AC Circuit Breakers Research Articles

Transient characteristics and protection coordination strategy of hybrid AC / DC system based on modular multilevel converters under AC/DC transmission lines touching fault

Rapid development of hybrid AC/DC transmission systems has increased the probability of the touching faults between AC and DC overhead lines. The mechanism of coupling interaction is so complex that the adaptability of the existing hybrid system protection is affected. Previous studies mainly focus on the line-commutated converter based high-voltage direct current (LCC-HVDC) system and AC system, this paper concentrates on the fault coupling characteristics analysis and coordination protection of modular multilevel converter based on high voltage direct current(MMC-HVDC) and AC transmission systems. The analytical expressions of fault current are deduced under the different operating states of the converters and the valve-side AC circuit breaker. Subsequently, the fault current coupling characteristics and generation mechanism are revealed. On this basis, the adaptability of existing hybrid system protection is assessed. Further, by comparing and analyzing the influence and risk of mal-operation of AC transmission line protection, the coordination reclosing protection strategies for hybrid system is proposed. Finally, the simulation tests were carried out based on PSCAD/EMTDC. The simulation results show that the proposed coordination of AC and MMC-HVDC system reclosing strategy is effective for improving the reliability of hybrid system.

An Overview of DC Microgrid Protection Schemes and the Factors Involved

The development of a suitable scheme for protection of DC microgrids is still a major challenge. This is due to the shortage of practical experience in appropriate protection guidelines and standards related to the uncertainty in the network topology, absence of a natural current zero-crossing point in DC systems, and dependence of fault current shape and magnitude on various factors. These factors include network topology, type of converters used in the network, type of fault and its location, fault impedance, and type of system grounding. In this paper, these factors and their impacts on the development of a comprehensive protection plan for DC microgrids have been investigated. Various algorithms that have been proposed in previous studies for fault detection and location are reviewed, and the technical challenges associated with the use of communication links in smart protection systems are described. In addition, the advent of new technologies for development of DC circuit breakers are reported, which are different from the conventional AC circuit breakers due to the differences between the faults in DC and AC microgrids and their respective clearance times. This comprehensive review covers all aspects of the DC microgrid protection including topology, converters, configurations, and types of faults.

The Prediction of Residual Electrical Life in Alternating Current Circuit Breakers Based on Savitzky-Golay-Long Short-Term.

In order to improve the accuracy of predicting the remaining electrical life of AC circuit breakers, ensure the safe operation of electrical equipment, and reduce economic losses caused by equipment failures, this paper studies a method based on the Savitzky-Golay convolution smoothing long short-term memory neural network for predicting the electrical life of AC circuit breakers. First, a full lifespan test is conducted to obtain degradation data throughout the entire life cycle of the AC circuit breaker, from which feature parameters that effectively reflect its operational state are extracted. Next, principal component analysis and the maximum information coefficient are used to remove redundancy in the feature parameters and choose the best subset of features. Subsequently, the Savitzky-Golay convolutional smoothing algorithm is employed to smooth the feature sequence, reducing the impact of noise and outliers on the feature sequence while preserving its main trends. Then, a secondary feature extraction is performed on the smoothed feature subset to obtain the optimal secondary feature subset. Finally, the remaining electrical lifespan of the AC circuit breaker is treated as a long-term sequence problem and the long short-term memory neural network method is used for precise time-series forecasting. The proposed model outperforms backpropagation neural networks and the gate recurrent unit in terms of prediction precision, achieving an impressive 97.4% accuracy. This demonstrates the feasibility of using time-series forecasting for predicting the residual electrical lifespan of electrical equipment and provides a reference for optimizing the method of predicting remaining electrical life.

Research on circuit breaker failure protection and the secondary accelerated fault isolation scheme of VSC-HVDC grids

When the direct current circuit breaker (DCCB) fails to operate, the fault range will further expand, endangering the safe operation of the power gird. Therefore, this study proposes a DCCB failure protection method and a secondary accelerated fault isolation scheme of voltage source converter-based high-voltage direct current (VSC-HVDC) grids. On the basis of considering the influence of DCCB disconnection, characteristics of fault current after the failure of the DCCB are analyzed, and a circuit breaker failure protection method based on “increase current” discrimination is proposed. By analyzing the logical relationship among failed breaker re-tripping, adjacent DCCB action, blocking of the converter station, and AC circuit breaker action, a secondary accelerated fault isolation scheme is proposed to minimize the fault isolation area. The scheme achieves the accelerated isolation of DC faults after the failure of the DCCB and avoids the trip of the AC circuit breaker effectively. Simulation results verify the effectiveness of the proposed scheme.

