parallel operation of distributed generators by virtual synchronous

Parallel Operation of Distributed Generators by Virtual Synchronous Generator Control in Microgrids

Jia Liu* and Toshifumi Ise Osaka University, Japan

October 21, 2016

Niagara 2016 Symposium on Microgrids October 20-21, 2016 Niagara, Canada

Contents

• Introduction

• Parallel Inverters

• Synchronous Generator + Inverter

• Conclusion

2

Contents

• Introduction



• Conclusion

3

4

Synchronous Generator

Swing Equation

Kinetic Energy of Rotating Mass

Frequency fluctuation under active power transition

limited by the inertia

DC/AC Inverter

Inertial feature of Synchronous Generators

No intrinsic relation

Undesirable frequency dynamics

5

Virtual Synchronous Generator (VSG) Control

Conventional Droop Control Load Sharing

Swing Equation Emulation

Smooth Transition between Islanding

and Grid-connection

Inertia Support

A New Concept of Inverter Control in AC Microgrid

Concept of VSG Control

6

To apply VSG control to DC-AC inverters in microgrids Conventional droop control

Objectives

• Two topics to be discussed • Parallel Inverters • Synchronous Generator + Inverter

What is the benefit?

7

With Frequency

Restoration

Without Frequency

Restoration

Less maximum frequency deviation

Frequency during Loading transition

Slower frequency variation rate

Less maximum frequency deviation Frequency during Three-phase ground fault cleared in 0.1 s

Contents

• Introduction



• Conclusion

8

Microgrid: Parallel Inverters 9

Sbase1=10kVA

Lf1:1mH

0.0942pu R*line1:0.035pu

L*line1:0.00375puDG1

BK3

Sbase2=5kVADG2

100m 22mm2

(0.14+j0.015)Ω

100m 22mm2

(0.14+j0.015)Ω

Grid

0.0471pu

Cf1:10μF66.31pu

R*line2:0.0175pu

L*line2:0.00188pu

Cf2:10μF33.16pu

BK1

BK2

Load1

Load2

.

.

BUS

MGCC

Vout1 Iout1

Vout2 Iout2

Vbus*_0 iP *

_0 iQ,

Lf2:1mH

2 parallel DGs equipped with VSG control

Fluctuated loading

Microgrid Central Controller (MGCC) Frequency and voltage restoration

SwingEquation Function

PowerMeter

PWM

1/s

Pin

Frequency Detector

Pout

EnergyStorage

DistributedGenerator

PI ++

+

Governor Model

Q DroopVbusQref

E0

ωm

Lf

-

Iout(αβ)Vout(αβ)

abc/αβ

Stator ImpedanceAdjuster

Vbus Estimator

Vout(abc) Iout(abc)

ωg

Vpwm

θpwm

θm

E

Cf BUS

Zline

^Q0

P0

QoutLPF.

. Zls

VSG Control Scheme (Basic Part)

10

Swing Equation

ω–P Droop Controller

V–Q Droop Controller

kp： ω–P droop coef. P0：Set value of active power

kq：V–Q droop coef. Q0：Set value of reactive power

J：Virtual inertia D：Damping factor ωm：Virtual rotor frequency

ωm

P0

Pin+

+pk−

+

-

ω0 (Constant) .

.

Vbus

Q0

Qref+

+qk−

+

-

E0

.

. (Constant)

^

VSG Control Scheme (Enhanced Part)

11


PowerMeter

PWM

1/s

Pin

Frequency Detector

Pout

EnergyStorage


PI ++

+

Governor Model

Q DroopVbusQref

E0

ωm

Lf

-


abc/αβ


Vbus Estimator

Vout(abc) Iout(abc)

ωg

Vpwm

θpwm

θm

E

Cf BUS

Zline

^Q0

P0

QoutLPF.

