With the ongoing focus on reducing emissions from energy production and use, renewable energy is being deployed at an increasing rate worldwide. Broadly speaking, renewable energy can be described as either centralized large-scale plants (e.g. wind farms, geothermal, large-scale hydro, marine, solar thermal, bioenergy) or distributed generation (e.g. photovoltaic, small-scale wind, micro-hydro, small-scale bioenergy). Distributed generation (DG) connects to the distribution network and can help defer network augmentation, reduce line losses and, being smaller, is more modular and so can be gradually installed as required. Of the distributed generation technologies, micro-hydro and bioenergy have very constant and predictable energy output and small-scale wind is relatively rare, and so they have little impact on the distribution network.

Where small-scale wind is used at higher penetrations, such as on remote mini-grids, well developed technologies such as battery storage and diesel generator backup are currently used. Photovoltaics on the other hand, are being rapidly deployed at an increasing rate and are often based on a source of energy that can fluctuate daily, hourly and even over shorter periods. Photovoltaics can also have significant power quality impacts if appropriate measures are not implemented. Nonetheless, the grid impacts discussed in this report capture all those that other DG technologies (such as wind) are likely to have. The potential impacts include voltage fluctuations, voltage rise and reverse power flow, power fluctuation, impacts on power factor, frequency regulation and harmonics, unintentional islanding, fault currents and grounding issues.

As a result of these potential problems, in countries where DG has been deployed at increasing levels of penetration, utilities as well as governments have developed standards to which DG systems are required to conform. These standards have often been developed as and when required, either by utilities themselves or in collaborative efforts between utilities, governments and industry associations.

This report found that the threshold where problems occur depends heavily on the configuration of the network, length of lines involved (and hence impedances) and the concentration and time dependence of the load and generation in the area. When penetration of DG rises above the minimum threshold to moderate levels of penetration (typically 10-20% of connected load) more significant issues arise in the network. More DG may be accommodated by making changes to the network such as minimizing VAr flows, power factor correction, increased voltage regulation in the network and careful consideration of protection issues such as fault current levels and ground fault over-voltage issues. In many countries, the level of penetration is already at this middle stage and significant network modification is under consideration to allow expansion of DG without taking the next significant step of major design and infrastructure change.

At high levels of penetration, a point is reached (which is very network dependent) where significant changes have to be made to accommodate these higher levels of DG. This will probably require significant overall design and communications infrastructure changes to accommodate coordinated protection and power flow control. This third stage is very much in the research area and, although there are a number of communications protocols developed for distributed generation, the use, coordination and the design philosophy behind this are very much under research and development, the microgrid concept being one example. The full use of microgrids within the wider electricity network is again still very much in the research and development stage.

To see recommendations and a full review of renewable energy and DG, download the full report.