Differential protection data self-synchronization method and system suitable for t-connected load feeder

Abstract

The application provides a differential protection data self-synchronization method and system suitable for a T-connection load feeder, which comprises the following steps: obtaining active and reactive power of each T-connection load branch in a protected feeder, and constructing a T-type equivalent model of the protected feeder; when a fault occurs, a protection device at each end of the protected feeder takes the time when the measured current of each end meets a protection starting criterion as a starting time, and calculates a fault current phasor of each end with the time as a reference; based on voltage and current sampling data in a preset time period before the starting time of the protection device at each end of the feeder, a comprehensive reference phasor of voltage and current of each end is calculated; according to the comprehensive reference phasors of voltage and current at each end and the T-type equivalent model of the feeder, a voltage reference phasor of an equivalent T-connection point of each end is obtained; and the fault current phasor is compensated according to the phase difference between the voltage reference phasors of the equivalent T-connection points at each end.

Description

Technical Field

[0001] This invention belongs to the field of power distribution network relay protection technology, and in particular relates to a differential protection data self-synchronization method and system applicable to feeders with T-connected loads. Background Technology

[0002] The statements in this section are merely background information related to the present invention and do not necessarily constitute prior art.

[0003] In recent years, an increasing number of distributed generation (DG) sources have been locally connected to medium-voltage distribution networks, making the fault current characteristics of feeders more complex. Furthermore, the diversification of distribution network operation modes has further altered the distribution of load current and fault current on feeders. In such distribution network feeders, the traditional three-stage current protection configuration is no longer applicable, necessitating new protection principles. With the development of distribution network communication technology, some cities both domestically and internationally have already adopted differential current protection in distribution network feeders in key urban areas.

[0004] Differential current protection is widely used in transmission line protection. It only needs to monitor whether the current flowing into the protected line satisfies Kirchhoff's current law, and is less affected by distributed generation (DG) fault characteristics and distribution network operation modes, exhibiting high reliability and absolute selectivity. Furthermore, the capacitor charging current in distribution network feeders is relatively small, which is highly advantageous for implementing differential protection. Therefore, some experts believe that differential current protection is the future development trend of distribution network feeder protection. However, data synchronization on both sides of the line must be ensured when applying differential protection. In transmission lines, a ping-pong algorithm is typically used for data synchronization, adjusting the synchronization clocks on both sides by measuring the round-trip delay of the symmetrical channel. However, distribution networks usually do not have dedicated symmetrical channels for protection, but instead communicate by reusing existing distribution network channels, thus making ping-pong algorithm-based data synchronization impossible.

[0005] Existing technologies propose differential protection data synchronization methods utilizing Global Navigation Satellite System (GNSS) timing. GNSS includes my country's BeiDou, the United States' GPS, Russia's GLONASS, and the European Union's GALILEO. While GNSS synchronization is not limited by communication channels, its reliability is susceptible to factors such as installation environment, extreme weather, and human interference, making it unsuitable as the sole synchronization method for differential protection. Current technologies typically use GNSS signal receivers in conjunction with chip-level atomic clocks / high-stability crystal oscillators. Considering the numerous and widespread nature of distribution networks, configuring GNSS signal receivers and chip-level atomic clocks at every protection installation point would significantly increase the construction cost of the distribution network. Therefore, this technology is difficult to widely implement in distribution networks.

[0006] Existing technologies also propose a self-synchronization method for feeder differential protection based on the fault time. This method assumes that the distribution network feeders are relatively short, and that the protection devices on both sides of the line can immediately detect the fault signal after a fault occurs. Therefore, each device uses its own fault detection time as a reference time to calculate protection-related information, thereby achieving approximately synchronous calculation of data on both sides. This method does not rely on symmetrical data channels or satellite timing signals, and has high economic efficiency. However, this method is greatly affected by the fault detection sensitivity. When the fault location is far from the load side or the fault point is accompanied by a large transition resistance, the measured current in the load-side protection device needs a long delay to reach the fault detection threshold. In this case, the error of this method is large, which will seriously affect the reliability of the differential protection.

[0007] Existing technologies have also proposed self-synchronization methods for differential protection data based on the current waveform characteristics at a specific moment before a fault. These methods assume that the capacitor charging current in the feeder is relatively small, and that the normal load current waveforms on both sides of the protected feeder are approximately symmetrical before the fault. Therefore, a reference phasor calculated based on the zero-crossing point or a specific time period of the current waveform before the fault is used as the synchronization reference. This type of self-synchronization method is unaffected by fault detection sensitivity, and its synchronization accuracy under various fault conditions meets the requirements of differential protection. However, in some distribution network feeders, there are T-connected load branches. The current shunting at the T-connected branches causes the normal load current waveforms on both sides to become asymmetrical, at which point this type of self-synchronization method will fail.

