Multipurpose controlled three-phase inverter formotor driving and grid power injection applications

Juan Morıs, Angel Navarro, Jose Marıa Cuartas, Bassam MohammedPolitechnical School of Engineering of Gijon

Department of Electrical, Electronic, Computers and Systems Engineering, University of OviedoCampus de Viesques s/n, Ed. Departamental Oeste, Gijon, Spain 33204

Emails: juanmorisg@gmail.com, angelnr23@gmail.com, jcuartasalonso@gmail.com, EngBassam@gmail.com

Abstract—This paper reports the development of a controlsystem for a three-phase power stage for an induction motor aswell as for an RLC load, able to synchronize with a virtual grid.Vector current control strategy has been applied in both cases,although Volts-Hertz control method is also available in the caseof driving the motor. The project includes the implementation ofa control software in a DSP board as well as the development ofa PCB that interfaces the inverter with such a platform. Severalexperimental results for both applications will be presented.

Index Terms—Current control, grid power injection, gridsynchronization, induction machine, phase lock loop, vectorcontrol, three-phase inverter, volts-hertz control.

I. INTRODUCTION

LOW-POWER low-voltage three-phase inverters arewidely use in distribution applications, either to drive

motors or to control generic loads. However, in many appli-cations this loads take part of a larger system, with lots ofthese kind of loads. Moreover, with the growth of distributedgeneration and smart grids, there is a need of perform aproper management of both active and reactive power in suchsub-systems. In this context, a multi-purpose controlled three-phase inverter is presented, able to deal with most of the low-power applications, and providing friendly and easy to handleinterface and control system.

II. EQUIPMENT AND REQUIREMENTS OF THE PROJECT

The system elements are listed below:• 1 Power stack consisting on an AC/AC converter formed

by a rectifier and an IGBT inverter.• 1 Three phase voltage sensor.• 1 LC low pass filter.• 1 Induction machine.• 1 Interface PCB created in the project scope.• 1 C2000 experimenter kit and a F28335 digital controller.• DC power sources of 12, 5, ±15V.The requirements of the project include:• A PCB interface (design and construction) to adapt the

signal levels from TI experimental kit to the power stackand also handle the measurement signals. Furthermore,the PCB must handle autonomously the charge of theDC-link and drive the relay in the power stack to connectto the grid.

• The development of a real-time program to implementthe control routine in the DSP platform, able to controlthe speed of the motor based on a reference given by apotentiometer.

III. DESCRIPTION AND PROPERTIES OF THE SYSTEMUNDER DEVELOPMENT

Fig. 1 shows the overall system under development. Asexplained before, it consist on an IGBT 3-phase inverter whichcan drive an induction motor or inject active and reactivepower to a grid, controlled through a digital system, whichin this case is a set composed by the F28335 chip and the TIC2000 experimenter kit. As shown in the diagram, the systemis divided in four main parts.

Tri

Grid

Relay

ScaleOffsetFilter

ScaleOffsetFilter

Scale

PWMGIOADC

D a t a B u s

SCI CPU RAM Flash

VoltageSensor

CurrentSensor

GatesDriver

Buffer

SetPoint

PC

Vdc

Reset Fault

En

ScaleFilter

BADC

A

Motor

LC

FilterLoad

Fig. 1. Diagram and setup of the system under development. Three mainelements compose the system, the comercial AC/AC converter (Top), theinterface board (Middle) and the digital controller system (Bottom).

978-1-4799-2911-5/13/$31.00 c©2013 IEEE

1) Power stage: It is constituted by the commercial threephase converter MTL-CBI0010N12IXFA manufactured byGuasch R©.

The converter consist on an AC/AC converter composed bya diode rectifier, a DC-Link, whose charge can be controlledwith a relay and an IGBT inverter, and provides the followingfeatures used in the project:

• 3 output current sensors valid for feedback.• A DC link voltage sensor.• 6 fault signals to indicate a failure in the IGBTs.• Input reset signal.• Input relay signal.• Driver system which adapts the signals of the IGBT gates

from an input PWM signal.The converter can be fed with both three phase voltage

or DC voltage applied directly to the DC-Link. In the finalapproach, the normal operation will be using three phase input.

