Single Phase Inverters on 208 3 Phase

[Carultch]

If we consider these two circuits as totally independent (not grid-connected), would it take (4) of the wye circuits, each with (3) 240 volt voltage sources, to come close to the same energy as the (1) delta with (3) 240 volt voltage sources?

In order for inverters to grid-form a WYE grid, you have to connect them in a WYE manner, or have a delta:wye transformer to form the neutral. Either directly through a delta:wye transformer, or indirectly with an auxiliary delta:wye transformer used as a grounding bank. Usually the latter is a more economical way of doing it.

The reason is that grid-forming inverters have to establish the voltage-to-ground of all three phases. This is not possible to do directly with inverter power stages connected in a delta formation. A delta formation can establish interphase voltage, but it can't establish where relative to the neutral point, the voltages are.

Most grid-tied inverters are grid-following, rather than grid-forming, which means they do not produce an independent voltage source. In an ideal textbook model of the grid, they'd be current sources. UL standards require that inverters stay off during a power outage, unless there is an air gap between the inverter and the public utility grid. In other words, a transfer switch that keeps the power and voltage on the customer's property, for the safety of the utility linesmen.

 

[wwhitney]

If we consider these two circuits as totally independent (not grid-connected), would it take (4) of the wye circuits, each with (3) 240 volt voltage sources, to come close to the same energy as the (1) delta with (3) 240 volt voltage sources?

Your question is underspecified, but if you fill in the details as I believe you intend, then no. For a fixed resistance and method of connection (L-N or L-L), it would take (3) of the circuits with delta connected sources to dissipate the same power as (1) of the wye version. See post #377.

Cheers, Wayne

 

[wwhitney]

Forget the fact that traditionally we think wye and delta should be certain voltages. These are the same circuits, with the same transformer, with one connected in a wye configuration (944 W) and one connected delta (2.8 kW).

You got this backwards, the power dissipation is higher with the wye-configured sources (given fixed source voltage and fixed load resistance and configuration).

Because the voltage in the delta configuration is sqrt(3) times higher

And I got this backwards as well, the L-N (also the L-L) voltages are sqrt(3) times higher in the wye configuration (of fixed voltage sources) than in the delta configuration.

Cheers, Wayne

 

[ggunn]

You got this backwards, the power dissipation is higher with the wye-configured sources (given fixed source voltage and fixed load resistance and configuration).


And I got this backwards as well, the L-N (also the L-L) voltages are sqrt(3) times higher in the wye configuration (of fixed voltage sources) than in the delta configuration.

Cheers, Wayne

There is no L-N voltage in a delta configuration because there is no N. Maybe I am reading you wrong, but a 480V delta has the same L-L voltages as does a 480/277V wye.

 

[wwhitney]

There is no L-N voltage in a delta configuration because there is no N. Maybe I am reading you wrong, but a 480V delta has the same L-L voltages as does a 480/277V wye.

There's no N as a conductor that you can use to power unbalanced loads unless you provide additional equipment (like a grounding transformer). But the N still exists as an abstract voltage point. And you can realize that voltage point by using wye-connected identical impedances. As per my first post today (#375).

Cheers, Wayne

 

[ruxton.stanislaw]

There's no N as a conductor that you can use to power unbalanced loads unless you provide additional equipment (like a grounding transformer). But the N still exists as an abstract voltage point. And you can realize that voltage point by using wye-connected identical impedances. As per my first post today (#375).

Exactly this. Think of a theoretical place, between the three voltages, that you can reach with the right equipment or balance of loads.

 

[ggunn]

Exactly this. Think of a theoretical place, between the three voltages, that you can reach with the right equipment or balance of loads.

Of course, but unless you add something to the circuit you cannot directly measure L-"N" if there is no N. But no matter; L-L voltages are the same whether wye or delta.

