Control of Power Converters for Distributed … › ems › iab2004 › Min ›...

10/22/200410/22/2004 11

Mechatronics Industrial Advisory Board 2004

Control of Power Control of Power Converters for Distributed Converters for Distributed

GenerationGenerationPh.D. Student: Min DaiPh.D. Student: Min Dai

Advisor: Prof. Ali Advisor: Prof. Ali KeyhaniKeyhaniDepartment of Electrical and Computer EngineeringDepartment of Electrical and Computer Engineering

The Ohio State UniversityThe Ohio State University

October 22, 2004October 22, 2004

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AgendaAgendaIntroductionIntroductionProblem statementProblem statementSystem DescriptionSystem DescriptionPower control of gridPower control of grid--connected connected invertersinverters–– Single unitSingle unit–– Multiple unitsMultiple units

TransfromerlessTransfromerless power converter unit power converter unit topology and control issuestopology and control issuesResearch PlanResearch Plan

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Introduction (1)Introduction (1)What is distributed generation (DG)?What is distributed generation (DG)?–– Distributed: 2Distributed: 2--50MW50MW–– Dispersed: <500kWDispersed: <500kW–– Close to end userClose to end user–– Wind, photovoltaic, fuel cells, micro gas turbinesWind, photovoltaic, fuel cells, micro gas turbines

Why DG?Why DG?–– Growth of power demandGrowth of power demand–– Limited transmission capabilityLimited transmission capability–– Increase of critical loadIncrease of critical load–– Availability of new sources and new technologyAvailability of new sources and new technology

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Introduction (2)Introduction (2)Centralized power plants will remain the major Centralized power plants will remain the major sourcesourceRole of DGRole of DG–– standby emergency powerstandby emergency power–– cogeneration (heat and power)cogeneration (heat and power)–– peak shavingpeak shaving–– grid supportgrid support–– stand alone onsite powerstand alone onsite power

Impacts of DG on power system stabilityImpacts of DG on power system stability–– Define: Define: Penetration = Pd/(P+Pd)Penetration = Pd/(P+Pd)××100%100%–– At small penetration: InsignificantAt small penetration: Insignificant–– At high penetration: Enhances system stabilityAt high penetration: Enhances system stability

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Introduction (3)Introduction (3)

Impacts of DG on power system Impacts of DG on power system protectionprotection–– Operation conflictsOperation conflicts

affects utility protective relayingaffects utility protective relayingvoltage sag if disconnectedvoltage sag if disconnectedprolongs fault arc and fails utility prolongs fault arc and fails utility reclosereclose

–– Possible solutionsPossible solutionsDG unit current protection DG unit current protection –– local, no utility local, no utility involvedinvolvedIntegrate protection with utilityIntegrate protection with utility

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How is DG going to be operated once How is DG going to be operated once connected to the grid?connected to the grid?–– No standard solution yet No standard solution yet –– in policy making in policy making

stagestage–– PossibilitiesPossibilities

Small units (e.g., residential size)Small units (e.g., residential size)-- no dispatchno dispatchLarge units run under higher level dispatchLarge units run under higher level dispatchHierarchical dispatchHierarchical dispatchNetworking technologyNetworking technology

–– Cost/profit issuesCost/profit issues

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SolidSolid--state power converter and state power converter and distributed generationdistributed generation–– Static switch is economical only for small unitsStatic switch is economical only for small units–– Parallel operation for large capacityParallel operation for large capacity

Local load issuesLocal load issues–– High profile load requires high power qualityHigh profile load requires high power quality–– Nonlinear loadNonlinear load–– Unbalanced loadUnbalanced load–– Dynamic disturbancesDynamic disturbances

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Distributed generation unit Distributed generation unit –– the the topologytopology–– With an isolation transformerWith an isolation transformer–– TransformerlessTransformerless

Control issuesControl issues–– Island mode: voltage controlIsland mode: voltage control–– GridGrid--connected mode: power controlconnected mode: power control–– Paralleled multiple units: load sharingParalleled multiple units: load sharing–– FrontFront--end DC bus end DC bus –– the rectifier controlthe rectifier control

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Problem Statement (1)Problem Statement (1)

Power control of gridPower control of grid--connected invertersconnected inverters–– Single unit with gridSingle unit with grid--connectionconnection