2) Virtual grid: Although in a real system, the injectionof current would be done while the output of the filter isconnected to a fixed 3 voltage grid, using the component idof the synchronous current vector for modifying the DC-Linkvoltage and so, the active power injection, and iq componentfor modifying just the power factor, in the system underdevelopment, the output will not be connected to any gridbecause of safety and security issues. Therefore, in order toemulate a synchronization with the grid, a virtual grid willbe considered, and the reference angle for the synchronousreference frame transformation will be obtained from thereadings of 2 voltage sensors connected to the rectifier’sinput of the power stage. Thus, the output current will besynchronized with the 3-phase input voltage.

3) Interface PCB: Since the signal levels of the voltageused for measuring the virtual grid voltage, and the currentand DC-Link sensors embedded in the power converter are notin the same range as the ADC of the F28335 board, a signalconditioning analog circuit should be design to adapt suchsignals. Moreover, a filter should be applied to the measuredsignals in order to avoid aliasing in the AD conversion.Also, PWM signals and other digital signals generated by theF28335 should be adapted to interact with the inverter driver.Thus, an interface PCB has been developed and included inthe system with the following elements:

• 3 signal conditioning which scale, introduce an offset andfilter the voltage sensors signals.

• 3 signal conditioning which scale, introduce an offset andfilter the current sensors signals.

• 1 signal conditioning which scale and filter the DC-Linkvoltage sensor signal.

• 1 integrated circuit 8 outputs buffer which adapts the6 PWM digital signals, 1 RESET signal and 1 RELAYsignal coming from the F28335.

• 1 wired-or which allows to manage the 6 FAULT digitalsignals of the power converter through the TripZone 6 ofthe F28335 ePWM module.

• 1 electronic circuit that allows to switch off the PWM,RELAY and RESET signals using a manual switch.

4) Digital Control board: The control system is imple-mented using the controller F28335 and the C2000 experi-menter kit from Texas Instruments. Such a control card willimplement a vector current control through the interaction withthe interface PCB and the user using the modules ePWM,ADC(Analog to Digital converter) A and B, GPIO(GeneralPurpose Input Output digital signals) and SCI (Serial Com-munication Interface).

The control software allows different modes of operationand enables the user to interact with the system in real timeusing the peripheral elements of the experimenter kit boardand the serial communication.

5) The load: Induction machine or electrical grid: Twokind of loads will be supported by the system dependingon the mode of operation. As the first mode consists on thesystem operation as a motor drive the load will be an InductionSquirrel cage Motor. On the other hand, in grid injection mode,the load will be a grid which in this case is emulated by anRLC filter due to safety and security issues.

IV. MOTOR DRIVE MODE: VOLTS-HERTZ CONTROL

First of all, let us briefly describe the fundamentals of theVolts-Hertz control strategy. As mentioned above, the voltageapplied to the motor will be proportional to speed, in order tomaintain the flux constant in the machine.

Neglecting the voltage drop across the stator resistance, thestator voltage in steady state is:

Vs ≈ jωλs (1)

where ω is the frequency of the stator voltage and Vs and λsare the phasors of stator voltage and stator flux, respectively.Thus, considering only the magnitude we get:

λs ≈Vsω

=1

2π

Vsf

(2)

It can be easily inferred from above that whenever the ratioVsf

remains constant, the stator flux λs then remains constant

as well.

A. Control routine

As control structure consist of V-Hz, the amplitude and thefrequency of the sinusoidal signals have to change propor-tionally with the frequency commanded. For that purpose theADC reads from a potentiometer in the DSP board, scalingthe measurement into two variables, amplitude and frequency,which will be used to build a sinusoidal voltage.

The amplitude limits will be set to the maximum voltageavailable in the DC-link, by reading the VDC measurement,whereas the frequency range is calculated from the voltage,multiplying by the Volts/Hertz rate, thus keeping the flux atthe rated value.

The motor will be able to rotate in both directions, depend-ing on the position of the knob. Thus, the sequence of thesinusoidal voltage will depend on position of the knob, beingzero in the middle point.

Motor

Directsequence

Indirectsequence

sinVrms*

f

CURRENTCONTROL

V/HZCONTROL

Fig. 2. Control scheme of the motor drive mode.

Once the amplitude and frequency are read, the threesinusoidal reference voltages are built, taking into accountthat they must be 120 phase shifted. Afterwards, all thereference voltages are moved to per unit values, consideringthe maximum voltage available. Finally, the duty cycles areworked out and sent to the PCB interface.

Figure 2 shows the control scheme under motor drive mode.

