Glossary

Maintenance
Maintenance involves carrying out inspections, maintenance and repair work. This guarantees that the facility is kept in its desired condition throughout its service life.
Medium load

Medium load is the amount of power that is consumed by an electrical load during the greater part of a day, predominantly from morning until evening.

Merit order

Merit order is a way of ranking generating units to show the order in which they are to be used. This is determined according to the variable costs of electricity generation. Starting with the generating units with the lowest marginal costs, power stations are brought on to the grid with increasing marginal costs until the demand is satisfied.

Meshing, degree of meshing

The degree of meshing indicates the number of other nodes that an individual grid node in the transmission network is connected to. In a highly meshed network, the grid nodes have a large number of direct connections to other nodes. A higher degree of meshing is the foundation for a high level of supply reliability in the transmission network. As a rule, all circuits in transformer stations and substations are connected to one another (‘coupled’) in order to reduce network losses, provided that other technical reasons such as the level of short-circuit capacity or stability requirements do not preclude this. If, however, specific circuits that are experiencing high loads they can be relieved of this load by a process known as ‘demeshing’, whereby the circuits in question are separated from the interconnections. This can be done in different way, e.g. by opening couplings or by directly connecting selected circuits using separate busbar sections within a facility.

Minimum output

The minimum output of a generating unit indicates a level where output must not fall below, due to facility-specific or equipment-based reasons. If the minimum output does not refer to long-term usage, but rather to a shorter time period, this must be clearly highlighted.

Must run

In addition to covering the demand for electrical power, the provision of power from generating facilities can also be determined by other factors. In these cases the infeed of energy into the electricity grid is independent from the actual level of demand. This includes facilities that, due to technical restrictions, must feed in energy at predetermined times – in particular, heat led CHP plants where electricity generation takes place depending on respective levels of local heat demand. Without the use of options to increase flexibility such as boilers, it is not possible to shut down these facilities without simultaneously restricting heat supply – the facilities ‘must run’. Further restrictions may arise, for example, if facilities supply industrial processes or provide the power plant site’s own energy supply (e.g. in lignite mining regions).