Droop control method and system under circuit breaker closing and reclosing working conditions

Abstract

The invention discloses a droop control method and system under the working conditions of circuit breaker closing and reclosing, and the method comprises the steps: building a four-port flexible DC power transmission system equipped with a high-voltage DC circuit breaker, and the four-port flexible DC power transmission system comprises a main circuit, a measurement system, a droop control module and a voltage and current double-closed-loop module; closing and reclosure of a high-voltage direct-current circuit breaker are equipped to complete fault line removal and converter station re-grid connection; inputting the actual voltage power of the four-port flexible direct-current power transmission system with the converter station quitting and re-operating into the droop control module to obtain a system power deviation value; and inputting the system power deviation value into a proportional integral controller, outputting a power regulation value, and inputting the power regulation value into a voltage and current double-closed-loop module to obtain final output voltage and power of the system. According to the technical scheme, the power fluctuation of the converter station can be reduced, the system power out-of-limit possibility is reduced, and the direct-current voltage deviation is reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of droop control of flexible direct current transmission systems, and in particular relates to a droop control method and system under circuit breaker closing and reclosing conditions. Background Art

[0002] At present, flexible direct current transmission technology is developing rapidly, providing strong technical support for the access of renewable energy, the complementarity of various forms of energy, and the flexible consumption of large-scale energy grid connection in my country. However, the flexible direct current system has low equivalent resistance. When a short circuit occurs in the direct current line, the line current will rise rapidly, which will have a great impact on the system and endanger its safety.

[0003] As the core equipment of the flexible DC transmission system, the high-voltage DC circuit breaker can quickly cut off the fault current, isolate the fault line, and ensure the normal operation of other converters. However, after the circuit breaker cuts off the fault, the voltage stability and power redistribution between converter stations are the problems that need to be solved urgently. Summary of the invention

[0004] To solve the above technical problems, the present invention provides a droop control method and system under circuit breaker closing and reclosing conditions, to ensure safe and stable operation of the power system after the circuit breaker cuts off the fault, and to improve power supply quality and availability.

[0005] In order to achieve the above object, the present invention adopts the following technical scheme:

[0006] A droop control method under circuit breaker closing and reclosing conditions, comprising:

[0007] Establishing a four-port flexible direct current transmission system equipped with a high-voltage direct current circuit breaker, wherein the four-port flexible direct current transmission system includes a main circuit, a measurement system, a droop control module, and a voltage-current dual closed-loop module;

[0008] Equipped with high-voltage DC circuit breaker closing and reclosing to complete fault line removal and converter station reconnection;

[0009] Inputting the actual voltage and power of the four-port flexible direct current transmission system with the converter station de-gridified and re-grid-connected into the droop control module to obtain a system power deviation value;

[0010] The system power deviation value is input into a proportional-integral controller to output a power adjustment value, and the power adjustment value is input into a voltage-current dual closed-loop module to obtain the final output voltage and power of the system.

[0011] A droop control system under circuit breaker closing and reclosing conditions, comprising:

[0012] Equipped with high-voltage DC circuit breakers, used to close and reclose high-voltage DC circuit breakers to complete fault line removal and reconnect converter stations to the grid;

[0013] A four-port flexible direct current transmission system equipped with a high-voltage direct current circuit breaker, the four-port flexible direct current transmission system comprising a main circuit, a measurement system, a droop control module and a voltage-current dual closed-loop module;

[0014] The droop control module is used to receive the actual voltage power of the four-port flexible direct current transmission system when the converter station exits and restarts, and obtain the system power deviation value;

[0015] The proportional-integral controller is used to receive the system power deviation value, output a power adjustment value, input the power adjustment value into the voltage-current dual closed-loop module, and obtain the final output voltage and power of the system.

[0016] The beneficial effect of the present invention is as follows: in a droop control method under circuit breaker closing and reclosing conditions disclosed in the present invention, based on the circuit breaker closing and reclosing conditions, a power function and an inverse tangent function are used to replace the traditional droop curve, and then an adaptive droop coefficient is used to replace the unchanging droop coefficient, which effectively reduces the deviation of the system DC voltage and reduces the power impact of the system, and effectively improves the stability of the system under fault state operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The accompanying drawings constitute part of the present invention and are used to provide a further understanding of the present invention. The schematic drawings and descriptions of the present invention are used to explain the present invention and do not constitute an improper limitation of the present invention. In the accompanying drawings:

[0018] Figure 1 This is the structural diagram of the four-port flexible DC system.

