AC Side Research Articles

Virtual Impedance Refined Inductor Current Observation and Current Sensorless Control for Grid-Connected Inverter

When facing the current sensor failures, the inverter system will be shut down or run in open-loop mode, and the controllability will be severely degraded. Therefore, the development of the current sensorless control strategy for the grid-connected inverter is worthy of academic attention. In order to solve this problem, a current observation method is proposed in this article, which is enhanced by the virtual impedance and operation data based error correction. The method combines the control scheme variables of dc side voltage and ac side grid voltage to observe the ac side inductor current. The output of current sensors can be replaced in the current closed-loop control, contributing to the current sensorless control. The research results show that the current observation method can effectively track the real-time current and the virtual impedance implementation ensures the accuracy of this current observation. Under the impedance refined current observations, dynamic variations in inductor current can be timely reflected, and the stable operation of the current sensorless closed-loop control can be ensured. Finally, the correctness and effectiveness of the proposed control strategy have been verified by MATLAB / Simulink simulation and experimental results.

Voltage Vector Redundancy Exploitation for Battery Balancing in Three-Phase CHB-Based Modular Energy Storage Systems

This article presents a flexible controller algorithm for bidirectional modular cascaded H-bridge (CHB)-based battery storage systems. The primary objectives are to maintain high-quality power on the ac side while balancing dc-side battery voltages under charging or discharging operations. The proposed algorithm is based on a finite-set model-predictive current control (FSMPC) technique, which performs in-phase and phase voltage equalization in two complementary balancing terms by taking advantage of intrinsic voltage vector redundancy characteristic of three-phase CHB converters. Additionally, the proposed modifications to the FSMPC provide lower demand on computational resources of the controller through offline reordering of vectors and adjacent vector introduction, which also brings the least differential-mode <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$dv/dt$</tex-math></inline-formula> as an added value. Effects of battery voltage variation in charge/discharge cycles and voltage drop caused by nonideal semiconductors are considered to ensure the most accurate performance during steady-state, transient, and common-mode voltage reduction operation. The proposed approach is an efficient easy-to-implement multiparameter controller, which handles all the tasks needless to any additional control loops. Simulations and experimental prototyping results validate the feasibility of the battery voltage balancer terms to be utilized in battery storage applications with no downside impact on key power exchange functions.

Distributed Event-Triggered Hierarchical Control to Improve Economic Operation of Hybrid AC/DC Microgrids

A Hybrid AC/DC microgrid (MG) can integrate distributed generation sources and distributed loads on the AC and DC side of the MG by eliminating many unnecessary power conversion devices, which is more flexible and efficient. However, to achieve reliable and economic operation of a hybrid AC/DC MG is challenging due to its complex structure. In this paper, a novel distributed event-triggered hierarchical control strategy is proposed to improve the economic operation of a hybrid AC/DC MG. For the primary control, distributed local controls of AC DGs, DC DGs, and interlinking converters (ICs) are realized by adopting the droop control method. For the secondary control, the distributed economic dispatch, distributed average bus voltage discovery, and distributed proportional power-sharing algorithms are first proposed; then, control objectives of voltage and frequency restoration and economic operation of the hybrid AC/DC MG are realized based upon the developed algorithms. Furthermore, the distributed secondary control is built upon an event-triggered mechanism developed in this paper, which can reduce the communication burden. The simulation results demonstrate the effectiveness of the proposed control strategy.

A Fault-Tolerant Method for Cascaded H-Bridge-Based Photovoltaic Inverters With Improved Active and Reactive Power Injection Capability in Postfault Condition

This article proposes a fault-tolerant strategy for the cascaded H-bridge-based photovoltaic (PV) inverters with a single failure in power switches. In the introduced method, the faulty leg is bypassed after a switch failure, and the faulty cell (FC) continues to operate with the remaining healthy leg. After bypassing the faulty leg, one voltage level is missed at the ac side of the FC, and if other cells operate normally, a dc offset will appear at the ac terminal voltage, leading to dc current injection to the grid. To tackle this problem, in one of the healthy cells, which is called halved H-bridge cell (HHC), the leg complementary to the faulty leg in the FC is bypassed. In this faulty condition, FC and HHC cannot operate in maximum power point. Therefore, the control system is modified to get the maximum possible active power from the existing PV arrays. Using the proposed configuration and the modified control system, all PVs are kept in use, and consequently, the drawn active power increases compared to the conventional bypassing methods and existing partially bypassing methods. Moreover, in the postfault condition, the ability of the system to inject reactive power to the grid is improved since the suggested method synthesizes higher voltage than the alternative methods at the ac side of the inverter. Mathematical equations describing the system behavior in the postfault condition are derived, and the improvement in active and reactive power injection capability is analytically proved. The effectiveness of the proposed method is verified through simulations and experiments.

