Medium Voltage DC Grid Research Articles

Multi-Terminal DC Transformer for Renewable Energy Cluster Grid Connection

An AC (alternative current) power integration of distributed energies faces multi-fold challenges such as synchronization and weak grid-induced instability. In this study, a multi-terminal DC transformer is proposed for renewable energy clusters grid connection. The DC transformer provides multiple DC input ports for renewable energy collection, while the load port is connected to the medium-voltage DC grid via a modular multilevel converter (MMC). The multi-port topology enables flexible power transfer between multiple input sources to the load without additional components. The carrier layer modulation strategy is implemented to balance the MMC module voltage; bidirectional power transmission between multiple input sources is achieved through the phase shift modulation (PSM) method. First, we provided a detailed introduction of the proposed topology and working principle. A simulation model was built using the SIMULINK, and the simulation results verified the effectiveness of the proposed converter modulation strategy and phase shift modulation method. A corresponding hardware experimental platform was designed and built, and the modulation and voltage equalization functions of modular multilevel rectifiers were presented as well as the measured results of power transmission modulation of the converter under single-input and multi-input conditions. The results indicate that the proposed transformer can achieve multiple DC inputs and has multi-channel power transmission capabilities, making it suitable for renewable energy clusters grid connection.

Continuous Time Simulation and System-Level Model of a MVDC Distribution Grid Including SST and MMC-Based AFE

Medium-voltage DC (MVDC) technology has gained increasing attention in recent years. Power electronics devices dominate these grids. Accurate simulation of such a grid, with detailed models of switching semiconductors, can quickly became very time-consuming, according to the number of connected devices to be simulated. A simulation approach based on interactions on a continuous time model can be very interesting, especially for developing a system-level control model of such a modern MVDC distribution grid. The aim of this paper is to present all the steps required for obtaining a continuous time modelling of a +/−10 kV MVDC grid case study, including a solid-state transformer (SST)- and modular multilevel converter (MMC)-based active front end (AFE). An additional aim of this paper is to supply educational content about the use of the continuous time simulation approach, thanks to a detailed description of the various devices modelled into the presented MVDC grid. The results of a certain number of simulation scenarios are eventually presented.

DC Cascaded Energy Storage System Based on DC Collector With Gradient Descent Method

With the continuous development of distributed energy, the energy storage system (ESS) is indispensable in improving power quality. Aiming at the application of large-capacity storage battery access to medium voltage dc power grid, a dc cascaded energy storage system based on the dc collector is proposed, and the characteristic, topology and control are presented in detail. In this scheme, the low-voltage storage batteries are accessed to medium voltage dc bus directly with dc collectors, which not only has higher efficiency, but also improves power density. Especially, the diversity of state of charge (SOC) is only reflected in the duty cycle of submodules (SMs), rather than the voltage deviation. Further, a variable angle carrier phase-shifted modulation with gradient descent method is discussed to eliminate the dominant current harmonics. By calculating the harmonic amplitude and solving the Jacobian matrix, the gradient that reduces harmonic current is obtained, and the optimal combination of the carrier phases is deduced by iterative calculation. Meanwhile, the method can be extended in the dc collector with any number of SMs to eliminate the dominant harmonics. Finally, the experimental prototype is built and the benefit of proposed solution is verified.

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.

Coordinate Control of Wind Turbines in a Medium Voltage DC Grid

In this paper a coordinate control strategy for wind turbines (WTs) and the collector grid in an all-DC wind generation system is proposed. The all-DC system is adopted from a previous work of the authors which consists of a high-voltage wind turbine generator (WTG) that uses a multiphase hybrid machine interfaced to a passive rectifier, a medium voltage DC (MVDC) collector grid, and a high-voltage DC (HVDC) transmission system. Two control schemes are implemented: 1) A local WTG controller that controls a maximum power point tracking (MPPT) for the hybrid generator using field current adjustment as the wind velocity varies, and 2) An aggregated controller that maintains the MVDC grid voltage corresponding to minimum wind velocity among WTs. The hybrid generator has two rotor elements which enables full control over the generator without the use of active power electronics. The hybrid generator is interfaced to the MVDC via an uncontrolled passive rectification stage. The MVDC grid collects the power from WTs and sends it to an offshore substation where DC-DC converters step-up the voltage to a HVDC grid with fixed voltage for transmission. A coordinate control of WTG and aggregated controller allows WTs to maintain their desired power via MPPT, while enabling a variable voltage MVDC grid operation. Simulation results under different conditions are performed to verify the effectiveness of the proposed control strategies. Experimental results from a scaled-down laboratory prototype 9-phase hybrid generator connected to a DC grid using a passive rectifier validate the simulation analyses.

