Current-limiting Reactors Research Articles

Resilient Protection of Medium Voltage DC Microgrids Against Cyber Intrusion

Differential current protection has been a reliable approach for line protection in DC systems. However, due to its dependence on communication for terminal current information, resiliency of such a protection approach is poor against external cyber intrusions (ECI) of different types, i.e., false data injection (FDI) and time synchronization attacks (TSA). As a result of which, the system is prone to collapse due to line tripping in the presence of coordinated ECIs. The scope of paper is to propose a resilient protection approach for medium voltage DC (MVDC) microgrids differentiating between the real faults and the ECIs in a system. The approach also detects ECIs on an adjacent line occurring simultaneous to a real fault where the system deploys current limiting reactors (CLR). Phase-modal transformation is carried out to obtain the mode voltages (line and zero) across these CLRs. The mode voltages across the faulty mode networks are utilised along with differential current protection to propose a resilient protection approach against ECIs, which is sensitive to high impedance faults (HIFs). Further, a sensitivity function is formulated which gives the variation of decisive variables i.e., mode voltages with respect to change in decisive parameters i.e., fault resistance and fault location.

Research on Comprehensive Active Current- Limiting Control Strategy Applied in Two-Port MMC-HVDC Transmission System

Aiming at the defect that the current-limiting strategy based on the modular multilevel converter (MMC)-high-voltage direct current (HVDC) system only focuses on the unilateral ac side or dc side, this article proposes a current-limiting strategy that is suitable for both ac-side ground fault and dc-side short-circuit fault. First, for the ground fault on the ac side, the outer loop controller with power state equation and the inner loop controller with passivity-based control (PBC) strategy are used to control it, and the genetic algorithm, which takes the absolute value of the difference between the reference current and the actual current as the fitness function, is used to fit the optimal injection damping parameters for the PBC method. Then, for the short-circuit fault on the dc side, the capacitor discharge duty ratio is introduced on the basis of the equivalent capacitance without considering the voltage drop, and its segmented control is combined with the current-limiting reactors to achieve current-limiting at different fault points. Finally, a two-port +500-kV HVDC transmission system was built on MATLAB/Simulink software, and the effectiveness of the current-limiting method proposed in this article was verified.

Multiterminal DC Fault Identification for MMC-HVDC Systems Based on Modal Analysis—A Localized Protection Scheme

For the purpose of dc fault identification in multiterminal modular multilevel converter (MMC) high-voltage dc (HVDC) systems, the proposed work addresses the issues of localized protection schemes. The proposed work is focused on selectivity issues, such as differentiating between forward external and internal faults, and the classification of the type of fault contingency, i.e., pole to pole (PTP) or pole to ground (PTG) in the system. The scheme uses an equivalent network of multiterminal MMC-HVDC systems for dc fault identification. Modal transformation is used to analyze line-mode and zero-mode voltages across the current-limiting reactor (CLR) for different possible contingencies in the system. A maloperation region has been defined to address the issue of selectivity of internal HIFs and external low-impedance faults (LIFs). The rate of change of dc voltage ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$dU_{\mathrm{ dc}}$ </tex-math></inline-formula> <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">/dt</i> ) is used as the mitigation technique to differentiate a HIF as high as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1000 \Omega $ </tex-math></inline-formula> . Furthermore, a sensitivity function is formulated, which gives the variation of decisive parameters i.e., line-mode voltage and zero-mode voltage with respect to change in fault resistance and value of CLR. PSCAD-/EMTDC-based simulations are used to validate the performance of the proposed scheme. The scheme is validated in terms of detection time, robustness to WGN in measurement, security for ac faults, dc circuit breaker (DCCB) operation, and power reversal (load transients).

