From the end of 2004 the Gui-Guang interconnector will deliver up to 3000 MW at ±500 kV DC from hydro and coal-fired power plants in the west of China to the densely populated areas around Guangzhou and Shenzen in the south-east of the country; a 940 km overhead power line will link the two converter stations in Guizhou and Guangzhou. Siemens is supplying and installing all HVDC technology, including converter valves, converter transformers, smoothing reactors, high voltage switchgear and control and protection systems. To facilitate problem-free commissioning of the interconnector in the shortest possible time, the entire installation was simulated in Germany so that the control and protection systems for both converter stations could be tested in advance.

Using the original hardware, Siemens assembled the two control centres of the

3000 MW link at Group headquarters in Erlangen and put the interconnectors control and protection technology through a series of exhaustive tests. This process involves real-time, digital simulation of the overhead transmission line, complete with all HVDC equipment including valves, transformers, filtering and smoothing, and all HV switchgear and the associated AC power systems, connected to the original control and protection system.

Reducing the risk of failure

Siemens maintains that this process reduces the commissioning time and the risk of malfunctions at site to a minimum. The company is investing in this technology, and in a new control and protection system called Win-TDC, because it believes that within the next twenty years demand for power worldwide will rise by more than seventy percent, implying enormous investment in power generation and transmission. It also sees a tendency towards a global energy market, which will result in a correspondingly increased need for both national and international grid interconnections. And, especially in China and India, centres of generation and consumption are frequently very far apart. China in particular is a rapidly growing market for power transmission projects and will be the world´s largest single market for HVDC transmission for the next 20 to 30 years.

Testing was completed in mid-August, after which all control and protection cabinets were dismantled and shipped to China to be reassembled exactly as in Germany. If all goes to plan, the first pole will be commissioned by June and the second pole by December 2004.

The engineers who designed and developed the control and protection software for this HVDC project also carried out function testing and system commissioning at the customer’s sites.

The test rig

50 electronics cubicles were installed for the control and protection gear at the Erlangen centre. The entire system, consisting of operator interface with around 20 workstations, communications, and alarm and diagnostics system, was put through its paces. State-of-the-art real-time simulators built up using fast parallel computers digitally simulated the process of HVDC current transmission throughout testing, together with the required power electronics for conversion and the filters, switchgear, transformers, lines and networks.

Protection system

A good HVDC protection system has to be fast, reacting typically in a time range from 2 ms to about 10 ms, it has to be reliable, and of a failsafe design ­ in other words, avoid false trips. In the event (for example) of a normal AC or DC fault, a trip should not occur. Only if the equipment is in a really dangerous situation must tripping take place. So spurious trips resulting from wrong settings or from over-sensitive protection equipment must be avoided. (See example, right).

The protection system must also discriminate among faults that require, for instance, routing through AC filters. And it should possess a high dgree of redundancy. All these parameters and settings have to be verified precisely, not just under normal conditions but also during transients.

Once all systems had been checked individually, the control and protection systems and the two HVDC controls were assembled and tested. When both control centres were functioning correctly, the actual transmission process was simulated by modelling a range of operating conditions; dynamic processes such as power system faults were also analysed.

The purpose of the exercise was to demonstrate that the system was stable and above all reliable, functioning without interruptions even if redundant systems went down. The series of tests and simulations was also used as a training opportunity for the commissioning engineers and the operator’s technicians.