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

Abstract

A control system for controlling parallel connected power converters determines, for each power converter, a frequency droop value (Δf droop_n ) and a frequency reference (f ref_n The data processing system (101-103) is configured to change the frequency reference (f) of each power converter according to the frequency droop value of the power converter. The data processing system forms an arithmetic mean of the frequency droop values, forms a correction value based on the arithmetic mean of the frequency droop values, and adjusts the frequency reference (f) of each power converter according to the correction value. ref_n ) to move the arithmetic mean towards zero. Moving the arithmetic mean towards zero reduces undesirable frequency drift in the power converter.

Description

[Technical field]

[0001] The present disclosure relates generally to the control of power converters that are part of an alternating current (AC) system, such as a microgrid. More particularly, the present disclosure relates to control systems and methods for controlling power converters having alternating current voltage terminals connected together. Further, the present disclosure relates to power converters. Furthermore, the present disclosure relates to computer programs for controlling power converters having alternating current voltage terminals connected together. [Background technology]

[0002] Often, alternating current (AC) systems, such as microgrids, are provided with power converters having AC voltage terminals connected together. Each of the power converters may be supplied with a direct current (DC) energy source, for example, through a direct current voltage link intermediate the power converters. The DC energy source may include, for example, a battery, a fuel cell, and / or a genset including a rectifier and a generator driven by an internal 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 of the DC voltage link into one or more AC voltages, for example, a three-phase AC voltage.

[0003] One reason for paralleling power converters in the above manner is that it is desirable to have redundancy so that in a failure situation in which one of the power converters is inoperable, the remaining power converter can continue to power the AC load, albeit with a reduced total power. Paralleling also allows for smaller power converters and therefore a smaller energy source to be used to supply the required power to the AC load.

[0004] The parallel connected power converters create an AC voltage grid and must maintain the frequency and voltage of the AC voltage grid at their set values ​​with sufficient accuracy. Typical set values ​​are, for example, a root mean square "RMS" line voltage of 400V 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 be able to advantageously parallel multiple power converters, the power converters need to equalize the load between the power converters or balance them in another desired manner. In many cases, balancing is required not only for real power but also for reactive power.

[0005] The conventional solution to balance the power between parallel connected power converters is to use a frequency droop where the frequency reference of each power converter varies with the frequency droop value of the power converter, which depends on the power delivered by the power converter, and the frequency reference decreases when the power flow direction is outward from the AC voltage terminals of the power converter. Since the sharing of power between the power converters does not depend on the absolute value of the frequency reference but on the difference between the frequency references of the power converters, there is a risk that the frequency of the AC voltage grid created by the parallel connected power converters will drift too much from the desired nominal frequency, e.g. 50 Hz.

[0006] In another conventional solution for balancing power among parallel-connected power converters, one of the power converters acts as a master that determines the AC voltage frequency of the system, while the other converters act as slaves that control their own frequency references such that the desired power sharing is achieved. If the master fails, the entire system becomes inoperable or the system requires a mechanism to dynamically change the active master in a reliable manner. A mechanism for dynamically changing the master may therefore entail unique technical challenges. Summary of the Invention [Means for solving the problem]

[0007] The following presents a simplified summary in order to provide a basic understanding of some aspects of various invention embodiments. This summary is not an exhaustive overview of the invention. It is not intended to identify key or critical elements of the invention, nor is it intended to 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 a more detailed description of example embodiments of the invention.

[0008] According to the present invention, a novel control system is provided for controlling a power converter having alternating current voltage terminals connected together, the power converters being capable of forming, for example, an alternating current (AC) microgrid.

[0009] The control system according to the present invention is a data processing system comprising: - for each of the power converters, forming a frequency droop value based on data indicative of a power of the power converter; - modifying a frequency reference of each of the power converters by a frequency droop value of the power converter, the frequency reference determining an AC voltage frequency of the power converter, and the frequency droop value decreasing the frequency reference when the power flow direction is outward from the AC voltage terminals of the power converter; - forming an arithmetic mean of the frequency droop value of the power converter; - forming a correction value based on an arithmetic average of the frequency droop values; - changing the frequency droop value of each of the power converters by the correction value so that the arithmetic mean of the frequency droop values ​​approaches zero; The present invention includes a data processing system configured to perform the above steps.

