TECHNICAL FIELD

The present disclosure generally relates to controlling a modular multilevel converter, and more particularly to a part of said controlling involving the selection of cells of the converter.

BACKGROUND

Modular multilevel converters (MMCs) are frequently employed as medium to high power converters owing to their low losses, low harmonics and their modularity. Typical for the MMC is the legs each having several, often a large number of, cells, where each cell can be considered as a switchable voltage source having two states, either being inserted, contributing with an amount of voltage to the total voltage of the leg, or being bypassed and thereby providing no contribution. Advanced Pulse Width Modulation (PWM) techniques and cell sorting methods have been provided to achieve very low switching frequencies, which are close to the fundamental frequency, to reduce converter switching losses without compromising on harmonic and cell voltage balancing performances. The cell sorting methods are used to select which cells to switch to be inserted or bypassed and when to perform the switching. The cell sorting methods are designed to achieve a cell voltage balancing at a rated current operation. For instance, in a STATCOM (Static synchronic compensator) application typically for 70%-90% of the time the converter is in a no load state and thereby its current is <10% of its rated, or nominal, current.

For instance, WO2014/082655 discloses a cell selection method based on both a reference voltage and a measured current through the cells and, in particular, detecting a zero crossing of the current and acting on it. When the current has a low magnitude the measurements of the current may become a source of error since temporary changes of the polarity of the current due to presence of undesired harmonics may be given an unproportional impact on the cell selection method. SUMMARY

In view of the foreoing, a concern of the present invention is how to improve the cell selection method at low currents.

To address at least this concern, in a first aspect of the present invention, there is provided a method of controlling an MMC comprising legs, wherein each leg comprises several cells connected in series, the method comprising controlling a group voltage of a group of cells constituting at least some of the cells of a leg; determining a reference group voltage; determining a reference group current; and detecting a present group current. The operation of controlling a group voltage comprises selecting cells within the group of cells to be switched in order to adapt the group voltage to the reference group voltage. The operation of selecting cells comprises choosing a group current as one or the other of the reference group current and the present group current. The operation of choosing a group current comprises comparing the magnitude of the reference group current with a threshold value, and if a magnitude of the reference group current is smaller than the threshold value then choosing the reference group current, else choosing the present group current. The operation of selecting cells further comprises estimating a polarity of the chosen group current. By choosing the reference group current instead of the actual current at low current magnitudes more reliable current values may be obtained and used for the cell selection.

The threshold value may be a fraction of the magnitude of a nominal group current.

The operation of estimating a polarity of the chosen group current may comprise generating samples of the chosen group current within a predetermined time period of the chosen group current, determining a sum of the samples, and determining the polarity on basis of the sum.

The operation of generating samples may comprise real time sampling the chosen current and storing the samples.

The operation of generating samples may comprise predicting future samples of the chosen current. Thereby, a further improvement of the accuracy of the control of the group voltage may be achieved. The operation of estimating a current polarity may comprise one of inverting a polarity of each sample before the sum is determined and inverting a polarity of the sum.

The operation of selecting cells may comprise detecting cell voltages for the cells in the group of cells, determining whether at least one cell is to be inserted or bypassed, and if it is determined that at least one cell is to be inserted or bypassed, then selecting the at least one cell in the group of cells on basis of the cell voltages and the estimated current polarity.

The operation of selecting cells may comprise, for each cell, estimating a polarity of a cell voltage and using the polarity of the cell voltage in combination with the polarity of the group current to detemine whether the cell should be inserted or bypassed.

According to another aspect of the present disclosure, there is provided a control system for controlling a modular multilevel converter comprising legs, where each leg comprises several cells connected in series, each cell being switchable between an inserted state where it provides a cell voltage contributing to a total voltage of the leg and a bypassed state where it does not contribute to the total voltage of the leg, and wherein each leg comprises a group of cells constituting at least some of the cells of the leg, the control system further comprising: a main controller configured to detect a present group current and a present group voltage, and configured to determine a reference group voltage and a reference group current; and a cell selection device connected with the main controller and configured to select cells to be switched in order to adapt the group voltage to the reference group voltage, the cell selection device comprising a polarity estimator, wherein the polarity estimator, in turn, comprises comprises a comparator configured to determine the magnitude of the reference group current with a threshold value and, if the magnitude of the reference group current is smaller than the threshold value then choose the reference group current, else choose the present group current group, and wherein the polarity estimator is configured to estimate a current polarity on basis of the chosen group current.

