Longitudinal differential protection method for line containing hybrid power flow controller

Abstract

The invention provides a line longitudinal differential protection method comprising a hybrid power flow controller. The method comprises the following steps: collecting power frequency current at two ends of each phase line and calculating to obtain each phase differential current and each phase braking current; acquiring a negative sequence current starting auxiliary criterion according to each phase of differential current, acquiring a ratio braking action criterion according to each phase of differential current and each phase of braking current, and performing asymmetric fault judgment in the line area in combination with the negative sequence current starting auxiliary criterion and the ratio braking action criterion; obtaining a symmetric three-phase fault criterion according to the differential current of each phase and carrying out symmetric three-phase fault judgment in the line area; obtaining a magnetizing inrush current criterion according to each phase of differential current, and carrying out magnetizing inrush current judgment according to the magnetizing inrush current criterion; and performing line longitudinal differential protection according to the asymmetric fault judgment result, the symmetric three-phase fault judgment result and the excitation surge current judgment result. According to the invention, line longitudinal differential protection maloperation and refusal operation caused by access of the hybrid power flow controller can be effectively solved, and the line protection reliability is improved.

Description

Technical Field

[0001] The invention relates to the technical field of relay protection of power systems, and in particular to a line longitudinal differential protection method. Background Art

[0002] With the large-scale development and utilization of renewable energy, such as wind power, solar power and other new energy sources, the structure and operation mode of the power system have changed significantly. The intermittent and unpredictable nature of these new energy sources makes the power flow distribution in the power grid more complex and dynamic. In this case, the traditional power network may encounter the problem of uneven power flow distribution, that is, some transmission lines may become bottlenecks due to overload, while other lines may have underutilized capacity.

[0003] To meet this challenge, the Hybrid Power Flow Controller (HPFC) has been introduced into the power grid as an advanced power electronic device. The hybrid power flow controller consists of components such as series transformers, parallel transformers, and cascaded H-bridges. It can achieve fast, continuous and flexible control of line power flow by adjusting the voltage and current amplitude and phase at both ends of the line. Such regulation capabilities help balance the load in the power grid, alleviate bottleneck problems, and improve the stability and efficiency of the entire power system.

[0004] However, the integration of hybrid power flow controllers has also brought new problems to the existing line differential protection. Traditional differential protection relies on detecting the differential current on the line to make fault judgments. Under the action of the hybrid power flow controller, even in normal operation, non-zero differential currents will appear on the line. This situation may cause the differential protection to malfunction or refuse to operate, thus affecting the safe operation of the power grid.

[0005] According to the characteristics of the hybrid power flow controller, it will produce a phase shift effect on the current on both sides. When the system is operating normally, under ideal circumstances, if current transformers No. 1 and No. 2 are still used to construct the longitudinal differential protection, the differential current will not be zero. At the same time, the braking current will be reduced to a certain extent, which will cause the protection to malfunction. The differential current and braking current are shown in the following formula.

[0006]

[0007] Braking ratio It can be seen that when the phase shift angle When the phase shift angle is small, k is small and there is no risk of false operation. When it is larger, the braking ratio k will increase accordingly, and it may enter the longitudinal differential protection action area and cause false operation.

[0008] The invention patent with the patent number CN112271707B discloses that the present application constructs the protection start criterion according to the change of the voltage gradient value of the DC line. After the protection start criterion is satisfied, the time domain energy value of the fault voltage reverse wave at both ends of the fault point is calculated. According to the time domain energy value of the fault voltage reverse wave at both ends, the fault identification criterion inside and outside the area and the fault pole selection criterion are constructed, which can determine whether the high-voltage DC transmission line is an inside-area fault or an outside-area fault. When it is determined to be an inside-area fault, combined with the fault pole selection criterion, the positive pole inside-area fault, the negative pole inside-area fault or the bipolar inside-area fault can be calculated. Therefore, there is no need to delay 500ms in the project to avoid misoperation due to outside-area faults, which improves the speed and reliability of DC transmission line protection. However, this invention has poor flexibility for the special protection requirements for the access of additional equipment.

