Search

Weather-related or other types of faults can negatively impact the continuity of the electricity delivery to the consumers and, thus, lead to poorer reliability scores. 90% of customer outage-minutes are due to events which affect local distribution systems. The radial configuration is simple and has a low cost design; however, when a fault occurs it often causes power outages beyond the location of the fault itself. Therefore, it is beneficial for utilities to automatically isolate the fault and, thus, restore power to as many consumers as possible. The benefits of fault isolation and service restoration include, but are not limited to: enhanced reliability and resiliency, reduced loss of revenue to the utility, and enhanced comfort and satisfaction for the utility’s consumers. This paper provides an algorithm to quickly isolate a fault and automatically restore power to as many nodes of the distribution system as possible by opening and closing switches already placed within the system. The expert model and centralized algorithm that are proposed in this paper guarantee an optimal solution and a short run-time regardless of the size of the distribution system. A mathematical proof of the optimal nature of the solution is provided. In addition, the model is validated in practice, solving two different-sized systems. [ABSTRACT FROM AUTHOR]

Modular multilevel converters (MMCs) in high- voltage applications contain large numbers of power switches, which increases the fault probability inside MMCs. This paper proposes a detection method for the faulty arm containing switch open-circuit failures in high-voltage MMCs to improve the reliable operations of MMCs. The proposed method uses the periodicity of the deviation between the maximum and minimum capacitor voltages to provide the detection reference. To yield a reliable reference, the repetitive signal generator is employed. Then, the fault is identified by the difference between the referenced and measured capacitor voltage deviations. An auxiliary criterion is also designated to avoid the false detection caused by the mutations in MMCs. The proposed method can reduce the difficulty in the selection of thresholds. Moreover, the proposed method is robust in high-voltage MMCs with inferior capacitor voltage balancing performance, which ensures detection rapidity and accuracy. Finally, the feasibility of the method is verified by simulation results in Matlab/Simulink. [ABSTRACT FROM AUTHOR]

The missing capability of DC fault current limitation is an inherent disadvantage in the typical half-bridge modular multilevel converter (MMC), which restricts its application in long-distance high voltage direct current (HVDC) systems. The full-bridge submodule (FBSM) and clamped double submodule (CDSM) topologies allow for fault current self-clearing, but also suffer from additional hardware cost and power losses. In this paper, an improved submodule topology utilizing the new reverse-blocking insulated gate bipolar transistor (RB-IGBT) is proposed to improve system economy without compromising its performance. The operating principles and DC fault characteristics of the proposed topology are analyzed in details, including comparison with various topologies in terms of cost, losses, fault current clearing speed and thermal. Compared with the traditional CDSM topology, the proposed RBSM can clear DC faults faster with reduced cost of switching devices. A DC fault protection strategy for MMC-HVDC system is also presented. Simulation results under normal operating, permanent and transient fault conditions are carried out to verify the effectiveness of the proposed topology. [ABSTRACT FROM AUTHOR]

The integration of overhead lines in future MTDC grids leads to a higher fault rate compared to cables. Most of the faults are considered to be non-permanent, so that the line can be reconnected after dielectric recovery. The auto-reclosing procedure in HVDC systems reveals new challenges since multiple restrikes are possible within a short period of time. This paper proposes an auxiliary circuit for DC Circuit Breakers (DCCB) which enables to use the DCCB internal capacitor for the line pre-energization. A comprehensive description of the switchgear operations during the pre-energization sequence is provided and the fault property identification method is defined. The method is validated by simulations in a MTDC grid and a sensitivity analysis is carried out using EMTP-RV. [ABSTRACT FROM AUTHOR]

DC transformer is an essential equipment for formation of multi-voltage-level DC grid. This paper proposes a novel non-isolated Boost-type alternate arm DC transformer (Boost-AADCT) for medium- and high-voltage level DC grid application. The Boost-AADCT is comprised of three submodule-cascaded arms, two thyristor-cascaded director switches, and two buffer capacitors. Bidirectional power transmission is achieved by alternatively charging and discharging of the intermediate arm following the Boost principle, while the pulsating power component caused by intermittent transmission is actively smoothened by grid-side buffer subcircuits formed by the buffer capacitors and grid-side arms. The active power buffering method increases the tolerance range of voltage ripple of the buffer capacitor, and the required energy storage capacity is resultantly reduced. Circuit configuration, operation principle, and mathematical model are first introduced. Then, the control strategy is proposed, the main circuit parameters are designed, and bidirectional fault-blocking capability is analyzed. Comparison with other DC-DC converters shows that the Boost-AADCT significantly reduces the requirement of semiconductors and has relatively lower synthetic cost. At last, performance of the Boost-AADCT and the proposed control strategies are evaluated by simulation tests conducted in MATLAB/Simulink environment. [ABSTRACT FROM AUTHOR]

The medium-voltage grid is often planned in loops, but operated in a radial structure to allow a robust protection scheme. This allows the reconfiguration of the grid. This paper focuses on reconfiguration after a fault to re-establish the service, also referred to as self-healing. It includes the localization of faults and finds new grid configurations based on the actual status of the grid. The solving of the reconfiguration problem is nonpolynomial difficult. Finding an acceptable solution to resupply nonfaulted grid parts under tight time limits is difficult. The innovative contribution of this paper is an approach that finds new configurations as local as possible to the faulty area, evolvingly increasing the search space. New configurations are generated based on a heuristic method relying on graph theory. An aggregated fitness function is used to evaluate the solutions. For the realization, a distributed control architecture in the form of an advanced substation automation is proposed. [ABSTRACT FROM PUBLISHER]

