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AC side impedance modelling of an HVDC converter station usually ignores the DC-side dynamics, instead, the dynamics characteristic of cable and another converter station is represented as a constant power source or a constant voltage source. The influence of DC-side power network dynamics for AC-side small-signal stability is not yet well analysed. This paper presents a three-port transfer matrix model of the three-phase voltage source converter (VSC) which intuitively characterizes both AC-and DC-side dynamics by relating AC-and DC-side current and voltages. This model not only derive the equivalent impedance of the DC-side power network but also easily obtain a modified AC impedance model of VSC considering the impedance characteristic of the DC-side network. With comparing the impedance characteristics of the modified impedance model and the original impedance model, this paper shows that if the DC-side dynamics are ignored in an HVDC system, it may lead to an incorrect result of stability assessment. The proposed three-port transfer matrix model simplifies the impedance modelling process of the HVDC transmission system and the modified AC impedance model provides accurate stability analysis of the HVDC transmission system. Simulation results are given to validate the correctness of the proposed impedance model and the effectiveness of the modified AC impedance model for stability analysis.

The alternate arm converter (AAC) is a promising candidate for HVdc converter stations due to its reduced number of devices and capability of blocking the dc fault current. This paper is focused on modeling and control of the AAC with an extended overlap period. First, a generic mathematical model considering all the switching state combinations of the AAC is developed, which governs the characteristic of the time-varying, three-phase imbalanced circuit parameters of the AAC. Based on the developed model, an ac-side current control strategy with a feed-forward compensation method to overcome the time-varying nature of the circuit parameters is proposed. Then, by introducing the so-called “double-overlap period,” a synthetic circulating current strategy is proposed, which enables both dc current active filtering and zero-current switching of the director switches. The validity of the model and effectiveness of the proposed strategies are confirmed by simulation studies on an HVdc system.

The acoustic noise generated by filter capacitors in a high voltage direct current (HVDC) converter station will have a serious negative impact on the residents' lives and the ecological environment. Though accurate enough, traditional modeling methods based on either boundary element method (BEM) or finite element method (FEM) are almost impossible to reconstruct sound field for whole HVDC station filter array because of their extremely high demand for computing resources. To deal with this problem, this paper proposes a novel Tikhonov Regularization Equivalent Source Method (TRESM)-based filter capacitor acoustic model. By constructing multiple point-sound source models, the acoustic characteristics of the actual filter capacitor are simulated and the original sound field is reconstructed. The accuracy of the proposed model is verified by the experimental results and the sound field distribution characteristics of the three-phase filter capacitor array are observed and analyzed using the proposed model.

In recent years, using Internet of Things (IoT) technology to build the intelligent functions in the Smart Grid has become one of the key development strategies of China State Grid Corporation. As a fundamental technology building block to IoT, wireless sensor network (WSN) technology faces greater technical challenges in power grids than in most of other domains. In the harsh deployment environment of a power grid, satisfying requirements such as low power consumption, good transmission coverage and high reliability are the key focuses for the WSN design. This paper presents our WSN design techniques that address the requirements for Smart Grid facilities. We demonstrate our WSN design in the supporting system of the LingZhou HVDC converter station with over 1600 wireless sensors installed. Our WSN system not only makes the converter station smarter, but also significantly reduce the cost of the installation and maintenance of the supporting system of the station.

High voltage DC electronic current transformer is the key equipment in HVDC system, and its transient step response is one of the most important indicators for the control and protection, which is significant to power quality and power supply reliability of the power network. This paper introduces the structure of DC electronic current transformer and the corresponding transient step response character and related influence factors. A closed-loop test method on transient step response for DC electronic current transformer is proposed. Polynomial fitting is used to calculate the delay time of the step response, the rise time error of the field testing results and other key indicators. The validity of the test method is proved at a HVDC converter station testing system.