[0019] Figure 2 It is a topological diagram of high voltage DC circuit breaker.

[0020] Figure 3 It is a system voltage waveform diagram of the traditional method and the droop control method under the circuit breaker closing and reclosing conditions in the example of the present invention.

[0021] Figure 4 (a) is the active power waveform of MMC1 under the traditional method and the droop control method under the circuit breaker closing and reclosing conditions, Figure 4 (b) is the active power waveform of MMC2 under different control methods, Figure 4 (c) is the active power waveform of MMC3 under different control methods, and Figure 4 (d) is the active power waveform of MMC4 under different control methods.

[0022] Figure 5 It is the current and voltage waveform diagram during the disconnection process of a temporary fault in the line. DETAILED DESCRIPTION

[0023] The present invention will be described in detail below with reference to the accompanying drawings and in conjunction with embodiments.

[0024] According to an embodiment of the present invention, a droop control method under circuit breaker closing and reclosing conditions is provided, comprising the following steps:

[0025] Establishing a four-port flexible direct current transmission system equipped with a high-voltage direct current circuit breaker, wherein the four-port flexible direct current transmission system comprises a main circuit, a measurement system, a droop control module, and a voltage-current dual closed-loop module;

[0026] Closing and reclosing of the high-voltage DC circuit breaker completes the removal of the faulty line and the reconnection of the converter station to the grid;

[0027] The actual voltage and power of the four-port flexible DC transmission system with the converter station de-gridified and re-grid-connected is input into the droop control module to obtain the system power deviation value;

[0028] The power deviation value is input into a proportional-integral controller to output a power adjustment value, and the power adjustment value is input into a voltage-current dual closed-loop module to obtain the final output voltage and power of the system.

[0029] The above-mentioned main circuit, measurement system and voltage-current dual closed-loop module are contents that can be understood by those skilled in the art based on the prior art knowledge, and this application will not elaborate on them.

[0030] Figure 1 The four-port flexible DC system structure diagram of the present invention includes four converter stations MMC1, MMC2, MMC3 and MMC4. The two converter stations MMC1 and MMC4 adopt voltage power droop control, and the droop coefficient =5, the system DC voltage command value is 100KV, the active power command value is 200MW, and the system rated power value is 400MW; the two converter stations MMC2 and MMC3 adopt traditional active-reactive power control, the active power command value is -200MW, and the reactive power command value is 0MW. At T=1s, a temporary bipolar short circuit fault occurred in the system DC line L3. After the fault was removed, the circuit breaker was reclosed at 1.3s, and the converter station MMC3 was reconnected to the system. Figure 1 B1, B2, B31, B32, B41, and B42 shown in the figure are circuit breakers.

[0031] Figure 2 The topological structure diagram of the high voltage DC circuit breaker of the present invention is shown in FIG. Figure 1 6 groups of circuit breakers.

[0032] The main body of the high-voltage DC circuit breaker is divided into a flow branch, a disconnecting branch and an energy-consuming branch. The flow branch is composed of a fast-disconnecting switch K1; the disconnecting branch includes three full-bridge submodules connected in series, a resonant capacitor, a resonant inductor and a fast-closing switch K2. The full-bridge submodule consists of four pairs of diodes and IGCTs and a charging capacitor (C0). One diode and one IGCT form a small module, and four small modules form an H-bridge module. The charging capacitor is connected in parallel in the middle of the H-bridge module; the energy-consuming branch consists of three lightning arresters. When the arrester action voltage is reached, the arrester starts to act and absorbs the short-circuit current energy.

