Non-unit two-zone protection method for dc transmission line of high voltage dc grid

Abstract

The application provides a non-unit two-section protection method for a high-voltage direct current power grid direct current transmission line, comprising the following steps: (1) in a normal state, real-time detection of current and voltage at the installation position of the non-unit protection is performed, and line mode voltage and zero mode voltage are obtained; when the current rate of change is greater than a threshold value, step (2) is performed; (2) fault polar selection is performed, and a pre-action instruction is then issued to a circuit breaker on a potential fault pole to perform first commutation; meanwhile, step (3) is performed; (3) a section I protection time window is started; if the voltage rate of change at the installation position of the non-unit protection meets section I protection criteria, a second commutation instruction is issued to a direct current circuit breaker to perform second commutation; otherwise, step (4) is performed; (4) a section II protection time window is started after a delay; if the line mode voltage rate of change at the installation position of the non-unit protection meets section II protection criteria, a second commutation instruction is issued to a direct current circuit breaker to perform second commutation; otherwise, a pre-action return instruction is issued to the direct current circuit breaker.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of power system DC transmission, in particular to a non-unit two-section protection method for DC transmission line of HVDC power grid. BACKGROUND

[0002] DC distribution network protection can be divided into non-unit protection and unit protection. Unit protection determines the fault section through the electrical information collected by the protection devices installed at different positions of the system, and requires communication between protection devices. Common unit protection includes differential protection and networked protection. Related research has reduced the adverse effects of communication synchronization time error to some extent, but still requires the transmission of a large number of non-logical signals and the existence of communication delay, so it is still difficult to be used as the main protection for flexible DC transmission lines at this stage.

[0003] Non-unit protection does not rely on communication between protection devices, and starts to act when the fault measurement value collected by the protection device reaches the action setting value. Non-unit protection of DC distribution network mainly includes overcurrent protection, current differential protection, parameter identification protection, etc. Non-unit protection only relies on single-end fault characteristic quantity, can quickly identify faults, and is very suitable for main protection of DC lines in HVDC power grid. However, most of the existing non-unit protection has insufficient sensitivity to remote high-resistance faults.

[0004] In order to solve the above problems, it is necessary to propose a non-unit two-section protection scheme for DC transmission line of HVDC power grid. SUMMARY

[0005] The purpose of the present application is to overcome the shortcomings of the prior art and provide a non-unit two-section protection method for DC transmission line of HVDC power grid.

[0006] In order to achieve the above purpose, the technical scheme adopted by the present application is:

[0007] The present application provides a non-unit two-section protection method for DC transmission line of HVDC power grid, comprising the following steps in sequence:

[0008] (1) In the normal state, the non-unit protection detects the current i and the voltage u at the installation position of the non-unit protection in real time, and obtains the line mode voltage u1 and the zero mode voltage u0 through the phase-to-mode transformation formula;

[0009] When the current rate of change di / dt is greater than the threshold value η, step (2) is performed;

[0010] (2) The zero mode voltage u0 at the installation position of the non-unit protection and the fault pole selection criterion are used to select the fault pole, and then a pre-action instruction is issued to the circuit breaker on the potential fault pole. After receiving the pre-action instruction, the DC circuit breaker performs the first commutation; At the same time, step (3) is performed;

[0011] (3) In the time window T1, if the voltage rate of change at the non-unit protection installation site meets the I-section protection criterion, the non-unit protection determines that the fault occurs in the area, and issues a secondary commutation instruction to the DC circuit breaker. After receiving the secondary commutation instruction, the DC circuit breaker performs secondary commutation to isolate the fault. If the I-section protection criterion is not met, step (4) is performed;

[0012] (4) The time window T2 of the II-section protection is started after a delay t delay If the line mode voltage rate of change at the non-unit protection installation site meets the II-section protection criterion, the non-unit protection determines that the fault occurs in the area, and issues a secondary commutation instruction to the DC circuit breaker. After receiving the instruction, the DC circuit breaker performs secondary commutation. If the II-section protection criterion is not met, the non-unit protection determines that the fault occurs outside the area, and issues a pre-action return instruction to the DC circuit breaker. After receiving the instruction, the DC circuit breaker returns to the normal conduction state.

