Closing delays for a domestic transfer switch. 2

[itsmoked]

We've talked a lot in here about power interruption delays and the 'fun' they can cause with motors involved. However, we're typically discussing industrial and commercial situations and 3 phase systems with multi HP motors.

What about domestic applications? Here you have mostly DC loads fed via don't-give-a-dang rectifiers and resistive loads like heaters and incandescent lighting and as for motors; single phase small 1/4hp or less refrigeration compressors and ~1/2HP furnace blowers? Do we have to delay reclosing on fear of residual fields?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Several dozen residential standby installations and never had a problem.
These were contactor based transfer switches.
A typical pull in time for that size contactor may be in the range of 100 milli-seconds. (Subject to correction. I haven't had to check the operating time of contactors for a few decades now.)
The drop-out time may be slightly more or less.
Many of the ATSs were configured so that when one contactor closed the N/C contacts the other contactor was energized.
No intentional delay when going from the generator to the mains.
EXCEPTION
There is one type of transfer switch that does a very fast transition. These switches typically have a sync check relay and only transfer when the standby phase is in sync with the grid phase.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[oldfieldguy]

Transfer switches up the wazoo - all industrial, but two considerations:

1. Transfer from utility to generator - give the generator time to warm up. some don't respond well to taking on load when cold.
2. Transfer back to utility - a time delay to make sure that the utility is REALLY available, not just some switching going on to see if the breaker holds in.

old field guy

 

[wayne440]

I have never seen an automatic residential switch that allowed an intentional "off time" when switching from generator to utility. The cheaper ones just bang in after the utility has been present for some preset time, the better ones may have a sync check function that can be used if desired. (edited to include "automatic")

 

[itsmoked]

Yeah Wayne I see that too as most are just those double pole double throw switches and there's no OFF in the middle.

I was planning to use one of those but really did want an OFF somewhere though that can be managed elsewhere. The bigger problem was that a lot of inverter systems want the utility neutral gone or they throw errors. That means a two-pole doesn't cut it as you need three poles total.

oldfg; That's a good system I agree. In my case I'm switching to an inverter that needs no warm-up at all. A contactor based transfer switch so my speed limit is the time it takes the utility contactor to drop out and the already called-for inverter contactor to to be engaged by the fully dropped out utility contactor's N.C. aux contacts. Summed up, probably about 50~70ms of fiddling about between sources.

Bill has me hoping for the best.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

My experience with sourcing transfer switches:
1. Cheapest and best. Order a transfer switch with the set. It doesn't have to match the set.
I have ordered 100 Amp ATSs for a set that normally came standard with a 40 Amp ATS. Forget the gen-set, I had to switch 100 Amp mains.
I found that my suppliers would supply an over-rated ATS at the same markup level as the standard ATS. (Mark-up, nil, they want the sale of the set.)
2. For one reason or another there was no ATS available with the gen-set. Roll your own. A pair of interlocked reversing contactors and a phase loss, voltage monitoring relay. It's been a while but I was able to find octal socket relays that were economically priced and would check for phase loss and phase rotation. Add a few timers if you want. Some seconds delay on re-transfer to the utility is a good idea. Consider also a cool down timer for the engine if it has a turbo.
3. Most expensive, Buy a new ATS "off the shelf".

I have several times seen all the electronic devices wiped out in a home when the neutral went open for some reason or other.
Anecdote alert: One EX customer wanted some work done in his home. I told him when I could be there and he said that that was ok.
Then he didn't wait. He got someone from the shallow end of the gene pool to do the work.
The guy crossed a hot with the neutral on a sub panel and burned up his new flat screen TV among other things.
We discussed the work still to be done and when I could be there. All OK.
Then he hired another technically challenged individual who opened the neutral with the power on. This time he lost his micro-wave oven and the rest of his home electronics.
Then he got mad at me. Over the years he was one of several customers that I "fired". grin

The point is that I will do what it takes to avoid switching a neutral.
Sometimes I have had to "Try Harder", but I have always found a way.
That said, I have been fortunate to never have encountered a situation where there was no alternative to switching the neutral.

If you are switching the neutral with a manual switch, it must have an early-make, late break neutral contact.
Contactors are available with an early-make, late-break pole.
A case that I investigated for the utility where a customer claimed that the utility was responsible for the damage to his electronics had a three pole manual transfer switch switching the neutral.
The code allows switching the neutral if all poles make and break simultaneously.
All three poles were equal but the neutral was a few thousandths of an inch less equal.
I explained the issue to the owner and solidly connected the neutral and made him happy. He quit claiming damages.
For a manual switch you may be able to shim the neutral pole or trim 1/8" or 1/16" from the hot blades to ensure the early-make, late-break.
For a contactor you may bend or shim the neutral contacts or put a dab of silver solder on the neutral contacts.
Both the line side and the standby side.
You see why I have always "Tried Harder".

OFG, with the greatest respect; I have never seen a residential set or a commercial set that had an intentional warm up time.
A couple of sets had a few seconds delay (less than 5 seconds) to allow the AVR to stabilize.
A standby set has a tough life. Most of the sets that I have installed and serviced went from cold to full speed and then had about a 300% block-load dropped on them. all within less than 10 seconds. Then they settled down with about 10% to 25% of rated load.
A standby set has a rough life.
Oh, and don't forget, a 110 KVA standby set will be rated at only 100 KVA as a prime power set.

I did have one industrial installation in a remote sawmill where we did need a warm-up time.
We needed two sets in parallel to start the big hammer hog motor.
One set had a hydraulic governor and one set had an electronic governor.
When the big motor when on-line the electronic governor would open faster and hog the load and trip off on overload.
If we started the unit with the hydraulic governor first and let it take the load for about 15 or 20 minutes and then start and paralleled the electronic unit, the oil would be thin enough in the governor that the motor could be started without tripping out.

A thought Keith.
An invertor may not be as forgiving as a generator winding in regards to back EMF of spinning motors.
70 ms is only about two revolutions. you may want to arrange an intentional delay of 100 ms or 200 ms or even allow a few seconds if there are reclosers on the distribution circuit. You don't want to subject your invertor to an intimate relationship with a recloser banging in and out about 3 or 5 times in a few seconds.
I have lots of experience applying ATSs to generators and none with invertors.
I did one a couple of invertor jobs but they were manual transfer switches.
Keep us in the loop.

Bill
--------------------
"Why not the best?"
Jimmy Carter