Ultra-high Voltage Direct Current Projects Research Articles

Negligible carbon costs of UHVDC infrastructure delivering renewable electricity

Ultra-high voltage direct current (UHVDC) transmission make it possible for efficient large-scale long-distance renewable energy deliver, while the infrastructure construction consumes significant amounts of material and energy. The carbon tradeoffs are often overlooked, however, hindering a holistic evaluation of its climate benefits. Here, by collecting field construction data from fifteen 500 kV transmission projects and applying the field data in a Life Cycle Assessment framework, we estimate the infrastructure-induced emissions of nine recently-constructed UHVDC projects in China to be 16.7 Mt CO2, which is negligible compare to an annual reduction of 305.2 Mt CO2 these projects contribute by delivering renewable electricity at full load, multiplied by a 40-yr service lifetime. The tradeoff results may evolve with advancing energy storage technology at exporting region or CCUS application at importing region. Low-carbon strategies for UHVDC construction are therefore favored, especially under China's commitment to expanding UHVDC construction along the Belt and Road Initiative.

Power grid practice of suppressing transient overvoltage of sending-end system by ultra high voltage direct current control parameters

The transient overvoltage off-grid problem of new energy in the sending-end network has become an important factor limiting the ultra high voltage direct current (UHVDC) transmission capacity. In this paper, the causes of transient overvoltage during the DC fault process is analysed firstly, then from the perspective of adjusting the reactive power consumption of the converter, taking the Qingyu UHVDC project in China as an actual simulation example, the influence of the DC control mode and current control parameters at the rectifier side on the transient overvoltage is deeply studied. Combined with the transient voltage evaluation results of the actual power grid, guiding suggestions for DC control strategies are given to improve the transmission capacity. The research results have important reference significance for improving the consumption of new energy and the transmission capacity of UHVDC projects.

Optimized isolation and restarting strategy for UHVDC converter faults

As an important method to solve the contradiction between energy distribution and demand, Ultra-high voltage direct current (UHVDC) power transmission has great advantages in long-distance, large-capacity, and cross-regional power transmission scenarios, and has been widely used. Aiming at the problems of long-time consumption, large power outage range, and power loss of the converter fault isolation and restarting process in a UHVDC project, an optimized strategy was proposed based on the analysis of the fault process. On the one hand, the converter protection area, as well as the connection mode of the breakers is optimized, and fast circuit breakers are added to speed up the process. On the other hand, the fault isolation and restarting strategy is optimized. By using the converter phase-shifting control to fast limit the fault current, using fast circuit breaker action to narrow the scope of fault isolation, the occurrence of pole isolation, pole restart, and large-scale machine and load shedding is avoided. Simulation verification shows that the optimized isolation and restarting strategy takes less than 0.5s in total, and only isolates the faulty converter to ensure the continuous power transmission of the unfaulty part, which significantly improves the reliability and economy of UHVDC transmission.

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.

Coupling Effect Investigation of Series-Connected Converters in MMC-Based Ultra-HVDC Station

The modular multilevel converter (MMC) technology is being utilized in ultrahigh voltage direct current (UHVDC) transmission for much larger power transmission and a series of multiple MMC converters are necessary to achieve ultrahigh voltage. Based on the scheme of the Wudongde multiterminal UHVDC project in China, this article develops a small-signal model (SSM) for MMC-based UHVDC station under constant active power-control mode. The dynamics of the high-end converter (HEC) and low-end converter (LEC) are modeled independently considering the internal harmonics. Then, the dynamic model of an MMC-based UHVDC station is developed by interfacing the models of HEC and LEC, considering the characteristics of parallel connection at the ac side and series connection at the dc side. In addition, a voltage-balancing control approach, which is necessary to balance the voltage between HEC and LEC, is also included in the dynamic model. After verifying the accuracy of the derived SSM, this article studies the coupling effect between HEC and LEC. The results show that the vector current controllers of HEC and LEC closely interact with each other, and the voltage-balancing controller makes the coupling between HEC and LEC more complex. Moreover, the circulating current suppressing controllers (CCSCs) of HEC and LEC are relatively independent.

Observation Research on the Effect of UHVDC Grounding Current on Buried Pipelines

In order to research the electromagnetic interference in buried oil and gas pipelines generated by the grounding current of a grounding electrode of Ultra High Voltage Direct Current (UHVDC) system, observation experiments of stray current and pipe-to-soil potential (PSP) in the pipeline were carried out. Monitoring devices were installed at two sites of the Dong-Huang oil pipeline before the commissioning of the Zalute–Qingzhou and Shanghaimiao–Shandong ±800 kV UHVDC projects. Monitoring data on the stray current and PSP of the two monitoring sites were obtained when the two UHVDC projects were operated in monopolar mode on 24 December 2017 and 2 January 2019 and the grounding current reached 6250 A. The amplitude characteristics of the stray current and PSP at different distances from the grounding electrode and the effects of the magnitude of the stray current and PSP on the cathodic protection system are analyzed herein. The results show that the effects of the grounding current on pipeline corrosion are not only closely related to the distance between the grounding electrode and the pipeline but are also related to the running state of the potentiostats of pipelines and the distance between insulation flanges. Optimizing the performance of potentiostats and the distribution of insulation flanges can reduce the effects of UHVDC grounding current on pipeline corrosion.

