# Phase Balancing and Reactive Power Support Services for Microgrids
## 1 Introduction  
Smart grids are proposed to drive forward the traditional electrical grids to a more flexible, secure, reliable and efficient networks. The development of smart grids requires complex frameworks which should be supported by the appropriate infrastructure (e.g., advanced metering, information and communication technology (ICT)). In addition, renewable energy sources (RES) are able to play a key role in the sustainability of future grids. However, their variable and unpredictable nature cannot be easily handled in a global context. Moreover, the electrification of thermal loads and the expected increase of electrical vehicles (EVs) will increase significantly the already rising demand. To satisfy this climbing demand, improvements will be required on the current infrastructure, adding significant costs to the distribution system operator (DSO) and grid owner. Alternating current (AC) microgrids (MGs) are introduced to deal with all these issues since a better coordination among the distributed energy resources (DERs) can be achieved to support the local loads and enable the more efficient utilization of existing infrastructure. MGs are expected to provide both environmental and financial benefits, however their implementation requires advanced control, power management systems and protection schemes [1; 2]. Hierarchical (primary, secondary and tertiary) control structures are proposed to deal with various objectives at different time scales in References [3; 4]. Decentralized and distributed control structures are also proposed for controlling MGs, however further research is needed to ensure stable operation under any grid conditions [5; 6]. At a primary level, RES and energy storage systems (ESSs) local controllers including inner current/voltage control loops should follow the reference values considering local measurements. The reference values are generated from the secondary control level in order to enhance the power system quality and to achieve fair power sharing [7] among resources. This state usually requires monitoring of the grid operating conditions and communication infrastructure to coordinate the RES local controllers. Finally, a tertiary control level is added on top of the previous two layers in order to optimize the operation of the whole MG in both technical and economical manners [8; 9]. According to the standard for the specification of the MG controller [10], MGs should be able to provide ancillary services (i.e., reactive power support, voltage control, frequency control, phase balancing) for enhancing efficiency, reliability and power quality of the distribution network. Reactive power consumption is considered among the major concerns for the system operators from the early beginning of the electrical networks since it reduces the available capacity for active power and increases the energy losses. In addition, most of the loads in low voltage distribution networks and MGs are single phase connected. Therefore, intense phase unbalances can be observed at the MG's point of common coupling (PCC) which can jeopardize the power quality of the distribution grid affecting the losses, the capacity of the lines and the performance of power electronic based DERs. These ancillary services can be provided by exploiting advanced capabilities and new functional operating modes of power electronics based DERs. Converter-interfaced DERs can be utilized to provide several control functionalities. For instance, the main requirement of grid tied inverters of photovoltaics (PVs) is the extraction of the maximum active power from the PV panels using advanced maximum power point tracking (MPPT) algorithms [11] and properly inject this power into the grid in synchronized manner. However, PVs' inverters can also operate in de-loaded mode; responding to frequency variations [12] and providing reactive power support. It should be noted that there are already commercial inverters providing such functionalities [13].