[0010] Bringing the arithmetic mean of the frequency droop value towards zero reduces undesirable frequency drift in the AC system formed by the power converters, even when no power converter acts as a master and / or there is no higher level controller to control the frequency references of the power converters in a centralized manner.

[0011] Because the power converters can be controlled in the same manner relative to each other, redundancy can be provided such that in a failure situation in which one of the power converters becomes inoperable, the total power will be reduced but the remaining power converter or converters can continue to supply power to the AC load.

[0012] In one advantageous and non-limiting embodiment of the control system, the aforementioned data processing system is constituted by the data processors of the power converters, and the control system includes a data transfer network configured to transfer the frequency droop values ​​of each power converter from the data processor of the power converter under consideration to one or more data processors of the other power converter or converters.

[0013] As used herein, the term "electrical power" refers to real power associated with AC systems, i.e., power as opposed to reactive or apparent power. The term "electrical power" is used because it is applicable in association with both AC and direct current "DC" systems.

[0014] According to the present invention, there is provided a novel power system, comprising: - power converters having alternating voltage terminals connected to each other, each of the power converters including a converter stage configured to form one or more alternating voltages, a driver stage configured to control the converter stage to form the one or more alternating voltages according to a frequency reference representative of a frequency of the one or more alternating voltages, and a line filter between the converter stage and the alternating voltage terminals; a control system according to the invention, configured to determine a frequency reference for a power converter; A new power system including:

[0015] According to the present invention, there is provided a novel power converter, comprising: - a converter stage configured to generate one or more alternating voltages; - a driver stage configured to control the converter stage to form one or more alternating voltages according to a frequency reference representative of a frequency of the one or more alternating voltages; a data processor configured to generate a local frequency droop value based on data indicative of a power of the power converter and to modify a frequency reference by the local frequency droop value, the local frequency droop value decreasing the frequency reference when a direction of power flow maintained by the one or more AC voltages is outward from the converter stage; and The data processor includes: - receiving one or more other frequency droop values ​​from a data transport network; - transmitting the local frequency droop value to a data transport network; forming an arithmetic mean of the local frequency droop value and one or more other received frequency droop values; - forming a correction value based on an arithmetic mean; - The local frequency droop value is changed by the correction value to move the arithmetic mean closer to zero. A novel power converter configured to:

[0016] The power converter according to an exemplary and non-limiting embodiment further includes a line filter connected to the converter stage. However, the line filter may also be part of an electrical system to which the power converter can be connected. The term "line filter" is intended to be interpreted broadly, whereby, for example, stray inductances on the primary and secondary sides of a transformer may constitute a line filter.

[0017] According to the invention there is also provided a new method for controlling a power converter having alternating voltage terminals connected together, the method according to the invention comprising the steps of: - for each of the power converters, forming a frequency droop value based on data indicative of a power of the power converter; - modifying a frequency reference of each of the power converters by a frequency droop value of the power converter, the frequency reference determining an AC voltage frequency of the power converter, and the frequency droop value decreasing the frequency reference when the power flow direction is outward from the AC voltage terminals of the power converter; - forming an arithmetic mean of the frequency droop value of the power converter; - forming a correction value based on an arithmetic average of the frequency droop values; - changing the frequency droop value of each of the power converters by the correction value so that the arithmetic mean of the frequency droop values ​​approaches zero; Includes.

[0018] According to the invention there is also provided a new computer program for controlling a power converter having alternating voltage terminals connected together. The computer program according to the invention comprises computer executable instructions for controlling a programmable data processing system to: - for each of the power converters, forming a frequency droop value based on data indicative of a power of the power converter; - modifying a frequency reference of each of the power converters by a frequency droop value of the power converter, the frequency reference determining an AC voltage frequency of the power converter, and the frequency droop value decreasing the frequency reference when the power flow direction is outward from the AC voltage terminals of the power converter; - forming an arithmetic mean of the frequency droop value of the power converter; - forming a correction value based on an arithmetic average of the frequency droop values; - changing the frequency droop value of each of the power converters by the correction value so that the arithmetic mean of the frequency droop values ​​approaches zero; The method includes computer-executable instructions for performing the steps of the method.

[0019] According to the present invention there is also provided a novel computer program product, comprising a non-volatile computer readable medium, for example a compact disc "CD", encoded with a computer program according to the present invention.

[0020] Exemplary and non-limiting embodiments are set forth in the accompanying dependent claims.

