Solid State Transformer Control Aspects For Various Smart Grid Scenarios Naga Brahmendra Yadav Gorla, Sandeep Kolluri and Sanjib Kumar Panda Department of Electrical and Computer Engineering National University of Singapore, Singapore-119077 Email: naga@u.nus.edu Abstract—The power distribution networks can be classified as active/passive and stiff/weak based on the nature of loads and network impedances. As an alternative to the conventional iron- and-copper based passive transformer, a solid state transformer (SST) can be used for interfacing such distribution networks to the medium voltage AC grids. Although the power architectures of SST for distribution grids are studied in detail, the control schemes for SST considering the nature of network is yet to be explored. In this paper, the control aspects for the popular three-stage cascaded multilevel solid state transformer (CMSST) are studied for the following grid scenarios: (i) SST interfacing two stiff grids, (ii) SST interfacing a stiff grid and a weak grid, and (iii) SST interfacing a stiff grid with a passive network. In this context, an average model of the CMSST is developed by replacing the basic switching cell (H-bridge) with its equivalent average model. It is shown that the control aspects are network specific and are not interchangeable. I. I NTRODUCTION Integration of distributed energy resources (DER) and elec- tric vehicles (EV) into the distribution networks has opened up many problems such as voltage instability, protection malfunction and unintentional islanding etc., [1]–[3]. Until now the distribution networks are connected to the medium voltage (MV) grids through an iron-and-copper based passive transformer at the distribution substation. Due to the passive nature of the conventional transformer, any disturbances on the distribution networks will be reflected onto the MV grid and eventually healthy networks will be affected. A solid state transformer (SST) can solve these problems in the distribution network by not only facilitating a controlled bidirectional flow of active and reactive powers, but also providing a stiff DC bus for decoupling the disturbance on both sides of transformer. The distributed energy storage (DES) devices, EV loads and DERs can be integrated into the DC bus of the SST. However, realizing an SST that can compete with the conventional transformer interms of efficiency and reliabity itself is a challenge [4], [5]. Various SST architectures viz., fully-modular, semi-modular and non-modular architectures are being explored keeping in mind the functionalities that it has to provide for distribution grids [6], [7]. A three-stage cascaded multilevel SST (CMSST) is a popular fully-modular SST architecture that is studied more often for distribution grid applications [8]–[10]. The three-stage CMSST architecture, which is hereafter referred to as an SST, has three stages. Stage-1 consists of a cascaded multilevel AC-DC rectifier to convert medium voltage AC 978-1-5386-4950-3/17/$31.0 c 2017 IEEE (MVAC) to medium voltage DC (MVDC). A high frequency transformer isolated dual active bridge (DAB) is used in stage- 2 to convert MVDC to low voltage DC (LVDC). Finally, a three-phase inverter in stage-3 converts LVDC to low voltage AC (LVAC). The reliability of this fully-modular SST archi- tecture can be improved by integrating extra healthy redundant modules and put them to operation in the event of any fault in the active modules [11]. The power conversion stages of the three-stage CMSST have been standardized by comparing various options, for example, resonant DC-DC converters are compared with non- resonant type DC-DC converters to find a suitable choice for isolation stage [12]. Similarly, various multilevel architec- tures viz., cascaded H-bridge, cascaded neutral point clamped (NPC) and flying capacitor (FC) multilevel configuations are compared and the cascaded H-bridge is preferred in the first stage of the CMSST especially for higher number of levels [13], [14]. However, the control aspects of the three-stage CMSST have not been explored in depth. In most of the SST experiments, the LVAC side is considered as a passive network and the SST is designed to act as a grid forming converter [15], [16]. In [17], the LVAC grid is modelled as a voltage source neglecting the impedance of the distribution network which is not practical because most of the distribution networks are either resistive or inductive depending on the length of feeder. Hence, the focus of this paper is to explore various grid scenarios, understand them and propose suitable control techniques for the SST. Power networks/grids can be classified as active and passive depending on the nature of loads connected to the network. Now-a-days, most of the distribution networks are becoming active due to the integration of DES and DER. Some of the unforeseen challenges with the active distribution networks are the reverse power flow at the distribution transformer, voltage raise, voltage flicker, harmonics, unexpected islanding and sympathetic tripping [2]. Although researchers have coined several solutions for these problems such as dynamic on-load tap changers and reactive control of DER etc., the solution are limited by the nature of the network [18]. The second classification of power grids is based on the impedance, whether it is a strong grid or a weak network. A detailed analysis on the classification of AC networks and loads is presented in Section II of this paper. Rest of the paper is organized as follows. The three-stage CMSST and