Functionalities of Power Electronic Converters in the context

of SmartGrids

Jon Are Suul

pos.doc. researcherDepartment of Electric Power Engineering

Norwegian University of Science and Technology

Japan‐Norway Energy Science Week 2015Tokyo, 27.05.2015

2

Outline

• Introduction to converters in power systems• Added functionalities in traditional power systems

– Contributions to primary frequency control– Control of voltage or reactive power– Damping of oscillations– Active filtering– Inertia emulation

• Operation in MicroGrids and isolated systems– Example: Virtual Synchronous Machine– Control for islanding with local loads– Resynchronization to large-scale power system

• Summary

3

Traditional grids vs. "SmartGrids"

F. Rahimi, A. Ipakchi, “Demand Response as a Market Resource Under the Smart Grid Paradigm,” in IEEE Transactions on Smart Grid, Vol. 1, No. 1, June 2010, pp. 82‐88

Traditional Utility grid

"SmartGrid" Scenario

4

Local actuation - Power Electronics

• Power Electronic converters has become the main enabling technology for grid integration of renewable energy sources and controllable loads

• Typical examples:– Variable speed wind turbines– Photovoltaic power generation– Regenerative loads– Electric vehicle battery chargers– Energy Storage Systems

• The local actuation and control in a Smart Energy System will be mainly performed by power electronic converters

Production/Load/Storage

DCC1LPCC

gV cVfV 1R cIgLgR

Grid equivalent source LCL Filter

2L2RDCV

Converter

Two main types of converter control systems

• Grid integrated operation– Depends on Phase Locked

Loop for synchronization to the grid voltage

– Usually operated with inner loop current control

– Stand-alone operation requires change of control system

• Stand-alone operation– Must be able to operate in

voltage control mode– Need control loops for sharing

of active and reactive power– Synchronization based on

power-frequency control characteristics

DCC

DCv

1L

,c abcI

PWMg

ci

1C

2L

sv

ActivePower

*v̂Reactive

0q

0p

p&qq

p

Modulation

Current

&Control

*ciVoltage

Control

PowerControl

Control

loadZ

,s abV

,c abI

1 6g

Synchronization

CurrentControl

&Modulation

DCC

ActivePowerControl

Reactive

Control

DCV

Power

*P

,c abI DCV

Grid information

Synchronizingsignals

1L

*Q

Current

Reference

Calculation

*I

,c abI

DCVGrid

6

Outline

• Introduction to converters in power systems• Added functionalities in traditional power systems

– Contributions to primary frequency control– Control of voltage or reactive power– Damping of oscillations– Active filtering– Inertia emulation

• Operation in MicroGrids and isolated systems– Example: Virtual Synchronous Machine– Control for islanding with local loads– Resynchronization to large-scale power system

• Summary

7

Power-frequency control

• Relevant for contribution to power system primary frequency regulation– All converters with controllable generation or load can apply

frequency-dependent power control– Traditional power-frequency droop is the simplest option

• Can be applied to most control schemes– Implies loss of generation or load

for units without energy storage• Can be used in addition to load

shifting or other techniques for improving the match between generation and load profiles

*rp* k

*p

grid

Frequency Droop

8

Reactive power or voltage control

• Inverters and regenerative rectifiers can control their reactive power flow– Free capacity can be used to regulate grid voltage in inductive

transmission systems

*ˆrv*q

qk

*v̂

f

fs

q

Droop-based Reactive Power Controller

mq

• Droop control can be applied to share load between parallel or nearby units

• Dual control option– Voltage control with reactive power

droop– Reactive power control with voltage

droop

• Dedicated compensation units are needed in case of generation units and loads without reactive power control capability

9

Consequences of reactive current injection

• Dependent on grid impedance

• Required incremental rating can be small

d

q

dI

qItI

t dI IM. Molinas, D. Moltoni, G. Fascendini, J. A. Suul, R. Ferranda, T. Undeland: “Investigation on the role of power electronic controlled constant power loads for voltage support in AC systems,” in Proceedings of the IEEE 39th Power Electronics Specialists Conference, PESC’08, Rhodes, Greece, 15‐19 June, 2008, pp. 3597‐3602J. A. Suul, M. Molinas; "Properties of Reactive Current Injection by AC Power Electronic Systems for Loss Minimization," in Proceedings of the 15th International Power Electronics and Motion Control Conference, EPE‐PEMC 2012 ECCE Europe, Novi Sad, Serbia, 3‐6 September 2012, 8 pp.

