LU101106B1

LU101106B1 - Method of controlling a multi-phase chain-link power converter using MPC, and a power converter controller.

Info

Publication number: LU101106B1
Application number: LU101106A
Authority: LU
Inventors: Marta IBANEZ; Federico Bertoldi; Jan Svensson; Aravind Mohanaveeramani
Original assignee: Abb Schweiz Ag
Priority date: 2019-01-18
Filing date: 2019-01-18
Publication date: 2020-07-20

Abstract

A multi-level chain-link converter (4), a controller (40) and a method (100) of controlling, or designing, the converter (4). The converter (4) comprises phase arms connected to a respective phase (A, B, C) of a power system (1). The converter (4) further comprises an energy transfer circuit (11B) that inter- connects the phase arms. The energy transfer circuit (11 B) comprises at least one energy transfer arm (10D-F). The controller (40) is configured to control the converter (4) in accordance with the method. The method includes obtaining (110) a voltage reference for a time step to be controlled, and selecting (130) a switching state that meet the voltage reference. Especially, the method comprises - performing (120) model predictive control, MFC, over a predictive horizon comprising a plurality of time steps. The performing (120) of MPC includes - satisfying 122 the voltage reference for each phase (A, B, C) in each time step, and - satisfying 124 an arm energy criterion over the predictive horizon. The selecting (130) includes selecting a switching state that satisfies (124) the arm energy criterion for each phase arm (10A-C) and each energy transfer arm (10D-F). The energy criterion constrains the electrical energy to remain unchanged over the total predictive horizon in order to counteract power system unbalance, without causing the converter (4) to become unbalanced.

Description

Title: Method of controlling a multi-phase chain-link power converter HU10T108 using MPC, and a power converter controller. Technical Field The present invention concerns multi-level chain-link power converters for electrical power systems, such as power transmission and distribution systems. Especially the present invention concerns control of multi-level chain-link power converters in multi-phase systems, especially three-phase systems. The invention is also related to design and dimensioning of multi-level chain-link converters. Background and Prior Art The present invention is related to multi-level power converters for multi-phase power systems, such as three-phase power systems. Especially the present invention concerns controlling chain-link power converters that comprises one arm of switching cells for each phase of the power system, which switching arms are interconnected (in the end opposite the power system) by means of an energy transfer circuit provided for transferring energy between the phases.

US 9,876,358 (E1) describes a power converter arrangement for a three-phase power system. The converter comprises three chain-link arms, or phase legs (10, 20, 30 in E1), one for each of the phases (A, B, C in E1) and an energy transfer circuit (40 in E1) for exchanging energy between the arms, i.e. phase legs (10, 20, 30 in E1). The energy transfer circuit (40 in E1) comprises energy storage elements such as capacitors (44) that are selectively connectable to each of the phase legs (10, 20, 30). The storage elements (e.g. 44) are used to provide the voltages of the phases (A, B, C) in combination with storage elements (11A-n, 21A-n, 31A-n in E1) of the chain-link phase legs (10, 20, 30 in E1).

WO 2017/140357 (E2) shows a chain-link power converter that is similar to the power converter of E1, where three arms, or phase legs (10A-C in fig. 1A of

E2), are interconnected by an energy transfer circuit, each of which phase legs pOTOTT08 (10A-C in E2) constitute a multi-level chain-link, and wherein the energy transfer circuit is configured as a delta circuit (2 in E2), i.e. a delta circuit having three chain-link legs, or arms, with switching cells (the blocks 9 in E2) (see abstract and fig. 1A of E2).

The present invention primarily concerns control of power converters similar to the three-phase multi-level chain-link power converter of E2 where three phase legs, or phase arms, are interconnected at their neutral sides opposite their power system facing sides, by means of an energy transfer circuit comprising three legs, or arms, configured in a delta, and wherein energy storage elements of each arm of the delta circuit are selectively connectable to two of the phase arms. The delta circuit of E2 comprises switching blocks (9 in E2) that include energy storage elements, e.g. capacitors, and bi-directional voltage blocking semiconductor switches, such as RB-IGBTs (Reverse-Blocking Insulated-Gate Bipolar Transistors). The multi-level chain-link arrangement of the power converter in E2 can be viewed as a WYE-connected chain-link where the neutral point has been substituted for by an interconnecting delta-circuit, especially a multi-level chain-link delta circuit having bi-directional current blocking capability. In this model, the delta circuit enables moving the neutral point of the WYE, thereby enabling changing the conduction path of the chain- link of each phase, so that any phase leg (10A, 10B, 10C) with the aid of an arm, or leg, (at 9 in E2) of the delta circuit (2 in E2) can provide a higher voltage level than its switching cells (12, 13 in E2) alone can provide.

The power converters of documents E1 and E2 are advantageous compared to the more common WYE-connected, or Delta-connected, multi-level chain-link power converters in that the energy transfer circuits allow energy transfer between the phases of the power system. The energy transfer circuit can for example be used to handle imbalances and fault currents in the power system by means of controlling the power converter to provide currents through the converter between the phases of the power system. Thus, the topology

| . a 3 provided in E1 and E2 enables counteracting negative sequence currents of a 191198 power system by means of providing currents between the phases of the power system using the energy transfer circuit of such a power converter. The topologies disclosed in E1 and E2 also enables more economic designs, since the energy transfer circuits enables designing multi-level chain-link converters with comparably fewer levels, so that the number of semiconductor switches and capacitors used can be lower than in more conventional WYE- or Delta- designs.

E1 and E2 do disclose several examples of chain-link power converter topologies with energy transferring capability, however do not provide specific details on control of fault currents, nor of dimensioning of the chain-link power converter in view of fault currents.

