An Introduction to Inverters for Photovoltaic (PV) Applications

Inverters belong to a large group of static converters, which include many of today’s devices able to “convert” electrical parameters in input, such as voltage and frequency, so as to produce an output that is compatible with the requirements of the load. 

Generally speaking, inverters are the devices capable of converting direct current into alternating current and are quite common in industrial automation applications and electric drives. The architecture and the design of different inverter types changes according to each specific application, even if the core of their main purpose is the same (DC to AC conversion). 

This article introduces the architecture and types of inverters used in photovoltaic applications.

 

Standalone and Grid-Connected Inverters

Inverters used in photovoltaic applications are historically divided into two main categories: 

1. Standalone inverters
2. Grid-connected inverters

Standalone inverters are for the applications where the PV plant is not connected to the main energy distribution network. The inverter is able to supply electrical energy to the connected loads, ensuring the stability of the main electrical parameters (voltage and frequency). This keeps them within predefined limits, able to withstand temporary overloading situations. In this situation, the inverter is coupled with a battery storage system in order to ensure a consistent energy supply. 

Grid-connected inverters, on the other hand, are able to synchronize with the electrical grid to which they are connected because, in this case, voltage and frequency are “imposed” by the main grid. These inverters must be able to disconnect if the main grid fails in order to avoid any possible reverse supply of the main grid, which could represent a serious danger. 

 

Figure 1 - Example of Standalone system and Grid-connected system.  Image courtesy of Biblus.  

 

Nowadays, the difference between standalone and grid-connected inverters is not as evident because many solar inverter are designed to work in both standalone or grid-connected conditions. In fact, some distribution system operators (DSO) allow, or even require, specific generators to stay active in the case of grid failure in order to supply energy to a specific area or load. This situation is called “island operation mode” and actually falls in the conditions described for the standalone application. 

 

PV Inverter Architecture

Let’s now focus on the particular architecture of the photovoltaic inverters. There are a lot of different design choices made by manufacturers that create huge differences between the several inverters models. Knowing this, we will present the main characteristics and common components in all PV inverters. 

Figure 2 shows the very simple architecture of a 3-phase solar inverter.

 

Figure 2 - Three-phase solar inverter general architecture 

 

The input section of the inverter is represented by the DC side where the strings from the PV plant connect. The number of input channels depends on the inverter model and its power, but even if this choice is important in the plant design, it does not affect the inverter operation. So let’s suppose, for the moment, that all the strings are coupled before the inverter with a pre-parallel box and the inverter has just two inputs: + and -. 

 

MPPT Converter

The first important area to note on the inverter after the input side is the maximum power point tracking (MPPT) converter. MPPT converters are DC/DC converters that have the specific purpose of maximizing the 1 power produced by the PV generator. Note that this specific device converts the characteristic of the electrical parameters at the input in the desired ones (typically it increases or decreases the input voltage) keeping them always in the direct current mode. In fact, the PV module’s power largely depends on the climatic conditions of the site (mainly irradiance and temperature). 

Each PV module (or string) can be characterized by an I-V curve (seen in Figure 3) where it is possible to determine the maximum power conditions (Imp, Vmp). As a standard rule, this curve is available in each PV module’s datasheet and is calculated according to the Standard Test Condition, STC: (1000 W/m2, 25 °C, IAM 1.5). To better understand IAM, read How Radiation and Energy Distribution Work in Solar PV.

Figure 3 - Example of I-V curve of a PV module. Image courtesy of PVEducation. 

 

As soon as temperature and irradiance differ from those of the STC, voltage and current change, resulting in I-V curves different from those of the STC. Figures 4 and 5 show how the I-V change according to temperature and irradiance. Obviously the maximum power point will also change, so the MPPT algorithm always looks for this point in order to maximize the power output. 

 

Figure 4 - I-V curve at different temperatures. Image courtesy of PV Education. 

 

Figure 5 - I-V curve and Power curve at different irradiations. Image courtesy of PV Education. 

 

The Perturb and Observe Method

The most common method to achieve the MPPT algorithm’s continuous hunting for the maximum power point is the “perturb and observe” method. Basically, with a predefined frequency, the algorithm perturbs the working conditions by changing the voltage and then checks if the new operating point actually corresponds to a higher power. If so, it continues in the same way by changing the voltage. If it is an increase, it keeps trying to increase. Otherwise, it comes back to the previous operating point. It’s continuous, very fast tracking and each voltage change is very small (less than 1V). 

 

Inverter Conversion Bridge 

Next, we find the “core” of the inverter which is the conversion bridge itself. There are many types of conversion bridges, so I won’t cover different bridge solutions, but focus instead on the bridge’s general workings. 

In Figure 2, a three-phase inverter is represented, and from each “leg” of the bridge are two switching devices, commonly MOSFET or IGBT — nowadays, 3 IGBT is the most popular solution for solar inverters. Control logic governs the switching behavior of the IGBT in such a way as to produce DC to AC conversion. The most common switching strategy for producing a sinusoidal waveform from a DC signal is pulse width modulation (PWM). 

 

The Inverter Filter

The last section of the inverter is the filter section, designed to compensate for the harmonic content produced by all the previous sections and clean up the output waveform. The switching of the IGBT is the main source of harmonics. It introduces waveforms at a higher frequency than the fundamental. 

 

How to Choose the Proper Solar Inverter for a PV Plant 

In order to couple a solar inverter with a PV plant, it’s important to check that a few parameters match among them.

 

Once the photovoltaic string is designed, it’s possible to calculate the maximum open-circuit voltage (Voc,MAX) on the DC side (according to the IEC standard). So, the first important check consists of verifying that the maximum open-circuit voltage that the inverter can tolerate is higher than the one produced by the PV field: 

V OC, MAXPV < V OC, MAXINV 

The second important check is the short circuit current match. It’s important to ensure that the maximum short circuit current of the PV field is lower than the maximum current allowed by the inverter. This rule is valid for each inverter input. 

ISC, MAXPV < IDC, MAXINV 

The last two important checks are related to the MPPT algorithm. This algorithm works in a predefined voltage range. In order to maximize the yield, it’s important to check that the maximum and minimum PV voltage at the MPP conditions (according to the site’s climatic conditions) stay within the MPPT voltage range. If that does not happen, the inverter will still work but the plant will not maximize its production. 

V MPP, MAXP V ≤ V MPPT, MAXINV 

V MPP, MIN PV ≥ V MPPT, MININV 

 

Checking Inverter Efficiency

Finally, it’s important to check the overall efficiency of the inverter. Nowadays, the efficiency of the inverters on the market is very high and some manufacturers declare values around 99%, while more common values are between 97%-98%. However, defining efficiency as a single peak value is not completely correct. The true efficiency depends on the load and the temperature. For this reason, it’s common to find the following three characteristics in an inverter’s datasheet: 

1. Peak efficiency 
2. Euro efficiency 
3. CEC efficiency (California Energy Commission) 

The peak efficiency corresponds to the efficiency at the maximum inverter power and is usually the nominal value in the datasheet. Euro and CEC efficiency take into consideration the different load conditions of the inverter according to specific site conditions — the continental European climate (for the Euro efficiency) and the climate in the southwest US regions (for the CEC efficiency). So, basically both approaches weigh the efficiency of the inverter at specific load conditions according to the times the inverter will work at that condition in that specific site. 

There are excellent resources available to further understand the formulas for Euro and CEC efficiency. These standard methods to calculate the overall inverter efficiency provide a much more realistic value than the stated peak efficiency.