. Zls

Eθm ρθ/αβ

iout_α

-+

iout_β

αβ/ρθeαeβ

-+

vpwm_α

vpwm_β

Lls

Vpwm

θpwm

ωm×

×

vZls_α

vZls_β

-+

++

Rls

Rls

+

+

Iout +-

Ithresh

0kZ

Lls0

ΔRls = Rls

ΔZls

2)/(1

1

XR+1/ω0

++

LlsΔLls

2)/(1

1

RX+

.

.

Stator Impedance Adjuster

Virtual Stator

Impedance Calculator

iout_α

-+

iout_β

αβ/ρθ-+

Llineωm

×

×

vline_α

vline_β

-+

++Rline

Rline

vout_α

vout_β

vbus_α

vbus_βVbus

^

^^


• Constant part - Transient power sharing - Increased damping

• Transient part - Overcurrent limiting

Bus voltage estimator

Estimate bus voltage for proper reactive power sharing

Control Parameters 12

Parameter Value Parameter Value 10 kVA

5 kVA

200 V 0.0125 pu

376.99 rad/s

8 s 1 pu

17 pu 0.05 s

1 pu 6.39 mH

0 pu 13.81 mH

20 pu 1.79 pu

5 pu 5

Simulation Results Loading Transition

13

Droop Control VSG Control

Reduced frequency deviation

Loading Transition Islanding Loading

Transition Islanding

Simulation Results Three-Phase Ground Fault

14

Droop Control VSG Control

Reduced frequency deviation

3P Ground Fault

3P Ground Fault

Fault Current During Three-Phase Ground Fault

15

Without Transient Stator Impedance With Transient Stator Impedance

Fault Current is limited by Transient Stator Impedance

Eθm ρθ/αβ

iout_α

-+

iout_β

αβ/ρθeαeβ

-+

vpwm_α

vpwm_β

Lls

Vpwm

θpwm

ωm×

×

vZls_α

vZls_β

-+

++

Rls

Rls

+

+

Iout +-

Ithresh

0kZ

Lls0

ΔRls = Rls

ΔZls

2)/(1

1

XR+1/ω0

++

LlsΔLls

2)/(1

1

RX+

.

.


Virtual Stator


16

Oscillation damped

Without Constant Stator Reactance With Constant Stator Reactance

Effect of Constant Stator Reactance

Simulation

Experiment

Oscillation eliminated

Loading Transition Islanding P0_2

Change Loading

Transition Islanding P0_2 Change

Loading Transition

Loading Transition

P0_2 Change

P0_2 Change

Oscillation damped Oscillation eliminated

State-Space Model of Islanded Microgrid

17

Loading Transition

Change of Active Power Set Value

Frequency Active Power

Disturbance:

Output：

• The damping ratio of oscillation in output parameters is determined by the eigenvalues of state matrix A

18

• Total output reactance X increases • ⇒Damping ratio ζ increases

Oscillation can be damped by increasing

output reactance

Relation between Output Reactance and Damping Ratio

Transient Load Sharing 19

• When the disturbance is a loading transition – If the total output reactance of each VSG is of the same

per unit value, poles are cancelled by zeros and oscillation is eliminated

Design of Constant Stator Reactance

20

Eθm ρθ/αβ

iout_α

-+

iout_β

αβ/ρθeαeβ

-+

vpwm_α

vpwm_β

Lls

Vpwm

θpwm

ωm×

×

vZls_α

vZls_β

-+

++

Rls

Rls

+

+

Iout +-

Ithresh

0kZ

Lls0

ΔRls = Rls

ΔZls

2)/(1

1

XR+1/ω0

++

LlsΔLls

2)/(1

1

RX+

.

.


Virtual Stator


21

Without Constant Stator Reactance With Constant Stator Reactance

Effect of Constant Stator Reactance

Simulation

Experiment

Loading Transition Islanding P0_2

Change Loading

Transition Islanding P0_2 Change

Loading Transition

Loading Transition

P0_2 Change

P0_2 Change

Reactive power sharing improved

Reactive power sharing improved

22

ω–P Droop V–Q Droop

If the input of droop controller is equal, reactive power should be shared properly

Cause of Poor Reactive Power Sharing

Bus Voltage Estimator for Proper Reactive Power Sharing

23


PowerMeter

PWM

1/s

Pin

Frequency Detector

Pout

EnergyStorage


PI ++

+

Governor Model

Q DroopVbusQref

E0

ωm

Lf

-


abc/αβ


Vbus Estimator

Vout(abc) Iout(abc)

ωg

Vpwm

θpwm

θm

E

Cf BUS

Zline

^Q0

P0

QoutLPF.