Power stations have a very complex electromagnetic environment. The situation is typically worse in an HVDC converter station, because the electromagnetic field (EMF) levels are more severe in proximity to HVDC equipment. Therefore, it is very important to address electromagnetic compatibility (EMC) issues and compliance in the design and operation of HVDC converter stations. This article analyses and discusses in detail HVDC convertor station EMC concerns by focusing on three aspects of the overall design process: analysis method, computer model simulation and EMF calculation. The main purpose of this work is to evaluate the impact of the HVDC station electromagnetic environment on personnel safety and specific equipment by examining the electromagnetic field of the entire converter station under various operating conditions. The analysis methods and procedures introduced in this article can provide guidance and reference for similar EMC analysis and design of offshore and onshore AC/DC converter stations.

Online condition monitoring for High Voltage Direct Current (HVDC) converters are getting more important. This could be achieved by embedding sensor nodes for monitoring the critical Multiphysics phenomena that indicate the healthy conditions of the components as well as the system. To facilitate the sensor node development, it would be beneficial to investigate the environmental condition, to which the development of sensor nodes as well as the energy harvester is subjected. Analysis of the electromagnetic field as well as the ambient power source are the focus of this paper. Moreover, it is shown how the electromagnetic field and ambient power source would affect the development of the sensor nodes

Aiming at the difficulty of CT polarity checking of MMC-HVDC converter station transformer, a method for CT polarity checking based on waveform comparison and instantaneous power theory is proposed. The transient current during the process of the converter transformer and the converter valve charging is analyzed, and the polarity of the zero-sequence differential protection and the longitudinal differential protection is checked by comparing the crossing transient current on both sides of the protection based on the longest common subsequence (LCSS) distance. At the same time, the instantaneous active power during the converter valve deblock is used to check the polarity of directional protection. This method can not only check the polarity of longitudinal differential protection and zero-sequence differential protection, but also can check the polarity of directional protection, and the reversed phase of CT can be found out. Moreover, it can withstand a certain degree of ratio error and phase error of CT. The simulation verifies the effectiveness and applicability of the method.

In recent years, Modular Multilevel Converter based High Voltage Direct Current (MMC-HVDC) transmission system has gradually developed, and the AC short-circuit current contributed by both the converter and AC system needs to be solved urgently. In order to solve this problem, the paper focuses on the transient process of phase current change on the AC side after a unipolar ground fault occurs on the DC side. Then, an equivalent model of the AC system short-circuit current is established. Therefore, a calculation method of the short-circuit current contribution of the AC system based on the arithmetic circuit is given. Finally, a simulation model of two-terminal project built based on PSCAD verifies the correctness of the proposed calculation method.

In order to solve the impact of photovoltaic volatility on power systems, flexible HVDC transmission technology is more and more used for photovoltaic grid connection. As the most important component of flexible HVDC transmission systems, the characteristics of the equipment in the flexible HVDC converter station are of great significance to the safe and stable operation of the HVDC transmission system. When the AC/DC side of the converter station fails or is struck by lightning, the converter station equipment will have an overvoltage. In severe cases, it will damage the equipment in the converter station and threaten the safe and stable operation of the system. Therefore, this paper comprehensively considers the various overvoltage conditions of a ±30kV converter station with modular multilevel converter (MMC), and uses PSCAD/EMTDC for simulation calculation. According to the calculation results, the arrester configuration of the converter station is analysed, and the results can be used for equipment selection and engineering design.

Comparing voltage and reactive power exchange before and after reactive power optimization, the effect of reactive power optimization of converter station is verified. This article presents reactive power balance method of converter station on account of reactive power compensation equipment at low voltage side of main transformer, and proposes a strategy of changing reactive power control region. Taking one local converter station as an example, the real operation condition is simulated in RTDS, and the two proposed methods are applied to reactive power control. The simulation results verify the beneficial effect of the proposed methods on optimizing the switching of reactive power equipment and improving the voltage control ability of converter station. It provides important reference for engineering application of reactive power control strategies optimization.

The study focuses on the water saving technology for external cooling system of DC transmission converter station equipment, a water saving design applicable to external cooling system of such key equipment cooled by water as converter valve and transformer in the converter station of the conventional/flexible DC transmission project. Current external cooling method for converter valve equipment in the converter station mostly combines the closed-circuit cooling tower and spray cooling. This method, however, has the disadvantages of, for example, high water consumption and large quantity of blowdown. In view of this, this paper proposes a water saving technology by sewage recycling through spray treatment system based on two-stage reverse osmosis. In this technology, the water in the converter station can be recycled, thus achieving the purpose of water saving and blowdown reduction. For the transformer cooled by external cooling with water, this paper provides an optimized design of common configuration in which the external cooling systems of converter valve and transformer share the same spray pool and spray treatment equipment. This design will not only save water resources, but also facilitate the operation, maintenance and comprehensive management of converter station. It is thus in accord with the green development philosophy of converter station.