Initiation of ac fault may lead to the commutation failure, and the high-voltage direct current (HVDC) system is still confronted with the risk of subsequent commutation failure (SCF) during recovery process prior to the fault clearance. In this paper, the mechanism of subsequent commutation failure is investigated through the analysis of controller response and ac-dc interaction under the symmetrical and asymmetrical faults. According to the modulation theory, a switching function method based on the line-to-line voltages is introduced to depict the voltage transfer characteristics of converters under asymmetrical faults. The early warning criterion and its corresponding early warning system for the subsequent commutation failure (SCF-EWS) are developed by taking the risks of controller regulation and the variations of electrical quantities into account. For the purpose of suppression, an emergency advance control strategy against the subsequent commutation failure (SCF-EAC) is also presented. The analysis and the proposed schemes are verified by the CIGRE benchmark model that is shipped with the PSCAD/EMTDC software under different fault conditions, and the simulation results demonstrate the feasibility and superior performance. [ABSTRACT FROM AUTHOR]

In some conditions, the flux in the core of current transformers (CT) increases. If this increase in flux leads to saturation of the protective core, it can create a lot of problems in power system protection. One of the hardware solutions to prevent the occurrence of core saturation in CT is the fixed switched resistor demagnetization method. Due to disadvantages such as dependence on CT parameters and inefficiencies in severe core saturation, the use of this method has not been expanded. This paper proposes a new method to improve the fixed switched resistor demagnetization method. In the proposed method, instead of using a fixed resistor in the circuit topology, an adjustable resistor with an auxiliary transformer is used. In the previous method, the control process is based on flux estimation from the secondary terminal voltage. The control process in the proposed method is based on the second derivative of CT secondary current. To verify the effectiveness of the suggested method, first simulation is done using COMSOL and MATLAB softwares and then all necessary electronic and control circuitry are designed and built. Experimental test is carried out on a 20 kV- 500/5 A CT. The simulation and experimental results show that this method prevents the CT from saturation in case of short circuit faults. The main advantages of this method are independent to CT parameters and efficient in severe core saturation. [ABSTRACT FROM AUTHOR]

Modular multilevel converter based high voltage direct current (MMC-HVDC) transmission is the prospective technology for large-scale wind farms integration. In this paper, a comprehensive coordinated control strategy for DC fault ride-through (FRT) of wind farms connected to the grid through bipolar MMC-HVDC transmission is proposed. To reduce the requirement of breaking speed and capacity of DC breakers, an active current-limiting control method is proposed to limit the rise rate of DC fault current, and the principle of parameter selection is designed. After DC breakers isolate fault, the control mode switching methods of bipolar MMC are designed to make the fault pole MMC operate as a STATCOM to support the AC grid and improve the active power transmission capability of the healthy pole MMC, which can reduce the impact range of unbalanced power between wind farms and converter station. In addition, a coordinated control strategy for wind farms load-shedding considering speed constraints is designed to realize the optimal distribution of load-shedding power among wind turbines, which can regulate unbalanced power together with the healthy pole MMC to realize DC FRT. Finally, the effectiveness and feasibility of the proposed control methods are verified by simulation results. [ABSTRACT FROM AUTHOR]

Pole-to-pole dc short circuit fault blocking capability is a major concern when modular multilevel converters (MMCs) are applied in high voltage direct current (HVDC) transmission systems. Traditional MMCs based on half-bridge submodules (HBSMs) cannot block the fault current, thus needing expensive direct current circuit breakers (DCCBs). There are MMCs based on improved submodule topologies that can block dc faults, but usually with significantly increased device cost. This paper proposes an improved HBSM-based MMC (HB-MMC) which needs fewer extra semiconductor devices. The improved HB-MMC is more competitive than other schemes when it comes to investment cost and loss. An H-bridge circuit composed of two director switches and two diode valves is installed around one arm of the HB-MMC, which will not change the direction of current during the normal operation, whereas it can change the direction of the fault current when a pole-to-pole dc short circuits fault is detected. After two director switches have been turned off, a reverse-biased voltage provided by one arm can block the fault current. The topology can be further improved to provide reactive power during faults. Simulation results are presented to validate the features of the proposed improved HB-MMC. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC) is one of the most attractive topologies for high-voltage and high-power applications. Reliability is an important challenge for MMCs due to large amounts of power switches in the phase leg of the MMC. To enhance the reliable performance of MMCs, an effective and accurate fault detection and localization strategy for MMCs with the nearest level modulation is proposed in this paper. According to the general characteristics of the MMC, the capacitor voltage of the faulty submodule (SM) is higher than the healthy ones’. Based on the sorting algorithm, the faulty SM will have the maximum voltage and it will be identified by the proposed fault detection method. The auxiliary monitoring is supplemented to the detection algorithm for the cases of faults occurring in multiple SMs. The faulty SM localization strategy is based on the differences of capacitor voltages. The faulty switches are located by the distinctions of capacitor voltage in different current intervals. The proposed strategy could be applied under the cases of single or multiple SMs failures and identifies three kinds of the power switches faults in the faulty SM. The validity of the proposed strategy is verified by the simulation results in MATLAB/Simulink. [ABSTRACT FROM AUTHOR]

Transient voltage and current stresses are essential for the design and protection of multi-terminal DC (MTDC) transmission systems, especially under the short-term dynamics after severe grid faults. In this paper, we propose a discrete-time model for the modular multilevel converter based MTDC (MMC-MTDC) system considering the DC-side disturbances in the system. Based on the piecewise linear modeling, the state variables such as the transient voltage on the sub-module (SM), arm current, etc., can be calculated analytically for the critical short term (within 10 ms) after the severe DC fault while the system protection is yet to be activated. Parameter sensitivity analysis is also performed to give a better system design guideline. The estimation results are validated by detailed electromagnetic (EMT) circuit simulations and the scaled-down experimental laboratory prototype. [ABSTRACT FROM AUTHOR]

