Method, Apparatus and System for regulating energy balancing of MMC for Power Oscillation Damping controlling under unbalanced condition

Abstract

The present invention relates to a modular multi-level converter energy balance control method, apparatus, and system for POD control in unbalanced situations, and more specifically, to a modular multi-level converter energy balance control method, apparatus, and system for POD control in unbalanced situations capable of performing reactive power compensation while simultaneously performing POD control in unbalanced fault situations. The present invention discloses a method for compensating reactive power while performing POD control in an unbalanced situation of a power system, wherein the method comprises: a control unit comprising an unbalanced situation recognition step for recognizing an unbalanced situation of the power system; a POD control step for performing POD control considering the state of the power system; and a reactive power compensation step for performing reactive power compensation by calculating a control value for reactive power compensation for the power system, wherein the control unit performs reactive power compensation simultaneously while performing the POD control.

Description

Technology Field

[0001] The present invention relates to a modular multilevel converter energy balance control method, apparatus, and system for POD control in unbalanced situations, and more specifically, to a modular multilevel converter energy balance control method, apparatus, and system for POD control in unbalanced situations capable of performing POD control and reactive power compensation simultaneously in an unbalanced fault situation of a power system. Background Technology

[0002] Recently, High Voltage Direct Current (HVDC) transmission systems are widely used to improve power control and stability in large-scale power grids.

[0003] High Voltage Direct Current (HVDC) transmission refers to a transmission method in which a substation converts alternating current (AC) power produced at a power plant into direct current (DC) for transmission, and then converts it back to AC at a receiving station to supply power. It offers advantages such as long-distance transmission, asynchronous grid interconnection, and the ability to use submarine cables.

[0004] Meanwhile, although there are various types of voltage converters used in High Voltage Direct Current (HVDC) transmission systems, Modular Multi-Level Converters (MMCs) have recently been receiving significant attention.

[0005] A Modular Multi-Level Converter (MMC) is a device that converts DC power into AC power using multiple sub-modules, and operates by controlling each sub-module into charging, discharging, and bypass states.

[0006] However, in the past, when an unbalanced fault occurred in a high-voltage direct current (HVDC) transmission system based on a modular multi-level converter (MMC), the response was typically to supply active power while performing Balanced Positive Sequence Control (BPSC) or Power Oscillation Damping (POD) control. Consequently, it was difficult to supply reactive power simultaneously with POD control in unbalanced situations, which could lead to a problem where the voltage of the modular multi-level converter (MMC) capacitors became unbalanced by phase.

[0007] More specifically, Figure 1 illustrates a case where an unbalanced situation is induced due to a grid fault in the power system. In this case, POD control is performed and the oscillation of active power can be eliminated. When only active power is supplied in the grid fault situation, the power (Idc) of each phase has the same value.

[0008] However, when reactive power is supplied to the grid, the power supplied by each phase may become unbalanced, which can lead to an imbalance in the DC current (Idc). Thus, when reactive power is supplied to the grid while performing POD control, a problem of phase power imbalance may occur.

[0009] Accordingly, in order to resolve the problems of the conventional technology described above, there is a need for a method to stably balance the DC current (Idc) inside the modular multilevel converter (MMC) and the voltage of the sub-module capacitor while simultaneously supplying reactive power and POD control in a modular multilevel converter-based high-voltage DC transmission (MMC-HVDC) system under unbalanced power system conditions; however, no appropriate alternative has yet been presented for this purpose. Prior art literature

[0010] Korean Published Patent Application No. 10-2017-0079758 (July 10, 2017) The problem to be solved

[0011] The present invention was devised to solve the problems of the prior art described above, and aims to provide a modular multilevel converter energy balance control method, apparatus, and system for POD control in unbalanced situations, which enables the DC current (Idc) inside the modular multilevel converter (MMC) and the voltage of the sub-module capacitor to be stably balanced while simultaneously performing POD control and reactive power supply in a modular multilevel converter-based high-voltage DC transmission (MMC-HVDC) system in unbalanced situations of the power system.

