Voltage source converter and control thereof

Patent metadata

Field	Value
Publication number	US-9866138-B2
Application number	US-201515128262-A
Country	US
Kind code	B2
Filing date	Mar 23, 2015
Priority date	Mar 25, 2014
Publication date	Jan 9, 2018
Grant date	Jan 9, 2018

Abstract

A voltage source converter of the controlled transition bridge type, having three phase limbs, each phase limb having a high &de director switch (Sw 1 Sw 3 , Sw 5 ) and a low side director switch (Sw 4 , Sw 6 , Sw 2 ) connecting a respective DC terminal (DC+, DC−) to an AC node for that phase limb. Chain-link circuits for each phase limb comprise a plurality of series connect cells, each cell having an energy storage element that can be selectively connected in series or bypassed. The chain-link circuits are operated in a voltage mode to provide a defined voltage transition at the AC node during a transition between one director switch being turned off and the other director switch being turned on. Chain-link circuits are connected to a common node such that, in use, a current can flow from one phase limb to another via the respective chain-link circuits.

First claim

The invention claimed is:

1. A voltage source converter comprising:
a plurality of phase limbs connected between high and low DC terminals; wherein each phase limb comprises:
a high side director switch connecting the high DC terminal to an AC node for that phase limb;
a low side director switch connecting the AC node to the low DC terminal; and
a chain-link circuit, a first end of the chain-link circuit being directly connected between the high side and low side director switches to be electrically coupled to the AC node;

first and second additional chain-link circuits, wherein first ends of the first and second additional chain-link circuits are connected to the high side DC terminal and low side DC terminals respectively; and
a converter controller configured to control the director switches of each phase limb in a power cycle and to control the chain-link circuits of each phase limb in a voltage mode to provide a controlled voltage transition at the AC node during the power cycle, wherein said converter controller is operable to control at least some of said chain-link circuits in a current mode to provide a defined current at times during the power cycle;
wherein each chain-link circuit comprises a plurality of series-connected chain-link modules, each of the chain-link modules comprising at least one energy storage element that can be selectively connected in series with the other modules or bypassed; and
wherein a second end of each of the chain-link circuits is connected to a common node such that, in use, a current can flow from one phase limb to another via the respective chain-link circuits.

2. The voltage source converter as claimed in claim 1 wherein the converter controller is configured to:
control the chain-link circuit of each phase limb in a voltage mode to provide a defined voltage transition at the AC node during a transition state between one director switch being turned off and the other director switch being turned on; and
operate the chain-link circuit of a phase limb in current mode for at least part of the time when that phase limb is not in a transition state.

3. The voltage source converter as claimed in claim 1 , wherein the converter controller is configured to determine the defined current flow for each chain-link circuit operating in current mode based on an indication of AC and DC current demand and the switch states of each of the phase limbs of the voltage source converter.

4. The voltage source converter as claimed in claim 1 , wherein the voltage source converter has three phase limbs and is operable in use in a repeating sequence of converter switch states comprising a first plurality of converter states in which:
one phase is in a high state with the high side director switch on and the low side director switch off;
one phase limb is in a low state with the low side director switch on and the high side director switch off; and
one phase limb is in a transition state,
wherein, when the voltage source converter is in one of said first plurality of converter states the converter controller:
operates the chain-link circuit of the phase limb in the high state in current mode to provide a defined chain-link current having a component based on a DC current demand minus an AC current demand for that phase; and/or
operates the chain-link circuit of the phase limb in the low state in current mode to provide a defined chain-link current having a component based on a DC current demand minus an AC current demand for that phase.

5. The voltage source converter as claimed in claim 1 , wherein the voltage source converter has three phase limbs and is operable in use in a sequence of converter switch states comprising a second plurality of converter states in which:
two phase limbs are in the same one of a high state, in which the high side director switch is on and the low side director switch is off, or a low state, in which the low side director switch is on and the high side director switch is off, and
the other phase limb is in the other of high state or the low state,
wherein, when the voltage source converter is in one of said second plurality of converter states, the converter controller:
operates one of the chain-link circuits of the two phase limbs which are in the same state to provide a defined chain-link current having a component based on a first fraction of a DC current demand minus an AC current demand for that phase, and
operates the other of the chain-link circuits of the two phase limbs which are in the same state to provide a defined chain-link current having a component based on a second fraction of the DC current demand minus the AC current demand for that phase; wherein the first and second fractions together equal a value of substantially one.

6. The voltage source converter as claimed in claim 5 wherein, when the voltage source converter is in one of said second plurality of converter states the converter controller operates the chain-link circuit of the phase limb in the other state to provide a defined chain-link current having a component based on a DC current demand minus an AC current demand for that phase.

7. The voltage source converter as claimed in claim 1 , wherein the voltage source converter has three phase limbs and is operable in use in a sequence of converter switch states comprising a third plurality of converter states in which:
two phase limbs are in a transition state; and
the other phase limb is in one of a high state, in which the high side director switch is on and the low side director switch is off, or a low state, in which the low side director switch is on and the high side director switch is off,
wherein the converter controller is configured, in use, to:
operate the first additional chain-link circuit to provide a defined current for at least part of a time when the voltage source converter is in one of said third plurality of converter states and one phase limb is in the low state; and/or
operate the second additional chain-link circuit to provide a defined current for at least part of a time when the voltage source converter is in one of said third plurality of converter states and one phase limb is in the high state.

8. The voltage source converter as claimed in claim 7 wherein, when the voltage source converter is in one of said third plurality of converter states, the converter controller operates the chain-link circuits of the phase limb in the high state or low state to provide a defined chain-link current having a component based on a DC current demand minus an AC current demand for that phase.

9. The voltage source converter as claimed in claim 7 , wherein said defined current of the first or second additional chain-link circuit has a component based on a DC current demand.

10. The voltage source converter as claimed in claim 1 , wherein the converter controller is configured to control each phase limb to generate a trapezoidal voltage waveform at the AC node.

11. The voltage source converter as claimed in claim 10 , wherein a duration of the controlled voltage transition from a midpoint voltage to a voltage of the high DC terminal or the low DC terminal corresponds to a difference in phase angle of greater than π/6.

12. The voltage source converter as claimed in claim 10 , wherein the trapezoidal voltage waveform has at least two different slopes during the controlled voltage transition.

13. The voltage source converter as claimed in claim 1 , wherein the energy storage elements of the chain-link modules comprise capacitors and wherein the converter controller comprises a capacitor balancing controller for maintaining a voltage of the capacitor

Assignees

• General Electric Technology Gmbh
• General Electric Company Gmbh

Inventors

• Oates Colin Donald Murray
• Dyke Kevin
• Trainer David Reginald

Classifications

• H02M2007/4835

  Electricity · mapped topic

• H02M5/458 Primary

  using semiconductor devices only · CPC title

• H02M7/219

  in a bridge configuration · CPC title

• H02M7/483

  Converters with outputs that each can have more than two voltages levels · CPC title

• H02M1/15 Primary

  using active elements · CPC title