Two key problems in meshed high-voltage direct current (HVDC) transmission grids are managing line power flows and protection against dc faults. Current flow controllers (CFCs) will be required to balance cable currents in meshed dc grids, in order to prevent individual line power capacity limits restricting overall power flow in the grid. Direct current circuit breakers (DCCBs) will be also required to protect HVDC grids from dc faults. This paper demonstrates that the current flow controller functionality can be added into a hybrid CB's design. This paper proposes integrating an interline CFC into the load commutation switch (LCS) of a hybrid DCCB. The integrated design LCS/CFC is analyzed and a state-space model is derived. The control of the CFC is designed and the performance of the LCS/CFC during normal operation is verified by means of MATLAB Simulink and PSCAD simulations. A comparison of the integrated LCS/CFC and the separate design is given. The case studies show a reduction in total power losses, and improved protection operation times can be achieved. [ABSTRACT FROM PUBLISHER]

A special stress for circuit breaker is the occurrence of evolving fault or parallel switching condition. These switching cases are not standardized by international standards, but may occur in service and may create problems for the breaker. In the past, it was a problem mainly with oil circuit breakers and some types of air-blast circuit breakers but lesser known with SF6 circuit breakers. In the meantime, with upcoming new switching technologies along with changed operating duties, some doubts exist that these switching cases may become an issue again. This paper explains in detail the background of these stresses and presents a method to evaluate the capability of SF6 circuit breakers for such situations. The theoretical evaluation involves network calculations, gas flow simulations, and dynamic dielectric analysis. It is important to note here that several tests have been done in the past for evolving and parallel switching faults for both puffer as well as self-blast circuit breakers with the knowledge available at that time. These test results are used to validate the outcome of the theoretical evaluation which will help in better prediction of the circuit breaker behavior before performing actual tests. It can be concluded that theoretical simulation may, in majority of the cases, serve as sufficient proof to show feasibility for such cases. [ABSTRACT FROM AUTHOR]

An increasing number of offshore wind farms are connected to the public grid with large-scale HVDC links. The electrical transients caused by the energization of the export transformers appear more significant in the converter-driven offshore AC grid with low short-circuit power, than in direct AC grid connections. This may lead to a violation of the 2% voltage dip criterion, thus seriously affecting the production of neighboring wind farms. Additionally, the transformer and the entire insulation system are exposed to mechanical and dielectric stress. This paper investigates the influence of three factors, namely excitation, switching angle, and remnant core magnetization, onto the inrush current. Electromagnetic transient simulations and field measurements show that with a smart use of the on load tap changer, point on wave switching, and a demagnetization of the core by a cable-shunt swing circuit, the three factors above can be used as degrees-of-freedom to effectively mitigate the inrush current phenomenon. [ABSTRACT FROM AUTHOR]

The standard alternative-arm converters (AAC) with the sweet spot or short overlap operation have faced a challenge of balancing submodule (SM) capacitor voltages, where this issue has not been paid much attention so far. In this paper, a new control strategy for the SM-capacitor voltage balancing of the AAC applied in high-voltage dc transmission systems is proposed. The suggested control technique is composed of three regulators, where a control for the average voltage of all SM capacitors, the voltage balancing among the converter arms, and the voltage balancing of the individual SM capacitor are performed. Based on the operation principle of the standard AAC, which activates only three of six converter arms simultaneously, the energy balancing of the three activated arms is performed every one sixth of the fundamental cycle by the converter common offset voltage. Band-stop filters with the central frequencies at the fundamental, second-, and third-order components of the line frequency are applied to obtain the average values of the SM capacitor voltages for the feedback controllers. The simple proportional and integral controllers are employed for this scheme and the voltages of SM capacitors are mostly balanced during the steady states and kept closely to the rating with overshoot of less than 7% under the transient conditions. The proposed control method is verified by simulation results for a 33-level AAC under both normal and fault conditions. [ABSTRACT FROM AUTHOR]

With the development of HVdc transmission technology, the isolation of dc faults is becoming increasingly important. As the most effective solution, hybrid HVdc circuit breaker (CB) is not mature enough, which has restricted its engineering application. Accordingly, this paper proposes an auxiliary dc CB scheme suitable for HVdc grid. In this scheme, the traditional half-bridge modular multilevel converter (MMC) is partially innovated to have the ability of auxiliary breaking operation. With the effect of the MMC auxiliary breaking (MAB) operation at both ends of the fault line, the dc fault current can be easily interrupted and isolated by low-cost CB unit installed in the dc transmission line. The equivalent circuit under MAB process is analyzed, and the fault isolation sequences are designed. Furthermore, the economic comparative analysis among three schemes is performed. Finally, a four-terminal bipolar HVdc grid test model is built in PSCAD/EMTDC; in addition, an MMC prototype with proposed scheme is also developed. The simulation and experimental results validate the effectiveness of the proposed scheme. [ABSTRACT FROM AUTHOR]

Future multi-terminal dc networks are envisioned for the large-scale integration of renewable energy sources into today's power systems. An essential aspect for the reliable operation of these systems is the fast and selective dc-side fault handling and separation of faulted lines. The objective of this paper is the development and evaluation of a multi-terminal dc protection strategy based on fault-blocking converters (e.g., full-bridge based modular multi-level converters) and high-speed switches. Both a suitable fault control and a new high-speed switch design for a fast separation of faulted dc lines are elaborated. The novel concept is intended to reduce the requirements on the high-speed switches compared to dc circuit breakers and to ensure a fast restoration of the active power transmission capability, as well as the dc voltage. The fault-clearing strategy is analyzed using electromagnetic transient (EMT) simulations under variation of the fault location, type, and resistance. The investigations are carried out in a cable-based multi-terminal HVdc system with full-bridge modular multi-level converters in symmetrical monopole configuration. [ABSTRACT FROM AUTHOR]

