Ancillary services

The term ancillary services is used to refer to a variety of operations beyond generation and transmission that are required to maintain grid stability and security. These services generally include, frequency control, operating reserves, voltage and reactive power control, voltage support in case of faults and various, coordinated actions related to system recovery after major disturbances. Ancillary services are managed by system and network operators. Most of them, however, are provided by network users and, in particular, generators. (For more details review the article on Ancillary Services at Wikipedia.)

Automatic generation control

Automatic generation control (AGC) is a system for adjusting the power output of multiple generators at different power plants, in response to changes in the (residual) load. (For more details review the article on AGC at Wikipedia.)

Curtailment

Curtailment describes an intended, temporary reduction of the power output of variable renewable energy plants below the actual potential offered by the resource. This may be, for example, motivated by network congestion or operational restrictions of the generation mix (e.g. minimum must run capacity from dispatchable plants).

Dispatchable plants

Dispatchable plants do not depend on fluctuating resources like wind and solar. Their generation output can be scheduled and set the to any value within the operational range, respecting possible restrictions. Restrictions may be related to technology (e.g. mimium loading of thermal plants). Also environmental factors affect operational restrictions: lack of cooling water (thermal plants) or depleted water reservoirs (hydro plants) may force reduced output.

Dynamic line rating

Dynamic line rating allows to load overhead lines above their nominal ratings in case of favourable meteorolical conditions. Overhead lines are heated by the current flowing through the wires. The maximum temperature is determined by the manufacturers component specifications and by the resulting sag of the line (minimum clearing distance to ground). However, in case of low ambient temperature or strong wind, the conductors of an overhead line are effectively cooled and, hence, with the same current their temperature is lower than under conditions assumed in standards and manufacturers specifications.

Energy Transition

A full decarbonisation of the energy system requires huge efforts in various sectors, including in the transport, industry and buildings sectors. Decarbonisation of the power sector is one (and a major) part of the transition of the worldwide energy systems.

Extra high / high / medium and low voltage

Power networks transport and distribute electricity from generators to consumers' loads. High voltage transmission lines allow to transport electricity over large distances with minimum losses. For distribution of electricity close to load, voltage is stepwise reduced.
Voltage levels depend on structure and scale of the networks. Terminology ('What is high voltage?') depends on national conventions.

Fault

High Voltage Direct Current (HVDC) Links

If power is transmitted over very long distances, the losses associated with alternating current (AC) increase considerably. In these cases, high-voltage direct current (HVDC) lines are an alternative.
At the ends of the line, dedicated converter stations convert AC to DC and back to AC. From one station to the other, DC currents can be transported over very long distances with low transmission losses. However, the converter stations introduce their own losses (and costs).
DC links allow coupling of systems which are not synchronised. Nominal frequency levels as well as phase angles may differ. Using power electronic converters , (state-of-the-art) HVDC links do not provide inertia. They have to be taken into account like vRES when assessing SNSP restrictions.

Imbalance

From a system operation point of view, imbalance is the difference between generation and load at a given moment in time. From a power market perspective, imbalance describes the difference between the energy delivery (or consumption) committed by a market participant ahead in the dispatch and its actual delivery (or consumption) in real time.

Inertia

Inertia is the resistance of any physical object to any change in its velocity (Wikipedia). Think of a riding train: it takes some meters (...) to stop.
In power systems, all rotating masses introduce inertia. This includes generators, but also rotating loads directly connected to the network.
In case of an imbalance between generation and load, system inertia supports the frequency. This is of particular importance immediately after a sudden loss of generation (or load). In the first seconds after a disturbance, the stabilising effect of system inertia allows the reserves to pick up.
vRES replace generation from rotating machinery with power electronic converters. With current designs, these do not provide inertia. Hence, overall system inertia may be reduced with increasing vRES penetration.

Power electronic converters, PV inverters

Power electronic converters convert electrical energy of any shape from a power source into a shape suitable for feeding into the network. Example: DC current from a solar PV array is converted into AC current with a frequency of 50 or 60 Hertz. In the case of PV, the power electronic converters often are called PV-inverters.
Power electronic converters use semiconductor switches to create the required sine shape of the current. Sometimes, these converters are also called static converters because they do not have any moving parts. As the shape of the current solely depends on the control of the switches and not on the rotational speed of the generator or the voltage at the network terminals, converter output is also called non-synchronous generation.
In some aspects, power electronic converters behave differently than synchronous generators:

• By design, they are not able to energise a network without some pre-existing voltage meeting the quality standards of the network;
• With nowadays designs, they represent a current source. Their peak current output is just slightly higher than the rated current. As a consequence:
  A) They do not improve voltage quality in case non-linear loads introduce harmonic distortion. This may affect the quality of supply for the customers.
  B) They have a lower short circuit capacity than synchronous generators. This may affect protection schemes.
• With current designs, they do not add inertia to the system.

Residual load

In the past, i.e. before introducing vRES, the complete system load was supplied by dispatchable plants only. If part of the load is covered by vRES, at any moment in time the remainder is called residual load. Simplifying:
residual load = total instantaneous load - instantaneous vRES generation.

Synchronous generators

Synchronous generators are electro-mechanical machinery. By rotating they convert mechanical in electrical energy. The voltage created is determined by physical laws and is close to a sine shape. The terminals of synchronous generators are directly connected to the network. Because of this direct connection, the rotational speed of the generator and the system frequency are linked very strictly (synchronism).
One important effect of synchronism: synchronous generators effectively contribute to system inertia and, thus, inherently stabilise the system frequency.

System Non-Synchronous Penetration - SNSP

System Non-Synchronous Penetration describes the share of supply from technologies relying on power electronic converters in the instantaneous power balance. This applies to vRES technologies like solar PV and wind but also to transmission interconnectors using High Voltage Direct Current technology.

Unit commitment, economic dispatch

The system or market operator in advance schedules power plants for electricity generation. This ex-ante scheduling of plants is known as unit commitment. If scheduling primarily is based on economic efficiency, it also called economic dispatch.