Automatic Voltage Control (AVC) of Danish Transmission System - Concept design

For more than 20 years it has been a consistent plan by all Danish governments to turn the Danish power production away from fossil fuels towards renewable energy. The result today is that 37% of the total Danish power consumption was covered by mainly wind energy in 2013 aiming at 50% by 2020. Another consequence is the public way of generally thinking green which have led to a national decision of undergrounding not only all of the Danish distribution system but also the future transmission system. These issues initiate the infrastructure constructions of the transmission system i.e. a large amount of overhead lines over 100 kV as well as the new planed transmission lines will be undergrounded; the transfer capacities will be enlarged by upgrading the interconnections. Large amounts of reactive power components will be placed in the system partly for cable compensation and partly for voltage security. Consequently, the ramping speed of the transits in the main corridors via the interconnections is foreseen to be increased. The voltage control based on the present system is thus becoming a challenging objective. This survey paper presents some of the existing AVC systems i.e. system structures, objectives, constraints, algorithms for optimal power flow and some special functions in particular systems, which inspires the concept design of a Danish AVC system to address the future challenges of voltage control. In the concept, the Danish AVC design is based on a centralized control scheme. All the buses are monitored where the voltage magnitudes are maintained continually. The loss minimization including switching cost is the objectives of the AVC system. The reactive power reserves as constraints are taken into account to limit the regulation capabilities of generators. The Day ahead and short-term forecast is to be implemented to reduce the working load of the real time system, as well as to minimize the numbers of switching times of the discrete components. The fallback controllers are to be installed in selected substations to enhance the system reliability and the robustness in case the substation loses the telecommunications to the control center. RPCs will be integrated to the AVC system as normative regulators in the later stage. Distributed generation units can be organized as virtual power plants and participate in voltage control at transmission level. Energinet.dk as the Danish TSO will implement the first stage of the AVC system by 2016.