DC Grid Research Articles

An efficient high-gain bidirectional interleaved boost converter for PV integration to DC microgrid.

The design of a power electronic interface for high voltage difference DC buses is a key aspect in DC microgrid applications. A multi-port non isolated interleaved high-voltage gain bidirectional converter, which facilitates bidirectional power transfer and islanded operation in a DC microgrid, is presented in this paper. The forward high-voltage transfer ratio is achieved using a voltage multiplier circuit, and the high-gain step-down power conversion is performed using a resonant power module. A novel power transfer selection algorithm is proposed to control power flow among the interfaces of the RES, ESS, and DC grid converters, which utilizes the net power difference as the basis for switching the converter. The proposed converter is simulated for a 24 V PV source, 12 V battery, and 400 V DC grid interface using MATLAB/SIMULINK. A 200 W hardware prototype is implemented. The simulation results for voltages, currents, and power flow among RES, ESS, and microgrid DC bus proved an excellent voltage regulation, efficient power conversion, and a feasible duty cycle range with high voltage gain. These observations are validated through equivalent experimental results. A comparison is made regarding achieved gain, component sizing, achievable power transfer modes, efficiency, and control complexity with existing converters for DC microgrid applications. The presented topology proved to be a better interface with multiple-mode support with high efficiency.

Analysis and Simulation of Three-Input DC-DC Converter with Bipolar Symmetric Outputs

This paper presents a three-input high-gain DC-DC converter with bipolar symmetric outputs. The advantages of a bipolar DC grid are higher reliability, flexibility, and efficiency as compared with the unipolar DC grid. Therefore, the proposed converter is designed to connect directly with the bipolar DC grid due to its bipolar symmetric outputs. The three input voltage sources to the converter are operated in a sequential manner from the first voltage source to the last one, with each source providing power at a level proportional to its duty cycle. Similarly, when the sources are equal, they operate in a sequential manner with their specific power. In addition, the proposed converter is designed to provide the required power to the grid under both simultaneous or individual from three input voltage sources. A double-loop PI controller is applied to regulate the converter output voltages and the control loop is analyzed and the transfer function relating reference voltage to output voltage is derived. Voltage balance control is provided to ensure that the outputs remain symmetrical despite changes in load conditions or input voltage. A converter with a power rating of 1000W and output voltage levels of 200V, -200V, and 400V is designed. The three supply voltage of 150V,100V, and 75V are used. The converter is tested under a variation of supply voltages and load using MATLAB/SIMULINK. The results show the feasibility and effectiveness of the proposed converter.

Integrated Equipment with Functions of Current Flow Control and Fault Isolation for Multiterminal DC Grids

Integrated Equipment with Functions of Current Flow Control and Fault Isolation for Multiterminal DC Grids

Auxiliary Branch for Hybrid DC Circuit Breaker in Flexible DC Grid to Reduce Fault Breaking Time and Arrester Capacity Demand

Auxiliary Branch for Hybrid DC Circuit Breaker in Flexible DC Grid to Reduce Fault Breaking Time and Arrester Capacity Demand

Synthesis and Comparison of Soft-Switched Operating Modes of a Series Resonant Balancing Converter for Bipolar DC Grids

Synthesis and Comparison of Soft-Switched Operating Modes of a Series Resonant Balancing Converter for Bipolar DC Grids

A high step-up multiphase interleaved boost converter for fuel cells power system

The fuel cells need the high step-up DC/DC converter with low current ripples to interface to the high voltage DC grid. A novel zero voltage switching high step-up multiphase interleaved boost converter (ZVS HSMIBC) is proposed in this paper. This new converter employs advantages of low input current ripples, high voltage step-up ratio, soft-switching operation, and low switching harmonics and noise. In this paper, the two-phase interleaved structure of the proposed ZVS HSMIBC is analyzed and researched in detail as a representative topology. The relevant conclusions also can apply to multiphase (3 and above) interleaved structure of the HSMIBC. Finally, an experimental prototype is built and test, the experimental results show that the proposed converter can operated under ZVS conditions with low input current ripples and low switching noise and harmonics. In addition, this converter can obtain high voltage step-up ratio up to 45 (mn = 8, d = 0.8) with high efficiency, and proportion of the experimental ratios to theoretical ratios is also high (not less than 0.8).