No load connectionNo load connectionDisconnectionDisconnectionRecloseReclose

–– Multiple unitsMultiple unitsParallel operation in island modeParallel operation in island modeConnect a 2Connect a 2ndnd unit to a gridunit to a grid--connected unitconnected unitDisconnectionsDisconnectionsRecloseReclose

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Problem Statement (2)Problem Statement (2)

Voltage and current control of a Voltage and current control of a transformerlesstransformerless inverter inverter –– a three a three phase four wire systemphase four wire system–– Control as a three phase system rather Control as a three phase system rather

than three single phase systemsthan three single phase systems–– Rectifier control to eliminate effects of Rectifier control to eliminate effects of

unbalanced inverter load currentunbalanced inverter load current

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System Description (1)System Description (1)

Power circuit topologyPower circuit topology–– Single unit (3Single unit (3--ph 3 wire inverter plus ph 3 wire inverter plus

transformer)transformer)

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Power Power circuit circuit topologtopologyy–– MultiplMultipl

e unitse units

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State space State space modelingmodeling

0,0,,

31

31

αβαβαβαβ

sif

invf

xfm

CCdtd

ITIV

−=

αβαβαβ

,,, 11

xfmf

invf

inv

LLdtd

VVI

−=

0,0,0, 11

αβαβαβ

outs

ss

out

CCdtd

IIV

−=

0,,0,0,0, 11

αβαβαβαβαβ

outT

xfmvT

sT

Ts

LLLR

dtd

VVTII

−−−=

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Operation in island modeOperation in island mode–– Performs voltage controlPerforms voltage control–– Control goals: Control goals:

Low steady state errorLow steady state errorLow total harmonic distortion (THD)Low total harmonic distortion (THD)Fast response to load disturbancesFast response to load disturbances

–– Challenges: Challenges: Unbalanced loadUnbalanced loadNonlinear loadNonlinear loadInstantaneous load disturbanceInstantaneous load disturbance

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Existing voltage control strategiesExisting voltage control strategies–– PI controlPI control–– State space based linear controlState space based linear control–– Sliding mode controlSliding mode control–– Internal model principle based controlInternal model principle based control

Repetitive controlRepetitive controlServo controlServo control

Our approachOur approach–– Robust servomechanism control for voltage Robust servomechanism control for voltage

looploop–– DiscreteDiscrete--time sliding mode for current looptime sliding mode for current loop

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PQ Control with GridPQ Control with Grid--Connection (1)Connection (1)

Real & reactive power Real & reactive power control in gridcontrol in grid--connected modeconnected modeOperating scenariosOperating scenarios--single unitsingle unit–– Connection with no load:Connection with no load:

Synchronization before Synchronization before closing the switchclosing the switchControl strategy change Control strategy change while closing the switchwhile closing the switchInsignificant transient Insignificant transient due to low inverter due to low inverter output impedance

InverterUnit

GridLocalLoad

jX

output impedance

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InverterUnit

GridLocalLoad

jX–– Disconnection with Disconnection with load:load:

Three phase opened Three phase opened one by one due to one by one due to zerozero--cross switchingcross switching

–– RecloseRecloseSynchronizationSynchronizationControl strategy Control strategy switchswitchP,Q transient exists P,Q transient exists due to nondue to non--zero X

InverterUnit

GridLocalLoad

jX

zero X

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Operating Operating scenariosscenarios--multiple multiple unitsunits–– Parallel in island Parallel in island

mode:mode:SynchronizationSynchronizationLoad sharing after Load sharing after connectionconnectionHarmonic load Harmonic load sharingsharing

–– Connect a 2Connect a 2ndnd unitunitSynchronizationSynchronizationLoad sharing Load sharing operationoperation

InverterUnit 1

GridLocalLoad

jX

InverterUnit 2

InverterUnit 1

GridLocalLoad

jX

InverterUnit 2

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–– Disconnections with Disconnections with load:load:

Three phase opened Three phase opened one by one due to one by one due to zerozero--cross switching

InverterUnit 1

GridLocalLoad

jX

InverterUnit 2cross switching

InverterUnit 1

GridLocalLoad

jX

InverterUnit 2

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–– RecloseRecloseSynchronization in Synchronization in parallelparallelControl strategy Control strategy switchswitchFundamental and Fundamental and harmonic load harmonic load sharing before and sharing before and after switchingafter switchingP,Q transient exists P,Q transient exists due to nondue to non--zero X

