A Family of Bidirectional Solid-State Circuit Breakers With Increased Safety in DC Microgrids

DC microgrids have attracted more and more attention with its universality, high efficiency, and potential application market. However, dc fault protection is still a challenge to the development of dc microgrids. In this article, a novel bidirectional dc solid-state circuit breaker based on coupled inductor is introduced. It can realize bidirectional energy flow and bidirectional isolation and interruption of short-circuit faults with fewer components. The working principle of the topology and the working process under different conditions are described, and some properties of the novel circuit breaker are explored, including minimum fault current ramp rate and load step analysis, which provide a reference for the selection of component parameters. Finally, the feasibility and performance of the novel bidirectional circuit breaker is evaluated by simulations and experiments.

Ensuring a protection scheme in a DC distribution system is more difficult to achieve against pole-to-ground faults than in AC distribution system because of the absence of zero crossing points and low line impedance. To complement the major obstacle of limiting the fault current, several compositions have been proposed related to mechanical switching and solid-state switching. Among them, solid-state circuit breakers (SSCBs) are considered to be a possible solution to limit fast fault current. However, they may cause problems in circuit complexity, reliability, and cost-related troubles because of the use of multiple power semiconductor devices and additional circuit configuration to commutate the current. This paper proposes a SSCB with a coupled inductor (SSCB-CI) that has a symmetrical configuration. The circuit is comprised of passive components like commutation capacitors, a CI, and damping resistors. Thus, the proposed SSCB-CI offers the advantages of a simple circuit configuration and fewer utilized power semiconductor devices than the other typical SSCBs in the DC microgrid. For the analysis, six operation states are described for the voltage across the main switches and fault current. The effectiveness of the SSCB-CI against the short-circuit fault is proved via simulation and experimental results in a lab-scale prototype.

Recently according to the development of DC transmission which is high efficiency, stability of DC grid has become a major concern. Therefore, the reliable DC SSCB (Solid-State Circuit Breaker) is required in order to secure high safety. The previous DC SSCB is unstable and many elements are required in order to perform the operating duty. This paper proposes a new DC SSCB. Since the proposed DC SSCB does not cause delay of breaking operation caused by charging operation, it can break an accident quickly. Also, since it can perform the operation duty with a few elements, it is economical. The operating characteristic of the proposed DC SSCB is verified by simulations on short circuit fault. Also, this paper suggests the design guidelines so that it can be applied to any other DC grid.

This paper proposes two novel active snubbers for direct current (dc) solid-state circuit breakers (SSCBs): metal-oxide-varistor with resistor-capacitor-switch (MOV-RCS) and active-MOV with resistor-capacitor-diode (AMOV-RCD). In the snubber branch, either half- or full-controlled switch can be applicable, leading to four novel topologies. There are two main contributions: 1) MOV is disconnected from the power line during SSCB OFF-state, which enhances reliability as neither voltage nor power appears on MOV; 2) voltage utilization rate η <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>v</i></sub> of the main switch is remarkably increased, which improves efficiency and power density, and reduces design cost. Experiments of five prototypes are conducted including four proposed snubbers and a comparison with conventional MOV-RCD snubber. Results show that, for 1.2 kV MOSFETs as the main switch, new snubbers can enhance η <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>v</i></sub> from 37.5% to 60%, achieving 183 A dc current interruption under 720 V dc bus voltage within 2.4 μs. Meanwhile, the voltage peak is limited to 1.112 kV.

We consider the problem of stabilizing voltages in Direct Current (DC) microgrids given by the interconnection of Distributed Generation Units (DGUs), power lines, and loads. We propose a decentralized control architecture where the primary controller of each DGU can be designed in a plug-and-play fashion, allowing the seamless addition of new DGUs. Differently from several other approaches to primary control, local design is independent of the parameters of power lines and the only global quantity used in the synthesis algorithm is a scalar parameter. Moreover, differently from the plug-and-play control scheme in [1], the plug-in of a DGU does not require to update the controllers of neighboring DGUs. Local control design is cast into a linear matrix inequality problem that, if infeasible, allows one to deny plug-in requests that might be dangerous for microgrid stability. The proof of closed-loop stability of voltages exploits structured Lyapunov functions, the LaSalle invariance theorem and the properties of graph Laplacians. Theoretical results are backed up by simulations in PSCAD.

