Control algorithm of a DC/AC converter

applied in a small wind turbine

Pawel Mlodzikowski, Adam Milczarek, Mariusz Malinowski

Warsaw University of Technology, Institute of Control & Industrial Electronics, Warsaw, Poland

mlodzikp@isep.pw.edu.pl, adam.milczarek@ee.pw.edu.pl, malin@isep.pw.edu.pl

Abstract-Paper in detail presents control algorithm of a DC/AC converter applied in a small wind turbine (SWT), which has grid

connected and a stand-alone operation mode. Common problems encountered SWT’s transient between modes are described. Algorithm was implemented using a digital signal processor (DSP)

control platform and tested using a lab setup with a 2.2 kW permanent magnet synchronous generator (PMSG). That type of generator is popular among SWTs. Results obtained from

experimental verification are included.

I. INTRODUCTION

Wind power plants are playing an important role in

a modern electrical grid. According to IEC 61400-02 [1]

SWT must fulfill requirement of area swept by rotor blades

higher than 2m2, but less than 200m

2. These kind of wind

turbines are a very promising supplement to full scale

commercial applications [10]. Moreover there are simple in

construction and could be more efficient supplying local

loads near SWT. Therefore many governments are

encouraging private sector to broaden the SWT market.

In Poland 95% of SWT is working in stand-alone mode.

Number of installations is growing, and according to many

SWT producers this growth is estimated between 15-30% this

year.

Fig. 1 illustrates a block scheme of a small wind turbine

suited for working in grid connected or stand-alone mode.

The SWT basically consists of:

� A wind turbine and PMSG connected directly with

each other (without a gearbox) [8],

� AC/DC converter which is mostly a six-pulse diode

rectifier with a DC/DC boost converter for

utilization of Maximum Power Point Tracking

(MPPT) [9],

� DC/AC converter for transferring energy to the grid

or local load (converter should be able to operate

with non-symmetrical load), what is discussed in this

paper

Many DC/AC converter topologies are considered in

SWT. Their prices, efficiency, functionality and control

methods of energy conversion are varying between types.

One of most simple, is a three-phase converter with ∆-Y

transformer (Fig. 1), which helps to control power

independently in every phase during stand-alone operation.

Additionally exist plenty of control methods for this kind of

converters which can be applied in low-cost micro-controller.

Fig. 1 Block scheme of a small wind turbine with three-phase DC/AC converter with ∆-Y transformer

A universal control algorithm for two modes of operation

is in detail explained and the transition process between grid

connected and stand-alone mode are analyzed (section II).

Proposed control method applied in DSP platform is verified

by experiment (section III).

II. CONTROL SCHEME FOR DC/AC CONVERTER

APPLIED IN SMALL WIND TURBINES

Control scheme for DC/AC converter for SWT must ensure

proper work in stand-alone and grid connected mode, also it

has to be able to fast switch between them without delays. In

scheme shown in Fig. 3 we can distinguish three control

blocks:

� Direct Power Control – Space Vector Modulated

(DPC-SVM) in grid connected mode of operation,

� Voltage control in stand-alone mode,

� Monitoring.

A. DIRECT POWER CONTROL IN GRID CONNECTED MODE

DPC-SVM method bases on controlling instantaneous

values of active and reactive powers [7],[2]. Fig. 2 (red

dashed line) illustrates a block scheme of DPC-SVM.

978-1-4244-9312-8/11/$26.00 ©2011 IEEE 1006

Given active (pref) set by outer dc-link controller and

reactive (qref=0 for unity power factor) powers are compared

with their estimated values described as:

�� = � ∙ ���� + ����� (1)

�� = � ∙ ����� − ���� (2)

where:

udc - dc link voltage; iLα, iLβ - phase currents in α-β coordinate

system; uLα, uLβ - phase voltages in α-β coordinate system.

Instantaneous active and reactive power errors are input

values of PI controllers. Their outputs generate signals in d-q

coordinate system that represent given voltages for SVM.

B. VOLTAGE CONTROL IN STAND-ALONE MODE

The most important criterion taken into consideration

while choosing a control method in stand-alone mode was

simplicity. A simple algorithm can be easily implemented

using a cost effective microcontroller. Subsystem shown in

Fig. 2 (blue dashed line) contains inner ac voltage control

loops in d-q coordinate system and outer dc voltage control

loop [4],[5]. Outer control loop is in charge for keeping udc

voltage at constant, given level.