[0021] Exemplary and non-limiting embodiments, both as to structure and method of operation, together with additional objects and advantages, will be best understood from the following description of specific exemplary and non-limiting embodiments when read in conjunction with the accompanying drawings.

[0022] The verbs "comprise" and "include" are used herein as open limitations and do not exclude or require the presence of unrecited features.

[0023] The features recited in the dependent claims are mutually freely combinable, unless expressly stated otherwise.

[0024] Furthermore, it is to be understood that the use of "a" or "an", i.e. the singular, does not exclude a plurality throughout this specification.

[0025] Exemplary and non-limiting embodiments and their advantages are explained in more detail below, by way of example, with reference to the accompanying drawings, in which: [Brief description of the drawings]

[0026] [Figure 1a] 1 illustrates a power system including a control system according to one exemplary and non-limiting embodiment for controlling a power converter of the power system. [Figure 1b] 1 illustrates a functional block diagram of a portion of a control system configured to control an nth one of the power converters of the power system illustrated in FIG. 1a. [Diagram 2]1 illustrates a power system including a control system according to one exemplary and non-limiting embodiment for controlling a power converter of the power system. [Diagram 3] 1 shows a flowchart of a method for controlling a power converter having alternating voltage terminals connected together, according to an exemplary and non-limiting embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] 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 this description are not all-inclusive unless expressly stated otherwise.

[0028] FIG. 1a illustrates a power system according to an exemplary and non-limiting embodiment. The power system includes power converters with alternating current (AC) voltage terminals connected to each other. In FIG. 1a, three of the power converters are indicated with reference numbers 104, 105, and 106. In this exemplary case, the power converters are each 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 solar panel system, and / or a generator set including a rectifier and a generator driven by an internal combustion engine, a wind turbine, a hydroelectric turbine, or any other prime mover. For example, if the DC energy source under consideration is a battery system that is charged by power provided by one or more other DC energy sources of the power system, one or more of the power converters may be capable of transferring power from the alternating current (AC) voltage terminals to the respective DC energy source.

[0029] 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_N1a , the number of phases of the AC load 112 may be less than three or more than three, and thus the number of phases of the converter stages of the power converter may be less than three or more than three. The converter stages of the power converter 104 are indicated by reference numeral 108 in FIG. 1a . Each converter stage of the power converter may be, for example, an inverter bridge. The inverter bridge may be implemented using controllable semiconductor components, such as, for example, insulated gate bipolar transistors “IGBT” or gate turn-off “GTO” thyristors, and possibly using diodes in anti-parallel with the controllable semiconductor components.

[0030] In the exemplary power system illustrated in FIG. 1a, the power converters each include a line filter between a converter stage and the AC voltage terminals of the power converter. 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.

[0031] The power converter uses the respective frequency reference f ref_1 , ...or f ref_N According to the respective reference voltage U ref_1 , ...or U ref_N Each of the power converters 104 includes a driver stage configured to control the converter stages to form a three-phase AC voltage according to. The driver stage of the power converter 104 is denoted by reference numeral 109 in FIG. 1a. For example, the three-phase AC voltage can be generated by forming a reference phase voltage and performing a pulse width modulation "PWM" according to the reference phase voltage. The PWM results in phase voltages that constitute the three-phase AC voltage. There are other techniques for forming a three-phase AC voltage using PWM, such as space vector modulation.

[0032] The power system includes a control system according to an exemplary and non-limiting embodiment. The control system controls a frequency reference f of the power converter. ref_1 , ..., fref_N The data processing system includes a data processing system configured to determine: In this exemplary case, the data processing system is constituted by 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 connected to each other via a data transfer network 107, which may be, for example, a Fieldbus.

[0033] FIG. 1b shows the frequency reference f of a power converter 105, which is the nth one of the power converters of the power system. ref_n 1 illustrates a functional block diagram of a portion of a control system that determines the frequency reference f for the other power converters. The functionality within block 113 may be implemented using the data processor 102. Without limiting generality, the following discussion may be limited to the data processor 102 only, since the frequency reference f for the other power converters may be limited to the data processor 102. ref_1 , ..., f ref_n-1 , f ref_n+1 , ..., f ref_N This is because the data processors of other power converters can be configured to implement the same functionality for determining