 

[bellington]

If I'm on the moon and have (3) single phase 240 volt generators that I could sync their outputs to maintain 120 degree phase shifts, and (3) single phase transformers, and wanted to run a 3-phase motor that needed 11.2 kW, how should I connect them? How much difference would it make to connect the generators to a delta arrangement on the generator side, or a wye, connected a L-N on the generator side, or a wye, connected L-L on the generator side?

 

[ggunn]

If I'm on the moon and have (3) single phase 240 volt generators that I could sync their outputs to maintain 120 degree phase shifts, and (3) single phase transformers, and wanted to run a 3-phase motor that needed 11.2 kW, how should I connect them? How much difference would it make to connect the generators to a delta arrangement on the generator side, or a wye, connected a L-N on the generator side, or a wye, connected L-L on the generator side?

You're gonna have trouble running a generator on the moon where there is no air.

 

[wwhitney]

If I'm on the moon and have (3) single phase 240 volt generators that I could sync their outputs to maintain 120 degree phase shifts, and (3) single phase transformers, and wanted to run a 3-phase motor that needed 11.2 kW, how should I connect them?

Note your 3 phase motor will only need 3 wires connected, just the line conductors, no neutral. So this is a different load configuration than your previous examples with resistive loads supplied L-N.

With that in mind, the answer depends on what voltage your 3 phase motor is designed to run on.

If it wants 240V 3-phase, then you don't need any transformers. You can connect your 240V sources in a delta configuration and supply the motors directly.

If it wants 416V 3-phase, then you don't need any transformers. You can connected your 240V sources in a wye configuration and supply the motors directly. With the 240V sources in a wye configuration, you 240V * sqrt(3) = 416V L-L.

If it wants some other voltage, then you need a transformer to change voltage.

Cheers, Wayne

 

[bellington]

Note your 3 phase motor will only need 3 wires connected, just the line conductors, no neutral. So this is a different load configuration than your previous examples with resistive loads supplied L-N.

With that in mind, the answer depends on what voltage your 3 phase motor is designed to run on.

If it wants 240V 3-phase, then you don't need any transformers. You can connect your 240V sources in a delta configuration and supply the motors directly.

If it wants 416V 3-phase, then you don't need any transformers. You can connected your 240V sources in a wye configuration and supply the motors directly. With the 240V sources in a wye configuration, you 240V * sqrt(3) = 416V L-L.

If it wants some other voltage, then you need a transformer to change voltage.

Cheers, Wayne

It needs 12.4 kV. How do we connect them?

 

[wwhitney]

It needs 12.4 kV. How do we connect them?

Seems surprising to me for a motor that you described as 11.2 kW (the motor current would be under 1A), but I don't know much about high voltage, so I'll go with it.

Then you will need a transformer whose secondary side is 12.4 kV L-L; not sure if that would typically be a 3 wire delta secondary, or a 12.4Y7.2kV secondary. And you just need to match your transformer primary side configuration with your source configuration.

So on the primary side, you could connect your 240V sources in a delta, and use a 240V delta primary transformer. Or you could connect your 240V sources in a wye, and use a 416V delta primary on the transformer. Or if your transformer has a wye secondary, you could connected your 240V sources in a wye, and use a 416Y/240V wye primary.

Again, this is for a 3 phase generator that makes 3 sets of 2 wire 240V sources with the correct phase relationships, and has nothing to do with grid-following inverters.

Cheers, Wayne

 

[bellington]

Seems surprising to me for a motor that you described as 11.2 kW (the motor current would be under 1A), but I don't know much about high voltage, so I'll go with it.

Then you will need a transformer whose secondary side is 12.4 kV L-L; not sure if that would typically be a 3 wire delta secondary, or a 12.4Y7.2kV secondary. And you just need to match your transformer primary side configuration with your source configuration.

So on the primary side, you could connect your 240V sources in a delta, and use a 240V delta primary transformer. Or you could connect your 240V sources in a wye, and use a 416V delta primary on the transformer. Or if your transformer has a wye secondary, you could connected your 240V sources in a wye, and use a 416Y/240V wye primary.