A significant amount of interest from the scientific community has been focused on Direct Current (DC) microgrids in recent years, as a direct result of the proliferation of appliances that run on DC power. Nevertheless, the acceptability of DC microgrids by power utilities is still restricted owing to the challenges connected with the construction of a dependable protection system. This is because obtaining dependable protection for DC microgrids might be difficult due to the large size of DC fault current, its quick rate of rise, and the lack of zero crossing. In addition, the intermittent nature of the power produced by non-traditional distributed generators necessitates adaptability in response to a wide range of climatic circumstances. The method used in this study to solve the problems outlined above is developing an Ensemble of Decision Tree-based protection solutions for a multi-terminal DC microgrid. The plan developed accommodates the intermittent nature of renewable energy sources. It only uses information that is local to the current and voltage signals, which allows it to execute the duties of fault detection and classification, and prevents the disadvantages associated with communication networks, such as data loss and delay.

Over the past decade, the high penetration of renewable-based distributed generation (DG) units has witnessed a considerable rise in electrical networks. In this context, direct current (DC) microgrids based on DGs are being preferred due to having less complexity for the establishment and control. At the same time, they offer higher efficiency and reliability compared to their alternating current (AC) counterparts. This paper proposes a new model predictive control (MPC)-trained artificial neural network (ANN) control strategy being an ANN-MPC instead of conventional cascaded-proportional-integral (PI)-trained ANN control for dynamic damping of photovoltaic (PV)-battery-based grid-connected DC microgrids. Unlike traditional controllers, the proposed control approach more rapidly attains generation-load power balancing under variable climate input (meteorological sensor data) and output (load demand), hence achieving quick DC-bus voltage damping. The proposed ANN-MPC scheme is examined under different operating conditions, and the results are compared with the ANN-based conventional PI controller. The results show the proposed control strategy's efficacy to lessen the instability issues and achieve effective attenuation of oscillations in DC microgrids.

The AC and DC power system structures need to be modernized to meet consumer demands. DC microgrids are suitably admired due to their high efficiency, consistency, reliability, and load sharing performance, when interconnected to DC renewable and storage sources. The main control objective for any DC microgrid is providing proper load–power balancing based on the Distributed Generator (DG) sources. Due to the intermittent nature of renewable energy sources, batteries play an important role in load–power balancing in a DC microgrid. The existing energy management strategy may be able to meet the load demand. However, that technique is not suitable forrural communities’ power system structure. This research offers an energy management strategy (EMS) for a DC microgrid to supply power to rural communities with solar, wind, fuel cell, and batteries as input sources. The proposed EMS performs the load–power balancing between each source (renewable and storage) in a DC microgrid for dynamic load variation. Here, the EMS handles two battery sources (one is used to deliver power to the priority load, and the other is utilized in the common DC bus) to meet the required demand. The proposed EMS is capable of handling load–power balancing using renewable energy sources with less consumption of non- conventional energy sources (such as a diesel generator). The performance of the system is analyzed based on different operating conditions of the input sources. The MATLAB/Simulink simulation model for the proposed DC microgrid with their EMS control system is developed and investigated, and their results are tabulated under different input and load conditions. The proposed EMS is verified through a laboratory real-time DC microgrid experimental setup, and the results are discussed.

Solid-State Circuit Breaker (SSCB) has been identified as a potential game-changing technology for DC distribution. However, fewer SSCB research and applications have been reported in AC due to the direct competition with the conventional electromechanical circuit breaker, which has a proven record as AC circuit protection device. This paper investigates the current protection and soft start functions of a three-phase SSCB in the motor control center (MCC). The proposed SSCB integrates application-specific protection functions of soft starter, magnetic contactor, circuit breaker, and thermal relay. The paper designs and implements a three-phase SSCB with a rated voltage of 380VAC and a rated current of 63A. Three key design considerations, including semiconductor device selection, metal oxide varistor (MOV) and snubber selection, and heat sink selection are introduced. The current fault protection of the SSCB was tested and verified at 200A by a pulse test platform with a microsecond-level fault current clearing time. Finally, the soft start function was analyzed and realized without zero current detection. Both simulation and experimental results validate the soft-starting function with significant reduction of motor inrush current. By combining the conventional contactor, circuit breaker, and soft starter functions into one, SSCB can offer competitive advantages in MCC applications.

Distributed consensus control for voltage tracking and current distribution in DC microgrid

In this letter we propose a new distributed control scheme, achieving current sharing and average voltage regulation in direct current (dc) microgrids. The considered dc microgrid is composed of several distributed generation units (DGUs) interconnected through resistive-inductive power lines. Each DGU includes a generic energy source that supplies a local current load through a dc-dc buck converter. The proposed distributed control scheme achieves current sharing and average voltage regulation, independently of the initial condition of the controlled microgrid. Moreover, the proposed solution requires only measurements of the generated currents, and is independent of the microgrid parameters and the topology of the used communication network, facilitating plug-and-play capabilities. Global convergence to a desired steady state is proven and simulations indicate a good performance.