In an HVDC system, the power line carrier noise filter, or the PLC filter for short, is mainly used to eliminate the interference of high-frequency harmonics generated by the converter valve to AC line carrier communication. There had been several burning accidents of PLC filter tuning elements in HVDC projects, which profoundly reduced the availability of the HVDC system. To solve this problem, in this paper, the operating parameters of PLC filter components were simulated and analyzed with the variation of harmonic power supply frequency firstly, with the fault weak points of tuned components being found. Then, according to the structures of the PLC communication filters and the tuning elements, three optimization schemes of the PLC filter were proposed. The feasibility of these schemes are compared by simulation and calculation from two aspects: the effect of reducing high-frequency harmonic interference and the influence on the running capacity of the equipment. Finally the risk of these schemes are pointed out.

For a HVDC converter station, the commonly used power loss determination methods are difficult to accurately reflect the changes of power loss of the converter in real time, given that the operating parameters of the converter station are dynamically changing when the converter station is running normally. Therefore, this paper proposes a method for predicting power loss of HVDC converters based on support vector regression. According to this method, firstly, the power loss data of a converter is analyzed. Then the appropriate feature in the power loss data is selected and thus a dataset of power loss samples can be obtained for further work. By applying the support vector regression algorithm to the dataset collected before, it is possible to predict the power loss of a converter for various operating parameters of the HVDC converter station. Finally, the cross-validation method was used to validate the stability of the prediction method. The result of the validation shows that the proposed method is able to accurately and stably predict the power loss of a converter of the HVDC converter station in real time.

This paper deals with verification of valve losses on High Voltage Direct Current(HVDC) converter stations in Korea. Technique of calculating losses is one of important verification process to evaluate systems performance, so customers have right to get accurate information of losses in project bidding process. However, it is not easy for utility company to verify maker’s documents because getting information is limited from makers.Losses of a converter station can be easily estimated from calculation of two main equipments, valve and transformer, because proportion of two equipments accounts for more than 80% of the total values in the HVDC converter station. In this paper, the authors firstly analyzed calculating process of valve losses and verified if the calculated results are accorded with the reports provided from maker in accordance with the IEC standards, IEEE standard, EPRI Handbook in Jeju No.2 converter station in Korea. After that, it was verified whether the results were satisfactory compared to KEPCO’s specification.

In order to evaluate corona-free performance of the key HV connecting electrodes in one ±320kV VSC-HVDC indoor converter station, the equivalent 3-D models of different electrode types for electric field simulation were built with finite-element method (FEM) according to the preliminary designs. Considering the influence of devices nearby, full-scale models of all the indoor parts of the converter station were established as the boundary conditions for electric field calculation, including the indoor arm reactor yard, valve hall and DC yard. The potential and electric field distribution of the full-scale models were calculated. The maximum electric field strength of each connecting electrode was obtained and an optimization design was proposed for the electrodes whose surface field strength exceeded the critical value for evaluation. The results show that the full-scale simulation models of indoor converter station could reflect the difference of electric field distribution of the connecting electrodes with the same shape and various installation positions. For the preliminary electrode designs, serious electric field distortion occurred at the surfaces of some busbar connectors and terminal electrodes under operating condition. After optimization, the maximum field strength could decrease from 43.6kV/cm to 21.5kV/cm and the corona-free performance could be predicted. The optimized design scheme passed the corona tests and the research method could provide references for the development of connecting electrodes design and corona-free performance evaluation for VSC-HVDC or other indoor station.

