Flexible DC converter control method and device based on island-networking mode conversion, medium and flexible DC converter

Abstract

The invention belongs to the technical field of direct-current power transmission, and particularly relates to a flexible direct-current converter control method and device based on island-networking mode conversion, a medium and a flexible direct-current converter. The method comprises the following steps: S1, acquiring a system frequency of a network side; s2, if the working mode of the flexible DC converter is an island operation mode or a networking operation mode, when the system frequency is greater than an island upper limit threshold value and less than a networking lower limit threshold value or the system frequency is greater than the networking upper limit threshold value and less than an island lower limit threshold value, keeping the working mode of the flexible DC converter unchanged; the island upper limit threshold value is smaller than the networking lower limit threshold value, the networking lower limit threshold value is smaller than the networking upper limit threshold value, and the networking upper limit threshold value is smaller than the island lower limit threshold value. According to the invention, the technical problem of poor system stability caused by frequent switching between the networking operation state and the island operation state in the prior art is solved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of direct current transmission, and particularly relates to a flexible direct current converter and a control method and device for island-network mode conversion of the flexible direct current converter and a medium. BACKGROUND

[0002] The flexible direct current transmission system has the characteristics of flexible control, rapid dynamic response and low harmonic content, can independently adjust active power and reactive power quickly, and has broad application prospects. The flexible direct current converter can be operated in parallel with the grid or can independently realize island power supply or new energy access to the grid. After the flexible direct current converter is connected to the power system, it can operate in island mode when the parallel AC line fails to maintain power supply to the load, and can be switched to network operation after the fault is recovered.

[0003] In the article "Island-grid control mode switching method of flexible direct current converter based on droop control" by Zou Changyue published in November 2022 in the South Power Grid Technology, it is proposed to add an "intermediate state" of mode switching in the island-grid switching control strategy, design a droop controller for the current inner loop based on the droop control strategy, and add a virtual impedance link to improve the dynamic response speed of the system. The proposed control structure does not depend on the phase-locked loop and the grid synchronization, and can stably operate in island and grid conditions. The controller structure does not switch during the closing process of the grid-side AC switch, thereby solving the coupling between the operation mode switching process and the position of the AC switch. However, the precondition of this method is that the AC field outgoing line of the converter station is connected to the AC grid. If the AC field outgoing line of the converter station is not connected to the AC grid, even if the AC field circuit breaker of the converter station is in the closed state, the system is still in island operation, and this method cannot be used.

[0004] The Chinese invention patent application publication with publication number CN1158000359A and publication date of March 14, 2023 proposes that when the converter state is switched from network operation to island operation, the actual value of active power after first-order smoothing filtering processing is used as the active power reference value to smoothly switch the converter control from network operation to island operation, thereby improving the reliability and stability of the flexible direct current transmission system. When the converter state is switched from island operation to network operation, the actual value of the speed of the virtual synchronous generator after first-order smoothing filtering processing is used as the speed reference value of the virtual synchronous generator to smoothly switch the converter control from island to network operation. The above-mentioned scheme can meet the needs of island or network operation without switching the flexible direct current controller when the flexible direct current is in island or network operation state, and can adapt to the two operation states. However, the precondition is that there is an external grid device such as "safe and stable device" to input "island or network operation state", and it cannot adapt to island-network conversion without relying on external device input.

[0005] The Chinese invention patent authorization announcement text with authorization announcement number CN105870967B and authorization announcement date 2020.06.16 proposes to phase-lock the remote AC voltage, obtain the phase-locked phase of the remote AC voltage; the remote AC voltage and the grid-side AC voltage are subjected to dq coordinate transformation; it is judged whether the remote AC voltage is normal, and in the case that the remote AC voltage is normal, the d and q components of the grid-side AC voltage are compared with the d and q components of the remote AC voltage respectively, and after the difference values of the d and q components are all less than the set values, the AC circuit breaker is closed, realizing the conversion from island to networking. The deficiency of the above-mentioned scheme is that the remote outgoing line AC voltage collection and transmission have uncertainty, and it is necessary to additionally increase the remote outgoing line AC voltage measuring point, which is difficult to realize in real engineering.

