Line fault restarting method and system of constant voltage operation direct current system

Abstract

The invention relates to a line fault restarting method and system of a constant voltage operation direct current system, and belongs to the technical field of power system stability analysis and control. When the direct current line fails, forced phase shift is carried out on the rectification side until the direct current is zero, and after processing is carried out on the inversion side, forced phase shift on the rectification side is canceled, and the trigger angle of the rectification side is recovered to a steady-state angle; when the direct current is zero after forced phase shift of the rectification side, the current margin is kept to be the current margin during steady-state operation, and the trigger angle of the inversion side is reduced until the lower limit of the trigger angle of the inversion side is reached; and after the forced phase shift at the rectification side is cancelled, the trigger angle at the inversion side is increased to a steady-state angle along with voltage recovery. The direct-current power recovery time is shortened, the direct-current line insulation breakdown risk is reduced, and transient stability of an alternating-current system is facilitated.

Description

Technical Field

[0001] The invention relates to a line fault restart method and system for a constant voltage operating DC system, belonging to the technical field of power system stability analysis and control. Background Art

[0002] Since the DC transmission lines in the DC transmission system are thousands of kilometers long and the geographical and climatic environment of the lines is complex, line faults are very likely to occur. DC transmission line faults are mostly instantaneous faults. The existing DC transmission line fault restart adopts phase shift restart. Figure 1 , Figure 2 As shown. When a line fault occurs at time t1, the rectifier side and the reverse side execute the following restart strategy: On the rectifier side, forced phase shifting is performed at time t1, and the trigger angle is moved to 160° to reduce the fault current to 0. After the deionization time, the forced phase shifting ends at time t3, and the rectifier side restores the trigger angle to the steady-state operation angle; on the inverter side, at time t1, as the DC current on the inverter side drops, the inverter side enters the constant DC current control; at time t2, the inverter side enters the VDCOL (Voltage Dependent Current Order Limiter, low voltage current limiting) link, and switches the current margin from 0.1pu to 0.7pu, the purpose of which is to ensure that the inverter side is in constant voltage control or fixed arc extinction angle control during the line fault recovery process, so as to ensure that the DC voltage recovers quickly; at time t3, the deionization time ends, the inverter side enters the fixed arc extinction angle control, and the DC voltage recovers quickly.

[0003] For projects using fixed arc extinction angle control, the recovery process is in fixed arc extinction angle control, and its trigger angle is close to the steady-state operating angle. Therefore, the DC voltage at the recovery moment is close to the stable voltage. However, for fixed DC voltage projects, if the recovery process is in fixed arc extinction angle control, the DC voltage on the inverter side is higher than the steady-state voltage, and the rectifier side needs to adjust the angle to overvoltage to establish current.

[0004] like Figure 2 As shown, in the existing line fault restart strategy, during the line fault recovery period, the inverter side maintains the fixed arc extinction angle control. When the fixed arc extinction angle is controlled, the inverter side trigger angle AMAX formula is as follows: (1) In the formula, is the reference value of the arc extinction angle (generally 17°), is the commutation reactance, is the current reference value before the fault, DC current rating, is the actual value of the no-load voltage of the commutation transformer, is the rated no-load voltage of the converter transformer, is the positive slope coefficient, is the actual value of DC current.

[0005] It can be seen from the fixed arc extinction angle limit AMAX formula that when the inverter side is in fixed arc extinction angle control, the DC voltage is higher than the steady-state voltage, so the rectifier side needs to have a trigger angle lower than the steady-state operation angle (minimum 5°), that is, the DC current can only be established after the DC voltage is higher than the rated value. This causes the rectifier side to enter the minimum trigger angle limit during the recovery process, and then the inverter side reduces the trigger angle as the voltage increases, and finally recovers to steady-state operation.

[0006] At present, new UHV DC transmission projects mostly adopt a fixed DC voltage control strategy. If the original DC line fault restart strategy is still used, there are two problems: first, the DC line recovery time is too long, which does not meet the line fault recovery response time requirement of "excluding the free time, the time required from the start of the fault to the transmission power of the pole to recover to 90% of the transmission power before the fault shall not exceed 120ms"; on the other hand, due to the large trigger angle of the inverter side during the recovery time, there is a short-term DC overvoltage in the recovery process. Since the line has just failed and the insulation has not been fully restored, the excessively high DC voltage is not conducive to DC restart, and the probability of restart failure increases due to the short circuit of the fault point again. Summary of the invention

[0007] The object of the present invention is to provide a line fault restart method and system for a constant voltage DC system, so as to solve the problem of long DC line recovery time and restart failure at the fault point.

