Robert M. Cuzner

The Status of DC Micro-Grid Protection

AC microgrids are a convenient approach to integrating distributed energy systems with utility power systems. On the other hand, DC micro-grids can lead to more efficient integration of distributed generation. They are the preferred topology for present shipboard, aircraft and automotive power systems and hold promise for future environmentally friendly office buildings, homes, rural areas and industrial power parks. However, standards, guidelines, practical experience and cost effective implementations for DC system protection are well behind practices in AC system protection. This paper presents a comprehensive overview of the body of research on protection of DC micro-grids, presented with a goal of identifying and advancing the field. The paper presents a discussion of the current status of dc micro-grid protection, including the use of electro-mechanical circuit breakers, solid state circuit breakers, protective system design, ground fault location and fault isolation.

Shipboard Solid-State Protection: Overview and Applications

This article presents an overview of the present art of low-voltage (LV) dc power distribution system protection using solid-state protective devices (SSPDs). It describes how IGBTand IGCT-based SSPDs are constructed and how the important feature of galvanic isolation can be included in them. The article outlines the advantages of SSPDs, which include reduced fault-current level, greatly reduced current interruption time, limitation of arc-flash energy, improved acoustic performance, and reduced maintenance. It also demonstrates protective coordination using three of these solid-state circuit breakers and discusses new paradigms to consider. Test results are presented validating the use of restraint signals to aid in proper protective coordination in a generic three-level power distribution system. The article also shows test results for two paralleled SSPD building blocks used to make higher-current-rated solid-state circuit breakers, showing very good dynamic current sharing.

Mitigating Circulating Common-Mode Currents Between Parallel Soft-Switched Drive Systems

A mathematical model that Is developed for a generalized drive system, including common-mode passive and active elements, is used to explore the issues of paralleling soft-switched resonant dc-link drive systems. Differences between the modulation pattern for each drive system cause common voltage disturbances, which lead to significant circulating currents between the drive systems. Control methods for actively compensating for common-mode circulating currents or reducing the common- mode voltage disturbances are investigated. Practical modifications to the drive system controls are implemented to reduce the circulating currents between paralleled systems.

Circuit breaker protection considerations in power converter-fed DC Systems

Considerations in applying circuit breaker protection to DC systems are capacitive discharge, circuit breaker coordination and impacts of double ground faults. Test and analysis results show the potential for equipment damage. Solutions are proposed at the cost of increased integration between power conversion and protection systems.

Active filtering for common mode conducted EMI reduction in voltage source inverters

As solid-state power converters become more numerous, the unwanted EMI effects created by these converters increase. To address this issue, more attention has recently been given to the reduction of electromagnetic emissions created by power electronics. The proliferation of solid-state inverters brings the cost of semiconductors down, making active filtering an attractive alternative to passive filter topologies. Active filtering may also realize higher efficiency than passive filtering. This paper focuses on an active filter topology that significantly reduces the common mode voltage created by voltage source inverters. The reduction of common mode voltage is achieved by introducing a novel inverter topology and a new hysteresis modulation strategy to control this inverter topology.

Implementation of a Non-Isolated Three Level DC/DC Converter Suitable for High Power Systems

This paper describes the implementation of a non-isolated three-level DC/DC buck converter that is suitable for high power systems. This topology has the advantage of achieving all of the design objectives for this classification of power converters. The authors have developed, built and tested a system which utilizes a general purpose power electronic module, which can be configured via software/firmware programming and modifications to its input/output power connections to implement various types of power systems. The work behind this paper has proven the usefulness of this topology and has addressed many of the issues associated with successful implementation and commercialization. The paper describes the control implementations and simulation results compared to measured results used in tuning and commissioning of the system. The final 500 V, 100 kW system is implemented in hardware and experimental results are provided showing its performance under step load and step short circuit conditions.

DC zonal electrical system fault isolation and reconfiguration

Fault isolation and re-configuration on a DC bus distributed through an electrical system is studied. This paper builds upon previously proposed zonal architectures that ensure power continuity during a fault and isolation of a fault with minimal impact to un-faulted portions of the system. The approach utilizes no load switches for fault isolation aided by multiple power electronic converters feeding the DC bus. Three electromechanical no load switches in a single assembly are utilized at the interface between electrical zones. The ability of the system to segregate the fault without loss of power to unfaulted zones is very limited without network communications between the power converter and switch components. A simulation model of the DC portion of the integrated power system is developed and simulation results demonstrate the effectiveness of the model in verifying the method for locating a fault and in predicting the impact of the fault to the external medium voltage AC distribution system and inter-zonal low voltage interfaces.

Implementation of a Four-Pole Dead-Time-Compensated Neutral-Point-Clamped Three-Phase Inverter With Low Common-Mode Voltage Output

This paper describes the implementation of a four-pole neutral-point-clamped (NPC) three-phase inverter that produces virtually no common-mode voltage. The low common-mode voltage output is achieved by constraining the switch states of the NPC inverter to only those states that produce zero common-mode voltage in dead-time compensation, which enhances the capability of the circuit to produce low common-mode voltage by compensating common-mode voltages produced by reverse diode commutations. The low common-mode voltage performance is achieved at the expense of reduced voltage utilization and loss of DC-link voltage balance control. In order to overcome this limitation, a fourth pole and associated control are added to balance the upper and lower dc-link voltages. This paper describes the control implementations and simulation results compared to measured results used in tuning and commissioning of the system. A 450-V 78-kW system is implemented in hardware, and experimental results are provided, showing its differential mode transient and steady-state harmonic performance as well as its common-mode output voltage.

Power-Dense Shipboard-Compatible Low-Horsepower Variable-Frequency Drives

A variable-frequency drive (VFD) having a 440-V front-end current source rectifier (CSR) interface to a voltage source inverter (VSI) feeding a Permanent-Magnet Axial-flux Air Core motor combination is a solution for low-horsepower pump and fan control that is both power dense and compatible with a shipboard environment. This paper describes the control and power platforms for the CSR/VSI and provides experimental results for a 250-V 3.3-kW system. Power density and efficiency comparisons are made between equivalent CSR/VSI- and voltage-source-conversion-based VFDs to demonstrate that the CSR/VSI-based VFD is more power dense.

Fault tolerant shipboard MVDC architectures

Medium Voltage DC (MVDC) architectures are identified from the literature search that are suitable for a highly survivable 20kVdc shipboard Integrated Power System (IPS). “Breaker-based” architectures enable fast fault isolation through the use of Solid State Protective Device (SSPD) technology. “Breaker-less” architectures require based generator power converter and Solid State Transformer (SST) interfaces that can fold back outputs and coordinate with no load switches to isolate faults. Estimated size/weights and survivability of various “breaker-based” and “breaker-less topologies are compared. “Breaker-Less”, Current Source Converter (CSC) based architectures have the highest power density but at the cost of lower survivability. Expanding the role of galvanically isolating converters within the system (i.e. SSTs) increases power density and survivability.