Hybrid Cascaded UHVDC Line Protection Method Based on Control Response Characteristics

Abstract

This invention discloses a hybrid cascaded ultra-high voltage direct current (UHVDC) line protection method based on control response characteristics. Specifically, it involves: setting a control strategy for the hybrid cascaded UHVDC transmission system; real-time acquisition of rectifier-side DC current, firing angle, and DC current command values; calculating the rectifier-side firing angle gradient and zero-sequence current; determining whether to activate protection based on the firing angle gradient; calculating the firing angle increment and control adjustment time difference after activation; constructing a control adjustment time difference-firing angle increment curve for different transition resistance faults outside the forward zone; calculating the values ​​of control adjustment time difference and firing angle increment under any fault condition and comparing them with the control adjustment time difference-firing angle increment curve; if the values ​​are above the curve, it is determined that there is a DC line fault; otherwise, it is determined that there is no DC line fault; if there is a DC line fault, the fault pole is further determined based on the zero-sequence current. This invention ensures the safe and stable operation of the high-voltage direct current transmission system and improves the DC transmission line protection system.

Description

Technical Field

[0001] This invention belongs to the field of power system relay protection technology, and particularly relates to a hybrid cascaded ultra-high voltage DC line protection method based on control response characteristics. Background Technology

[0002] Traditional HVDC transmission technology based on thyristors and line commutated converters (LCCs) offers advantages such as large transmission capacity, low manufacturing cost, and mature and reliable technology. However, it suffers from commutation failure issues at inverter stations. Modular multilevel converters (MMCs), based on fully controllable devices, offer advantages such as power supply to passive systems, independent control of active and reactive power, and no commutation failure, making them a promising technology. However, their construction cost is higher for the same voltage rating. Therefore, hybrid HVDC transmission technology, which combines the advantages of both technologies, is gaining popularity.

[0003] Currently, there is considerable research on the structure of hybrid DC transmission systems, and many hybrid DC transmission system structures have been applied in engineering, such as hybrid DC transmission with one pole consisting of cascaded LCC and MMC. Therefore, configuring fast and reliable protection for DC transmission lines is of great significance for ensuring the safe and stable operation of high-voltage DC transmission systems.

[0004] Typically, the operating time of traveling wave protection for HVDC transmission lines is within tens of milliseconds, while the operating time of current-coupled differential protection for HVDC transmission lines is as high as 1.1 seconds. This creates a significant protection gap between traveling wave protection and current-coupled differential protection, which may lead to other protection systems operating before the current-coupled differential protection in the event of a high-resistance fault on the HVDC transmission line. Therefore, it is necessary to propose a hybrid cascaded UHVDC transmission line protection method that considers the system control characteristics to improve the HVDC transmission line protection system. Summary of the Invention

[0005] In view of the above situation, the present invention provides a hybrid cascaded ultra-high voltage DC line protection method based on control response characteristics.

[0006] The present invention provides a hybrid cascaded ultra-high voltage direct current line protection method based on control response characteristics, comprising the following steps:

[0007] Step 1: Set the control strategy for the hybrid cascaded DC transmission system: The dual 12-pulse LCCs on the rectifier side adopt constant DC current control and constant minimum firing angle control; the single 12-pulse LCC on the high-voltage side of the inverter side adopts constant DC voltage control, constant turn-off angle control, and current deviation control; the three parallel MMCs on the low-voltage side adopt master-slave control, with MMC1 selected as the master station and adopting constant DC voltage control, and MMC2 and MMC3 adopting constant active power control; among them, the LCCs on the rectifier side and the inverter side both adopt low-voltage current limiting control in the control loop.

[0008] Step 2: Real-time acquisition of rectifier-side DC current, real-time acquisition of rectifier-side firing angle and DC current command value, and calculation of firing angle gradient. As shown in the following formula:

[0009]

[0010] In the formula, α R n is the firing angle of the rectifier-side converter. c Using the sampling point, determine the gradient of the trigger angle. If the value exceeds the start threshold ε, then activate protection and record the start time as t. s Otherwise, continue to collect the firing angle and DC current command values ​​on the rectifier side.

