HVDC Converter Research Articles

Exploration of Zr-doped PtSe2 monolayer for sensing C2H2 and C2H4 in oil-immersed transformers of HVDC converter station: A first-principles investigation

Using the first-principles theory, this work purposes Zr-doped PtSe2 (Zr-PtSe2) monolayer as a potential sensing material upon two typical dissolved gases (C2H2 and C2H4), in order to evaluate the operation status of the oil-immersed transformers. Zr-doping on the pristine PtSe2 monolayer is firstly analyzed, yielding the formation energy of −3.12 eV. The Zr-PtSe2 monolayer conducts chemisorption upon C2H2 and C2H4 with the adsorption energy of −1.40 and −1.02 eV, respectively. The electronic property analysis of the gas adsorbed systems reveal the much higher sensing response upon C2H2 molecule, and the recovery property analysis uncovers the reusability for the purposed material. Moreover, the work function is also analyzed to explore the sensing potential of Zr-PtSe2 monolayer based on its change after gas adsorption. The findings pave the way to explore the PtSe2-based material as gas sensors, providing informative guidance for gas sensing material exploration in the electrical engineering.

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

Sensitivity analysis of commutation failure in multi-infeed HVDC systems: Exploring the impact of ac system representations and fault types

This paper investigates commutation failure (CF) in line commutated HVdc converters within multi-infeed HVDC systems and thereby provides valuable insights for the operation and design of HVdc systems. CF susceptibility across various operational scenarios, fault types, and ac system representations at different frequencies is explored. An Electromagnetic Transients (EMT) model is constructed and parameter changes are applied using Monte-Carlo Simulation. The process is fully automated with the use of a controlling program written in Python, which varies parameter values and facilitates result retrieval and post-simulation analysis. A typical analysis results in several hundred thousands of simulation runs, requiring the procedure to be implemented on a parallel computing platform. The results show that two-phase faults are the most critical CF triggers, an aspect often overlooked in previous literature focused on three-phase or single-phase-to-ground faults. Furthermore, the frequency dependent characteristics of the ac network can result in very different CF susceptibilities.

Design and validation of a novel semiconductor area optimised 3300 V SiC half bridge for MMC

AbstractThis article presents the design and experimental validation of a novel semiconductor area optimised 3300 V half bridge with Silicon Carbide (SiC) MOSFETs for HVDC converters. Based on a loss simulation, the problem statement is provided. On this results, a mathematical derivation for the optimised semiconductor area design is executed. After this step, a system loss simulation shows the performance in efficiency and specific output power. Finally, a proof of concept was provided by a scaled hardware test setup to characterise the dynamic behaviour of the novel SiC half bridge design compared to the conventional SiC half bridge.

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.

Failure analysis of polymeric outdoor insulators used in HVDC converter stations

This study presents the performance of polymeric outdoor insulators used in high-voltage direct current (HVDC) converter stations. These insulators typically encounter voltage polarity reversal based on the power flow direction, which may occur within days or months. The accelerated aging investigations are conducted on composite insulators adopting the rotating wheel experimental facility and considering the voltage polarity reversal effect. A two-dimensional model for the evaluated insulators is developed and simulated in COMSOL Multiphysics to comprehensively understand their performance under various environmental conditions. It is observed that the voltage polarity reversal significantly impacts the degradation and aging of composite insulators—the leakage current increases after each polarity reversal with higher surface temperatures. Additionally, unique patterns were observed on both sides of the housing sheds. However, these patterns were only observed on one side in the case of a single polarity. The failure of insulators was noticed after 625 h with multiple consecutive flashovers, indicating the severe deterioration of the silicone rubber housing. In addition, physicochemical analyses were conducted on the aged samples collected from various locations to identify their degradation level. Results obtained indicate a significant deterioration of silicone rubber housing, especially near the shank areas. Moreover, aged samples showed a substantial deterioration of the mechanical and hydrophobicity properties.

