A method for line longitudinal differential protection with hybrid power flow controller

Abstract

The application provides a line longitudinal differential protection method containing a hybrid power flow controller, and the steps are as follows: collecting power frequency currents at two ends of each phase line and calculating each phase differential current and each phase braking current; obtaining a negative sequence current starting auxiliary criterion according to each phase differential current, obtaining a ratio braking action criterion according to each phase differential current and each phase braking current, and combining the negative sequence current starting auxiliary criterion and the ratio braking action criterion to determine an asymmetric fault in a line area; obtaining a symmetrical three-phase fault criterion according to each phase differential current and determining a symmetrical three-phase fault in the line area; obtaining an excitation inrush current criterion according to each phase differential current, and determining an excitation inrush current according to the excitation inrush current criterion; and performing line longitudinal differential protection according to the asymmetric fault determination result, the symmetrical three-phase fault determination result and the excitation inrush current determination result. The application can effectively solve the misoperation and refusal of the line longitudinal differential protection caused by the hybrid power flow controller, and improve the reliability of the line protection.

Description

Technical Field

[0001] This invention relates to the technical field of power system relay protection, and in particular to a method for line longitudinal differential protection. Background Technology

[0002] With the large-scale development and utilization of renewable energy sources, such as wind and solar power, the structure and operation mode of the power system have undergone significant changes. The intermittent and unpredictable nature of these renewable energy generation sources makes power flow distribution in the power grid more complex and dynamic. In this situation, traditional power networks may encounter problems of uneven power flow distribution, meaning that some transmission lines may become bottlenecks due to overload, while other lines may experience underutilization of capacity.

[0003] To address this challenge, the Hybrid Power Flow Controller (HPFC), an advanced power electronic device, has been introduced into the power grid. Composed of components such as series transformers, parallel transformers, and cascaded H-bridges, the HPFC enables rapid, continuous, and flexible control of power flow across lines by adjusting the voltage and current amplitudes and phases at both ends of the line. This regulatory capability helps balance loads in the grid, alleviate bottlenecks, and improve the stability and efficiency of the entire power system.

[0004] However, the integration of the hybrid power flow controller also brings new challenges to existing line differential protection systems. Traditional differential protection relies on detecting differential current on the line for fault diagnosis. However, under the control of the hybrid power flow controller, even under normal operating conditions, non-zero differential current will appear on the line. This situation may cause the differential protection system to malfunction or fail to operate, thereby affecting the safe operation of the power grid.

[0005] Based on the characteristics of the hybrid power flow controller, it will induce a phase shift in the current on both sides. When the system is running normally, under ideal conditions, 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 magnitude of 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] Make braking ratio It can be seen that when the phase shift angle When the phase angle is small, k is small and there is no risk of malfunction; when the phase angle is small... When the value is large, the braking ratio k will increase accordingly, which may cause false tripping when entering the longitudinal differential protection operating zone.

[0008] The invention patent with patent number CN112271707B discloses a protection start criterion constructed based on the change in DC line voltage gradient. After the protection start criterion is satisfied, the time-domain energy value of the fault voltage backflow across the fault point is calculated. Based on the time-domain energy value of the fault voltage backflow across the fault points, a fault identification criterion for both inside and outside the fault zone and a fault polarity selection criterion are constructed. This can determine whether the high-voltage DC transmission line is experiencing an inside-zone fault or an outside-zone fault. When determined to be an inside-zone fault, combined with the fault polarity selection criterion, it can calculate whether the fault is within the positive polarity zone, the negative polarity zone, or the bipolar polarity zone. Therefore, it eliminates the need for a 500ms delay in engineering to prevent false tripping due to outside-zone faults, improving the speed and reliability of DC transmission line protection. However, this invention has poor flexibility due to the special protection requirements of additional equipment integration.

[0009] Therefore, a new longitudinal differential protection method is needed for lines containing hybrid power flow controllers. This method should be able to accurately identify fault conditions by improving the protection algorithm and logic while keeping the original current transformer positions unchanged, thus avoiding protection maloperation or failure to operate due to the presence of hybrid power flow controllers. Summary of the Invention

[0010] To address the technical problem that existing technologies can easily lead to maloperation or failure to operate protection systems when connected to devices such as hybrid power flow controllers, this invention proposes a line differential protection method incorporating a hybrid power flow controller. This method improves the protection algorithm and logic while keeping the position of the original line differential protection current transformer unchanged, thereby enabling correct fault identification.

[0011] To achieve the above objectives, the technical solution of the present invention is implemented as follows:

[0012] A method for line differential protection incorporating a hybrid power flow controller includes 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 based on the differential current of each phase, obtain the ratio braking action criterion based on the differential current of each phase and the braking current of each phase, and combine the negative sequence current starting auxiliary criterion and the ratio braking action criterion to determine the asymmetrical fault in the line area.

