Wind power technology - Wind Power Integration: Connection and System Operational Aspects - page 85

giving

s

2
C

ð P C dt

V DC ¼ ð 3 : 41 Þ

By substituting P C in (3.37) in (3.41), the DC-link voltage V DC can be expressed in
terms of the generator output power, P S gen , and the power transmitted to the grid,
P S grid ,as

s

2
C

ð

V DC ¼ ð P S gen P S grid Þ dt ð 3 : 42 Þ

Equation (3.42) calculates the actual value of V DC . The reference value of the
active power, P ref

S grid , to be transmitted to the grid is calculated by comparing the
actual DC-link voltage, V DC , with the desired DC-link voltage reference, V DC ref .
The error between these two signals is processed by a PI controller, whose output
provides the reference active power P ref

S grid , as shown in Figure 3.29. It should be
noted that in a physical implementation the actual value of the DC-link voltage,
V DC , is obtained from measurements via a transducer.
To implement the load angle controller the reference value of the reactive
power, Q ref

S grid , may be set to zero for unity power factor operation. Hence, the
magnitude, V S , and angle, d , required at the terminals of the grid-side converter are
calculated using (3.29) and (3.30) as

P ref

S grid X grid
V S V R

d ¼ ð 3 : 43 Þ

Q ref

S grid X grid
V S

S grid ¼ 0

ð 3 : 44 Þ

Q ref V S ¼ V R þ ;

Figure 3.30 illustrates the implementation of the load angle control scheme for the
grid-side converter with unity power factor. If the reactive power exchange with the
network is not zero it should be noted that (3.44) is transcendental.

Integrator

ref
S grid

V DC

P

P S gen

PI
controller

2
C

∫ -

+

- +

V DC ref

P S grid

Calculation of active power reference, P ref
S grid

Figure 3.29 (suitable for simulation

purposes)