[0012] Other detailed objectives of the present invention will be self-evident and understandable to experts or researchers in the art through the specific details described below. means of solving the problem

[0013] A method for compensating reactive power while performing POD control in an unbalanced situation according to one aspect of the present invention for solving the above problem is a method for compensating reactive power while performing POD control in an unbalanced situation of a power system, comprising: a control unit comprising an unbalanced situation recognition step of recognizing an unbalanced situation of the power system; a POD control step of performing POD control considering the state of the power system; and a reactive power compensation step of performing reactive power compensation by calculating a control value for reactive power compensation for the power system, wherein the control unit performs reactive power compensation simultaneously while performing the POD control.

[0014] At this time, in the reactive power compensation step, the control unit can compensate for reactive power such that one or more of the DC current inside the modular multilevel converter (MMC) or the voltage of the sub-module capacitor equipped in the modular multilevel converter-based high-voltage DC transmission (MMC-HVDC) system are balanced within a predetermined range.

[0015] Here, the reactive power compensation step may include a step of calculating a modulation index for phases A, B, and C by considering the POD control and reactive power compensation.

[0016] Additionally, the reactive power compensation step may include a step of calculating a zero sequence value such that one or more of the DC current inside the modular multi-level converter (MMC) or the voltage of the sub-module capacitor are balanced within a predetermined range.

[0017] Furthermore, the reactive power compensation step may include a phase selection step for selecting a phase to inject the zero sequence by considering the modulation index of the A, B, and C phases.

[0018] Furthermore, in the phase selection step, a signal corresponding to the zero sequence can be injected into a phase within an allowable range by considering the magnitude of the modulation index of the A, B, and C phases so that over-modulation does not occur.

[0019] In addition, a control device for compensating reactive power while performing POD control in an unbalanced situation according to one embodiment of the present invention is a control device for compensating reactive power while performing POD control in an unbalanced situation of a power system, comprising: an unbalanced situation recognition module that recognizes the unbalanced situation of the power system; a POD control module that performs POD control considering the state of the power system; and a reactive power compensation module that calculates a control value for reactive power compensation for the power system and performs reactive power compensation; wherein the control device is characterized by performing the reactive power compensation simultaneously while performing the POD control.

[0020] At this time, the reactive power compensation module can compensate for reactive power such that one or more of the DC current inside the modular multilevel converter (MMC) or the voltage of the sub-module capacitor equipped in the modular multilevel converter-based high-voltage DC transmission (MMC-HVDC) system are balanced within a predetermined range. Effects of the invention

[0021] Accordingly, in the modular multilevel converter energy balance control method, apparatus, and system for POD control in an unbalanced situation according to one embodiment of the present invention, reactive power supply is performed simultaneously with POD control in a modular multilevel converter-based high-voltage DC transmission (MMC-HVDC) system in an unbalanced situation of a power system, thereby enabling the DC current (Idc) inside the modular multilevel converter (MMC) and the voltage of the sub-module capacitor to be stably balanced.

[0022] In addition, in the modular multilevel converter energy balance control method, apparatus, and system for POD control in an unbalanced situation according to one embodiment of the present invention, grid coordination control is possible for a large-capacity transmission facility based on a modular multilevel converter (MMC) connected to a power grid.

[0023] In addition, in the method, device, and system for controlling energy balance of a modular multilevel converter for POD control in an unbalanced situation according to one embodiment of the present invention, the dynamic state inside the modular multilevel converter (MMC) is improved to prevent deterioration and damage of the device.

[0024] In addition, in the modular multilevel converter energy balance control method, device, and system for POD control in unbalanced situations according to one embodiment of the present invention, the operational portfolio of a power system operator can be effectively expanded. Brief explanation of the drawing

