How Are Units in a VPP Selected to Provide Ancillary Services?

by Nils Quak / 23. July 2020

Selection of VPP units through the algorithm of the control system In a Virtual Power Plant, thousands of systems provide balancing services every day. But how exactly are the individual units selected for the provision of balancing reserves? Which components are included in the evaluation?

At the very beginning of the process are the individual parameters and restrictions each participating unit has, which are both a given by the design of the respective system itself as well as communicated by each operator. These restrictions include the following parameters:

  • Reaction time to signals for changing power generation resp. power consumption (this also results in the selection of the fitting products of balancing services that the unit subsequently will bid into)
  • (Flexible) capacity of the plant
  • Size of the gas storage/heat storage tank, if applicable
  • Location of the plant
  • Availability of the system (whether there is a repair or other maintenance, for example)

These parameters form the basis from which the pool is compiled, which is to provide the bundled ancillary services of the Virtual Power Plant. From this pool, the flexibility that can be offered in the individual balancing energy auctions is derived. Among other things, this pool is formed from the operation of the individual plants and the availability of the previous days. If there is an explicit deregistration for the corresponding day, the plant will of course not be considered.

Composition of the Pool

Contractual, economic and technical aspects play an equal role in the composition of the available plants for the individual types of balancing services.

Since ancillary services products are interdependent both in terms of time and the amount of potential revenue, the balancing power volume is compiled in correlation to the balancing power products. Next Kraftwerke anticipates the market before the auctions and derives the quantities for the individual auctions from this. A possible approach would be to first compile the plants for FCR, then for aFRR and in the last step for mFRR in order to know at the end for each balancing power product which quantities can be bid in the auctions. If the aFRR auction is awarded for a smaller quantity than the quantity of aFRR-eligible installations in the pool, the "surplus" installations could be transferred to the mFRR pool, for instance.

In order to enable detailed planning for the provision, a forecast for the coming week is made from the current operation mode and past operational schedules. Various factors such as weather or electricity market forecasts are taken into account.

Of course, redundancies are also planned for the provision of ancillary services in order to compensate for short-term downtimes of individual units. In order to minimise any possible bottlenecks, additional redundancy is also planned into the provision of balancing services.

Activation of Ancillary Services by the VPP Algorithm

If the Virtual Power Plant receives the signal from the transmission system operator to deliver balancing power, the algorithm of the VPP selects the appropriate plants from the pool. Technical and economic criteria play a role in this selection. Both the energy costs of a plant and the technical requirements are included in the activation process. As with the selection for the provision of balancing power, a random factor also plays a role in the activation of balancing power in order to guarantee fair distribution within the pool. The calls are distributed statistically across the individual installations so that the same installations are not always called up.

If an activation is communicated by the transmission system operator, the required quantity of flexible reserves is distributed among several units, even if a single unit could also provide the required quantity on its own. If a large unit were to fail to deliver, the effort to compensate for this failure would be greater than if a small unit failed. For this reason, it is also more desirable for several units to run at partial load than for one unit to be switched off completely (when negative balancing power is called up). The reasons are easy to understand: On the one hand, the full start of a unit takes longer than if one unit had to ramp up from partial load to full load again. On the other hand, when a unit is switched off completely, there is a greater risk of complications that could be occurring during the start. During the activation, the quality of the service is constantly monitored. If a unit does not meet the set target value, further units are activated to provide the desired amount of balancing energy.

In short: Since there is a high number and diversity of units in the virtual power plant, the underlying algorithms try to form this heterogeneity into a coordinated swarm with precise control commands and detailed planning data.

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Nils Quak is working at the Next Kraftwerke marketing and takes care of the corporate brand communication. Furthermore, he's also interested in new traffic concepts and urban development.


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