Power quality - impact of solar self-consumption

From Electrical Installation Guide

Photovoltaic systems create some limited disturbances in the electrical installation, coming mainly from the operation of the solar inverters.

Those are:

Photovoltaic systems are 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 and harmonics. Their integration in the electrical installation, if not done correctly, can also be a cause of unbalance or power factor degradation.

Harmonics

Solar inverters generate harmonics, usually limited to an acceptable level for the building electrical installation

Like all other forms of electronic equipment, photovoltaic inverters inject harmonics into the electrical installation. To convert the DC power produced by the solar panels into controlled AC power, photovoltaic inverters use pulse width modulation switching. This method allows the control of the magnitude and the frequency of the inverter output and eliminates 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 mitigation if needed) of the harmonics in the installation is recommended

It is recommended to measure the overall harmonics level in the electrical installation. If it exceeds the acceptable limits, harmonic mitigation measures such as active or passive filters can be used.

Fig. P45 – Screenshot of a power monitoring system providing a status on the main power quality disturbances, based on real-time measurements

Unbalance

Single phase PV inverters can generate unbalance if not connected evenly across the three phases

Unbalance can be generated if single phase photovoltaic inverters are connected to a three-phase electrical installation. To avoid unbalance, it is recommended to connect the photovoltaic inverters evenly across the three phases, as shown in the example below.

Residual DC Current

Solar inverters may 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. This depends on the inverters technology regarding 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.

Select the appropriate Residual Current Device for photovoltaic applications

Appropriate protection measures must be taken to avoid damaging equipment and more importantly to avoid safety concerns.

In particular, as stated by IEC 60364-7-712, 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 Residual_Current_Devices_(RCDs)

Fig. P47 – Residual current device type B for photovoltaic application
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