V. GRID INJECTION MODE: VECTOR CONTROL TOPOLOGYUSED IN THE SYSTEM IMPLEMENTATION

The control topology used in the project is a current vectorcontrol in synchronous reference frame which allows thecontrol of id and iq independently, and so, the magnitud andpower factor. Fig. 3 shows the followed control scheme andits relation with each element of the power stage.

Three main parts can be differentiated in the scheme, thePark transformation, the angle synchronization and the dqcurrent regulation using PIs. The control consist on obtainingthe dq currents from Park’s transform using grid angle, appliedto a PI whose output is the voltage vector reference. Suchvoltage is transformed in an inverse Park’s transformation andthe resulting 3-phase voltage references are applied to the

Fig. 3. Control scheme of the current vector control for the RLC load mode.

inverter through a PWM.

A. Park’s synchronous reference frame transformation

If a 3-phase balanced source defined by sinusoidal signals(cosine) separated 120 between phases is supposed, the dqcomponents can be obtained as shown in equations (3) and(4), [1] [2].

fq =2

3[facos(θe) + fbcos(θe −

2π

3) + fccos(θe +

2π

3)] (3)

fd =2

3[fasin(θe) + fbsin(θe −

2π

3) + fcsin(θe +

2π

3)] (4)

Where f could be voltage or currents, θ is the angle oftransformation, which in this case is the grid angle. Thistransformation allows the synchronization with the grid anda proper situation for the regulators as 2 DC variables insteadof 3 AC ones are treated.

B. Grid synchronization: Phase Lock Loop (PLL)

The angle tracking allows to extract the 3-phase grid anglein order to generate grid synchronized signals through thePark’s transform. In order to perform the angle tracking, aPLL has been included in the control system in order to reachan accurate grid synchronization. Fig. 4 shows the followeddiagram of PLL for its implementation in the digital controller.In the final implementation a feed-forward is included, toimprove the dynamic response, by adding the rated frequencyof the grid in rad/s to the output of the PI [3].

Va

Vb

Vc

Vd

Vq

Vo

PIController

ParkTransform

Fig. 4. Implementation of Phase Lock Loop technic in synchronous referenceframe.

The PI has been tuned according to the widely used Wienermethod defined by (5) where ζ = 0.7071 and ωn = 2 ∗ π ∗BW (Hz) [3].

Kp = 2 · ζ · ωn, T i =2 · ζωn

(5)

The parameters of PI are:• BW = 15 Hz

• ζ = 0.7071

• Kp = 133.28

• Ti = 0.015

C. Tuning the PI regulatorsFor the design of PI regulator values, the zero-pole can-

cellation method has been used. The electric system to becompensated is defined by:

TF =1

Lf · s+Rt(6)

Where Lf is the filter inductance while Rt is the additionof filter resistance Rf and a resistive load Rz .

As the load is balanced, the two PIs will present the sameparameters. Such parameters are calculated as (7)

Kp = 2 · π · L ·BW ; Ti =L

R(7)

The bandwidth BW has been set to 200Hz in the designsystem.

D. PI digital implementationAs the control will be implemented digitally in the F28335,

the Z transform has to be applied to the PI transfer functionobtaining the expression 8 which defines the digital equationof the PI, where E is the error and U the control action.

U [k] = E[k] · b0 + E[k − 1] · b1 + U [k − 1] · a1 (8)

The parameter a1 is 1 while b0 and b1 are defined belowas:

b0 = Kp · 1

Fs · 2 · Ti+ 1 (9)

b1 = Kp · 1

Fs · 2 · Ti− 1 (10)

VI. HARDWARE DESIGN

Considering the ranges of both, sensors and AD converter ofthe digital controller, various signal conditioning circuits weredesign to be included in the interface PCB as well as othercircuitry needed in such board. In this section, the design ofthe different elements is explained.

A. Sensor signal conditioning

A total of 7 signal conditioning circuit should be imple-mented, 3 for the current sensors, 3 for the voltage sensorsand 1 for the DC-Link sensor. Therefore, 3 different circuitsshould be design as just a repeated model is necessary for the3 current sensors and also for the 3 for the voltage sensors.

The conditioning circuits should scale and introduce anoffset if necessary, in order to adapt the range of each sensor tothe 0-3V range of the analog to digital converter of the digitalcontroller. For that purpose, 3 operational amplifier circuitsbased on inverting scaling summer topology have been used.