Resonance based fault location for DC photovoltaic integration system

Photovoltaic (PV) power DC integration systems can be applied in remote areas in which conventional AC cable integration is inhibited by PV collection and transmission distance. However, unpredictable DC fault transients create challenges for traditional fault location methods. To address this issue, a high-frequency resonance estimation based fault location technique is proposed in this paper. Series resonance is generated by discharging the DC capacitor through a current-limiting reactor, which weakens the attenuation of high-frequency signals at the line boundary. The relationship between resonance current and fault distance can then be obtained by analyzing the fault current loop, which is immune to the states of the converter. Compared with the traditional fault location method, the proposed method does not require additional high-frequency injection sources and very high sampling frequency. A flexible DC photovoltaic integration system is also built in PSCAD/EMTDC and the simulation results verify the accuracy of the proposed location method.

A kind of DC fault current-limiter with instantaneous by-pass of current-limiting reactor

With the rapid development of DC power grids, the protection of DC power grids is particularly important. Therefore, this paper proposes a kind of DC fault current-limiter with instantaneous by-pass of current-limiting reactance, analyses its working principle and theoretical calculations, and verifies the correctness of its principle through simulation experiments. At the same time, the experimental results show that the current-limiter proposed in this paper can not only instantaneously and greatly limit the fault current when a short-circuit fault occurs in the power grid, but not affect the normal operation of the power grid. A new device is provided for DC gird fault current-limiting protection.

К вопросу значимости влагостойких свойств бетона пристроительстве токоограничивающих реакторов подстанции

Switchgears are an integral part of the energy industry of the Republic of Kazakhstan, which include substations, electrical installations, power transmission lines and other facilities. Currently, they are undergoing physical and moral wear and tear. The growth of production capacities of industry and the population emphasizes the need for intensive development of the energy complex using the technologies of the XXI century, while maintaining a technological balance between traditional energy and renewable energy sources [4]. The concept of development of the fuel and energy complex of the Republic of Kazakhstan until 2030 fully reflects these tasks and development approaches. In addition, the construction and reconstruction of distribution networks are due to the growth of a total load of consumers, which, even now, continues to increase rapidly. Concrete used in construction is an integral part of all types of construction and installation work. Thus, the concrete mixture used in the construction of energy facilities must have special properties and moisture-resistant characteristics, allowing you to solve specific energy problems, which include: reliability, durability, low electrical conductivity and other characteristics of concrete structures. High indicators of the above requirements are imposed in the places of installation of current-limiting reactors at electrical substations.

An Improved DC Fault Protection Scheme Independent of Boundary Components for MMC Based HVDC Grids

For modular multilevel converter (MMC) based DC grids, current-limiting reactors (CLRs) are mainly employed to suppress the fault current and provide boundary effects to detect internal faults. Thus, most existing protection schemes are highly dependent on the larger CLRs to guarantee high selectivity. However, in existing MMC based HVDC projects, the size of CLRs is restrained by the cost, weight, and system stability under normal state. Thus, boundary protections may fail to detect high-resistance faults and pole-to-ground faults under weak boundary conditions. To overcome these shortcomings, this paper proposes a fast and selective DC fault detection scheme independent of boundary components. The propagation characteristics of line-mode backward traveling-waves (TW) are analyzed to identify external and internal faults. The polarities of zero-mode backward TWs are employed to select faulted poles. The directional overcurrent based pilot protection is adopted as a complementary criterion to detect remote faults. The proposed method can be applied in MMC-HVDC systems with small CLRs that cannot provide strong boundary conditions. Besides, the detection speed is fast (less than 1.5ms). Moreover, it is robust to fault resistance and immune to noise. Various simulation results in PSCAD/EMTDC verifies the effectiveness of the proposed scheme.

