Grid-Connected MPPT Control for MG Operating under Partially Shaded Condition - Intelligent Computing for Sustainable Energy and Environment - page 52

L

L D

A
C

ac1

to T T

C

1 4

C T

C

L
i
n
e

ac1

PV

2

u

i ac

ac

sin θ

PWM PI

SPWM PLL

*
i ac

I

pv

*
pv

U

U

U pv

dc

MPPT PI

PI

U

*
dc

Fig. 4. The whole control system configuration of MG under grid-connected mode

In figure 4, the first converter is the Boost converter and adopts voltage loop
control, which is used to achieve the MPPT control through adjusting the output
voltage of the PV array. The second converter is connected with the first converter by
a large capacitor C 2 , and the output current of the first converter is the control target
in the back converter.
DC bus line control is also necessary. Although MPPT control can make the PV
array track the maximum power point, the different level of insolation will affect the
power output, and the power change will cause the DC bus line drift. If the PV output
energy increases sharply, the power delivery of DC bus line will be increased if there
is no converter or load to consume the extra energy. On the contrary, if the PV output
energy is decreased, and cannot satisfy AC line voltage peak value, the converter
cannot work, therefore it is necessary to keep the DC bus line balanced.
The second part has two loops, that is the outer voltage loop and inner current loop.
The function of the voltage loop is to keep the DC bus line balanced, it is controlled
by a PI controller by comparing the actual value U dc and the given value U * dc as the
error for driving the PI controller. The output of the outer loop PI controller produces
the AC given current value, which is multiplied by the sine output signal of the PLL
of the AC voltage u ac to produce the given current i * ac . The aim of the inner current
loop is to realize the AC current control. The given current i * ac is compared with the
actual i ac , producing the error and is controlled by the inner loop PI controller. The
output of the inner loop PI controller is compared with the triangle wave for
generating the PWM signal to control T 1 to T 4 , and thus produce the AC current
output, which has the same frequency and phase with the grid side.

4

Simulation Results

Firstly, regarding to the influence of partial shading condition of PV, the paper
studied two conditions: one is the slightly PSC, the other is the condition of heavy