High Voltage Direct Current Converter Transformers Research Articles

A Novel Integrated Switching Strategy for Reliable Starting of Two Terminal MMC HVDC Transmission System

If the initialization protocol of the Modular Multilevel Converter (MMC) system is not adhered to, the primary winding of the system transformer will experience a surge in inrush current, while the secondary winding on the converter side will exhibit significant starting current draw to charge the capacitors of the MMC-H-bridge. This pronounced initial current surge can trigger the activation of protective measures, leading to the disconnection of the High Voltage Direct Current (HVDC) line and thereby engendering power system reliability and stability concerns. This paper presents a starting methodology for a two-terminal MMC HVDC system. The proposed approach entails employing controlled switching techniques to mitigate inrush currents in the HVDC converter transformer, coupled with the utilization of pre-insertion resistance circuit breakers to diminish starting currents on the converter side. The efficacy of the aforementioned strategy is validated through simulations conducted on a two-terminal MMC HVDC system modeled using PSCAD/EMTDC, with the results confirming its effectiveness.

Influence of Thermal Aging on Dielectric Properties of Natural Ester Oil-Impregnated Paper

Mineral oil is widely used as a dielectric liquid in high voltage direct-current (HVDC) converter transformers because of its excellent electrical insulating and cooling properties. Due to the low biodegradability and high fire risk of mineral oil, natural ester oil is gaining considerable interest as a potential alternative for HVDC converter transformers. This requires more information thorough understanding of the electrical behaviours of natural ester oil under various conditions. This paper compares the impact of thermal ageing on the space charge characteristics of natural ester oil-impregnated paper to mineral oil-impregnated paper. During thermal ageing, although the increased acidity caused by the natural ester oil led to more charge accumulation in natural ester oil-impregnated paper compared to the mineral oil, the natural oil-impregnated paper showed a higher DC breakdown voltage.

3‐D thermal analysis for the valve‐side bushings of HVDC converter transformer and application in the evaluation of current‐carrying connections defects

To analyze the overheating problem of the valve-side bushings of high voltage direct current (HVDC) converter transformer, which is mainly caused by current-carrying connections defects at copper–aluminium transition region in the bushing, this paper set up a 3-D electromagnetic-thermal analysis model for the valve-side bushings, and a method was proposed to calculate the thermal distribution of the valve-side bushings under the influence of current-carrying connections defects. Based on this model, the details of an example were shown in this paper, and the numerical results agreed with the measured results within 1%. Then, the temperature field distribution characteristics of valve-side bushing under different working current I, defect degree k, and ambient temperature Tamb were determined. According to the characteristics, the temperature difference ΔT and working current I of the bushing were applied to evaluate contact resistance at the copper–aluminium transition region of the bushing, which can be provided by a novel method to evaluate for current-carrying connections defects in the valve-side bushings.

Evaluation of lightning overvoltage at neutral point of HVDC converter transformer based on EMTP

In order to ensure the safe and stable operation of high voltage direct current (HVDC) systems, this paper studies the lightning overvoltage at the neutral point of the HVDC converter transformer. The work first introduces the basic types of lightning overvoltage based on the CIGRE Benchmark 500 kV HVDC transmission system which is modeled in the electromagnetic transient program (EMTP) software. The high-frequency model of the HVDC transmission system is considered where lightning overvoltage at the converter transformer neutral point is studied at different values of the small reactance connected at neutral point grounding and different lightning strike locations. Although increasing the small reactance value can effectively suppress the lightning current, the lightning overvoltage will gradually increase. When the neutral point is grounded by 11 Ω small reactance, the lightning overvoltage is most serious, where the peak value can reach 415 kV. Also, the farther the lightning strike position from the converter transformer, the smaller the lightning overvoltage.

Design Requirement and DC Bias Analysis on HVDC Converter Transformer

The converter transformer plays a critical role in the high-voltage direct current (HVDC) transmission system, which connects but separates the AC grids and the converter. This paper briefly introduced the essential design requirement of the HVDC converter transformer in a system at ±800kV level and analyzed the DC bias influence on the transformer. The DC bias current effect simulations are analyzed in MATLAB and ANSYS Maxwell in this paper, respectively.

Analysis and Disposal of Typical Breakdown Failure for Resin Impregnated Paper Bushing in the Valve Side of HVDC Converter Transformer

This paper presents analysis, diagnosis and disposal with a typical internal breakdown failure of the resin impregnated paper (RIP) valve side bushing in high voltage direct current (HVDC) converter transformer. Based on the analysis of fault current characteristics at the time of the RIP valve side bushing failure, and field test results of insulation parameters, a method of diagnosing typical breakdown failures of valve side bushings is proposed. Through disassembly inspection of the internal overheating and arcing traces on the failure bushing, the root cause of this typical breakdown failure is found, which is upper axial flashover along the RIP condenser/SF6 interface caused by the abnormal contact of two current-carrying conductive tubes. Temperature distribution inside the bushing with an abnormal contact resistance between the copper conductive tube and aluminum conductive tube under different load current is simulated by using the finite element method. An special device is also developed for repairing defective bushing on-site, and 75 bushings with conductive contact defects have been repaired on the premise of not pushing converter transformers away from the valve hall and even without pulling out defective bushings.

