DC Power Distribution System Research Articles

Weighted Double Sampling to Obtain the Average Value of Triangular Current for Accurate Droop Control in DC Power Distribution Systems

Droop control is broadly used in dc power distribution systems, e.g., dc microgrids. Implementing droop control for switch mode power supplies requires the measurement of average current. Synchronous sampling is used in digital control systems to obtain the average of a triangular inductor waveform. However, the dead time, semiconductor turn- on and turn- off delays, and signal conditioning delays shift the sensed waveform and change the effective duty cycle, resulting in a misalignment and measurement error from the average value. This paper proposes a duty cycle weighted averaging method based on double sampling. By sampling twice per switching period and averaging the results using duty cycle as the weighting factor, the error due to waveform misalignment is theoretically eliminated. The result always equals the average value of the triangle. This method does not rely on the knowledge of circuit delays, making it easy to implement and robust to parameter variations. Simulations and experimental measurements validate the superior performance compared with conventional single and double sampling methods. The experiment on a droop-controlled dc–dc converter demonstrates a more accurate droop characteristic.

Data-Driven Control of LVDC Network Converters: Active Load Stabilization

This paper addresses the (model-free) data-driven control of power converters, acting as distributed generators, in low voltage direct current (LVDC) networks (e.g., DC microgrids, DC distributed power systems, DC buses wit multiple sources and loads, etc.). Since traditional stand-alone control design, cannot guarantee stability when converters are connected to a network, it is proposed a deterministic solution that does not require the network model–an approach purely based on measurement data. This is a suitable way to overcome common issues when using a model-based approach, e.g., the use of an excessive number of variables and equations, the presence of un-modeled dynamics, unknown parameters and/or the lack of first principle model equations. To corroborate the advantages of the proposed approach, the present work addresses an extreme but also realistic scenario: weak networks with active loads, such as constant power loads (CPLs). It is also shown that the proposed scheme guarantees stability in a rigorous deterministic way–using a Lyapunov approach based on coefficient matrices directly constructed from data. Simulation results using a multi-bus LVDC distribution network, based on PSCAD/EMTDC, are provided as proof of concept.

Wide Load Range ZVS Three-level DC-DC Converter: Modular Structure, Redundancy Ability, and Reduced Filters Size

In future dc distributed power systems, high performance high voltage dc-dc converters with redundancy ability are welcome. However, most existing high voltage dc-dc converters do not have redundancy ability. To solve this problem, a wide load range zero-voltage switching (ZVS) three-level (TL) dc-dc converter is proposed, which has some definitely good features. The primary switches have reduced voltage stress, which is only Vin/2. Moreover, no extra clamping component is needed, which results simple primary structure. Redundancy ability can be obtained by both primary and secondary sides, which means high system reliability. With proper designing of magnetizing inductance, all primary switches can obtain ZVS down to 0 output current, and in addition, the added conduction loss can be neglected. TL voltage waveform before the output inductor is obtained, which leads small volume of the output filter. Four secondary MOSFETs can be switched in zero-current switching (ZCS) condition over wide load range. Finally, both the primary and secondary power stages are modular architecture, which permits realizing any given system specifications by low voltage, standardized power modules. The operation principle, soft switching characteristics are presented in this paper, and the experimental results from a 1 kW prototype are also provided to validate the proposed converter.

An Impedance-Based Stability Assessment Methodology for DC Distribution Power System With Multivoltage Levels

Impedance-based methods are effective for stability assessment of cascaded systems. However, the dc distribution power system (DPS) is becoming increasingly complex, incorporating multivoltage levels, many dc buses, and numerous converters. Consequently, conventional stability criteria, such as Middlebrook criterion and its extensions, may not have the ability to judge stability very succinctly. Thus, more research work related to stability evaluation of dc DPS with multivoltage levels needs to be carried out. This article proposes an impedance-based stability assessment methodology. First of all, a unified form of the system is obtained regardless of its structures and operating modes, based on classification of any converter as either a bus voltage-controlled converter or a bus current-controlled converter. Then, according to the two-port small-signal model, the equivalent loop gain of the system is derived precisely. It should be noted that intermediate bus converter could adopt output current or output voltage control modes, which will have influence on the equivalent loop gain expression. After that, the stability requirement of the system is given by introducing the Nyquist criterion to the equivalent loop gain. Finally, an experimental prototype is established to validate the effectiveness of the proposed criterion.

