tutorial april 2016- - psim software · 3 phase grid connected pv inverter - 3 - 3 phase grid...

3 Phase grid connected PV inverter

Tutorial –April 2016-

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3 Phase grid connected PV inverter Tutorial V2.0 – April 2016 www.powersmartcontrol.com

Table of Contents

1. Introduction ............................................................................................. 3

2. AC Analysis in PSIM .................................................................................. 5

3. Import transfer function to SmartCtrl ................................................... 6

4. Control loop design with SmartCtrl ........................................................ 8

4.1. Select the sensor type: ............................................................................... 8

4.2. Select the regulator type: .......................................................................... 9

5. Closed loop performance ...................................................................... 13


- 3 -


1. Introduction

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.

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

+-


- 4 -


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.

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


- 5 -


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.

2. 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:

AC sweepcontrol

AC source


- 6 -


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).

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

3. Import transfer function to SmartCtrl

1.1 Open your SmartCtrl Software.

1.2 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.

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)


- 7 -


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

loaded. The corresponding window is shown below:

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.


- 8 -


4. Control loop design with SmartCtrl

4.1. 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.

Switching frequency


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4.2. 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.

Sensor gain

Pole frequency


- 10 -


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.

Modulator gain

R11 value


- 11 -


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.


- 12 -


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)

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.

Regulator components


- 13 -


5. 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


- 14 -


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 SmartCtrl). 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

tutorial april 2016- - psim software · 3 phase grid connected pv inverter - 3 - 3 phase grid...

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