A Single-End Protection Scheme for Hybrid MMC HVDC Grids Considering the Impacts of the Active Fault Current-Limiting Control

In the hybrid modular multilevel converter (MMC) based high voltage direct current (HVDC) systems, the fault current can be actively suppressed by the converter itself, which endows a smaller requirement for current-limiting reactors (CLR) and a larger time margin for fault detection algorithms, comparing with the half-bridge MMC. But the robustness to fault resistance and noise disturbance of existing boundary protection schemes will be deteriorated with small CLRs. Moreover, the fast response of the fault current-limiting control will change the output DC voltage of hybrid MMC, which affects the fault characteristics and may cause mal-operation of existing protection algorithms. Thus, a single-end protection scheme considering the impacts of the active current-limiting control is proposed for the hybrid MMC based DC grids. The traveling-wave characteristics under different fault stages are analyzed to evaluate the impacts of the fault current-limiting control. In addition, a coordination protection strategy versus different fault conditions is adopted to improve reliability. Various cases in PSCAD/EMTDC are simulated to verify that the proposed method is robust to fault resistance, fault distance, power reversal, AC faults, and immune to noise.

UPFC fault ride-through strategy based on virtual impedance and current limiting reactor

When a fault occurs on the line equipped with Unified Power Flow Controller (UPFC) based on Modular Multilevel Converter (MMC), it may cause the blocking of MMCs and then make UPFC out of operation. To avoid the non-function of UPFC under the condition of external fault, a UPFC fault ride-through (FRT) strategy is proposed based on the analysis of fault characteristic. The FRT strategy is realized by adding virtual impedance (VI) loop in the Shunt Side MMC control process and connecting a Current Limiting Reactor (CLR) on the DC bus to limit the valve-side current of shunt side and the fault current fed through the DC bus from the Series Side MMC. Furthermore, the evaluating indicators and the value selected method for VI and CLR are provided, and the values of VI and CLR are discussed. Finally, simulation results based on PSCAD/EMTDC verify the fault characteristic and the effect of proposed FRT strategy.

An Adaptive Reclosing Scheme for MMC-HVDC Systems Based on Pulse Injection From Parallel Energy Absorption Module

Modular multilevel converter (MMC) based high voltage direct current (HVDC) transmission system is one of the important development trends of DC power grid. After fault isolation, the traditional reclosing method of MMC-HVDC system is directly reclosing DC circuit breaker (DCCB). Once the fault point still exists, DCCB will be tripped again. This method will make the DC system suffer from the secondary strike of fault in a short time, and seriously endanger the service life of DCCB. In order to overcome the drawbacks of the traditional reclosing schemes, this paper proposes a novel adaptive reclosing scheme based on pulse injection from parallel energy absorption module. When dc fault current is interrupted by DCCB, the fault energy stored in current-limiting reactors (CLR) and dc lines is transferred to energy absorption module automatically. By switching on IGBTs embedded in absorption module for a short time, a voltage pulse is injected directly into the faulty line without interference from CLR. Then the nature of fault is identified and the fault distance can also be calculated in the case of permanent faults. The simulation results verify the effectiveness of the proposed reclosing scheme.

Lifetime estimation for hybrid HVDC breakers

As an important aspect of breaker reliability analysis, the lifetime of a hybrid high-voltage direct current (HVDC) breaker is estimated in this paper. Based on the equivalent thermal network, the static temperature profiles of the breaker components during normal operation are solved for using an iterative method, and the components’ transient temperature profiles during short-circuit faults are computed using an infinitesimal method. Then the computed temperatures are verified via finite element analysis in COMSOL Multiphysics. Considering the influence of aging on the thermal resistances of insulated gate bipolar transistor (IGBT) modules, the lifetimes of the press-pack IGBT modules and arresters in the breaker are estimated using lifetime models and cumulative damage models. In case studies, the weakest link in the breaker and the influences of the fault location, the reclosing and the current-limiting reactor on the breaker lifetime are discussed. The results show that the arresters are the weakest breaker components, and that the fault location, reclosing and current-limiting reactor all affect the breaker lifetime.

