Thyristor Valves Research Articles

Development of SyntheticTest System for Thyristor Voltage Monitoring of Converter Valve

Thyristor voltage monitoring is one core equipment for a light-controlled converter valve, and its performance directly affects the reliability, stability, and efficiency of the whole power system. However, the field test equipment for thyristor voltage monitoring is limited by import, which is disadvantageous to the operation and maintenance of the converter valve. Thus, developing a synthetic test system for thyristor voltage monitoring is particularly urgent. The synthetic test system for thyristor voltage monitoring is developed based on the operational principle of thyristor voltage monitoring and the technical standard IEC 60700-1, which is subsequently applied to the thyristor valve test. The simulation and test demonstrate this synthetic test system’s reasonability, meeting the need for engineering applications.

Diagnosis of Loose Core Fault in Saturable Reactor of Thyristor Valve Based on Vibration Signal Time-Frequency Analysis and CNN

The saturable reactor is one of the core components of the thyristor valve, which plays a role in suppressing current changes and balancing voltage distribution. Due to long-term operation under complex working conditions, the vibration of the reactor can cause the core to loosen. To detect faults in time, a fault diagnosis method is proposed for loose cores in saturable reactors of thyristor valves based on time-frequency analysis of vibration signals and convolutional neural networks. The two-dimensional time-frequency spectrum of the vibration signal is obtained using the short-time Fourier transform method. A convolutional neural network model structure is designed to extract time-frequency spectrum features and diagnose different levels of core looseness faults through learning and training. An experiment was carried out to collect core fault information under variable working conditions. The effectiveness of the method proposed in this paper was verified based on experimental data validation. Through comparative experiments, it was found that the proposed method is superior to existing fault diagnosis methods.

Development and Validation of a Smart Architecture for Thyristor Valves

A high voltage thyristor converter is realized by many valve sections, whose volume is approximately occupied for only the 10% by thyristors and for the 10% by the relevant gate drivers. The remaining 80% is taken by passive auxiliary circuits, needed to protect thyristors during turn-on and turn-off commutations. This work represents a preliminary validation of an innovative architecture that aims to reduce the auxiliary circuit cost, volume, and weight of the overall valve, through the investigation of active, instead of passive solutions. The work starts from investigation of the phenomena behind the valve transient behaviors and proceeds with simulations and experimental tests, using a reduced scale circuit prototype. The match between the obtained results validates the investigated active configurations, confirming that the proposed solution can be used for improving thyristor valves.

A Time-Domain Piecewise Analytical AC Harmonic Current Calculation Method Suitable for Capacitor Commutated Converter

The Capacitor Commutated Converter (CCC) is constructed as a conventional Line Commutated Converter (LCC) modified with a series capacitor between the converter transformer and thyristor valves in each phase. This paper proposes a time-domain piecewise analytical AC harmonic current calculation method for the CCC system. Compared to the existing calculation methods, the proposed method can calculate both characteristic and non-characteristic harmonics simultaneously when various asymmetric factors are considered. Firstly, the overlap angles are calculated considering the asymmetric factors after partitioning the piecewise intervals in a fundamental period. Secondly, the AC harmonic currents are calculated considering the DC side harmonic currents based on the Thevenin equivalent model of the DC circuit. Then, the effectiveness of the proposed calculation method is validated by simulation results in PSCAD/EMTDC. The comparison result shows that the proposed method is high-efficiency and high-accuracy for the AC harmonic current calculation of the CCC system and can meet the requirement of CCC filter design in practical projects. Finally, the influence of the asymmetric factors on the overlap angle calculation and the influence of the commutation capacitor variation on the commutation parameters are studied.

A Hybrid Five-Level Modular Multilevel Converter With High Efficiency and Small Energy Storage Requirements for HVDC Transmission

This article presents a hybrid five-level converter (H5LC) with cascaded full-bridge submodules (FBSMs) for high voltage direct current (HVdc) transmission. A five-level converter operates at a fundamental switching frequency to generate symmetric five-level square wave output voltages which are shaped into smooth sinusoidal voltages by the ac-side FBSMs. Using a proposed third-order harmonic voltage injection scheme, the FBSMs’ total blocking voltage is limited to one-eighth of the dc-side voltage, reducing the H5LC's losses, footprint, and capital costs. dc pole-to-pole fault blocking is enabled using a bypass branch consisting of bidirectional thyristor valves and fast mechanical switches to suppress the ac-side fault contribution within half a fundamental cycle. Full-scale HVdc simulation studies show the H5LC's ability to control real and reactive powers and additionally, its dc fault resilience. A lab-scale hardware implementation of an H5LC with 30 FBSMs is presented to verify its operating principle. In contrast to the half-bridge submodule based modular multilevel converter, the H5LC has improved efficiency, fewer semiconductor devices, and significantly reduced energy storage requirements.

