Safe Bus Transfer Scheme for High Inertia Motors

Hi Jhonson,

It depends on your application need and the process time constant (the dead time that your process can afford). If you are running a critical continuous process, then fast transfer would work best for you provided you have two independent synchronized sources available at all times.

We have worked with providing bus transfer systems for power plants and process industries for more than 25 years now. Please visit www.aartechsolonics.com or email me at amitraje@aartechsolonics.com for more info.

Amit.

Thanks for your reply,

Its a critical continuous process, does the fast transfer check the phase angle of the motor voltage and aux voltage before closing? Its it safe to parallel (no fault on the main) in 1 - 2 cycles which out checking for synchronism?

What do you mean by two independent synchronized sources?

Yes, the fast transfer checks the phase angle between the motor bus and the incoming source before closing. It also checks the bus voltage to be above a certain threshold. 20 degrees / 80% is quite typical. ANSI C50.41(1982/2000) provides for 1.33 p.u. |delta V/F| across the incoming breaker as acceptable which can provide a different latitude as far as permissibles for fast transfer is concerned.

NO - It is not safe to parallel without checking for synchronism. Infact, the synchronism better be a high speed sync-check with very sensitive dropout (<20ms) to handle dynamic system conditions that may arise even if there are no electrical faults.

It will be good to have your Single Line Diagram and operating philosophy to better comment on your actual application. I am assuming the bus transfer is required at Medium Voltage levels as the application considerations differ slightly as per different voltage levels. Switchman's comments and queries are also pertinent in this regard.

Best,

Amit.

There are some things missing that need to be filled in, in order to be able to give a better discussion on this.

What are your sources of power and at what voltage? Is your alternate source a generator or a second live feed? If it's a live feed is there any phase shift due to transformers?
Are you laying out a new system or retrofitting an existing application? Do you need to comply with NEC 700, 701, or 702?

In order for you to figure out the best switching method you'll need to go at it from two directions... source failure and hot-to-hot switching.

I'm not really sure what you meant by residual voltage transfer...all these years I haven't ran into that term. Motor loads are notorious for hard hitting...as well as lightly loaded transformers.

Looking at your second post I may be misunderstanding... "phase angle of the motor voltage and aux voltage" can you explain this a bit more?

A typical "Y" transfer switch sitting on source A will see an outage once the voltage drops below the dropout setpoint. Most controllers will ignore any back emf from the load ..from an in-phase monitor standpoint... and transfer immediately once the alternate source is acceptable. If you have a high inertia motor with a long wind down there's not a heck of a lot you can do other than delay closing in on it or taking the hit. And that hit can be really wicked bus shaking...I've seen easily upwards of 15x to 18x motor fla.

Going hot-to hot, whether for testing or retransfer, is altogether different. In-phase monitors work just fine. Switching is delayed until the voltage differential is close to minimum. Some work as a very basic 25 sync check and some are more advanced even taking into account the slip difference between sourses and leading some to close flat in phase.
Closed transition and delayed transition are options.. but each has its own little nuances one has to be aware of.

Here's a couple links to look at from ASCO. There's some good info there and at minimum a starting point.

http://www.asco.com/ascofacts/jdieeev34no6.pdf

http://www.asco.com/cgi-bin/redirect.pl?/ascofacts/facts.html