Effect of Choke Placement on Common-Mode Noise in Three-Phase Variable Speed Drives

Three-phase variable speed drives play a key role in energy conversion systems. To comply with the electromagnetic compatibility standards for the 0–2 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\text{kHz}$</tex-math></inline-formula> frequency range, magnetic chokes are typically placed at the dc or ac sides of the drive systems. Recently, compatibility levels have been defined at 9–150 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\text{kHz}$</tex-math></inline-formula> for the public network in accordance with IEC 61000-2-2. At this frequency range, common-mode noise is an important factor affecting electromagnetic compatibility. This article analyzes the effect of choke placement on common-mode noise at the 9–150 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\text{kHz}$</tex-math></inline-formula> frequency range. In particular, a comparative study on choosing either two dc-link choke inductors or three ac-line inductors is carried out via mathematical calculations. To this end, single-phase common-mode equivalent models are extracted for each placement of dc and ac chokes. Subsequently, by calculating the transfer functions of the currents in the single-phase equivalent circuits, the attenuation rate of common-mode currents from the motor to the grid side is analytically modeled for each choke configuration. The developed theories prove that ac-line inductors are more effective at suppressing low-frequency common-mode contents, while dc chokes are useful in attenuating high-order noise contents. Laboratory test results are used to prove the validity of the presented theoretical modeling and analysis.

Investigation of common mode voltages of single stage boost inverter for five phase induction motor drive

In this manuscript, the single stage boost inverter is proposed to eliminate the common-mode voltage (CMV) and electromagnetic interference (EMI) in the motor drives. The proposed topology has the capability of eliminating the effects of intrinsic CMV without any additional filters, active or passive elements, and modified pulse width modulated (PWM) techniques. This topology is proposed with the rearrangement of filter components from ac side to mid place in such a way that the capacitor constantly clamped to the DC bus, and the inductor is placed at the mid-point of half-bridge module and supply to achieve the high gain. This arrangement is also eliminating the effect of switching natured common-mode voltage, which generally occurs in any conventional PWM inverter. Conventional PWM techniques can be implemented to this topology without any modification in the control strategy. The proposed topology is simulated in a MATLAB Simulink to verify the operation and CM reduction capabilities with the results of experimental prototype. In the manuscript, the proposed topology is compared with the traditional inverters and modulation schemes in terms of the CMV effect elimination to explain its effectiveness.

DC-Link Voltage Stability Analysis of Grid-Tied Converters Using DC Impedance Models

With the integration of renewable energy sources into the power grid, a number of power electronic converters need to be connected together in parallel. Due to this interconnection among the power converters with a common DC bus, the equivalent impedance of the DC network, i.e., DC network impedance (DCNI) of these parallel converters, may vary and can cause oscillations in the DC link voltage (DCLV). In the literature, impedance models of grid-tied converters (GCs) based on the AC side are well reported without including these variations in DCNI. In addition, the dynamics of a phase-locked loop (PLL) play a significant role in GC system stability. To evaluate these stability issues, this paper proposes small signal impedance models viewing from the DC side of a three-phase GC operating under different control modes considering the PLL dynamics and the DCNI variations. Using the proposed DC impedance models (DCIM), DCLV stability analysis is evaluated for a GC. It is verified through bode plots that the interaction between the proposed DCIM and DCNI leads to unstable operation of the closed-loop converter near the PLL bandwidth when the phase difference between DCIM and DCNI is more than 180 degrees. Finally, the analytically developed models are validated using hardware in-the-loop (HIL) testing.

Analysis of rectification techniques and autonomous hybrid power plants potential for railway power supply systems

The paper considers optimization approaches for AC and DC traction supply systems. The recent developments in the area of traction supply systems operation optimization mean the application of modern energy storage systems, renewable energy sources, and AI-based control systems considering traction system operation features. The authors provide a traction substation layout with an autonomous hybrid power plant and its components. The main requirements for the control system of an autonomous hybrid power plant are formulated. The operational areas and modes of the proposed system are investigated: voltage stabilization, external power supply volume decreasing, voltage quality improving. The modernization methods for AC and DC side of the power system is analyzed for improvement of the power supply. Also, the paper deals with developing a multi-pulse rectifier for DC traction systems. The key features of the rectifier and its performance vs pulse number are analyzed. The influence of different rectifier structures on voltage quality and the harmonic spectrum was examined. The optimal rectifier layout for DC traction systems is proposed and experimentally verified with a physical model. The theoretical data on the proposed solution is verified with the physical model.

