Voltage Direct Current Transmission Research Articles

Two-Level Surrogate-Assisted Transient Parameters Design Optimization of a Wound-Field Synchronous Machine

The rapid development of ultra-high voltage direct current transmission systems has placed greater demands on the design optimization of transient parameters for large wound-field synchronous machines (WFSMs) in power systems, such as large synchronous condenser for dynamic reactive power compensation. This paper proposes a novel two-level surrogate-assisted multi-objective optimization method for WFSMs that can optimize transient parameters including per-unit reactance and time constants rapidly and accurately. To facilitate the manufacture of a prototype for verification, the proposed method is applied to the optimization of a WFSM for scale-down generator application. This method is employed to deal with the excessive number of design parameters, issues of multi-objective evaluation and strong coupling between objectives that either result in inaccuracy or become time-consuming by using conventional methods. To generate the training set for the surrogate model efficiently by finite element analysis, an improved arbitrary rotor position standstill time response (SSTR) method is employed to identify all transient parameters simultaneously in a single test, while Latin hypercube design is used to capture data features. The optimal WFSM designed by using the proposed method is prototyped and measured. It confirms that the proposed method can achieve high validity and high optimization efficiency simultaneously.

HVDC Transmission and its Potential Application in Remote Communities: Current Practice and Future Trend

High voltage direct current (HVdc) transmission systems play an essential role in our modern power grids, not only providing bulk power transmission and asynchronous ac connection, but also enabling renewable energy integration into power grids. Renewable energy integration through HVdc transmission systems leads to the creation of multiterminal HVdc (MT-HVdc) grids. Recently, the idea of electrifying remote communities by HVdc transmission systems has emerged, and multi-terminal small-scale HVdc grids have been investigated in such applications. Despite numerous applications and significant potential of HVdc transmission systems, various technical and economic challenges in utilizing these systems still persist. In this article, an extensive literature review is conducted on HVdc transmission systems, particularly focusing on their role in renewable energy integration, and their potential application in remote communities. Future research directions are also recommended.

Research on negative sequence current coordinated control strategy for severe asymmetric offshore ac faults of wind power transmission system based on mmchvdc

When there is a severe asymmetric offshore ac fault in the grid-connected system for full-scale type-4 wind turbines via the high voltage direct current (HVDC) transmission system based on modular multilevel converters (MMC), there will be transient overcurrent in the MMC and the grid side converters (GSC) of wind turbines. In order to prevent transient overcurrent, a negative sequence coordinated current control strategy is proposed in this paper. The method of decoupling circuit characteristics and control characteristics is adopted to study the asymmetric fault characteristics of this system. The levels of the negative sequence current during severe asymmetric ac faults are analysed. The key influencing factor of the negative sequence currents is extracted. And the feature that the negative sequence currents of the MMC and the GSC cannot be suppressed to 0 simultaneously is revealed. Based on the above reasons, the control strategy that the negative sequence current of the MMC is suppressed to zero and the GSC cooperate to reduce the positive sequence current thus reducing the current stress of the bridge arm is proposed. Finally, an electromagnetic transient model was built on PSCAD to verify the effectiveness of the control strategy.

Stability Control of Flexible High Voltage Direct Current (HVDC) Transmission Based on DC Circuit Breaker Protection

In order to control the stable operation of flexible DC transmission system and ensure the uninterrupted power supply of flexible DC transmission system, the stability control method of flexible DC transmission system based on DC circuit breaker protection is studied. Taking support vector machine as the basic classifier, the two parameters of support vector machine classifier and support vector machine classifier are optimized by bird swarm algorithm, The sample weight is adjusted in real time according to the classification error of support vector machine classifier; The corresponding kernel function is selected and the bird swarm algorithm is used to find the optimal parameters; Through the fault location method of flexible DC transmission system based on wemtr, the fault location of flexible DC transmission line is realized; According to the fault location of flexible DC transmission system, the optimal coordination strategy of DC circuit breaker based on protection line is adopted to realize the stability control of flexible DC transmission system. The simulation results show that this method can realize the stability control of flexible DC transmission in a short time. Under the control of this method, the fluctuation amplitude of voltage and current of flexible DC transmission system becomes smaller, which has application value.

