Helder Leite

Control Strategies for AC Fault Ride Through in Multiterminal HVDC Grids

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.

Increasing distributed generation using automatic voltage reference setting technique

The increasing connection of distributed energy generation in a distribution network may lead to unacceptable voltage rise. An automatic voltage reference setting technique, acting on the tap change control of the transformer supplying the network, can be used to address this problem. By measuring essential voltage points along the distribution network, an automatic voltage reference setting technique is applied to the automatic voltage control (AVC) relay. The AVC relay then controls an on-load tap changing (OLTC) transformer in order to maximise distributed generation while maintaining the feeder voltage within limits. The automatic voltage reference setting technique and its implementation are described. A closed-loop testing facility, based on a real time digital simulator (RTDS), was constructed and used to test the technique. The paper presents and discusses the results obtained and shows how this technique may be used to maximise the distributed generation that can be connected to a distribution feeder.

A new topology of fast solid-state HVDC circuit breaker for offshore wind integration applications

Emerging new voltage source converter based HVDC applications demand fast short-circuit fault current interruption. Fast dc circuit breakers are identified as the feasible solution to handle the dc fault current. Switching overvoltage across the dc circuit breakers is destructive for the interrupter device and also for the other components of the system. A new topology of fast solid-state circuit breaker for HVDC applications is proposed in this paper. Instead of conventional approaches, a pre-charged capacitor is used for soft switching. Different modes of operation of proposed circuit breaker are analysed and also design process of circuit parameters are described. Finally, simulation results are presented.

A Holistic Approach to the Integration of Battery Energy Storage Systems in Island Electric Grids With High Wind Penetration

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.

Technical and economic assessment for optimal sizing of distributed storage

The introduction of renewable sources poses new challenges to power systems operation. Their impact on distribution networks urges the need of a more flexible grid that can ensure high reliability and power quality as well as operational efficiency. Distributed storage systems may address those emerging challenges by covering operational needs of power, response time and energy. This paper describes a methodology for optimal sizing of distributed storage through technical and economic assessment and storage scheduling optimization. A case study synopsis of voltage regulation and renewable energy time-shift application to LV and MV networks is presented.

Feed-in tariffs for wind energy in Portugal: Current status and prospective future

The question about how wind energy can be remunerated after the end of feed-in tariff schemes still remains unanswered in Portugal. Presently, the majority of wind energy promoters are connected through a point of common coupling, with a defined capacity attribution. In return, these promoters are rewarded with a premium rate (i.e., a feed-in tariff) for a limited amount of time or energy delivered. Therefore, promoters are foreseeing novel ways to return their investments and maximise their income, assuming feed-in tariff schemes will eventually come to a halt. This paper addresses the current Portuguese regulatory and commercial frameworks, especially the feed-in tariff mechanisms in force for supporting wind energy. The current level of revenues obtained by wind energy promoters in Portugal (provided by the feed-in tariff scheme) is compared with the hypothetical income they could obtain on a market environment. The non-firm access to the distribution networks is presented as a solution for integrating further generation capacity and thus increasing profitability for their promoters.

A directional protection strategy for multi-terminal VSC-HVDC grids

Protection issue is identified as the main drawback of the emerging multi-terminal HVDC grids. Multi-terminal HVDC grids demand fast short-circuit fault current interruption. The fast DC circuit breakers as a promising solution can be implemented as either bidirectional or unidirectional devices. In additional to less implementation cost, the unidirectional DC circuit breakers have less power losses as compared to the bidirectional ones. A protection strategy for multi-terminal HVDC grids 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 four-terminal MMC-HVDC grid model. Furthermore, the impacts of unidirectional protection strategy on the power converters and also the current interruption and surge arrester ratings of the DC circuit breakers are discussed.

Operation and control of multiterminal HVDC grids following the loss of an onshore converter

A fully operational Multi-Terminal DC (MTDC) grid will play a key role for the creation of AC systems interconnection and to integrate offshore wind farms. Disturbances (at both AC and DC side) may culminate in the sudden disconnection of onshore HVDC-VSC (High Voltage Direct Current - Voltage Source Converter). To continue operating the DC grid under these conditions, the development of control functionalities is required. A communication-free advanced control scheme is proposed to be used as a supplementary local control acting at VSC level and aiming on providing fast active power accommodation in the DC grid, culminating on the mitigation of the resulting DC overvoltage. The implementation of the proposed control mechanisms exploits a set of coordinated local control rules at the converter stations and at wind turbines (WT) level. The performance of the proposed strategies is discussed and assessed through numerical simulation in the paper.

Reclosers to Self-Healing schemes in distribution networks: A techno-economic assessment

A methodology to assess the economic benefits of deploying reclosers in overhead distribution networks allowing Self-Healing strategies is advanced in this work. The developed methodology comprises a technical assessment of the service restoration feasibility through network reconfiguration and an economic assessment based on a cost-benefit analysis (CBA) over the equipments life cycle. The economic benefit is assessed opposing the capital and the operational expenditures (CAPEX, OPEX) against the savings due to the reduction with the cost of energy not supplied (CENS). Additionally, penalties reduction or incentives increase are considered due to the incentive to quality of service (IQS), as well as the reduction in the total compensation (TC) to customers due to the violation of individual indices. A case study to validate the methodology is performed on a real distribution network in Portugal, defining the optimal number and location of reclosers according to reliability improvement and investment cost effectiveness.

Applying Reliability Centered Maintenance to a digital protective relay

The implementation of Reliability Centered Maintenance (RCM) method can assist in achieving cost effectiveness, and allow a greater understanding of the risk level that organizations presently manage. This systematic method identifies the most applicable and effective maintenance plan to avoid each failure mode of the equipment. The work presented here describes how the RCM methodology can be applied to a digital protective relay installed in a primary distribution substation. RCM can estimate failure before it occurs with predictive testing and inspection tools, as well as organize maintenance plans according to the impacts of protective relay failure on system reliability.