Digital Control Strategy for Harmonic Compensation of Dynamic Voltage Restorer - Intelligent Computing for Sustainable Energy and Environment - page 12

The low order harmonic current of the load current i o of the inverter unit of
the H Bridge is a disturbance in the continuous time model. It will distort the
output voltage u o when it passes through 1 /Cs . The action of current inner loop
harmonic PR controller on the inverter unit inhibits the influence of low order
harmonic current on the system. That ensures the DVR to track and compensate
the voltage dips and harmonic voltage quickly and accurately.

T

i

*

i

i i

u 1

z

T

1
+

1
Cs

u

K K

o

p ph

Ls r

Rz
1

() Rz

3

()

R () z

5

Rz
7

()

T

Fig. 1. Dual-loop digital resonant controller

3 Discretization of Proportion Resonant Controller

According to the principle of reference [11-13], the transfer function of PR con-
troller can be shown as formula (1).

K r s
s 2 + ω n

PR ( s )= K p + (1)

Where: K p is the radio coecient of the proportional control link, K r the reso-
nant coecient, and ω n the resonant angle frequency.
That is, the discussion about the discrete method of the PR controllers is
converted into that about the discrete method of resonant controllers, shown as
formula (2).

K r s
s 2 + ω n

R ( s )= (2)

However, in the resonant frequency formula (2) will introduce the 90 ◦ lagging
phase angle, and an compensation for lagging phase angle is necessary in order
to increase the stability of phase margin [11-13]. Resonant coecient K r can
be neglected during the discussion of the discrete method of PR controller. So
formula (2) can be rewritten as:
R ( s )= s cos θ n −

ω n sin θ n
s 2 + ω n

(3)

Where: θ n in above formula refers to the compensation angle of phase angle lag.
Formula (3) can be achieved by the virtual LC methods in reference [13], its
output equation can be expressed as:

R o ( s )=[ I L ( s )cos θ n − U C ( s )sin θ n ] /ω n (4)