Using Matlab/Simulink Software Package to Investigate Fault Behaviors in HVDC System

Existing studies show that several performance issues will arise in the HVDC link during the three phase-to-ground fault at the side of the inverter and that the DC voltage will oscillate around zero and will not affect the rectifier of the AC system though the inverter of the AC system, and the AC voltages will become zero and the AC currents will show high amplitude as well as minor disturbances. It has also been argued that when the fault is applied on a single-phase to ground fault at the inverter side on the AC side, the voltage will decrease. In this paper, we focus on single line-to-ground fault, double line-to-ground fault, and three phase-to-ground fault at the inverter of the AC system and their behavior on the DC link as well as on the AC system of the rectifier with detailed simulations. A high voltage direct current (HVDC) Monopolar system is modeled using a Matlab/Simulink software package for the research. The results show that during the three phase-to-ground fault at the AC system of the inverter, the DC voltage will increase with a bogus waveform and the currents of the AC system at the rectifier will collapse to zero.At the double phase-to-ground fault level, the DC voltage will experience an increase in waveform while the currents of the AC system of the rectifier will experience different disturbances. At the single phase-to-ground fault level, the DC voltage will remain stable and the rectifier side of the AC system will also experience a stable state for both currents and voltages.

Mitigating Misfire and Fire-through Faults in Hybrid Renewable Energy Systems Utilizing Dynamic Voltage Restorer

Recently, there was a great focus on integrating renewable energy sources (RESs) into electrical power systems (hybrid systems) due to their many environmental and economic advantages. The output of most of these RESs is DC; some power electronic devices, including inverters, must be used to integrate these RESs into the electrical grid. Any maloperation, faults, or improper control in these power electronic devices will enormously affect these hybrid systems’ performance. This paper aims to mitigate the misfire and fire-through faults that occur at the switching of the inverter that connects three renewable sources: PV, wind, and the fuel cell to the grid. This mitigation of such inverter faults (misfire and fire-through) is performed through optimal tuning of the PI controller driving a dynamic voltage restorer (DVR) connected at the system’s AC side. The optimization technique used is particle swarm optimization (PSO). While mitigating these two inverter faults using the PI-PSO controller for the DVR, improved system performance through voltages, currents, and powers waveforms is achieved. Besides, the three renewable sources were kept in continuous operation without disconnection from the system during these faults.

Three‐vector model predictive power control for three‐level T‐type rectifier based on dead‐beat control

This paper presents a three-vector model predictive power control with neutral-point voltage balance (TVMPPC-NPVB) and a dead-beat control for three-phase three-level T-type rectifier. Three vectors are adopted to reduce the current ripple, balance the neutral-point voltage and achieve unity power factor on the AC side simultaneously, while the conventional model predictive power control suffers from large current ripple, difficulty in designing the weighting factor and computational burden. In the proposed strategy, two cost functions are derived to select the optimal space vector sequence and reduce the computational burden. The action time of each vector is introduced in detail. The traditional PWM rectifier utilizes PI controller to regulate the output voltage. But PI controller tends to have a long transition time and big variation on output voltage when the load changes suddenly. The proposed dead-beat control is capable of solving these problems caused by conventional PI controller. Both simulation and experimental results are shown to confirm the effectiveness of the proposed method.

An AC Fault Ride Through Method for MMC-HVDC System in Offshore Applications Including DC Current-Limiting Inductors

The current-limiting inductors (CLIs) are commonly used to suppress DC fault current to safely trip DC circuit breakers in MMC-HVDC systems, but it significantly jeopardizes DC dynamics and causes instantaneous power mismatch between AC and DC sides of islanded rectifier station (RS), exposing MMC internal states to large disturbance. This paper firstly provides a detailed analysis of the influence of CLIs on AC fault, establishes RS mathematical representation including AC side dynamic, and reveals the mechanism of DC voltage oscillation, modulation saturation and PCC waveform distortion. The analysis shows that the AC and DC fault characteristics are decoupled under unsaturated AC modulation, but coupled under excessively low submodule capacitor voltage, where AC modulation is saturated and PCC waveforms are distorted. To solve this issue, a novel RS AC fault ride through method for MMC-HVDC system with CLIs is proposed to stabilize RS DC voltage, desaturate AC modulation and decouple fault characteristics by regulating the zero-sequence modulation of circulating current suppression. In addition, a current reference disturbance and an AC energy dissipation device coordinated control are designed, which adaptively recovers the submodule capacitor voltage. Finally, the feasibility of the proposed method is verified by simulation results.