DC protection coordination for multi‐terminal HB‐MMC‐based HVDC grids

AbstractFast progress of High Voltage Direct Current (HVDC) technology has been considerable over the past decades. Emerging Modular Multilevel Converter (MMC) provides power systems with promising solutions to integrate large‐scale renewable energy sources, as well as multiple asynchronous power grids. Consequently, implementation of a multi‐terminal HVDC transmission system has been easier using MMC converters. However, protection of multi‐terminal MMC‐based HVDC grids under DC fault conditions is a challenge for the industry. This study sheds light on concept of system‐level DC protection coordination for multi‐terminal HVDC grids. Although, there are different protection methods for HVDC transmission systems including multi‐terminal ones, there are no well‐defined protection coordination strategies for transmission links protection and belonging HVDC converters under DC fault conditions. The proposed protection coordination prevents unnecessary tripping of MMCs in the multi‐area HVDC grids and supports the system with minimum effect on healthy parts. For the first time, different HVDC Converter Tripping (HCT) profiles are presented based on a fully selective protection strategy and their characteristics are discussed comprehensively. Simulation results demonstrate the effectiveness of the proposed strategy for multi‐terminal MMC‐based HVDC grids. The proposed coordination can be applied to DC grid code requirements of meshed HVDC grids in the future.

Robust and efficient control strategy for single-phase two-stage PV grid-tied system with improved power quality

In the last couple of decades, the integration of photovoltaic (PV) systems with the utility grid has gained significant attention due to its potential to harness renewable energy and contribute to sustainable power generation. However, the integration of the PV system into the grid leads to issues related to power quality and system stability under varying solar irradiation, dc offset, and distorted grid conditions. Therefore, this paper focus on the robust and efficient control of a single-phase, two-stage PV grid-tied system to improve power quality and stable system operation under abnormal grid conditions. These objectives can be achieved using the synchronization method, dc link voltage control, and grid current control. The hybrid PLL is proposed in this paper for synchronization and to mitigate the effect of dc offset and lower-order harmonics in the estimated phase and frequency. Further, the sliding mode voltage controller and integral sliding mode current controller are proposed for dc link voltage regulation and grid current tracking. The stability of voltage and current control loops is analyzed using the Lyapunov stability theory. Furthermore, the proposed schemes are validated using MATLAB simulation and experimental analysis. The proposed approach outperforms conventional methods and exhibits better disturbance rejection capability under both dynamic and steady-state conditions, making it a robust and efficient control strategy for grid-tied systems.

Synchronous Generator Feeding DC Grid Having Brushless Excitation System With Induction Machine Operating in Deep Plugging Mode

This paper proposes a novel excitation control for a recently proposed synchronous generator (SG) entitled Brushless Induction Excited Synchronous Generator (BINSYG) for wind energy applications. In BINSYG, one SG and another Induction machine (IM) are embedded in the same machine frame to make the overall system brushless. Here, IM acts as the excitation machine and regulates the dc excitation for SG by rectifying the rotor-induced currents through a rotating rectifier. SG feeds power to dc grid via a two-level six-switch Voltage Source Rectifier that is operated at unity power factor at the SG terminal. The excitation developed by IM is regulated using a new frequency modulation control strategy for a two-level Voltage Source Inverter connected to the stator of IM. The excitation machine helps operate SG at variable wind speeds with reduced flux distortions. The proposed system is speed/position sensorless and controlled without using any voltage or current sensors on the IM side. The system with its proposed control algorithm is modeled in MATLAB/Simulink and experimental verification for a small-scale laboratory prototype BINSYG feeding power to a 220 V dc grid are presented to validate the effectiveness of the controller.

Analysis and Design Guidelines of the Isolated Modular Multilevel DC–DC Converter With the Impact of Magnetizing Inductance

The Isolated Modular Multilevel DC-DC (IMM DC-DC) converter is one of the potential candidates for interconnection of medium and high voltage DC grids. One of the key components for realization of the IMM DC-DC converter is its transformer whose magnetizing inductance depends on the core material, dimensions, and airgap. Depending on the transformer design, the magnetizing inductance may vary over a wide range, adding to the complexity of the converter design. In this paper, the impact of the magnetizing inductance on the converter real and reactive powers, transformer primary and secondary side currents, arm current, differential-mode current, capacitor voltage ripple, and soft-switching operation is investigated. The presented analysis is validated by time-domain simulation in the Matlab/Simulink software environment and experiments on a scaled-down prototype.

