SCRs in Arc Flash studies

But, we also know SCRs are power storing devices, so do we have to include this knowledge in any possible fault down stream?

This isn't correct, an SCR is just an electronic switch. In the event of a worst-case failure it can short circuit allowing fault current to pass thru it, or blow up, but that is about it (from an arc flash study point of view). It doesn't store any energy.

With regard to the study, are you talking about assessing the arc flash hazard at the point of the SCRs or are you talking about assessing the arc flash hazard downstream of the SCRs (where the SCR-connected lines could potentially add to the overall fault current and hence increase the available arc flash energy)?

Are you talking about grid transmission and distribution systems? If so then you will find that the vast majority of the fault current is going to come from the generation side, with any downstream energy storage devices (such as motors and capacitors) only providing a very, very small amount of the overall current and power to the fault (further limited by the long lengths of transmission cable and distribution transformer impedance). So small in fact (orders of magnitude) that it is ignored in standard fault current and arc flash calculations. There are exceptions, so it does depend on the system you are trying to model.

Can you provide more information on the systems you are trying to model?

Thanks for your response. I was not sure how much information I needed to include.

Our models need to assess the system from the sub-station down to the load, which could be resistive (heater) or inductive (motor). Well, we start out with the protective device (breaker) on the sub-station, then we usually have one more device on the control cabinet, which is usually just a disconnect; but could be another protective device. Inside the cabinet is where the SCR controller is and possibly a motor contactor. Finally, you get to the load, which again is usually a motor or a heater.

To say that SCR's are "power storing devices" is not exactly accurate. SCR's block current until they're turned on, which is not the same as storage of power/energy as a capacitor or inductor does. I'm also not sure what you mean when you say that, "there is never a need to contribute it's (SCR's) load to an upstream fault." There can be a fault such that SCR's fail due to a fault between SCR's and their loads, which, potentially, can contribute to the energy of an arc flash.

I have never run across any arc flash study which includes SCR's as components as a factors, however their loads are included, including induction motors.

I guess I did not think about that Bill, since I just assumed since SCRs are rectifiers that current could only move one way; but standard rectifiers can fail and we are talking about fault conditions.

Thanks.

... or if the SCR happens to be conducting when the fault occurs...

Just because the term "SCR" has the word "Rectifier" in it does not mean they are always used as rectifiers. In fact in AC power controls, such as heater controllers and soft starters, the SCRs are used in anti-parallel pairs so there is one that can technically conduct in each direction, so they are NOT rectifying. For that reason, I like to use the term "Thyristors" top describe their use in this manner. So when a fault occurs downstream from a Thyristor control devices, you must consider than load's potential contribution to the fault study, because for all you know, the controller was On when the fault happens, whether it failed or not.

Are you mistaking SCR for Variable Frequency Drives or Adjustable Speed Drives that have capacitors and store a charge?

I'll butt out now--K T

Probably, but the issue of basic (non-regen) VFDs contributing to faults in studies is a controversy in some circles. One argument says no, because of the rectification to DC acting as a buffer in keeping the motor from contributing back into an upstream fault. But the other argument is that one must always consider the failure mode, and it is at least theoretically possible for a VFD to short from line-to-load.

I don't happen to be in the latter camp, because to my thinking, if the VFD components DID short, the fact that the DC link is only 2 conductors makes it essentially a bolted fault itself, and therefore the spinning motor is NOT contributable to any up-stream fault.