System services for frequency maintenance
The frequency within a power grid is either 50 or 60 Hertz. It is important that the frequency be within a defined range so that the power grid does not collapse. In Europe, this is a positive or negative deviation of max. 0.2 Hertz from 50 Hertz. Therefore, power feed-in and power consumption have to be in balance at all times. This also applies in cases of unforeseen events, such as power plant outages, a strong wind period with the resulting high feed-in of wind power, or a sudden increase in electricity consumption. The responsibility for a stable frequency lies with the balancing group managers , who must keep their balancing groups in balance, and with the grid operators , who are responsible for stabilizing the frequency in case balancing groups – and subsequently the frequency itself – are out of balance.
In the narrower sense, the spinning reserve is not part of the ancillary services , since it is already created by the inertia of rotating masses: If a heavy mass is set in rotation like a flywheel, it continues for a while, even after the energy supply has been stopped. For the sake of completeness, however, the spinning reserve is often mentioned in connection with ancillary services and is a kind of preliminary stage to balancing energy.
Due to the kinetic energy of their rotors, power plant generators are able to compensate for a short-term power failure. Renewables do not yet provide a spinning reserve. However, it is conceivable that the kinetic energy of large wind turbines could be used for this purpose.
Frequency control / balancing energy
If the effect of the spinning reserve cannot balance the frequency sufficiently, balancing energy is used to restore equilibrium . In some countries, balancing energy is obtained via a market. This is often based on an auction procedure to keep costs low. In countries with a vertically integrated energy industry, such as China or parts of the USA, this task is performed directly by the grid operator. Balancing energy is often divided into different products depending on how fast they have to react. In Germany, for example, balancing energy is divided into three products: Frequency Containment Reserve (FCR : reaction time < 30 seconds), Automatic Frequency Restoration Reserve (aFRR: reaction time < five minutes), and Manual Frequency Restoration Reserve ( mFRR: reaction time < 15 minutes).
Switchable loads, sometimes also called “sheddable loads”, are loads that are deactivated for a short time in order to guarantee network stability . Who provides these loads and in which procedure the needed power is tendered, varies from country to country. Oftentimes, heavy industries with a high degree of power consumption and automation in their production lines, such as aluminum or steel manufacturers, provide sheddable loads.
System services for voltage compensation
In addition to the frequency, voltage must also be kept within a certain range in order to ensure the safety of persons, equipment, and final consumption devices. In the event of a voltage drop, it is the task of the network operators to contain the voltage instability.
Power factor correction
For alternating current (AC) to maintain its real power, active power and reactive power need to be out of phase by 90°. Transformers, the grid itself or various consumption processes can destabilize the phase relation between active and reactive power, which leads to a loss in voltage. Grid operators need to take measures to ensure stable voltage range , such as providing additional load.
Energy losses occur during the transport of energy. Grid operators have to forecast this power loss and purchase additional amounts of energy on the electricity market accordingly. The amount of power dissipated is influenced by the capacity utilization of the networks and represents one of the largest costs of system services. Loss energy is a by-product of power production or a system efficiency problem, which the TSOs and DSOs try to counteract.