Helder Leite @FEUP

Abstract: Abstract Voltage Source Converter (VSC) based High Voltage Direct Current (HVDC) can control voltage and power flow on ac transmission lines with the priority of reactive current support during the fault as suggested in the grid codes for HVDC connections. Furthermore, the fault current contribution from the converter is limited in magnitude and modified in the waveform in order to protect the power electronic devices during the fault, which on the other hand affects the apparent impedance seen by the distance relay. This paper addresses the challenges faced by the distance protection of transmission lines connected with VSC-HVDC system by performing a closed-loop test of a commercial distance relay using Real Time Digital Simulator (RTDS). The power system with VSC is modeled in detail under a more realistic scenario in RTDS so that the performance of the distance protection can be tested under near to actual field conditions. The responses of the commercial distance relay for balanced and unbalanced faults are analyzed. The oscillograph data of the voltage, current and protection bits recorded by the relay are extracted and compared with the simulation results. Keywords: Closed-loop test; Distance relay; HVDC; RTDS; Transmission line; Voltage Source Converter

(http://www.sciencedirect.com/science/article/pii/S0378779617302742)

Abstract

Abstract: This paper proposes a new n-port hybrid dc circuit breaker for offshore multi-terminal HVDC grid application. The n-port dc circuit breaker can substitute n-1 hybrid dc circuit breakers at a dc bus with n-1 adjacent dc transmission lines. The system level behavior of the proposed multi-port hybrid dc circuit breaker is similar to the behavior of other hybrid dc circuit breakers. The operation principles of the proposed multi-port dc circuit breaker are introduced, analyzed and compared to the existing solution in this work. The components ratings are compared to the existing solution and the functionality of proposed device is verified by simulation.

keywords: {Circuit breakers;Circuit faults;Fault currents;Insulated gate bipolar transistors;Interrupters;Switches;Topology;Fault Protection;Multi-port Device;Multi-terminal dc Grid;dc Circuit Breaker},

Available at: <http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=7956212&isnumber=4387790>

Abstract

The priority to reactive power contribution from the Voltage Source Converter (VSC) based High Voltage Direct Current (HVDC) connection to support the grid during faults as suggested by the modern Network Code (NC) for HVDC affects the distance protection of transmission lines. Moreover, suppressing the negative sequence current during an unbalanced condition also interferes with the proper operation of the distance relays. This is because the current contribution from the converter is limited in magnitude and modified in the waveform in order to protect the power electronic devices during the fault in comparison to the synchronous generator fault current characteristics. This paper discusses the cause as well as the severity of the problems faced by the distance protection of transmission lines connected to the VSC based HVDC system by analyzing the apparent impedance analytically and in the simulation. The response of the relay to balanced and unbalanced faults lying on transmission lines is investigated. It is shown that the VSC limited reactive support and suppressed negative sequence current affect fault detection, forcing the relay to malfunction. The results of this paper can be used as a reference for understanding the effects of VSC-HVDC system on the operation of the distance protection during faults.

Keywords: Distance relay; HVDC; Transmission line; Voltage Source Converter

Available at: <http://www.sciencedirect.com/science/article/pii/S0378779616303583>

Abstract

The complexity of DC side short circuit fault current interruption has been identified as a major drawback of realization of multi-terminal HVDC grids. Although application of superconducting fault current limiters (SFCLs) in protection of HVDC systems based on slow protective schemes has been studied in the literature, its application in meshed HVDC systems based on modern and fast protective schemes has not been investigated, yet. This paper scrutinizes the SFCL application in the meshed HVDC grids protected by the fast DC protective schemes and fast hybrid DC circuit breakers. The purpose of SFCL application is to reduce the current interruption requirement of the HVDC circuit breaker and size of its elements. This research analyzes the requirements of suitable SFCL considering the fault operation behaviors of local protective relays and the hybrid DC circuit breakers. The results show that the appropriated SFCL can reduce fault current peak, size of current limiting inductor and also fault identification time. The application of SFCL not only reduces the size of surge arrester in the hybrid DC circuit breaker, but also can significantly decrease the current stress on the hybrid DC circuit breaker.

Keywords: DC circuit breaker; Multi-terminal HVDC grid; Fault protection; Fault currents; SFCL

Available at: <http://www.sciencedirect.com/science/article/pii/S0378779616303583>

Abstract

Protection issue is identified as the main drawback of emerging multi-terminal HVDC grids. Multi-terminal HVDC grid demands fast short circuit fault current interruption. Fast DC circuit breakers as a promising solution can be implemented as either bidirectional or unidirectional devices. In addition to less implementation cost, the unidirectional DC circuit breakers have less power losses as compared to the bidirectional devices. A protection strategy for multi-terminal HVDC grid based on unidirectional breaking devices is discussed and assessed in this paper. The performance of unidirectional protection strategy is examined under different fault scenarios in a fourterminal MMC-HVDC grid model. Furthermore, the impacts of unidirectional protection strategy on power converters and also current interruption and surge arrester ratings of the DC circuit breakers are discussed.

