Multilevel high voltage frequency converters

General characteristics

High-voltage converter is designed for start-up and speed control of high-voltage motor.
The high-voltage converter made according to a “cell” scheme features a high efficiency and its output voltage curve shape is near-sine (next figure).

6kV converter output current and voltage shape

This frequency converter is a multi-level converter with serial connection of 1-phase bridge cells. The topology of the converter power circuit is presented in Figure 2. Each motor phase has 6 series connected cells for 6kV or 9 for 10kV. Three phase groups of cells are star-connected with a “floating” neutral. Each cell is powered by an isolated secondary winding of the input transformer.
Six series connected cells in each phase ensure control of the converter output line voltage from 0 to 6000 V, nine cells respectively – from 0 to 10000V.
There are 13 levels of phase voltage for 6kV and 19 levels for 10kV, linear voltage has 25 and 37 levels respectively. The ability to set up many voltage levels allows for a very accurate approximation to a sinusoidal output voltage.
The power transformer has 18 three-phase secondary windings for an output voltage of 6kV and 27 – for an output voltage of 10kV. The secondary windings are shifted on 5 electric degrees relative to each other. This scheme is equivalent to 36-pulse rectification scheme and features high energy performance. The converter power factor is not less than 0.96, and the total harmonic distortion of input current (THD I) is not exceeding 5%.

Functional diagram of multi-level high voltage FC

The frequency converter ensures:

  • Motor start and speed control;
  • Acceleration, deceleration, shutdown;
  • Motor and converter protection;
  • “Pick up” in motion;
  • Technological parameter PI-control;
  • Preset speed parameters;
  • Auto-reclosing;
  • Linear or S-shaped velocity controller.

Protection system

  • protection against overcurrent at the converter input (internal fault);
  • protection against prohibited current overload and output short-circuit;
  • protection against phase failure of mains supply:
  • power system overvoltage or undervoltage protection;
  • surge voltage on main circuit elements;
  • overtemperature of semiconductor power devices;
  • short circuits to ground protection.

Functional structure of  the frequency converter includes:

  • Input multi-wound step-down transformer;
  • 18 (27) cells of single-phase frequency converters, each consisting of:
    • input fuses;
    • three-phase diode rectifier;
    • capacitive filter in DC link;
    • single phase bridge inverter on IGBT;
    • cell control system.
  • Control system.

Installation of additional input filters is not required for the converter to protect the network against generated disturbances, and the sinus shape of output voltage curve allows for connection of a standard electric motor without imposing restrictions on the power cable length without using additional output filters.

By using the converter you will be able to achieve:

  • cut down network energy consumption in pumping, compression and other units operating under variable load conditions;
  • increase service life of electric motor and actuator by optimization of their performance in a wide range of loads;
  • eliminate water hammer during pump unit start-up as well as dynamic overload in the pipelines;
  • reduce operating costs for control systems of pumps, fans, centrifuges, etc .;
  • build closed systems for asynchronous electric drive with the ability to accurately maintain the specified technological parameters.

The control systems are consistent across the whole range of high-voltage converters and have:

  • embedded diagnostics system and accident “track” recording;
  • full-featured protection system
  • high-speed two-wire interfaces for communication with top-level equipment – ProfiBus DP, CAN, others (upon request).

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