Method and control system for controlling parallel connected power converters - Patents.com

Abstract

A control system for controlling parallel-connected power converters includes a data processing system (101-103), which calculates, for each power converter, the reactive power (Q _n ) based on the data showing the reactive power index (I react_n ) and the arithmetic mean of the reactive power index of the power converter (I react_ave ) and for each of the power converters, determining the difference between the reactive power index of the power converter and the arithmetic mean (I react_n -I react_ave ) based on the imbalance index (e dm_n ) and forming an AC voltage amplitude reference (U AC,ref_n ) based on at least the unbalance index of the power converter. The use of the arithmetic mean reduces undesirable voltage drift of the power converter when balancing the reactive power of the power converter.

Description

[Technical field]

[0001] The present disclosure relates generally to the control of power converters forming part of alternating current (AC) systems, such as microgrids. More particularly, the present disclosure relates to control systems and methods for controlling power converters whose AC voltage terminals are connected to each other. Further, the present disclosure relates to power converters. Further, the present disclosure relates to computer programs for controlling power converters whose AC voltage terminals are connected to each other. [Background technology]

[0002] Often, alternating current (AC) systems, e.g., microgrids, include power converters whose AC voltage terminals are connected together. Each of the power converters may be supplied with a direct current (DC) energy source, e.g., through an intermediate direct current voltage link of the power converter. The DC energy source may include, e.g., a generating set including a battery, a fuel cell, and / or a rectifier and a generator driven by a combustion engine, a wind turbine, a hydro turbine, or some other prime mover. Each of the power converters is configured to convert the DC voltage from its DC voltage link to one or more AC voltages, e.g., three-phase AC voltages.

[0003] One reason for paralleling power converters as described above is the need for redundancy so that in a fault condition that renders one of the converters inoperable, the remaining converters can continue to deliver power to the AC loads at a reduced total power. Paralleling also allows the use of smaller power converters and smaller energy sources to supply the power required by the AC loads.

[0004] The parallel connected power converters form an AC voltage grid, whose frequency and voltage must be maintained at their set values ​​with sufficient accuracy. Typical set values ​​are, for example, a line-to-line voltage of 400V root mean square voltage “RMS” and a frequency of 50Hz. The AC loads connected to the AC voltage grid draw real and reactive power from the AC voltage grid. To allow advantageous parallel connection of several power converters, the power converters should balance their loads equally between the power converters or in another desired manner. In many cases, the need to balance is related not only to real power but also to reactive power.

[0005] In a conventional solution for balancing reactive power among parallel-connected power converters, one of the power converters acts as a master that determines the AC voltage of the AC voltage grid, while the other converter acts as a slave that controls its AC voltage reference so that the desired share of reactive power is achieved. If the master fails, the whole system will become inoperable. To prevent this, the system should have a mechanism to ensure that the master is dynamically changed during operation. Therefore, a mechanism to dynamically change the master may come with its own technical challenges. Summary of the Invention [Means for solving the problem]

[0006] The following presents a simplified summary in order to provide a basic understanding of some aspects of various embodiments of the invention. This summary is not an exhaustive overview of the invention. It does not identify key or critical elements of the invention, nor does it delineate the scope of the invention. The following summary is merely intended to present some concepts of the invention in a simplified form as a prelude to the more detailed description of example embodiments of the invention.

[0007] According to the present invention, there is provided a novel control system for controlling a power converter having alternating current voltage terminals connected together, the power converter may, for example, comprise an alternating current (AC) microgrid.

[0008] The control system according to the present invention includes a data processing system, - for each of the power converters, forming a reactive power indicator based on data indicative of the reactive power of the power converter; - forming an arithmetic mean of the reactive power indicators of the power converter; - forming, for each of the power converters, an imbalance indicator based on a difference between a reactive power indicator of that power converter and an arithmetic mean of the reactive power indicators of the power converter; - controlling an AC voltage amplitude reference of each conversion stage of the power converter based on at least an unbalance index of the power converter, each conversion stage of the power converter being connected to an AC voltage terminal of the power converter via a line filter; The present invention is configured to:

[0009] The amplitude of the real AC voltage at the parallel-connected AC voltage terminals is not substantially affected by balancing the reactive powers of the power converters, because the above-mentioned unbalance index is based on the deviation of the reactive power index from the arithmetic mean. Since the deviations from the arithmetic mean cancel each other out, the effect of the sum of the deviations on the amplitude of the real AC voltage may be small, and undesirable drift of the amplitude of the real AC voltage can be avoided.