Control of multi-level chain-link power converters can be used in view of measurements in the power system, such as voltage levels and load currents, wherein a reference for the output voltage is provided on the basis of the measurements. The selection of the specific switching cells used for providing the desired reference voltage can for example be made in view of the current state of the converter in order to limit switching or conduction losses, or in view of the actual voltage levels of individual capacitors in order to balance the capacitor voltages.

Among control techniques, so called Model Predictive Control (MPC) has been suggested for controlling the switching of multi-level chain-link converters. In a typical MPC, the control of a process, such as current control of a converter connected to a power system, is performed in view of a model of the system for each control, or time, step in view of some constraints or criteria, so called cost functions in MPC, in order to optimize the control. The optimization is performed in view of the cost function over a sequence of time steps. The sequence of control, or time, steps is referred to as a predictive horizon. Documents E3-E5 in the following provides some examples of using MPC for controlling multi-levelchain-link converters, however none of which MPC is adapted specifically for Hootie chain-link converters that includes energy transfer circuits, such as those described in E1 and E2.

US 8,737,103 (E3) describes a method of predicting pulse-width modulated switching patterns for a multi-phase multi-level converter. A plurality of possible switching sequences is determined and in view of an optimizing goal one sequence is selected (see abstract of E3).

US 9,705,420 (E4) suggests using MPC with reference tracking (title) for controlling a power converter (abstract) in an electric power system (abstract, page 1, line 15-20, line 39-50 in E4).

US 2017/0237331 (E5) describes a method of controlling a multi-level converter that includes evaluating each switching cell as two separate switching legs, i.e. each full bridge cell is viewed as two legs, in order to limit the computational load (818 in E5). The method of E5 is especially aimed at limiting the computational load when using MPC (815 of ES) including predicting behavior of load currents and using a cost function for optimizing the control of the system.

An object of the present invention is to counteract the disadvantages of the prior art, and facilitating counteracting of fault currents in power systems.

Summary of Invention In view of the above, a first aspect of the present invention provides a method of controlling a multi-level chain-link converter. The multi-level chain-link converter is a converter that comprises a first, a second and a third phase arm, where each phase arm is connected at a respective first end to a respective phase of a multi-phase power system, e.g. a three-phase system. The multi-level chain-link converter further comprises an energy transfer circuit that inter-connects the respective second ends of the phase arms, and which energy transfer circuitcomprises at least one energy transfer arm. The energy transfer arm, or arms, Hootie is selectively connectable to the phase arms in order to contribute to an output voltage at the phases. Preferably, each phase arm and each energy transfer arm are a switching arm that comprises a plurality of switching cells, each 5 switching cell comprising a respective energy storing element, and switches for selective connection of the energy storage elements to provide an output voltage. The control method includes - obtaining a voltage reference for a time step to be controlled, said voltage reference comprising a voltage reference for each phase, and - selecting a switching state for the time step that meet the voltage reference. Especially the control method includes: - performing model predictive control, MPC, over a predictive horizon comprising a plurality of time steps in order to evaluate different voltage references for each phase arm and each energy transfer arm, which performing of MPC includes - satisfying the voltage reference for each phase of the power system in each time step, and - satisfying an arm energy criterion for each phase arm, and for each energy transfer arm, over the predictive horizon. The energy criterion allows the electrical energy in each arm to vary between time steps but constrains the electrical energy in the arm to remain unchanged over the total predictive horizon. Thus, the voltage references for the phase arms as well as for the energy transfer arms are evaluated in the MPC over the whole predictive horizon. The voltage references over a predictive horizon can be referred to as reference voltage waveforms, since these reference voltages vary over the predictive horizon.

In an embodiment of the first aspect, the selecting of switching state is performed in accordance with a modulation algorithm and a voltage balancing algorithm.

In an embodiment of the first aspect, the energy transfer circuit comprises three energy transfer arms arranged in a delta circuit, wherein the delta circuit is connected at each of its three corners to a respective one of the phase arms.

In an embodiment of the first aspect, each phase arm and each energy transfer arm comprises a plurality of switching cells comprising respective energy storing elements, and switches for selective connection of the energy storage elements to contribute to the output voltage.

In an embodiment of the first aspect, a fundamental period of the power system, or a multiple of said fundamental period, is used as the predictive horizon. In an embodiment of the first aspect, the method further includes determining sequence components of the voltage reference and using the sequence components when performing the MPC, especially when satisfying the arm energy criteria. According to a second aspect, the present invention provides a controller for a multi-level chain-link converter comprising a first, second and a third phase arm. Each phase arm of the converter being configured to be connected at a respective first end to a respective phase of a multi-phase power system. The multi-level chain-link converter further comprises an energy transfer circuit that inter-connects the respective second ends of the phase arms, which energy transfer circuit comprises at least one energy transfer arm. Each phase arm and each energy transfer arm comprise a plurality of switching cells comprising respective energy storing elements, and switches for selective connection of the energy storage elements to provide an output voltage at each phase of the power system. The controller including - an input stage for obtaining a voltage reference, - an MPC block for performing MPC, and

- output stages for selecting a switching state, wherein the controller is 191198 configured to perform the method according to the first aspect.

According to a third aspect, the present invention provides multi-level chain-link converter comprising the controller of the second aspect.

According to a fourth aspect, the present invention provides a method of designing a multi-level chain-link converter.

The designed multi-level chain-link converter comprises a first, second and a third phase arm.

Each phase arm is connectable at a respective first end to a respective phase of a multi-phase power system, such as and preferably a three-phase system. the multi-level chain-link converter further comprises an energy transfer circuit that inter-connects the respective second ends of the phase arms, and which energy transfer circuit comprises at least one energy transfer arm.

The at least one energy transfer arm is selectively connectable to the phase arms in order to contribute to an output voltage at the phases.

