WO2014154290A1

WO2014154290A1 - Method for controlling a chain-link converter

Info

Publication number: WO2014154290A1
Application number: PCT/EP2013/056742
Authority: WO
Inventors: Jean-Philippe Hasler
Original assignee: Abb Technology Ltd
Priority date: 2013-03-28
Filing date: 2013-03-28
Publication date: 2014-10-02

Abstract

The present disclosure relates to a method for controlling a chain-link converter, said converter comprising three phase legs, each of which phase legs comprising a plurality of series-connected converter cells, each of which cells comprising a DC capacitor; the method comprising controlling the converter by means of a closed-loop control function adjusting a voltage reference for each cell of each phase leg. The closed-loop control function comprises, for each cell of the phase leg: obtaining a DC capacitor voltage of the cell; computing a control error by comparing the obtained DC capacitor voltage with the mean voltage of all DC capacitors of the phase leg, said mean voltage of all DC capacitors of the phase leg being used as the voltage reference for the cell; detecting a sign of a product of a current over the whole phase leg and a voltage reference for the whole phase leg; computing a gain of the third closed-loop control function, based on the current reference of the phase leg; obtaining a control output as the product of the control error, the gain and the sign; and adding the control output to the voltage reference for the cell, thereby updating said voltage reference for the cell.

Description

METHOD FOR CONTROLLING A CHAIN-LINK CONVERTER

TECHNICAL FIELD

The present disclosure relates to a method for controlling a chain-link converter, said converter comprising three phase legs, each of which phase legs comprising a plurality of series-connected converter cells, each of which cells comprising a DC capacitor. Disclosed are also a control apparatus and a computer program for controlling a chain-link converter.

BACKGROUND

A three-phase chain-link converter consists of three phase legs where each phase consists of a number of series-connected cell converters. The phase legs of series-connected cell converters can be connected either in delta- or in wye-configuration.

Each cell of the converter includes a direct current (DC) capacitor where the energy stored in the capacitor should optimally be constant. This implies that the phase currents should be in quadrature to the phase voltages of each leg such that the mean power in each phase is zero. In order to satisfy this condition, a high level control coordinates the current orders based on measured converter voltages and converter current references. This control level is of feed-forward type and it requires additional controls of closed-loop type to fine control the cell DC capacitors.

Closed-Loop control of the cell DC capacitor voltage is achieved by means of controlling the mean value of all DC capacitor cells of a phase leg by adjusting the phase leg current reference, thus balancing the phase legs.

CN 2009 10062883.8 (publication number CN 101599708) discloses a DC-side power balance control method for a cascaded multilevel inverter, comprising an upper layer control and a lower layer control for each phase link. The upper layer control allocates a same modulation wave to each full-bridge conversion unit. The lower layer control adjusts the modulation wave allocated to each full-bridge conversion unit. The lower layer control is performed by (1) measuring a DC-side capacitor voltage of each full-bridge conversion unit in the phase link, and performing mean filtering thereon, (2) summating and averaging the filtered DC-side capacitor voltages of the full-bridge conversion units, to obtain a DC voltage balance reference value of the phase link, (3) performing proportional control using the DC voltage balance reference value as a reference and the filtered DC-side capacitor voltage of each full-bridge conversion unit as a feedback, respectively, and multiplying a result of the proportion control and an instantaneous current of the phase link, to obtain a fine-tuning amount of the modulation wave for each full-bridge conversion unit, and (4) superposing the fine-tuning amount of the modulation wave for each full-bridge conversion unit with the modulation wave allocated by the upper layer control, to generate an actual modulation wave for each full-bridge conversion unit. The document thus discloses a two-level closed loop control, where the second, lower, level and is based on the product of the DC voltage deviation of a controlled cell from a reference. This will produce a change in the mean power exchanged between the controlled cell and the other cells. However, at a low current level, harmonic currents are produced and fed into the feeding network.

SUMMARY

It is an objective of the present invention to at least alleviate the problem with the prior art in balancing the voltages of chain-link converter.