Single-phase autoreclosing (SPAR) is among the alternatives for mitigating the many harmful effects that a line-to-ground fault in a transmission line (TL) may cause. This procedure basically consists of opening the compromised phase, waiting a preset time span (dead time), and recomposing the previously isolated phase. In the vast majority of SPAR implementations, the open phase switching proceeds with no guarantee of whether the TL is still faulted. Switching the open phase into a fault might represent a more severe threat to the system than simply disconnecting the TL in the first place. Hence, in an effort to ensure higher levels of operational reliability to SPAR, the present paper offers a new methodology to check the soundness of the open phase during the dead time before its reclosing. First, the method's fundamental concepts are exposed. Second, a huge number of steady-state tests are presented, for which both the system topology and its operative condition were varied. To conclude, the SPAR is simulated in the time-domain with the most critical condition found in the steady-state analysis. The proposed strategy was capable of correctly identifying whether the system was faulted in every performed simulation. [ABSTRACT FROM AUTHOR]

This paper proposes a modified bridge switch-type flux-coupling nonsuperconducting fault current limiter (BSFC-NSFCL) for suppressing the fault current and voltage sag. A flux-coupling reactor and a bidirectional bridge switch are combined to perform the functions of steady-state line current sharing and fault current suppression. The modified BSFC-NSFCL mainly utilizes the magnetic flux cancellation effect to make the limiter behave as a short circuit on the circuit during normal operation. Hence, there is almost no impact on the power system when the limiter is used. When a fault event occurs, the bridge switch turnsoffimmediately, and then, the magnetic flux cancellation effect disappears. Thus, the impedance of primary coil inserts into the circuit to restrict the fault current and voltage sag phenomena. Once the fault is removed, the bridge switch turnsonagain, and the BSFC-NSFCL recovers to the normal operation. The modified NSFCL is more cost-effective and easy to be implemented than the flux-coupling superconducting fault current limiter. Theoretical analysis for the BSFC-NSFCL has been fully developed and proven by MATLAB software. Finally, the analytical, simulated, and experimental results have been carried out to verify the feasibility and performance of the proposed BSFC-NSFCL. [ABSTRACT FROM AUTHOR]

In this paper, a static synchronous compensator (STATCOM) operation scheme of the modular multilevel converter adopting clamp double submodules (CDSM-MMC) during a pole-to-pole dc fault is proposed. By analyzing the current paths of a CDSM, this paper proposes three new operation states of the CDSM, which allow the CDSM to provide bipolar voltages when the arm current that flows through it is negative. Through control of the CDSMs to operate in these new states along with the blocked state, the arms of the CDSM-MMC can operate alternately in two different modes, that is, the conducting mode and blocked mode even when the dc-link voltage drops to zero. Thus, the CDSM-MMC is capable of working as a STATCOM during a dc fault. Moreover, a control strategy of the dc-fault STATCOM operation mode is designed to control the reactive power provided by the CDSM-MMC, to limit the dc fault current, and to reserve and balance the energies stored in capacitors. Simulation results conducted in PSCAD/EMTDC demonstrate the validity of the proposed STATCOM operation scheme and control strategy of the CDSM-MMC during a pole-to-pole dc fault. [ABSTRACT FROM AUTHOR]

This paper presents an economic study to find the switches that should participate in a switch upgrade plan, based on the results of reconfiguration. The traditional reconfiguration problem trend considers only one loading condition (mostly maximum demand), in order to find the optimum reconfiguration. However, as the distribution loading condition has hourly and daily variations, the optimum configuration continuously changes. Consequently, considering only one loading condition might lead to inefficiency of the results. In this paper, daily load curves of different types of distribution consumers, during various types of days (weekdays and holidays) and seasons (summer and winter), are used to obtain the best reconfiguration hours during a day. Then, genetic algorithm (GA) is used to obtain the optimum configuration during each time interval. The objective function applied to GA consists of loss and energy not supplied. The switches that contribute to reconfiguration should be remotely controlled in order to have the capability of immediate mode alteration. In order to evaluate the feasibility of automated switch installation, the benefit-to-cost ratio is calculated. The entire procedure is applied to a test distribution system, and the results are discussed in this paper. [ABSTRACT FROM AUTHOR]

This paper presents a breaker arrangement concept, the multiline breaker (MLB), for the protection of multiterminal high voltage dc (MTdc) networks. Based on the design of a hybrid breaker, the MLB is an economically attractive solution for the protection of multiple dc lines in nodal connection using a single main breaker path. By using commutation units, the MLB directs the fault current through the main breaker in a unidirectional way, irrespective of the fault location. Hence, this study presents the design requirements for the MLB, regarding both hardware and control, and evaluates its operation within a grid. For this reason, a four-terminal half-bridge multilevel modular converters-based MTdc grid in radial configuration was used and pole-to-ground dc fault conditions were investigated. The dc fault response of the grid with one MLB at the central node is compared to the respective response of the grid when one hybrid breaker is employed at each dc line. The simulations show that the MLB is feasible and that the overall MTdc grid fault response for the two protection systems is very similar. As a result, the design advantages of the MLB make it a promising solution for the dc fault isolation in MTdc grids. [ABSTRACT FROM AUTHOR]