-0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 0.60.70.80.91 1.11.21.31.41.51.61.71.81.92

iq [pu]

i [pu

]

Characteristics of system with 1.0 pu generation, xeq = 0.25, req = 0.05

id(iq) for vg = 1.0

itot(iq) for vg = 1.0

id(iq) for vg = 0.8

itot(iq) for vg = 0.8

2 2

t d qI I I

Active Damping of oscillations• Basic concept

– Inject voltage or current component in counter-phase with the measured oscillations

• Effectively cancels poorly damped oscillations– Oscillatory components usually isolated by high-pass filter

• Can be applied for wide range of oscillation phenomena– LC oscillations in filters or transmission cables– Low frequency oscillations in mechanical components

• Can require corresponding energy storage capability

AD

ADs,o dqv *

,dq ADvADk

dq

Active Damping of LCL filter oscillations

Example: Active Damping of LC oscillations• Hysteresis controlled converter

– Wide spectrum harmonics generated by hysteresis control initiates oscillations in LCL filter

– Active damping is a simple way of alleviating this problem

• Experimental results without and with active damping

• Fixed frequency PWM can also trigger LC oscillations– Can be damped using the same

approach

J. A. Suul, K. Ljøkelsøy, T. Midtsund, T. Undeland, "Synchronour Reference Frame Hysteresis Current Control for Grid Converter Applications, in Proceedings of the International Conference on Power Electronics and Motion Control, EPE‐PEMC 2010, Ohrid, Republic of Macedonia, 6‐8 September 2010, 9 pp

Active Power Filtering

• Conventional approach:– Dedicated converter

compensating for distortions from non-linear loads

• High control bandwidth is required

• High switching frequency

• Possibility:– Active filtering as auxiliary

service from converter for generation, load or energy storage

H. Akagi, E. Watanabe, M. Aredes, "Instantaneous Power Theory and Applications to Power Conditioning," Wiley/IEEE Press, 2007

13

Inertia emulation

• Conventional approach– Based on frequency derivative

• Operation depends on the presence of conventional generation with physical inertia and power-frequency droop control

gIE IE

dp J

dt

G. Delille, B. François, and G. Malarange, "Dynamic Frequency Control Support by Energy Storage to Reduce the Impact of Wind and Solar Generation on Isolated Power System’s Inertia" in IEEE Transactions on Sustainable Energy, Vol. 3, No. 4, October 2012, pp. 931‐939

14

Outline

• Introduction to converters in power systems• Added functionalities in traditional power systems

– Contributions to primary frequency control– Control of voltage or reactive power– Damping of oscillations– Active filtering– Inertia emulation

• Operation in MicroGrids and isolated systems– Example: Virtual Synchronous Machine– Control for islanding with local loads– Resynchronization to large-scale power system

• Summary

15

MicroGrids and isolated systems

• Requirements for power converters in isolated systems– Distributed control of:

• Frequency and active power sharing• Voltage and reactive power sharing

– Possibility to operate without physical inertia– Possibility to operate in mixed systems with generation from both

synchronous machines and converter-interfaced sources

• Additional requirements for MicroGrids– Converter control systems must also be adapted to grid-connected

operation– Seamless transition between grid-connected and islanded operation– Smooth reconnection to the main power system

16

Virtual Synchronous Machine (VSM)

• Power Electronic converter controlled to emulate the behavioral characteristics of synchronous machines

• Concept first introduced about one decade ago – Several possible implementations

• Parameters are not limited by physical design constraints

• Emulated characteristics:– Inertia and damping– Power-balance-based grid

synchronization mechanism– Active and reactive power control

mechanisms• Can operate in both grid

connected and islanded modes

S. D'Arco, J. A. Suul, "Virtual Synchronous Machines –Classification of Implementations and Analysis of Equivalence to Droop Controllers for Microgrids," in Proceedings of IEEE PES PowerTech 2013, Grenoble, France, 16-20 June 2013, 7 pp.

