AC Terminals Research Articles

Impedance-Based AC/DC Terminal Modeling and Analysis of the MMC-BTB System

Impedance-based small-signal stability analysis is widely applied in practical engineering with modular multilevel converters (MMCs). However, the deficiencies of existing impedance models (IMs) and the idealized extension for the single MMC influence the analyses in multiterminal systems. In this paper, these gaps are filled by focusing on an MMC-based back-to-back system. To obtain the steady-state harmonics of the system, a numerical method is initially proposed based on Newton–Raphson iteration in the frequency domain. Then, by substituting the shared terminal dynamics with active or passive devices, theoretical AC/DC IMs based on typical control loops with pure time delays are directly established via multiharmonic linearization. Further aided by the derived IMs, two neglected aspects in the current literature are investigated, including the influence of power transformers on low-frequency impedance characteristics and the rationality of using simplified IMs for high-frequency resonance studies. The stability of interlinking systems needs to be comprehensively analyzed at both AC and DC terminals. The analyses help to position the instability source, obtain the stability margin, and guide the supplementary control strategy. All IMs and analyses are verified via frequency scans and simulations in PSCAD.

Control and analysis of a fault-tolerant offshore HVDC link fed by a bipolar DC collection network

This paper presents a comprehensive investigation of a fault-tolerant bipolar HVDC link with electrodes utilized to interconnect an offshore wind farm. The wind farm’s generating units are interconnected in series through their dc terminals, which represents an emerging configuration for offshore wind farms. This novel approach allows achieving high transmission voltages without the need for a bulky offshore substation with transformers and rectifiers for direct current transmission. The proposed configuration involves the connection of two modular multilevel converters in a bipolar arrangement at the onshore substation. To evaluate its performance and robustness, extensive digital simulations were conducted using the PSCAD/EMTDC software. Various faults were applied to both the dc and ac terminals to demonstrate the flexibility and robustness of the studied topology. The results obtained from the simulations demonstrate exceptional capability of the bipolar HVDC system to maintain continuous operation, even in the event of a cable fault to the ground at the dc link. This research contributes valuable insights into the feasibility and advantages of employing a series-dc connection for offshore wind farms, offering a promising approach for future high-voltage transmission projects.

Autonomous Control of Soft Open Point for Distribution Network Reliability Enhancement

Power-electronics-interfaced soft open points (SOPs) emerge as a promising solution for the fault-tolerant operation of distribution networks. The conventional SOPs interface two AC terminals with a back-to-back circuit configuration. Nevertheless, the SOP will break down if a fault occurs and interferes with the converter responsible for DC-link voltage regulation. To address this problem, a smart SOP with enhanced reliability is developed in this paper. The proposed SOP can protect the system from the fault, which occurs at any of the two AC terminals. More importantly, the control of SOP is fully autonomous and does not need remote monitoring, manual activation, or operation mode transformation. This merit makes the proposed SOP more reliable and intelligent than the conventional SOP and normally-open point (NOP). Finally, the effectiveness of this proposal is validated by simulation and experimental results.

Blackstart Capability of Modular Multilevel Converters From Partially-Rated Integrated Energy Storage

This paper discusses blackstart provision enabled by Modular Multilevel Converters (MMC) with integrated partially-rated Energy Storage (ES) systems. It is demonstrated that ES units integrated in an MMC for ancillary service provision can be utilised to energise the auxiliary power supply electronics of the submodules (SMs), fully charge the converter capacitors, and establish voltage at the ac and dc terminals. This feature can prove especially useful during the converter start-up phase in passive grids or following a post-fault disconnection event. For an MMC with integrated ES, the added functionality of charging the converter capacitors and starting-up the adjacent networks requires no additional infrastructure – only an updated control scheme. The charging algorithm goes through successive steps: ES-SM charging, <styled-content style="color:black" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">activation of the Auxiliary Power Supplies in the SMs</styled-content> , active charging of all SMs, <styled-content style="color:black" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">and finally,</styled-content> connection to the grid. The effectiveness of the method is confirmed by simulation results where a fully disconnected MMC charges nominally within 2 seconds, and connects to <styled-content style="color:black" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a passive</styled-content> ac network. <styled-content style="color:black" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">By using a grid forming control scheme, the converter ramps up the voltage at the ac terminals and supplies 5% of active and reactive power with only 4% of ES-SMs per stack.</styled-content>

