Hybrid storage system composed of battery energy storage systems (BESSs) and supercapacitors is a promising solution to mitigate the high frequency power fluctuations of pulsed power loads (PPLs) in ...medium voltage DC (MVDC) shipboard power systems. Due to the presence of multiple storage units, a power sharing algorithm needs to be considered within the energy management system (EMS) in the shipboard power system. In this paper, an optimal power flow problem is formulated for MVDC shipboard power systems with hybrid energy storage systems. Battery energy storage systems (BESSs) and conventional generation units are enhanced with virtual resistive droop controllers to share the steady-state power fluctuations. Supercapacitors are enhanced with virtual capacitive droop controllers to share the high frequency fluctuations in the load. The optimal flow accounts for minimizing the operational cost of the generation units in the shipboard power system considering the constraints of network, load balance, voltage profile, and power/energy limits. Second-order cone programming (SOCP) relaxation is used to approximate the nonconvexity of the optimal flow formulation and necessary conditions for global optimality of the solution are discussed. Results confirm the effectiveness of the proposed formulation in load support and managing the energy between storage units.
DC transformers based on power electronics are key items of equipment for medium voltage dc (MVDC) distribution systems. Both high and low step-ratio dc-dc conversions are required to interface dc ...links at different voltages to form an integrated dc distribution system. The transformer-coupled resonant modular multilevel dc-dc converter (RMMC) is well-suited to high step-ratio connection between an MVDC network and a low voltage dc network but it is not suitable in low step-ratio conversion for linking two MVDC networks with similar but not identical voltages. This article presents a circuit evolution of the high step-ratio transformer-coupled RMMC into its low step-ratio transformer-less RMMC counterpart. These two RMMCs share the same structure and the same resonant process within the resonant submodule stack giving rise to the same operational advantages. Also, the circuit design experience can be readily transferred from one to the other. From these two base RMMC circuits, a family of RMMCs with further configurations is elaborated that provide a wider variety of connection options for MVDC distribution systems. In this circuit family, each high step-ratio transformer-coupled RMMC has a low step-ratio transformer-less RMMC counterpart and one can be transformed into the other via the circuit evolution process presented. The theoretical analysis for both high and low step-ratio RMMCs has been verified through full-scale simulations of medium voltage examples and further verified through down-scaled experiments on laboratory prototypes.
The cascaded three-level neutral-point-clamped (3L-NPC) converter and the modular multilevel converter (MMC) are attractive solutions for medium-voltage direct-current (MVdc) applications. Due to ...their low cost compared to MMCs, cascaded 3L-NPC converters have been adopted in ANGLE-DC-a 30-MVA MVdc link demonstration project in North Wales, U.K. The dc voltage imbalance across submodules (SMs) is a common challenge for both types of MVdc converters. Such imbalance is topology dependent and remains underresearched for cascaded 3L-NPC converters. In this article, a small-signal model-based analysis has been done to reveal that the dc voltage imbalance in cascaded 3L-NPC converters is caused by an unstable system pole. Two voltage balancing methods are presented. The first method is based on PI controllers to precisely regulate SMs' voltages without influencing output power. However, it relies on communication between a central controller and local controllers within SMs. The second method uses inverse-droop-based control to take over the dc voltage regulation upon loss of communication. Both balancing methods are experimentally validated using a 30-kVA testbed based on the ANGLE-DC project. It has been demonstrated that the dc voltages of SMs can be effectively balanced with both methods during changes of load conditions and dc bus voltages.
This paper presents a design methodology for RC snubbers to address Dynamic Voltage Imbalance (DVI) in series-connected 10 kV SiC JBS (Junction Barrier Schottky) diodes, utilized in a 3-Level (3 L) ...Neutral Point Clamped (NPC) converter. The proposed method considers the effect of bus bar inductance, unequal diode junction capacitance, base plate capacitance, and common mode choke tied between the heat sink and midpoint of the DC link capacitor on snubber losses and turn-off voltage mismatch across series-connected diodes. Additionally, the effect of hard-switched turn-on across complementary SiC MOSFETs has been considered on snubber loss and Dynamic Voltage Balancing (DVB) across SiC JBS diodes. The following steps have been used to simplify the R-C snubber design: (1) Modeling the switching transition across series-connected 10 kV SiC JBS diodes and 10 kV SiC MOSFETs on two-level Clamped Inductive Switching (CIS) test setup, (2) Modification of snubber capacitor and switching transition model for 3L-NPC topology, using the experimental data, (3) Modification of snubber resistor using experimental data from 3L-NPC setup with common mode(CM) choke. Key design parameters obtained from the proposed model have been compared with the experimentally obtained data, which validates the accuracy of the model. Experimental results at the 7 kV DC bus show that the designed R-C snubber constrains the turn-off voltage mismatch, snubber loss, and turn-on transition time within the specified limit.