[0009] Therefore, a new longitudinal differential protection method is needed for lines with hybrid power flow controllers. This method should be able to accurately identify fault conditions by improving protection algorithms and logic while keeping the original current transformer positions unchanged, and avoid protection misoperation or refusal to operate due to the presence of hybrid power flow controllers. Summary of the invention

[0010] In view of the technical problem in the prior art that the connection of equipment such as a hybrid power flow controller easily leads to false operation or refusal of protection, the present invention proposes a line differential protection method containing a hybrid power flow controller, which can improve the protection algorithm and logic on the basis of keeping the position of the original line differential protection current transformer unchanged, so as to correctly judge the fault.

[0011] In order to achieve the above object, the technical solution of the present invention is achieved as follows:

[0012] A line longitudinal differential protection method including a hybrid power flow controller comprises the following steps:

[0013] S1: Collect the power frequency current at both ends of each phase line and calculate the differential current and braking current of each phase;

[0014] S2: Obtain the negative sequence current starting auxiliary criterion according to the differential current of each phase, obtain the ratio braking action criterion according to the differential current of each phase and the braking current of each phase, and make asymmetric fault judgment in the line area by combining the negative sequence current starting auxiliary criterion and the ratio braking action criterion;

[0015] S3: Obtain symmetrical three-phase fault judgment criteria according to the differential current of each phase and perform symmetrical three-phase fault judgment in the line area;

[0016] S4: obtaining the excitation inrush current criterion according to the differential current of each phase, and performing the excitation inrush current determination according to the excitation inrush current criterion;

[0017] S5: Perform line longitudinal differential protection based on the asymmetric fault determination results, the symmetrical three-phase fault determination results and the excitation inrush current determination results.

[0018] Furthermore, the power frequency current described in step S1 includes the power frequency current of the primary side sending end of the A-phase series transformer Power frequency current at the primary side of the B-phase series transformer The power frequency current of the primary side sending end of the C-phase series transformer A-phase series transformer primary side receiving end power frequency current Power frequency current at the primary receiving end of the B-phase series transformer The power frequency current of the primary side of the C-phase series transformer

[0019] Furthermore, the calculation method of the differential current of each phase is:

[0020]

[0021] Among them, I r_a is the differential current of phase A; I r_b is the B phase differential current; I r_c is the C phase differential current;

[0022] The calculation method of the braking current of each phase is:

[0023]

[0024] Among them, I res_a A is the phase braking current; I res_b is the braking current of phase B; I res_c is the C phase braking current.

[0025] Further, the method for obtaining the negative sequence current starting auxiliary criterion in step S2 is: using the A phase differential current I r_a , B phase differential current I r_b and C phase differential current I r_c Calculate the negative sequence component I of the three-phase differential current phasor r_2 : Define the auxiliary criterion for negative sequence current starting as:

[0026] I r_2 >nI E

[0027] Among them, I E is the rated current of the primary winding of the series transformer, and n is the threshold coefficient.

[0028] Further, the ratio braking action criterion described in step S2 is:

[0029]

[0030] Among them, I op.min is the minimum operating current; I res.g is the inflection point current; K is the ratio braking coefficient.

[0031] Furthermore, the method for determining an asymmetric fault in the line area is as follows: if any phase among the three phases satisfies the ratio braking action criterion and also satisfies the negative sequence current starting auxiliary criterion, it is determined that an asymmetric fault occurs in the line area.

[0032] Further, the symmetrical three-phase fault criterion described in step S3 is:

[0033]

[0034] Where m is the threshold.

[0035] Further, the method for obtaining the excitation inrush current criterion according to the differential current of each phase described in step S4 is: using FFT fast Fourier transform to extract the second harmonic component in the differential current vector of each phase, and comparing it with the power frequency component of the differential current vector to obtain the excitation inrush current criterion:

[0036] |I r2_a |>K d |I r_a0 |

[0037] |I r2_b |>K d |I r_b0 |

[0038] |I r2_c |>K d |I r_c0 |

[0039] Among them, I r2_a ,I r2_b and I r2_c are the second harmonic components of the differential current of phases A, B and C, I r_a0 ,I r_b0 ,I r_c0 are the power frequency components of the differential current vectors of phases A, B and C, respectively, K d is the second harmonic braking coefficient.