The polarity estimator may comprise an array generator connected with the comparator, and configured to generate an array of samples of the chosen current within a predetermined time period of the chosen current, an adder connected with the array generator and configured to determine a sum of at least some of the samples of thearray, and a polarity generator configured to determine the current polarity on basis of the sum. The polarity estimator may comprise a polarity inverter configured to perform one of inverting a polarity of each sample before the sum is determined and inverting a polarity of the sum.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplifying embodiments of the present disclosure will be described below with reference to the accompanying drawings, in which:

Figure 1 schematically shows examples of general converter structures;

Figure 2 is a schematic block diagram of an MMC including an embodiment of the control system according to the present disclosure;

Figure 3 is a schematic block diagram of an embodiment of the control system according to the present disclosure;

Figure 4 is a schematic block diagram of a polarity estimator, which is a part of an embodiment of the control system according to the present disclosure;

Figure 5 is a schematic block diagram of an embodiment of the control system according to the present disclosure;

Figure 6 illustrates a group current; and

Figure 7 is a flowchart of an embodiment of the method of controlling an MMC according to the present disclosure.

DETAILED DESCRIPTION

The present disclosure will be described hereinafter with reference to the accompanying drawings, in which exemplifying embodiments are shown. The present disclosure should however not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this description will convey the scope of the present disclosure to those skilled in the art.

To facilitate the understanding of the figures and description some general definitions are as follows. In all embodiments described below there is an MMC and a control system comprising a main controller. There are several different MMC topologies, such as Y-connected MMCs or delta-connected MMCs, and the MMCs are used in different couplings, such as the STATCOM, conversion between AC and DC, etc. However, the MMC has a general structure comprising several groups of cells. For the purposes of this application a group of cells is defined to have a group current l _G flowing through the cells of the group, and a group voltage U _G being a total voltage across the group. The main controller controls one or more of those groups of cells, and it may control all groups of cells in the MMC or the control system comprises several main controllers controlling one or more groups of cells each.

A Y-connected MMC 100 including a control system 101 according to embodiments of the present disclosure is most schematically illustrated in Fig. 1. The MMC 100 is used as a STATCOM, and is connected to a three-phase AC structure 102 having three phase lines a, b, and c. As mentioned above, the MMC 100 can have any topology and this is just one non-limiting example thereof. The number of phases can be different as well, as understood by the person skilled in the art. The MMC 100 comprises three legs 104, 105, 106, connected, at one end thereof, to each respective phase line a, b, c of the AC structure, and interconnected at the other end thereof. Some kind of load 109 is connected to the phase lines a, b, c too downstream of the MMC 100. The MMC is arranged to balance the load by either absorbing or generating reactive power. Each leg 104, 105, 106 is typically connected to the AC structure 102 via a transformer 103. Each leg 104, 105, 106 comprises several cells 108 connected in series. In this example, all the cells 108 of a leg constitutes one group of cells 107. The cells 108 can be of any suitable type, e.g. half-bridge or full-bridge IGBT elements and many more as known to the person skilled in the art. Fig. 1 illustrates full-bridge cells. As shown, each cell 108 comprises a voltage provider, which may be, or may be symbolized by, a cell capacitor. Due to the direction of the current through the cell 108, the cell capacitor is either being charged or discharged when the cell is in its inserted state. The number of cells 108 can be from a few cells 108 to tens or hundreds of cells 108 in each leg 104-106. The control system 101 may connected to the MMC 100 at several positions in order to be able to detect voltages and currents on several levels of the MMC structure from overall converter level down to cell level and to control the cells, as schematically exemplified with broken lines in Fig. 2, and as will be further described below. The control system 101 is configured to control all legs 104-106 of the MMC 100, by controlling all the cells 108. In order to simplify this description the control of one group of cells of the MMC 100 will be described in greater detail, while there may well be more groups of cells as explained above. They will, however, be controlled in the same way.

