Interlock vs. throwing the main circuit breaker for generator powering a house

[CobyRupert]

I don't think the transformers have much loss, but you have the consumer loads on the other secondary sides that would certainly load your generator down. I was actually quite amazed to learn that those big filled cans, really only have a little square transformer in them.

Right, transformers don't have much loss, but the inrush (magnetizing current) on reverse feeding the 30kVA or 50kVA transformer that supplies your 240V/200A service might be on the order of 10-12x (10-12 times) it's rated current (120amps-200amps, around here, utility typically under-sizes transformers for the service, knowing you don't typically draw 200 amps on 200amp service).

And that's just your transformer, never mind all the other ones connected to the primary line.

Unless your genny has a huge flywheel, a quick governor, a slow breaker, and lots of reserve power to supply this 1200 amps to 2400 amps of inrush, it's going to trip or stall.

 

[LouNY]

I don't know if this information will survive and be readable;
I tried to copy from a transformer web page

Transformer Protection Guide – Typical Inrush Current Limiters for Select Transformers
Transformer, Single phase Input Voltage, Continuous Current, Inrush Current, Impedence X, Inductance Xl
KVA VAC Amps Amp (Ω) (µH)
.50 120 4.16 104 1.63 4328
1.0 240 4.16 104 3.26 8642
2.0 240 8.33 208 1.63 4328
3.0 240 12.5 312 1.09 2881

A typical home is supplied by a 10-15 kva transformer;
so the inrush current is quite a bit more then even a 3.0 kva unit

 

[Industrial Toys]

How long is the inrush?

As I mentioned, I have a transformer behind my transfer switch used as an auto transfomer so the generators are only connected as 220. I have never noticed it being in circuit when connecting the generator as apposed to not being there. Not 12 , 7.5 or even the little air cooled Yanmar 3.7KW.


I think our rural lines are 4800. It does amaze me how many customers they can put on that. Or how little the TS is to serve all of us.

 

[CobyRupert]

Neutral referenced to ground or not . It will still be 120V line to neutral and 240V line to line . An open neutral part way between the loads and the utility neutral is where the 150V and 90V occurs .

Only if the neutral is connected to the center tap (neutral) of the generator's winding will you maintain 120V/120V split on a ungrounded neutral.
It's more the source (generator or transformer) that needs it's center tap of its 240V winding to be designated "neutral" to maintain 120V/120V split.
Now if that neutral is not grounded, and it's floating (relative to earth, so it may be 0 volts compared to earth or 30V, 60V, etc.. compared to earth, who knows, and one may not care ) but you still have 120V/120V split between L1-N & L2-N, and 240V between L1-L2.

We may be saying the same thing different ways.

 

[CobyRupert]

I don't know if this information will survive and be readable;
I tried to copy from a transformer web page

Transformer Protection Guide Typical Inrush Current Limiters for Select Transformers
Transformer, Single phase Input Voltage, Continuous Current, Inrush Current, Impedence X, Inductance Xl
KVA VAC Amps Amp (Ω) (ｵH)
.50 120 4.16 104 1.63 4328
1.0 240 4.16 104 3.26 8642
2.0 240 8.33 208 1.63 4328
3.0 240 12.5 312 1.09 2881

A typical home is supplied by a 10-15 kva transformer;
so the inrush current is quite a bit more then even a 3.0 kva unit

Those are typical control transformers, and you can see for a 3.0kva (12.5amps) that the inrush is 312 amps (25 times!) its rated current. I think 8-12 times is typical for bigger transformers, but then reverse feeding them may be worse. Fuses supplying transformers are usually selected with a "time-current curve" that can handle 25 times rated current for .01 seconds and 12 times rated current .1sec.

How long is the inrush?

As I mentioned, I have a transformer behind my transfer switch used as an auto transfomer so the generators are only connected as 220. I have never noticed it being in circuit when connecting the generator as apposed to not being there. Not 12 , 7.5 or even the little air cooled Yanmar 3.7KW.


I think our rural lines are 4800. It does amaze me how many customers they can put on that. Or how little the TS is to serve all of us.

The inrush is worse the first 1/2 cycle (of a 60 hz system) or for about .01 seconds (actually .0083 sec.). From there it recedes. Impedance of the transformer and how much iron is in the core will be a factor.
I don't understand what your autotransformer does? Boost 220V to 240V? 120V to 240V?

 

[Industrial Toys]

I only use half the transformer. So I don't care what is on the other side, as long as I have 110/220 windings. My generators go across the 220 with no neutral and my transfer swich connects to the same 220 but with a neutral off the centre. When running my little 3.7 generator, it's always fully balanced and I can take the full load off just one side. You naturally have to be mindful of the total load, but not what leg you are on or iif it is balanced. It's a nice system. Also, my 3000 Watt Telecom inverter only puts out 220 so it helps with that as well. I think or feel that having extra inductance in the circuit is a plus but I have nothing to back that up.

I actually have a second manual transfer switch behind the Automatic ASCO but that's a different story. Allows me to switch between a Diesel Generator and lets say the inverter for night time use. Too bad the power never goes out!

 

[fried1765]

Aircraft are designed as much as possible to 吐ail safe? No shame in making your gen set connection 擢ail safe ?

I agree with "no shame" ....but in my opinion..... also not necessary, if using fail safe (checklist) procedures.

 

[Industrial Toys]

That's kind of funny. I would figure my method of conveyance to be failsafe when sitting solidly on the ground, not moving and preferably not on fire!

 

[otlski]

The term fail safe refers to a process and/or device utilizing several (more than two different and independent types) layers of redundancy. A single point of failure, such as a person running a check list, is not fail safe.

As far as the "you cannot back feed the whole town/neighborhood with your tiny genny " argument goes; you don't have to power the whole town. We have had multiple ice storms where our street had 7 out of 10 poles down. There were multiple local breaks in the lines. One directly in front of my house and one 400 feet down the road. A improper hook up can indeed power that small section. It is not unprecedented - I have seen storms this bad a half dozen times in my 56 years, and while living in different places. Add to it fatigue, cold, wet, wind, etc, and line crews make mistakes. Why add yourself to the mix?

 

[Industrial Toys]

I guess that is why an airplane has two wings. I never thought of that. One falls off, you land on the other.