[0006] The Chinese invention patent application publication text with application publication number CN117353361A and application publication date 2024.01.05 discloses a control method of a flexible DC power transmission system, comprising: detecting the frequency of the flexible DC power transmission system in a networking operation mode; in the case that the frequency deviation is greater than or equal to the frequency set value, the flexible DC power transmission system is converted from the networking operation mode to the island operation mode. The above-mentioned mode determines the operation mode of the system according to the system frequency, but when the system frequency deviation fluctuates near the set value, the switching between networking and island operation states is too frequent, thereby causing poor system stability. SUMMARY

[0007] The purpose of the present application is to provide a kind of based on island-networking mode conversion's flexible converter control method, device, medium and flexible converter, to solve the technical problems that the networking-island operation state between switching too frequent in prior art, in turn, cause poor system stability.

[0008] To solve the above technical problems, the technical scheme of a kind of based on island-networking mode conversion's flexible converter control method provided by the present application is as follows: a kind of based on island-networking mode conversion's flexible converter control method, the method comprises:

[0009] S1, the system frequency of grid side is obtained;

[0010] S2, if the working mode of flexible converter is island operation mode or networking operation mode, when the system frequency is greater than the upper limit threshold of island and less than the lower limit threshold of networking or the system frequency is greater than the upper limit threshold of networking and less than the lower limit threshold of island, the working mode of flexible converter remains unchanged;

[0011] The upper limit threshold of island is less than the lower limit threshold of networking, the lower limit threshold of networking is less than the upper limit threshold of networking, and the upper limit threshold of networking is less than the lower limit threshold of island.

[0012] The beneficial effects of the above technical solutions are: the technical scheme of the VSC control method based on island-networking mode conversion belongs to an improved invention. The VSC control method compares the real-time detected controller output AC frequency with the set networking / island operation frequency setting value, and sets a frequency hysteresis. In the corresponding interval (i.e. island upper threshold to networking lower threshold interval or networking upper threshold to island lower threshold interval), no mode switching is performed, avoiding frequent networking-island operation mode switching, and greatly improving the operation stability of the control system. The VSC control method solves the technical problem that the networking-island operation state switching is too frequent in the prior art, thereby causing poor system stability.

[0013] Further, S2 further comprises:

[0014] When the system frequency is greater than the networking lower threshold and less than the networking upper threshold, the VSC works in the networking operation mode.

[0015] Further, S2 further comprises:

[0016] When the system frequency is less than the island upper threshold or greater than the island lower threshold, the VSC works in the island operation mode.

[0017] Further, when the working mode of the VSC is the networking operation mode, the control strategy of the VSC comprises power-phase angle control:

[0018]

[0019] Wherein, P ref is a system active power reference value; P is an actual system active power value; f is a system real-time frequency; f0 is a frequency reference value; T j is an inertia time constant; D is a damping coefficient; and θ is an actual output phase angle of the VSC.

[0020] Further, when the working mode of the VSC is the island operation mode, the control strategy of the VSC comprises power-phase angle control:

[0021]

[0022] Wherein, P ref is a system active power reference value; P is an actual system active power value; f is a system real-time frequency; f0 is a frequency reference value; T j is an inertia time constant; D is a damping coefficient; k f is a power-frequency droop coefficient; and θ is an actual output phase angle of the VSC.

[0023] Further, the frequency reference value f0 is obtained by first-order inertia filtering of the system real-time frequency f.

[0024] Further, the networking lower threshold and / or the networking upper threshold are determined according to a power grid frequency reference value.

[0025] The application also provides a technical solution of a VSC control device based on island-networking mode conversion: a VSC control device based on island-networking mode conversion, comprising a processor for executing a computer program to realize the steps of the VSC control method based on island-networking mode conversion as described above.

[0026] The application also provides a technical solution of a VSC: a VSC comprising a VSC controller, wherein the VSC controller comprises a processor for executing a computer program to realize the steps of the VSC control method based on island-networking mode conversion as described above.