A fault ride through strategy of unified power flow controller and its coordination with protection

When a fault occurs on the line equipped with Unified Power Flow Controller (UPFC) based on Modular Multilevel Converter (MMC), it may cause the blocking of MMCs and then make UPFC out of operation. To avoid the non-function of UPFC under the condition of external fault, a UPFC fault ride through (FRT) strategy, which can convert UPFC to Static Synchronous Compensator (STATCOM), is proposed. The FRT strategy is realized by connecting the Current Limiting Reactor (CLR) on the DC bus to limit the fault current flowing through the DC bus, which is good for the shunt side MMC being unblocked. On the other hand, the FRT strategy may cause the series side MMC be unblocked under some fault cases, which has adverse effects on the distance protection due to the vibration of the measured impedance. To solve this problem, the coordination of the FRT strategy of UPFC with the distance protection is provided, the High Voltage Breaker (HVB) across the series transformer of UPFC is forced to close if the series side MMC is unblocked during FRT, thus the performance of the distance relay is guaranteed. Simulation results based on PSCAD/EMTDC verify the proposed FRT strategy and coordinationi scheme.

A DC fault identification scheme for multi-terminal VSC-LVDC system based on voltage of current-limiting reactor

The rapid and effective identification of DC faults is one of the key technologies for the development of multi-terminal DC (MTDC) system based on voltage source converter (VSC). The DC system fault current is characterized of high increasing speed and wide influence range. At present, the peak current and the rising speed of the fault current can be limited by current-limiting circuits, which are serially connected at both ends of the DC line. In this paper, the DC fault characteristics of a three-terminal DC system with DC line current-limiting reactors are analysed. On this basis, a fast identification method for DC faults in MTDC system according to the voltage across the line current-limiting reactor is proposed. The proposed scheme can quickly identify the fault location, the fault type and the fault pole. Finally, the effectiveness of the proposed fault detection scheme is verified by simulations. The simulation results show that the proposed method is still effective when the fault resistance and fault distance change, and is able to realize the rapid and accurate detection of DC faults.

Specific Application Features of Current-Limiting Devices Based on High-Temperature Superconductivity in High-Voltage Electric Networks

The specal features of the use of current-limiting devices based on high-temperature superconductivity (HTSC CLD) in high-voltage electrical grids are presented. Separate installation of an HTSC CLD and jointly with a current-limiting reactor (CLR) is considered, and the advantages and disadvantages of an HTSC CLD installation were noted.

Study of the state of the issue of increasing the quality of electric energy in the power supply systems

This article examines the importance of ensuring the quality of electricity and considers the possibility of using a system for monitoring the quality of electricity in the power supply system. Based on the results of the study, it was found that the use of contactless switching devices as switches of sections of windings of power transformers allows you to abandon current-limiting reactors and resistors. This is achieved by the fact that the switching of the windings occurs at the moment the load current passes through zero.

Improvement of the Method of Calculation of Steady-State Modes of Urban Electric Networks Taking into Account Consumer Energy Sources

The method of calculation of the steady-state mode of the open city distribution electric network with small generation sources is offered. It is shown that the city electric network consists of passive and active physical elements. Passive elements include power transformers and linear regulators, overhead and cable power lines, current-limiting reactors, etc. Active elements of networks are power sources (transformer inputs of low voltage of feeding substations and distributed small generation sources) and loads, mathematical models of which can be various. Power sources in urban distribution networks of 10 kV and below are classified by type and power and can be of two kinds, viz. power feeding centers of the network and small generation sources. Consumer loads are modeled by current sources connected to network nodes. The parameters of the current sources are usually graphs of the change of the module of the actual current value and the power factor in time. The values of these parameters in single-line substitution schemes of 6–10 kV distribution networks with isolated neutral are assumed to be average for three phases. The power centers are transformer inputs of low voltage power substations of the main electric networks of 35 kV and above, equipped with digital devices of the account of the electric power connected to the automated system of control and accounting of power resources. The developed technique allows receiving the refined balanced calculation model of the steady mode of the open city network for the set time of the daily schedule. It includes the sequential distribution of the specified power of each network power supply between its loads, followed by clarification of flows and power losses in the sections, as well as voltages and actual loads in the nodes of the scheme by the overlay method.