Fault Detection and Zonal Protection Strategy of Multi-Voltage Level DC Grid Based on Fault Traveling Wave Characteristic Extraction

Currently, DC breakers are commonly used in mainstream protection schemes for DC grids to eliminate faults. However, the cost of high voltage DC (HVDC) breakers is high, and equipping each DC line with DC breakers is expensive. In order to minimize the number of DC breakers while ensuring the reliability of the power supply, a zonal protection strategy suitable for multi-voltage level DC grids is proposed. Subsequently, the qualitative impact of partial power interruption caused by fault DC areas is analyzed in the system. The basic zonal principle of the multi-voltage level DC grid is formulated, taking into account unbalanced power, the mode of system-level control, and the type of converters. Additionally, a time sequence coordination strategy is derived in detail based on the characteristics of DC breakers, AC/DC converters, DC/DC converters, AC breakers, high-speed switches, and other fault removal components. Finally, a seven-terminal DC grid is modeled in the PSCAD/EMTDC simulation platform. According to the simulation analysis, the DC grid can adopt the converter with fault clearing ability or an AC circuit breaker to cooperate with the fast disconnector (FD) to complete fault clearing in the DC fault area under the proposed zonal protection coordination strategy.

Intersystem fault between MMC‐HVDC and AC systems and its impact on DC/AC protection

Flexible DC and AC transmission lines can be arranged across vertically or sharing transmission corridor. Under this condition, the possibility of intersystem fault increases significantly, which endangers the safety of both DC and AC systems. Here, the intersystem fault between bipolar half-bridge MMC-HVDC system and AC system is analysed. The reliability of DC wavefront protection after intersystem fault is analysed based on initial traveling wave, while the reliability of DC intersystem fault protection and AC protection is analysed based on 50 Hz component characteristics. Since wavefront protection cannot be guaranteed to operate reliably, intersystem fault protection based on 50 Hz components is applied to DC system to reliably detect intersystem faults and isolate the fault by commanding DC circuit breaker to trip. Although current differential relaying and zero-sequence overcurrent relaying in AC system can reliably detect intersystem faults, a large DC offset prevents effective fault isolation of AC circuit breaker. This highlights the necessity of MMC-HVDC system to reliably isolate intersystem faults.

Aging Characteristics of Contact Electrodes of Low Voltage DC Switches

With the present state of the direct current (DC) distribution market, securing the safety of the DC distribution system is emerging as a major issue. Like AC distribution systems, DC switches and circuit breakers are one of the main means to ensure safety. However, in the DC system, since there is no current zero point in the load current, the phenomenon occurring when the circuit is cut off is different from that of the AC system, so technical research is required to cope with this. In this study, the aging characteristics of the contact electrode of a 400 V class low voltage DC (LVDC) switch is studied for the development of wall-mount switches or circuit breakers for residential houses. As an arc extinguishing method to break DC load current, a prototype experimental circuit breaker that uses a magnetic extinguishing method that is effective for blocking low voltage low power DC is invented, and an automated experiment system is established. The DC switch test repeats the operation of turning it on and off 13,000 times, and continuously evaluates the performance of the electric contacts by calculating the voltage drop between the electrode contacts and the corresponding Ohmic resistance value when conducting every 500 times. This paper tests six contact materials to compare the aging characteristics of them by evaluating contact resistance during the test period. AW18-Cu composite material showed the most stable and excellent contact performance for LVDC switches during the entire test operation period.

IoT Technology-Based Protection Scheme for MT-HVDC Transmission Grids With Restoration Algorithm Using Support Vector Machine

This paper proposes a protection scheme for multi-terminal high voltage direct current (MT-HVDC) transmission grids. The proposed scheme is based on three protection levels; di/dt overcurrent protection as the main protection, dv/dt undervoltage protection as a backup one protection. If the main and backup one fails to detect and clear the fault current, then the fault tripping and clearing is accomplished using the overcurrent protection by AC circuit breakers (ACCBs) in the AC grid side after a coordinated time. The tripping in the main and backup one protection levels are based on using fault isolation devices (FIDs) tripping. Support vector machine (SVM) is proposed for the fault detection and discrimination in the MT-HVDC using the backup two protection system. The Internet of things (IoT) has been used for monitoring and coordination between the AC grid and MT-HVDC grid. The proposed protection scheme is applied and verified on the MT-HVDC network under different transmission system faults using the MATLAB/Simulink <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">®</sup> environment. The results show that the proposed scheme's effectiveness as a fast and reliable scheme in fault detection and discrimination for the MT-HVDC grids.