[0027] The application also provides a technical solution of a computer readable storage medium: a computer readable storage medium, wherein the computer readable storage medium internally stores a computer program, and the computer program is used to be executed by a processor to realize the steps of the VSC control method based on island-networking mode conversion as described above. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 Control strategy block diagram for the networking operation mode in the embodiment of the VSC control method based on island-networking mode conversion of the application;

[0029] Figure 2 Control strategy block diagram for the island operation mode in the embodiment of the VSC control method based on island-networking mode conversion of the application;

[0030] Figure 3 Island-networking operation state identification schematic diagram in the embodiment of the VSC control method based on island-networking mode conversion of the application. DETAILED DESCRIPTION

[0031] The application compares the real-time detected controller output AC frequency with the set networking / island operation frequency setting value, and sets a frequency hysteresis, and does not perform mode switching within the corresponding interval (i.e. the interval from the island upper threshold to the networking lower threshold or the interval from the networking upper threshold to the island lower threshold), thereby avoiding frequent switching between the networking-island operation modes, and greatly improving the operation stability of the control system. The application solves the technical problem that the switching between the networking-island operation states is too frequent in the prior art, thereby causing poor system stability.

[0032] Embodiment of the VSC control method based on island-networking mode conversion:

[0033] In practical applications of flexible DC engineering, the output frequency reference value f of the flexible DC system is... base It is definite. For example, the frequency of my country's AC power grid is 50Hz. To ensure the safe operation of the system, the safe power grid frequency range is f. LL1 ~f LU1 (f LL1 <f base <f LU1 The actual frequency value of the flexible DC system is in f. LL1 ~f LU1 Within this range, the system is considered to be in a network-connected operating state; outside this range, it is considered to be in an isolated operating state.

[0034] In this implementation, to avoid excessively frequent switching between networked and islanded operation states, frequency hysteresis can be added; that is, during islanded operation, frequency f > f LU2 or f < f LL2 Among them, f LL2 f is the upper limit threshold for islands; LL1 f is the lower limit threshold for network connectivity. LU1 The upper limit threshold for network connectivity; f LU2 This is the lower threshold for islands.

[0035] like Figure 3 As shown, when the flexible DC converter is in grid-connected operation mode, when the frequency f > f LU2 or f < f LL2 When the flexible DC system enters islanded operation mode; when the flexible DC converter is in islanded operation mode, when the frequency f LL1 <f<f LU1 At that time, the flexible DC system enters the grid-connected operation mode.

[0036] When f LL2 <f<f LL1 or f LU1 <f<f LU2 At this time, regardless of whether the flexible DC converter is in network operation mode or island operation mode, there is no need to switch modes; the control strategy in the current mode can be maintained.

[0037] In other words, if the flexible DC converter operates in grid-connected mode, then when f LL2 <f<f LU2 When f > f, the flexible DC converter remains in grid-connected operation mode; LU2 or f < f LL2 At this time, the flexible DC converter switches to islanded operation mode.

[0038] If the flexible DC converter operates in islanded mode, then when f > f LU1 or f < f LL1At that time, the flexible DC converter remains in islanded operation mode; when f LL1 <f<f LU1 At that time, the flexible DC converter switches to network operation mode.

[0039] In network operation mode, with f LL2 and f LU2 This is the frequency setting for mode switching / holding; in islanded operation mode, it is set to f LL1 and f LU1 This is the frequency setting for mode switching / holding.

[0040] Specifically, in grid-connected mode, power-phase angle control is adopted. The power-phase angle control block diagram of the flexible DC converter in grid-connected mode is as follows: Figure 1 As shown, with frequency reference f0 and active power reference P... ref The phase angle θ and active power P are controlled as system reference parameters. Figure 1 The control strategy shown is represented by the following formula:

[0041]

[0042] Among them, P ref P is the system active power reference value; f is the system active power actual value; f is the system real-time frequency; f0 is the frequency reference value; T j θ is the inertia time constant; D is the damping coefficient; θ is the phase angle of the actual output of the converter.

