High Voltage Direct Current Converter Research Articles

State of the Art and Future Trends of Unconventional PD-Measurement at Power Transformers

This paper discusses the current state of the art of diagnostics at power transformers. A special focus is set on the UHF-PD-measurement (ultra-high-frequency partial discharge measurement) because at power transformers, this diagnostic method has become more important in recent years. The current state, basics and principles of operations, proceedings as well as advantages of PD-measurement methods are covered. Furthermore problems and proposed solutions are discussed. Bushings and tap changers are not discussed in detail. In many cases, one single diagnostic method does not have the ability to sufficiently evaluate a power transformer. Therefore, a variety of diagnostic methods came up over times, which are commonly used by now. To expand the evaluation opportunities of power transformers, science strives to develop new diagnostic methods as well as to improve the existing ones. Furthermore, environmentally friendly and hardly inflammable ester liquids are examined for the use at power transformers and PD-measurement at HVDC (high voltage direct current) converter transformers as well. Potential diagnostic options and respectively current developments and findings in the field of oil-paper-insulation systems are outlined conclusively.

A combined AC/DC optimal power flow algorithm for meshed AC and DC networks linked by VSC converters

Summary A new method to optimize the power flow for a meshed alternating current (AC) network in parallel with a meshed direct current (DC) network in a single optimization run is proposed. Both networks are coupled by voltage-sourced converters high voltage direct current converters. The approach for this algorithm is the extension of the standard AC optimal power flow definition by an additional set of linear and non-linear constraints to define all DC network equations. The systematic method to describe the DC system equations provides the needed flexibility to define DC networks with a high number of nodes, voltage-sourced converters, and DC network meshes. The method is applicable for all optimization solvers. The implementation and validation of the method is performed using MATPOWER (Power Systems Engineering Research Center - PSERC), an open source license-based power flow package. Copyright © 2014 John Wiley & Sons, Ltd.

Control of high voltage direct current links with overall large‐scale grid objectives

When a high voltage direct current (HVDC) link connects two distinct power systems, the latter are usually modelled by two infinite buses. However, in some context of use, the HVDC links are no longer separating two systems but they are embedded into meshed AC systems in parallel with other AC lines and interact with other elements of the power system. This led us to enhance the control objectives by considering stability requirements. In this context, the previous modeling assumption presents serious limitations. Instead, a more efficient control model is used. It is developed such that the concerned dynamics of the system are taken into account during the synthesis. It is shown how the controllers of the HVDC converters can be synthesised using the control model mentioned above such that global grid performances are ensured, in addition to the local objectives like power and DC voltage control. In order to avoid remote measures, an output feedback control which uses only local measures is developed. The new controller is tested in comparison with the standard vector control and a non-linear feedback linearisation control via simulation tests. The tests showed that the controller synthesised using the new control model contributes to a better coordination of the control actions and thus improve grid performances. In particular, the transient stability of the neighbour zone is a priori taken into account at the synthesis level.

PSCAD based multi-infeed HVDC system simulation validated by a recorded fault

More and more high voltage direct current (HVDC) converters are being located in a load area in the Yangtze River Delta Region in China. It’s large transmission capacity and intensive placement are hardly seen in other countries. Accurate and reliable study tools and methods are extremely needed for power system engineers and researchers to deal with such problems which were never met before and otherwhere. A novel approach on electromagnetic modeling of alternating current / direct current (AC/DC) system is proposed and simulation is carried out to replay a real AC 3-phase to ground fault on a multi-infeed high voltage direct current (MIHVDC) system. AC system is specially simplified. Dynamic models and real parameters are adopted in main AC/DC equipments concerned and retained. Modeling and simulation are based on the power system computer aided design/electro-magnetic transient in direct current (PSCAD/EMTDC) system software package. Comparisons between simulation results and the records from the fault recorders are studied including both AC and DC quantities.

A Series-Connected Topology of a Diode Rectifier and a Voltage-Source Converter for an HVDC Transmission System

In this letter, a novel converter topology of high-voltage direct current (HVDC) transmission for integrating the offshore wind farm (WF) with the grid is proposed, which consists of a series connection of the 12-pulse diode rectifiers and a voltage-source converter (VSC). For this topology, the PCC (point of common coupling) voltage of the offshore WF side is regulated at a constant magnitude and constant frequency by the WFVSC. Also, the current at the PCC becomes almost sinusoidal since the WFVSC functions as an active power filter for the 11th- and 13th-order harmonic current components. By virtue of the diode rectifier, the cost and power loss of HVDC converters is reduced compared with the case of the conventional fully rated VSC, whereas the performance is kept almost the same. The PSCAD/EMTDC simulation results for a 500-MW HVDC transmission system have been shown to verify the validity of the proposed scheme.

