Hybrid cascade extra-high voltage direct current line protection method based on control response characteristics

Abstract

The invention discloses a hybrid cascading extra-high voltage direct current line protection method based on control response characteristics, and the method specifically comprises the steps: setting a hybrid cascading extra-high voltage direct current transmission system control strategy, and collecting a rectification side direct current, a trigger angle and a direct current instruction value in real time; a rectification side trigger angle gradient and zero sequence current are calculated, whether protection is started or not is judged through the trigger angle gradient, and a trigger angle increment and a control adjustment time difference are calculated after starting; constructing a control adjustment time difference-trigger angle increment curve when different transition resistors outside the forward region have faults; calculating the numerical values of the control adjustment time difference and the trigger angle increment under any fault working condition, judging the numerical values with a control adjustment time difference-trigger angle increment curve, judging that the direct-current line has a fault if the numerical values are positioned above the curve, and otherwise, judging that the direct-current line has no fault; and if the DC line has a fault, further determining a fault pole according to the zero-sequence current. Safe and stable operation of the high-voltage direct-current power transmission system is guaranteed, and a direct-current power transmission line protection system is perfected.

Description

Technical Field

[0001] The present invention belongs to the technical field of relay protection of power systems, and in particular relates to a hybrid cascaded ultra-high voltage direct current line protection method based on control response characteristics. Background Art

[0002] Although the traditional HVDC transmission technology based on thyristor-based line commutated converter (LCC) has the advantages of large transmission capacity, low manufacturing cost, mature and reliable technology, there is a commutation failure problem in the inverter station; while the modular multilevel converter (MMC) based on fully controlled devices has the advantages of supplying power to passive systems, independently controlling active / reactive power, and no commutation failure, and has a good development prospect, but the construction cost is higher under the same withstand voltage level. Therefore, the hybrid DC transmission technology that combines the advantages of the two DC transmission technologies is highly favored.

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

[0004] Usually, the action time of the traveling wave protection of the HVDC transmission line is within tens of milliseconds, while the action time of the current longitudinal differential protection of the HVDC transmission line is as high as 1.1 seconds. There is a long protection blank period between the traveling wave protection and the current longitudinal differential protection of the HVDC transmission line, which may cause other protections to act before the current longitudinal differential protection of the DC transmission line when a high-resistance fault occurs in the HVDC transmission line. Therefore, it is necessary to propose a hybrid cascade UHVDC transmission line protection method in combination with the system control characteristics to improve the DC transmission line protection system. Summary of the invention

[0005] In view of the above situation, the present invention provides a hybrid cascade UHV DC line protection method based on control response characteristics.

[0006] A hybrid cascade UHV DC line protection method based on control response characteristics of the present invention comprises the following steps:

[0007] Step 1: Set the control strategy of the hybrid cascaded DC transmission system: the dual 12-pulse LCC on the rectifier side adopts constant DC current control and fixed minimum trigger angle control; the single 12-pulse LCC on the high-voltage end of the inverter side adopts constant DC voltage control, fixed turn-off angle control, and current deviation control; the three parallel MMCs on the low-voltage end adopt master-slave control, MMC1 is selected as the master station to adopt constant DC voltage control, and MMC2 and MMC3 adopt constant active power control; the LCC on the rectifier and inverter sides both adopt low-voltage current limiting control in the control link.

[0008] Step 2: Collect the DC current on the rectifier side in real time, collect the trigger angle and DC current command value on the rectifier side in real time, and calculate the gradient of the trigger angle As follows:

[0009]

[0010] In the formula, α R is the trigger angle of the rectifier side converter, n c is the sampling point, and the gradient of the trigger angle is determined Is it greater than the start threshold ε? If so, the protection is started and the start time is recorded as t s , otherwise continue to collect the trigger angle and DC current command value on the rectifier side.

[0011] Step 3: Calculate the increment of the trigger angle Δα (n c )=α(n c +1)-α(n c ), if there exists Δα(n c ) is greater than 0 and Δα(n c +1) is less than 0, record sampling point n c +1 corresponds to the trigger angle α max , calculate the trigger angle as α max With the 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 DC current command value on the rectifier side, from the start time t s Start detecting the time t corresponding to the first time when the DC current command value is less than 1.0pu d , and calculate the control adjustment time difference between the moment when the DC current command value starts to decrease and the moment when the protection starts as ΔT z =t d -t s .

[0013] Step 5: The trigger angle increment Δα when the transition resistance outside the forward region increases from 10Ω to 500Ω in steps of 10Ω z The time difference between the control and adjustment is ΔT z , construct the trigger angle increment and control adjustment time difference curve of the fault outside the forward zone of the DC transmission line.

