IEEE Transactions on Power Delivery

A general formulation of the feeder reconfiguration problem for loss reduction and load balancing is given, and a novel solution method is presented. The solution uses a search over different radial configurations created by considering switchings of the branch exchange type. To guide the search, two different power flow approximation methods with varying degrees of accuracy have been developed an... View full abstract»

In this paper, a low-voltage (LV) DC microgrid protection system design is proposed. The LV DC microgrid is used to interconnect distributed resources and sensitive electronic loads. When designing an LV DC microgrid protection system, knowledge from existing DC power systems can be used. However, in most cases, these systems use grid-connected rectifiers with current-limiting capability during DC... View full abstract»

The modular multilevel converter (MMC) is a newly introduced switch-mode converter topology with the potential for high-voltage direct current (HVDC) transmission applications. This paper focuses on the dynamic performance of an MMC-based, back-to-back HVDC system. A phase-disposition (PD) sinusoidal pulsewidth modulation (SPWM) strategy, including a voltage balancing method, for the operation of ... View full abstract»

This paper describes a modified phase-shifted carrier-based pulsewidth-modulation (PSC-PWM) scheme for modular multilevel converters (MMC). In order to reduce the average device switching frequency, a reduced switching-frequency (RSF) voltage balancing algorithm is developed. This paper also proposes a circulating current suppressing controller (CCSC) to minimize the inner circulating current in a... View full abstract»

Thermal and mechanical behavior of ZnO varistors used in surge arresters is simulated for current pulses of various magnitude and duration. Time dependence of the temperature profile and of the distribution of thermal stresses is computed by solving heat transfer equations for varistor disks with nonuniform electrical properties. The simulations identify failure processes and determine energy abso... View full abstract»

This paper presents a new approach in adaptive out-of-step (OOS) protection settings in power systems in real time. The proposed method uses extended equal area criterion (EEAC) to determine the critical clearing time and critical clearing angle of the system, which are vital information for the OOS protection setting calculation. The dynamic model parameters and the coherency groups of the system... View full abstract»

In this paper, a method for predicting pressure drop and flow distribution in disc-type transformer windings in an oil-forced and directed cooling mode is proposed. First, dimensional analyses are conducted to identify the independent dimensionless variables that affect pressure drop over and flow distribution in the winding. Next, parametric sweeps are performed with computational fluid dynamics ... View full abstract»

With the progressive exhaustion of fossil energy and the enhanced awareness of environmental protection, more attention is being paid to electric vehicles (EVs). Inappropriate siting and sizing of EV charging stations could have negative effects on the development of EVs, the layout of the city traffic network, and the convenience of EVs' drivers, and lead to an increase in network losses and a de... View full abstract»

This paper explains the working principles, supported by simulation results, of a new converter topology intended for HVDC applications, called the alternate arm converter (AAC). It is a hybrid between the modular multilevel converter, because of the presence of H-bridge cells, and the two-level converter, in the form of director switches in each arm. This converter is able to generate a multileve... View full abstract»

A control technique is proposed in this paper for control of modular multilevel converters (MMC) in high-voltage direct current (HVDC) transmission systems. Six independent dynamical state variables are considered in the proposed control technique, including two ac currents, three circulating currents, and the dc-link voltage, for effectively attaining the switching state functions of MMCs, as wel... View full abstract»

A direct approach for unbalanced three-phase distribution load flow solutions is proposed in this paper. The special topological characteristics of distribution networks have been fully utilized to make the direct solution possible. Two developed matrices-the bus-injection to branch-current matrix and the branch-current to bus-voltage matrix-and a simple matrix multiplication are used to obtain lo... View full abstract»

Control and operation of a dc microgrid, which can be operated at grid connected or island modes, are investigated in this paper. The dc microgrid consists of a wind turbine, a battery energy storage system, dc loads, and a grid-connected converter system. When the system is grid connected, active power is balanced through the grid supply during normal operation to ensure a constant dc voltage. Au... View full abstract»

