Operation mode of direct-current power transmission system

Abstract

The invention discloses a direct-current power transmission system operation mode, which comprises that a direct-current power transmission system comprises an HCC converter valve, when commutation failure does not occur, the direct-current power transmission system operates according to a fixed extinction angle control mode, the operation extinction angle of the HCC converter valve is less than 8 degrees, and a device for executing on / off in the HCC converter valve is a full-control device. The problem of commutation failure can be effectively avoided based on active turn-off of a full-control power device, the HCC converter valve is designed to operate in a small extinction angle mode, the blocking recovery speed is high, firstly, the requirement for reactive power is reduced, and the requirements for reactive power and filtering facilities of an alternating current field of a converter station are reduced; and thirdly, the voltage fluctuation range of the alternating-current power grid is small, the risk of causing direct-current power transmission commutation failure is reduced, and the direct-current power transmission system is safer and more stable in operation.

Description

Technical Field

[0001] The present invention relates to the technical field of direct current power transmission, and in particular to an operating mode of a direct current power transmission system. Background Art

[0002] High-voltage direct current transmission is an important means to achieve the cross-regional transmission of renewable energy. At present, there are many high-voltage direct current transmission projects in my country that have been put into operation or are under construction. LCC (Line Commutated Converter) has been widely used in the construction of high-voltage direct current transmission projects due to its large transmission capacity and low construction cost. LCC relies on the grid voltage to achieve phase change. When the grid is disturbed or fails, its voltage fluctuation or drop will cause LCC phase change failure. Phase change failure will cause problems such as reduced DC voltage, reduced DC transmission power, increased current, and DC bias of the converter transformer. When a phase change failure occurs at one end of the multi-terminal DC feed-in system, it may cause a chain phase change failure, which will seriously affect the safe and stable operation of the power grid. The power device of LCC is a thyristor. The thyristor has a slow recovery blocking speed and a long time. Correspondingly, the required arc extinction time is longer, that is, the arc extinction angle is larger. In order to avoid phase change failure, the receiving-end converter station of the existing project is designed to operate at a larger arc extinction angle, generally about 17°. Although this type of solution can alleviate the problem of commutation failure to a certain extent, its essence is to increase the arc extinction angle margin by advancing the trigger angle to avoid commutation failure. This results in large reactive power demand and high harmonic content when the system is running, and a large number of reactive compensation and filtering measures need to be configured. Summary of the invention

[0003] In order to solve the above problems, the inventors have made the present invention, and provide a DC transmission system operation mode through specific implementation methods.

[0004] An embodiment of the present invention provides a DC transmission system operation mode, including:

[0005] The DC transmission system includes a Hybrid Commutated Converter (HCC) valve. When no commutation failure occurs, the system operates in a fixed arc extinction angle control mode. The arc extinction angle of the HCC valve is less than 8°, and the device that performs opening / closing in the HCC valve is a fully controlled device.

[0006] All thyristors in the LCC converter valve are replaced with fully controlled devices to obtain the HCC converter valve.

[0007] Optionally, when a commutation failure occurs in the DC power transmission system, the system switches to a constant current control mode, and under low-voltage current limiting control, the HCC converter valve is gradually restored to the operating point before the commutation failure as the fault is cleared.

[0008] Optionally, when the HCC converter valve recovers to the operating point before the commutation failure, the DC power transmission system switches to a fixed arc extinction angle control mode, and the HCC converter valve operates at an arc extinction angle less than 8°.

[0009] Optionally, when the DC power transmission system operates in a fixed arc extinction angle control mode, a given value of the arc extinction angle is 5°.

[0010] Optionally, when the DC power transmission system switches between the constant current control mode and the fixed arc extinction angle control mode, the advance trigger angles output by the constant current control mode and the fixed arc extinction angle control mode are compared, and the larger value is determined as the advance trigger angle given value.

[0011] Optionally, the output current of the low-voltage current limiting control is compared with the main control stage current, and the smaller value is determined as the given value of the current transmitted from the rectifier side to the inverter side of the direct current transmission system.

[0012] Optionally, the operation is performed in a fixed arc extinction angle control mode, including the following steps:

[0013] The minimum value of the periodic arc extinction angle on the DC power transmission system is collected, and the minimum value, the current deviation control output value, and the arc extinction angle given value are calculated to obtain the arc extinction angle calculated value;

[0014] The calculated arc extinction angle value is compared with the arc extinction angle change value, and the larger value is input into the fixed arc extinction angle control mode PI controller.

