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This paper proposes a new modulated model predictive control method for the control of modular multilevel converters (MMCs) in voltage source converter-high voltage dc systems. The proposed method retains the advantages of the conventional finite control set-model predictive control methods by programing the nonlinear properties of the MMC into the design calculations while minimizing the ac line and circulating current ripples and steady-state error by generating modulated switching signals with a fixed switching frequency. In this paper, first, the predictive modeling of the MMC is provided. Next, the proposed control method is described. Then, the application of the proposed method to an MMC system is detailed. Finally, experimental results from an MMC system connected to a three-phase grid are provided to validate the theoretical outcomes. [ABSTRACT FROM AUTHOR]

Piezoelectrically actuated fast mechanical switches provide a low-loss conduction path in hybrid circuit breakers for protecting medium-voltage direct-current systems. As the desired actuation speed approaches the driving limit of a piezoelectric actuator, excessive vibration starts to dominate its underdamped travel curves, resulting in insulation failures and delayed switching in the fast mechanical switch. To improve the underdamped travel curves into critically damped travel curves, several switching motion control schemes with active damping filters are proposed in this paper. The switching motion controllers are based on an electromechanical model of a fast mechanical switch with experimentally identified parameters. According to hardware implementation tests, the proposed closed-loop control schemes optimized the switch travel curves with a 56% reduction in the undershoot and a 75% reduction in the settling time. Equipped with the critically damped travel curves, the piezoelectrically actuated fast mechanical switch is better suited for sequential submodule operations to reduce the interruption time, the peak current and the fault energy to be absorbed in hybrid circuit breakers. [ABSTRACT FROM AUTHOR]

Different from typical circuit breakers, breakers used to switch AC filters in convertor stations have to operate frequently according to the real-time operation mode and daily load changes. Therefore, the performance of filter breakers decreases very quickly, which leads to frequent accidents. Both preinsertion resistors and controlled switching devices can suppress switching transients. In this paper, a comparative analysis of whether a controlled switching device and preinsertion resistor should be adopted is carried out using PSCAD. The transient is calculated as the combined result of the switching angle and preinsertion resistor value, and the optimal configurations are proposed. After determining the resistor value and whether a controlled switching device should be included, the reference voltage of the arrester should be reevaluated. [ABSTRACT FROM AUTHOR]

Condition assessment of Medium Voltage switchgears requires effective, but inexpensive solutions to become a common practice. On the other hand, failure of a switchgear component can cause cascade effects and outage of other apparatus, in addition to significant economic losses due, e.g., to lack of energy availability. A major diagnostic property to evaluate switchgear health and reliability is partial discharges, being a direct cause of failure of organic insulation systems. A new approach to automatic, self-assessment of switchgear reliability based on partial discharge, PD, acquisition and processing, as well as a new type of PD sensor solution are presented in this paper. The sensor is based on the capacitive divider existing in most bushings to indicate the presence or absence of voltage. The innovative automatic software is able to provide automatic noise and PD separation, noise rejection and type of PD-source identification. The embedded bushing sensor has been tested on switchgears in the presence of defective components as cable terminations and bushings. Results show that the sensor, coupled with the automatic software, is able to provide good sensitivity compared to other sensors, and good capability to locate the cabinet and identify the type of source generating partial discharges. [ABSTRACT FROM AUTHOR]

This paper presents a new method based on piecewise analytical formulas to evaluate the valve losses of the modular multilevel converter based high-voltage direct current (MMC-HVDC) transmission systems. According to the generating mechanism, the total valve losses of the MMC are divided into three parts: 1) conduction losses; 2) essential switching losses; and 3) additional switching losses. Due to the large number of arm submodules in MMC–HVDC links, the harmonic components in the arm voltages and currents are omitted. Therefore, the conduction losses and the essential switching losses can be precisely represented by analytical formulas. The additional switching losses are described by an analytical upper limit. The case studies are performed on a 400-MW\pm 200-kV MMC-HVDC system under 12 typical operating conditions. Both the relative errors of the conduction losses and the essential switching losses between the analytical and the simulated results are within \pm4%. The analytical upper limits cover almost all of the simulated results for additional switching losses. Finally, an acceptable number for the analytical method is described based on the simulation results, which proves the effectiveness of this method and clarifies the potential to take advantage of this method in engineering applications. [ABSTRACT FROM AUTHOR]

