In this post we will develop how the flexibility provided by elements such as electric vehicles, heat pumps, storage systems, etc., can give added value to different agents in the electricity system
We will review what services the aggregation of flexibility can provide to the different agents involved in the system. Some of these services are compatible with each other and therefore aggregation of flexibility could be offered two or more compatible services simultaneously to more than one agent. In other cases, the services offered could be incompatible and the local regulation, the provider of aggregation or the market will have to choose. In the following paragraphs, the products that aggregation of flexibility can offer to the different agents of the system and the added value will be explained for each specific customer.
Flexibility value for distribution system operators
The extent to which electricity distribution companies can control what is happening in their networks is acceptable at a high, medium voltage level but really low or non-existent in many cases in low voltage networks. In some circumstances, the power transformer stations that connect the low voltage network with the medium voltage network have tap changers that allow them to regulate the output voltage of the transformer to a certain degree depending on the load on the low voltage lines they serve. However, the number of operations that this type of device can perform is very limited since in the majority of the cases, they need to be done manually and once a tap is changed, it remains in that position for a long period of time, so this mechanism is not acceptable in a context where the dynamics of load variation causes rapid fluctuations in the voltage level. Something similar could be said about the use of capacitor banks that inject reactive power into the system raising its voltage. On one hand its use is not very extended and its control mechanisms do not allow a fast and continuous regulation of the injected reactive power.
Another issue to take into account is the possible overloading of lines and power transformer. It must be taken into account that in many cases low voltage distribution network in urban environments has a very complex topology which allows to be reconfigured, thus distributing the load from some power transformer stations to others, i.e. a certain line can be fed (not simultaneously) from two or more power transformer stations, thus allowing some congestion to be resolved. However, once again, in the majority of cases, this reconfiguration is manual and once it is done it tends to remain static for months or years, so this technique does not respond to the dynamic control requirements necessary to manage distributed resources like electrical vehicles, solar panels, batteries… with high variability either.
In this respect, it must be said that distributed resources not only provide flexibility in the sense that they can vary their power or shift their consumption over time, but a common feature is that they are connected to the grid through converters whose catalogue of functions is very extensive. Both electric vehicles with simple or bidirectional charge, as well as heat pumps, accumulation systems, photovoltaic systems and other resources are connected to the network through converters that allow a very fast variation of the working conditions. Regulations such as the one proposed by the IEEE in its 1547 Standard on the requirements for converters for interconnecting distributed resources to the network , or the advances made by working group WG17 of IEC’s technical committee TC57 to adapt the IEC 61850 standard on systems for the automation of power utilities already include this type of functionality for smart converters . A detailed description of the above-mentioned functionalities goes beyond the scope of this document and can be found in the literature in documents such as the one proposed by the (Electric Power Research Institute) EPRI in its report on “Common Functions for Smart Inverters” . In summary, we could say that the functionalities described are the enabling technologies for the implementation of the whole set of products and services that the flexible resources can offer to distributors and also to the rest of the system’s agents. In this way the value that the aggregation of flexibility can offer to a distribution system operation are the following:
- Congestion management: In the event that the aggregated resources are concentrated in a specific geographical distribution area, the aggregation of flexibility could coordinate the resources in such a way as to guarantee a point of operation for the distributor below the overload. An example would be the coordinated charging of electric vehicles which would avoid peak loads and thus reduce the need for investments in network maintenance and upgrading by the distributor. Normally the peak demand in a low voltage network, depending on the consumer profile, can occur either at midday or at night. The distributor must have the necessary infrastructure to deal with these peaks of load, which normally last for a very short time, and this means operating for most of the time with an oversized infrastructure. The aggregation of flexibility would make it possible to resolve infrastructure congestion by coordinating flexibility and thus contribute to increasing the quality of supply, while also allowing distribution companies to save on infrastructure investment.
- Voltage control: As mentioned above, the voltage regulation mechanisms currently available to distributors are very few and have a slow dynamic at best. In this case, it should be mentioned that the distributed devices once again have an under-utilized asset which makes it possible to regulate the voltage, that is the converter present in solar generation systems, accumulation systems, in some electric vehicle recharging systems, etc. Grid connection converters use the rated current for very short periods of time, in the case of solar generators, for example, at most they will use that maximum current to inject active power at the hour of maximum radiation on a sunny day, the rest of the time they will be operating below their nominal capacity. In this case this capacity can be used to inject reactive power “free of charge” for the owner of the asset so that local voltage control is exercised in the distribution network. It should be noted that the example of the solar converter has been given because in case of a peak demand in a cloudy day or at night which causes large voltage drops, the full capacity of the solar converters could be used to inject reactive and raise the grid voltage. In addition, in the case of distribution networks, given the high R/X ratio (resistance/reactance), there is also a high correlation between consumption/injection of active power with the voltage level, so a Volt-Watt type control could also be implemented in the converters. This control would prevent excessive increase or decrease in voltage due to over-injection or overconsumption of distributed resources and in this case, this type of control could and should be installed in vehicle recharging systems, V2G systems, heat pumps, etc.
