HVDC Converter Transformers Research Articles

Thermal ageing study on palm oil methyl ester-impregnated thermally upgraded kraft paper insulation for reliable application in HVDC converter transformers

Thermal ageing study on palm oil methyl ester-impregnated thermally upgraded kraft paper insulation for reliable application in HVDC converter transformers

Design and testing of an arc resistant power transformer with a reinforced turret

This paper presents the design and testing of a full-scale 510-kV, 415-MVA HVDC converter transformer prototype designed to withstand the resulting pressure of a 20 MJ low-impedance fault in oil occurring inside a bushing turret, which is one of the major causes of transformer fires. In addition, there is currently few studies and recommendation regarding this failure mode, which is the main motivation for initiating this study. In total, two full-scale units were designed and built: First consisting of a reinforced turret and a transformer tank designed for arc resistance using nonlinear static finite-element analysis with design pressures retrieved from explicit dynamic simulations, and a second reference unit, with a conventional turret design. Both units were then experimentally tested with a methodology based on injecting pressurized air into the water-filled units, with test equipment dimensioned and calibrated to achieve a linear injection of energy equivalent to a 23 MJ arc in oil in 100 ms. The reinforced turret and transformer tank was able to contain the arc energy and performed as predicted by the nonlinear static and explicit dynamic simulations. The energy levels used in tests are, to our knowledge, the highest reported for a low-impedance fault occurring inside a turret, demonstrating the feasibility of designing a transformer to safely withstand mentioned faults without rupturing. The suggested reinforced turret is a robust and maintenance free solution offering passive protection against internal arcing, without the need of additional relive devices. It will significantly reduce the risk of fires, improve overall safety of the substation, and has the potential to be optimized for different arc energy levels and voltage class for turrets, cable boxes and chimneys.

Research progress and prospect of insulation characteristics of HVDC converter transformer bushing

The paper mainly analyzes the development of the high voltage bushing is closely related to the development of power industry. With the gradual rise of the voltage level in power industry, bushing has experienced the development process from low voltage, high voltage, ultra-high voltage to ultra-high voltage. In order to meet operation requirements under the different voltage levels, the pure porcelain bushing, metal shielded bushing and capacitive bushing have also appeared in the bushing body structure, and the core material has gradually changed from the oil impregnated paper and the adhesive paper to adhesive impregnated paper. In the process of the continuous development of bushing structure and main insulation materials, the research on the high-voltage bushing mainly focuses on the material performance test of bushing, the improvement of the main insulation structure design method, and the simulation analysis of bushing voltage and heat sharing.

Multi-Objective Design Optimization for HVDC-LCC Converter Transformers: Analytical and FEA-Based Comparison

The emerging HVDC technology has been used for long-distance power transmission, increasing flexibility to the power systems, handling asynchronous interconnections, crossing long-distance submarine cables, unusual loading, and generation profiles, and improving energy market relations. The HVDC converter transformers are designed based on system parameters, that directly affect the core and windings geometries, thus the weight and operating losses. In a standard design process, the designer adjusts the active part dimensions, until the constructive and specified aspects, moreover, the technical standard restrictions, are satisfied. The paper’s main contribution is to formulate analytic equations, in a way that losses and weight can be obtained for several options of core and windings geometry. However, weight and losses are opposite objectives in the search for an optimal solution. Understanding this compromise between opposing goals is relevant to the equipment learning process. In this context, another significant contribution of the paper is to carry out a formal optimization process through the analytical formulation developed. To minimize the weight and operating losses, and subject to IEC standards and constructive restrictions, the multi-objective Genetic Algorithm has been used to search for the Pareto Frontier. Far beyond the chosen solution, the non-dominated frontier obtained for each transformer design, allows the designer to learn about the equipment and its operation, leading to a continuous improvement of the proposed methodology. The analytical formulation is validated by an alternative numerical methodology for winding harmonic losses and short-circuit impedance verification, providing meaningful confidence for the applied method.

