This project will use smart meter data to provide improved visibility of the existing capacity headroom along the length of feeders, and to improve the targeting in location and time of active and reactive power management of V2G, (also known as Volt/Var or Volt/Watt control techniques), while improving the confidence that assets will remain within thermal and voltage limits.
Benefits
The NGED SILVERSMITH NIA project assessed the impacts of LCTs on LV assets, proposing a series of archetype networks that were modelled to determine whether voltage or thermal constraints would apply, and how they might be mitigated if so. Uptake of LCTs was modelled based on NGED’s Distribution Future Energy Scenarios and included exports from solar PV and a limited number of battery storage installations, but not V2G. The export capacity from V2G is therefore additional to the capacity shown in these results.
The plot below shows a modified version of the timeline for the LV6 archetype, representing a typical suburban underground radial feeder serving domestic customers with 3- or 4- bedroom houses. The grey line shows predicted maximum export capacities form the LV feeder, and other dotted lines show how investments in the voltage rise and thermal loading capacity would be needed so that the predicted export can be accommodated. The line in red has been added to incorporate an uptake of V2G of up to 45% of customers based on figures from the National Grid ESO Future Energy Scenarios (‘Leading the Way’), with 33 customers on the LV feeder and with coordinated exports of 7 kW V2G chargers. This assumes no diversity in the V2G operation, which is plausible as a response to an aggregator request will tend to align these exports in time. The figures are also conservative as they also ignore the likely occurrent of clustering and the possibility that customers will have three-phase chargers. Exports including V2G now exceed the available voltage rise capacity much earlier (2028 rather than 2032) and also exceed the thermal constraints around 2037.
Taking this archetype as an example, the voltage rise investment of £9.5k per LV feeder would be brought forward by 4 years due to V2G. However, the peak exports causing this investment are likely to occur rarely, only when demand is low and both the PV and V2G exports occur simultaneously. The V2G dynamic headroom methods therefore avoid bringing forward this investment, while retaining the customer value of V2G exports for the majority of discharging events when not all exports occur at the same time.
Avoiding bringing this investment forward saves £368 per LV feeder, based on a £2,495 cost to provide the additional voltage rise capacity through manual tap changes, assuming a discount rate of 3.5% and time period of 4 years. If this avoided early investment applies to 10% of feeders, then there is a saving of £1.9m over the 54,064 feeders that this archetype represents.
Over the GB distribution system, this saving can approximately be scaled by the number of domestic households served, 28 million across GB compared to 8 million for NGED, suggesting a GB-wide saving of £6.6m. Clearly this estimate involves significant assumptions, but it is also conservative in as calculations here are only based on the LV6 archetype, covering 16% of the LV feeders in the NGED license, but the techniques could be applied throughout, wherever V2G customers are likely to occur. It has also been assumed that the savings only apply in 10% of feeders within one archetype but savings could also apply more widely, albeit at a later date.
There is a further risk that V2G would necessitate bringing forward the thermal capacity upgrades from 2037 to 2035. The V2G dynamic headroom can also mitigate this, although the thermal capacity upgrades would still be required in the longer term.
Learnings
Outcomes
As indicated throughout this report, there are moderate benefits that can be achieved by employing volt-watt and vot-var control techniques but further investigation is needed to provide firm conclusions.
The final results and recommendations, including considerations for further trials, will be provided upon completion of the project and in the next ENA annual Summary Report.
Lessons Learnt
The key lessons learnt from Work Package 1: Simulation Study are:
- Initial results suggest that volt-watt control is more effective than volt-var control in limiting extreme voltage deviations. The models so far assume that all EV chargers participate in V2G export events for a half-hour period around midday when PV exports are also high and when demand is low. EV chargers do not participate in the export event if they are already charging. Applying volt-var control has so far demonstrated less of a reduction in voltage rise than volt-watt control. From the network perspective, volt-watt control is more effective.
o This finding has set the directions for WP2. In particular, the project will aim to understand whether modifications to the volt-var control method could improve effectiveness, or whether it will be effective if more appliances, such as PV inverters, can also participate.
