# Application of Surge Arrester in Limiting Voltage Stress at Direct Current Breaker
## 1 Introduction  
With the advances in renewable energy systems and DC mechanisms, the DC grid has attracted much attention. The availability of DC generation in low- and medium-voltage levels has led to the expansion of DC grids. DC grids possess many advantages such as the absence of reactive power, lower path impedance, compatibility with renewable sources, etc. However, fault clearance is one significant barrier in these systems due to the lack of a current zero-crossing and higher fault currents due to lower path impedance. High-voltage DC (HVDC) systems have been utilized since the 1970s [1; 2; 3; 4]. The advent of low-loss semiconductor technologies, rectifiers, inverters, and DC-DC converters made the integration of multi-terminal direct current (MTDC) systems possible, facilitating energy production using new generation units, such as wind farms, solar panels, and tidal-wave generators at different voltage levels. Furthermore, deploying direct current lines in AC power grids enhances power flow control, system voltage and frequency stability and has economic benefits [5; 4]. Managing bidirectional power flow is achieved via DC microgrids at low and medium voltage levels. In addition, managing various DC distributed power sources and loads is detailed in references [6; 7; 8; 9; 10]. Despite the many advantages of DC grids, fault-current interruption and the safety of DC grids is still an ongoing challenge and a subject of research. DC grids have extremely low impedance compared to AC networks, leading to considerable short-circuit currents [5; 4; 6]. Hence, DC breakers must break the fault current at a much faster speed [7], ideally in the range of a few milliseconds [2]. Second, unlike in AC networks, there is no natural zero-crossing of the current in DC circuits. Therefore, one way to disrupt the fault current is to artificially generate a current zero-crossing via an oscillatory circuit in the DCCBs so that the current can be interrupted with a mechanical switch [5]. Currently, the available DC breakers fall into three categories, namely mechanical DCCBs, solid-state DCCBs, and hybrid DCCBs. The mechanical DCCBs have negligible series resistance due to their metal contacts and are ideal for steady-state high-current applications; however, current interruption causes significant arcing and heat generation in this type of DCCB. Solid-state DCCBs present rapid and arc-free current interruption; however, a significant amount of energy loss occurs in the steady-state operation due to the higher series path resistance. Hybrid DCCBs combine solid-state and mechanical switches, where mechanical contacts conduct the current in a steady state. Upon current interruption, the current is rerouted through a parallel solid-state switch until separation occurs between the mechanical contacts. Subsequently, the solid-state switch interrupts the current with no arc. Despite this advantage, most hybrid DCCBs involve a wait time for the mechanical contacts to separate, which delays the fault interruption, leading to significant fault-current generation unless complex rerouting circuits are used (including inductors, transformers, and/or high-current IGBTs). Table 1 presents various types of DCCBs. In [11], a bipolar DC breaker was introduced that generates a current zero-crossing with a simple structure and rapid fault-interruption capability while restricting the switching voltage to that of the source voltage. The proposed topology [11] employs a bipolar configuration that uses both DC line polarities and can reroute the fault current back to the source using two mechanical switches and two diodes. In addition, the switches are equipped with bypassing capacitors to provide a zero-voltage switching (ZVS), as shown in Figure 1b (excluding the surge arrester). This combination causes a voltage rise across the mechanical switches up to the source voltage level. In addition, via rerouting the current through the diodes, a reverse supply voltage will help damp the fault current and return it to the source.