The controllable device is one of the main heat sources in the HVDC converter station, and the controllable devices have a high requirement for temperature. This paper studies the heat and the cooling problems of thyristors in the converter station. Taking the thyristor on the rectification side of ± 800 kV /10000 MW HVDC as an example, firstly, the loss power of a single thyristor is calculated by simulation based on the datasheet of the thyristor and the actual circuit of the project. Then the thermal model of the thyristor and valve tower of the converter station are built in the Ansys environment. And the junction temperature characteristic of the thyristor at different power is analyzed. What is more, the temperature and airflow distribution of the valve tower are given. Thirdly, the temperature distribution characteristics of thyristors with series and parallel cooling modes are compared. Finally, the temperature characteristics of the thyristors in the converter station are summarized. The method of heat dissipation to ensure the uniform temperature distribution of thyristor is also given. And the study in the paper can provide technical reference for the safe operation of the converter station.

The controllable device is one of the equipments that are most affected by electromagnetic field. This paper mainly studies the electromagnetic interference (EMI) between controllable devices in the converter station. Taking the thyristors on the rectifier side of ± 800 kV/10 000 MW HVDC as an example, firstly, the paper analyzes the operation characteristics of a single thyristor on the rectifier side. And the du/dt and di/dt are calculated when the thyristor switches based on the actual circuit. Secondly, the relevant converter station valve tower model is established based on the thyristor model which is built in the Ansys environment. And the influences of EMI generated by the thyristor on different thyristors during the turn-on and turn-off process are analyzed by the combined field and circuit simulation method. It is also pointed out that the EMI may cause the device to be misconducted under certain conditions, and the corresponding suppression measures are given. Then the distribution characteristics of the valve tower are analyzed. The research in this paper can provide a reference for the analysis and design of HVDC converter stations.

Compared with AC substation, DC substation incorporates more kinds of electrical devices emitting high level of noise, which leads to more complicated acoustic filed and exerts negative influences to ambient environment and residents. Filter capacitors and reactors reveal to be the main noise sources in HVDC converter stations. In order to control and restrict the noise emission, it is necessary to study the noise characteristics of filter capacitor and reactor and figure out the maximum noise contribution. In this paper, the noise generating mechanism of AC filter capacitor and reactor are theoretical introduced firstly. Then the sound signal processing array technique based on beam forming algorithm is investigated and applied to build the noise positioning and measurement system. By applying the novel system to measure and positioning the noise emission in different types of filter equipment fields, the general noise distribution and the spectral characteristic of filter capacitor and reactor are obtained. The measurement results illustrated that the sound signal processing array technique based on beam forming algorithm is very useful for noise positioning and extraction in filter equipment field. The maximum noise source could be found and the accuracy of noise abatement project could be largely improved.

The analysis of electromagnetic interference (EMI) noise of power electronic circuits involves both the transient characteristics of power semiconductor devices and the wideband stray parameters of passive equipment. Modular multilevel converters (MMCs) used in high-voltage direct current (HVdc) transmission systems contain thousands of submodules (SMs), which makes it considerably challenging to perform device-level simulation on the traditional commercial software. This article presents an accurate and fast method for wideband modeling and simulation of MMC-HVdc system for the assessment of conducted EMI during the design stage. Physical characteristics of the semiconductor devices, parasitic parameters of the insulated-gate bipolar transistor (IGBT) packages, and stray capacitances of the SMs are all taken into consideration, and massively parallel transient simulation of the wideband MMC model is carried out on the graphics processor (GPU). The accuracy and efficiency of the GPU-based parallel algorithm are validated by the comparison with the experimental measurement of an 11-level full-bridge MMC prototype. Furthermore, the stray capacitance network of the valve tower in HVdc project is extracted, and a matrix partition method based on the shielding plate configuration is utilized to conduct the computation in a fully parallelized manner. The developed GPU program is used to run the large-scale case of a 201-level two terminal MMC-HVdc system, and the primarily affected frequency range by various factors is analyzed. Execution time test is conducted for different level topology, and it is demonstrated that the GPU can achieve a remarkable speedup over multicore CPUs, especially when the system scale is more substantial.

