An Inverter System

8/18/2019 An Inverter System

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An Inverter System

DC-DC

vbus

(t )v pv

(t )

Inverter EMI

vac

(t )

d inv

!

iref (t )

PV

d

H 2

H 1 Phase-locked loop

vac

(t )

PWM

+–

Sinusoidunit amplitude

phase-locked to vac

H 3

iac(t )

+ –

Gc3

(s)

Gc2(s)V ref-bus

ibus

PWM

Gc3

(s)

+ –

MPPT

V ref-pv

vbus

(t )

v pv(t )


4/20

Standards

IEEE 1547: standard for connecting a renewable energy source to the utility grid• Current harmonic limit (THD < 5%)• Anti-islanding (detect loss of grid, shut down within 1 sec)

• Disconnection when grid frequency or grid voltage is out of boundsNational Electric Code

UL 1741

Weighted Efficiency standards: California Energy Commission (CEC)

Power level,

% of rated

Weight

100% 0.05

75% 0.53

50% 0.21

30% 0.12

20% 0.05

10% 0.04

• Provides a way to compare productsof different companies

• Weightings reflect typical distributionof array power experienced inCalifornia


5/20

Microinverters

One inverter per panel

•

Mounted on or near the panel—on roof

• MPPT on per-panel basis

•

Conventional AC wiring reduces

Balance-of-system cost

•

Straightforward expandability

• Reliability? Efficiency?

Rated temperature?

Enphase

microinverter,2008

Ascension Tech. microinverter, 1998


6/20

Elements of a Microinverter System

ACutility

Transientprotection

ACdisconnect

Communications

Smart gridComputer

Central box

Roof AC

Microinverter

Power stage

MPPT Currentcontrol

Anti-islanding

Microinverter

Power stage

MPPT Currentcontrol

Anti-islanding

DC-ACInverter

DC-DCConverter

v(t )

i(t )

PVCells

Energy

storage

+

vac

–

iac

PacPdc

iac(t ) t

vac(t )

+

v

–

i

i

v

PacPdc

+

vcap

–

Microinverterpower train:

• DC-DC converter(high boost ratio)

• Energy storage

capacitor

•

Inverter

Rooftop system

• Microinverters

include most or allof grid interface

control

•

Central box


7/20

Microinverter Approaches

H-bridge inverter

Buck converter plus unfolder

Unfolder: similar to bridgerectifier, but power flows inreverse direction. Implementedusing transistors that switch atac line frequency


8/20

Inverter sinewave synthesis approaches

We can employ any of the approaches we have already discussed for PWMrectifier systems:

•

Average current control• Peak current control

• Boundary conduction mode•

Hysteretic control

•

Discontinuous conduction mode control

• Cycle-by-cycle control

(and there are a few we didn’t discuss, most notably harmonic elimination, thatcould be employed for either rectifiers or inverters)


9/20

Synthesizing a Sinusoidal Current:

Boundary Conduction Mode (BCM)

Inductor current waveform, BCM

Loss components at different solarirradiance levels, BCM(300 W, 240 Vac example)


10/20

Discontinuous conduction mode (DCM)

•

Higher conduction loss • Lower switching loss • A net improvement in

CEC efficiency

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Weighted efficiency vs. inductorsize, DCM vs. BCM 300 W, 240 Vac example


11/20

Measured Results: 300 W Microinverter Prototype

CEC Efficiency

CEC

powerlevel

weightaverage AC

power Pac

Average

loss overAC cycle

average AC

efficiency

100 % 0.05 300 W 2.6 W 99.13 %

75 % 0.53 225 W 1.97 W 99.12 %

50 % 0.21 150 W 1.26 W 99.16 %

30 % 0.12 90 W 0.7 W 99.22 %

20 % 0.05 60 W 0.45 W 99.24 %

10 % 0.04 30 W 0.24 W 99.2 %

Overall weighted CEC efficiency = 99.15 %

AC line voltage

Current reference

Filtered inverter current

Instantaneous inductor current


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18/20

Example MPPT: Perturb and Observe

• A well-known approach

• Works well if properly tuned

• When not well tuned, maximum power

point tracker (MPPT) is slow and can

get confused by rapid changes in

operating point

• A common choice: control is switch

duty cycle

Basic algorithm

Measure power Loop:

• Perturb the operating point insome direction

• Wait for system to settle

• Measure power

• Did the power increase?

Yes: retain direction for next

perturbation

N: reverse direction for next

perturbation

Repeat


19/20

Control Issues:MPPT by Perturb-and-Observe

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Typical experimental data

( ) α

( ) α →

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An Inverter System

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