To guarantee that the grid’s frequency is stable, there must a balance between injections and withdrawals to/from the grid at all times. The mechanism in place for this purpose is load frequency control.
Graphic representation of load frequency control
In the event of a major load fluctuation such as a power station outage or other unpredictable variations in injection and withdrawal in the ENTSO-E grid, the control area operators’ primary reserve is activated; this is available almost immediately. Contributions of balancing capacity are voluntary. The amounts are expressed by an agreed formula based on generation output. This system maintains frequency stability.
Where there is only a brief power deficit or surplus, primary control is sufficient to stabilise the system. Primary control is defined as “an automated reestablishment of the balance between generation and consumption … within no more than 30 seconds following such imbalance” in section 7(58) of the Elektrizitätswirtschafts- und -organisationsgesetz
(Electricity Act) 2010. If a longer disturbance occurs, secondary control is activated after a defined period (max 30 secs) or simultaneously with primary control, in order to relieve the burden on primary control and free it to perform the above function. While primary control ensures system stability across control areas, secondary control stabilises imbalances within a control area. Secondary control operates for periods of several minutes.
If an imbalance persists even longer than that (>15 minutes), tertiary control takes over or is activated alongside secondary control. This is done either automatically or manually; tertiary control must be ready for dispatch no later than 15 minutes after the imbalance starts. Secondary control covers a period of up to 15 minutes; tertiary control does not necessarily have to be concluded after this time. The incident reserve available must at least be equal to the largest generating unit.
Balancing capacity is provided by technical units that have successfully completed a qualifying procedure conducted by the control area operator. They can then participate in the regular tenders for balancing capacity. Most balancing capacity is provided by power stations that can increase or decrease their input. Consumers can also participate in this mechanism by regulating their electricity off-take. Smaller units can participate in the balancing market as pools.
The responsibility of the control area operator to guarantee system stability lies with the transmission system operator. It lives up to this responsibility by carrying out load frequency control (section 7(60) Electricity Act 2010).
The control area operator procures the balancing services needed in regular tenders. All market players that fulfil the requisite technical conditions and have signed a corresponding framework agreement can participate in these tenders.
Costs and cost allocation
In line with section 68(1) Electricity Act 2010, costs for primary control are borne collectively by all electricity producers with a maximum capacity of more than 5 MW. They are distributed according to annual output.
Secondary control causes costs for the availability of balancing capacity and for the actual supply of balancing energy. Section 69 Electricity Act 2010 provides that 78% of these costs is covered by the system services charge, payable by all producers with a maximum capacity of more than 5 MW, while the residual 22% are reimbursed through the imbalance costs that are paid by the balance groups.
Physical balancing services vs calculated imbalances
The overall purpose of the balancing system is to maintain injections and withdrawals at the same level to physically guarantee that the system remains stable. A deviation from forecast supply or demand in a balance group, e.g. as a result of an outage, gives rise to an imbalance (section 7(3) Electricity Act 2010). The net imbalances across all balance groups in a control area is the balancing energy demand that the control area operator must meet. The total imbalances may be many times the balancing energy, as the balance groups’ needs may offset each other.
- Balancing energy is the deviation from forecast in a control area;
- Imbalances are deviations from forecast inside balance groups.
The balancing services are invoiced to the balance groups on the basis of the quantities recorded and the calculated costs (cf. also the general terms and conditions for the clearing and settlement agent). The manner in which these costs are passed on to suppliers and consumers is for the market players to decide.