A delta-connected MMC 200 including a control system 201 according to embodiments of the present invention is most schematically illustrated in Fig. 2. The MMC 200 converts between AC and DC and is connected to an AC structure 202 at the AC side and to a DC structure 203 at the DC side. There are many feasible AC and DC structures, such as the AC grid, power plants generating AC or DC voltages and currents, a High Voltage Direct Current (HVDC) power transmission system, etc. The MMC 200 may be a three-phase converter comprising three legs 204, 205, 206, connected in parallel. It should be noted that alternatively there may be a different number of legs, such as two legs. Each leg 204-206 comprises several cells 208 connected in series. The AC structure 202 is connected to each one of the legs 204- 206 halfways between its ends dividing the leg 204-206 in two arms 204a, 204b, 205a, 205b, 206a, 206b. The control system 201 may be connected to the MMC 200 at several positions in order to be able to detect voltages and currents on several levels of the MMC structure from overall converter level down to cell level and to control the cells, as schematically exemplified with broken lines in Fig. 2, and as will be further described below. The control system 201 is configured to control all legs 204-206 of the MMC 200, by controlling all the cells 208. The control is divided into control of groups of cells 207, where all cells 208 in an arm constitute one group of cells 207.

According to embodiments of the invention, as shown in Fig. 3, the control system 101, 201 may comprise a main controller 110, a cell selection device 111 connected with the main controller 110, and gate units 112 connected with the cell selection device 111 and connected with the cells 108, one gate unit 112 per cell 108. The main controller 110 is a high level control device that is configured to provide one or more reference voltages and reference currents to other parts of the control system 101, 201 in order to control the MMC 100, 200 to generate the required converter output votlage. The operation of the main controller 110 is based on one or more values of voltages and currents on the converter inputs/outputs, and within the MMC 100, 200 However, since a focus of this disclosure is the control of the groups of cells 107, 207, where the actual cell selection is performed, and in order to simplify this description, the control of a single group of cells 107, 207 will be described in more detail. Therefore, the main controller 110 is illustrated to receive a group voltage and a group current, while, typically, the main controller receives group voltages UG and group currents IG for all groups 107, 207 that it controls, in conjunction with other values needed for the control, as mentioned above. However, such other values will not be exhaustively described here since the use thereof is general knowledge within the present field of technology. The cell selection device 111 may be configured to select cells 108, 208 to be switched among all the cells of the MMC 100, 200. However, due the typically large number of cells, the control system 101, 201 usually comprises several cell selection devices, where each cell selection device acts on a group of cellsl07, 207. In other words, the cell selection device 111 is configured to select cells to be switched within a group of cells 107, 207. In Fig. 2 the group of cells 207 is exemplified as all the cells of a phase arm 204a. Thus, in this example the control system 201 shown in Fig. 2 comprises six cell selection devices 111 in total, while the control system 101 shown in Fig. 1 comprises three cell selection devices 111. However, as explained above, for reasons of simplicity only one of them is indicated in the drawings. In order to provide the cell selection device 111 with control information, the main controller 110 is configured to detect a present group current IGA« through the group of cells 107, 207, and to determine a reference group voltage UcRef across the group of cells 107, 207 and a reference group current l _G Ref- The values of the currents may be transformed to vector representation, which will simplify comparisons and other calculations. The cell selection device 111 is configured to select cells 108 to be switched, either from the bypassed state to the inserted state or vice versa, in order to adapt the group voltage UG to the reference group voltage UcRef. The cell selection device 111 comprises a PWM (Pulse Width Modulation) generator 114, a cell selector 115, and a polarity estimator 116.

The PWM generator 114 is connected with the main controller 110 to receive the reference group voltage UcRef and the present group current IGA«. The PWM generation unit 114 determines how many cells 108 are to be inserted or bypassed on basis of the reference group voltage UcRef, the present state of all cells 108, the presenet group current IGA« and cell voltage Uceii(i:N) feedback from all N cells 108 of the group of cells 107, 207.

