We just finished construction on a new wind farm in Minnesota. We were testing the power management unit (PMU) to verify its correct operation. The PMU measures the total wind farm active and reactive power, and it sends this signal to the wind farm owner and to the utility. It can also regulate the output of each individual wind turbine in case of any power limitation requested from the utility. Yet something was wrong with the PMU. It wasn't measuring the correct power.
The PMU measures the power by receiving inputs from current and voltage transformers (CTs and VTs) located close to the main breaker in the switchyard of the wind farm. The power measured by the PMU must be slightly lower than the sum of the power from all wind turbines, because there are always some losses in the power lines from the turbines to the switchyard.
We noticed that the PMU was measuring active power about 10 percent above the turbines' output, but not always. We also noticed that the reactive power was completely wrong. For example, one day, each wind turbine was generating about 1MW, so 10 turbines should deliver a total output of about 10MW or slightly less. But the PMU showed about 11MW. At the same time, the sum of reactive power as measured at each turbine was about 6MVar, but the PMU would show hardly any MVars. What was going on?
After weeks of checking for faults and not finding anything wrong, I decided to check what was happening to the apparent power. This is the hypotenuse in the right-angled triangle made by active and reactive power -- a simple Pythagorean equation. I was surprised to find that the apparent power was magically almost the same between the PMU and the sum of all turbines.
Next I noticed that the difference between the sharp angles of the two triangles (also known as the power factor angle) made by the active and reactive power in the PMU and from the sum of turbines was exactly 30 degrees. I checked this at different times of day, with different generation values, and it was always 30 degrees. Why? Someone on the pour team mentioned that 30 degrees is the difference between three-phase systems connected in Star (Y) or Delta (Δ) connections. From there, it didn't take us long to realize that the CTs and VTs were connected in Delta, and the PMU was expecting a Star connection.
Pythagoras was probably looking at us and laughing from above. Never underestimate what you learned, even in high school. It will come in handy one day, and you'll be glad you listened.
This entry was submitted by Amir Zohar and edited by Rob Spiegel.
Amir Zohar has more than five years of experience as an electrical engineer, supporting the sales, construction, and operation of wind turbines. He works for REpower USA Corp., a wind turbine manufacturer.
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