[0011] Step 3: Calculate the increment Δα(n) of the firing angle after the start-up time. c )=α(n c +1)-α(n c If there exists Δα(n) c ) is greater than 0 and Δα(n) c If +1) is less than 0, record the sampling point n. c The trigger angle corresponding to +1 is α. max The trigger angle is calculated as α. max With start time t s Corresponding trigger angle α ts The difference is the firing angle increment Δα z =α max -α(t s ).

[0012] Step 4: For the collected rectifier-side DC current command value, from the start time t s The moment t corresponds to the first time the DC current command value is less than 1.0 pu. d And calculate the control adjustment time difference ΔT between the moment when the DC current command value begins to decrease and the moment when the protection is activated. z =t d -t s .

[0013] Step 5: Based on the firing angle increment Δα when the transition resistance outside the forward region starts from 10Ω and increases in steps of 10Ω to 500Ω. z The time difference between control and adjustment is ΔT z The trigger angle increment and control adjustment time difference curves for faults outside the forward zone of DC transmission lines were constructed.

[0014] Step 6: For any fault condition, calculate the trigger angle increment and control adjustment time difference, and determine the positional relationship between the trigger angle increment and control adjustment time difference curves when there is a fault outside the protection zone. If it is above the curve, it indicates a fault in the DC transmission line; otherwise, it indicates that no fault has occurred in the DC transmission line. The protection criterion is as follows: Where ΔT x Let Δα be the control adjustment time difference under any fault condition. x This represents the trigger angle increment under any fault condition.

[0015] Step 7: Based on polar mode transformation Calculate the zero-mode current I0(t), where I p (t), I n (t) represent the positive and negative currents, respectively, and I0(t) and I1(t) represent the zero-mode and line-mode components after current transformation, respectively. Based on the zero-mode fault current, a fault pole identification criterion is constructed. If the zero-mode current I0(t) is less than the negative fault pole selection threshold, the fault is determined to be located in the positive pole line. If the zero-mode current I0(t) is greater than the fault pole selection threshold, the fault is determined to be located in the negative pole line. Otherwise, it is determined to be a bipolar line fault.

[0016] Furthermore, in step 2, the sampling frequency for the rectifier-side DC current, firing angle, and DC current command value is no less than 10kHz.

[0017] Furthermore, in step 4, the target control adjustment time difference ΔT z When a minor fault occurs in a DC transmission line, if the DC voltage on the inverter side does not drop below 0.9 pu, from the protection start time t s The system begins by detecting whether the DC current command value decreases within 100ms. If the DC current command value does not decrease, the control adjusts the time difference ΔT. z The value is 100ms.

[0018] Furthermore, in step 7, the fault selection threshold value is set to 0.01 pu.

[0019] The beneficial technical effects of this invention are as follows:

[0020] This invention avoids the problem of a long protection gap between the traveling wave protection and the current longitudinal differential protection of traditional DC transmission lines, effectively ensuring the safe and stable operation of the high voltage DC transmission system and improving the DC transmission line protection system. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the single-pole full-voltage operation mode of the hybrid cascaded DC transmission system in an embodiment of the present invention.

[0022] Figure 2 This is a schematic diagram of the control external characteristic curves of the hybrid cascaded DC transmission system in an embodiment of the present invention (where (a) is the LCC of the rectifier station and inverter station, and (b) is the MMC1-3 of the inverter station).

[0023] Figure 3 This is the firing angle increment and control adjustment time difference curve for different transition resistance fault conditions occurring outside the positive zone of a DC line in this invention.

[0024] Figure 4 This is a simulation result diagram of simulation condition one in this invention.

[0025] Figure 5 This is a simulation result diagram of simulation condition two in this invention. Detailed Implementation

[0026] The invention will now be described in further detail with reference to the accompanying drawings and simulation conditions.

