System and method for controlling a back-to-back three-level converter with voltage ripple compensation

Patent metadata

Field	Value
Publication number	US-10224830-B2
Application number	US-201815958381-A
Country	US
Kind code	B2
Filing date	Apr 20, 2018
Priority date	Apr 21, 2017
Publication date	Mar 5, 2019
Grant date	Mar 5, 2019

Abstract

A method for controlling a back-to-back three-phase three-level converter having a grid-side AC/DC converter and a machine-side DC/AC converter connected by a split DC link which defines a DC link midpoint. The method includes controlling the grid-side converter to convert AC power from a grid into DC power of the DC link, controlling the machine-side converter to convert DC power from the DC link to AC power to feed a low frequency machine, and concurrently performing common mode voltage injection for the machine-side converter so as to at least partially compensate midpoint voltage ripple caused by the machine-side converter. The method further includes performing common mode voltage injection for the grid-side converter so as to at least partly further compensate the portion of the midpoint voltage ripple which remains uncompensated by controlling the machine-side converter. A control system implementing the control method and a power conversion system utilizing same.

First claim

The invention claimed is:

1. A method for controlling a back-to-back three-phase three-level converter having a grid-side alternating current (AC) to direct current (DC) converter and a machine-side DC/AC converter connected by a split DC link which defines a DC link midpoint, the method comprising:
controlling the grid-side converter to convert AC power from a grid into DC power of the DC link, including a positive, a negative, and a neutral voltage potential, the neutral potential being at the DC link midpoint;
controlling the machine-side converter to convert DC power from the DC link to AC power to be output to a machine,
wherein the controlling the machine-side converter is adapted to perform common mode voltage injection for the machine-side converter so as to at least partially compensate midpoint voltage ripple caused by the machine-side converter; and
determining a value (i MP Ext Ref ) of the midpoint current which is uncompensated by controlling the machine-side converter,
wherein the controlling the grid-side converter is adapted to perform common mode voltage injection for the grid-side converter based on the determined uncompensated value (i MP Ext Ref ) of the midpoint current so as to at least partly further compensate the uncompensated portion of the midpoint current, which is uncompensated by controlling the machine-side converter.

2. The method of claim 1 , wherein the back-to-back converter is configured for low frequency applications, wherein the grid-side converter is controlled to operate at a constant grid frequency of 50 Hz or 60 Hz and the machine-side converter is controlled to operate at nearly full modulation depths at nominal frequencies below 10 Hz and wherein the midpoint voltage ripple is dominated by the 3 rd harmonic component of the operating frequency of the machine-side converter.

3. The method of claim 1 , wherein controlling the machine-side converter comprises:
dynamically calculating, in an actual operational point, the natural converter midpoint current injection (i MP M nat );
mapping a functional relationship between the midpoint current and common mode voltage injections (i MP M =f(v 0 M ); v 0 M =f(i MP M )); and
calculating a reference (i MP Ref ) for the capacitor midpoint current for the machine-side converter for use in its control using the pre-calculated map.

4. The method of claim 1 , wherein calculating a reference (i MP Ref ) for the capacitor midpoint current comprises:
calculating a first part (i MP M Ref FF ) of the midpoint current reference of the machine-side converter, which is preferably minimum practically realizable midpoint current and which can be used for feedforward control of the machine-side converter;
optionally calculating a second part (i MP M Ref FB ) of reference for the capacitor midpoint current of the machine-side converter for compensating an average value of the midpoint voltage drift preferably using a feedback based control; and
calculating a composite reference (i MP M Ref ) for the capacitor midpoint current for the machine-side converter to be used for its control.

5. The method of claim 3 , wherein controlling the machine-side converter comprises:
determining limits (i MP M min ; i MP M max ) within which the converter midpoint current injection can be controlled via available range of the common mode voltage injection in the machine-side converter;
limiting the reference or the composite reference (i MP M Ref ) for the capacitor midpoint current for the machine-side converter to stay between the pre-calculated limits; and;
calculating reference (v 0M Ref ) for the common mode voltage injection for the machine-side converter based on the limited reference or the composite reference (i MP M Ref ) using the pre-calculated map.

