RET_2015-01-02-03-04_Flipbook

Grid voltage control with wind turbine inverters by using grid impedance estimation – Jonas De Kooning
with a signal (active) or only observe the grid (passive), and if they measure transient
effects or steady-state phenomena.
The method which is the easiest in practical application and does not require additional
measurements or components, is the (online active steady-state) non-characteristic
current injection, [2]. In this method, a harmonic current is injected into the grid by the
converter, a discrete Fourier transform (DFT) of both voltage and current is calculated
and the estimated grid impedance results.
4 SIMULATIONS
Simulations are performed where the grid voltage control as presented in this article is
activated. The droop parameters kp and kq are varied to demonstrate the effect of the
control on the grid voltage. Three different settings for the droop parameters
demonstrate an important feature of the grid voltage control. The control can be tuned
towards more effective grid voltage control (high kp and kq), less sacrifice of active
power (lower kp) or less reactive power injection (lower kq). The setting of these
parameters can depend on the preference of the grid operator.
Fig. 1: Wind turbine system
The droop parameters of both the active and reactive power controller are tuned to
high values. This tuning of parameters results in a fierce control of the grid's voltage
where both active and reactive power have influence. The grid voltage for this tuning is
depicted on Fig. 2 by the black squares. It is shown that the overvoltage decreases for
more resistive grids. This is due to the fact that the controllers are proportional (droop)
controllers: as the overvoltage is larger, the effect of the control is larger as well. Also, a
higher grid impedance ratio R/X imposes more active power control.
The droop parameters are tuned to achieve a less fierce active power control as
depicted on Fig. 2 by the gray dots. For resistive grids, it is clear that the voltage is
controlled less. In comparison to the previous tuning, the remaining overvoltage is
higher in this region. However, less renewable energy is sacrificed since the active
power output is closer to its maximum. This is the trade-off between a low and a higher
kp-value. While the first sacrifices less renewable energy, the latter improves the power
quality more.
Revue E Tijdschrift – 131 ste jaargang/131 e année – n° 1-2-3-4-2015 (publication mars/publicatie maart 2017) 5