What is meant by grid connection?
The existing electricity distribution network includes many different types of equipment that are looked after by various distribution network operators (known as DNOs), for example the National Grid, SSE, SPEN, and UKPN. The higher voltage levels 275kV and above are generally operated by transmission system operators, such as National Grid and SP Transmission.
A new grid connection is required when a new power plant is built - such as a solar farm, wind farm, gas generator or even storage devices such as BESS. Every new power plant needs to be connected to the grid to ensure that it can safely access electricity and power while meeting the requirements of the grid to ensure system stability.
What is a grid connection study?
EREC G99 sets the requirements for connecting to the grid. The connection process for transmission system connections is broadly similar although compliance with Grid Code is required instead. A grid connection study ensures that every new system can connect to the grid without causing and issues to the network.
A grid connection study generally involves four different studies to ensure the plant complies with the criteria set out by the network association. These requirements are defined individually in a set of standard documents including the G99, P28 (voltage fluctuation studies), P29 (unbalanced voltage), G55 (harmonic distortion) etc.
Based on these standards a grid connection study will include all or some of the following, actual elements depend on the G99 declared type and generation type:
- G99 or grid code
- Reactive power capability: Every generating plant is required to deliver an amount of reactive power based on the export capacity.
- Fault ride through capability and Fast Fault Current Injection: During an external grid fault the generator should maintain the connection to support the grid voltage. This is inherent with synchronous generation; Power Park Modules have the additional requirement to inject an amount of reactive current.
- FSM, LFSM-O and LFSM-U: The generator’s response to grid frequency events should evaluated, for example, under LFSM-O, when the grid frequency increases above 50.4Hz, the generator must reduce its output active power according to its droop setting.
- Voltage control and reactive power stability: the generator’s voltage controller should respond to step changes in voltage by modifying its reactive power output. This is carried out for small and large steps (to maximum reactive power output limit)
- Model verification: the frequency controller must respond according to its droop setting for a fixed pattern frequency injection. The voltage controller must demonstrate a quick and stable step response.
- Power system stabiliser tuning: For systems with a power system stabiliser, dynamic simulations are carried out. The open and closed loop frequency domain response is evaluated with and with out the PSS and system stability is evaluated.
- Fault Infeed study: Increasing fault levels on some parts of the network pose a threat to existing equipment. This study evaluates the new sites short-circuit contribution in the event of symmetrical and asymmetrical faults in the distribution network.
- Voltage fluctuation assessment (P28): The grid’s standards will recommend flicker and voltage fluctuation limits for new connections that should not be exceeded.
- Harmonic distortion (G55): an assessment of the impact of a new installation on the system harmonic. G55 has recently replaced the G54 assessment. The full requirements depend on the site and connection voltage. The most detailed assessment utilises an equivalent representation of the grid for all configurations at the point of connection. This avoids the requirement to model an extend part of the distribution network but relies on a much large number of harmonic load flow simulations. The site is assessed against an incremental distortion limit, accounting for new harmonic injections and a total distortion limit, which accounts for new distortions and the modified network impedance that could lead to modification of existing harmonic distortions.
What are the different sources of electricity you can connect to the grid?
Power plants can include one or more sources of power. There are a variety of electricity sources that can be connected to the grid, including:
- PV farm / solar farm
- Wind farm
- Gas generation
- Steam
- Tidal
- Storage – including BESS, flywheel and pumped storage.
How do PSE 2 carry out a grid connection study?
When we work with a new customer the first step is to send them a Request For Information (RFI form) asking a series of questions about the proposed connection, such as where their site will be, connection voltage and how much power they plan to import/export etc. Also dependent on the stage of the development, we would ask for equipment specifics, so that we can create a detailed model of the site.
Once we have this information, we are able to carry out the grid connection study, as described above. We use software such as PowerFactory, ERACS, ETAP, PSCAD, COMSOL, and Cableizer to carry out these studies. We also utilise automation where possible through custom scripting to both decrease the turnaround time for studies and remove human error.