High‐frequency resonance in HVDC and wind systems: Root causes and solutions

AbstractThis paper presents methods to model and solve high‐frequency resonance problems in HVDC and wind power systems. Control and digital PWM delays are identified as a common root cause for such resonances. A systematic method to include such delays in small‐signal sequence impedance models is presented. The developed models are fully validated and used to characterize the effects of delay on system stability. In addition to full models that can predict the associated impedance and negative damping behaviour accurately, simplified approximate models are also presented to gain insights. The method is applied to type‐III and type‐IV turbines as well as HVDC converters based on modular multilevel converters (MMC). In addition to resonance of MMC and wind turbines with overhead transmission lines, the paper also identifies several other modes of high‐frequency resonance, including resonance between wind turbines and MMC HVDC converters, among different wind turbines, and between MMC HVDC converters and cables. Different methods to damp high‐frequency resonances are also discussed and demonstrated.

Adaptive fault ride through control of VSM Grid-forming converters

Recently, many high voltage direct current (HVdc) transmission lines are being constructed with the intention of becoming “backbones” of future power grids. Grid-forming (GFM) controlled Voltage Source Converters (VSCs) are anticipated to provide significant stability advantages compared with Grid-following (GFL) VSCs in HVdc converters connected to weak ac grids or to grids with high penetration of renewable inverter-based resources (IBRs). This paper proposes an adaptive fault ride through controller for Virtual Synchronous Machine (VSM) GFM control, which has significantly improved fault ride through capability over the more conventional GFM control with cascaded or switchable current limiting. The performance of the proposed adaptive control is investigated using Electromagnetic Transient (EMT) simulation. The results show that the proposed adaptive control has superior post disturbance recovery from ac faults as well as phase angle shifts, and overcomes the instability reported in GFM converters connected to strong ac networks.

Metrological Qualification of PD Analysers for Insulation Diagnosis of HVDC and HVAC Grids.

On-site partial discharge (PD) measurements have turned out to be a very efficient technique for determining the insulation condition in high-voltage electrical grids (AIS, cable systems, GIS, HVDC converters, etc.); however, there is not any standardised procedure for determining the performances of PD measuring systems. In on-line and on-site PD measurements, high-frequency current transformers (HFCTs) are commonly used as sensors as they allow for monitoring over long distances in high-voltage installations. To ensure the required performances, a metrological qualification of the PD analysers by applying an evaluation procedure is necessary. A novel evaluation procedure was established to specify the quantities to be measured (electrical charge and PD repetition rate) and to describe the evaluation tests considering the measured influence parameters: noise, charge amplitude, pulse width and time interval between consecutive pulses. This procedure was applied to different types of PD analysers used for off-line measurements, sporadic on-line measurements and continuous PD monitoring. The procedure was validated in a round-robin test involving two metrological institutes (RISE from Sweden and FFII from Spain) and three universities (TUDelft from the Netherlands, TAU from Finland and UPM from Spain). With this round-robin test, the effectiveness of the proposed qualification procedure for discriminating between efficient and inappropriate PD analysers was demonstrated. Furthermore, it was shown that the PD charge quantity can be properly determined for on-line measurements and continuous monitoring by integrating the pulse signals acquired with HFCT sensors. In this case, these sensors must have a flat frequency spectrum in the range between several tens of kHz and at least two tens of MHz, where the frequency pulse content is more significant. The proposed qualification procedure can be useful for improving the future versions of the technical specification TS IEC 62478 and the standard IEC 60270.

Analysing the performance of incremental quantity based directional time-domain protection near HVAC cables and VSC HVDC converters

New grid elements such as Voltage Source Converters (VSC) and cables are being increasingly used in today’s power grid. These new grid elements behave differently compared to synchronous machines and overhead lines during transients and faults, impacting the present system’s legacy protection. An increased share of underground cables affects the system’s resonance frequencies whereas VSCs introduce actively controlled current phase shifts during short-circuit faults. Existing research mainly focuses on VSC impacts on phasor-based legacy protection. This paper studies the impact of these new grid elements on incremental quantity-based directional protection, which operates in the time domain, using EMT-type models and hardware experiments. First, cables introduce low frequency oscillations for remote, single phase faults and additionally under weak grid conditions. At low frequencies, these oscillations are not filtered out by the protection, which may result in a decrease of dependability margin (23% of the tested cases) and potential security issues. For the majority of studied cases, the relays worked correctly. Second, fast reactive current injection by a VSC can cause malfunction of incremental quantity protection. Malfunction occurred in 1 case and a decrease of dependability in 10% of the tested cases. This is especially problematic for grids with relatively low synchronous infeed.