[0033] Closing of the circuit breaker: When the circuit breaker detects a fault, the quick-disconnect switch K1 starts to disconnect, and the quick-closing switch K2 starts to close. When the quick-closing switch K2 reaches the just-closed position, the disconnect branch is connected to the flow branch. The pre-charge capacitor C0 charges the resonant capacitor C through the full-bridge submodule, and the resonant capacitor resonates with the resonant inductor, and generates a large reverse resonant current that is superimposed on the fault current. When the current on the quick-disconnect switch K1 is 0, the flow branch is completely disconnected from the DC line. The fault current charges the resonant capacitor through the disconnect branch. When the voltage across the arrester reaches the threshold voltage, the fault current is transferred to the energy consumption branch to consume energy. The arrester continues to absorb energy until the energy absorption is completed, the circuit breaker completes the breaking process, and the fault current is successfully isolated.

[0034] Circuit breaker reclosing: Close the fast-opening switch K1 and the fast-closing switch K2, the full-bridge submodule IGCT is locked, the resonant capacitor C resonates with the resonant inductor L, and the pre-charge capacitor C0 is charged through the diode until the resonant inductor voltage drops to 0, the fast-closing switch K2 is disconnected, the reclosing process ends, and waits for the next fault to occur.

[0035] The improved droop control method adopted by the two converter stations MMC1 and MMC4 also includes: The PV droop control equation is:

[0036] ,

[0037] In the formula, is the active power reference value, is the active power, is the system DC voltage rating, is the actual DC voltage of the system, is the adaptive droop coefficient.

[0038] Furthermore, the adaptive droop coefficient in the improved droop control equation is:

[0039] ,

[0040] In the formula, is the droop coefficient, It is the rated active power of the converter station that is reconnected to the grid.

[0041] After the circuit breaker is closed and reclosed, when the system voltage encounters a small disturbance, especially In this case, the droop coefficient remains unchanged, and the disturbance is suppressed by improving the droop curve itself to make the system enter a stable state; when the system encounters a large disturbance, In this case, not only the droop curve itself is improved, but also the voltage difference is reduced and the power fluctuation is suppressed through adaptive droop coefficient.

[0042] The traditional droop function is replaced by the power function and the inverse tangent function, and an adaptive droop control coefficient is proposed, which can reduce the power fluctuation of the converter station, reduce the possibility of system power exceeding the limit, and reduce the DC voltage deviation.

[0043] The power deviation value is passed through a proportional-integral controller to obtain a power adjustment value, and then the power adjustment value is used as a current signal reference value in the voltage-current dual closed-loop module. The current signal reference value and the output voltage value obtained by the measurement module are subtracted to obtain an offset current, and the offset current value is input into the proportional-integral controller to obtain a modulated wave. Finally, the modulated wave is approximated by the nearest level to obtain the final output voltage and power.

[0044] Figure 3 1 is a system voltage waveform diagram of different control methods in an example of the present invention.

[0045] like Figure 3 As shown in the figure, after the circuit breaker completes the opening operation, the system power exceeds the limit, causing the system DC voltage to rise to consume the excess power. The maximum DC voltage using traditional droop control is 107KV, and the voltage value after stabilization is 105KV; while the maximum DC voltage using traditional droop control is 105KV, and the voltage value after stabilization is 102KV. The voltage deviation value is significantly reduced, and the voltage fluctuation after the circuit breaker reclosing is also smaller than the traditional droop control under the improved droop control.

[0046] FIG. 4 is a diagram showing power waveforms of converter stations in different control methods in an example of the present invention.

[0047] As shown in Figure 4 (a) and Figure 4 (d), after the circuit breaker is closed, the converter station MMC3 is out of operation and the system power is in excess. At this time, the converter stations MMC1 and MMC4 adopt droop control, and their power slowly decreases to 100MW. After the circuit breaker is reclosed, the converter station MMC3 is reconnected to the system, and the power of the converter stations MMC1 and MMC4 returns to the initial value. During this period, it can be seen from Figure 4 (a) and Figure 4 (d) that the power impact of the improved droop control is significantly smaller than that of the traditional droop control, and the value is 50MW lower.

[0048] As shown in Figure 4 (b) and Figure 4 (c), since the converter station MMC2 adopts active-reactive power control, the value has basically not changed and is stable at the rated value of -200MW; due to the exit and reconnection of the converter station MMC3, the active power also changes from -200MW to 0MW and finally to -200MW.