An Improved Three-Level Neutral Point Clamped Converter System With Full-Voltage Balancing Capability for Bipolar Low-Voltage DC Grid

An Improved Three-Level Neutral Point Clamped Converter System With Full-Voltage Balancing Capability for Bipolar Low-Voltage DC Grid

Modular high conversion ratio soft-switching DC-DC converters with extended multi-ports structure for multi-energy storage systems interconnection

This paper focus on the advanced multi-energy storage systems interconnection by DC smart grids with high efficiency and high compactness. A non-isolated modular high conversion ratio bidirectional soft switching DC-DC converter and its extended multi-ports structure are proposed in this paper. In each power module of the converter, an auxiliary zero-current transition (ZCT) cell is integrated to achieve the zero-current turn-on and turn-off for all semiconductor devices. Therefore, the switching losses are reduced greatly and the efficiency is improved considerably. The presented converter is suitable for the small and medium power scale interconnection in smart DC grids, and can realize the flexible multi-energy storage system interaction owing to its extended multi-ports structure. Besides, various control schemes are adopted to optimize the control system of this converter, which can improve the performance on energy interaction. The operation principles and design considerations are discussed in details and a 5 kW experimental prototype is constructed to justify the validity of the theoretical analysis.

Grounding and Isolation Requirements in DC Microgrids: Overview and Critical Analysis

DC microgrids, along with existing AC grids, are a future trend in energy distribution systems. At the same time, many related issues are still undefined and unsolved. In particular, uncertainty prevails in isolation requirements between AC grids and novel microgrids as well as in the grounding approaches. This paper presents a critical technical analysis and an overview of possible grounding approaches in DC systems and the feasibility of avoiding isolation between AC and DC grids.

A new hybrid multilevel thyristor-based DC-DC converter

The rapid growth in HVDC grids is becoming inevitable for long-distance power transmission. Therefore, the idea of interconnection between the point-to-point links becomes essential. However, these point-to-point connections face several challenges such as the requirement of DC fault blocking capability, interfacing of different grounding schemes, offering multi-vendor interoperability, and difficulty to achieve high DC voltage stepping. DC-DC converters are considered the optimum solution to tackle these challenges in DC grids interconnection. In this paper, a new hybrid modular DC-DC converter is proposed that achieves a low number of semiconductors, low losses, and cost in comparison to other DC-DC converters due to the utilization of thyristors. The new DC-DC converter consists of two hybrid MMC bridges connected through an isolating transformer. Each MMC bridge is comprised of half bridge submodules and bidirectional thyristors. Detailed mathematical analysis, design, and control are illustrated. A comparison is carried out between different topologies in terms of semiconductor count, power loss, and cost. Also, both simulation model and experimental test rig are built to validate the proposed hybrid modular DC-DC converter under different scenarios. Finally, another variant of the hybrid-thyristor based converter (version two) is proposed for multiport DC-Hub application to achieve DC fault blocking without turning off all connected bridges.

Practical Dead-Time Control Methodology of a Three-Phase Dual Active Bridge Converter for a DC Grid System

An effective dead-time control strategy for the three-phase dual active bridge (3P-DAB) converter of a distribution system is studied to reduce the switching losses of power switches and improve the under-light-load power conversion efficiency. Because of the advantages of a dual-active bridge converter, such as an inherent zero-voltage switching (ZVS) capability without any additional resonant tank and a seamless bi-directional power transition, this is an attractive topology for bi-directional application. The 3P-DAB converter is apt for high-power applications such as aircraft due to an interleaved structure, which can reduce conduction losses. However, the design of the dead time depends on engineering experience and empirical methods. In order to overcome the conventional practicality of the dead-time design method, the effective control of dead time is proposed based on the theoretical analysis. In this paper, the overall explanation of the 3P-DAB converter is shown with operation principles. In addition, the dead-time effect of the 3P-DAB converter is examined and the practical variable dead-time control strategy is studied. Finally, experimental results validate the proposed variable dead-time control strategy using a 25 kW prototype 3P-DAB converter.

High pass filter based traveling wave method for fault location in VSC-Interfaced HVDC system

Due to the difficult topography and bad weather conditions across large distances, dc line breakdown is a serious danger to the safety of high-voltage direct-current systems (HVDC). Thus, there is a need of an accurate and reliable fault location method to safely isolate the dc grid in the occurrence of a dc fault. Traveling wave (TW) fault location techniques have gained in popularity over the decades, with the expectation that power networks will become considerably more credible and accurate throughout. However, in the existing traveling wave based location, the signal processing methodology for extracting the wave front consumes too many computer resources, necessitating the development of a new location method with a fast response and high accuracy. In this paper, a new algorithm of TW based on high pass filter is proposed, it can differentiate the TW peak more easily by removing the oscillation in a signal. To ensure more accuracy double ended method (Type D) is used to obtain the location for the fault. The theory behind this new algorithm is presented in detail. High pass filter is designed for this purpose and its validity tested on point to point (P2P) dc simulation model which is created in PSCAD software to gather the voltage data from both terminals during transient. Following that the proposed fault location algorithm is executed in the MATLAB environment to get the fault location results. A comparison of the proposed algorithm with two popular methods namely the wavelet transforms (WT) and the single ended method highlights the superiority.

