After this overview of the solar inverters and their topologies, it is important to look at the various parameters and characteristics of this technology. The choice of the inverters' topology for implementation depends entirely on the system needs, size, and the budget. While choosing an inverter for your PV system, what are the requirements for a good solar inverter?
Inverters are designed to operate within a voltage range, which is set by the manufacturer's specification datasheet. In addition, the datasheet specifies the maximum voltage value of the inverter. Both the maximum voltage value and operating voltage range of an inverter are two main parameters that should be taken into account when stringing the inverter and PV array. PV designers should choose the PV array maximum voltage in order not to exceed the maximum input voltage of the inverter. At the same time, PV array voltage should operate within the input voltage range on the inverter to ensure that the inverter functions properly.
Aside from the operating voltage range, another main parameter is the start-up voltage. It is the lowest acceptable voltage that is needed for the inverter to kick on. Each inverter has a minimum input voltage value that cannot trigger the inverter to operate if the PV voltage is lower than what is listed in the specification sheet.
Why is start-up voltage different from the minimum operating voltage for an inverter?
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Power electronics switching devices need slightly more voltage to kick on when they start up in the morning. However, they are designed to allow lower voltage once they are in “ON” mode, and that is what we mean by the minimum operating voltage range.
As power is processed and converted from one shape to another, the solar inverters are expected to perform these tasks with the highest possible efficiency. This is because we wish to deliver maximum PV generated power to the load or the grid. Typical efficiencies are in the range of more than 95% at rated conditions specified in the datasheet.
Inverter efficiency is discussed in EME 812 (11.5. Efficiency of Inverters).
Depending on the topology, most modern inverters have built-in MPP trackers to insure maximum power is extracted from the PV array. Each inverter comes with a voltage range that allows it to track the maximum power of the PV array. It is recommended to match that range when selecting the inverter and the PV array parameters.
Inverter MPPT is discussed in EME 812 (11.3 DC/DC Conversion).
In most applications, the solar inverters are exposed to ambient conditions such as solar radiation, temperature, and humidity. Inverters must comply with the conditions of the location to make sure they can work under ambient conditions listed in the specification sheet.
Since grid-tied inverters pump power into the grid, they are expected to maintain a very high quality of power to guarantee that the acceptable power flows into the grid. For that reason, inverters are expected to have a very low harmonic content on the line currents. Furthermore, grid-tied inverters are expected to have active islanding detection capability per IEEE 1547.
Islanding refers to the situation in which the inverters in a grid-tied setup continue to inject power from the PV system even though the power from the grid operator has been restricted due to fault of scheduled maintenance. Due to safety concerns, islanding needs to be prevented. Therefore, inverters are expected to detect and respond immediately by switching their output so that no more power flows into the grid. This is also referred to as anti-islanding capability.
Inverter grid features are discussed in EME 812 (11.4. Grid connection and role of inverters).
There is also ongoing work to increase the lifespan of the inverter. A good inverter will probably reach, under favorable conditions, around 10-12 years of lifetime. This is a bottleneck in the PV system lifetime, especially considering the fact that PV modules can last over 25 years.
What are the solutions to the lower inverter lifetime when compared to the PV module lifetime?
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ANSWER: Inverters need to be changed two to three times during the lifetime of the PV system. More research is being conducted to push inverters' lifetimes to longer periods.
Each inverter comes with a maximum recommended PV power, or sometimes is referred to as "DC-AC Capacity factor," which is defined as the percentage of DC power over the inverter's max power. We will use "DC to AC ratio" when we refer to this specific term throughout this calss.
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