[0025] The accompanying drawings, which are included as part of the detailed description to aid in understanding the present invention, provide embodiments of the present invention and explain the technical concept of the present invention together with the detailed description. FIG. 1 is a diagram illustrating POD control in an unbalanced situation according to the prior art. FIG. 2 is a diagram illustrating the configuration of a conventional modular multi-level converter-based high-voltage DC transmission system (10). FIG. 3 is a diagram illustrating the configuration of a conventional modular multi-level converter (11). FIG. 4 is a flowchart of a method for compensating reactive power while performing POD control in an unbalanced situation according to one embodiment of the present invention. FIG. 5 is a detailed flowchart of a reactive power compensation step in a method for compensating reactive power while performing POD control in an unbalanced situation according to one embodiment of the present invention. FIGS. 6a to 6d are drawings illustrating the operation of a method for compensating reactive power while performing POD control in an unbalanced situation according to an embodiment of the present invention. FIG. 7 is a diagram illustrating a control algorithm for a high-voltage DC transmission system based on a modular multi-level converter according to an embodiment of the present invention. FIG. 8 is a block diagram of a control device that compensates for reactive power while performing POD control in an unbalanced situation according to one embodiment of the present invention. Specific details for implementing the invention

[0026] The present invention is capable of various modifications and may have various embodiments. Therefore, specific embodiments will be described in detail below based on the attached drawings.

[0027] The following examples are provided to facilitate a comprehensive understanding of the methods, devices, and / or systems described herein. However, they are merely illustrative and the invention is not limited thereto.

[0028] In describing the embodiments of the present invention, if it is determined that a detailed description of known technology related to the present invention may unnecessarily obscure the essence of the present invention, such detailed description will be omitted. Furthermore, the terms described below are defined in consideration of their functions within the present invention, and these may vary depending on the intentions or practices of the user or operator. Therefore, such definitions should be based on the content throughout this specification. The terms used in the detailed description are intended merely to describe the embodiments of the present invention and should not be limiting in any way. Unless explicitly stated otherwise, expressions in the singular form include the meaning of the plural form. In this description, expressions such as "include" or "comprise" are intended to refer to certain characteristics, numbers, steps, actions, elements, parts thereof, or combinations thereof, and should not be interpreted to exclude the existence or possibility of one or more other characteristics, numbers, steps, actions, elements, parts thereof, or combinations thereof other than those described.

[0029] Additionally, terms such as first, second, etc., may be used to describe various components, but said components are not limited by said terms, and said terms are used only for the purpose of distinguishing one component from another.

[0030] Below, exemplary embodiments of a modular multilevel converter energy balance control method, apparatus, and system for POD control in an unbalanced situation according to one embodiment of the present invention will be described in turn with reference to the attached drawings.

[0031] In a modular multilevel converter energy balance control method, apparatus, and system for POD control in an unbalanced situation according to one embodiment of the present invention, reactive power supply is performed simultaneously with POD control in a modular multilevel converter-based high-voltage DC transmission (MMC-HVDC) system (10) in an unbalanced situation of a power system, thereby enabling the DC current (Idc) inside the modular multilevel converter (MMC) (11) and the voltage of the sub-module capacitor to be stably balanced.

[0032] More specifically, FIG. 1 illustrates a standard Modular Multilevel Converter (MMC)-based High Voltage Direct Current (HVDC) system (10). As can be seen in FIG. 1, in the Modular Multilevel Converter-based High Voltage Direct Current (MMC-HVDC) system (10), AC power supplied through a transformer (13) connected to a power grid (12) is converted into DC power by a first Modular Multilevel Converter (11a) and transmitted through a DC cable (14), and then converted back into AC power by a second Modular Multilevel Converter (11b) and supplied back to the power grid (12). Accordingly, AC power is converted into DC power for transmission and then converted back into AC power to supply power, thereby providing advantages such as long-distance transmission, asynchronous grid connection, and the ability to use submarine cables.

[0033] Additionally, FIG. 2 illustrates a configuration diagram of the modular multi-level converter (MMC) (11). As can be seen in FIG. 2, the modular multi-level converter (MMC) (11) may be configured to include a plurality of sub-modules (SM) (11c). At this time, the modular multi-level converter (MMC) (11) converts DC power into AC power or converts DC power into AC power while controlling each of the plurality of sub-modules (SM) (11) to a charging, discharging, or bypass state.

[0034] However, in the past, when power oscillation damping (POD) control was performed and reactive power supply was simultaneously performed in a modular multi-level converter (MMC)-based high-voltage direct current transmission (HVDC) system (10) in the event of an unbalanced fault, problems such as the voltage of the capacitors of the modular multi-level converter (MMC) (100) becoming unbalanced by phase could occur.

