DC distribution is gaining increasing interest in the maritime industry for modern shipboard microgrids (SMGs), as it supports the ongoing decarbonization and electrification of the sector by facilitating the integration of converter-based resources, such as energy storage systems, propulsion drives, and diesel generators. DC distribution typically relies on voltage regulation governed by a voltage–power relationship, which simplifies system control by removing the fixed-speed constraint of diesel generators (DGs), allowing them to operate at their maximum-efficiency point and thereby improving efficiency and fuel savings. Energy storage systems (ESSs) are increasingly adopted in shipboard power systems (SPSs) to support power generation and assist diesel generators, providing voltage regulation during both transient load variations and steady-state operation. Different types of ESSs, such as batteries, supercapacitors, and superconducting magnetic energy storage, may be employed. In addition, hybrid configurations combining high-energy and high-power ESSs can be adopted, relaxing spinning-reserve requirements and further enhancing the efficiency and fuel savings of DGs. While attractive from an operational perspective, DC distribution systems also introduce protection and stability challenges. From a protection perspective, DC short-circuit faults are characterized by steep current rise rates and the absence of natural current zero crossings, requiring fast interruption with solid-state circuit breakers (SSCBs), often equipped with current-limiting inductors (CLIs) to limit the rate of change of the fault current. While SSCBs enable sub-millisecond fault clearing, their speed introduces challenges in achieving selective operation. This aspect is particularly critical for marine systems with stringent fault-tolerance requirements, where any fault must be isolated by disconnecting the minimum portion of the system, while the fast DC fault dynamics and the limited fault current supplied by converter-interfaced sources leave a very narrow margin for achieving selectivity. From a stability perspective, DC SPSs can be affected by the constant power load (CPL) behavior of electric propulsion. Indeed, tight speed control of the propeller results in an approximately constant power absorption seen from the main DC distribution, thus exhibiting negative incremental resistance and potentially affecting system stability through interactions with source impedances. In addition, the CLIs of SSCBs may introduce additional resonances with the filter capacitors used by converters, potentially amplified to instability by the CPLs. Therefore, the present thesis focuses on three distinct yet interconnected topics. First, control strategies for the sources connected to the DC distribution are addressed, with particular focus on ESS control schemes. Two control architectures are proposed to enable proportional power sharing among sources and ensure accurate DC-bus voltage regulation, both relying on a hierarchical subdivision of control layers. The first is a centralized secondary control that allocates power across high-power and high-energy ESSs, allowing a reduction of DG contribution during load transients. The second is a distributed secondary regulation based on cross-current compensation, which improves power sharing and mitigates the steady-state voltage drop introduced by droop-based regulation. Second, impedance-based stability analysis is performed in different scenarios. Starting from a single-busbar configuration, where the impact of CPL behavior on system stability is addressed through the Nyquist criterion applied to the impedance ratio between the aggregated source and load impedances seen at the bus, the analysis is extended to multi-bus configurations with SSCB-protected bus-ties. In this scenario, the conventional impedance-ratio approach is no longer valid; therefore, a MIMO impedance-based stability criterion is proposed to address these limitations. Third, the protection of DC SMGs is addressed. After a review of the state of the art of DC protection, a detailed SSCB model is developed, calibrated on experimental data, and exported as a functional mock-up unit (FMU) model, enabling its use in different simulation environments while preserving manufacturer confidentiality. To characterize the fault current that this device must interrupt, the short-circuit response of ESSs interfaced through power electronic converters (PECs) is then analyzed, highlighting the dominant role of filter-capacitor discharge within the SSCB intervention timescale. On this basis, the analysis is extended to the system level for a dynamic-positioning vessel with closed bus-ties and a three-level protection (3LP) scheme, in order to assess fault-current propagation and the selectivity conditions of the SSCBs as a function of the current-limiting inductances and connection lengths.