Sliding-Mode Control for Interleaved Operation of Parallel-Connected Three-Phase Active Front-Ends

A converter system is considered that consists of parallel-connected two-level voltage source converters that share the same ac and dc terminals. A controller design is proposed that, in a first stage, controls the grid currents using hysteresis controllers. To this end, augmented grid currents are introduced, which feature decoupling between the phases. As a consequence, a switching action in one phase does not affect the derivatives of these currents in the other phases. In a second stage, the converter currents are controlled using hysteresis controllers such that a desired current distribution among the parallel converters may be ensured. This implicitly sets the circulating currents. The proposed approach is verified by measurements on an industrial megawatt-scale testbench with ten parallel converters. The results are compared with a benchmark approach, with the proposed approach showing lower semiconductor losses and grid current harmonic distortion.

Fault Tolerant Control of Advanced Power Generation Center for More-Electric Aircraft Applications

This article presents a control scheme for a recently reported aircraft advanced power generation center (APGC) during postfault operation conditions. Within the APGC, two electrical generators extract power from two separate engine shafts and supply electrical power to a common high-voltage direct current (HVDC) bus through their dedicated main ac/dc power converters. An extra back-to-back (BTB) converter is also used to connect the ac terminals of the two generators. This architecture provides merits of fault tolerance capabilities of the APGC. In the case of main ac/dc power converter failure, the system can be reconfigured and the BTB converter provides an extra power flow path from the generators to the HVDC bus. This ensures electrical power generation capabilities onboard. This article discusses in detail about the control of the APGC under fault conditions with one main ac/dc converter failure. A seamless transition scheme from normal operation to postfault conditions is proposed using a voltage command initialization technique within the BTB converter. Both experimental and simulation results have verified the fault tolerance improvement and control performances of the APGC.

Multi-port admittance model and SISO stability criterion for oscillatory stability analysis of VSC-based AC/DC interconnected renewable systems

The voltage-source converter (VSC) is physically a multi-port circuit consisting ac and dc terminals. The impedance/admittance models (IMs/AMs) of VSC are generally developed considering the frequency-couplings at its ac port, while couplings between ac and dc ports are paid less attention. The ac AM cannot directly reflect oscillatory risks between dc grid and VSC’s dc port, and vice versa. This paper proposes a multi-port admittance modeling-based framework to simplify the above modeling and analysis procedure. First, a VSC is viewed as a three-port circuit modeled by a three-dimensional admittance matrix (3DAM). Both ac and dc port dynamics are incorporated into 3DAM with couplings among ports explicitly expressed by off-diagonal elements. Then, the dc and ac grids are easily modeled into 3DAM by sequentially terminating the VSC’s three ports with grid impedances. In doing so, the three-port circuit is reduced into a one-port circuit. The principle and steps to transform the 3DAM to a single-input–single-output (SISO) equivalent form are introduced. The resulting SISO model preserves all coupling information in 3DAM and facilitates a more intuitive stability analysis procedure, where a single Nyquist curve suffices. Finally, the 3DAM and SISO model are applied to identify the dominant factors of oscillatory instability. Time-domain simulations validate the effectiveness of the proposed modeling and analysis methods.Index Terms: Admittance matrix, circuit theory, stability criterion, transfer function.

Effects of rare earth Sc2O3 doping on the phase composition, diffusion rate and conductivity of CaHf1−xScxO3−δproton conductor

To explain the electrochemical performance of Sc2O3 doped orthogonal CaHfO3, in the temperature range of 973 to 1473 K, the conductivity of this series of electrolyte samples in oxygen rich atmosphere and hydrogen rich atmosphere was measured by using the two terminal AC method. The conductivity first increases and then decreases with the increases of Sc2O3 doping amount; it reaches a maximum at the Sc3+ concentration of x = 0.18. Furthermore, doping with Sc3+ improves the conductivity of the electrolyte CaHf1−xScxO3−δ and reduces its conductivity activation energy. By examining the H / D isotope effect and the electromotive force of electrolyte samples, it is found that CaHf1−xScxO3−δ is a pure proton conductor between 973 and 1273 K. Furthermore, the chemical diffusion coefficients of the CaHf1−xScxO3−δ are about 1.25 × 10−6 to 1.61 × 10−5 cm2 s−1 in the temperatures range of 973 to 1473 K, that is to say, the response speed of the CaHf1−xScxO3−δ is very sensitive. Study found that proton is the major carrier in this electrolyte in both a hydrogen rich atmosphere and an oxygen rich atmosphere. Moreover, at higher temperatures of 1373 to 1473 K, positive holes are the primary carriers in an oxygen rich atmosphere.

