Flexible HVDC Research Articles

Design of Transient Energy Regulation and Reuse Device of Flexible HVDC Transmission for Offshore Wind Power

Design of Transient Energy Regulation and Reuse Device of Flexible HVDC Transmission for Offshore Wind Power

Frequency Stability Control Strategy for Voltage Source Converter-Based Multi-Terminal DC Transmission System

The voltage source converter-based multi-terminal DC transmission (VSC-MTDC) system can use additional frequency control to respond to the frequency change of faulty AC system. However, the control coefficient of traditional additional frequency control is mostly fixed, and the control flexibility is insufficient, so it cannot be adjusted adaptively according to the frequency change of the system. Therefore, a frequency control strategy of the VSC-MTDC system based on fuzzy logic control is proposed. Based on the DC voltage slope controller, this strategy introduces an additional frequency controller based on fuzzy logic control, takes the frequency deviation and frequency change rate as the additional controller input, and dynamically adjusts the control quantity through the fuzzy logic control link to realize the adaptive adjustment of the VSC-MTDC system to the AC system’s frequency. Finally, a three-terminal flexible HVDC system is built on the PSCAD/EMTDC simulation platform for simulation verification. The results show that the proposed control strategy can effectively use the flexible DC system to support the frequency of the AC system and significantly improve the frequency stability of the faulty AC system.

Non‐uniform voltage distribution characteristics of metallized film capacitors in flexible HVDC projects

AbstractThe problem of non‐uniform voltage distribution in the internal element of metallized film capacitors in flexible HVDC transmission systems exists, the influence of electrical parameters of dielectric and metal copper row are not considered in the existing metallized film capacitor equivalent circuit model. In this paper, the conductance of biaxially oriented polypropylene film (BOPP) for metalized film capacitors is measured at different electric field intensity and dielectric properties at different frequencies. The current density distribution and voltammetric properties of the metalized film capacitor elements under AC and DC superimposed voltages are calculated by the finite element method, and the voltammetric properties of the copper row considering the skin effect are derived. The equivalent circuit model of the metalized film capacitor under the AC‐DC superposition voltage is proposed. The simulation results of the capacitor equivalent circuit model show that the voltage distribution of the internal elements of the capacitor is uniform at low frequency AC voltages (<102 Hz), and the voltage amplitude and phase difference between the elements of the capacitor gradually increases with increasing AC voltage frequency. At an AC voltage frequency of 105 Hz, the difference in magnitude between the two capacitor elements on the same copper row was up to 2.01% and the phase difference was 13.5%, while the difference in magnitude between capacitors at the same horizontal position on different copper rows was up to 56.8% and the phase difference was 10.44%. The results of this paper provide some guidance for the modeling of the internal components of metalized film capacitors, the calculation of voltage distribution and the structural optimization design.

A Fault Current Limiter Self-Adapting Activation Strategy for Flexible HVDC System Based on Fault Severity Assessment

In the flexible HVDC system, fault current limiters (FCLs) need to be configured to reduce the rising rate of fault current. However, the FCL configured according to the bipolar near-end metallic short-circuit faults may cause the fault current to be unnecessarily over-suppressed and consume equipment life. This study develops a method to activate FCLs of the flexible HVDC system adaptively to follow changes in fault severity. Based on analyzing the relationship between the fault current and the transition resistance at the initial moment of the fault, a method to predict the fault condition based on the single-ended instantaneous information is proposed, which realizes the fault severity assessment. Considering mutual coupling of FCLs action sequences, a fast action sequence generate (FASG) method of FCLs based on fault current limiting contribution is proposed, which realizes the coordination between multi-terminal FCLs without communication. A FCL self-adapting activation strategy (SAAS) for flexible HVDC systems based on fault severity assessment and FASG is constructed. This strategy matches the severity of the fault by adjusting the FCL action sequence, improving the current limiting effect of severe faults and the life of FCLs. The accuracy of the SAAS is validated on a scaled-down two-terminal system in the laboratory and the simulation.

