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Photovoltaic (PV) systems have been gaining popularity among distribution network engineers. The integration of these small-scale sources has a variety of benefits for the distribution network. One of the most important benefits is the power loss reduction. However, a high penetration level of PV systems may disrupt the protection coordination of conventional overcurrent relays of the distribution network, reducing the profit of the electricity distribution company (EDC) due to healthy feeder de-energization. This paper determines the optimal penetration level of PV systems in the distribution network considering both power loss reduction profit and protection miscoordination cost to maximize the profit of the EDC. The merits of the proposed methodology are verified through several case studies on a simulation model of the IEEE 33-bus test system in the Electrical Transient Analyzer Program (ETAP) environment considering the reliability data of the IEEE Roy Billinton Test System.

Electricity generation is one of the major concerns of today's world due to increasing dependence on this energy. Continuity in power supply and service to network consumers may be disrupted by various disturbances; thus, the power grid requires a reliable and fast protection system. The protection of distribution networks is conventionally performed by overcurrent protective devices which preserving their coordination is a complex task due to bidirectional power flow resulting from the presence of distributed generation (DG) units. This paper presents an adaptive scheme to restore protection coordination in the distribution networks with a high penetration level of DG units. Using the multi-agent system (MAS) structure, the main protection technique monitors the current of the lateral fuse by an intelligent electronic device (IED) to estimate the operating time of the fuse and sends this data to the upstream recloser IED. Then, the recloser IED determines the recloser operating time to preserve the protection coordination. In the case of failing the main technique, the backup technique protects the distribution feeder by using a definite time piecewise linear curve for the recloser based on the proposed recloser current coefficient. The backup technique is independent of communication links. The reliable performance of the developed scheme is assessed on the ETAP simulation model of the IEEE 33 bus test system.

Miscoordination and faulty operation are the two main protection challenges of renewable Distributed Generations (DGs) in Distribution Networks (DNs) and Micro‐Grids (MGs). Although various approaches have already been presented in the literature, none of them has proposed a solution for simultaneous impact of DN/MG reconfigurations and DGs’ protection challenges. Topology‐change being a usual method in decreasing power loss or voltage deviations, it would be preferable to have an online adaptive scheme considering all protection challenges together. Hence, the ordinary overcurrent relays using the IEC 61850 protocol capabilities are utilized in an algorithm capable of definitely assigning which relays are responsible for the on‐going fault and which ones are not. That is, the algorithm is applied on a peer‐mode multi‐agent system (MAS) structure. First, the relay agents are classified into radial and loop Virtual Local Area Networks (VLANs). When a relay detects the fault, the MAS will find the responsible VLAN and instantly isolate other VLANs, including their agents. Next, the main and backup relays using the proposed communication map are assigned within the responsible VLAN. Last, a suitable protection algorithm is applied on the selected relays, according to direction of fault current.

Despite various advantages of integrating distributed generation (DG) units into distribution networks such as power loss reduction and voltage profile improvement, they may disrupt the performance of the conventional protection system due to bidirectional power flow and change of short‐circuit power. With the development of multi‐agent systems (MASs), intelligent protection schemes have been introduced that operate on the basis of data processing by a central controller. However, the possibility of communication failure and cyber attacks reduce reliability of these protection systems. To address this problem, this paper proposes an intelligent protection scheme that relies on peer‐to‐peer communication between agents in the first layer of the MAS. In the case of communication failure, the agents react to the fault condition by using the available data for similar conditions. The main advantages of the proposed scheme are reduction of number of employed agents by using only relay agents and no need for communicating with higher layers of MAS. Implementation of the proposed scheme in the ETAP simulation model of a 14 bus active distribution network shows that data exchange between relay agents properly modifies the operating time of the relays, resulting in protection coordination between main and backup protections.

