HVDC Converter Research Articles

Optimization of Lead Insulations for HVDC Converter Transformers

This paper explains design considerations for lead and lead-exit insulations of HVDC converter transformer for the purpose of automated design optimization. A HVDC converter transformer should at least deal with AC, DC, and DC polarity reversal stresses, which complicate designing process of insulation structures. Design considerations for those insulations are the types of electrical stresses and design parameters of insulation shapes. The design objective of the optimization is to ensure insulation robustness with reduced size of insulation structures. Optimization results showed 16.72% and 29.07% of length reduction with sufficient robustness, for lead and lead-exit insulations, respectively.

VSC HVDC active damping for the Sylwin 1 offshore wind power plants

SylWin1 is a VSC HVDC link connecting 864 MW offshore wind power generation to Germany's high voltage network. This paper presents a harmonic performance analysis of the offshore network carried out during the design and commissioning phase with a focus on the distortions at the 155 kV and 33 kV offshore networks caused by the amplification of harmonics generated by the HVDC and wind turbine converters. Low frequency harmonic amplification problems are mitigated by introducing active damping in the HVDC converter controls in order to achieve a cost effective solution with the aim of reducing offshore harmonic current and voltage distortions. The analysis was carried out with the help of detailed frequency domain model offline simulations of the offshore network and on-site measurements.

Research on Reliability Assessment of Thyristor in HVDC Converter Valve

With the development of HVDC transmission technology, the efficiency and reliability of the operation of the converter valve have been widely concerned. Because the core device of the converter valve is thyristor, safe and effective operation of the thyristor is the prerequisite for efficient operation of the converter valve. In this paper, the thermal impedance model is established according to the radial heat dissipation direction of the thyristor. The basic electric heating correspondence is used to describe the rate of heat rise per unit time by the rate of change of the heat capacity temperature at each node. Through simulations and calculations, the heat change of each part of the thyristor during converter valve is in operation is analyzed, and the parts where the thyristor of the converter valve is easily damaged are analyzed. This method takes the influence of temperature on thyristor into consideration and provides a new angle to describe the reliability of the operation of thyristor.

Comparison of Current Limiting Effect of an HTS DC Cable Under Different Short-Circuit Conditions

Power transmission with HTS dc cables has the characteristics of high capacity, high efficiency, more compact size, and environmental friendly. It has become an alternative and revolutionary upgrade of the traditional dc power transmission. The self-acting limit function during its quenching for fault current is unique to HTS dc cables. Fault-current-limiting capability of the HTS DC cable in a line-commutated converter (LCC) HVDC system is therefore analyzed with HTS current-limiting features in different short-circuit situations to identify the HTS dc transmission system design and operation features and verification achieved with the HTS dc cable characteristics and parameters, especially the peak value of the fault currents in the LCC HVDC systems.

Zero-Current Switching Control of the Alternate Arm HVdc Converter Station With an Extended Overlap Period

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.

An Enhanced Strategy to Inhibit Commutation Failure in Line-Commutated Converters

Failure of the commutation process is a serious malfunction in line-commutated high-voltage direct current (HVdc) converters, which mainly occurs due to inverter ac faults and may lead to outage of the HVdc system. In this paper, an improved strategy is developed that functions based on the SIEMENS HVdc control system under normal conditions and switches to a designed commutation failure inhibition module (CFIM) during an inverter ac fault. From the response speed point of view, since the designed CFIM does not require any proportional-integral controller, the inverter control system has a quick performance in prevention of the commutation failure. This is achieved by direct measurement of the overlap area using the waveforms of the valves anode-cathode and commuting voltages. In addition, from the accuracy aspect, the proposed method has a superior performance in comparison with the existing strategies. It is because of the fact that by direct measurement of the overlap area, variations of both direct current and the commutation inductance are considered, and hence, the unnecessary increase of the inverter reactive power consumption during the fault and the repetitive commutation failures are prevented. The practical performance and feasibility of the proposed strategy is validated through the laboratory testing, using the real-time Opal-RT hardware prototyping platform. The experimental results demonstrate that the proposed strategy can effectively inhibit the commutation failure or repetitive commutation failures under different fault types by considering the lowest possible reactive power consumption.

Sediment deposition model of grading electrode in water cooling circuit of HVDC converter valve

Sediment deposition model of grading electrode in water cooling circuit of HVDC converter valve

FPGA implementation of impedance‐compensated phase‐locked loop for HVDC converters

FPGA implementation of impedance‐compensated phase‐locked loop for HVDC converters

Electrical Tree Growth Characteristics in Epoxy Resin With Harmonic Superimposed DC Voltage

HVDC converters can generate harmonic voltage, which distorts the DC voltage quality and threatens epoxy resin insulation reliability. In this paper, the electrical tree growth was investigated in epoxy resin under harmonic superimposed DC voltage. The DC voltage ranged from -20 to +20 kV and harmonic frequency ranged from 50 to 450 Hz. This experiment was carried out with needle-plate electrodes system. The effects of DC amplitude and harmonic order were characterized by electrical tree length and accumulated damage. Results show that the electrical tree varies with different combinations of DC amplitudes and harmonic orders. The tree length and accumulated damage experience a non-linear trend with the rise of harmonic order. The low-order harmonics bring more damage to epoxy resin under superimposed voltage. The time to breakdown shows a minimum value at 3 rd harmonic superimposed voltage. The DC amplitude has an acceleration on the electrical treeing process. With higher DC amplitude, the electrical tree breakdown happens immediately when the electrical tree reaches the plate electrode. The charge transport process accounts for the tree initiation characteristics under different superimposed DC amplitudes and harmonic orders. Meanwhile, “reverse tree” has been found in epoxy resin sample. The field-driven tree growth model has been employed to interpret reverse tree growth from the perspective of dynamic electric field distribution.