[0015] Optionally, determining the current deviation control output value comprises the following steps:

[0016] The DC current measurement value of the DC power transmission system is input into a low-pass filter to obtain a filtered DC current measurement value, and a given current value on the rectifier side of the DC power transmission system is subtracted from the filtered DC current measurement value to obtain a current deviation control output value.

[0017] Optionally, determining the input current of the constant current control mode includes the following steps:

[0018] The DC current measurement value of the DC power transmission system is input into a low-pass filter to obtain a filtered DC current measurement value. After the filtered DC current measurement value is calculated with the given current value on the rectifier side of the DC power transmission system, 0.1 per unit of the current is subtracted to obtain the input current of the constant current control mode PI controller.

[0019] The beneficial effects of the above technical solution provided by the embodiment of the present invention include at least:

[0020] The present invention proposes an operation mode under a small arc extinction angle of an HCC converter valve based on a fully controlled power device. The fully controlled power device can withstand high voltage and high current. It is applied to a high-voltage direct current transmission system. While realizing large-capacity power transmission, it can also effectively avoid the problem of commutation failure due to its good shutdown capability. The active shutdown based on the fully controlled power device can effectively avoid the problem of commutation failure. The HCC converter valve is designed to operate in a small arc extinction angle mode, and the recovery and blocking speed is fast. First, it reduces the demand for reactive power and reduces the demand for reactive power and filtering facilities in the converter station AC field. Second, it reduces the impact of DC power fluctuations on the AC grid voltage, thereby improving the friendliness of the DC transmission system to the AC grid. Third, the small fluctuation range of the AC grid voltage in turn reduces the risk of causing DC transmission commutation failure, making the operation of the DC transmission system safer and more stable.

[0021] Other features and advantages of the present invention will be described in the following description, and partly become apparent from the description, or understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained by the structures particularly pointed out in the written description, claims, and drawings.

[0022] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The accompanying drawings are used to provide a further understanding of the present invention and constitute a part of the specification. Together with the embodiments of the present invention, they are used to explain the present invention and do not constitute a limitation of the present invention. In the accompanying drawings:

[0024] Figure 1 This is a flow chart of the operation of the DC power transmission system in an embodiment of the present invention;

[0025] Figure 2 A schematic diagram of the control logic of the HCC converter valve in an embodiment of the present invention;

[0026] Figure 3 1 is a topological diagram of a double-terminal direct current transmission system in an embodiment of the present invention. DETAILED DESCRIPTION

[0027] The exemplary embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. Although the exemplary embodiments of the present disclosure are shown in the accompanying drawings, it should be understood that the present disclosure can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, these embodiments are provided to enable a more thorough understanding of the present disclosure and to fully convey the scope of the present disclosure to those skilled in the art.

[0028] In order to solve the problems existing in the prior art, the embodiment of the present invention provides a DC transmission system operation mode, and its control process is as follows: Figure 1 As shown, including:

[0029] The DC power transmission system includes an HCC converter valve. When no commutation failure occurs, the system operates in a fixed arc extinction angle control mode. The operating arc extinction angle (or extinction angle, corresponding to the time required to establish a forward voltage) of the HCC converter valve is less than 8°. The devices that partially or completely perform opening / closing in the HCC converter valve (Hybrid Commutated Converter, HCC) are fully controlled devices, such as IGCT (Integrated Gate-Commutated Thyristor) devices, which are one of the fully controlled devices. The minimum arc extinction angle of the thyristor valve in the high-voltage DC power transmission system is generally in the range of 8 to 12°. Since the present embodiment uses an HCC converter valve with better anti-commutation failure performance, it can effectively avoid the problem of commutation failure, has a fast recovery blocking speed, and requires a small arc extinction angle, so it can operate at a smaller arc extinction angle. Operating at a smaller arc extinction angle reduces the demand for reactive power and the demand for reactive and filtering facilities in the converter station AC field compared to the existing DC transmission system, and reduces the impact of DC power fluctuations on the AC grid voltage, thereby improving the friendliness of the DC transmission system to the AC grid. The small fluctuation range of the AC grid voltage in turn reduces the risk of causing DC transmission commutation failure, making the operation of the DC transmission system safer and more stable.

[0030] HCC DC transmission system refers to a DC transmission system that uses HCC converter valves. HCC converter valves are based on LCC converter valves, but by upgrading the switching devices, they have better resistance to commutation failure.

[0031] In some optional embodiments, part or all of the thyristors in the LCC converter valve may be replaced with fully controlled devices to obtain the HCC converter valve.