An analytic method for sequential state estimation of integrated AC/LCC-HVDC system

In this paper, a sequential algorithm for state estimation of an AC system embedded with LCC-HVDC link is presented. In the proposed method, the estimation of the states of the AC system and the DC links is carried out separately. The proposed method consists of two stages. In the first stage, the state estimation of the AC system is carried out. In the second stage, utilising the estimated voltage magnitudes of the HVDC terminal AC buses, the state variables corresponding to the DC link are estimated. Further, an analytical approach, without the necessity of any iterative algorithm, is developed for estimating the state variables of the DC links. As a result, the proposed method does not suffer any degradation of the convergence characteristics of the standard state estimation algorithms. Moreover, the developed method does not require any measurement from the DC side. The effectiveness of the proposed algorithm has been tested on IEEE 30 bus and IEEE 118 bus systems involving multiple LCC-HVDC links.

Adaptive Droop Control of the MTDC System With High-Capacity Energy Storage Based on Dynamic and Static Power Decoupling Method

In a multiterminal DC (MTDC) system with a large number of different types of energy storage devices, the AC terminals and the energy storage devices need to cooperate to maintain the stability of the DC bus voltage. Due to the difference in the dynamic and static power capability of each energy storage unit, the dynamic and static power should be distributed separately. To solve the above problems, an adaptive droop control strategy based on the dynamic and static power decoupling is proposed in this paper. The impact of the virtual impedance values on the dynamic and static power flows between the DC voltage regulating terminals operating with the RC droop method is analyzed. Through optimized virtual capacitance and adaptive virtual resistance, the dynamic power and static power can be distributed according to the PCS capacity and the available charge–discharge battery capacity, respectively. In addition, a simple secondary control method is adopted to compensate the static deviation of the DC bus voltage. Finally, a six-terminal MTDC system model is established in Matlab/Simulink, and the simulation results verify the feasibility and effectiveness of the proposed control strategy.

New Sub-Module with Reverse Blocking IGBT for DC Fault Ride-Through in MMC-HVDC System

When integrating multi-grid renewable energy systems, modular multi-level converters (MMCs) are promising for high-voltage DC (HVDC) transmission. Because of the characteristics of the system, however, it is more difficult to prevent a fault at the DC terminal than at the AC terminal of the MMC. Accordingly, a fault ride-through (FRT) strategy for the operation of the MMC in the DC terminal is required for stable system operation. In this paper, a solution for closed-circuit overcurrent caused by a permanent line-to-line DC fault is proposed. This method is able to reduce the fault current through the adjustment of the slope of the total voltage in the system by operating a sub-module having lower switching losses and fewer passive devices compared with existing topologies. Additionally, through the equivalent circuit of the proposed scheme in a sub-module in case of a fault, the FRT mechanism for the fault current is explained. The feasibility of this proposed technique was verified through time-domain simulations implemented by Powersim, Inc.

Single-Phase Hybrid Switched-Capacitor Interleaved AC-DC Boost Converter

In this paper a single-phase boost PFC multilevel interleaved high-voltage gain based on the hybrid switched-capacitor concept is presented. The proposed converter merges interphase transformers and switched-capacitor cells to obtain current sharing and high voltage gain. These features allow for both the reduction of voltage and current stresses on the semiconductors. Furthermore, the interleaving method allows for obtaining multilevel voltages at AC terminals, thereby reducing the weight and bulk of the input inductor. PFC operation is guaranteed through both input current control and output voltage control. Both steady-state and dynamic analyses were conducted. Experimental results for a 1.25 kW, 127 V to 800 V laboratory prototype are presented and discussed.