[0008] To achieve the above object, the solution of the present invention includes: The present invention provides a line fault restart method for a constant voltage operating DC system, comprising the following steps: When a DC line fault occurs, the rectifier side performs forced phase shifting until the DC current is 0. After being processed by the inverter side, the forced phase shifting is cancelled on the rectifier side, and the trigger angle on the rectifier side is restored to the steady-state angle. When the DC current on the inverter side is 0 after the forced phase shift on the rectifier side, the current margin is maintained at the current margin during steady-state operation, and the inverter side trigger angle is reduced until the lower limit of the inverter side trigger angle is reached; after the forced phase shift on the rectifier side is cancelled, the inverter side trigger angle increases to the steady-state angle as the voltage recovers.

[0009] Furthermore, after the forced phase shift is cancelled on the rectifier side, the trigger angle on the rectifier side does not need to be moved to an angle lower than the steady-state operation, and can be directly restored to the steady-state angle.

[0010] Furthermore, the line fault restart is used as a condition to shield the current margin switching strategy so as to maintain the current margin at the current margin in steady-state operation.

[0011] Furthermore, the current margin during steady-state operation is 0.1 pu.

[0012] Furthermore, the lower limit of the inverter side trigger angle is adaptively changed according to the operating condition setting: the lower limit of the inverter side trigger angle is the steady-state operation trigger angle minus a margin preset according to the steady-state angle based on experience.

[0013] The present invention also provides a line fault restart system for a constant voltage DC system, comprising a processor, wherein the processor is used to execute instructions for implementing the following steps: When a DC line fault occurs, the rectifier side performs forced phase shifting until the DC current is 0. After being processed by the inverter side, the forced phase shifting is cancelled on the rectifier side, and the trigger angle on the rectifier side is restored to the steady-state angle. When the DC current on the inverter side is 0 after the forced phase shift on the rectifier side, the current margin is maintained at the current margin during steady-state operation, and the inverter side trigger angle is reduced until the lower limit of the inverter side trigger angle is reached; after the forced phase shift on the rectifier side is cancelled, the inverter side trigger angle increases to the steady-state angle as the voltage recovers.

[0014] Furthermore, after the forced phase shift is cancelled on the rectifier side, the trigger angle on the rectifier side does not need to be moved to an angle lower than the steady-state operation, and can be directly restored to the steady-state angle.

[0015] Furthermore, the line fault restart is used as a condition to shield the current margin switching strategy so as to maintain the current margin at the current margin in steady-state operation.

[0016] Furthermore, the current margin during steady-state operation is 0.1 pu.

[0017] Furthermore, the lower limit of the inverter side trigger angle is adaptively changed according to the operating condition setting: the lower limit of the inverter side trigger angle is the steady-state operation trigger angle minus a margin preset according to the steady-state angle based on experience.

[0018] The present invention proposes a line fault restart method for a constant voltage DC system. Compared with the original DC line fault recovery strategy, the DC power recovery time is accelerated, and the time for the DC power to recover to 90% is shortened from 280ms to 120ms, which meets the line fault recovery response time requirement and is beneficial to the transient stability of the AC system. At the same time, since the rectifier side does not need to reduce the trigger angle below the steady-state operation angle, there is no DC overvoltage in the entire recovery process. For a DC line that has just suffered a ground short circuit fault, the possibility of DC line insulation breakdown caused by excessive DC voltage is reduced, and the success rate of line fault restart is increased.

[0019] The line fault restart system of the constant voltage operation DC system can achieve the same beneficial effects as the line fault restart method of the constant voltage operation DC system mentioned above. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1It is the line fault recovery waveform under the original restart strategy; Figure 2 It is a two-station recovery flow chart for line failure under the original restart strategy; Figure 3 It is a schematic diagram of the two-station restoration process of a DC line fault after optimization of the present invention; Figure 4 This is a waveform diagram comparing the effects before and after the optimization of the strategy of the present invention (blue is before optimization, red is after optimization); Figure 5 This is the original current margin switching schematic diagram; Figure 6 It is a schematic diagram of line fault shielding current margin switching of the present invention; Figure 7 It is a schematic diagram of the lower limit adjustment of the line fault restart adaptive trigger angle of the present invention. DETAILED DESCRIPTION

[0021] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention is described in detail in a clear and complete manner in conjunction with the accompanying drawings and embodiments.