[0013] Based on the above, the calculation method of the line mode voltage u1 and the zero mode voltage u0 is as follows:

[0014]

[0015] Wherein, u p and u n are the positive voltage and negative voltage of the DC transmission line at the non-unit protection installation site.

[0016] Based on the above, the fault polarity selection criterion in step (2) is as follows:

[0017]

[0018] Wherein, u 03 is the zero mode voltage amplitude in the time window T3; u set0 is set to be greater than the maximum value of the zero mode voltage under a bipolar fault.

[0019] Based on the above, the time window T3 is selected to be 100 µs.

[0020] Based on the above, the I-section protection criterion is as follows:

[0021]

[0022] M is the absolute value of the voltage rate of change in the time window T1; u th1 is the threshold value; D is the maximum absolute value of the voltage rate of change of all possible external faults in the T1 time window; K1 is the reliability coefficient; i0 is the normal current value under a non-fault state; ∆i is the current increment; I set is the current increment threshold value.

[0023] Based on the above, the selection method of the time window T1 is: taking the time point when the current rate of change di / dt is greater than the threshold value η as the reference time point, and selecting each 100 µs before and after the reference time point as the time window T1.

[0024] Based on the above, the time window T1 and the time window T2 satisfy the following relationship:

[0025] (T1 / 2+t delay )<Δt min

[0026] (T1 / 2+t delay +T2)>Δt max

[0027] Wherein, Δt min and Δt max are the minimum value and the maximum value of the time difference between the fault traveling wave and the secondary traveling wave generated by the action of the opposite DC circuit breaker to reach the non-unit protection respectively;

[0028] The value formula of Δt is:

[0029]

[0030] Wherein, v1 is the propagation speed of the line mode traveling wave, l line is the length of the transmission line, t I is the fault detection time of the I section protection, t CB is the action time of the DC circuit breaker.

[0031] Based on the above, the II section protection criterion is constructed by using the line mode voltage rate of change, and is:

[0032]

[0033] Wherein, P is the line mode voltage rate of change during the time window T2; u th2 and u th3 are the threshold values designed under the single-pole grounding fault and the double-pole grounding fault respectively; K2 is a reliability coefficient; σ1 and σ2 are the minimum line mode voltage rates of change in the time window T2 under all possible external single-pole grounding faults and double-pole grounding faults respectively.

[0034] The present application has outstanding substantial features and significant progress compared with the prior art, in particular:

[0035] The application provides a non-unit two-section type protection method for a high-voltage direct current power grid DC transmission line, which uses the current change rate to distinguish between fault and non-fault states. The amplitude and decay rate of the traveling wave caused by external faults are more obvious than those of the traveling wave caused by internal low-resistance faults, and the amplitude and decay rate are not obvious compared with internal high-resistance faults, so the I-section protection can detect internal low-resistance faults of the DC transmission line and external low-resistance faults of the transmission line by using the voltage change rate. When the first wave head of the secondary traveling wave caused by the action of the opposite end DC circuit breaker arrives, the line mode voltage of the local protection sharply decreases under internal faults. According to this feature, the II-section protection uses the line mode voltage change rate to identify external high-resistance faults of the transmission line, thereby effectively improving the sensitivity of the non-unit protection to high-resistance faults on the DC side, the algorithm is simple, and the sampling frequency is low. BRIEF DESCRIPTION OF DRAWINGS

[0036] Figure 1 is a flowchart of the method of the application.

[0037] Figure 2 is a wiring diagram of a four-terminal bipolar DC power grid system constructed using the PSCAD / EMTDC platform.

[0038] Figure 3 is a simulation result of the non-unit protection R1 protection using the method of the application. DETAILED DESCRIPTION

[0039] The technical solutions of the application will be further described in detail below through specific embodiments.

