Frequency conversion technology - a user's guide
There are many potential applications for frequency converters – such as a cruise liner wanting to supply its hotel loads from the harbour network, or an industrial company that needs to power a process line purchased from a country with a different network frequency to the local supply. Jon Clews, ABB UK’s Power Conversion specialist, reviews the options.Why does a ship need frequency conversion?
A rotary or static frequency converter allows equipment to be connected to a grid which has a different frequency from which it was designed, without the need to change the motors and other wound components. This is particularly needed in shore-to-ship power applications (otherwise known as ‘cold-ironing’), which helps ships in dock to connect to the local power grid so they can stop their onboard generators, reducing fuel use and cutting pollution.
Most power distribution networks generate at 50 Hz. But for historic reasons, most ships use a 60 Hz (Hertz) system. Using a rotary or static frequency converter ensures that the vessel is compatible with the grid requirements of the port where it is berthed.
What are rotary frequency converters?
The traditional frequency conversion technique uses a rotary frequency converter (RFC). This comprises a motor-generator set for speed control of the driven motor. It achieves the frequency change using a 10-pole motor with a 12-pole generator, or simply a fixed ratio gearbox.
While reliable, the RFC requires regular maintenance and has a conversion efficiency of less than 85 percent. RFCs suffer fixed losses, which are inherent in any electromechanical system. Spare parts can also be difficult to obtain for older equipment.
What are static frequency converters?
A static frequency converter (SFC) requires only a single annual service and has a much higher efficiency at 96 percent, the result of lower losses from this purely electrical system. The SFC’s power consumption is also more closely related to the connected load.
SFCs have lower standby losses and can be brought on-line and provide full output in a matter of seconds. They can automatically synchronise to an existing network, significantly reducing stresses on connected equipment.
SFCs offer a viable, modern and efficient alternative to RFCs, but can also be used in conjunction with traditional technology to provide a highly reliable hybrid solution.
Can frequency converters reduce noise levels?
SFC at Croatia's Lenac shipyard
Ships need to ensure they produce low noise levels when docked. In certain parts of the USA it is compulsory for vessels to switch off their auxiliary generators and connect to shore power systems. The rotating parts of RFCs produce noise from the motor, generator and, if used, gearbox. The system may require noise attenuation in order to comply with local regulations.
SFCs have none of these problems. They can work in temperatures up to 45°C, or higher if chillers are used. Most are air-cooled, although larger units utilise liquid cooling which further reduces ambient noise. An SFC can be installed in switchrooms or supplied in transportable, weatherproof, ISO style containers. This means that connecting to a shore power supply using a SFC reduces the noise levels in ports.
Can they provide redundant supplies?
Ports that offer a 60 Hz power supply may have a number of RFCs connected to a common bus. This allows a reasonable level of redundancy, but also means that the generators can be kept to a practical size. In practice it also means that at least two RFCs are always in service to ensure availability of power should an RFC fail. System losses are higher as a result.
It is feasible to use SFCs in conjunction with RFCs to provide a highly efficient ‘duty/assist’ system. In this way a RFC can be allowed to run at full load and maximum efficiency, with the SFC providing ‘top up’ power to the system.
Other features can include auto synchronisation and load sharing.
Can variable-speed drives be used?
The most common frequency converter in use in industry is a variable-speed drive (VSD) using IGBT semiconductor technology. These are designed to rotate induction motors with output frequencies in excess of 300 Hz.
Unfortunately the output waveform from a VSD is unsuitable for connection to the type of loads found on board a ship. In addition, VSDs are designed to be connected to single loads with a large induction, and cannot be connected to multiple loads which may regularly be switched on and off. In fact, a conventional VSD may incorrectly interpret a switched load as either an under- or over-current and shutdown as a result.
Therefore, this application requires specifically designed equipment, with a suitable output waveform and a very low total harmonic distortion (THD) of <2 percent. Otherwise, the connected loads may malfunction, suffer damage, or become stressed.
This is why it is important to use equipment which is specifically designed for the application – effectively a generator simulator with identical characteristics. This ensures that the high starting currents of large fixed speed motor loads are not interpreted as a short circuit.
What solutions does ABB offer?
ABB has developed a portfolio of state-of-the-art frequency converters including the PCS 100 SFC and PCS 6000 to cover applications ranging from 120 kVA (kilovoltamp) up to 120 MVA (megavoltamp).