高压柔性直流换流站控制保护系统的设计与研究

This paper first puts forward the technical requirements for the control and protection system of HVDC project; secondly, it describes the hierarchical structure of the control and protection system of HVDC Converter substation, including system monitoring and control layer, control protection layer and field I/O layer, and discusses the functional requirements and configuration scheme of each layer.

Upgrading of the Existing Bi-Pole to the New Four-Pole Back-to-Back HVDC Converter for Greater Reliability and Power Quality

Back-to-back high-voltage direct current systems are used to transfer electrical power between two asynchronous AC systems. The existing bi-pole back-to-back system (2PBTBS) can be converted into the four-pole back-to-back system (4PBTBS) to save in the required infrastructure for proposed installations. This upgrading provides four parallel 12-pulse DC circuits instead of only two 12-pulse DC circuits as in the existing 2PBTBS. The power transfer capability of each DC circuit in the proposed system is 25% of the original system capacity instead of 50% as in the existing 2PBTBS. The reliability of the proposed 4PBTBS is improved twice, and the line-to-line DC voltage levels are reduced to 50% in comparison with the existing 2PBTBS. In this study, the 4PBTBS and existing 2PBTBS are simulated using MATLAB/Simulink. Simulation result shows that the proposed 4PBTBS has four parallel 12-pules DC circuits at lower line-to-line DC voltage and higher power quality compared with the existing 2PBTBS. These results validate the performance of the proposed system obtained from upgrading the existing 2PBTBS.

Implications of Capacitor Voltage Imbalance on the Operation of the Semi-Full-Bridge Submodule

Future meshed high-voltage direct current grids require modular multilevel converters with extended functionality. One of the most interesting new submodule topologies is the semi-full-bridge because it enables efficient handling of dc-side short circuits while having reduced power losses compared to an implementation with full-bridge submodules. However, the semi-full-bridge submodule requires the parallel connection of capacitors during normal operation, which can cause a high redistribution current in case the voltages of the two submodule capacitors are not equal. The maximum voltage difference and resulting redistribution current have been studied analytically by means of simulations and in a full-scale standalone submodule laboratory setup. The most critical parameter is the capacitance mismatch between the two capacitors. The experimental results from the full-scale prototype show that the redistribution current peaks at 500 A if the voltage difference is 10 V before paralleling and increases to 2500 A if the difference is 40 V. However, neglecting very unlikely cases, the maximum voltage difference predicted by simulations is not higher than 20–30 V for the considered case. Among other measures, a balancing controller is proposed, which reduces the voltage difference safely if a certain maximum value is surpassed. The operating principle of the controller is described in detail and verified experimentally on a down-scaled submodule within a modular multilevel converter prototype. It can be concluded that excessively high redistribution currents can be prevented. Consequently, they are no obstacle for using the semi-full-bridge submodule in future HVdc converters.

The Application of a Novel Infrared Temperature Measurement System in HVDC Converter Valve Equipment Connector Overheat Failure Prevention

Converter valve is the most important equipment in HVDC transmission system, but during the operation and maintenance of HVDC projects, electrical connectors overheat failures in converter valve equipments caused several unexpected trip events, which affects the safe and reliable operation of HVDC projects. Portable infrared temperature instrument is effective to surpervise the temperature of eletrical equipments in valve hall and prevent the overheat failure, but it is carried out manually. The electric company arranges oeprators to inspect the temperature of certain important equipments, which requires a lot of labor and has inspection blind aera. Therefore, on line temperature measurement system based on infrared radiation thermometer is proposed. The infrared temperature instrument performance is affected by ambient temperature seriously, in order to improve the accuracy of the measurement, this paper designs a novel infrared temperature measurement system with a silicon temperature sensor device embedde to provide valve hall temperature. This paper introduces the situation of converter valve equipments overheat failure and infrared temperature measurement technology, then a novel infared temperature measurement system applied in converter valve hall is proposed, both hardware and software design are addressed. At last, experiments are carried out to test the performance, which shows the embedded temperature measurement has high accuracy.