The modular multilevel converter-based unified power flow controller (MMC-UPFC) is able to operate under unbalanced grid conditions with symmetric component decoupling. However, the constraint of the voltage limit of UPFC is not considered and no protection schemes are investigated to protect the UPFC from overmodulation under unbalanced grid conditions. To solve this problem, this paper proposes the cascaded control scheme for MMC-UPFC based on voltage limit control and symmetric component decoupling to balance the ac current of the transmission line. With appropriate transformer connections for MMC-UPFC, the negative- and zero-sequence currents are suppressed by the corresponding inner current loops. Considering the voltage limit of MMC, the operating ranges of UPFC-MMC under balanced and unbalanced grid conditions are investigated in both mathematical derivation and point scanning methods. The results are depicted and analyzed in 3-D vision. Based on the analysis of operating regions, the voltage limit control is proposed to protect the MMC from overmodulation and to maximize the controllable region under unbalanced grid conditions. Finally, the final cascaded control structure is constructed. The simulation results obtained in PSCAD/EMTDC are provided to validate the effectiveness and robust performance of the proposed control strategies. [ABSTRACT FROM AUTHOR]

This paper studies the dc fault development stages and analyzes the fault dynamics in a point-to-point multilevel modular converters (MMC)-based dc connection. First, the effect of the dc grid configuration on the normal operation was investigated by comparing an asymmetric monopole with metallic return and a symmetric monopole. Then, the main parameters that affect the dc fault response of a grid, such as the fault type, impedance and converter blocking, were examined. Compared to previous studies, which are based on simulation results, the analysis is performed hereby both in theory, deriving the equations that describe the dc fault stages, as well as using experimental results obtained in the designed laboratory setup. The setup consists of two MMC terminals connected to two ac sources representing independent ac grids. These terminals are connected using a simple dc link based on pi-section equivalent of dc cables. The obtained results, which verified the theoretical analysis, can be used to determine the protection function thresholds of the MMC, as well as to estimate the developed stresses on dc lines during fault conditions and to define the design requirements for dc breakers. [ABSTRACT FROM PUBLISHER]

High-voltage direct current (HVDC) grids will require the development of dc protections that provide fast fault isolation and minimize the disturbance caused to the existing ac power networks. This paper investigates how the dc fault recovery performance of a half-bridge modular multilevel converter (HB-MMC) is impacted by different dc protection design choices. An HB-MMC point-to-point HVDC system that is protected with dc circuit breakers (CBs) is simulated on a real-time digital simulator using detailed switch models of the converters and switch gear. A dc CB controller has been developed and implemented in a software-in-the-loop fashion, and has been made available free for download. A novel blocking scheme for the HB-MMC is proposed, which limits the prospective dc-side fault current, benefiting dc switch gear. A comparison of circulating current controllers shows that the standard d —q controller is likely to be unsuitable for fault studies. Finally, benchmarking shows that a 48% reduction in power-flow recovery time and a 90% reduction in the energy dissipated in the circuit breaker can be achieved, along with other benefits, depending on the protection design. [ABSTRACT FROM PUBLISHER]

The ground return current of a high voltage direct current system will intrude the transformers adjacent to the grounding electrode in the case of monopole operation, leading to the half-cycle saturation of transformers. To mitigate the adverse effect, placing dc blocking devices (BDs) in transformer neutrals is popularly advocated. The conventional switching method of BDs may redirect the dc flow from one transformer with a BD to neighboring transformers without BDs. In this case, the dc currents through the neutrals of those transformers may possibly exceed the threshold undesirably. This problem is due to the lack of guidance for dc current distribution from the global view by means of finite BDs. Thus, this paper presents an optimization approach for the switching of BDs based on the gravitational search algorithm. A mathematical model for investigating the dc global distribution is established with the equivalent resistive network of an ac system, the location distribution of substations and grounding electrodes. Then, an optimal switching scheme to minimize the adverse impact of dc bias is presented, and the principle and the concrete implementation process are detailed. The feasibility and the advantage of the proposed scheme are demonstrated by numerous comparative case studies. [ABSTRACT FROM AUTHOR]

Insulation monitoring is crucial to personal safety and equipment operation in the power distribution system. Recently, with the rapid development of distributed generators, the three-phase voltage-source PWM converter (VSC) is widely used as the bidirectional interface between the dc microgrids and the distribution networks. This paper presents the relationship between the response of insulation monitoring device (IMD) and the neutral-point voltage deviation (NPVD) of VSC, by analyzing the disturbances of the switching function and the dc-side insulation balancing. Then, a method of detecting the NPVD for grid-connected VSC is proposed, based on the dc signal injection and the Duffing oscillator. Furthermore, the proposed method takes the mean and spectrum of the neutral-point voltage as the primal characteristics. Thus, the dc signal injection is adopted to detect the mean of the neutral-point voltage, and the Duffing oscillator is adopted to determine the frequency disturbance of the neutral-point voltage, respectively. The test result confirms the existence of the neutral-point voltage deviation and verifies the availability of the proposed method. Hence, the proposed method could be used to improve the response of IMD. [ABSTRACT FROM AUTHOR]

Voltage source converter (VSC) based high voltage direct current (HVdc) transmission draws more attention in recent years for the grid connection of large-scale offshore wind farms (WF). This paper proposes nine switch converter (NSC) based configuration and control strategy for the VSC-HVdc connected offshore WFs to enhance the fault ride through operation of the system. The configuration allows having both shunt and series interfaces to the onshore grid, which allows the isolation of the faulty part of the network, continuous power delivery to the healthy portion, and significant reduction of the fault current. Reduced switch count NSC is utilized at the onshore station to provide simultaneous shunt and series compensation to the electric grid. The proposed control strategy ensures full power evacuation and the dc-link voltage regulation during network disturbances. The comparative analysis between the proposed and conventional VSC-HVdc systems carried out in MATLAB/ Simulink environment revealed the former's enhanced transient performance. [ABSTRACT FROM PUBLISHER]

