AC Fault Conditions Research Articles

A Novel Commutation Failure Mitigation Method Based on Transformer-Side Magnetic Coupling Injection

To mitigate the commutation failure (CF), the topology of the line-commutated converter high-voltage direct current (LCC-HVDC) needs to be modified. However, the power loss and capital cost of the modification schemes proposed in the existing literature are relatively high. A novel commutation failure mitigation method based on transformer-side magnetic coupling injection (TMCI) module is proposed in this paper. The topology and working principle of the TMCI module are presented. Considering the CF prediction and turn-off time of the thyristor, the control strategy of the TMCI module is designed. For the TMCI module, the voltage of the capacitor and voltage-current stress of thyristors are analysed, and the method to select parameters is designed. Then, a test system with the TMCI module is developed based on the CIGRE HVDC benchmark model. The simulation results show that the TMCI module can effectively mitigate CFs under AC fault conditions. Furthermore, compared with the enhanced capacitor commutated converter (ECCC) and the evolutional line-commutated converter (ELCC) proposed in the existing literature, the TMCI module can not only offer the best CF immunity with the least capital cost, but also has lower steady-state active power losses and less impact on the system reliability.

An improved four‐layer capacitor voltage control strategy combined with arm current control for transformerless modular multilevel converters

AbstractIn order to improve the performance of the transformerless modular multilevel converters (MMC) under AC and DC grid fault conditions, this paper proposes an improved four‐layer capacitor voltage control strategy combined with arm current control. The proposed control strategy consists of overall submodule capacitor voltage averaging control, leg‐balancing control with flexible weight factor W, arm‐balancing control with zero‐sequence suppression, and inner‐arm individual balancing control. The new concept of weight factor W for leg‐balancing control is also originated, considering negative‐sequence current and DC circulating current jointly from power point of view. The proposed approach effectively realizes the capacitor voltage balance, ac‐side current tracking, and unwanted circulating current elimination. The practical implementation of the proposed control strategy and the experimental results from a downscale three‐phase transformerless MMC prototype are presented to validate the performance of the proposed four‐layer capacitor voltage control strategy under grid faults.

Overcurrent Suppression Control for Hybrid LCC/VSC Cascaded HVDC System Based on Fuzzy Clustering and Identification Approach

The hybrid LCC/VSC cascaded HVDC (HC-HVDC) scheme, in which the LCC is adopted as rectifier, and one LCC in series with multiple paralleled VSC based MMCs is adopted as inverter, combines the advantages of both LCC and VSC-MMC technologies. Presently, a HC-HVDC project is planned to deliver the hydro power from Baihetan to Jiangsu in China. One of the major challenges is to suppress the DC overcurrent issue of VSC-MMCs especially under commutation failure condition of LCC at inverter side. Based on Baihetan-Jiangsu project under construction, the model of HC-HVDC system is developed in PSCAD/EMTDC. In the system, the rectifier employs a dual 12-pulse LCC, while the inverter adopts a hybrid scheme, i.e., one LCC converter in series with three paralleled VSC-MMC converters. This paper addresses the main cause and feature of the DC overcurrent, then proposes an overcurrent suppression control method for HC-HVDC system based on fuzzy clustering and identification approach. And the system transient performances with the proposed control approach are evaluated under AC fault conditions. The results show that the proposed approach can not only suppress the DC overcurrent following the commutation failure to a certain extent, but also improve the post-fault recovery performance of the overall system.

A Method of Demarcating Critical Failure Impedance Boundary of Multi-Infeed HVDC Systems Based on Minimum Extinction Angle

A method of rapidly demarcating the critical commutation failure (CF) region of a multi-infeed high-voltage direct-current (HVDC) system is proposed. Based on the nodal impedance matrix and nodal voltage interaction factor, for different AC fault conditions—both balanced and unbalanced—a method of calculating the extinction angles of converters in multi-infeed HVDC systems is deduced in detail. First, the extinction angles of convertor stations under single-phase, double-phase, and three-phase ground faults and line-to-line faults occurring at any bus in an AC system are calculated. The minimum extinction angle serves as a CF criterion. If the calculated extinction angle for a certain bus is smaller than the minimum extinction angle, then a fault at that bus will cause CF of the HVDC system and put that bus into a failed bus set. The critical failure impedance boundaries of the topology diagram can therefore be demarcated by examining every bus in the AC system. The validity and accuracy of the proposed index and the method were verified by calculation results based on the three-infeed HVDC system model of the IEEE 39-bus system. Finally, the critical failure impedance boundary was demarcated in the IEEE 118-bus system to demonstrate the application in a wider range of systems.