A Selective Fault Clearing Scheme for a Hybrid VSC-LCC Multi-Terminal HVdc System

A selective fault clearing scheme is proposed for a hybrid voltage source converter (VSC)-line commutated converter (LCC) multi-terminal high voltage direct current (HVdc) transmission structure in which two small capacity VSC stations tap into the main transmission line of a high capacity LCC-HVdc link. The use of dc circuit breakers (dc CBs) on the branches connecting to VSCs at the tapping points is explored to minimize the impact of tapping on the reliability of the main LCC link. This arrangement allows clearing of temporary faults on the main LCC line as usual by force retardation of the LCC rectifier. The faults on the branches connecting to VSC stations can be cleared by blocking insulated gate bipolar transistors (IGBTs) and opening ac circuit breakers (ac CB), without affecting the main line’s performance. A local voltage and current measurement based fault discrimination scheme is developed to identify the faulted sections and pole(s), and trigger appropriate fault recovery functions. This fault discrimination scheme is capable of detecting and discriminating short circuits and high resistances faults in any branch well before 2 ms. For the test grid considered, 6 kA, 2 ms dc CBs can easily facilitate the intended fault clearing functions and maintain the power transfer through healthy pole during single-pole faults.

Studies of commutation failures in hybrid LCC/MMC HVDC systems

A hybrid of line commutated converters (LCCs) and modular multi-level converters (MMCs) can provide the advantages of both the technologies. However, the commutation failure still exists if the LCC operates as an inverter in a hybrid LCC/MMC system. In this paper, the system behavior during a commutation failure is investigated. Both half- bridge and full-bridge MMCs are considered. Control strategies are examined through simulations conducted in PSCAD/ EMTDC. Additionally, commutation failure protection strategies for multi-terminal hybrid LCC/MMC systems with AC and DC circuit breakers are studied. This paper can contribute to the protection design of future hybrid LCC/MMC systems against commutation failures.

Enhanced Control of Offshore Wind Farms Connected to MTDC Network Using Partially Selective DC Fault Protection

In recent years, several dc fault clearance schemes have emerged, in which the reduced number of fast-acting dc circuit breakers (DCCBs) and ac circuit breakers (ACCBs) are used to clear dc faults. In offshore dc grids, such approach entails the opening of the ACCBs that connect the wind farms to the offshore high-voltage dc (HVDC) stations that control the offshore ac voltages and frequencies, potentially leading to uncontrolled offshore voltage and frequency. Existing studies show that the loss of offshore converter due to blocking or sudden opening of ACCBs can cause significant overvoltage and overfrequency in the offshore ac grid, which could necessitate an immediate shutdown of the wind farm. Enhanced control for wind turbine converters (WTCs) of the offshore wind farm is proposed to enable retention of ac voltage and frequency control when the offshore converter is lost, in which seamless transition of the WTCs between grid following and forming modes is facilitated. The viability of the proposed control is demonstrated in a wider context of partially selective dc fault protection in an illustrative meshed dc grid, which includes detailed implementations of dc fault clearance, system restart, and power transfer resumption. The presented simulation results confirm the effectiveness of the proposed WTC control in preventing the excessive rise of offshore ac voltage and frequency and facilitating dc fault ride-through using the reduced number of DCCBs.

A Reliable Protection Scheme for Fast DC Fault Clearance in a VSC-Based Meshed MTDC Grid