[0008] The inventors discovered that existing data synchronization methods for feeder differential protection all suffer from drawbacks such as high cost, susceptibility to fault detection algorithms, or inapplicability to branches with T-connected loads. There is no low-cost data synchronization method for feeder differential protection in distribution networks that is unaffected by fault detection algorithms and applicable to branches with T-connected loads. Summary of the Invention

[0009] To overcome the shortcomings of the prior art, the present invention provides a differential protection data self-synchronization method and system applicable to feeders with T-connected loads. The scheme only utilizes the comprehensive measurement information (i.e., voltage and current) of the protection device, without requiring external GNSS signals, and has low investment costs. In addition, the method is not affected by fault detection algorithms and can also be applied to distribution network feeders with T-connected load branches.

[0010] According to a first aspect of the present invention, a method for self-synchronization of differential protection data applicable to feeders with T-connections is provided, comprising:

[0011] Obtain the active and reactive power of each T-connected load branch inside the protected feeder, and construct the T-type equivalent model of the protected feeder.

[0012] When a fault occurs, the protection devices at both ends of the protected feeder take the moment when their respective measured currents meet the protection start criteria as the start time, and use that moment as a reference to calculate the fault current phasor at each end.

[0013] Based on the voltage and current sampling data within a preset time period before the start-up time of the protection devices at both ends of the feeder, the comprehensive reference phasor of voltage and current at each end is calculated respectively.

[0014] Based on the integrated reference phasors at both ends and the feeder T-type equivalent model, the voltage reference phasors of the equivalent T-junctions at each end are obtained respectively.

[0015] The fault current phasor is compensated based on the phase difference of the voltage reference phasors at the two equivalent T-junctions.

[0016] Furthermore, the construction of the T-type equivalent model of the protected feeder is specifically as follows: based on the protection devices at both ends of the protected feeder, the active and reactive power of each T-connected load branch inside the feeder is obtained; the equivalent impedance of each T-connected load branch is calculated according to the line rated voltage and power data; and then the T-type equivalent model of the protected feeder is calculated according to the star-angle inverse transformation of the equivalent impedance.

[0017] Furthermore, the determination of the occurrence of the fault is based on the three-phase current change rate as the starting criterion for detecting whether a fault has occurred in the protected feeder by the protection devices at both ends of the protected feeder.

[0018] Furthermore, based on the voltage and current sampling data within a preset time period before the start-up time of the protection devices at both ends of the feeder, the comprehensive reference phasors of voltage and current at each end are calculated respectively. Specifically, based on the maximum detection delay of the protection start-up criterion not exceeding one cycle and the data window length of the full-cycle Fourier algorithm, the starting point for calculating the voltage and current reference phasors is selected so that the data window for calculating the voltage and current reference phasors is completely located before the fault occurs.

[0019] Furthermore, the protection devices at both ends of the protected feeder transmit the calculated values ​​of their respective fault current phasors and comprehensive reference phasors to each other.

[0020] Furthermore, based on the comprehensive reference phasors at both ends and the feeder T-type equivalent model, the voltage reference phasors of the equivalent T-junctions at each end are obtained respectively. Specifically, for any protection device at any end, the voltage reference phasor of the equivalent T-junction is equal to the voltage reference phasor of the current end minus the product of the current reference phasor of the current end and the positive sequence impedance of the equivalent T-junction at the current end.

[0021] Furthermore, the compensation of the fault current phasor based on the phase difference of the voltage reference phasors of the equivalent T-junctions at both ends is specifically as follows: if the protection start times at both ends are synchronized, the fault current phasors are also synchronized, and the voltage reference phasors of the equivalent T-junctions derived from the data at both ends are equal; if the protection start times at both ends are not synchronized, the fault current phasors are also not synchronized, and the voltage reference phasors of the equivalent T-junctions derived from the data at both ends are not equal. The phase difference is the synchronization error of the fault current phasors at both ends. The fault current phasors at the other end are corrected based on this phase difference to achieve phase angle synchronization of the fault current phasors at both ends.

[0022] According to a second aspect of the present invention, a differential protection data self-synchronization system suitable for load feeders with T-connections is provided, comprising:

[0023] The data acquisition unit is used to acquire the active and reactive power of each T-connected load branch inside the protected feeder and to construct the T-type equivalent model of the protected feeder.

[0024] The start-up time determination unit is used when a fault occurs, and the protection devices at both ends of the protected feeder use the time when their respective measured currents meet the protection start-up criteria as the start-up time, and use this time as a reference to calculate the fault current phasor at each end.

[0025] The voltage and current phasor calculation unit is used to calculate the comprehensive reference phasor of voltage and current at each end based on the voltage and current sampling data within a preset time period before the start-up time of the protection devices at both ends of the feeder.

[0026] The synchronization error compensation module is used to obtain the voltage reference phasor of the equivalent T-junction at each end based on the comprehensive reference phasor at both ends and the feeder T-type equivalent model; and to compensate the fault current phasor based on the phase difference of the voltage reference phasors of the equivalent T-junction at both ends, so as to realize the phase angle synchronization of the fault current phasors at both ends.

