# Overload Control in Smart Transformer-Fed Grid
## 1 Introduction  
The integration of new loads (e.g., electric vehicles) challenges distribution grid management in several aspects, such as phases imbalance, power quality issues, and transformer overload. The last aspect is important concerning the installation of electric vehicle (EV) charging stations [1]. Even in the case of the limited integration of EVs, the transformer overloads more frequently than the purely passive load case [2]. Of particular concern is uncontrolled EV charging. In a possible initial implementation scenario, the overload conditions can occur in up to 40% of the cases. Under controlled charging, the overload cases can be reduced to 20%. The conventional transformers are designed for handling higher power than the nominal one. Their overload capability depends on the highest temperature (hot spot), and short term overload is permitted for a few hours per day [3; 4]. The smart transformer (ST) is a power electronics-based transformer [5] that aims not only at adapting the voltage level from medium voltage (MV) to low voltage (LV), but at providing new services to the distribution grids: load identification and control [6; 7], reverse power flow controller [8], reactive power support in MV grid [9], power management in the DC grid [10], and grid hosting capacity increase for photovoltaics (PV) and the integration of electric vehicle charging stations [11]. The ST--in contrast to the conventional transformer--has a limited overload capability. Its overload capability has been accounted for the 120% of nominal current rating for a short time window in [12]. However, a current higher than the nominal one may impact on the ST lifetime. In this paper, a ST overload control is proposed based on the interaction with the local loads and generators in an LV grid by means of voltage and frequency signals. The idea was already proposed in [13; 14]; however, in this paper, it is described in more detail and verified by means of PSCAD/EMTDCTM simulations and lab experiments. When approaching the overload condition,the ST decreases the frequency in order to increase the local generators power output, if equipped with droop controllers. In the case of insufficient power handling capability of these resources, the ST resorts to the voltage controller. Decreasing the voltage, the load reduces the power absorption, avoiding the overload condition. The sensitivity of load active power to voltage is assumed unknown a priori throughout this study. It should be noted, however, that this can be measured on-line using the method introduced in [6; 7]--in which case the active load control through voltage setpoint manipulation becomes more accurate. In this paper, the assumption of constant impedance load is made in order to simplify the experimental validation of the method by means of a lab setup. This assumption does not affect the generality of the approach, once the load coefficient sensitivity are computed following [6]. Similar methods have been applied for energy saving purposes, such as the Conservation Voltage Reduction (CVR) method [15; 16]. Unlike the proposed overload controller, the CVR is applied in steady state and not during transients, and the current value is not controlled during the voltage variation. A possible solution to solve the conventional transformer overload is given in [17]: the transformer tap changer and the implementation of \(P/V\) droop controller in the Distributed Generation (DG) has been used to modify the voltage and following the power injection of DG in a microgrid. The current literature on ST is more focused on fault handling more than in the overload transients. To the authors' knowledge, no extensive research has been performed until now regarding the overload control of the ST or solid state transformer (SST) LV side. Particular emphasis has been given in the existing literature to the ST MV converter protection, more than in the overload condition.