A multi-terminal high voltage DC (MTDC) grid, is an optimum and cost-effective transmission network to minimize the energy crisis worldwide largely. However, the core demand for a fast DC protection scheme with an extraordinary strict fault clearance time of a few milliseconds, is a key research gap in this network, holding back its development and scalability. To bridge this gap, this paper proposes a novel protection scheme for a meshed MTDC grid. The main goals of the scheme include accurate discrimination of a faulty line, rapid fault detection, fault location estimation, significant fault current reduction, and fully selective isolation of only the faulted line, while continuing normal power flow in the healthy grid zones. Reliability of the scheme for long and extra-long-distance power transmission is increased by aiding the differential protection and Type-D traveling wave (TW)-based algorithms utilizing the distributed optical current sensing technology with the other auxiliary methods and backup plans. These auxiliary methods include independent discrete wavelet transform (DWT), current derivative polarity principles with a minimum sample (short time) window, overcurrent relays, and AC circuit breakers (ACCBs). A faulty segment of a transmission line is accurately discriminated from the healthy ones by measuring a series of multi-point differential currents on it. A faulty line at a particular DC node is accurately discriminated using the differential protection by measuring the current flowing into or out of each line from each side at every node to obtain the algebraic sum. The current sum of a real-time local transient data and a delayed remote data is compared to a preset threshold level. DC fault current is significantly reduced below the breakable levels by coordinating bidirectional hybrid DC circuit breakers (HDCCBs) with the active and passive fault current limiters (FCLs) and the half bridge-VSC-based modular multilevel converters (MMCs). The proposed concepts are successfully verified by the simulation results under a variety of fault scenarios and are found to be accurate.

Investigation and Numerical Simulation of a High-Current AC Circuit Breaker

The article is devoted to the study of the high-current AC circuit breaker. The results of the study are presented for various configurations of the arc divider. The study includes methods of spectral diagnostics and high-speed camera shooting synchronized with the electrical characteristics of the circuit breaker (current, voltage) in time. The obtained results allow to determine the composition of the plasma and dynamics of changes in the composition of the discharge in time. Calculation of the plasma composition and properties is made according to the obtained data, which makes it possible to take into account the products of circuit breaker materials ablation in numerical simulation. Non-stationary two-dimensional mathematical model with a moving mesh is developed. The obtained results allow to correct and verify the developed mathematical model of the circuit breaker operation. The evaluation of the arc divider influence is presented in the article.

Improving Low-Voltage DC Circuit Breaker Performance Through an Alternate Commutating Circuit

Current interruption in dc circuits is more challenging than in their ac counterparts due to the absence of natural current zero-crossing and resistive nature of the dc grids. This article summarizes the current dc breaker technologies and proposes new alternate methods for low-voltage applications. The proposed mechanisms utilize two switches, of which one generates zero crossing with an alternate oscillatory circuit for the other one, which can be a conventional ac breaker and is used in the main circuit. This is different than the conventional single-switch commute-and-absorb method used in high-voltage dc systems. It is shown that the proposed oscillatory circuit improves the fault current extinction and significantly reduces the voltage slew rate of conventional mechanical and solid-state switches. Thus, the proposed mechanism is capable of interrupting high dc currents with a minimal arc through a less expensive ac circuit breaker. Simulation and hardware results are provided to show the efficiency of the proposed breakers.

Numerical simulation of heat transfer processes of the circuit breaker contact system

The article is devoted to the study of heat transfer processes of the high-current AC circuit breaker contact system. The creation of a mathematical model is based on the application of well-known physical laws such as the Joule-Lenz and Fourier laws, which describe physical processes in the form of partial differential equations. Cooper-Mikic-Yovanovich correlation is used for evaluation of thermal and electrical contact conductance. The developed mathematical model is verified by the results of experimental studies. The calculation of the composite material properties for numerical simulation of heat transfer processes is presented. The calculation is presented for the actual design and materials of the circuit breaker.

Double-Thyristor-Based Protection for Valve-Side Single-Phase-to-Ground Faults in HB-MMC-Based Bipolar HVDC Systems

The valve-side single-phase-to-ground (SPG) faults in the half-bridge modular multilevel converter (HB-MMC)-based bipolar high-voltage direct-current (HVdc) systems induce some special fault consequences: overvoltage on the submodule (SM) capacitors in converter upper arms and nonzero-crossing currents in the grid-side ac circuit breaker (ACCB). In this letter, a protection strategy based on converter-embedded double-thyristors is proposed to address these issues. The double-thyristors are installed in parallel with each SM in the lower arms of the converter. By triggering the double-thyristors, the SPG fault is converted into a three-phase short-circuit that ensures current zero-crossings in the grid-side ACCB. Moreover, as the voltages of converter ac buses are clamped by the ground through the triggered double-thyristors, the upper arm overvoltage is mitigated. The effectiveness of the proposed protection strategy has been verified in a bipolar HB-MMC HVdc link built in PSCAD/EMTDC.

