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Adaptive Capacitor Switching for Wind Energy Generation

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Srijib Mukherjee's picture
Principal Engineer Mott MacDonald

Dr. Mukherjee is an experienced electrical power engineer and academic teaching professor. He brings over 25 years of experience working with electric utilities, universities and established...

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  • Jun 20, 2019 4:43 am GMT
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This item is part of the Special Issue - 2019-06 - US Wind Power, click here for more

At last month’s American Wind Energy Association’s WINDPOWER Conference, Mott MacDonald has the privilege of presenting a poster on our work in adaptive capacitor switching in wind energy generation. As a follow up, we also wanted to share our findings and insights with the Energy Central community.

Why are capacitors needed in wind farms?

Wind farms are typically required to be able to operate within a power factor of +/- 0.95. In order to achieve this range of operation, switched capacitor banks are used to supply bulk reactive power to the system when the generators approach their reactive power limits.

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One traditional approach to a capacitor control scheme would find fixed open and closed thresholds for the capacitors, an approach that does not adapt to changes in the wind farm. On the other hand, an adaptive capacitor control scheme can be carefully chosen to optimize the use of the switched capacitors for all operating conditions as the power output and number of online generators changes.

Example wind farm layout

For example, say you have 50 wind turbine generators with a capacity of 2.5 MW each. This system could have a 34.5 kV collector system with one main 138/34.5 kV step up transformer that uses a load tap changer (LTC) to regulate the secondary (34.5 kV) bus with a bandcenter of 100% nominal voltage and a bandwidth of +/- 0.5%.

This example wind farm could come with 5 miles of 795 kcmil ACSR transmission line that connects the collection station to the point of interconnection (POIC). As a result, the reactive power compensation is located at the collection station and includes, for example, two 14 MVAR switched capacitor banks, which are connected to the 34.5 kV bus. The generators are then operating in a voltage control mode.

What can adaptive capacitor switching do for this example wind farm?

To develop an algorithm that adapts to changes in the operating state of the wind farm, a few simple equations will need to be developed to predict the behavior of several key components of the system:

Wind Turbine Generator Capability

To model the wind turbines at various operating conditions the reactive power capability limits of the generators must be modeled.  The reactive power capability of a generator is typically a function of the real power output from the generator and the voltage at the generator.  The manufacturer should be able to provide the capability curves for a generator, which look like the following:

Then you plot the reactive power capability of the generators at 0 kW output in comparison to the voltage at the generator to get the following:

These analyses allow us to determine what the reactive power capability limit of the generator is at any normal operating point.

Generator VAR Export Voltage Sensitivity

To develop an adaptive control algorithm for the capacitors a function that can predict their operating voltage is needed.  When a generator is operating in a voltage control mode the reactive power output of the generator becomes a function of the difference in voltage between the regulated bus at the collector station and the generator voltage set point.  When the voltage of the generator is set to operate at a point above that of the collector bus, the generator will need to export reactive power to increase the voltage at its bus.  If the wind farm control system wants to increase the net reactive power export of the wind farm, the voltage set points of the generators is increased. Using load flow software, the sensitivity of the reactive power export of the generators to changes in the voltage set point of the generators can be determined, with the following figure showing the strong linear relationship between the voltage difference at the generator and the reactive power export of each generator:

Generator Voltage Set Point Estimation

Using the voltage sensitivity model developed in the previous section above, the voltage set point of the generators can be estimated.  The following figures show how the generator voltage set point will need to be adjusted as the generators real power output is increased with a power factor of ± 0.95 at the POI.  The non-linearity of the curve is due to operations of the LTC:

Reactive Power Export Analysis

Combining the generator reactive power capability function and the generator voltage set point function gives the following graphical prediction of the reactive power operating limit of the wind farm:

Generator Cut-Out Analysis

The reactive power capability equation can be updated to include the number of generators online by repeating the load flow tests used to develop the voltage sensitivity function with various numbers of generators disconnected and recording how the function constants change:

Adaptive Capacitor Switching Logic

By fitting a linear function to the 70% capability limit line and performing a sensitivity analysis based on the number of online generators, the following equations adapt to operating conditions of the system:

Capacitor Operation with Adaptive Switching        

The dashed blue line below shows how the reactive power demand from the generators will decrease when the capacitors turn on during a ramp up in real power event, and capacitors are closing:

The dashed green line shows how the reactive power demand from the generators will decrease when the capacitors turn on during a ramp down in real power event, and capacitors are opening:

Conclusions

The purpose of this capacitor switching study was to develop an algorithm that adapts to the changes in a wind farm to ensure the generators can always meet their reactive power requirements.

The following capacitor control scheme is recommended for this example wind farm. A time delay on operations of the capacitor should be used to allow the generators reactive power output to ride though operations of the LTC and previous operations of the capacitor.  The duration of this time delay can be determined from the wind farm control system parameters.  Each 14 MVAR capacitor has its own circuit switcher.  Assign the capacitor circuit switchers as Sequence 1 and Sequence 2.  Whenever the Sequence 1 capacitor opens, switch the sequence assignment so that the other capacitor now becomes the Sequence 1 capacitor for the next operation.  This will cause each capacitor to wear uniformly.  The Sequence 1 capacitor will be the first capacitor to close and the last to open.  The Sequence 2 capacitor will be the last capacitor to close and the first to open.  The Sequence 2 capacitor will be disabled until the Sequence 1 capacitor is closed.  The Sequence 1 capacitor will not open until the Sequence 2 capacitor is open.  The wind farm control system should continually calculate and update the capacitor close and open threshold.

The adaptive switching algorithm developed in this study optimizes the use of the switched capacitors for all operating conditions and prevents repetitive switching as the generators react to the capacitors reactive power generation.

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