Ultra-high Voltage Direct Current Converter Research Articles

Analysis of green energy-saving retrofit for air-conditioning system in UHVDC converter stations

Abstract The air-conditioning system in ultra-high-voltage direct current (UHVDC) converter stations uses hydrofluorocarbons (HFCs) as refrigerants currently, which pose a certain environmental hazard. Moreover, the cooling system of the converter valves contains a great deal of low-grade thermal energy, with waste heat being directly discharged into the air, resulting in significant energy wastage and air pollution. In light of the functional characteristics and energy wastage situation of current converter station buildings, a green energy-saving retrofit scheme for the air-conditioning system in UHVDC converter stations is proposed. It adopts a transcritical CO2 cycle system for building cooling and introduces an ejector into the system to improve its performance. Simultaneously, the heat pump technology is used to elevate the grade of waste heat from the converter valves, which is then utilized for heating in the control building. Simulation results demonstrate that this scheme is effective in enhancing ecological and environmental benefits while promoting green development of the converter station.

Performance Analysis of an Ejector-Enhanced Heat Pump System for Low-Temperature Waste Heat Recovery Using UHVDC Converter Valves

This article proposes a heating method based on heat pump technology to address the large amount of low-grade waste heat generated by a certain type of ultra-high voltage direct current (UHVDC) converter valve. Thermal performance calculations for two systems, a basic vapor compression heat pump system (BVCHPS) based on thermal expansion valve throttling and an ejector-enhanced heat pump system (EEHPS) are analyzed. The research results show that the EEHPS exhibits superior COP and exergy efficiency when generating hot water above 80 °C using a heat source below 50 °C. Additionally, mathematical modeling analysis identifies optimal structural parameters such as nozzle throat diameter, throat area ratio, and nozzle outlet diameter for the ejector in its design state. The low-temperature waste heat recovered from the UHVDC converter valves can be further used in engineering applications such as heating, refrigeration, seawater desalination, and sewage treatment.

Reactive Power Output Modeling of Synchronous Condenser in UHVDC Converter Station Based on Interlaced Superposition CNN-BiLSTM

To guarantee stable power system operation, a synchronous condenser (SC) is configured in an ultra-high voltage direct current (UHVDC) converter station to provide dynamic reactive power support to the power system. The research on the reactive power output modelling of a SC in an UHVDC converter station has important theoretical significance and practical value for the reactive power control of a SC in an UHVDC converter station. Focusing on the reactive power regulation system of the SC with strong coupling, multivariable, and nonlinear features, it is difficult for the universal analytic method to build the SC reactive power output model. A novel reactive power output model of the SC in the UHVDC converter station based on interlaced superposition convolutional neural network-bidirectional long short-term memory (CNN-BiLSTM) is proposed, in which a novel interlaced superposition CNN composed of convolution units with two different structures is built to increase the depth of the network and avoid over-fitting. Particularly the branch channels of convolution units with two different structures are connected by a convolution layer and a skip connection respectively. The interlaced superposition CNN-BiLSTM model is that the combination of the interlaced superposition CNN model and BiLSTM model for improving the model accuracy and generic capability. The Bayesian optimization method is used to optimize its hyperparameters. The application of interlaced Superposition CNN-BiLSTM in SC reactive power output modelling is a new technology. The excitation voltage and excitation current of the SC are used as inputs for the training and testing sampled data, and the reactive power of the SC is used as outputs for the training and testing sampled data. Therefore, the reactive power output model of a SC in an UHVDC converter station based on interlaced superposition CNN-BiLSTM is obtained, and it attains a low root mean square error (RMSE = 0.126750) and a high determination coefficient (R2 = 0.999999).

Analysis of Electromagnetic Transient Characteristics on the Winding of UHVDC Converter Transformers Based on an Improved Broadband Model

This work proposes an improved broadband equivalent circuit model of the winding system of an ultrahigh-voltage direct current (UHVDC) converter transformer and the validation experiment of the model conducted on a tapped transformer. The matrices of the inductance, capacitance, and resistance parameters in the improved model were all obtained by the finite element method according to the winding system structure of a UHVDC transformer. Then, the potential distributions along the windings under impulse voltages with different waveforms were studied in detail. Finally, the relationship between the potential gradient and waveforms was obtained. This work provides a convenient method to acquire the broadband electromagnetic transient characteristics of the winding systems within UHVDC converter transformers.