The introduction of “smart grid” solutions imposes that cyber security and power system communication systems must be dealt with extensively. These parts together are essential for proper electricity transmission, where the information infrastructure is critical. The development of communication capabilities, moving power control systems from “islands of automation” to ... View full abstract»

We intend to prove that phasor-measurement-unit (PMU)-based state estimation processes for active distribution networks exhibit unique time determinism and a refresh rate that makes them suitable to satisfy the time-critical requirements of protections as well as the accuracy requirements dictated by faulted line identification. In this respect, we propose a real-time fault detection and faulted l... View full abstract»

The problem of capacitor placement on a radial distribution system is formulated and a solution algorithm is proposed. The location, type, and size of capacitors, voltage constraints, and load variations are considered. The objective of capacitor placement is peak power and energy loss reduction, taking into account the cost of the capacitors. The problem is formulated as a mixed integer programmi... View full abstract»

Using voltage-source converter high-voltage direct current (VSC-HVDC) to interconnect weak ac power systems has drawn much attention. Under the traditional vector control for the VSC that the response of AC voltage loop cannot match that of the active power loop may limit power transmission capability of the VSCs that interconnect with weak power systems. To enhance the power transmission capabili... View full abstract»

Generation is shifting from a centralized power generating facility having large synchronous generators to distributed generation involving sources of smaller capacity. Most of these sources require inverters on the front end while being connected to the grid. Lower available kinetic energy, coupled with less short-circuit current ratio compared to large synchronous generators, compromises the tra... View full abstract»

The continuously increasing demand for electric power and the economic access to remote renewable energy sources such as off-shore wind power or solar thermal generation in deserts have revived the interest in high-voltage direct current (HVDC) multiterminal systems (networks). A lot of work was done in this area, especially in the 1980s, but only two three-terminal systems were realized. Since th... View full abstract»

A VSC-MTDC (multi-terminal dc) system consists of voltage-source converters (VSCs) connected to a dc network at their dc terminals. The MTDC is most vulnerable to a dc fault which paralyses all the VSCs until the dc fault is cleared. As dc circuit breakers are expensive, this paper proposes a solution based on extinguishing the dc fault current by opening all the ac-circuit breakers (ac-CBs) which... View full abstract»

This paper presents a transient-based algorithm for high-impedance fault identification on distribution networks. It uses the discrete wavelet transform to monitor high- and low-frequency voltage components at several points of the power system, being able to indicate the most likely area within which the disturbance has occurred, without requiring data synchronization nor the knowledge of feeder ... View full abstract»

This paper presents an enhanced control for HVDC systems employing hybrid modular multilevel converters (MMCs) during asymmetrical dc voltage faults. The characteristics of pole-to-ground (PTG) fault under different system grounding schemes are analyzed. Then, the principle of enhanced control strategy to deal with the PTG fault is presented for a hybrid-MMC-based HVDC system with symmetrical mono... View full abstract»

DC line faults are major issues for a multiterminal high-voltage direct current (HVDC) system based on voltage-source converter (VSC). The fault current increases quickly along with a large peak, and complete isolation of the faulted system is not a viable option. Therefore, protection with high selectivity and accuracy is essential. In this paper, a new protection scheme for dc line in multitermi... View full abstract»

This paper investigates (i) preplanned switching events and (ii) fault events that lead to islanding of a distribution subsystem and formation of a micro-grid. The micro-grid includes two distributed generation (DG) units. One unit is a conventional rotating synchronous machine and the other is interfaced through a power electronic converter. The interface converter of the latter unit is equipped ... View full abstract»

Because traditional electric power distribution systems have been designed assuming the primary substation is the sole source of power and short-circuit capacity, DR interconnection results in operating situations that do not occur in a conventional system. This paper discusses several system issues which may be encountered as DR penetrates into distribution systems. The voltage issues covered are... View full abstract»

Intentional frequency perturbation by recently researched active islanding detection techniques for inverter based distributed generation (DG) define new threshold settings for the frequency relays. This innovation has enabled the modern frequency relays to operate inside the non-detection zone (NDZ) of the conventional frequency relays. However, the effect of such perturbation on the performance ... View full abstract»