[0040] Embodiment 1

[0041] As shown in Figure 1 , the embodiment provides a non-unit two-section type protection method for a high-voltage direct current power grid DC transmission line, comprising the following steps in sequence:

[0042] (1) In the normal state, the current i and the voltage u at the installation place of the non-unit protection are detected in real time, and the line mode voltage u1 and the zero mode voltage u0 are obtained through the phase mode transformation formula;

[0043] The calculation method of the line mode voltage u1 and the zero mode voltage u0 is as follows:

[0044]

[0045] Wherein, u p and u n are the positive voltage and the negative voltage of the DC transmission line at the installation place of the non-unit protection, respectively.

[0046] When the current rate of change di / dt is greater than threshold η, non-unit protection is started, where the threshold η is selected to ensure that the triggering criterion is sufficiently sensitive to internal high resistance faults.

[0047] (2) Fault polar selection is performed using the zero-mode voltage u0 at the non-unit protection installation and the fault polar selection criterion, and then a pre-action command is issued to the circuit breaker on the potential fault pole. After the DC circuit breaker receives the pre-action command, the first commutation is performed. At the same time, step (3) is performed.

[0048] The fault polar selection criterion is:

[0049]

[0050] where u 03 is the zero-mode voltage amplitude within the time window T3; u set0 is set to be greater than the maximum value of the zero-mode voltage under bipolar fault; once the fault polar selection criterion in step (2) is activated, it indicates that the non-unit protection detects a potential internal fault, and the DC circuit breaker starts the first commutation. The time window T3 is selected to be 100 µs.

[0051] (3) The protection time window T1 of section I is started. If the voltage rate of change at the non-unit protection installation meets the section I protection criterion, the non-unit protection determines that the fault occurs in the zone, and issues a secondary commutation command to the DC circuit breaker. After the DC circuit breaker receives the secondary commutation command, the second commutation is performed to isolate the fault. If the section I protection criterion is not met, step (4) is performed.

[0052] The time when the current rate of change di / dt is greater than threshold η is selected as the reference time, and 100 µs before and after the reference time is selected as the time window T1.

[0053] The section I protection criterion is:

[0054]

[0055] M is the absolute value of the voltage rate of change within the time window T1; u th1 is the threshold; D is the maximum absolute value of the voltage rate of change of all possible external faults within the time window T1; K1 is a reliability coefficient, which can be set to 1.2; i0 is the normal current value under non-fault state; Δi is the current increment; I set is the current increment threshold, which is selected to be a small positive value to avoid protection misoperation in reverse fault.

[0056] To ensure that the first wave head of the secondary traveling wave is within the time window T2, there is

[0057] (T1 / 2 + t delay ) < Δt min

[0058] (T1 / 2+t delay +T2)>Δt max

[0059] where Δt min and Δt max are the minimum and maximum values of the time difference between the fault traveling wave and the secondary traveling wave caused by the action of the DC breaker at the opposite end reaching the non-unit protection, respectively.

[0060] The formula for the value of Δt is:

[0061]

[0062] where v1 is the propagation speed of the line mode traveling wave, l line is the length of the transmission line, t I is the fault detection time of the I-section protection, and t CB is the action time of the DC breaker.

[0063] (4) The II-section protection time window T2 is started after a delay t delay If the rate of change of the line mode voltage at the installation site of the non-unit protection meets the II-section protection criterion, the non-unit protection determines that the fault occurs in the area and issues a secondary commutation instruction to the DC breaker. After receiving the instruction, the DC breaker performs secondary commutation. If the II-section protection criterion is not met, the non-unit protection determines that the fault occurs outside the area and issues a pre-action return instruction to the DC breaker. After receiving the instruction, the DC breaker returns to the normal conduction state.

[0064] To reduce the risk of II-section protection misoperation, the II-section protection time window T2 should not be too long, and a few milliseconds are sufficient.

[0065] The II-section protection criterion is constructed using the rate of change of the line mode voltage, and is:

[0066]

[0067] where P is the rate of change of the line mode voltage during the time window T2; u th2 and u th3 are the threshold values designed for single-pole and double-pole ground faults, respectively; K2 is a reliability coefficient, which can be set to 1.2; σ1 and σ2 are the minimum line mode voltage change rates within the time window T2 for all possible external single-pole and double-pole ground faults, respectively.