Superconductors for power applications: an executable and web application to learn about resistive fault current limiters

High-temperature superconductors (HTS) can be superconducting in liquid nitrogen (77 K) at atmospheric pressure, which holds immense promises for our future such as nuclear fusion, compact medical devices and efficient power applications. In a power system, high short-circuit currents can exceed the operational current by more than ten times, putting many parts of the system at risk. Superconducting fault current limiters (SFCLs) can limit the prospective fault current without disconnecting the power system, and are thus becoming increasingly attractive for future grids. With a growing interest in modeling and commercializing SFCL, the question of how to teach and to explain their operation to students has arisen. In order to help students visualize the potential use and benefits of an SFCL, we created an executable and a web application using COMSOL Multiphysics. This executable allows students to investigate the electro-thermal response of a resistive SFCL. The executable solves a 1D electro-thermal model of the SFCL under AC fault conditions, evaluating important figures of merit such as the limited current, the prospective current and the maximum temperature reached within the tape. Finally, the geometrical parameters as well as the superconducting properties of the device can be modified. The importance of the amount of silver stabilizer necessary to protect the device from over-heating occurring during a fault current can be investigated. In addition, the effects of having a sharp nonlinear transition from the superconducting to the normal state (intrinsic property of the superconductor) to obtain a current limitation can be explored. The executable allows the users to learn about the benefits of superconductors in real-life applications, without the prerequisite of extensive modeling or experimental setup. The executable can be downloaded from the HTS modeling website and run on the most commonly used operating systems.

Improved Coordinated Control Approach for Evolved CCC-HVDC System to Enhance Mitigation Effect of Commutation Failure

The evolved capacitor commutated converter (ECCC), embedded with anti-parallel thyristors based dual-di-rectional full-bridge modules (APT-DFBMs), can effectively reduce commutation failure (CF) risks of line-commutated con-verter-based high voltage direct current (HVDC) and improve the dynamic responses of capacitor-commutated converter-based HVDC. This paper proposes an improved coordinated control strategy for ECCC with the following improvements: 1 under normal operation state, series-connected capacitors can accelerate the commutation process, thereby reducing the overlap angle and increasing the successful commutation margin; 2 under AC fault conditions, the ability of ECCC to mitigate the CF issue no longer relies on the fast fault detection, since the capacitors inside the APT-DFBMs can consistently contribute to the commutation process and further reduce the CF probability; 3 the inserted capacitors can output certain amount of reactive power, increase the power factor, and reduce the required reactive power compensation capacity. Firstly, the proposed coordinated control approach is presented in detail, and the extra commutation voltage to mitigate the CFs provided by the proposed control approach and an existing approach is compared. Secondly, the mechanism of the improved control approach to accelerate commutation process and improve the power factor is analyzed theoretically. Finally, the detailed electromagnetic transient (EMT) simulation in PSCAD/EMTDC is conducted to validate the effectiveness of the proposed coordinated control. The results show that the proposed approach can present a further substantial improvement for ECCC, especially enhancing the CF mitigation effect.

Hybrid AC/DC hub for integrating onshore wind power and interconnecting onshore and offshore DC networks

A hybrid AC/DC hub is proposed in this study, where a modular multilevel converter and a line-commutated converter are paralleled at the AC side to integrate onshore wind power, and connected in series at the DC sides to interconnect two DC networks with different voltages. The hybrid AC/DC hub facilitates wind power integration and DC network interconnection with reduced converter ratings and power losses when compared with the ‘conventional’ approach using DC–DC converters. To investigate the design requirement and performance of the hybrid AC/DC hub, power flow analysis is assessed to evaluate the converter power rating requirement. To ride through DC faults at either side of the interconnected DC networks, a coordinated DC fault protection for the hybrid AC/DC hub is proposed and studied. Simulation results in PSCAD/EMTDC verify the feasibility and effectiveness of the proposed control and protection of the hybrid AC/DC hub under power flow change, AC and DC fault conditions.