Research on Vibration Data-Driven Fault Diagnosis for Iron Core Looseness of Saturable Reactor in UHVDC Thyristor Valve Based on CVAE-GAN and Multimodal Feature Integrated CNN

The imbalance of data samples and fluctuating operating conditions are the two main challenges faced by vibration data-driven fault diagnosis for the iron core looseness of saturable reactors in UHVDC thyristor valves. This paper proposes a vibration data-driven saturable reactor iron core looseness fault diagnosis strategy named CVG-MFICNN based on CVAE-GAN and MFICNN to overcome the two challenges. This strategy uses a novel 1-D CVAE-GAN model to produce generated samples and expand the training set based on imbalanced training samples. An MFICNN model structure is designed to allow the simultaneous processing of multimodal features such as the SST time-frequency spectrum, time-domain vibration sequence, frequency-domain power spectrum sequence, and time-domain statistics. Using these multimodal features and the MFICNN model, the hidden fault information in vibration data can be effectively mined. An experiment is conducted to collect vibration data of saturable reactors with different faults. Models based on the proposed strategy and other methods are trained and tested using the collected data. The comparison results show that the performance of the proposed CVG-MFICNN approach is significantly superior to that of single-feature CNNs, traditional machine learning methods, and classical image classification CNNs in the application of UHVDC thyristor valve saturable reactor iron core looseness fault diagnosis.

A Hybrid Modular Interline Current Flow Controller for Meshed HVDC Grids

In meshed high-voltage direct current (HVdc) grid, the current flows through the lines cannot be controlled with sufficient freedom without additional power electronics-based devices, namely current flow controller (CFC). In this article, a novel hybrid modular interline CFC is proposed based on H-bridge submodules and thyristor valves. Due to its modularity, the proposed CFC is particularly suitable for applications requiring high voltage and large power capacity. The circuit structure, operation principle, and parameter design are presented. In addition, a control strategy is developed for the proposed CFC. Simulations are carried out using a four-terminal meshed HVdc grid to verify the effectiveness of the topology and its control strategy. A downscaled prototype is built, which further validates the proposed CFC.

Real-Time Virtual Measurement Technique for the Thyristor Valve Current Based on the Converter Terminal Current

The safety of converters is key to the stable operation of AC/DC hybrid power grids after AC faults. Thyristor valve current is a direct manifestation of the converter observability, but there are no measuring elements inside the converter, so the thyristor valve current can be reflected only indirectly through external electrical quantities. This paper proposes a virtual measurement scheme based on the temporal features of the converter terminal current. First, under the action of the trigger pulse, the polarity characteristics of the AC current and temporal features between the valves are utilized to predictive estimate the valve state. Then, when commutation failures occur and the bypass paths form, the conduction state of valve is constructed based on the relationship between AC and DC current. Last, when the valve is to be shut-off in the bypass path, the blocking state of the valve is corrected based on the unidirectional conductance characteristics of the valve. According to the above analysis, combined with the topological current equation of the converter, the valve current can be calculated in the whole process. Through the simulation of the different valve states and bypass paths and analysis of the field record data, the results show that the valve current can still be accurately calculated based on the virtual measurement scheme.