Non-unit traveling wave protection for flexible HVDC lines based on the ratio of the waveform fitting residual

Non-unit traveling wave protection for flexible HVDC lines based on the ratio of the waveform fitting residual

Exploration of NLM Based on MMC on Offshore Wind Power

Nowadays, the world is facing a common problem of resource scarcity, so the search for a new clean energy source is the trend of the future. China is rich in marine resources and is close to the centers of electricity consumption. Offshore wind power as a future clean energy source is thus a new avenue to pursue today. Now, one of the technical issues limiting the expansion of offshore wind power is the reliable and economical transmission of long-range electricity to onshore power grids. However, traditional high-voltage DC transmission is no longer suitable for large, long-distance offshore wind farms due to its economics, the difficulty of compensating for offshore reactive power, and the complexity of large-scale system coupling. The flexible HVDC transmission method based on MMC (Modular Multilevel Converter) has become the development direction for HVDC transmission due to its high output quality and economic advantages. MMC is the core device for flexible HVDC transmission. In order to improve the output voltage quality and ensure the stability of the transmission process, the selection of a suitable modulation method is a basic condition. Among the existing applied modulation methods, this paper took NLM (nearest level modulation) based on MMC as the research object. It was verified through principal analysis and simulation that this method could not only achieve multi-level output but also that the more sub-module (SM) investment, the higher the output waveform quality.

Research on life prediction control of MMC submodule based on fiber sensing

With the rapid development of energy, power electronic equipment plays a crucial role in renewable energy. Modular multilevel converter (MMC) has turned into cutting-edge inverter topology in flexible HVDC and offshore wind power situations. However, under the conditions of high humidity and high voltage, the internal submodules lifetime of MMC might be affected. The system reliability mainly depends on good running of the submodules, so stable performance of the submodules serious threat to the normal operation of the inverter. Therefore, this paper proposes a model based on Peck accelerated life predictable model, and model predictive control (MPC) strategy is improved, at the same time without affecting the external characteristics can better control the submodule voltage fluctuations. What is more, real time monitoring of the working environment of the converter through the optical fiber sensing module. Finally, the experimental and simulative outcomes show that the proposed control strategy can effectively reduce the voltage fluctuations in the small module, and the life parameters of the submodules can be obtained in real time by bringing the results into the proposed life prediction model. By comparing the results, the results can be obtained the proposed control strategy can effectively increase the reliability of the submodule.

Fully Distributed Energy and Reserve Scheduling for Hybrid AC/DC Systems

Preparing spin reserve power in advance is an efficient approach to address the intermittency of renewable energy. To promote renewables penetration levels and increase the cost-efficiency of bulk power system, a novel flexible HVDC model that allows both energy and reserve transmitted across regions is proposed. Based on this, a risk based dynamic reserve schedule (RBDRS) model is introduced for the investigating systems. In addition, renewable generation and load centers locate separately in an extensive spatial range and are usually managed by the different system operators in practice. Therefore, a fully distributed calculation method which is based on the alternating direction method of multipliers (ADMM) is proposed. The proposed decomposition method adaptively updates its parameters during the iteration process which guarantees calculation efficiency and convergence properties. Moreover, the dynamic updating strategy only requires limited information from the direct neighbor, therefore the privacy operational information of each regional AC grid is well protected while the fully distributed application is realized. The effectiveness and robustness of the proposed model and algorithm are validated through case studies based on a 2-area 6-bus system and a 3-area 354-bus system.

Research on junction temperature monitoring technology of IGBT modules

Modular Multilevel Converter (MMC) is widely used in flexible HVDC transmission systems, IGBT module as its core power device, its failure process seriously affects the operation reliability of MMC. The high junction temperature is an important reason for the failure of the IGBT module, so monitoring the junction temperature of IGBT module is an important means to ensure the stable operation of the system. According to the topological structure of IGBT module, the current junction temperature monitoring methods of IGBT module are summarized and compared. According to the shortcomings of the current research and the current research status, the research direction of IGBT module junction temperature monitoring is prospected.

Analysis of typical fault characteristics of modular multilevel converter for flexible HVDC transmission

Modular multilevel converter (MMC) has been widely used in VSC-HVDC system for its smooth output waveform, low harmonic content and low switching loss. But compared with the conventional LCC-HVDC system, modular multilevel converter has a short development history, few engineering applications, high failure rate and lacks of very effective protection strategies, which limits its further development. Therefore, this paper firstly introduces topological structure of MMC and its working principle. Then the commonly used control policy of dual-terminal MMC-HVDC project is elaborated. Finally, the transient electrical characteristics of the typical faults are detailedly analyzed through the combination of theoretical analysis and simulation, such as three phase short circuit of converter valve, single phase grounding of converter valve, and the bridge arm short circuit. The conclusion provides an essential theory for the research of the converter protection strategy in the MMC-HVDC project.