[0068] Simulation verification

[0069] A four-terminal bipolar DC power grid is constructed using the PSCAD / EMTDC platform, and the system connection mode is as follows: Figure 2As shown, the rated voltage of the DC grid is ±400kV. Both ends of the line are equipped with 50mH current-limiting inductors and DC circuit breakers. The detailed MMC model is taken from the standard model provided by the Cigre working group, and the line uses a frequency-dependent parameter model. The DC grid parameters are shown in Table 1.

[0070] Table 1. Main parameters of high voltage DC power grid

[0071]

[0072] The specific implementation method of the non-unit two-stage protection scheme in this embodiment includes the following steps:

[0073] (1) The metal single-pole grounding fault f 11 and high-resistance single-pole grounding fault f 12 (R f =500Ω) is set at the end of line l4;

[0074] (2) Simulation results of non-unit protection R1 are as follows Figure 3 As shown, Figure 3 In the middle, (a) is f 11 The voltage u under fault conditions, (b) is f 11 under fault conditions, du / dt, (c) is f 11 The zero-mode voltage u0,(d) under fault conditions is f 12 The voltage u,(e) under fault conditions is f 12 The voltage du / dt under fault conditions, (f) is f 12 The zero-mode voltage u0,(g) under fault conditions is f 12 The line-mode voltage u1,(h) under fault conditions is f 12 du1 / dt under fault conditions;

[0075] (3) Real-time detection of current i and voltage u at non-unit protection R1, and obtaining line-mode voltage and zero-mode voltage through phase-mode transformation formula. Figure 3 From (a) and (d), we can see that the fault f 11 and fault f 12 When this occurs, after the fault voltage traveling wave arrives, the voltage at the non-unit protection R1 drops. The current change rate di / dt is greater than the threshold η (set to 1.01kA / ms), and the non-unit protection is activated.

[0076] (4) Fault pole selection is performed using the zero-mode voltage u0 at non-unit protection R1 and the fault pole selection criterion. Figure 3 From (c) and (f), we can see that in f 11 and f 12 After the fault traveling wave arrives, the zero-mode voltage u0 during period T3 is less than u. -set0 (Set as -5kV), it can be known that the fault f at this time11 and f 12 are positive pole ground faults. Then pre-action instructions are issued to the circuit breakers on the fault poles, and the DC circuit breaker performs first commutation after receiving the pre-action instructions; meanwhile, step 5 is performed;

[0077] (5) The protection time window T1 of the I section is started, and Figure 3 (b) shows that M reaches the maximum value 7693.95 kV / ms at 1.68 ms, which is greater than the threshold value u th1 (set as 5467.54 kV / ms), and the I section protection criterion is met. According to step (4), the protection identifies the fault f 11 as an internal positive pole ground fault, and issues a second commutation instruction to the DC circuit breaker, which performs second commutation after receiving the instruction. As shown in Figure 3 (e), M reaches the maximum value 3245.23 kV / ms at 1.68 ms, which is less than the threshold value u th1 , and it is known that the fault f 12 does not meet the I section protection criterion. The protection time window T3 of the II section is started after the delay t delay (set as 2.5 ms). As shown in Figure 3 (h), P reaches the minimum value -2130.50 kV / ms at 4.7 ms, which is less than the threshold value u th2 (set as -873.99 kV / ms), and it is known that the fault f 12 meets the II section protection criterion. Therefore, the non-unit protection R1 determines that the fault f 12 is an internal positive pole ground fault, and issues a second commutation instruction to the DC circuit breaker, which performs second commutation after receiving the instruction.

[0078] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application and not to limit it; although the present application has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the specific embodiments of the present application can be modified or some technical features can be replaced by equivalent replacements; without departing from the spirit of the technical solutions of the present application, they should be covered in the technical solution range of the present application claimed.