Available at: <https://teee.eu/index.php/TEEE/article/view/71>

Abstract:

This paper details an optimization tool for the planning and operation of battery energy storage systems (BESS) in island power systems with high wind penetration. The selection of the most suitable battery technology, its sizing and location is achieved through a comparative analysis of the operational and capital expenditure of the islanded system during the planning horizon with and without the deployment of the BESS solution. An operational algorithm is developed consisting on sequentially closer to time of delivery optimization stages in order to provide a robust quantification of the technical, environmental, and economic impacts of the battery system. The developed methodology is assessed and validated in a real-world case study of a Portuguese island with a high share of wind generation. Results show that BESS enhances the flexibility of the islanded power system thus ensuring a higher accommodation of wind energy with significant economic benefits.

URL: http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=7339481&isnumber=5433168

Abstract:

This study develops a methodology for coordinated operation of distributed energy storage systems in distribution networks. The developed methodology considers that energy storage resources can contribute to their owners’ inherent activities and to a more flexible and efficient distribution network operation. The optimisation tool based on mixed-integer linear programming is developed to maximise the technical and economic value of distributed storage taking into account their multifunctional potential and the presence of intermittent renewable sources. The methodological developments are validated in a case study of a real medium-voltage distribution network with two storage systems coupled with a wind park and an industrial ‘prosumer’ (i.e. energy consumers who are producing their own energy). Results make evident the robustness of the methodology and enable the assessment of the technical and economic impacts of the integration of distributed storage. Moreover, opportunity costs for distributed storage to perform services to the distribution system operator are demonstrated.

URL: http://digital-library.theiet.org/content/journals/10.1049/iet-gtd.2015.0398

Abstract:

Permission to connect a generator to the distribution system is generally obtained on the basis that the generator’s effect is limited and that the network voltages and currents remain acceptable at all times. The capacity of generation that can be connected is limited by this firm access connection policy. Energy storage systems can facilitate the integration of additional generation that cannot be dispatched. The coordination of independent non-dispatchable generators and energy storage systems connected to the same circuit is presented. The coordination aims to maximise the production of the generators without exceeding the power limit of the circuit, so as to maximise their income. Energy storage systems are asked to change their schedule to allow generators to maximise their production. Additional income is obtained from additional energy production. The additional income obtained is allocated so that both generators and energy storage systems have an economic incentive to be operated in a coordinated manner.

URL: http://www.sciencedirect.com/science/article/pii/S014206151400204X

Abstract:

A fully operational multiterminal dc (MTDC) grid will play a strategic role for mainland ac systems interconnection and to integrate offshore wind farms. The importance of such infrastructure requires its compliance with fault ride through (FRT) capability in case of mainland ac faults. In order to provide FRT capability in MTDC grids, communication-free advanced control functionalities exploiting a set of local control rules at the converter stations and wind turbines are identified. The proposed control functionalities are responsible for mitigating the dc voltage rise effect resulting from the reduction of active power injection into onshore ac systems during grid faults. The proposed strategies envision a fast control of the wind turbine active power output as a function of the dc grid voltage rise and constitute alternative options in order to avoid the use of classical solutions based on the installation of chopper resistors in the MTDC grid. The feasibility and robustness of the proposed strategies are demonstrated and discussed in the paper under different circumstances.

URL: Ieee.org

Abstract:
This article describes a time-domain transmission line model based on distributed parameters for transient analysis. This model is based directly on the differential equations for the basic transmission line without any previous simplification. The solution presented here for these differential equations results in a more detailed time-domain model than others models currently in use, and with certain structural similarities with the distributed parameter frequency-domain model for long transmission lines. The deduction of a general time-domain transmission line model for fundamental frequencies parameters and single-phase line are presented in this article, but the model can also be extended to cases with multiconductor and frequency-depended parameters. In order to validate the model, a comparative test is presented to facilitate the analysis about the main similarities and differences between this and other models.

URL: http://www.jpier.org/pierb/pier.php?paper=13041804

Abstract: Present distribution network voltage design practice limits the distributed generation capacity that can be connected to 11 kV networks. It has been shown previously that adoption of a more active approach to network voltage control can significantly increase distributed generation capacity. One way to do this is to control the target voltage of automatic voltage control relays at primary substations. A basic design for a controller to do this has been created, comprising three algorithms. A statistical state estimation algorithm estimates the voltage magnitude at each network node supplied by the primary substation, using real-time measurements, network data and load data. The estimate accuracy depends on the number and placement of real-time measurements. Studies using an 11 kV feeder model showed that an acceptable accuracy could be obtained with one or two measurements. The state estimator uses pseudo measurements for unmeasured loads. A load model was constructed using load profiles to calculate the pseudo measurements. The calculated pseudo measurements were inaccurate, but it was found that acceptable state estimate accuracy could be obtained with inaccurate pseudo measurements. A control algorithm alters the AVC relay target voltage, based on the maximum and minimum node voltage magnitude estimates. A simulation on a four-feeder network showed that the algorithm enabled the generator power export to be more than doubled.

URL: http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=1281016&isnumber=28621