## 3 Electricity Distribution Network Development
### Ancillary Services and Reserves  
Ancillary services (ASs) support the reliable delivery of electricity and the operation of the transmission systems. The supply and demand must be balanced at every moment, ensuring that frequency, voltage, and power load remain within certain limits. When demand changes, the adjustments and corrections in the power grid are completed with ASs. These services include frequency stability support, power balance, voltage control, supply restoration, and system management, as presented in Table 1[49].  
Frequency support services are applied for normal operations: power system balancing and the disturbance situations or unexpected events such as power plant outages or an unforeseen increase in consumption. For example, peak shaving by load shedding is a frequency support method, often provided by heavy industry loads that are deactivated for a short time to guarantee network stability. The spinning reserve compensates for the short-term power failures due to the kinetic energy of power plant generators. REGs do not yet provide spinning reserves, but large wind turbines (WT) might be used [50].  
Voltage support services ensure the quality and safety of the supply. With power factor correction, the relation between active and reactive power is adjusted so that voltage is within the operational range and stabilized. The electricity transmission and distribution cause power losses that affect the voltage magnitudes in the grid. The use of the network's capacity (particularly in the cabled network) and capacitors affect energy losses, and consequently, voltage management [50].  
After a power failure, supply restoration uses services that power plants can offer with automatic black start capabilities without external energy supply, such as hydroelectric or gas power plants. In addition, electricity storage can be used to ensure the black start capability [50].  
ASs can be used for the power grid's operational and bottleneck management. The rise of RES increases demands for the grid operators to manage the network congestion, aiming to avoid the foreseeable potential grid bottlenecks. In congestion management, the power plant operators are instructed to shift the planned electricity generation, called re-dispatch. Another method is the feed-in management for regulating the supply in the case of a power surplus from RES. In addition, the available capacities and capacity mechanisms can be used as ASs. Such services can be for the grid reserves or the reserves to guarantee the reliability of power plants [50].  
TSOs are responsible for managing the power balance in the transmission grid in each operating situation. The quality criteria for frequency and voltage must be met in normal operation and abnormal operations. For constant control of frequency in power systems, the frequency containment reserve (FCR) programs are used. Next, the frequency restoration reserve (FFR) programs are implemented for returning the frequency to its normal range and release the FCR back to use. Further, the replacement reserves (RR) can be released to activate the FFR to stand by in case of new disturbances (not used in the Nordic system) [51].