[0010] The power converters may be controlled identically relative to each other so that in a fault condition that renders one of the power converters inoperable, there may be redundancy such that the remaining power converter or converters can continue to deliver power to the AC load at a reduced total power.

[0011] In one advantageous and non-limiting embodiment of the control system, the aforementioned data processing system is configured with a data processor of the power converter, and the control system includes a data transfer network configured to transfer the reactive power indicators of each of the power converters from the data processor of the target power converter to one or more data processors of one or more other power converters.

[0012] In accordance with the present invention, there is also provided a novel power system, comprising: - power converters, whose AC voltage terminals are connected to each other, each of the power converters including a conversion stage configured to form one or more AC voltages, a driver stage configured to control the conversion stage to form the one or more AC voltages according to an AC voltage amplitude reference representative of the amplitude of the one or more AC voltages formed by the conversion stage, and a line filter between the conversion stage and the AC voltage terminals; a control system according to the invention, configured to determine an AC voltage amplitude reference for a power converter; Includes.

[0013] According to the present invention there is also provided a novel power converter, comprising: a conversion stage configured to generate one or more alternating voltages; - a line filter between the converter stage of the power converter and the alternating voltage terminals; - a driver stage configured to control the conversion stage to form one or more AC voltages according to an AC voltage amplitude reference representative of an amplitude of the one or more AC voltages; - a data processor configured to generate a local reactive power indicator based on data indicative of reactive power supplied by the power converter to an electrical system external to the power converter; The data processor includes: - receiving one or more other reactive power indicators from a data transfer network; - transmitting the local reactive power indicators to a data transfer network; - forming an arithmetic average of the local reactive power indicator and one or more other received reactive power indicators; - forming an imbalance indicator based on a difference between the local reactive power indicator and an arithmetic average; controlling an AC voltage amplitude reference of the conversion stage based on at least the imbalance indicator; The present invention is configured to:

[0014] The power converter according to an exemplary and non-limiting embodiment further includes a line filter connected to the conversion stage. However, the line filter may also be part of the electrical system to which the power converter is connectable. The term "line filter" should be understood broadly, such that, for example, the primary and secondary stray inductances of a transformer may constitute a line filter.

[0015] According to the invention there is also provided a novel method of controlling a power converter, the alternating voltage terminals of which are connected together. The method according to the invention comprises the steps of: - for each of the power converters, forming a reactive power indicator based on data indicative of the reactive power of the power converter; - forming an arithmetic mean of the reactive power indicators of the power converter; - forming, for each of the power converters, an imbalance indicator based on a difference between a reactive power indicator of that power converter and an arithmetic mean of the reactive power indicators of the power converter; - controlling an AC voltage amplitude reference of each conversion stage of the power converter based on at least an unbalance index of the power converter, each conversion stage of the power converter being connected to an AC voltage terminal of the power converter via a line filter; Includes.

[0016] According to the invention there is also provided a novel computer program for controlling a power converter, the alternating voltage terminals of which are connected together. - for each of the power converters, forming a reactive power indicator based on data indicative of the reactive power of the power converter; - forming an arithmetic mean of the reactive power indicators of the power converter; - forming, for each of the power converters, an imbalance indicator based on a difference between a reactive power indicator of that power converter and an arithmetic mean of the reactive power indicators of the power converter; - controlling an AC voltage amplitude reference of each conversion stage of the power converter based on at least an unbalance index of the power converter, each conversion stage of the power converter being connected to an AC voltage terminal of the power converter via a line filter; The method includes the steps of: (a) providing a programmable data processing system having a programmable data processing system;

[0017] According to the invention there is also provided a novel computer program product comprising a non-volatile computer readable medium, such as a compact disc "CD", encoded with a computer program according to the invention.

[0018] The accompanying dependent claims set forth exemplary and non-limiting embodiments.

[0019] The exemplary and non-limiting embodiments, both as to structure and method of operation, together with further objects and advantages thereof, will be best understood from the following description of specific exemplary and non-limiting embodiments, read in conjunction with the accompanying drawings, in which:

[0020] In this specification, the verbs "comprise" and "include" are used as open-ended limitations that do not exclude or require the presence of unrecited features.

[0021] The features recited in the dependent claims may be freely combined with one another, unless expressly stated otherwise.