[0040] Furthermore, the method for determining a symmetrical three-phase fault in the line area is as follows: if any one of the three phases satisfies the symmetrical three-phase fault criterion, it is determined that a symmetrical three-phase fault has occurred in the line area; the method for determining an excitation inrush current is as follows: if any one of the three phases satisfies the excitation inrush current criterion, it is determined that an excitation inrush current exists in the three phases.

[0041] Further, the method for performing line longitudinal differential protection described in step S5 is: if the excitation surge current determines that there is an excitation surge current in the three phases, then the line longitudinal differential protection action signal is locked; if the excitation surge current determines that there is no excitation surge current in the three phases, the asymmetric fault determination result and the symmetrical three-phase fault determination result are detected, and if there is an asymmetric fault or a symmetrical three-phase fault, the line longitudinal differential protection action is performed, otherwise the line longitudinal differential protection action is not performed.

[0042] The beneficial effects of the present invention are as follows: the present invention is applicable to lines containing hybrid power flow controllers. Under the premise of not changing the original protection measurement point configuration, the present invention avoids the problems of false operation and refusal of line differential protection caused by the access of the hybrid power flow controller, realizes the correct judgment and action of line faults, and improves the reliability of protection. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0044] Figure 1 is a flow chart of the method of the present invention.

[0045] Figure 2 It is a topological diagram of a two-terminal power supply system including a hybrid power flow controller.

[0046] Figure 3 The action logic diagram of the line differential protection containing a hybrid power flow controller is given.

[0047] Figure 4 This is a diagram showing the longitudinal differential protection action during normal operation at different gears in the simulation of the present invention.

[0048] Figure 5 It is a diagram of the differential current waveform of each phase and its second harmonic content when the excitation surge current occurs in the simulation of the present invention. DETAILED DESCRIPTION

[0049] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0050] The topology diagram of the two-terminal power supply system with hybrid power flow controller is as follows: Figure 2As shown in the figure, a 220kv double-ended single-circuit line system simulation model is built, and a hybrid power flow controller is connected in series to the system for operation. By adjusting its different gears, the current and voltage phase angles on both sides of the hybrid power flow controller are changed to regulate the line power flow.

[0051] A line differential protection method including a hybrid power flow controller, such as Figure 1 As shown, the steps include:

[0052] S1: Collect the power frequency current at both ends of each phase line and calculate the differential current and braking current of each phase.

[0053] In the differential protection mechanism, by comparing the power frequency current magnitude and direction measured at both ends of the line at the same time, it is possible to detect whether there is a fault. Ideally, if no fault occurs and the influence of factors such as line impedance is ignored, the inflow and outflow currents at both ends of the line should be equal and opposite in direction. When the difference between the two is detected to exceed the preset threshold, it may mean that a short circuit or other type of fault has occurred. At this time, the protection device will trigger the corresponding action to isolate the faulty part to avoid greater damage.

[0054] The specific operation is as follows: Use the current transformer 1 on the sending end busbar side to collect the power frequency current of the primary side sending end of the A-phase series transformer Power frequency current at the primary side of the B-phase series transformer The power frequency current of the primary side sending end of the C-phase series transformer The power frequency current of the primary side of the A-phase series transformer is collected by using the current transformer 2 on the sending side busbar side. Power frequency current at the primary receiving end of the B-phase series transformer The power frequency current of the primary side of the C-phase series transformer

[0055] Using the power frequency current of the primary side sending end of the A-phase series transformer The power frequency current of the primary side of the transformer connected in series with phase A Calculate the differential current I of phase A r_a :

[0056]

[0057] Calculate the braking current I of phase A res_a :

[0058]

[0059] Using the power frequency current of the primary side sending end of the B-phase series transformer The power frequency current of the primary side receiving end of the B-phase series transformer Calculate the B phase differential current I r_b :

[0060]

[0061] Calculate the B phase braking current I res_b :

[0062]

[0063] Using the power frequency current of the primary side sending end of the C-phase series transformer The power frequency current of the primary side of the C-phase series transformer Calculate the C phase differential current I r_c :

[0064]

[0065] Calculate the C phase braking current I res_c :

[0066]

[0067] The differential current is used to determine whether a fault has occurred in the zone, and the braking current is used to prevent false protection caused by external short circuit.