InverterUnit 1

GridLocalLoad

jX

InverterUnit 2

zero X

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Control goalsControl goals–– Low steady state PQ tracking errorLow steady state PQ tracking error–– Relatively fast transient responseRelatively fast transient response–– Steady state decoupling between P and Q Steady state decoupling between P and Q

controlscontrols–– Small PQ transient at Small PQ transient at reclosereclose–– Load sharing in island mode including Load sharing in island mode including

harmonic load sharingharmonic load sharing–– Load sharing in gridLoad sharing in grid--connected modeconnected mode–– As less control interconnections between the As less control interconnections between the

units as possibleunits as possible

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Power control Power control principleprinciple–– Power flow between Power flow between

two two soucessouces jX

Vout/_δ E/_0ILine

δsinXEV

P out=

XEVV

Q outout δcos2 −=

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–– V, V, δδ impacts on P, Qimpacts on P, Q

–– δδ --> P, > P, V V --> > QQ

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Basic methodology Basic methodology –– Add one more feedback control loop Add one more feedback control loop –– the the

power looppower loop–– The power controller generates voltage The power controller generates voltage

commandcommand

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Existing approachesExisting approaches–– Integral control based techniqueIntegral control based technique

–– Parameter based feedforwardParameter based feedforwardV, V, δδ, E, X , E, X →→ P, QP, QPPrefref, , QQrefref →→ VVrefref, , δδrefref

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The problemsThe problems–– The integral controlThe integral control

Slow when P, Q error is largeSlow when P, Q error is large–– Parameter based feedforwardParameter based feedforward

Without coupling inductor Without coupling inductor –– Line impedance X unknownLine impedance X unknownWith a coupling inductorWith a coupling inductor

–– Extra componentExtra component–– Local load voltage rippleLocal load voltage ripple

To utilize the filter inductor/transformerTo utilize the filter inductor/transformer–– Existence of capacitors causes complexity in analytical Existence of capacitors causes complexity in analytical

solutionsolution–– Parametric inaccuracyParametric inaccuracy–– Unknown power of local loadUnknown power of local load–– Control plant structure changeControl plant structure change

–– MultiMulti--units power control problems not reportedunits power control problems not reported

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Proposed research directionProposed research direction–– Single unit caseSingle unit case

Integral control plus a good inner voltage loopIntegral control plus a good inner voltage loop–– Robust servomechanism plus discreteRobust servomechanism plus discrete--time sliding modetime sliding mode

A new feedforward power controller to eliminate PQ A new feedforward power controller to eliminate PQ transients at transients at reclosereclose

–– MultiMulti--unit caseunit caseLoad sharing between multiple units with girdLoad sharing between multiple units with gird--connection connection and local load, on the basis of and local load, on the basis of technique for load sharing in technique for load sharing in island modeisland modeSynchronization before Synchronization before recloserecloseFeedforward issueFeedforward issue

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Simulation results Simulation results –– single unitsingle unit

Grid connected,

P,Q step responseIsland Mode, V-I control

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Simulation results Simulation results –– single unitsingle unit

Grid connected,

RecloseGrid connected,

Line current conditioning

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The 3The 3--Ph 4Ph 4--Wire Inverter Control (1)Wire Inverter Control (1)

The topologyThe topology

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Control goals Control goals –– voltage controlvoltage control–– Low steady state errorLow steady state error–– Low THDLow THD–– Robust to load disturbancesRobust to load disturbances–– Fast transient responseFast transient response

ChallengesChallenges–– Nonlinear loadNonlinear load–– Unbalanced loadUnbalanced load–– Transient load disturbancesTransient load disturbances

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Existing approachesExisting approaches–– Treat it as 3 independent single phase Treat it as 3 independent single phase

inverter (inverter (halfhalf--bridgebridge))–– Control conducted in ABC reference Control conducted in ABC reference

frameframe–– Or synchronous reference frame for PI Or synchronous reference frame for PI

controllerscontrollers–– Performs SinePerforms Sine--triangle PWM (SPWM)triangle PWM (SPWM)

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ProblemsProblems–– PI control in synchronous framePI control in synchronous frame

Designed for fundamental, poor in Designed for fundamental, poor in harmonicsharmonics