Analysis of the Monitoring Data of UHVDC Grounding Current Interference in a Buried Pipeline

The transmission power and rated current of the ±800 kV ultrahigh-voltage direct current (UHVDC) project is so high that the grounding current aggravate the corrosion of buried pipelines during the monopole operation period, which is a critical issue that needs to be studied. The stray current and the pipe-soil potentials (PSPs) synchronous monitors have been installed on a buried pipeline near the receiving end of the Zhalute-Qingzhou ±800 kV UHVDC transmission project and the monitoring experiment for the effect of the grounding current of the receiving ground electrode is carried out. The monitoring data of stray current and PSP offset in relation to the grounding current of the ground electrode are obtained for the first time all over the world, during the UHVDC project commissioning in October 10th and December 24th, 2017. The results of data analysis show that the pipeline 150 km away from the ground electrode is also affected by the UHVDC project, its PSP offset exceed the limit value required by the national standard for discharge measures and the dc stray current is produced in the pipeline. According to the data and calculation, the characteristics and rules of the influence of UHVDC grounding current on pipelines are acquired.

Evaluation of the Interference Effects of HVDC Grounding Current on a Buried Pipeline

The stray current created by the grounding current of HVdc ground electrode will accelerate the corrosion of the oil and gas pipeline when the HVdc is operate in single-pole mode. The magnitude of the stray current and the pipe-soil potentials (PSP) offset are indexes to evaluate the corrosion situation of the pipeline. An algorithm of the stray current and PSP of an oil pipeline generated by the Gaoqing ground electrode of Zhalute-Qingzhou ±800-kV ultrahigh voltage direct current project is put forward, the influence of potentiostat output, defects of insulation coating, and the insulation joint are considered for the first time. The stray current and PSP of the pipeline is calculated. The correctness of the algorithm is verified by the comparison of the calculation results and the monitoring data. The results show that the magnitude of the stray current and the PSP offset of the pipeline near an oil transmission station is the largest, the PSP offset is beyond the limit value prescribed by the national standard, and the potentiostat cannot provide an effective protection for the pipeline when the HVdc is operated in single-pole mode with the transmission load reaches to 2500 MW.

Construction of an example system for AC/DC hybrid power grid with high‐proportion renewable energy

Large-scale wind and solar power generations have got rapid development in recent years in China and abroad. They always connect to load centres through high-voltage direct current (HVDC) transmission or ultra HVDC (UHVDC) transmission lines. Such systems are so large and it is necessary to make appropriate equivalence according to the purpose of different research. At present, the analysis model of this kind of real system is few, and simplified single-machine infinite-bus system or IEEE example system is adopted most of the time, which may affect the accuracy of analysis results. Here, the UHVDC project from a practical renewable energy base to load centres in China is chosen as an example, the actual structure and parameters are used to establish a typical example system for AC/DC hybrid power grid with high-proportion renewable energy. The new-generation synchronous condenser with large capacity and the supporting synchronous generators are all considered. First, the installed power generation, power grid structure, and load configuration of example system are introduced. Then the load flow distribution and transient stability characteristics are analysed by using DIgSILENT PowerFactory software. Finally, the feasibility of the proposed example system is shown by simulation results.

Impact of Space Charges From Direct Current Corona Discharge on the Measurement by the Rotating Electric-Field Meter

The ionized field is one of significant parameters to evaluate the electromagnetic environment of ultra-high-voltage direct current (UHVDC) transmission lines. The field mill is usually utilized to measure the ionized field on the ground level under the UHVDC power lines. In order to investigate the distortion characteristics of the ionized field with the presence of spaces charges near the probe of the field mill, Poisson's equation and current continuity equation are solved, respectively, based on the finite-element method and Gauss's law in the cylindrical coordinate system. The ionized fields on the surface of the probe are calculated under different ion current densities. An experimental apparatus with corona wire net and ground plate is designed and used to measure the ionized field and ion current density. Experimental results have shown that the measured data are consistent with the calculated ones, which verify the validity of the algorithm presented. The impact of the space charges on the field mill may provide valuable reference for the measurement of the electromagnetic environment of UHVDC projects.

Application of GWS-1 Remote Equipment in the UHVDC Projects

The present situation of the UHVDC (ultra high voltage direct current) power transmission project is introduced, based on ±800kV UHVDC power transmission project put into commercial operation, the function design and application of GWS-1 remote equipment are carried out, the communication capability on IEC61850 is supported, the temperature problem of HP rx2660 is resolved, the GWS-1 remote equipment could meet the requirement of the UHVDC power transmission project is approved, and there is guiding significance on the research of RCI of UHVDC power transmission project.