Stability Analysis of MgB2 Coils for SMES Application Consisting of Large-Scale Rutherford Cables

MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> wires have been provided by several manufacturers, showing enough critical current (Ic) for practical applications in relatively low-field. Because the MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> has critical temperature above boiling temperature of hydrogen, dc power distribution system with low carbon emission using MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> superconducting magnetic energy storage (SMES), named advanced superconducting power conditioning system has been proposed. For the system, the MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> coil production technology obtaining 30 kJ stored energy the investigation about the SMES coil consists of 600 A, 1.7-T Rutherford-type conductors made of commercially-available MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> wires. Due to strain sensitivity before/after heat treatment for MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> production, the proper designs of the large-scale twisted conductors both in wind and react, react and wind methods are needed, choosing optimized twist pitches and cable compaction factors. To demonstrate the SMES coil performance, we have been carried out the test campaign of conductors and small prototype coils in various temperature and background field conditions. These results are used for a computer simulation for estimating full size double pancake coil performance of the system, based on the non-steady state heat conduction analysis. The calculated result seems to be a good tool for predicting coil performance for the large capacity energy storage operation.

Investigation of Nonlinear Droop Control in DC Power Distribution Systems: Load Sharing, Voltage Regulation, Efficiency, and Stability

Linear droop faces the design tradeoff between voltage regulation and load sharing due to cable resistances and sensing errors. Using a larger droop resistance improves load sharing, but requires a wider droop voltage range. In the nonlinear droop, droop resistance is a function of the converter's output current, and its value increases when the output current increases. As a result, the impacts from sensors and cables are reduced. In this paper, the design of nonlinear droop in dc power distribution systems is studied with special emphasis on load sharing, voltage regulation, system efficiency, and stability. After discussing the piecewise linear and nonlinear droop control, a generic polynomial expression is presented to unify different droop equations. The impact of droop on dc system efficiency is analyzed by evaluating cable and power converter losses. The converter's output impedance using nonlinear droop is modeled to analyze the system stability with constant power loads. The selection and design guidelines of nonlinear droop are summarized, considering both the static performance and interaction with load systems. The analysis is verified in 400-V multi-source dc systems. The nonlinear droop is fully distributed as it only needs local information.

A Novel DC Power Distribution System Stabilization Method Based on Adaptive Resonance-Enhanced Voltage Controller

DC power distribution systems typically consist of several source converters supplying load converters through single or multiple dc buses. Such complex systems are prone to stability issues caused by the interactions among feedback-controlled converters. Additionally, dc interconnected systems typically undergo considerable changes in their operating points. This can cause a system that is well-stabilized at a certain operating point to go unstable for another operating condition. Therefore, a stabilization method that is capable of adapting to variations in system operating conditions is highly desirable. This paper presents an adaptive stabilization method implemented on the source-side. The system stability is periodically analyzed by real-time measurement of the system bus impedance, which is dominated by the source impedance. Accordingly, the source converter control is modified by the proposed method to guarantee stability and high performance of the entire system after system changes. The approach has several advantages, including ease of design and implementation, minimal deviation from the nominal controller, and robustness due to the real-time adaptive implementation. Simulation and experimental results are presented that confirm the effectiveness of the proposed method.

An Isolated Multilevel Quasi-Resonant Multiphase Single-Stage Topology for 380-V VRM Applications

In order to increase the efficiency of modern microprocessor power supplies used in data centers, 380-V dc power distribution systems have been attracting attention due to their high efficiency and high reliability. This paper presents an innovative single-stage approach for 380-V voltage regulator modules (VRMs) based on a quasi-resonant multilevel topology constant on -time operation. The proposed topology inherently integrates the multiphase approach, providing fast phase shedding and flat high-efficiency curves even at light-load conditions. This is a unique advantage, which is not possible to establish in the two-stage approach, which is very important in server architectures, and where high efficiency is required even at light-load conditions. This paper analyzes the circuit topology and proposes a control architecture for fast transient response, including current-sharing capabilities. The digital controller has been implemented in 0.16- $\mu$ m lithography together with a digital pulsewidth modulation with a 195-ps resolution and a 40-MS/s 7-bit analog-to-digital converter. Experimental results show an efficiency of 93% for a 120-A 380–1.8-V VRM power supply.