[0068] S2: Obtain the negative sequence current starting auxiliary criterion according to the differential current of each phase, obtain the ratio braking action criterion according to the differential current of each phase and the braking current of each phase, and make a judgment on asymmetric faults in the line area by combining the negative sequence current starting auxiliary criterion and the ratio braking action criterion.

[0069] Using the A phase differential current I r_a , B phase differential current I r_b and C phase differential current I r_c Calculate the negative sequence component I of the three-phase differential current phasor r_2 :

[0070]

[0071] The auxiliary criterion for negative sequence current starting is:

[0072] I r_2 >nI E

[0073] Among them, I E It is the rated current of the primary winding of the series transformer. The rated currents of the two windings on the primary side are the same, and the coefficient n is 0.1~0.2.

[0074] When an asymmetric fault occurs in the zone, the asymmetry of the phase parameters makes the three-phase differential current unequal, thus generating a negative-sequence differential current component. During normal operation, although the protection may enter the action zone due to the access of the hybrid power flow controller, the three-phase parameters are symmetrical and there is no negative-sequence component. Therefore, adding a negative-sequence current starting criterion can effectively determine whether an asymmetric fault occurs in the zone or the normal operation state.

[0075] Ratio braking action criteria:

[0076]

[0077] Among them, the minimum operating current I op.min =0.1~0.2I E ; Inflection point current: I res.g =0.3~0.6I E ; Ratio braking coefficient: K = 0.5 ~ 1.0. If the above formula is satisfied, the longitudinal differential protection will send out an action signal, and the action characteristics are shown in the figure.

[0078] Determine an asymmetric fault in the line area: If any of the three phases meets the ratio braking action criterion and falls into the action area while the sequence current starting auxiliary criterion is activated, it is determined that an asymmetric fault has occurred in the line area.

[0079] S3: Obtain symmetrical three-phase fault judgment criteria based on the differential currents of each phase and perform symmetrical three-phase fault judgment in the line area.

[0080] The fault criterion for phase A in case of symmetrical three-phase fault is:

[0081] I r_a >mI E

[0082] The fault criterion for phase A in case of symmetrical three-phase fault is:

[0083] I r_b >mI E

[0084] The fault criterion for phase A in case of symmetrical three-phase fault is:

[0085] I r_c >mI E

[0086] The coefficient m is generally 1 to 2, according to the maximum phase shift angle of the hybrid power flow controller. Determine. The differential current amplitude in normal operation is:

[0087]

[0088] Then the maximum differential current during normal operation will not exceed The coefficient m is determined based on the maximum phase shift angle with a certain margin.

[0089] Conduct symmetrical three-phase fault judgment within the line area: If any of the three phases meets the symmetrical three-phase fault judgment criteria, it is determined that a symmetrical three-phase fault has occurred within the line area.

[0090] S5: Obtain the excitation inrush current criterion according to the differential current of each phase, and perform excitation inrush current determination according to the excitation inrush current criterion.

[0091] Extracting the A-phase differential current vector using FFT fast Fourier transform Second harmonic component I r2_a ; The second harmonic component I r2_a With the differential current vector The power frequency component I r_a0 By comparison, the A phase excitation inrush current criterion is obtained, where the differential current vector The power frequency component I r_a0 The differential current vector Using FFT extraction,

[0092]

[0093] If the A-phase excitation inrush current criterion is met, it is determined that the A-phase series transformer has an excitation inrush current;

[0094] Extracting the B-phase differential current vector using FFT Second harmonic component I r2_b ; The second harmonic component I r2_b With the differential current vector The power frequency component I r_b0 By comparison, the criterion for the magnetizing inrush current of phase B is obtained:

[0095] |I r2_b |>K d |I r_b0 |

[0096] If the B-phase excitation inrush current criterion is met, it is determined that the B-phase series transformer has an excitation inrush current;

[0097] Extracting C-phase differential current vector using FFT fast Fourier transform Second harmonic component I r2_c ; The second harmonic component I r2_c With the differential current vector The power frequency component I r_c0 By comparison, the criterion for the C-phase excitation inrush current is obtained:

[0098] |I r2_c |>K d |Ir_c0 |

[0099] If the C-phase excitation inrush current criterion is met, it is determined that the C-phase series transformer has an excitation inrush current.

