A control method for suppressing commutation failure caused by faults in the sending-end AC system

Abstract

The present invention discloses a control method for suppressing commutation failure caused by a fault in a sending-end AC system. Specifically, the method comprises: a high-voltage direct current transmission system control and protection device collects in real time the effective value of the AC bus voltage on the rectifier side, the DC current, the advance trigger command value of the constant current control and the advance trigger angle command value of the constant turn-off angle control on the inverter side, and the current control command value transmitted from the inverter side to the rectifier side; calculates the voltage drop value of the AC bus on the sending side at the current moment t and its maximum value within one cycle; utilizes the AC bus voltage information on the rectifier side before and after the fault to implement fault detection; calculates the current command value output by the current limiting link and simultaneously detects the control interaction on the inverter side; inputs the calculated current limiting link command value into the original constant current control link to control the DC current. At the same time, the current margin on the inverter side is reduced to reduce improper control interaction. The present invention can effectively reduce commutation failure caused by faults in the sending-end AC system and ensure the safe and stable operation of the system.

Description

Technical Field

[0001] The present invention belongs to high voltage direct current (HVDC) transmission technology, and in particular relates to a control method for suppressing commutation failure caused by a sending-end AC system fault. Background Art

[0002] High-voltage direct current (HVDC) transmission technology is widely used for long-distance power transmission due to its advantages, including large transmission capacity, long transmission distances, low line losses, and no synchronization issues. However, the converters at both ends of the HVDC system use thyristors without self-shutoff capability, making commutation failures a common problem in DC transmission. Commutation failures can cause DC current surges, DC voltage drops, and even DC system shutdown, significantly impacting the safe and stable operation of the system.

[0003] Current research on commutation failures has largely focused on commutation failures caused by AC system faults at the receiving end. However, the sending and receiving ends of a HVDC transmission system have a close electrical coupling and control coordination relationship. Faults in the sending AC system can still affect the operation of the receiving AC system through system control responses and parameter transfer, and may even cause commutation failures. However, conventional commutation failure suppression measures currently rely primarily on the receiving AC bus voltage amplitude as a criterion. However, during the failure and recovery process of the sending AC system, the receiving AC bus voltage amplitude does not change significantly. In other words, conventional commutation failure suppression measures cannot effectively prevent commutation failures caused by sending AC system faults, which seriously threatens the safe and stable operation of the DC system. Therefore, it is urgent to propose commutation failure suppression measures for sending AC system faults. Summary of the Invention

[0004] In view of the problems existing in the prior art, the present invention provides a control method for suppressing commutation failure caused by a fault in a sending-end AC system.

[0005] A control method for suppressing commutation failure caused by a fault in a sending-end AC system of the present invention comprises the following steps:

[0006] Step A: Data collection:

[0007] The HVDC system control and protection device collects the DC current I in real time at a sampling frequency of 10kHz. di (t), RMS value of AC bus voltage at sending end U rms (t), inverter side constant current control advance trigger command value β cc (t) and fixed turn-off angle control advance trigger angle command value β cea (t), the current control command value I transmitted from the inverter side to the rectifier side ord (t), where t is the sampling time.

[0008] Step B: Calculate the AC bus voltage drop value:

[0009] The control protection device calculates the sending end AC bus voltage drop value ΔU at the current time t rec (t):

[0010] ΔU rec (t)=|(U rms (t)-U N |

[0011] Among them, U N It is the value of the rated voltage of the AC bus at the sending end.

[0012] Step C: Fault detection:

[0013] Step C1: calculate the voltage drop value ΔU of the sending-end AC busbar at the current time t and the previous 39 sampling times. rec (t), ΔU rec (t-1), ΔU rec (t-2), ..., ΔU rec (t-37), ΔU rec (t-38), ΔU rec (t-39), forming the sending-end AC bus voltage drop value sequence ΔU(t) at the current time t, and obtaining the maximum value ΔU of the sending-end AC bus voltage drop value sequence ΔU(t) recmax .

