Challenges to the Integration of Renewable Resources at High System Penetration

Successfully integrating renewable resources into the electric grid at penetration levels to meet a 33 percent Renewables Portfolio Standard for California presents diverse technical and organizational challenges. This report characterizes these challenges by coordinating problems in time and space, balancing electric power on a range of scales from microseconds to decades and from individual homes to hundreds of miles. Crucial research needs were identified related to grid operation, standards and procedures, system design and analysis, and incentives, and public engagement in each scale of analysis. Performing this coordination on more refined scales of time and space independent of any particular technology, is defined as a “smart grid.” “Smart” coordination of the grid should mitigate technical difficulties associated with intermittent and distributed generation, support grid stability and reliability, and maximize benefits to California ratepayers by using the most economic technologies, design and operating approaches

Effects of energy storage systems grid code requirements on interface protection performances in low voltage networks

The ever-growing penetration of local generation in distribution networks and the large diffusion of energy storage systems (ESSs) foreseen in the near future are bound to affect the effectiveness of interface protection systems (IPSs), with negative impact on the safety of medium voltage (MV) and low voltage (LV) systems. With the scope of preserving the main network stability, international and national grid connection codes have been updated recently. Consequently, distributed generators (DGs) and storage units are increasingly called to provide stabilizing functions according to local voltage and frequency. This can be achieved by suitably controlling the electronic power converters interfacing small-scale generators and storage units to the network. The paper focuses on the regulating functions required to storage units by grid codes currently in force in the European area. Indeed, even if such regulating actions would enable local units in participating to network stability under normal steady-state operating conditions, it is shown through dynamic simulations that they may increase the risk of unintentional islanding occurrence. This means that dangerous operating conditions may arise in LV networks in case dispersed generators and storage systems are present, even if all the end-users are compliant with currently applied connection standards

Distributed Generation: Issues Concerning a Changing Power Grid Paradigm

Distributed generation is becoming increasingly prevalent on power grids around the world. Conventional designs and grid operations are not always sufficient for handling the implementation of distributed generation units; the new generation may result in undesirable operating conditions, or system failure. This paper investigates the primary issues involved with the implementation of distributed generation and maintaining the integrity of the power grid. The issues addressed include power flow, system protections, voltage regulation, intermittency, harmonics, and islanding. A case study is also presented to illustrate how these issues can be addressed when designing distributed generation installation on an existent distribution system. The case study design is performed on the campus distribution system of California Polytechnic State University, San Luis Obispo, with the design goal of implementing renewable energy sources to make the campus a net zero energy consumer

Island operation capability in the Colombian electrical market: a promising ancillary service of distributed energy resources

Hoy en día, la mayoría de los generadores distribuidos no están diseñados para operar bajo condiciones de isla. El servicio complementario de capacidad de operación por islas se presenta como un servicio de soporte técnico con la capacidad de aumentar la resiliencia, confiabilidad, seguridad y flexibilidad de un sistema eléctrico de distribución. Sin embargo, la operación por islas presenta unos desafíos técnicos, económicos y sociales que deben discutirse y analizarse durante la etapa de planificación.En este artículo, se presenta una comparación entre una operación intencional por islas y una isla planificada previamente, así como una descripción de los principales desafíos y beneficios de la operación por islas. Además, se realiza una evaluación económica de la confiabilidad del servicio de energía eléctrica al implementar la capacidad de operación por islas como un servicio complementario. Dicha evaluación muestra que la operación por islas tiene el potencial de minimizar la energía no suministrada hasta en un 50%. Además, se muestra un análisis técnico para la implementación del servicio complementario de capacidad de operación por islas. Luego, se estudia un sistema eléctrico de distribución existente con pequeñas centrales hidroeléctricas como un estudio de caso, con el fin de identificar los requisitos técnicos establecidos tanto para el sistema de distribución como para la fuente de generación. Finalmente, y teniendo en cuenta las normativas y regulaciones vigentes, se esboza una propuesta para la implementación del servicio complementario de capacidad de operación por islas en el sistema eléctrico colombiano.Nowadays most distributed generators are not designed to operate under islanded conditions. The ancillary service of islanded operation capability is proposed as a technical support service with the ability to increase the reliability, security and flexibility of an electrical distribution system. Nevertheless, island operation entails technical, economic and social issues that must be discussed and analyzed during the planning stage.This article compares an intentional island project to a preplanned island and describes the main issues and benefits of islanded operation. Additionally, the reliability of the service to implement the islanded operation capability as an ancillary service is economically assessed, which shows that island-based operation has the potential to minimize non-supplied energy up to 50%. Also, a technical analysis of implementing island operation capability ancillary service is presented. Subsequently, an existing electrical distribution system with small hydropower plants is considered as a study case in order to identify the technical requirements set out for both, the distribution system and the generation source. Finally, taking into account the current Law and regulations, a proposal is outlined for the implementation of the island operation capability ancillary service in the Colombian electrical system