[0022] Furthermore, it is to be understood that the use of "a" or "an", ie the singular, does not exclude a plurality throughout this specification.

[0023] Exemplary and non-limiting embodiments and their advantages are explained in more detail below, by way of example and with reference to the accompanying drawings, in which: [Brief description of the drawings]

[0024] [Figure 1a] 1 illustrates a power system including a control system that controls a power converter of the power system, according to one exemplary and non-limiting embodiment. [Figure 1b] 1 shows a functional block diagram of a portion of a control system configured to control one of the power converters of the power system shown in FIG. 1a. [Diagram 2] 1 illustrates a power system including a control system that controls a power converter of the power system, according to one exemplary and non-limiting embodiment. [Diagram 3] 1 illustrates a flowchart of a method for controlling a power converter having alternating voltage terminals connected to each other, according to an exemplary and non-limiting embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] The specific examples provided in the following description should not be construed as limiting the scope and / or applicability of the appended claims. The lists and groups of examples provided in the following description are not exhaustive unless expressly stated.

[0026] FIG. 1a illustrates a power system according to an exemplary and non-limiting embodiment. The power system includes power converters whose alternating current voltage terminals are connected to each other. Three of the power converters are indicated in FIG. 1a with reference numbers 104, 105, and 106. In this exemplary case, each of the power converters is configured to transfer power from a direct current "DC" energy source to an alternating current "AC" load 112. Each DC energy source may include, for example, a battery system, a fuel cell system, a photovoltaic panel system, and / or a generating set including a rectifier and a generator driven by a combustion engine, a wind turbine, a hydroelectric turbine, or some other prime mover. It may also be possible that one or more of the power converters may transfer power from their respective alternating current voltage terminals to their respective DC energy sources. This is the case, for example, when the DC energy source in question is a battery system, which is charged with power provided by one or more other DC energy sources of the power system.

[0027] Each of the power converters is connected to the DC voltage U of the DC voltage link of the power converter. DC_1 , ..., U DC_N 1a , the number of phases of the AC load 112 and therefore the number of phases of the conversion stages of the power converter may be less than three or more than three. The conversion stages of the power converter 104 are indicated in FIG. 1a with reference numeral 108. Each conversion stage of the power converter may be, for example, an inverter bridge implemented with controllable semiconductor components, such as insulated gate bipolar transistors "IGBT" or gate turn-off "GTO" thyristors, and possibly with anti-parallel diodes implemented with those controllable semiconductor components.

[0028] In the exemplary power system shown in FIG. 1a, each of the power converters includes a line filter between the conversion stage of the power converter and the AC voltage terminals. The line filter may be, for example, an inductor-capacitor-inductor "LCL" filter. The line filter of the power converter 104 is indicated by reference numeral 110.

[0029] Each of the power converters includes a driver stage configured to control the conversion stage to form a three-phase AC voltage, the control including controlling a respective alternating current (AC) voltage amplitude reference U representing the amplitude of the three-phase AC voltage. AC,ref_1 , ...or U AC,ref_N and the respective frequency standards f ref_1 , ...or f ref_N The driver stage of the power converter 104 is shown in FIG. 1a with reference number 109. The three-phase AC voltage can be generated, for example, by forming a phase voltage reference and performing a pulse width modulation "PWM" according to the phase voltage reference. As a result of the PWM, phase voltages that constitute the three-phase AC voltage are obtained. There are also other techniques for forming a three-phase AC voltage using PWM, for example space vector modulation.

[0030] The power system includes a control system according to an exemplary and non-limiting embodiment. The control system controls an AC voltage amplitude reference U of the power converter. AC,ref_1 , ..., U AC,ref_N The data processing system includes a data processing system configured to determine: In this exemplary case, the data processing system consists of data processors of the power converters. In Fig. 1a, the data processors of the power converters 104, 105 and 106 are indicated with reference numerals 101, 102 and 103. The data processors of the power converters are communicatively coupled to one another by a data transfer network 107, which may be, for example, a fieldbus.

