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A Review and Proposal on Controller Design for a DC/AC Power Converter

Xinxin Wang

Control and Power Group

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Outline

1. Background

2. The two-loop control system review

3. A new discrete voltage controller design and switching procedure design

4. DSP implementation

5. Conclusion

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1. Background

• The distributed generation (DG) is developing rapidly.

• Power converters, such as IGBTs, are used as the interfaces between DGs and local loads.

• H∞ repetitive control theory is used to design a controller for the DC/AC power converter.

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• System Modelling• Two-loop control system• Repetitive control system• Formulation of the H-infinity problem• Calculation of active and reactive power• Active and reactive power controller

2. Two-loop control system review

The work in this section has been done before. Two published papers, ‘H∞ Repetitive Control of DC-AC Converters in Microgrids’, G. Weiss, et. al. (2004) and ’Decoupling control of the active and reactive power for a grid-connected three-phase dc-ac inverter’, J. Liang, et. al. (2003), are related to this section.

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• System Modelling

PWMIGBT bridge

DC source

u

DCV

fu

fR fL

fr

1i fi ci

cV cS

C

outV

di

L

Rr

i3i

local loads

harmonicdistortion

grid

gV

2i

gL

grgR

gi

grid interface inductor

micro-grid

neutral

filterinductor

inverter

gS

back

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Decoupling power controller Plant

Repetitive voltage controller

Calculation of P and Q

PLL

refP

refQ

gPgQ

di gv

refv

u

ci

e

gi

gv

quadv

gVwtwt ),cos(),sin(

• Two-loop control system

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• Repetitive control system

plant

Stablizing compensator

sdesw )(

e

+

ic

P

C

p

u

id

Vg

Vref

winternal model

M

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• Formulation of the H-infinity problem

1v

2vw~

w

u

+

+

z~

y~

P~

ci

e

P

C

WuW

1~z

2~z

1~y

2~y

a b

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• Calculation of active and reactive power

],cos)2[cos()cos(2cos2)()()( wtVIwtIwtVtitvtp

T quad

T

VIdttitvT

Q

VIdttitvT

P

0

0

,sin)()(1

,cos)()(1

We assume that the grid voltage is the reference phasor, with angle zero:

, , gygxggg IjIIVV

, , ggyggx VIQVIP

,)( ggygxggggzggref ZIjIVZIVVVV

From the figure of the system modeling,

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,sincosgg zz

g

g jZ

Zn

Assume that the we know the angle of the equivalent impedance ,gz

, , gg

Qgg

P ZV

QVZ

V

PV

,nVjnVVV QPgref

wtjVVwtVVVvgggg zQzPzQzPgref cos))cos()sin((sin))sin()cos((

,gg

gg

gref ZV

QjZ

V

PVV

Here, Zg is the equivalent impedance of the grid interface inductor and short distance of the transmission line.

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• Active and reactive power controller

PIform

reference voltages

PI

Pref

Qref

QgPg

vref

+-

+-

))((

))((

s

KKQQV

s

KKPPV

IQPQrefQ

IPPPrefp

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• Shortcoming of the previous voltage controller

• Formulation of H∞ problem for the new controller

• Simulation results of the new discrete controller• Switching procedure design and simulation

results

3. The new discrete voltage controller design and switching procedure design

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• Shortcoming of the previous voltage controller

back

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• Formulation of H∞ problem for the new controller

1v

2vw~

w

u

+

+

z~

y~

P~

ci

gi

e

P

C

WuW

gW

1~z

2~z

3~z

1~y

2~y

3~y

a b

The new block is Wg which is of PI type.

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• Simulation results

0 0.5 1 1.5 2 2.5 3 3.5-250

-200

-150

-100

-50

0

50

100

150

200

250

1.85 1.9 1.95 2 2.05 2.1

-3

-2

-1

0

1

2

3

There is no DC component in the tracking error. Compared to the previous tracking error, this new controller has a better performance.

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• Switching procedure design

loads

converter

Sc Sg Vg

In some cases, Sg and Sc should be switched on and off.