[0027] The structure of the hybrid cascaded DC transmission system with single-pole full-voltage operation mode adopted in this invention is as follows: Figure 1 As shown, the high-voltage converter on the left is a 12-pulse LCC, and the low-voltage converter is a 12-pulse LCC. The two 12-pulse converters are connected in series to form a dual 12-pulse converter. The high-voltage converter on the right is a 12-pulse LCC, and the low-voltage converter uses three MMCs connected in parallel, which are MMC1, MMC2, and MMC3 from left to right. Figure 2 This is a schematic diagram of the control external characteristic curves of a hybrid cascaded DC transmission system, where U dcref This is a DC voltage reference value. The hybrid cascaded UHVDC transmission system uses dual 12-pulse LCCs on the rectifier side with constant DC current control and a constant minimum firing angle α. minThe inverter station's 12-pulse LCC employs constant DC voltage control, constant turn-off angle control, and current deviation control. The three parallel MMCs in the inverter station use master-slave control, with MMC1 selected as the master station and using constant DC voltage control, while the other two MMCs use constant active power control. Both the rectifier station and inverter station LCCs have low-voltage current limiting mechanisms. After a fault occurs in the high-voltage DC transmission line, the DC voltage decreases and the DC current increases. The rectifier station and inverter station LCCs will enter the low-voltage current limiting control mechanism. At this time, the active power of inverter station MMC2 and MMC3 decreases due to the reduced DC current command value, while MMC1 maintains a constant DC output voltage, thus supporting the DC voltage of the inverter station's high-voltage end LCC. This invention provides a hybrid cascaded UHVDC line protection method based on control response characteristics, specifically including the following steps:

[0028] Step 1: Set the control strategy for the hybrid cascaded DC transmission system: The dual 12-pulse LCCs on the rectifier side adopt constant DC current control and constant minimum firing angle control; the single 12-pulse LCC on the high-voltage side of the inverter side adopts constant DC voltage control, constant turn-off angle control, and current deviation control; the three parallel MMCs on the low-voltage side adopt master-slave control, with MMC1 selected as the master station and adopting constant DC voltage control, and MMC2 and MMC3 adopting constant active power control; among them, the LCCs on the rectifier side and the inverter side both adopt low-voltage current limiting control in the control loop.

[0029] Step 2: Real-time acquisition of the rectifier-side DC current, firing angle, and DC current command values. The acquisition frequency of the rectifier-side DC current, firing angle, and DC current command values ​​should be no less than 10kHz. Calculate the gradient of the firing angle. As shown in the following formula:

[0030]

[0031] In the formula, α R n is the firing angle of the rectifier-side converter. c Using the sampling point, determine the gradient of the trigger angle. If the value exceeds the start threshold ε, then activate protection and record the start time as t. s Otherwise, continue to collect the firing angle and DC current command values ​​on the rectifier side.

[0032] Step 3: Calculate the increment Δα(n) of the firing angle after the start-up time. c )=α(n c +1)-α(n c If there exists Δα(n) c ) is greater than 0 and Δα(n) c If +1) is less than 0, record the sampling point n. c The trigger angle corresponding to +1 is α. max The trigger angle is calculated as α. maxWith start time t s Corresponding trigger angle α ts The difference is the firing angle increment Δα z =α max -α(t s ).

[0033] Step 4: For the collected rectifier-side DC current command value, from the start time t s The moment t corresponds to the first time the DC current command value is less than 1.0 pu. d And calculate the control adjustment time difference ΔT between the moment when the DC current command value begins to decrease and the moment when the protection is activated. z =t d -t s .

[0034] Target control adjustment time difference ΔT z When a minor fault occurs in a DC transmission line, the DC voltage on the inverter side may not drop below 0.9 pu, and the DC current command value on the rectifier side will not decrease. This situation is addressed from the protection start-up time t. s The system begins by detecting whether the DC current command value decreases within 100ms. If the DC current command value does not decrease, the control adjusts the time difference ΔT. z The value is 100ms.

[0035] Step 5: Based on the firing angle increment Δα when the transition resistance outside the forward region starts from 10Ω and increases in steps of 10Ω to 500Ω. z The time difference between control and adjustment is ΔT z The trigger angle increment and control adjustment time difference curves for faults outside the forward zone of DC transmission lines were constructed.

[0036] Step 6: For any fault condition, calculate the trigger angle increment and control adjustment time difference, and determine the positional relationship between the trigger angle increment and control adjustment time difference curves when there is a fault outside the protection zone. If it is above the curve, it indicates a fault in the DC transmission line; otherwise, it indicates that no fault has occurred in the DC transmission line. The protection criterion is as follows: Where ΔT x Let Δα be the control adjustment time difference under any fault condition. x This represents the trigger angle increment under any fault condition.