[0031] FIG. 1b shows an AC voltage amplitude reference U of a power converter 105, which is the nth power converter of the power system. AC,ref_n 1 shows a functional block diagram of a portion of a control system that determines the AC voltage amplitude reference U of the other power converters. The functionality in block 113 may be implemented in the data processor 102. Without limiting generality, the following discussion may be limited to the data processor 102. This is only in accordance with the understanding that the data processor of the other power converters may determine the AC voltage amplitude reference U of the other power converters. AC,ref_1 , ..., U AC,ref_n-1 , U AC,ref_n+1 , ..., U AC,ref_N, as they may be configured to perform the same functionality with respect to determining

[0032] The data processor 102 calculates the reactive power Q supplied by the power converter 105. _n Based on the data showing the reactive power index I react_n The reactive power index I react_n may be directly proportional to the reactive current supplied by the power converter 105, for example. The reactive power index of each of the power converters may also be directly proportional to the reactive power supplied by the power converter. The data processor 102 may calculate the reactive power index I react_n The calculation of may be configured to be performed, for example, according to the following formula: I react_n =Im{u act_grid i AC_n *} / U act_grid (1) Here, u act_grid is the space vector of the real three-phase AC voltages at the parallel-connected AC voltage terminals of the power converter, and i AC_n * is the complex conjugate of the space vector of the AC current supplied by the power converter 105, and U act_grid is the amplitude of the real three-phase AC voltage at the parallel-connected AC voltage terminals of the power converter (i.e., U act_grid is the space vector u act_grid ), and Im{} denotes the imaginary part of a complex number.

[0033] The data processor 102 transmits the reactive power indicators I of the other power converters via the data transfer network 107. react_1 , ..., I react_n-1 , I react_n+1 , ..., I react_N The data processor 102 further includes a second data interface TX configured to transmit the reactive power indicator Ireact_n to other power converters via a data transfer network 107. The data processor 102 also includes a first data interface RX configured to receive the reactive power indicator Ireact_n from the power converter 102. react_nThe arithmetic mean of all reactive power indices of the power converter, including I react_ave The data processor 102 is configured to form a difference I between the reactive power index of the power converter 105 and the arithmetic mean of the reactive power index of the power converter. react_n -I react_ave Based on the imbalance indicator e dm_n and at least the imbalance indicator e dm_n Based on the AC voltage amplitude reference U of the conversion stage of the power converter 105 AC,ref_n and controlling the

[0034] Amplitude U of the real three-phase AC voltage act_grid is not substantially affected by balancing the reactive powers of the power converters because each unbalance index is based on the deviation of the corresponding reactive power index from the arithmetic mean of the reactive power indexes. The amplitude U of the real three-phase AC voltage is reduced because the deviations from the arithmetic mean cancel each other out. act_grid The effect of the sum of the deviations on the real AC voltage amplitude U can be small and can be measured even in the absence of a power converter acting as a master and / or a higher level controller centrally controlling the AC voltage amplitude reference of the power converters. act_grid Therefore, undesirable drift of the signal can be avoided.

[0035] In a control system according to an exemplary and non-limiting embodiment, the data processor 102 calculates an AC voltage amplitude reference U according to the following equation: AC,ref_n The method is configured to determine: I react_ave =Σ i=1...N I react_i / N (2), e dm_n =I react_n -I react_ave (3), and U AC,ref_n =U0+p(U0-αe dm_n -U act_grid )+(1 / T I )∫(U0-αe dm_n -U act_grid )dt (4) where N is the number of power converters and Ireact_ave is the arithmetic mean of the reactive power index of the power converter, and I react_n is the reactive power index of the power converter 105, and e dm_n is the unbalance index of the power converter 105, U0 is the base value of the AC voltage amplitude, e.g., the line-to-line voltage √2×400 V, and U act_grid is the amplitude of the real three-phase voltage at the AC voltage terminals of the power converter, α is the control parameter of the unbalance index, p is the proportionality coefficient of the proportional-integral “PI” controller, and T I is the integral time of the PI controller.

[0036] In the exemplary control system shown in Figure 1b, the data processor 102 is configured to limit the integral term of the PI controller to at least a predetermined lower limit I_Min and at most a predetermined upper limit I_Max. In Figure 1b, the limiting of the integral term is indicated by limiter block 114. The data processor 102 is advantageously configured to implement an anti-windup functionality that prevents the absolute value of the time integral 115 from increasing in situations where the limiter block 114 is active.