Topology of Wind Power DC Collection Converter Based on Double Bus Structure and Its Power Balancing Control Strategy

Wind power collection technology is the key to grid-connected wind power generation. Compared with AC wind integration, DC wind power collection is more advantageous. Using a triple active bridge (TAB) converter as a submodule, this paper proposes a wind power DC converter topology and its control strategy for medium-voltage dc (MVDC) grid connection. The converter adopts a double bus structure, with the MVDC bus connected to the grid and the low-voltage dc (LVDC) bus handling the mismatched power. It has independent maximum power point tracking (MPPT) control, power balancing, and autonomous voltage balancing on the output side. The converter system is simulated and verified on the MATLAB/SIMULINK software platform. The simulation results show the effectiveness of the control strategy and the ability of the converter to operate stably under multiple conditions.

Multiport Hybrid Converter for Electrified Transportation Systems

Compact and efficient power converter solutions are seen to be the backbone of future transportation systems in order to cope with the ongoing transition towards greener systems. Such systems usually comprise a main load section, in which one or more propulsion or traction motors are connected, in addition to an auxiliary load, which might comprise the hotels and air conditioning for example. This auxiliary load can be as low as 5-10% of the main load power. Therefore, it can be challenging to drive this power from a typical high power system that employs a medium voltage (MV) DC (MVDC) grid, which is typical in high power systems. In such MVDC-integrated systems, neutral point-clamped (NPC) and active NPC (ANPC) converters are commonly used, where the auxiliary load converter is over-rated in this case, resulting in a bulky and inefficient power system. Thus, in order to enable a lighter and efficient transportation power system, a multiport hybrid converter (MHC) is presented in this paper. This converter can feed the main MV motor, in addition to two auxiliary LV loads. Compared to the state-of-the-art ANPC converter, the proposed MHC utilizes only two extra switches per phase-leg in order to achieve this multiport operation along with increasing the voltage rating of another two switches. The proposed MHC is analysed in this paper, where its operation, modulation, and mathematical derivation are presented. These analyses are supported by simulation and experimental results utilizing a reduced-scale 5 kW system.

A Single-Stage ZCS Boost Module for MVDC Collection Converters

To accomplish the large-scale renewable energy collection to the medium-voltage dc grid within one single stage, an interleaved current-fed zero-current-switching (ZCS) boost module for input-parallel–output-series dc–dc converter is proposed in this article. In the proposed module, two interleaved current-fed arms are responsible to achieve a high step-up ratio, while two reverse series switches in the resonant tank path are employed to regulate the equivalent duty cycle. The full ZCS in the whole load range is achieved for all the semiconductors. The operation principles and parameters design are analyzed in detail. Finally, all theoretical analysis and characteristics are verified on a 30–70 V/600 V/1 kW prototype.

A Peak Current Reducing Method for Input-Independent and Output-Series Modular Converters With LC-Branch-Based Power Balancing Unit

Input-independent and output-series (IIOS) modular converters are characterized by high step-up ratio and independent maximum power point tracking and can therefore be used to integrate renewable energy into medium voltage dc grids. However, the associated power mismatch can lead to voltage imbalance of the submodules(SMs). At present, the only solution using no extra active switch involves the insertion of an <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</i> -branch-based power balancing unit (PBU) between every two adjacent SMs (the SM and PBU share the switches). However, the considerable peak current of the shared switches threatens the security of the switches and increases the power loss. Therefore, this article considered the peak-current problem. A dual active bridge (DAB) converter was employed as the SM of the IIOS converter. The basis for the large peak current associated with the shared switches was analyzed and a peak current reducing method using the DAB inner-phase-shift control was proposed. The contribution of this article is that the current stress can be reduced and the power efficiency can be increased. A two-SM and 1000-W down-scaled experimental prototype was developed for verification of the theoretical analysis results. The peak currents and efficiencies with and without the proposed method were compared with experimental results.