Control, Stability and Protection of DC Microgrids
SIVORI, FABRIZIO
2026-07-27
Abstract
DC distribution is gaining increasing interest in the maritime industry for modern shipboard microgrids (SMGs), as it supports the ongoing decarbonization and electrification of the sector by facilitating the integration of converter-based resources, such as energy storage systems, propulsion drives, and diesel generators. DC distribution typically relies on voltage regulation governed by a voltage–power relationship, which simplifies system control by removing the fixed-speed constraint of diesel generators (DGs), allowing them to operate at their maximum-efficiency point and thereby improving efficiency and fuel savings. Energy storage systems (ESSs) are increasingly adopted in shipboard power systems (SPSs) to support power generation and assist diesel generators, providing voltage regulation during both transient load variations and steady-state operation. Different types of ESSs, such as batteries, supercapacitors, and superconducting magnetic energy storage, may be employed. In addition, hybrid configurations combining high-energy and high-power ESSs can be adopted, relaxing spinning-reserve requirements and further enhancing the efficiency and fuel savings of DGs. While attractive from an operational perspective, DC distribution systems also introduce protection and stability challenges. From a protection perspective, DC short-circuit faults are characterized by steep current rise rates and the absence of natural current zero crossings, requiring fast interruption with solid-state circuit breakers (SSCBs), often equipped with current-limiting inductors (CLIs) to limit the rate of change of the fault current. While SSCBs enable sub-millisecond fault clearing, their speed introduces challenges in achieving selective operation. This aspect is particularly critical for marine systems with stringent fault-tolerance requirements, where any fault must be isolated by disconnecting the minimum portion of the system, while the fast DC fault dynamics and the limited fault current supplied by converter-interfaced sources leave a very narrow margin for achieving selectivity. From a stability perspective, DC SPSs can be affected by the constant power load (CPL) behavior of electric propulsion. Indeed, tight speed control of the propeller results in an approximately constant power absorption seen from the main DC distribution, thus exhibiting negative incremental resistance and potentially affecting system stability through interactions with source impedances. In addition, the CLIs of SSCBs may introduce additional resonances with the filter capacitors used by converters, potentially amplified to instability by the CPLs. Therefore, the present thesis focuses on three distinct yet interconnected topics. First, control strategies for the sources connected to the DC distribution are addressed, with particular focus on ESS control schemes. Two control architectures are proposed to enable proportional power sharing among sources and ensure accurate DC-bus voltage regulation, both relying on a hierarchical subdivision of control layers. The first is a centralized secondary control that allocates power across high-power and high-energy ESSs, allowing a reduction of DG contribution during load transients. The second is a distributed secondary regulation based on cross-current compensation, which improves power sharing and mitigates the steady-state voltage drop introduced by droop-based regulation. Second, impedance-based stability analysis is performed in different scenarios. Starting from a single-busbar configuration, where the impact of CPL behavior on system stability is addressed through the Nyquist criterion applied to the impedance ratio between the aggregated source and load impedances seen at the bus, the analysis is extended to multi-bus configurations with SSCB-protected bus-ties. In this scenario, the conventional impedance-ratio approach is no longer valid; therefore, a MIMO impedance-based stability criterion is proposed to address these limitations. Third, the protection of DC SMGs is addressed. After a review of the state of the art of DC protection, a detailed SSCB model is developed, calibrated on experimental data, and exported as a functional mock-up unit (FMU) model, enabling its use in different simulation environments while preserving manufacturer confidentiality. To characterize the fault current that this device must interrupt, the short-circuit response of ESSs interfaced through power electronic converters (PECs) is then analyzed, highlighting the dominant role of filter-capacitor discharge within the SSCB intervention timescale. On this basis, the analysis is extended to the system level for a dynamic-positioning vessel with closed bus-ties and a three-level protection (3LP) scheme, in order to assess fault-current propagation and the selectivity conditions of the SSCBs as a function of the current-limiting inductances and connection lengths.
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