The polarity estimator 116 is connected with the main controller 110 to receive the reference group current l _G R _e f and the present group current IGA« from the main controller 110. It should be noted that, typically, the main controller 110 samples the detected group current and provides the samples as the present group current IGA« to the polarity estimator 116. It goes without saying that the operations performed by the cell selection device 111 are generally performed on sampled values. The polarity estimator 116 is configured to choose a group current IG as one or the other of the reference group current l _G R _e f and the present group current IGA«. The choice is made in dependence of a magnitude of the reference group current iGRef. More particularly, the polarity estimator 116 is configured to choose a group current IG on basis of a comparison of the magnitude of the reference group current IGREF with a threshold value T, and if a magnitude of the reference group current l _G Ref is smaller than the threshold value T then choose the reference group current l _G R _e f, else choose the present group current IGA«. Further, the polarity estimator 116 is configured to estimate the polarity of the thus chosen group current l _G and provide the cell selector 115 with the result.

The cell selector 115 is configured to select which cel l/cel Is 108, 208 within the group of cells 107, 207 is/are to be inserted or bypassed. As a basis for the selection the cell selector 115 may use information about how many cells to be inserted or bypassed received from the PWM generator 114, the individual cell voltages Uceii(i:N), and the polarity of the group current l _G received from the polarity estimator 116. Further, the cell selector 115 is configured to provide switching instructions SSceii(i:N) to the gate units 112 to perform the necessary switching actions to achieve the required Switching State (SS) of the desired cells 108, 208.

Embodiments of a method of controlling an MMC will be described in the following with reference to the MMCs 100, 200 described above. However, the method is applicable to any MMC having the necessary structure as understood from the operations being performed. Thus, as illustrated in the flowchart of Fig. 7, the method may comprise determining a reference group voltage U _G R _e f and a reference group current l _GRe f, in box 701. Further, in box 702, a present group current IGA« is detected. A group current l _G is chosen to be used for further calculations by comparing the magnitude of the reference group current IcRef with a threshold value T. If the magnitude is smaller than the threshold value T then the reference group current IcRef is chosen as the group current IG, otherwise the present group current IGA« is chosen as the group current IG, boxes 703, 704, 705. Then, in box 706, a current polarity is estimated on basis of the chosen group current l _G . The current polarity estimation is included in an operation of selecting cells 108, 208 to be switched in order to adapt the group voltage UG to the reference group voltage U _G Ref and thereby controlling the group voltage U _G , box 707.

The threshold value T may be a fraction of the magnitude of a nominal group current IcNom, such as e.g. about 10% of the nominal current. The nominal current may be as high as in the order of kA.

The estimation of the current polarity may comprise generating samples of the chosen group current IG within a predetermined time period of the chosen group current, determining a sum of the samples, and determining the current polarity on basis of the sum.

The final selection of individual cells 108 to be switched may be performed by the cell selector 115, by detecting cell voltages Uceii(i:N) for the N cells 108, 208 in the group of cells 113, determining whether at least one cell 108 is to be inserted or bypassed, and, if it is determined that at least one cell 108 is to be inserted or bypassed, then selecting the at least one cell 108 in the group of cells 113 on basis of the cell voltages Uceii(i:N) and the estimated current polarity, and sending a switching command, constituting the desired switching state SSceii(i:N) to the gate unit/units 112 associated with the chosen cel l/cel Is 108 to be switched. Thus, the cell selector 115 detects a present switching state for all cells 108 in the group of cells 113.

The operation of the cell selector 115 may be based on any known sorting algorithm to generate the final switching instructions to the gate units 112. Due to its simplicity in implementation a conventional voltage sorting algorithm is often preferred, i.e. choosing the cell 108, 208 to be switched based on the cell voltage level and current polarity, indicating charging or discharging the cell capacitor. However, by additionally using the polarity estimator 116 to determine the current polarity of the chosen group current IG the cell selection operation is improved at low group currents l _G . At low current magnitudes the harmonics present in the current could lead to an incorrect estimation of the current polarity, which, in tun, causes inaccuracy in the group voltage due to an erroneous switching. An example current waveform is presented in Figure 6 to exemplify this problem. When a cell needs to be switched at a first time instant ti indicated in Figure 6, the cell selection algorithm would determine that the current has a negative polarity and chose a cell based on that information. But in reality, for most of the next half-fundamental cycle, e.g. between the first time instant ti and a second time instant t2, the current will be in positive polarity and hence would bring an adverse effect to the cell voltage balancing. This effect is pronounced when the group current is low, such as during no load operation, since the ratio of harmonic current to fundamental current is high.