According to an aspect of the present invention, there is provided a method for controlling a chain-link converter, said converter comprising three phase legs, each of which phase legs comprising a plurality of series-connected converter cells, each of which cells comprising a DC capacitor. The method comprises: controlling the converter by means of a first closed-loop control function, controlling a mean voltage of all DC capacitors of all three phase legs by adjusting a current reference for each of the phase legs; and for each phase leg: controlling the converter by means of a feed-forward control function based on a measured converter AC voltage and on the current reference, controlling a mean voltage of all DC capacitors of the phase leg by means of (adjusting) the current reference for the phase leg, controlling the converter by means of a second closed-loop control function, controlling the mean voltage of all DC capacitors of the phase leg by adjusting the current reference for the phase leg, and controlling the converter by means of a third closed-loop control function adjusting a voltage reference for each cell of the phase leg. The third closed-loop control function comprising, for each cell of the phase leg: obtaining a DC capacitor voltage of the cell; computing a control error by comparing the obtained DC capacitor voltage with the mean voltage of all DC capacitors of the phase leg, said mean voltage of all DC capacitors of the phase leg being used as the voltage reference for the cell; detecting a sign of a product of a current over the whole phase leg and a voltage reference for the whole phase leg; computing a gain of the third closed-loop control function, based on the current reference of the phase leg; obtaining a control output as the product of the control error, the gain and the sign; and adding the control output to the voltage reference for the cell, thereby updating said voltage reference for the cell.

According to another aspect of the present invention, there is provided a control apparatus for controlling a chain-link converter, said converter comprising three phase legs, each of which phase legs comprising a plurality of series-connected converter cells, each of which cells comprising a DC capacitor. The control apparatus comprises a processor; and a storage unit storing instructions that, when executed by the processor, cause the apparatus to: control the converter by means of a first closed-loop control function, controlling a mean voltage of all DC capacitors of all three phase legs by adjusting a current reference for each of the phase legs; and for each phase leg: control the converter by means of a feed-forward control function based on a measured converter AC voltage and on the current reference, controlling a mean voltage of all DC capacitors of the phase leg by means of the current reference for the phase leg, control the converter by means of a second closed-loop control function, controlling the mean voltage of all DC capacitors of the phase leg by adjusting the current reference for the phase leg, and control the converter by means of a third closed-loop control function adjusting a voltage reference for each cell of the phase leg. The third closed-loop control function comprising, for each cell of the phase leg:

obtaining a DC capacitor voltage of the cell; computing a control error by comparing the obtained DC capacitor voltage with the mean voltage of all DC capacitors of the phase leg, said mean voltage of all DC capacitors of the phase leg being used as the voltage reference for the cell; detecting a sign of a product of a current over the whole phase leg and a voltage reference for the whole phase leg; computing a gain of the third closed-loop control function, based on the current reference of the phase leg; obtaining a control output as the product of the control error, the gain and the sign; and adding the control output to the voltage reference for the cell, thereby updating said voltage reference for the cell.

According to another aspect of the present invention, there is provided a computer program for controlling a chain-link converter, said converter comprising three phase legs, each of which phase legs comprising a plurality of series-connected converter cells, each of which cells comprising a DC capacitor. The computer program comprising computer program code which is able to, when run on a processor of a control apparatus for the converter, cause the control apparatus to: control the converter by means of a first closed-loop control function, controlling a mean voltage of all DC capacitors of all three phase legs by adjusting a current reference for each of the phase legs; and for each phase leg: control the converter by means of a feed-forward control function based on a measured converter AC voltage and on the current reference, controlling a mean voltage of all DC capacitors of the phase leg by means of the current reference for the phase leg, control the converter by means of a second closed-loop control function, controlling the mean voltage of all DC capacitors of the phase leg by adjusting the current reference for the phase leg, and control the converter by means of a third closed-loop control function adjusting a voltage reference for each cell of the phase leg. The third closed-loop control function comprises, for each cell of the phase leg: obtaining a DC capacitor voltage of the cell; computing a control error by comparing the obtained DC capacitor voltage with the mean voltage of all DC capacitors of the phase leg, said mean voltage of all DC capacitors of the phase leg being used as the voltage reference for the cell; detecting a sign of a product of a current over the whole phase leg and a voltage reference for the whole phase leg; computing a gain of the third closed-loop control function, based on the current reference of the phase leg; obtaining a control output as the product of the control error, the gain and the sign; and adding the control output to the voltage reference for the cell, thereby updating said voltage reference for the cell. By the third closed-loop control function of the present invention, the chain link converter can better controlled and balanced also at low current levels and the harmonic currents fed into the feeding network can be reduced. The cell voltage reference is adjusted based on the estimated or measured cell DC voltage, on the DC voltage reference corresponding to the mean voltage of the DC capacitor voltage of all cells in the same phase, and on the sign of the phase current.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, apparatus, component, means, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated. The use of "first", "second" etc. for different features/components of the present disclosure are only intended to distinguish the features/components from other similar features/components and not to impart any order or hierarchy to the features/components.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will be described, by way of example, with reference to the accompanying drawings, in which: Fig l is a schematic diagram of an electrical system comprising an

