In August 2011, power conversion specialist Converteam, reinforced the role it has been playing in the Chinese pumped storage market since the first schemes were developed in the early 1990s. The company signed a contract with NARI Technology Development Company, a major player in the Chinese electric power industry, to supply static frequency converters (SFC) for the 1500MW Liyang pumped storage power plant (PSPP).

Liyang will be the largest pumped storage plant in Jiangsu Province when operational in 2015. Scope of supply for the project includes two sets of 18MW water-cooled static frequency converters, equipped with a high-performance control platform. The SFCs will be used to start, and quickly run up in pump mode, six sets of reversible hydro turbine motor/generators, rated at 250MW each.

Converteam’s SFCs feature many advantages for power plant applications. As a four-quadrant operation drive it allows unit braking in turbine or condenser mode by regenerating power to the grid, thus reducing the need for mechanical brakes. It also offers the flexibility to be used for rotor balancing, bearing inspection and maintenance of the shaft. Compared with a fixed speed solution, this variable speed drive solution can furthermore save up to 50% of energy consumption.

Liyang is the third pumped storage plant in Jiangsu. One of these three, Shahe PSPP, was previously equipped by Converteam in 2000. All three schemes are part of the state’s major programme to answer growing demands for electricity in the east of China; improve quality of the power supply and to mitigate the environmental impacts of coal based power plants.

Liyang powerhouse is located underground in a cavern 150m below the lower reservoir bed. It is accessed through a 1200m long road tunnel. This requires huge civil engineering works which are currently under execution. Installation of the SFC equipment will not start until 2013.

There is no specific difficulty in installing the SFCs but it is a highly skilled operation to coordinate them with all the other equipment. The heaviest equipment, ie the generator/motors, will be assembled on site. They will then interconnect with the other systems using cables. Auxiliary circuits such as water cooling, ventilation and lighting are also managed. The complete systems are to be commissioned at the end of 2014, and the PSPP will enter into commercial operation in 2015.

“After our successful involvement in the Shahe PSPP, Converteam is once again recognised as an expert in power conversion,” comments Bell Huang, Sales and Marketing Director at Converteam China. “Today, we are proud to work with NARI in this major project and bring our expertise in this field. With the impressive growth of wind power generation capacity in China, more and more power storage will be required in order to ensure grid stability. Today pumped storage plants represent the largest opportunity for power storage.”

“Converteam’s tailor-made SFC technology featuring a high degree of design flexibility, as well as the high-performance control platform, gives us full confidence in the successful execution of the project,” adds Lian Fan, Vice President of NARI Electrical Control Company.

Converteam has extensive experience of SFCs technology for pumped storage, and has been working in this field since the 1970s. The company states that its technology is designed to be fuseless and robust, offering the most advanced technology in terms of control and protection. It offers a broad range of static frequency converters with power rated between 1MW and 40MW. Converteam’s other hydro installations include:

• The 1200MW Guangzhou project in Guangdong Province, China. This is the largest in the world, of which four units of 300MW are operated by a 20MW de-ionised water-cooled SFC.

• The 1060MW Goldisthal in Germany is one of the world’s most advanced hydroelectric power plants. It is equipped with four rotating machines of 300MW each. Converteam supplied the following systems:

• 100MVA converters for the variable speed asynchronous machines.

• Two SFCs with 18kV/40MW to start up all machines.

• Two static excitation equipment units for the synchronous machines.

Every day, Converteam’s SFCs successfully start up more than 25GW of installed pumped storage capacity worldwide. Converteam has a large number of SFC references:

• China: more than 54 units have been installed since 1989. For the past 12 years, China has invested a lot in pumped storage, and today has the largest capacity in the world. Converteam is focusing particularly on this growing market, with its European units working with a strong local team.

• Rest of Asia: more than 27 units have been installed since 1989.

• Germany: more than 15 units have been installed since 1990.

• Rest of Europe: more than 25 units have been installed since 1974.

• Africa: more than 6 units have been installed since 2002.

All of the installations are reported as being reliable and having a very good track record. The oldest SFCs have not yet been refurbished. However, it should be mentioned that in China, the SFC control systems in Guangzhou and Tian Huang Ping PSPPs have been upgraded in the past few years to improve their performance.

Appropriate tools

Following the increased uptake of renewable energy, particularly wind and solar which provide unpredictable power into the grid, there is a need to balance out such unpredictability. This is made possible by increasing pumped storage capacity as well as by improving the flexibility and performance of the existing plants.

A variable speed PSPP is the appropriate ‘tool’ to ensure grid stability and safety. SFC technology is increasingly being replaced by another type of variable speed drive, based on the IGBT transistor, which can also be used as AC excitation for the generator/motor. Converteam is a leading supplier of this new technology, with highly advanced research and development.

Converteam is a GE Energy business. In September 2011 GE successfully completed a US$3.2B acquisition of the company.

Getting static

The purpose of the static frequency converter (SFC) is to start up the synchronous motor/generator in order to connect it to the grid by adjusting the voltage and frequency. A SFC is made of two thyristor Graetz bridges connected together in series through an intermediate DC circuit. The bridge on the network-side operates under a steady voltage and frequency. It converts AC power into DC.
The bridge on the machine-side operates under variable frequency and a variable voltage. It converts DC power into AC. In a PSPP such as Liyang, the SFC is an auxiliary device used to start the units turn-by-turn in pump mode and launch them softly to the grid, making the generator/motor work as a motor on variable speed.
The SFC is also called a “˜soft starter”™ as it prevents the implementation of a starting motor with hydraulic coupling, which would make the machine shaft much more complex. For large unit power above 50MW, it is the most economical solution. They are highly reliable, highly efficient, and have very low operational costs as they do not require any maintenance.