[0014] Step 6: For any fault condition, calculate the trigger angle increment and the control adjustment time difference, and determine the position relationship between the trigger angle increment and the control adjustment time difference curve when the fault occurs outside the zone. If it is above the curve, it indicates that the DC transmission line is faulty. Otherwise, it indicates that the DC transmission line is not faulty. The protection criterion is: Where ΔT x is the control adjustment time difference under any fault condition, Δα x is the trigger angle increment under any fault condition.

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

[0016] Furthermore, in step 2, the acquisition frequency of the rectifier-side DC current, the trigger angle and the DC current command value is not less than 10 kHz.

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

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

[0019] The beneficial technical effects of the present invention are:

[0020] The present invention avoids the problem of a long protection blank period between the traditional DC transmission line traveling wave protection and the current longitudinal differential protection, effectively ensures the safe and stable operation of the high-voltage DC transmission system, and improves the DC transmission line protection system. BRIEF DESCRIPTION OF THE DRAWINGS

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

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

[0023] Figure 3 The present invention is a curve of trigger angle increment and control adjustment time difference under different transition resistance fault conditions outside the forward zone of the DC line.

[0024] Figure 4 It is a simulation result diagram of simulation condition 1 in the present invention.

[0025] Figure 5 It is a simulation result diagram of the simulation condition 2 in the present invention. DETAILED DESCRIPTION

[0026] The present invention is further described in detail below in conjunction with the accompanying drawings and simulation conditions.

[0027] The structure of the hybrid cascade DC transmission system monopolar full voltage operation mode adopted by the present invention is as follows Figure 1 As shown, the converter on the high-voltage end on the left is a 12-pulse LCC, and the converter on the low-voltage end is a 12-pulse LCC. The two 12-pulse converters are connected in series to form a double 12-pulse converter; the converter on the high-voltage end on the right is a 12-pulse LCC, and the low-voltage end uses three MMCs in parallel, which are MMC1, MMC2, and MMC3 from left to right. Figure 2 is a schematic diagram of the control external characteristic curve of the hybrid cascaded DC transmission system, where U dcref The dual 12-pulse LCC on the rectifier side of the hybrid cascade UHV DC transmission system adopts constant DC current control and a constant minimum trigger angle α minControl, the pulsating LCC of the inverter station 12 adopts fixed DC voltage control, fixed shut-off angle control, and current deviation control. The three parallel MMCs of the inverter station adopt master-slave control. MMC1 is selected as the master station to adopt fixed DC voltage control, and the other two MMCs adopt fixed active power control. Both the rectifier station and the inverter station LCC have a low-voltage current limiting link. After a high-voltage DC transmission line fails, the DC voltage decreases and the DC current increases. The rectifier station and the inverter station LCC will enter the low-voltage current limiting control link. At this time, the active power of the inverter station MMC2 and MMC3 is reduced due to the reduction of the DC current command value, and MMC1 maintains the DC outlet voltage unchanged to support the DC voltage of the LCC at the high-voltage end of the inverter station. A hybrid cascade ultra-high voltage DC line protection method based on control response characteristics of the present invention specifically includes the following steps:

[0028] Step 1: Set the control strategy of the hybrid cascaded DC transmission system: the dual 12-pulse LCC on the rectifier side adopts constant DC current control and fixed minimum trigger angle control; the single 12-pulse LCC on the high-voltage end of the inverter side adopts constant DC voltage control, fixed turn-off angle control, and current deviation control; the three parallel MMCs on the low-voltage end adopt master-slave control, MMC1 is selected as the master station to adopt constant DC voltage control, and MMC2 and MMC3 adopt constant active power control; the LCC on the rectifier and inverter sides both adopt low-voltage current limiting control in the control link.

[0029] Step 2: Collect the DC current on the rectifier side in real time, and collect the trigger angle and DC current command value on the rectifier side in real time. The acquisition frequency of the DC current, trigger angle and DC current command value on the rectifier side shall not be less than 10kHz. Calculate the gradient of the trigger angle As follows:

[0030]

[0031] In the formula, α R is the trigger angle of the rectifier side converter, n c is the sampling point, and the gradient of the trigger angle is determined Is it greater than the start threshold ε? If so, the protection is started and the start time is recorded as t s , otherwise continue to collect the trigger angle and DC current command value on the rectifier side.