[0014] Step C2: Determine ΔU recmax Greater than the threshold ΔU set Is it true? If so, go to step D; if not, return to step C1.

[0015] Step D: Current limiting link control:

[0016] Step D1: Calculate the current command value ΔI output by the current limiting link according to the following formula: ord :

[0017]

[0018] Where: ΔU L , ΔU H They are respectively the low-pressure starting value and high-pressure starting value of the current limiting link, ΔI H It is the maximum output value of the current limiting link.

[0019] Step D2, calculate I ord_new (t) = I ord (t)-ΔI ord , and I ord_new (t) is put into constant current control on the rectifier side, and I ord_new(t) The input duration is a milliseconds, and the current on the rectifier side of the HVDC transmission is controlled.

[0020] Step E: Determine β cc (t) is greater than β cea (t) Is it true? If so, change the current margin value of the inverter side constant current control to I M1 , and the changed duration is a milliseconds, the current on the inverter side of the high-voltage direct current transmission is controlled; if not, return to step C2.

[0021] Furthermore, ΔU in step C2 set The value is 0.15pu.

[0022] Furthermore, ΔU in step D1 L The value is 0.15pu, ΔU H The value is 0.65pu, ΔI H The value is 0.3pu.

[0023] Furthermore, in step D2 and step E, the value of a is 12.

[0024] Furthermore, in step E, M1 The value is 0.05pu.

[0025] The beneficial technical effects of the present invention are:

[0026] 1. The present invention constructs a current limiting link based on the change in the sending-end AC bus voltage, which can quickly detect sending-end faults and limit the DC current during the fault recovery phase. Compared with other commutation failure suppression measures, it can more quickly and effectively reduce commutation failures caused by sending-end faults.

[0027] 2. The parameter selection of the present invention takes into account a series of issues such as communication delay, and is more reasonable than other methods.

[0028] 3. After detecting a fault in the sending-end AC system, the present invention further detects improper control interaction at the receiving end and quickly reduces the current margin control after detecting improper interaction, which can effectively reduce the control interaction time and help the system recover quickly.

[0029] 4. This invention does not require changes to the hardware structure of the DC transmission system. Instead, it simply collects the three-phase voltage signals of the inverter-side AC busbar in real time based on existing system measurement information and performs simple operations such as multiplication, division, addition, subtraction, and comparison to implement control functions. This system has low hardware and software requirements, offers rapid response, and is suitable for practical engineering applications. DETAILED DESCRIPTION

[0030] The present invention is further described in detail below in conjunction with specific implementation methods and simulation experiments.

[0031] A control method for suppressing commutation failure caused by a fault in a sending-end AC system of the present invention comprises the following steps:

[0032] Step A: Data collection:

[0033] The HVDC system control and protection device collects the DC current I in real time at a sampling frequency of 10kHz. di (t), RMS value of AC bus voltage at sending end U rms (t), inverter side constant current control advance trigger command value β cc (t) and fixed turn-off angle control advance trigger angle command value β cea (t), the current control command value I transmitted from the inverter side to the rectifier side ord (t), where t is the sampling time.

[0034] Step B: Calculate the AC bus voltage drop value:

[0035] The control protection device calculates the sending end AC bus voltage drop value ΔU at the current time t rec (t):

[0036] ΔU rec (t)=|(U rms (t)-U N |

[0037] Among them, U N It is the value of the rated voltage of the AC bus at the sending end.

[0038] Step C: Fault detection:

[0039] Step C1: calculate the voltage drop value ΔU of the sending-end AC busbar at the current time t and the previous 39 sampling times. rec (t), ΔU rec (t-1), ΔU rec (t-2), ..., ΔU rec (t-37), ΔU rec (t-38), ΔU rec (t-39), forming the sending-end AC bus voltage drop value sequence ΔU(t) at the current time t, and obtaining the maximum value ΔU of the sending-end AC bus voltage drop value sequence ΔU(t) recmax .

[0040] Step C2: Determine ΔU recmax Greater than the threshold ΔU set (ΔU set If yes, go to step D; if no, return to step C1.