[0032] In some optional embodiments, such as Figure 1 As shown, when the DC power transmission system fails to commutate, it switches to the constant current control mode. Under the low-voltage current limiting control, as the fault is cleared, the HCC converter valve is gradually restored to the operating point before the commutation failure. The constant current control mode refers to keeping the DC pole line current at a set value. Low-voltage current limiting control (voltage-dependent current order limiter, VDCOL) refers to the control function that automatically reduces the set value of the DC current regulator when the DC voltage is lower than a certain value under certain fault conditions, so that the DC current is reduced, and automatically restores its set value after the DC voltage is restored, so that the DC current is restored.

[0033] In some optional embodiments, such as Figure 1As shown, when the HCC converter valve recovers to the operating point before the commutation failure, the DC power transmission system switches to the fixed arc extinction angle control mode, and the HCC converter valve operates at an arc extinction angle less than 8°.

[0034] In some optional embodiments, when the DC power transmission system operates in a fixed arc extinction angle control mode, the arc extinction angle set value is 5°.

[0035] In some optional embodiments, the HCC converter valve control method is as follows: Figure 2 As shown, when the DC power transmission system switches between the constant current control mode and the fixed arc extinction angle control mode, the leading trigger angle β output by the constant current control mode and the fixed arc extinction angle control mode is compared. i and β γ , and determine the larger value as the advance trigger angle given value β. Figure 2 Middle,U di is the DC voltage measurement value, I di is the DC current measurement value, I dref is the main control level current instruction, I drref is the given value of the rectifier side current, K is the gain coefficient; γ is the arc extinction angle, γ ref is the given value of the arc extinction angle; β i is the leading firing angle given by the constant current control mode, β γ It is the leading trigger angle given by the fixed extinction angle control mode. Δγ represents the change of the extinction angle, and 0.1pu represents 0.1 per unit value. represents a low-pass filter, G represents the filter gain coefficient, T represents the time coefficient, and s represents the Laplacian operator.

[0036] In some optional embodiments, such as Figure 2 As shown, the output current of the low-voltage current limiting control is compared with the main control level current, and the smaller value is determined as the given value of the current transmitted from the rectifier side to the inverter side of the DC power transmission system.

[0037] In some optional embodiments, such as Figure 2 As shown, the operation according to the fixed arc extinction angle control mode includes the following steps:

[0038] The minimum value of the periodic arc extinction angle on the DC power transmission system is collected, and the minimum value, the current deviation control output value, and the arc extinction angle given value are calculated to obtain the arc extinction angle calculated value;

[0039] Compare the calculated value of the arc extinction angle with the change value of the arc extinction angle, and input the larger value into the PI controller of the fixed arc extinction angle control mode. The PI (proportional integral) controller is a linear controller that controls the controlled object by forming a control deviation based on the given value and the actual output value, and forming a control quantity by linearly combining the proportion and integral of the deviation.

[0040] In some optional embodiments, such as Figure 2 As shown, determining the current deviation control output value includes the following steps:

[0041] The DC current measurement value of the DC power transmission system is input into a low-pass filter to obtain a filtered DC current measurement value, and a given current value on the rectifier side of the DC power transmission system is subtracted from the filtered DC current measurement value to obtain a current deviation control output value.

[0042] In some optional embodiments, such as Figure 2 As shown, determining the input current of the constant current control mode includes the following steps:

[0043] The DC current measurement value of the DC power transmission system is input into a low-pass filter to obtain a filtered DC current measurement value. After the filtered DC current measurement value is calculated with the given current value on the rectifier side of the DC power transmission system, 0.1 per unit of the current is subtracted to obtain the input current of the constant current control mode PI controller.

[0044] A typical implementation case is Figure 3 shown. Figure 3 The figure shows a double-terminal DC transmission system with a DC voltage level of ±800kV. The converters use HCC converters HCC1 and HCC2 with active shutdown capabilities. HCC1 is located on the rectifier side of the DC transmission system, and HCC2 is located on the inverter side of the DC transmission system. The DC circuit breaker (DCB) uses a hybrid DC circuit breaker. Pos and Neg represent the positive and negative DC busbars, respectively. The DC circuit breaker is set between the DC busbar and the DC side of the converter. The AC side of the converter is connected to the AC grid. The converter uses Figure 2 The control strategy shown is that when the DC transmission system is operating normally, the converter operates in the fixed arc extinction angle control mode, and the arc extinction angle is set to γ ref =5°. If the converter fails to commutate, the control system switches to the constant current control mode. Under the low voltage current limiting control (VDCOL), the converter returns to the operating point before the fault. The control system switches to the fixed arc extinction angle operation mode, and the HCC converter re-operates in the small arc extinction angle mode. The system operation flow chart is as follows: Figure 1 The HCC converter operating at a small arc extinction angle can significantly reduce the demand for reactive power and has a better support capability for the AC grid voltage.