17

Overview of implementation• Synchronous Reference Frame cascaded voltage and current controllers• Virtual Impedance • Active damping of LC filter• Local frequency tracking by Phase Locked Loop only needed for damping emulation • Power Controller including Virtual Swing equation ensuring inertial characteristics• Reactive Power Controller

DCC

DCv

fL

cvI

PWMg

cvi

fC

gZ

ov

VSM

gV

Inertia

Power

*ˆrvReactive*q

*p

q

pCurrentControl

*,cv dqiVoltage

ControlVSMPWM

*,o dqvVirtual

Impedance

PowerControl

dq

dqm

oi

Virtual

Control

and

Measurement

Processing

,cv dqi,o dqv,o dqi

abcm

ActiveDamping

PhaseLockedLoop

*,AD dqv

*VSM

PLL

PLL

,o dqv*v̂

lZ

S. D'Arco, J. A. Suul, O. B. Fosso, "Small‐signal modelling and parametric sensitivity of a Virtual Synchronous Machine in islanded operation," in International Journal of Electric Power and Energy Systems, Vol. 72, November 2015, pp. 3‐15

18

Response to sudden islanding• VSM operated with power reference in grid connected mode• Sudden islanding to a local load

• Transition is immediate and smooth while maintaining voltage on the local load• Local frequency settles to a value depending on the steady-state frequency droop

0 0.5 1 1.50.35

0.4

0.45

0.5

0.55

0.6

0.65

0.7

0.75

Time [s]

Power [pu]

p [p

u]

0 0.5 1 1.5 2 2.5 3 3.50.995

1

1.005

1.01

1.015

1.02

1.025

Time [s]

VSM speed

VS

M [p

u]

S. D'Arco, J. A. Suul, O. B. Fosso, "Small‐signal modelling and parametric sensitivity of a Virtual Synchronous Machine in islanded operation," in International Journal of Electric Power and Energy Systems, Vol. 72, November 2015, pp. 3‐15

19

Resynchronization to main grid

• Amplitude, frequency and phase of the voltage in the isolated system must be aligned with the grid voltage before reconnection

• A centralized controller can act on the references to the local controllers for eliminate the voltage across the breaker

lZ

gVgZ ,B gV ,B lV ,1oV

fC

,2oV

fC

,1cV,1cI

,2cV,2cI

Source/Storage

DCCDCV

Source/Storage

DCCDCV

Local Bus VSM 1

VSM 2

SynchronizingController

1L2L

1L2L

S. D'Arco, J. A. Suul, " A Synchronization Controller for Grid Reconnection of Islanded Virtual Synchronous Machines," in Proceedings of the 6thInternational Symposium on Power Electronics for Distributed Generation, PEDG 2015, Aachen, Germany, 22‐25 June 2015, 8 pp.

20

Incremental VSM frequency reference

• Acting on frequency reference to be compatible with different VSM implementations as well as synchronous machine controllers

DCC

DCv

fL

cvI

PWMg

cvi

fC

gL

ov

VSM

Inertia

Power

*ˆrvReactive*q

*p

q

pCurrentControl

*,cv dqiVoltage

ControlVSMPWM

*,o dqvVirtual

Impedance

PowerControl

dq

dqm

oi

Virtual

Control

and

Measurement

Processing

,cv dqi,o dqv,o dqi

abcm

ActiveDamping

PhaseLockedLoop

*,AD dqv

*

PLL

PLL

,o dqv*v̂

*VSM

*VSM

Smooth reconnection• In islanded operation the phase

of the voltage across the breaker is drifting with the frequency difference

• Synchronization controller must eliminate the frequency and phase difference across the breaker

• When phase difference is eliminated the breaker can be closed

• VSM is smoothly transferred into grid-connected operation without any change of internal control system

• VSM can start tracking the power reference

0 5 10 15 20 25 30 35 40-0.06

-0.04

-0.02

0

0.02

Time [s]

[pu]

0 5 10 15 20 25 30 35 40-pi

-pi/2

0

pi/2

pi

Time [s]

[rad]

0 5 10 15 20 25 30 35 400

0.10.20.30.40.50.60.7

Time [s]

Act

ive

pow

er -

p o [pu]

0 5 10 15 20 25 30 35 400

0.10.20.30.40.50.60.7

Time [s]

Cur

rent

- |i o| [

pu]

S. D'Arco, J. A. Suul, " A Synchronization Controller for Grid Reconnection of Islanded Virtual Synchronous Machines," in Proceedings of the 6thInternational Symposium on Power Electronics for Distributed Generation, PEDG 2015, Aachen, Germany, 22‐25 June 2015, 8 pp.