[0049] Figure 5 It is a simulation diagram of the circuit breaker action process in the example of the present invention.

[0050] like Figure 5 As shown in the figure, the system fails at 1s. After 2ms, the disconnecting branch is connected to the flow branch. The resonant capacitor resonates with the resonant inductor, and a large reverse resonant current is generated to superimpose the fault current. At 1.0025s, the current on the fast disconnect switch K1 crosses zero, and K1 is turned off. The fault current charges the resonant capacitor through the disconnecting branch. When the voltage across the arrester reaches the threshold voltage, the fault current is transferred to the energy consumption branch to consume energy. At 1.0074s, the arrester continues to absorb energy until the energy absorption is completed. The circuit breaker completes the breaking process and the fault current is isolated successfully. At 1.3s, the circuit breaker begins to reclose, the flow branch is reconnected to the DC line, the arrester voltage drops to 0, the converter 3 resumes operation, and the line current rises to the rated current value of 2kA. Figure 5 In the figure, IK represents the current on the quick-disconnect switch K1, and IO represents the line fault current.

[0051] According to an embodiment of the present invention, there is also provided a droop control system under the circuit breaker closing and reclosing conditions, comprising:

[0052] Equipped with high-voltage DC circuit breakers, used to close and reclose high-voltage DC circuit breakers to complete fault line removal and reconnect converter stations to the grid;

[0053] A four-port flexible direct current transmission system equipped with a high-voltage direct current circuit breaker, the four-port flexible direct current transmission system comprising a main circuit, a measurement system, a droop control module and a voltage-current dual closed-loop module;

[0054] The droop control module is used to receive the actual voltage and power of the four-port flexible direct current transmission system when the converter station is disconnected from the grid and reconnected to the grid, and obtain the system power deviation value;

[0055] The proportional-integral controller is used to receive the system power deviation value, output a power adjustment value, input the power adjustment value into the voltage-current dual closed-loop module, and obtain the final output voltage and power of the system.

[0056] In the description provided herein, a large number of specific details are described. However, it is understood that embodiments of the present invention can be practiced without these specific details. In some instances, well-known methods, structures and techniques are not shown in detail so as not to obscure the understanding of this description.

[0057] Although the present invention has been described according to a limited number of embodiments, it will be apparent to those skilled in the art, with the benefit of the above description, that other embodiments may be envisioned within the scope of the invention thus described. In addition, it should be noted that the language used in this specification is primarily selected for readability and instructional purposes, rather than for the purpose of explaining or limiting the subject matter of the present invention.

Claims

1. A droop control method under circuit breaker closing and reclosing conditions, characterized in that: include: Establishing a four-port flexible direct current transmission system equipped with a high-voltage direct current circuit breaker, wherein the four-port flexible direct current transmission system includes a main circuit, a measurement system, a droop control module, and a voltage-current dual closed-loop module; Equipped with high-voltage DC circuit breaker closing and reclosing to complete fault line removal and converter station reconnection; Inputting the actual voltage and power of the four-port flexible direct current transmission system with the converter station de-gridified and re-grid-connected into the droop control module to obtain a system power deviation value; The system power deviation value is input into a proportional-integral controller to output a power adjustment value, and the power adjustment value is input into a voltage-current dual closed-loop module to obtain the final output voltage and power of the system.

2. The droop control method under the circuit breaker closing and reclosing conditions according to claim 1, characterized in that: The main body of the high-voltage DC circuit breaker is divided into a current-carrying branch, a disconnecting branch and an energy-consuming branch. The current-carrying branch is composed of a fast-opening switch (K1); the disconnecting branch includes three full-bridge sub-modules connected in series, a resonant capacitor, a resonant inductor and a fast-closing switch (K2), wherein the full-bridge sub-module is composed of four pairs of diodes and IGCT and a charging capacitor (C0); the energy-consuming branch is composed of three lightning arresters. When the arrester action voltage is reached, the arrester starts to act and absorbs the short-circuit current energy.