[0037] In the exemplary control system shown in FIG. 1a, the data processor of the power converter generates a frequency reference f that defines the instantaneous frequency of the three-phase AC voltage generated by the conversion stage of the power converter. ref_1 , ..., f ref_N The frequency reference may be determined, for example, by a frequency droop such that the frequency reference of each power converter varies by the frequency droop value of the power converter. The frequency droop value depends on the real power delivered by the power converter, and when the power flow direction is outward from the power converter towards the AC load 112, the frequency reference is decreased by the frequency droop value. Correspondingly, when the power flow direction is reversed, the frequency reference is increased by the frequency droop value such that the AC voltage terminals of the power converter receive real power. The frequency reference f ref_1 , ..., f ref_NIt may also be possible to determine the frequency reference in some other suitable manner. The present invention is not limited to any particular method of determining the frequency reference.

[0038] The implementation of each data processor of the power converter, e.g., data processors 101-103, may be based on one or more analog circuits, one or more digital processing circuits, or a combination thereof. Each digital processing circuit may be a programmable processor circuit provided with appropriate software, e.g., a dedicated hardware processor such as an Application Specific Integrated Circuit "ASIC" or a configurable hardware processor such as a Field Programmable Gate Array "FPGA". Furthermore, the data processor may include one or more memory circuits, each of which may be, e.g., a Random Access Memory "RAM" circuit.

[0039] The exemplary control system shown in FIG. 1a has a distributed architecture, and the data processing system is implemented by the data processors of the power converters, e.g., data processors 101-103. FIG. 2 illustrates a power system including a control system according to another exemplary and non-limiting embodiment. The power system includes power converters, the AC voltage terminals of which are connected to each other. Three of the power converters are shown in FIG. 2 with reference numbers 204, 205 and 206. In this exemplary case, each of the power converters is configured to transfer power from a direct current "DC" energy source to an alternating current "AC" load. The control system controls the AC voltage amplitude reference U of the power converters. AC,ref_1 , ..., U AC,ref_N In this exemplary case, the data processing system has a centralized architecture and the data processing system includes an AC voltage amplitude reference U for all the power converters. AC,ref_1 , ..., U AC,ref_N The control system further includes a data processor 201 configured to determine an AC voltage amplitude reference U AC,ref_1 , ..., U AC,ref_NThe data processor 201 includes a data transfer network 207 configured to transfer data indicative of the AC voltage amplitude reference U from the data processor 201 to the respective power converters and to transfer data indicative of the reactive power of the power converters from the power converters to the data processor 201. AC,ref_1 , ..., U AC,ref_N The determination may be configured to be performed, for example, such that n=1, . . . , N in the equations (1)-(4) already shown.

[0040] 2 may be based on one or more analog circuits, one or more digital processing circuits, or a combination thereof. Each digital processing circuit may be a programmable processor circuit provided with appropriate software, such as a dedicated hardware processor, e.g., an application specific integrated circuit "ASIC," or a configurable hardware processor, e.g., a field programmable gate array "FPGA." Additionally, data processor 201 may include one or more memory circuits, each of which may be, for example, a random access memory "RAM" circuit.

[0041] 3 shows a flow chart of a method for controlling a power converter, the AC voltage terminals of which are connected to each other, according to an exemplary and non-limiting embodiment. The method includes the following actions: Action 301: forming, for each of the power converters, a reactive power indicator based on data indicative of the reactive power of the power converter; Action 302: forming an arithmetic mean of reactive power indicators of the power converter; Action 303: forming, for each of the power converters, an imbalance indicator based on a difference between a reactive power indicator of the power converter and an arithmetic mean of the reactive power indicators of the power converter; and Action 304: Controlling an AC voltage amplitude reference of each conversion stage of the power converter based on at least an unbalance index of the power converter.

[0042] In a method according to an exemplary and non-limiting embodiment, the reactive power index of each of the power converters is directly proportional to the reactive current supplied by the power converter. The reactive power index of each of the power converters may also be directly proportional to the reactive power supplied by the power converter.

[0043] In a method according to one exemplary and non-limiting embodiment, the AC voltage amplitude reference of each of the power converters is decreased when the reactive power index of the power converter exceeds the arithmetic mean of the reactive power index of the power converter, and the AC voltage amplitude reference of the power converter is increased when the reactive power index of the power converter falls below the arithmetic mean of the reactive power index of the power converter.

