Grid Tie Solar Inverters, types and functions

Inverters are an important part of any solar PV installation that acts as the brain of the system. Although the inverter’s main job is to convert DC power produced by the solar PV modules into AC power, its role is only expanding and they provide a number of other capabilities and services to ensure that the inverter can operate at an optimal performance level.

Inverters enable monitoring so installers and PV power plant owners are increasingly taking on decision-making and control functions. With the growth of solar added with storage (hybrid), inverters are also taking on responsibility for battery management. They provide:

  • data on how the PV power plant is performing;
  • diagnostic information to help O&M team to identify and fix issues.
  • improved grid stability and efficiency.

Types of Grid-tied solar inverters

Inverters are categorised based on their features, capacity and the size of PV power plant installation. There are three common types of grid-tied solar inverters:

  • String Inverters,
  • Central Inverters, and
  • Micro Inverters

The functioning of a solar inverter and various types are well explained in the video by EnergyX: Sustainable Energy: Design A Renewable Future.

Video copyright and property of EnergyX: Sustainable Energy: Design A Renewable Future (Youtube)

1. String Inverters

A string inverter is a type most commonly used in home and commercial solar PV power plants. PV modules are installed in rows, each on a “string.” For example, if you have 25 PV modules you may have 5 rows of 5 modules. Multiple strings are connected to one string inverter. Each string carries the DC power the PV modules produce to the string inverter where it’s converted into AC power consumed as electricity. Depending on the size of the installation, one may have several string inverters and maybe more than one string inverter present.

Common String inverter advantages 

  • Allows for high design flexibility
  • High efficiency
  • Robust
  • 3 phase variations available
  • Low cost
  • Inbuilt Remote system monitoring capabilities
  • Can be easily mounted on the wall

String inverter disadvantages

  • No Module-level Maximum Power point tracking
  • No module-level monitoring.

2. Central Inverters

Central inverters are similar to string inverters but they are much larger and can support more strings of Solar PV Modules. Instead of strings running directly to the inverter, as with string models, the strings are connected together in a common combiner box that runs the DC power to the central inverter where it is converted to AC power.

Central inverters require fewer component connections but require a combiner box. They are best suited for large installations (solar parks, groud mount and large commercial rooftop PV installations) with consistent production across the PV modules array.

Some advantages of a central inverter

  • Low capital price per Wp
  • High efficiency
  • Comparative ease of installation – a single unit with large capacity in comparison to String Inverters

Disadvantages of a central inverter

  • Large Size
  • High Noise
  • A single potential point of entire system failure

3. Micro Inverters

Microinverters are also becoming a popular choice for residential and commercial installations these days. Microinverters are module-level inverters and are installed for each PV module. It consists of a small box installed very close/usually backside of the PV module.

Microinverters convert DC power to AC right at the PV module level and so don’t require a string inverters. Also, because of the module level conversion, if one or more modules are shaded or are performing on a lower level than the others, the performance of the remaining PV modules won’t be impacted.  Microinverters also monitor the performance of each individual module, while string inverters show the performance of each string.  This makes microinverters good for installations with shading issues or where installation demands facing various directions. Systems with microinverters can be more efficient, but these often cost more than string inverters.

Microinverter advantages:

  • PV Module level Maximum Power Point Tracking
  • Increase system availability – malfunctioning of a PV module will not have such an impact on the entire PV array
  • PV module-level monitoring
  • Lower DC voltage, increasing safety. No need for DC cabling and conduits
  • Allows for increased design flexibility, modules can be oriented in different directions
  • Better generation in overshadowed locations, as one shadowed PV module doesn’t impact down a whole PV string

Microinverter disadvantages

  • Higher costs as compared to string inverters
  • Physical data checking via LCD screen is not available
  • Given their positioning in an installation, some micro-inverters may have issues in extreme heat
  • Increased maintenance costs due to there being multiple units in a PV installation.

Grid-tie Inverter Specification

The inverter shall have local LCD (Liquid crystal display) and keypad for monitoring instantaneous parameters, event logs and data logs. The display should be simple and self-explanatory, and should indicate:

  1. Instantaneous DC power input
  2. DC input voltage
  3. DC Current
  4. Instantaneous active AC power output
  5. Instantaneous reactive AC power output
  6. AC voltage (all the 3 phases and line)
  7. AC current (all the 3 phases and line)
  8. Units (kWh) Produced during the entire day
  9. Total kWh produced during its lifetime

The inverter should also have the following minimum protection against various possible faults:

  1. Ground fault monitoring
  2. Gird monitoring
  3. DC surge protection, Type II (if the inverter does not have integrated DC surge arrestors, then the surge arrestors shall be separately installed in the DC junction box on the positive and negative DC inputs)
  4. DC reverse polarity
  5. AC short-circuit capability
  6. Over-voltage and over-current
  7. Anti-islanding (as per IEEE 1547/UL 1741/ equivalent BIS standard)
  8. Manual DC isolator

Common mandatory technical standard compliance for Grid Tie Inverters

 

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