Medium voltage drives at the steel shop
When converting hot metal to steel in the converter, many dusty flue gases are released. We remove these gases with large ventilators. During the blowing process, the flue gases are extracted using the ventilator of the primary dust removal unit.
During the charging of the converter and the pouring of steel and slag, the gases are extracted by the ventilators of the secondary dust removal unit. Thanks to a drive, both dust removal installations can create the required underpressure. (see figure 1)
Figure 1: operator
Making steel is a batch process. The drive gives us great flexibility regarding energy saving.
We use a 3.6MW ventilator for primary dust removal and 3 x 2.5MW ventilators for secondary dust removal. We can only achieve such power using medium voltage drives: for a drive capacity of 3MW, at a voltage of 690V a current of approximately 3100A is required, at 6000V this is a mere 350A. For cabling, this corresponds with 6 cables of each 300mm˛, or one cable of just 120mm˛ respectively.
For primary dust removal, we use drives with IGBT technology (Insulated Gate Bi-polar Transistor). This offers a simple and reliable configuration with significant advantages:
- Power semiconductors without snubber network (these ensure stable power distribution when controlling the power component), fewer components in the drive.
- Low power to control the gate.
Due to its modular construction, the drive is very low-maintenance.
Figure 2: IGBT as the basis for the medium voltage drive, assembled on sliding drawers
For secondary dust removal we chose a supplier who makes use of IGCT’s (Integrated Gate Commutated Thyristor). These have the big advantage that they have low conductivity losses in comparison with IGBT’s.
Both systems have a comparable construction:
Figure 3: schematic composition of a drive
The drives are fitted as standard with a 12-pulse diode rectifier. This results in an excellent Cosφ.
The 3-level inverter ensures an optimum output voltage characteristic so the motors are less loaded by so-called spikes.
Different drive components have to be cooled with water for our power range. This is no simple matter in combination with such voltages.
Only de-ionised water comes into consideration for use. This is water of which the conductivity is less than 0.5 µS/cm (as a comparison: demineralised water for steam irons has 10 µS/cm). With these low values the water functions as an isolator. A complete deionisation circuit with the necessary safety devices is present for each group of drives.
Figure 4: deionisation takes place in a deionisation bottle filled with resin
Control of the drive takes place using fast processors that are all connected to the basic computer system using a Profibus network. Control of the power electronics takes place over a glass fibre network.
Figure 5: processor cards
In this way, we can perfectly monitor the medium voltage drive from the control room.
Figure 6: drive monitoring using a tool developed ourselves