Wind power technology - Wind Power Integration: Connection and System Operational Aspects - page 84

P S gen

P S grid

P C

Generator
output
power

+ Power

transmitted
to the grid

V DC

C

-

Figure 3.28 Power flow in the DC link

A widely used converter control method is also the load angle control techni-
que, where the grid-side converter is the sending source ( V S ffd ), and the grid is the
receiving source ( V R ff 0). As the grid voltage is known it is selected as the reference,
hence the phase angle d is positive. The inductor coupling these two sources is the
reactance X grid .
For simulation purposes the reference value for the active power, P ref

S grid , that
needs to be transmitted to the grid can be determined by examining the DC-link
dynamics with the aid of Figure 3.28. This figure illustrates the power balance at
the DC link:

P C ¼ P S gen P S grid ð 3 : 37 Þ

P C is the power across the DC-link capacitor, C , P S gen is the active power output of
the generator (and transmitted to the DC link), and P S grid

is the active power

transmitted from the DC link to the grid.
The power flow across the capacitor is given as

P C ¼ V DC I DC

C dV DC
dt

ð 3 : 38 Þ

¼ V DC

From (3.38) the DC-link voltage V DC is determined as follows:

C dV DC

C

2 2 V DC dV DC

P C ¼ V DC

dt ¼ dt

C
2

dV DC
dt

ð 3 : 39 Þ

¼

Rearranging (3.39) and integrating both sides of the equation

ð P C dt

2
C

V DC ¼

ð 3 : 40 Þ