Novel Converter Topology With Reduced Cost, Size and Weight for High-Power Medium-Voltage Machine Drives: 3x3 Modular Multilevel Series Converter

The modular multilevel converter is considered the state-of-the-art power electronic solution for high-power high-voltage applications that require a DC stage and that operate with fixed frequency at their AC terminals. However, in high-power AC-AC variable-frequency applications, it has been proved that this converter topology presents an unnecessarily large number of components and an intolerably high submodule-capacitor voltage ripple at low frequencies. The modular multilevel matrix converter was proposed as a promising alternative for high-power medium-voltage electrical-machine drives since it presented improved performance at low frequencies and a reduced number of components in comparison to the modular multilevel converter resulting in reduced costs and improved efficiency. Even though, the modular multilevel matrix converter is one of the converter solutions that stands out the most for high-power machine drives, it presents some operational drawbacks such the unstable behavior under unbalanced grid conditions and the extremely complex internal control and modulation. Recently, a new converter topology named modular multilevel series converter was proposed presenting attractive operational characteristics such as the high performance at low frequencies, high performance under unbalanced grid conditions, and straightforward control and modulation, which are important advantages compared to the modular multilevel converter and the modular multilevel matrix converter. Nonetheless, as demonstrated in this paper, the modular multilevel series converter presents large component count and poor efficiency. Thus, in this paper, a new converter solution is proposed, which is an enhanced version of the modular multilevel series converter that preserves the previously mentioned advantageous operational characteristics and that presents an improved efficiency and a considerably reduced number of components in comparison to the other three converter topologies, resulting in reduced cost, size and weight.

Impact of Optimal Control of Distributed Generation Converters in Smart Transformer Based Meshed Hybrid Distribution Network

A smart transformer (ST) based meshed hybrid distribution network is realized by extending ST low voltage dc (LVDC) link to form a LVDC line which connects dc buses of existing distributed generation (DG) converters. This paper proposes a method for optimal operation in such ST based meshed hybrid distribution network. A CIGRE LV residential distribution network with DG sources is connected at ST low voltage ac (LVAC) and LVDC terminals. All the DG sources are proposed to be connected at LVDC line. The power management scheme and method of determining power flow solution for the considered distribution network are proposed. An optimal problem is formulated for determining the active and reactive power references of DG converters while maintaining the load bus voltages within the grid limits and considering the DG converters constraints. The minimization of energy drawn from ST medium voltage (STMV) grid is considered as objective function and solved using genetic algorithm. To know the impact of proposed optimal control DG converters various performance indicators i.e., energy loss, operating energy costs, voltage profile and sizing of ST converters are considered and compared with existing literature.

Multi-Port DC-DC and DC-AC Converters for Large-Scale Integration of Renewable Power Generation

Numerous research studies on high capacity DC-DC converters have been put forward in recent years, targeting multi-terminal medium-voltage direct current (MVDC) and high-voltage direct current (HVDC) systems, in which renewable power plants can be integrated at both medium-voltage (MV) and high-voltage (HV) DC and AC terminals; hence, leading to complex hybrid AC-DC systems. Multi-port converters (MPCs) offer the means to promote and accelerate renewable energy and smart grids applications due to their increased control flexibilities. In this paper, a family of MPCs is proposed in order to act as a hybrid hub at critical nodes of complex multi-terminal MVDC and HVDC grids. The proposed MPCs provide several controllable DC voltages from constant or variable DC or AC voltage sources. The theoretical analysis and operation scenarios of the proposed MPC are discussed and validated with the aid of MATLAB-SIMULINK simulations, and further corroborated using experimental results from scale down prototype. Theoretical analysis and discussions, quantitative simulations, and experimental results show that the MPCs offer high degree of control flexibilities during normal operation, including the capacity to reroute active or DC power flow between any arbitrary AC and DC terminals, and through a particular sub-converter with sufficient precision. Critical discussions of the experimental results conclude that the DC fault responses of the MPCs vary with the topology of the converter adopted in the sub-converters. It has been established that a DC fault at high-voltage DC terminal exposes sub-converters 1 and 2 to extremely high currents; therefore, converters with DC fault current control capability are required to decouple the healthy sub-converters from the faulted one and their respective fault dynamics. On the other hand, a DC fault at the low-voltage DC terminal exposes the healthy upper sub-converter to excessive voltage stresses; therefore, sub-converters with bipolar cells, which possess the capacity for controlled operation with variable and reduced DC voltage over wide range are required. In both fault causes, continued operation without interruption to power flow during DC fault is not possible due to excessive over-current or over-voltage during fault period; however, it is possible to minimize the interruption. The above findings and contributions of this work have been further elaborated in the conclusions.