Bidirectional Power Flow Control in AC/DC Hybrid System under AC and DC Fault Conditions

This paper presents a hybrid AC-DC microgrid to reduce the process of multiple conversions in an individual AC microgrid or DC microgrid. The proposed hybrid microgrid compose of both AC microgrid and DC microgrid connected together by bidirectional interlink converter (BIC). Utility grid, 150kVA diesel generator (DG) and 100kW AC load are connected in AC microgrid. DC microgrid is composed of 100 kW photovoltaic array (PV), 20kW battery energy storage system (BESS) and 20kW DC load. The droop control technique is applied to control the system for power sharing within the sources in AC/DC hybrid microgrid in proportion to the power rating. When the faults occur at AC bus, protection signal applied to breaker for isolating the healthy and faults system. DC faults occur at DC bus, DC breaker isolate the AC and DC bus. The system performance for power flow sharing on hybrid AC-DC microgrid is demonstrated by using MATLAB/SIMULINK.

Design and Transient Studies on Multi-Terminal VSC-HVDC Systems Interconnecting Offshore Wind Farms

In recent years, offshore wind energy has increased significantly. The continuous increase in the offshore wind power generation level brings the requirement of the offshore wind farms (OWFs) integration with an AC grid. The multi-terminal (MT) voltage source converters (VSC)-based high voltage direct current (HVDC) transmission system is an emerging technology and also the best option to interconnect the large-scale OWFs to the AC grid. This paper presents the design, modeling, and control of MT VSC-HVDC transmission system linked offshore wind farms. Different cases of MT VSC-HVDC transmission systems are developed, and its simulation studies are carried out using PSCAD/EMTDC. The test results show the transient performance of the MT VSC-HVDC transmission systems under various AC and DC fault conditions. The studies also include the influence of wind variabilities as in the form of gust and ramp pattern during steady state and fault conditions.

Decentralized and autonomous voltage balancing control approach for hybrid multi-terminal Ultra-HVDC system

The hybrid multi-terminal Ultra-HVDC (UHVDC) transmission system combines the advantages of line commutated converter (LCC) and voltage source converter (VSC). It can meet long-distance and large-capacity power transfer requirements from multiple power supplies to multiple power receiving system. To balance the voltages between series-connected high-end converter (HEC) and low-end converter (LEC) of the power-controlled VSC station in hybrid multi-terminal UHVDC system, this paper investigates the inherent mechanism of voltage imbalance issue in the power-controlled VSC station, and then proposes a decentralized and autonomous voltage balancing control approach, which can be easily implemented in the practical engineering. Then, a hybrid three-terminal UHVDC system is developed in PSCAD/EMTDC, based on the scheme of the under-constructed Wudongde hybrid three-terminal UHVDC project (±800 kV, 8000 MW), and the effectiveness of the proposed voltage balancing control approach is investigated under steady-state, power step-change and AC fault conditions. The results show that, with the proposed control approach, the voltage-balance between series-connected converters in the power-controlled VSC station can be realized under different operating conditions.

A Novel Series-Connected Asymmetric Hybrid Modular Multilevel Converter for HVDC Tapping Application

In this paper, a three-phase series-connected asymmetric hybrid modular multilevel converter (SCAH-MMC) tap is proposed. The proposed SCAH-MMC reduces the requirement of switches by connecting phases in series to withstand the dc-bus voltage and possesses the dc fault ride-through capability by partially employing full bridge sub-modules. The system structure and operation principle of SCAH-MMC are presented in detail, which reveals the immunity of the SCAH-MMC from dc faults. In order to solve the third harmonic problem of SCAH-MMC, the interactions of harmonic components in arms are investigated. Based on that, two feed-forward third harmonic suppression methods are proposed. Furthermore, a control strategy for the SCAH-MMC is introduced to maintain dc voltage balancing in each phase under unbalanced ac fault conditions. The effectiveness of third harmonic suppression methods and the proposed control strategies of SCAH-MMC are validated by simulations using PSCAD/EMTDC. The proposed SCAH-MMC is compared with other MMC topologies in terms of cost and control to show the merits of it.

