Rating of CB for protecting PF Cap Banks
Rating of CB for protecting PF Cap Banks
(OP)
I have a question about a low voltage PowerPack J frame CB that will be used to protect a PF correction CAP Bank.
I was instructed to find the "capacitor interrupting rating" for this CB. So I'm thinking... what the heck is the capacitor interrupting rating of a CB? I was told (briefly) a CB has trouble detecting an abnormal condition and also extinguishing the arc when the PF is 0.
For me, when current is flowing, the CB will be able to detect an abnormal condition by the:
a) Thermal effects caused from electron flow
b) magnetic effects caused by the change of electron flow
How does the PF effect this? What about arcing current, how can arcing becoming problematic when the current leads or laggs the voltage by 90?
What about if current was being detecting via a CT and relay, would the PF then become an issue?
Finally, negleting the issues of a capacitor high energizing currents, short circuit rating and continuous rating, what is also required for consideration when sizing a CB for protecting a PF cap bank?
I was instructed to find the "capacitor interrupting rating" for this CB. So I'm thinking... what the heck is the capacitor interrupting rating of a CB? I was told (briefly) a CB has trouble detecting an abnormal condition and also extinguishing the arc when the PF is 0.
For me, when current is flowing, the CB will be able to detect an abnormal condition by the:
a) Thermal effects caused from electron flow
b) magnetic effects caused by the change of electron flow
How does the PF effect this? What about arcing current, how can arcing becoming problematic when the current leads or laggs the voltage by 90?
What about if current was being detecting via a CT and relay, would the PF then become an issue?
Finally, negleting the issues of a capacitor high energizing currents, short circuit rating and continuous rating, what is also required for consideration when sizing a CB for protecting a PF cap bank?






RE: Rating of CB for protecting PF Cap Banks
RE: Rating of CB for protecting PF Cap Banks
Thanks for the help. :)
However, what I'm trying to visualize is the CB's max capacitor interrupting rating. How is setting a limit on the current flowing through the protection device reducing the chance of a restrike?
The ability of an interrupter to interrupt current
without restriking is determined by its contact material,
contact design, and gap dielectric field strength. IE: arching is a voltage issue over contact area, not current.
B) How does PF come into play with all of this?
RE: Rating of CB for protecting PF Cap Banks
RE: Rating of CB for protecting PF Cap Banks
The interrupting ability is dependent on both current and the transient voltage. Clearly, a breaker would have no problem interrupting a circuit where the load was switched off at the next device. The current interrupted would be very small, but almost totally capacitive, the capacitance of the cable.
The transient voltage across the contacts is related to the power factor. The current is interrupted at a current zero point. With 100% power factor, the voltage is also zero at that point. With 0% power factor, the voltage is at a maximum.
RE: Rating of CB for protecting PF Cap Banks
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Refer to 5.1 Restrike on Page 2 which talks about how the current and voltage angle affects restriking.
The pdf refered me to "IEEE Guide for the Protection of Shunt Capacitor Banks" IEEE Std C37.99-2000
Thanks for helping me understand about the restrike problem with CB's and Cap Banks. My last question is my original... what is the Capacitor interuption Current Rating of a CB and how does it tie into all of this.
RE: Rating of CB for protecting PF Cap Banks
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Basic Insulation level 250 kV peak BIL (min)
Rated Maximum Voltage 46.6 kV rms
Rated Continuous Current 1200 A
Fault Interrupting Current 20 kA
Rated Momentary Current 54 kA peak
Capacitor current Interrupting Duty 370 A rms
Rated Transient Recovery Voltage duty for restrike free
capacitor switching
113 kV peak
Rated Transient Inrush current with 0.5 mH series reactor 4.42 kA peak@1.4 kHz
RE: Rating of CB for protecting PF Cap Banks
RE: Rating of CB for protecting PF Cap Banks
1) Continuous Current = 1200 A
2) Current Interrupting Duty = 370 A rms
I'm reviewing the IEEE Guide for the Protection of Shunt Capacitor Banks IEEE Std C37.99-2000 to see if I can find an answer or at least clues.
