Soft Starts and fault contribution

[simonfr]

Im currently designing a network, where i would have like to put 3 off 740kW motors with VSD's. This would have been kinder to the network upon start up, but also provide a 'barrier' to stop these motors contributing to any fault which occurs on the network.

However, due to factors outwith my control, we cannot have VSD's on the network, but the fault levels seen on the MCC busbars go up to a level just over the designed fault rating of the installed busbars (using ETAP computer model, busbars rated for 100kA 1 sec).

Do softstarts give the same protection?
When the bypass contactor is in operation there is short-circuit current contribution from the connected motor to the input side.
But newer types of softstart do not utilise a conventional mechanical bypass contactor. Some use solid state/electronic means to fulfill the bypass duty.

The Allen Bradley SMC-Flex has the 'integral' bypass, but via solid state/electronic means.

Has anyone ever encountered this type of problem before?

Simon

 

[itsmoked]

Not sure why you would consider an SS to be different from a VFD on your networks? A VFD could even be less abusive to the the network than an SS.

If an SS doesn't have a mechanical bypass then it really has NO bypass. It is just using the SCRs it uses for the reduced voltage start ON fully all the time.

I believe they have internal electromechanical bypass's or the option for an external one.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

ALL bypass contactors on soft starters are mechanical. What is different in newer versions (such as the SMC-Flex) is just that the control of the bypass contacts are solid state, meaning that the same PCB in the soft starter is also directly controlling the bypass contacts now, instead of having external electro-mechanical relay control of them. The bypass modules of the SMC-Flex, although integral to the power stack, are just a set 3 separate 1 pole electro-mechanical contactors inside each power pole.

So from a fault study standpoint, the motor CAN contribute to the fault current when connected by a soft starter running in bypass, unlike when controlled by a VFD. It is no different from that standpoint than any other contactor. The reason it doesn't contribute through the VFD is because the VFD is rectifying and converting, which is (typically) a one-way process.

That said, there are "regenerative" VFDs which do re-convert motor energy back to the line (typically for braking or harmonic mitigation), but I am not clear as to whether or not that would be considered in a fault study as a contribution because it is a controlled situation, independent of the effects of the upstream fault.

 

[LionelHutz]

Yup, all soft-starters with a bypass use a mechanical contact.

A regenerative VFD would not contribute much more than about 1.5 times it's rated current to a fault. It should also trip very quickly on low line or over current.

A soft-starter can contribute to the fault even if running on the SCR's. It depends on the firing pattern of the SCR's.

 

[simonfr]

Cheers for the replies.

I called Rockwell about the SMC Flex.
I assumed (wrongly) as it had an integral bypass it was electronic in some form and therefore may differ from a standard seperate bypass contactor. Therefore may not contribute to the fault levels as much as the DOL starter.

Looks like i'll be DOL starting the 740kW motors as im not allowed to purchase VSD's

Simon

 

[jraef]

Again, if someone at Rockwell is telling you those are "solid state bypass" devices, it shows that they do not know of what they speak. it's so hard for manufacturers to find good help now days...

The SCRs of the soft starter itself are solid state, but you need electro-mechanical bypass devices because all solid state switching devices reject heat, and the reason for the bypass device is to avoid the heat rejection into the starter enclosure. There is no such thing as some magical miracle solid state bypass device that is as efficient at carrying current as an electro-mechanical contact. If there were, they would just make the soft starter out of it!

I also don't understand why you would now consider DOL starting something as big as 740kW. I would use the soft starter if I were you. Sure, it will not help out with the fault current contribution issue, but there are plenty of reasons to use soft starters besides that.

Good luck with that.