The method of designing a multi- level chain-link converter according to the fourth aspect of the present invention especially comprises: - obtaining a voltage reference for a time step to be controlled during operation of the designed multi-level chain-link converter, - selecting a design for the time step that meet the voltage reference, and - performing model predictive control, MPC, over a predictive horizon comprising a plurality of time steps in order to evaluate different voltage references of each phase arm and evaluate voltage references of each energy transfer arm.

The performing of MPC of the fourth aspect includes - satisfying the voltage reference for each phase in each time step, and - satisfying an arm energy criterion for each phase arm, and for each energy transfer arm, over the predictive horizon, which energy criterion allows the electrical energy in each arm to vary between time steps but constrains the electrical energy in the arm to remain unchanged over the total predictive horizon.

Especially, the selecting of a design includes selecting a design topology that HU10T108 satisfy the voltage reference for each phase and satisfy the arm energy criterion for each phase arm as well as for each energy transfer arm.

In an embodiment of the fourth aspect, the selecting a design comprises evaluating a cost function for the design topology over the predictive horizon. Preferably, in this embodiment, - evaluating the cost function for the topology includes evaluating number of voltage levels, the number of energy storage elements and/or the number of semiconductor switches, of the multi-level chain-link converter, and - the selecting of design includes selecting the least costly design according to the cost function.

In an embodiment of the fourth aspect, the energy transfer circuit comprises three energy transfer arms arranged in a delta circuit having a respective corner connected to each one of the phase arms.

In an embodiment of the fourth aspect, the topology of said designed multi.level chain-link converter comprises a plurality of H-bridge switching cells arranged in series in each phase arm and in each energy transfer arm.

Brief Description of Drawings Figure 1 illustrates an overview of a voltage source converter connected to an electric power system.

Figure 2 illustrates a simplified view of a voltage source converter provided with an energy transfer circuit in form of a delta circuit in fig. 2a, and provided with an energy transfer circuit having only one switching arm in fig. 2c. Figure 2b illustrates a switching cell and a bi-directional switch for the voltage source converters of figures 2a, 2c.

Figure 3 illustrates different control states over a predictive horizon in accordance with a control method using MPC in accordance with anembodiment of the present invention. A first state in fig. 3a, a second state in HU10TT08 fig. 3b, a third state in fig. 3c and an overview of the states during a predictive horizon of 360 degrees. Figure 4 illustrates a method of controlling a VSC in accordance with embodiments of the present invention. Fig 4a illustrates a control method, fig.

4c a design method and fig. 4b illustrates a more detailed embodiment of the control method and the design method. Figure 5 illustrates an embodiment of a controller for multi-level chain-link converter according to the invention.

Description of Embodiments Figure 1 illustrates a power system 1 comprising a power generating side exemplified as an equivalent generator 2 connected, and supplying power, to a power consuming side, exemplified as a total load 3 via distribution or transmission lines 7. A multi-level chain-link converter 4 is connected to the transmission line 7 in order to control the power between the generating side and consuming side of the power system 1. The general arrangement of the multi-level chain-link converter 4 and the power system 1 are similar to those found in prior art arrangements. The power system 1 is a three-phase system and means 5 for measuring a load current are arranged at each phase A, B, C of the transmission line 7. Also, means 6 for measuring the output terminal voltage and current of the multi-level chain-link converter 4 are arranged in the connection between multi-level chain-link converter 4 and transmission line 5. In this way, the converter 4 is configured to monitor the load current, the output voltage and the output current. The converter 4 is also configured to monitor the voltage levels of its energy storage elements (14 in figure 2). The converter 4 includes a controller 40 and is controlled by the controller 40 based on these measurements of load current, output voltage and current and voltage levels of its energy storage elements 14.

Figures 2a-c illustrate multi-level chain-link converters suitable for control in LUTOT106 accordance with the invention. The multi-level chain-link converter 4 comprises an outer circuit 11A for connection to the power system 1 comprising three phase arms 10A-C, one phase arm 10A-C for each phase A, B, C of the power system 1, and an inner circuit, or energy transfer circuit 11B that includes at least one switching arm, or energy transfer arm 10D. Each phase arm 10A-C constitutes a switching arm dedicated for one of the phases A, B, C. Each energy transfer arm 10D-F constitute a switching arm of the energy transfer circuit 11B.

Figure 2a illustrates a type of multi-level chain-link converter 4 previously disclosed in document E2 (WO 2017/140357), wherein the energy transfer circuit 11B comprises three switching arms for energy transfer, i.e. the energy transfer arms 10D-F, which are arranged in a delta circuit.

Figure 2c shows a multi-level chain-link converter 4 that comprises an energy transfer circuit 11B having only one switching arm for energy transfer, i.e. the energy transfer arm 10D. An energy transfer circuit 11B with three switching arms (10D-F) is a preferred embodiment, however, the invention can be utilized for controlling converters having only one switching arm (10D) in the energy transfer circuit 11B. Figure 2b illustrates an example of a switching cell 9 and suitable connection switches 13 of the energy transfer circuit 11B, which are equivalent to switching cells and delta circuit connecting switches of document E2. The switching cell 9 of figure 2b comprises an energy storage element 14 in the form of a capacitor and semiconductor switches 12 arranged in an H-bridge, i.e. full bridge, wherein the semiconductor switches 12 comprises IGBTs with reverse diodes (Insulated-Gate Bipolar Transistor). The connection switches 13 between each phase arm 10A, 10B, 10C and the energy transfer circuit 11B are bi-directional switches, suitably RB-IGBTs (Reverse-Blocking IGBTs), for example.