Current-transformer (CT) saturation causes severe distortion in the measured current waveform which may lead to maloperation of the protective devices. This paper proposes a low-cost, power-electronic device to prevent the CT from saturation. The proposed compensator is inserted in series with the relay in the CT secondary circuit and acts as a controlled voltage source (CVS). The proposed CVS generates a time-varying voltage to cancel the voltage developed across the CT burden; therefore, the CT magnetic flux remains almost constant and undistorted during the power system transients. It will be shown that this device can precisely compensate fault current, inrush current, and other probable transient currents despite its simplicity. The proposed device can be employed to compensate the already in-service CTs connected to nondigital and digital relays. Comprehensive computer simulations are carried out to validate the effectiveness of the proposed compensator. The performance of a sample compensator of this type is validated through special high-current laboratory experiments (carried out over several hundred amperes up to 1.6 kA), and the obtained results illustrate the capability of the proposed compensator to prevent CT saturation. [ABSTRACT FROM PUBLISHER]

Without additional circuitry, the half-bridge modular multilevel converter (HB-MMC) is endangered under dc-side faults. Typically, a bypass thyristor is augmented to each HB cell to take up fault current until ac circuit breakers interrupt the dc fault. This paper proposes a dc fault protection concept for HB-MMC stations that requires insignificant extra silicon area relative to the thyristor bypass concept. Herein, bypass thyristors of typical HB cells are rearranged such that an independent modular shadow rectifier bridge (SRB) is formed. A low-loss switch assembly is utilized to immediately isolate the MMC following fault detection and the SRB suppresses the fault current by injecting a reverse dc voltage. Among several advantages, the proposed arrangement incurs insignificant losses in steady state, and in some arrangements the MMC is capable of operating in STATCOM mode briefly after fault inception to support ac grid voltage. The proposed concept may be suitable for clearing temporary faults on overhead HVdc lines. Several structural variations will be viewed and discussed. Applicability for a two-level voltage source converter will be addressed. The concept is validated by detailed numerical simulations of a ±200-kV HB-MMC station under dc fault. [ABSTRACT FROM PUBLISHER]

A challenge raised in today's power system is assessing the system protection security and dependability after the topology has been changed due to relay operation upon the occurrence of cascading faults or intentional operator switching action. This paper proposes an automated setting calculation module which could be used to review the adequacy of the distance relay settings following network topology changes. The calculation procedure is broken down into blocks which could be processed in parallel in order to improve the computation speed. A new concept called distance of impact is proposed to reduce the computational burden by conducting the calculations on a limited portion of the network affected by the topology change. The module performance is tested on a synthetic IEEE 118-bus and real-life Alberta transmission operator systems. A sensitivity analysis in the form of N-2 contingency impact on the network relay settings coordination is conducted on the test systems. [ABSTRACT FROM PUBLISHER]

This paper presents a novel algorithm to recognize magnetizing inrush condition in power transformers by analyzing average differential power signal. The algorithm is independent of the value of transformer parameters and consumed power and relies only on the waveshape properties of the average power signal. These distinctive properties are obtained from the power signal by determining its instantaneous frequency. Extensive simulation studies presented in this paper demonstrate the merits of the proposed technique in discriminating between fault and inrush cases accurately in less than a quarter cycle. The accuracy and speed of the method is unaffected by changing parameters of the transformer or for different operating conditions of the power system, since it is based on a fundamental difference between iron-core saturation and other operating conditions. Since the employed signatures are extracted from the low-frequency spectrum of the power signal, the algorithm is robust against high-frequency noise. [ABSTRACT FROM PUBLISHER]

In this paper, a novel solid-state fault current-limiting circuit breaker (SSFCLCB) based on a series resonance LC tank is proposed. Since the series configuration of the resonance structure (a capacitor and a reactor) of the SSFCLCB is invisible during normal operation mode, it shows negligible impedance in the line. In fault conditions, the SSFCLCB topology changes to a rectifier bridge and feeds the resonance structure with rectified ac voltage. With the rectified voltage, the series capacitor is charged and, as a result, the faulty line is opened and the fault current is seized. Hence, the suggested SSFCLCB cannot only limit the fault current but can also open the faulty line and acts as a circuit breaker. For confirmation of these aspects, simulations are performed using MATLAB software. Also, a prototype structure is designed and built for results confirmation. This experimental setup is established to prove that SSFCLCB has excellent performance in its startup, normal operation, and current-limiting/breaking conditions. The results show that the SSFCLCB has the ability to improve distribution network reliability and it can decrease the network fault current level successfully. [ABSTRACT FROM PUBLISHER]

This paper proposes a novel bridge-type solid-state fault current limiter (BSSFCL) based on one series reactor which operates in ac and dc modes. The proposed BSSFCL includes a rectifier bridge with a reactor. This reactor is used as a dc reactor in normal operation mode and as an ac reactor in fault conditions. The advantages of the proposed BSSFCL over the existing dc reactor-type FCLs are its negligible impedance in normal operation mode and its high impedance during a fault interval using simple and novel switching. In other words, during the normal operation mode, the proposed BSSFCL operates in dc mode and in the fault interval, its topology is changed to the ac mode. This switching decreases the switching transient recovery voltage and introduces considerable impedance during the fault period. MATLAB/Simulink software is used for simulations and a prototype is designed and tested for results verification, and to show the performance of the proposed BSSFCL. [ABSTRACT FROM PUBLISHER]