In a high voltage direct current (HVDC) transmission converter station, the purpose of the Reactive Power Control (RPC) is to facilitate the switching of alternating current (ac) filters to meet the reactive power demand with the ongoing dc power flow. In addition to it, the RPC will also ensure that the required ac filters are connected to prevent harmonics from entering into the AC system. In a comparatively smaller grid like North Eastern Region (NER) of India, with RPC in automatic mode, the switching in of ac filter sub banks of HVDC system due to sudden rise of transmitted dc power through the HVDC link during oscillations in the connected ac grid causes significant rise in ac voltage was a challenge and this challenge has been solved practically. By using the manual scheme of RPC without compromising the quality of power and by connecting bus reactors the problem of sudden rise of ac voltage during oscillations have been resolved.

This paper proposes the concept of static inverter circuit (DC to AC conversion). Here a transformerless single phase having DC to AC step up converter is designed by the use of PWM (Pulse-Width Modulation) scheme. Further a new technique has been proposed which produces an output voltage of desired magnitude with 400 Hz frequency which eliminates harmonic distortion. A DC-DC booster and full bridge converter are contained in the circuit implemented here. The proposed circuit is used for high ignition unit in aircraft and is designed according to a given input- output voltage, current and frequency. In this paper PSIM simulation software is used for the entire simulation task.

The synchronous condenser is used to supply reactive power for HVDC converter station through the power transformer. The DC power that invades the power transformer biases the operating point of the magnetic field and produces harmonic components on the side of the synchronous condenser. In this paper, the analysis method of big data is adopted to analyze rotor loss of synchronous condenser under the DC bias of the transformer for the goal of accurate calculation of rotor loss. Lots of high-frequency harmonics in terminal voltage of synchronous condenser are generated by DC-bias of transformer. The rotor loss is calculated and compared under lagging phase and leading phase of synchronous condenser. The big data method is used to analyze the harmonic which produce the largest rotor loss. Some conclusions about the rotor loss caused by stator harmonic current are obtained.

DC grounding fault is a typical fault for HVDC system. In order to obtain smoother, faster, and more secure transient process, the DC protection block strategy is used to remove and isolate the converter to prevent the accident enlargement and reduce the impact on devices and power grid when the grounding fault is detected. Two main strategies are commonly taken in this situation: the X block for not >±500 kV and the S block for ±800 kV. There is no specific principle on the strategy choosing for ±1100 kV UHVDC system with hierarchical connection mode. Changji–Guquan ±1100 kV UHVDC transmission project adopting hierarchical connection mode at receiving end. In this study, the effects of different DC block strategies when grounding faults occurred in both converter stations are compared based on PSCAD simulation. The simulation results indicate that the overvoltage at rectifier side has two stages while the overvoltage at inverter side has only one. Taking S block strategy can cause lower overvoltage but longer oscillation duration on grounding current. The proper block strategy for ±1100 kV UHVDC grounding faults is adopting S block at rectifier side and X block at inverter side.

Modular Multilevel Converters (MMC) HVDC converter station topologies require the use of high-impedance grounding reactors on the converter side of power transformers. The main requirement for such units is the ability to withstand simultaneous AC and DC loading. The aim of this paper is to give insight on how to determine the DC capabilities of such units both through calculation and measurements. Additionally, the purpose of this paper is to show what the possibilities of different core designs in terms of DC capability are and what values can be expected and obtained.

An H-bridge hybrid modular converter (HBHMC) is proposed for high-voltage direct current (HVDC) applications. It uses a wave-shaping circuit consisting of series-connected full-bridge submodules (FBSMs) at the output of the main H-bridge converter. For a three-phase system, three HBHMCs are connected either in series (series-HBHMC) or in parallel (parallel-HBHMC) across the dc-link. The operating modes of HBHMC, novel modulation strategies for voltage balancing of FBSMs, and control of HBHMC-based HVDC system are presented in this paper. A detailed comparison between HBHMC and other hybrid topologies is performed on the basis of required number of switches and capacitors. The HBHMC has the features of dc fault blocking capability, lower footprint structure, and extra degree of freedom for submodules capacitor voltage balancing. The efficacy of the HBHMC-based HVDC system for three-phase balanced and unbalanced grid conditions and its fault-tolerant capability are validated using PSCAD simulation studies. Furthermore, the feasibility of the proposed converter under normal and dc fault conditions and of the proposed capacitor voltage control scheme is validated experimentally by using a three-phase grid-connected HBHMC laboratory prototype. The results demonstrate the effectiveness of the proposed HBHMC topology, control techniques, and satisfactory responses of the HBHMC-based HVDC system.

