# Compliance of Distribution System Reactive Flows with Transmission System Requirements
## 1 Introduction
### Literature Review  
Several works have investigated the topic of reactive power exchange between transmission and distribution grids in the past. In [2; 3], novel methods are presented for assessing the range of controllable reactive power available at the transmission node for any level of active power exchange with the distribution network. In [4], particle swarm optimisation is used to exploit wind farm active and reactive power control capability to achieve a reactive power exchange level. In [5], the objectives of maintaining safe voltage levels and respecting the reactive power exchange limits in distribution networks are achieved via a ruled-based method and the use of an iterative process. In [6], Model Predictive Control (MPC) is employed for achieving multiple objectives, among which is the minimisation of reactive power exchange, and in [7] the method is expanded to include the potential for DSOs to provide a range of reactive power set-points as a service to the TSO, according to DN capabilities. In [8], a rule-based central controller is calculating the reactive power compensation level from Distributed Generators (DG), to which level the DG comply according to their capabilities. In [9], a two-stage optimisation method is employed for controlling both voltage levels and reactive power exchange. The tap positions of capacitor banks are optimised on a daily basis, and reactive power support from DG on an hourly basis.  
Overall, the proposed methodologies can be divided into those that have as an objective to minimise deviations from set-points [4; 6; 7] and those that have as a constraint to keep the reactive power exchange within certain limits [5; 8; 9]. This paper falls in the second category, and its objective is to keep reactive power exchange between transmission and distribution grids within the limits specified by the TSO. Specifically, the limits of the Finish TSO, Fingrid [10]. The recent requirements of the Finnish TSO do not define a simple static upper and lower limit for reactive power exchange, but a complex structure that depends on active power exchange.  
Previous attempts to solve this problem have been made in the past. Different requirements for the reactive power flow at the TN and DN Point of Interconnection (POI), for the Finnish distribution grid case, and tested at the Sundom Smart Grid, is presented in [11]. This paper considers requirements for (1) European Commission's conditions for reactive power management for the transmission grid-connected distribution systems [12], (2) the conditions set by Fingrid [13] and (3) the Non-Detection Zone (NDZ) requirements for microgrids [14; 15; 16]. It presents the \"future reactive power window\"--developed for the Sundom Smart Grid--and a reactive power controller for a 3.6 MW Wind Turbine (WT) unit connected to the MV bus with a full-scale converter that controls reactive power according to the Fingrid and NDZ requirements. Further, techno-economic case studies controlling the reactive power flow at the POI of the developing Sundom Smart Grid according to the Fingrid requirements are presented in [3]. The reactive power software controller development and implementation into the lightweight Intelligent Electronic Device (IED) microcontroller, based on IEC61850 GOOSE, and testing the IED in the real-time Controller Hardware-In-the-Loop (CHIL) platform are presented in [17; 18]. The potential of accelerating the long-term simulation and CHIL testing is beneficial to the developing the DER controls for AS provision and is demonstrated in [19]. In all of these papers, the reactive power controllers were operating according to the last measurements of active and reactive power at the POI. Acceptable results were obtained by setting more tight reactive power limits than Fingrid requires. The work in [17] provides evidence for the need of developing a predictive controller.