[0044] In a method according to one exemplary and non-limiting embodiment, an AC voltage amplitude reference for each of the power converters is formed by the following formula, n=1, . . . , N, the number of the power converter: I react_ave =Σ i=1...N I react_i / N, e dm_n =I react_n -I react_ave , and U AC,ref_n =U0+p(U0-αe dm_n -U act_grid )+(1 / T I )∫(U0-αe dm_n -U act_grid )dt Here, I react_ave is the arithmetic mean of the reactive power index of the power converter, and I react_n is the reactive power index of the nth power converter, and e dm_n is the unbalance index of the nth power converter, U0 is the base value of the AC voltage amplitude, and U AC,ref_n is the AC voltage amplitude reference of the nth power converter, and U act_grid is the amplitude of the real voltage at the AC voltage terminals of the power converter, α is the control parameter of the unbalance index, p is the proportionality coefficient of the proportional-integral “PI” controller, and T I is the integral time of the PI controller.

[0045] A method according to one exemplary, non-limiting embodiment includes limiting an integral term of a PI controller to at least a predetermined lower limit I_Min and at most a predetermined upper limit I_Max.

[0046] A method according to one exemplary and non-limiting embodiment includes preventing an increase in absolute value of a time integral of a PI controller in response to a situation in which the integral term of the PI controller is limited to an upper limit I_Max as described above or a lower limit I_Min as described above.

[0047] A computer program according to one exemplary and non-limiting embodiment includes computer executable instructions for controlling a programmable data processing system to perform actions associated with a method according to any of the exemplary and non-limiting embodiments described above.

[0048] A computer program according to an exemplary and non-limiting embodiment includes a software module for controlling a power converter, the power converter having alternating voltage terminals connected to each other. The software module includes: - for each of the power converters, a reactive power index I based on data indicative of the reactive power of the power converter; react_n and forming - arithmetic mean of reactive power indicators of power converters I react_ave and forming - forming, for each of the power converters, an imbalance indicator based on a difference between a reactive power indicator of that power converter and an arithmetic mean of the reactive power indicators of the power converter; - AC voltage amplitude reference U of each conversion stage of the power converter AC,ref_n based on at least an imbalance index of the power converter. The method includes the steps of:

[0049] The software modules may be, for example, subroutines or functions implemented by programming tools suitable for a programmable data processing system.

[0050] A computer program product according to one exemplary and non-limiting embodiment includes a computer readable medium, such as a compact disc "CD", encoded with a computer program according to an exemplary embodiment of the present invention.

[0051] A signal, according to one exemplary and non-limiting embodiment, is encoded to carry information that defines a computer program, according to an exemplary embodiment of the present invention.

[0052] The specific examples provided in the above description should not be construed as limiting the scope and / or applicability of the appended claims. The lists and groups of examples provided in the above description are not exhaustive unless expressly stated.

Claims

1. A control system for controlling power converters (101-106, 201-206), the AC voltage terminals of which are connected to each other, comprising: for each of the power converters, the reactive power of the power converter (Q ＿ｎ ) based on the data indicating the reactive power index (I ｒｅａｃｔ＿ｎ ) and the arithmetic mean of the reactive power indices of the power converters (I ｒｅａｃｔ＿ａｖｅ ) and for each of the power converters, the difference between the reactive power index of the power converter and the arithmetic mean of the reactive power index of the power converter (I ｒｅａｃｔ＿ｎ -I ｒｅａｃｔ＿ａｖｅ ) based on the imbalance index (e ｄｍ＿ｎ ) and In a control system including a data processing system (101-103, 201) configured to perform - AC voltage amplitude reference (U ＡＣ，ｒｅｆ＿ｎ ) based on at least the unbalance index of the power converter, wherein the conversion stage of each of the power converters is connected to the AC voltage terminals of the power converter via a line filter (110). A control system configured to:

2. The reactive power index (I ｒｅａｃｔ＿ｎ 2. The control system of claim 1, wherein the reactive current supplied by the power converter is directly proportional to the reactive current supplied by the power converter.

3. The data processing system calculates the AC voltage amplitude reference (U ＡＣ，ｒｅｆ＿ｎ ) when the reactive power index of the power converter exceeds the arithmetic mean of the reactive power index of the power converter, and ＡＣ，ｒｅｆ＿ｎ 3. The control system of claim 1, configured to increase the reactive power index of the power converter when the reactive power index of the power converter falls below the arithmetic mean of the reactive power index of the power converter.