Series LC DC circuit breaker

The article proposes a mechanical DC circuit breaker (CB) based on a series LC circuit. It requires two switches (a fast disconnector and an AC circuit breaker), an inductor and a capacitor, and therefore the cost is expected to be low. A series LC circuit is analysed and it is concluded that fault current will always have natural zero-current crossings which enable use of simple AC CBs. The current commutation into a capacitor is investigated since this is important for successful operation. A number of analytical conditions are derived for the voltage stress across disconnector contacts which enable arc-less contact opening. Experimental results on a 900 V laboratory prototype LC DC CB illustrate successful DC fault clearing, with commutation of 130 A and peak DC current of 190 A. A detailed PSCAD model for 320 kV LC DC CB is developed and DC fault clearing is evaluated in order to understand the possible benefit for high-voltage direct current applications. Further comparisons with the commercialised hybrid DC CB and mechanical DC CB on 320 kV system illustrate some benefits in terms of performance and simplicity. The mechanical LC DC CB operates very fast because of early capacitor insertion, and this results in low peak current and energy dissipation.

Analysis of Single-Phase-to-Ground Faults at the Valve-Side of HB-MMCs in HVDC Systems

Although the probability of occurrence of station internal ac grounding faults in modular multilevel converter (MMC)-based high-voltage direct-current systems is low, they may lead to severe consequences that should be considered when designing protection systems. This paper analyzes the characteristics of valve-side single-phase-to-ground (SPG) faults in three configurations of MMC systems. Fault responses for symmetrical monopole MMCs are first studied. Upper arm overvoltages and ac-side nonzero-crossing currents arising from SPG faults in asymmetrical and bipolar configurations are then investigated. DC grounding using an LR parallel circuit is employed to create current zero-crossings, which will enable the operation of grid-side ac circuit breakers. The theoretical analysis is verified through simulations performed in PSCAD/EMTDC, with simulation results and the theoretical analysis showing a good agreement. The studies in this paper will be valuable for the design of protection systems for station internal ac grounding faults.

Investigation of Fault Ride-Through Capability of Hybrid VSC-LCC Multi-Terminal HVDC Transmission Systems

In this paper, clearing of DC faults in a hybrid multi-terminal HVDC transmission system consisting of line commutated converters (LCCs) and voltage source converters (VSCs) implemented using half-bridge modular multilevel converter (MMC) technology is investigated. While the hybrid HVDC system has several possible configurations, this paper focuses on two of them: 1) a half-bridge MMC-HVDC link piggy-backing on the transmission line of a LCC-HVDC link and 2) LCC-HVDC link tapped by half-bridge MMC inverters. The proposed dc fault recovery strategy employs a high rating series diode valve placed at each VSC inverter pole to block fault currents; AC circuit breakers to isolate the faulty VSC rectifier pole; and force retardation applied at LCC rectifier to extinguish the arc. Detailed simulations demonstrate fast fault recovery performance with the proposed fault recovery procedure. In the case where a single transmission line is shared by the LCC and VSC links, the VSC rectifier is subjected to considerably high current for a period of few hundreds of milliseconds and the ac side voltage dips momentarily. During a single pole fault, interrupting power flow on the healthy pole of VSC rectifier may be necessary to maintain smooth operation.

Protection and post‐fault recovery of large HVDC networks using partitioning and fast‐acting DC breakers at strategic locations

DC fault protection arrangements for a large multi-terminal HVDC (MTDC) network are proposed where fast-acting DC circuit breakers are only used at strategic locations with the large DC network to be operated interconnected but partitioned into islanded DC zones in case of any DC fault events in one of the DC zones. Each DC network zone can be protected using low cost, slow protection devices such as AC circuit breakers (ACCBs) coordinated with DC switches or slow mechanical type DC circuit breakers. This ensures the maximum ‘loss-of-infeed’ for any AC networks connected to the large HVDC system is kept within acceptable limits with reduced investment in protection cost as expensive fast-acting DC circuit breakers are kept to a minimum. A post-fault recovery method of the faulty section is proposed including the reconnection with the healthy part of the network to ensure reliable and smooth restoration of the large MTDC network. A detailed pre-fault and post-fault power flow analyses are also conducted in a MTDC network with DC voltage droop control. The proposed protection arrangements and post-fault recovery method are validated by simulation of a two-zone, six-terminal DC network with respective radial and meshed configurations.