A Modular Bidirectional Solid-State DC Circuit Breaker for LV and MVDC Grid Applications

Direct current (dc) microgrids are increasingly gaining attention in industrial applications due to their simpler and more efficient integration with renewable energy resources and energy storage elements. The dc grid demands a faster, compact, cost-effective, and fault-tolerant protective system for reliable operation. To address the above challenges, this article proposes a bidirectional solid-state dc circuit breaker topology that guarantees reliable operation of dc grids [low voltage dc (LVDC) and medium voltage dc (MVDC)]. A modular extension of the proposed circuit breaker is also presented, resulting in better reliability, scalability, and fault-tolerant operation. The circuit breaker is derived using power semiconductor devices [silicon-controlled rectifiers (SCRs) and insulated-gate bipolar transistors (IGBTs)], with SCR acting as a main power interruption device. Salient features of the proposed topology include modularity, use of low-power rated devices, low-current rated sensors, and pre-fault interruption. A detailed mathematical analysis validating the design and operation of the proposed modular circuit breaker is presented. Moreover, the article also highlights the merits and limitations of the proposed concept. Finally, a laboratory prototype is developed with a system specification of 400 VDC/14 A to validate the performance of the proposed circuit breaker with single and modular operations, which is in line with the obtained simulation results. To verify current sharing between the modules, a few non-ideal conditions such as the use of non-identical main SCRs and turn-on delay are considered and tested on the developed prototype.

A Constant-Frequency High-Voltage Gain Resonant Converter Module With Semiactive Phase-Shifted Voltage Multiplier for MVdc Distribution

A constant-frequency step-up LLC resonant converter module that employs a new phase-shifted semiactive voltage multiplier technique is proposed in this article for a medium-voltage dc distribution grid. In the proposed converter, a wide voltage gain for different loading conditions can be realized by adaptively controlling the phase shift of the auxiliary switch in a voltage quadrupler-based semiactive voltage multiplier. Zero-voltage switching turn-on is always achieved for all the primary side switches with a constant-frequency operation. In the proposed semiactive voltage multiplier, zero-current switching (ZCS) turn-on is achieved for the auxiliary switch with ZCS operations provided for all the output diodes over a wide range of voltage gain and loading conditions. The operating principles of the proposed circuit are discussed in this article. Simulation results on a 120-kW, 10-kV output and experimental results on a modular laboratory-scale silicon carbide (SiC)-based 5-kW (per-module), 240–440-V/5-kV prototype are provided to demonstrate the performance of the proposed converter.

Overvoltage Suppression Scheme for Minimized Snubber Requirements in MVDC Solid-State Breakers With Series-Connected IGBTs

Solid-state dc breakers are the ultimate technology for protecting medium-voltage dc grids having equipment vulnerable to excessive fault-current stress, due to the very short fault-clearing times. A typical implementation of such breakers is based on insulated gate bipolar transistors (IGBTs). However, as the rated voltage of the grid increases, series connection of IGBTs is inevitable and impose challenges on transient and steady-state blocking voltage distribution. This article presents a novel voltage-balancing scheme that combines passive resistive-capacitive-diode (RCD) snubber circuits and a gate driver containing a gate-current balancing magnetic-core. The proposed concept aims at minimizing the required snubber capacitances, which may be high. The impact of the proposed configuration on the breaker design and its performance are studied under different fault scenarios. Using the proposed breaker configuration, the snubber capacitance has been reduced by up to 60%. Besides, both simulations and experimental results reveal the mitigation of high peak voltage stress across the early turned-off IGBTs under gate signal propagation delays.

A Step-Up Reconfigurable Multimode LLC Converter Module With Extended High-Efficiency Range for Wide Voltage Gain Application in Medium Voltage DC Grid Systems