[0015] S3: Obtain the symmetrical three-phase fault criteria based on the differential current of each phase and perform symmetrical three-phase fault determination within the line area.

[0016] S4: Obtain the inrush current criterion based on the differential current of each phase, and determine the inrush current based on the inrush current criterion;

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

[0018] Furthermore, the power frequency current mentioned in step S1 includes the power frequency current at the primary side of the A-phase series transformer. The primary side power frequency current of the B-phase series transformer is supplied with power frequency. The primary side power frequency current of the transformer connected in series with phase C The primary side receiving-end power frequency current of the A-phase series transformer B-phase series transformer primary side receiving end power frequency current The primary side receiving-end power frequency current of the C-phase series transformer

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

[0020]

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

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

[0023]

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

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

[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] Furthermore, the ratio braking action criterion mentioned in step S2 is:

[0029]

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

[0031] Furthermore, the method for determining asymmetrical faults within the line area is as follows: if any one of the three phases satisfies the ratio braking action criterion and also satisfies the negative sequence current starting auxiliary criterion, then it is determined that an asymmetrical fault has occurred within the line area.

[0032] Furthermore, the symmetrical three-phase fault criterion mentioned in step S3 is as follows:

[0033]

[0034] Where m is the threshold.

[0035] Furthermore, the method for obtaining the inrush current criterion based on the differential current of each phase in step S4 is as follows: The second harmonic component of the differential current vector of each phase is extracted using FFT (Fast Fourier Transform), and compared with the power frequency component of the differential current vector to obtain the 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 These are the second harmonic components of the differential currents in phases A, B, and C, respectively. r_a0 I r_b0 I r_c0 These are the power frequency components of the differential current vectors for phases A, B, and C, respectively, and K. d This is the second harmonic braking coefficient.

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

[0041] Furthermore, the method for performing line differential protection in step S5 is as follows: if inrush current is determined to exist in the three phases, the line differential protection operation signal is blocked; if inrush current is determined to not exist in the three phases, the asymmetrical fault determination result and the symmetrical three-phase fault determination result are detected. If an asymmetrical fault or a symmetrical three-phase fault exists, the line differential protection operation is performed; otherwise, the line differential protection operation is not performed.

[0042] The beneficial effects of this invention are as follows: This invention is applicable to lines containing hybrid power flow controllers. Without changing the original protection measurement point configuration, it avoids the problems of false tripping and failure to trip of line longitudinal differential protection caused by the access of hybrid power flow controllers, realizes the correct identification and action of line faults, and improves the reliability of protection. Attached Figure Description

[0043] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0044] Figure 1 This is a flowchart of the method of the present invention.

[0045] Figure 2 This is a topology diagram of a dual-ended power supply system including a hybrid power flow controller.

[0046] Figure 3 Provide the operational logic diagram for the line longitudinal differential protection system that includes a hybrid power flow controller.

[0047] Figure 4 This diagram illustrates the operation of the longitudinal differential protection during normal operation at different gears in the simulation of this invention.

[0048] Figure 5 This is a diagram showing the differential current waveforms and second harmonic content of each phase when inrush current occurs in the simulation of this invention. Detailed Implementation

[0049] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0050] The topology of a dual-ended power supply system including a hybrid power flow controller is shown below. Figure 2As shown, a simulation model of a 220kV double-ended single-circuit system is built. A hybrid power flow controller is connected in series in the system. By adjusting its different levels, the phase angle of the current and voltage on both sides of the hybrid power flow controller is changed, thereby regulating the power flow of the line.

[0051] A line differential protection method incorporating 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 differential protection mechanisms, the presence of a fault can be detected by comparing the magnitude and direction of the power frequency current measured at both ends of the line at the same time. 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 theoretically be equal and opposite. When the difference between the two exceeds a preset threshold, it may indicate a short circuit or other type of fault. In this case, the protection device will trigger corresponding actions to isolate the faulty section, thereby preventing 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. The primary side power frequency current of the B-phase series transformer is supplied with power frequency. The primary side power frequency current of the transformer connected in series with phase C The power frequency current at the receiving end of the primary side of the A-phase series transformer is collected using the current transformer 2 on the sending-end bus side. B-phase series transformer primary side receiving end power frequency current The primary side receiving-end power frequency current of the C-phase series transformer

[0055] Using the primary side power frequency current of the A-phase series transformer The primary side receiving end power frequency current 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] Utilizing the primary side power frequency current of the B-phase series transformer The primary side receiving end power frequency current of the transformer connected in series with phase B Calculate the differential current I of phase B. r_b :

[0060]

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

[0062]

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

[0064]

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

[0066]

[0067] Differential current is used to determine whether a fault has occurred within the zone, and braking current is used to prevent maloperation of the protection system caused by external short circuits.

