Inverters – Central, String and Micro

A solar inverter is a key component in a grid-tied solar power system.  There are 3 types of grid tied inverters – central, string and micro inverters.  This article defines how an inverter works and compares the advantages and disadvantages of each inverter type.

What is an inverter?

An inverter converts the direct current (DC) from a generator into alternating current (AC) for grid and appliance use.

DC power is a unidirectional flow of electrons. As a result, electrons work like a line of ants, marching along with packets of energy. A basic flashlight, for example, runs on DC  power and the electrons march along an unbroken loop linking the DC battery, a lamp, and a switch. 

AC power is the electric supply that comes from the outlet in your wall. Alternating current describes the flow direction of the power.  In practice, the flow switches direction as much as 50–60 times per second (e.g. at a frequency of 50–60 Hz). When you flick on a light switch, all the electrons fill the light’s cable and then vibrate back and forth.  The same vibration takes place on the lamp’s filament, creating heat that makes the bulb glow. The electrons don’t have to run in circle to transport energy: in AC, they simply “run on the spot.”

Inverters interrupt DC flow and create alternating flow.  They use physical or electromagnetic switches that flick on and off at high speed to reverse current direction. Inverters like this produce an output shaped as a square-wave: the current is either flowing one way or the opposite way, instantly swapping between the two states.

Square wave output can be quite brutal for some forms of electrical equipment due to the sudden reversal.  In normal AC power, the current gradually swaps from one direction to the other.  Consequently, the result is AC power with a sine-wave pattern:

So inverters do two things: convert DC to AC and convert the shape of the AC current. Specifically, they use electronic components called inductors and capacitors to make the output current rise and fall more gradually than the abrupt, on/off-switching square wave output you get with a basic inverter.

How does the inverter work?

An old-style mechanical inverter is a simple switching unit connected to a transformer. In a mechanical inverter, an electric motor (or some other kind of switching tool) flips the incoming direct current back and forth in the primary.  Flow switching is achieved by reversing the contacts, and that produces alternating current in the secondary.

For example, in the animation above, DC current feeds into the primary winding (pink zig-zag wires on the left side) of a transformer (brown donut), through a spinning plate (red and blue) with criss-cross connections. As the plate rotates, it also switches over the connections to the primary winding, switching the direction of the flow so the transformer receives AC as its input instead of DC. There is also a step-up transformer with more windings in the secondary (yellow zig-zag, right-hand side) than the primary, so it boosts a small AC input voltage into a larger AC output. The speed at which the disk rotates governs the frequency of the AC output. Most inverters don’t work anything like this; this simply illustrates the concept.

 Advantages of Central Inverters

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The classic system architecture of utility-scale solar has been to locate a central inverter close to the medium voltage transformer at the point of gfrid connection. As a result, more than 70% of all large scale projects in the US rely on central inverters.  The same design is shown at right with the inverter depicted in yellow.

The advantages are simple:

  • lower CapEx cost as measured by $/WAC;
  • reduced CapEx cost as measured by installation labor and relative ease of installation;
  • lower CapEx as measured by balance of system AC wiring;
  • reduced CapEx as measured by replacement costs given longer service life (20 years);
  • lower OpEx as measured by service costs;
  • improved warranty terms typical of larger capital equipment;
  • higher performance efficiency per unit;
  • centralized power conditioning with only one SCADA interface for command and control.

Disadvantages of Central Inverters

The disadvantages of the centralized inverter are straightforward:

  • larger size and more noise;
  • higher CapEx for site preparation and civil engineering1;
  • increased CapEx as measured by balance of system DC wiring;
  • higher CapEx linked to transformers 2;
  • higher OpEx given the single point of failure3;
  • no ability to directly optimize panel or string level power performance
  • higher OpEx linked to long-lead time supply chains;
  • higher DC cable losses
  • obsolescence if factory support ends due to rapid industry change.

tAdvantages of String Inverters

Click to enlarge

Historically, string inverters have been more costly, as shown below. However, relative pricing has narrowed and string inverters accounted for 12% of the market in 2016, up from 5% since 2012. Of the 12% total, the majority were dedicated to residential systems and only only 5% of the utility scale projects used string inverters.  In comparison, over 40% of the large projects in China now use string inverters.

The advantages of string inverters include:

  • no single point of failure
  • lower CapEx in site preparation and civil engineering costs;
  • lower CapEx as measured by balance of system DC wiring;
  • more flexible design options
  • equipment mounting on solar module mounting structures;
  • 3-phase variations available
  • ability to manage string level power performance separately from total performance
  • impact of module mismatches and partial shading is minimized
  • can monitor voltage, current and power by string
  • modestly better power generation throughout the day
  • Increased recognition of performance advantages in bank financing

Disadvantages of String Inverters

The disadvantages of string inverters are primarily related to the number of individual units that are required to create large capacity systems

  • higher CapEx as measured by balance of system AC wiring;
  • increased CapEx as measured by labor installation time (complex wiring)
  • higher OpeX associated with shorter service life, more replacements
  • Risk that fit for purpose replacements may not be available given rapid market change
  • relatively weaker warranty terms
  • high number of control and communication points (SCADA)
  • no panel level monitoring or control
  • increased safety requirements for high voltage

Advantages of Micro Inverters

Click to enlarge

In addition, the micro inverter consists of a small box located on the back of or situated very close to a solar panel. Its role is to manage the DC/AC conversion of a single solar panel, to provide panel level monitoring control.  A simple architecture is shown at right. In general the benefits are performance related, but at much higher cost.  This technology is not yet used in utility-scale solar. and is found on municipal lighting systems and the residential market.

The advantages include:

  • panel level monitoring;
  • panel level power control;
  • increased system availability;
  • lower DC voltage, increasing safety;4
  • allows for increased design flexibility;5
  • increased yield from sites that suffer from overshadowing, as one shadowed module doesn’t drag down a whole string;
  • no need to calculate string lengths – simpler to design systems;
  • ability to use different makes/models of modules in one system, particularly when repairing or updating older systems.

Disadvantages of Micro Inverters

  • CapEx costs as measured by $/WAC can be as high as twice the cost of string inverters;
  • Increased complexity in design and installation;
  • Some micro-inverters may have issues in extreme heat, depending on location;
  • Increased maintenance costs due to there being multiple units in an array.

Show 5 footnotes

  1. for the skids the equipment sit on and the cement foundation under the skids
  2. for galvanic isolation as part of the grid protection strategy
  3. for plant-wide production losses
  4. No need for ~ 600 V DC cabling requiring conduits
  5. modules can be oriented in different directions
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