Fall 2009

Module 8

Inverters

Inverters

• Converts the DC power produced by the solar modules into AC power used on the utility grid

History

• Inverters have been the weaker link in PV systems

• Early inverters were unreliable

• Early inverters were inefficient which penalized an entire system

Inverter Types

• Square Wave

• Modified square wave

• Sine wave

Square wave

• Step-function or “square” alternating current output– Considerable harmonic distortion

• For small resistive heating loads

Modified Square Wave

• Complex circuits used to switch DC into AC.

• More appropriate for operating a wide variety of appliances

• Still noise will be picked up by sensitive equipment

Sine Wave

• High quality wave form

• Most common inverters today

• Little harmonic distortion

The wave…comparison

Inverter manufacturers

• SMA (Sunny Boy)

• Fronius

• Xantrex

• Outback

• PV Powered

• Kaco

• Enphase – Micro Inverters

The Guts

Some examples

Enphase Micro-Inverter

All Installed

Installing SMA

Optional Inverter Features

• Remote control operation and data-monitoring

• Load transfer switch-allows system to meet critical loads during power failure

• Series and Parallel connectivity for multiple inverters

• Individual module performance monitoring

Ideal Inverter Features – now standard

• Maximum power point tracking (MPPT)

• Low harmonic distortion

• Near perfect Sine wave shape

• Ground Fault Protection

• AC/DC disconnects

• Weatherproof enclosure

Specifying an inverter

• Grid Tied:Watts Output AC-computed STC rated watts of PV panels

• Stand-Alone: based on peak load

• Input Voltage-usually between 75 and 480 VDC, called the window or range– Insure that the voltage will not drop or exceed

this range because of climatic conditions or inverter will not function

• Output Voltage:120, 240, 208, 480

• Frequency: U.S. is 60Hz

Specifying an inverter

• Grid Tied:Watts Output AC-computed STC rated watts of PV panels

• Stand-Alone: based on peak load

• Input Voltage-usually between 75 and 480 VDC, called the window or range– Insure that the voltage will not drop or exceed

this range because of climatic conditions or inverter will not function

• Output Voltage:120, 240, 208, 480

• Frequency: U.S. is 60Hz

Grid Tied Inverter Sizing

• Sized based on the amount of watts we pass through the system at any one time– Size of inverter is the labeled watts

• an SMA 3000 is a 3000 watt inverter– The most power that will pass through this inverter is 3000

watts– If the output is 240 volts, the current will be 12.5 amps

» 3000W ÷ 240v = 12.5 amps

String Sizing Software

And sized by…

• PV system component package (grid tied)

• The Grid Tied PV system will be based on other factors– Budget– Roof Size– Rebate/Tax incentives