Switching of shunt capacitors can cause transient overvoltage. The voltages are traditionally evaluated with electromagnetic transients (EMT) simulation programs. It is a very time-consuming process mainly due to the need to convert a power-flow (or short-circuit) case to the EMT case. This paper proposes an idea to conduct capacitor switching studies directly inside commercial power-flow/short-circuit tools such as PSS/E, thus avoiding the data-conversion process. The idea is to conduct switching studies in the frequency domain. The frequency response of the study network is determined using the short-circuit solution engine of the host commercial software. This paper presents the theory, unique issues, and implementation details of the idea. The effectiveness of the proposed method is verified by comparing the results with those of the EMT simulations. [ABSTRACT FROM PUBLISHER]

This paper describes the application of a controlled switching device to energize a 500 kV/230 kV/46 kV, 1200 MVA autotransformer in Manitoba. The development of a detailed electromagnetic-transient (EMT) simulation model of the transformer formed the basis of the work described. The transformer model includes an accurate representation of hysteresis and remanance, which was validated using recorded waveforms and manufacturer data. This paper discusses a feasibility study conducted using this transformer model to evaluate the suitability of controlled switching to energize this transformer. A series of real-time hardware in the loop tests was conducted using a real-time simulator as part of precommissioning tests. The real-time simulation (RTS) tests and phase 1 of commissioning tests conducted in October 2014 showed that the controlled switching device would produce inaccurate residual flux estimates when it integrates the secondary voltage waveforms of a capacitive voltage transformer. Based on these RTS test results and EMT simulations, the installation of a wound potential transformer (PT) was recommended. This paper also discusses simulation studies conducted to manually program the controller until the PT was commissioned. During phase 2 of testing conducted in October 2015, a wound PT was commissioned and the controller was tested. The recorded waveforms showed good agreement with simulation results. The controller has accurately predicted residual flux and minimized inrush currents during transformer switching events. [ABSTRACT FROM PUBLISHER]

Zero-missing is a phenomenon in shunt-compensated cable systems, in which the current through the line breaker does not cross the zero point for several cycles. This paper deals with a thorough investigation on countermeasures of the zero-missing phenomenon in transmission systems and determines the requirements, benefits, and risks of applying each method. The effectiveness of countermeasures is studied on a simulated cable project with different cable lengths in an actual grid model of the Dutch 380 kV transmission system. Results are analyzed based on three criteria related to the IEC standards and the Dutch grid code. In addition, the switching sequence of circuit breakers is specified to maximize the effectiveness of the countermeasures. A statistical switching analysis is performed for the insulation coordination study since the application of some countermeasures increases the probability of high transient switching overvoltages. Moreover, the closing variation threshold of circuit breakers is calculated as a function of the circuit impedance and the shunt compensation degree. [ABSTRACT FROM PUBLISHER]

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]

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]

Recently, a modular multilevel converter (MMC) has become one of the most popular converters in medium/high-power applications. In this paper, a new technique is presented to decrease the computational time of the MMC by using switching state grouping for the predictive controller. The proposed technique has the ability to control the output current, minimize the circulating currents, and balance all capacitor voltages, simultaneously. For a predefined number of MMC submodules, the number available switching states is known. The proposed technique uses two stages of predictive modules. In the first stage, the switching states of the MMC are divided equally into a number of groups, controlled simultaneously by the same number of predictive modules. The optimal number of the switching state groups in the first stage is obtained. In the second stage, only one predictive module is used to obtain the optimal switching state of the MMC without any other complexities or additional sorting controllers. The effectiveness of the proposed control technique has been verified by using MATLAB/SIMULINK software considering steady state and dynamic analysis. The results of the proposed technique are compared with the conventional one in terms of computation time, verifying better performance of the proposed method. [ABSTRACT FROM PUBLISHER]

During the switching of disconnectors in gas-insulated switchgear (GIS), repeated strikes occur in the contact gap and lead to very fast transient overvoltage (VFTO). Severe VFTO may bring damage to transformers and other equipment. The switching speed of the disconnector is one of the important factors which influences VFTO. There are still different opinions on this factor. Very little quantitative study was done before this paper. In this paper, the whole repeated strike process associated with the switching operation of the disconnector was simulated and analyzed. The polarity and the probability characteristics of breakdown voltage were analyzed. The impacts of the switching speed of the disconnector on VFTO were studied by statistical methods. Comparisons were made between simulated and measured results. The simulation result shows that within a certain range of speed, disconnectors with a lower switching speed generate lower trapped charge voltage after opening operation and lower VFTO magnitude during the opening and closing operations. [ABSTRACT FROM AUTHOR]

