Grid Linked Systems

Grid Linked Systems

Key Facts

The UK's national electricity supply is fragmented into many companies.
A few net metering schemes are available (1), whereby the amount paid for producing electricity is the same as the cost of using the electricity. More typical is import export metering (2)
Getting paid for generating electricity is complicated by the role of Renewable Obligation Certificates (ROCs) and Levy Exemption Certificates (LECs). (3) Also, consider the role of Renewable Energy Guarantees of Origin (REGO) (4)
Inverters convert DC to AC voltage. To attach the AC supply to the grid, the inverter must be able to synchronise with the grid, must produce the same voltage and waveform, and must be able to detect the loss of the grid. Inverters should be sized to 80-90% of system's potential. It is also important to correctly size cables.
Grid based photovoltaic (PV) systems are one of the most expensive ways of producing energy as carbon costs are not included in price structures.

There are many players in the UK electricity generation and supply industry, each of whom fulfill one or more roles, such as domestic network operator (DNO), large generator, large industrial and domestic consumers, and public electricity supply companies.

The relative value of ROCs (5) and LECs (6) indicate that there is little pressure on the major generators to substantially reduce their carbon emissions. At time of writing (October 2007), even with climate change and global warming an almost daily news item, prices are £49.27 for each ROC (MWh) and a maximum of £4.41 for each LEC (which is set by legislation) respectively.

The physical grid structure has 5 layers at different voltages with step changes occurring between the layers, starting at the super grid at 400kV, steeping down to 132kV, 32kV, 11kV and finally the common domestic voltages of 230V/400V. The supply is split into three phases, which can be split before reaching the domestic household.

Different engineering standards apply (7). For PV systems up to 16A per phase, G83/1 is used. For installations of 5kW and upwards, G59/1 applies. This standard requires on-site test of inverter and protection to ensure voltage and frequency remain with the National Grid allowable ranges. Islanding standards ensures that should the National Grid fail, everything is switched off. This prevents the potential for electrocutions as power is restored.

Connecting PV electricity generation systems to the national grid can be an attractive option. The main advantages are that they are modular, so virtually any size can be installed, there is no requirement for batteries resulting in lower installation costs and little on-going environmental problems, and there is a huge potential resource with good summer performance. Disadvantages are mainly cost of installation at around £800 per KW/h per annum produced, which results in a high cost of electricity of around 30p/kWh, and that as there is no storage, power cuts are still possible (8). Current technologies in UK conditions require 6 to 8 m² of cells to produce 1 kWh.

Standalone PV systems are more complex than grid connected systems. It is important to ensure that the power supply is sufficient to meet requirements, therefore energy efficiency is paramount. Make the best use of the power generated and oversize any critical systems. Often, standalone systems can not be justified because of the additional costs of batteries and the probable need to oversize the system to overcome downturns in electricity generation.

An easy reckoner for calculating the daily useful energy (Wh) is Total irradiation (kWh) × Performance Ratio (<= 0.8) × PV array peak power (W_p).

Further Information