Research on Influence of Winding and Core Vibration Under Harmonic of Hvdc Converter Transformer

Research on Influence of Winding and Core Vibration Under Harmonic of Hvdc Converter Transformer

Feasibility analysis of neutral grounding by small reactor of HVDC converter transformer

Nowadays, the scale of power grid is remarkably increasing and there is a continuous reinforcement of AC–DC hybrid systems. Thus, the single-phase grounding fault currents on the AC grid-side of rectifier and inverter stations increase significantly, which may exceed the capacity limit of the circuit breakers. The large short-circuit currents would result in problems during equipment selection and threaten the safety of equipment and personnel. To solve such issue, the effect of the converter transformer on the single-phase grounding fault current on the grid-side is analyzed. The mechanism of high short-circuit current in DC near region is revealed. Also, the feasibility of neutral point earthing via small reactor at HVDC converter transformer to suppressing the single-phase grounding fault current is verified. In addition, the principle of selecting the suitable small reactor is summarized. The results show that the neutral point earthing via small reactor at converter transformer can effectively limit the single-phase grounding fault current on the AC-side. However, it would lead to an increase of overvoltage at the neutral point of HVDC transformer. Therefore, neutral grounding by small reactor might be implemented when the insulation level is satisfied.

Research on hidden dangers of oil-flow relay in HVDC converter transformer

HVDC converter transformer oil-flow relay plays an important role in realizing non-power protection of converter transformer on-load tap changer, which can act accurately and directly affect the safe and stable operation of the whole HVDC program as well as the converter transformer itself. In this paper, the research revolves the hidden dangers of HVDC converter transformer and its identification. The paper begins with the analysis of the converter transformer operating principles of two German companies: MR and EMB. Then, the current application status and related inspection requirements of oil-flow relay are expounded. Finally, pertinent hidden danger identification and modification schemes are provided based on analysis of the danger in actual operation of oil-flow relay and its causes. The feasibility and effectiveness of the scheme are verified by the actual operation on site.

Residual flux impact in controlled switching of HVDC converter transformer

Abstract Converter transformers are an interface point of HVDC frameworks and are basic hardware from activity and reliability view point. Converter transformers are turned off intentionally under planned support and un-deliberately during fault conditions. Random energization of HVDC Converter transformer delivers high inrush currents due to core saturation asymmetries and endanger equipment health. In this paper, charging approach has been proposed. By utilizing point on wave exchanging for moderation of charging drifters during charging of 888 MVA 400/325 kV Converter Transformer of ±800 kV HVDC Sub-station. The proposed procedure has been applied on HVAC electrical Circuit Breaker furnished with Pre-Insertion Resistor considering diverse degree of residual flux. Performance of proposed philosophy has been affirmed by field tests and reproduction in PSCAD programming package and is found very viable. Near investigation is done in this paper and shows that with the use of proposed system, the switching transient during stimulation and field tests of coupled three phase converter transformers is decreased to worthy level.

Research on distribution characteristics of vibration signals of ±500 kV HVDC converter transformer winding based on load test

The converter transformer is important for the high-voltage direct current transmission system. The distribution characteristics of the vibration signal of the tank are significant for the operation status monitoring and the judgment basis for the fault condition of the converter transformer. However, most present research on vibration detection technology focused on ordinary power transformers rather than the HVDC converter transformers. Moreover, the influence of the special structure and the heat accumulation effect of the load current on the vibration signal distribution characteristics is not fully considered. To solve the above problems, this paper analyzes the intensity distribution of vibration signals at different positions on the tank surface and valve side bushings based on the offline load test. At the same time, to study the influence factors of vibration signal distribution, the influence of current heat accumulation on winding characteristics and transformer oil temperature is analyzed. The results show that vibration intensity is related to location and the load current. And the vibration intensity of the tank surface increases with the time of current flowing through the winding, nevertheless, the vibration intensity tends to decrease near the valve-side bushing. The test methods and research on distribution Characteristics in this paper can provide new thoughts for the optimization of the layout of converter transformer vibration detection sensors, and provide a research basis for the application of vibration signal fault diagnosis methods.

Electro-thermal comprehensive analysis method for defective bushings in HVDC converter transformer valve-side under multiple-frequency voltage and current harmonics

The insulation failure mechanism of the resin impregnated paper (RIP) valve-side bushings of HVDC converter transformer under high ambient temperature and strong electromagnetic field conditions is explored in this study via a novel electro-thermal comprehensive analysis method. Considering that the bushings are operating under Multiple-Frequency Voltage and Current Harmonics, a superposition analysis method is established for voltage and current with five frequencies based on a Fourier spectrum analysis of the rated voltage waveform and full-load current waveform. Calculation methods for the heat loss, temperature field, and electric field of RIP bushings at the converter transformer valve-side are then established under two defect conditions: (1) overheating defect caused by an abnormal spring contact at the copper-aluminium conductive tube transition joint and (2) electric potential suspension caused by disconnection between the outmost condenser’s foil layer and grounded flange. An overheating defect in a valve-side bushing with spring contact at the conductive tube is successfully identified by the proposed method to prove its effectiveness. This method can be used to optimize bushing insulation and current-carrying structures as well as to diagnose insulation failures in RIP valve-side bushings.