The modular multilevel converter (MMC) has become one of the most promising converter topologies for medium/high-power applications. Since the MMC is structured based upon stacking up a number of series-connected identical submodules (SMs), to improve its fault tolerance and reliability, SM failure detection and location is of significant importance. In this paper, the impact of open-circuit switch failures of the SMs on the operation of the MMC is analyzed. Based on the analysis under SM failure conditions, two SM failure detection and location methods are proposed, that is, a clustering algorithm (CA)-based method and a calculated capacitance (CC)-based method. In the proposed CA-based method, a pattern-recognition-based fault diagnosis approach, which employs the clustering algorithm to detect and locate the faulty SMs with open-switch failures through identifying the pattern of 2-D trajectories of the SM characteristic variables, is developed. The proposed CC-based method is based on the calculation and comparison of a physical component parameter, that is, the nominal SM capacitance, and is capable of failure detection, location, and classification within one stage. The performance of the proposed failure detection methods for an MMC system is evaluated based on time-domain simulation studies in the PSCAD/EMTDC software environment. The reported study results demonstrate the capabilities of the two proposed methods in detecting and locating any SM failure under various conditions accurately and efficiently. [ABSTRACT FROM PUBLISHER]

This paper presents a novel mitigation technique to reduce the effects of geomagnetically induced currents (GICs) on high-voltage power systems. The method consists of connecting switching devices at the neutral grounding connection point of transformer banks. Only one transformer bank needs to be grounded through a switch to reduce GIC in a two-terminal system. For multiterminal systems, n-1 switches are necessary and the operation is independent from each other. The switching frequency and the duty cycle are selected from a tradeoff between the effectiveness of the method and the detection of fault currents. Transient simulations on a two-bus 230/500 kV system show that the proposed switching at the neutral successfully mitigates the GIC. This proposed technique opens the door to a new family of GIC mitigation methods. [ABSTRACT FROM AUTHOR]

The high voltage direct current transmission system based on modular multilevel converter (MMC-HVDC) technology is now studied and used widely. But traditional half bridge topology of sub module (HBSM) of MMC can't block dc link fault current because of the freewheeling effect of diodes. In order to solve this problem, firstly, this paper improves HBSM topology, a series connected double sub module (SDSM) is designed and dc link fault current blocking capability is realized without tripping ac circuit breaker, so MMC-HVDC with this improved topology can immediately rebuild dc link voltage and resume power transmission. Secondly, the self starting and fault disposing procedure are also designed. At last, simulations in the software PSCAD/EMTDC are carried out to verify its effectiveness and feasibility. [ABSTRACT FROM AUTHOR]

This paper compares how a dc fault affects a multiterminal dc (MTdc) network depending on the HVDC transmission system topology. To this end, a six-step methodology is proposed for the selection of the necessary dc fault protection measures. The network consists of four voltage-source converters converters radially connected. The converters natural fault response to a dc fault for the different topologies is studied using dynamic simulation models. For clearing of the dc faults, four different dc breaker technologies are compared based on their fault interruption time, together with a current direction fault detection method. If necessary, the converters are reinforced with limiting reactors to decrease the peak value and rate of rise of the fault currents providing sufficient time for the breakers to isolate the fault without interrupting the MTdc network operation. The study shows that the symmetric monopolar topology is least affected by dc contingencies. Considering bipolar topologies, the bipolar with metallic return exhibits better fault response compared to the one with ground return. Topologies with ground or metallic return require full semiconductor or hybrid breakers with reactors to successfully isolate a dc fault. [ABSTRACT FROM AUTHOR]

In Nordic countries, distribution networks are traditionally unearthed and increasingly compensated. For such networks, switching their neutral point to the earth is practically applied through a resistor for better selectivity functions of earth faults. In this paper, the neutral switching and, consequently, an arrival time of the aerial mode traveling wave reflected from the fault point are utilized to accurately determine the earth fault distance. A concept to create traveling waves is implemented by earthing the neutral via a controlled thyristor that provides a short period of high fault current and produces traveling waves to estimate the fault distance. Much higher transient signals are generated by earthing through an opposite charged capacitor. A capacitor-resistor divider is utilized to measure the reflected surge over a heterogeneous distribution feeder. An adaptive setting is proposed for stamping the arrival surge. The results provide evidence of the efficacy of the proposed fault distance estimation. [ABSTRACT FROM AUTHOR]

The applications of power semiconductors in circuit-breaker (CB) technology can help achieve considerable improvement in its performance and possible new capabilities. In this paper, new trends in power electronics for the applications in CBs are presented. It also summarizes and reviews the appropriate hybrid mechanical-static CB topologies. Various conventional and derived topologies for ac as well as dc applications are described. Discussions about common and future trends in this technology development are presented. This study will provide a useful framework and point of reference for the future development of hybrid CBs for various different applications. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC) has become the most promising converter technology for high-voltage direct current (HVDC) transmission systems. However, similar to any other voltage-sourced converter-based HVDC system, MMC-HVDC systems with the half-bridge submodules (SMs) lack the capability of handling dc-side short-circuit faults, which are of severe concern for overhead transmission lines. In this paper, two new SM circuit configurations as well as a hybrid design methodology to embed the dc-fault-handling capability in the MMC-HVDC systems are proposed. By combining the features of various SM configurations, the dc-fault current path through the freewheeling diodes is eliminated and the dc-fault current is enforced to zero. Several MMC configurations based on the proposed hybrid design method and various SM circuits, that is, the half-bridge, the full-bridge, the clamp-double, and the five-level cross-connected SMs, as well as the newly proposed unipolar-voltage full-bridge and three-level cross-connected SMs, are investigated and compared in terms of the dc-fault-handing capability, semiconductor power losses, and component requirements. The studies are carried out based on time-domain simulation in the PSCAD/EMTDC software environment for various SM configurations and dc-fault conditions. The reported study results demonstrate the proposed hybrid-designed MMC-HVDC system based on the combination of the half-bridge and the proposed SM circuits is the optimal design among all evaluated systems in terms of the dc-fault-handing capability, semiconductor power losses, and component requirements. [ABSTRACT FROM PUBLISHER]