[0035] In this regard, in the method, apparatus, and system for controlling energy balance of a modular multilevel converter for POD control in an unbalanced situation according to one embodiment of the present invention, in an unbalanced situation of a power system (12), the reactive power supply is performed simultaneously with POD control in a modular multilevel converter-based high-voltage DC transmission (MMC-HVDC) system (10), and control is made so that the DC current (Idc) inside the modular multilevel converter (11) and the voltage of the sub-module (11c) capacitor can be stably balanced.

[0036] More specifically, FIG. 4 illustrates a flowchart of a method for compensating reactive power while performing POD control in an unbalanced situation according to an embodiment of the present invention. As can be seen in FIG. 4, the method for compensating reactive power while performing POD control in an unbalanced situation according to an embodiment of the present invention comprises, in the method for compensating reactive power while performing POD control in an unbalanced situation of a power system (12), an unbalanced situation recognition step (S110) in which a control unit (100) recognizes an unbalanced situation of the power system (12), a POD control step (S120) in which POD control is performed considering the state of the power system (12), and a reactive power compensation step (S130) in which a control value for reactive power compensation for the power system (12) is calculated and reactive power compensation is performed, wherein the control unit (100) performs reactive power compensation simultaneously while performing the POD control.

[0037] At this time, in the reactive power compensation step (S130), the control unit (100) can compensate for reactive power such that one or more of the DC current inside the modular multilevel converter (11) or the voltage of the sub-module (11c) capacitor in the modular multilevel converter-based high-voltage DC transmission (MMC-HVDC) system (10) are balanced within a predetermined range.

[0038] Furthermore, as can be seen in FIG. 5, the reactive power compensation step (S130) may include a step (S131) ​​of calculating a modulation index for phases A, B, and C in consideration of the POD control and reactive power compensation.

[0039] Additionally, the reactive power compensation step (S130) may include a step (S132) of calculating a zero sequence value such that one or more of the DC current inside the modular multilevel converter (11) or the voltage of the sub-module (11c) capacitor are balanced within a predetermined range.

[0040] Additionally, the reactive power compensation step (S130) may include a phase selection step (S133) for selecting a phase to inject the zero sequence by considering the modulation index of the A, B, and C phases.

[0041] At this time, in the phase selection step (S133), a signal corresponding to the zero sequence can be injected into a phase within an allowable range so that over-modulation does not occur, by considering the magnitude of the modulation index of the A, B, and C phases.

[0042] Furthermore, the reactive power compensation step (S130) may also include a step (S134) of simultaneously performing the POD control and the reactive power compensation.

[0043] Accordingly, in the method, apparatus, and system for controlling energy balance of a modular multilevel converter for POD control in an unbalanced situation according to one embodiment of the present invention, in an unbalanced situation of a power system, it is possible to control the DC current (Idc) inside the modular multilevel converter (11) and the voltage of the sub-module (11c) capacitor so that they can be stably balanced while simultaneously performing POD control and reactive power supply in a modular multilevel converter-based high-voltage DC transmission (MMC-HVDC) system (10).

[0044] In this regard, FIGS. 6a to 6d illustrate drawings explaining the operation of a modular multilevel converter energy balance control method, apparatus, and system for POD control in an unbalanced situation according to one embodiment of the present invention.

[0045] More specifically, referring to FIGS. 6a through 6d, in step A (A in FIG. 6c), phase A is ground faulted (voltage reduction rate 75%), and in step B (B in FIG. 6c), the reactive power reference is changed from 0 [MVAR] to 100 [MVAR]. Subsequently, in step C (C in FIG. 6c), Asat(wt+theta) is injected into VrefB (Ib_de_zero in FIG. 6a and 6b).

[0046] At this time, as can be seen in FIG. 6d, in the modular multilevel converter energy balance control method, apparatus, and system for POD control in an unbalanced situation according to one embodiment of the present invention, it can be confirmed that each leg voltage operates within the normal range through balance control for Idc.

[0047] In addition, Figure 7 illustrates a control algorithm for a high-voltage DC transmission system based on a modular multi-level converter according to one embodiment of the present invention.