–– Any control algorithmAny control algorithmSPWMSPWM

–– Higher THD compared to space vector PWM Higher THD compared to space vector PWM (SVPWM)(SVPWM)

–– OverOver--modulation causes pulse “drop out”modulation causes pulse “drop out”

If the control is not done in ABC frame, If the control is not done in ABC frame, extra reference frame transformation is extra reference frame transformation is neededneeded

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–– If the frontIf the front--end is powered by a end is powered by a controlled rectifier, unbalanced inverter controlled rectifier, unbalanced inverter load causesload causes

Unbalanced input currentUnbalanced input currentRipple on DC bus voltageRipple on DC bus voltage

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Proposed research directionProposed research direction–– Utilize SVPWMUtilize SVPWM

It has to be modifiedIt has to be modifiedAdd 0Add 0--sequence control capabilitysequence control capability

–– Perform control in stationary Perform control in stationary αβαβ0 0 reference framereference frame

–– Eliminate impacts of unbalanced Eliminate impacts of unbalanced inverter load on frontinverter load on front--end rectifier end rectifier control and make the input current control and make the input current balanced.balanced.

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Research Plan (1)Research Plan (1)1.1. Analyze the traditional three phase three wire Analyze the traditional three phase three wire

inverter plant in theoretical domain. Choose an inverter plant in theoretical domain. Choose an effective existing voltage control technique to be effective existing voltage control technique to be the lower level controller for power control.the lower level controller for power control.

2.2. Analyze the load sharing problem in island mode Analyze the load sharing problem in island mode in theoretical domain. Choose an effective in theoretical domain. Choose an effective existing load sharing control technique for existing load sharing control technique for parallel operation of multiple DG units in island parallel operation of multiple DG units in island mode and the harmonic load sharing part should mode and the harmonic load sharing part should be used in gridbe used in grid--connected mode.connected mode.

3.3. Implementation of the single unit voltage control Implementation of the single unit voltage control technique in simulation.technique in simulation.

4.4. Implementation of the multiImplementation of the multi--unit load sharing unit load sharing control technique in simulation.control technique in simulation.

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Research Plan (2)Research Plan (2)5.5. Apply basic power control on single unit with grid Apply basic power control on single unit with grid

connection and implement the control technique in connection and implement the control technique in simulation.simulation.

6.6. Develop a feedforward power controller which can estimate Develop a feedforward power controller which can estimate the the TheveninThevenin parameters of the utility grid and help to find parameters of the utility grid and help to find the correct operating point of the grid connected DG. In the correct operating point of the grid connected DG. In presence of the knowledge of the operating point, seamless presence of the knowledge of the operating point, seamless transition should be able to achieved in transition should be able to achieved in reclosereclose operation. operation. Demonstrate the effectiveness of the technique in Demonstrate the effectiveness of the technique in simulation.simulation.

7.7. Conduct case study for gridConduct case study for grid--connected single unit in connected single unit in simulation.simulation.

8.8. Connect a second inverter to a gridConnect a second inverter to a grid--connected inverter and connected inverter and achieve harmonic load sharing. Demonstrate the result in achieve harmonic load sharing. Demonstrate the result in simulation.simulation.

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Research Plan (3)Research Plan (3)9.9. Conduct case study for gridConduct case study for grid--connected multiple units connected multiple units

based on the proposal in simulation.based on the proposal in simulation.10.10. For a three phase four wire system, develop an For a three phase four wire system, develop an αβαβ0 0

reference frame based control technique plus a modified reference frame based control technique plus a modified space vector PWM which can perform 0space vector PWM which can perform 0--sequence control. sequence control. Demonstrate the result in simulation under different types Demonstrate the result in simulation under different types of load.of load.

11.11. For a three phase four wire system, analyze the impact on For a three phase four wire system, analyze the impact on the DC bus voltage by unbalanced inverter load. Develop a the DC bus voltage by unbalanced inverter load. Develop a rectifier control technique based on the analysis result to rectifier control technique based on the analysis result to yield balanced frontyield balanced front--end input current. Demonstrate the end input current. Demonstrate the performance in simulation.performance in simulation.

12.12. Get familiar with existing code of the DSP controllers in the Get familiar with existing code of the DSP controllers in the laboratory setup.laboratory setup.