An Evolved Capacitor-Commutated Converter Embedded With Antiparallel Thyristors Based Dual-Directional Full-Bridge Module

This paper proposes an evolved capacitor commutated converter (ECCC), embedded with antiparallel thyristors based dual-directional full-bridge module (APT-DFBM), to mitigate the commutation failure of line-commutated converter-based-high-voltage direct-current (LCC-HVdc) transmission and enhance the control flexibility and dynamic response of capacitor-commutated converter-based HVdc (CCC-HVdc). The operating modes and parameter selection approach of the APT-DFBMs are provided, and the coordinated control approach between the APT-DFBMs and converter valves is presented. Then, an ECCC-HVdc system, adopting an LCC station as the rectifier and an ECCC station as the inverter, is developed in PSCAD/EMTDC. The transient responses including commutation failure probability, voltage and current stress, and fault recovery performance are compared under ac fault conditions among the proposed ECCC-HVdc, LCC-HVdc, and CCC-HVdc. The results show that the proposed ECCC-HVdc can effectively reduce the commutation failure risks of LCC-HVdc, and enhance the control flexibility and improve the dynamic responses of CCC-HVdc.

Control design of Modular Multilevel Converters in normal and AC fault conditions for HVDC grids

This paper describes a control design strategy of Modular Multilevel Converters (MMC) for High Voltage Direct Current (HVDC) applications to operate during normal and AC fault conditions. First, a steady state analysis of the converter is performed to identify the uses of the current components within the control strategy. Based on the initial stationary study, a complete converter control structure is proposed, which enables full control of the MMC internal energy during normal and AC fault conditions. A detailed design procedure is included for the current and energy regulators, in order to be able to ensure a dynamic response under any grid condition. Finally, theoretical developments are validated through simulation results by means of a detailed model in normal operation and during an AC voltage sag.

Common‐mode voltage injection‐based nearest level modulation with loss reduction for modular multilevel converters

This study focuses on the possibility of modular multilevel converter (MMC) loss optimisation by modulation without any additional circuit, where a common-mode voltage is injected into a nearest level modulator. Owing to the common-mode voltage injection, the proposed modulation strategy increases the fundamental voltage. Therefore, for a given voltage, operation is possible with the submodule (SM) voltage of at least one SM inserted each arm. This reduces the number of SMs required, resulting in reduction of conduction losses and leading to significant cost savings and/or returns on investment for facilities. Furthermore, the injection of common-mode voltage also decreases the total SM transitions, resulting in reduction of switching losses. The proposed modulation strategy is valid under both normal and unbalanced ac fault conditions for MMCs. The common-mode voltage extraction method is provided and its harmonic components are extracted. Then, the SM saving capability of the injection-based nearest level modulation is discussed and the SM saving limit is presented in relation with modulation indexes. The optimisation of both conduction losses and switching losses is analysed. The effectiveness of the proposed modulation strategy under both normal and unbalanced ac fault condition is verified by simulations of a 101-level three-phase MMC in MATLAB/Simulink.

Analytical circuit oriented modelling and performance assessment of modular multilevel converter

This study presents a detailed time domain model of a modular multilevel converter (MMC) to provide an alternative form of evaluation of the converter dynamic performance, for example, during AC or DC fault conditions. The purpose of the model is to accurately reflect the converter behaviour to be used in control systems design. It can also be beneficial to predict the steady state or transient state of the converter with fixed or variable inputs. In an MMC, the capacitor voltages are modulated using pulse-width modulation (PWM) to fabricate a sinusoidal output voltage. With time, and because of the PWM action, the capacitors get charged or discharged. Therefore the circuit behaves as a time-variant circuit. The analysis is based on the equivalent circuit derived from the converter differential expressions at any time instant in a piecewise fashion. The usefulness and accuracy of the theoretical study is validated for the three-phase MMC topology by simulation results with respect to normal operation, start-up process, cell short-circuit fault, line-to-ground and DC-side fault conditions. Experimental tests are carried out through a scaled-down laboratory prototype.