Design optimization for the insulation of HVDC converter transformers under composite electric stresses

This paper proposes a design optimization method for the insulation structure of a High Voltage Direct Current (HVDC) converter transformer, which is exposed to AC, DC, and DC polarity reversal (DCPR) stresses. Since DC and DCPR stresses provide significantly different electric field distributions, conventional transformer insulations designed for AC systems cannot guarantee safety against electrical breakdown. Focusing on main-winding and end-winding insulation including pressboard barriers and oil between windings, this paper presents a design solution based on genetic algorithm (GA). By exploiting modern computing power, the proposed approach can save a considerable amount of effort and time compared to conventional approaches which depend on experimental data and human experience. From the comparison with example reference insulation structures, we verified that the optimized structures maintain the required insulation performance with the reduction of length by 2.8% for the main-winding insulation and by 13.8% for the end-winding insulation.

Calculation method of winding eddy‐current losses for high‐voltage direct current converter transformers

In this paper, a correction method is introduced to make a more accurate calculation of winding eddy-current losses of high-voltage direct current (HVDC) converter transformer. The assumption that winding eddy-current losses of HVDC converter transformers follow the frequency with the exponent 2 proposed by IEC 61378-2 Std. is thought to be inappropriate and the skin effect should be taken into consideration. The frequency characteristic of winding eddy-current losses is proposed by analysing the distribution of magnetic flux in conductors, in which skin effect is taken into consideration. The correction method with a correction factor is then introduced to correct the assumption. To verify the validity of the correction method, a two-dimension axisymmetric finite element analysis of a HVDC converter transformer is conducted. The geometrical complexity of windings and electrical connection are fully taken into account, it is shown that the correction method shows a significant advantage. Influence of different methods is revealed by applying the actual non-sinusoidal load current and it is shown that the error of correction method is the least. At last, harmonic losses measurements are conducted on three coils to investigate the limits of the correction method.

Simulation of the Electromagnetic Response Characteristic of an Inductively Filtered HVDC Converter Transformer Using Field-Circuit Coupling

A field-circuit coupling computation method is proposed in this paper to reveal the special electromagnetic characteristic of a novel inductive filtering (IF) method applied in high-voltage direct current (HVDC) converter station. The new converter transformer is modeled by using 3-D finite-element method, and the surrounding fully tuned branches, for creating precondition of implementing the IF method, are modeled by using the coupled circuits. The harmonic characteristic of the load-side current is modeled by using mixed harmonic current sources. The parameter design of the coupled circuit is also performed for the coupling model. Following this, based on the established coupling model, the multiphysical characteristics (e.g., electric field, magnetic field, excitation characteristic, and core losses) are analyzed in detail. The system simulation and the experimental results validate the modeling method. All of the research results indicate that the IF method can greatly reduce the harmonic currents in the grid winding, can improve the excitation performance, can improve the electromagnetic environment, and can reduce the harmonic losses of the HVDC converter transformer.

On-line monitoring of partial discharges in a HVDC station environment

This paper deals with the on-line measurement of partial discharges (PD) in a high voltage direct current (HVDC) converter station. The HVDC station is a particularly challenging environment for measurement due to elevated interference levels caused from the switching of thyristor-controlled converters. In this work, online PD measurements were performed on two specimens; one is an HVDC converter transformer and the other an HVDC converter wall bushing. The measurements were performed in the high-frequency range from 400 kHz to 30 MHz with modern wideband PD measurement instrumentation. Results demonstrate that on-line measurement of PD in an HVDC station environment is possible and that a combination of input filtering and modern signal processing methods for feature extraction can be used to mitigate the converter interference. The feature extraction method used plots partial discharge data on a time-frequency classification map. The map enabled isolation of individual PD phenomena, and demonstrated that trending to monitor insulation degradation is possible with online partial discharge measurement. Furthermore, specific strategies for PD measurement and analysis are developed for each of the transformer and bushing specimens.

A Two-Dimensional Cloud Model for Condition Assessment of HVDC Converter Transformers

Converter transformers are the key and the most important components in high voltage direct current (HVDC) power transmission systems. Statistics show that the failure rate of HVDC converter transformers is approximately twice of that of transformers in AC power systems. This paper presents an approach integrated with a two-dimensional cloud model and an entropy-based weight model to evaluate the condition of HVDC converter transformers. The integrated approach can describe the complexity of HVDC converter transformers and achieve an effective assessment of their condition. Data from electrical testing, DGA, oil testing, and visual inspection were chosen to form the double-level assessment index system. Analysis results show that the integrated approach is capable of providing a relevant and effective assessment which in turn, provides valuable information for the maintenance of HVDC converter transformers.

Reliability assessment of 3-phase three winding converter transformer of HVDC transmission system

High voltage direct current (HVDC) is today recognized as an effective and efficient means of transmitting bulk power over long distances through over head lines or cables. The reliability of HVDC links have always been of primary concern in planning and operating of power systems. The existence of spares and maintenance practices do affect the availability of HVDC stations. The HVDC converter transformer plays a relevant roll in HVDC converter station, not only due to the direct cost associated to them but also causes its availability affects the HVDC transmission capability (or) capacity. In this paper the effect of converter transformer failure rate, repair time on expected capacity level of three phase three winding converter transformer with and without spares is analyzed and compared. It also effect of number of spares on expected capacity level is presented. Key words: HVDC transmission system, converter transformer, unavailability, Morkov modeling.