Photovoltaic (PV) and energy storage(ES) hybrid systems are widely used in microgrids due to their high efficiency, environmental friendliness, and flexible regulation capabilities. As platforms for integrating multiple types of distributed power sources, energy storage devices, and loads, AC/DC hybrid microgrids have attracted extensive attention. Droop control is widely used in AC/DC hybrid microgrid. By equivalent conversion of AC frequency droop and DC voltage droop coefficients, the static power can be reasonably distributed between the AC and DC microgrids, and the steady-state stability of DC voltage and AC frequency can be maintained. However, drop method cannot realize the effective allocation of dynamic power. Transient overloads of partial ES units and instantaneous overlimit of AC frequency or DC voltage may occur when system is subjected to power steps. The essence of the above problem is how to reasonably allocate instantaneous power according to the dynamic changes of DC voltage and AC frequency, or the cooperative inertia response between DC and AC microgrid. Through power and energy balance analysis, this paper establishes intrinsic equivalence relationships between AC torque and DC current, AC frequency and DC voltage, AC inertia and DC inertia, AC inductance and DC inductance. According to the equivalent relationship, an inertial cooperative control strategy for ILC is proposed, which realizes the complementary support of AC and DC side dynamic and static power, as well as the stability of AC frequency and DC voltage. Simulation results verify the correctness and effectiveness of the proposed strategy.

Nowadays, DC microgrids have become more popular than AC microgrid systems due to their high efficiency, as they can reduce the requirement of the conversion stages. Also, they can integrate with renewable energy sources easily. Additionally, DC microgrids are more stable, reliable, and less complex in design. Nevertheless, DC microgrid fault detection and identification present a significant challenge due to the limited research available in this field. Out of the several faults in the DC microgrid (like pole to pole, pole to ground, and arc fault), arc fault is a more serious fault because Arc discharge raises the temperature, which can harm the circuit and increase the risk of a fire. The DC microgrid system's ability to operate safely will be impacted when a High Impedance Series Arc Fault arises, so to provide protection to the line from the arc fault, we need to detect and identify the fault accurately. However, because the fault current generated in the DC microgrid by an arc fault is nonlinear, it is difficult to detect the fault in a DC network using conventional techniques, such as CBs or fuses. In order to detect and diagnose the major arc fault more effectively and precisely, a new approach is presented here that builds an observer model using the Geometric Control Theory's idea of subspaces, such as unobservable and invariant subspaces, among others. This paper aims to design an observer that will generate residual signatures for each of the faults and show zero results for a consecutive fault.

We analyze the stability of meshed DC microgrids using the proposed Probabilistic Analysis of Deterministic Systems (PADeS). By sampling a random distribution of networks as well as disturbances, we make probabilistic statements valid for two- (ring) and three-valent (meshed) networks. The producing nodes are droop controlled sources and the consuming nodes are modeled as constant power loads (CPLs). We introduce basin stability and survivability as probabilistic stability measures within the PADeS framework and illustrate that it has a large potential for understanding the properties of DC microgrids and beyond. To show this, we run numerical experiments with 128-node networks and 1000 samples for the system's probability (a) to return to a stable equilibrium point after a perturbation (basin stability) and (b) to remain within defined operational bounds for disturbances such as sudden load power increase, line outages and short circuits at random nodes or power lines (survivability).

This paper addresses load current sharing and circulating current issues of parallel-connected dcdc converters in low-voltage dc microgrid. Droop control is the popular technique for load current sharing in dc microgrid. The main drawbacks of the conventional droop method are poor current sharing and drop in dc grid voltage due to the droop action. Circulating current issue will also arise due to mismatch in the converters output voltages. In this work, a figure of merit called droop index (DI) is introduced in order to improve the performance of dc microgrid, which is a function of normalized current sharing difference and losses in the output side of the converters. This proposed adaptive droop control method minimizes the circulating current and current sharing difference between the converters based on instantaneous virtual resistance <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\bm{R}_{{\bf droop}}$</tex-math> </inline-formula> . Using <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\bm{R}_{{\bf droop}}$</tex-math> </inline-formula> shifting, the proposed method also eliminates the tradeoff between current sharing difference and voltage regulation. The detailed analysis and design procedure are explained for two dcdc boost converters connected in parallel. The effectiveness of the proposed method is verified by detailed simulation and experimental studies.