Parallel Operation of Distributed Generators by Virtual Synchronous ...

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

Parallel Operation of Distributed Generators by Virtual Synchronous Generator Control in Microgrids Jia Liu* and Toshifumi Ise Osaka University, Japan October 21, 2016

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Contents • Introduction • Parallel Inverters • Synchronous Generator + Inverter • Conclusion

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Contents • Introduction • Parallel Inverters • Synchronous Generator + Inverter • Conclusion

Inertial feature of Synchronous Generators Synchronous Generator

4

Kinetic Energy of Rotating Mass Swing Equation

Frequency fluctuation under active power transition limited by the inertia

DC/AC Inverter No intrinsic relation Undesirable frequency dynamics

5

Concept of VSG Control Conventional Droop Control

Load Sharing

Smooth Transition between Islanding and Grid-connection

Swing Equation Emulation Inertia Support

Virtual Synchronous Generator (VSG) Control

A New Concept of Inverter Control in AC Microgrid

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Objectives

Conventional droop control

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

To apply VSG control to DC-AC inverters in microgrids

What is the benefit? Without Frequency Restoration

Slower frequency variation rate

Frequency during Loading transition With Frequency Restoration

Frequency during Three-phase ground fault cleared in 0.1 s

Less maximum frequency deviation

Less maximum frequency deviation

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Contents • Introduction • Parallel Inverters • Synchronous Generator + Inverter • Conclusion

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Microgrid: Parallel Inverters Fluctuated loading 100m 22mm2 (0.14+j0.015)Ω

.

Lf1:1mH

BK1 Load1

0.0942pu DG1 Sbase1=10kVA

Vout1 Iout1

R*line1:0.035pu L*line1:0.00375pu

Cf1:10μF 66.31pu 100m 22mm2 (0.14+j0.015)Ω

Lf2:1mH

BK2 Load2

* Vout2 Iout2 R* line2:0.0175pu L line2:0.00188pu

0.0471pu DG2 Sbase2=5kVA

2 parallel DGs equipped with VSG control

Cf2:10μF 33.16pu

P0*_ i , Q0*_ i

MGCC

Vbus BUS

Grid

BK3

.

Microgrid Central Controller (MGCC) Frequency and voltage restoration

VSG Control Scheme (Basic Part) .

V^ bus

E0 (Constant) Q0 + − kq + +

Qref

Swing Equation

.

V–Q Droop Controller .

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

Q0 ^ Vbus Q Droop

Distributed Generator Qref + -

Zls

Energy Storage PI +

+ E0

Governor Pin Swing Model Equation ω 1/s m P0 ωg Function Vbus Estimator LPF

Zline

Lf Vout(abc)

Cf

Iout(abc)

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

BUS

E Stator Vpwm PWM Impedance Adjuster θpwm θm

Vout(αβ)

abc/αβ Iout(αβ)

Frequency Detector Pout Qout

Power Meter .

.

ωm

10

ω0 (Constant) P0 + − kp + +

ω–P Droop Controller Pin .

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

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VSG Control Scheme (Enhanced Part)

Stator Impedance Adjuster

.

Distributed Generator Q0 ^ Vbus Q Droop

Qref + -

Zls

Energy Storage PI +

+ E0

Governor Pin Swing Model Equation ω 1/s m P0 ωg Function Vbus Estimator

Lf Vout(abc)

Cf

Iout(abc)

.

1 BUS

Iout + 0 Ithresh

E Stator Vpwm PWM Impedance Adjuster θpwm θm

Vout(αβ)

Iout(αβ)

θm

Power Meter .

vout_α + vout_β iout_α ωm iout_β

Rline ×

+ -

v^ bus_β

Rline

ρθ/αβ

ωm iout_β

+ - vline_α

eβ

+ vZls_α -

Rls ×

Stator Impedance Adjuster

Lls × Rls

+ vline_β +

Estimate bus voltage for proper reactive power sharing

Virtual ΔRls = Rls Stator Impedance Calculator ΔLls+ Lls 1/ω0 + Lls0

vpwm_α Vpwm + + + + vpwm_β αβ/ρθ θpwm -

+ vZls_β + .