[0100] The above K d It is the second harmonic braking coefficient, which is generally between 0.15 and 0.2.

[0101] Since the hybrid power flow controller is connected in series with the system and operates, when an excitation inrush current occurs, the longitudinal differential protection will malfunction, so an excitation inrush current criterion needs to be added.

[0102] The magnetizing inrush current is determined according to the magnetizing inrush current criterion: if any phase among the three phases satisfies the magnetizing inrush current criterion, it is determined that there is a magnetizing inrush current in the three phases.

[0103] S6: Perform line longitudinal differential protection based on the asymmetric fault determination results, the symmetrical three-phase fault determination results and the excitation inrush current determination results.

[0104] If the excitation surge current determines that there is an excitation surge current in the three phases, the line longitudinal differential protection action signal will be locked; if the excitation surge current determines that there is no excitation surge current in the three phases, the asymmetric fault determination result and the symmetrical three-phase fault determination result will be detected. If there is an asymmetric fault or a symmetrical three-phase fault, the line longitudinal differential protection action will be performed, otherwise the line longitudinal differential protection action will not be performed.

[0105] The present invention distinguishes asymmetric faults (single-phase grounding, phase-to-phase, two-phase grounding) from symmetrical three-phase faults. Since the three-phase symmetry of the system has no negative-sequence current component during normal operation, and a negative-sequence current component will be generated when an asymmetric fault occurs in the line, this characteristic is utilized to add a negative-sequence current starting criterion on the basis of the original longitudinal differential protection. Only when the two signals are simultaneously actuated can it be determined that an asymmetric fault has occurred in the area; a protection action constant is set for a symmetrical three-phase fault, and when the differential current of one phase is greater than the constant, it is determined that a symmetrical three-phase fault has occurred in the area. In addition, since the hybrid power flow controller is connected in series to the system for operation, when an excitation surge current occurs, the longitudinal differential protection will malfunction, so an excitation surge current locking criterion needs to be added. The protection algorithm and logic can be improved on the basis of the original line longitudinal differential protection current transformer position remaining unchanged to correctly distinguish the fault.

[0106] The present invention simulates and analyzes the longitudinal differential protection action during normal operation at different gears (i.e. different phase shift angles). Figure 4 As shown, it can be seen that the higher the gear position (that is, the larger the phase shift angle), the higher the false operation rate. At this time, the negative sequence current starting criterion can be used to prevent the protection from false operation.

[0107] In addition, when the hybrid power flow controller generates an excitation inrush current, the differential current waveforms of each phase and their second harmonic content are as follows: Figure 5The differential current waveforms of the three phases A, B, and C, as well as the changes in the second harmonic content of each phase are shown. When the second harmonic content is detected to exceed a certain threshold, the protection device will consider it as an excitation inrush current, thereby locking the longitudinal differential protection to avoid its malfunction. Therefore, the excitation inrush current locking criterion of the present invention can effectively prevent the malfunction of the longitudinal differential protection.

[0108] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A line longitudinal differential protection method including a hybrid power flow controller, characterized in that: Includes steps: S1: Collect the power frequency current at both ends of each phase line and calculate the differential current and braking current of each phase; S2: Obtain the negative sequence current starting auxiliary criterion according to the differential current of each phase, obtain the ratio braking action criterion according to the differential current of each phase and the braking current of each phase, and make asymmetric fault judgment in the line area by combining the negative sequence current starting auxiliary criterion and the ratio braking action criterion; S3: Obtain symmetrical three-phase fault judgment criteria according to the differential current of each phase and perform symmetrical three-phase fault judgment in the line area; S4: obtaining the excitation inrush current criterion according to the differential current of each phase, and performing the excitation inrush current determination according to the excitation inrush current criterion; S5: Perform line longitudinal differential protection based on the asymmetric fault determination results, the symmetrical three-phase fault determination results and the excitation inrush current determination results.

