Dynamic power factor correction
Power quality at the docks
China’s largest shipyard relies on EPCOS products for dynamic power factor correction. They allow energy quality and costs to be significantly improved. The China State Shipbuilding Corporation (CSSC) plans to become the world’s largest shipbuilder by the year 2015. To achieve this lofty aim, its shipbuilding capacity will rise from the current figure of four million gross tons (GT) to 14 million GT by 2015. Its range of high-tech products will be extended to include vessels, not least luxury liners, powered by liquefied gas. Offshore installations for research, conveying and development operations will consequently also be built.
The CSSC was founded on July 1, 1999 as a state-authorized investment company under the direct control of the Chinese central government. A total of 60 major and associated companies are united under the umbrella of the CSSC, among them large shipbuilding and ship repair yards, research and development institutes, manufacturers of equipment for the maritime sector as well as trade companies in China.
The CSSC Changxing Shipbuilding Base is located on the southern shore of the island of Changxing in the immediate vicinity of Shanghai, where the Yangtse flows into the sea. This is an ideal location for the shipbuilding industry with its eight kilometers of coast and water depths of between 12 and 16 meters. At Changxing seven docks with a capacity of eight million GT are scheduled for completion by 2015. The total investment volume will be over a billion euros. At the present extension stage, the maximum capacity is three million GT.
The problem of nonlinear loads
Like all shipyards, CSSC also faces the problem of nonlinear loads caused by the use of welding equipment. Its large transformers and clocked power supplies produce enormous inductive reactive powers. This problem is aggravated by the fact that the welding equipment is always being switched on and off. As a result, the supply network is continuously stressed by alternating power factors, transients and harmonics.
Reactive power is a particularly undesirable phenomenon in power networks because it stresses generators, power lines and transformers, thus causing additional losses. It is usual practice to use capacitors for power factor correction (PFC) to compensate the phase shift between voltage and current. PFC capacitors improve not only the power factor but also the overall quality of the available power supply. So a reduction of the harmonic component, for instance, also stabilizes the voltage and thus reduces electrical losses.
Conventional PFC systems consist of a power factor controller and PFC capacitors which are connected to the power line via mechanical capacitor contactors. The reaction time between the individual switching operations is more than 60 seconds due to the discharge time of the capacitors – a process that in turn impacts the response time of the entire system.
Part 1 of the IEC831 standard for low-voltage PFC capacitors stipulates that they should not exceed 5000 switching operations annually. Conventional PFC systems are thus designed for slowly changing load conditions where only few switching operations take place per day. Typical examples are large machine tools or drives with high duty ratios.
Rapid reaction thanks to dynamic PFC
Loads that are continuously switched on and off, such as welding equipment, are in particular need of dynamic PFC in real time. Such systems use electronic switches instead of electromechanical ones. The thyristor modules allow an unlimited number of switching operations, but also assure a short reaction time to load changes. Extremely short reaction times of 5 ms can be achieved, depending on their design. The use of thyristor switches prevents high inrush currents because the thyristors switch at zero crossing (Fig. 1). No inrush currents are produced, thus avoiding destructive power-line reactions and solving one of the greatest problems occurring in the conventional switching of capacitors.
|FIGURE 1: IMAGE OF DYNAMIC SWITCHING|
Dynamic PFC is characterized by the following benefits:
- The reactive power drawn from the line is eliminated. Power costs are reduced even with rapidly fluctuating loads. Load cycles of only a few hundred milliseconds may be achieved.
- High load cycles allow fast switching without the delays caused by discharging.
- Significant cost savings can be made: Investments in new equipment are avoided because peak loads are reduced.
- The line voltage is stabilized. This leads to qualitative improvement of the welding process, for instance, while simultaneously accelerating it.
- Flickering is avoided.
- Avoiding powerline reactions: smooth switching ensures that no voltage or current transients occur.
- Contactless switching both at the switching element and the capacitors prolongs operating life.
- Improved reliability.
- The reaction time is very short at 5 - 40 ms.
For the first stage of expansion of the CSSC shipyard, EPCOS will supply PFC systems with a total (dynamic) PFC output of 6920 kvar (Fig. 2). All systems are designed for 400 V, 50 Hz and are 7 percent detuned in order to avoid resonance phenomena due to harmonics and minimize the degree of distortion (THD-V). Donald Tang, Marketing Manager Power Capacitors in Shanghai, is proud to have implemented this project against the tough national and international competition on the PFC market. It is particularly pleasing that this is the first dynamic PFC project in China. Tang: “With this very complex solution, EPCOS has implemented a good reference installation in China that clearly demonstrates the benefits of dynamic PFC. This is an important step toward power quality solutions.”
|FIGURE 2: DYNAMIC PFC INSTALLATIONS FOR CSSC|
|THE INSTALLED PFC UNITS IN DETAIL|