embodiment of a chain link converter in a wye (Y) half-bridge configuration, of the present invention.

Fig 2 is a schematic diagram of an electrical system comprising an

embodiment of a chain link converter in a wye (Y) full-bridge configuration, of the present invention.

Fig 3 is a schematic diagram of an electrical system comprising an

embodiment of a chain link converter in a delta (Δ) full-bridge configuration, of the present invention. Fig 4 is a schematic diagram illustrating a hierarchy of different control functions in an embodiment of the present invention.

Fig 5 is a schematic flow chart of an embodiment of a closed-loop control function of the present invention.

Fig 6 is a schematic diagram illustrating an embodiment of a closed-loop control function of the present invention.

Fig 7 is a schematic diagram illustrating a gain calculation in an embodiment of the present invention.

DETAILED DESCRIPTION

Embodiments will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments are shown.

However, other embodiments in many different forms are possible within the scope of the present disclosure. Rather, the following embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art. Like numbers refer to like elements throughout the description.

According to the invention, the converter is controlled on three levels: over the whole converter (total voltage), over each phase (balancing the phases in relation to each other), and on cell level (balancing each cell by relating its DC converter voltage to a cell specific voltage reference). The first closed-loop control function relates to the control level over the whole converter. The feed-forward control function and the second closed-loop control function relates to the control level over each phase, and the third closed-loop control function relates to the individual control of each cell.

The current reference of each phase leg (especially for a delta configured converter) is primarily a of zero-sequence type and secondarily of negative- sequence type. Each phase has its own current reference. The current reference of each phase is processed by a current control apparatus of the electrical system which e.g. calculates corresponding voltage references for each phase which can be compared with measured or estimated voltages over the phase or over a cell of the phase (i.e. DC capacitor voltage).

The gain is the amplification of the calculated control error for adjusting the voltage reference for the respective cell. It is noted, however, that since the gain is multiplied with the detected sign, the control output will be zero if the sign is zero.

Figure 1 schematically illustrates an embodiment of a chain-link converter 1 of the present invention. In the embodiment of figure 1, the chain-link converter 1 is in a Y configuration and in a half-bridge configuration. The converter 1 is configured to transform DC current to three-phase AC current. The converter 1 comprises three phase legs 2a-c. Each of the phase legs 2 comprises a plurality of series-connected converter cells 3. The phase legs 2 can in different embodiments be connected in Y-form or in delta-form. A control apparatus or controller 10 is associated with the converter 1 in order to control the operation of the converter 1. The controller 10 comprises a processor 11 and a storage unit 12, as well as other circuitry which may be appropriate. In accordance with the present invention, the controller 10 implements a plurality of control functions on the converter 1 for optimizing the operation of the converter 1. Figure 2 illustrates another embodiment of a chain-link converter 1, now in a full-bridge configuration. The chain-link converter 1 of figure 2 is however still in a Y configuration as in figure 1.

Figure 3 illustrates another embodiment of a chain-link converter 1, now in a delta and full-bridge configuration.