Low-voltage dc (LVdc) microgrids facilitate the integration of renewable energy systems and modern loads. However, they suffer from the lack of a sensitive, selective, reliable, and fast protection strategy. The low fault current of high-resistance faults makes fault detection and faulty zone identification challenging tasks for protection engineers. This article proposes a protection scheme that is effective for both bolted and high-resistance faults at the LVdc feeders. The developed strategy relies on two intelligent electronic devices at both ends of the protected zone, calculating the normalized superimposed component of the line current. By adding this component to the input of the periodically forced harmonic oscillator (PFHO), both fault occurrence and fault direction can be simultaneously determined based on the oscillator state variable with only two possible values during the stable and unstable modes. It eliminates the need for the selection of various protection thresholds for different LVdc microgrid structures/topologies. The proposed protection strategy can classify pole-to-ground and pole-to-pole faults and is effective for both grid-connected and islanded LVdc microgrids. The reliable performance of the developed PFHO-based protection is assessed on the simulation model of a ±750 V meshed LVdc microgrid with the TN-S grounding system.

With the increasing penetration of distributed generations, preserving the protection system coordination is of great importance for the distribution network operation. This paper presents a supplementary protection strategy for the multi-agent system (MAS)-based hierarchical protection scheme (HPS). The primary level of MAS-based HPS clears the fault and preserves the protection coordination while the higher levels update the relay settings. The key advantage of the proposed supplementary scheme is that the primary level can autonomously and quickly preserve the protection coordination when the main relay is during the time-consuming process of updating settings in the case of network operating condition change. By adopting converter-based DGs as the only MAS agents, the proposed supplementary scheme divides the distribution network into several zones that communicate together using point-to-point communication. Without communicating with higher protection levels, the supplementary protection limits the influence of DGs on the fault current by controlling the produced power of each zone. The effectiveness of the proposed strategy is validated through several simulation case studies on the Isfahan distribution network.

Penetrating distributed generation units into the radial distribution networks has been making the protection schemes become ever so complex. The microgrid modes of operation have also increased this complexity. Moreover, the ‘loop-based microgrid’ concept has made it extremely difficult or even impossible to utilise the available radial protection schemes. Although redesigning a new protection structure solves the problem, there should be an affordable way for surviving today's protection systems; accordingly, this study equips the ordinary overcurrent relays with intelligent electronic devices to act as an agent within a multi-agent framework. Given these devices are equipped with an efficient communication infrastructure for industrial applications, named IEC 61850 standard, the proposed approach uses a communication-based strategy, named token, to securely protect the system without any conflict in relays’ operation. The simulation results prove the effectiveness of the proposed method for any DG's penetration, network configuration and fault location. This kind of flexible strategies can affordably differ the expensive protection system replacement in the future revolutionary network upgrades.

Modern distribution system including Distributed Generation (DG) requires reliable and fast islanding detection algorithms in order to determine the grid status. In this paper, a new multi-model classification-based method is proposed, in order to detect islanding condition for photovoltaic units. Decision tree is chosen as the classification algorithm to classify input feature vectors. The final result is based on voting among three decision tree algorithms. First order derivatives of electrical parameters are employed to construct feature vectors. To cover intermittent nature of renewable sources, different generating states for PV unit are assumed. Probable events are simulated under different system operating states to generate classification data set. The pro­po­sed method is tested on typical distribution system including the PV unit, different loads, and synchronous generator. This study sh­o­wed that this method succeeds in highly fast islanding det­ec­tion. This quick response can be used in micro-grid application as well as anti-islanding strategy. The results revealed that the proposed vot­ing-base algorithm could classify instances with very high acc­ur­a­cy which leads to reliable operation of distributed gene­rat­i­on units.