An improved switching scheme for static VAr compensators (SVCs) that have fixed capacitor-thyristor-controlled reactors (FC-TCRs) for load balancing and power factor improvement has been addressed in this paper. The proposed control scheme can eliminate negative-sequence current caused by unbalanced load while improving the source power factor simultaneously. Thus total current supplied by the source can be minimized along with improvement of the load voltage profile. As the switching of TCR can introduce additional harmonics in the line, an optimized switching function is adopted in the proposed scheme for minimum injected harmonics without using an external filter. The gravitational search algorithm has been used for obtaining the optimized switching angle. The proposed scheme has been verified through proper simulations backed by suitable experimental results on a practical system. [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]

In this paper, a framework for distribution system optimization is proposed. In this framework, different control variables, such as switchable capacitors, voltage regulators, and system configuration can be optimally determined to satisfy objectives, such as loss minimization and voltage profile improvement. Linearized power-flow equations are used in the optimization, and the problem is formulated as mixed-integer quadratic programming (MIQP), which has a guaranteed optimal solution. Existing efficient solution algorithms developed for MIQP problems facilitate the application of the proposed framework. System operational constraints, such as feeder ampacities, voltage drops, radiality, and the number of switching actions are considered in the model. The performance of the proposed framework is demonstrated using a variety of distribution test systems. [ABSTRACT FROM PUBLISHER]

Shunt power capacitors are essential to system operation and reliability as a critical reactive power source to increase system capacity, support voltage magnitude, improve power quality, and reduce reactive current flow. To mitigate capbank switching transients, the synchronous switching with preinsertion resistor technology is developed and used at an extra-high voltage transmission system. In this paper, the coupling relationship between several key factors that contribute to switching transients is studied. Optimization solutions are developed to improve the switching process. Field operations with an updated setting are compared with original setting records to prove the correctness of the study. [ABSTRACT FROM PUBLISHER]

Switching operation of the disconnector in gas-insulated switchgear (GIS) results in very fast transient overvoltage in GIS pipes. In a smart substation, this process induces disturbance voltage on the port of the intelligent component fixed on GIS pipes or placed in the local cabinet nearby, which then may lead to the failure of the equipment. The experimental results of the port disturbance voltage during the switching operation via a 1100 kV test circuit are first reported in this paper. The test arrangement, including both 70 opening operations and 70 closing operations, had been performed. The influences of different grounding schemes of cable shield on port disturbance had been investigated. Three different cases of the terminal load had also been considered: open circuit, short circuit, and 50 Ω. Over 100 000 micro-pulses had been extracted from the disturbance voltage waveforms. The waveform characteristics including peak voltage, rise-time, oscillation frequency, and decaying had been obtained by statistical analysis of these micro-pulses. Furthermore, the differences between measured waveforms and the IEC 61000-4-18 immunity test standard waveform had been discussed. [ABSTRACT FROM PUBLISHER]

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]

A control technique is proposed in this paper for control of modular multilevel converters (MMC) in high-voltage direct current (HVDC) transmission systems. Six independent dynamical state variables are considered in the proposed control technique, including two ac currents, three circulating currents, and the dc-link voltage, for effectively attaining the switching state functions of MMCs, as well as for an accurate control of the circulating currents. Several analytical expressions are derived based on the reference values of the state variables for obtaining the MMC switching functions under steady state operating conditions. In addition, dynamic parts of the switching functions are accomplished by the direct Lyapunov method to guarantee stable operation of the proposed technique for control of MMCs in HVDC systems. Moreover, the capability curve of MMC is developed to validate maximum power injection from MMCs into the power grid and/or loads. The impacts of the variations of MMC output and dc-link currents on the stability of dc-link voltage are also evaluated in detail by small-signal analysis. [ABSTRACT FROM PUBLISHER]