Real-Time Simulation of Hybrid Three-Level and Modular Multilevel Converter Based on Complete Equivalent Model for High Voltage Direct Current Transmission System

Real-Time Simulation of Hybrid Three-Level and Modular Multilevel Converter Based on Complete Equivalent Model for High Voltage Direct Current Transmission System

Fault Ride-Through (FRT) Behavior in VSC-HVDC as Key Enabler of Transmission Systems Using SCADA Viewer Software

The world’s energy consumption and power generation demand will continue to rise. Furthermore, the bulk of the energy resources needed to satisfy the rising demand is far from the load centers. The aforementioned requires long-distance transmission systems and one way to accomplish this is to use high voltage direct current (HVDC) transmission systems. The main technical issues for HVDC transmission systems are loss of synchronism, variation of quadrature currents, amplitude, the inability of station 1 (rectifier), and station 2 (inverter) to either inject, or absorb active, or reactive power in the network in any circumstances (before a fault occurs, during having a fault in network and after a fault cleared), and the variations of power transfer capabilities. Additionally, faults impact power quality such as voltage dips and power line outage time. This paper presents a method of overcoming the aforementioned technical issues using voltage-source converter (VSC) based HVDC transmission systems with SCADA VIEWER software and dynamic grid simulator. The benefits include having a higher capacity transmission system and proposed best method for control of active and reactive power transfer capabilities. Simulation results obtained using MATLAB validated the experimental results from SCADA Viewer software. The results indicate that the station’s rectifier or inverter can either inject or absorb either active power or reactive power in any circumstance. Also, the reverse power flow under different modes of operation can ride through faults. At a 100.0% power transfer rate, the rectifier injected 775.0 W into the network. At a 0.0% power transfer rate, the rectifier injected 164.0 W into the network. At a −100.0% rated power, the rectifier injected 1264.0 W into the network and direction was also changed.

A Dual Arm Complementary Hybrid Modulation Strategy Combining NL-SPWM for MMC Applications

In the medium voltage direct current (MVDC) transmission system, a small number of MMC sub modules will reduce the power quality. In this paper, based on the research background of Photovoltaic Medium Voltage Direct Current (PV-MVDC) system, a Hybrid Modulation Strategy based on the Decoupled Double Synchronous Reference Frame (DDSRF) control strategy is proposed. The dual arm complementary hybrid modulation combining nearest-level-SPWM (NL-SPWM) can keep the number of SMs in the ON state constant. Then, the corresponding voltage sharing control algorithm of sub module (SM) is introduced. Through theoretical calculation, the modulation strategy can be found to stabilize the DC voltage and reduce the harmonic content. A 32-level MMC system has been developed to verity that the proposed hybrid modulation strategy and its SM voltage sharing algorithm have the advantages of restraining circulating current and maintaining capacitor voltage balance.

Research on Self-Adaptive Algorithm of Transient Performance Analysis for DC Electronic Instrument Transformer Calibration

DC electronic instrument transformers (DCEIT) are widely used in the voltage source converter based high voltage direct current transmission (VSC-HVDC) system to measure the bus voltage or current, and should be of accurate and quick transient response. The transient performance of the DCEIT need to be calibrated using a calibrator. The existing schemes are difficult to recognize different transient step responses automatically, and usually require human intervention, which limits their flexibility and applicability. Besides, the accuracy of the calculated results is affected by human factors. A self-adaptive algorithm of transient performance analysis is proposed. It recognizes the characteristics of the transient step response based on wavelet analysis, and distinguishes the steady-state and step intervals automatically. Then it evaluates and finds the stable data segments in the steady-state intervals adaptively to calculate the high and low steady-state values. Finally, the data in the step interval are interpolated to search the feature points of the waveform, and then the transient response parameters are calculated according to the coordinates of feature points. In the validation stage, six kinds of step response waveforms are generated by MATLAB to test the performance of the proposed algorithm. Compared with the boundary window method, the proposed algorithm could recognize the characteristics of the tested waveforms automatically and achieve a more accurate analyses of transient performance. In addition, a calibrator prototype based on the proposed algorithm is developed, and some tests are carried out at the National Center for High Voltage Measurement (NCHVM) in China, whose results verify the feasibility of the algorithm.

