3 ENERGY TRANSITION
4.4 Ancillary services and reserves
The aim of ASs is to support the reliable electricity supply and the transmission system operation. The demand and supply must be balanced constantly in the transmission system by ensuring that frequency, voltage, and power remain within certain limits. The adjustments and corrections in the power grid are provided by the ASs, including frequency stability support, power balance, voltage control, supply restoration, and system management (Chuang & Schwaegerl, 2009), as
presented in Table 3. This thesis focuses on frequency stability, power balance and voltage control ASs, which are explained below.
Table 3. Ancillary services.
Ancillary Service Type Means
Frequency stability support Frequency control of power, regulation, and operating reserves Power balance Scheduling and dispatching of balancing
energy
Voltage control Tap-changer control
Reactive power control Supply restoration Black start capability
Island operation
System management Power quality assurance operation Asset management
Frequency stability support services apply to normal operations of the grid in power system balancing and disturbance situations (power plant outages) or unexpected events (an unforeseen increase in consumption). The services supporting the power balance in the network aim to avoid foreseeable potential grid congestion or bottlenecks. Voltage support services ensure the quality and safety of electricity. Power factor correction aims to adjust the relationship between active and reactive power to stabilise voltage within the operational range.
Supply restoration services use power plants (for example, hydroelectric or gas) able to offer automatic black-start without external energy supply after a power outage. (Kraftwerke, 2020)
The means to control the power system’s frequency and power balance are generally upward regulation with a reserve that can increase electricity generation or reduce electricity consumption, and downward regulation with a reserve that can reduce electricity generation or increase electricity consumption. Operating reserves can be provided from the part-load operated and the standing but fast-starting power plants, controllable loads, wind power plants’ downregulation, and energy storages (Chuang & Schwaegerl, 2009). These operating reserves are divided into the following (Denholm et al., 2019):
1) Frequency-responsive reserves, activated automatically by frequency changes
2) Regulating Reserves 3) Contingency Reserves 4) Ramping Reserves
5) The normal operation provided by energy and capacity
The time association of frequency regulation and operating reserves for the frequency stability support AS in Europe is illustrated in Figure 19 that was created based on Chuang & Schwaegerl (2009), Dattaray et al. (2019), ENTSO-E (2013), Fingrid (2018), Kaushal & Van Hertem (2019), Tamrakar et al. (2017), TUT et al.
(2020), Yap et al. (2019), and You et al. (2017).
Figure 19. Time association of frequency regulation and operating reserves.
Various and random deviations in generation and load or disturbances can cause a frequency deviation in the power system to which the power system’s primary control must react by the involved generators. The power equilibrium is immediately restored with the primary control by maintaining the system frequency within the permissible limits. If the frequency still exceeds the limits, additional measures outside the scope of primary control, such as automatic load-shedding, are executed to maintain interconnected operation. When the balance is restored, the system frequency stabilises and remains at a quasi-steady-state (restoration range) differing from the frequency set-point value. Then, the secondary control manages the remaining deviations (after 15 – 30 s), restoring power in cross-border exchanges to set-point values and the system frequency to its set-point value and completed (after 15 min). If the demand exceeds generation continuously, actions must be made to restore the balance despite the reserve
capacity. The tertiary control replaces the secondary control reserve to restore a sufficient secondary control band by standby supplies and contractual load variation (load shedding). (ENTSO-E, 2004; Hodel et al., 2019)
The primary control is the frequency containment process (FCP), and the secondary control is the frequency restoration process (FRP) (Yap et al., 2019).
The power systems’ frequency control is based on the harmonised services across Europe. Frequency containment reserves (FCR) aim to maintain the frequency as close as possible to 50 Hz. Further, FCR reserves are divided according to the normal operation (FCR-N) and disturbance situation (FCR-D) reserves. Next, the frequency restoration reserves (FRR), realised automatically (aFRR) or manually (mFRR), aims to free activated FCR to take care of the subsequent frequency deviation and to restore frequency to the normal range. Further, the RRs release the activated FRRs back to a ready state for new disturbances. In the future, low inertia situations are to be managed faster than the FCR operates with the fast frequency reserves (FFR) (Fernández-Muñoz et al., 2020). FFR is procured only for low inertia situations, coping with a fast drop in frequency after the dimensioning failure. (TUT et al., 2020)
Electricity markets are developing for integrating DER and DR, aiding to maintain the flexibility and reliability of power systems. However, a review of AS marketplaces is excluded from this thesis since they relate to the economic perspective.
Voltage control at a grid node can be performed by adjusting reactive power. The voltage is raised by feeding in and reduced by intaking of reactive power. TSOs specifies reference voltages for the generation units and the distribution networks in the transmission grid connection points. The controlled reactive power exchange allows the voltage at the connection point to be settled to specified reference limits. Power plants directly connected to the transmission grid are obligated to participate in voltage support actively. Distribution grids are participating partially active in the voltage support (EU, 2016).
AS provided from DG (or BESS) can be voltage control, regulation, load following, spinning reserve, supplemental reserve (non-spinning), backup supply, harmonic compensation, network stability, seamless transfer, and peak shaving (Campbell, 2005). In developing power systems, ASs provided by DERs can be carried out for both transmission (system-wide) and distribution system level. The distribution system level AS is called local AS in this research. ASs to the transmission grid can be the frequency response, voltage/reactive power control, and smoothing of fluctuating power, and the local ASs can be reactive power control, smoothing of fluctuating power, harmonic mitigation, and fault-clearing and fault ride-through
(FRT) capability (Demoulias et al., 2018; Oureilidis et al., 2020)(Oureilidis et al., 2020). Microgrids provide an excellent option for AS operations (Laaksonen et al., 2021).
The next section focuses on a study case of local AS provided by a large-scale DG unit. A coordinated reactive power ADNM scheme is developed for the DSO to manage power flow at distribution/transmission network POI.