The distributed secondary control is applied in an inverter-dominated microgrid (MG) for compensating the voltage and frequency deviations caused by the operation of the primary droop-based control. Despite the flexibility and reliability of the controllers, the low-inertia nature of voltage source inverters threatens the system stability against probable disturbance. Therefore, it is necessary to analyze the stability of islanded MG by focusing on the hierarchical control structure. The effect of the distributed secondary control on the stability margin of the MG using bifurcation analysis when parameters change is investigated in this article. To this end, this article firstly derives a precise dynamic model of the MG with the secondary control level. Next, the eigenvalues of the system state matrix are analyzed to determine the dominant parameters affecting the stability. Then, the bifurcation theory is used to predict the parameter stability margin with/without secondary control. By changing the parameters, the Hopf and saddle-node bifurcations are detected in the system. Finally, the simulation and experimental results confirm the effectiveness of the distributed secondary control in preventing the occurrence of bifurcation.

The network asymmetries have led to the conventional droop methods that have significant reactive power sharing error during islanding operation of a microgrid. That is why the expandability, modularity, flexibility and communication-free nature of droop-based methods do not let the researchers to easily replace them with another kind of solution. Accordingly, this paper proposes a visually driven nonlinear droop curve making it possible to improve the reactive power sharing accuracy. Of course, due to the probability of the voltage drop at the output terminals of distributed generations, a voltage restoration mechanism should be also presented to maintain the voltage within an acceptable range. In this way, the proposed method can successfully improve both the reactive power sharing accuracy and voltage profile even during fault conditions. Both simulation and experimental case studies using various scenarios are conducted to demonstrate the merits of the proposed scheme. According to the obtained results, the proposed scheme ensures the voltage regulation of MG buses at all times. For instance, the accuracy of reactive power sharing in heavy load conditions has been increased to 96.5% which indicates 21.2% increase in comparison with the conventional droop strategy.

This paper proposes a bi-level multi agent system-based protection scheme with the aim of maintaining the coordination of over current relays in the presence of Distributed Generation (DG) resources. In the proposed scheme, the distribution network protection has been divided into two levels so that the first level protection is implemented through relay agents which are participated in the protection scheme via exchanging information to each other. The second protection level is performed by generation agents when the fault still remains and the first level protection is not effective. To this end, the generation agents are categorized into the main and distributed zones and perform the protection through sharing information together. The effectiveness of the proposed model is examined through several simulation studies on a practical distribution network.

Development of an efficient protection strategy is one of the main barriers in paving the way for the implementation of inverter-based microgrids. The limited fault current of voltage-sourced converter (VSC) units as the sole fault feeding sources in the islanded mode of operation makes the fault detection difficult for conventional overcurrent-based protection system. On the other hand, bidirectional power flow due to the presence of distributed energy resource units and looped configuration of microgrids results in miscoordination of the overcurrent relays. To address these problems, this paper proposes a protection strategy for islanded inverter-interfaced looped microgrids. By monitoring the fault-imposed component of VSC current, the proposed scheme quantifies the severity of fault condition as the fault detection criterion. To preserve the protection coordination among available overcurrent relays, the proposed protection scheme adaptively changes the limiting level of hybrid reference frame limiter of the VSC control system proportional to their distances to the fault point. The proposed protection scheme is local, and there is no need for any change in the commercially available overcurrent protection relays. The reliable performance of the proposed protection strategy is assessed on the simulation model of a looped microgrid network.

Despite the increasing attention paid to multi-agent systems (MASs), their applications in protection schemes of distribution networks would face serious challenges when the generation level of photovoltaic (PV) systems is significantly increased, i.e. the inherent variable generation of PV systems would not only increase the communication burden, but it can also cause protection miscoordination. To solve this problem, this study first classifies the protection tasks into two hierarchical categories as the ‘first-control-level’ and ‘second-control-level’ functions, where the first group is responsible for the urgent task of fault clearing, and the second group updates protection settings in the event of network/generation changes. Given clearing the fault should be accomplished as soon as possible, the first-control-level functions are designed to require the least possible data communication. Presenting a penetration-free approach, the study next describes the mechanism of managing the various generation-change events through a first-control-level function. Therefore, communication failure, as well as protection miscoordination risks, would be mitigated. Finally, the effectiveness of the proposed method is demonstrated using a practical PV-integrated distribution network. This study tackles an important challenge in the protection of distribution systems with a high level of distributed generation penetration where MAS-based schemes are applied.