A Single Gate Driver Based Solid-State Circuit Breaker Using Series Connected SiC MOSFETs

Semiconductor devices based solid-state circuit breakers (SSCBs) are promising in the dc power distribution system as protective equipment for their ultrashort action time. This letter proposes a topology of SSCB using series connected silicon carbide (SiC) metal oxide semiconductor field effect transistors (mosfet s), which only requires a single isolated gate driver. The SSCB has very low cost and high reliability because it only has 13 components including passive components and diodes apart from two SiC mosfet s to achieve both balanced voltage distribution during short-circuit interruption duration and reliable positive gate voltage during on -state. The SSCB prototype is built and experimentally verified to interrupt 75 A short-circuit current under the dc-bus voltage of 1200 V within 1.5 μs.

A Bidirectional High-Efficiency Transformerless Converter With Common-Mode Decoupling for the Interconnection of AC and DC Grids

This paper presents an interface ac–dc converter to connect the 380 V bipolar dc microgrid with the 240 V split-phase single-phase ac utility. The design targets are high efficiency, leakage current attenuation, and symmetric bipolar dc bus generation. A transformerless two-stage topology is proposed to decouple the common-mode (CM) voltage between the connected ac and dc systems. The two-stage structure also reduces the required capacitance in single-phase power conversion by allowing a large voltage ripple on the intermediate dc link. Adaptive dc-link voltage control and interleaving of state-of-the-art silicon carbide (SiC) mosfet s are adopted to achieve high efficiency. Interleaving angles are optimized to minimize the total volume of both differential-mode and CM magnetics. An active CM control method is incorporated to regulate the dc and low-frequency CM voltages on the dc buses with respect to the ground. The resultant dc bus voltages are rendered symmetric to the ground, and the leakage current is suppressed. The generated ±190 V dc bus is suitable for bipolar 380 V dc power distribution systems. Pluggable phase-leg modules with embedded driving and protection circuit are designed and tested, based on which a 10 kW all-SiC converter prototype is built achieving an efficiency >97%.

DC current bus distributed power system and its stability analysis

DC distributed power systems (DPSs) have attained much attention because of its characteristics of high efficiency, high power density and flexible configuration. However, when converters are interconnected in a system, adverse interactions may occur due to the converter sensitivity to the external impedance, these interactions may degrade converters transient performance or cause stability issues. Currently, research mainly focuses on DPS with a DC voltage bus, including stability analysis and approaches to improve system stability. This study is to establish stability criteria for DPS with a DC current bus. Firstly, basic current type DC/DC converters and their small signal models are introduced. Then, the current bus DPS is derived, discussed, and the stability criteria are obtained by using two-port small signal model. The proposed criteria are verified by comparing the results of theoretical calculations and simulations. Finally, an experimental prototype is built to validate the theoretical analysis and simulation.

Research on Parameter Collaborative Configuration of DC Circuit Breaker and DC Fault Current Limiter

Fault current suppression is the key technology to ensure the safe operation of the DC power distribution system. In order to realize the parameter collabora-tive configuration of the DC circuit breaker and the DC current limiter and improve the fault current suppression capability, the fault current suppression mechanism of the DC power distribution system is revealed based on the circuit model. Then, based on the mathematical model of the DC breaker, the characteristic parameters of DC breaking are extracted, and then the influence of different characteristic parameters on the breaking characteristics of fault current is studied. Finally, the mathematical model of the collaborative process between DC circuit breaker and DC current limiter is established. The charac-teristic parameters of fault current collaborative suppression are extracted. The coupling effects of different characteristic parameters on the fault current col-laborative suppression are studied. The principle of collaborative configuration of DC circuit breaker and DC current limiter is proposed, and the collaborative suppression ability of DC circuit breaker and DC current limiter to fault current is fully exploited to ensure the safe and reliable operation of the DC power distribution system.

