Open versus Closed Transitioning on Medium Voltage Switches

[Redskinsdb21]

In my research on open versus closed transition switching, I fount this:

Open Transition Transfer Switches
An open transfer switch’s connection with a generator is established before the utility is shut off, and then the swap is made quickly once the connection has been safely established. Open transition switches are often regarded as the most cost-effective and widely used systems, as they’re compatible and viable for most business operations and needs.

Closed Transition Transfer Switches
A closed transition transfer switch performs a similar but slightly different function. With a closed system, the two power sources (utility and generator/backup) are allowed to briefly overlap, which limits or removes the momentary stutter in power that open systems can occasionally create.

My question is, with open do you momentarily loose power but with closed transitioning you do not due to the brief overlap?

Any other differences known?

 

[Overvoltage]

Yes, you have a momentary "blink" with open transition. It may or may not impact the load (such as if it's on a UPS the load won't notice but the UPS will). Lighting will likely blink although depending on the speed of the transfer it may or may not be noticable.

Closed transition it is very important to make sure that the sources are in sync as close as possible. Often a closed transition switch will have a limited range within which it will complete the transition and a limited time period to complete the transition before it defaults to an open transition (or fails to transfer).

Both have their uses, although open transition is more common and less expensive if it is "good enough".

 

[jghrist]

Usually, the transition from utility to generator is made when the utility fails. There is no closed transition. The issue of closed transition comes when the utility source is restored. A closed transition will avoid an outage, but the generator has to be synchronized with the utility before making the transition.

 

[Redskinsdb21]

Closed transition it is very important to make sure that the sources are in sync as close as possible. Often a closed transition switch will have a limited range within which it will complete the transition and a limited time period to complete the transition before it defaults to an open transition (or fails to transfer).

In the case of MV Duplex switch, how is this "sync" accomplished? Is there a sync check relay typically installed?

 

[Overvoltage]

Not familiar with a MV duplex switch specifically, but I would assume there is a sync check relay or a relay of some sort performing a sync check function. At MV, probably an electronic relay (SEL/ABB/etc) as opposed to a simpler relay configuration (such as a simple coil relay blocking close based on the voltage difference between the two sources).

Many ways to skin a cat...

 

[Overvoltage]

Usually, the transition from utility to generator is made when the utility fails. There is no closed transition. The issue of closed transition comes when the utility source is restored. A closed transition will avoid an outage, but the generator has to be synchronized with the utility before making the transition.

Very true, jghrist. I was thinking more of during testing (hopefully the customer does more testing than they see actual outages). But yes, during an outage situation the only difference will be the behavior on restoration.

 

[Redskinsdb21]

Appreciate the feedback Overvoltage and jghrist.

I am looking to replace several MV duplex switches. A couple are fed from utility and generator as the 2 sources but the others are fed from two different utility loops within the site. The purpose of the closed transitioning spec. is to not loose power at the medical facilities on the site regardless if the duplex switches are to be switched manually or if they switch automatically due to loss of power from primary source.

 

[waross]

You should recheck the available fault current when both switches are closed.
There may be equipment rating and/or arc flash issues with a closed transition.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[bacon4life]

Open transitions can discourage customers from routine testing, perhaps even going years without flopping over. With closed transitions, it is much easier to schedule monthly testing.

Open transitions inrush transients every time the switch operates. Occasionally stuff breaks due to inrush. Closed transitions typically have very tiny transients...but closing out of sync will cause a catastrophic transient.

 

[crshears]

Your supplying utility and/or local Authority Having Jurisdiction may very well prohibit closed transfers, so this may not be an option for you.

If the AHJ does allow them, it will most likely be with conditions attached; the utility I worked for had specific levels of Technical Interface Requirements pertaining to such instances.

If the generator is provided for back-up purposes only, the requirements are generally less stringent in recognition of the fact that the period of closed transition is relatively brief.

If on the other hand the generator installation is designed to remain in connection with the grid whilst providing load displacement, tighter regulations would typically apply, and if oversized beyond site load so as to generate onto the grid as an electrical market participant, tighter still.

For load displacement installations, net metering will generally suffice, depending on the AHJ; for market participants, four quadrant bidirectional metering is generally required.

Addition via Edit: Incidentally, I would strongly suspect most AHJs would disallow using any type of switch for closed transitions, and would likely insist on the use of appropriate circuit breakers instead; the utility I worked for had a specific instruction enjoining either the connection or disconnection of rotating equipment from the grid using air break switches, but would almost certainly take a dim view of using any type of switch for this purpose.