A strategy for grid power peak shaving and valley filling using vehicle-to-grid systems (V2G) is proposed. The architecture of the V2G systems and the logical relationship between their sub-systems are described. An objective function of V2G peak-shaving control is proposed and the main constraints are formulated. The influences of the number of connected EVs and the average value of the target cu... View full abstract»

The change rate of the dc reactor voltage with predefined protection voltage thresholds is proposed to provide fast and accurate dc fault detection in a meshed multiterminal HVDC system. This is equivalent to the measurement of the second derivative of the dc current but has better robustness in terms of electromagnetic-interference noise immunization. In addition to fast dc fault detection, the p... View full abstract»

Voltage-source-converter (VSC) technologies present a bright opportunity in a variety of fields within the power system industry. New modular multilevel converters (MMCs) are expected to supersede two- and three-level VSC-based technologies for HVDC applications due to their recognized advantages in terms of scalability, performance, and efficiency. The computational burden introduced by detailed ... View full abstract»

The protection system has been facing new challenges with the distribution systems with distributed generation (DG), which may be successfully accomplished through the combination of the existing protections, such as overcurrent protection, with modern protections based on emerging technologies. By using conventional and modern protection trends, the aim of this paper is to propose the overcurrent... View full abstract»

Microgrids have been proposed as a way of integrating large numbers of distributed renewable energy sources with distribution systems. One problem with microgrid implementation is designing a proper protection scheme. It has been shown that traditional protection schemes will not work successfully. In this paper a protection scheme using digital relays with a communication network is proposed for ... View full abstract»

Renewable energy resources (RES) are being increasingly connected in distribution systems utilizing power electronic converters. This paper presents a novel control strategy for achieving maximum benefits from these grid-interfacing inverters when installed in 3-phase 4-wire distribution systems. The inverter is controlled to perform as a multi-function device by incorporating active power filter ... View full abstract»

The number of semiconductor switches in a modular multilevel converter (MMC) for HVDC transmission is typically two orders of magnitudes larger than that in a two or three level voltage-sourced converter (VSC). The large number of devices creates a computational challenge for electromagnetic transient simulation programs, as it can significantly increase the simulation time. The paper presents a m... View full abstract»

The scope of this working group study is to review current practices, technical information, and safety criteria to assist in developing factors to size protective ground cable systems to protect workers during work on de-energized transmission and distribution lines. These guidelines are proposed to be included as part of “Guide for Protective Grounding of Power Lines”, P1048, at the ... View full abstract»

This paper presents the experimental determination of the ZIP coefficients model to represent (static) modern loads under varying voltage conditions. ZIP are the coefficients of a load model comprised of constant impedance Z, constant current I, and constant power P loads. A ZIP coefficient load model is used to represent power consumed by a load as a function of voltage. A series of surveys was p... View full abstract»

This paper presents the design, development, control, and supervision of a hardware-based laboratory multiterminal-direct-current (MTDC) testbed. This work is a part of the TWENTIES (transmission system operation with large penetration of wind and other renewable electricity sources in networks by means of innovative tools and integrated energy solutions) DEMO 3 European project, which aims to dem... View full abstract»

These days, distributed generators (DGs), such as photovoltaic, wind turbine, and gas cogeneration systems have attracted more attention than in the past. DGs are often connected to a grid by power inverters. The inverters used in DGs are generally controlled by a phase-locked loop (PLL) in order to be synchronized with the grid. In a stability point of view, the power system will be significantly... View full abstract»

This paper focuses on a comparative study of the modeling and simulation of the first CIGRE HVDC benchmark system using two simulation tools PSCAD/EMTDC and PSB/SIMULINK; an interface between them (PSCAD-SIMULINK) has also been implemented and used as a simulator. The CIGRE HVDC system and its controller has been carefully modeled in all three simulation environments so that the differences are mi... View full abstract»

