3 Phase grid connected PV inverter - PSIM Electronic ... · 3 Phase grid connected PV inverter - 3 - Due to the axis decoupling, the conventional control scheme requires an implementation

3 Phase grid connected PV inverter

- 1 - www.powersimtech.com

SmartCtrl Tutorial




SmartCtrl1 is a general-purpose controller design software specifically for power electronics

applications.

This tutorial is intended to guide you, step by step, to design the inner control loop in dq axis

of a three phase grid connected PV inverter from its imported frequency response. In this

tutorial have not been included the outer control loop and the MPPT.

There are a lot of possibilities to design the control loop of the aforementioned system. One of

them is the conventional control using axis decoupling by means of feedforward loops as

shown in the next figure.

1SmartCtrl is copyright ©2009-2012 by Carlos III University of Madrid, GSEP Power Electronics Systems Group, Spain

PI

PI a

b

c

d

q

PWM Modulator

P*

Q*

Id*

Iq*

+

+

-

-

id

iq

a

b

c

d

q

223

2

qd

qd

dvv

vqvpi

223

2

qd

dq

qvv

vqvpi

VrsL

1GV+

-

dd

L

di

VrsL

1GV

+

-

qd qi

L

+REFdi _

ˆ

+

-

REFqi _ˆ

d

scK

dsc iK ˆ

-

dRi

scK

1

GV

L

scK

1

GV

L

+

+q

qRi

scK

qsc iK ˆ

+-



Due to the axis decoupling, the conventional control scheme requires an implementation with

a high computational cost mainly for low cost applications. For these applications (low cost µP,

PIC, DSP or small FPGA) a simplified control strategy is proposed and shown in the next figure.

As it can be seen in this figure, a direct PI control (without feedforward loops for axis

decoupling) has been proposed.

The plant (grey colored area) can be rewritten as it is shown in the next figure. The obtained

transfer functions can be classified as follows:

Mutual transfer functions:

d

q

q

d

d

I

d

I

ˆ

ˆ and

ˆ

ˆ

Self transfer functions

d

d

q

q

d

I

d

I

ˆ

ˆ and ,

ˆ

ˆ

Taking into account the symmetry of the balanced three phase inverter, two identical PI

controllers must be tuned. In order to design this PI compensator, two different possibilities

can be selected; it can be considered as converter plant the mutual transfer function or the

self one.

Due to the higher bandwidth and stable (together enough phase margin and stable operation)

is preferable to use the self transfer function.

The first step will be validate the theoretical self and mutual transfer functions by means of a

PSIM AC analysis of the three phase grid connected inverter.

PI

PI

+

+

-

-

22

·

···5.0

LrsL

rsLVDC

++

dI*

dIdd

22·

··5.0

LrsL

LVDC

*

qI

22

·

···5.0

LrsL

rsLVDC

qd

+-

22·

··5.0

LrsL

LVDC

qI



1. AC Analysis in PSIM

The frequency response of the plant is obtained from the averaged model form of the three

phase inverter in dq axis by means of the PSIM AC analysis tool. In the next figure the

aforementioned converter is shown.

The obtained frequency response is included below:

Before proceeding with the control loop design, the Bode diagram in the next figure seeks to

compare the frequency response obtained with PSIM (green trace) and the aforementioned

theoretical model (red trace).

AC sweepcontrol

AC source



Once the frequency response has been obtained, it must be imported to SmartCtrl.

2. Import transfer function to SmartCtrl

2.1 Open your SmartCtrl Software.

2.2 Import the frequency analysis.

Press on button to select text file imported data (current mode control) or select

the corresponding option within the Data menu.

Next, the .txt file containing the frequency response from the PSIM AC Analysis is loaded.

The corresponding window is shown below:

10 100 1 103

1 104

20

0

20

40

60

dBIDPsim1 Freci

MG Freci

Freci

10 100 1 103

1 104

300

200

100

0

100

FaseIDPsim1 Freci

FG Freci

Freci

(dB

)

Frecuency (Hz)

de

gre

es

Frecuency (Hz)



The loaded transfer function is automatically shown:

Complete the switching frequency data in the left lower corner of the window and click

OK to continue.



3. Control loop design with SmartCtrl

3.1 Select the sensor type:

The current sensor specification window is automatically shown. It is necessary to set the

sensor gain and the bandwidth (pole frequency) of the Hall effect sensor. Then, press OK.



3.2 Select the regulator type:

The PI regulator specification window is automatically shown.

For an equivalent analog implementation of the compensator, it is necessary to set the

modulator gain and the value of the resistance “R11”. In this case the resistance value is

not important because the PI regulator to be implemented in PSIM is given by the values

of the proportional gain and the time constant. Once the desired values have been

introduced press OK.



3.3 Once all the control loop transfer functions have been defined, the crossover

frequency and the phase margin must be selected. Click on the "Set.." button and the

solutions map will be displayed.

Now, select the crossover frequency and the phase margin of the loop just by clicking

within the white zone and click OK to continue. The selected pair (PM and fcross) will

provided stable operation. Optimization process could be carried out in the main

SmartCtrl window.



Once the crossover frequency and the phase margin have been selected, the solution

map will be shown on the right side of the input data window. If, at any time, the two

aforementioned parameters need to be changed, just click on the shown solution map.

(See next figure)

3.4 Confirm the design. Now accept the selected configuration and confirm the

design, the program will automatically show the performance of the system in terms of

frequency response, etc...

The regulator components to be introduced into PSIM are the Kp and Kint values.



4. Closed loop performance

In order to check the closed loop performance of the regulator calculated with SmartCtrl, a

closed loop simulation with PSIM has been carried out. Both the simulation schematic circuit

and the simulation results are included below.

vd*

vq*

abs

vd*

vq*abs

Id*

abs

Iq*

vq*

vd*

V

P_ref

VQ_ref

V

V

Id_ref

V

Iq_ref15k

300m

K

2/3

K

2/3

b

a

c

d

q

o

K

-1

wt

Vmod_a

Vmod_b

Vmod_c

165m

15k

P

5k

Id_fil

vd*

Iq_fil

K

1.5 V p_dq V q_dq

Id_fil

vq*

Iq_fil

K

1.5

vq* vd*

Id*

Iq*

Iq_fil

Id_fil

Power References Generation

Direct PI control

PI8.24078m1.89538m

PI

1.89538m8.24078m

PI components calculated by

SmartCtrl



As it can be seen in the previous simulation results, the system has a good performance

(without the use of feedforward decoupling loops). Therefore, the control loop design with the

proposed method by means PSIM and SmartCtrl is valid to connect a three phase PV inverter

to the grid.

Note than more detailed power stages can also been controlled by means this strategy (PSIM

plus SmarrtCtrl). LCL filters, digital implementation with delay effects and every other parasitic

effect can be included in PSIM before the AC analysis to obtain a plant transfer function as

realistic as it will be required.

0.0

-50.00

-100.00

50.00

100.00

Ia_fil Ib_fil Ic_fil

0.0K

-10.00K

-20.00K

10.00K

20.00K

30.00K

p_dq P_ref

0.0K

-5.00K

5.00K

10.00K

15.00K

20.00K

q_dq Q_ref

0.0 0.10 0.20 0.30 0.40

Time (s)

0.0

-100.00

-200.00

100.00

200.00

Va Ia_fil

Active Power Step

Reactive Power Step

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3 Phase grid connected PV inverter - PSIM Electronic ... · 3 Phase grid connected PV inverter - 3 - Due to the axis decoupling, the conventional control scheme requires an implementation