# Dimensioning of Reactive Power Compensation in an Autonomous Island System
## 1 Introduction  
The autonomous power system of Rhodes Island, Greece, has recently undergone a major transformation, including the implementation of a new thermal power plant (TPP), the reinforcement and upgrade of the HV network transmission system from 66 kV to 150 kV, and the installation of a new GIS HV/MV substation fed by underground HV cable lines to supply the city of Rhodes. These developments have altered the fundamental electrical characteristics of the local power system and created challenges that had to be overcome to secure its operation. One major challenge was related to the management of excess reactive power during low demand periods, an issue typically addressed by shunt reactors, as is the case in modern power systems incorporating significant lengths of cable lines [1]. To properly size reactive compensation, voltage rise and power transfer capability limitations are taken into account in typical interconnected system planning studies [1; 2; 3; 4]. HVAC overhead and cable lines produce substantial amounts of reactive power which may have to be transmitted over long distances, if not consumed locally, as is the case in lightload conditions. This, in turn, will impact active power transmission capability, increase losses, and give rise to significant overvoltages. VQ curves can help determine the amount of reactive compensation needed, while secondary voltage control (if applied) is in charge of shunt compensation switching, with the objective of maintaining reactive reserves on generators to face incidents [5]. Reactive compensation can significantly improve voltage stability, and the effect of its allocation and amount on the voltage stability margin is often analyzed with sensitivity analysis methods [6; 7]. Traditionally, the locations for placing new VAr sources are either simply estimated or directly assumed. The optimal allocation and determination of the types and sizes of installed reactive compensation is known as reactive power planning. Recent research has presented some rigorous optimization-based methods to address reactive power planning as a security-constrained optimal power flow problem in large power systems [8; 9; 10; 11]. In autonomous systems, excess reactive power will have to be consumed by voltage-regulating units, typically synchronous generators operating in an underecitated mode, a situation usually characterized by a reduced rotor angle stability margin and prevented by means of underecitation limiters (UELs) [12]. However, the application of underecitation limiters in autonomous systems should be approached with caution, as this may result in overvoltage instability, even though there may be no imminent risk of loss of synchronism [13; 14; 15]. An incident of this type occurred in Rhodes in 2016 and is analyzed in detail in [16]. As it will be further analyzed in this paper, in dealing with a small autonomous system, the main driver when sizing reactive power compensation is not voltage rise concerns in certain parts of the network, but rather the reactive power absorption capability of local generators. An investment-oriented approach would foresee reactive power compensation facilities large enough to consume any excess VAr in the system, ensuring that reactive power issues never interfere with unit commitment and economic dispatch (UC-ED). However, such an approach would inevitably result in over-dimensioning compensation facilities. An operational approach, on the other hand, would seek a suitable unit commitment schedule, securing online units with sufficient reactive power absorption capabilities to manage excess VAr in the system. Even if a feasible UC solution does exist, such an approach would come at the detriment of operating efficiency, as reactive power constraints would impact the generation cost of the system and potentially hamper renewable energy penetration by enforcing the operation of a high number/capacity of conventional units.