110 / 116
108
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pril
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).
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:
• 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.
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).
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
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.
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).
fi
Figure 5
:
IGBT basic unit
€
Figure 6
:
IGBT converter
for a medium-frequency
furnace
›
Figure 4
:
Power consumption of a 10,000kg coreless furnace
›
Figure 7
:
OCP sensor cable