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.

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Erik Hoff, Tore Skjellnes, prof. Lars Norum

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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

NORPIE 2004

Consumers

DC

Energy storage

Renewable energy source

3-phase inverters

Grid

Capacitor

Battery

Sun

=/~

Focus today

Internal communication system

Wind

=/~

External communication system

:

:

Overview of lab setup to be built

NORPIE 2004

Outline of Presentation

Simulation of two 3-phase inverters using a virtual resistor

LC filter dampening

Load sharing

Simulation results

NORPIE 2004

Load

1.3mH

0.3mH

=/~

1mH

65µF

Inverter A

1.3mH

1mH

=/~

Diode rectifier load

65µF

Inverter B

Problem: LC filter resonance

Solution: Negative current feedback

→ Virtual resistor

NORPIE 2004

VC,αβ

IL,αβ

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

NORPIE 2004

Solution: Band-pass filtering

High load problem

Series resistor will give voltage drop!

NORPIE 2004

Still a resistor at 50Hz?

Band pass filtering of inductor current

NORPIE 2004

L

R

VE

VS

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

NORPIE 2004

Pure inductance

Pure resistance

Is,pR

Is,p

Is,q R

IsR

Vs

Is

Is,q

VE

Is,p

Is,q

Is

VE

IsjX

Is,p jX

Vs

Is,qjX

Phasor diagram – Inductance and resistance contributions to load sharing

NORPIE 2004

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

Control assuming an inductive transmission line.

P: 4% frequency droop

Q: 24% voltage droop

+ Good load sharing

- Poor dampening

Control assuming a resistive transmission line

P: (24%) voltage droop

Q: 4% frequency increase

+ Good dampening

- 20% load sharing difference

NORPIE 2004

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)