[0037] Step 7: Based on polar mode transformation Calculate the zero-mode current I0(t), where I p (t), I n(t) represent the positive and negative current quantities, respectively, and I0(t) and I1(t) represent the zero-mode and line-mode components after current transformation, respectively. The fault pole identification criterion is constructed based on the zero-mode fault current. If the zero-mode current I0(t) is less than the negative fault pole selection threshold value, the fault is determined to be located in the positive pole line. If the zero-mode current I0(t) is greater than the fault pole selection threshold value (the fault pole selection threshold value is taken as 0.01pu), the fault is determined to be located in the negative pole line. Otherwise, it is determined to be a bipolar line fault.

[0038] The following describes specific exemplary embodiments using actual working conditions.

[0039] It should be noted that, using the Baihetan-Jiangsu hybrid cascaded UHVDC transmission project in my country as an example to verify the method, an electromagnetic transient simulation model of the hybrid cascaded UHVDC transmission system was built. The simulation covered ground fault conditions with different transition resistances occurring outside the forward direction zone of the DC line. The transition resistance started at 10Ω and increased in steps of 10Ω to 500Ω, obtaining the firing angle increment Δα for each transition resistance fault condition outside the forward direction zone. z The time difference between control and adjustment is ΔT z Numerical value, firing angle increment Δα under different transition resistance conditions. z The time difference between control and adjustment is ΔT z Connecting points on the resulting two-dimensional plane forms a line that constitutes the control adjustment time difference-trigger angle increment curve, such as... Figure 3 As shown.

[0040] Simulation Condition 1:

[0041] The simulation included scenarios such as a 300Ω ground fault occurring 500km from the rectifier side on the positive pole of a DC transmission line, a 400Ω ground fault occurring 1400km from the rectifier side on the negative pole, a bipolar short-circuit fault occurring 1000km from the rectifier, and an A-phase metallic ground fault occurring on the LCC AC bus on the inverter side. The simulation yielded the control regulation time difference ΔT. x With the firing angle increment Δα x The numerical value is given, and its position in the control adjustment time difference-fire angle increment curve is also given. Figure 4 As shown in the figure. Specifically, the calculated ΔT is obtained when a 300Ω ground fault occurs 500km from the rectifier side of the positive line of the DC transmission line. x It is 38.6 ms, Δα x The value is 0.5979; the calculated ΔT is obtained when a 400Ω ground fault occurs on the negative pole of a DC transmission line 1400km away from the rectifier side. x It is 46.6 ms, Δα x The value is 0.5139; the calculated ΔT when a bipolar short-circuit fault occurs 1000km from the rectifier side of the bipolar line. xIt is 20.7 ms, Δα x The value is 0.7696; the ΔT calculated when a phase A metallic ground fault occurs on the inverter-side LCC AC bus. x It is 7.6 ms, Δα x The value is 0.8462. The simulation results show that when a fault occurs within the DC transmission line area, the value is determined by ΔT. x and Δα x The point formed is always located at ΔT x -Δα x Above the curve, a DC line fault is identified; while when a phase A metallic ground fault occurs on the inverter-side LCC AC bus, it is determined by ΔT. x The point formed by Δα and ΔT is located in ΔT. x -Δα x Below the curve, it is determined that there is no fault in the DC line. Based on ΔT x and Δα x The point formed and ΔT x -Δα x The positional relationship of the curves can accurately identify DC line faults.

[0042] Simulation Condition Two:

[0043] The simulations were conducted on ground faults occurring at distances of 200km, 1000km, and 2086km from the rectifier side of the DC line, and on the positive line outside the forward zone, with transition resistances of 50Ω, 100Ω, 300Ω, and 500Ω, respectively. The simulations obtained the ΔT value under different fault conditions. x and Δα x In ΔT x -Δα x Position on the curve as follows Figure 5 As shown. From Figure 5 It can be seen that when faults with different transition resistances occur at different locations on a DC transmission line, the ΔT under various operating conditions... x and Δα x The points formed are all located at ΔT x -Δα x Above the curve, when a DC transmission line experiences a fault outside the forward zone, ΔT varies depending on the transition resistance. x and Δα x The points formed are all located at ΔT x -Δα x On the curve. Faults in DC transmission lines can be accurately identified under different transition resistance conditions.