Despite the fast, reliable and multi-purpose operation of MAS-based protection schemes, the inherent delay or failure of communication system must not affect the vital role of fault clearing, i.e., the ever-increasing renewables in distribution networks as well as network size increase has resulted in a heavy communication burden yielding to delay or even failure in communication. This fact will necessitate having an auxiliary algorithm to save protection system against any load curtailment. To affordably achieve this goal, this paper proposes a self-healing approach having the ability to temporarily replace the common MAS algorithms during emergency conditions. Accordingly, this paper, first, classifies the protection tasks into two hierarchical control levels; the first is responsible for clearing the fault while the second pursues some technical or economical goals. Next, using some less-considered capabilities of the common relays, a penetration-free algorithm is proposed to perform the first-control-level duties without MAS aid. Finally, the effectiveness of the proposed is proved on a practical distribution network. In general, this method can be a complement for the common MAS-based protection schemes to avoid any electricity interruption due to protection failure.

Investigating partial discharge (PD) is a critical issue in monitoring the condition of power transformers. For analyzing the transient state behavior of transformer winding, the availability of a model that accurately presents its behavior at high frequencies is necessary. The conventional detailed model is among the models used to perform PD studies. One of the underlying issues of the conventional detailed model is the low frequency validity range of the model, which is limited to about 1 MHz. In this paper, an improved detailed model is presented that overcomes the mentioned problem by adding stray capacitances to the conventional detailed model. A general state-space algorithm is proposed to calculate the reaction of the winding towards the application of the PD signal. In the present study, the frequency-dependent parameters of the improved detailed model have been calculated via the finite element method (FEM). The new methods and ideas are implemented and validated on a real 20 kV distribution transformer winding as well as a 63 kV, 30MV winding. Comparing the results of the proposed model with the waveforms obtained from using PD pulses to both windings in the laboratory environment shows the accuracy and the increased frequency validity range of this model.

The mismatched feeder impedance of the voltage-sourced converter (VSC) units may result in reactive power sharing error in the islanded microgrids. This study presents a reactive power sharing strategy for single-phase inverter-based microgrids that eliminates the circulating currents and reduces the coupling between real and reactive powers. The proposed controller is based on an adaptive complex virtual impedance that aims to equalise the E - Q characteristic output of VSCs. The resistive term regulates the virtual voltage drop using the proposed ΔR v - P droop characteristic, while the inductive one controls the phase angle of VSC current and increases the X/R ratio of the microgrid. The proposed strategy does not require a central controller and knowledge of the microgrid lines and loads data and is applicable for the various microgrid topologies. Furthermore, it employs the unidirectional low bandwidth communication link and is robust against the delay and failure of communication channels. The proposed method is implemented using the conventional droop control scheme and presents the medium plug and play capability. Both simulation and experimental case studies using various microgrid topologies are conducted to demonstrate the merits of the proposed scheme.

The conventional droop methods can not accurately share reactive power among DG units in an islanded Micro-Grid (MG) because of the mismatched line impedances. This paper proposes an enhanced decentralized control strategy for an MG with inverter-based voltage sources to improve reactive power sharing accuracy. This method is based on the conventional droop curves corrected following a change in the DGs’ operating points. Thus, first, any significant load change, DG switching or mode-transfer of the MG as the main reasons for operating point variation is monitored to trigger the local correction process. Next, for each DG units, according to the injected real power disturbance, the reactive power sharing error is estimated. Simultaneously, an integration term is added to voltage droop control to eliminate the error within a transient period. The proposed local correction procedure modifies the slope or y-intercept of the Q - V curve to cancel the impedance mismatch effects and lead DGs to proper active and reactive power sharing. The simulations and experimental tests prove the accuracy of the proposed method. The MGs can easily be equipped with this technique to affordably improve the power quality and reliability of the network.