[0027] According to a third aspect of the present disclosure, an electronic device is provided, including a memory, a processor, and a computer program stored in the memory and running on the memory, wherein the processor executes the program to implement the aforementioned differential protection data self-synchronization method applicable to feeders with T-connections.

[0028] According to a fourth aspect of the present disclosure, a non-transitory computer-readable storage medium is provided, on which a computer program is stored, which, when executed by a processor, implements the aforementioned method for self-synchronization of differential protection data applicable to feeders with T-connected loads.

[0029] The above one or more technical solutions have the following beneficial effects:

[0030] (1) This invention provides a method and system for differential protection data self-synchronization suitable for feeders with T-connection loads. The scheme utilizes the measurement data of the protection devices at both ends to achieve differential protection data synchronization, without the need for external GNSS timing signals and dedicated symmetrical channels for protection. It is applicable to the differential protection of feeders in distribution networks that reuse the existing communication network of the distribution network SCADA system (Supervisory Control And Data Acquisition system, i.e., data acquisition and monitoring control system), and is economical and easy to implement.

[0031] (2) The solution described in this invention uses the comprehensive reference phasor at a specific moment before the fault to calculate the equivalent T-junction voltage reference phasor of the feeder, and performs synchronous correction of the fault current phasor based on the phase difference between the two ends. It is not affected by the sensitivity of the fault detection algorithm or the T-junction load branch, and has a wider range of applications.

[0032] Advantages of additional aspects of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0033] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0034] Figure 1 This is a schematic diagram of differential protection configuration for a load feeder with a T-connection as described in an embodiment of the present invention;

[0035] Figure 2 This is a schematic diagram of the current waveforms at both ends of a load feeder with a T-connection as described in an embodiment of the present invention.

[0036] Figure 3 This is a schematic diagram of the equivalent model of the load feeder with T-connection described in an embodiment of the present invention;

[0037] Figure 4 This is a schematic diagram of the star-angle inverse transformation of the T-connected load impedance described in this embodiment of the invention;

[0038] Figure 5 This is a schematic diagram of the simplified T-type equivalent model of the feeder described in this embodiment of the invention;

[0039] Figure 6 This is a schematic diagram of the current and voltage waveforms at both ends of a load feeder with a T-connection as described in an embodiment of the present invention.

[0040] Figure 7 This is a schematic diagram of the distribution network feeder simulation model described in the embodiments of the present invention;

[0041] Figure 8This is an overall flowchart of the differential protection data self-synchronization method applicable to load feeders with T-connections, as described in this embodiment of the invention. Detailed Implementation

[0042] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0043] It should be noted that the terminology used herein is for the purpose of describing particular implementations only and is not intended to limit the exemplary implementations of the present invention.

[0044] Where there is no conflict, the embodiments and features in the embodiments of the present invention can be combined with each other.

[0045] Based on the description of the background technology, Figure 1 The limitations of existing feeder differential protection data self-synchronization methods are illustrated using a load feeder with a T-connection as an example.

[0046] The fault-time self-synchronization method uses the activation time of the protection device as a synchronization reference. When a fault occurs inside the protected feeder MN, the protection devices R at both ends... M and R N The measured phase current will change abruptly. Since the distribution network feeder length is relatively short and the propagation speed of fault electromagnetic waves is close to the speed of light, the protection devices R at both ends can be approximated as... M and R N Able to detect fault signals without delay, R M and R N By calculating the protection-related phasors with their respective start-up time (fault detection time) as the reference time, the synchronous calculation of data at both ends can be achieved.

[0047] However, the start-up time in the above-mentioned fault self-synchronization method is not the actual fault occurrence time; in reality, the start-up delay of the protection device is unavoidable. The commonly used phase current surge start-up criterion in current digital relay protection devices is as follows:

[0048] ||i(k)-i(kN)|-|i(kN)-i(k-2N)||≥K S I N

[0049] In the formula, i(k) represents the phase current sampled value at the k-th sampling point, N is the number of sampling points in one power frequency cycle, and K S I is the startup threshold coefficient. N This is the rated current.

[0050] This criterion indicates that the delay in protection activation depends on the rate of change of the phase current amplitude after the fault. When a fault occurs within the protection zone, due to the large short-circuit current supplied by the system power supply, the system-side protection device R... M It can start quickly; the fault current on the load side is relatively small. If the fault point is far from the load side or contains a large transition resistance, the load side protection device R... N The startup delay is relatively long. When R N When the startup delay is large, the startup times of the two ends will differ significantly, and the self-synchronization method at the fault time will result in a large synchronization error.

[0051] Figure 2 For one in Figure 1 The figure shows a simulated fault current waveform when the load side (N side) of the distribution network feeder has a long start-up delay. In the figure, t... d.M and t d.N R respectively M and R N The start-up time.

