Grid Connected Photovoltaics

Grid Connection of PV Power Systems

Conclusions

Utility-interactive PV power systems mounted on residences and commercial buildings are likely to become a small, but important source of electric generation in the next century. As most of the electric power supply in developed countries is via centralised electric grid, it is certain that widespread use of photovoltaics will be as distributed power generation interconnected with these grids. This is a new concept in utility power production- a change from large-scale central examination of many existing standards and practices to enable the technology to develop and emerge into the marketplace.

Regulations

There is no regulation nowadays in the UK for the grid connection of PV. There are some engineering recommendations but they are not specific for PV applications. We can mention:

G. 5/3 Limits for Harmonics in the United Kingdom Electricity Supply System.
G. 59/1 Connection of Embedded Generating Plant to the Regional Electricity Companies' Distribution System.

These recommendations are complicated to understand and apply. The government has started to prepare some guidelines specific for grid connected PV applications but a draft of these guidelines, makes references to previous recommendations and it looks also quite complicate. But experience is necessary to develop and apply proper regulations. Regulations in USA are clearer and more helpful than the recommendations of the UK. It is obvious that they have been working on it during many years. We think that UK regulations should take into account developments in regulations from other countries.

Grid connection of PV Systems is a new issue for the utilities in the UK. They don't have any experience integrating PV systems into the grid and they don't know how to face the situation due to the lack of regulation. At the end of the day, the decision of taking a PV system into the supply network is theirs.

One of the most important technical issues of the grid connection of generation plants is the Power Quality. For any grid-connected system, power factor and harmonic consideration are important. The electricity supplier dictates the requirements for the grid-connected system, and each utility may impose a unique set of requirements. The main grid interface criteria which should be checked with the utility are the following:

Voltage regulation
Frequency regulation
Harmonic distortion in the operating load range:
- Total of all current harmonics (usually 5% maximum)
- Any single current harmonic (usually 3% maximum)
- Total of all voltage harmonics (usually 5% maximum)
Power factor and reactive power consumption
Protection and operation criteria
- Inverter disconnect criteria in the event of a grid failure.
- Inverter reconnect criteria.
- Adequate safeguard against islanding

However, the grid-interface criteria vary with the utility companies and have yet to be standardised internationally.

In this situation, we decide to carry out an analysis of power quality issues related to Grid Connected PV Systems, concentrating on a Harmonic Distortion created by the Inverters used in the systems. The first stage was to get familiar with the technical concepts involved by analysing the output signal from a zero cost inverter in the laboratory. The next step was to apply the knowledge gained to a small Power Quality Case Study. The analysis scoped a harmonic distortion analysis in a weak supply network.

Lab tests
Power Quality Case study

Conclusions

There are no major technical barriers to the implementation of dispersed PV generating systems connected to the supply network. While more technical research is needed in some specific areas, the remaining barriers are fundamentally price and policy.