[0044] Obviously, the described embodiments are only a part of the embodiments of this application, not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this application without inventive effort are within the scope of protection of this application.

Claims

1. A hybrid cascaded UHVDC line protection method based on control response characteristics, characterized in that, Includes the following steps: Step 1: Set the control strategy for the hybrid cascaded DC transmission system: The dual 12-pulse LCCs on the rectifier side adopt constant DC current control and constant minimum firing angle control; the single 12-pulse LCC on the high-voltage side of the inverter side adopts constant DC voltage control, constant turn-off angle control, and current deviation control; the three parallel MMCs on the low-voltage side adopt master-slave control, with MMC1 selected as the master station and adopting constant DC voltage control, and MMC2 and MMC3 adopting constant active power control; among them, the LCCs on both the rectifier side and the inverter side adopt low-voltage current limiting control in the control loop. Step 2: Real-time acquisition of rectifier-side DC current, real-time acquisition of rectifier-side firing angle and DC current command value, and calculation of firing angle gradient. As shown in the following formula: In the formula, α R n is the firing angle of the rectifier-side converter. c Using the sampling point, determine the gradient of the trigger angle. If the value is greater than the start threshold ε, then activate protection and record the start time as t. s Otherwise, continue to collect the firing angle and DC current command values ​​on the rectifier side; Step 3: Calculate the increment Δα(n) of the firing angle after the start-up time. c )=α(n c +1)-α(n c If there exists Δα(n) c ) is greater than 0 and Δα(n) c If +1) is less than 0, record the sampling point n. c The trigger angle corresponding to +1 is α. max The trigger angle is calculated as α. max With start time t s Corresponding trigger angle α ts The difference is the firing angle increment Δα z =α max -α(t s ); Step 4: For the collected rectifier-side DC current command value, from the start time t s The moment t corresponds to the first time the DC current command value is less than 1.0 pu. d And calculate the control adjustment time difference ΔT between the moment when the DC current command value begins to decrease and the moment when the protection is activated. z =t d -t s ; Step 5: Based on the firing angle increment Δα when the transition resistance outside the forward region starts from 10Ω and increases in steps of 10Ω to 500Ω. z and control adjustment time difference ΔT z Construct the trigger angle increment and control adjustment time difference curve for faults outside the forward zone of DC transmission lines; Step 6: For any fault condition, calculate the trigger angle increment and control adjustment time difference, and determine the positional relationship between the trigger angle increment and control adjustment time difference curves when there is a fault outside the protection zone. If it is above the curve, it indicates a fault in the DC transmission line; otherwise, it indicates that no fault has occurred in the DC transmission line. The protection criterion is as follows: Where ΔT x Let Δα be the control adjustment time difference under any fault condition. x The firing angle increment under any fault condition; Step 7: Based on polar mode transformation Calculate the zero-mode current I0(t), where I p (t), I n (t) represent the positive and negative current quantities, respectively, and I0(t) and I1(t) represent the zero-mode and line-mode components after current transformation, respectively; Based on the zero-mode fault current, a fault pole identification criterion is constructed. If the zero-mode current I0(t) is less than the negative fault pole selection threshold, the fault is determined to be located in the positive pole line. If the zero-mode current I0(t) is greater than the fault pole selection threshold, the fault is determined to be located in the negative pole line. Otherwise, it is determined to be a bipolar line fault.

2. The hybrid cascaded UHVDC line protection method based on control response characteristics according to claim 1, characterized in that, In step 2, the sampling frequency of the rectifier-side DC current, firing angle, and DC current command value is not less than 10kHz.

3. The hybrid cascaded UHVDC line protection method based on control response characteristics according to claim 1, characterized in that, In step 4, the target control adjustment time difference ΔT z When a minor fault occurs in a DC transmission line, if the DC voltage on the inverter side does not drop below 0.9 pu, from the protection start time t s The system begins by detecting whether the DC current command value decreases within 100ms. If the DC current command value does not decrease, the control adjusts the time difference ΔT. z The value is 100ms.

4. The hybrid cascaded UHVDC line protection method based on control response characteristics according to claim 1, characterized in that, In step 7, the fault selection threshold value is set to 0.01 pu.

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