Figure 3d illustrate the voltage of the three phases A, B, C of the power system HU10TT08 1 over a fundamental line period, i.e. a period of 2*7 or 360°. The fundamental line period is marked in degrees and divided into six sectors from 0 to 360°, each sector extending over 60°. Figures 3a-d illustrate switching states of the preferred embodiment where the voltage source converter 4 comprises an energy transfer circuit 11B arranged as a delta circuit, as in figure 2a. At any time instant of the fundamental line period, two of the phase arms 10A-C will be connected to a respective switching arm 10D-F of the energy transfer circuit 11B and provide the respective phase voltage together with the energy storage elements 14 of the energy transfer circuit, whereas the third phase arm (10A, 10B, or 10C) of the phase arms 10A-C will provide its respective phase voltage by itself. Figure 3a-c illustrates the corresponding preferred switching of the connecting switches 13 between outer circuit 11A and energy transfer circuit 11B, wherein the two phase arms 10A-C that should provide the largest phase voltages, i.e. having the largest deviation from zero, are connected to a respective energy transfer arm 10D-F of the energy transfer circuit 11B. This can be seen as moving a neutral point N between the phase arms 10A-C six times each fundamental line period between the three corner points of the delta circuit.

The neutral point N faces the second phase arm 10B in “state 1” when the voltage of phase B is the lowest, i.e. closest to zero, in figure 3a, which corresponds to a first sector of the fundamental line period between 0° and 60° degrees, and a fourth sector of the fundamental line period between 180° and 240°, as illustrated in figure 3d. Similarly, the neutral point N faces the first phase arm 10A in “state 2” when the voltage of phase A is the lowest, in figure 3b, corresponding to “sector 2” and “sector 5” in figure 3d, between 60° and 120°, and between 240° and 300°, respectively. Also, the neutral point N faces the third phase arm 10C in “state 3” in “sector 3” and “sector 6”, from 120° to 180°, and 300° to 360°.

The present invention will now be discussed in terms of methods. In these LJT01706 methods it is preferred to make a multi-level chain-link converter 4 that have phase arms 10A-C interconnected by means of an energy transfer circuit 11B that has a delta configuration the subject of the methods. In these methods of controlling the converter 4 with energy transfer circuit 11B in delta configuration, it is preferred that the connection switches 13 that are used to connect the energy storages 14 of a phase arm 10A-C in series with the energy storage elements 14 of an energy transfer arm 10D-F are controlled in the three states shown in figures 3a-c and that the fundamental line period of the power system 1 is divided into six sectors, each of 60°, where the neutral point N is moved in accordance with figures 3a-d. However, such strict selection of switching states between phase arms 10A-C and energy transfer circuit 11B is in general not required even for the delta configured energy transfer circuit 11B.

Figures 4a illustrates a method for controlling a multi-level chain-link converter 4 with energy transfer circuit 11B. Figure 4c illustrate a method of designing such a multi-level chain-link converter 4. Figure 4b illustrates a method of controlling such a converter 4 during operation in more detail on the left-hand side of the figure. The right-hand side of figure 4b illustrates a variant of the method used for designing such a converter 4. Referring first to figures 4a and 4b describing control 100 of multi-level chain- link converter 4 with energy transfer circuit 11B. For ease of understanding, the control method is illustrated in three main control phases 110, 120, 130. The control method starts with a “first phase” of operation of obtaining reference signals for the control including obtaining 110 a voltage reference for the output of the VSC 4, i.e. including a phase voltage reference for each output terminal voltage. (Figure 4b illustrates further steps of this first phase that will be described in more detail.).

The control methods of figures 4a and 4b ends with a “third phase” of operation, wherein a switching state is selected 130 for each switching arm 10A-F, andcontrol signals, i.e. gate signals, are provided 140 for the switches of the VSC. HUTOTI08 The third phase includes providing 140 control signals to the switches 12 of the switching cells 9 of the phase arms 10A-C, control signals to the switches 12 of the switching cells 9 of the energy transfer circuit 11B, and control signals to the connection switches 13 that connects the switching arms 10D-F of the energy transfer circuit 11B to the phase arms 10A-C. An intermediate “second phase” of operation comprises performing 120 a model predicted control (MPC). The method uses a predictive horizon for the MPC that preferable is equal to one fundamental line period of the power system 1, i.e. 2*7 Rad or 360 degrees. The MPC predicts the operational state of the total system including the converter 4 and the power system 1, including loads 3, for a number of time steps over a predictive horizon. The operational state that relates the electrical entities of the power system, such as load current and voltage levels to the operation, e.g. output voltage and current, of the converter 4, are predicted from the measurements and the different voltages being evaluated in the MPC.

Each phase voltage reference is provided for the full time period of the predictive horizon and basically defines a sinus-shaped voltage for each phase. The phase voltage reference may comprise an amplitude level for a sinus- shaped voltage thereby defining a voltage level for each time step of the predictive horizon in view of the position of the time step, i.e. control step, along the fundamental period. In general, the steps of the first phase and third phase of operation, i.e. obtaining 110 voltage reference, and selecting 130 switching state, are similar to how voltage source converters of the prior art are controlled.

The intermediate second phase of operation includes performing 120 an MPC HU10T108 that use the voltage reference including a reference for each phase voltage as input, and provide reference arm voltages as output, i.e. a reference voltage level for each switching arm 10A-F of the VSC. These reference voltages comprise waveforms defining voltage levels for the predictive horizon, e.g. the fundamental line period. Thus, the output of the MPC comprises a respective reference voltage for each switching arm 10A-F that varies (not a single level) and extends over the predictive horizon. For clarity, the term “switching arm” 10A-F used here, and in the following, refers to the phase arms 10A-C as well as the energy transfer arms 10D-F. In more detail, each voltage reference can be viewed as a waveform that includes a reference voltage level for the present time step and the future timesteps of the predictive horizon. The performing 120 of the MPC aims at ensuring an uncompromised operation of the VSC, especially avoiding instability, also in cases of an unbalanced load condition in the power system 1 or fault condition in the power system 1. The performing 120 of the MPC is performed in view of a phase voltage criterion (step 122) for the output terminal 18 voltages and an energy criterion (step 124) for each switching arm 10A-F of the VSC.