[0041] Step D: Current limiting link control:

[0042] Step D1: Calculate the current command value ΔI output by the current limiting link according to the following formula: ord :

[0043]

[0044] Where: ΔU L , ΔU H They are respectively the low-pressure starting value and high-pressure starting value of the current limiting link, ΔI H is the maximum output value of the current limiting link (ΔU L The value is 0.15pu, ΔU H The value is 0.65pu, ΔI H The value is 0.3pu).

[0045] Step D2, calculate I ord_new (t) = I ord (t)-ΔI ord , and I ord_new (t) is put into constant current control on the rectifier side, and I ord_new (t) The duration of the input is a milliseconds (a is 12), and the current on the rectifier side of the high-voltage direct current transmission is controlled.

[0046] Step E: Determine β cc (t) is greater than β cea (t) Is it true? If so, change the current margin value of the inverter side constant current control to I M1 (I M1 The value is 0.05pu), and the changed duration is a millisecond, and the current on the inverter side of the high-voltage direct current transmission is controlled; if not, return to step C2.

[0047] The principle and basis of the present invention for suppressing commutation failure caused by a fault in the sending-end AC system are as follows:

[0048] The mechanism by which commutation failures caused by sending-end AC system faults are primarily caused by a surge in DC current and improper inverter-side control interaction during the recovery process. During this process, the sending-end AC bus voltage drops significantly, while the receiving-end AC bus voltage changes slightly. Therefore, the receiving-end low-voltage current limiting mechanism cannot effectively limit the surge in DC current. Therefore, the present invention constructs a current limiting mechanism based on the sending-end AC bus voltage drop, reduces the current margin during the fault recovery process, and thereby improves improper control interaction, reducing the risk of commutation failure during the fault recovery period.

[0049] When the AC bus voltage drop value at the sending end is detected to be less than the high voltage starting value of the current limiting link, that is, ΔU H <ΔU HAt this time, the current limiting link does not start, the current limiting link output is 0, and the system operates normally according to the original constant current control mode; when it is detected that the AC bus voltage drop value at the sending end is greater than the minimum starting value of the current limiting link but less than the maximum starting value of the current limiting link, the current limiting link starts and the current limiting link output is And input to the constant current control mode to limit the growth of DC current; when it is detected that the voltage drop of the AC busbar at the sending end is greater than the maximum starting value of the current limiting link, the current limiting link will output the maximum value of the DC current command value ΔI H , input to the constant current control module at the sending end to limit the DC current surge. At the same time, if the inverter side control interaction is detected during this process, that is, β cc (t)>β cea At (t), the inverter-side control system will quickly send a signal to reduce the current margin control from 0.1pu to 0.05pu. This ultimately allows for a smooth recovery of the sending-end AC system fault and reduces the risk of commutation failure.

[0050] Essentially, DC current surges and improper control interactions are the root causes of commutation failures. Based on fault detection, DC current is limited based on the sending-end AC bus voltage drop, and the current margin is reduced when control interactions are detected. This can quickly reduce DC current surges and control interaction time, preventing commutation failures caused by sending-end AC system faults.

[0051] Simulation experiment:

[0052] To verify the effectiveness of the proposed control method in suppressing commutation failures caused by faults in the sending-end AC system, a CIGRE standard test system under PSCAD / EMTDC was used as a simulation model. Various fault types were set at the rectifier-side AC busbars using the proposed method and the existing CIGRE control strategy models. The fault inductance ranged from 0 to 0.8 h, with smaller fault inductance indicating more severe faults. The fault duration was 2.0 s, and the fault duration was 0.1 s. Tables 1 and 2 show the commutation failures that occurred in the system under different fault conditions using the proposed strategy and the CIGRE control mode.

[0053] Table 1 Commutation failure when three-phase ground fault and single-phase ground fault occur in the rectifier side AC system

[0054]

[0055]

[0056] Table 2 Commutation failure when two-phase ground fault and two-phase fault occur in the rectifier side AC system

[0057]

[0058] Note: 0 means no commutation failure occurs, 1 means one commutation failure occurs.