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.

This paper presents a comprehensive analysis and a novel solution for the discharge problem on the half-bridge submodules’ (HBSMs’) capacitors in the hybrid modular multilevel converter (MMC)-based high-voltage direct current transmission systems under a long-term (e.g., several minutes or longer) pole-to-ground fault condition. The mechanism of this issue is analyzed, and then a solution based on fundamental-frequency reactive circulating current (FFRCC) injection is proposed. The influence of FFRCC injection is analyzed in terms of the fluctuated arm capacitor energy and the current stress of semiconductors. Moreover, an enhanced arm voltage modulation method based on HBSMs and full-bridge submodules coordinate control is proposed to accelerate the capacitor voltage balance of HBSMs. Finally, the performance of the proposed control strategy is verified by the simulation results.

This paper proposes an evolved capacitor commutated converter (ECCC), embedded with antiparallel thyristors based dual-directional full-bridge module (APT-DFBM), to mitigate the commutation failure of line-commutated converter-based-high-voltage direct-current (LCC-HVdc) transmission and enhance the control flexibility and dynamic response of capacitor-commutated converter-based HVdc (CCC-HVdc). The operating modes and parameter selection approach of the APT-DFBMs are provided, and the coordinated control approach between the APT-DFBMs and converter valves is presented. Then, an ECCC-HVdc system, adopting an LCC station as the rectifier and an ECCC station as the inverter, is developed in PSCAD/EMTDC. The transient responses including commutation failure probability, voltage and current stress, and fault recovery performance are compared under ac fault conditions among the proposed ECCC-HVdc, LCC-HVdc, and CCC-HVdc. The results show that the proposed ECCC-HVdc can effectively reduce the commutation failure risks of LCC-HVdc, and enhance the control flexibility and improve the dynamic responses of CCC-HVdc.

DC grid technology is regarded as a promising solution for future electric networks integrating a great amount of renewable energies. It calls for high-efficiency dc–dc converters with high voltage step-up ratio to interconnect medium-voltage (MV) dc distribution grids and high-voltage (HV) dc transmission grids. This paper presents an isolated bidirectional soft-switching dc–dc converter combining two-level converters in parallel on the MV side and a modular multilevel converter (MMC) on the HV side. A dedicated control method of the proposed converter is presented. By the proposed method, a certain reactive current is injected into the MV side by the MMC to ensure soft-switching on the MV side. The proposed converter presents low power-semiconductor total device rating and low semiconductor losses over a wide power range at variable input and output voltages. Simulation of a 50 kV/400 kV, 400 MW converter is conducted to evaluate semiconductor losses and verify the validity of this work.

This paper presents a novel protection method for monopolar high-voltage direct current (HVDC) converters. This method uses the secondary and tertiary currents of the converter transformers and based on dc and harmonic contents of the currents decides for the security of the converter. The new protection method detects the location of the fault inside the converter and based on that issues a signal for single-phase disconnection in the ac system to eliminate the fault. It will be shown that the new method is not only faster compared to the conventional protection methods but also can be easily applied to the existing HVDC systems without imposing any significant requirement. The Hydro-Quebec HVDC test system is used to present the simulation results and to ensure the validity and functionality of the proposed protection method.

This paper provides a detailed analysis of the temporal development of fault currents in a multiterminal high-voltage direct-current (MT-HVDC) grid composed of a bipolar converter configuration. The sequence of events following the occurrence of a pole-to-ground fault is identified, divided into three distinct periods; namely, submodule capacitor discharge, arm current decay, and ac in-feed periods. The critical parameters that have a significant impact on the fault current in each period are discussed. The impacts of various parameters of the HVDC grid such as the size of the current limiting reactor, ac grid strength, as well as the location of the fault within the grid are studied through PSCAD/EMTDC simulation. Then, a fault current interruption process using models of various HVDC circuit breaker technologies and the resulting stresses are studied. Both serve as important inputs to define test procedures. It is found that the HVDC circuit breakers are subjected to not only dc current and voltage stresses but also energy stress. These stresses are translated into test requirements.

