AC Port Research Articles

Power quality improvement in single-phase transformerless semi-quasi-Z-source inverters for off-grid photovoltaic systems

The transformerless single-phase semi-quasi-Z-source inverter (SqZSI) provides several advantages over classical PWM converters. Reduced or eliminated leakage current, high power density, and low cost are important features making this topology an attractive choice for PV micro-inverter applications. Two drawbacks of this topology are DC and second harmonic injection in the output AC port, which in some cases prevent the fulfillment of standard requirements. To alleviate this power quality issue, four novel harmonic mitigation algorithms based on PI and PR controllers are proposed, analyzed, and experimentally verified with the 90 W SqZSI in this paper. Moreover, a modified modulation reference waveform strategy is introduced. Consequently, the converter performance, in terms of total harmonic distortion (THDV) is achieved at about 1.27% under full power load. Also, in this paper, the non-ideal state-space model of the SqZSI with the effect of the equivalent series resistance of inductors is developed and the generation origin of DC and second harmonic component is analyzed based on the obtained model. The achieved results confirms the significant THD reduction by more than 75%, the DC component suppression, the second harmonic mitigation, and the power quality improvement.

Operation and Control of Bipolar-Type Modular Solid State Transformer With Active Circulating Current Injection and Arm Voltage Adjusting Method

Modular multilevel converter (MMC) based solid state transformer (M-SST) shows promise in hybrid ac/dc distribution grids. To further meet low voltage dc (LVdc) users’ requirements, a bipolar-type M-SST (BM-SST) is developed in this article, which not only has medium voltage ac and dc ports but also directly provides a bipolar low voltage dc (BLVdc) port for various LVdc sources and loads such that an extra SST or voltage balancer for achieving similar outcomes could be saved. Featuring various ac and dc ports, easy voltage conversion, and flexible power control capability, BM-SST is most suitable for multiple ac and dc grids interconnection as an energy hub. The most crucial issue of BM-SST is how to deal with the unbalanced power caused by the bipolar LVdc port which leads to submodule capacitor voltage deviation and uncontrollable circulating current between upper and lower arms of MMC. An arm voltage regulation (AVR) combing with active circulating current injection (ACCI) method is proposed in this article to eliminate the unbalanced power and its negative effects. Based on detailed circuit analysis of the BM-SST, the coordinated control of the BM-SST with optimum AVR&ACCI is designed. Moreover, the effects of AVR&ACCI method on current stress, voltage stress, and power loss of BM-SST are evaluated. The feasibility of the proposed BM-SST has been verified by experimental results.

Guest Editorial Special Issue on Smart Solid State Transformers for AC/DC Hybrid Power Grids

Solid state transformers (SSTs) feature a series of functional advantages, such as voltage level conversion, electrical isolation, power regulation and control, multiple ac and dc ports, power quality control, and communications with other intelligent devices. In recent years, smart SST, also called as smart transformer (ST), has been developed based on the integration with advanced information technology. Smart SST has good application prospects in ac/dc hybrid distribution networks and microgrids for renewable energy integration, electric vehicle charging stations, data centers, rail, and other fields.

Single AC/DC fault current limiter for both side of hybrid AC/DC microgrid

ABSTRACT Increasing short-circuit fault currents is one of the critical challenges in microgrids (MGs). In this paper, a novel AC/DC fault current limiter (ADFCL) is proposed to limit short circuit current of both DC and AC sides in hybrid AC/DC MGs (ADMGs). It includes two AC and DC ports, installed between AC MG (ACMG) and DC MG (DCMG). Also, it has only one common limiting resistor for limiting short circuit current in faulty MG without affecting the performance of the healthy MG. The proposed ADFCL power circuit, its principle of operation and control system are presented. Then, the feasibility of the ADFCL is verified by simulation in PSCAD/EMTDC environment and finally, a prototype is built. Both experimental and simulation results reveal that the ADFCL can limit the faulty MG by using only one limiting resistor without affecting the performance of the healthy one, which reduces the cost of using FCL in ADMGs.