Modelling and Simulation of Monopolar HVDC Transmission System Feeding a Strong AC Network with Firefly Algorithm based Optimal PI Controller

Transmission of electrical power through high voltage direct current (HVDC) has attracted the attention of a number of researchers in the recent years. For economic design and optimal operation, HVDC system requires a detailed simulation model. Therefore, in this paper a detailed Matlab simulation model of line commutated converter (LCC) based monpolar HVDC system, feeding a strong AC network, with a fixed capacitor (FC) as a reactive power compensator is presented. Firefly algorithm based optimal proportional integral (PI) controller has been proposed for the rectifier and the inverter control. The transient performances of the HVDC system under various AC and DC fault conditions were studied. The results show the supremacy of the firefly algorithm based optimal PI controller over the conventional PI controller. The harmonic analysis is also carried out under steady state operation to assure the quality of power supply on the inverter AC side.

An adaptive neuro fuzzy inference system controlled space cector pulse width modulation based HVDC light transmission system under AC fault conditions

Abstract In HVDC Light transmission systems, converter control is one of the major fields of present day research works. In this paper, fuzzy logic controller is utilized for controlling both the converters of the space vector pulse width modulation (SVPWM) based HVDC Light transmission systems. Due to its complexity in the rule base formation, an intelligent controller known as adaptive neuro fuzzy inference system (ANFIS) controller is also introduced in this paper. The proposed ANFIS controller changes the PI gains automatically for different operating conditions. A hybrid learning method which combines and exploits the best features of both the back propagation algorithm and least square estimation method is used to train the 5-layer ANFIS controller. The performance of the proposed ANFIS controller is compared and validated with the fuzzy logic controller and also with the fixed gain conventional PI controller. The simulations are carried out in the MATLAB/SIMULINK environment. The results reveal that the proposed ANFIS controller is reducing power fluctuations at both the converters. It also improves the dynamic performance of the test power system effectively when tested for various ac fault conditions.

Erratum to Central European Journal of Engineering, Volume 4, Issue 1

AbstractPaper by M. Ajay Kumar, N. V. Srikanth, et al. “An adaptive neuro fuzzy inference system controlled space cector pulse width modulation based HVDC light transmission system under AC fault conditions” in Volume 4, Issue 1, 27–38/March 2014 doi: 10.2478/s13531-013-0143-4 contains an error in the title. The correct title is presented below

Grid connection of large offshore wind farms using HVDC

AbstractThis article describes the use of high‐voltage DC (HVDC) transmission systems for connection of large offshore wind farms using doubly fed induction generators (DFIGs) to the main grid. HVDC systems based on voltage source converters (VSC transmission) and on line‐commutated converters (LCC HVDC) are discussed. The article describes proposed system configurations, operating principles and controls for the two technologies. PSCAD/EMTDC simulations are presented to demonstrate the robust performance of the proposed systems during variation of generation and onshore AC fault conditions. Copyright © 2005 John Wiley & Sons, Ltd.

Hybrid HVDC System for Power Transmission to Island Networks

A hybrid system, comprising a line commutated thyristor high-voltage direct current (HVdc) converter and a static VAR compensator, is proposed in this paper for the connection of networks with no synchronous generation to a main grid. The proposed system combines the robust performance and low capital cost and power loss of a line-commutated HVdc converter, with the fast dynamic performance of a voltage-source-converter (VSC)-based transmission system. The paper describes the principles and control strategies of the proposed system. PSCAD/EMTDC simulations are presented to demonstrate the robust performance and to validate the proposed system during various operating conditions such as black-start, variations of load or generation, and ac fault conditions.