This paper presents a new hybrid diverter design for onload tap changers. The design uses “active-shunt” current diversion principles. At its core, the design employs a low-voltage high-current switch-mode amplifier to divert current out of the mechanical contacts and into a pair of anti-parallel thyristors. Commutation between transformer taps may then be performed by the thyristors. The amplifier and thyristors are placed outside the normal load current path and only conduct during a tap change, producing efficiency savings and improving robustness when compared to previous hybrid onload tap changer implementations. An amplifier control loop that autonomously produces zero-current conditions at switch opening and zero-voltage conditions at switch closure is demonstrated. Experimental results investigating the wear characteristics of contacts operated under the new hybrid diverter are presented, along with comparison results from a passive-type switching scheme. Contact lifetime of more than 25 million operations is demonstrated under the new scheme. [ABSTRACT FROM PUBLISHER]

This paper addresses the capacitor bank switching back to back to underground cables. The high currents recorded during the capacitor bank switching occur due to the traveling waves in the underground cables. The detailed analytical study of the capacitive inrush currents is done. The closed-form expressions of the inrush current waveforms due to the initial current surges as well as due to the superposition of the initial and reflected current waves are derived. The formulas for the peak inrush current and its maximum rate of rise are obtained. The case study of the 24-kV capacitor bank switching back to back to the underground cables is described. The results of the field measurements are compared with the results of Electromagnetic Transients Program simulation. The dependence of the inrush current peak on the number of the cables, and the equivalent inductance between the capacitor bank and the substation busbar as well as on the cable length are analyzed. The inrush currents caused failures of the capacitor bank circuit breakers. Since the described capacitor inrush current significantly exceeds the “inrush current of the isolated capacitor bank,” the selection of the capacitor bank switchgear should be taken into account. [ABSTRACT FROM PUBLISHER]

This paper describes a new approach for the maintenance of high-voltage devices in switching substations that is based upon upgraded reliability-centered maintenance (RCM). In addition to periodical maintenance, the Slovenian Transmission System Operator ELES also uses an application that enables RCM. The strategic maintenance concept was developed on the basis of ongoing monitoring maintenance. It takes into consideration not only the well-known indices regarding technical conditions and importance, but also the indices for risk and environment. This new approach is demonstrated using the example of a Podlog 400-kV switching substation before and after its overhaul. [ABSTRACT FROM AUTHOR]

This paper describes two new approaches for the determination of the direction of switched capacitor banks as well as an efficient technique for estimating the distance of switched capacitors from the monitoring location in distribution systems. At first, two new indices based on branch voltage and current changes immediately after the capacitor switching instant are proposed for determining the direction of switched capacitor banks. The wavelet transform is used to detect the exact instant of switching. Then, a new technique based on linear circuit theory is proposed to estimate the distance of the switched capacitor bank from the monitoring location using capacitor switching transient data. Simulation results show that the error of the estimated distance is acceptable. A combination of the approaches for direction finding and the technique for distance estimation provide a novel method for determining the location of switched capacitor banks in real distribution systems. The method is valid in all normal, back-to-back, and abnormal switching of wye- and delta-configured capacitor banks. Also, the size of the capacitor bank has no effect on method accuracy. The efficacy of the proposed method is validated by using the IEEE 13-bus and 34-bus distribution systems. [ABSTRACT FROM AUTHOR]

Modeling and analysis of power and power-electronic systems' transients using digital programs enables testing new design concepts in modern electric grid and many industrial and commercial products and applications. This paper gives an overview of dynamic average-value modeling techniques for representing static switching converters for the system-level studies. Concepts and desirable properties of averaged models for conducting large-signal time-domain transient studies and small-signal frequency-domain analysis for control design tasks are discussed. Basic approaches for developing average models for dc/dc and dc/ac converters are briefly reviewed and summarized. Finally, the desirable properties of the average-value models are demonstrated through an example system. [ABSTRACT FROM PUBLISHER]

Replacing SF6, the most potent greenhouse gas, with an alternative gas is a challenge faced by medium-voltage load break switches (MV-LBS). Air is a possible alternative, but there are some challenges regarding low dielectric strength leading to high arcing time and dissipated energy. Therefore, understanding the switching processes for both interruption and making operations in air MV-LBS is crucial to designing efficient compact switchgear. This work focuses on making operations in air MV-LBS. A synthetic test circuit and a model switch are designed based on the standards to simulate making operations under fault conditions similar to a real test. The test condition is set to achieve high pre-strike arcing time and energy. The results show that the most destructive impact of the making short-circuit current occurs in the first half-cycle of the load current when the pre-strike arc is formed. For an average short-circuit current with a peak of 22 kA and a breakdown voltage of 20 kV, the switch failed to re-open due to the arcing contacts welding after four successive making operations without main contacts and seven successive operations with main contacts. It has been shown that the total arcing contact mass loss occurs mainly during the pre-arcing time.Increasing the closing speed could be a possible solution to minimize the impact of arcing. [ABSTRACT FROM AUTHOR]