Potential application of Six Sigma method in operation and maintenance management of UHVDC converter station

Abstract The ultra high voltage direct current (UHVDC) technology plays an important role in the long-distance and large-capability transmission of renewable energy. Power grid company has to improve the reliability and efficiency of UHVDC transmission systems to support renewable energy utilization. In this paper, Six Sigma method is considered for improving the operation and maintenance (O&M) performance of UHVDC converter stations. The working tasks for the operation team and the maintenance team are introduced at first, respectively. The limitation of current O&M management is analyzed. A five-stage implementation of Six Sigma method is discussed in detail. The formed negative feedback ensures the sustainability of implementation and realizes closed-loop management. A representative equipment disposal case is used to demonstrate the adaptability and operability of the potential application of Six Sigma method.

Dynamic reactive power optimisation of UHVDC converter station considering coordination of synchronous condenser and system filtering requirements

The reactive power compensation of large-capacity synchronous condensers in a ultrahigh-voltage direct current (UHVDC) converter station is not yet fully utilised, and there is no method for achieving effective coordination between synchronous condensers and DC systems. To realise their coordinated control of reactive power and voltage, a dynamic reactive power optimisation model that considers the coordination of synchronous condenser and filtering requirements of system is proposed in this study. The model considers the constraints of the variable steady reactive power capacity of synchronous condenser by taking into account its dynamic reserve demand, the minimum filter capacity of the system, and the switching sequence of AC filters and shunt capacitors. The model is solved by a two-stage hybrid intelligent algorithm based on the interior point method and the differential evolution algorithm. Further, the effectiveness of the proposed method is verified by the example of a UHVDC sending converter station in Northeast China. The simulation results show that the proposed model can greatly reduce the number of times discrete devices enter operation in a converter station under satisfying the dynamic reactive power reserve of the synchronous condenser, providing a basis for the coordinated optimal control of synchronous condensers and DC systems.

Strengthening technique on firestop system of valve hall for existing UHVDC converter stations

The ultra-high voltage direct current (UHVDC) converter station is the core of the UHVDC project to realize the mutual conversion of alternating current (AC) and direct current (DC). Its operational reliability directly affects the safety of personnel and equipment and the stable operation of the grid. The firestop system is key part of the protective system in converter station, and it faces the threat of fire and explosion of converter transformer. In this paper, in order to improve the safety performance of the firestop system of the valve hall of the existing converter station, the blast resistant design target of the firestop structure and a fireproof and blast resistant strengthening scheme are proposed; For a typical UHV converter station, the dynamic response process under different explosion conditions is further analyzed by numerical model. The results show that the strengthening scheme proposed in this paper can effectively meet the requirements of blast resistant design targets, and greatly improve the safety performance of the firestop system of the valve hall of the converter station under fire and explosion.

Fire and blast resistance analysis of the firestop system of valve hall in UHVDC converter station

The ultra-high voltage direct current (UHVDC) converter station is the core of the UHVDC project to realize the mutual conversion of alternating current (AC) and direct current (DC). Its operational reliability directly affects the safety of personnel and equipment and the stable operation of the grid. The firestop system is key part of the protective system in converter station, and it faces the threat of fire and explosion of converter transformer. In this paper, in order to analyze the shortcomings of the existing firestop system of valve halls, numerical models for fire and blast resistance analysis of typical firestop system are established. For a typical UHVDC converter station, the temperature distribution under hydrocarbon heating curve and dynamic response under different blast loads are analyzed by using the established numerical model. And the results show that the existing firestop system has good heat insulation performance, while the blast resistance cannot meet the demand.