[0032] Step 3: Calculate the increment of the trigger angle Δα (n c )=α(n c +1)-α(n c ), if there exists Δα(n c ) is greater than 0 and Δα(n c +1) is less than 0, record sampling point n c +1 corresponds to the trigger angle α max , calculate the trigger angle as α maxWith the 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 DC current command value on the rectifier side, from the start time t s Start detecting the time t corresponding to the first time when the DC current command value is less than 1.0pu d , and calculate the control adjustment time difference between the moment when the DC current command value starts to decrease and the moment when the protection starts as ΔT z =t d -t s .

[0034] Target control adjustment time difference ΔT z When a weak fault occurs in the DC transmission line, the DC voltage on the inverter side may not drop below 0.9pu, and the DC current command value on the rectifier side will not decrease. s Start to detect whether the DC current command value decreases within 100ms. If the DC current command value does not decrease, control the adjustment time difference ΔT z The value is 100ms.

[0035] Step 5: The trigger angle increment Δα when the transition resistance outside the forward region increases from 10Ω to 500Ω in steps of 10Ω z The time difference between the control and adjustment is ΔT z , construct the trigger angle increment and control adjustment time difference curve of the fault outside the forward zone of the DC transmission line.

[0036] Step 6: For any fault condition, calculate the trigger angle increment and the control adjustment time difference, and determine the position relationship between the trigger angle increment and the control adjustment time difference curve when the fault occurs outside the zone. If it is above the curve, it indicates that the DC transmission line is faulty. Otherwise, it indicates that the DC transmission line is not faulty. The protection criterion is: Where ΔT x is the control adjustment time difference under any fault condition, Δα x is the trigger angle increment under any fault condition.

[0037] Step 7: Transform according to the polar mode Calculate the zero-mode current I0(t), where I p (t), I n(t) represent the positive and negative currents respectively, I0(t) and I1(t) represent the zero-mode and line-mode components after current transformation respectively; the fault pole identification criterion is constructed according to the zero-mode fault current. If the zero-mode current I0(t) is less than the negative fault pole selection threshold value, it is determined that the fault is located in the positive line; if the zero-mode current I0(t) is greater than the fault pole selection threshold (the fault pole selection threshold value is 0.01pu), it is determined that the fault is located in the negative line; otherwise, it is determined to be a bipolar line fault.

[0038] The following is a specific exemplary embodiment described with reference to actual working conditions.

[0039] It should be noted that, taking the hybrid cascaded DC line system and the Baihetan-Jiangsu hybrid cascaded UHV DC project in my country as an example to verify the method, an electromagnetic transient simulation model of the hybrid cascaded UHV DC transmission system was built. The ground short circuit fault conditions with different transition resistances outside the forward zone of the DC line were simulated. The transition resistance started from 10Ω and increased in steps of 10Ω to 500Ω, and the trigger angle increment Δα under each transition resistance fault outside the forward zone was obtained. z The time difference between the control and adjustment is ΔT z Value, trigger angle increment Δα under different transition resistance z The time difference between the control and adjustment is ΔT z The points on the two-dimensional plane are connected into lines to form a control adjustment time difference-firing angle increment curve, such as Figure 3 shown.

[0040] Simulation condition 1:

[0041] The simulation sets up a 300Ω ground fault on the positive line of the DC transmission line 500km away from the rectifier side, a 400Ω ground fault on the negative line 1400km away from the rectifier side, a bipolar line short circuit fault 1000km away from the rectifier side, and a metallic ground fault on the A-phase LCC AC busbar on the inverter side. The simulation obtains the control adjustment time difference ΔT x With the firing angle increment Δα x The value of the control adjustment time difference-firing angle increment curve is given as follows Figure 4 When a 300Ω ground fault occurs in the positive line of the DC transmission line 500km away from the rectifier side, the calculated ΔT x is 38.6ms, Δα x is 0.5979; when a 400Ω ground fault occurs in the negative line of the DC transmission line 1400km away from the rectifier side, the calculated ΔT x is 46.6ms, Δα x =0.5139; ΔT calculated when a bipolar short circuit occurs at a distance of 1000 km from the rectifier side x20.7ms, Δα x is 0.7696; the ΔT calculated when a metallic ground fault occurs on the A-phase of the inverter-side LCC AC busbar x 7.6ms, Δα x It is 0.8462. From the simulation results, it can be seen that when a fault occurs in the DC transmission line area, ΔT x and Δα x The points formed are always located at ΔT x -Δα x When the DC line fault occurs above the curve, the inverter side LCC AC busbar A phase metallic ground fault is determined by ΔT x The point formed by Δα is located at ΔT x -Δα x Below the curve, it is determined that there is no fault in the DC line. x and Δα x The points formed and ΔT x -Δα x The positional relationship of the curves can accurately identify DC line faults.