. Zls

iout_α

-+

iout_β

αβ/ρθ-+

Llineωm

×

×

vline_α

vline_β

-+

++Rline

Rline

vout_α

vout_β

vbus_α

vbus_βVbus

^

^^

Bus voltage estimator

Estimate bus voltage for proper reactive power sharing

V–Q Droop Controller kq：V–Q droop coef. Q0：Set value of reactive power

Vbus

Q0

Qref+

+qk−

+

-

E0

.

. (Constant)

^

Contents

• Introduction



• Conclusion

24

Microgrid: SG + Inverter 25

• Stand-alone microgrid in remote area • SG (10 kVA): Round-rotor moved by gas engine • DG (10 kVA): Photovoltaic panels • Load: Three-phase loads and single-phase Loads

Issues of Small Rating SG 26

Parameter Value Parameter Value

200 V 1 pu

10 kVA 0 pu

376.99 rad/s 20 pu

0.16 s 5 pu

20 pu 1 s

0.025 s AVR LPF cut-off frequency 20 Hz

Impedance Model

0.219 pu 0.027 pu 0.01 pu

6.55 s 0.039 s 0.85 s 0.071 s

SG Controller SG Parameters

Poor inertia

Slow governor response

Simulation of Single SG Operation

27

Initial loading : 3P 1 kW, 0.5 kvar Connected loading : 1P 4.8 kW, 2.1 kvar

Rotor frequency decreased rapidly due to poor inertia

Slow governor response cannot restore this frequency drop immediately

Ripples due to 3P unbalance introduced by 1P loading

Control Requirements of DG 28

• 1. Reduce SG rotor frequency deviation – If SG rotor frequency drop below 90%, SG may be unstable – Virtual Synchronous Generator (VSG) Control

• Generate virtual inertia to provide transient frequency support • Share active and reactive power with the SG

• 2. Eliminate Negative-Sequence Current from SG – Prevent SG from overheating and torsional stresses – Active power filter (APF)

• 3. Both 1+2 should be realized in ONE inverter

Proposed Modified VSG Control 29


PWM

1/s

Pin_dg

Pout_dg

EnergyStorage


PI ++

+

Qout_dg

Governor Model

Vout_dg

Zls

Qref_dg

E0

ωm_dg

Lf

-

Vcomp(αβ)

abc/αβ

Iout_dg(αβ)


Vout_dg(abc)

Iout_dg(abc)ωg_dg

Vpwm

θpwmθm_dg

Edg

Cf BUS

Zline

Vout_sg(abc)

Iout_sg(abc)

−)(_ dqsgoutI

SG Neg.-Seq. Compensation

DDSRF

Q Droop

Iout_dg

Q0

P0

LPF

θg_sg

.

.

DDSRF 30


PWM

1/s

Pin_dg

Pout_dg

EnergyStorage


PI ++

+

Qout_dg

Governor Model

Vout_dg

Zls

Qref_dg

E0

ωm_dg

Lf

-

Vcomp(αβ)

abc/αβ

Iout_dg(αβ)


Vout_dg(abc)

Iout_dg(abc)ωg_dg

Vpwm

θpwmθm_dg

Edg

Cf BUS

Zline

Vout_sg(abc)

Iout_sg(abc)

−)(_ dqsgoutI


DDSRF

Q Droop

Iout_dg

Q0

P0

LPF

θg_sg

.

.

