Mysterious CB Trip on Pre-Charge Capacitor Inrush

[ThePunisher]

Hi all,

We have a Siemens Perfect Harmony VFD feeding three motors rated 3 x 4,000 HP, 6.9 kV. The VFD is having isolation transformer with 12.5 MVA primary wye winding and 13 MVA secondary delta winding (18 x windings connected to individual power cells (6 windings per phase).

The VFD pre-charging circuit is protected locally at the VFD by a 250A MCCB whose line side is further fed from a 100A MCCB at the MCC at 600V.

We are aware that the upstream MCC (100A) is under-rated and not matching the VFD local breaker (250A.

However, what bothers us is that the inrush is tripping both or either circuit breakers. With an amplitude of approx. 364Apk-pk or approx. RMS = 128A at approx. 3.5 sec. the 250A VFD MCCB tripped.

There is also case where the 100A circuit breaker at the MCC tripped in approx. 4 secs.

Looking into just the 100A MCCB TCC, it will take a current of approx. 300A to hit the bottom thermal element at 10 secs.

Therefore, it is quite a mystery if either of both breakers trip on a short time while the amplitude is not high enough.

Does anyone had the same experience and if what mitigations were done?

We assume to replace the 100A MCCB with 200A and set both VFD breaker and upsized MCC breaker to an instantaneous settings to override the inrush. Siemens was claiming the high loading on pre-charging will not exceed 60A.

 

[waross]

So, you have a 250 Amp breaker and a 100 Amp breaker tripping on a rated load of 60 Amps.
You suspect that there is something wrong with both breakers or their settings.
Is it possible that there is a problem internally that is causing the breakers to trip?
The breakers may be doing the job that they were designed and intended to do.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[FreddyNurk]

What waross said. The chances of two separate, differently rated breakers both being faulty or otherwise incorrectly tripping are a lot less than the possibility that both are correctly tripping and there's an issue downstream.

EDMS Australia

 

[dpc]

If these are standard thermal magnetic breakers, the magnetic trip element will respond to any type of current - including dc offset, harmonic current, asymmetrical current. How are you measuring the current?

The fact that both are tripping would tend to indicate that the magnetic element is causing the trip, not the thermal element. My initial suspicion would be some type of transient event with a high magnitude current for short time.

 

[itsmoked]

Perhaps your pre-charge is malfunctioning since they say "not to exceed 60A" and you're seeing more than double that.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[che12345]

Dear Mr. Punisher
Q. "... what bothers us is that the inrush is tripping both or either circuit breakers. With an amplitude of approx. 364Apk-pk or approx. RMS = 128A at approx. 3.5 sec. the 250A VFD MCCB tripped. There is also case where the 100A circuit breaker at the MCC tripped in approx. 4 secs.
A. a) since both MCCBs are tripping at 3.5s and 4s respectively, it is very "unlikely" that both are faulty,
b) based on the tripping time at 3.5s and 4s respectively, it is "unlikely" due to the [inrush current] as the inrush is unlikely to last for that length of time. It may trip instantaneously on instantaneous or magnetic trip, if it is due to the [inrush current]. The time delay of 3.5s and 4s respectively is an indication that tripping is "likely" due to the [thermal over-load] element.
c) be aware that the MCCB [thermal tripping characteristic] is a "band". It is not "one line curve". It also differs depending on whether in "cold" or "hot" state.
As an example: a) a "typical" 250A MCCB may trip on "hot state" within 7s to 40s at 3x250A,
b) a "typical" 100A MCCB may trip on "hot state" within 4s to 15s at 3x100A.
d) Taking the tripping time into consideration, the 100A MCCB may have to be increased. Check their current settings. The 250A MCCB should be in order.
e) suggest to investigate other (fault) possibilities instead of the MCCB, besides increase the 100A breaker rating.
Che Kuan Yau (Singapore)