[0042] Simulation condition 2:

[0043] The simulation was conducted for the positive line of the DC line at 200km, 1000km, and 2086km from the rectifier side, and the positive line outside the positive zone had a ground fault with transition resistance of 50Ω, 100Ω, 300Ω, and 500Ω, respectively. The ΔT under different fault conditions was obtained by simulation. x and Δα x In ΔT x -Δα x The position on the curve is Figure 5 As shown. Figure 5 It can be seen that when faults with different transition resistances occur at different locations of the DC transmission line, ΔT under various working conditions x and Δα x The points formed are all located at ΔT x -Δα x Above the curve, when the DC transmission line fails outside the forward zone, ΔT under different transition resistance conditions x and Δα x The points formed are all located at ΔT x -Δα x The DC transmission line fault can be accurately identified under different transition resistance conditions.

[0044] Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in the field without creative work are within the scope of protection of the present application.

Claims

1. A hybrid cascade UHV DC line protection method based on control response characteristics, characterized in that: The following steps are involved: Step 1: Set the control strategy of the hybrid cascaded DC transmission system: the dual 12-pulse LCC on the rectifier side adopts constant DC current control and fixed minimum trigger angle control; the single 12-pulse LCC on the high-voltage side of the inverter side adopts constant DC voltage control, fixed turn-off angle control, and current deviation control; the three parallel MMCs on the low-voltage side adopt master-slave control, and MMC1 is selected as the master station to adopt constant DC voltage control, and MMC2 and MMC3 adopt constant active power control; the LCC on the rectifier side and the inverter side both adopt low-voltage current limiting control in the control link; Step 2: Collect the DC current on the rectifier side in real time, collect the trigger angle and DC current command value on the rectifier side in real time, and calculate the gradient of the trigger angle As follows: In the formula, α R is the trigger angle of the rectifier side converter, n c is the sampling point, and the gradient of the trigger angle is determined Is it greater than the start threshold ε? If so, the protection is started and the start time is recorded as t s , otherwise continue to collect the trigger angle and DC current command value on the rectifier side; Step 3: Calculate the increment of the trigger angle Δα (n c )=α(n c +1)-α(n c ), if there exists Δα(n c ) is greater than 0 and Δα(n c +1) is less than 0, record sampling point n c +1 corresponds to the trigger angle α max , calculate the trigger angle as α max With the 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 Start detecting the time t corresponding to the first time when the DC current command value is less than 1.0pu d , and calculate the control adjustment time difference between the moment when the DC current command value starts to decrease and the moment when the protection starts as ΔT z =t d -t s ; Step 5: The trigger angle increment Δα when the transition resistance outside the forward region increases from 10Ω to 500Ω in steps of 10Ω z The time difference between the control and adjustment is ΔT z , construct the trigger angle increment and control adjustment time difference curve of the fault outside the forward zone of the DC transmission line; Step 6: For any fault condition, calculate the trigger angle increment and the control adjustment time difference, and determine the position relationship between the trigger angle increment and the control adjustment time difference curve when the fault occurs outside the zone. If it is above the curve, it indicates that the DC transmission line is faulty. Otherwise, it indicates that the DC transmission line is not faulty. The protection criterion is: Where ΔT x is the control adjustment time difference under any fault condition, Δα x is the trigger angle increment under any fault condition; Step 7: Transform according to the polar mode Calculate the zero-mode current I0(t), where I p (t), I n (t) represents the positive and negative currents, respectively, I0(t) and I1(t) represent the zero-mode and line-mode components after current transformation, respectively; The fault pole identification criterion is constructed according to the zero-mode fault current. If the zero-mode current I0(t) is less than the negative fault pole selection threshold, the fault is judged to be located in the positive line; if the zero-mode current I0(t) is greater than the fault pole selection threshold, the fault is judged to be located in the negative line; otherwise, it is judged to be a bipolar line fault.

2. A hybrid cascade UHV DC line protection method based on control response characteristics according to claim 1, characterized in that: In the step 2, the acquisition frequency of the rectifier-side DC current, the trigger angle and the DC current command value is not less than 10 kHz.

3. The hybrid cascade UHV DC line protection method based on control response characteristics according to claim 1, characterized in that: In step 4, the target control adjusts the time difference ΔT z When a weak fault occurs in the DC transmission line, if the DC voltage on the inverter side does not drop below 0.9pu, the protection will start at t s Start to detect whether the DC current command value decreases within 100ms. If the DC current command value does not decrease, control the adjustment time difference ΔT z The value is 100ms.

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

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