[0034] The data processor 102 calculates the actual power p of the power converter 105. act_n Based on the data showing the frequency droop value Δf droop_n The actual power p act_n is, for example, the DC current I supplied to the converter stage of the power converter 105. DC_n and DC voltage U DC_n Based on p act_n =U DC_n ×I DC_n The data processor 102 can compute the frequency droop value Δf droop_n By the frequency reference f ref_n, so that the frequency droop value varies with the frequency reference f when the power flow direction is outward from the AC voltage terminals of the power converter 105, i.e., when the power flow direction is from the power converter 105 to the AC load 112. ref_n Correspondingly, the frequency droop value Δf droop_n is the frequency reference f when the power flow direction is opposite. ref_n Increase the frequency reference f ref_n is the frequency droop value Δf droop_n Dependence on the power p act_n When the AC voltage frequency of the power converter 105 is greater than the operating frequency of the AC load 112, this frequency difference increases the power angle between the power converter 105 and the AC load 112, so that the power p act_n In this example situation, the droop feedback described above reduces the AC voltage frequency of the power converter 105, which in turn reduces the power p act_n Accordingly, when the AC voltage frequency of the power converter is less than the operating frequency of the AC load 112, this frequency difference reduces the power angle mentioned above, so that the power p act_n In this example situation, the droop feedback described above increases the AC voltage frequency of the power converter 105, which in turn decreases the power p act_n Increase the

[0035] The data processor 102 transmits the frequency droop value Δf of the other power converter via the data transfer network 107. droop_1 , ..., Δf droop_n-1 , Δf droop_n+1 , ..., Δf droop_N Further, the data processor 102 transmits the frequency droop value Δf to other power converters via the data transfer network 107. droop_n The data processor 102 includes a second data interface TX configured to transmit its own frequency droop value Δf droop_nThe arithmetic mean Δf of all frequency droop values ​​of the power converter, including droop_ave The data processor 102 is configured to form a correction value Δf based on the arithmetic mean of the frequency droop values. droop,corr and a correction value Δf droop,corr The frequency droop value Δf of each power converter is calculated by droop_n Change the arithmetic mean of the frequency droop value Δf droop_ave The arithmetic mean Δf is configured to approach zero. droop_ave Moving ω toward zero reduces undesirable frequency drift of the AC load 112 without any power converter acting as a master and / or without a high level controller to control the frequency references of the power converters in a centralized manner.

[0036] In one exemplary and non-limiting embodiment of the control system, the data processor 102 receives a frequency reference f ref_n is configured to determine according to the following formula: Δf droop_ave =Σ i=1...N Δf droop_i / N (1), Δf droop,corr =k f,corr ∫(Δf droop_ave )dt (2), Δf droop_n =k droop p act_n -Δf droop,corr (3), and f ref_n =f0-Δf droop_n (4) where N is the number of power converters and k f,corr is the correction coefficient, k droop is the power p act_n According to the frequency standard f ref_n is the droop coefficient for changing the frequency of the power converter, and f0 is the base value of the frequency reference of the power converter, for example 50 Hz. droop The correction coefficient k may be in the range of 0.01 to 0.1, for example, 0.04. f,corr can be, for example, in the range of 0.01 to 0.1, for example 0.05.

[0037] In one exemplary and non-limiting embodiment of the control system, the data processor 102 determines at least a predetermined lower limit Δf droop,corr_min and at most a given upper limit Δf droop,corr,max So, the correction value Δf droop,corr In FIG. 1b, the correction value Δf droop,corr The limit of is depicted in limiter block 114. The data processor 102 is advantageously configured to implement an anti-windup function. This anti-windup function reduces the correction value Δf droop,corr is the upper limit Δf droop,corr,max Or the lower limit Δf mentioned above droop,corr_min In response to the situation reaching the arithmetic mean Δf droop_ave This prevents the absolute value of the time integral of from increasing.

[0038] In the exemplary control system illustrated in FIG. 1a, the data processor of the power converter controls the reference voltage U ref_1 , ..., U ref_N The reference voltage can be determined, for example, based on a given voltage setpoint, for example an RMS line voltage of 400V, and the desired behavior of reactive powers Q_1, ..., Q_N delivered to the AC load 112 by the power converter. ref_1 , ..., U ref_N can be determined using any suitable method known in the art, for example. The present invention is not limited to any particular method of determining the reference voltage.

[0039] The implementation of each of the data processors 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.