ωg

T +Vαβ

-

+

T +2

θg

θg

LPF−

dqfV

−dqV

T –2 LPF+

dqfV

+dqV

T – +

T +Iαβ-

+

T +2

θg

LPF−dqfI

−dqI

T –2 LPF+dqfI

+dqI

T – +

PLL+qv

.

.

• Applied to both DG and SG voltage and current to extract positive and negative sequence components

• Output power, voltage and current of DG are calculated from only positive sequence components

• Prevent ripples due to negative sequence from entering the controller

LPF: 1st order low pass filter (cut-off frequency 40 Hz)

Double Decoupled Synchronous Reference Frame (DDSRF) Decomposition

31



PWM

1/s

Pin_dg

Pout_dg

EnergyStorage


PI ++

+

Qout_dg

Governor Model

Vout_dg

Zls

Qref_dg

E0

ωm_dg

Lf

-

Vcomp(αβ)

abc/αβ

Iout_dg(αβ)


Vout_dg(abc)

Iout_dg(abc)ωg_dg

Vpwm

θpwmθm_dg

Edg

Cf BUS

Zline

Vout_sg(abc)

Iout_sg(abc)

−)(_ dqsgoutI


DDSRF

Q Droop

Iout_dg

Q0

P0

LPF

θg_sg

.

.



Control SG Neg.-Seq. to be 0

Parameter Design of Inverter 32

Parameter Value Parameter Value 200 V 1 pu

10 kVA 0 pu

376.99 rad/s 20 pu

0.16 s 5 pu

8.7 pu 0 s

1.122 mH 0.69 pu

1.836 mH 5

PI Controller for Reactive Power

0.05 pu

PI Controller for SG Neg.-Seq. Compensation

0.1 pu 0.01 s

20 Hz

All parameters in red should be set equal to SG in per unit value to ensure proper transient and steady power sharing

Design of Parameters in blue will be discussed in detail

33

Set equal to SG in order to share transient active power?

Low frequency oscillation indicated by oscillatory conjugated eigenvalues

Constant Virtual Stator Inductance

34

Output reactance of DG and SG should be set equal to share transient power, but how about the value?

Connection of DG

Simulation of 1P Loading Transition

35

Initial loading : 3P 1 kW, 0.5 kvar Connected loading : 1P 4.8 kW, 2.1 kvar

Improved SG rotor frequency deviation

(Frequency Support of VSG)

SG only

SG + VSG

Initial loading : 3P 2 kW, 1 kvar Connected loading : 1P 9.6 kW, 4.2 kvar

(2x of SG only case)

Simulation of 1P Loading Transition

36

Ripples due to unbalance have been eliminated

w/o DDSRF and SG Neg. Seq.

with DDSRF and SG Neg. Seq.

Phase Current 37

w/o DDSRF and SG Neg. Seq.

Balanced SG current

with DDSRF and SG Neg. Seq.

Contents

• Introduction



• Conclusion

38

Conclusion 39

• VSG control can provide inertia support for microgrids, leading to less fluctuant frequency

• Parallel inverters and SG + inverter operations were established, and several related issues were solved

Conclusion

• Operation of Multiple SGs + Multiple inverters

Future Plan

Thank you for your kind attention!

• J. Liu, Y. Miura, H. Bevrani, and T. Ise, “Enhanced virtual synchronous generator control for parallel inverters in microgrids,” IEEE Transactions on Smart Grid., doi: 10.1109/TSG.2016.2521405.

• J. Liu, Y. Miura, T. Ise, J. Yoshizawa, and K. Watanabe, “Parallel operation of a synchronous generator and a virtual synchronous generator under unbalanced loading condition in microgrids,” 8th International Power Electronics and Motion Control Conference (IPEMC-ECCE Asia), Hefei, China, 2016, pp. 3741-3748.

• J. Liu, Y. Miura, and T. Ise, “Power quality improvement of microgrids by virtual synchronous generator control,” 10th Electric Power Quality and Supply Reliability Conference (PQ2016) , Tallinn, Estonia, 2016.

For more details, please refer to:

parallel operation of distributed generators by virtual synchronous

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