The reignition probability of switching overvoltage caused by vacuum circuit breaker (VCB) switching off shunt reactor (SR) in offshore wind farm (OWF) is studied. Firstly, a model of OWF is developed in the environment of electromagnetic transient program. According to the switching time of VCB, a current period is divided into non-reignition zone, reignition zone and current chopping zone. Based on the analysis of overvoltage mechanism, a method for calculating critical arcing time and reignition probability is proposed. Then, the effects of resistor-capacitor (RC) snubber on reignition probability are discussed. The capacitance of RC snubber is optimized based on switching dispersibility of VCB and critical arcing time, and a new structure of RC snubber is developed. Finally, the hybrid method of RC snubber with new structure and phase-controlled switching is presented to suppress the switching overvoltage. The results indicate that the method can eliminate the reignitions and weaken the impact of overvoltage on the insulation of power apparatus. [ABSTRACT FROM AUTHOR]

Insulation failures in dry-type distribution transformers occur frequently and severely when 12-kV vacuum circuit breakers are applied to switch shunt reactors. However, the reason for this phenomenon is unclear. Based on the test platform for the failure circuit of a running 10-kV power system, a series of field tests was conducted to investigate the overvoltage mechanism of this type of failure. Field tests successfully recorded the prestrike and reignition waveforms during the transient processes of switching shunt reactors, based on which transient process and phenomena were analyzed. The prestrike probability was as high as 60% during the transient process of switching on shunt reactors. However, the probability of reignition was significantly lower than that of prestrike. The phase-to-ground and interphase maximum overvoltages during the switching transient processes have approximately the same amplitude. The prestrike caused phase-to-ground overvoltages beyond 6.0 p.u., whereas the reignition caused overvoltage beyond 8.0 p.u. The mechanisms of overvoltage generation of the switching transient processes were analyzed based on circuit analysis. Multiprestrikes and multireignitions, cable length, switching conditions, and system configurations were the main reasons for severe overvoltages. [ABSTRACT FROM AUTHOR]

An approach for controlled reclosing of transmission lines with shunt reactive compensation is proposed here to reduce switching surge overvoltages. It consists of applying Karrenbauer's transformation as a matrix operator for the analysis of voltage signals required to control switching operations. The procedure also determines the arc extinction time for single and double-phase-to-ground faults which may avoid unsuccessful reclosing. A device to control the line reclosing was implemented and evaluated through real-time simulations. An offline version was implemented for parametric and sensitivity analysis through the Electromagnetic Transients Program using actual parameters from the Brazilian Power System Grid. Several field oscillographic data, obtained from Brazilian power transmission lines with different transposition schemes, were used to validate the proposed approach, and the results attest its efficiency. [ABSTRACT FROM AUTHOR]

During the process in which a vacuum circuit breaker (VCB) switches off the shunt reactor (SR), an overvoltage with high amplitude and high frequency (HF) will occur because of current chopping and multireignition. To reproduce this switching overvoltage faithfully, the voltage and current characteristics of the VCB were analyzed in this work, based on the field test data from a 220-kV substation in Chongqing, China. Then, typical parameters of three basic factors mentioned in Helmer's VCB model, chopping current value, dielectric dynamic insulation strength, and HF current quenching capability, were selected after statistical analysis and an arc reignition model of VCB was established. Based on this presented model, the influences of the surge arrester, the resistor–capacitor (RC) snubber, and the installation positions and quantities of these protective devices on overvoltage suppression performance were studied in simulation. In addition, the field test using the surge arrester and the RC snubber as the suppression measures was performed again in the same substation. Finally, we derived the conclusion that the most suitable suppression scheme for overvoltage caused by VCB switching off 10-kV SR is installing two 12/32.4 kV surge arresters at the terminals of the cables for the SR and the substation transformer (ST). [ABSTRACT FROM AUTHOR]