[0040] The exemplary control system illustrated in FIG. 1a has a distributed architecture such that the data processing system is implemented with the data processors of the power converters, such as 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 having alternating current voltage terminals connected to each other. In FIG. 2, three of the power converters are labeled with reference numerals 204, 205, and 206. In this exemplary case, the power converters are each configured to transfer power from a direct current "DC" energy source to an alternating current "AC" load. The control system controls the frequency reference f of the power converters. ref_1 , ..., f ref_N In this exemplary case, the data processing system includes a data processing system configured to determine a frequency reference f ref_1 , ..., f ref_N The control system has a centralized architecture to include a data processor 201 configured to determine a frequency reference f ref_1 , ..., f ref_N and transfers power p act_1 , ..., p act_NThe data processor 201 includes a data transfer network 207 configured to transfer data indicative of the frequency reference f, for n=1,...,N, according to equations 1-4 above, for example. ref_1 , ..., f ref_N The method may be configured to determine:

[0041] 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, 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, data processor 201 may include one or more memory circuits, each of which may be, for example, a random access memory "RAM" circuit.

[0042] 3 shows a flow chart of a method for controlling a power converter having alternating voltage terminals connected to each other according to an exemplary and non-limiting embodiment. The method comprises the following actions: Action 301: for each of the power converters, forming a frequency droop value based on data indicative of the power of the power converter; action 302: modifying a frequency reference of each of the power converters, the frequency reference determining the AC voltage frequency of the power converter, by a frequency droop value of the power converter, the frequency droop value decreasing the frequency reference when the power flow direction is outward from the AC voltage terminals of the power converter; Action 303: forming an arithmetic mean of the frequency droop value of the power converter; Action 304: forming a correction value based on an arithmetic average of the frequency droop values; Action 305: modifying the frequency droop value of each of the power converters by the correction value so that the arithmetic mean of the frequency droop values ​​approaches zero; Includes.

[0043] In a method according to one exemplary and non-limiting embodiment, the frequency reference for each of the power converters is formed according to the following formula, for n=1, . . . , number N of power converters: f droop_ave =Σ i=1...N Δf droop_i / N, Δf droop,corr =k f,corr ∫(Δf droop_ave )dt, Δf droop_n =k droop p act_n -Δf droop,corr , and f ref_n =f0-Δf droop_n , In the formula, Δf droop_ave is the arithmetic mean of the frequency droop value of the power converter, and Δf droop_n is the frequency droop value of the nth power converter, and Δf droop,corr is the correction value, and k f,corr is the correction coefficient, and p act_n is the power of the nth power converter, and k droop is the droop factor for changing the frequency reference according to the power, and f ref_n is the frequency reference of the nth one of the power converters, and f0 is the base value of the frequency reference of the power converter.

[0044] According to one exemplary and non-limiting embodiment, the method includes determining at least a predetermined lower limit Δf of the correction value. droop,corr_min and at most a predetermined upper limit Δf of the correction value droop,corr,max So, the correction value Δf droop,corr This includes limiting

[0045] According to one exemplary and non-limiting embodiment, the method includes: droop,corr is the upper limit Δf droop,corr,max Or the lower limit Δf mentioned above droop,corr_min In response to the situation reaching the arithmetic mean Δf droop_aveThis includes preventing the absolute value of the time integral of from increasing.

[0046] 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.

[0047] A computer program according to an exemplary and non-limiting embodiment includes a software module for controlling a power converter having alternating voltage terminals connected together, the software module being computer-executable instructions for controlling a programmable data processing system to: - for each of the power converters, the power p act,n Based on the data showing the frequency droop value Δf droop_n Forming - frequency reference f of each of the power converters ref_n modifying a frequency reference that determines an AC voltage frequency of the power converter by a frequency droop value of the power converter, the frequency droop value decreasing the frequency reference when the power flow direction is outward from the AC voltage terminals of the power converter; - The arithmetic mean Δf of the frequency droop value of the power converter droop_ave Forming - Correction value Δf based on the arithmetic mean of the frequency droop values droop,corr and - The frequency droop value Δf of each of the power converters by the correction value droop_n to move the arithmetic mean of the frequency droop values ​​towards zero.

[0048] A software module may be, for example, a subroutine or function implemented using programming tools suitable for a programmable data processing system.

[0049] 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 one exemplary embodiment of the present invention.