•

Bus voltage estimator

Lline ×

Vbus

1 + (R / X )2

eα

^

αβ/ρθ

1 + ( X / R) 2 1

E

iout_α

v^ bus_α

kZ

ΔZls

abc/αβ

Frequency Detector Pout Qout

LPF

Zline

•

Constant part ‐ Transient power sharing ‐ Increased damping Transient part ‐ Overcurrent limiting

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Control Parameters Parameter

Value

Parameter

Value

10 kVA 5 kVA 200 V

0.0125 pu

376.99 rad/s 8s

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

Droop Control

VSG Control

Reduced frequency deviation

Islanding

Loading Transition

Islanding

Loading Transition

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Simulation Results Three-Phase Ground Fault Droop Control

VSG Control

Reduced frequency deviation

3P Ground Fault

3P Ground Fault

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Fault Current During Three-Phase Ground Fault Without Transient Stator Impedance

With Transient Stator Impedance

.

1

Iout + 0 Ithresh

kZ

iout_α ωm iout_β

1 + (R / X )2

eα ρθ/αβ

eβ

vpwm_α Vpwm + + + + vpwm_β αβ/ρθ θpwm -

Fault Current is limited by Transient Stator Impedance

+ vZls_α -

Rls ×

Stator Impedance Adjuster

Lls × Rls

1 + ( X / R) 2 1

E θm

ΔZls

Virtual ΔRls = Rls Stator Impedance Calculator ΔLls+ Lls 1/ω0 + Lls0

+ vZls_β + .

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Effect of Constant Stator Reactance Without Constant Stator Reactance

With Constant Stator Reactance

Simulation Oscillation eliminated

Islanding

Loading Transition

P0_2 Change

Islanding

Loading Transition

Oscillation damped P0_2 Change

Experiment Oscillation damped

Oscillation eliminated Loading Transition

P0_2 Change

Loading Transition

P0_2 Change

State-Space Model of Islanded Microgrid

Loading Change of Active Transition Power Set Value Disturbance: Frequency

Active Power

Output：

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

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Relation between Output Reactance and Damping Ratio

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

Oscillation can be damped by increasing output reactance

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Transient Load Sharing

• 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

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Design of Constant Stator Reactance

.

1

Iout + 0 Ithresh

kZ

iout_α ωm iout_β

1 + (R / X )2

eα ρθ/αβ

eβ

vpwm_α Vpwm + + + + vpwm_β αβ/ρθ θpwm -

+ vZls_α -

Rls ×

Stator Impedance Adjuster

Lls × Rls

1 + ( X / R) 2 1

E θm

ΔZls

Virtual ΔRls = Rls Stator Impedance Calculator ΔLls+ Lls 1/ω0 + Lls0

+ vZls_β + .

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Effect of Constant Stator Reactance Without Constant Stator Reactance

Simulation

With Constant Stator Reactance

Reactive power sharing improved

Islanding

Loading Transition

P0_2 Change

Experiment

Islanding

Loading Transition

P0_2 Change

Reactive power sharing improved

Loading Transition

P0_2 Change

Loading Transition

P0_2 Change

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Cause of Poor Reactive Power Sharing

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

ω–P Droop

V–Q Droop

Bus Voltage Estimator for Proper Reactive Power Sharing .

V^ bus

E0 (Constant) Q0 + − kq + +

.

Qref

Q0 ^

.

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

Distributed Generator

Vbus

Qref + Q Droop -

Zls

Energy Storage PI +

+ E0

Governor Pin Swing Model Equation ω 1/s m ω g Function P0 Vbus Estimator

Zline

Lf Vout(abc)

Cf

Iout(abc)

BUS

E Stator Vpwm PWM Impedance Adjuster θpwm θm

abc/αβ

Vout(αβ)

Iout(αβ)

Frequency Detector Pout Qout

LPF

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Power Meter .

vout_α + vout_β iout_α ωm iout_β

Rline ×

v^ bus_α + -

v^ bus_β

^

αβ/ρθ

+ - vline_α

Bus voltage estimator

Lline × Rline

Vbus

+ vline_β +

Estimate bus voltage for proper reactive power sharing

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Contents • Introduction • Parallel Inverters • Synchronous Generator + Inverter • Conclusion

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Microgrid: SG + Inverter

• • • •

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

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Issues of Small Rating SG SG Controller

SG Parameters 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

1s

0.025 s

AVR LPF cut-off frequency

20 Hz

Impedance Model

Poor inertia

0.219 pu 6.55 s

0.027 pu 0.039 s

Slow governor response

0.01 pu 0.85 s

0.071 s

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Simulation of Single SG Operation Initial loading : 3P 1 kW, 0.5 kvar Connected loading : 1P 4.8 kW, 2.1 kvar Ripples due to 3P unbalance introduced by 1P loading Slow governor response cannot restore this frequency drop immediately Rotor frequency decreased rapidly due to poor inertia

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Control Requirements of DG • 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

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Proposed Modified VSG Control .