4. The data processing system comprises: I ｒｅａｃｔ＿ａｖｅ =Σ ｉ＝１．．．Ｎ I ｒｅａｃｔ＿ｉ / N、 e ｄｍ＿ｎ =I ｒｅａｃｔ＿ｎ -I ｒｅａｃｔ＿ａｖｅ , and U ＡＣ，ｒｅｆ＿ｎ =U ０ +p(U ０ -αe ｄｍ＿ｎ -U ａｃｔ＿ｇｒｉｄ )+(1 / T Ｉ )∫(U ０ -αe ｄｍ＿ｎ -U ａｃｔ＿ｇｒｉｄ )dt The AC voltage amplitude reference (U ＡＣ，ｒｅｆ＿ｎ ), where N is the number of power converters, and I ｒｅａｃｔ＿ａｖｅ is the arithmetic mean of the reactive power index of the power converter, and I ｒｅａｃｔ＿ｎ is the reactive power index of the nth one of the power converters, and e ｄｍ＿ｎ is the unbalance index of the nth one of the power converters, and U ０ is the base value of the AC voltage amplitude, and U ＡＣ，ｒｅｆ＿ｎ is the AC voltage amplitude reference of the nth one of the power converters, and U ａｃｔ＿ｇｒｉｄ is the amplitude of the real voltage at the AC voltage terminals of the power converter, α is a control parameter of the imbalance index, p is a proportionality coefficient of a proportional-integral controller, and T Ｉ 3. The control system of claim 1, wherein: is the integral time of the proportional-integral controller.

5. The data processing system includes data processors (101-103) each configured to control one of the power converters, each of the power converters being controlled by one of the data processors, and the control system includes a data transfer network (107), the data transfer network (107) transmitting the reactive power index (I) of each of the power converters. ｒｅａｃｔ＿ｎ 3. The control system of claim 1, configured to transfer a signal from the data processor controlling the power converter to one or more other of the data processors controlling one or more other of the power converters.

6. The data processing system includes a data processor (201) configured to control the power converter, and the control system includes a data transfer network (207), the data transfer network (207) transmitting the AC voltage amplitude reference (U ＡＣ，ｒｅｆ＿１ , . . . , U ＡＣ，ｒｅｆ＿Ｎ ) from the data processor to the power converter; and transferring the reactive power (Q ＿１ ,... ,Q ＿Ｎ and transferring the data indicative of the power converter temperature from the power converter to the data processor.

7. 1. An electric power system comprising: power converters (104 to 106, 204 to 206), whose AC voltage terminals are connected to one another, each of said power converters comprising a conversion stage (108) configured to form one or more AC voltages and an AC voltage amplitude reference (U) representing the amplitude of said one or more AC voltages formed by said conversion stage (108); ＡＣ，ｒｅｆ＿ｎ a driver stage (109) configured to control the conversion stages to form the one or more AC voltages according to a voltage level of the power converter (104-106, 204-206), and a line filter (110) between the conversion stages and the AC voltage terminals; A control system according to claim 1 or 2, configured to determine the AC voltage amplitude reference of the power converter; Electric power system including.

8. A power converter (104), a conversion stage (108) configured to form one or more alternating voltages; a line filter (110) between said conversion stage of said power converter and the AC voltage terminals; an AC voltage amplitude reference (U) representing the amplitude of said one or more AC voltages; ＡＣ，ｒｅｆ＿１ a driver stage (109) configured to control the converter stage to form the one or more AC voltages according to - reactive power (Q) supplied by the power converter to the electrical system outside the power converter ＿１ ) based on the data indicating the local reactive power index (I ｒｅａｃｔ＿ｎ a data processor (101) configured to form a Including, - one or more other reactive power indicators (I ｒｅａｃｔ＿２ ,... ,I ｒｅａｃｔ＿Ｎ ) from the data transmission network; - the local reactive power index (I ｒｅａｃｔ＿１ ) to the data transfer network; the arithmetic mean (I) of the local reactive power indicator and the one or more other reactive power indicators received; ｒｅａｃｔ＿ａｖｅ ), and the difference between the local reactive power index and the arithmetic mean (I ｒｅａｃｔ＿１ -I ｒｅａｃｔ＿ａｖｅ ) based on the imbalance index (e ｄｍ＿１ ) to form In the power converter (104), the data processor - determining whether the AC voltage amplitude reference (U) of the conversion stage is based on at least the unbalance indicator; ＡＣ，ｒｅｆ＿１ ) to control A power converter (104) configured to:

