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How solar production affects power quality

How does the addition of a photovoltaic system affect the power quality of an electrical installation? Frankly, it depends on the details of the installation. But don’t worry – it’s something you can control.

Photovoltaic systems represent the so-called inverter-based type of generators. They consist of photovoltaic panels generating direct current (DC) power and an inverter that continually transforms the DC power into alternating current (AC) power. That inverter is what allows the photovoltaic system to be connected to an AC electrical installation.

Because the photovoltaic system is composed of DC source and electronic equipment, it can indeed be the origin of some power quality issues, such as residual DC current, harmonics or unbalance

Most inverters potentially generate residual DC current

One possible power quality disturbance due to photovoltaic production is the presence of a DC component in the AC circuit.

Photovoltaic inverters may provide a current path through which DC residual current can pass to the AC side of the electrical installation, but this depends upon their technology, specifically as it relates to electrical isolation.

  • Inverters with isolation between the DC side and the AC side do not inject DC residual current on the AC side.
  • Inverters without isolation can pass DC residual current to the AC side, unless specific measures are taken by the manufacturer to prevent this flow.

Most of the photovoltaic inverters available on the market do not have transformers, and thus do not provide isolation between the DC side and the AC side. So, the presence of DC residual currents in installations with photovoltaic production is not unlikely.

Presence of DC component on the AC side? Select the appropriate Residual Current Device

Appropriate protection measures must be taken to avoid damaging equipment and more importantly to avoid safety concerns. In particular, as stated by international standards, residual current devices (RCDs) installed on the AC side of the photovoltaic system must be type B, unless:

  • there is an isolation between the AC and the DC side, or
  • the inverter manufacturer’s instructions indicate that type B is not required

Type B RCDs are used in cases where the application may create smooth DC residual current or contain frequencies higher than 50 Hz. For additional background information on RCD types, see “Why Four Types of Residual Current Devices? It’s all About the Load.” An in-depth explanation of the underlying technology and operation is available in the white paper “Why to Choose Type B Earth Leakage Protection for Safe and Efficient People Protection.”

Solar inverters generate harmonics, although they usually are limited to an acceptable level for the installation

Just like all other forms of electronic equipment, photovoltaic inverters inject harmonics into the electrical installation. And just as other sources of harmonics can lead to overheating and other electrical system problems, so can photovoltaic inverters.

Indeed, the way photovoltaic inverters convert the DC power produced by the solar panels into controlled AC power is by using pulse width modulation switching.

This method allows the control of the magnitude and the frequency of the inverter output and eliminates some low order harmonics. On the other hand, it generates high frequency harmonics.

To limit the injection of these harmonics, photovoltaic inverters are equipped with filters so that the total harmonic distortion (THD) of their output is usually limited to acceptable values for the installation.

Even so, the overall THD in the electrical installation can be higher because the harmonics injected by the photovoltaic inverters are combined with those generated by the loads.

Supervision (and control if needed) of the harmonics in the installation is recommended

Experts recommend various measures to control the harmonics in the electrical installation. If the overall harmonic level exceeds acceptable limits, harmonic mitigation measures such as active or passive filters can be used.

The video below, which is part of series prepared by Schneider Electric’s technical communication group, explains the three main power quality disturbances generated by photovoltaic systems in demand side electrical installations: DC component presence on the AC side, harmonics, and unbalance.

Video

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