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.

Application Analysis of MMC-HVDC AC Tie Line Transmission in New Energy Power Generation

In order to solve the problem of new energy power generation, the author proposes an application analysis method based on MMC-HVDC AC tie line transmission in new energy power generation. This method analyzes the research status of the operation, protection, and control of HVDC flexible transmission systems in various countries and summarizes the advantages of flexible HVDC transmission systems compared with traditional DC, the research focus of the system protection control is explored from the research methods and research ideas. Experimental results show that: the dynamic response of the average value of the capacitor voltage when the two control systems are subjected to external disturbances, the disturbance is set as the DC side voltage rise, and the amplitude is 5%. Conclusion. MMC-HVDC is beneficial to improve the stability and reliability of system operation.

Line Pilot Protection of Flexible DC Grid Based on Traveling-Wave JS Divergence

The proportion of renewable energy mainly including hydropower, photovoltaic and wind power is growing rapidly. DC transmission technology has become the main tool of new energy transmission. The protection scheme of the voltage source converter-based high voltage DC (VSC-HVDC) power grid is the prerequisite for the DC system. But, problems such as high resistance faults, distributed capacitance, and noise interference reduce the reliability of existing protection schemes. To solve the problem of resistance and noise interference in existing protection schemes, a double-ended protection scheme based on traveling wave JS divergence is proposed. First, the flexible HVDC transmission system is analyzed, according to which the expression of traveling wave power is derived, and its fault characteristics are summarized. Subsequently, a combined algorithm of the JS divergence based on the energy of the S-transform was proposed, and the modified algorithm was used to express the fault characteristics of traveling wave power. Finally, a flexible DC transmission system model is built. The performance of the scheme is verified by setting different interference factors (resistance, noise, distributed capacitance, synchronization error).

Coordinated Control Strategy of Flexible HVDC Transmission based on New Energy Grid Connection

In order to improve the quality of photovoltaic power injected into the grid, a high-voltage flexible direct current (Voltage Source Converter-High Voltage Direct Curren, VSC-HVDC) grid-connected scheme based on a micro-synchronous phasor measurement unit (μPMU) coordinated control strategy is proposed. The four-terminal topology of photovoltaic power plants connected to the grid via VSC-HVDC is designed, and two photovoltaic power plants are connected to the grid via VSC-HVDC model using Matlab/Simulink simulation software to simulate when the light intensity is changed, the photovoltaic power plant is out of operation and the grid voltage occurs In the case of sudden changes, the system voltage, power, and AC side frequency can still maintain stable operation, which verifies the feasibility and effectiveness of the transmission scheme, and improves the stability and power quality of the power grid.

A pilot protection scheme for flexible HVDC transmission lines based on modulus power

Existing protection methods for Flexible HVDC transmission lines easily fail to detect the fault location fast and reliably. Concerning this problem, a modulus power based pilot protection scheme is proposed. By analyzing the dynamic switching process of sub-modules in flexible DC converter station, the expression of converter station DC voltage taking into account the influence of AC system is derived. On this basis, the modulus voltage and modulus current are calculated by phase-modulus transformation, and the modulus power calculation model containing DC components alone is built. Then, the expressions of modulus power in different fault cases are derived, and a protection criterion based on the polarity difference characteristics of modulus power is established. Finally, the RT-LAB is used to build the model of a flexible DC system for simulation verification. The results of simulation tests verify that, the proposed protection scheme can detect the fault line and fault pole fast and accurately, with easy threshold setting and low demand on communication synchronization. Besides, it is unaffected by the fault resistance and the specific boundary conditions of current-limiting reactor, etc.