Low-voltage ride through of multi-port power electronic transformer

A low-voltage ride-through (LVRT) control strategy for the multi-port power electronic transformer (PET) based on power co-regulation is proposed. During the sag and recovery of the grid-side voltage of the medium-voltage ac (MVac) port, the grid-connected active power of the low-voltage ac (LVac) port, rather than the power from external renewable energy sources (e.g., photovoltaic (PV)), is adjusted quickly to rebalance the power flowing across all ports, thereby preventing overcurrent and overvoltage. Moreover, a power-coordinate-frame-based LVRT mode classification is designed, and a total of six LVRT modes are classified to meet the LVRT requirements in all power configuration scenarios of the PET. In this way, the PET is endowed with the LVRT capability in both power-generation and power-consumption states, which is significantly different from traditional power generation systems such as PV or wind power. Furthermore, by optimizing the active power regulation path during LVRT transition, the overcurrent problem caused by the grid-voltage sag-depth detection delay is overcome. Finally, the effectiveness of the proposed control scheme is verified by experiments on a hardware-in-the-loop platform.

Dual-AC-port modular multilevel converter for hybrid frequency integration of offshore wind power: Modeling and control

Low frequency AC transmission system (LFAC) is a potential solution for deep-sea offshore wind power utilization besides HVAC and HVDC. Multi-port converter is the key device for the interconnection of hybrid frequency wind farms and power grid. An improved multi-port modular multilevel converter with dual AC ports and single DC port (DASD-MMC) is proposed as the key device for the interconnection among HVAC/HVDC/LFAC. The in-depth topology and working principle analysis are presented. The partitioned nearest level modulation method is proposed for high-voltage applications providing submodule capacitor voltage self-balancing ability. The mathematical model and the corresponding decoupling control strategy are presented afterward to realize the free power transfer among the three ports. To solve the output performance issues for hybrid HVAC and LFAC scenarios, an output current distortion suppression strategy is proposed with a novel cross-decoupling harmonic extraction method. Circulating current control strategy is presented with consideration of essential component analysis and calculation. Experiment and simulation validation and discussion are given, which show that the proposed DASD-MMC could provide a high quality hybrid frequency solution for offshore wind power collection and transmission.

Four-Port, Single-Stage, Multidirectional AC–AC Converter for Solid-State Transformer Applications

Solid-state transformers (SSTs) have emerged as an interesting solution for replacing the traditional low-frequency transformers in power distribution systems owing to the capacity of performing ancillary services and providing the integration of multiple energy sources. This work proposes a four-port, single-phase, multi-directional ac-ac converter for SST applications. Considering the integration of conversion stages, it is possible to achieve higher efficiency and lower component count when compared with the typical three-stage topologies reported in the literature. The introduced structure is based on the interleaving concept associated with the well-known dual active bridge (DAB) converter. The medium-voltage (MV) side employs a multilevel neutral-point-clamped (NPC) converter. The voltages across the ac ports present five levels, whereas the operating frequency of the filter elements is twice the switching frequency. Experimental results are presented and thoroughly discussed to validate the theoretical assumptions.

Determination of Conducted EM Emissions on DC–AC Power Converters Based on Linear Equivalent Thevenin Block Circuit Models

This article deals with circuit modeling of power systems and more particularly of DC–AC converters used for controlling electric motors. The objective of the model is to be able to assess conducted emission currents at both inputs and outputs for fixed operating power in order to apply appropriate mitigation design rules, such as filtering. First, we propose a linear Thevenin block model of the converter considering it as a five-port linear black box allowing possible interaction between the DC and AC ports. To this extent, the block model is made of an impedance matrix and a Thevenin voltage generators vector. We also propose theoretical relationships allowing the processing of data to generate this model from measurements. Then, we test the linear simplifications on a real DC–AC converter and compare the results to measurements. For this, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> -parameter measurements in <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> -mode and currents at the five ports in <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</small> -mode are used for the characterization of the model on a first test setup. The model is then applied and checked by comparing circuit simulated and measured current responses on a modified test setup for which the lengths of the connection cables are modified. Finally, the capability of this block model to extrapolate by calculation the response of other installation configurations is shown considering a filter and its insertion at input/output ports of the converter with much longer cables.