Claims

1. A non-unit two-stage protection method for DC transmission lines in a high-voltage DC power grid, characterized in that, The steps include the following sequence: (1) Under normal conditions, the non-unit protection detects the current i and voltage u at the installation location of the non-unit protection in real time, and obtains the line mode voltage u1 and zero mode voltage u0 through the phase mode transformation formula; When the rate of change of current di / dt is greater than the threshold η, step (2) is executed. (2) Use the zero-mode voltage u0 at the non-unit protection installation location and the fault pole selection criterion to select the fault pole, and then issue a pre-operation command to the circuit breaker on the potential fault pole. After receiving the pre-operation command, the DC circuit breaker performs the first commutation; at the same time, execute step (3). (3) Within the time window T1, if the voltage change rate at the non-unit protection installation location meets the protection criteria of the first stage, the non-unit protection determines that the fault occurs within the zone and issues a secondary commutation command to the DC circuit breaker. After receiving the secondary commutation command, the DC circuit breaker performs secondary commutation to isolate the fault; if the protection criteria of the first stage are not met, then step (4) is executed. The protection criteria for segment I are: M is the absolute value of the rate of change of voltage within time window T1; u th1 The threshold is denoted by D; the maximum absolute value of the voltage change rate under all possible external faults within the time window T1 is denoted by D; K1 is the reliability coefficient; i0 is the normal current value under non-fault conditions; ∆i is the current increment; I set The current increment threshold; (4) The protection time window T2 of segment II is delayed by t delay If the line mode voltage change rate at the non-unit protection installation location meets the Stage II protection criterion, the non-unit protection determines that the fault occurs within the zone and issues a secondary commutation command to the DC circuit breaker. The DC circuit breaker performs secondary commutation upon receiving the command. If the Stage II protection criterion is not met, the non-unit protection determines that the fault occurs outside the zone and issues a pre-action return command to the DC circuit breaker. The DC circuit breaker returns to the normal conduction state upon receiving the command. Time window T1 and time window T2 satisfy the following relationship: (T1 / 2+t delay )<Δt min (T1 / 2+t delay +T2)>Δt max Where, Δt min and Δt max These are the minimum and maximum values ​​of the time difference between the fault traveling wave and the secondary traveling wave generated by the operation of the DC circuit breaker at the opposite end reaching the non-unit protection, respectively. The formula for determining the value of Δt is: Where v1 is the propagation speed of the linear mode traveling wave, l line t is the transmission line length. I For the fault detection time of stage I protection, t CB This refers to the operating time of the DC circuit breaker. The protection criterion for stage II is constructed using the line-mode voltage change rate, and is as follows: Where P is the rate of change of the line-mode voltage during time window T2; u th2 and u th3 σ1 and σ2 are the threshold values ​​designed for single-pole and double-pole grounding faults, respectively; K2 is the reliability coefficient; σ1 and σ2 are the minimum line-mode voltage change rates within time window T2 under all possible external single-pole and double-pole grounding faults, respectively.

2. The non-unit two-stage protection method for DC transmission lines in high-voltage DC power grids according to claim 1, characterized in that, The calculation methods for the line-mode voltage u1 and the zero-mode voltage u0 are as follows: Among them, u p and u n These are the positive and negative voltages of the DC transmission line at the non-unit protection installation location, respectively.

3. The non-unit two-stage protection method for DC transmission lines in high-voltage DC power grids according to claim 1, characterized in that, The fault selection criterion in step (2) is: In the formula, u 03 The zero-mode voltage amplitude within time window T3; u set0 It is set to be greater than the maximum value of the zero-mode voltage under bipolar fault conditions.

4. The non-unit two-stage protection method for DC transmission lines in high-voltage DC power grids according to claim 3, characterized in that, The time window T3 is set to 100µs.

5. The non-unit two-stage protection method for DC transmission lines in high-voltage DC power grids according to claim 1, characterized in that, The method for selecting the time window T1 is as follows: take the moment when the current change rate di / dt is greater than the threshold η as the reference time, and select 100µs before and after the reference time as the time window T1.

Images