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Research Plan (4)Research Plan (4)13.13. Experimental test for single unit power control Experimental test for single unit power control

with girdwith gird--connection. Go through the case connection. Go through the case studies.studies.

14.14. Experimental test for multiExperimental test for multi--unit power control unit power control with girdwith gird--connection. Go through the case connection. Go through the case studies.studies.

15.15. Experimental test for the proposed control Experimental test for the proposed control technique for three phase four wire inverter technique for three phase four wire inverter under different types of load.under different types of load.

16.16. Experimental test for the proposed control Experimental test for the proposed control technique for three phase four wire rectifier technique for three phase four wire rectifier under unbalanced inverter load.under unbalanced inverter load.

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The Experimental Setup (1)The Experimental Setup (1)

Load240VMain

A B C D

E

CircuitBreaker

M1Contactor

M2

CircuitBreaker

M2

208V Main

Load240VMain

A’ B’ C’ D’

E’

ContactorM4

CircuitBreaker

M4

CircuitBreaker

L1

CircuitBreaker

L2

ContactorL1

ContactorL2

Measurements:A: 2C, 2V; A’: 2C, 2V;B: 1C, 1V; B’: 1C, 1V;C: 2C, 2V; C’: 2C, 2V;D: 3C, 3V; D’: 3C, 3V;E: 3C, 3V; E’: 3C, 3V;

Total: 22C + 22V = 44 Channels

Unit 1

Unit 2CircuitBreaker

M3

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The Experimental Setup (2)The Experimental Setup (2)CB 4.2mH

InductorRectifier Inverter 1.8mH

Inductor

240VL-LMain

120VL-NMain

CB

4.2mHInductor

∆/YTransformer

CB

LoadCB

Signal Conditioning Module

Input

DC

Inverter &Transformer

Primary

Load &TransformerSecondary

Bypass

Ain

Bin

Cin

-in

+in

Ain

Bin

Cin

Ain

Bin

Cin

Nin

Ain

Bin

Cin

Nin

Aout

Bout

Cout

-out

+out

Aout

Bout

Cout

Aout

Bout

Cout

Nout

Aout

Bout

Cout

Nout

A

B

C

N

55µF ∆ Capacitor

Bank 5µF YCapBank

-

+

LegendThick wire with big banana plugs at both endsThin wire with a big banana plug at one end

and a small banana plug at the other end

Thin wire with a capacitor connector at one end

Testing wire with small banana plugs at both ends

CB 4.2mHInductor

Rectifier 1.8mHInductor

240VL-LMain

120VL-NMain

CB

4.2mHInductor

CB

CB

Signal Conditioning Module

Input

DC

Inverter &Transformer

Primary

Load &TransformerSecondary

Bypass

Ain

Bin

Cin

-in

+in

Ain

Bin

Cin

Ain

Bin

Cin

Nin

Ain

Bin

Cin

Nin

Aout

Bout

Cout

-out

+out

Aout

Bout

Cout

Aout

Bout

Cout

Nout

Aout

Bout

Cout

Nout

55µF ∆ Capacitor

Bank 5µF YCapBank

-

+Inverter

∆/YTransformer

A B CN

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The Experimental Setup (3)The Experimental Setup (3)

VoltageDivider

Hall EffectCurrentSensor

SignalConditioning

TMS320LF2407A

DSP

OpticalIsolation

Host PC

PWM To GateDrive

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Recent PublicationsRecent PublicationsMin Dai, Mohammad N. Marwali, Jin-Woo Jung, and Ali Keyhani, “A PWM rectifier control technique for three phase double-conversion UPS under unbalanced load,” IEEE APEC’05, accepted for oral presentation.Jin-Woo Jung, Min Dai, and Ali Keyhani, "Modeling and control of a fuel cell based Z-source converter,” IEEE APEC'05, Austin, TX, USA, March 6-10, 2005, accepted for oral presentation.Min Dai, Mohammad N. Marwali, Jin-Woo Jung, and Ali Keyhani, “Power flow control of a single distributed generation unit with nonlinear local load,” IEEE PSCE’04, Oct. 2004, New York, NY, PSCE2004-000574.pdf.J. W. Jung, M. Dai, and A. Keyhani, "Optimal control of three-phase PWM inverter for UPS systems," IEEE Power Electronics Specialist Conference (PESC'04), pp. 2054-2059, Aachen Germany, June 20-24, 2004.

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