3 phase and Grid tie... what will happend?

[Sunking]

can you tell me what happens if the load is not balanced between the three phases say 1 amp on one leg, 2 amps on the second and 3 amps on the third?

Russ I am going to give you the simple answer. It means you will have neutral current flowing of about 1.5 amps. If all three phases are perfectly balanced, you will not have any neutral current flowing in a 4 wire wye system.

Think of it like this: We have three horses bridled together to a common point pulling in 3 different directions. They are pulling 120 degrees adjacent to each other. Think a pie cut in thirds. If they apply the same force, they go no where. If one or two are pulling harder than one, the center of the circle travels at x degrees at Y speed. The math is very complicated and beyond the scope of this discussion.

But to get to what you want to know the Power adds to PT = P1 +P2 +P3 assuming there is no power factor involved with pure resistive load. Again the math is very complicated if we factor in Power factor which means angles again.

So in your questions lets assume the power factor = 1, or a purely resistive load like a resistor and then we can use Ohm's Law. The voltage is assumed to be 208 volts for a 208 delta system. So you have 1 amp in phase A and the power in Phase A = 208 volts x 1 amp = 208 watts. Phase B power = 208 volts x 2 amps = 416 watts. Phase C power = 208 volts x 3 amps = 624 watts Total power consumed = PA + PB + PC = 208 + 416 + 624 = 1248 watts.

So in a 3-phase 208/120 circuit to deliver 1248 watts the maximum current is 3 amps at this degree of unbalance. If balance so all 3 phase current are equal the current in each phase would be 2 amps in each phase. To do the same with a single phase circuit at 240 volts would require 5.2 amps, or 10.4 amps @ 120 volts.

Hopefully now you see why 3-phase and higher voltages are the way to go for higher power demand.

Here is an example: I supply a single phase 240/120 service rated at 200 amps at your house, and next door I deliver the same 200 amp service at the equivalent voltage 0f 208/120. How much power can each service deliver at a given moment in time.

Single phase can deliver 240 volts x 200 amps = 48,000 watts or 48 Kw
3-phase can deliver 208 volts x 200 amps x 3 phases = 124,800 watts or 124.8 Kw

Understand?

[Kengine7]

Europe, huh?

If you have three phase and the line to line voltage is within the inverter specs I would think that the inverter would present as a source parallelled with those two lines. I have parallelled single phase generators to individual legs of a 120/208 wye genset. The application was a 3 phase motor and a 15 amp single phase load. To offset the imbalance and reduce heat from a single phase I paralled a 1500 watt 120-vac generator to the loaded leg. Before you say anything, the small generator could not handle 15 amps continuous and the 3 phase was too expensive to risk the heat.
Based on experimentation consider the inverter on the highest loaded lines.

[Kengine7]

How about 200 x 120 x 3 or 208 x 200 x 1.73?

[Kengine7]

Russ I am going to give you the simple answer. It means you will have neutral current flowing of about 1.5 amps. If all three phases are perfectly balanced, you will not have any neutral current flowing in a 4 wire wye system.

Think of it like this: We have three horses bridled together to a common point pulling in 3 different directions. They are pulling 120 degrees adjacent to each other. Think a pie cut in thirds. If they apply the same force, they go no where. If one or two are pulling harder than one, the center of the circle travels at x degrees at Y speed. The math is very complicated and beyond the scope of this discussion.

But to get to what you want to know the Power adds to PT = P1 +P2 +P3 assuming there is no power factor involved with pure resistive load. Again the math is very complicated if we factor in Power factor which means angles again.

So in your questions lets assume the power factor = 1, or a purely resistive load like a resistor and then we can use Ohm's Law. The voltage is assumed to be 208 volts for a 208 delta system. So you have 1 amp in phase A and the power in Phase A = 208 volts x 1 amp = 208 watts. Phase B power = 208 volts x 2 amps = 416 watts. Phase C power = 208 volts x 3 amps = 624 watts Total power consumed = PA + PB + PC = 208 + 416 + 624 = 1248 watts.

So in a 3-phase 208/120 circuit to deliver 1248 watts the maximum current is 3 amps at this degree of unbalance. If balance so all 3 phase current are equal the current in each phase would be 2 amps in each phase. To do the same with a single phase circuit at 240 volts would require 5.2 amps, or 10.4 amps @ 120 volts.

Hopefully now you see why 3-phase and higher voltages are the way to go for higher power demand.

Here is an example: I supply a single phase 240/120 service rated at 200 amps at your house, and next door I deliver the same 200 amp service at the equivalent voltage 0f 208/120. How much power can each service deliver at a given moment in time.

Single phase can deliver 240 volts x 200 amps = 48,000 watts or 48 Kw
3-phase can deliver 208 volts x 200 amps x 3 phases = 124,800 watts or 124.8 Kw

Understand?