Reference values ud_ref and uq_ref compared with uLd and

uLq become input for PI regulators. Signal uq_ref is equal to

zero, therefore vector of converter voltage is aligned with

d-axis and amplitude of AC voltage depend on ud_ref only,

what allows to simplify the control.

Output signals from inner PI regulators after

transformation:

������� = �cos (�) −sin (�)sin (�) cos (�) � ∙ �������� (3)

are given values for Space Vector Modulator (SVM).

Important part of the control algorithm in this mode of

operation is energy dissipation. When local load will be lower

than power generated by turbine this circuit should be

activated, what helps maintain dc voltage at a constant level.

Energy dissipation circuit consists of a transistor and

a resistor (or a water boiler so that dissipated heat can be

utilized) which power rating should be not less than generator

nominal power. Control is very simple because generated

power (pdc) described as:

�� = !� ∙ � (4)

is compared with instantaneous power of the three-phase load

pload, which can be calculated with help of

�"#$� = ��$�$ + ��%�% + �� � (5)

where:

uLa, uLb and uLc are estimated values based on duty cycles of

converter’s control pulses and dc link voltage describe as

��$ = &'() (2+, − +- − +.) (6)

��% = &'() (2+- − +, − +. ) (7)

Fig. 2 Chosen control scheme involving voltage control, monitoring and DPC-SVM

1007

Fig. 4 Phase synchronization process: a) converter not-synchronized,

b) converter synchronized

22

βα

β

LL

L

UU

U

+

22

βα

α

LL

L

UU

U

+

Fig. 3. Implemented PLL’s scheme

�� = &'() (2+. − +, − +-) (8)

After calculations, control signals are introduced to

a proportional controller, with gain given by:

/0 = 123

(9)

where:

PN – nominal generator power.

Output signal from this proportional controller is compare

with carrier signal, which amplitude is equal to PN.

In results, there is calculated duty cycle of gate signal for Td.

C. MONITORING

Monitoring contains two main blocks:

� voltage and frequency verification

� phase locked loop (PLL)

When the grid faults appears, control algorithm is switched

to stand-alone mode of operation as well as PLL is working

in open loop (∆Θ=0) (Fig. 3).

If voltage recover, firstly the algorithm is testing if grid

voltage and frequency fulfils standards PN-EN 50160:2008,

what means that phase voltage can vary ±10% and frequency

±1%. Moreover, monitoring is checking if zero sequence

component of grid voltages described as:

��4 = 15 (��$ + ��% + �� ) (10)

is not higher than ±3% (selected experimentally) by

comparison to phase voltage. Then system make decision to

continue stand-alone mode or switch to grid connected mode

of operation.

If system decides about grid connected mode, then PLL is

applied to phase synchronization of converter and grid

voltages, what provide smooth transition between two modes

of operation (Fig. 3). Idea based on assumption, that

converter can be connected to the grid if ∆Θ described as [5]:

For (6 − �)>π:

∆Θ ≅ 9:(6 − �) = 9: 6 ∙ ;<9 � − ;<9 6 ∙ 9: � (11)

For (6 − �)<π:

∆Θ ≅ sin(� − 6) = ;<9 6 ∙ 9: � − sin 6 ∙ cos � (12)

is equal zero (Fig. 4).

Unfortunately it can happens, when sine argument (6 − �)

is equal “0” (correctly) as well as “π” (incorrectly). Therefore

if (6 − �)=π, small value is added to ∆Θ, to protect against

this undesirable situation.

III. EXPERIMENTAL VERIFICATION

View of laboratory setup is presented in Fig.5 and the particular parts of system are shown in block scheme (Fig. 6).

Tests includes:

� steady state operation of grid connected converter with

DPC-SVM (Fig. 7),

� steady state operation of stand-alone mode with one

and two-phase load (Fig. 8 and 9),

� step change of the load in stand-alone mode (Fig. 10),

� transient from stand-alone to grid connected and from

grid connected to stand-alone mode (Fig. 11 and 12).

Fig. 5 View of a experimental laboratory setup

1008

Fig. 6 Laboratory setup

Fig. 7 Steady state of converter working in grid connected mode

(DPC-SVM algorithm). From the top: phase voltage uLa, phase current iLa, active power p, reactive power q

Fig. 8 Steady state of converter working in stand-alone mode and supplying

single-phase load. From the top: measured phase voltage uLa, phase currents iLa, iLb,, iLc

Fig. 9 Steady state of converter working in stand-alone mode and supplying

two-phase load. From the top: phase voltage uLa, phase currents iLa, iLb, iL

Fig. 10 Step change of the load for converter working in stand-alone mode.

