RET_2015-01-02-03-04_Flipbook

Grid voltage control with wind turbine inverters by using grid impedance estimation – Jonas De Kooning
method is confirmed to have desirable results by using both a simulation model and a
test set-up.
1 WIND TURBINE SYSTEM
A small wind turbine system consists of: a generator, machine-side converter, DC-bus,
grid-side converter and filter, as depicted in Fig. 1. The machine-side converter is
generally a passive rectifier with boost chopper which performs Maximum Power Point
Tracking (MPPT) . The DC-power output of this inverter depends on the wind speed and
the resulting MPP. The grid-connected inverter controls the DC-bus voltage by
controlling its current output into the grid. This control balances the DC-power being
injected into the bus and the AC-power being drawn out of the bus. The active power
output of the wind turbine is thus not determined by the grid-connected inverter but by
the machine-side converter.
Fig. 1: Wind turbine system
The grid voltage control explained in this article is performed by the grid-connected
inverter. This converter controls active and reactive power and estimates the grid
impedance.
2 GRID VOLTAGE CONTROL
A The grid voltage is controlled by active power in very resistive grids and by reactive
power in very inductive grids. The opposite is true for the grid's frequency. In resistive
grids, an active power output difference of a DG-unit results in a proportional change in
the voltage difference. In inductive grids, the effect of active and reactive power output
is the opposite of those in resistive grids.
The frequency of the grid is not easily controlled by a DG-unit. Unlike the grid voltage
which is affected locally, the frequency is a global parameter and is primarily controlled
by large power plants. In this article, the wind turbine is a grid-following unit and thus
frequency droop control is omitted.
The droop parameters determine the fierceness of active and reactive power on a grid
voltage difference to its nominal. The setting of these parameters have an important
effect on the grid voltage control and allows for great flexibility. A high kp and kq results
Revue E Tijdschrift – 131 ste jaargang/131 e année – n° 1-2-3-4-2015 (publication mars/publicatie maart 2017) 3