The Cost and Benefit of Enhancing Cybersecurity for Hybrid AC/DC Grids

As critical interfaces of AC grids and DC grids inside a hybrid AC/DC grid, the voltage-sourced-converter (VSC) has been demonstrated to be vulnerable to false data injection (FDI) cyber-attacks. As a result, the cyber-attack-induced AC grid frequency deviations and DC grid voltage deviations threaten the secure operation. To enhance cybersecurity in a not only feasible but also cost-effective manner, this paper proposes a cost-benefit-based cyber-defense strategy for a hybrid AC/DC grid. First, this paper establishes a spatial-temporal dual cyber-attack evaluation model, in which the cyber-attack-induced frequency and voltage deviations are modelled in both a spatially and temporally dual manner. Then, the proposed cost-benefit-based cyber-defense strategy is modelled as a VSC commitment problem to achieve the trade-off between maximizing the cyber-defense benefits and minimizing the cyber-defense costs. The VSC commitment problem is then mathematically convexified into a mixed-integer second-order cone programming (MISOCP) problem, which could be efficiently solved in an event-triggered manner against unfolding cyber-attack events. Simulation results on a test hybrid AC/DC grid verified the feasibility and the cost-effectiveness of the proposed cyber-defense strategy.

A hybrid DC-DC modular multilevel converter with capacitors parallel connectivity for arm energy balancing

The integration of renewable energy sources in Medium-/High-voltage DC grids has called up continuous research to develop DC-DC conversion systems. This paper presents a hybrid modular multilevel DC-DC converter topology which successfully overcomes the issue of capacitors voltage imbalance during DC-DC conversion. In the proposed topology, single MMC leg is employed with half-bridge submodules in the upper arm and full-bridge submodules in the lower arm along with soft-switched high-voltage valves. The proposed hybrid modular multilevel converter has inherited arm energy equalization with the help of the parallel connectivity of involved arms’ capacitors. During the equalization period, the series-connected upper arm capacitors are connected in parallel to the series-connected lower arm capacitors to transfer the energy between the involved arms which results in operating with balanced capacitors voltages. The proposed modular DC-DC converter is a bidirectional converter which allows power flow from high-voltage side to low-voltage side and vice versa, i.e., it can be operated successfully as a DC transformer. The operational concept of the proposed configuration, control strategy, and parameters design are presented. Simulation results are presented along with experimental results of a scaled-down prototype to validate the proposed approach. Simulation and experimental results show the viability of the suggested approach.

Current Ripple Reduction Employing the Reactor of DC–DC Converter to Have Variable Inductance through Magnetic Saturation

When DC distribution grids with different voltage levels are connected, harmonic currents flow into the grid under light load conditions, which is a significant cause of grid instability. In this paper, we proposed a method to reduce the current ripple at light load by designing a bidirectional DC–DC converter reactor to have an inductance variable according to the load power capacity. The inductance is varied using magnetic saturation and implemented in three ways: two magnetic circuits in one magnetomotive force (MMF) circuit, a single magnetic circuit in one MMF circuit, and two magnetic components with different magnetic permeability in one MMF circuit. The feasibility is verified through simulation and experiments targeting the 750 and 380 V DC grid connection. The operating characteristics and efficiency are compared and analyzed by applying the existing and proposed variable reactor to a bidirectional DC–DC converter.

Real-Time Hierarchical MPC Applied to an MMC in Grid-Forming Mode: Implementation and Validation in Power Hardware-in-The-Loop

Modular Multilevel Converters (MMC) are now expected to fulfill AC and DC grid stability functions while at the same time they must ensure their own operation. In this paper, we propose a new MIMO control structure to coordinate the AC/DC power exchange and MMC internal energy control using a hierarchical MPC. This controller is tested on an AC/DC system where the MMC works in grid-forming mode on the AC side and controls the DC-grid voltage at the same time. This MPC solution is implemented in a real-time controller to operate a physical mockup of a 6kVA MMC which contains 60 submodules and interfaces a 400V DC grid and a 120V (line-to-ground) AC grid. The MPC solution shows similar tracking performance with the state-of-the-art dual PI control solution based on two PI controllers working in parallel, and exhibits a better disturbance rejection capacity than this dual PI.