The performing 120 of the MPC includes satisfying 122 the phase voltages for each control, or time, step of the predictive horizon of the MPC. The predictive horizon includes a plurality of individual control steps, and preferable the predictive horizon is equal to a fundamental period of the power system 1 (i.e. 2*tT Rad or 360 degrees). The performing 120 of the MPC includes ensuring that the control of the VSC will satisfy 124 an energy criterion for each switching arm 10A-F over the predictive horizon. The energy criterion for each switching arm 10A-F is that the energy of the switching arm does not change over the predictive horizon, such as over a fundamental period of the power system. The energy criterion (in 124) for each switching arm 10A-F means that the input power and the output powershould be equally large over the predictive horizon. Thus, the two main criteria 191198 for the MPC can be described as: - satisfying (in 122) the (three) phase voltages at the output terminals 18 of the VSC in each time step, and - satisfying (in 124) zero change in energy level in each switching arm, such as in each of the six switching arms 10A-F, over the predictive horizon. Satisfying 122 the phase voltage criterion The phase voltages provided at the output terminals 18 will be a combination of the voltages in the phase arms 10A-C and the voltages in the energy transfer arm, or energy transfer arms, 10D-F of the energy transfer circuit 11B. The phase voltages will therefore depend on the state of the connection switches 13 that selectively connects the switching cells 9 of the energy transfer arms 10D-F of the energy transfer circuit to the phase arms 10A-C.

It should be noted that when a phase arm 10A-C, during a sequence of time steps, provides the full output voltage at the terminal 18, the voltage of the phase arm 10A-C will be sinusoidal. However, when a phase arm 10A-C together with a switching arm 10D-F of the energy transfer circuit 11B, during a sequence of time steps, provides the output voltage at the terminal 18, the voltage level of the phase arm need not be sinusoidal, and the voltage level of the energy transfer arm need not be sinusoidal even though the combined voltage of the phase arm 10A-C and the energy transfer arm 10D-F will be sinusoidal.

Mathematically, the relation between the phase voltages and the voltages of the phase arms 10A-C and energy transfer arms 10D-F vary with the sector along the fundamental line period (as illustrated in fig. 3) and can be expressed in the following relation: Vphase = f (A, Vam, sector)

For the preferred converter 4, of figure 2a, comprising a delta configured energy LU101106 transfer circuit 11B, the three phase voltages Vphase is a function of a matrix A with elements 1, 0, and -1, the sector (along the fundamental line) and the six switching arm voltages Vam.

In view of figure 3, the relations between the six switching arm voltages and the phase voltages can be expressed in table format as: STATE 1 STATE 2 STATE 3 Sector 1: 0° to 60° Sector 2: 60° to 120° Sector 3: 120° to 180° and and and Sector 4: 120° to 180° | Sector 5: 180° to 240° | Sector 6: 240° to 360° Vy1 + Va1 =Va Vy1 +0 =Va Vy1 - Vas =Va Vy2 +0 =Vo Vy2 -Va1 =Vb Vy2 + Vdz =Vb Table 1 showing the constraints for the voltages Vy1, Vy2, Vys, of the phase arms 10A-C and for the voltages of Vai, Vaz, Vas, of the energy transfer arms 10D-F, where Va, Vb, Vc are the individual phase voltages Vphase for the phases A, B, C.

For clarity, the subscript “d” is for “delta” of the delta circuit, and the “y” is for “wye”, since the preferred converter 4 of figure 2a can be viewed as a WYE- connected converter wherein the phase arms 10A-C of the WYE are interconnected by means of a delta circuit.

In table 1, “Va1” is the voltage of energy transfer arm 10D, “Vaz” is the voltage pf energy transfer arm 10E, and “Vaz” is the voltage of energy transfer arm 10F of figure 2a.

Thus, table 1 illustrates the constraints provided by each phase voltage in each control, or time, step, which the MPC use as criterion for the combinations ofthe voltages of the phase arms 10A-C and the voltages of the energy transfer LUTOT106 arms 10D-F.

It should be noted that each switching arm 10A-F has a maximum voltage level and that the method also is constrained by the maximum available voltage levels.

Preferably, the phase arms 10A-C are dimensioned by equal nominal voltage levels, or maximum voltage, and the energy transfer arms 10D-F are dimensioned with mutual equal nominal voltage levels, or maximum voltage.

Using the “bwye” as maximum voltage in the phase arms 10A-C, and “Ddetta” as maximum voltage of the energy transfer arms 10D-F of the example using a delta circuit as energy transfer circuit 11B, the following defines further constraints in the MPC: -Dwye $ Vy1 < bwye -Dwye < Vy2 < Dwye -Dwye € Vy3 < Dwye -Ddeita < Vd1 < Ddeita -Pdeita € Vaz < betta -Pdeita € Vas < Ddelta

Satisfying 124 the arm energy criterion The method 100 for controlling, and the design method, also includes determining whether the solutions satisfy 124 an arm energy criterion over the predictive horizon that provides a constraint for the energy level of each of the switching arms 10A-F.

Such constraint will ensure that power can be moved through the energy transfer circuit 11B without causing voltage balance problems in the energy storage elements 14 of the switching arms 10A-F of the multi-level chain-link converter 4. Mathematically the energy constraint for each switching arm 10A-F can be written as:

on LU101106 E = | V * I dwt 0 where E is the energy in the switching arm, V is the voltage provided by the switching arm, | is the current through the switching arm. Thus, the total energy V*| dwt received in the switching arm during all the control steps of the predictive horizon, which in this case is a fundamental line period of 21, should equal the total energy V*I dwt transferred from the switching arm 10A-F during all the control steps of the predictive horizon; in this case a fundamental period of 2*7.