Transient overvoltage characteristics in submarine cables of multi-terminal flexible HVDC transmission system

The overvoltage transient characteristics of submarine cables are one of the key technologies for submarine power transmission. This article takes the submarine cable of Zhoushan Multi-terminal Flexible HVDC Transmission Project as the research object. The MMC-HVDC simulation models at both ends of converter stations are built in PSCAD. The parameters and equivalent models of the flexible HVDC cables are determined, and the submarine DC cable operating overvoltage is calculated by using statistical methods. The simulation results show that the maximum overvoltage of the submarine cable, that is, the maximum overvoltage of the conductor to sheath, the sheath to the armour, and the armour to the ground are 400 kV, 7.27 kV and 0.45kV respectively. The maximum overvoltage when it occurs faults on grid-side or valve-side appears in the middle of the submarine cable. Maximum overvoltage of HVDC single pole grounding fault appears at the connection point of land cable and submarine cable. The simulation results and the discovery of features in this article provide reference in the design, selection, and testing of submarine power transmission equipment.

Artificial Intelligence Technology in Flexible Dc Transmission Monitoring System

The flexible DC transmission system is a grand project being practiced by the State Grid Corporation of China and China Southern Power Grid. The flexible DC transmission monitoring system is particularly important, and the use of artificial intelligence related technologies to solve such problems is considered an effective solution. This article mainly introduces the application analysis of artificial intelligence technology in the flexible DC transmission monitoring system. This paper uses artificial intelligence technology to perform real-time detection on the flexible DC transmission monitoring system and establishes the SC-HVDC system model. The model is solved by neural network, and the status of the transmission system is evaluated, and the model is revised using historical data to improve the accuracy of the evaluation of the transmission safety status. The experimental results of this paper show that the SC-HVDC system model reduces the accident rate of flexible HVDC transmission by 16%. Finally, artificial intelligence technology has a good effect on improving the stability and safety of the flexible DC transmission monitoring system.

Analysis of Fault Waveforms of Key Electrical Parameters in Hybrid Multi-terminal DC System

LCC-VSC hybrid multi terminal HVDC system combines the advantages of traditional HVDC and flexible HVDC technology, and becomes one of the new directions of HVDC development. In this paper, taking the Baihetan Jiangsu HVDC transmission project as an example, under the appropriate simplified model, the critical electrical stresses of the DC side faults of a typical LCC-VSC hybrid multi-terminal DC system are analyzed. The simulation results verify the correctness of the simplified analysis in this paper.

Quench recovery time of an optical fiber encapsulated resistance-type DC superconducting fault current limiter coil determined by a Raman-based distributed temperature sensing system

The superconducting fault current limiter (SFCL), as a very important power facility for high voltage DC system, can consume fault current energy and make DC breakers cut off fault transmission lines successfully. For the moment of reclosing DC breakers, the quench recovery time of the SFCL must be determined in advance. In this article, according to the design requirement of a 160 kV/1 kA resistance type DC SFCL in the Nan’ao multi-terminal flexible HVDC transmission demonstration project, the quench recovery time of a SCFL coil wound by two optical fiber encapsulated high temperature superconducting (OFE-HTS) tapes was checked by our optimized Raman-based distributed temperature sensing (Rm-DTS) system. The experimental results show that the used OFE-HTS tapes can withstand a 1500 A/100 ms AC short circuit current impact and are qualified with the SFCL. Furthermore, the quench recovery time of the SFCL coil measured by the Rm-DTS was 2930 ms. During the quench, the highest temperature zone occurred near the center of the SFCL coil.

Research on the protection scheme of MMC‐MTDC based on high frequency energy

The development of flexible HVDC transmission technology requires the improvement of the theory of relay protections. The protection method based on the time domain has poor resistance to transition resistance and is easily affected by interference factors. The protection based on frequency domain analysis can better reflect the essential characteristics of the fault, and it is also an urgent problem for relay protections. The blocking effect of boundary elements on both sides of the line on high-frequency components is an important entry point for current protection research, but high-frequency protections are very sensitive to noise signals. Therefore, this paper analyses the frequency characteristics of the voltage at the measuring point from the perspective of the traveling wave transmission process, and uses the S transform to design a protection method based on high-frequency energy to realize the identification of internal and external faults. Finally, the ability to withstand transition resistance and anti-noise of the protection proposed here is verified by using PSCAD/EMTDC. The protection can complete fault identification within 2 ms, which means that it has both reliability and quick action.