Effect of Voltage Waveforms of HVDC Converter Transformer on Lifetime Characteristics

The converter transformers are reported to fail generally on the thyristor-valve side, where complex alternating voltage waveforms superimposed with dc voltages would occur. The effect of these waveforms on the failure and endurance of the valve side winding insulation of the transformers is not yet understood, which is investigated here. In this paper, the voltage waveforms occurring in a converter transformer are obtained through PSCAD/EMTDC for a typical double pole ±500 kV, 1000 MW DC-link HVDC transmission system based on the CIGRE benchmark system. The waveforms are then amplified using a High Voltage amplifier for experimental breakdown investigations. Stepped-stress damage equalization method has been used for obtaining the lifetime characteristics from failure data. A comparison with life curve under pure sinusoidal and actual alternating voltages with superimposed dc and harmonic voltage has been made to understand why the converter transformers generally failed on the valve side. The results first time give fundamental reasons for the possible failure of the converter transformers and put forth important design considerations.

A comprehensive analyses of aging characteristics of oil-paper insulation system in HVDC converter transformers

This paper investigates the dissolved gases evolution in transformer oil under combined DC/AC electrical-thermal stress. Dissolved gases detected in transformer aged insulation oil reveal that oil under AC electric field combined with thermal stress can produce more dissolved gases than oil under DC electric field with thermal stress but less than the gases produced in oil under distorted AC or combined AC/DC voltage stress. This is attributed to the divergent migration properties of the charged components under different types of electric fields. To further understand this behavior, carrier recombination coefficient is proposed to explain the oil DGA results under various aging stresses. Results show that diagnostic parameters such as breakdown voltage, oil interfacial tension, and moisture content in pressboard should be used along with DGA results in order to accurately diagnose the insulation condition within converter transformers that impose a combined AC/DC voltage stress on the insulation system.

3D modeling of an HVDC converter transformer and its application on the electrical field of windings subject to voltage harmonics

Converter transformers are key facilities in HVDC systems. The operation condition of converter transformers is more complex than that of conventional AC transformers. Because of the superposition of the voltage harmonics, the valve voltage wave is more serrated than a pure AC voltage. As a result, the insulation system withstands severe stress. Several tasks are accomplished in this study to obtain a better understanding of the electrical field distribution in converter transformers as well as analyze the effects of voltage harmonics on the main insulation of converter transformers. First, an HVDC system with a voltage of ±800 kV is simulated and the voltage harmonics at the valve side are obtained. The simulation is verified based on the measurement in a real ±800 kV HVDC substation. The harmonics in valve winding are mainly characterized by components with frequencies of 6k, 12k, 12k ± 1, and 6k ± 1. Then, a full-scaled 3D converter transformer model is proposed. The electric field of the proposed model is calculated under AC, DC, and polarity reversal voltages. Compared with conventional 2D models, the inhomogeneity of the electric field distribution is obtained, and the difference can reach 30%. Finally, the valve voltage waveform is applied on the 3D model to calculate the electric field using finite element method. Results show that the voltage harmonics in valve side further deepen the inhomogeneity of electric field in insulation structures. Moreover, a high value of voltage changing rate (dV/dt) with value 2.96 kV/μs is observed in simulation. The influence of defects on partial discharge within insulation materials under the voltage harmonics is also analyzed.

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.

Study of the Stray Losses Calculation in Structural Parts for HVDC Converter Transformers Based on the TEAM Problem 21 Family

The stray losses calculation method proposed by the IEC 61378-2 standard is analyzed in this study. The fundamental stray losses in different currents and the harmonic stray losses of the TEAM Problem 21 Family members are analyzed to verify the IEC method. It is shown that stray losses are approximately proportional to the square of the current, but do not follow the frequency to the power of 0.8. The IEC method typically produces large errors. A corrected calculation method is thus proposed in this study, in which the magnetic-field distribution is considered. The corrected method is verified in comparison with experimental results and the IEC method.