Predictive Control Applied to a Single-Stage, Single-Phase Bidirectional AC-DC Converter

This work aims to implement the control system of a single-phase, multi-port AC-DC converter with high-frequency isolation based on the dual active bridge (DAB) topology associated with the three-state switching cell (3SSC) applied to a distributed generation system. The converter has one ac port and two dc ports, this being a multi-input, multi-output (MIMO) system in which there is a coupling between the input and output variables. The generalized predictive control (GPC) is chosen to regulate the dc-link voltages. GPC is a well-consolidated approach in the literature, which can deal with MIMO coupled systems while presenting friendly-tuning rules. The current loop employs a traditional proportional plus resonant (P+R) controller because it has a single-input, single output (SISO) characteristic, as advanced controllers are unnecessary. Experiments are performed on a 1-kW prototype to validate the performance of the developed controller and show its advantages compared with traditional control strategies.

Extended Nodal Admittance Matrix Based Stability Analysis of HVDC Connected AC Grids

Motivated by emerging oscillation issues of high-voltage direct-current (HVDC) transmission system resulted from the interaction with AC grids, impedance-based stability analysis becomes an attractive approach in the de-risking studies due to the capability of black-box modelling and clear physical meaning. However, impedance-based stability assessment of HVDC is typically conducted either from the AC side or from the DC side, without fully considering both AC and DC grid dynamics, thus impairs evaluation accuracy. In this paper, the nodal admittance matrix based resonance mode analysis method is extended to include both AC and DC dynamics through formulating a hybrid AC-DC admittance matrix. To achieve this goal, HVDC interfacing-converters are modelled as three-port admittance networks with one DC port and two AC ports (in dq-frame) integrating both the AC side frequency coupling and the AC-DC dynamic coupling. In contrast to the widely adopted Nyquist-based methods, the proposed method does not requires knowing the number of right half-plane poles of system minor-loop gain, thus simplifies stability analysis, and moreover, the participation factors of both AC and DC grid nodes in critical resonance modes can be identified through eigenvalue-decomposition analysis, which is in favor of the design of mitigation measures. The effectiveness of the proposed method is validated by EMT-simulations in MATLAB/Simulink.

Development and Efficiency Test of a 3 MVA Four-port Power Electronic Transformer

The development of a 3 MVA four-port, i.e. 10 kV ac port, 380 V ac port, ±750 V dc port, and ±375 V dc port, power electronic transformer (PET) for an integrated energy system is described. For this four-port PET, the efficiency test is a complex problem as each port-to-port efficiencies should be tested. To solve this problem, this paper proposes a data fitting method using only the converter efficiency in each port. The experimental results show a high accuracy of the proposed testing method. The test results of this four-port PET are also analyzed and the efficiency under different power flow directions is explained.

A MMC-Based Multiport AC–N–DC Converter for Hybrid AC/DC Systems

Multiport converters are going to be the critical equipments in future hybrid AC/DC systems. Based on this, a MMC-based multiport AC <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$- {N} -$ </tex-math></inline-formula> DC converter topology is proposed in this brief, where multiple upper arms and one shared lower arm are configured to obtain one AC port and multiple DC ports. By designing the submodule (SM) number in multiple upper arms, the voltage levels at multiple DC ports can be flexibly configured. Moreover, the DC port can be further expanded by adding extra upper arm. Therefore, the proposed multiport AC <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$- {N} -$ </tex-math></inline-formula> DC converter has good extensibility and configuration flexibility. Due to the utilization of shared lower arm, the multiple ports are in direct electrical connection, and only one power conversion stage is needed among the multiple ports. Thus, a large number of SMs and passive components can be saved. In addition, the experimental results are provided to verify the feasibility of proposed multiport AC <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$- {N} -$ </tex-math></inline-formula> DC converter.

