Supporting Capacity-Building Project Characteristics Test of Zhoushan Multi-Terminal HVDC Flexible Demonstration Project

In this paper, a flexible Zhoushan multiterminal HVDC test demonstration projects supporting capacity-building project for the study, the DC cables through the flexible testing ground field measurements of its surrounding environment, the DC cables of a flexible environment affect the characteristics of the test site. The results show: the project on the surrounding environment has little effect of electromagnetic radiation, which affect the field strength generated by the project on the environment is negligible, the project generated frequency electric field, power frequency magnetic fields, DC magnetic field, radio interference strength, noise are smaller than the corresponding standard limits.


Introduction
The flexible direct current transmission technology is a new generation of DC transmission technology featuring fully controlled power electronics, voltage source converters and new modulation technologies. Compared with traditional DC transmission technologies, it has no need for reactive power compensation and no phase change. Failure problems, simultaneous adjustment of active power and reactive power, and low harmonic levels are suitable for the construction of multi-terminal DC systems and other obvious advantages [1]. They are widely used in grid-connected renewable energy, isolated islands, urban power supply, and wind power generation. Measure the weather and choose sunny weather [2][3].
This article selects Zhoushan multi-port flexible HVDC transmission demonstration project supporting test capacity construction project as the research object, and through the field measurement of the surrounding environment of the flexible DC cable test site, the environmental impact characteristics of the flexible DC cable test site are discussed.

Overview of Supporting Experimental Capacity Building Project of Zhoushan Multi-Station Flexible HVDC Transmission Demonstration Project
Zhoushan ±200kV five-terminal flexible DC transmission project is the first five-terminal flexible DC transmission project that has been successfully put into operation in the world. It is also the world's most flexible, direct-current DC project with the highest voltage rating, the largest number of terminals, and the largest single-ended capacity. The research object belongs to Zhoushan ±200kV five-terminal flexible DC transmission engineering technology reserve project. At present, the first DC 200kV flexible DC cable pre-assessment test has been carried out in the flexible DC test site of the Zhoushan multi-terminal flexible DC transmission demonstration project. The test aims to strengthen the islands under the jurisdiction of electrical contacts, enhance the structure of the grid, improve the reliability of power supply, solve the flexible access of new energy such as offshore wind power, cable charging power and impact stability and power quality problems.
The main equipment in the test field of flexible DC cables is one set of 1200kV/50mA DC high voltage test equipment. It is mainly used for power cable type test, pre-qualification test, polarity reversal test and DC superimposed impulse voltage test, which can be 500kV and below. DC power transmission equipment withstand voltage test to provide power; Another thermal cycle system 1 set, can provide heating power for the cable system, mainly used for the heating of the cable sample conductor core. Among them, the 1200kV/50mA DC generator bears the heavy responsibility of providing test line voltage, it will be used for testing the corona characteristics and environmental impact of the line segment, as well as the on-line assessment of some equipment. It is one of the key test equipments for the base. The corona of the DC generator itself is closely related to the accuracy of future experimental results. The intensity of the corona depends directly on the surface field distribution of the generator components [4].

Environmental Background Value
The monitoring results of the maximum synthetic field intensity in this project are 0.25-0.45kV/m, the 80% synthetic field monitoring results are 0.19-0.36kV/m, the DC magnetic field strength is between 48.1-49.4µT, and the radio interference frequency is 0.5MHz. The value is 39.4~52.4dB(µV/m) [5].

Measurement Items
This study selected power frequency electric field, power frequency magnetic field, synthetic field strength, DC magnetic field, radio interference, and noise as measurement items.

Measuring Instruments
Measurement instruments and corresponding technical parameters are shown in Table 1.

Measuring Weather Conditions
Choose good weather measurement in sunny days to avoid the influence of strong wind, rain, snow, hail and other weather [6]. Corresponding standard limit requirements 4 100 . Figure 3. Power frequency magnetic field measurement, limit value.     Corresponding standard limit 10000 20m to the northwest 0.5 40. 8 4 Outdoor test site 20m away 0.5 48. 8 5 Corresponding standard limit requirements 0.5 53   Corresponding standard limit requirements 65 55

Summary Analysis of Monitoring Results
From Table 4 to Table 8, Figure 2 to Figure 9, it can be seen that the strength of the composite Field around the test field of the flexible DC cable is basically the same as the background value of the environment.
The power frequency electric field, power frequency magnetic field, DC magnetic field strength, radio interference intensity and noise around the test field of the flexible DC cable are all less than the corresponding standard limit requirements [7-10].

Conclusion
As the first pre-qualification test of flexible DC cables in China, the results obtained from the actual measurement of the surrounding environment of the flexible DC test site in the supporting experimental construction project of the Zhoushan multi-terminal flexible HVDC transmission demonstration project show that: The flexible DC test field has less influence on the surrounding electromagnetic radiation environment. The influence of the synthetic field intensity generated by the project on the surrounding environment is negligible; the power frequency electric field, power frequency magnetic field, DC magnetic field, radio interference intensity, and noise generated by the project are much smaller than the corresponding standard limit values.