99.3% Efficient Three-Phase Buck-Type All-SiC SWISS Rectifier for DC Distribution Systems

DC power distribution systems for data centers, industrial applications, and residential areas are expected to provide higher efficiency, higher reliability, and lower cost compared to ac systems and have been an important research topic in recent years. In these applications, an efficient power factor correction (PFC) rectifier, supplying the dc distribution bus from the conventional three-phase ac mains, is typically required. This paper analyzes the three-phase, buck-type, unity power factor SWISS Rectifier for the realization of an ultrahigh-efficiency PFC rectifier stage with a 400-V rms line-to-line ac input voltage and a 400-V dc output voltage. It is shown that the mains current total harmonic distortion of the rectifier can be improved significantly by interleaving two converter output stages. Furthermore, the dc output filter is implemented using a current-compensated integrated common-mode coupled inductor, which ensures equal current sharing between the interleaved half bridges and provides common-mode electromagnetic interference (EMI) filter inductance. Based on a theoretical analysis of the coupled inductor's magnetic properties, the necessary equations and the design procedure for selecting semiconductors, magnetic cores, the number of turns, and the EMI filter are discussed. Based on these results, an ultrahigh-efficient 8-kW 4-kW $\cdot$ dm $^{-3}$ (66-W $\cdot$ in $^{-3}$ ) laboratory-scale prototype converter using 1.2-kV SiC MOSFETs is designed. Measurements taken on the prototype confirm a full power efficiency of $\text{{99.16}{\%}}$ and a peak efficiency of $\text{{99.26}{\%,}}$ as well as the compliance to CISPR 11 Class B conducted emission limits.

Research on the transformerless connection mode for DC power distribution system

The connection mode of the flexible DC distribution network and the AC network is not only the basis of the system design but also is one of the key technologies in the DC distribution. This paper demonstrates the feasibility of the transformerless system using modular multilevel converters (MMC) in the power distribution system. Here, in allusion to the DC power distribution network without transformer, the influence of AC system fault on DC system and the influence of DC system fault on AC system are analysed in detail separately. By establishing the simulation model with the PSCAD/EMTDC software, the transformerless system proposed is verified feasible at the appropriate voltage and the appropriate earthing way of AC system. The research proposed a solution of saving economic cost and space which is the critical issue in the conventional DC power distribution system with the transformer.

A Low-Complexity Trajectory Controller for Reduced Conduction Losses in Series-Resonant Dual Half-Bridge Converters

This paper describes a low-complexity trajectory controller for near-optimal minimization of conduction losses in series-resonant, dual half-bridge converters (SR-DHB). The technique builds on the concept of minimum current trajectory (MCT) of a resonant converter, and formulates a piecewise-linear approximation of the MCT (PWL-MCT) for an SR-DHB topology, which can be implemented digitally via standard arithmetic operations. The low-complexity realization of such technique makes it suitable to be easily implemented in a commercial microcontroller, or even inside a custom-designed digital IC, depending on the target application requirements. Even more importantly, the MCT-based control trajectory can be formulated independently of the converter parameters, resulting in a control approach of the broadest applicability. When equipped with such simple trajectory controller, the SR-DHB converter becomes an efficient, controllable bidirectional dc–dc power unit, which can be employed in a number of scenarios, including dc power distribution systems and battery-to-battery power interfaces. Compared with traditional phase shift modulation, the proposed trajectory controller yields higher efficiency at intermediate-to-light load levels, and equal efficiency at heavy load. Furthermore, the proposed PWL-MCT concept can be implemented to accommodate wide variations in the converter voltage conversion ratio, making the resulting power unit versatile and voltage-programmable. The proposed approach is validated on a 800 W, 200 V-to-145 V, SR-DHB converter prototype. Efficiency comparisons are discussed highlighting the benefits of the PWL-MCT controller over traditional modulation approaches, and the strong efficiency improvement achievable at voltage conversion ratios away from the nominal one.

Stability analysis of high power factor Vienna rectifier based on reduced order model in d-q domain

For a DC distributed power system, system stability can be predicted by dividing it into source and load subsystems, and then applying the Nyquist criterion to the impedance interaction between the source and load model. However, the generalized Nyquist criterion is extremely complicated and cannot directly reveal effective control strategies to reduce interaction problems of cascade three-phase AC systems. Specifically, as a current force rectifier, this characteristic makes it difficult to judge the stability of a cascade three-phase Vienna AC system. To deal with the aforementioned problems, a simplified small signal stability criterion is presented for an AC distributed power system. Based on the criterion, the small signal model and impedance based on the reduced order model in the d-q domain are studied theoretically. For the instability issue, an impedance regulator design method is presented. The correctness of the simplified stability criterion and the effectiveness of the proposed impedance regulator method are validated by extensive simulation and experiment.