Table I summarize the characteristics and features of eachsignal conditioning.

TABLE ISIGNAL CONDITIONING FEATURES

Element Description SensorRange

Current Scaling summer Input:±10Acircuit Inputs: 3V ref and sensor output. Output:±7.5V

DC-Link Implemented with an inverting amplifier Input:0-750Vcircuit Inputs: sensor output. Output:0-7.5V

Voltage Implemented with a scaling summer Input:±600Vcircuit Inputs: 3V ref and sensor output. Output:±22.72mA

The output of all these circuits is comprised in the rangebetween 0 and -3V.

B. Anti-aliasing filter

The anti-aliasing filter, serves two purposes, filter the signalto remove noise and avoid aliasing in the AD conversion andalso invert the signal coming from the signal conditioningadapting the range from 0 to 3V. This filter has been designusing the tool “FilterPro” from Texas Instruments with thefollowing characteristics:

• Lowpass Multiple Feedback Butterworth topology, order2.

• Passband Frequency of 2.5kHz.• Quality Factor Q=0.71.• Attenuation at 50Hz: 0dB.• Phaseshift at 50Hz: 178,611 (Inversion+1.389).

C. PWM and GPIO signals

3 channel PWM with a total of 6 PWM signals, and 2general purpose digital signals (GPIO) of the digital controllerF28335 are necessaries. The 2 GPIO signals are used to controlthe relay signal and the reset of the power stack. All the signalsare rated between 0 and 3.3V. In order to adapt these signals,an IC buffer is used to rise them to the range 0-5V.

In order to disable the modulation and disconnect the signalsconnected to the inverter driver, a simple circuit was designedwhich allows to enable and disable the IC buffer using amanual switch and indicating the state with a LED diode.

D. Fault signals management

The comercial power stack includes 6 output logic signalwhich indicates a failure in each IGBT. This signal generate alogic 0 when a fault occurs and are disposed in open collectoroutput, therefore, as just 1 fault is enough to considered afailure, a wired or is used to manage the signal, connected toa pull up resistor and a 3.3V reference, which is connected tothe TripZone 6 pin in the F28335.

VII. EXPERIMENTAL RESULTS

A. Motor drive mode results

Once the commissioning of the PCB interface has beenfinished, several results have been performed to test the wholesystem under V-Hz strategy.

Now the power stage is connected to the grid, startingto charge the DC-link until 585 V. Once the DC bus isproperly charged, the modulation is enabled, and the motoris accelerated up to rated speed. Then, the voltage sequence isreversed hence the motor changes the sense of direction. Theresults for phase currents can be appreciated in figure 5:

35.2 35.4 35.6 35.8 36 36.2

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

Time (s)

Iuvw

(A

)

V-Hz performance

iu

iv

iw

Fig. 5. Motor currents while changing direction of rotation in grid connection.

1) Current control: A current control scheme has beenimplemented in the stationary reference frame. The frequencyand the amplitude of the current reference will be set by twoindependent potentiometers in this case, allowing to controlthe magnetization level of the machine as well as the speed.Both variables are scaled within the rated values.

The machine parameters are obtained by applying a steppedvoltage to the motor, giving the results in table II.

Then the system transfer function to consider will be:

TABLE IIMOTOR PARAMETERS

Parameter ValueRs 4.08 ΩLs 16.1 mH

G =1

Ls · s+Rs(11)

Once the system has been identified, a PI controller is setusing the pole-zero cancellation strategy:

C = Kp ·(

1 +Ki

s

)(12)

Kp = 2π ·BW · Ls (13)

Ki =Rs

Ls(14)

The bandwidth has been set to 300 Hz, giving the followingparameters gathered in table III:

TABLE IIICONTROLLER PARAMETERS

Parameter ValueBW 300 HzKp 30.3769Ki 253.1710

In order to test the controller, a current step has beencommanded up to the magnetizing current value, and 7 Hzfrequency approximately. The results are shown in stationaryreference frame in figure 6:

0.4 0.6 0.8 1 1.2 1.4 1.6

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

Current Control performance

Time (s)

Cu

rre

nt

(A)

iα

iβ

iα*

iβ*

Fig. 6. Current control response in stationary reference frame.

B. Grid injection mode results

1) System performance: The system performance, bothdynamic and steady state, is near the expected results. Fig.7 shows the dynamic response of Id and Iq under differentstep responses, verifying a proper response.