High-voltage direct-current (HVdc) transmission lines have been implemented in many countries, including Australia, Brazil, China, and Sweden, and the safety concerns as the result of the high electromagnetic-radiation underneath the HVdc lines have garnered increased public attentions. Here, we report on the model, design, and testing of field-mill electric field sensors to measure the electric field at the ground level under the HVdc transmission lines. This study utilized a finite-element analysis method to establish numerical simulation results based on the electrical and mechanical parameters to achieve optimal designs with experimental calibrations. Afterward, these sensors were successfully tested and utilized at the national high-voltage test base.

Transient stability analysis is a traditional yet significant topic in power systems. The switching control schemes of the HVDC converter station after fault bring new challenges to the direct method of transient stability analysis in AC/DC power systems. To apply the direct method to investigate the transient stability of power systems accommodating the switching characteristics of the line commutated converter (LCC) HVDC, continuous piecewise Lyapunov functions are constructed for a whole AC/DC system model including several classical LCC-HVDC control modes, and the continuity of Lyapunov functions is studied and guaranteed in transitions between control modes. Then, the Sum-of-Squares optimization is employed to find the optimal continuous piecewise Lyapunov functions. Finally, the transient stability domain is estimated for a sample system including the main control loops of LCC-HVDC, a three-order generator model, and a one-order exciter model. Tests in MATLAB environment are conducted to demonstrate the feasibility of this method.

The use of direct current (dc) power networks, either at high voltage or at medium voltage, is being increasingly seen in modern smart grids. This is due to the flexible control possible with dc and its ability to transmit and distribute power under circumstances where alternating current (ac) networks are either unable to, or less economic. This paper provides an overview of the evolution of high-voltage dc (HVDC) transmission from early Thury systems, to modern ultrahigh-voltage dc and multiterminal voltage–source converter systems. The operation of both current–source and voltage–source systems is discussed, along with modeling requirements. The paper provides a snapshot of the state of the art of HVDC with copious references to enable in-depth reading. Key developments over the last 20 years are highlighted. Issues surrounding multiterminal operation and dc protection are explained, along with drivers in economics and policy. This will inform HVDC integration into smart grids.

The priority to reactive power contribution from the Voltage Source Converter (VSC) based High Voltage Direct Current (HVDC) connection to support the grid during faults as suggested by the modern Network Code (NC) for HVDC affects the distance protection of transmission lines. Moreover, suppressing the negative sequence current during an unbalanced condition also interferes with the proper operation of the distance relays. This is because the current contribution from the converter is limited in magnitude and modified in the waveform in order to protect the power electronic devices during the fault in comparison to the synchronous generator fault current characteristics. This paper discusses the cause as well as the severity of the problems faced by the distance protection of transmission lines connected to the VSC based HVDC system by analyzing the apparent impedance analytically and in the simulation. The response of the relay to balanced and unbalanced faults lying on transmission lines is investigated. It is shown that the VSC limited reactive support and suppressed negative sequence current affect fault detection, forcing the relay to malfunction. The results of this paper can be used as a reference for understanding the effects of VSC-HVDC system on the operation of the distance protection during faults.

For the control of high-voltage dc (HVdc) systems, especially for that of the multiterminal HVdc (MTdc) systems, the voltage source converter (VSC) is a good option because of its high controllability. These days, different types of VSC converters have been realized such as the two/three level converter and modular multilevel converter. However, VSC converters are vulnerable against dc faults because the paralleled diodes may experience large fault currents. In order to maintain the sustainability of electricity delivery, efforts have been paid on protecting the HVdc networks, such as the novel converter topologies with the capability to tolerate faults and the dc circuit breaker. Among which, the concept of the inductor-capacitor-inductor circuit (LCL)-VSC converter aims at enhancing the ability of converter to ride through dc faults, which limits currents flowing from the ac side to dc side. The proposed method in this paper optimizes the control of LCL-VSC for partial load so that the power loss can be drastically decreased. In addition, the preferable working range for the LCL converter is introduced to guarantee the ability of restraining fault currents. The method is verified on the PSCAD/EMTdc platform.