2. The line longitudinal differential protection method including a hybrid power flow controller according to claim 1, characterized in that: The power frequency current in step S1 includes the power frequency current at the primary side of the A-phase series transformer Power frequency current at the primary side of the B-phase series transformer The power frequency current of the primary side sending end of the C-phase series transformer A-phase series transformer primary side receiving end power frequency current Power frequency current at the primary receiving end of the B-phase series transformer The power frequency current of the primary side of the C-phase series transformer 3. The line longitudinal differential protection method including a hybrid power flow controller according to claim 2, characterized in that: The calculation method of the differential current of each phase is: Among them, I r_a is the differential current of phase A; I r_b is the B phase differential current; I r_c is the C phase differential current; The calculation method of the braking current of each phase is: Among them, I res_a A is the phase braking current; I res_b is the braking current of phase B; I res_c is the C phase braking current.

4. The line longitudinal differential protection method including a hybrid power flow controller according to claim 3, characterized in that: The method for obtaining the negative sequence current starting auxiliary criterion in step S2 is: using the A phase differential current I r_a , B phase differential current I r_b and C phase differential current I r_c Calculate the negative sequence component I of the three-phase differential current phasor r_2 : Define the auxiliary criterion for negative sequence current starting as: I r_2 >nI E Among them, I E is the rated current of the primary winding of the series transformer, and n is the threshold coefficient.

5. The line longitudinal differential protection method including a hybrid power flow controller according to claim 4, characterized in that: The ratio braking action criterion described in step S2 is: Among them, I op.min is the minimum operating current; I res.g is the inflection point current; K is the ratio braking coefficient.

6. The line longitudinal differential protection method including a hybrid power flow controller according to claim 5, characterized in that: The method for determining an asymmetric fault in the line area is as follows: if any phase among the three phases satisfies the ratio braking action criterion and also satisfies the negative sequence current starting auxiliary criterion, it is determined that an asymmetric fault occurs in the line area.

7. The line longitudinal differential protection method including a hybrid power flow controller according to any one of claims 3 to 6, characterized in that: The symmetrical three-phase fault criterion described in step S3 is: Where m is the threshold.

8. The line longitudinal differential protection method including a hybrid power flow controller according to claim 7, characterized in that: The method for obtaining the excitation inrush current criterion according to the differential current of each phase described in step S4 is: using FFT fast Fourier transform to extract the second harmonic component in the differential current vector of each phase, and comparing it with the power frequency component of the differential current vector to obtain the excitation inrush current criterion: |I r2_a |>K d |I r_a0 | |I r2_b |>K d |I r_b0 | |I r2_c |>K d |I r_c0 | Among them, I r2_a ,I r2_b and I r2_c are the second harmonic components of the differential current of phases A, B and C, respectively, I r_a0 ,I r_b0 ,I r_c0 are the power frequency components of the differential current vectors of phases A, B and C, respectively, K d is the second harmonic braking coefficient.

9. The line longitudinal differential protection method including a hybrid power flow controller according to claim 8, characterized in that: The method for determining a symmetrical three-phase fault in the line area is as follows: if any one of the three phases satisfies the symmetrical three-phase fault criterion, it is determined that a symmetrical three-phase fault has occurred in the line area; the method for determining an excitation inrush current is as follows: if any one of the three phases satisfies the excitation inrush current criterion, it is determined that an excitation inrush current exists in the three phases.

10. The line longitudinal differential protection method including a hybrid power flow controller according to claim 9, characterized in that: The method for performing line longitudinal differential protection described in step S5 is: if the excitation surge current determines that there is an excitation surge current in the three phases, then the line longitudinal differential protection action signal is locked; if the excitation surge current determines that there is no excitation surge current in the three phases, the asymmetric fault determination result and the symmetrical three-phase fault determination result are detected, and if there is an asymmetric fault or a symmetrical three-phase fault, the line longitudinal differential protection action is performed, otherwise the line longitudinal differential protection action is not performed.

Images