This is the simplified circuit.

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• Grid disconnected and connected while the converter is working

• ‘Grid up’ : The breakdown is over. Then set the reference voltage Vref = Vg

1, which is the fundamental component of the grid voltage Vg . Now, Sc is closed and Sg is open.

• ‘Grid connected’ : Set active and reactive power reference Pref = 0, Qref = 0. Connect the grid to the micro-grid. Now both Sc and Sg are closed.

• ‘Delayed grid connected’ : Set Pref and Qref to desired values.

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• Simulation results

control signals

tracking error

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• Converter disconnected and connected while the grid is working

• ‘Converter up’ : Assume the converter is not connected. Now, Sg is closed and Sc is open. Measure the active and the reactive power Pm, Qm.

• ‘Converter connected’ : Set active and reactive power reference Pref = Pm, Qref = Qm. Connect the converter to the micro-grid. Now both Sc and Sg were closed.

• ‘Delayed converter connected’ : Change Pref and Qref to desired values.

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• Simulation results

control signals

tracking error

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• Two DSPs introduction. • The controller structure with two DSPs.• Parallel communication between the two DSPs.• Layout of the printed circuit board in Protel.

4. DSP implementation of the controller

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• Two digital signal processors (DSPs) from TITM, a fixed-point DSP LF2407A and a high speed floating-point DSP C6713, are used.

• The power controller, voltage controller and neutral-point controller are implemented by C6713. The LF2407A is used to implement PLL, monitor the protection and transmit the values of voltages and currents.

• External memory interface (EMIF) of LF2407A and host port interface (HPI) of C6713 are used to do the parallel communication between the two DSPs.

• Introduction of the two DSPs

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• The controller structure with two DSPs.

EVM LF2407A

Power Reference Start/Stop

Control

DSK 6713

HP

I

EM

IF

SCI

A/D

PC

Power Control

Neutral-Point Control

H_inf Repetitive

Control

PLL

IGBT, VDC & Overcurrent

Protection and Internal Monitoring

Switch Logic Control

Digital Output Board

15VPWM

Sc, Sg and Sd

Transducersand

Conditioning Board

VDC

VgABC

VCABC

IgABC

ICABC

f, V, I, P, Q Signals

PC

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• Parallel communication between the two DSPs

GND

TMS320C6713Host Port Interface (HPI)

TMS320LF2407 (host)External Memory Interface

HCNTL[1:0]

HHWIL

HR/W

HD[15:0]

HDS1

HDS2

HCS

HAS

HRDY

HINT

A[3:2]

A[1]

R/W

D[15:0]

RD

WE

STRB

READY

VIS_OE

EDA-144

Vcc

Vcc

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• Layout of the printed circuit board in Protel

Three connectors are used. One is for the connection to HPI, and the other two are for the connection to EMIF.

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• Summary of the work to date• Future work

5. Conclusion

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• Review the voltage and power controller design.

• Design a new discrete voltage controller for the converter system.

• Design the switching procedures of the grid or converter disconnected and connected to the local loads.

• Do the parallel communication between the two DSPs.

• Summary of the work to date

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• Finish DSP implementation of the control system in the experiment.

• Design a power controller using dq transformation.

• Design control system for converters in parallel.

• Future work

Short term:

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Gear

Doubly-fedinduction generator

ACDC

DCAC

Grid

Pitch

Rotor-sideconverter

Grid-sideconverter

Wind

sP

Stator power

• Finish the embedding of the whole system which includes the wind turbine, the DFIG, the back to back converters and the grid.

A wind turbine blade experiences a variety of loads which occur at specific frequencies, leading to the output power variations. Such as:

• Tower shadow;

• Frictions due to the gearboxes and drive train.

Long term:

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P

C

Mw

u

e p

y

. . . . ,1 ,)(1

1)(

,1 and 0 ,11

NnsWe

sM

MM

sn

N

nnn

N

nnn

n

• Generalization of the internal model principle. Such model can reject (track) different disturbances (references) with different fundamental frequencies.

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