Harmonic Performance Analysis of Static Var Compensator Connected to the Power Transmission Network

The static var compensator (SVC) is a device which is designed to compensate reactive power, increase voltage stability and to reduce voltage fluctuations. Thyristor controlled reactors (TCRs) are composed of reactors in series with bidirectional pair of thyristors. Current through reactors can be continuously controlled by changing the firing angle of thyristor valves, thus
 the inductive power can be easily controlled. Typical applications of TCRs in AC systems are voltage stabilization and temporary overvoltage reduction, stability improvement, damping of power oscillations and load balancing. In this paper, harmonic performance analysis of SVC equipped with TCRs is presented. SVCs utilizing TCRs generate harmonic currents and therefore it is necessary to determine the effect of harmonics generated by the SVC on the power system and its elements. This includes interaction of the SVC with the system, the SVC performance under balanced and unbalanced operating conditions and finally, evaluation of countermeasures such as installation of harmonic filters. In order to carry out these analysis, it is necessary to determine harmonic characteristics of the network at the point of SVC connection, existing levels of harmonics, and to know appropriate standards regarding acceptable harmonic levels in the power system. Since harmonic distortions in the system are caused by the interaction between SVC and the system, all system contingencies which may affect system’s frequency response should be evaluated. Detailed power system model should be considered to make sure that parallel resonance points of system do not directly coincide with characteristic harmonics from the SVC. Harmonics generated by SVCs are largely dependent on the operating point within the SVC characteristic. A conservative approach is to use the maximum values of harmonics generated within the spectrum irrespective of the operating point. The results of harmonic performance analysis are important for appropriate design of SVC. Harmonic performance analysis related to SVC application which are presented in this paper include the determination of: frequency response of the transmission network impedance required for the specification and design of filters; the effects of SVC generated harmonics on the power system; the overall filter requirements and countermeasures to reduce harmonics to acceptable levels.

Research on Static Potential Distribution Characteristics of Thyristor Valve of UHV Controllable Arrester

The ultra-high voltage(UHV) AC thyristor valve-type controllable arrester can realize the access and exit of the controllable unit through the action characteristics of the thyristor, dynamically change the volt-ampere characteristics of the arrester connected to the system, to reliably and deeply suppress the operating overvoltage. Under continuous operating voltage, stray capacitance will cause the uneven distribution of axial potential at the high-voltage end of the thyristor valve. In severe cases, the thyristor with higher voltage may be triggered by mistake, which endangers the system security. Due to the more changeable potential distribution of thyristor valve under different ambient temperatures and charge rates, higher requirements are put forward for the design and application of the thyristor valve. Therefore, based on the test data, this paper simulates and analyzes the influence of the voltage and temperature changes of the thyristor and resistor on the potential distribution of the thyristor valve under the continuous operating voltage, analyzes and compares the configuration modes of various voltage equalizng capacitors, and obtains the voltage equalizng measures with better optimization effect, which provides a reference for the voltage equalizng optimization of the thyristor valve.

Dynamic behavior of multilayer suspension equipment and adjacent post insulators with elastic‐viscous connections

In an electrical substation or converter station, suspension equipment is connected to an adjacent post insulator by a rigid busbar or flexible conductors if the horizontal displacement is significant. In order to investigate the coupling effect between suspension and post equipment connected by a rigid busbar, a scaled-down model based on a multilayer suspended thyristor valve and an adjacent post insulator was tested. Additionally, an analytical model, which took into account the out-of-plane and torsional motion, was established. The test results showed that the damping of the suspended valve approximated to stiffness-proportional damping. The rigid busbar significantly reduced the seismic responses of both the valve and the post insulator by changing the vibration modes and increasing the damping ratio. Based on the analytical model, the rigid busbar connection is recommended for stiff insulators with a low fundamental frequency. A hard soil site, chain connection, low damping ratio, and an appropriate layout all contribute to the advantages of a rigid busbar. A telescopic busbar using additional rotational and axial damping also can improve the system's seismic performance. Rather than automatically installing flexible conductors, it is recommended that equipment parameters be used to determine an appropriate connection type between suspension equipment and an adjacent post insulator.

Opportunities for Embedded High-Voltage Direct Current: Evaluating the Benefits for the Legacy ac Grid