[0022] Embodiment of the method for restarting a DC system after a line fault at a constant voltage operation: The present invention proposes a line fault restart method for a constant voltage DC system, which improves the original fault restart method. Figure 2 and Figure 3 In the method, when the DC current on the inverter side is 0, the switching current margin is changed from 0.7pu to maintaining the current margin in steady-state operation. According to experience, the current margin is usually 0.1pu in steady-state operation, and the trigger angle change process changes from increasing to decreasing. The trigger angle limit changes from the inverter side trigger angle limit AMAX to the inverter side trigger angle lower limit. After the forced phase shift is cancelled on the rectifier side, the inverter side trigger angle returns to the steady-state angle. This improved method omits the step of moving the rectifier side trigger angle to 5° after the forced phase shift is cancelled on the rectifier side in the original method.

[0023] The effect of optimizing the line fault restart method of the DC system running at constant voltage is as follows: Figure 4 As shown in the figure, the waveforms from top to bottom in the figure mean: effective value of AC voltage on the rectifier side; effective value of AC voltage on the inverter side; DC voltage; DC current; trigger angle on the inverter side. After adopting the new strategy, during the fault process, the adjustment trend of the trigger angle on the inverter side is corrected from the original increase to the upper limit (151.6°) to the lower limit of the trigger angle (steady-state angle -10°, about 120°~130°), and the trigger angle on the inverter side is reduced, avoiding DC overvoltage at the recovery time. At the same time, the DC current is established faster, and the overvoltage phenomenon of the AC system is suppressed faster.

[0024] A line fault restart method for a constant voltage operating DC system of the present invention comprises the following steps: When a DC line fault occurs, the rectifier side performs forced phase shifting until the DC current is 0. After being processed by the inverter side, the forced phase shifting is canceled on the rectifier side, and the trigger angle on the rectifier side is restored to a steady-state angle. The inverter side processes the DC current to 0 after forced phase shifting on the rectifier side, and the current margin is maintained at 0.1pu, reducing the trigger angle on the inverter side until the lower limit of the trigger angle on the inverter side is reached. After the forced phase shifting is canceled on the rectifier side, the trigger angle on the inverter side increases to a steady-state angle as the voltage recovers.

[0025] The specific line fault restart as a conditional shielding current margin switching strategy is as follows: As the current is 0 and the line fault restart is started, the current margin on the inverter side is maintained at 0.1pu. The original current margin switching steps are as follows: Figure 5 As shown, when the current is 0, the switching current margin is 0.7pu (that is, NSW.I input is 1 to select NSW.X2); after the current is 0 signal disappears, the current margin is maintained for 500ms (PDF is a delayed shutdown, that is, after the input signal PDF.X1 disappears, PDF.Y continues to maintain PDF.X2 for 500ms), and after 500ms, the switching current margin is 0.1pu. This logic ensures that the current margin is maintained at 0.7pu during the fault (current is 0) and fault recovery process (500ms after the current disappears from 0), and the inverter side is maintained in fixed arc extinction angle control. Compared with the original current margin switching, the steps of shielding the current margin switching under the condition of line fault are as follows Figure 6 As shown, the dotted box is the modified part. The current 0 signal input in the original current margin switching step is modified to an RS trigger. If a non-DC line fault causes a current of 0, the original current margin switching logic is still maintained (such as an AC system fault, a commutation failure fault, etc.). After the current is 0 for 500ms, the current margin switching signal is restored (RSR.Y output is 0 and NSW.X1 is selected); if a DC line fault occurs, the current margin switching signal is restored through the DC line fault (RSR.Y output is 0), the current margin is no longer switched (NSW.Y always selects to output NSW.X1), the inverter side enters constant current control, and the trigger angle output by the PI controller is maintained at the lower limit, which is conducive to the establishment of the current at the recovery moment and suppresses the DC transient overvoltage at the beginning of the recovery.

[0026] The specific implementation method of adjusting the lower limit of the trigger angle on the inverter side is to set an adaptive change value according to the operating conditions. Since the current margin is kept unchanged, the trigger angle on the inverter side is reduced during the de-freeing time, but a trigger angle that is too low will cause the DC voltage to be too low when the line fault is restored, thereby affecting the current reference value output of the VDCOL link on the rectifier side (too low DC voltage limits the recovery of the DC current reference value), and will also increase the time it takes for the DC current to recover to a steady-state level. Therefore, it is necessary to set a suitable lower limit of the trigger angle on the inverter side, and the line fault restart adaptive trigger angle lower limit adjustment strategy. Since the trigger angle on the inverter side varies greatly at different power operation angles, it is considered to set an adaptively changing lower limit of the inverter side trigger angle according to the operating conditions, that is, the steady-state operation trigger angle that is latched 500ms ago when the line fault starts, and set the margin X preset based on the steady-state angle based on experience according to the steady-state angle. The margin X range is usually 10°~15° to ensure that the DC voltage is not higher than the steady-state voltage at the time of recovery, that is: the lower limit of the inverter side trigger angle ALP_LL=steady-state operation trigger angle ALP-X. Figure 7 As shown in the figure, when the line fault start is 0, the steady-state operation trigger angle ALP is used as the output, and the output is latched at the same time (that is, NSW.I is 0, the output NSW.X1 is selected, and the value of the output NSW.Y is assigned to NSW.X2), and the value of the steady-state operation trigger angle ALP is latched for 500ms (SAMP is the delay value, that is, after the input signal SAMP.X1 disappears, SAMP.Y continues to maintain the SAMP.X2 value for 500ms). When the line fault start is 1, the steady-state operation trigger angle ALP 500ms ago minus the margin X is the lower limit of the inverter side trigger angle (that is, NSW.I is 1, the output NSW.X2 is selected, and NSW.X2 is the steady-state operation trigger angle ALP 500ms ago), and the PI controller outputs the lower limit of the inverter side trigger angle.