The control method 100 may preferably include calculating or evaluating 126 a cost function for different voltage references of each switching arm 10A-F when performing 120 the MPC in order to select the most appropriate voltage reference among those voltage refences that have been found to satisfy the constraints of satisfying 122 the phase voltage criterion and satisfying 124 the arm energy criterion over the predictive horizon. The method of designing the converter preferably uses 127 a cost function for ranking different topologies. The suggested use of cost functions is, however, not necessary for providing a balanced control of the converter 4 or for designing a converter 4 that can contribute to power control in an unbalanced power system 1. The cost function for the control 100 of the converter 4 may suitable include penalties for losses, such as penalties for output harmonics, energy ripple in the switching arms and total power loss. The harmonics, e.g. THD (total harmonic distortion), the energy ripple and power loss can be estimated from the reference voltage waveform over the predictive horizon. The cost function for designing a converter may suitably penalize expensive solutions, such as penalizing the number of switching cells 9 in the switching arms 10A-F, i.e. penalizing number of voltage levels, and/or the number of energy storage elements 14 and/or the number of semiconductor switches 12.

The output from the MPC will provide at least one possible voltage reference for each switching arm, wherein each reference voltage is a waveform defining a voltage level for each time step of the predictive horizon. The control method 100 will then go through a modulation 132 and voltage balancing algorithm 134, including sorting 134A and power direction determining 134B algorithm, were the control signals for the switching is selected, i.e. applying 140 the gate signals and control signals of interconnecting switches 13. The modulating 132 and voltage balancing 134 will define the actual switching state provided for the converter 4. Figure 4b illustrates a more detailed embodiment of the control method, and includes illustrating how the control method 100 can be used also for designing a converter 4. The first phase that includes obtaining 110 voltage references may include obtaining 112 measurements of the power system 1, such as load currents, voltage level of the power system at the converter connection, the output current of the converter 4, and voltage levels of the energy storage elements 14 of the converter 4. These measurements are used to calculate 114 the voltage reference, including the phase voltage reference of each phase A-C.

In the design process, the power system 1 may instead be simulated 111, together with the converter 4, in order to provide load currents, voltage levels, converter output current and voltage levels of the energy storage elements. Thus, electrical operation properties that corresponds to a step of obtaining 112 measurements in the control method 100, should be provided through a step of simulating 111 the power system 1 and the converter 4 when the converter 4 is designed. The design may suitably include simulating 111 the system in “worst case”-scenarios, such as an unbalanced power system 1 or a power system 1 experiencing large fault currents or over-voltages.

The control method 100 may also include detecting, or determining, 116 sequence components in the power system 1, such as positive and negativesequence components of the load current and/or the voltage at the connection Hootie of the converter 4 output to the power system 1. The determined sequence components facilitate the calculation of arm energy when the MPC 120 determines whether the reference voltage waveform of an arm satisfy 124 the arm energy criterion.

Other ways of determining arm energies are of course possible, however using sequence components is a preferred embodiment.

These sequence components may suitably be handled separately in the MPC process, although such separate control is not necessary.

A separate control of the sequence components is for example advantageous in predicting the need for power transfer between the phases.

The method of control 100 and design, may suitably include a step of detecting 118 the sector along the fundamental line period that the present time step, or control step, concerns.

Detecting 118 the sector is especially suitable for the converter 4 that includes an energy transfer circuit 11B of delta configuration.

The switching of the connection switches 14 (see step 140 in figure 4b) between phase arms 10A-C and energy transfer circuit 11B may suitable be based on the detected sector of the present control step.

This detecting 118 of sector is also a suitable step in the case of energy transfer circuits 11B with only one energy transfer arm 10D.

As disclosed with reference to figures 4a and 4c, the control method 100 may preferably include determining or evaluating 126 a cost function over the predictive horizon.

When there are a plurality of reference voltage waveforms for a switching arm (10A-F) that satisfy the arm energy criterion (124), a cost function can be used to select a preferred waveform, e.g. the waveform that correspond to the most cost-efficient selection that can be predicted.

The cost function may include several variables, each variable being provided with a corresponding weight function, thus variables like THD and switching losses may be assigned different weight functions each of which weight function can be varied in view of the current operational state of the converter 4 and/or the power system 1.

For the design method, a calculation or evaluation 127 of costs that mirrors different design topologies is also suitable. Calculating the costs 126, 127 over the predictive horizon may suitably be used in order to select the most cost- efficient solution as output from the MPC 120. The MPC 120 suitable provides the reference voltage waveform for each switching arm 10A-F based on the result from the cost function calculation 126,

127.

The selection 130 of switching states suitably includes a modulating 132 step and a voltage balancing 134 step for the switching cells 12, i.e. balancing the voltages of the energy storage elements. The modulation algorithm used in the modulation 132 may for example be PWM (Phase Width Modulation), such as phase-shifted PWM, level-shifted PWM or Space-vector PWM. The balancing algorithm used when the balancing 134 the voltages of the energy storage elements 12 may include sorting 134A the energy storage elements in accordance with their respective voltage level, and determining 134B the direction of the current to select storage elements in dependence of whether the phase receives or provides energy in view of their need for adjusting voltage. The control method 100 further includes applying 140 control signals to the switching cells 9, e.g. gate signals to the semiconductor switches 12 of the switching cells 9 in the phase arms 10-C as well as in the energy transfer arms 10D-F; and control signals to the connection switches 13. The applying 140 of the control signals is based on the selected reference voltage of each switching arm 10A-F for the time, or control, step. The design method ends with selecting 150 a design, especially a design that satisfies the arm energy criterion (124) and the phase voltage criterion (122) for the requirements of the power system 1 that may be specified as grid codes by the proprietor of the power system 1 in order for the multi-level chain-linkconverter 4 to handle unbalanced operation scenarios.