6. The method of claim 1 , wherein the uncompensated portion (i MP Ext Ref (out) ) of the midpoint current produced by the machine-side converter is scaled by a factor between 0 and 1, inverted in sign and used as external reference (i MP Ext Ref (in)) for the control of the grid-side converter.

7. The method of claim 1 , wherein controlling the grid-side converter comprises:
dynamically calculating, in actual operational point, the natural converter midpoint current injection (i MP G nat );
mapping a functional relationship between the midpoint current and common mode voltage injection (i MP G =f(v 0 G ); v 0 G =f(i MP G )); and
calculating a first part (i MP G Ref FF ) of the midpoint current reference for feedforward control of the grid-side converter which is preferably set to the natural midpoint current n(i MP nat ).

8. The method of claim 7 , wherein controlling the grid-side converter further comprises:
calculating a second part (i MP G Ref FB ) of reference for the capacitor midpoint current of the grid-side converter for compensation of the average value of the common midpoint voltage drift preferably using a feedback based control; and
calculating a composite reference (i MP G Ref ) for the capacitor midpoint current for the grid-side converter, including an external reference (i MP G Ext Ref (in)) indicating a scaled uncompensated portion of the midpoint current produced by the machine-side converter, for compensation of the midpoint current produced by the machine-side converter.

9. The method of claim 6 , wherein controlling the grid-side converter further comprises:
determining limits (i MP G min ; i MP G max ) within which the converter midpoint current injection can be controlled via available range of the common mode voltage injection in the grid-side converter;
limiting the composite reference (i MP G Ref ) to stay between the determined limits (i MP G min ; i MP G max ); and
calculating the reference (v 0G Ref ) for common mode voltage injection for the grid-side converter based on the limited composite reference (i MP G Ref ) using the pre-calculated map.

10. The method of claim 1 , comprising midpoint voltage ripple compensation by feedforward control of the grid-side and machine-side converters and midpoint average voltage drift compensation by feedback control of at least one of the grid-side and machine-side converters.

11. A system for controlling a back-to-back three-phase three-level converter having a grid-side alternating current (AC) to direct current (DC) converter and a machine-side DC/AC converter connected by a split DC link which defines a DC link midpoint, the system comprising:
a first controller for controlling the grid-side converter to convert AC power from a grid into DC power of the DC link, including a positive, a negative, and a neutral voltage potential, the neutral potential being at the DC link midpoint; and
a second controller for controlling the machine-side converter to convert DC power from the DC link to AC power to be output to a machine; and
at least one control unit configured for:
performing common mode voltage injection for the machine-side converter so as to at least partially compensate midpoint voltage ripple caused by the machine-side converter;
determining a value (i MP Ext Ref ) indicating the portion of the midpoint current which is uncompensated by controlling the machine-side converter; and
performing common mode voltage injection for the grid-side converter based on the value (i MP Ext Ref ) so as to at least partly further compensate the portion of the midpoint current, which is uncompensated by controlling the machine-side converter.

12. The system of claim 11 , further comprising calculating means for calculating, in an actual operational point, midpoint current, its limits and required midpoint current and common mode voltag

Assignees

• Ge Energy Power Conversion Technology Ltd

Inventors

• Basic Duro
• Rothenhagen Kai Alexander
• Weitendorf Niels

Classifications

• H02M7/487 Primary

  Neutral point clamped inverters · CPC title

• H02M1/143

  using compensating arrangements (for reducing noise from the supply in transmission systems H04B15/005) · CPC title

• H02M5/458 Primary

  using semiconductor devices only · CPC title

• H02M1/0025

  Arrangements for modifying reference values, feedback values or error values in the control loop of a converter · CPC title

• H02M5/4585

  having a rectifier with controlled elements · CPC title