The biggest problem in designing the electromagnetic environment is the lack of a standard. The dc resultant electric field, ion flow, dc magnetic field, radio interference (including television interference) and audible noise have to be taken into account for the electromagnetic environment of dc projects. Based on intensive study, the following standards were adopted for the Yunnan-Guangdong project:
- Maximum resultant electric field strength at the ground level under lines is 30 kV/m
- Maximum resultant electric field strength near buildings is 25 kV/m and 80% of its measurements shall not be greater than 15 kV/m
- Ion flow density below lines is 100 nA/sq m
- Control index of the dc magnetic field is 10 mT
- Control index of the radio interference for 800-kV dc transmission lines is 55 dBmV/m
- Audible noise is 50 dBA in ordinary areas under good weather conditions and is checked against 45 dBA in densly populated areas.
Converter valves are designed with 127-mm (5-inch) light-triggered thyristors. These valves are installed using air-insulated, water-cooled, suspension-type, indoor double-valve structures. In addition to the new structural design, the greatest technical challenge in terms of converter valves was the type tests, including the multiple-valve insulation test, operational test, and testing other items and criteria. Furthermore, the current capacity of traditional 127-mm (5-inch) light-triggered thyristor valves is 3,000 A; however; the rated dc current of the Yunnan-Guangdong project is 3,125 A. Therefore, the technical performance of the thyristors required optimization.
A total of 28 single-phase, double-winding converter transformers are provided for the two poles of each station with four serving as standby. The delta-star converter transformer was the most difficult equipment to manufacture.
External lead-out devices are used at the valve side to accommodate transportation limits. Specifically, width could be no greater than 3.5 m. The site installation tolerance of lead-out devices at the valve side is small and the process is complex. Therefore, a special mounting platform and tools were required.
Vacuum on-load tap changers are used for converter transformers for the first time. Their structural dimensions are relatively small, which can help relieve the difficulties in designing the dimensions of the converter transformers.
Dry-type bushings are installed on the valve side, as the length of bushings on the valve side manufactured for the ±800-kV converter transformers is almost up to the maximum design length.
Dry-Type Smoothing Reactor
Four dry-type, air-cored and naturally air-cooled smoothing reactors are installed per pole in each converter station, being series-connected to the pole bus bars and neutral bus bars with an additional one as standby. Compared with the previous oil-immersed smoothing reactors, this dry-type structure can reduce design and manufacturing difficulties significantly.
A Foundational Project
The total cost of the Yunnan-Guangdong project was 15.4 billion yuan (US$2.4 billion), of which 7.9 billion yuan (US$1.23 billion) was invested in converter station equipment. Since commissioning the Yunnan-Guangdong project in June 2010, the HVDC system has operated in a stable and smooth manner. This is a measure of the high standards adopted in the various stages of system studies, design, manufacturing, installation, commissioning and operation. The Yunnan-Guangdong project has effectively alleviated the power-supply shortage of the Guangdong power grid, rendering good economic and social benefits.
This project provides a solid foundation for subsequent projects in terms of the research, manufacturing, development, design and operation of ultrahigh-voltage equipment. Furthermore, it serves as a demonstration for subsequent planned projects that will transmit electricity from the large hydropower stations at Nuozhadu and Xiluodu to the load centers. The successful construction of the Yunnan-Guangdong project represents an important milestone in China and even in the world’s electric power industry.
Qi Da-Cai (email@example.com) was awarded a Ph. D. by Tsinghua University in 2004 and, during his career at China Southern Power Grid Co. Ltd., he has held engineering posts in power project construction and power system operation before embarking on his managerial career. Since 2002, Dai-Cai, who is a professorate senior engineer, has served as vice-general manager of CSG.
Lu Jin-Zhuang (firstname.lastname@example.org) was awarded a Ph.D. by the North China Electric Power University in 2004 and, during his career at China Southern Power Grid Co. Ltd., he has conducted over-voltage and insulation coordination research on the HVDC and UHVDC power transmission project.
NARI-Relays Electric| www.nrelect.com
TBEA Shenyang Transformer Group| www.tbea-sb.cn
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Xi’an XD Power System | www.xdps.cn
XJ Group | www.xjgc.com