[0052] from Figure 2 As can be seen, due to the large abrupt change in the current waveform on the system side (M side) at the moment of the fault, the current surge rapidly exceeded the start-up threshold, R M It can start quickly; the current change on the N side is not obvious, and the current change cannot reach the start-up threshold in a short time after the fault. N The startup delay is relatively long. At this time, t... d.M and t d.N The time difference between them is about 3.9ms. If the fault-time self-synchronization method is used, there will be a synchronization error of about 70.2°, which will seriously affect the reliability of the current differential protection.

[0053] Data-driven self-synchronization methods based on current waveform characteristics at a specific moment before a fault assume that the load currents at both ends are the same current with opposite reference directions during normal operation, and the waveforms are approximately symmetrical about the zero axis. This type of method can only be used for feeders without T-connected loads. For feeders with internal T-connected load branches, due to the current shunting effect of the T-connected loads, the load current waveforms at both ends are no longer symmetrical about the zero axis. Taking the zero-crossing synchronization method as an example, this method uses the zero-crossing moment of the current waveforms at both ends before the fault occurs as the synchronization reference. Figure 2 As can be seen, for feeders with T-connected loads, the current waveforms at both ends before a fault are not symmetrical, and the zero-crossing times differ significantly. Therefore, the data self-synchronization method based on the current waveform characteristics at a specific moment before a fault is not applicable to feeders with T-connected loads.

[0054] Based on the above problems, this invention proposes a differential protection data self-synchronization scheme suitable for feeders with T-connections. The overall idea of ​​the scheme is as follows:

[0055] This invention first collects power data of each T-connected load and derives the T-type equivalent model of the protected feeder based on impedance star-angle inverse transformation. When a fault occurs, each protection device uses the start time as the starting time for calculating the fault current phasor of its own end, and then uses the voltage and current sampling data 40ms before the start time of each end to calculate the voltage and current reference phasors. Then, the voltage reference phasor of the equivalent T-connection is derived using the comprehensive reference phasors of both ends. Finally, the fault current phasor is corrected based on the phase difference of the equivalent T-connection voltage reference phasors derived from both ends to eliminate the error caused by the asynchronous start time in calculating the fault current phasor, thereby achieving data synchronization of differential protection.

[0056] Example 1:

[0057] The purpose of this embodiment is to provide a method for self-synchronization of differential protection data applicable to feeders with T-connections.

[0058] A method for self-synchronization of differential protection data applicable to feeders with T-connections includes:

[0059] Obtain the active and reactive power of each T-connected load branch inside the protected feeder, and construct the T-type equivalent model of the protected feeder.

[0060] When a fault occurs, the protection devices at both ends of the protected feeder take the moment when their respective measured currents meet the protection start criteria as the start time, and use that moment as a reference to calculate the fault current phasor at each end.

[0061] Based on the voltage and current sampling data within a preset time period before the start-up time of the protection devices at both ends of the feeder, the comprehensive reference phasor of voltage and current at each end is calculated respectively.

[0062] Based on the integrated reference phasors at both ends and the feeder T-type equivalent model, the voltage reference phasors of the equivalent T-junctions at each end are obtained respectively.

[0063] The fault current phasor is compensated based on the phase difference of the voltage reference phasors at the two equivalent T-junctions.

[0064] For ease of understanding, the solution described in this embodiment will be explained in detail below with reference to the accompanying drawings. The solution specifically includes the following steps:

[0065] Step 1: The protection devices at both ends collect the three-phase voltage and current at their respective ends in real time.

[0066] Step 2: The protection device acquires the power data of each T-connected load inside the protected feeder.

[0067] Step 3: Based on the power data and impedance star-angle inverse transformation of each T-connected load, derive the T-type equivalent model of the protected feeder.

[0068] Specifically, the process of deriving the T-type equivalent model of the protected feeder is as follows:

[0069] For each T-connected load, based on its active power (P) TL ), reactive power (Q) TL ) and line rated voltage (U n Calculate its equivalent impedance as follows:

[0070]

[0071] For a feeder with n T-connected load branches, its circuit equivalent model is as follows: Figure 3 As shown. Where Z T1 …Z Tn Z represents the equivalent impedance of each T-connected load. T1T2 …Z T(n-1)Tn The positive sequence impedance between any two adjacent T-connected branches; Z MT1 Z is the positive sequence impedance of the line from bus M to the first T-connection branch. NTn The positive sequence impedance of the line from bus N to the nth T-connection branch; and These are the reference phasors for the voltage and current at both ends, respectively. The calculation process is shown in step 6 below.

[0072] By using the inverse star-angle transformation of impedance, the impedances of two adjacent T-connected load branches can be simplified into a single T-connected branch. Figure 3 Taking the first two T-connected load branches as an example, the simplification steps are as follows: Figure 4 As shown.

[0073] Among them, Z T1T2.L Z T1T2.R and Z T1T2.C The calculation process is as follows:

[0074]

[0075] In this context, .L represents left, .R represents right, and .C represents center.