Modeling and Analysis of HVDC Converter by Three-Phase Dynamic Phasor

A three-phase dynamic phasor (TPDP) model of high-voltage direct current (HVDC) systems, especially the converter, is proposed. The proposed model can rapidly solve the dynamic characteristics of the converter for symmetrical and asymmetrical operating conditions: 1) the symmetrical faults occur at the ac side or the dc side and 2) the asymmetrical faults occur at the ac side of the converter. Switch functions are used to indicate the commutation process of the converter. By using the sequence component method, it is observed that the vd should contain the 0th and 2nd components and ia,ib,ic should contain the 1st and 3rd components. The TPDP model of the HVDC converter is proposed based on these observations. The interface and the details to establish the proposed model are presented and a method to represent the commutation failure in the TPDP model is also proposed. Simulation results show that the proposed model is effective and acceptable.

Research on the Whole Condition Loaded Noise Measurement Circuit of Power Capacitor

Power capacitor is one of the key equipments of the high-voltage direct current (HVDC) transmission system. It is also the main noise source difficult to control. Presently in the world, there are none explicit requirement and regulation on the evaluation and measurement of the noise generated by the power capacitor, which has seriously restricted the development of the further research on the noise control in power capacitor. So the research on the noise measurement method and technology has extremely important significance in scientifically evaluating the noise of power capacitor. Power capacitor in HVDC Converter Station under practical conditions has apparent fundamental frequency and rich harmonic frequency components. In order to simulate this condition, a whole condition loaded bridge circuit of noise measurement on power capacitor has been designed and studied. Two arms of the bridge are used to load fundamental frequency and a parallel resonance compensation inductance is used to reduce the fundamental current, while the other two arms are used to load harmonic frequency. The harmonic frequency load power supply composed of harmonic signal generator and power amplifier which can simultaneously load any combination of harmonics under 48th to simulate the various actual working conditions. The tests indicated that the noise spectrum measured under the designed circuit completely accorded with the one in HVDC Converter Station.

S-Transform-Based Classification of Converter Faults in HVDC System by Support Vector Machines

Based on Support Vector Machines (SVM) and S-transform, a novel approach to detect and classify various types of high voltage direct current (HVDC) converter faults is presented. An electro-magnetic transient state simulation software PSCAD/EMTDC was used to set up a simulation model of HVDC system to investigate the typical converter faults. For the good time-frequency characteristic of S-transform, S-transform is applied to obtain useful features of the non-stationary fault signals. Then fault types are identified through the pattern recognition classifier based on SVM classification tree. Numerical results show that the proposed classification method is an effective technique for building up a pattern recognition system for converter fault signals.

Hybrid converter with ac side cascaded H‐bridge cells against H‐bridge alternative arm modular multilevel converter: steady‐state and dynamic performance

This study explorers the operating principles of the hybrid converter with ac side cascaded H-bridge cells and the H-bridge alternative arm modular multilevel converter, where the main objective is to compare their steady-state and transient performance. This comparison focuses on steady-state aspects such as active and reactive power control and semi-conductor losses and the transient performance of these converters during ac and dc network faults. To facilitate this comparison, detailed switch models of both high-voltage direct current (HVDC) converters are developed and simulated in Matlab-Simulink. The major outcomes are discussed and strengths and weakness associated with each converter are highlighted.

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.

HVDC System 적용 Double-tuned 필터의 설계 방법 연구

The ac side current of an high voltage direct current (HVDC) converter is characterized by highly non-sinusoidal waveform. If the harmonic current is allowed to flow in the connected ac system, it may cause unacceptable levels of distortion. Therefore, ac side filters are required as part of the total HVDC converter station, in order to reduce the harmonic distortion of the ac side current and voltage to acceptably low levels. The ac side filters are also employed to compensate network requested reactive power because HVDC converters also consume substantial reactive power. Among different types of filters, double-tuned filters have been widely utilized for HVDC system. This paper presents two design methods of double-tuned filter; equivalent method and parametric method. Using a parametric method, in particular the paper proposes a new design algorithm for a realistic system. Finally, the performance of the design algorithm is evaluated for a 80㎸ HVDC system in Jeju island with PSCAD/EMTDC program. The results cleary demonstrate the effectiveness of proposed design method in harmonics elimination and steady-state stability.

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.

AC/DC interconnectors stability and torsional interaction

Higher levels of power transfers across AC transmission circuits (also called interconnectors when they connect two electrical systems or companies) can result in system stability issue (rotor angle and voltage stability) and prevent the use of circuits’ full thermal capacity. Due to increased amount of renewables there is also a need to reinforce the thermal capacity of such circuits. A high voltage direct current (HVDC) link parallel to these circuits can resolve the stability issue as well as provide the required additional transmission capacity. However the current controller of HVDC converters can introduce negative damping under certain operating conditions. If there are thermal generators close to the HVDC converter stations, and the negative damping exceeds the positive damping of generators, there will be a risk of sub-synchronous torsional interaction (SSTI) between the electrical and mechanical shaft system of the turbine-generators. When such an interaction is not mitigated, generators shafts could be damaged, putting generators out of service and possible loss of supply to customers – a major cost implication for power stations and system operator. This study discusses the above issues, and present ideas that have been considered in this project to enhance system stability, thermal capacities, and mitigate the SSTI issue.