Quadrilateral Current Mode Paralleling of Power MOSFETs for Zero-Voltage Switching

This article proposes a generic zero-voltage switching (ZVS) scheme for parallel power MOSFETs. Uncoupled or inversely coupled differential-mode commutation inductors are added to the midpoints (ac terminals) of parallel MOSFET half-bridges, and a time-delay-based control scheme is applied, generating a circulating current flowing through these commutation inductors. Thus, the inductor currents are reshaped as quadrilaterals, which enable all the parallel transistors to achieve ZVS. The mode of operation of the proposed paralleling technique is entitled quadrilateral current mode (QCM) due to the quadrilateral-shaped commutation inductor currents. The operating principle of the QCM-paralleling technique is detailed mathematically, yielding accurate closed-form analytical expressions for modulation parameters. Finally, simulations and experimental results of a QCM-enabled synchronous buck dc-dc converter are presented to validate the theoretical considerations.

Model predictive control of nine-switch converter with output filter for independent control of two loads

This paper proposes a model predictive control (MPC) algorithm designed for nine-switch converter with output filter (NSCOF) to control two three-phase AC loads independently. A state function of each leg of nine-switch converter (NSC) is established to characterize the internal conditions of the NSC because the traditional model of the NSCOF does not involve the interior model of the NSC. Based on the NSCOF model with the state function of each leg, a prediction model of NSCOF is constructed. The two AC terminals of NSCOF are treated as one module when the MPC is designed. The proposed MPC achieves controlling two three-phase AC loads working under different frequency modes. Furthermore, this algorithm is independent of the modulation method for reducing the difficulty of the controller design and its limits. The simulation and experimental results reveal that the steady-state and dynamic response performance of NSCOF are substantially improved using MPC.

A Power-Decoupled Current-Source Inverter for PV Energy Harvest and Grid Voltage Regulation

The single-phase microinverter (SMI) is becoming one of the most cost-efficient interfaces for bridging photovoltaics (PV) and power systems. However, in the maximum power point tracking operation, the SMI injects an intermittent active power to the grid, which leads to voltage variations at the point of common coupling (PCC). This certainly dampens the power quality of grids. Besides, the power mismatch between the dc and ac terminals of SMIs will render a double-line-frequency pulsating power at the dc terminal, which inevitably increases the power ratings of the circuit components of the SMIs. To address these issues simultaneously, a power-decoupled current-source inverter (PD-CSI) is proposed in this article. The PD-CSI can provide the required reactive power compensation for regulating the PCC voltage. Meanwhile, it is imbedded with an active power decoupling circuit, which can actively store the double-line-frequency pulsating power in a film capacitor so that the PV can be operated at a constant dc current. The state-space model and steady-state operation of the PD-CSI are elaborated. The controllers are designed for achieving the active power decoupling operation and the PCC voltage stabilization. Experimental and simulation results in an 110-V ac microgrid have been provided to validate the simultaneous functionalities of the active power decoupling and grid voltage regulation for the proposed PD-CSI.

Triac: Principle, Structure, Working with Application Circuits

Triac is a three terminal AC switch which is different from the other silicon-controlled rectifiers in the sense that it can conduct in both the directions that is whether the applied gate signal is positive or negative, it will conduct. Thus, this device can be used for AC systems as a switch.

On Power Scalability of Modular Multilevel Converters: Increasing Current Ratings Through Branch Paralleling

The modular multilevel converter (MMC) presented in Figure 1 found its place within highvoltage and medium-voltage (MV) applications owing to the possibility of effortlessly meeting the imposed voltage requirements by stacking the so-called submodules (SMs) in a series [1]. Therefore, high-quality voltage waveforms can be synthesized across the MMC ac terminals, while using the power devices of a lower voltage rating, resulting in a very modest or even no filtering requirement on the grid side. Depending on the application, the SMs are mainly half bridge (HB) or full bridge (FB), not including the other types of SMs reported in the literature [2]. According to Figure 1, a cluster of SMs in a series connection with an inductor L br will be referred to as the branch , whereas two branches comprise the leg .