[0076] After multiple star-angle inverse transformations, a feeder with n T-connected load branches can be simplified into an equivalent model with only one T-connected branch, such as... Figure 5 As shown. Where Z MT.eq and Z NT.eq Z represents the positive sequence impedance of the lines from bus M and bus N to the equivalent T-junction, respectively. TL.eq The equivalent T-connected branch impedance is given by .eq, where .eq represents the equivalent impedance.

[0077] Step 4: Monitor whether the three-phase current at the protection device meets the start-up criterion, and define the time when the start-up criterion is met as the reference time t. d .

[0078] In this embodiment, the sudden change in phase current is used as the starting criterion for detecting whether a fault has occurred.

[0079] Specifically, the protection devices on both sides calculate the current phasor starting from the moment when their respective measured currents meet the start criterion for the sudden change in phase current. The start criterion is as follows:

[0080] ||i(k)-i(kN)|-|i(kN)-i(k-2N)||≥K S I N

[0081] In the formula, i(k) represents the phase current sampled value at the k-th sampling point, N is the number of sampling points in one power frequency cycle, and K S I is the startup threshold coefficient. N This is the rated current.

[0082] Step 5: The protection devices at both ends of the protected line calculate the fault current phasor at their respective starting times.

[0083] It should be noted that the method calculates the fault current phasor with the start time as the starting time in order to simplify the overall process. Under certain fault conditions, the start times of the protection devices at both ends may differ significantly, and they need to be synchronized and corrected in subsequent steps.

[0084] Step 6: The protection devices at both ends calculate the voltage and current comprehensive reference phasor of their respective ends based on the voltage and current sampling values ​​at a specific moment before their respective start-up time.

[0085] Specifically, the process of calculating the reference phasor by selecting the sampled value at a specific time before the start-up time is as follows:

[0086] To ensure calculation accuracy, a full-cycle Fourier algorithm can be used to calculate the phasors. To avoid the influence of transient processes after a fault, it is essential to ensure that the data window for phasor calculation is entirely prior to the fault occurrence. Considering that the maximum detection delay of the protection start-up criterion will not exceed one cycle and the data window length of the full-cycle Fourier algorithm, in this invention, the starting point for calculating the reference phasors of the voltage and current at both ends of the protection devices is 40ms prior to their respective start-up times. Figure 2 Taking the typical fault shown as an example, the selection of the comprehensive reference phasor data window is as follows: Figure 6 As shown. The formula for calculating phasors using the full-cycle Fourier algorithm is as follows:

[0087]

[0088]

[0089]

[0090]

[0091] In the formula, X real and X imag θ represents the real and imaginary parts of the voltage or current phasor, respectively. X and X mag These represent the phase angle and amplitude of the voltage or current phasors, respectively. N represents the number of sampling points within one cycle, and x(n) represents the sampled value of the nth voltage or current sampling point.

[0092] Step 7: The two protection devices at both ends transmit the fault current phasor and the comprehensive reference phasor to each other. This step prepares for the calculation in Step 8.

[0093] Step 8: Derive the voltage reference phasor of the equivalent T-junction from the integrated reference phasor at both ends and the equivalent model of the feeder T.

[0094] Specifically, the process of deriving the equivalent T-junction voltage reference phasor from the data at both ends is as follows:

[0095]

[0096]

[0097] Step 9: Synchronously correct the fault current phasor in the differential protection criterion based on the phase difference of the equivalent T-contact voltage reference phasor derived at both ends.

[0098] Specifically, the principle of synchronously correcting fault current phasors is as follows:

[0099] The fault current phasors at both ends are calculated based on their respective protection activation times, and may be asynchronous. The reference phasors, however, are calculated based on a specific time prior to their respective activation times. Therefore, if the fault current phasors at both ends are asynchronous, the reference phasors at both ends will also be asynchronous, and the degree of asynchrony (synchronization error) will be the same. (Step 8...) and These are the same phasors calculated from data at both ends. Ignoring measurement and model errors, when the data at both ends are synchronized... and The phase angles should be equal. If they are not equal, it means that the data at both ends are not synchronized and the synchronization error is related to the phase difference between the two. The fault current phasor needs to be synchronized and corrected according to the phase difference.

[0100] Phase difference of the equivalent T-junction voltage reference phasor derived at both ends The calculation process is as follows:

[0101]

[0102] Without loss of generality, taking the basic current differential protection criteria with braking characteristics as an example, the criteria before synchronous correction are as follows:

[0103]

[0104] In the formula, and These are the fault current phasors at both ends, K rel This is the differential protection braking coefficient.

[0105] The current differential protection criteria after synchronous correction using this method are as follows:

[0106]

[0107] Where the superscript j denotes the imaginary unit, and e is the natural constant. When e is the base and j (or -j) is the exponent, it is the rotation factor.

[0108] Since the synchronization error of the fault current phasor calculated based on the protection start time is equal to the phase difference between the T-junction voltage reference phasor derived from both ends, the synchronous comparison of the data at both ends in the differential protection criterion can be guaranteed after correction by this method.