Again, this is for a 3 phase generator that makes 3 sets of 2 wire 240V sources with the correct phase relationships, and has nothing to do with grid-following inverters.

Cheers, Wayne

The transformers are single phase 7200 to 240. Let's connect them in a wye formation and connect the single phase generators N-L. Will that work okay?

 

[wwhitney]

The transformers are single phase 7200 to 240. Let's connect them in a wye formation and connect the single phase generators N-L. Will that work okay?

Transformer primary/secondary notation can be ambiguous; as the independent sources are on the 240V side, I will call the 240V side the primary. Notation and answers would differ if you have a 7200V grid connection as the primary source, and grid-tied inverters on the 240V side of the 7200V:240V transformers. Then the 240V side would be the secondary.

With single phase 240:7200 transformers, and (3) 240V single phase sources that are synced to be 3 phase, you will need to match the 240V single phase source connectivity to the transformer connectivity. That connectivity can be either wye or delta. The secondary (7200V) connectivity needs to be wye to get the 12400V L-L for your 12400V motor.

So yes, you can do a 416Y/240V source to a 416Y/240V : 12400Y/7200V transformer to a 12400V 3P3W motor. Or you could do a 240V Delta source to a 240V Delta: 124700Y/7200V transformer arrangement with the same equipment you've described. Either will work.

But this is fairly far removed from your recent questions; what are you driving at?

Cheers, Wayne

 

[bellington]

Transformer primary/secondary notation can be ambiguous; as the independent sources are on the 240V side, I will call the 240V side the primary. Notation and answers would differ if you have a 7200V grid connection as the primary source, and grid-tied inverters on the 240V side of the 7200V:240V transformers. Then the 240V side would be the secondary.

With single phase 240:7200 transformers, and (3) 240V single phase sources that are synced to be 3 phase, you will need to match the 240V single phase source connectivity to the transformer connectivity. That connectivity can be either wye or delta. The secondary (7200V) connectivity needs to be wye to get the 12400V L-L for your 12400V motor.

So yes, you can do a 416Y/240V source to a 416Y/240V : 12400Y/7200V transformer to a 12400V 3P3W motor. Or you could do a 240V Delta source to a 240V Delta: 124700Y/7200V transformer arrangement with the same equipment you've described. Either will work.

But this is fairly far removed from your recent questions; what are you driving at?

Cheers, Wayne

The motor is pulling a 15 HP load. It stopped working. All we have is a motor that runs on 6235 volts. How much HP can we expect from the motor with the generators now wired L-L?

 

[wwhitney]

The motor is pulling a 15 HP load. It stopped working. All we have is a motor that runs on 6235 volts. How much HP can we expect from the motor with the generators now wired L-L?

Is this a present real world situation, or are you just making this up? If the latter, why don't you jump to the idea or question you seem to be driving at?

Motors are not my forte, but I know enough not to energize a 6235V motor at 12400V, and I imagine 7200V would also be a problem.

Cheers, Wayne

 

[Carultch]

The motor is pulling a 15 HP load. It stopped working. All we have is a motor that runs on 6235 volts. How much HP can we expect from the motor with the generators now wired L-L?

It seems very unrealistic for a 15 HP motor to require medium voltage to power it directly. I wouldn't expect a motor to require medium voltage until it has at least a 3-digit horsepower rating.

 

[jim dungar]

I think this thread has jumped the track. Is it time to close it?

 

[bellington]

Can someone show me a different situation in which L-L feeding a 3-phase wye transformer is as effective as effective as feeding phase to phase?

 

[bellington]

Is this a present real world situation, or are you just making this up? If the latter, why don't you jump to the idea or question you seem to be driving at?

Motors are not my forte, but I know enough not to energize a 6235V motor at 12400V, and I imagine 7200V would also be a problem.

Cheers, Wayne

What if we connected the 240 L-L on the wye?