The hybrid simulation system of hydropower generation units in islanding AC grid and its applications

It is a trend for the large hydropower bases to transport the electricity to consuming center with High Voltage Direct Current (HVDC) transmission lines. The fault of HVDC has great significance impact on hydropower units in islanding sending AC grid. In this paper, a hybrid simulation system is established to analyze the dynamic characteristic of hydropower units in islanding system. The simulation system is based on Real Time Digital Simulator (RTDS) and Digital Signal Processor (DSP). The simulation of electromagnetic transient process in the generator, the load, the network and HVDC is completed by RTDS. The Hydroturbine Real Time Simulator (HRTS) based on DSP completes the simulation of hydraulic-mechanical-electrical transient process in hydroturbine, division system and governing system. The real time data communication between RTDS, HRTS and external equipment ensures the real time calculation of simulation. The simulation results show high coincidence with field data. The hybrid simulation system is an effective approach for the research of islanding system at sending terminal with HVDC.

Modelling and simulation analysis of MMC‐HVDC system based on transient energy flow

In the high voltage direct current (HVDC) transmission system composed of modular multilevel converters (MMC), considering the distribution of transient energy flow (TEF) contributes to the transient modelling and fault characteristic analysis when dc short-circuit fault occurs. For single MMC, based on transient energy conservation, the capacitors of half bridge sub-modular (HBSM) can be equivalent to time-varying capacitor. And the MMC transient model is established, in which dynamic switching of HBSM and ac import are considered. By solving state equations of the model, the fault and TEF characteristics of MMC are obtained. For MMC-HVDC grid, a TEF analysis method based on electromagnetic transient simulation results is proposed, which can not only reveal the evolution of dc fault, but also realize quantitative analysis of fault current through recursive calculation. Based on characteristics of TEF, an energy limitation strategy considering the fluctuation range of capacitor voltage in HBSM is proposed, which can delay the blocking time effectively. According to the simulation results in PSCAD, the correctness of the transient model and the energy limitation strategy is verified, meanwhile, the TEF analysis method is applied to the four-terminal MMC-HVDC grid to obtain its fault characteristics.

Enhancement of Transmission Efficiency and Voltage Profile in the Bauchi Axis of Nigerian Power Grid Using a VSC-HVDC System

Increasing electricity demand occasioned by increased population and advancement in economic activities makes it mandatory to upgrade the Nigerian power grid. The ongoing construction of a new hydropower plant at Mambila Plateau in Taraba State of Nigerian is a response to this call. Performance evaluation of the Nigerian power grid expanded to include the new power plant has revealed the occurrence of poor voltage profile accompanied by colossal power losses mostly in the Bauchi region. This paper explores the techniques for addressing this challenge by the integration of voltage source controlled high voltage direct current (VSC-HVDC) transmission system into the expanded Nigerian 41- bus 330kV transmission grid network at the weakest areas. The HVDC converter is introduced into the power grid as an extra bus, and all impedance elements in the HVDC system transformed to the grid side. This action merges the two converter stations into a single bus to produce an entirely high voltage alternating current (HVAC) system, thereby making it possible to apply conventional power flow techniques. This model was verified by the simulation of the modified network in Matlab Simulink environment. Performance evaluation of the expanded network prior to the connection of HVDC system identified the transmission line between the Jalingo bus and the Mambila power plant as the weakest area, which was consequently selected as the optimal location for the proposed VSC-HVDC system. Power flow analysis of the modified network with the inclusion of HVDC system revealed a remarkable improvement in voltage magnitude at the weak buses, and minimized power losses in the system. This implies that such a project is worth undertaking. Therefore, the enhancement of voltage profile using the VSC-HVDC transmission system is a panacea for curbing the expected line losses that may arise while evacuating power from the proposed Mambila plant.

Stability Improvement of The Iraqi Super Grid (400kV) using High Voltage Direct Current (HVDC) Transmission

This research analyzes the level of the short circuit effect of the Iraqi super network and decides the suitable location for the High Voltage Direct Current (HVDC) connections in order to obtain the best short circuit reduction of the total currents of the buses in the network. The proposed method depends on choosing the transmission lines for Alternating current (AC) system that suffers from high Short Circuit Levels (SCLs) in order to reduce its impact on the transmission system and on the lines adjacent to it and this after replacing the alternating current (AC) line by direct current (DC) line. In this paper, Power System Simulator for Engineering (PSS/E) is used to model two types of HVDC lines in an effective region of Iraqi networks and to perform comparative studies to test the location of Short Circuit Levels (SCLs) between an actual AC and AC/DC case study in a portion of the Iraqi national network. The results proved the effectiveness of this method in eliminating severe faults and unwanted short currents, and the results showed that the bipolar type is better in reducing Short Circuit Levels of the Iraqi network.