In this article, a newmodular multimode reconfigurable step-up resonant converter that is capable of extending very high efficiency from full load to reduced load conditions is proposed for wide voltage gain application in medium voltage dc (MVdc) grid system. The proposed converter module is able to switch from a hybrid control scheme that consists of variable frequency and phase shift control to pulsewidth modulation (PWM) control in the auxiliary switch of the output voltage quadrupler (VQ). In addition, the input bridge can also be reconfigured to a half-bridge mode to suit high input voltage range while utilizing a hybrid control technique that consists of variable frequency and asymmetrical PWM control. With the proposed approach, the converter module is able to achieve constant output voltage regulation without requiring a wide spectrum of switching frequency or phase shift control. Hence, close-to resonance operation with very high efficiency can be maintained for a wide range of input voltage and loading conditions. Soft switching operations are always guaranteed for all the primary side switches, the output diodes and the auxiliary switch in the VQ. The steady-state and dynamic performance of the proposed modular multimode step-up converter are validated through simulation results on a silicon carbide (SiC) based 360 V–1 kV/16 kV, 80-kW system and experimental results on a proof-of-concept 150–400 V/6.6 kV, 10-kW SiC laboratory prototype. Results confirmed that the efficiency is maintained between 97.8% and 99.1% from full load to at least 20% load condition for the specified input voltage range.

A Sharing-Branch Modular Multilevel DC Transformer With Wide Voltage Range Regulation for DC Distribution Grids

As the key equipment in dc distribution grids, dc transformer (DCT) suffers from low power density caused by a large number of submodules (SMs). Hence, this article proposes a novel DCT for the interconnection of medium-voltage dc (MVdc) and low-voltage dc (LVdc) distribution grids, which adopts a series-connected half-bridge SM branch (SSB) on MV-side to achieve low voltage stress of devices and reduce the number of SMs. Quasi square wave modulation strategy and phase-shifted control are employed to realize bidirectional power transmission control and zero-voltage switching (ZVS) for all switches. Furthermore, in order to ensure low current stress and ZVS within wide voltage range, a certain number of SMs are constantly inserted or bypassed to achieve voltage matching. Besides, smooth transition control strategy is employed to avoid current spikes caused by abruptly bypassing or inserting SMs. Based on the theoretical analysis, design consideration and control strategy are elaborated. Finally, a 500&#x00A0;kW&#x002F;6&#x2013;12&#x00A0;kV&#x002F;750&#x00A0;V simulation model and a 2&#x00A0;kW prototype are built to verify the proposed DCT.

Standalone Clamping Switched Capacitor-Based Modular DC Transformer for MVdc Application

This article introduces a standalone clamping switched capacitor-based dc transformer (CSCDCT) for the medium-voltage dc power grid. The CSCDCT owns the functions of the dc fault isolation and the online redundant design, and hence, the power distribution reliability can be guaranteed. It is also shown that the additional advantages of the CSCDCT are the matching regulation of the high-frequency ac voltages and the voltage self-balance of the dc capacitors. The coupling effect of ac voltage amplitudes and dc voltages can be eliminated by the matching regulation, and the action of dc voltages will not alter the amplitude of high-frequency ac voltages in CSCDCT, improving the soft-switching performance, efficiency, and control stability of the DCT system. Besides, by the self-balance capability, the dc capacitor voltage balancing mechanism is not required for the individual unit although two capacitors are used in each submodule. This simplifies the control system of the CSCDCT. The topology, operating mode, voltage matching modulation, current characteristic, switching performance, and power loss of the CSCDCT are investigated in detail. The correctness of the analysis is validated through the experimental and simulation results.

Evaluation of high step-up power conversion systems for large-capacity photovoltaic generation integrated into medium voltage DC grids

With the increase of dc based renewable energy generation and dc loads, the medium voltage dc (MVDC) distribution network is becoming a promising option for more efficient system integration. In particular, large-capacity photovoltaic (PV)-based power generation is growing rapidly, and a corresponding power conversion system is critical to integrate these large PV systems into MVDC power grid. Different from traditional ac grid-connected converters, the converter system for dc grid interfaced PV system requires large-capacity dc conversion over a wide range of ultra-high voltage step-up ratios. This is an important issue, yet received limited research so far. In this paper, a thorough study of dc-dc conversion system for a medium-voltage dc grid-connected PV system is conducted. The required structural features for such a conversion system are first discussed. Based on these features, the conversion system is classified into four categories by series-parallel connection scheme of power modules. Then two existing conversion system configurations as well as a proposed solution are compared in terms of input/output performance, conversion efficiency, modulation method, control complexity, power density, reliability, and hardware cost. In-depth analysis is carried out to select the most suitable conversion systems in various application scenarios.