Power system engineers and planners often consider high-voltage dc (HVdc) systems for bulk power transmission over long distances. There are two technologies within HVdc: line-commutated converters (LCC-HVdc) and voltage-source converters (VSC-HVdc). Each type has its own suitable range of applications. LCC-HVdc is a mature technology that has been available since the 1970s and uses thyristor valves. LCC-HVdc is commonly applied for high-capacity power transmission between two strong power systems (high short circuit ratio) over long distances with limited dynamic requirements (e.g., features such as fast power reversal, dynamic reactive power support, or black start are not available or are available only with limited performance). Currently installed systems reach ratings of 12 GW at a transmission voltage of ?1,100 kV and may cover distances beyond 3,000 km. The introduction of VSC-HVdc in the late 1990s initiated the use of self-commutated valves employing insulated-gate bipolar transistors with a significant improvement in dynamic performance. Presently, the ratings of VSC-HVdc a relower than those of LCC-HVdc. VSC-HVdc systems are now installed with a rating of 3,000 MW at a transmission voltage of ±525 kV, and higher ratings are under development. For special applications such as the connection of offshore wind power plants or connection to/between weak grids, VSC-HVdc often offers the only technoeconomically viable solution.

A Unidirectional Hybrid HVDC Transmission System Based on Diode Rectifier and Full-Bridge MMC

To reduce the cost of bulk power transmission using voltage source converter high-voltage DC (HVDC) technology, a unidirectional hybrid converter is proposed, where a diode rectifier (DR) and a modular multilevel converter (MMC) based on full-bridge (FB) submodules are connected in series on DC side. The FB-MMC controls its DC voltage to regulate the transmitted power. The majority of the power transmission is via the DR considering its cost and efficiency superiority and only low power rating FB-MMC is required. A thyristor valve is equipped at the DC side of the FB-MMC to prevent potential overcharge of the FB submodules during DC faults. Compared to conventional MMCs, losses can potentially be reduced by around 20%. An active power controller is proposed to regulate the DC voltage of the FB-MMC so as to control the transmitted power. With the inverter station controlling its DC terminal voltage constant, the FB-MMC increases the output DC voltage to increase the transmitted power and vice versa. To alleviate overvoltage of the HVDC link during AC grid faults of the inverter station, a dynamic DC voltage limiter is designed to actively reduce the DC output voltage of the FB-MMC. Simulation results confirm the proposed converter operation and control.

Mitigation of commutation failure method in LCC converter based on HVDC systems by mean of modeling and simulation

Commutation failures is the commonest perturbation can appear in the inverter valves during or after the faults. Therefore, understanding the system fault is very important for regulation and protection design. Simulation by means of digital real time simulator can provide quickly the simulation results, and also represents the most effective methods to find the system fault behavior. This paper proposes a simplified method to detect and avoid the commutations failures in an HVDC inverter. Besides, the inverter is connected to the AC system which is considered as a weak AC grid having a low short circuit ratio. The results are provided by means of the digital real time simulator using the RT-lab platform and achieved by the simulation in the loop approach. Although the real time simulation allows for testing the behavior of these kinds of systems under adverse conditions that rarely occur in the field but are important to evaluate. 12 pulses line-commutated converter HVDC system is used in this study (both stations the rectifier and the inverter are based on thyristor valves). The function of the protection function is validated against the real time simulation of the HVDC inverter following different kind of faults, both kind of fault are the single phase to ground AC fault and a short circuit at the inverter valve. The obtained results show clearly the effectiveness of the proposed method to avoid the commutation failure by ensuring and maintaining a safe commutation process on the inverter station in the worst case.

Development of a pulsed power supply with tens of milliseconds flattop time.

In order to generate a current pulse with tens of milliseconds flattop time on an inductive load for pulse magnetic window technology research, a prototype of pulsed power supply is fabricated and tested. It is composed of several pulse forming units with capacitor banks and a thyristor switch and can generate a flattop current pulse by triggering sequentially the thyristor valve of pulse forming units at certain time or adjust a current waveform by altering triggering time of units. The results show that the prototype with four pulse forming units can generate a current pulse with 5 ms flattop time and 35 kA peak current, and its energy transfer efficiency is significantly higher than that of conventional circuits such as parallel capacitor banks or pulse forming networks.

Gate Current and Snapback of 4H-SiC Thyristors on N+ Substrate for Power-Switching Applications

High-power switching applications, such as thyristor valves in a high-voltage direct current converter, can use 4H-SiC. The numerical simulation of the 4H-SiC devices requires specialized models and parameters. Here, we present a numerical simulation of the 4H-SiC thyristor on an N+ substrate gate current during the turn-on process. The base-emitter current of the PNP bipolar junction transistor (BJT) flow by adjusting the gate potential. This current eventually activated a regenerative action of the thyristor. The increase of the gate current from P+ anode to N+ gate also decreased the snapback voltage and forward voltage drop (Vf). When the doping concentration of the P-drift region increased, Vf decreased due to the reduced resistance of a low P-drift doping. An increase in the P buffer doping concentration increased Vf owing to enhanced recombination at the base of the NPN BJT. There is a tradeoff between the breakdown voltage and forward characteristics. The breakdown voltage is increased with a decrease in concentration, and an increase in drift layer thickness occurs due to the extended depletion region and reduced peak electric field.