9. 1. A method of controlling a power converter, the AC voltage terminals of which are connected together, comprising: for each of the power converters, the reactive power of the power converter (Q ＿ｎ ) based on the data indicating the reactive power index (I ｒｅａｃｔ＿ｎ ) (301); the arithmetic mean of the reactive power indices of the power converters (I ｒｅａｃｔ＿ａｖｅ ) (302); for each of the power converters, the difference between the reactive power index of the power converter and the arithmetic mean of the reactive power index of the power converter (I ｒｅａｃｔ＿ｎ -I ｒｅａｃｔ＿ａｖｅ ) based on the imbalance index (e ｄｍ＿ｎ ) (303) In a method comprising: - AC voltage amplitude reference (U ＡＣ，ｒｅｆ＿ｎ ) based on at least the unbalance index of the power converter, wherein the conversion stage of each of the power converters is connected to the AC voltage terminals of the power converter via a line filter. The method further comprising:

10. The reactive power index (I ｒｅａｃｔ＿ｎ 10. The method of claim 9, wherein the reactive current supplied by the power converter is directly proportional to the reactive current supplied by the power converter.

11. The AC voltage amplitude reference (U ＡＣ，ｒｅｆ＿ｎ ) is reduced when the reactive power index of the power converter exceeds the arithmetic mean of the reactive power index of the power converter, and the AC voltage amplitude reference (U ＡＣ，ｒｅｆ＿ｎ 11. The method of claim 9 or 10, wherein the reactive power index of the power converter is increased when the reactive power index of the power converter is below the arithmetic mean of the reactive power index of the power converter.

12. The AC voltage amplitude reference (U ＡＣ，ｒｅｆ＿ｎ ) has the following formula: ｒｅａｃｔ＿ａｖｅ =Σ ｉ＝１．．．Ｎ I ｒｅａｃｔ＿ｉ / N, e ｄｍ＿ｎ =I ｒｅａｃｔ＿ｎ -I ｒｅａｃｔ＿ａｖｅ , and U ＡＣ，ｒｅｆ＿ｎ =U ０ +p(U ０ -αe ｄｍ＿ｎ -U ａｃｔ＿ｇｒｉｄ )+(1 / T Ｉ )∫(U ０ -αe ｄｍ＿ｎ -U ａｃｔ＿ｇｒｉｄ )dt where N is the number of power converters, and I ｒｅａｃｔ＿ａｖｅ is the arithmetic mean of the reactive power index of the power converter, and I ｒｅａｃｔ＿ｎ is the reactive power index of the nth one of the power converters, and e ｄｍ＿ｎ is the unbalance index of the nth one of the power converters, and U ０ is the base value of the AC voltage amplitude, and U ＡＣ，ｒｅｆ＿ｎ is the AC voltage amplitude reference of the nth one of the power converters, and U ａｃｔ＿ｇｒｉｄ is the real voltage amplitude at the AC voltage terminals of the power converter, α is a control parameter of the imbalance index, p is a proportionality coefficient of a proportional-integral controller, and T Ｉ 11. The method of claim 9 or 10, wherein is the integral time of the proportional-integral controller.

13. 1. A computer program for controlling a power converter, the power converter having alternating current voltage terminals connected to each other, the computer program comprising: for each of the power converters, the reactive power of the power converter (Q ＿ｎ ) data (u ａｃｔ＿ｇｒｉｄ , i ＡＣ＿ｎ ) based on the reactive power index (I ｒｅａｃｔ＿ｎ ) and the arithmetic mean of the reactive power indices of the power converters (I ｒｅａｃｔ＿ａｖｅ ) and for each of the power converters, the difference between the reactive power index of the power converter and the arithmetic mean of the reactive power index of the power converter (I ｒｅａｃｔ＿ｎ -I ｒｅａｃｔ＿ａｖｅ ) based on the imbalance index (e ｄｍ＿ｎ ) and 1. A computer program comprising computer-executable instructions for controlling a programmable data processing system to perform - AC voltage amplitude reference (U ＡＣ，ｒｅｆ＿ｎ ) based on at least the unbalance index of the power converter, wherein the conversion stage of each of the power converters is connected to the AC voltage terminals of the power converter via a line filter (110).

10. A computer program product comprising computer-executable instructions for controlling said programmable data processing system to perform the steps of:

14. 14. A non-volatile computer readable medium encoded with the computer program of claim 13.

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