Bidirectional Buck-Boost and Series LC-Based Power Balancing Units for Photovoltaic DC Collection System

The input-independent and output-series (IIOS) structure is often used in the high step-up ratio converters required for medium-voltage dc (MVdc) grid interface of photovoltaic (PV) systems, which achieves high voltage gain and ensures high efficiency. However, due to the characteristics of IIOS structure, when some PV arrays are shadowed or malfunction, the generated power of these modules will be mismatched. Mismatched power will result in output capacitor voltages deviating from the rated value, which is not conducive to the standardized multimodule design. Besides, voltages of some PV arrays will deviate from their maximum power point (MPP) value, leading to a decrease in the total generated power. In order to balance the module output voltages of the IIOS structure, this article proposes bidirectional buck-boost and series <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC-</i> based power balancing units (LC-PBUs) for PV dc collection systems. Compared with a traditional solution, nearly half of the switches are saved and half of the inductors are displaced by small inductors. Power balance can be achieved through the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</i> -PBUs by controlling the duty ratio and the phase of the pulse-width modulation (PWM) signal. Simulations and experiments are carried out to verify the effectiveness of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</i> -PBU under power sags.

Real Time Simulation of Power Electronics Medium Voltage DC-Grid Simulator

Power electronics medium-voltage (MV) systems must comply with the requirements defined in grid codes. These systems’ compatibility with the standards can be validated by specialized testing equipment: grid simulators. This paper presents a hardware in the loop (HiL) implementation and the simulation results of a MV multiphase DC/DC converter designed for MV DC grid emulation. By using ABB’s reliable, patented power converter hardware topology (US 10978948 B2) and by applying advanced control algorithms, the presented system can be used for special purposes, such as the emulation of fault events in a DC-grid used for the certification of other devices, or for other research goals. The presented concept of a power electronics DC-grid simulator (PEGS-DC) is characterized by high power capability and high voltage quality. In this paper, the general idea of a power electronics grid simulator applied for the testing of MV electrical systems is discussed. Then, details related to the PEGS-DC, such as its hardware topology and the applied modulation method are presented. Subsequently, the HiL setup is described. The main scope of this article focuses on model the description and presenting recorded HiL simulations.

DAB Based DC-DC High Frequency Link PET for Interconnecting MVDC-LVDC Grids

This paper proposes dual active bridge (DAB) based high frequency power electronic transformer (PET) for interconnecting medium voltage dc (MVDC) and low voltage dc (LVDC) grids for dc power distribution. The above proposed concept works on dual active phase shift principle and square wave HF modulation technique for bidirectional power transfer. Compared to the traditional dc transformer scheme, The proposed power electronic transformer (PET) can disconnect from LVDC distribution grid effectively as a dc breaker when a short circuit fault occurs in the distribution grid. The isolated DC-DC PET topology with a wide range of voltage conversion ratio is useful for High Voltage DC tapping. The DAB based on switched capacitor is connected to the medium voltage DC side and acts as an inverter. The proposed topology has the ability to transfer higher power, and lower circulating power, lower high frequency link voltage, and RMS current and peak values with the same transmission power in the MVDC side. This paper analyzes the topology, voltage and power characterization, control strategy in detail. Increase in the intermediate AC frequency will reduce the size of the transformer and other passive elements significantly in the circuit. The theoretical analysis is supported by MATLAB simulation.

Arm current reversal‐based modular multilevel DC‐DC converter

Nowadays, most of the converters used in high-power high-voltage (HV) applications are the conventional modular multilevel converters (MMC). However, in the case of DC-DC conversion, an imbalance of the capacitor voltages occurs and the conventional MMC fails to operate correctly. This paper introduces an arm current reversal-based modular multilevel DC-DC converter, which successfully provides balance among the capacitor voltages while operating in DC-DC conversion. The proposed configuration is used in medium voltage DC grids to feed DC loads or to interconnect between two DC grids of different voltage levels. The proposed converter is a two-stage DC-DC modular converter, which consists of a single-phase half-bridge MMC with half-bridge submodules (HBMMC) followed by a single-phase H-bridge MMC with half-bridge submodules (SMs). The operational concept of the proposed converter is based on reversing the arm current direction and reversing the output terminals with the help of the H-bridge MMC stage, which ensures the same direction of the voltage at the load terminals. The proposed converter provides a high conversion ratio, bidirectional power flow, simple architecture, and a simple control scheme. Detailed illustrations, analysis, and design of the proposed converter are presented. Besides, MATLAB-based and Opal RT-based simulation results and experimental results are presented to validate the proposed configuration claims.