GaN-Based High-Power-Density AC–DC–AC Converter for Single-Phase Transformerless Online Uninterruptible Power Supply

This article introduces a new transformerless ac-dc-ac converter topology suitable for a high-power-density high-frequency single-phase online uninterruptible power supply (UPS) with a common neutral between the input and output ac ports. The proposed converter comprises an input power factor correction rectification (ac-dc) stage which is followed by an inversion (dc-ac) stage. The rectification stage operates in boost mode during the input positive half line cycle and buck-boost mode during the input negative half line cycle, while the inversion stage operates in buck and buck-boost modes in the output positive and negative half line cycles, respectively. The rectification stage utilizes boundary conduction mode control enabling soft-switching and allowing high-frequency operation. The inversion stage is operated in continuous conduction mode, wherein a digital controller regulates the output voltage of the converter across both resistive and reactive loads. The proposed online UPS utilizes a single (nonsplit) dc-bus between the rectification and inversion stages, resulting in a 50% reduction in dc-bus capacitance requirement compared to conventional split-bus online UPS topologies. Additionally, two battery interface solutions are also investigated which provide a trade-off between passive volume and additional switch-count. To verify the performance and control of the proposed online UPS, a GaN-based electrolytic-free 1-kVA prototype online UPS is designed, built, and tested. The prototype ac-dc-ac converter achieves a peak efficiency of 95.2% and maintains a high efficiency of above 92.3% across the full output power range. The electrolytic-free prototype ac-dc-ac converter for the online UPS achieves a power density of 26.4 W/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> .

Battery energy storage system with functions of wide AC voltage range and battery current filtering

ABSTRACT This paper develops a power conversion interface for a battery energy storage system (BESS). The power conversion interface is composed of a bidirectional DC-DC power converter and a three-port DC-AC power converter, and it has the functions of wide AC voltage range and battery current filtering. The three-port DC-AC power converter has a high-voltage DC port, a low-voltage DC port and an AC port. The high-voltage DC port and low-voltage DC port are, respectively, connected to the output of the DC-DC power converter and the battery set, and the AC port is connected to the grid. The three-port DC-AC power converter generates a five-level voltage or a three-level AC voltage at the AC port according to the amplitude of grid voltage. The bidirectional DC-DC power converter has two functions. One is to generate a higher DC voltage to the high-voltage DC port, and the other is to perform a DC active filter function for attenuating the low-frequency ripple of the charging/discharging current for the battery set. All or part of the charging/discharging power for the battery set is only processed by the three-port DC-AC power converter, which can improve the efficiency of the BESS. A hardware prototype with a digital signal processor controller is developed to verify the performance of the developed power conversion interface for the BESS.

Ripple Power Steering of HFL-TPC Connected VSI for PV-BESS/EV Applications

This paper presents low frequency ripple power steering control strategies for a power conversion topology that comprises of a high frequency linked three port converter (HFL-TPC) and a voltage source inverter (VSI) for PV-BESS/EV applications. The proposed topology has three interfaces which can be connected with a photovoltaic (PV) port, battery energy storage systems (BESS) and/or electric vehicles (EVs) port, and a four-quadrant ac port. The proposed topology inherently exhibits double line frequency (DLF) power fluctuation that needs to be decoupled and steered away from the PV port. In this paper, we have developed and demonstrated two control strategies to deal with the DLF ripple. The first control strategy utilizes loop gain of the closed loop control system to decouple the DLF power, however a residual DLF power still exists at the ac port. To completely decouple the DLF fluctuations, an improved sequence based control strategy is proposed where the DLF voltage components are converted into positive and negative sequence components and controlled separately on their respective synchronous reference frames. To verify the practical feasibility of the proposed low frequency ripple power steering control strategies, a 40 V, 400 W HFL-TPC is built and tested. Experimental results show that the proposed ripple power steering control strategies can decouple the DLF power fluctuation and selectively steer it to the BESS/EV port as DLF power fluctuation is undesirable at the PV port. A comparative results reveal that sequence based control strategy show higher degree of DLF ripple suppression.