Erosion Corrosion Behavior of Aluminum in Flowing Deionized Water at Various Temperatures.

To optimize the operating temperature and flow velocity of cooling water in a high voltage direct current (HVDC) thyristor valve cooling system, the erosion corrosion characteristics of aluminum electrodes in deionized water at various temperatures were studied. With increasing water temperature, the corrosion current of the aluminum electrode gradually increases and the charge transfer impedance gradually decreases, thus, the corrosion of aluminum tends to become serious. The aluminum electrode in 50 °C deionized water has the most negative corrosion potential (−0.930 V), the maximum corrosion current (1.115 × 10−6 A cm−2) and the minimum charge transfer impedance (8.828 × 10−6 Ω), thus, the aluminum corrosion at this temperature is the most serious. When the temperature of deionized water increases, the thermodynamic activity of the ions and dissolved oxygen in the deionized water increases, and the mass transfer process accelerates. Therefore, the electrochemical corrosion reaction of the aluminum surface will be accelerated. The corrosion products covering the aluminum electrode surface are mainly Al(OH)3. With increasing water temperature, the number of pits and grooves formed by corrosion on the aluminum surface increased. In this paper, the molar activation energy Ea and the equilibrium constant K of the aluminum corrosion reaction with various temperatures are calculated. This clarifies the effect of temperature on the aluminum corrosion reaction, which provides a basis for protecting aluminum from corrosion. The results of this study will contribute to research that is focused on the improvement of production techniques used for HVDC thyristor valve cooling systems.

Research on 1100-kV/5500-A Ultra-High Voltage Thyristor Valve Key Technology and Its Application

As direct current (dc) voltage and current increase to 1100 kV and 5455 A, respectively, ultra-high voltage (UHV) thyristor valve will be in complicated electric, magnetic, thermal, and mechanical environments. First, we analyzed the internal mechanism of damping characteristics and accurate internal relationship between electromagnetic transients of damping circuit and proposed its determination method. We developed a correction function of reverse recovery charge for thyristor valve under different working conditions, subsequently. Based on that, we proposed a calculation method for thyristor junction temperature and constructed a model for analyzing thyristor stress. Dielectric test was performed using dummy load and a method was proposed for determining dummy load parameters. The designed UHV thyristor valve successfully passed the full set of tests according to IEC60700-1 standards. Additionally, it has been applied in the Changji-Guquan HVdc project, which possesses the highest voltage level and largest transmission capacity in the world.

HVDC Light Transmission System with Fuzzy Logic Based PI Controller

In the recent times there are many advancements in the sector of power electronic equipment. HVDC transmission system is further developed into HVDC light transmission system by replacing of thyristor valves with IGBT valves. There are a lot of advantages with HVDC light system compared to normal HVDC. When HVDC light transmission is confronted with fault, then the voltage and current vary, and they takes more time to come back to their steady state with PI controller. In this paper fuzzy logic based PI controller is proposed to reduce the time to reach their normal state, and improve the system steady state. The recommended controller is compared with traditional PI (proportional integral) controller for different control requirements, DC voltage, and reactive power under sudden disturbances such as three phase to ground fault. The performance of HVDC light system with traditional PI and fuzzy-PI shall be tested on under three-phase faults using MATLAB-Simulink software.

Upgrading of the Vyborg Converter Substation: Analysis of Lightning Impulse Voltage Withstand Capability

The basic design of the high-voltage thyristor valve developed for the Vyborg converter substation is described. The parameters of the design impulse voltage across the valve and the model of the converter bridge valve assembly are justified. It is established that when subject to a design lightning impulse voltage, the designed valve ensures safe operation in terms of the voltage amplitude and voltage rise rate in the thyristors. It is shown that the difference between the detailed and simplified model does not exceed 3%.