This paper describes a method to apply a generalized systematic approach, useful to identify the faults' effect on load points' reliability accounting for intentional islanding, with either random or sequential Monte Carlo simulation (RMCS, SMCS) in order to assess distribution system reliability in smart grids. Moreover, a simple RMCS based on an annual branch fault probability, which correlates with the branch fault frequency, is described as well. A study case is presented to validate the results (in terms of expected reliability indices) of the proposed method in comparison with the analytical one. Further, other useful information, such as the best and worst results as well as the probability distribution of load-point annual outages number and duration is presented too. Finally, the results show that islanding positively affects reliability and then it could became a standard practice provided that the main technical issues are addressed. [ABSTRACT FROM PUBLISHER]

This paper proposes an LCL dc-dc converter concept which is capable of achieving very high stepping ratios with megawatt-level power transfers. The converter can find potential application in connecting high-power dc sources to high-voltage dc transmission including future dc transmission networks. This converter is based on two ac/dc insulated-gate bipolar transistor-based converters and an internal passive LCL circuit without internal ac transformers. The LCL circuit is designed to enable voltage stepping without any reactive power circulation and with potentially soft switching operation which minimizes the switching losses. The designed converter has the ability to achieve current regulation even under extreme external dc faults and, therefore, the converter can operate through dc faults. The switch utilization is better than similar topologies and losses are reasonably low. A dual-active-bridge transformer-based converter design is presented to compare with the proposed LCL converter. A detailed PSCAD model confirms conclusions using a 100-MW 20-kV/300 kV test system. An LCL 200-W 20-V/100-V dc/dc prototype converter is built to validate the proposed topology. [ABSTRACT FROM AUTHOR]

A high-voltage direct current system using modular multilevel converter (MMC-HVDC) is a potential candidate for grid integration of renewable energy over long distances. The dc-link fault is an issue MMC-HVDC must deal with, especially for the nonpermanent faults when using overhead lines. This paper proposed a protection scheme to implement fast fault clearance and automatic recovery for nonpermanent faults on dc lines. By employing double thyristor switches, the freewheeling effect of diodes is eliminated and the dc-link fault current is allowed to freely decay to zero. Then, the dc arc can be naturally extinguished and the insulation on the short-circuit point can be restored. The thyristor switches convert the dc-link fault into an ac short circuit of the ac grid through MMC arms. The ac short-circuit current can be cleared simply by turning off all thyristor switches. Since circuit breakers are not tripped during fault clearance, MMC can immediately and automatically rebuild the dc-link voltage and restart power transmission. Simulation results using PSCAD/EMTDC have verified the validity of the proposed protection scheme. [ABSTRACT FROM AUTHOR]

In this paper, a new noncommunication-based scheme is presented for the protection of three-terminal transmission lines. Using this scheme, simultaneous operation of circuit breakers (CBs) at all terminals is provided for all internal faults. This scheme can discriminate between external and internal faults based on initiated transients by faults, considering the high-stop filtering property of busbars through modeling their stray capacitances and employing a newly defined mathematical morphology (MM) filter to analyze the captured transients. Moreover, the transients generated by lightning or switching are identified and prohibited to enter the algorithm. The presented scheme has been applied on one of three-terminal lines in Iran's Transmission Grid. The effects of fault resistance, fault inception angle, (internal) fault location, and fault type on the performance of the proposed scheme are evaluated, where the high dependability, accuracy, and speed are verified. [ABSTRACT FROM AUTHOR]

This paper designs and implements an expert system adopting a newly proposed adaptive load transfer strategy and inference methodology as an integrated problem-solving solution of determining an appropriate load transfer strategy when diverse events occur in a distribution system under heavy load or severe overload conditions. In particular, to enhance the efficiency of search, the proposed inference engine is designed so that a solution for the load balancing and a solution for the minimization of the outage load can be inferred at the same time by searching the goal nodes and the fail nodes based on the local minimum tree search method. [ABSTRACT FROM AUTHOR]

This paper presents an innovative generalized systematic approach and related analytical formulation to evaluate distribution system reliability in smart grids where islanded operation of microgrids helps to improve local and overall reliability. In order to do this, the analytical formulation involves the adequacy calculation of conventional and renewable distributed generators supplying microgrids by using probabilistic models. Adequacy is computed by means of a new general analytical expression which takes into account load shedding (user load disconnection) and curtailment (user load reduction) policies. [ABSTRACT FROM PUBLISHER]

Automation is acknowledged by distribution utilities as a successful investment strategy to enhance reliability and operation efficiency. However, practical approaches that can handle the complex decision-making process faced by decision makers to justify the long-term financial effects of distribution automation have remained scarce. An automated and remote-controlled sectionalizing switch play a fundamental role in an automated distribution network. This paper introduces a new optimization approach for distribution automation in terms of automated and remotely controlled sectionalizing switch placement. Mixed-integer linear programming (MILP) is utilized to model the problem. The proposed model can be solved with large-scale commercial solvers in a computationally efficient manner. The proposed sectionalizing switch placement problem considers customer outage costs in conjunction with sectionalizing switch capital investment, installation, as well as annual operation and maintenance costs. The effectiveness of the proposed approach is tested on a reliability test system and a typical real size system. The presented results indicate the accuracy and efficiency of the proposed method. [ABSTRACT FROM PUBLISHER]

This paper addresses the subject of reducing outage durations and autorestoration applications within the distribution substation. Employment of intelligent electronic devices (IEDs) can aid in the reduction of the fault duration time, and also it enhances the protection, control, and monitoring at the distribution level. Further, communication among these IEDs can provide faster autorestoration, remote data analysis, event recording, etc. Many recent issues related to distribution substation protection are discussed and techniques to address these issues using current technologies are proposed. [ABSTRACT FROM AUTHOR]