Wind Power, Current on Each Phase

Beat me to it... again.

10% imbalance would be bad for a fully loaded system, but at 60% capacity it's not too much of a worry. Current imbalance in an AC generator creates extra heat in the windings because of what are called Negative Sequence Currents, but at 60% load you have a lot more head room than you would at 100% or 90% loading. I would check it out at peak loadings and if it's still more than 5% I'd look at taking measures to balance your loading better.

Negative sequence currents are generated more in rotor body and very less in rotor winding. No negative sequence current in generated in stator winding. This current is generally induced in rotor body and/or in damper windings. Generally rotor windings are located in the deeper slots of rotor and due to lesser depth of penetration (being double the frequency), remain on the outer surface, wedges if metallic, damper of the rotor. Each manufacturer provides a curve of negative sequence current withstand capability of the generator. The same should be taken as the guide.

What are the difference between the case of a AC generator and an AC motor?

These aspects needs to be clarified.

In case of generator, the generated or induced EMF will not have any negative sequence voltage.

In case of Motor, supply voltage can have negative sequence voltage due to unbalance in supply voltage.

In case of generator, negative sequence currents are due to, unbalance in the external loads. Consequently, generator terminal voltages can be unbalanced, even though the induced voltages are balanced.

In case of motor, negative sequence currents are due to unbalanced supply voltage.

The protection in both case however uses the negative sequence current measurement from the stator [supply lines] winding sides only.

GA

You beat me too!!

As others have suggested, current differences in the generation supply are due to the consumer load's. You can't produce power if it is not being used.

Your wind farm will have been designed to a max capacity but if the energy is not required it would not be produced. Now I am getting outside my comfort zone, I am not sure that what I will say next is correct, but i think it is!

If you wind farm is only running at about 60% load then it, the energy required via wind to turn the generator would be less than that required to run at 100% capacity, not that this is of concern to you anyway.

Cheers

Joe

summary:

The unbalance is due to loads and not due to generation

Unbalanced load causes negative sequence currents, which has twice the frequency of positive sequence current [in reverse direction] and hence causes overheating, which is much more significant compared to heating caused by positive sequence current of the same magnitude.

Additional points:

The frequency of wind mill is never the system frequency [or rarely the system frequency]. To couple this to grid, one needs AC to DC and DC to AC conversion before connecting to the grid [Just like HVDC systems for Asynchronous interconnection]

Power Generated from Windmills are usually small compared to major generators in the system.

If loading on each phase is unbalanced, you can connect the lesser loaded phase(s) with additional loads such as resistive heaters in an effort to minimize the imbalances. This should not cause much loss of load to grid while keeping problems associated with negative sequencing currents under control. Of course, monitoring of the system and adjustments would have to be taken into consideration and made. This process could be automated as well.

The problem is

1. Wind power is usually, simply connected to grid at nearest point, from where it is evacuated. The loads connected to the grid is not under control of wind generation company.

2. Wind power is usually small compared to grid power and may not be monitored [or is not monitored].

3. AC loads have both resistive and reactive power components which needs to be adjusted and hence complex to control the unbalance [both magnitude and phase shift needs to be adjusted]. Using reactive power compensation is common to control power factor, improve voltages, minimize losses, but, resistive loads usually depend on consumer demand which cannot be controlled.

In many three-phase installations where the power is split to supply single phase loads, the phase that has the heaviest load is usually the one in the distribution board that is on the top left. The reason? That's just the way that electricians work when connecting single phase loads willy-nilly.

It's got nothing to do with the way the generator is configured, solar, diesel, coal, nuclear/steam or whatever.

One note to add to this...The wind farm has a DVAR system in place as well.

The current on the phase is dependent on the load. The voltage you generate provides the 'push' for the power, but the loading provides the path for the current to flow.

In my experience as a user of three phase power, I have some equipment that uses all three phase legs (blower motors, and high voltage DC power supplies), but I also have single phase loads that are both 110V and 220V. Because the 220V line is connected to two of the three incoming legs, those two legs always have more current in them, than the other 'wild' leg.