[0109] To verify the effectiveness of the solution described in this embodiment, the following verification examples are provided:

[0110] The differential protection data self-synchronization method proposed in this invention, applicable to feeders with T-connected loads, was verified by constructing a distribution network feeder with a T-connected load branch using PSCAD / EMTDC simulation software.

[0111] 1) Simulation model

[0112] Simulation model structure as follows Figure 7 As shown in the figure. In this model, the rated voltage of the system power supply is 10kV; the feeder length between bus 1 and bus 2 is 8km, and the line parameters per unit length are shown in Table 1; the load capacity at the end of the feeder is (1.8 + j0.88)MVA, and DG is an inverter power supply with a maximum power of 2MW; the distances between the T-connected loads TL1, TL2, and TL3 inside the feeder and bus 1 are 2km, 4km, and 7km respectively, with a power factor of 0.9; protection devices R1 and R2 are deployed on both sides of the feeder, with a default sampling frequency of 10kHz, and the sensitivity coefficient K in the activation criterion... S Take 0.1.

[0113] Table 1 Line Parameters

[0114]

[0115]

[0116] 2) Simulation verification

[0117] a) The effectiveness of the synchronization method proposed in this embodiment when different T-connected loads are connected.

[0118] Since the proposed data self-synchronization method uses pre-fault data to calculate the comprehensive reference phasor, it only needs to consider the voltage and current waveforms during normal operation. In principle, it is not affected by factors such as fault type, fault location, fault initial phase angle, and transition resistance. Therefore, the simulation verification focuses on studying the effectiveness of the proposed scheme when the internal T-connected loads are different. Eleven typical cases with different numbers, locations, and capacities of T-connected loads were selected for simulation verification. The capacities of the T-connected loads in these cases are shown in Table 2.

[0119] Table 2. Capacity of T-connected load in different fault cases

[0120]

[0121]

[0122] Since traditional fault detection self-synchronization methods have large synchronization errors when the transition resistance is high, a series of single-phase grounding faults with 100Ω transition resistances were set at the midpoint f of the feeder. The simulation results of the proposed method in different cases are shown in Table 3.

[0123] Table 3 Simulation results of the proposed methods in different fault cases

[0124]

[0125]

[0126] Simulation results show that when the number, location, and capacity of the T-connected loads inside the protected feeder differ, the voltage and current during normal operation vary significantly. Therefore, the voltage and current reference phasors of this method also differ considerably in different fault cases. However, the final calculation results show that the synchronization error of the proposed method is very small under different conditions, with a maximum error of only 5.972 μs and a corresponding phase angle of less than 0.108°, which is sufficient to meet the data synchronization requirements of current differential protection.

[0127] b) The impact of the sampling frequency of the protection device on the proposed synchronization method

[0128] To verify the impact of sampling frequency on the proposed method, the sampling frequency of the protection device was reduced to 1kHz, and the aforementioned typical fault case was re-enacted. At this point, the simulation results were almost identical to those in Table 3. This is because the proposed method calculates the reference phasor using data prior to the fault, and is therefore unaffected by transient fault components. Ignoring noise, harmonics, and other factors, the voltage and current during normal operation contain only power frequency components; therefore, the sampling frequency of the protection device only needs to satisfy Shannon's sampling theorem to guarantee the synchronization accuracy of the proposed method.

[0129] c) The impact of noise on the proposed synchronization method

[0130] The measured current of the protection device contains noise due to electromagnetic interference, therefore the measured voltage and current during normal operation are not standard sinusoidal quantities. To study the impact of noise on the proposed method, Gaussian white noise with different signal-to-noise ratios (SNR) was superimposed on 11 typical cases, and then the synchronization error was calculated according to the method proposed in this invention. The results are shown in Table 4. Since the noise generation is random, noise with the same SNR was superimposed 10 times for each fault. The data recorded in Table 4 represent the largest synchronization error among the 10 calculation results.

[0131] Table 4 Synchronization errors of the proposed method under different levels of noise.

[0132]

[0133]

[0134] As shown in Table 4, noise increases the synchronization error of the proposed method, and the higher the noise level, the greater the error. However, because the proposed method uses a full-cycle Fourier algorithm to calculate the reference phasor, it has strong filtering capabilities. Even in a strong noise environment of 20dB, the maximum synchronization error is less than 70μs, and the corresponding phase angle error is only 1.26°, which is sufficient to meet the requirements of feeder current differential protection. In contrast, other existing self-synchronization methods require pre-filtering of the sampled data before use in a strong noise environment of 20dB.

[0135] The simulation results above demonstrate that the proposed data self-synchronization method for differential protection feeders with T-connections overcomes the limitation of data self-synchronization methods based on pre-fault current waveform characteristics, which are unsuitable for feeders with T-connections, and is unaffected by fault detection algorithms. Simulation results show that the synchronization accuracy of this method can reach the microsecond level under ideal conditions, and even in strong noise environments, the maximum synchronization error still meets the requirements of differential protection. Furthermore, the proposed method has low requirements for the sampling frequency of the protection device.