- The simulation results indicate that exported power on one phase causes voltage rise on that phase, but if exports are unbalanced, will cause voltage drops on other phases. Similarly, high loads on one phase can cause voltage rise on other phases. Where exports are unbalanced, the non-linear nature of volt-watt control can cause exports on one phase to be constrained more so than on other phases and this can accentuate unbalance.
o This observation has set directions for WP2. Highlights the impact of phase unbalance, particularly where volt-watt or volt-var control is applied at specific thresholds, creating a non-linear behaviour.
- The simulation model currently assumes that recharging from V2G export events takes place immediately after the half-hour export period has ended. Since exports are assumed limited to 3.68 kW, and imports may use the full rated 7 kW, the impact of the recharge for voltage drop may be more significant than the impact of the exports on voltage rise.
o This finding has set directions for WP2. Further discussion with stakeholders and ideally also aggregators will help to understand how the customers may recharge EV batteries after a discharge event.
- In initial models, no diversity was applied in the timings of the recharge periods. Where volt-watt control is applied, export powers are reduced relative to the permitted 3.68 kW export power. These power reductions occur throughout the half-hour export period. As a result, the total exported energy is reduced by the volt-watt control. There is a corresponding reduction in the energy needed for recharging. However, the model currently assumes that this reduction allows for the duration of the recharge period to be reduced rather than moderating the recharging power. As a result, although the volt-watt control can mitigate voltage rise during the export periods, it does not mitigate voltage drop during the recharge periods. The model has demonstrated a scenario where the exports on two phases were entirely switched off due to volt-watt control, such that they had no requirement for recharging. This resulted in significant unbalance during the recharging period, with a high voltage drop on the phase with EV chargers that had continued to export. This also caused a voltage rise on the other phases.
o This highlights the need for a diversified approach to the recharging after a V2G export event. Exports are necessarily coordinated in response to either a requirement for grid support or a common tariff. However, the timing of recharging can be diversified. This is particularly necessary where recharging uses a higher power than permitted exports.
- Although volt-watt control is more effective from the network perspective, this is achieved through a greater reduction in the exported energy. There is therefore a reduced benefit to the wider grid from V2G, likely associated with reduced revenue to the individual customers providing exports. Volt-var control provides less mitigation of voltage rise but does so without any significant impact on the exported energy.
o This fairness of constrained exports will be addressed together with the use of smart meter data.
- The models have assumed V2G export powers being limited to 3.68 kW, whereas EV charging imports are at 7 kW. Assuming that the power electronics therefore has a rated power transfer capability of 7 kVA, volt-watt control with reactive power at 60% of the inverter capacity, as in TS129, can be accommodated without any reduction in active power.
o Modelling in Work Package 2 will look at combined volt-watt and volt-var control.
- Modelling has also included scenarios where a diversity factor was applied to the recharging after V2G export events. This is very effective in mitigating the worst-case voltage drop caused if all participating EV chargers replace the exported energy as soon as the export event has ended. V2G implementations would ideally be designed so that either i) recharging at full import power is diversified, or ii) synchronised recharging uses a lower power to allow for the combined impact of nearby devices recharging at the same time.
o The diversified approach will be prioritised in future modelling as the non-diversified method creates severe short-term voltage drops that would not be acceptable for network operation within voltage limits
- The market for grid support services from V2G may become saturated if the number of participating devices becomes very large. In this case, customer revenue may be more likely to arise from demand peak-shaving and arbitrage. V2G would then be more likely to export in the evening peaks, and typically exports would only occur once the import demand had already been met. Exports are therefore determined by the residual energy once these demands have been met. The voltage rise impact of V2G exports is expected to be much less in this scenario as the exports will be more diversified and would occur at times when local demand from customers without EV chargers is also expected to be high.
o The modelling will still consider the grid support event as a worst-case scenario for voltage rise, but consideration of the impacts on fairness need to reflect that any inequality in the scope for exports is mitigated by the reduced contribution of these export events to the overall financial case for customers to invest in a V2G capability.