The ever-increasing penetration of photovoltaic (PV) systems in distribution networks has resulted in proposing a new control strategy to mitigate the negative impacts of PVs on the protection coordination. However, disconnecting DGs, limiting penetration level, using Fault Current Limiter (FCL) and executing communication-based adaptive approaches are not such affordable solutions. In this paper, an applicable communication-free adaptive technique is proposed to prevent miscoordination problems in distribution networks. It is not only cost effective, but also immune to PV penetration-changes. Firstly, according to the amount of fault current variation in the beginning of the feeder, the fault location is estimated. Next, using a new-added auxiliary control mode, the fault current contributions of PV systems are decreased. Given the parameters are locally measured in each amount of penetration, the proposed methodology needs no communication link to be adapted against penetration variations. The effectiveness of the proposed method for different types of fault scenarios, variations in the photovoltaic penetration rates and different fault resistances is evaluated via detailed simulation case studies. The results clearly show that the proposed adaptive approach can successfully prevent miscoordination problems.

The rapid growth of small-scale photovoltaic (PV) units in distribution grids brings new challenges in the protection coordination of overcurrent relays. So far, various adaptive techniques have been addressed to solve DG units’ mis-coordination problem, however, the un-predictable penetration of installed PV units highlights the importance of a penetration-free methodology. This study presents a novel offline, quick, applicable and cheap solution which guarantees protection coordination for any penetration and location of PV units. To do so, first, under different PV penetration levels and locations, the conventional protection performance is studied to discover the worst mis-coordination cases. Next, according to relays standards, the characteristic curve of the back-up relay is modified such that it can maintain coordination in all worst cases. The proposed methodology is applied to a practical power distribution network equipped with numerous small-scale PV systems. The results successfully prove the effectiveness of the proposed method. Since this technique only applies some changes in characteristic curves of the back-up relays, lack of programmable relay or adaptive protection requirements cannot decrease its performance. This fact makes this method an attractive option for distribution systems with PV units.

Photovoltaic (PV) systems, as distributed energy resources, can adversely affect the coordination of the protection system in distribution circuits. Various approaches have been presented in the literature to solve this problem, but none of them has proposed a solution for the staircase current waveforms seen in PV-dominated distribution systems. As there is insufficient time to apply new settings to a protection device following a fault, the appropriate solution should be to adaptively modify the characteristic curves in advance of the fault. Accordingly, using the voltage profile as a pre-fault index for current waveform, this study suggests an adaptive approach to efficiently modify the protection settings before the fault occurrence. To this end, first, an offline technique is used to calculate the best appropriate group setting corresponding to each voltage profile. Second, in real time and according to the voltage profile, an online algorithm applies the predefined group settings to the corresponding protection devices. Simulation results demonstrate the effectiveness of the proposed method on a practical network such that by applying the proposed technique, settings on programmable protection devices can be readily modified to improve on their coordination.

The installation of photovoltaic (PV) systems is gaining great attention due to the matured PV technology and the lowered price of PV modules. With an increasing penetration of PV systems, the selectivity of distribution network protection may be affected which results in undesirable de-energisation of loads, damage of network equipment, and reduction of reliability. This study presents a method to preserve the protection coordination considering future PV systems installation with any penetration level and different locations along the distribution feeder. Depending on the accessibility of protection device settings or PV control parameters, the proposed method modifies the existing characteristic curve of overcurrent devices or limits the output current of PV sources, respectively. The proposed strategy does not change the structure of existing distribution network protection system and can also be implemented in the old and non-programmable relays. Meanwhile, it does not need the communication links. The merits of the proposed method are demonstrated through several case studies using the Isfahan distribution network.