A Phase Angle Self-Synchronization Topology for Parallel Operations of Multi-Inverters in High Frequency AC Distribution

—High frequency ac (HFAC) power distribution system (PDS) has been known as the effective alternative of dc power distribution system (DC PDS) because of its excellent advantages such as high power density, high efficiency, fast dynamic response and fewer conversion stages. Multiple HFAC inverters are operated in parallel to satisfy the source side requirements like high power grade, safety margin and reliability. In order to eliminate circulation current caused by the amplitude and phase differences among the inverters, the amplitude and phase of individual inverter are required to be synchronous. However, parallel controls become extremely complicated as amplitude and phase are coupled together in full-bridge inverter with traditional phase-shift modulation. In this paper, a controllable half-bridge LCLC resonant inverter and dual-PWM modulation are presented with phase angle self-synchronization. Control strategy is given for voltage adjusting and phase-amplitude decoupling. Besides, the resonant network normalization parameters and control angle is summarized to ensure soft-switching. The simulation model and experimental prototype are implemented with the output frequency of 25 kHz. The experiment results and theoretical analysis agree well to verify the features of the proposed inverter.

Inertial and Damping Characteristics of DC Distributed Power Systems Based on Frequency Droop Control

With high penetration of renewable energy, DC distributed power systems (DDPSs) need to improve the inertia response and damping capacity of the power grid. The effects of main circuit parameters and control factors on the inertia, damping and synchronization of the DDPS were studied in this paper. Firstly, the dynamic model of DDPSs based on frequency droop control is established in the DC voltage control (DVC) timescale. Then, a static synchronous generator (SSG) model is used to analyze the parameters that affect the inertial level, damping effect and synchronization capability of the DDPS. The analysis results show that an optimal design of the frequency droop coefficient and proportional integral (PI) parameters of the DC bus voltage control loop can equivalently change the characteristics of inertia and damping when the frequency droop control strategy is applied to the DC/DC converter and the DC bus voltage control strategy is used in the grid-tied inverter. Simulation results verify the correctness of the conclusions. This paper helps to design an effective control strategy for DDPSs to enhance the inertial level and damping effect of the power grid and to improve the stable operation capability of renewable energy systems.

Minimum-Backflow-Power Scheme of DAB-Based Solid-State Transformer With Extended-Phase-Shift Control

As key component for the flexible dc distributed power system, the dual active bridge (DAB)-converter-based solid-state transformer (SST) with high efficiency for a wide operating range is essential. However, with the traditional phase-shift control, high backflow power and current stress will significantly affect the conversion efficiency. In this paper, the backflow power characteristics in both sides of DAB-based SST converters are comprehensively analyzed. On this basis, complete transmission power, backflow power, and peak current mathematical models are established. Then, a minimum-backflow-power-based extended-phase-shift control strategy is proposed with the determination of optimal phase-shift pairs by using the Karush–Kuhn–Tucker function for various scenarios. The backflow power and current stress curves with different algorithms are compared. It shows the proposed control can improve the output power regulation flexibility, minimize the backflow power, and improve the efficiency in wide operating range. Finally, a DAB-based SST prototype was developed and the experimental results verified the effectiveness of the proposed control strategy.

Research on Stability Improvement of the Cascaded DC–DC Converters Based on AC Signal Sampling Control Method

Cascaded instability of the cascaded power supply system has restricted the development of the dc distributed power system for a long time. In this paper, by analyzing the cascaded instability essential cause of the dc–dc cascaded system, a control method based on output voltage ac signal component sampling feedback is presented to improve the cascaded stability. Meanwhile, the small signal analysis interprets the physical meaning of the cascaded instability, and then analyzed the control method from the frequency characteristics and control loop point of view. Finally, based on the above-mentioned analysis and discussion, a cascaded dc–dc system comprising two Buck converters has been built and tested. The simulation and experimental results verify the validity and reliability of the proposed ac signal sampling component control method on cascaded stability improvement.