Management of Islanded Operation of Microgirds

Distributed generations with continuously growing penetration levels offer potential solutions to energy security and reliability with minimum environmental impacts. Distributed Generations when connected to the area electric power systems provide numerous advantages. However, grid integration of distributed generations presents several technical challenges which has forced the systems planners and operators to account for the repercussions on the distribution feeders which are no longer passive in the presence of distributed generations. Grid integration of distributed generations requires accurate and reliable islanding detection methodology for secure system operation. Two distributed generation islanding detection methodologies are proposed in this dissertation. First, a passive islanding detection technique for grid-connected distributed generations based on parallel decision trees is proposed. The proposed approach relies on capturing the underlying signature of a wide variety of system events on a set of critical system parameters and utilizes multiple optimal decision tress in a parallel network for classification of system events. Second, a hybrid islanding detection method for grid-connected inverter based distributed generations combining decision trees and Sandia frequency shift method is also proposed. The proposed method combines passive and active islanding detection techniques to aggregate their individual advantages and reduce or eliminate their drawbacks. In smart grid paradigm, microgrids are the enabling engine for systematic integration of distributed generations with the utility grid. A systematic approach for controlled islanding of grid-connected microgrids is also proposed in this dissertation. The objective of the proposed approach is to develop an adaptive controlled islanding methodology to be implemented as a preventive control component in emergency control strategy for microgrid operations. An emergency power management strategy for microgrid autonomous operation subsequent to inadvertent islanding events is also proposed in this dissertation. The proposed approach integrates microgrid resources such as energy storage systems, demand response resources, and controllable micro-sources to layout a comprehensive power management strategy for ensuring secure and stable microgrid operation following an unplanned islanding event. In this dissertation, various case studies are presented to validate the proposed methods. The simulation results demonstrate the effectiveness of the proposed methodologies

Intentional Islanding of Active Distribution Networks by GenSets: An Analysis of Technical Constraints and Opportunities

The willingness to improve the security and reliability of power supply to end-users, often pushed by prescriptions of national regulatory authorities, is bringing considerable challenges for distribution system operators. Islanding a portion of the public distribution network after a fault is considered a measure to mitigate the effects of service interruptions. This procedure is usually carried out by counterfeeding the grid through a generator set (GenSet). Even if this approach is widely adopted around the world, reenergizing the grid and keeping the electric island stable is not a trivial task. In this framework, the scope of this paper is to provide a set of technical guidelines for the usage of GenSets to supply public grids in emergency conditions. The goal is to highlight the static and dynamic limits of the GenSet operations and simplify their exploitation for the grid operators. The numerical analyses, which have been carried out through the RMS simulation tool of the DigSilent PowerFactory software, also aim to evaluate the technical constraints in the case of active networks, which involve distributed generation implementing regulations according to ENTSO-E and Italian technical standards

A Real Multitechnology Microgrid in Venice: A Design Review

Electrical grids are evolving rapidly toward smart, self-regulating systems capable of managing distributed generation from intermittent renewable sources. Apart from hydroelectric, the large majority of them are photovoltaic (PV) systems grasping the fluctuating solar radiation and wind turbines (WT) capturing fickle wind energy, but other sources, which are at different stages of development, also generate energy with predictable or unpredictable intermittency. Several investigations have highlighted that, when power production from intermittent sources exceeds 20% of the total generation, the grid may face instabilities that can evolve into blackouts. Energy storage (ES) is a measure to balance source-load mismatches and to avoid such occurrence, but it can also provide a number of additional services which are part of the smart-grid paradigm. The operation of energy storage systems (ESSs) depends on the interface converters that manage the power flow and on the supervisors that control them according to the ESS, grid, and load features. Furthermore, the transmission system operator (TSO) may impose constraints on the ESS operation such as the obligation of contributing to primary regulation. Several numerical analyses have been developed to investigate the behavior of electrical grids provided with energy generation from renewable sources and energy storage, either islanded or connected to the national/transnational grid (macrogrid)

Effect of Islanding and Telecontrolled Switches on Distribution System Reliability Considering Load and Green-Energy Fluctuations

To improve electrical distribution network reliability, some portions of the network could operate in autonomous mode, provided that the related technical issues are addressed. More specifically, when there is not a path from those portions to the primary substation due to a fault in the network, such portions could be disconnected from the main network and supplied by local generation only. Such a mode of operation is known as "intentional islanding" and its effectiveness, in terms of adequacy, depends on the ability of the local generation to meet the island's load. In fact, the ratio between the available local generation and load demand can frequently change during islanding due to load variations and, especially, due to the strongly irregular behavior of the primary energy sources of renewable generators. This paper proposes an analytical formulation to assess local generation adequacy during intentional islanding, accounting for the aforementioned variations. More specifically, the fluctuations of load and green-energy generators during islanding are modeled by means of Markov chains, whose output quantities are encompassed in the proposed analytical formulation. Such a formulation is used by the analytical equations of load points' outage rate and duration. The evaluation of the reliability indices accounts for a protection scheme based on an appropriate communication infrastructure. Therefore, a brief overview on the telecommunications technologies has been presented with reference to their suitability for the specific application. In particular, distribution network safety issues have been considered as the main concern. The results show that neglecting load and generation fluctuations leads to a strong overestimation of the ability of distributed generators to meet the island load. Through a case study it is observed that the error on the load point outage rate is greater than the one affecting the outage duration