978-1-5386-5186-5/18/$31.00 ©2018 IEEE Control and Voltage Stability of a Medium Voltage DC Micro-grid Involving Pulsed load Hassan H. Eldeeb, Student Members IEEE, and Osama A. Mohammed, Fellow, IEEE Energy Systems Research Laboratory, Florida International University, Miami, FL 33174, USA helde002@fiu.edu, mohammed@fiu.edu Abstract— Recent increase in the demand of medium voltage DC (MVDC) systems is due to the high penetration of distributed generators, pulsed loads and energy storage systems (ESS). This paper develops control techniques to improve the voltage stability of an islanded multi-terminal MVDC ship-board. The ship-board accommodate three synchronous generators interfaced via bi directional AC-DC converters as well as pulsed load. The control system has two objectives; 1) limiting the current flow in the MVDC ship-board cables, and 2) Support voltage profile stability even under pulsed load condition. This was achieved through two stages; 1) operating the ship-board converters in master-slave mode in nominal operation and 2) utilizing ultracapacitor (UC) to support the voltage stability in pulsed load conditions. The dynamics of the control loops were investigated in the PSCAD/EMTDC environment. The results proved the effectiveness of the control technique and the employment of the UC to support voltage stability. Index Terms—voltage stability, islanded MVDC ship-board, Ultracapacitor, pulsed load, coordinated control. I. INTRODUCTION Medium voltage DC (MVDC) ship-board power systems require simpler control than their AC analogous. Moreover MVDC are more reliable as most of the the energy storage systems (ESS) are DC [1]. Consequently, MVDC ship-boards support more efficient, reliable and less expensive platform to integrate the DERs as well as heavy pulsed loads [2], [3]. The pulsed load is defined to be the power load that require high power input frequently [4]. The sudden high current withdrawal by the pulsed load subjects the voltage profile of the hosting ship-board to the hazard of instability [5]. With proper sizing and control of high power density ESS (i.e.: flywheels), DC ship-boards could host the pulsed loads without the hazard of voltage profile collapsing [6]. Owing to the plentiful merits offered by the DC systems [7], the ship power system industry started to adopt DC distribution system with medium voltage (MV) levels [8]. Thus, the islanded medium voltage DC (MVDC) ship-boards operation, control and stability require more deep and detailed investigation. Despite the numerous pros of DC ship-boards, its implementation faces plentiful obstacles [9]. Unlike the conventional AC ship-boards whose control and energy management is ubiquitous in literature [10], [11], the control algorithms of the DC ship-boards are yet to be developed. Moreover, the power flow in the DC systems is mainly determined by the two factors: 1) the voltage difference (ΔV) between the DC terminals of the ship-board, and 2) the resistance of the interlinking cables between the terminals. Thereby, it is burdensome to achieve a safe operational point in which none of the cables is overloaded or operating near its thermal stability limit [12]. The problem become more significant when the MVDC ship-board is physically on small scale. Par instance, in the shipboard power systems, the cables interconnecting the ship-board terminals are short and of low impedance. Thus, the system would be more vulnerable to voltage mismatch between the terminals of the ship-board, which could result in system instability [13]. In DC ship-boards, the inductances of the cables have no effect on limiting the current flow among the ship-board. Moreover, the shipboard power system has pulsed loads of intermittent nature. For instance, the launching systems of air crafts, electromagnetic propulsion weapons and electron laser gun hosted by the shipboard, [14], represent additional burdens on MVDC system control and stability. Consequently, the energy in the MVDC ship-board should be managed in way that ensure the system’s stability with respect to voltage as well as current flow control. The protection and cyber security of such systems is another issue to consider [15]- [19]. Few attempts exists in the literature that address problem of managing the energy in islanded DC ship-boards hosting pulsed loads. A coordinated control strategy based on integrator current sharing was proposed in [20]. In that research the circulating current between ship-board terminals through eliminating the deviation in the current sharing term. However, the DC ship-board under study in [20] is connected to the AC grid, hence it is not islanded. Moreover, the effect of pulsed load was not investigated. A real-time coordinated control was proposed in [21] for a microgrid hosting pulsed load. The effect of the pulsed load was mitigated by a lithium- ion battery bank. However, the system under study in that research was hybrid AC/DC system, and using batteries to compensate the effect of pulsed load in not adequate. Furthermore, subjecting the battery bank to severe power pulses because of the pulsed loads results in operating the batteries at very high discharging currents. Thus, forcing the battery to operate in micro-cycles rather than complete cycles which adversely affects the life time of battery as verified in [22], for various batteries technologies. Ultracapacitor (UC) is more preferable than batteries to mitigate the pulsed load effect in DC [23]. A Hierarchical control of DC microgrids with renewable energy intermittent resources as well as lead- Part of this work was supported by grants from the office of Naval Research and the US Department of Energy. The authors are with the Energy System Research Laboratory, ECE Department. Florida International University, Miami, FL, 33174 USA. (E-mail: mohammed@fiu.edu).