts on this topic but would apprecaite, if you could advise if I got it right.

Please advise, if I am missing any thing here.

1. HV/MV Breaker: Modern microprocessor relays only measure symmetrical rms value of current becasue they filter out the DC Component. Asymmetrical components are only used for Breaker duty studies.

2. Molded case breakers: All TCC curves for fuses and MCCB are symmetrical rms curve. There is no filtering happening in case of thermal magnetic trip unit, Electronic(LSIG) trip unit.

So that means if I am doing coordination study my breaker or fuse will actually trip earlier than I predicted.

3. Why do molded case breakers have such a big band for trip units. Technically MCCB will also trip earlier than before then band ends.

I would appreciate, if you could drop a word.

Thanks

 

[dpc]

1. in general, yes, but this varies depending on the relay. In some cases the relay response to asymmetrical and harmonic currents is selectable. Many relays respond to fundamental current, not rms.

2. No, I would not say the MCCB curves are strictly based on symmetrical current. The TCCs show current, not necessarily symmetrical current. This is only an issue for the instantaneous trip portion of the curve. For the thermal portion, the dc offset current has decayed to a negligible value.

3. MCCBs have wide tolerance bands because they are cheaply made. You get what you pay for.

 

[Elmir]

Hi Sobeys,

With regards to MCCB's and fuses:

Let-through charts exist for the devices that show peak let through current and RMS-symetrical current.

The square of peak let through current relates to maximum mechanical forces and the square of the RMS-symetrical current is relates to thermal energy.

When acessing medium and high level fault conditions reference to the let through charts may be helpful as the TCC's do not show performance of the protection device during these conditions. TCC's are used to study low level and overload conditions.

 

[dpc]

TCC's are used to study low level and overload conditions.

This is incorrect. TCCs generally extend out to the maximum available fault current.

Let-through charts are used for a different purpose and only apply to current-limiting devices. MCCBs are not generally current-limiting, unless specifically designed to be.

 

[sobeys81]

dpc,

During 1/2 cycle, ASymmetrical component when the current has DC component, How can a relay read this value.

AC value is riding on DC component. Will CT transfer this component on the secondary side in to relay. How relay reacts to these DC components.
I spoke to GE and ABB people unfotrtunatley they had no basic engineering clue about this.
I would apprecaite, if you could elaborate/

 

[dpc]

You are probably not talking with the right people at GE and ABB, I'd say.

The dc transient can appear on the CT secondary, at least until the CT saturates. The CT responds to changing current, not just sine waves.

Some relays see the dc transient inherently (the old induction cup type instantaneous elements), while some do not. A digital relay samples the current so it sees whatever current comes through the CT. The relay uses some type of algorithm to determine the rms current, and this can include the dc offset or it can filter it out (in software). The dc transient filtering is slower because more samples are needed to separate out the dc and ac components.

MCCB instantaneous units do respond to asymmetrical current, at least in my experience.

GE and SEL both have some white papers on CT response and application. You can search their websites. I'm sure ABB must have something similar.

Dave

 

[sobeys81]

dpc,
Thanks for such a quick response. I spoke to the Manager Testing for ABB in NewJersey and that is where I got this response.
Last night, while reading red book, Chap2. I read that Digital relays and electromechnical relays dont consider DC component on ther other hand. Fuses and MCCB being quick acting that HV CB do see Asymmetrical current.

I am sure I can trust your response. GE F35 relay during instantenous region only reads fundamental component. There is no option for selection of rms or phasor value as avialble in TOC value.


So when I am seeing fault at a bus and I want to set my Instantenous element, What should I use.

ASymmetrical rms half cycle or Symmetrical rms half cycle.
Could you kindly drop a word here.

 

[EddyWirbelstrom]

Low Voltage Breakers
Is the instantaneous element of a thermal/magnetic MCCB calibrated in symmetrical rms current or asymmetrical rms current ?
How is the instantaneous element of MCCBs and ACBs with electronic trip units ( LSI ) calibrated ?
From experience I know that the Inst element of current limiting MCCBs must be set at approx. 15 x motor FLC to avoid trips on motor full-voltage (DOL) start.

Overcurrent Relays
The old attracted armature type instantanous relay responds to asymmetrical current. Where such instantaneous relays are on the primary side of a transformer, they must be set to avoid operation for a fault on the secondary side of the transformer. This involves consideration of the dc offset and is expressed as 'transient over-reach' of the relay. The Inst element is typically set at 1.3 x the bolted fault on the secondary to ensure non-operation for a secondary side fault.
Does anyone know why this practice is continued with modern instantaneous relays which have a very low 'transient over-reach' ?

Transient Over-reach = ( Is - If )/ If
where for a given setting of the Inst relay:
Is = symmetrical rms current required to operate the inst relay.
If = rms value of fully offset current required to operate the relay

 

[jghrist]

mnewman,

Good point on the transient overreach. An ABB CO instantaneous trip (IIT) will have a transient overreach of 1.23 pu on a circuit with a power factor angle of 80 degrees. A Schweitzer SEL-351 has a transient overreach of ±5% of pickup. I'm not sure how Schweitzer determines this since the ± part doesn't make sense to me for overreach and the relay does filter out the dc component. Maybe this is just a different way of expressing pickup accuracy of the instantaneous element.

 

[dpc]

sobey,

If the F35 does not respond to dc component (dc filtering), you can set the instantaneous pickup based on the momentary symmetrical fundamental current, plus a safety factor of about 10% to allow for CT error, etc.