TPT March 2007

consumes about 400kWh/t, a unit of identical size but powered via a medium-frequency converter system with a nominal rating of 4,000kW at 120Hz will require only 350kWh/tonne (see figure 4).

These advanced IGBT converters are characterised in particular by:

• Extremely high inherent protection against load peaks • Consistently high power factor (cosinus phi) of nearly 1.0 • Electrically isolated cooling • Simple and easy maintenance In addition to the successful use in power supply to coreless medium-frequency furnaces, IGBT converter systems are used for channel-type induction furnaces. Figure 6 shows an IGBT converter system for a medium- frequency furnace.

Their use for induction billet heater plants is described in a different paper (by Willi Johnen). € Figure 6 : IGBT converter for a medium-frequency furnace

› Figure 4 : Power consumption of a 10,000kg coreless furnace

Safety and reliability Dependable protection of the induction coil against overheating and, more importantly, against contact with molten metal is vital for ensuring safe and reliable furnace operation. Addressing this requirement, various technical solutions were proposed and implemented in the past but an optimum solution has not yet been found. Otto Junker’s Optical Coil Protection System (OCP) sets a new standard in coil monitoring. OCP is a latest-generation temperature measuring and monitoring system using fibre-optical sensors that are particularly suitable for interference-free monitoring in induction melting furnace applications. They provide direct and independent temperature field

2.4 Innovations Converter technology

Apart from the proven thyristor-based converter technology, the successful development of special IGBT (Insulated Gate Bipolar Transistor) converters for use in electro-thermal applications has gained increasing significance. Numerous successful IGBT converter systems testify the performance, reliability and flexibility of this technology. The Otto Junker IGBT converters are noted for their standardised, modular design. The inverters and dc link circuit capacitors form one integral unit. This unit can be used in a variety of circuit configurations. Typical examples are:

data. The system includes the sensor cable as well as an evaluator and dis- play for visualisation of the measured temperature fields. Figure 7 shows the arrangement of the OCP sensor cable in a coreless induction furnace.

• Independent inverters for several furnaces • Several inverters for induction billet heater coil sections

• Parallel connection for power increase • Series connection for voltage increase

High-power converters for induction melting and heating applications are normally cooled by water. As the IGBTs are mounted on electrically isolated water-cooled heat sinks there is no need for cooling water treatment as in the case of thyristor-based converters.

This significantly reduces the demands made on the water recooler as well as its maintenance requirements. The converter system is based on one and the same fundamental unit, irrespective of its application. This basic unit is shown in figure 5. The output range of the Otto Junker IGBT converter systems reaches up to several MW and the operating frequency covers the full range of applications.

fi Figure 5 : IGBT basic unit

› Figure 7 : OCP sensor cable

The advantages of this system are outlined below:

• Very high local resolution and temperature measurement with an absolute accuracy of +5°C • Recording and visualisation of temperature profile over the entire crucible • Direct optical temperature measurement not based on electrical circuitry • Early warning of impending critical temperature conditions. Normal lining wear and local erosion/cracks are reliably detected and localised as shown in figure 8 (typical example of crack formation in the crucible).

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