[0068] S2: Obtain the negative sequence current starting auxiliary criterion based on the differential current of each phase, obtain the ratio braking action criterion based on the differential current of each phase and the braking current of each phase, and combine the negative sequence current starting auxiliary criterion and the ratio braking action criterion to determine the asymmetrical fault in the line area.

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

[0070]

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

[0072] I r_2 >nI E

[0073] Among them, I E The rated current of the primary winding of the series transformer is given, and the rated currents of the two primary windings are the same. The coefficient n is taken as 0.1 to 0.2.

[0074] When an asymmetrical fault occurs within the zone, the asymmetry of the phase parameters causes the three-phase differential currents to be unequal, resulting in a negative sequence differential current component. During normal operation, although the protection may enter the operating zone due to the connection of the mixed 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 distinguish whether an asymmetrical fault has occurred within the zone or the normal operating state.

[0075] Criteria for ratio braking action:

[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 issue an operating signal, and the operating characteristics are shown in the figure.

[0078] To determine the asymmetrical fault within the line zone: if any one of the three phases meets the ratio braking action criterion and falls into the action zone, and the simultaneous current starting auxiliary criterion is activated, then it is determined that an asymmetrical fault has occurred within the line zone.

[0079] S3: Obtain symmetrical three-phase fault criteria based on the differential current of each phase and determine symmetrical three-phase faults within the line area.

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

[0081] I r_a >mI E

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

[0083] I r_b >mI E

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

[0085] I r_c >mI E

[0086] As mentioned above, the coefficient m is generally taken as 1 to 2, based on the maximum phase shift angle of the hybrid power flow controller. Confirmed. The differential current amplitude during normal operation is:

[0087]

[0088] Therefore, the maximum differential current during normal operation will not exceed The coefficient m is determined by increasing the maximum phase shift angle by a certain margin.

[0089] Perform symmetrical three-phase fault determination within the line area: If any one of the three phases meets the symmetrical three-phase fault criterion, it is determined that a symmetrical three-phase fault has occurred within the line area.

[0090] S5: Obtain the inrush current criterion based on the differential current of each phase, and determine the inrush current based on the inrush current criterion.

[0091] The differential current vector of phase A is extracted using Fast Fourier Transform (FFT). Middle second harmonic component I r2_a ; the second harmonic component I r2_a With differential current vector power frequency component I r_a0 By comparison, the inrush current criterion for phase A is obtained, where the differential current vector... power frequency component I r_a0 By differential current vector Obtained using FFT,

[0092]

[0093] If the inrush current criterion for phase A is met, then it is determined that inrush current has occurred in the series transformer of phase A;

[0094] The differential current vector of phase B is extracted using Fast Fourier Transform (FFT). Middle second harmonic component I r2_b ; the second harmonic component I r2_b With differential current vector power frequency component I r_b0 By comparison, the inrush current criterion for phase B is obtained:

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

[0096] If the inrush current criterion for phase B is met, then it is determined that inrush current has occurred in the phase B series transformer.

[0097] Extracting the C-phase differential current vector using FFT (Fast Fourier Transform). Middle second harmonic component I r2_c ; the second harmonic component I r2_c With differential current vector power frequency component I r_c0 By comparison, the inrush current criterion for phase C is obtained:

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

[0099] If the inrush current criterion for phase C is met, then it is determined that inrush current has occurred in the phase C series transformer.

[0100] The above K d This is the second harmonic braking coefficient, typically taken as 0.15 to 0.2.

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

[0102] The inrush current is determined according to the inrush current criterion: if any one of the three phases meets the inrush current criterion, it is determined that there is an inrush current in the three phases.

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

[0104] If inrush current is detected in all three phases, the line differential protection signal is blocked. If inrush current is detected in all three phases, the results of asymmetrical fault determination and symmetrical three-phase fault determination are checked. If an asymmetrical fault or symmetrical three-phase fault exists, the line differential protection is activated; otherwise, the line differential protection is not activated.

[0105] This invention distinguishes between asymmetrical faults (single-phase grounding, phase-to-phase, and two-phase grounding) and symmetrical three-phase faults. Since the system has no negative sequence current component during normal operation, but a negative sequence current component is generated when an asymmetrical fault occurs, this characteristic is utilized to add a negative sequence current initiation criterion to the original differential protection. Only when both signals operate simultaneously is it identified as an asymmetrical fault within the fault zone. For symmetrical three-phase faults, a protection action setting is set; when the differential current of one phase exceeds this setting, it is identified as a symmetrical three-phase fault within the fault zone. Furthermore, since the hybrid power flow controller operates in series with the system, the differential protection may malfunction when it experiences inrush current. Therefore, an inrush current blocking criterion is also required. The protection algorithm and logic can be improved to correctly identify faults without changing the position of the original line differential protection current transformer.