In this case it must be taken into account that in some cases the aggregation of flexibility may not be compatible with congestion management services provided to the distributor, i.e. the BRP may demand an increase in consumption from the aggregator but this increase in consumption is incompatible with avoiding congestion in a given distribution area. In these cases, there must be mechanisms for prioritizing the power increase or decrease orders of an aggregator when it offers products simultaneously to BRPs and distributors .
Value of flexibility for balancing
As per the nature of flexibility, the services that the aggregation of flexibility can provide to balancing service providers (BSPs) are clear:
- Primary control: for this model, the aggregation of flexibility could configure the distributed resources to respond very quickly and automatically to changes in network frequency. This does not pose any technical difficulty since the converters used to interface the distributed resource with the grid have PLL (Phase Locked Loop) systems that allow them to synchronize with the network and therefore estimate variations in frequency. In the case of electric vehicles, in the event of detecting a drop in the grid frequency, the charger could automatically cut off the charge and thus assist in restoring the frequency and supporting the system. If the vehicles were also equipped with V2G technology, they could also react to a drop in the frequency by injecting power. It should be borne in mind that the energy managed in the primary regulation is relatively small because its duration does not exceed a few seconds so the charging times of the vehicles in the example described would not be affected.
- Secondary control: the activation signal for flexibility would be automatically generated in a central control system which would impose the activation of power increase or decrease by zones. In this case the power to be increased or decreased should also be maintained for short periods of time determined by the duration of the imbalance settlement periods (15 minutes) so that the activation of this service would not, in principle, entail a relevant loss of comfort for the providers of flexibility in the vast majority of cases. In other words, the mere fact of delaying by 15 minutes the turning on or off of a heat pump or the charging of an electric vehicle would have practically no impact on the end user.
- Tertiary control: The business model of providing flexibility to participate in tertiary regulation is very similar to that described in for secondary regulation but in this case the response time is longer and the power to rise or fall must be maintained over time for a longer period. The one managing large amounts of flexibility could stagger short duration orders of power increase/decrease to the different devices that it manages, making the action last globally the two hours required but without causing losses of comfort to the end owners of flexibility.
In the case of services provided for balancing, there may be incompatibilities with services provided to distribution system operators in case the services are offered simultaneously to both agents..
Specific products for prosumers or active consumers
The prosumer is the key player and ultimate provider of the flexibility.
Apart of any benefits that the prosumers could have for providing the services mentioned before, managing the flexibility can benefit directly the the prosumers. These services are as follows [4; 5]:
- Time-of-Use optimization: The tariffs that retailers offer to their customers have, in some occasions, variable prices in the different periods of the day and in some cases, these prices may even vary dynamically in real time. The flexibility can be used to understand the customer’s capabilities and habits to shift as much as load as possible from high to low price periods and thus make the consumer get the most benefit of its flexibility by reducing its energy cost.
- Maximum Power control: A very important term in the majority of electricity tariffs offered by retailers is the so-called power term or the capacity fee which represents the cost of the availability of In other words, the user will pay more the higher the peak power it can consume. In many cases, the power term represents a very important part of the tariff and the user only reaches this power in specific situations, i.e. charging the vehicle to coincide with a consumption peak in the home. Let us assume that a user has a photovoltaic generator, an accumulation system and an electric vehicle. Let us also suppose that the surplus energy generated by the PV panel during the day is used to charge the battery and the battery begins to discharge at 7 pm because the electricity consumption of the home begins to increase. At 10 pm, the electric vehicle starts to charge but the battery is already discharged so that all the power of the vehicle plus that of the house has to be obtained from the network, which means a high peak and implies the need to increase the power term of the tariff and with it the total cost. Managing this flexibility would allow to charge the battery during the day but would not start discharging it at 7 pm but would wait until the vehicle demanded power at 10 pm and would discharge the battery against the vehicle, in this way the impact of the vehicle would be less and the user could reduce the power term of the tariff. This is just one of countless examples that can be given.
- Self-balancing: Similar to the previous product and available to those prosumers who have significant flexible capacity, self-balancing techniques would allow all the prosumer’s flexible resources to be managed in an integrated and optimal way, taking into account energy purchase and sales prices, capacities and consumer habits.
- Controlled islanding: In the case of weak networks with power quality problems such as micro-outages or undervoltage or overvoltage problems, the aggregation of flexibility could intentionally isolate the consumer from the network and make it self-sufficient for a certain period of time.
 “IEEE Draft Standard Conformance Test Procedures for Equipment Interconnecting Distributed Energy Resources with Electric Power Systems and Associated Interfaces,” IEEE P1547.1/D9.8, December 2019, pp. 1–283, 2019.
 T. C. 57 IEC, “IEC 61850: Communication networks and systems for power utility automation,” International Electrotechnical Commission Std, vol. 53, p. 54, 2010.
 Common Functions for Smart Inverters: 4th Edition, EPRI, Palo Alto, CA, 2016, 3002008217.
 P. Olivella-Rosell, P. Lloret-Gallego, Munné-Collado, R. Villafafila-Robles, A. Sumper, S. Ottessen, J. Rajasekharan, and B. Bremdal, “Local Flexibility Market Design for Aggregators Providing Multiple Flexibility Services at Distribution Network Level,” Energies, vol. 11, p. 882, 2018.