[0050] A signal, according to one exemplary and non-limiting embodiment, is encoded with information that defines a computer program, according to one exemplary embodiment of the present invention.

[0051] 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 groupings of examples provided in the above description are not exhaustive unless expressly stated otherwise.

Claims

1. A control system for controlling power converters (101-106, 201-206) whose AC voltage terminals are connected to each other, comprising: for each of the power converters, the power (p ａｃｔ，ｎ ) data (U ＤＣ＿ｎ , I ＤＣ＿ｎ ) based on the frequency droop value (Δf ｄｒｏｏｐ＿ｎ ) and - a frequency reference (f ｒｅｆ＿ｎ ) according to the frequency droop value of the power converter, wherein the frequency reference determines an AC voltage frequency of the power converter, and the frequency droop value decreases when a power flow direction is outward from the AC voltage terminals of the power converter. the arithmetic mean of the frequency droop values ​​of the power converter (Δf ｄｒｏｏｐ＿ａｖｅ ) and - a correction value (Δf ｄｒｏｏｐ，ｃｏｒｒ ) and - the correction value reduces the frequency droop value (Δf ｄｒｏｏｐ＿ｎ ) to cause the arithmetic mean of the frequency droop value to move towards zero.

2. The data processing system comprises: f ｄｒｏｏｐ＿ａｖｅ =Σ ｉ＝１．．．Ｎ Δf ｄｒｏｏｐ＿ｉ / N、 Δf ｄｒｏｏｐ，ｃｏｒｒ =k ｆ，ｃｏｒｒ ∫(Δf ｄｒｏｏｐ＿ａｖｅ )dt、 Δf ｄｒｏｏｐ＿ｎ = k ｄｒｏｏｐ p ａｃｔ＿ｎ −Δf ｄｒｏｏｐ，ｃｏｒｒ 、及びf ｒｅｆ＿ｎ = f ０ −Δf ｄｒｏｏｐ＿ｎ wherein Δf ｄｒｏｏｐ＿ａｖｅ is the arithmetic mean of the frequency droop value of the power converter, and Δf ｄｒｏｏｐ＿ｎ is the frequency droop value of the nth one of the power converters, N is the number of the power converters, and Δf ｄｒｏｏｐ，ｃｏｒｒ is the correction value, and k ｆ，ｃｏｒｒ is a correction coefficient, and p ａｃｔ＿ｎ is the power of the nth one of the power converters, and k ｄｒｏｏｐ is a droop factor for modifying the frequency reference according to the power, and f ｒｅｆ＿ｎ is the frequency reference of the nth one of the power converters, and f ０ 2. The control system of claim 1, wherein: is a base value of the frequency reference for the power converter.

3. 3. The control system of claim 1, wherein the data processing system includes data processors (101-103) each configured to control one of the power converters, and each of the power converters is controlled by one of the data processors, and the control system includes a data transfer network (107) configured to transfer the frequency droop value of each of the power converters 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.

4. 3. The control system of claim 1, wherein 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) configured to transfer the frequency reference from the data processor to the power converter and to transfer the data indicative of the power from the power converter to the data processor.

5. The data processing system adjusts the correction value (Δf) so as to be at least a predetermined lower limit of the correction value and at most a predetermined upper limit of the correction value. ｄｒｏｏｐ，ｃｏｒｒ 3. The control system of claim 1, configured to limit the

6. 1. An electric power system comprising: power converters (104-106, 204-206) whose alternating voltage terminals are connected to one another, each of said power converters comprising a converter stage (108) adapted to generate one or more alternating voltages and a frequency reference (f) for controlling said converter stage and representing the frequency of said one or more alternating voltages; ｒｅｆ＿ｎ a power converter (104-106, 204-206) including a driver stage (109) configured to form the one or more AC voltages according to A control system according to claim 1 or 2, configured to determine the frequency reference of the power converter; Electric power system including.

