An Adaptive Overcurrent Coordination Scheme to Improve Relay Sensitivity and Overcome Drawbacks due to Distributed Generation in Smart Grids

Distributed Generation (DG) brought new challenges for protection engineers since standard relay settings of traditional system may no longer function properly under increasing presence of DG. The extreme case is coordination loss between primary and backup relays. The directional overcurrent relay (DOCR) which is the most implemented protective device in the electrical network also suffers performance degradation in presence of DG. Therefore, this paper proposes the mitigation of DG impact on DOCR coordination employing adaptive protection scheme (APS) using differential evolution algorithm (DE) while improving overall sensitivity of relays . The impacts of DG prior and after the application of APS are presented based on interconnected 6 bus and IEEE 14 bus system. As a consequence, general sensitivity improvement and mitigation scheme is proposed

Impact of distributed generation on protection and voltage regulation of distribution systems : a review

During recent decades with the power system restructuring process, centralized energy sources are being replaced with decentralized ones. This phenomenon has resulted in a novel concept in electric power systems, particularly in distribution systems, known as Distributed Generation (DG). On one hand, utilizing DG is important for secure power generation and reducing power losses. On the other hand, widespread use of such technologies introduces new challenges to power systems such as their optimal location, protection devices' settings, voltage regulation, and Power Quality (PQ) issues. Another key point which needs to be considered relates to specific DG technologies based on Renewable Energy Sources (RESs), such as wind and solar, due to their uncertain power generation. In this regard, this paper provides a comprehensive review of different types of DG and investigates the newly emerging challenges arising in the presence of DG in electrical grids.fi=vertaisarvioitu|en=peerReviewed

Voltage Rise Problem in Distribution Networks with Distributed Generation: A Review of Technologies, Impact and Mitigation Approaches

Energy demand has constantly been on the rise due to aggressive industrialization and civilization. This rise in energy demand results in the massive penetration of distributed generation (DG) in the distribution network (DN) which has been a holistic approach to enhance the capacity of distribution networks. However, this has led to a number of issues in the low voltage network, one of which is the voltage rise problem. This happens when generation exceeds demand thereby causing reverse power flow and consequently leading to overvoltage. A number of methods have been discussed in the literature to overcome this challenge ranging from network augmentation to active management of the distribution networks. This paper discusses the issue of voltage rise problem and its impact on distribution networks with high amounts of distributed energy resources (DERs). It presents different DG technologies such as those based on conventional and unconventional resources and other DERs such as battery storage systems and fuel cells. The study provides a comprehensive overview of approaches employed to curtail the issue of voltage increase at the point of common coupling (PCC), which includes strategies based on the network reinforcement methodology and the active distribution network management. A techno-economic comparison is then introduced in the paper to ascertain the similarities and dissimilarities of different mitigation approaches based on the technology involved, ease of deployment, cost implication, and their pros and cons. The paper provides insights into directions for future research in mitigating the impact of voltage rise presented by grid-connected DGs without limiting their increased penetration in the existing power grid

Analysis and Mitigation of Temporary Over-Voltage (TOV) Phenomenon in Unintentionally Islanded Grid-Connected Photovoltaic (PV) Inverters

Grid-connected photovoltaic (PV) solar systems, like other inverter-based distributed generators, can cause temporary over-voltages (TOVs), especially subsequent to faults and unintentional islanding incidents, and can damage equipment and customers within the host distribution network. Thus, this thesis aims to study the phenomenon and propose corrective measures for it. Thus, the thesis first presents detailed models for a conventional single-stage PV system and a modified single-stage PV system. The conventional system uses a Δ/YG isolation transformer, whereas the modified system, proposed in the literature, assumes a Y/YG isolation transformer that is effectively grounded by an additional half-bridge leg energized by the dc-link of the voltage-sourced inverter (VSI) of the PV system. Moreover, the thesis proposes two TOV mitigation schemes that augment the basic controls of the conventional and modified single-stage PV systems, respectively. Further, the thesis models a two-stage PV system that adopts the same TOV mitigation scheme as that proposed for the conventional single-stage system. Then, the TOV caused by the two-stage system is evaluated, with and without the TOV mitigation scheme. It is shown that the proposed TOV mitigation schemes are effective. The thesis also compares the TOVs caused by the three aforementioned PV systems, with and without the TOV mitigation schemes, and concludes that a two-stage PV system without a TOV mitigation scheme produces smaller TOVs than its single-stage counterparts without TOV mitigation schemes. Similarly, a two-stage PV system with its TOV mitigation scheme produces smaller TOVs than its single-stage counterparts with their respective TOV mitigation schemes

