By Uwe Armonics
Siemens AG, Power Transmission and Distribution Group, Germany
The 940 km long Guizhou-Guangdong HVDC power link in China will deliver power from power stations in south-west China to the south-east of the country. Rated at 3000 MW, the link will help to ensure the economic development of Guangzhou and Shenzhen.
In late 2001, the State Power Corporation of China awarded Siemens Power Transmission and Distribution (PTD) group a contract to help develop a 940 km-long HVDC transmission link between Guizhou and Guangdong provinces. Construction of the link is already underway, and commissioning is scheduled for 2004.
Tian Guang quadruple valves
The link, known as Gui-Guang, will form an essential part of China’s developing power grid, and of the country’s economic growth plans. Rated at 3000 MW, the link will deliver bulk power to the rapidly growing economic areas of Guangzhou and Shenzhen.
Gui-Guang will complement another HVDC link developed by Siemens PTD – Tian-Guang. Delivering 1800 MW across 960 km, Tian-Guang is one of the largest transmission links of its kind in China. It was completed in 2001, and also supplies Guangzhou and Shenzhen with bulk power. Together, these HVDC links are of huge importance to the Southern China electric power network. Reliable HVDC links among the interconnected AC systems will increase the power transmission capacity between and within the involved networks.
Schematic of the main circuit
The Gui-Guang ±500 kV DC transmission system will deliver 3000 MW (Bipolar 2 x 1500 MW) over 940 km from the Anshun converter station in Guizhou province to the Zhaoqing converter station in Guangdong province. Both converter stations will be connected to the respective 500 kV AC networks.
The Gui-Guang HVDC scheme will allow the bi-directional control of power interchange and will also improve the reliability and dynamic performance of both connected AC systems. Additionally , the converter stations give the advantage of controlling the reactive power exchange with the connected AC grids and thus the AC system voltage.
The contract for delivery of the converter station equipment was effected by the State Power South Company and Siemens AG – the contractor – on November 1, 2001.
Siemens’ scope comprises the design studies, architectural design, design and manufacturing of electrical equipment and components, delivery to site, supervision of on site installation and commissioning. State Power South Company is responsible for execution of civil works, provision of the AC switchyard at both converter stations, construction of the DC overhead transmission line and both corresponding ground electrodes near the converter stations.
The project execution will be performed according to the following time schedule. The first pole will be in commercial operation 36 months from the effective date of contract, while the second pole will follow six months later. Therefore, the start of the erection of converter building is scheduled for January 2003.
Erection of equipment in the AC and DC yards will start in the second half of 2003. The joint efforts of all the involved project parties during design and manufacturing of the converter equipment, together with smooth project execution and an intensive test programme, conducted within a short time period, and the experience of the co-operation from the Tian-Guang HVDC project will guarantee the high performance of the Gui-Guang HVDC power link.
Power transmission capacity: The 940 km long distance HVDC transmission system is carried out as a ±500 kV bipolar scheme.
The bipolar system is rated for a continuous power of 3000 MW (±500 kV, 3000 A) at the DC terminals of the rectifier converter station for power direction from Anshun to Zhaoqing. The converter stations are designed to transmit full rated power up to a maximum ambient dry bulb temperature of 40°C without the redundant cooling system in service. With redundant cooling a continuous overload of 110 per cent of rated load is achievable. Additionally the DC transmission permits a 3s overload of 1.4 per unit power. For low ambient temperature conditions increased overload ratings are possible.
The HVDC interconnection scheme is capable of continuous operation at a reduced DC voltage of 400 kV (80 per cent) and 350 kV (70 per cent) from minimum current up to the rated DC current of 3000 A with redundant cooling equipment not in service. In order to optimize the filter and converter transformer design the capability of the valves to operate at high firing angles combined with an extended range of the tap-changer is used for such special modes of operation.
Modes of operation: The Gui-Guang HVDC system is designed for operation in the following configurations:
- Bipolar mode
- Monopolar metallic return mode
- Monopolar ground return mode
In order to meet the overall availability requirements, the transfer between any mode of operation will be conducted without transmission interruption. This requires installation of transfer switches in the DC switch yards. One MRTB (Metallic Return Transfer Breaker) and one MRS (Metallic Return Switch) are installed at the converter station in Anshun. The main task of the MRTB is to transfer current from the low impedance ground return path to the relatively high impedance metallic return path, whereas the main task of the MRS is to transfer current from the high impedance metallic return path to the low impedance ground return path.
Performance requirements: A high degree of energy availability is a major design objective. This design goal will be achieved mainly by minimizing the downtimes using high quality products, fast fault detection as well as effective repair and maintenance strategies. Fault-tolerant control systems, redundancy, spare components and quality assurance will ensure high component and system reliability.
To provide the highest level of reliability and availability and hence quality of the HVDC control and protection system intensive off-site tests (e.g. functional performance test) will be performed. The converter pole energy availability for both stations together is guaranteed to be more than 99.5 per cent, and respectively a forced energy unavailability of less than 0.5 per cent. The pole forced outage rate shall be less than five outages per pole and year.
Reactive power requirements: According to the demand of reactive power of the HVDC converters and the connected AC Networks, reactive power equipment (AC filter and c-shunt banks) will be installed at the converter stations. These reactive power elements are individually switchable and configured in filter banks, which are connected to the station bus bar. High performance double (DT) and triple (TT) tuned filters will be installed at both converter stations.
Major technical features
Thyristor valves: Continuous R&D and many years of experience have led to a design of thyristor valves which meets the highest requirements with respect to fire resistance, seismic robustness, corrosion-free water cooling as well as high reliability and easy maintenance.
Each valve for the Gui-Guang DC project will comprise 78 thyristor levels (three redundant) and 12 non-linear reactors per valve distributed to three thyristor modules. Four valves will be arranged in a quadruple valves, similar to the Tian-Guang valves. Three quadruple valves form one 12puls group of one converter pole.
Figure 1 shows the thyristor valves of the Tian-Guang HVDC Project, fully assembled quadruple valves or multiple valve unit (MVU) installed in a valve hall.
Standard features of Siemens HVDC thyristor technology will be complemented with the use of modern, yet proven thyristor technology of 8 kV blocking voltage, current carrying capability of up to 4000 A, direct-light-triggering and integrated forward overvoltage protection. Logic circuits and auxiliary power requirements at the thyristor level will be reduced to an absolute minimum.
Converter transformer: Single-phase two-winding transformers (12 units plus two spares at each station) will be used (see Table 1).
Smoothing reactor: Oil immersed smoothing reactors of 270 mH per pole will be installed.
Control and protection: The control and protection systems in each converter station of the HVDC transmission system will be designed according to the modern decentralized concept. The central controls are located in the control room, and all local controls are within relay houses in the switchyard.
The communication between the control room and the relay houses will be performed using a redundant fieldbus. The central controllers are interconnected via a local area network. The HVDC system can be operated either remotely via the Remote Terminal Unit or locally via the Initiation and Monitoring workstations.
Tailor-made protection systems for HVDC applications co-ordinated with the pole control and the AC systems protection are responsible for selective fault clearing, respectively prevention of damage of HVDC components. Diagnosis of faulty conditions is assisted by the Sequence of Events Recording and Transient Fault Recorder time synchronized via a GPS controlled master clock.