[0027] Example of line fault restart system for constant voltage operation DC system: This embodiment provides a technical solution for a line fault restart system of a constant voltage operating DC system. The line fault restart system of a constant voltage operating DC system includes a processor, which is used to execute a computer program to implement the steps of the above-mentioned line fault restart method of the constant voltage operating DC system.

[0028] Since the specific implementation process and principle of the line fault restart system for a constant voltage operating DC system of this embodiment have been described in detail in the embodiment of the line fault restart method for a constant voltage operating DC system, no further details will be given here.

Claims

1. A method for restarting a DC system with a constant voltage after a line fault, characterized in that: The method comprises the following steps: when a DC line fault occurs, the rectifier side performs forced phase shifting until the DC current is 0, and after being processed by the inverter side, the forced phase shifting is cancelled on the rectifier side, and the trigger angle on the rectifier side is restored to a steady-state angle; When the DC current on the inverter side is 0 after the forced phase shift on the rectifier side, the current margin is maintained at the current margin during steady-state operation, and the inverter side trigger angle is reduced until the lower limit of the inverter side trigger angle is reached; after the forced phase shift on the rectifier side is cancelled, the inverter side trigger angle increases to the steady-state angle as the voltage recovers.

2. The line fault restart method of a constant voltage DC system according to claim 1, characterized in that: After the forced phase shift is cancelled on the rectifier side, the trigger angle on the rectifier side does not need to be moved to an angle lower than the steady-state operation angle, and can be directly restored to the steady-state angle.

3. The line fault restart method of a constant voltage DC system according to claim 1, characterized in that: The line fault restart is adopted as a condition to mask the current margin switching strategy so as to keep the current margin at the current margin in steady-state operation.

4. The line fault restart method of a constant voltage DC system according to claim 1, characterized in that: The current margin during the steady-state operation is 0.1 pu.

5. The line fault restart method of a constant voltage DC system according to claim 1, characterized in that: The lower limit of the trigger angle on the inverter side is adaptively changed according to the operating condition setting: the lower limit of the trigger angle on the inverter side is the steady-state operation trigger angle minus a margin preset according to the steady-state angle based on experience.

6. A line fault restart system for a constant voltage DC system, comprising a processor, characterized in that: The processor is used to execute instructions to implement the following steps: When a DC line fault occurs, the rectifier side performs forced phase shifting until the DC current is 0. After being processed by the inverter side, the forced phase shifting is cancelled on the rectifier side, and the trigger angle on the rectifier side is restored to the steady-state angle. When the DC current on the inverter side is 0 after the forced phase shift on the rectifier side, the current margin is maintained at the current margin during steady-state operation, and the inverter side trigger angle is reduced until the lower limit of the inverter side trigger angle is reached; after the forced phase shift on the rectifier side is cancelled, the inverter side trigger angle increases to the steady-state angle as the voltage recovers.

7. The line fault restart system of a constant voltage DC system according to claim 6, characterized in that: After the forced phase shift is cancelled on the rectifier side, the trigger angle on the rectifier side does not need to be moved to an angle lower than the steady-state operation angle, and can be directly restored to the steady-state angle.

8. The line fault restart system of a constant voltage DC system according to claim 6, characterized in that: The line fault restart is adopted as a condition to mask the current margin switching strategy so as to keep the current margin at the current margin in steady-state operation.

9. The line fault restart system of a constant voltage DC system according to claim 6, characterized in that: The current margin during the steady-state operation is 0.1 pu.

10. The line fault restart system of a constant voltage DC system according to claim 6, characterized in that: The lower limit of the trigger angle on the inverter side is adaptively changed according to the operating condition setting: the lower limit of the trigger angle on the inverter side is the steady-state operation trigger angle minus a margin preset according to the steady-state angle based on experience.

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