[0043] In islanded mode, power-phase control is also used. The power-phase control block diagram of the flexible DC converter in islanded mode is as follows: Figure 2 As shown. The difference between the control strategy and the network mode lies in the introduction of a power-frequency droop control loop (i.e., Figure 2 k in f (f0-f) part). Figure 2 The control strategy shown is represented by the following formula:

[0044]

[0045] Among them, P ref P is the system active power reference value; f is the system active power actual value; f is the system real-time frequency; f0 is the frequency reference value; T j D is the inertia time constant; D is the damping coefficient; k f θ is the power-frequency droop factor; θ is the phase angle of the actual output of the converter.

[0046] Specifically, the frequency reference value f0 is obtained by filtering the system's real-time frequency f through a first-order inertial element, as shown in the following formula:

[0047]

[0048] Where t is the inertial time constant of the filter; s is the Laplace operator.

[0049] In other words, in grid-connected operation mode, the flexible DC converter uses Figure 1 The control strategy shown operates; in islanded operation mode, the flexible DC converter operates as follows: Figure 2 The control strategy shown is in operation. The control of the flexible DC converter in network operation mode is disabled. Figure 2 k in f (f0-f) part.

[0050] In islanded operation mode, when f > f LU2 At that time, the flexible DC converter will adjust according to... Figure 2 The control strategy shown will control the AC system frequency at f LU2 When f < f LL2 At that time, the flexible DC converter will adjust according to... Figure 2 The control strategy shown will control the AC system frequency at f LL2 This means stabilizing the system in two different islanded operating modes based on the frequency.

[0051] Implementation method of flexible DC converter control device based on island-to-network mode conversion:

[0052] A flexible DC converter control device based on islanded-network mode switching includes a processor for executing a computer program to implement the steps of the flexible DC converter control method based on islanded-network mode switching as described above. The specific flexible DC converter control method based on islanded-network mode switching has been described in sufficient detail in the above-described embodiments and will not be repeated here.

[0053] Specifically, a processor can be a CPU, or other general-purpose processors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. A general-purpose processor can be a microprocessor or any conventional processor. A processor can also be a processor that supports the Advanced Reduced Instruction Set Machine (ARM) architecture.

[0054] Implementation of flexible DC converter:

[0055] A flexible direct current converter comprises a converter controller, the converter controller comprising a processor configured to execute a computer program to implement the steps of the island-connected mode transition based flexible direct current converter control method as described above. The specific island-connected mode transition based flexible direct current converter control method has been described in sufficient detail in the above embodiments of the island-connected mode transition based flexible direct current converter control method and will not be repeated here.

[0056] In particular, the processor can be a CPU, and can also be other general-purpose processors, Digital Signal Processer (DSP), Application Specific Integrated Circuit (ASIC), Field Programmable Gate Array (FPGA) or other programmable logic device, discrete gate or transistor logic device, discrete hardware component, etc. The general-purpose processor can be a microprocessor or any conventional processor, etc. The processor can also be a processor supporting Advanced RISC Machines (ARM) architecture.

[0057] Embodiment of computer readable storage medium:

[0058] A computer readable storage medium, the computer readable storage medium internally storing a computer program, the computer program being configured to be executed by a processor to implement the steps of the island-connected mode transition based flexible direct current converter control method as described above. The specific island-connected mode transition based flexible direct current converter control method has been described in sufficient detail in the above embodiments of the island-connected mode transition based flexible direct current converter control method and will not be repeated here.

[0059] Specifically, the computer readable storage medium can be a volatile memory or a non-volatile memory, or can include both volatile and non-volatile memories. Among them, the non-volatile memory can be a Read Only Memory (ROM), a Programmable ROM (PROM), an Erasable PROM (EPROM), an Electrically EPROM (EEPROM), or a flash memory. The volatile memory can be a Random Access Memory (RAM) used as an external cache. For example, a Static RAM (SRAM), a Dynamic Random Access Memory (DRAM), a Synchronous DRAM (SDRAM), a Double Data Rate SDRAM (DDR SDRAM), an Enhanced SDRAM (ESDRAM), a SynchLink DRAM (SLDRAM), or a Direct Rambus RAM (DRRAM).