New Synthetic Partial Discharge Calibrator for Qualification of Partial Discharge Analyzers for Insulation Diagnosis of HVDC and HVAC Grids.

A synthetic partial discharge (PD) calibrator has been developed to qualify PD analyzers used for insulation diagnosis of HVAC and HVDC grids including cable systems, AIS, GIS, GIL, power transformers, and HVDC converters. PD analyzers that use high-frequency current transformers (HFCT) can be qualified by means of the metrological and diagnosis tests arranged in this calibrator. This synthetic PD calibrator can reproduce PD pulse trains of the same sequence as actual representative defects (cavity, surface, floating potential, corona, SF6 protrusion, SF6 jumping particles, bubbles in oil, etc.) acquired in HV equipment in service or by means of measurements made in HV laboratory test cells. The diagnostic capabilities and PD measurement errors of the PD analyzers using HFCT sensors can be determined. A new time parameter, "PD Time", associated with any arbitrary PD current pulse i(t) is introduced for calibration purposes. It is defined as the equivalent width of a rectangular PD pulse with the same charge value and amplitude as the actual PD current pulse. The synthetic PD calibrator consists of a pulse generator that operates on a current loop matched to 50 Ω impedance to avoid unwanted reflections. The injected current is measured by a reference measurement system built into the PD calibrator that uses two HFCT sensors to ensure that the current signal is the same at the input and output of the calibration cage where the HFCT of the PD analyzer is being calibrated. Signal reconstruction of the HFCT output signal to achieve the input signal is achieved by applying state variable theory using the transfer impedance of the HFCT sensor in the frequency domain.

Design of Nanosecond-Level Transient Electric Field Sensor and Its Application in HVDC Converter Station

In this paper, a nanosecond-level transient electric field (E-field) sensor based on a monopole electrically small rod antenna is proposed. The working principle and design process of the sensor are analyzed in detail while a finite element simulation is carried out to verify the response characteristics of the sensor both in time and frequency domain, whose upper cut-off frequency can reach 680 MHz. For the purpose of assessing the actual detection capability of the sensor in HVDC converter stations, the sensor designed in this paper was used to detect the transient radiation E-field in the valve hall during the artificial short-circuit test in the Zhoushan 1200 kV five-terminal flexible DC transmission project. The measurement results of the transient E-field indicates that the sensor is able to respond instantaneously to the radiated E-field generated by the operation of the key power electronic equipment in the valve hall. It demonstrates that the designed sensor can be effectively applied in the scenario of the measurement of transient electromagnetic environment and non-invasive situational awareness technology for high-voltage devices in HVDC systems.

An MMC based HVDC system with optimized AC fault ride-through capability and enhanced circulating current suppression control

Modular multilevel converter (MMC) is a proven technology for HVDC applications due to its salient features such as modularity and excellent power quality. To ensure best possible grid support, recent grid codes require incorporating fault ride-through (FRT) strategies so that HVDC converter stations remain connected and maintain reliable operation under various symmetrical and asymmetrical AC faults. In this paper, a communication-free enhanced fault ride-through technique without the need of DC chopper has been proposed. The proposed FRT strategy ensures quick post fault recovery operation and can effectively manage DC link and capacitor voltages within safe limits. Along with proposed FRT strategy, in order to avoid high circulating current (CC) inside an MMC, this paper has proposed an optimal circulating current control approach based on proportional resonant and PI controllers in an abc reference frame. The suggested technique lowers the ripple in capacitor voltages while reducing the magnitude of the CC. Under both balanced and unbalanced ac grid conditions, the ripple in the dc link voltage is also reduced without the use of dual synchronous reference frame or any additional controllers. Simulation results confirm the effectiveness of the proposed FRT and CC suppression techniques for a 580-kV, 850-MW MMC-based HVDC system.