HVDC multi-infeed analysis of the Brazilian transmission system and possible mitigation methods

The Brazilian transmission system is facing challenging problems with the distance between its generation areas and consumer centers that will be partly solved by the use of HVDC point-to-point systems. In the near future, the southeast subsystem will have a large amount of power injected through HVDC Systems in multiple points with relatively close electrical proximity. Therefore, the effects of a multi-infeed system are expected to influence the performance and operation of the network. Extensive studies and simulations will play an important role in determining the extension of the interactions among HVDC converters and determine if such interactions cause multiple commutation failures, thus disturbing the dynamic stability of the system. The use of CCC (Capacitor Commutated Converter) HVDC systems will also be assessed and is expected to diminish the need for a strong AC network (high short circuit level) and, therefore, mitigate multi-infeed interactions. The southeast subsystem of the Brazilian Power System currently has four LCC inverters, two of them belong to the Madeira power plant and the other two are from the Itaipu power plant. By the year 2024, four other HVDC systems will be arriving in the same region. This paper discusses the HVDC multi-infeed phenomena regarding the relevance of using synchronous machine models to represent important power plants and the application of mitigation methods regarding the 2020 network model, where six HVDC links will be present.

Research on protection block strategy of grounding faults in HVDC converter station

Research on protection block strategy of grounding faults in HVDC converter station

Multipoint Synchronized Recordings in Offshore Wind Farms With Continuous Measurement Power Quality Meters

This paper presents measurement techniques for multipoint synchronized recordings in offshore wind farms (OWFs). Wind farms in the German North Sea are clustered and connected to the public grid via large-scale HVdc converters. One of the main risks for the OWF operators is the quality of the grid connection provided by the transmission system operator. Dynamic influences originated in the transmission system or neighboring wind farms cause power quality (PQ) disturbances, and may seriously affect the production of a wind farm up to damages of the equipment. This paper shows how to treat the challenges of continuous multipoint synchronized PQ recordings in the OWFs concerning the expected phenomena, meter placement, event source allocation, instrument transformer usage, and time synchronization. Furthermore, it describes the different device categories and how the records of continuous recording high-resolution PQ devices are compared to the ones of a common oscilloscope. After the description of the applied measurement method, with the product standard and the project specific requirements, the catalog of requirements for the measurement system is defined. Linking the PQ data with the theoretic active power production enables the wind farm operator to measure the influence of the grid connection on the yearly energy yield and therewith can figure out the technical and operational improvements.

Innovative mitigation measures for electrostatic charge build-up on surfaces of dry-type air-core reactors for HVDC application

Equipment installed in the d.c. yards of HVDC converter stations is subject to the electrostatic charge build-up phenomenon. This can result in pollution accumulation on the surfaces of the high voltage equipment operating at high d.c. potential. The charge build-up on the reactor surface may cause discharges across the surface and through the encapsulation of the winding packages. In case of dry-type air-core HVDC reactors the observed pollution build-up takes the form of an irregular pattern of varying sized black spots.

Impedance-based analysis of harmonic instabilities in HVDC-connected Offshore Wind Power Plants

HVDC-connected Offshore Wind Power Plants (OWPPs) with a large number of power converters are prone to instabilities due to interactions with converter controllers. This paper analyzes harmonic instabilities in OWPPs connected through HVDC due to interactions with the offshore HVDC converter and the wind turbine converters. An impedance-based model is used to represent a detailed OWPP with fully-rated converter based wind turbines, including the offshore HVDC converter, wind turbine converters, submarine cables and transformers of the offshore AC grid. Harmonic stability is evaluated both from the grid connection point of the offshore HVDC converter and the WT grid side converters. As a result, the influence that parameters of converter control systems and the offshore grid configuration have on harmonic resonant frequencies and stability conditions is analyzed in depth through a sensitivity analysis. Active damping implemented as a feed-forward control is coordinated in all converters to mitigate the harmonic instabilities. Examples of harmonic instabilities and active damping application are validated with time domain simulations in PSCAD/EMTDC.

Impact of Wind Generators Number and Location on the Resonance Risk of Wind Farm Integration through Flexible HVDC System

Resonances or instability phenomena may occur in wind farms integration through a flexible HVDC transmission system. The relationship between the resonance risk and wind turbines number under different lengths of the line from wind farms to the flexible HVDC converter station is studied in this paper. First, the effect of the line length on the impedance characteristics of interconnected system is analyzed, and the aggregated RLC circuit approach is applied for the quantitative assessment of potential resonance risk. Then, the variation trend of system impedance characteristics caused by different wind turbines number is revealed, and resonance risk affected by the number of wind turbines integrated from the different location is evaluated by calculating system resonance damping around the resonant frequency. Finally, the simulation results on the RT-LAB platform validate the theoretical analysis.

Modelling and resonance analysis of wind power integration via flexible HVDC considering wind speed and wind farm location

Field experience has shown that harmonic resonance may occur in wind farms integration through a flexible HVDC transmission system. The impact on resonance characteristics from wind speed and length of the connection line from wind farm to flexible HVDC converter station is studied in this paper. First, the system model of wind farm integration through flexible HVDC system is established, including the whole structure of electrical model, control structures of converters and the impedance measurement model. Then, the effect of the line length and wind speed on the impedance characteristics of interconnected system is analyzed, and the aggregated RLC circuit approach is used to evaluate the risk of potential resonance quantitatively. Finally, the simulation results on the RT-LAB platform validate the theoretical analysis.