Distributed Generator Q0 Vout_dg Q Droop

Zls

Energy Storage

Qref_dg + -

PI

Cf

BUS

+ + E0

Edg

Stator Vpwm PWM Impedance Adjuster θpwm

Governor Pin_dg θm_dg Swing Model 1/s Equation ω m_dg Vcomp(αβ) Function P0 SG Neg.-Seq. Compensation Pout_dg ωg_dg − I out _ sg ( dq) θg_sg

Iout_dg Iout_dg(αβ)

Iout_dg(abc) Vout_dg(abc)

DDSRF LPF

Zline

Lf

abc/ Iout_sg(abc) αβ Vout_sg(abc)

Qout_dg .

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.

Distributed Generator Q0 Vout_dg Q Droop

Zls

Energy Storage

Qref_dg + -

PI

Cf

DDSRF

BUS

+ + E0

Edg

Stator Vpwm PWM Impedance Adjuster θpwm

Governor Pin_dg θm_dg Swing Model 1/s Equation ω m_dg Vcomp(αβ) Function P0 SG Neg.-Seq. Compensation Pout_dg ωg_dg − I out θg_sg _ sg ( dq)

Double Decoupled Synchronous Reference Frame (DDSRF) Decomposition

Iout_dg Iout_dg(αβ)

θg ωg

.

Iout_dg(abc) Vout_dg(abc)

DDSRF LPF

Zline

Lf

Vαβ

abc/ Iout_sg(abc) αβ Vout_sg(abc)

T

+

Qout_dg

+ T

.

• 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

Iαβ

T

–

T

+

− Vdqf

–2

+ Vdqf

+ -

LPF

+

Vdq− I dq+

T +2

T T

LPF

+

θg –

Vdq+

+2

θg T

PLL

vq+

–2

− I dqf

+ I dqf

LPF

LPF

I dq− .

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

SG Neg.-Seq. Compensation .

Distributed Generator Q0 Vout_dg Q Droop

Zls

Energy Storage

Qref_dg + -

PI

Cf

BUS

Stator Impedance Adjuster

+ + E0

Edg

Stator Vpwm PWM Impedance Adjuster θpwm

Governor Pin_dg θm_dg Swing Model 1/s Equation ω m_dg Vcomp(αβ) Function P0 SG Neg.-Seq. Compensation Pout_dg ωg_dg − I out θg_sg _ sg ( dq)

Iout_dg Iout_dg(αβ)

Iout_dg(abc) Vout_dg(abc)

DDSRF LPF

Zline

Lf

abc/ Iout_sg(abc) αβ Vout_sg(abc)

Qout_dg .

SG Neg.-Seq. Compensation

Control SG Neg.-Seq. to be 0

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Parameter Design of Inverter Parameter

Value 200 V

Parameter

Value 1 pu

10 kVA

0 pu

376.99 rad/s

20 pu

0.16 s

5 pu

8.7 pu

0s

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

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

Low frequency oscillation indicated by oscillatory conjugated eigenvalues

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

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Simulation of 1P Loading Transition Initial loading : 3P 1 kW, 0.5 kvar Connected loading : 1P 4.8 kW, 2.1 kvar SG only

Improved SG rotor frequency deviation (Frequency Support of VSG)

SG + VSG

Initial loading : 3P 2 kW, 1 kvar Connected loading : 1P 9.6 kW, 4.2 kvar (2x of SG only case)

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Simulation of 1P Loading Transition

w/o DDSRF and SG Neg. Seq.

with DDSRF and SG Neg. Seq.

Ripples due to unbalance have been eliminated

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Phase Current

w/o DDSRF and SG Neg. Seq.

with DDSRF and SG Neg. Seq.

Balanced SG current

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Contents • Introduction • Parallel Inverters • Synchronous Generator + Inverter • Conclusion

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Conclusion Conclusion • 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 Future Plan • Operation of Multiple SGs + Multiple inverters

Thank you for your kind attention!

For more details, please refer to: • 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.