[0045] In the above embodiment, the present invention proposes an operation mode under a small arc extinction angle of an HCC converter valve based on a fully controlled power device. Fully controlled power devices can withstand high voltage and high current. They are applied to high-voltage direct current transmission systems. While realizing large-capacity power transmission, they can also effectively avoid commutation failure problems due to their good shutdown capability. The active shutdown based on fully controlled power devices can effectively avoid commutation failure problems. The HCC converter valve is designed to operate in a small arc extinction angle mode, and the recovery and blocking speed is fast. First, it reduces the demand for reactive power and reduces the demand for reactive power and filtering facilities in the converter station AC field. Second, it reduces the impact of DC power fluctuations on the AC grid voltage, thereby improving the friendliness of the DC transmission system to the AC grid. Third, the small fluctuation range of the AC grid voltage in turn reduces the risk of causing DC transmission commutation failure, making the operation of the DC transmission system safer and more stable.

[0046] Those skilled in the art can change the above sequence without departing from the protection scope of the present disclosure.

[0047] Any modifications, supplements and equivalent substitutions made within the scope of the principles of the present invention shall still fall within the patent scope of the present invention.

Claims

1. A DC transmission system operation mode, characterized in that: The DC power transmission system includes an HCC converter valve, which operates in a fixed arc extinction angle control mode when no commutation failure occurs. The HCC converter valve operates with an arc extinction angle less than 8°, and the device that performs opening / closing in the HCC converter valve is a fully controlled device.

2. The DC power transmission system operation mode according to claim 1, characterized in that: All thyristors in the LCC converter valve are replaced with fully controlled devices to obtain the HCC converter valve.

3. The DC power transmission system operation mode according to claim 1, characterized in that: When a commutation failure occurs in the DC power transmission system, the system switches to a constant current control mode, and under low voltage current limiting control, the HCC converter valve is gradually restored to the operating point before the commutation failure as the fault is cleared.

4. The DC power transmission system operation mode according to claim 3, characterized in that: When the HCC converter valve recovers to the operating point before the commutation failure, the DC power transmission system switches to the fixed arc extinction angle control mode, and the HCC converter valve operates at an arc extinction angle less than 8°.

5. The DC power transmission system operation mode according to any one of claims 1 to 4, characterized in that: When the DC power transmission system operates in a fixed arc extinction angle control mode, the arc extinction angle set value is 5°.

6. The DC power transmission system operation mode according to claim 3 or 4, characterized in that: When the DC power transmission system switches between the constant current control mode and the fixed arc extinction angle control mode, the leading trigger angles output by the constant current control mode and the fixed arc extinction angle control mode are compared, and the larger value is determined as the given value of the leading trigger angle.

7. The DC power transmission system operation mode according to claim 3, characterized in that: The output current of the low-voltage current limiting control is compared with the main control level current, and the smaller value is determined as the given value of the rectifier-side current of the direct current transmission system.

8. The DC power transmission system operation mode according to claim 1 or 4, characterized in that: Operation in fixed arc extinction angle control mode includes the following steps: The minimum value of the periodic arc extinction angle on the DC power transmission system is collected, and the minimum value, the current deviation control output value, and the arc extinction angle given value are calculated to obtain the arc extinction angle calculated value; The calculated arc extinction angle value is compared with the arc extinction angle change value, and the larger value is input into the fixed arc extinction angle control mode PI controller.

9. The DC power transmission system operation mode according to claim 8, characterized in that: Determining the current deviation control output value comprises the following steps: The DC current measurement value of the DC power transmission system is input into a low-pass filter to obtain a filtered DC current measurement value, and a given current value on the rectifier side of the DC power transmission system is subtracted from the filtered DC current measurement value to obtain a current deviation control output value.

10. The DC power transmission system operation mode according to claim 3, characterized in that: Determining the input current of the constant current control mode includes the following steps: The DC current measurement value of the DC power transmission system is input into a low-pass filter to obtain a filtered DC current measurement value. After the filtered DC current measurement value is calculated with the given current value on the rectifier side of the DC power transmission system, 0.1 per unit of the current is subtracted to obtain the input current of the constant current control mode PI controller.

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