Converter transformers for rectifying units and inverter installations

Purpose

The usage of power electronics of different purpose is constantly growing. In this regard there is a demand in special-purpose transformers operating with rectifying units and inverter installations. Converter transformers transfer power between AC mains and DC mains circuits.

Field of application of such transformers includes high-powered wires of industrial plants (for example roll mills), high-power engine drives, frequency-regulated drives, soft starters, excitation systems, cell plants and etc. Such converter transformers are required in power distribution schemes of renewable energy sources, such as wind-powered generators and solar batteries.

Apart from the task of power transfer between different electric systems (of alternate and rectified current) converter transformers shall perform the following tasks:

  • Provide for galvanic isolation between AC and DC (rectified current) circuits;
  • Increase reactive impedance of AC mains as a power supply source. It is required to decrease SC currents on the DC side and to level down current fronts on semiconductor switches.
  • Provide for voltage control by means of branch lines.

The transformer for rectifying unit is a transformer, the secondary winding of which is connected with semiconductor switches having unidirectional conductance (rectifying cells — thyristors, IGBT-transistors and others), which convert the AC into the DC (rectified current).

Depending upon the rectifying unit operational diagram, smoothing filter diagram and DC load features the operating conditions of such transformer may change considerably. Simple rectification circuits are three-phase (three-phase with neutral point, three-phase bridging), more complicated and modern circuits are multiphase (6- and 12-phase), which requires the transformer special design. When using six-phase circuit with current balancing reactor the transformer and reactor may be implemented as a single apparatus on the common base — transformer with current balancing reactor.

Inverter installations has connection circuits similar to the rectifying ones but with reverse switching: semiconductor switches implement reverse conversion (DC to AC). The current generated at the output of inverter circuit is non-sinusoidal. Unsinusoidality degree may vary as follows: from almost rectangular pulses of alternating pulse ratio (in case of thyristors phase control) up to step-like curve (in case of multilevel conversion). Although inverter installations circuits provide for smoothing filters, nonetheless transformers operating conditions in inverter circuits are similar to the rectifying ones.

Design Features

One of the transformer windings may be made as multiphase or split-off for usage in complicated rectifier circuits. As referenced above it is possible to make converter transformer with compensating winding on the common base.

Raised requirements are applied to converter transformers in terms of electric insulation level (owing to constant voltage on one of the windings), as well as in terms of overload capacity. The latter is caused by the fact that windings heating on the rectified voltage side is higher than heating on the winding of sinusoidal current of commercial frequency of the same power. Windings on the side of semiconductor rectifiers shall have improved mechanical strength as well. Characteristic features of rectified current circuits include the following: current ripples, unbalanced load on phases, non-sinusoidal currents, containing a number of higher harmonics, zero shift (which causes forced saturation of core).

Due to design features the air-barrier monolith is well accommodated for a number of specified effects. In particular transformers manufactured under this technology have improved mechanical strength of the windings, good load-voltage characteristics (overload capacity). Accommodation for other factors includes insulation reinforcement between the windings and magnetic core insulation reinforcement, magnetic core and yoke section increase.

Converter transformers are special-purpose electrical machines, that is why their fabrication requires detailed agreement of specifications.