22

Summary• Power electronic converters will be among the main actuators of the

emerging SmartGrid scenario• High controllability of grid converter enables provision of additional

functions • Active and reactive power control for contribution to the power system

operation• Damping of oscillations• Harmonic filtering• Inertia emulation for increasing equivalent rotating mass of the power system

• Additional control system requirements apply to converters in isolated power systems and MicroGrids• Virtual Synchronous Machines presented as an example of control

systems that can ensure:• Operation in both grid-connected and islanded modes with smooth transitions

and without need for on-line control system reconfiguration• Operation in isolated systems with both power converters and machines• Active and reactive power control with inertial characteristics in both grid-

connected and islanded operation

Thank you for your attention!

Questions?

24

References• S. D'Arco, J. A. Suul, O. B. Fosso, "Small-signal modelling and parametric sensitivity of a Virtual Synchronous Machine in islanded

operation," in International Journal of Electric Power and Energy Systems, Vol. 72, November 2015, pp. 3-15• S. D'Arco, J. A. Suul, O. B. Fosso, "A Virtual Synchronous Machine Implementation for Distributed Control of Power Converters in

SmartGrids," in Electric Power System Research, Vol. 122, May 2015, pp. 180-197• S. D'Arco, J. A. Suul, O. B. Fosso, "Small-Signal Modeling and Parametric Sensitivity of a Virtual Synchronous Machine," in Proceedings

of the 18th Power Systems Computation Conference, PSCC 2014, Wrocław, Poland, 18-22 August 2014, 9 pp.• S. D'Arco, J. A. Suul, "Equivalence of Virtual Synchronous Machines and Frequency-Droops for Converter-Based MicroGrids," in IEEE

Transactions on Smart Grid, Vol. 5, No. 1, January 2014, pp. 394-395• S. D'Arco, J. A. Suul, " A Synchronization Controller for Grid Reconnection of Islanded Virtual Synchronous Machines," in Proceedings of

the 6th International Symposium on Power Electronics for Distributed Generation, PEDG 2015, Aachen, Germany, 22-25 June 2015, 8 pp.• J. A. Suul, S. D'Arco, G, Guidi, "Virtual Synchronous Machine-based Control of a Single-phase Bi-directional Battery Charger for Providing

Vehicle-to-Grid Services," in Proceedings of the 9th International Conference on Power Electronics, ICPE – ECCE Asia, Seoul, Korea, 1-5 June 2015, 8 pp.

• J. A. Suul, S. D'Arco, G. Guidi, "A Single-Phase Virtual Synchronous Machine for Providing Vehicle-to-Grid Serviced from Electric Vehicle Battery Chargers," in Proceedings of the International Electric Vehicle Technology Conference & Automotive Power Electronics, EVTeC & APE Japan, Yokohama, Japan, 22-24 May 2014, 7 pp.

• S. D'Arco; J. A. Suul; O. B. Fosso, "Control System Tuning and Stability Analysis of Virtual Synchronous Machines," in Proceedings of the 2013 IEEE Energy Conversion Congress and Exposition, ECCE 2013, Denver, Colorado, USA, 15-19 September 2013, pp. 2664-2671

• S. D'Arco, J. A. Suul, "Virtual Synchronous Machines – Classification of Implementations and Analysis of Equivalence to Droop Controllers for Microgrids," in Proceedings of IEEE PES PowerTech 2013, Grenoble, France, 16-20 June 2013, 7 pp.

• M. Molinas, D. Moltoni, G. Fascendini, J. A. Suul, R. Ferranda, T. Undeland: “Investigation on the role of power electronic controlled constant power loads for voltage support in AC systems,” in Proceedings of the IEEE 39th Power Electronics Specialists Conference, PESC’08, Rhodes, Greece, 15-19 June, 2008, pp. 3597-3602

• J. A. Suul, M. Molinas; "Properties of Reactive Current Injection by AC Power Electronic Systems for Loss Minimization," in Proceedings of the 15th International Power Electronics and Motion Control Conference, EPE-PEMC 2012 ECCE Europe, Novi Sad, Serbia, 3-6 September 2012, 8 pp.

• J. A. Suul, K. Ljøkelsøy, T. Midtsund, T. Undeland, "Synchronous Reference Frame Hysteresis Current Control for Grid Converter Applications, in Proceedings of the International Conference on Power Electronics and Motion Control, EPE-PEMC 2010, Ohrid, Republic of Macedonia, 6-8 September 2010, 9 pp.