Five-level hybrid-clamped (5L-HC) converter is a newly proposed topology which is suitable for high-performance medium-voltage high-power conversions without switches directly connected in series. ...The critical issue of this converter is that two flying capacitor voltages of each phase and two neutral-point potentials of the dc-link need to be balanced. This paper presents a decoupled voltage balancing method for this 5L-HC converter based on modified phase-shifted pulse width modulation (PS-PWM) and optimal zero-sequence voltage injection. The voltages across the central dc-link capacitor and two flying capacitors are balanced first by adjusting the width of four PWM signals. Second, the relationship between the neutral-point currents and the output phase voltage is studied and the upper and lower dc-link capacitor voltages are balanced by zero-sequence voltage injection. Both steady-state and dynamic-state simulation and experimental results are presented to confirm the validity of this method.
Medium voltage (MV) ac-dc hybrid distribution network attracts much interest due to its high reliability, high power quality, and high compatibility with distributed generation and dc loads. As the ...key enablers to interconnect ac and dc grids, the converter stations usually adopt the modular multilevel converter (MMC) topology due to its superior modularity and scalability. Unlike the conventional stations that are implemented with multiple MMCs to accommodate multiple ac feeders, a novel multi-ac-port MMC station (MACP-MMC), which evolves from the MMC topology, is proposed in this article to interconnect different ac feeders and facilitate flexible power exchange in the MV hybrid distribution network. In addition to the benefits inherited from the MMC, the proposed MACP-MMC also features easy multiport extension, bidirectional power flow adjustment, high power density, and low cost. The operation principle, energy balance mechanism, and control strategies are elaborated. Finally, its effectiveness is verified by both simulation results in a 3-MVA case and experimental results obtained from a 3-kVA scaled-down lab prototype.
One of the most widely used reliability measures for the electric grid is the System Average Interruption Duration Index (SAIDI), i.e. how many minutes the average customer is without power per year. ...Primary substation failures may significantly increase SAIDI, since they affect a large number of customers. This may be avoided by improving the service restoration process, which requires solving complex non-convex combinatorial problems. Hence, this paper proposes an algorithm for smart, SAIDI-optimizing service restoration in meshed MV grids after primary substation failures. We first formulate service restoration as a Mixed-Integer Linear Programming (MILP) problem by using a linearization of the power flow equations that is proper for meshed grids and by applying exact Big M relaxations for linearizing other involved non-convexities. The decisions include among others the travel routes of the field crew and account for the grid security constraints. Then, we propose a low complexity heuristic iterative method that optimally solves the MILP problem.
Voltage-to-power sensitivities play a key role in the control and operation of distribution networks. To estimate these sensitivities without network information, data-driven estimation methods have ...been studied. However, conventional methods do not consider mutual dependency (MD) of the sensitivities and thus the estimation can be inaccurate. Thus, this paper proposes a new data-driven estimation method of the sensitivities, which considers MD of the sensitivities in medium voltage distribution networks. In the proposed method, via MD analysis, the sensitivities are estimated by solving nonlinear least square problems and thus the accurate estimation can be achieved. The effectiveness of the proposed method is verified using the real-time platform.
This letter presents the impact of 6.5 kV silicon carbide (SiC) planar MOSFET negative gate bias to their reverse recovery behavior at room temperature to 175°C. The difference between low- and ...medium-voltage (MV) SiC device 3 rd -quadrant characteristics under different gate bias is introduced. The impact of negative gate bias on conductivity modulation and reverse recovery is discussed. A custom clamped-inductive load (CIL) switching test setup is developed to evaluate the MV SiC device's body diode performance. Results show that the reverse recovery worsens under negative gate bias at increasingly higher temperatures. The negative gate bias leads to an increase in reverse recovery charge and energy.
DC power grid is considered as the future trend for distribution networks. However, the pure medium voltage DC (MVDC) grid faces great challenges in designing power supplies with high voltage input ...but small power output requirements. The continuous power supply from the MVDC bus typically requires power converters with complex circuit structures and control algorithms. This paper presents a power supply with a simple circuit structure for medium-voltage DC systems. The proposed power supply is comprised of cascaded standardized submodules to cope with high voltage input and reduce the isolation requirement, and the voltage balance of all DC link capacitors can be ensured with submodules operating independently. A start-up strategy and a simple self-powering strategy for the proposed converter are proposed in this paper, and the average current model is established to characterize the proposed power supply. To verify the analysis, an average model and a switching model are constructed and simulated in Simulink and a 600 W prototype comprised of five submodules is built and tested to validate the design, and the maximum input voltage can reach 2700 V.
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