Singular Value Decomposition Based Pilot Protection for Transmission Lines With Converters on Both Ends

The modular multilevel converter-based high voltage direct current transmission (MMC-HVDC) technology has become an effective integrating method for large-capacity offshore wind farms. However, if AC side faults occur between the MMC and the wind farm, conventional current differential protection methods based on phasor measurements might not detect the large fault current at the wind farm and the MMC, resulting in poor protection performance. In the worst case, MMC blocking may occur during faults near the MMC station, causing relay refusal. The fault currents provided by both converters are analyzed in detail. Due to the fault-ride-through control objectives, it is proved that the singularity of fault current between wind farms and MMC is different. Therefore, a novel singular value decomposition (SVD) based pilot protection is proposed to solve this problem. A Hankel matrix is established for the short-circuit current at each end of the AC line. The Hankel matrix rapidly detects the singularity of the signal and its derivative, and the SVD method uses a short time window to extract the waveform features. This approach enables the detection of the fault features before MMC blocking in the worst-case fault scenario and prevents the incorrect fault identification caused by the minor amplitude of fault current provided by the converters. A detailed model of an offshore wind farm connected to the MMC-HVDC system is established in the real-time digital simulator (RTDS). A protection device compliant with industry standards is used to implement the SVD-based protection algorithm. Hardware-in-loop dynamic experimental tests are conducted to verify the performance of the proposed protection method. The experimental results demonstrate that the SVD-based protection method has excellent performance during various fault scenarios.

A Weighted Efficiency Enhancement Method for Coupled Inductor Filter-Based H-Bridge Inverters

This article presents a weighted efficiency enhancement (WEE) method for H-bridge inverters based on coupled inductor filter (CIF). The proposed WEE method includes coupled inductor optimization design and switching frequency optimization (SFO) strategy. First, all the energy losses of the H-bridge inverter are elaborated in detail, and the energy loss analysis model of the inverter is constructed based on the dominant energy losses. By installing a CIF on the ac side of the inverter and reasonably selecting the switching frequency, zero-voltage switching (ZVS) is realized for all switches. In addition, an IEEI-type coupled inductor structure and the efficiency-oriented coupled inductor optimization design principle are presented. Then, an SFO strategy is proposed to minimize the total loss of the inverter under full range of loads. Compared with the inverter without the WEE method, the inverter with the WEE method significantly improves the weighted efficiency. Finally, a laboratory prototype is built to verify the validity of the proposed WEE method.

The Tolerance Control Method Based on Zero-Sequence Circulating Current Suppression for Parallel T-Type Three-Level Rectifiers

Zero-sequence circulating current (ZSCC) suppression and fault-tolerant control (FTC) are important guarantees for the safe and reliable operation of parallel T-type three-level rectifiers <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$({\mathrm {PT}}^{2}3{\mathrm {LRs}})$ </tex-math></inline-formula> . However, none of the existing literature can solve the FTC under the ZSCC suppression of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\mathrm {PT}}^{2}3{\mathrm {LRs}}$ </tex-math></inline-formula> when the open switch fault (OSF) occurs in neutral-point (NP) switches. In order to solve these issues, a novel FTC method based on ZSCC suppression is proposed in this article. First, an improved finite-time controller is constructed to suppress the ZSCC by adjusting the difference of zero-sequence duty ratio between two rectifiers, completely. Then, based on the principle of area equivalence, the switching sequence is reconstructed by changing the state switching mode of the fault phase in real-time. As a result, the FTC under the OSF of NP switches is realized without affecting the suppression of ZSCC. Compared with the existing methods, the proposed method not only realizes the suppression of the ZSCC of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\mathrm {PT}}^{2}3{\mathrm {LRs}}$ </tex-math></inline-formula> but also completes the FTC under the OSF of NP switches, which guarantees the ZSCC suppression ability under the OSF of NP switches. Consequently, the power quality of the ac side of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\mathrm {PT}}^{2}3{\mathrm {LRs}}$ </tex-math></inline-formula> is improved significantly during the normal operation and the operation under the OSF of NP switches. Thus, the safe and reliable operation of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\mathrm {PT}}^{2}3{\mathrm {LRs}}$ </tex-math></inline-formula> is guaranteed effectively. Finally, the feasibility and correctness of the proposed method are verified by experiment results.