Improvement of high voltage direct current material properties upon tailoring the morphology of crosslinked polyethylenes

Crosslinked linear low-density polyethylene (XLPE) is the most common polymeric cable insulation material for high-voltage applications with a high number of operating hours in high voltage alternating current (HVAC) systems. High voltage direct current (HVDC) power transmission and polymeric cable systems play a major role in the future and raise, besides numerous systemic benefits, challenges in the design of material properties. The main issue is injection and trapping of space charges in insulation materials under DC-fields. The objective of this work is to increase knowledge of the interplay between microstructure and material performance of XLPE under DC by tailoring its morphology beyond the capabilities of “common crystallization kinetics” upon constrained crystallization at certain elongations. It was found that the tailored oriented morphology influences the energetic depth of traps and a significant reduction of space charge density occurs. Moreover, the optimized oriented morphology leads to a significant reduction of field enhancement for field strengths ELaplace ≥ 20 kV mm−1 compared to unoriented XLPEs with spherulitic morphology. It is shown that this way of morphology tailoring results in a considerable, material dependent reduction of field exposure by a factor of 4, which promises a significant improvement in the electrical life time of polymeric insulation material used.

Analysis on the Influence of Remote 110kV Substation on Transformer DC Bias

High voltage direct current (HVDC) transmission system has the advantages of large capacity, long distance and low loss. Relying on the advantages of HVDC transmission, China has established a large-scale energy transmission channel in the west, effectively solving the problem of uneven local energy distribution. However, HVDC transmission will generate ground current when it is operating in a single pole, which will cause the DC bias effect of the transformer. At this time, the core magnetic circuit of the transformer is saturated. Transformers will experience increased vibration, increased noise, local overheating, and surge in harmonics. This paper, based on the Jinhua HVDC transmission project as a background, established a power grid topology model in Zhejiang area based on the field-circuit coupling method. The focus is on calculating the DC bias of Shenze transformers in Jinhua area, and proposing treatment measures based on the calculation results. According to the actual measurement of the project, the accuracy of the calculation result is verified, which reflects the effectiveness of the governance measures.

Optimization of parabolic trough based concentrated solar power plant for energy export from Saudi Arabia

In this work optimization of the parabolic trough (PT) based concentrated solar power (CSP) system is analyzed for solar energy export from Saudi Arabia to European and Asian countries based on their peak load hours. Initially, Al-Ahsa, in east, and Tabuk, in the west, are compared for a PT based CSP plant for electrical energy generation as these locations are used to export solar energy from Saudi Arabia to Pakistan, India, and China in east and Greece, Germany and UK in the west. The analysis is performed to sell electrical energy generated by the CSP plant to the customers in their peak load hours to reduce their load factor and keep the capital cost minimum. A high voltage direct current (HVDC) transmission system is employed for this study as an undersea cable for Asian or European countries. The feasibility of 2 GW solar energy export by PT based CSP system transmitted by 2 GW bipolar 660 kV HVDC transmission line is based on the net present value of the project (NPV), real levelized cost of energy (LCOE), and the payback period of the project where Karachi, Pakistan has the highest NPV of 1866 MUSD with LCOE of 10.76 cents/kWh and a payback period of 12.7 years.

Coordinated power control of electrochemical energy storage for mitigating subsequent commutation failures of HVDC

Commutation failure is one of the most common faults in high voltage direct current (HVDC) transmission system. Subsequent commutation failures (SCFs) caused by commutation failure will cause large voltage and current swing. Several SCFs may cause DC transmission power interruption, which seriously threatens the safe and stable operation of AC/DC hybrid power grid. With the construction and commissioning of grid-side electrochemical energy storage (EES), it is possible to mitigate SCFs of adjacent HVDC transmission lines using EES with fast power response characteristics. In this paper, the influence mechanism of active and reactive power output of EES on commutation conditions is studied by combining the evolution of cascading outages and SCFs. It is found that reactive power mainly affects AC voltage and active power mainly affects DC current. Based on the mechanism analysis, a coordinated power control strategy for EES is presented. This strategy, combined with EES capacity constraints, can control EES active and reactive power output instructions adaptively according to the degree of AC failure at different stages of SCFs to fully release the transient support capacity of EES. Case studies are undertaken on the Henan multi-infeed UHVDC system and CIGRE-HVDC standard test system, respectively. The simulation results verify the correctness of the proposed EES power control strategy and the effectiveness of the strategy for SCFs mitigation in different systems, different fault types, and different fault degrees.