Transient potential distribution on transformer winding considering the effect of core lamination stack

The potential distribution of windings under impulse voltage is very important for the design of transformer inter-turn insulation especially for large capacity transformers such as ultra-high voltage direct current (UHVDC) converter transformer. Quite a lot of equivalent circuit models for transformer winding have been proposed for the potential distribution calculation assuming that the influence of magnetic core is negligible at frequencies higher than 10 kHz. However, lightning impulse or VFTO waveforms usually contain abundant frequency components higher than 10 kHz. At above situations the magnetic core plays an important role during the transient procedure. To obtain a more comprehensive model and also to provide a more accurate potential distribution of transformer winding, in this paper, a wide frequency magnetic properties of silicon steel sheet were measured and the relationship between relative permeability of lamination stack and frequency is studied and implemented in the calculation of frequency-dependent parameters such as resistance, self- and mutual-inductances. Then the equivalent circuit model of UHVDC converter transformer is established considering the properties of core lamination stack. Coding the program in MATLAB to solve the matrix equation and the potential distribution properties are extracted from the calculation results under lightning situation. The inter-turn potential distribution is also analyzed and the results may provide more accurate information for transformer inter-turn insulation design.

An Efficient Modeling Method for Large-Scale Complex Finite-Element Electric Field Numerical Calculation

In the process of solving electric field of the large-scale complex finite-element models, the fine control of the mesh on the outer surface of metal entity models is often difficult. Therefore, considering the basic geometric structure of the equipment, an efficient finite-element modeling method based on multilayer external surround entities (MESE) is proposed. Taking a simple 2-D model as an example, the algorithm of MESE generation, the principle of region division and related operation flow are introduced. The correctness of the method is validated through the typical case of parallel cylindrical conductors model, and the number of MESE is discussed as well. The proposed method is applied to the surface electric field calculation of the ±1100 kV ultrahigh-voltage direct current converter station valve hall. The calculated maximum electric field strength is 14.2 kV/cm. Furthermore, compared with the submodel method, the accuracy and efficiency of the proposed method are verified.

A novel coast-down no-load characteristic test and curve conversion method for large-scale synchronous condenser

Large-scale synchronous condensers (LSCs) are synchronous motors that have no prime movers and that operate in a continuous no-load condition. They will soon be widely used as the main dynamic reactive power compensation equipment for transmission systems in ultra-high voltage direct current (UHVDC) transmission projects across China. As the standard no-load characteristic test procedure is not applicable when the LSCs are installed at UHVDC converter stations, this paper presents a novel coast-down no-load characteristic test and curve conversion method that has been successfully applied to a 300 Mvar LSC. For the proposed test, the LSC is set to run a coast-down process and its excitation current is incrementally increased. In order to get an accurate no-load characteristic curve, a series of data processing methods, including frequency conversion, wavelet denoising and a hybrid optimization algorithm for saturation correction are used. To validate the proposed method, the test results were compared to the no-load characteristic curve obtained by the standard method.

Research on the design principle of the contact terminal in large DC current loop of ±800 kV, 10,000 MW UHVDC converter station

The abnormal heating phenomena of contact terminal in large DC current loop of the ultra-high voltage direct current (UHVDC) converter station became one of the most serious problems, which caused serious impact on the operation reliability of the power grid. Based on existing codes and standards of contact terminal temperature rise and the results of large current temperature rise test, the design principle of the contact terminal in large DC current loop of DC voltage ±800 kV, capacity 10,000 MW converter station is put forward and introduced detailedly in this study. Then, taking the design of three typical contact terminals in large DC current loop of Taizhou ±800 kV converter station as an example, the infrared temperature measurement results of contact terminals on the large current operation condition are analysed. The infrared temperature measurement results indicate that the design principle of the contact terminal in large DC current loop proposed in this study is able to meet the requirements of existing codes and standards of the temperature (temperature rise) limit of the contact terminal. This design principle is significant to improve the maintenance ability and operation reliability of the UHVDC converter station.

Defect Pattern Recognition Based on Partial Discharge Characteristics of Oil-Pressboard Insulation for UHVDC Converter Transformer

The ultra high voltage direct current (UHVDC) transmission system has advantages in delivering electrical energy over long distance at high capacity. UHVDC converter transformer is a key apparatus and its insulation state greatly affects the safe operation of the transmission system. Partial discharge (PD) characteristics of oil-pressboard insulation under combined AC-DC voltage are the foundation for analyzing the insulation state of UHVDC converter transformers. The defect pattern recognition based on PD characteristics is an important part of the state monitoring of converter transformers. In this paper, PD characteristics are investigated with the established experimental platform of three defect models (needle-plate, surface discharge and air gap) under 1:1 combined AC-DC voltage. The different PD behaviors of three defect models are discussed and explained through simulation of electric field strength distribution and discharge mechanism. For the recognition of defect types when multiple types of sources coexist, the Random Forests algorithm is used for recognition. In order to reduce the computational layer and the loss of information caused by the extraction of traditional features, the preprocessed single PD pulses and phase information are chosen to be the features for learning and test. Zero-padding method is discussed for normalizing the features. Based on the experimental data, Random Forests and Least Squares Support Vector Machine are compared in the performance of computing time, recognition accuracy and adaptability. It is proved that Random Forests is more suitable for big data analysis.