[0106] This invention uses simulation analysis to examine the operation of the longitudinal differential protection under normal operation at different speeds (i.e., different phase shift angles). The results are as follows: Figure 4 As shown, the higher the gear (i.e., the larger the phase shift angle), the higher the false trip rate. At this time, the negative sequence current starting criterion can be used to prevent the protection from tripping.

[0107] Furthermore, when the hybrid power flow controller experiences inrush current, the differential current waveforms of each phase and their second harmonic content are as follows: Figure 5As shown, the differential current waveforms of phases A, B, and C, as well as the changes in the second harmonic content of each phase, are displayed. When the detected second harmonic content exceeds a certain threshold, the protection device will consider it to be an inrush current, thereby blocking the longitudinal differential protection to prevent it from malfunctioning. Therefore, the inrush current blocking criterion described in this invention can effectively prevent the longitudinal differential protection from malfunctioning.

[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 principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A line differential protection method incorporating a hybrid power flow controller, characterized in that, Including the following 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 based on the differential current of each phase, obtain the ratio braking action criterion based on the differential current of each phase and the braking current of each phase, and combine the negative sequence current starting auxiliary criterion and the ratio braking action criterion to determine the asymmetrical fault in the line area. S3: Obtain symmetrical three-phase fault criteria based on the differential current of each phase and determine symmetrical three-phase faults within the line area; S4: Obtain the inrush current criterion based on the differential current of each phase, and determine the inrush current based on the inrush current criterion; S5: Perform line longitudinal differential protection based on the results of asymmetrical fault judgment, symmetrical three-phase fault judgment, and inrush current judgment. The method for obtaining the negative sequence current start-up auxiliary criterion in step S2 is as follows: using the differential current of phase A. B-phase differential current and C phase differential current Calculate the negative sequence components of the three-phase differential current phasors Define the negative sequence current starting auxiliary criterion as follows: ； in, is the rated current of the primary winding of the series transformer, and n is the threshold coefficient; The method for obtaining the inrush current criterion based on the differential current of each phase in step S4 is as follows: The second harmonic component of the differential current vector of each phase is extracted using FFT (Fast Fourier Transform), and compared with the power frequency component of the differential current vector to obtain the inrush current criterion. ； in, , and These are the second harmonic components of the differential currents in phases A, B, and C, respectively. , , These are the power frequency components of the differential current vectors for phases A, B, and C, respectively. This is the second harmonic braking coefficient.

2. The line differential protection method with a hybrid power flow controller according to claim 1, characterized in that, The power frequency current mentioned in step S1 includes the power frequency current at the primary side of the A-phase series transformer. The primary side power frequency current of the B-phase series transformer The primary side power frequency current of the transformer connected in series with phase C The primary side receiving-end power frequency current of the A-phase series transformer The primary side receiving end power frequency current of the B-phase series transformer The primary side receiving-end power frequency current of the C-phase series transformer .

3. The line differential protection method with a hybrid power flow controller according to claim 2, characterized in that, The calculation method for the differential current of each phase is as follows: ； in, This refers to the differential current of phase A. This refers to the differential current of phase B. This refers to the differential current of phase C. The calculation method for the braking current of each phase is as follows: ； in, A is the phase braking current; This is the braking current for phase B; This is the braking current for phase C.

4. The line differential protection method with a hybrid power flow controller according to claim 3, characterized in that, The ratio braking action criterion mentioned in step S2 is: ； in, Minimum operating current; This is the inflection point current; This is the ratio braking coefficient.

5. The line differential protection method with a hybrid power flow controller according to claim 4, characterized in that, The method for determining asymmetrical faults within the line area is as follows: if any one of the three phases satisfies the ratio braking action criterion and also satisfies the negative sequence current starting auxiliary criterion, then it is determined that an asymmetrical fault has occurred within the line area.

6. The line differential protection method with a hybrid power flow controller according to any one of claims 3 to 5, characterized in that, The symmetrical three-phase fault criterion mentioned in step S3 is as follows: ； Where m is the threshold.

7. The line differential protection method with a hybrid power flow controller according to claim 6, characterized in that, The method for determining symmetrical three-phase faults within the line area is as follows: if any one of the three phases meets the symmetrical three-phase fault criterion, then it is determined that a symmetrical three-phase fault has occurred within the line area; the method for determining inrush current is as follows: if any one of the three phases meets the inrush current criterion, then it is determined that there is inrush current in the three phases.

8. The line differential protection method with a hybrid power flow controller according to claim 7, characterized in that, The method for performing line differential protection in step S5 is as follows: if inrush current is detected in the three phases, the line differential protection action signal is blocked; if inrush current is detected in the three phases, the asymmetrical fault determination result and the symmetrical three-phase fault determination result are detected. If an asymmetrical fault or a symmetrical three-phase fault exists, the line differential protection action is performed; otherwise, the line differential protection action is not performed.

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