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Team SBIn a solar power system, the solar inverter converts the generated DC current to AC current, which is ideally in a sinusoidal waveform.
Ideally, the waves generated by inverters are continuously consistent. But due to many practical factors, these waves might not emerge in an ideal pattern as expected. This issue entails harmonic distortion and total harmonic distortion. We will elaborate on it in the following content.
What Is Total Harmonic Distortion in Solar Inverters?
Simply put, harmonic distortion in solar inverters refers to the deviation from the ideal sinusoidal waveform of the electrical voltage and current output by the inverters.
This distortion refers to the individual unwanted harmonics present in a wave or signal, which are frequencies that are integer multiples of the fundamental frequency of the system (e.g., if the fundamental frequency is 60 Hz, harmonics would be at 120 Hz, 180 Hz, etc.).
Though harmonic distortion and total harmonic distortion (THD, for short) are related, they are distinct concepts.
Opposite to harmonic distortion, total harmonic distortion (THD) is a single, aggregate measure of the overall level of harmonic distortion in the signal, factoring in all the individual harmonic components.
It is typically expressed as a percentage, indicating how much of the signal is made up of unwanted harmonics compared to the fundamental frequency. This measurement is publicly disclosed by manufacturers and provides users with an overall measure of the signal’s distortion level.
Now, THD has become an important criterion to assess the quality of solar inverters. The latest IEEE 519-2022 standard provides guidelines for acceptable levels of THD in power systems, including solar inverters.
Understanding the Origins of Harmonic Distortion
The main causes of harmonic distortion IN solar inverters include the switching actions of the power electronic devices (such as IGBTs or MOSFETs) within the inverters. These devices switch on and off rapidly to create an approximation of AC power. This switching process inherently produces non-sinusoidal waveforms that contain harmonic frequencies.
Every solar inverter has a designed total harmonic distortion limit (some may have particular limits for linear and non-linear loads). The THD mirrors the inverter’s capability to regulate harmonic distortion and the maximum amount of harmonic distortion it could potentially output.
However, beyond the hardware and software/algorithm configurations of the inverter, various external factors can negatively impact the inverter's performance and bring about harmonic distortion.
These factors may include:
- Unstable Photovoltaic Power Input: The output of solar power systems can fluctuate, largely attributed to the weather and irradiation conditions. If the input side of a solar inverter is directly connected to the system rather than the energy storage module, fluctuations in the input DC will amplify the inverter's harmonics during the conversion process.
- The Type and Nature of the Load: The load connected to the inverter, depending on its type and nature, can have an indirect effect on the inverter's harmonic distortion. Non-linear loads, which draw a current that is not proportional to the voltage, can more easily increase harmonic distortion in turn. Plus, when the inverter is operating at a low load, it tends to produce more harmonic distortion as the inverter's internal switching circuits are less efficient at low loads.
Furthermore, improper installation or maintenance of the inverter might result in increased harmonic distortion. Finally, when the inverter's electrical components deteriorate over time, harmonic distortion may gradually increase as well.
What Are the Adverse Effects of Harmonic Distortion?
Harmonic distortion in solar inverters can lead to several adverse effects on electrical systems and components they connect to.
Here are some of the main consequences caused by harmonic distortion in inverters:
Decreased Efficiency & Degradated Power Quality
Harmonic distortion can naturally reduce the overall efficiency of the solar inverter, leading to reduced energy output and increased energy losses.
On the other hand, the harmonics can degrade the overall power quality, which refers to the stability and reliability of the electrical power supply. Poor power quality would then cause inefficient operation of electrical equipment and increased operational costs.
Overheating of Electrical Components
Harmonic current generated can cause additional heat generation in electrical components it connects to, such as transformers, cables, and motors. This can lead to overheating, reduced efficiency, and a decreased lifespan of the equipment; this is due to the harmonics and their negative impact on the system.
Harmonic Instability in Nearby Inverters
For those large solar power systems, multiple solar inverters are setup to coordinate the DC-AC conversion process.
The presence of harmonic distortion in one solar inverter can induce harmonic instability in nearby inverters. The harmonic distortion not only affects the productivity of these inverters but also impacts their connected loads.
In some cases, the harmonics in these inverters together, can potentially result in the disconnection of the complete solar generation system.
Compliance Issues
Many utility companies and regulatory bodies have strict limits on the allowable levels of harmonic distortion fed into the grid by solar systems. Exceeding these limits can bring on penalties or the requirement to install additional filtering equipment.
Ways to Mitigate the Impact of Harmonic Distortion
For system owners, the negative impacts of harmonic distortion ultimately erode the ROI of their system, and induce extra losses in the operation of connected loads.
One should bear in mind that a scientific system configuration is fundamental to minimizing the impact of harmonic distortion, which should begin with evaluation of the climate and irradiation conditions of the site.
Ensuring that the inverter is correctly sized and appropriately integrated into the power system can reduce harmonic impacts. This includes careful selection of components and layout to minimize the paths for harmonic propagation.
If budget allows, presciently leveling up the configuration of solar inverters whether for a new project or replacement is recommended. This includes opting for inverters with more advanced semiconductor devices, pulse width modulation (PWM) technology, filters and control algorithms. The use of multilevel inverters with increased levels can also help reduce harmonic distortion.
For existing projects, system owners can consider integrating separated filters, passive filters or active filters, into the system to lower the harmonic distortion level.
Finally, regular monitoring and maintenance are crucial.
Regular monitoring of the harmonic levels in the system can help in early detection of issues, allowing for timely corrective actions before any significant damage occurs. While routine maintenance of both the solar inverters and/or associated filtering systems ensures that all components function optimally and continue to effectively mitigate harmonics
Find Word
Solar inverter is part of a complete solar power system. Any harmonic distortion in inverters can affect their own performance as well as that of their associated devices.
Choosing a solar inverter with low total harmonic distortion (THD) lays the groundwork for maintaining the overall harmonic distortion at an ideal level. It is wise to be aware that investing in a quality inverter means lower risks of potential damage to connected loads.