Research on the standardization of lightning arresters layout at AC side of ±800kV UHVDC converter station

UHVDC converter station is the key position of UHVDC power system. Once lightning damage accident occurs, it may cause large-area power outage and have a very serious consequence. Therefore, UHVDC converter station requires reliable lightning protection measures. At present, the electrical distance between AC side arrester and protected equipment in UHVDC converter station has not been standardized. This paper will give the critical value of safe electrical distance between AC side arrester and protected equipment by simulation calculation. Firstly, the lightning intruding wave overvoltage level on the equipment on the AC side of UHVDC converter station under typical operation mode will be given by simulation. Secondly, the critical value of safety distance between AC side arrester and protected equipment of converter station will be studied and determined. Thirdly, for the condition that the distance between the AC side arrester and the protected equipment of UHVDC converter station exceeds the critical value of safety distance, the strengthening measures of lightning protection of UHVDC converter station are proposed. The research results will provide data support for the standardized layout of lightning arresters in UHVDC converter stations.

Fault interaction analysis approach for hybrid AC/DC power grids

Heretofore, the comprehensive fault analysis approach for AC/DC hybrid grids has not been sufficiently investigated. Inspired by the conventional symmetrical components method, a novel “sequence component network” model based on the mapping relation is proposed to study the characteristics of AC (DC) side after DC-side (AC-side) faults in AC/DC hybrid grids. Subsequently, the AC-side electrical characteristic after DC-side faults was discussed employing the “sequence component network” model. Finally, taking into account the influence of different grounding modes, a DC-type integration system model of PV stations was established in PSCAD/EMTDC to verify the proposed approach.

Application of three-phase voltage source PWM converter in mine electric vehicle

The PWM converter has many excellent characteristics, in order to reflect the importance of three-phase voltage source PWM converters for the development of mine electric vehicle ,the working principle of the PWM converter is analyzed,the PWM converters can realize the AC and DC side control functions,V2G charging and discharging simulation model is built by using PSCAD software,the specific operating characteristics of PWM converter during charging and discharging are studied,through the analysis of the experimental data, the results show that the PWM converter can achieve unity power factor operation, and the harmonic pollution is small.

Non-characteristic harmonics in modular multilevel converters—An analytical approach

This paper analyzes the harmonic content of the arm currents and voltages of the modular multilevel converter (MMC) by using the dynamic phasor (DP) theory. DP modeling of the MMC facilitates a study without relying on simplifying assumptions on the harmonic content of the currents and voltages. In doing so, the DP-MMC model used in this paper reveals generalized conditions under which non-characteristic harmonics can exist. These conditions offer a detailed insight into the transient behavior of harmonics in the MMC. An open-loop DP model of the MMC is developed, which has the harmonics of the arm currents and voltages as independent state variables. The linearized state-space matrices of the DP model are assessed under different grounding options at the ac side of the MMC, and the relationship between characteristic and non-characteristic harmonics of the MMC voltages and currents is discussed.

An improved data-driven based model predictive control for zero-sequence circulating current suppression in paralleled converters

To address the requirement of improved power capacity, reliability and operation, parallel converters have been widely applied in micro-grid. However, the connections between DC and AC sides of different parallel converters may lead to the surge of zero-sequence circulating current (ZSCC), which would result in the deterioration of system functionality. Regarding the stated problem above, this paper proposes an improved data-driven based model predictive control for parallel converters. Specifically, a model-free adaptive control (MFAC), which is based on the measured input/output data, is incorporated into the finite control-set model predictive control (FCS-MPC) framework with multi-objective constraints, thus allowing for the suppression of ZSCC and the improvement of robustness performance. Furthermore, to avoid the tuning of weighting factors, a novel cost function is constructed based on the multi-objective decision making method. The main contribution of the proposed methodology relies on the fact that no knowledge of any model parameters and weighting factors in whole control process are required, which leads to a significant enhancement in the robustness and reliability of the control system in the presence of parametric uncertainties. Furthermore, by this proposal, the accurate current tracking and ZSCC suppression can be achieved. Finally, the validity of the proposed FCS-MPC scheme is verified by comprehensive case studies.