The impact of inverter side PV plant on HVDC commutation failures

M.Phil. (Electrical Engineering in Power and Energy Systems)Abstract: The high-voltage direct current (HVDC) system is a crucial technology in transmission; however, this system suffers from commutation failure. Commutation failure is defined as an adverse dynamic event that occurs when a converter valve that is supposed to turn off continues to conduct without transferring its current to the next valve in the firing sequence. Commutation failure disturbs the power transfer, yields a large overcurrent in the converter, and causes a voltage drop in an alternating current (AC) network. Although commutation failure in HVDC systems has been studied using many other compensating devices, academic researchers have not given enough attention to evaluating the impact of distributed generation (DG) on the power system. Within this gap and based on the publications researched, no published material could be found regarding the impact of a photovoltaic (PV) plant on HVDC commutation failures. This research project seeks to focus on the impact of an inverter side PV plant on HVDC commutation failures. In this dissertation, the objective is to evaluate the impact of the inverter side PV plant on HVDC commutation failures. The case studies are done by considering the commutation failure severity, the magnitudes of the remaining voltages after different types of faults occurring, and the recovery time required to clear a fault. Case studies are performed in a network with a PV plant and also without a PV plant. The network was set up in Power System Computer-Aided Design (PSCAD) software to find the critical voltages. Further simulations were done in this study using Power System Simulator for Engineering (PSS/E) software. A network by Conférence Internationale des grandes réseaux..

Voltage stability of power systems with renewable-energy inverter-based generators: A review

© 2021 by the authors. The main purpose of developing microgrids (MGs) is to facilitate the integration of renewable energy sources (RESs) into the power grid. RESs are normally connected to the grid via power electronic inverters. As various types of RESs are increasingly being connected to the electrical power grid, power systems of the near future will have more inverter-based generators (IBGs) instead of synchronous machines. Since IBGs have significant differences in their characteristics compared to synchronous generators (SGs), particularly concerning their inertia and capability to provide reactive power, their impacts on the system dynamics are different compared to SGs. In particular, system stability analysis will require new approaches. As such, research is currently being conducted on the stability of power systems with the inclusion of IBGs. This review article is intended to be a preface to the Special Issue on Voltage Stability of Microgrids in Power Systems. It presents a comprehensive review of the literature on voltage stability of power systems with a relatively high percentage of IBGs in the generation mix of the system. As the research is developing rapidly in this field, it is understood that by the time that this article is published, and further in the future, there will be many more new developments in this area. Certainly, other articles in this special issue will highlight some other important aspects of the voltage stability of microgrids

Distributed Control Methods for Integrating Renewable Generations and ICT Systems

With increased energy demand and decreased fossil fuels usages, the penetration of distributed generators (DGs) attracts more and more attention. Currently centralized control approaches can no longer meet real-time requirements for future power system. A proper decentralized control strategy needs to be proposed in order to enhance system voltage stability, reduce system power loss and increase operational security. This thesis has three key contributions: Firstly, a decentralized coordinated reactive power control strategy is proposed to tackle voltage fluctuation issues due to the uncertainty of output of DG. Case study shows results of coordinated control methods which can regulate the voltage level effectively whilst also enlarging the total reactive power capability to reduce the possibility of active power curtailment. Subsequently, the communication system time-delay is considered when analyzing the impact of voltage regulation. Secondly, a consensus distributed alternating direction multiplier method (ADMM) algorithm is improved to solve the optimal power ow (OPF) problem. Both synchronous and asynchronous algorithms are proposed to study the performance of convergence rate. Four different strategies are proposed to mitigate the impact of time-delay. Simulation results show that the optimization of reactive power allocation can minimize system power loss effectively and the proposed weighted autoregressive (AR) strategies can achieve an effective convergence result. Thirdly, a neighboring monitoring scheme based on the reputation rating is proposed to detect and mitigate the potential false data injection attack. The simulation results show that the predictive value can effectively replace the manipulated data. The convergence results based on the predictive value can be very close to the results of normal case without cyber attack

Control of Four-Wire Inverter-Interfaced DGs for Accurate Fault Type Classification

Inverter-interfaced distributed generators (IIDGs) generate fault currents that are different from those generated by conventional synchronous generators (SGs). As a result, commercial relays—that utilize current-angle-based phase selection measurements—misidentify faulty phase(s), which adversely impact the grid resiliency and reliability. In this thesis, a new control scheme is proposed to regulate the sequence components of the IIDG currents during unbalanced faults to ensure accurate fault type classification by commercial relays. The proposed controller controls the positive-sequence and negative-sequence currents in the dq-frame with a decoupled synchronous reference frame (DDSRF) based phase-locked loop (PLL) for components extraction and synchronization. It also uses a second order generalized integrator (SOGI) based PLL to synchronize the zero-sequence components. This scheme forces the angles of the negative-sequence and zero-sequence fault IIDG currents to behave like those of an SG while preserving the inverter’s current limits. This leads to proper fault type classification. The proposed control scheme pertains to three-wire IIDGs as well as four-wire IIDGs, which are common in low-voltage distribution networks. A performance evaluation using time-domain simulations is used on a benchmark network to confirm the success of the proposed control scheme under different fault conditions

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