This work proposes a new Modulated Model Predictive Control (M2PC) method for control of Modular Multilevel Converters (MMCs) in Voltage Source Converter-High Voltage DC (VSC-HVDC) systems. The proposed method retains the advantages of the conventional Finite Control Set-Model Predictive Control (FCS-MPC) methods by programing the nonlinear properties of the MMC into the design calculations while ... View full abstract»

A multi-terminal dc (MTDC) grid has a number of advantages over traditional ac transmission. However, dc protection is still one of the main technical issues holding back the expansion of point-to-point dc links to MTDC networks. Most dc protection strategies are based on dc circuit breakers (DCCBs); however, DCCBs are still under development and their arrival to the market will come at an unclear... View full abstract»

Modular multilevel converters (MMC) are presently the converter topology of choice for voltage-source converter high-voltage direct-current (VSC-HVDC) transmission schemes due to their very high efficiency. These converters are complex, yet fast and detailed electromagnetic transients simulation models are necessary for the research and development of these transmission schemes. Excellent work has... View full abstract»

The least erroneous knowledge on fault location in distribution systems helps the restoration process, expedites maintenance, and reduces power outage duration. A fault-location method using the probe power unit (PPU) in dc microgrid assumes that the natural frequency of the system is equal to the damped resonant frequency of probe current. This assumption leads to prominent error in fault-locatio... View full abstract»

Although virtually every overhead HVdc line is based on line commutated converter (LCC) technology, hardly any work has been reported on modeling and analysis of the impact of inertia and effective short circuit ratio on control of frequency in weak grids interfacing LCC-HVdc and onshore doubly fed induction generator (DFIG)-based wind farms. This paper develops a comprehensive modeling and stabil... View full abstract»

This paper presents (i) an equivalent model of the half-bridge modular multilevel converter (HB-MMC), (ii) a hybrid CPU/FPGA-based architecture for real-time simulation of electromagnetic transients of systems which include HB-MMC and (iii) a modified odd-even bubble sorting method for voltage balancing of MMC sub-module capacitors. The Adam-Bashforth (AB) method is used for numerical integration ... View full abstract»

A test stand to evaluate the radial buckling strength of power transformer windings under the influence of electromagnetic forces is presented. The resulting conductor deformation can be measured in parallel to the sinusoidal test current. The proposed test results focus on the inception of forced buckling for continuously transposed conductors (CTCs), which are a special type of conductor often u... View full abstract»

In this paper, distributed energy-storage systems (ESSs) are proposed to solve the voltage rise/drop issues in low-voltage (LV) distribution networks with a high penetration of rooftop photovoltaics (PVs). During the peak PV generation period, the voltages are mitigated by charging the ESSs, and the stored energy is discharged for voltage support during the peak load period. The impact of storage ... View full abstract»

This paper presents a transformerless high-voltage DC-DC converter based on cascaded sub-modules. It is intended for interconnecting high-voltage or medium-voltage DC grids. The proposed DC-DC converter is composed by two phase legs, each of which consists of an upper arm and a lower arm with their middle nodes crossly connected through a branch of active sub-modules. By properly controlling the o... View full abstract»

A fault protection and location method for a dc bus microgrid system is presented in this paper. Unlike traditional ac systems, dc bus systems cannot survive or sustain high-magnitude fault currents. And if a fault causes the dc bus to de-energize completely, it makes locating faults very difficult. The main goal of the proposed scheme is to detect and isolate faults in the dc bus without de-energ... View full abstract»

Differential protection is widely used for transmission lines, but its performance is affected by distributed capacitance current and current-transformer (CT) saturation. Travelling-wave (TW) differential protection is immune to these factors and can achieve ultra-high-speed operation. These characteristics are essential for long-distance extra/ultra-high-voltage transmission-line protection. Howe... View full abstract»

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 bac... View full abstract»

In this paper, the use of DC power system to supply sensitive electronic loads is treated. First, general design issues regarding DC power systems are discussed, and then the measurement results from a scaled laboratory setup are presented. The results show that it is possible to supply sensitive electronic loads through an AC/DC interface, and to keep them online during grid transients. The use o... View full abstract»