The present invention is relevant for any chain-link converter 1 configuration, such as Y or delta configuration, and half-bridge or full-bridge configuration. Apart from the different configurations, the discussion relating to figure 1 is also relevant for figures 2 and 3. Figure 4 is a schematic hierarchy chart illustrating an embodiment of a converter control method of the present invention. The method controls the converter 1 on three different levels. The first (top) level is the

control/balancing over the whole converter including all phase legs 2. The second level is the individual control/balancing of each phase leg 2a, 2b and 2c. The third (bottom) level is the individual control/balancing of each converter cell 3 of each of the phase legs.

In the first control level, a first closed-loop control function 101 is employed. By means of said first closed-loop control function 101 the converter is controlled by controlling a mean voltage of all DC capacitors of all three phase legs by adjusting a current reference for each of the phase legs. Thus, a voltage may be measured over each of the cell DC capacitors, and the mean voltage of all these measurements is controlled. There is a current reference, and thus a voltage reference, for each phase leg 2 set by the controller 10 or other control circuitry. These current references are adjusted in order to control the mean voltage by means of the first closed-loop control function 101.

In the second level, two control functions are employed for each of the three phase legs 2. A feed-forward control function 102 is employed to control the converter 1 based on a measured converter AC voltage and on the current reference for respective phase leg. A mean voltage over all DC capacitors of the phase leg is controlled by means of the current reference for the phase leg. A second closed-loop control function 103 is employed to control the converter by controlling the mean voltage of all DC capacitors of the phase leg by adjusting the current reference for the phase leg. Thus, in contras to the control function 101 of the first level, the current reference for the phase leg may be adjusted to control the mean voltage in the phase leg (not the mean voltage in the whole converter).

In the third level, a third closed-loop control function 104 is employed for each of the cells 3 individually. A cell individual AC voltage reference is adjusted for each cell by means of the third closed-loop control function 104. The third closed-loop control function 104 is further discussed with reference to figures 5 to 7.

Figure 5 is a schematic flow chart illustrating an embodiment of the third closed-loop control function 104 of the present invention. The third closed- loop control function 104 is performed for each cell 3 in a phase leg 2 of the converter 1. A DC capacitor voltage of the cell is obtained 201. The DC capacitor voltage may e.g. be measured over the capacitor and the

measurement is forwarded to the controller 10 for processing. Optionally, the obtained 201 DC capacitor voltage is filtered 202 for removing a harmonic component of said voltage. A control error is calculated 203 by comparing the obtained 201 DC capacitor voltage with the mean voltage of all DC capacitors of the phase leg, said mean voltage of all DC capacitors of the phase leg being used as a DC voltage reference for the cell. A sign of a product of a current over the whole phase leg and a voltage reference for the whole phase leg is detected 204. A gain of the third closed-loop control function is computed 205, based on the current reference of the phase leg 2. A control output is obtained 206 as the product of the control error, the gain and the sign.

Finally, the AC voltage reference for the cell 3 is updated 207 by adding the control output to the AC voltage reference for the cell. Figure 6 is a schematic diagram illustrating an embodiment of the third closed-loop control function 104 of the present invention. The different values (measurements and references etc.) inputted into the function 104 are:

UAC REF = the voltage reference 41 of the phase leg

la = the current 42 over the whole phase leg

K_p ceil DC unbalance = the computed 205 gain 43

UDC Ref = the mean voltage 44 over all DC capacitors of the phase leg

UDC = the obtained 201 DC voltage 45 over the cell 3

UDC High = the control DC voltage protective level to force the current into the DC capacitor in order to reduce the DC voltage.

