14/06/2004 NORPIE 2004 1

Paralleling of 3-phase inverters a part of my PhD study:

"Control and monitoring for distributed power supply“

which is a part of the project "Technologies for Reliable Distributed Generation of Electrical

Power from Renewable Energy Sources”

Erik Hoff,

Tore Skjellnes,

prof. Lars Norum

14/06/2004 NORPIE 2004 2

Overview of lab setup to be built

Capacitor

Battery

Sun

Wind

:

Renewable energy source

=/~

=/~

:

3-phase invertersDC

Energy storage

Grid

Consumers

Internal communication system

External communication system

Focus today

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Outline of Presentation

Simulation of two 3-phase inverters using a virtual resistor

»LC filter dampening»Load sharing»Simulation results

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Problem: LC filter resonance

Inverter A

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Inverter B

1.3mH 0.3mH

1.3mH 1mH

1mH65µF

65µFDiode rectifier load

Solution: Negative current feedback

→ Virtual resistor

Load

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Where to place the virtual resistor?

Two possible locations:– R1 is most common, but expensive

– R2 is used here, because the inductor current IL,αβ

already is available due to overcurrent protection

VE

R2

17

VS

R1

L11.3mH

C165uF IL,αβ

VC,αβ

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High load problem

Series resistor will give voltage

drop!

Solution: Band-pass

filtering

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Band pass filtering of inductor current

Still a resistor at 50Hz?

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Load sharing between two voltage sources

An inductor L is necessary to decrease the disturbance (load current) frequency

A virtual (series) resistor R can be added for improved load sharing

VE VS

L R

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Phasor diagram – Inductance and resistance contributions to load sharing

1 2 1 22 2 2 2

sin( ) cos( )X U U R U UP

R X R X

Vs Is,q R

Is

IsR

Is,pR

Is,p

VE

Is,q

Vs

Is,p jX

Is

IsjX

Is,qjX

Is,q

VE

Is,p

Pure inductance

Pure resistance

1 2 1 2 1 12 2 2 2 2 2

cos( ) sin( )X U U R U U X U UQ

R X R X R X

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Simulations results for traditional (increased gain by 4x) and virtual resistor based load sharing vector

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35-6000

-4000

-2000

0

2000

4000

6000

8000

10000

12000

Time [s]

Pow

er

[W / V

Ar]

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35-4000

-2000

0

2000

4000

6000

8000

10000

Time [s]

Pow

er

[W /

VA

r]

Control assuming an inductive transmission line.

P: 4% frequency droop

Q: 24% voltage droop

+ Good load sharing

- Poor dampening

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35-3000

-2000

-1000

0

1000

2000

3000

4000

5000

6000

Pow

er

[VA

r /

W]

Time [s]0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

-3000

-2000

-1000

0

1000

2000

3000

4000

5000

6000

7000

Time [s]

Pow

er

[W /

VA

r]

Control assuming a resistive transmission line

P: (24%) voltage droop

Q: 4% frequency increase

+ Good dampening

- 20% load sharing difference

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Conclusion

A band pass filter makes it possible to use a virtual resistor in series with the load for LC-filter dampening.– Cheaper and simpler than a virtual resistor in series with the filter

capacitor. The use of a virtual resistor requires rotation of the load

sharing vector.– Good oscillation dampening, but load sharing may be uneven

The traditional control method assuming inductive transmission line has poor active power dampening.– Some special ”tricks” may be used to decrease response time:

Adding a phase angle in addition to the frequency (Patent US6,693,809 by Alfred Engler)

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Thank you for your attention