[0048] More specifically, the modular multi-level converter internal controller (MMC Internal Controller) (210) is equipped with a pulse width modulation (PWM) module (211) and a balancing algorithm (212) to perform circulating current control (213) and arm voltage control (214).

[0049] In addition, the inner control loop (220) is equipped with a current limiting block (221) and a zero-sequence determination and injection algorithm (222) to perform power oscillation damping control (POD) (223) or balanced positive-sequence control (BPSC) (224).

[0050] Additionally, the outer control loop (230) performs reactive power control (231), active power control (232), or DC link voltage control (223).

[0051] Finally, the Data Collection & Extraction unit (240) is equipped with a DQ Transformation unit (241), a DSOGI PLL and PNSC (242), and a Data Acquisition Station (243) to collect and convert data required in a high-voltage DC transmission system control system based on a modular multi-level converter according to one embodiment of the present invention.

[0052] In addition, a computer program according to another aspect of the present invention is characterized as being a computer program stored on a computer-readable medium to execute each step of a method for compensating reactive power while performing POD control in the aforementioned unbalanced situation on a computer. The computer program may include not only a computer program containing machine code generated by a compiler, but also a computer program containing high-level language code that can be executed on a computer using an interpreter, etc. In this case, the computer is not limited to personal computers (PCs) or notebook computers, but includes any information processing device equipped with a central processing unit (CPU) capable of executing a computer program, such as a server, smartphone, tablet PC, PDA, or mobile phone.

[0053] The above medium may continuously store a program executable by a computer, or temporarily store it for execution or download. Additionally, the medium may be various recording or storage means in the form of a single or multiple hardware components, and is not limited to a medium directly connected to a computer system but may exist distributed over a network. Examples of the medium may include magnetic media such as hard disks, floppy disks, and magnetic tapes; optical recording media such as CD-ROMs and DVDs; magneto-optical media such as floptical disks; and media configured to store program instructions, including ROM, RAM, and flash memory. Furthermore, other examples of the medium may include recording or storage media managed by app stores that distribute applications or sites and servers that supply or distribute various other software.

[0054] In addition, FIG. 8 illustrates a block diagram of a control device (100) that compensates for reactive power while performing POD control in an unbalanced situation according to one embodiment of the present invention.

[0055] The above control device (100) may be configured to be included within the above modular multi-level converter-based high-voltage DC transmission system (10), but the present invention is not necessarily limited thereto.

[0056] The above control device (100) can be easily implemented by referring to the modular multi-level converter energy balance control method, device, and system for POD control in an unbalanced situation according to one embodiment of the present invention described above. Therefore, detailed descriptions that are redundant are omitted below, and the core configuration is described in detail.

[0057] At this time, as can be seen in FIG. 8, a control device (100) that compensates for reactive power while performing POD control in an unbalanced situation according to one embodiment of the present invention may be configured to include an unbalanced situation recognition module (110), a POD control module (120), and a reactive power compensation module (130).

[0058] At this time, the above-mentioned unbalanced situation recognition module (110) recognizes the unbalanced situation of the above-mentioned power system (12).

[0059] In addition, the POD control module (120) performs POD control by taking into account the state of the power system (12).

[0060] In addition, the reactive power compensation module (130) calculates a control value for reactive power compensation for the power system (12) and performs reactive power compensation.

[0061] At this time, the control device (100) is characterized by performing the reactive power compensation while simultaneously performing the POD control.

[0062] Furthermore, the reactive power compensation module (120) can compensate for reactive power such that one or more of the DC current inside the modular multilevel converter (11) or the voltage of the sub-module (11c) capacitor in the modular multilevel converter-based high-voltage DC transmission (MMC-HVDC) system (10) are balanced within a predetermined range.

[0063] Accordingly, in the method, apparatus, and system for controlling energy balance of a modular multilevel converter for POD control in an unbalanced situation according to one embodiment of the present invention, in an unbalanced situation of a power system (12), reactive power supply is performed simultaneously with POD control in a modular multilevel converter-based high voltage DC transmission (MMC-HVDC) system (10), and the DC current (Idc) inside the modular multilevel converter (11) and the voltage of the sub-module (11c) capacitor can be controlled to stably balance.

