Mehdi Monadi

A protection strategy for fault detection and location for multi-terminal MVDC distribution systems with renewable energy systems

A fault location method and a fault clearance strategy are presented in this paper for medium voltage dc (MVDC) distribution system. MVDC systems are applicable for connection between microgrids (MGs) and integration of renewable energy systems (RESs) to distribution systems. Due to the specifications of fault current in dc systems, it is difficult to coordinate the over current (O/C) relays based on the time inverse grading. Hence, in this paper, a communication link between O/C relays is used to diagnose the fault location. On the other hand, the fault clearance is done by the operation of dc circuit breakers (DCCB) and isolator switches. In this protection strategy, O/C relays detect the faulty part using communication links and after the fault extinguishing by DCCBs, the dc switches isolate the faulty part. Finally, the sound parts of the system re-energize when DCCB are re-closed. Moreover, data transmission by communication links is based on the standard messages of IEC61850 protocol.

Centralized protection strategy for medium voltage DC microgrids

This paper presents a centralized protection strategy for medium voltage dc microgrids. The proposed strategy consists of a communication-assisted fault detection method with a centralized protection coordinator and a fault isolation technique that provides an economic, fast, and selective protection by using the minimum number of dc circuit breakers. The proposed method is also supported by a backup protection that is activated if communication fails. This paper also introduces a centralized self-healing strategy that guarantees successful operation of zones that are separated from the main grid after the operation of the protection devices. Furthermore, to provide a more reliable protection, thresholds of the protection devices are adapted according to the operational modes of the microgrid and the status of distributed generators. The effectiveness of the proposed protection strategy is validated through real-time simulation studies based on the hardware in the loop approach.

Decoupled voltage stability assessment of distribution networks using synchrophasors

This paper presents a real-time voltage stability assessment in distribution networks (DNs) capable of separating the effects of the transmission and distribution network by using synchronized phasor measurements. The method aims to assist transmission and distribution system operators to quantify the need of their services in different parts of the DN in order to provide adequate voltage support to their specific stakeholders. The method uses data from phasor measurement units (PMUs) to estimate models for both DN and transmission network (TN). A Thévenin equivalent model is estimated for the TN and a T-model for the DN. By applying the superposition theorem on these two models, the contribution of each network to the overall system voltage stability can be distinguished at a specific bus (equipped with a PMU). The method has been validated by a hardware-in-the-loop setup which consists an OPAL-RT real-time simulator and three PMUs.

A communication-assisted protection for MVDC distribution systems with distributed generation

In this paper, a communication-assisted protection scheme is proposed for medium voltage dc (MVDC) distribution networks. MVDC grids are applicable for connection between microgrids, upgrading the transmission capacity of ac lines by the conversion to dc, and integration of renewable energy systems to the distribution grids. However, protection issues are one of the main challenges in the development of the VSC-based dc networks. Due to the special behavior of the dc fault currents, it is almost impossible to coordinate the overcurrent relays based on the time inverse grading. Hence, in the proposed scheme, to provide a fast and selective protection, each proposed relay communicates with two other relays to operate as the main protection for a dc feeder and the backup protection of the adjacent feeder. Hardware in the loop simulation approach by use of the OPAL-RT real time simulator is used to verify the performance of the proposed method.

Distributed FLISR algorithm for smart grid self-reconfiguration based on IEC61850

This paper proposes a distributed algorithm for Fault Location, Isolation, and Service Restoration (FLISR) to be implemented as part of the intelligent software for controlling the operation of each circuit breaker. The paper proposes an event driven finite-state machine design that assures the self-reconfiguration of the distribution power grid when a fault occurs on a certain section of a feeder. The benefits of such an approach are first of all the modularity, as each breaker is controlled by the same algorithm and requires data exchange only with the neighboring protection devices. Secondly, by using the event driven technique, the FLISR algorithm is perfectly suited for taking advantage of the features and benefits defined by the IEC 61850 protocol. Using GOOSE messages for transferring status information between the neighboring circuit breakers, the proposed distributed algorithms act in a collective manner for reconfiguring the distribution grid.

Analysis of ferroresonance effects in distribution networks with distributed source units

The connection of Distributed Generation (DG) units to existing networks may give rise to harmful effects on the network and the DG units itself. Therefore, due to increasing penetration of DG units, it is important to consider these effects in order to avoid some of harmful events. Ferroresonance is one of these destructive phenomena. Over voltages due to this phenomena can damage devices which are connected to the network. In DG systems, due to the use of capacitor banks which are used for VAR compensation, grid connection filters, etc. ferroresonance may appear. Also, special operations like islanding, may lead these systems to ferroresonance. In traditional systems ferroresonance normally appear after there is an unbalance in the system. Nevertheless, for networks with significant penetration of DG units, the situation that gives rise to ferroresonance may be different from traditional systems and they can happen without any unbalancing occurrence. In this paper a distribution system integrating wind (WT) and photovoltaic (PV) power systems has been evaluated in order to analyze the performance of ferroresonance in such systems. Finally, the most appropriate techniques to prevent its appearance, and the associated over voltages, have been studied and implemented using PSCAD/EMTDC.