As shown in figure 6, the obtained 201 DC capacitor voltage 41 may be filtered 202 before being compared with the mean voltage 44 for calculating 203 the error. The calculated 203 error is then multiplied with the gain 43 (i.e. an amplification multiplier) as well as with the sign 204 to obtain 206 the control output, which is then compared with the voltage reference 41 of the phase leg to obtain the updated AC voltage reference 48 for the individual

In accordance with some embodiments of the present invention, the sign is detected by means of a 3-state comparator 47 (within the dashed line in figure 6). the output of the comparator is +1 when the product of the current over the whole phase leg and the voltage reference for the whole phase leg is above a first threshold, -1 when the product of the current over the whole phase leg and the voltage reference for the whole phase leg is below a second threshold, and o when the product of the current over the whole phase leg and the voltage reference for the whole phase leg is between the first and the second thresholds.

If the DC voltage is above the control protective level at 49, the phase leg current is forced to flow through the DC capacitor in order to reduce the DC voltage. This is achieved by forcing the control signal UUDC_BAL to - lFigure 7 is a schematic diagram illustrating a gain 43 calculation in an embodiment of the present invention. In figure 7, IA is the current 42 over the whole phase leg, ABS(IA) is the absolute value of the current 42 over the whole phase leg. i₃min is the minimum phase leg current (3^rd harmonic) circulating inside the delta converter (valid for delta converter 1). Kpi_{p u} is the Gain Reference at 1 p.u. phase leg current.

According to another aspect of the present invention, there is provided a method for controlling a chain-link converter, said converter comprising three phase legs, each of which phase legs comprising a plurality of series- connected converter cells, each of which cells comprising a DC capacitor; the method comprising controlling the converter by means of a closed-loop control function adjusting a voltage reference for each cell of each phase leg. The closed-loop control function comprises, for each cell of the phase leg: obtaining a DC capacitor voltage of the cell; computing a control error by comparing the obtained DC capacitor voltage with the mean voltage of all DC capacitors of the phase leg, said mean voltage of all DC capacitors of the phase leg being used as the voltage reference for the cell; detecting a sign of a product of a current over the whole phase leg and a voltage reference for the whole phase leg; computing a gain of the third closed-loop control function, based on the current reference of the phase leg; obtaining a control output as the product of the control error, the gain and the sign; and adding the control output to the voltage reference for the cell, thereby updating said voltage reference for the cell.

The present disclosure has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the present disclosure, as defined by the appended patent claims.

Claims

CLAIMS l. A method for controlling a chain-link converter (l), said converter comprising three phase legs (2), each of which phase legs comprising a plurality of series-connected converter cells (3), each of which cells comprising a DC capacitor, the method comprising: controlling the converter by means of a first closed-loop control function (101), controlling a mean voltage of all DC capacitors of all three phase legs by adjusting a current reference for each of the phase legs; and for each phase leg (2): controlling the converter by means of a feed-forward control function (102) based on a measured converter AC voltage and on the current reference, controlling a mean voltage of all DC capacitors of the phase leg by means of the current reference for the phase leg, controlling the converter by means of a second closed-loop control function (103), controlling the mean voltage of all DC capacitors of the phase leg by adjusting the current reference for the phase leg, and controlling the converter by means of a third closed-loop control function (104), adjusting an AC voltage reference (41) for each cell (3) of the phase leg (2), the third closed-loop control function (104) comprising, for each cell (3) of the phase leg (2): obtaining (201) a DC capacitor voltage (45) of the cell; computing (203) a control error by comparing the obtained DC capacitor voltage (45) with the mean voltage (44) of all DC capacitors of the phase leg, said mean voltage of all DC capacitors of the phase leg being used as a DC voltage reference (44) for the cell; detecting (204) a sign of a product of a current (42) over the whole phase leg and a voltage reference (41) for the whole phase leg; computing (205) a gain (43) of the third closed-loop control function, based on the current reference (52) of the phase leg; obtaining (206) a control output as the product of the control error, the gain (43) and the sign; and adding the control output to the AC voltage reference (41) for the cell (3), thereby obtaining an updated (207) AC voltage reference (48) for the cell.

2. The method of claim 1, wherein the sign is detected by means of a 3- state comparator (47), where the output of the comparator is +1 when the absolute value of the product of the current (I) over the whole phase leg and the voltage reference for the whole phase leg is above a threshold and I has a positive sign , -1 when the absolute value of the product of the current (I) over the whole phase leg and the voltage reference for the whole phase leg is above a threshold and I has a negative sign, and o when the absolute value of the product of the current (I) over the whole phase leg and the voltage reference for the whole phase leg is below the threshold.