From the top: phase voltage uLa, phase current iLa, voltage in dc link UDC

YPMSGDCM

Danfoss 5.5 kW

3-ph converter

Danfoss 5.5 kW

3-ph converter

used as diode rectifier

P=2.2 kW,

n=1750 rev/min,

T=12 Nm,

U=3x220V,

pp=3

Grid simulator

California Instr.

15003iX

P=15 kW

U=3x400 V

Transformer

3x400/3x400

P=5kW per phase

PC

3x L=5 mH

3xC=3µF

Digital

Aquisition & ControlCard

1103

Master: PowerPC 604e

Slave: DSP TMS320F240

PCI RS-232

LEM

voltage & current

converters

with conditoning circuits

thyristor based

controled rectifier

direct

current

motor

control

Control

I. II. III. IV. V.

VI. VII.

Load

1009

Fig. 11 Transient from stand-alone to grid connected mode. From the top:

signal of switching command, phase voltage uLa, phase current iLa, voltage in

dc link UDC

Fig. 12 Transient from grid connected to stand-alone mode. From the top:

signal of switching command, phase voltage uLa, phase current iLa, voltage in

dc link UDC

CONCLUSIONS

This paper focus on control algorithm of a DC/AC

converter applied in a small wind turbine. Main control

blocks and monitoring are described in detail. Shown

experimental results prove, that converter is working

correctly in grid connected as well as stand-alone mode of

operation. Presented method of monitoring and converter

synchronization to the grid provides a smooth transient

between modes without affecting continuity of power supply

to local loads.

ACKNOWLEDGMENT

Described problems are the parts of the project number N

R01 0015 06/2009 “Complex solution for low speed small

wind turbine with energy storage module for distributed

generation systems” and sponsored by The National Centre

for Research and Development.

REFERENCES

[1] IEC 61400-02 Design requirements for small wind turbines, IEC, (2007)

[2] M. Malinowski, “Sensorless Control Strategies for Three – Phase PWM Rectifiers”, Ph.D. Thesis, Warsaw University of Technology, (2001)

[3] A. Milczarek, “Control of Three-phase PWM Converter for Small Wind Turbine in Stand-Alone and Grid-Connected Mode”, M.Sc. Thesis (in Polish), Warsaw University of Technology, (2010)

[4] M. Fatu., L. Tuteaea, R. Teodorescu, F. Blaabjerg, I. Boldea, ”Motion Sensorless Bidirectional PWM Converter Control with Seamless Switching from Power Grid to Stan Alone and Back”, Power Electronics Specialists Conference, pp. 1239-1244,(2007)

[5] R. Teodorescu, F. Blaabjerg, “Fexible Control of Small Wind Turbines Turbines With Grid Failure Detection Operating in Stand-Alone and Grid-Connected Mode”, IEEE transaction on power electronics, vol. 19, pp. 1323-1332, (2004)

[6] PN-EN 50160:2008, Polish Grid Codes, (2008) [7] M. Malinowski, M. Jasinski, M. P. Kazmierkowski, “Simple Direct

Power Control of Three-Phase PWM Rectifier Using Space Vector Modulation”, EPE-PEMC`02, vol.4, pp. 1114 – 1118, (2002)

[8] Z. Goryca, M. Ziolek, M. Malinowski, „Cogging Torque of the Multipolar Generator with Permanent Magnets”, (in Polish) "Maszyny Elektryczne Zeszyty Problemowe nr 88", pp. 53-56, (2010)

[9] Kot R., “Design, Investigation and Development of the Rectifier and DC/DC Converter with Maximum Power Point Tracking Algorithm for Optimum Operation of PMSG”, M.Sc. Thesis (in Polish), Warsaw University of Technology, (2010)

[10] Spagnuolo G., Petrone G. Araujo S.V. Cecati, C., Friis-Madsen, E., Gubia E., Hissel, D., Jasinski, M., Knapp, W., Liserre, M., Rodriguez, P., Teodorescu, R., Zacharias, P., , "Renewable Energy Operation and Conversion Schemes: A Summary of Discussions During the Seminar on Renewable Energy Systems," Industrial Electronics Magazine, IEEE , vol.4, no.1, pp.38-51, (2010)

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