[0064] In addition, in the method, device, and system for controlling energy balance of a modular multi-level converter for POD control in an unbalanced situation according to one embodiment of the present invention, grid coordination control is enabled for a large-capacity transmission facility based on a modular multi-level converter (MMC) connected to a power grid, and the deterioration and damage of the equipment can be prevented by improving the dynamic state inside the modular multi-level converter (MMC), and furthermore, the operational portfolio of the power grid operator can be effectively expanded.

[0065] The foregoing description is merely an illustrative explanation of the technical concept of the present invention, and those skilled in the art to which the present invention pertains will be able to make various modifications and variations within the scope of the essential characteristics of the present invention. Accordingly, the embodiments described in the present invention are intended to explain, not limit, the technical concept of the present invention, and are not limited to such embodiments. The scope of protection of the present invention shall be interpreted by the claims below, and all technical concepts within an equivalent scope shall be interpreted as being included within the scope of rights of the present invention. Explanation of the symbols

[0066] 10: Modular Multilevel Converter-Based High-Voltage DC Transmission System 11, 11a, 11b: Modular multilevel converter 11c : Submodule 12: Power System 13: Transformer 14 : DC cable 100 : Control unit (control device) 110: Unbalanced Situation Recognition Module 120: POD control module 130: Reactive Power Compensation Module

Claims

Claim 1 A method for compensating reactive power while performing POD control in an unbalanced power system in a modular multilevel converter-high voltage DC transmission (MMC-HVDC) system configured based on a modular multilevel converter (MMC) that converts DC power into AC power by having multiple sub-modules, wherein the control unit comprises: an unbalanced situation recognition step for recognizing the unbalanced situation of the power system; and a POD control step for performing POD control considering the state of the power system. A method for compensating reactive power while performing POD control in an unbalanced situation, comprising: a reactive power compensation step for calculating a control value for reactive power compensation for the power system and performing reactive power compensation; wherein the control unit performs reactive power compensation while performing POD control simultaneously; wherein in the reactive power compensation step, the control unit compensates by supplying reactive power so that one or more of the DC current inside the modular multi-level converter (MMC) or the voltage of the sub-module capacitor are balanced within a predetermined range; and wherein the control unit selects a phase to which a zero sequence is to be injected based on the magnitude of the modulation index of phases A, B, and C calculated by simultaneously considering the POD control and reactive power compensation, and injects a signal corresponding to the zero sequence into a phase within an allowable range so that over-modulation does not occur. Claim 2 delete Claim 3 A method for compensating reactive power while performing POD control in an unbalanced situation, wherein, in claim 1, the reactive power compensation step comprises the step of calculating a modulation index for phases A, B, and C in consideration of the POD control and reactive power compensation. Claim 4 A method for compensating reactive power while performing POD control in an unbalanced situation, characterized in that, in paragraph 3, the reactive power compensation step comprises a step of calculating a zero sequence value such that one or more of the DC current inside the modular multi-level converter (MMC) or the voltage of the sub-module capacitor are balanced within a predetermined range. Claim 5 delete Claim 6 delete Claim 7 A control device for compensating reactive power while performing POD control in an unbalanced power system in a modular multilevel converter-high voltage DC transmission (MMC-HVDC) system configured based on a modular multilevel converter (MMC) that converts DC power into AC power by having multiple sub-modules, comprising: an unbalanced situation recognition module that recognizes the unbalanced situation of the power system; and a POD control module that performs POD control considering the state of the power system. A control device for compensating reactive power while performing POD control in an unbalanced situation, comprising: a reactive power compensation module that calculates a control value for reactive power compensation for the power system and performs reactive power compensation; wherein the control device performs reactive power compensation while simultaneously performing POD control, and the reactive power compensation module compensates by supplying reactive power so that one or more of the DC current inside the modular multi-level converter (MMC) or the voltage of the sub-module capacitor are balanced within a predetermined range, and selects a phase to inject a zero sequence based on the magnitude of the modulation index of phases A, B, and C calculated by simultaneously considering the POD control and reactive power compensation, and injects a signal corresponding to the zero sequence into a phase within an allowable range so that over-modulation does not occur. Claim 8 delete

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