Space charge characteristics in oil and oil-impregnated pressboard and electric field distortion after polarity reversal

The formation of space charge in the oil/paper insulation under HVDC condition could influence the electric field distribution. In the case of HVDC converter transformer, the distortion of the electric field may affect its performance. It is, therefore, important to analyze factors that can affect space charge formation and dissipation characteristics such as temperature, moisture, electric stress and ageing process. This paper mainly focuses on the effect of electric stress and oil ageing on space charge characteristics in oil/paper insulation system. The pulsed electroacoustic technique (PEA) method was used to monitor space charge dynamics. This data was further incorporated into a model established using COMSOL software to estimate space charge impact on electric field distribution, with an emphasis on the electric field after polarity reversal operation. The results show that the electric field arising from the space charge formation is much higher compared to the average electric field after the polarity reversal operation. Moreover, the electric field caused by space charge decreases faster for the aged oil-impregnated pressboard compared to the fresh one, which could be related to different space charge dissipation characteristic related to the condition of the oil.

Transient electric field characteristics in oil-pressboard composite insulation under voltage polarity reversal

Polarity reversal generally enhances the electric field intensity in oil-pressboard composite insulation, leading to partial discharge and even insulation flashover or breakdown of the HVDC converter transformer. In an attempt to study the transient electric field in oil at polarity reversal, the present research adopted the Kerr electro-optic effect technique to conduct real-time measurement of the oil electric fields at the polarity reversal time of 10, 60 and 120 s respectively. Dielectric interface conditions were also adopted to capture the dynamic development of interface space charges. Research results indicate that (i) negative charges, under the absorption of insulation pressboard, are more likely to accumulate at the oil-pressboard interface. As a result, the development of electric field in oil at polarity reversal demonstrates prominent polarity effect, with faster electric field attenuation under negative voltage; (ii) the development of electric field in oil shows asymmetric features under double polarity reversals. The first reversal witnesses a high magnitude of electric field in oil that is about 1.47 times of the initial capacitive electric field, whilst the second reversal sees only 85% of the initial capacitive electric field. Such asymmetric development is mainly attributed to different accumulation performances of the positive and negative charges; iii) the shorter the polarity reversal time is, the higher the oil electric field strength will be. The maximum magnitudes of the field strength after polarity reversals with different time, however, are not much different from one another.

English

4 ABSTRACT: This paper intends to detect the inter-turn fault by performing impulse test on HVDC Converter transformer by Coherence Function method. The inter-turn in a transformer (due to insulation failure) is located in impulse test by neutral current method. In the neutral current method, a record of the impulse current flowing through a resistive shunt between the neutral and ground point is used for detecting the fault. Neutral current so measured or calculated is in time domain. Theoretically the calculated neutral current, both for healthy winding and turn-fault winding is subjected to FFT to convert time domain into frequency domain of real and imaginary values. The real and imaginary values are used to calculate the coherence function. Coherence function compares both with and without fault neutral current in frequency domain, so that fault can be identified. Coherence function compares input and output at each frequency.It displays output i.e coherence value as 1 if input and output at that frequency is same other wise less than 1. 1 indicates that there is no fault at that frequency and decreasing coherence value (<1) indicates fault at that frequency.

Step Size Selection for Tap Changers in Converter Transformers for Line Commutated HVDC Systems

This paper deals with the tap changer design for line-commutated HVDC converter transformers. It examines the impacts of tap changer switching to provide criteria for selecting an optimal step size. When switching the tap changer the firing angles in the converter change and with them the reactive power consumption. The dependence of the changes on the step size of the tap-changer is linear, so that a maximum step size of the tap changer can be calculated based on criteria for maximum allowable changes of firing angle and reactive power. For implementation into an existing AC system the analysis of tap changer switching has to consider the interaction between converter reactive power and AC voltage iteratively to derive a stable operating point of the HVDC after switching. Both effects are influenced by the reaction of the AC-voltage.

Dielectric properties of transformer oils for HVDC applications

The knowledge of the behavior of electric conductivity in mineral oils for possible applications in HVDC converter transformers is of paramount importance for proper design of their insulation system. This study presents the results of measurements of dielectric properties of various transformer oils by means of frequency response technique as well as time domain measurements, including measurements of ion mobility using the reversal polarity method. Influences imposed by varying the measurement voltage level and temperature are investigated. Some important parameters of the investigated oils, e.g. their conductivities, ion mobilities, ionic concentrations and effective ionic radiuses are compared and discussed. The results show that despite of similarities in various physical parameters of insulating mineral oils available on the market, the dielectric behavior and especially ionic conduction vary greatly between different oil types. Changes of these properties with temperature are characterized by different activation energies.