A new converter fault discrimination method for a 12-pulse high-voltage direct current system based on wavelet transform and Hidden Markov Models

The progressive development in high-voltage direct current (HVDC) transmission enhances the need to implement an efficient protection scheme to distinguish the minimum faulty part of the system and to relieve the stressed equipment. This paper proposes a new classification method based on Hidden Markov Models and wavelet transform to discriminate HVDC converter faults. In the proposed technique, probabilistic characteristics of signals discriminate fault signals without any deterministic index, so more flexible classification in different system conditions is achieved. Based on this method, high-speed protection decisions with small computational time could be performed in approximately 5 ms for severe system faults, and in 12.5 ms for faults that are restricted by protective control decision. PSCAD/EMTDC software simulations demonstrate suitable performance of this scheme for different fault types and system conditions in the International Council for Large Electric Systems (CIGRE) HVDC benchmark. All simulation results validate the stability and robustness of proposed scheme in different conditions, such as noisy systems.

Application of SMES Unit in Improving the Performance of an AC/DC Power System

This paper investigates the use of a superconducting magnetic energy storage (SMES) unit to improve the performance of an ac/dc power system. In this context, investigations have been conducted on a large turbine-generator unit connected to a high-voltage direct current (HVDC) system. The impact of HVDC converter station faults on the torsional torques induced in turbine-generator shafts with and without using an SMES unit is elaborated. Faults considered are fire-through, misfire, short circuit across the inverter station, flashover, and a three-phase short circuit in the ac system. These investigations are studied using an electromagnetic transient program power system simulation/electromagnetic transients including DC (PSCAD/EMTDC) and the results are presented in the form of typical time responses as well as harmonic analysis.

Theoretical aspects of fault isolation on high-power direct current lines using resonant direct current/direct current converters

This study investigates direct current (DC) fault current limiting and interrupting capability of a multi-megawatt resonant DC/DC converter, which is proposed for applications with high-voltage direct current (HVDC) systems. The converter can be used as a DC circuit breaker (CB) or as a multi-functional unit: DC transformer, regulating element and DC CB. The study is primarily concerned with the inherent converter responses, in the first several milliseconds after the fault, and prior to any controller action. A detailed converter design is given to prevent fault propagation through the converter even for most severe faults on low-voltage or high-voltage terminals. The analytical modelling proves that the converter will internally reduce power transfer during fault conditions. The simulation on PSCAD/EMTDC shows that converter operates uninterrupted through most severe faults, the switch turn-off time is not violated and inherent stabilising properties prevent any overvoltage or extreme currents. The converter can respond like high-impedance circuit on the unfaulted terminals. The impact of unbalanced DC line faults on a bipolar DC system is also discussed. The detailed PSCAD tests with a 200 MW DC/DC converter interconnecting 44 and ±250 kV DC lines demonstrate the ability to prevent fault propagation even for most severe DC line faults. The interaction with other HVDC converters in case of faults is also demonstrated.

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.

Dynamic Performance of a Modular Multilevel Back-to-Back HVDC System

The modular multilevel converter (MMC) is a newly introduced switch-mode converter topology with the potential for high-voltage direct current (HVDC) transmission applications. This paper focuses on the dynamic performance of an MMC-based, back-to-back HVDC system. A phase-disposition (PD) sinusoidal pulsewidth modulation (SPWM) strategy, including a voltage balancing method, for the operation of an MMC is presented in this paper. Based on the proposed PD-SPWM switching strategy, a mathematical model for the MMC-HVDC system, under both balanced and unbalanced grid operation modes, is developed. Dynamic performance of the MMC-based back-to-back HVDC converter system, based on time-domain simulation studies in the PSCAD/EMTDC environment, is then evaluated. The reported time-domain simulation results show that based on the adopted PD-SPWM switching strategy, the MMC-HVDC station can respond satisfactorily to the system dynamics and control commands under balanced and unbalanced conditions while maintaining voltage balance of the dc capacitors.

Simplified parasitic capacitance extraction of shield in HVDC converter system with boundary element method

It is critical to build a wide-band circuit model to conduct research on the characteristics of the electromagnetic disturbance source during the localization of high voltage direct current (HVDC) technology. Parasitic capacitance is most essential for modeling the equivalent circuit, so a fast and accurate computation of capacitance parameters plays a vital role. Because of the large size and complex structure of the converter equipment, it is impossible to obtain capacitance parameters by means of measurement or simulating calculation with finite element software. In this paper, a simplified method of capacitance extraction based on boundary element method is proposed, which can provide an efficient means of establishing simulation models. In the method presented, simulation model of the shield may not be chamfered. Consequently, the edge and corner of the shield do not need to be handled with a sphere, cylinder and other curved surface model. The availability of this method is demonstrated by comparing the capacitance parameters of chamfered shield with that of non-chamfered shield.