Inverters for PhotoVoltaic Systems

© AH, ARJ 2002

Evolution & Applications of Inverters

Square Wave:– mechanical switching– now extinct

Modified Square-Wave

Modified Sine Wave (mid-70’s):– low cost, high reliability, easy service.– Control Average Voltage.– not good for some TV’s, ballasts, laser printers,

energized coils/transformers.

Sine Wave Inverters

Sine wave: Series I and II (early-90’s):– higher cost, added features & complexity– handle surge currents, >2:1– 1st line tie capability, higher efficiency, less

heat, increased size/weight

Added Features• Battery Charging

– full-bridge rectifier– 3-stage advantages

• Power Management– Relays for generator starting, exercise, charge timing,

selling power back to utility– voltage-driven, or clock-driven

• Temperature Management– Heat sinking, variable-speed fan control, high

efficiency is your friend

High Frequency Switching Sine Wave Inverters

• Light-weight and with a very high quality sine-wave.

• Early units did not have the surge capability of the SW inverters.

Line-Tie Inverters

• Sine wave inverters dedicated or modifies to feed power back to the Utility Line.

• Stringent safety requirements to protect the inverter and utility.

• High efficiency, relatively low cost.

Inverter Classifications• Uninterruptable Power/Stand-Alone: Battery powered;

independent of electric utility; can often charge batteries with AC hybrid source.

• Utility-Interactive or Grid-Connected: Must detect the waveform and consume power (nighttime) from the electric utility to function; synchronizes voltage, frequency and phase with utility; controls the PV array directly.

• Stand-Alone with Utility-Interactive : Can operate in conjunction with utility but provide power independently if power fails.

Inverter Comparison

Stand-alone• Inverter creates voltage

waveform.

• Load current may distort waveform or shift phase.– Motor current lags voltage

– Computers demand current at voltage waveform peaks

Utility-interactive• Grid creates voltage

waveform - inverter unlikely to distort.

• Inverter behaves like negative load, feeding current back into grid.

• Loads respond to grid voltage, not inverter current.

Inverter Comparison con’t.

Stand-alone• Inverter has no control

over battery charging via PV, wind, hydro, etc.

• Amount of energy extracted from PV array is a function of battery voltage .

Utility-interactive• Maximum Power Point

Tracking (MPPT) eliminates separate controller & extracts max power from PV array at temperature.

Codes for Inverters

UL 1741 and IEEE 929-2000:• Avoid “islanding” when utility-interactive:

inverter powering loads on disconnected section of grid.

• Avoid dumping power into a short.

• Inverter to restart 5 min. after grid specifications restored (voltage and frequency).

Utility Defined Distributed Generation

Separate, Emergency or Standby Generation- “Non-parallel System”

• No possibility of electrical connection.

• “Break before make” transfer switch required for main service box.

• Or move critical loads onto separate service box.

Interconnected Generation - “Parallel System”

• Connected to electrical bus common with utility.

• “Visible open” disconnect switch required, protective relaying, metering, etc.

• Agreements with utility.

Selection Tree

Power Requirement

Less than 10kW(Residential / Small Commercial

120 / 240 VAC 80A max.)

More than 10kW(Commercial

3 phase, 480 / 208 VAC)

Residential / Small Commercial

Power /Use / Cost Requirement

Sine Wave Modified Sine Wave

Residential / Small Commercial

Sine Wave

Grid-Tie Standalone

Residential / Small Commercial

Grid-Tie

PV Hybrid

Sunny Boy ST/XR SW

Sunny Boy

• High Voltage / high efficiency.

• Can be used in 3-phase circuits.

• Available as 1.8kW and 2.5kW.

Xantrex SunTie XR

• 48VDC Line Tie Inverter

• Built-in system display

• Optional Remote Display

• 1kW, 1.5kW, 2kW & 2.5kW

• Early ST inverters had problems which seem to have been solved in the XR series

Components

SW Series Inverter

• Xantrex /Trace SW Inverter / Battery Charger. Can be used stand-alone or Grid-Tied (with interface)

• This is the workhorse inverter for many standalone systems.

• Available in 4kW and 5.5kW

• Inverters can be combined to provide 240VAC, or stacked in parallel to increase power.

Residential / Small Commercial

Standalone

Charger No Charger

Exeltech ProsineAEI MM-5000

SW

Exeltech

• Innovative design that provides redundancy and upgrade capabilities by adding modules.

• No Battery charging capability.

• Very high quality sine wave.

ProSine Series

• Small, light inverters, often used in the RVs and boats.

Residential / Small Commercial

Modified Sine Wave

UXHeartDR Rail Mount etc.

DR Series Inverter

• Modified Square Wave.

• High Reliability.

• Relatively Cheap.

Commercial

Power

PV Hybrid

AESAdvanced Energy Systems

Australia

Xantrex PV Series

PV Series Inverter

• High Voltage High Power PV only Inverter.

• Available from 10kW – 225kW.

• Requires additional transformer if not behind a Utility fence.

AES Hybrid Installation

Decision Tree