3. The method of any preceding claim, wherein the gain (43) is limited when the current reference (42) for the phase leg (2) is above and/or below a threshold, e.g. below 0.05 per unit.

4. The method of any preceding claim, wherein the DC capacitor voltage (45) of the cell (3) is obtained (201) by measuring a voltage over an electronic switch of the cell, or by measuring the DC voltage over the capacitor of the cell.

5. The method of any preceding claim, wherein the third closed-loop control function (104) further comprises, for each cell (3) of the phase leg (2): filtering (202) the obtained DC capacitor voltage (45) for removing a harmonic component of said voltage.

6. A control apparatus (10) for controlling a chain-link converter (1), said converter comprising three phase legs (2), each of which phase legs comprising a plurality of series-connected converter cells (3), each of which cells comprising a DC capacitor, the control apparatus comprising: a processor (11); and a storage unit (12) storing instructions that, when executed by the processor (11), cause the apparatus (10) to: control the converter by means of a first closed-loop control function (101), controlling a mean voltage of all DC capacitors of all three phase legs by adjusting a current reference for each of the phase legs; and for each phase leg (2): control the converter by means of a feed-forward control function (102) based on a measured converter AC voltage and on the current reference, controlling a mean voltage of all DC capacitors of the phase leg by means of the current reference for the phase leg, control the converter by means of a second closed-loop control function (103), controlling the mean voltage of all DC capacitors of the phase leg by adjusting the current reference for the phase leg, and control the converter by means of a third closed-loop control function (103), adjusting a voltage reference for each cell of the phase leg, the third closed-loop control function (104) comprising, for each cell (3) of the phase leg (2): obtaining a DC capacitor voltage (45) of the cell; computing a control error by comparing the obtained DC capacitor voltage (45) with the mean voltage (44) of all DC capacitors of the phase leg, said mean voltage of all DC capacitors of the phase leg being used as the voltage reference for the cell; detecting a sign of a product of a current (42) over the whole phase leg and a voltage reference (41) for the whole phase leg; computing a gain (43) of the third closed-loop control function, based on the current reference (52) of the phase leg; obtaining a control output as the product of the control error, the gain (43) and the sign; and adding the control output to the voltage reference (41) for the cell, thereby obtaining an updated voltage reference (48) for the cell (3).

7. A computer program for controlling a chain-link converter, said converter comprising three phase legs, each of which phase legs comprising a plurality of series-connected converter cells, each of which cells comprising a DC capacitor, the computer program comprising computer program code which is able to, when run on a processor of a control apparatus for the converter, cause the control apparatus to: control the converter by means of a first closed-loop control function, controlling a mean voltage of all DC capacitors of all three phase legs by adjusting a current reference for each of the phase legs; and for each phase leg: control the converter by means of a feed-forward control function based on a measured converter AC voltage and on the current reference, controlling a mean voltage of all DC capacitors of the phase leg by means of the current reference for the phase leg, control the converter by means of a second closed-loop control function, controlling the mean voltage of all DC capacitors of the phase leg by adjusting the current reference for the phase leg, and control the converter by means of a third closed-loop control function adjusting a voltage reference for each cell of the phase leg, the third closed-loop control function comprising, for each cell of the phase leg: obtaining a DC capacitor voltage of the cell; computing a control error by comparing the obtained DC capacitor voltage with the mean voltage of all DC capacitors of the phase leg, said mean voltage of all DC capacitors of the phase leg being used as the voltage reference for the cell; detecting a sign of a product of a current over the whole phase leg and a voltage reference for the whole phase leg; computing a gain of the third closed-loop control function, based on the current reference of the phase leg; obtaining a control output as the product of the control error, the gain and the sign; and adding the control output to the voltage reference for the cell, thereby updating said voltage reference for the cell.

8. A computer program product comprising a computer program according to claim 8 and a computer readable means on which the computer program is stored.