To start high-voltage motors with a power of up to 1 MW, it is economically feasible to use a two-transformer switching circuit instead of classical high-voltage frequency converters, using a serial frequency converter with a rated voltage of 400/690 V in the heart of the circuit.
The operation of the frequency converter is based on sequential transformation of the supply voltage.
First, the voltage from the 6/10 kV network is supplied to a step-down transformer with a secondary winding of 400/690 V. The transformer rating is selected taking into account: load power, efficiency of the step-up transformer, efficiency of the frequency converter, efficiency of the transformer itself.
The resulting voltage of 400/690 V is supplied to a low-voltage frequency converter. The voltage from the output of the converter with the required characteristics is supplied to a step-up transformer, which increases the converted voltage of the required frequency to the initial 6/10 kV.
The frequency converter is selected taking into account: load power, efficiency of the step-up transformer, reserve for extinguishing emerging excitations. To suppress and prevent the occurrence of peak overvoltages with a frequency from several kilohertz to several tens of kilohertz, installation of sine filters is provided. Sine filters are selected according to the manufacturer's recommendations for the selected frequency converter.
The step-up transformer is equipped with a blowing system to cool the transformer core.
The two-transformer circuit has a number of advantages:
The system provides all the protections implemented by the frequency converter: from short circuit, from overcurrent, from phase loss at the input/output, from motor jamming, motor underload (dry running), converter overheating, loss of control and others.
The system can be controlled: from the frequency converter control panel, remote control devices (for example, remote control), using industrial network protocols, or a personal computer.
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