Analysis of research and area of application of self-resetting liquid metal fuse

The article shows that the level of research to date, both in Russia and abroad, does not yet allow the development of industrial samples of liquid metal self-resetting fuses. However, the high speed and current-limiting properties of the self-resetting liquid metal fuse are of interest in continuing research. The use of the self-resetting liquid metal fuse as a current limiter competitive with current-limiting reactors received a more complete scientific substantiation and development. It has been found that a common disadvantage of all structures of self-resetting liquid metal fuses is that during the switching process the channel of the dielectric bushing of the self-resetting liquid metal fuse is subjected to arc erosion with each triggering, which reduces its switching resource. There are proposals to increase the switching resource through the use of materials more resistant to the effects of an electric arc, constructive and circuit solutions. From the point of view of speed and current-limiting properties, it is preferable to use fusible alkali metal inserts in the self-resetting liquid metal fuse. It is noted that the amount of research that has been conducted makes it possible to create a current-limiting device based on a competitive self-resetting liquid metal fuse with current-limiting reactors. To do this, it is necessary to combine the efforts of all interested industrial and scientific organizations, to create and explore a mock-up sample.

A Pilot Protection Scheme of DC Lines for Multi-Terminal HVDC Grid

Multi-terminal HVDC grid has extremely high demand for protection speed of DC lines, consequently, the sampling time window of the main protection is very short, which affects its selectivity and reliability. To make up for the insufficiency of the non-unit main protection, this paper proposes a pilot protection scheme based on the voltage polarities of current-limiting reactors at the ends of DC lines. Analysis shows that the voltage polarities of terminal reactors at the two ends of the DC lines are both positive when internal fault occurs, while they are opposite for external faults. Based on the aforementioned characteristic difference, a pilot protection criterion is built to discriminate between internal and external faults. To avoid mal-operation of the protection on healthy pole generated by the coupling effect, the polarity mutation of reactor voltage on healthy pole due to coupling effect is analyzed when single-pole-to-ground fault occurs. Based on the analysis, the fault voltage of current-limiting reactor is integrated, and its polarity can be identified based on the integration value. At last, a four-terminal modular multilevel converter (MMC)-based HVDC grid model is built in PSCAD/EMTDC, and the effectiveness of the protection scheme is verified by extensive simulations.

Research on Coordination and Optimal Configuration of Current Limiting Devices in HVDC Grids

This paper proposes a collaborative optimal configuration method of current limiting reactors (CLRs) and fault current limiters (FCLs) in the modular multilevel converter (MMC)-based high voltage direct current (HVDC) grids. The calculation methods of dc line fault currents and bridge arm fault currents considering the collaboration of CLRs and FCLs have high accuracy. The multi-objective optimization configuration model is established, in which the fault current limiting effect, the total inductances of CLRs and the number of FCLs are used as objective functions while the DCCBs current breaking capacity, the converter valves overcurrent protection, the boundary of the dc line protection area, the dynamic performance of the dc system and the converter station outlet dc voltage are used as constraints. In this paper, the multi-objective optimization algorithm is used to solve multi-objective optimal configuration problem for the CLRs and FCLs. Finally, the proposed method is tested with a symmetrical monopole four-terminal MMC-HVDC grid in PSCAD/EMTDC. The simulation results verify that the proposed optimal configuration method can ensure the continuous and reliable operation of the dc grid under dc faults.

Fast single‐end line protection method for the meshed multi‐terminal HVDC grid

In view of the short-circuit fault characteristics, the protection technique of the dc lines limits the development of the modular multi-level converter (MMC)-based HVDC grid in a large extent. This study derives the calculation formulas of the transient voltages of internal and external faults and proposes a single-end protection scheme based on the transition period (TP) from the abrupt changing point to the first extreme point of the fault voltage. Due to the current-limiting reactors, the TP of the internal fault is much shorter than that of the external fault, which is used to discriminate internal fault from external fault. A four-terminal symmetrical bipolar HVDC grid based on MMC is modelled using PSCAD/EMTDC, and simulation results demonstrate the validity of the new proposed dc transmission line protection method.