A: Estimating the slope is an art. The positive-sequence slope for most solar and type-4 wind plants is between 1.0 and 3.0. The best way to estimate the slope is to look at the progression of the power-factor angle, as follows. If at a certain per-unit voltage, V1, the power factor angle approaches -90 degrees and stops changing appreciably for voltages lower than V1, then:
•The slope is 3 if V1 is about 0.7pu
•The slope is 2 if V1 is about 0.5pu
•The slope is 1 if v1 is about 0.1pu
As for the negative-sequence slope, the value recommended by the German grid code is 2. EPRI research suggested that the negative-sequence slope should be set no higher than the positive-sequence slope.
You can also estimate the negative-sequence slope based on this formula:
Magnitude of I2 injection in pu = negative-sequence slope * V2
In this formula I2 and V2 are the negative-sequence current and voltage in pu. In the test results, look for a voltage V2 where the magnitude of I2 is approximately 1 pu. The negative-sequence slope is 2 if V2 is about 0.5pu.
Note: If the negative-sequence voltage is not shown in the tabula data, then we suggest you enter a negative-sequence slope of 2 because 2 is the value recommended by the German grid code.
The negative-sequence current phasor from the converter should be 90 degrees ahead of the negative-sequence voltage phasor. This angle is designed to imitate the negative-sequence voltage/current relationship in the output of a conventional synchronous generator.
Note: We provided a different recipe for calculation the slopes in previous versions of this manual. That method was based on the assumption that the dynamic reactive-power injection follows the “classical formula” strictly. We discovered from experience that the method does not work well in practice.