A cost-efficient design LUT0T106 may be selected among several designs satisfying handling of unbalanced operations, such as a design selected also in view of costs estimated by calculating 127 a cost function that penalize costly constructions, such asnumber of switching cells, switches, and/or capacitors.

Figure 5 illustrates an embodiment of a controller 40 for a multi-level chain-link converter 4 according to the invention.

The controller 40 is configured to perform the methods of the invention, including the embodiments of themethods for control and design of the multi-level chain-link converter 4 as described in relation to figures 4a-c.

The controller 40 should be connected to a converter 4, such as directly, or e.g. via a communication network.

The controller 40 comprises a current controller 41 for the converter 4, and receives input signals, such as measurements of the converter current Iconv, the loadcurrent load, the voltage at the connection to the power system Upcc, and the voltage levels of the energy storage elements 14, e.g. capacitors, Ucar, ii.

The controller 40 further includes a PLL 42, Phase Locked Loop, for following the voltages of the converter output.

The converter 40 also includes at least one output stage 49, exemplified as three output stages 49A-C, for providing controlsignals for the switching of the converter 4. A first output stage 49A for control signals to the interconnecting switches 13 between the phase arms 10A-C and the energy transfer circuit 11B, a second output stage configured to provide gate signals to the switching cells 9 of the energy transfer circuit 11B and a third output stage 49C configured to provide gate signals to the switching cells 9 ofthe phase arms 10A-C.

Thus, input and output means similar to conventional converters are provided in combination with output stages for interconnecting switches 13 and energy transfer circuit 11B.

The controller 40 further includes a sequence detector 43 configured todetermine the sequence components of the load current and of voltage at the PCC on the basis of the measured, or simulated, load current and voltage at the power system connection point, i.e. the PCC.

The controller 40 preferably alsocomprises a operation condition unit 44 configured to determine an operating LU101106 condition, including such conditions as a nominal operation, a fault current operation, an unbalanced operation and/or for example a process for designing a converter 4. The operation condition unit 44 is especially configured to determine the operation condition from the sequence components, but may also be configured to receive and use further input, such as indicating a design process. The operation condition unit 44 outputs the determined operating condition to a weighting coefficients decision maker 46 provided for a cost function of an MPC block 48 of the controller 40. The MPC block 48 also comprises a sector detector 45, receiving input from the PLL 42 that identifies the position along the fundamental line period, which in an embodiment is used to determine the sector of operation, see sector definition of table 1, but may in other embodiments be used to determine only the position along the fundamental line period; the phase angle. The MPC block 48 also comprises the MPC algorithm controller 47 configured to perform the MPC including verifying the voltage and energy constraints by means of a constraint controller 47A and performing the evaluation of the cost function by means of a cost function controller 47B. The cost function controller 47B is configured to receive weighting coefficients from the decision maker 46, which coefficients depend on the operation condition as determined by the operation condition unit 44. The MPC block is configured to provide respective voltage waveforms, for the phase arms 10A-C and the energy transfer arm, or arms, 10D-F, to the second output stage 49B and third output stage 49C.

A multi-level chain-link converter 4, a controller 40 and a method 100 of controlling the converter 4 have been described in embodiments. The converter 4 comprises phase arms connectable to a respective phase A, B, C of a power system 1. The converter4 further comprises an energy transfer circuit 11B that inter-connects the phase arms. The energy transfer circuit 11B comprises at least one energy transfer arm 10D-F. The controller 40 is configured to control the converter 4 in accordance with the method. The method includes obtaining 110 a voltage reference for a time step to be controlled, and selecting 130 aswitching state that meet the voltage reference. Especially, the method LU107106 comprises performing 120 model predictive control, MPC, over a predictive horizon comprising a plurality of time steps. The performing 120 of MPC includes satisfying 122 the voltage reference for each phase A, B, C in each time step, and satisfying 124 an arm energy criterion over the predictive horizon. The selecting 130 includes selecting a switching state that satisfies 24) the arm energy criterion for each phase arm 10A-C and each energy transfer arm 10D-F.

The energy criterion constrains the electrical energy to remain unchanged over the total predictive horizon in order to counteract power system unbalance, without causing the converter 4 to become unbalanced.

The invention is however not restricted to the embodiments described, but may be varied by a person skilled in the art within the scope of the claims.

Claims

1. Method (100) of controlling a multi-level chain-link converter (4), the multi-level chain-link converter (4) comprising a first (10A), second (10B) and a third (10C) phase arm, each phase arm (10A-C) being connected at a respective first end (17) to a respective phase (A, B, C) of a multi-phase power system (1), the multi-level chain-link converter (4) further comprising an energy transfer circuit (11B) that inter-connects the respective second ends (18) of the phase arms, and which energy transfer circuit (11B) comprises at least one energy transfer arm (10D-F), wherein the at least one energy transfer arm (10D-F) is selectively connectable to the phase arms (10A-C) in order to contribute to an output voltage at the phases (A, B, C), the method includes - obtaining (110) a voltage reference for a time step to be controlled, - selecting (130) a switching state for the time step that meet the voltage reference, characterized in - performing (120) model predictive control, MPC, over a predictive horizon comprising a plurality of time steps in order to evaluate different voltage references of each phase arm (10A-C) and evaluate voltage references of each energy transfer arm (10D-F), which performing (120) of MPC includes - satisfying 122 the voltage reference for each phase (A, B, C) in each time step, and - satisfying 124 an arm energy criterion for each phase arm (10A-C), and for each energy transfer arm (10D-F), over the predictive horizon, which energy criterion allows the electrical energy in each arm (10A-F) to vary between time steps but constrains the electrical energy in the arm (10A-F) to remain unchanged over the total predictive horizon, and wherein the selecting (130) of a switching state includes selecting a switching state that satisfy (122) thevoltage reference for each phase (A, B, C) and satisfy (124) the arm energy LU101106 criterion for each phase arm (10A-C) and each energy transfer arm (10D-F).