Application of a simple method for determining the equilibrium resistivity of dielectric luiquids

For the design of the electrical insulation of an UHVDC (Ultra High Voltage Direct Current) converter transformer the resistivity of the pressboard and mineral oil has to be known. For solid insulation the resistivity is virtually constant, however, in liquids it is dependent on, e.g., applied electric stress and time of electrification. Several methods are discussed in the standards. One way to characterize the oil is the measurement of the equilibrium resistivity, in contrast to an apparent resistivity for oil under high electric stress. Measurements using a low voltage triangular shaped voltage form allow calculation of equilibrium resistivity. The oils investigated show equilibrium resistivity in the range 5E11 Ohm·m to 5.96E12 Ohm·m, which are typical values for transformer oil. There is good agreement between these results and reference measurements using dielectric response

Parameter Extraction and Wideband Modeling of $\pm $ 1100 kV Converter Valve

As the key equipment of ultra-high-voltage direct-current (UHVDC) transmission systems, the converter valve would bear different forms of overvoltage. Consequently, a wideband model for the $\pm $ 1100 kV UHVDC converter valve is essential for the converter system to make overvoltage analysis, insulation design, and electromagnetic disturbance prediction. This paper proposed a wideband modeling method for the $\pm $ 1100 kV converter valve. First, the physics-based wideband model for the key component in the converter valve module is built, of which the parameters could be obtained through genetic algorithm. Second, the parasitic capacitances are obtained through a simplified boundary-element method according to the given structure of the $\pm $ 1100 kV converter valve. By combining wideband models for key components in the converter valve module and the parasitic capacitances of the valve tower, the integrated wideband model for the converter valve could be obtained. The systematic modelling method proposed in this paper could also be of reference value to the modelling of other large-scale complex high-voltage power-electronic equipment. Finally, the experiments for converter valve module and converter tower are carried out, and the corresponding results justified the effectiveness and correctness of the proposed wideband model for the converter valve tower.

Study on Short Circuit Current Calculation of Power System with UHVDC and New Energy Source

With the development of power system, the level of short circuit current will increase accordingly. In general, the influence of the HVDC system and the new energy source is not considered in the calculation of the short circuit current. For the power grid that short circuit current level closes to the interrupting capacity of circuit breaker, it’s necessary to fully consider all kinds of influence factors, careful checking, so as to obtain more accurate calculation results of short circuit current. In 2018, two ±800 kV high-power Ultra High Voltage Direct Current (UHVDC) transmission projects will be connected with Shaanxi power grid, accompanied by a lot of concomitant fossil-fuel generating plants as power resource, and also a large number of new energy source, includes wind power generation and photovoltaic power generation. Around one of the UHVDC converter stations, short circuit current may exceed the withstand limit of some certain circuit breakers. In order to get more accurate short circuit current calculation results, three measures are used: 1) contrastive calculation and analysis by algorithm based on schemes and algorithm based on power flow; 2) analysis the influence of UHVDC by electromagnetic transient and electromechanical transient hybrid simulation; 3) considered a detailed model of Doubly Fed Induction Generator (DFIG) with low voltage ride through characteristics. The calculation results shows that: in the typical operation mode of Shaanxi power grid of 2018, the original calculation results by conventional calculation method are coincident with the results by considering the influence of algorithms, UHVDC and DFIG in large, in which: the results of the algorithm based on power flow are smaller than that of the algorithm based on schemes about 2 - 8 kA; the steady values of the short circuit current provided by UHVDC converterstation (includes rectifiers and smoothing capacitors) are about 0 - 3 kA; the steady values of the short circuit current provided by DFIG are about 0 - 5 kA. The calculation results can provide reference for the selection of the circuit breaker, and it can be verified by fault recording data in the future.