[0136] This embodiment takes into account the characteristics of T-connected load branches in distribution network feeders and proposes a differential protection data self-synchronization method suitable for feeders with T-connected loads. First, the T-type equivalent model of the protected feeder is derived based on the power data of the T-connected load branch. Then, the fault current phasor is calculated based on the start-up time of the protection devices at both ends. Next, the comprehensive reference phasor is calculated using the voltage and current sampling data 40ms before the start-up time of each end. The voltage reference phasor of the equivalent T-connection is derived from the comprehensive reference phasor of both ends. Finally, the fault current phasor is synchronously corrected based on the phase difference of the voltage reference phasors of the T-nodes at both ends to achieve synchronous comparison of the phasors at both ends.

[0137] PSCAD simulation results show that the data synchronization method proposed in this invention is applicable to distribution network feeders with different T-connected loads, unaffected by the number, location, and capacity of the T-connected loads. It has low requirements for the sampling rate of the protection device and strong noise immunity, exhibiting high synchronization accuracy under various fault scenarios. Furthermore, the differential protection data synchronization method proposed in this invention, applicable to feeders with T-connected loads, utilizes only the voltage and current measurement information at both ends of the feeder, eliminating the need for GNSS signal receivers at each protection installation point and a dedicated protection channel, thus significantly reducing the cost of applying current differential protection in distribution network feeders.

[0138] Example 2:

[0139] The purpose of this embodiment is to provide a differential protection data self-synchronization system suitable for load feeders with T-connections.

[0140] A differential protection data self-synchronization system suitable for load feeders with T-connections includes:

[0141] The data acquisition unit is used to acquire the active and reactive power of each T-connected load branch inside the protected feeder and to construct the T-type equivalent model of the protected feeder.

[0142] The start-up time determination unit is used when a fault occurs, and the protection devices at both ends of the protected feeder use the time when their respective measured currents meet the protection start-up criteria as the start-up time, and use this time as a reference to calculate the fault current phasor at each end.

[0143] The voltage and current phasor calculation unit is used to calculate the comprehensive reference phasor of voltage and current at each end based on the voltage and current sampling data within a preset time period before the start-up time of the protection devices at both ends of the feeder.

[0144] The synchronization error compensation module is used to obtain the voltage reference phasor of the equivalent T-junction at each end based on the comprehensive reference phasor at both ends and the feeder T-type equivalent model; and to compensate the fault current phasor based on the phase difference of the voltage reference phasors of the equivalent T-junction at both ends, so as to realize the phase angle synchronization of the fault current phasors at both ends.

[0145] Furthermore, the system described in this embodiment corresponds to the method described in Embodiment 1, and its technical details have been described in detail in Embodiment 1, so they will not be repeated here.

[0146] In further embodiments, the following is also provided:

[0147] An electronic device includes a memory and a processor, as well as computer instructions stored in the memory and running on the processor. When executed by the processor, the computer instructions perform the method described in Embodiment 1. For brevity, further details are omitted here.

[0148] It should be understood that in this embodiment, the processor can be a central processing unit (CPU), or it can be other general-purpose processors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), programmable gate arrays (FPGAs), or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. The general-purpose processor can be a microprocessor or any conventional processor, etc.

[0149] Memory may include read-only memory and random access memory, and provides instructions and data to the processor. A portion of memory may also include non-volatile random access memory. For example, memory may also store information about the device type.

[0150] A computer-readable storage medium for storing computer instructions, which, when executed by a processor, perform the method described in Embodiment 1.

[0151] The method in Embodiment 1 can be directly implemented by a hardware processor, or implemented by a combination of hardware and software modules within the processor. The software modules can reside in readily available storage media in the art, such as random access memory, flash memory, read-only memory, programmable read-only memory, electrically erasable programmable memory, or registers. This storage medium is located in memory; the processor reads information from the memory and, in conjunction with its hardware, completes the steps of the above method. To avoid repetition, a detailed description is not provided here.

[0152] Those skilled in the art will recognize that the units, i.e., algorithm steps, of the various examples described in connection with this embodiment can be implemented in electronic hardware or a combination of computer software and electronic hardware. Whether these functions are implemented in hardware or software depends on the specific application and design constraints of the technical solution. Those skilled in the art can use different methods to implement the described functions for each specific application, but such implementation should not be considered beyond the scope of this disclosure.

[0153] The above embodiments provide a method and system for self-synchronization of differential protection data applicable to feeders with T-connections, which can be implemented and has broad application prospects.

[0154] The above description is merely a preferred embodiment of this disclosure and is not intended to limit this disclosure. Various modifications and variations can be made to this disclosure by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this disclosure should be included within the scope of protection of this disclosure.