Design of a centralized protection technique for medium voltage DC microgrids

This paper presents a communication-assisted protection technique for dc microgrids. The technique, consist of a centralized fault detection, by use of the differential method; and a fault isolation strategy by coordinated operation of dc breakers and isolator switches. Hardware in the loop simulation is used to evaluate the proposed protection.

Implementation of the differential protection for MVDC distribution systems using real-time simulation and hardware-in-the-loop

Development of Medium Voltage DC (MVDC) distribution systems require using proper protection schemes whereas due to the behavior of dc fault current, the conventional fault detection methods for ac distribution systems cannot provide a selective protection for these networks. In this paper, a differential based protection method using Ethernet communication has been implemented for a radial MVDC distribution system with integrated PV and WT. This method must be fast enough to detect the faults before involving the main converters and separate only the faulty feeder in order to increase the reliability of the power system. The experimental validation is performed by using Hardware-in-the-Loop (HIL) approach and communication based on the IEC61850 protocol. A real time simulator (OPAL-RT) and a development board (DK60) are used to evaluate the method and to calculate the time delay of this fault detection algorithm.

DC Distribution Networks: A Solution for Integration of Distributed Generation Systems

The employment of renewable energy systems (RES) such as wind energy systems and photovoltaic (PV) systems is increasing in electrical networks. The integration of these RESs and the other types of distributed generations (DGs) has introduced new opportunities and challenges in conventional distribution networks. Most renewable energy-based DGs are interfaced to alternating current (AC) grids through power electronic converters that include an AC-to-DC conversion stage. Therefore using direct current (DC) distribution systems can contribute to reduce the total cost and loss of the system, as the power conversion stages are decreased. On the other hand, consequent to the recent developments in power electronics devices, new efficient converters have developed that facilitate the construction of DC switchgears and networks. Consequently, due to the advantages of DC distribution systems, they have attracted considerable attention over the last few years and they are introduced as an alternative for AC systems in future commercial and industrial grids. Besides the advantages of DC networks, there are serious concerns about their worldwide usage. Protection, control, and network designing are the main key issues associated with these grids. In this chapter, the specifications of DC distribution networks and their advantages for DGs integration are described. Furthermore, the issues related to the protection and control of these networks and the already presented solutions are discussed. The chapter also includes several numerical examples to facilitate the understanding of the presented concepts.The employment of renewable energy systems (RES) such as wind energy systems and photovoltaic (PV) systems is increasing in electrical networks. The integration of these RESs and the other types of distributed generations (DGs) has introduced new opportunities and challenges in conventional distribution networks. Most renewable energy-based DGs are interfaced to alternating current (AC) grids through power electronic converters that include an AC-to-DC conversion stage. Therefore using direct current (DC) distribution systems can contribute to reduce the total cost and loss of the system, as the power conversion stages are decreased. On the other hand, consequent to the recent developments in power electronics devices, new efficient converters have developed that facilitate the construction of DC switchgears and networks. Consequently, due to the advantages of DC distribution systems, they have attracted considerable attention over the last few years and they are introduced as an alternative for AC systems in future commercial and industrial grids. Besides the advantages of DC networks, there are serious concerns about their worldwide usage. Protection, control, and network designing are the main key issues associated with these grids. In this chapter, the specifications of DC distribution networks and their advantages for DGs integration are described. Furthermore, the issues related to the protection and control of these networks and the already presented solutions are discussed. The chapter also includes several numerical examples to facilitate the understanding of the presented concepts.

Enhanced performance of SVC via using rotor speed deviation signal (RSDS)

Transient stability enhancement through using a Static Var Compensator (SVC) and Rotor Speed Deviation Signal (RSDS) of synchronous machine are thoroughly investigated in this paper. Speed deviation signal of generator as a remote signal is sent to SVC in order to achieve more transient stability. Studies are carried out on Kundurs four machine two-area test system which SVC is implemented on midway of transmission system. The simulation results verify the effectiveness of the proposed method to improve rotor angle stability versus different kinds of disturbances scenarios. The standard benchmark model is used for the analysis in the MATLAB/Simulink environment.