Grid-Connected MPPT Control for MG Operating under Partially Shaded Condition - Intelligent Computing for Sustainable Energy and Environment - page 51

When a PV array is series-parallel structure, a diode will be added to each series
branch. If all the branches are in the shadow, then the expression of the output voltage
is as follows:

N

P

(7)



I = I

scA scA x

x = 1

where I scA x is the short current of the x th branch.
Therefore, the whole output current of a PV array is given as [24]:

N N

   

qU ( + RI − U )

 

P P

(8)



A x ocA x

I = I = I 1exp −

AS

 

 

A A x scA x

AKT N ( − N )

   

 

x = 1 x = 1

S D

where I A x is the output current of the x th branch.
Equations (6), (7) and (8) produce the I - V and P - V curves shown in Fig. 3, where
non-dotted segments are the curves under PSC.

3.3 MG Unit Control under PSC

To control the MG configured in Fig. 1, we first aim to achieve the PV MPPT control
and improve the efficiency of the PV array power conversion under partial shading
conditions. Then, we use DC/DC converter to make the output current and the grid
voltage following the same phase and frequency to realize the unit power factor
control.
An important target in MPPT control is to track the global maximum power point
under partial shading condition. One of the key issues is to identify the partial shading
conditions, and in this paper, we adopt the following partial shading judgment criteria
which was originally proposed in [11]

() ( )

 Δ= − −<Δ UUnUn 1 U

pv pv pv set



(9)

() ( )
( )

( )

Δ I I n − I n − 1 I n − 1



pv pv pv pv

= <Δ ≈− I



( )

set

In − 1 In − 1 N



pv pv p

Δ Δ where U pv is the voltage change of PV array, U set is a preset voltage variation

Δ Δ

limit of PV array, I pv is the current change of PV array and I set is a preset current

voltage variation limit of PV array.
The overall configuration of the control system for MG under PSC is shown in
figure 4, and it has two separate parts that can be controlled individually. The first
part incorporates the global maximal power point tracking into the inverter to connect
DC power from the PV array to the MG. The back part is the inverter for controlling
the integration of MG to the AC line. In this configuration, the key element is the
MPPT control of the PV system with a PSC judging mechanism, which is described
as follows.