Hi,

I have consulted the IEEE 1584 standard but have not seemed to find any discussion regarding the impacts of PFC units to the incident energy.

From IEEE 551, it states that during fault conditions, the capacitor discharge takes place in the initial 1/30–1/8 cycles. Since the breaker protective device and contacts cannot operate in this time frame, capacitor discharge currents will have no effect on breaker parting or clearing operations. I take from this that the discharge current does not sustain long enough to effect the tripping of breakers, thus not having an effect on the arc duration.

But I wonder if the discharge contribution during the first cycle/s significantly impact the arc current, hence the incident energy?

Referring to IEEE 1584 again, they state that motors smaller than 50 HP can be ignored since they are not large enough to sustain their fault current for any meaningful length of time. Would this principle be applied for PFC units too? Also when modelling the system, are PFC units ignored?

I would appreciate your thoughts on this.

Kind regards,

studentengineer

Here is a response I got from EasyPower a while back concerning power factor caps:

I wanted to mention to you that the Capacitor banks are not considered as sources for short circuit studies and are only used in the Power Flow. With this in mind it is not as important to have the capacitor information filled out for short circuit studies.

Hi Barry,

Thanks for sharing the response you got from EasyPower. I wonder why they're not considered sources in short circuit studies? Is it that they're insignificant on their impact to short circuit faults?

For the case of work on generator rotor brushgear, I have seen papers consider the stored energy in the rotor inductance as contributing to a brush arc flash as 1/2 Li^2. Similarly, the capacitor stored energy of 1/2 CV^2 must be dissipated in the circuit. The portion that is contributed in arc energy as incident energy could be estimated based on appropriated shape and absoption factors presumably.

Welcome to the ArcFlashForum!
The IEEE 551 Standard - Calculating Short-Circuit Currents in Industrial and Commercial Power Systems addresses capacitors in Chapter 7. This chapter provides many simulations involving capacitors for motors, harmonic filters, power factor correction. The following is the summary from this standard.

During fault conditions, the capacitor discharge takes place in the initial 1/30–1/8 cycles,
depending on the time constant of the system. Since the breaker protective device and
contacts cannot operate in this time frame, the discharge takes place into closed contacts.

The electro-magnetically induced forces of the discharge current are instantaneously
proportional to the current squared. Since the close and latch (momentary) rating of a
breaker is the maximum fundamental frequency rms fault current the breaker can
withstand, it can also be considered a measure of the forces which may be safely imposed
on the various physical members of the breaker during a rated frequency (i.e., 60 Hz) fault
condition.

Based on the simulations shown in this chapter, capacitor discharge currents will have no
effect on breaker parting or clearing operations. Some small additional stresses may be
imposed for the closing and latching duty for very large capacitor banks. However, it
should be noted that the models developed in this chapter were sized larger than standard
design practices in order to determine any potential problems.

At this point, the standard cannot recommend that capacitors be added to system
simulations for breaker duty calculations. The existing ANSI C37 series fault calculation
methodologies remain adequate for the determination of breaker, fuse, and switch duties.

Source: IEEE 551 - 2006 Calculating Short-Circuit Currents in Industrial and Commercial Power Systems, (C) 2006 Institute of Electrical and Electronics Engineers, Inc.

Short circuit and arc flash incident energy calculations both use rms values. Since the rms calculation depends on a periodic waveform, the idea of a 1/30 or 1/8 cycle rms current is meaningless. Yes, the duration is too short to add significantly to incident energy.