Check your math below.

[Sunking]

Kengine7 my reference is to purely resistive power to keep it simple where PT = P1 + P2 + P3.

If you want to take power factor into th equation the math is a little more complicated and I do not think many here would understand: P = √3(V * I * cos(f)) or P = 3(Vp * Ip * cos(f))

[russ]

@ Sunking - Thanks for the explanation with details and you are right - the math quickly gets quite confusing!

What I was trying to determine is how the unbalanced load would affect my power bill. Would it be any less if balanced. As I understand from your explanation the 3 phases are only cumulative and it would not.

Best Regards,
Russ

[Sunking]

What I was trying to determine is how the unbalanced load would affect my power bill. Would it be any less if balanced. As I understand from your explanation the 3 phases are only cumulative and it would not.

Unbalanced does not affect the bill, but you might have overlooked something.

As I said earlier 3-phase is typically used for industrial and commercial users. With those customers they typically have very large motor and non linear loads which results in low power factor. Here in the USA utilities do not charge residential customers for reactive power, only resistive or the real power because in a home you just do not have large reactive loads that a electric utility is concerned about.

However utilities do charge commercial and industrial users for reactive power. The lower the power factor (more reactive), the more the utility charges. The higher the power factor (less reactive), the less they charge. This is why large industrial users have capacitor banks to correct the power factor, it saves them large amounts of cash, and save the utility a lot of generating capacity. In other words the utilities penalize the users for low power factors. Both benefit when the power factor is closer to 1.

To take it one step further, the power factor correction devices you see marketed to residential customers is fraud because the utilities do not charge residential customers for VAR's.

[russ]

Here as a residential customer we pay per kWh - not for reactive power.

When the electrician connected the loads to the various phases he just went one after the other with no questions about use or thought about it either I am sure.

Thanks again!
Russ

[ionized]

Well, I have 3ph in my 2000 sq ft McMansion in the USA. In my neighborhood, in the 1950s, any AC installation 3-ton or bigger got 3ph. Many people have heard of 3ph

[miket]

It depends on the country, i've heard small 3 phase sytems are popular in france.

[Sunking]

I know this thread is about Europe, but I wanted to clear up some misconceptions about 3ph and McMansions. O.K.?

I understand that, and I did not mean to imply there are no 3-phase systems in residential. It is just rare in the USA, and you will see some in older homes like yours.

From the POCO point of view it is not economical to install 3-phase to a residence for small loads. The reason is simple, it takes 3 transformers for a 3-phase system and an extra conductor (4), versus a single transformer and 3 wires for single phase.

As for the Europeans, they actually have a better electrical architectural than the USA in my professional opinion. For residential they use 240 volt at 50 Hz. Only down side, and it is minor, by operating at 50 Hz requires physically large transformers, but 50 Hz is a little more efficient than 60 Hz. The big advantage is at balanced 240 volts you can double the power for a given size current. That means a lot fewer circuits you have to run from your breaker box. In addition since it is balanced power you do not have the harmonics and electrical noise problems you get with unbalanced 120 volt systems we use in the USA.

[miket]

or use a single 3 phase transformer and those 4 transmission wires use less copper than 3 or 2 wires!

I guess there might be some basis to that argument because in rural Australia they run only a single wire and use the earth as a return for single phase. The have to use isolation transformers and some efficiency loss but the cost is less is less the extra wire.

60hz is more efficient in motors not 50hz, electronics requires smaller transformers and caps etc. 50hz was chosen because it was half of 100. 120v was chosen here because it was good for primitive light bulbs and primitive insulation. Neither system is optimal. A few countries like south korea run 60hz and 240v. To bad thats not going to happen here oh and sockets that are receesed into the wall are safer and stronger than flush mounted. Round pins plugs dont bend as easily as our flat ones.

Yes 50hz is better than 60hz for veery long transmission lines but then HVDC is better for those.

[Sunking]

I guess there might be some basis to that argument because in rural Australia they run only a single wire and use the earth as a return for single phase. The have to use isolation transformers and some efficiency loss but the cost is less is less the extra wire.

That is called a SWER (Single Wire Earth Return) system and only used to my knowledge in Australia and Alaska in extremely remote areas, a long ways from the generation plants with very little population density.

[Sunking]

Yes 50hz is better than 60hz for very long transmission lines but then HVDC is better for those.

Well from a POCO POV transmission efficiency is everything. You can even apply that to a country point of view also. Less loss is better for everyone. The difference in motor winding, transformer, and capacitor sizes is minimal compared the the transmission losses. If size were that important we would use 400 Hz like aircraft where transformers, generators, motors are 1/5 the size and weight of their 50 and 60 Hz cousins.

[miket]

Im not sure how efficiency minded they really are. Does the transformer on your street have an amorphous metal core? Why dont we see SWER in the rural areas of the lower 48?