Ultra-high Voltage Direct Current Power Research Articles

Multiobjective optimal operations for an interprovincial hydropower system considering peak-shaving demands

Large-scale power transmission in China poses a challenge for the operation of an interprovincial hydropower system (IHS) connected by ultrahigh-voltage direct current (UHVDC) lines. This study develops a multiobjective optimization model for the monthly operation of an IHS considering daily peak-shaving demands. This model is first transformed into a sequence of single-objective subproblems with different peak-to-valley ratio constraints. A multiphase multigroup coordination strategy is proposed in which the plant group for each month is determined based on the hydrological seasons of the different rivers. A multidimensional search method is then utilized to optimize the monthly generation schedules, where the energy target for a typical day is proportional to the daily load share in each month. Finally, a load-reconstruction-based strategy is used to determine feasible hourly generation schedules and obtain the peak-to-valley ratios to evaluate the monthly operation schemes. The above model and method are applied to a real-world IHS related to the Xiluodu-Zhejiang UHVDC power transmission project. The obtained Pareto solutions reveal the interaction between the energy production and peak-shaving objectives and show a significant improvement in peak shaving with a small decrease in energy production. Moreover, in typical inflow scenarios, all operation schemes enhance the total energy generation from local hydropower plants while leaving that from Xiluodu almost unchanged compared with the conventional operation results.

Calculation and application of surface nominal electric fields of ±1100 kV UHVDC power lines

Electric field strength is one of fundamental electromagnetic environmental parameters for the design of ultra-high voltage direct current (UHVDC) power lines and towers when considering human exposure and transient shock. In order to investigate the impacts of power lines’ geometric parameters on the nominal electric fields on the surfaces of ±1100 kV UHVDC sub-conductors, charge simulation method is used for calculations. The algorithm is verified by both verification nodes’ potentials in the process and measurement experiments in the laboratory. The geometric parameter of sub-conductor radius has the most obvious effect on the field. When the radius varies by 10%, the electric field strength on the sub-conductor’s surfaces may change approximately by 8%. Calculation results demonstrate that increasing sub-conductor radius may be most effective measure to restrict the electric field on the ground.

Error assessment method of electromechanical–electromagnetic hybrid simulation

With ultra high voltage direct current (UHVDC) and flexible alternating current transmission systems (FACTS) have been widely applied in power system, the AC/DC power grid is constantly extending which makes its composition more complicated. The independent electromechanical transient simulation and electromagnetic transient simulation cannot meet the need of rapidly developing modern power system in simulation technology. Electromechanical–electromagnetic hybrid simulation not only has the advantages of analysing large systems, but also accurate description of power electronic components. Analysing errors in hybrid simulations can help them to quantitatively analyse different hybrid simulation interface technologies to optimise simulation settings. Concerned fault types, fault location, and point fault cleaning time, an electromechanical–electromagnetic hybrid simulation scenarios probabilistic model based on Monte Carlo is put forward. Then adopt Prony method to quantitatively analyse the hybrid simulation errors. Through a typical example of UHVDC power grid in Shandong province, the error evaluation method of the hybrid simulation is demonstrated in detail. The research shows that this kind of assessment method can well analyse the numerical validity of hybrid simulation.

Research and application of UHV power transmission in China

The power demand increases rapidly in China; however, the areas of huge power demands are of long distance from most areas of abundant energy resource in the country. Therefore, China put in great effort to develop ultrahigh voltage (UHV) power transmission systems to optimise its energy allocation. This includes (i) systematically developing key technologies such as overvoltage suppression, external insulation design, and electromagnetic environment control, and (ii) developing key equipment such as UHV alternative current (UHVAC) transformers, circuit breakers, and series compensated equipment, and UHV direct current (UHVDC) converter transformers, smoothing reactors, converter valves, and DC transmission control and protection systems. Eight AC UHV projects and 11 DC UHV projects have been built in China, which play an important role in the optimal allocation of energy. Plus there are one more UHVAC and three more UHVDC transmission projects in construction, while UHVAC gas-insulated lines will be applied in the UHVAC line crossover the Yangtze River and the ±1100 kV UHVDC power transmission technology is in development. Here, the development and application of UHV power transmission technologies in China are described, some main challenges the UHV projects encountered are discussed, and experiences obtained from the operation of UHV systems are introduced. It is concluded that China obtained mature experience in developing, constructing, and operating UHV systems and successfully realised long-distance, large-capacity power transmission, and the UHV power transmission technology has become an important measure for energy allocation in large areas.

Characteristics of space charge distribution in epoxy-paper composite insulation system

The HVDC dry-type bushing, with its main insulating medium being epoxyimpregnated- paper composite, is a key device in the ultra-high voltage direct current power transmission system. The HVDC bushing is under the combined effect of electricity, heat, and mechanical stress when it is in operation, in which case the electric field strength in the core of bushing is high, the temperature in the central conductor is high, and the interfacial effect between the epoxy layer and insulating paper is strong. Consequently, space charge will accumulate in the epoxy-paper composite insulation. Previous studies mainly focuses on the characteristics of space charge distribution in polymer-micro-nano filler, whereas the studies on that in epoxy-insulating-paper composite are quite limited. This paper, based on the HVDC bushing made up of epoxy-insulating-paper composite, establishes and a space charge measuring system which is suitable for the pulse electro-acoustic (PEA) method at high temperature (⩽110 °C). A modified PEA method which is applicable to the measurement of space charge in the multi-dielectric-layer composite with different acoustic impedance is put forward. By controlling the thicknesses of dielectric layers in the composite sample, the effect of sound wave reflection on the measuring system can be eliminated. An in-depth study is conducted on the effects of temperature and field strength on the characteristics of space charge distribution in epoxy-paper composite.

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.