Inverters play a central role in photovoltaic systems, whether it’s a 4kW domestic system or a 10-MW solar park. PV Inverter’s function is to “invert” the direct current (DC) output from the PV modules into alternating current (AC) for powering appliances in the home and commercial premises.

When installing an inverter it is important to consider the size of the PV array, the type of modules, orientation and pitch, location and potential shading problems.

There are two main types of inverters, String inverters or Micro inverters

String Inverters

A string inverter is the type most commonly used in home and commercial solar power systems. Depending on the size of the installation, there may be more than one string inverter required.

The Diagram above shows how a string inverter is stringed in series with PV modules

How does the string inverter work?

Solar panels are connected in a series (string connection) which increases the voltage across the inverter connection, higher DC voltages increase the efficiency of the inverter and the total energy generated when converted from DC to AC current through the inverter. There maybe one or several strings, depending on the current and voltage parameters of the inverter used.  

The main limitation of a string inverter is the overall module performance as the strings output is limited by the lowest performing solar module, however string inverters are the most cost effective method of installing PV systems. 

Summary of Advantages and Disadvantages of string inverters

Advantages

·         Low cost

·         High Efficiency

·         High design flexibility

·         Robust

·         Inverter level Monitoring options

Disadvantages

·         Strings performs to the lowest performing panel

·         Performance loss due to module mismatching and shading

·         No Module Level Monitoring

·         High Voltage levels

Additionally there is the option of using string inverters in conjunction with an optimiser such as Tigo which prevents the performance issues of module mismatching and shading and also offer module level monitoring. Recently modules such as the Jinko Smart module have been designed with the Optimiser built in thus substantially reducing the cost of the system.  

Micro inverters

Micro inverters are smaller inverters that are attached to each individual panel or in some manufacturer cases, one per two modules. They convert the DC directly to AC for each individual module. This has several advantages as it prevents problems such as module mismatching and shading issues affecting the performance of the system. The have the added advantage of also offering module level monitoring.

A diagram of the stringing of a microinverter system

Micro Inverter Advantages summary:

·         Increased system power output

·         Module Level monitoring

·         Increased Safety no DC current running into the property

·         Flexibility of system design

·         Retrospective expandable systems

·         Reliability of system

Disadvantages:

·         Cost

·         Increased maintenance costs due to there being multiple units in an array.

Multi Point Power Tracking (MPPT)

Common to both string and micro inverters is the MPPT control – this is the operating parameters of the inverter voltage and at which values it will convert DC to AC voltage. MPPT is control usually operated between upper and lower voltages beyond which the inverter switches off. Therefore it is important that when sizing any inverter it will operate within the string voltage without switching off prematurely.

Summing up

No PV system will work without an inverter; the choice is usually down to cost or flexibility. Each inverter type is designed to provide solutions to different issues on installation but the choosing the correct inverter is paramount to an efficient system.

Warranty is also worth consideration and 5 years is not uncommon with the possibility of extension to 10 years.