[0059] As shown in Table 1, when the system uses the original CIGRE control mode, commutation failure occurs when a three-phase ground fault occurs on the rectifier side AC system with a fault inductance between 0 Hz and 0.5 Hz, and when a single-phase ground fault occurs between 0 and 0.1 Hz. Furthermore, in the case of a three-phase ground fault, commutation failure also occurs when the fault severity is relatively mild, such as 0.5 Hz. However, under these corresponding fault conditions, commutation failure is prevented using the control method proposed in this invention.

[0060] Similarly, in Table 2, when a two-phase ground fault (fault inductance of 0-0.3H) or a two-phase unground fault (fault inductance of 0-0.2H) occurs on the rectifier-side AC bus, commutation failure occurs in the system under CIGRE control mode. However, under the corresponding control strategy proposed by the present invention, commutation failure does not occur. As shown in Tables 1 and 2, the control method proposed by the present invention can effectively avoid commutation failure under different fault conditions, reducing the probability of commutation failure.

Claims

1. A control method for suppressing commutation failure caused by a fault in a sending-end AC system, characterized in that: The following steps are involved: Step A: Data collection: The HVDC system control and protection device collects the DC current I in real time at a sampling frequency of 10kHz. di (t), RMS value of AC bus voltage at sending end U rms (t), inverter side constant current control advance trigger command value β cc (t) and fixed turn-off angle control advance trigger angle command value β cea (t), the current control command value I transmitted from the inverter side to the rectifier side ord (t), where t is the sampling time; Step B: Calculate the AC bus voltage drop value: The control protection device calculates the sending end AC bus voltage drop value ΔU at the current time t rec (t): ΔU rec (t)=|(U rms (t)-U N | Among them, U N is the value of the rated voltage of the AC bus at the sending end; Step C: Fault detection: Step C1: calculate the voltage drop value ΔU of the sending-end AC busbar at the current time t and the previous 39 sampling times. rec (t), ΔU rec (t-1), ΔU rec (t-2), ..., ΔU rec (t-37), ΔU rec (t-38), ΔU rec (t-39), forming the sending-end AC bus voltage drop value sequence ΔU(t) at the current time t, and obtaining the maximum value ΔU of the sending-end AC bus voltage drop value sequence ΔU(t) recmax ; Step C2: Determine ΔU recmax Greater than the threshold ΔU set Is it true? If so, go to step D; if not, return to step C1; Step D: Current limiting link control: Step D1: Calculate the current command value ΔI output by the current limiting link according to the following formula: ord : Where: ΔU L , ΔU H They are respectively the low-pressure starting value and high-pressure starting value of the current limiting link, ΔI H is the maximum output value of the current limiting link; Step D2, calculate I ord_new (t) = I ord (t)-ΔI ord , and I ord_new (t) is put into constant current control on the rectifier side, and I ord_new (t) The duration of input is a milliseconds, and the current on the rectifier side of the HVDC transmission is controlled; Step E: Determine β cc (t) is greater than β cea (t) Is it true? If so, change the current margin value of the inverter side constant current control to I M1 , and the changed duration is a milliseconds, the current on the inverter side of the high-voltage direct current transmission is controlled; if not, return to step C2.

2. A control method for suppressing commutation failure caused by a fault in a sending-end AC system according to claim 1, characterized in that: ΔU in step C2 set The value is 0.15pu.

3. The control method for suppressing commutation failure caused by a fault in a sending-end AC system according to claim 1, characterized in that: ΔU in step D1 L The value is 0.15pu, ΔU H The value is 0.65pu, ΔI H The value is 0.3pu.

4. The control method for suppressing commutation failure caused by a fault in a sending-end AC system according to claim 1, characterized in that: In step D2 and step E, the value of a is 12.

5. The control method for suppressing commutation failure caused by a fault in a sending-end AC system according to claim 1, characterized in that: In the step E, M1 The value is 0.05pu.

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