7. A power converter (104), a converter stage (108) configured to generate one or more alternating voltages; - controlling said converter stage to provide a frequency reference (f ｒｅｆ＿１ a driver stage (109) configured to generate the one or more AC voltages according to - the power of the power converter (p ａｃｔ，１ ) data (U ＤＣ＿１ , I ＤＣ＿１ ) based on the local frequency droop value (Δf ｄｒｏｏｐ＿１ ), and the local frequency droop value determines the frequency reference (f ｒｅｆ＿１ a data processor (101) configured to modify a local frequency droop value, the local frequency droop value decreasing the frequency reference when a direction of power flow maintained by the one or more AC voltages is outward from the converter stage; In a power converter (104) including: - one or more other frequency droop values ​​(Δf ｄｒｏｏｐ＿２ , . . . , Δf ｄｒｏｏｐ＿Ｎ ) and - transmitting the local frequency droop value (Δf ｄｒｏｏｐ＿１ ) and the arithmetic mean (Δf ｄｒｏｏｐ＿ａｖｅ ) and - a correction value (Δf ｄｒｏｏｐ，ｃｏｒｒ ) by said correction value; and ｄｒｏｏｐ＿１ ) to bring the arithmetic mean closer to zero; A power converter (104) configured to:

8. 1. A method for controlling a power converter having alternating current voltage terminals connected together, comprising: for each of the power converters, the power (p ａｃｔ，ｎ ) data (U ＤＣ＿ｎ , I ＤＣ＿ｎ ) based on the frequency droop value (Δf ｄｒｏｏｐ＿ｎ ) (301); - the frequency reference (f ｒｅｆ＿ｎ ) by the frequency droop value of the power converter, wherein the frequency reference determines the AC voltage frequency of the power converter, and the frequency droop value decreases the frequency reference when power flow direction is outward from the AC voltage terminals of the power converter. In a method comprising: the arithmetic mean of the frequency droop values ​​of the power converter (Δf ｄｒｏｏｐ＿ａｖｅ ) (303); and - a correction value (Δf ｄｒｏｏｐ，ｃｏｒｒ ) (304); - the correction value reduces the frequency droop value (Δf ｄｒｏｏｐ＿ｎ ) to move the arithmetic mean of the frequency droop values ​​towards zero (305); The method further comprising:

9. The frequency reference for each of the power converters is calculated using the following formula: f ｄｒｏｏｐ＿ａｖｅ =Σ ｉ＝１．．．Ｎ Δf ｄｒｏｏｐ＿ｉ / N、 Δf ｄｒｏｏｐ，ｃｏｒｒ =k ｆ，ｃｏｒｒ ∫(Δf ｄｒｏｏｐ＿ａｖｅ )dt、 Δf ｄｒｏｏｐ＿ｎ = k ｄｒｏｏｐ p ａｃｔ＿ｎ −Δf ｄｒｏｏｐ，ｃｏｒｒ 、and f ｒｅｆ＿ｎ = f ０ −Δf ｄｒｏｏｐ＿ｎ where Δf ｄｒｏｏｐ＿ａｖｅ is the arithmetic mean of the frequency droop value of the power converter, and Δf ｄｒｏｏｐ＿ｎ is the frequency droop value of the nth one of the power converters, N is the number of the power converters, and Δf ｄｒｏｏｐ，ｃｏｒｒ is the correction value, and k ｆ，ｃｏｒｒ is a correction coefficient, and p ａｃｔ＿ｎ is the power of the nth one of the power converters, and k ｄｒｏｏｐ is a droop factor for modifying the frequency reference according to the power, and f ｒｅｆ＿ｎ is the frequency reference of the nth one of the power converters, and f ０ 9. The method of claim 8, wherein: is a base value of the frequency reference of the power converter.

10. 1. A computer program for controlling a power converter having alternating current voltage terminals connected to one another, the computer program comprising: for each of the power converters, the power (p ａｃｔ，ｎ ) data (U ＤＣ＿ｎ , I ＤＣ＿ｎ ) based on the frequency droop value (Δf ｄｒｏｏｐ＿ｎ ) and - the frequency reference (f ｒｅｆ＿ｎ a frequency droop value of the power converter, the frequency reference determining an AC voltage frequency of the power converter, and the frequency droop value decreasing the frequency reference when power flow direction is outward from the AC voltage terminals of the power converter; the arithmetic mean of the frequency droop values ​​of the power converter (Δf ｄｒｏｏｐ＿ａｖｅ ) and - a correction value (Δf ｄｒｏｏｐ，ｃｏｒｒ ) and - the correction value reduces the frequency droop value (Δf ｄｒｏｏｐ＿ｎ ) to cause the arithmetic mean of the frequency droop values ​​to move towards zero.

11. 11. A non-volatile computer readable medium encoded with the computer program of claim 10.

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