[0060] The present application has the following characteristics:

[0061] The present application compares the output AC frequency of the real-time detection controller with the set networking / island operation frequency setting value, identifies the flexible DC system in networking or island operation mode in real time, and sets a frequency hysteresis to avoid frequent switching between networking and island operation modes. When the flexible DC system is in island operation, the AC system frequency can be controlled to be stable at the island operation frequency setting value, avoiding the output AC frequency of the flexible DC power transmission system deviating too much from the frequency reference value, which leads to instability of the connected AC power grid system. The present application accurately identifies the networking and island operation modes through the self-controller without increasing external equipment, and realizes adaptive switching between island and networking operation modes, improving the transient response capability and system reliability and stability of the flexible DC system.

[0062] Finally, it should be noted that the above description is only the preferred embodiments of the present application, and is not intended to limit the present application. Although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art will appreciate that modifications can be made to the technical solutions described in the foregoing embodiments without departing from the spirit and principle of the present application, or some technical features thereof can be replaced by equivalent features. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A method for control of a flexible direct current converter based on island- grid mode transition, characterized by, The method comprises: S1, acquiring a system frequency of a grid side; S2, if the working mode of the HVDC converter is an island operation mode or a grid-connected operation mode, keeping the working mode of the HVDC converter unchanged when the system frequency is greater than an upper threshold of the island operation mode and less than a lower threshold of the grid-connected operation mode or the system frequency is greater than an upper threshold of the grid-connected operation mode and less than a lower threshold of the island operation mode. The upper threshold of the island operation mode is less than the lower threshold of the grid-connected operation mode, the lower threshold of the grid-connected operation mode is less than the upper threshold of the grid-connected operation mode, and the upper threshold of the grid-connected operation mode is less than the lower threshold of the island operation mode.

2. The island-grid mode conversion based VSC control method of claim 1, wherein, S2 further comprises: keeping the HVDC converter working in the grid-connected operation mode when the system frequency is greater than the lower threshold of the grid-connected operation mode and less than the upper threshold of the grid-connected operation mode. 3.The island-grid mode conversion based HVDC converter control method of claim 1, wherein, S2 further comprises: keeping the HVDC converter working in the island operation mode when the system frequency is less than the upper threshold of the island operation mode or greater than the lower threshold of the island operation mode.

4. The island-grid mode transition based VSC control method of claim 1, wherein, When the working mode of the HVDC converter is the grid-connected operation mode, the control strategy of the HVDC converter comprises power-phase angle control. Wherein, P ref is the system active power reference value; P is the system active power actual value; f is the system real-time frequency; f0 is the frequency reference value; T j is the inertia time constant; D is the damping coefficient; θ is the phase angle of the actual output of the converter.

5. The island-grid mode transition based flexible HVDC converter control method of claim 1, wherein, When the working mode of the HVDC converter is the island operation mode, the control strategy of the HVDC converter comprises power-phase angle control. Wherein, P ref is the system active power reference value; P is the system active power actual value; f is the system real-time frequency; f0 is the frequency reference value; T j is the inertia time constant; D is the damping coefficient; k f is the power-frequency droop coefficient; θ is the phase angle of the actual output of the converter.

6. The island-grid mode conversion based VSC control method of claim 4 or 5, wherein, The frequency reference value f0 is obtained by first-order inertia filtering of a real-time system frequency f.

7. The island-grid mode transition based flexible direct current converter control method of claim 1, wherein, The lower threshold and / or the upper threshold of the grid-connected operation mode are determined according to a grid frequency reference value.

8. An HVDC converter control device based on island-grid mode transition, comprising a processor, characterized in that, The processor is configured to execute a computer program to implement the steps of the HVDC converter control method based on island-grid-connected mode conversion according to any one of claims 1-7.

9. A flexible direct current converter comprising a converter controller, the converter controller comprising a processor, characterized in that, The processor is configured to execute a computer program to implement the steps of the HVDC converter control method based on island-grid-connected mode conversion according to any one of claims 1-7.

10. A computer readable storage medium, internally storing a computer program, characterized in that, The computer program is configured to be executed by the processor to implement the steps of the HVDC converter control method based on island-grid-connected mode conversion according to any one of claims 1-7.

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