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07−2

−1

0

1

2

D axis current response for multiple steps

time (s)

Cu

rre

nt(

A)

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07−2

−1

0

1

2

Q axis current response for multiple steps

time (s)

Cu

rre

nt(

A)

Id

idref

Iq

iqref

Fig. 7. Current controller dynamic performance. Id and Iq responses undermultiple steps commands. DC-Link 50V. Filter and load of 10 ohms.

Fig. 8 shows the performance of Id and Iq comparedwith two of the measured output currents, demonstrating thecoherence between results in the F28335 and the real output.

1.1 1.105 1.11 1.115 1.12 1.125 1.13 1.135−1

0

1

2

3

D axis current response

time (s)

Cu

rre

nt

Id(A

)

1.1 1.105 1.11 1.115 1.12 1.125 1.13 1.135−0.5

−0.3

−0.1

0.1

0.3

0.5

Q axis current response

time (s)

Cu

rre

nt

Iq (

A)

−0.06 −0.04 −0.02 0 0.02 0.04 0.06

−2

−1

0

1

2

Output Currents filtered with the scope

time (s)

Cu

rre

nt(

A)

Ia

Ib

Fig. 8. Current control loop performance of the system for low voltage DC-link 50V condition and 10ohms resistive load add to the LC filter. Id referenceis 2A, Iq reference is 0A. Bottom graph shows the 2 of the output currentsmeasured with the scope.

2) Improve grid synchronization methods: PLL perfor-mance: The dynamic response of the PLL is shown in Fig.9, tracking the angle in less than two cycles. Fig. 10 showsthe PLL result compared with a simple tracking method usingarctangent, observing a noticeable improvement as the PLLremove the disturbances and noise.

0.47 0.48 0.49 0.5 0.51 0.52 0.53 0.54 0.55 0.56 0.570

2

4

6

Grid Angle obtained using PLL

time (s)

An

gle

(ra

d)

20ms50Hz

Fig. 9. Implemented Phase Lock Loop performance. Dynamic and steadystate response

0.49 0.5 0.51 0.52 0.53 0.54 0.55 0.560

2

4

6

Grid angle extraction using PLL

time (s)

An

gle

(ra

d)

0.49 0.5 0.51 0.52 0.53 0.54 0.55 0.560

2

4

6

Grid angle extraction using arctangent

time (s)

An

gle

(ra

d)

Fig. 10. Grid tracking methods comparison. Arctangent compare with PLLperformance.

3) Grid synchronization and power control demonstration:The grid synchronization and correct control of power factorand current magnitud is shown in Fig. 11, where one of theoutput current phases is compared with one of the phases ofthe 3-phase virtual grid.

VIII. CONCLUSION

This paper has presented both the control of a grid-connected RLC load and an induction motor. For that purposea single PCB interface has been built, able to connect the DSPboard with the power stage and suitable for both applications.Once the design process has been explained, results for bothapplications have been presented.

This setup allows the possibility to manage both genericloads and motors with the same hardware and slight changesin the control software. Thus, a wide range of applications canbe covered such as active and reactive power management,motor control and even other applications using the DC input

0 0.01 0.02 0.03 0.04 0.05 0.06−3

−2

−1

0

1

2

3

Ib Output Current and Virtual grid. 2A PF=1

time (s)

Cu

rre

nt(

A)

Vo

lta

ge

(pu

)

Ib

Vb

0 0.01 0.02 0.03 0.04 0.05 0.06−3

−2

−1

0

1

2

3

Ib Output Current and Virtual grid. 2.5A PF=0

time (s)

Cu

rre

nt(

A)

Vo

lta

ge

(pu

)

Ib

Vb

0 0.01 0.02 0.03 0.04 0.05 0.06−3

−2

−1

0

1

2

3

Ib Output Current and Virtual grid. 0.8A PF=0.3

time (s)

Cu

rre

nt(

A)

Vo

lta

ge

(pu

)

Ib

Vb

Fig. 11. Grid synchronization performance. Figures show the the outputcurrent phase b compared with the grid voltage b under different power factorsand magnitudes. Top chart shows 2A peak and power factor 1, middle chart2.5A peak and power factor 0 and bottom chart 0.8A and power factor 0.3.

of the power stage such as battery charging and more. Inaddition, the communication system allows to manage thecontrol commands, become the system a ready-to-use packagewith high versatility.

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