Interdependencies in HVDC grid protection: Impact of converter parameters and controls on DC fault‐ride‐through capabilities and protection system design

AbstractFor future multi‐terminal HVDC networks, protection systems based on DC circuit breakers (DCCBs) are envisioned to limit the loss of power transmission towards AC grid(s). Yet, HVDC protection system design is considered a major challenge—especially in a multi‐vendor setup. In particular, protection systems shall enable HVDC converters to ride through DC faults without protective blocking. Several studies have performed sensitivity analyses to determine the DCCB properties and current‐limiting inductor properties that are required to enable converter fault‐ride‐through (FRT), but the HVDC converters themselves have been simplified and kept identical. However, in a multi‐vendor setup, the exact converter design is not known at the time of DC protection planning. Therefore, this paper investigates the impact of different converter properties on the DC‐FRT behaviour, and reveals implications on HVDC protection design. Firstly, analytical approaches are proposed to mathematically derive potential impact factors on the converters’ FRT characteristics, also considering different converter control architectures. Secondly, a comprehensive EMT study demonstrates a significant impact of the converter parameters on the FRT behaviour. Analytical approaches and EMT results are compared with regard to limitations and applicability, such that the paper's findings can support the development of functional requirements for multi‐vendor multi‐terminal HVDC networks.

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.

Research on abnormal discharge of exhaust valve of HVDC converter valve

The MMC converter valve adopts the water-cooled heat dissipation method. The automatic exhaust valve is installed at the upper part of the main water pipe, and the manual ball valve is installed between the automatic exhaust valve and the main water pipe, which is made of insulating material. During laboratory tests, it was found that the switch state of the manual ball valve may lead to an abnormal discharge of the manual ball valve. When the manual ball valve is closed, it is filled with air, and the insulating material bears high voltage, and the air inside will be broken down. This paper studies the abnormal discharge phenomenon of the manual ball valve, analyses the discharging principle, evaluates the influence on converter valve equipment, and proposes an optimal design scheme.

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.

On Fault-Ride-Through Performance in MMC-HVDC Applications Controlled as a Virtual Synchronous Machine

Due to decreasing inertia within the German transmission grid, HVdc transmission links are recently planned as a grid-forming asset in order to contribute to the overall system stability. Here, virtual synchronous machine implementations may be favoured as they enable inertia supply and operation at different grid strengths. However, their fault-ride-through capability is linked to several challenges such as current limitation, the preservation of transient stability and dynamic reactive current injection, especially, while providing high inertia. Moreover, diverse effects occur at the point of connection, such as voltage drop and phase jump effects, which can be traced back to the fault impedance characteristic during a symmetrical fault event and affect the transient stability. Therefore, this article investigates these occurring effects in detail. Based on this analysis, the performance of the virtual synchronous machine is evaluated with respect to different design aspects - namely the reactive power control, the inertia model and design of the phase locked loop - resulting in a recommendation for a design compromise. Finally, observed relations are classified according to their applicability within a typical design of an HVdc converter equipped with half-bridge submodules. Derived design recommendations are validated by simulations in electromagnetic transients software.

Harmonic characteristics of HVDC transmission system and its suppression method system

In this paper, firstly, the generation of characteristic harmonics and non-characteristic harmonics, the content of each harmonic and its amplitude varying with the frequency of HVDC converter are analyzed theoretically, then, the harm of harmonics to HVDC transmission lines is introduced, and some restraining measures are put forward, such as increasing converter pulsation number, installing and improving AC filter to suppress the generation of specific frequency harmonics and reduce their amplitude, so as to increase the stability of the system. Secondly, through simulation based on PSCAD/EMTDC, it is verified that 6K±1 harmonics (where K is odd) can be eliminated by increasing the converter from 6 to 12 pulsations, so as to reduce the harmonic content of transmission lines. Finally, a simulation model is built to verify that the harmonic amplitude of 12K±1 (K is a positive integer) is greatly reduced when the hybrid tuning filter is installed at 12 pulsations.

Research on Flexible HVDC Converter Valve and VBC Redundancy Reliability Improvement

Flexible HVDC transmission play an important role in building a strong smart grid, and it is also very important to design a reliable redundant system for the converter valves and the valve-base controller (VBC). In order to improve the redundancy of the current valve control system, firstly, this paper proposed a new redundant configuration scheme for flexible HVDC converter valve and VBC. Secondly, this paper proposed a switching logic and switching time sequence for the redundant link, in order to make all VBC board hot-swappable when the HVDC system is in operation. Finally, depend on the “Baihetan-Jiangsu” HVDC project, the RTLAB-based test platform was established, which verified the correctness of the redundant system design. The test result shows that the designed scheme improves the redundant system reliability of the flexible HVDC system, and has well-referenced value.