The research and development work toward a compact SF $_6$ -free load break switch for the medium voltage range has led to several design proposals. The interruption capability of three different nozzles and gas flow concepts with atmospheric air as the interrupting medium is compared and assessed. The three test switches are installed in circuits corresponding to the mainly active load and switch-fuse test duties of the 24 kV/630 A load break switch standard. A pressure tank is used to provide different air flow rates, and the interruption capabilities of the different flow concepts are compared with basis in the tank pressure required to give successful interruptions. Two hundred and seventy current interruption tests were carried out. Air flows directed radially onto the arc or swirling along the arc turn out to result in a substantially better interruption performance than when the air flows straight and parallel to the arc. Air flows corresponding to upstream over-pressures of a few tenths of a bar seem to be sufficient for an air-based load break switch rated for 24 kV/630 A. [ABSTRACT FROM AUTHOR]

The use of standard HVac disconnectors for bus-transfer switching in future HVdc gas-insulated switchgear (GIS) has been demonstrated successfully in past research through simulation and testing. Prolonged arcing times through reignitions caused by high frequency harmonics as well as the proposed extension of GIS disconnector switching cases, however, create an HVdc-specific demand for more advanced disconnector concepts capable of withstanding higher switching stresses and increased recovery voltages. A spring-loaded disconnector designed for use in HVac mixed technology switchgear was found to shorten the time to first current zero through an accelerated increase of the arcing voltage compared to standard disconnectors. Despite its increased switching performance, the reignition characteristics of this fast-acting disconnector were found to largely follow the previously established extinction criterion, which enables further simulation of similar technologies for HVdc applications. To additionally combat the issue of contamination of HVdc GIS by electrode erosion products, a prototype disconnector was improved using permanent magnets to evoke arc rotation. Erosion tests in air have shown an effective reduction of electrode erosion for long arcing times at currents typical for disconnector switching, whereas bus-transfer tests revealed promising enhancements of the reignition behavior. [ABSTRACT FROM AUTHOR]

The present study analyzed the characteristics of switching overvoltage, and discussed in detail the main influencing factors and worst-case conditions of switching overvoltage in the \pm 1100-kV ultra-high-voltage direct-current (UHVDC) converter station. The results showed that ac bus overvoltage was produced by oscillation, and would not be a threat to the equipment in the converter station. The results also showed there were two cases for overvoltage on related valves. In the first case, the valve overvoltage was on converter valves in the upper three-pulse group of the 12-pulse converter, and was caused by the voltage difference between the dc line and ac bus at the valve side under a specific fault. In the second case, the overvoltage was on other valves, and was caused by a phase-to-phase switching surge that penetrated into the valve hall. Other results were that overvoltage on the dc bus was mainly caused by the full reflection of the wave, and overvoltage on the neutral bus was caused by high-voltage sources, such as the dc source creating stress on the neutral bus under specific faults, or the freewheeling current of the metal return line and the smoothing reactors. [ABSTRACT FROM AUTHOR]

Very-fast transient overvoltage (VFTO) generated during switching of the disconnector in gas-insulated switchgear (GIS) may threaten the safety operation of electrical equipment. As the rated voltage of GIS increases, the difference between the level of VFTO and its lightning impulse withstand voltage gradually decreases. Hence, VFTO becomes the main dielectric stress to switchgear optimization and insulation coordination for ultra-high voltage (UHV) systems. To obtain the characteristics of VFTO in UHV GIS, two full-scale 1100-kV GIS test circuits were established in China. Two types of capacitive sensors were developed to measure VFTO generated in two test circuits. The measurement systems based on both types of the sensors were calibrated by a special calibration device in laboratories and onsite. The measurable frequency range of the systems is 0.1 Hz \sim 100 MHz and their division ratios are stable. More than 2000 tests by switching the disconnector in GIS had been carried out in the test circuits, and VFTO waveforms were captured by the measurement systems. From the test results, the waveform properties, the striking number, the predominant frequency, and distribution of the trapped charge voltage for full-voltage waveforms, the typical characteristics and oscillation coefficient of single-strike waveform, the distribution of VFTO magnitude was analyzed. The mitigation of VFTO by a damping resistor in the disconnector was illustrated. The maximum magnitude of VFTO was 2.27 p.u., and it was mitigated to 1.33 p.u. by a damping resistor of 500 \Omega. The achieved characteristics of VFTO in the UHV GIS have become the basis of VFTO simulation and insulation coordination for engineering design. [ABSTRACT FROM AUTHOR]