Thus, the control method proposed in this disclosure improves the cell voltage balancing at low valve current even with conventional voltage sorting algorithm without increasing its control complexity.

According to embodiments of the present control system 101, and as shown in Fig. 4, the polarity estimator 116 may comprise a comparator 117, receiving the present group current IGA« and the reference group current IcRef from the main controller 110, an array generator 118 connected with the comparator 117, an adder 119 connected with the vectorizer, a polatity inverter 120 connected with the adder 119, and a polarity generator 121 connected with the polarity inverter 120. The polarity estimator 116 may further comprise a high-pass filter 122 at the input of the comparator 117 to be passed by the group currents to remove any DC noise from the group currents IGA«, IcRef.

The polarity estimator 116 operates as follows. The comparator 117 is configured to sample the received present group current IGA« and reference group current IcRef, and compares the magnitude of the reference group current het, i.e. the peak value obtained from the vector representation of the current, with a threshold T, which constitutes a fraction of the nominal group current IcNom. The array generator 118 is configured to generate an array of samples, within a predetermined time period of the chosen group current. The time period is represented by a number of samples P denoted by IG( k-P:k). The adder 119 is configured to determine the sum of the P samples in the array IG( k-P:k), or the sum of at least some of the P samples. The polarity generator 121 is configured to determine the current polarity on basis of the sum and feed the resulting current polarity information to the cell selector 115. However, due to negative symmetry in the waveform from past, before being input to the polarity generator 121 the sum is multiplied by -1 in the polarity inverter 120, i.e. the polarity of the sum is inverted. Alternatively, the polarity inverter 120 may be arranged between the array generator 118 and the adder 119, or the adder 119 may be arranged to perform the polarity inversion, and may then be regarded as comprising a polarity inverter.

The operation of choosing which current to use as the basis for the polarity estimation improves the performance of the cell selection operation, since harmonics caused by the switching may cause inaccuracy of the polarity estimation at low currents. The reference group current has no harmonics, and thus its magnitude is more stable.

A further improvement of the polarity estimation may be obtained by predicting future sample values of the chosen group current. Therefore, as an alternative to using the last P samples of the chosen group current l _G , i.e. l _G Act or l _G Ref, an actual prediction of a number of samples Q during a future time period and generation of a corresponding array of samples l _G ( k: k+Q) may be performed. Then the sum determined by the adder 119 is based on the Q predicted future samples. The prediction may use several samples, present and previous, and predict the future samples by means of linear or higher order prediction methods known as such to the person skilled in the art. The duration of the future time period is dependent on the effective cell switching frequency. For instance, the duration may be one half-period of the period corresponding to the cell switching frequency. Another choice of the duration may be 1/4-period of the period corresponding to the cell switching frequency. The cell switching frequency may be close to the fundamental frequency, which in turn may be e.g. 50 Hz or 60 Hz.

According to embodiments of the present control system 101, the PWM generator 114 is configured to predict the cell switching states for all the cells 108 of the group 113 for said future time period, as shown in Fig. 5. The duration of the future time period depends on the effective switching frequency of the cells 108 as explained above regarding the current. More particularly, the PWM generator 114 predicts the future cell voltages Uceii(i:N)(k: k+Q), denoted as UPWM in Fig. 5. The polarity estimator 116 estimates the polarity of the cell voltage U _C eii(i:N)(k:k+Q) in addition to the polarity of the chosen group current l _G and provides both estimates Pol. (IG), Pol. (UPWM) to the cell selector 115. Thereby, the estimation of the polarity is further improved. Additionally, if a cell 108, 208 exceeds minimum or maximum cell voltage limits during no load operation, then a disturbance signal Lbst is generasted by a voltage limit detector 123 in the DC voltage control loop and input to the main controller 110 see Fig. 5, in an attempt to restore the cell voltage balance. In this case the main controller 110 comprises a DC voltage controller receiving the disturbance signal.

Although features and elements may be described above in particular combinations, each feature or element may be used alone without the other features and elements or in various combinations with or without other features and elements.

Additionally, variations to the disclosed embodiments may be understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the words "comprising" and "including" does not exclude other elements, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain features are recited in mutually different dependent claims does not indicate that a combination of these features cannot be used to advantage.