Analysis and Control of a Four-Port Megawatt-Level High-Frequency-Bus-Based Power Electronic Transformer

Aiming for mutual isolation between ports, this article proposes a four-port high-frequency-bus-based power electronic transformer (HFB-PET), the core of which is the modular multiactive bridge (MMAB). In addition to port isolation, it also has the advantages of modularity, scalability, and reduced power conversion stages. First, the configuration of the proposed PET is introduced and compared with the existing solutions. Then, based on the mathematical model, a power coordinated control scheme is presented, including the power decoupling control for the cascaded H-bridge converter of the medium-voltage ac port to handle unbalanced grid voltages and active loads, the cell-power balance control for the medium-voltage dc port to achieve dc-link voltage equalization, and the power cross-coupling suppression control employed to improve the dynamic performance of the MMAB. On top of that, a hierarchical control framework with the global synchronous clock is constructed. Besides, a soft startup scheme considering the power balance between cells is introduced. Finally, the effectiveness of the proposed control scheme under the bidirectional power flow for the dc ports and the four-quadrant power flow for the ac ports is verified by experiments conducted on the laboratory scale-down prototype and the 10 kV/2 MVA industrial prototype.

Low voltage ride through function for the high voltage AC ports of four-port power electronic transformers

Low voltage ride through function for the high voltage AC ports of four-port power electronic transformers

An adjustable gain three port converter for battery and grid integration in remote location microgrid systems

The battery-grid integration (BGI) is the crux of microgrid systems that not only performs the energy management but also coordinates distributed energy resources and loads. The prevailing BGIs are either individual power converters based, which cause low efficiency and high cost, or multiport converter (MPC) based, which involve a higher component count and/or the lower voltage limit restriction in the DC port. With the increased penetration of distributed energy resources, the demand for MPCs with a wider buck-boost voltage gain capability is unavoidable. They can be used to interface low voltage sources like fuel cells, photovoltaic panels, batteries, etc. To perform the basic functionalities in BGI converters, like the power transfer from the storage to the grid and vice versa at different working conditions, the bidirectional capability in DC and AC ports is mandatory. This paper proposes a high voltage gain, bidirectional buck-boost port based hybrid MPC, called as an energy cushion multiport converter (ECMPC), for the integration of batteries with the single-phase AC microgrid (SPACMG). The ECMPC has three ports, among them one is the bidirectional AC port for the grid, the second one is the bidirectional DC port for the battery, and the last one is a DC output port for the common usage. The modes of operation with an appropriate switching sequence is detailed and a control scheme is also suggested. The performed comprehensive simulation analysis guides the component selection and the design of the converter. The critical comparison of the suggested converter with the prevailing single-phase bidirectional DC-AC converters being employed in the BGI is also presented. The theoretical study is supported by a laboratory corroboration. The system is managed for a minimal energy wastage through anew suggested minimal wastage energy management system (MWEMS). The system in the entirety is implemented and controlled by an ALTERA field-programmable gate array, ALTERA QUARTUS II 12.0SP2 processor.

A Dual Active Bridge Converter With Multiphase Boost Interfaces for Single-Stage Bidirectional DC–AC Conversion

A novel bidirectional dual active bridge based dc-ac converter is proposed in this article. The converter has multiphase boost interfaces at both the dc and ac ports. The interleaved operation of the multiphase units cancels the ripple currents and eliminates the input filter requirement. The dc to ac conversion is carried out in a single stage using only eight switches. The converter utilizes the low-frequency and high-frequency components in the inverter output to achieve both power conversion and power transfer across the high-frequency transformer simultaneously. A thorough analysis of the converter is carried out for the proper design of the components. Based on the results, a detailed design method is discussed. A multiresonant controller based phase-shift control is suggested to eliminate the low-frequency ripple in the input dc current. Finally, the effectiveness of the proposed converter is validated using results obtained from a 3-kW hardware prototype.

Power flow calculation of AC / DC hybrid distribution network considering energy router

Energy router (ER) is the key equipment in the construction of AC / DC hybrid distribution network in the future. Based on the characteristics of AC and DC ports and topological structure of ER, a steady-state model of ER with multi ports access was established, and a power flow model of AC / DC hybrid distribution network considering ER was constructed. Newton Raphson method combined with alternating iteration method was used to solve the problem. IEEE-33 was selected to calculate and compare the circular distribution network considering ER and the traditional AC distribution network. The results show that ER can improve the voltage level of nodes and reduce network loss in hybrid distribution network.