3. The droop control method under the circuit breaker closing and reclosing conditions according to claim 2, characterized in that: The process of closing the high-voltage DC circuit breaker includes: when the high-voltage DC circuit breaker detects a fault, the quick-disconnect switch (K1) starts to disconnect, and the quick-closing switch (K2) starts to close. When the quick-closing switch (K2) reaches the just-closed position, the disconnecting branch is connected to the flow branch, and the charging capacitor (C0) charges the resonant capacitor (C) through the full-bridge submodule. The resonant capacitor resonates with the resonant inductor and generates a large reverse resonant current that is superimposed on the fault current. When the current on the quick-disconnect switch (K1) is 0, the flow branch is completely disconnected from the DC line, and the fault current charges the resonant capacitor through the disconnecting branch. When the voltage across the arrester reaches the threshold voltage, the fault current is transferred to the energy consumption branch to consume energy. The arrester continues to absorb energy until the energy absorption is completed. The circuit breaker completes the breaking process and the fault current is successfully isolated. The process of circuit breaker reclosing includes: closing the fast opening switch (K1) and the fast closing switch (K2), locking the IGCT of the full-bridge submodule, resonating the resonant capacitor and the resonant inductor, charging the charging capacitor (C0) through the diode until the resonant inductor voltage drops to 0, the fast closing switch (K2) is disconnected, the reclosing process ends, and waits for the next fault to occur.

4. The droop control method under circuit breaker closing and reclosing conditions according to claim 1, characterized in that: The PV droop control equation used in the droop control module is: ， In the formula, is the active power reference value, is the active power, is the system DC voltage rating, is the actual DC voltage of the system, is the adaptive droop coefficient.

5. The droop control method under the circuit breaker closing and reclosing conditions according to claim 4, characterized in that: The expression of the adaptive droop coefficient is: ， In the formula, is the droop coefficient, It is the rated active power of the converter station that is reconnected to the grid.

6. The droop control method under circuit breaker closing and reclosing conditions according to claim 1, characterized in that: The power offset is passed through a proportional-integral link to obtain a power adjustment value, which is then used as a current signal reference value in the voltage-current dual closed-loop module. The current signal reference value is subtracted from the measured output voltage value to obtain an offset current, which is input into the proportional-integral controller to obtain a modulated wave. Finally, the output voltage and power are obtained by approximating the modulated wave to the nearest level.

7. A droop control system under circuit breaker closing and reclosing conditions, characterized in that: include: Equipped with high-voltage DC circuit breakers, used to close and reclose high-voltage DC circuit breakers to complete fault line removal and reconnection of converter stations to the grid; A four-port flexible direct current transmission system equipped with a high-voltage direct current circuit breaker, the four-port flexible direct current transmission system comprising a main circuit, a measurement system, a droop control module and a voltage-current dual closed-loop module; The droop control module is used to receive the actual voltage and power of the four-port flexible direct current transmission system when the converter station is disconnected from the grid and reconnected to the grid, and obtain the system power deviation value; The proportional-integral controller is used to receive the system power deviation value, output a power adjustment value, input the power adjustment value into the voltage-current dual closed-loop module, and obtain the final output voltage and power of the system.

8. The droop control system under circuit breaker closing and reclosing conditions according to claim 7, characterized in that: The main body of the high-voltage DC circuit breaker is divided into a current-carrying branch, a disconnecting branch and an energy-consuming branch. The current-carrying branch is composed of a fast-opening switch (K1); the disconnecting branch includes three full-bridge sub-modules connected in series, a resonant capacitor, a resonant inductor and a fast-closing switch (K2), wherein the full-bridge sub-module is composed of four pairs of diodes and IGCT and a charging capacitor (C0); the energy-consuming branch is composed of three lightning arresters. When the arrester action voltage is reached, the arrester starts to act and absorbs the short-circuit current energy.

9. The droop control system under the circuit breaker closing and reclosing conditions according to claim 7, characterized in that: The PV droop control equation used in the droop control module is: ， In the formula, is the active power reference value, is the active power, is the system DC voltage rating, is the actual DC voltage of the system, is the adaptive droop coefficient.

10. The droop control system under the circuit breaker closing and reclosing conditions according to claim 9, characterized in that: The expression of the adaptive droop coefficient is: ， In the formula, is the droop coefficient, It is the rated active power of the converter station that is reconnected to the grid.

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