A smooth startup strategy for hybrid MMC based HVDC systems

Hybrid modular multilevel converter (HMMC) topology consisting of full-bridge submodules (FBSMs) and half-bridge submodules (HBSMs) is one of the promising technical solutions for long-distance high voltage direct current (HVDC) power transmission systems. However, the HMMC has very different charging circuit characteristics from the conventional half-bridge MMC (HB-MMC) topology and suffers from low and unbalanced precharge capacitor voltages and surge current when starting from the AC-side, due to its hybrid submodule configuration. In this paper, an AC-side startup scheme for the HMMC is proposed, including an additional semi-controlled charging process and an improved controlled charging method, which allows the HMMC to smoothly charge its submodule capacitors and establish the DC-link voltage without requirements of auxiliary sources. The improved controlled charging method is composed of a group modulation and a group voltage balancing control strategies, which enables the HMMC to operate stably with enhanced AC voltage output capability under the condition of low and unbalanced capacitor voltage in the HBSM group and FBSM group, thereby eliminating converter overmodulation and surge current during the startup procedures. In addition, DC-side startup strategy is also discussed in this paper. PSCAD/EMTDC simulation results based on a practical ±200 kV/750 MW HMMC-HVDC system demonstrate the effectiveness of the proposed method.

Interaction between damping characteristics and the initial‐stage short‐circuit current of MMC‐HVDC transmission systems

This paper studies the interaction between the damping characteristics and the initial-stage short-circuit current of modular multilevel converter based high voltage direct current (MMC-HVDC) transmission system. First, the MMCs' input–output characteristics are obtained through the operator approach, and the influences of both linear part and nonlinear part are analysed. Second, an input–output characteristics based short-circuit current calculation method is proposed. Third, an eigenvalue-based comprehensive index is proposed to evaluate the damping characteristics of the MMC-HVDC transmission system. Finally, the interaction between the damping characteristics and the initial-stage DC short-circuit current is analysed under different system parameters and DC system structures. The analysis results show that there is a negative correlation between the system damping characteristics and the average rising rate of the short-circuit current. The proposed index G can effectively evaluate the short-circuit current of MMC-HVDC and can provide guidance for the planning of DC system structure, the selection of operation mode and system parameters.

Design and performance tests of a 160 kV/1.0 kA DC superconducting fault current limiter

Superconducting fault current limiter (SFCL), which is used in voltage source converter based high voltage direct current (VSC-HVDC) transmission systems to limit fault currents and protect the grid together with DC circuit breakers, could improve system safety and stability. The first transmission voltage DC SFCL is developed in China, and it has been applied in the 160 kV VSC-HVDC transmission system in Nan’ao. The electromagnetic design as well as insulation analysis of the current limiting unit is presented, where the current limiting unit is composed of 96 bifilar coils made of stainless steel reinforced YBCO tapes. The liquid nitrogen at the temperature of 77 K is used as the cooling and insulation medium, and the liquid nitrogen is cooled by a closed-loop cryogenic system consists of 3 G-M refrigerators. The 160 kV DC SFCL prototype has been fabricated, and the prototype has passed the steady-state current test of 1 kA, the DC withstand voltage test of 252 kV within 1 min, the lightning impulse test of 550 kV/1.2 µs/50 µs and the operating impulse test of 450 kV/250 µs/2500 µs. Power switching test results show that the developed SFCL could generate the desired limiting resistance, and the maximum fault current duration time could be as high as 60 ms. The developed prototype has been installed in the Nan’ao VSC-HVDC transmission system, and put into operation on August 17, 2020.

DC‐presaturated fault current limiter for high voltage direct current transmission systems

DC‐presaturated fault current limiter for high voltage direct current transmission systems