RE: Rating of CB for protecting PF Cap Banks
Regards
Marmite
RE: Rating of CB for protecting PF Cap Banks
Lower ratings for cap bank switching are normal for any type of circuit breaker. This is most difficult task for any breaker due to the leading power factor of the load. It is just a physical reality.
The NEC Article 460 requires conductors and disconnecting means for capacitors be rated at not less than 135% of the cap bank's rated current to account for harmonic currents, etc.
RE: Rating of CB for protecting PF Cap Banks
I'm looking for the reason why the CB continous rating has to be derated for use in Cap Banks. Probably because of the 0 pf and the potential re-striking issue, but why is the current being derated? How does reducing the amount of current flowing through a CB reduce re-stricking? My guess is that by de-rating the CB for Capacitor bank application, you are essentially choosing a bigger CB for switching duty. Hence the CB's contact point would have more surface area thus reducing the chance of arching since it is a function of voltage over contact area.
However, that is my guess... something I came up with. Is it right or wrong? I am looking for the answer.
I read IEEE Std C37.99-00, now I shall read C37.06.
RE: Rating of CB for protecting PF Cap Banks
Load-break is different than fault-break as the internal mechanisms are different. A piece of equipment has a fault interrupting rating (also stated at a maximum X/R ratio) and a close & latch rating which are different but it's the same device and the same current carrying contacts.
Does this address your question?
RE: Rating of CB for protecting PF Cap Banks
I understand what you mean, but as you mentioned: "the magnetizing current." The X/R ratio that we speak of, isn't that the ratio of inductive impedance/resistance?
It is simple to visualize the difficulty when interupting the current of an inductive load due to the fact that I=di/dt can not instantaneously change. As the X/R becomes largers and larger, that is why you are required to derate the CB.
But for a CB that shall be applied for a capacitor bank, I don't see how current will be an issue because the flow can instantly stop (assuming L of the system ~ 0)
That's where my question lies: why for a CB must the capacitor's current be derated?
Note: Another thing, I was talking to Schneider this morning, and they said the LV CB's used for Capacitor bank application have no issues in regards to this (which is great, as my application is for 600V).
It is only when MV and HV CB are used, that you it becomes more application specific where specific duty CB are required for Capacitor Banks. IEEE C36.07-2000 address various CB continous ratings and their dereated current ratings used for Capacitor Bank.
RE: Rating of CB for protecting PF Cap Banks
RE: Rating of CB for protecting PF Cap Banks
It explains the cause and derives equation to use for calculations. But it didn't explain why a HV CB would require specific duty ratings and a LV CB does not. Nor could I find an explain why derating a CB's continuous current would solve these problems.
Maybe I'll have to wait until I am at some sort of circuit breaker presentation and ask an expert.
RE: Rating of CB for protecting PF Cap Banks
4.7.1 Capacitor Bank Current.
Circuit breakers are to be applied according to the actual capacitance current they are required to interrupt. The rating
should be selected to include the following effects.
1) Voltage Factor. The nameplate reactive power rating of the capacitor bank, in kilovars, is to be multiplied by
the ratio of the maximum service voltage to the capacitor bank nameplate voltage when calculating the
capacitance current at the applied voltage. This factor can be as large as 1.1, since capacitors can be operated
continuously up to 10 percent above the capacitor rated voltage.
2) Capacitor Tolerance. The manufacturing tolerance in capacitance is -0 to +15 percent with a more frequent
average of -0 to +5 percent. A multiplier in the range of 1.05 to 1.15 should be used to adjust the nominal
current to the value allowed by tolerance in capacitance.
3) Harmonic Component. Capacitor banks provide a low-impedance path for the flow of harmonic currents.
When capacitor banks are ungrounded, no path is provided for zero-sequence harmonics (third, sixth, ninth,
etc), and the multiplier for harmonic currents is less. A multiplier of 1.1 is generally used for a grounded
neutral bank and 1.05 for an ungrounded neutral.
In the absence of specific information on multipliers for the above factors, it will usually be conservative to use a total
multiplier of 1.25 times the nominal capacitor current at rated capacitor voltage for ungrounded neutral operation and
1.35 times the nominal current for grounded neutral operation.
See also the attached Cahier Technique from Schneider.These are available on Schneiders website, but you have to register so I've attached the pdf.
Regards
Marmite
RE: Rating of CB for protecting PF Cap Banks