Hope this helps.



CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[waross]

I installed a number of residential standby switches that used very fast open transfers when gong to the grid from the generator.
The transfer was so fast that it had to be made in sync with a sync check relay.
An out of sync transfer could cause damage to motors and could cause a system to system flash-over.
The generators operated in droop mode.
This meant that the frequency varied with the load over a 3% range. 61.8 Hz to 60 Hz.
At times the generator frequency would match the grid frequency.
At such times it may take minutes for the generator to drift into sync with the grid.
You had to wait or drop some load. The very fast transfer switches did not have a center open position and an open transition was not possible with these transfer switches.
You may have a similar issue with closed transitions.
You may consider some means to tweak the generator frequency a few Hz, so as to sync faster at times.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[bacon4life]

I suppose what utilities require of customers can be very different from what utilities do themselves. Many of the utility prohibitions against closed transitions seem to date back decades to before interconnected distributed generation became common.

In my region is it common to use PMH style padmounted switches with an SEL-351 controller as ATS on the utility MV system. This setup uses switches performing the ATS function, with upstream breakers/fuses providing fault protection.

Based on discussion with a couple technicians who have installed backup generators, it seems like lots of customer installations get converted from open to closed transition after the initial permitting is complete.

 

[crshears]

Hmmm . . . are there very fast transfer schemes that operate at medium voltage? I'm not questioning the truth of any of what's being posted in response, just its relevance to the ask in the OP.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[Redskinsdb21]

I am going to follow up with some vendors to see what options may be available at MV levels. I think some good points are made and it may very well be that some AHJ's will not give a permit for closed transitioning at MV level or maybe even LV.

 

[Overvoltage]

Don't forget your utility also. They may have requirements that apply also (whether not allowed at all or regarding requirements to do so).

 

[waross]

are there very fast transfer schemes that operate at medium voltage?

Probably not, cr.
The point of my post was the possibility of the need to wait for some time for a generator to drift into sync for a closed transition.
When the generator frequency matches the grid frequency, and is out of sync, the choices may be:
1. After a suitable delay, on open transition.
2. After a suitable delay, tweak the generator frequency.
3. Wait for a long time.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[bacon4life]

Eaton refers to the scheme Waross describes as an Open in-phase transition . I don't know if these are used as medium voltage levels.

 

[waross]

Actually, bacon4life, the switches that I encountered were much faster than that.
Eaton shows a transfer time of 150 milliseconds.
That is about 9 cycles.
There are motor starting type contactors available which can give that speed of transfer.
I have been unable to find a transfer time for the ASCO switches, but they are much faster.
Imagine a double throw relay with a movable contact between two fixed contacts.
Many of these relays are NOT approved for opposite polarity.
Now scale that up to 100 Amps or 200 Amps, with close contact spacing that will allow opposite polarity to be present but not switched.
The second part is a small fly wheel that may rotate 180 degrees.This flywheel has a protruding arm.
A solenoid is energized momentarily to slap the arm.
The flywheel rotates 1899 degrees to the opposite stop.
A second solenoid is positioned to return the flywheel to the first position.
The flywheel is linked to the contacts in such a way that only a few degrees of rotation will move the contacts from one source to the other source.
A delay of 150 milliseconds may result in a momentary flicker of some lights.
These very fast transfers do not cause noticeable flickering of lights.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[bacon4life]

Waross- Cummins lists their open-in-phase ATS as operating in 30-50 milliseconds and ASCO shows a ~40 millisecond gap on page 8. It is a little weird that Eaton mentions the max transfer time of 150 milliseconds without listing typical transfer times. I wonder if Cummins/ACSO are actually that much faster than Eaton products or if Eaton just the list the max possible time. For closed transfers the maximum allowable parallel time is often specified as 100 milliseconds, but the ATS usually operates much faster 100 milliseconds.

 

[waross]

Thanks for the research, bacon4life.
I suspect that Cummins is using a similar system as ATCO for there very fast transfer switches.
The Eaton time is probably based on a pair of fast acting contactors.
In a previous life, I researched the operating times of motor starting contactors while preparing a course on motor controls.
It was a long time ago, but as I recall, some of the faster contactors were capable of doing a transfer in 150 milliseconds.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!