Claims

1. A method for self-synchronization of differential protection data applicable to feeders with T-connections, characterized in that, include: Obtain the active and reactive power of each T-connected load branch inside the protected feeder, and construct the T-type equivalent model of the protected feeder. When a fault occurs, the protection devices at both ends of the protected feeder take the moment when their respective measured currents meet the protection start criteria as the start time, and use that moment as a reference to calculate the fault current phasor at each end. Based on the voltage and current sampling data within a preset time period before the start-up time of the protection devices at both ends of the feeder, the comprehensive reference phasors of voltage and current at each end are calculated respectively. Specifically, based on the maximum detection delay of the protection start-up criterion not exceeding one cycle and the data window length of the full-cycle Fourier algorithm, the starting point for calculating the voltage and current reference phasors is selected so that the data window for calculating the voltage and current reference phasors is completely located before the fault occurs. Based on the integrated reference phasors at both ends and the feeder T-type equivalent model, the voltage reference phasors of the equivalent T-junctions at each end are obtained respectively; The fault current phasor is compensated based on the phase difference of the voltage reference phasors of the equivalent T-junctions at both ends. Specifically, if the protection start times at both ends are synchronized, the fault current phasors are also synchronized, and the voltage reference phasors of the equivalent T-junctions derived from the data at both ends are equal. If the protection start times at both ends are not synchronized, the fault current phasors are also not synchronized, and the voltage reference phasors of the equivalent T-junctions derived from the data at both ends are not equal. The phase difference is the synchronization error of the fault current phasors at both ends. The fault current phasors at the other end are corrected based on this phase difference to achieve phase angle synchronization of the fault current phasors at both ends.

2. The differential protection data self-synchronization method applicable to feeders with T-connections as described in claim 1, characterized in that, The construction of the T-type equivalent model of the protected feeder is specifically as follows: based on the protection devices at both ends of the protected feeder, the active and reactive power of each T-connected load branch inside the feeder is obtained, the equivalent impedance of each T-connected load branch is calculated according to the line rated voltage and power data, and then the T-type equivalent model of the protected feeder is calculated according to the star-angle inverse transformation of the equivalent impedance.

3. The differential protection data self-synchronization method applicable to feeders with T-connections as described in claim 1, characterized in that, The determination of whether a fault has occurred is made by the protection devices at both ends of the protected feeder using the sudden change in three-phase current as the activation criterion for detecting whether a fault has occurred in the protected feeder.

4. The differential protection data self-synchronization method applicable to feeders with T-connections as described in claim 1, characterized in that, The protection devices at both ends of the protected feeder transmit the calculated values ​​of their respective fault current phasors and comprehensive reference phasors to each other.

5. The differential protection data self-synchronization method applicable to load feeders with T-connections as described in claim 1, characterized in that, The voltage reference phasor of the equivalent T-junction at each end is obtained based on the comprehensive reference phasor at both ends and the feeder T-type equivalent model. Specifically, for any protection device at any end, the voltage reference phasor of the equivalent T-junction is equal to the voltage reference phasor of the current end minus the product of the current reference phasor of the current end and the positive sequence impedance of the equivalent T-junction at the current end.

6. A differential protection data self-synchronization system suitable for load feeders with T-connections, characterized in that, include: The data acquisition unit is used to acquire the active and reactive power of each T-connected load branch inside the protected feeder and to construct the T-type equivalent model of the protected feeder. The start-up time determination unit is used when a fault occurs, and the protection devices at both ends of the protected feeder use the time when their respective measured currents meet the protection start-up criteria as the start-up time, and use this time as a reference to calculate the fault current phasor at each end. The voltage and current phasor calculation unit is used to calculate the comprehensive reference phasor of voltage and current at each end based on the voltage and current sampling data within a preset time period before the start time of the protection devices at both ends of the feeder. Specifically, based on the maximum detection delay of the protection start criterion not exceeding one cycle and the data window length of the full cycle Fourier algorithm, the starting point for calculating the voltage and current reference phasor is selected so that the data window for calculating the voltage and current reference phasor is completely located before the fault occurs. The synchronization error compensation module is used to obtain the voltage reference phasor of the equivalent T-junction at each end based on the comprehensive reference phasor at both ends and the feeder T-type equivalent model. The fault current phasor is compensated based on the phase difference of the voltage reference phasors of the equivalent T-junctions at both ends. Specifically, if the protection start times at both ends are synchronized, the fault current phasors are also synchronized, and the voltage reference phasors of the equivalent T-junctions derived from the data at both ends are equal. If the protection start times at both ends are not synchronized, the fault current phasors are also not synchronized, and the voltage reference phasors of the equivalent T-junctions derived from the data at both ends are not equal. The phase difference is the synchronization error of the fault current phasors at both ends. The fault current phasors at the other end are corrected based on this phase difference to achieve phase angle synchronization of the fault current phasors at both ends.

7. An electronic device comprising a memory, a processor, and a computer program stored in the memory and running thereon, characterized in that, When the processor executes the program, it implements a differential protection data self-synchronization method as described in any one of claims 1-5, applicable to feeders with T-connections.

8. A non-transitory computer-readable storage medium having a computer program stored thereon, characterized in that, When executed by the processor, the program implements a differential protection data self-synchronization method as described in any one of claims 1-5, applicable to feeders with T-connected loads.

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