Chaz Andrews, Doepke Technical Manager, discusses RCD circuit protection associated with EV vehicle-to-grid installations.
Octopus energy have announced the first commercially available Vehicle-to-Grid (V2G) tariff, offering free home charging, for compatible vehicles. Although the jury’s still out on the economic viability of disparate domestic V2G technology, installers should be able to answer basic customer questions. The possibilities associated with V2G and V2H (Home), raises questions about existing switchgear and RCD requirements.
V2* Circuit Protection / Switchgear general points
Basic V2G technology is only marginally more complex than smart charging, allowing for bi-directional current flow (see below) and the existing power export agreement process. However V2H adds a degree of complexity to individual domestic installations, when included with PV and battery storage. The associated IET Codes of Practice (CoP) covering V2* Charging Ch.10 offer an authoritative guide to this subject. Be extremely wary of YouTube so called “experts” unless you have sufficient knowledge to critic the information provided, or the person has previous industry standing e.g. independent John Ward, ECA or similar technical support team, established Manufacturer, member of a Standards Working Group.
Functionality: The switchgear design must encompass the required operating modes, direct supply, reverse supply and or Island mode. Customers can assume incorrectly that V2G automatically encompasses island mode facilities. BS7671 Annex B82 gives pictorial overview of PEI functionality. The CoP for EV charging 5th edition section 10 & 11 gives detailed requirements for V2*.
Functionality (within the CP) must include a method of limiting the customers export capacity to under 16A (ENA G98), or the DNO may specify that the installation complies with G100 – see section 10 point 2. In the medium term widespread (all DNOs) automatic asset registration (AAR), is a prerequisite to mass market adoption of V2G technology and other small scale energy assets.
Rating: Conventional domestic units (BS61439-3) may be continuously rated <100A within a daily average maximum temperature range - refer to manufacturers documentation. Adding additional generation in parallel with existing supply capacity may result in the conventional switchgear being overloaded, leading to permanent damage /deterioration of insulation properties. Additional heat (temperature rise) can be transferred into adjacent or connected components, manifesting in unexpected tripping events and or premature failure.
Consequently, the rating of the switchgear, physical layout and method of controlling the maximum loading on any individual unit must take account of BS7671 551.7.2 (v). See examples given in CoP for EV charging 5th edition section 10 point 1 & 2. Where island mode operation is required, the maintained (essential) loads must be isolated from the non-essential loads based on available battery capacity – see CoP scheme Fig 10.4 and note 6 relating to the OCPD device.
V2* RCD selection
The following is based on reference to: BS 7671 clause 826.1.1.1 and 551.4.3.3.2. IET CoP EV Charging 5th Ed. 10.6, 10.7.3. and IET CoP EES Systems 2nd Ed. 9.12 9 (3rd edition due for publication at time of writing).
Type of RCD: Compatible with the inverters mounted in the associated chargers (EV and Battery). EES Systems Table 9.4 illustrates the selection process / Type of RCD e.g. if the inverter equipment does not include simple separation (preventing the flow of dc fault current), Type B required. RCDs providing automatic disconnection on the output of the inverters* must operate reliably with any DC components flowing in circuit.
*10.7.3 “It is likely, particularly for dwellings and smaller commercial/industrial premises, that disconnection times can only be achieved using RCDs at the output of the inverter(s)”.
Conclusion
Consider the guidance given in the associated IET CoPs for safe installation designs, before deciding on the final design of the installation. With innovative technologies the “State of the Art” is an important concept to consider in relation to COPs and the year of publication. Designers require sufficient and appropriate experience to consider existing advice and any additional available information relating to safety issues, design, and equipment / system developments when working with innovative technologies.