2. Method (100) according to claim 1, wherein the selecting (130) of switching state is performed in accordance with a modulation algorithm (132) and a voltage balancing algorithm (134).

3. Method (100) according to claim 1 or 2, wherein the energy transfer circuit (11B) comprises three energy transfer arms (10D-F) arranged in a delta circuit, wherein the delta circuit is connected at each of its three corners to a respective one of the phase arms (10A-C).

4. Method (100) according to any of claims 1 to 3, wherein each phase arm (10A-C) and each energy transfer arm (10D-F) comprises a plurality of switching cells (9) comprising respective energy storing elements (14), and switches (12) for selective connection of the energy storage elements (14) to contribute to the output voltage.

5. Method (100) according to any of claims 1 to 4, wherein a fundamental period of the power system (1) is used as the predictive horizon.

6. Method (100) according to any of claims 1 to 4, wherein an integer multiple of the fundamental period of the power system (1) is used as the predictive horizon, said integer being 2, 3, or higher.

7. Method (100) according to any of claims 1 to 6, further including determining (116) sequence components of the voltage reference and using the sequence components when performing (120) the MPC, especially when satisfying (124) the arm energy criteria.

8. A controller (40) for a multi-level chain-link converter (4) comprising a first (10A), second (10B) and a third (10C) phase arm, each phase arm (10A-C)

being configured to be connected at a respective first end (17) to a respective LU101106 phase (A, B, C) of a multi-phase power system (1), said multi-level chain-link converter (4) further comprising an energy transfer circuit (11B) that inter- connects the respective second ends (18) of the phase arms, and which energy transfer circuit (11B) comprises at least one energy transfer arm (10D-F), each phase arm (10A-C) and each energy transfer arm (10D-F) comprising a plurality of switching cells (9) comprising respective energy storing elements (14), and switches (12) for selective connection of the energy storage elements (14) to provide an output voltage, said controller (40) including - an input stage (41 — 44) for obtaining (110) a voltage reference, - an MPC block (48) for performing (120) MPC, and - output stages (49A-C) for selecting (130) a switching state, said controller being configured to perform the method of any of claims 1-7.

9. A multi-level chain-link converter (4) comprising a first (10A), second (10B) and a third (10C) phase arm, each phase arm (10A-C) being configured to be connected at a respective first end (17) to a respective phase (A, B, C) of a multi-phase power system (1), said multi-level chain-link converter (4) further comprising an energy transfer circuit (11B) that inter-connects the respective second ends (18) of the phase arms, and which energy transfer circuit (11B) comprises at least one energy transfer arm (10D-F), wherein each phase arm (10A-C) and each energy transfer arm (10D-F) comprises a plurality of switching cells (9), each switching cell (9) comprising respective energy storing elements (14), and switches (12) for selective connection of the energy storage elements (14) to provide an output voltage, said multi-level chain-link converter (4) further comprising a controller (40) according to claim 8 for controlling the multi-level chain-link converter (4).

10. Method (100B) of designing a multi-level chain-link converter (4), the multi-level chain-link converter (4) comprising a first (10A), second (10B) and a third (10C) phase arm, each phase arm (10A-C) being connectable at arespective first end (17) to a respective phase (A, B, C) of a multi-phase power LU107106 system (1), the multi-level chain-link converter (4) further comprising an energy transfer circuit (11B) that inter-connects the respective second ends (18) of the phase arms, and which energy transfer circuit (11B) comprises at least one energy transfer arm (10D-F), wherein the at least one energy transfer arm (10D-F) is selectively connectable to the phase arms (10A-C) in order to contribute to an output voltage at the phases (A, B, C),

the method of designing a multi-level chain-link converter (4), being characterized in comprising:

- obtaining (110) a voltage reference for a time step to be controlled during operation of the multi-level chain.link converter (4), - selecting (150) a design for the time step that meet the voltage reference,

- performing (120) model predictive control, MPC, over a predictive horizon comprising a plurality of time steps in order to evaluate different voltage references of each phase arm (10A-C) and evaluate voltage references of each energy transfer arm (10D-F), which performing (120) of MPC includes - satisfying 122 the voltage reference for each phase (A, B, C) in each timestep, and - satisfying 124 an arm energy criterion for each phase arm (10A-C), and for each energy transfer arm (10D-F), over the predictive horizon, which energy criterion allows the electrical energy in each arm (10A-F) to vary between time steps but constrains the electrical energy in the arm (10A-F) to remainunchanged over the total predictive horizon, and wherein the selecting (150) of a design includes selecting a design topology that satisfy (122) the voltage reference for each phase (A, B, C) and satisfy (124) the arm energy criterion for each phase arm (10A-C) and each energy transfer arm (10D-F).

Lo. . Un . LU101106

11. Method (100) of designing a multi-level chain-link converter (4), according to claim 10, wherein the selecting (150) a design comprises evaluating (127) a cost function for the design topology over the predictive horizon.

12. Method (100) of designing a multi-level chain-link converter (4), according to claim 11, wherein - evaluating (127) the cost function for the topology includes evaluating number of voltage levels, the number of energy storage elements 14, or the number of semiconductor switches 12, of the multi-level chain-link converter (4), and -the selecting (150) of a design includes selecting the least costly design according to the cost function.

13. Method (100) of designing a multi-level chain-link converter (4), according to any of claims 10-12, wherein the energy transfer circuit (11B) comprises three energy transfer arms (10D-F) arranged in a delta circuit having a respective corner connected to each one of the phase arms (10A-C).

14. Method (100) of designing a muiti-level chain-link converter (4), according to any of claims 10-13, wherein the topology of said multi.level chain.link converter (4) comprises a plurality of H-bridge switching cells (9) arranged in series in each phase arm (10A-C) and in each energy transfer arm (10D-F).