Fault detection method and system for T-type access line of optical storage and charging micro-grid

Abstract

The invention discloses a fault detection method and system for a T-type access line of an optical storage and charging micro-grid. According to the method, voltages and currents of three ends of a T-type access line of the optical storage and charging micro-grid are collected, fault phases are determined through a fault phase selection element, then three-end measurement impedance is calculated, and the measurement impedance is transmitted among three-end fault detection devices through a communication network. And the fault detection device at each end constructs characteristic impedance according to the measured impedance and the line impedance of each end of the T-shaped line, and judges a fault branch by utilizing the difference of characteristic impedance values when different branches have faults. Finally, differential impedance and braking impedance of different fault branches are constructed, the fault position is judged according to the relation between the differential impedance and the braking impedance, and fault detection of the optical storage and charging micro-grid T-type access line is achieved. Fault detection is realized based on impedance characteristics, the sensitivity is high, the method is not influenced by a system operation mode, and a detection blind area of T node faults can be eliminated.

Description

Technical Field

[0001] The present invention belongs to the access technology of an AC-DC hybrid microgrid composed of photovoltaic, energy storage, and charging loads, and particularly relates to a high-sensitivity fault detection and location method when a photovoltaic-energy storage-charging hybrid microgrid is connected to a distribution network through a T-type line. Background Technique

[0002] Developing a photovoltaic-energy storage-charging AC-DC hybrid microgrid is an important technical route for realizing large-scale distributed consumption of renewable energy. The photovoltaic-energy storage-charging microgrid is connected to the distribution network in a T-type connection mode, which has many advantages such as short outgoing lines, flexible access positions, and land saving. However, it also causes problems such as a more complex power supply network structure and the failure of traditional fault detection methods due to the bidirectional flow of current in the T-connected branch line, which restricts the engineering application of the photovoltaic-energy storage-charging microgrid connected to the distribution network in a T-type connection mode. To solve the problems of the degradation of the performance of the fault detection method and the occurrence of missed detections and false detections after the photovoltaic-energy storage-charging microgrid is T-connected to the distribution network line, the existing fault detection methods are specifically improved.

[0003] (1) Fault detection technology based on the overcurrent principle

[0004] To address the problem of the failure of the overcurrent fault detection technology caused by the boosting effect of the current in the photovoltaic-energy storage-charging microgrid, the improved technology accurately distinguishes internal faults in the photovoltaic-energy storage-charging microgrid and distribution network line faults by adding a power direction element, or adjusts the overcurrent threshold adaptively according to the fault type to improve the sensitivity of fault detection. In addition, the improved technology based on the inverse-time overcurrent principle corrects the delay characteristic of fault detection by introducing a voltage factor, increases the action time difference of fault detection between adjacent devices, and avoids the expansion of the fault influence range. However, the voltage quantity is greatly affected by the fault type, and the detection sensitivity in the case of minor faults needs to be improved. The fault characteristics of distributed power sources in the photovoltaic-energy storage-charging microgrid are different from those of traditional synchronous power sources, and their fault current characteristics are more complex. The fault detection technology based on the overcurrent principle has large defects in sensitivity and accuracy.

[0005] (2) Fault detection technology based on the impedance principle

[0006] Compared with the overcurrent principle that relies entirely on fault current characteristics, the fault detection technology using the impedance principle has improved detection performance. The measured impedance can reflect the fault distance, is not affected by the system operation mode, and has directionality, making it more suitable for the T-connected lines of the optical storage and charging microgrid where fault current flows in multiple directions. However, the T-connected branch lines of the optical storage and charging microgrid will cause the branch coefficient to be uncertain when calculating the impedance on the power supply side of the distribution network system, resulting in the inapplicability of the impedance detection method with a fixed threshold. The improved technology calculates the branch coefficient based on the complex sequence network using positive sequence current and negative sequence current, realizing the adaptive adjustment of the threshold, which can reduce the impact of the T-connected lines of the optical storage and charging microgrid on the impedance principle fault detection technology. However, the adaptive adjustment of the branch coefficient increases the calculation amount and the complexity of program compilation.

[0007] (3) Fault detection technology based on multi-terminal electrical quantity comparison

[0008] Compared with the overcurrent principle and impedance principle that utilize electrical quantities at a single location, the fault detection technology based on multi-terminal electrical quantities has absolute selection ability, can accurately determine the faulty equipment, and achieve rapid tripping. For the T-type access line, the three-terminal current differential principle uses the three-terminal current to calculate the differential current and braking current for fault detection, but its performance is easily affected by the selection of the braking coefficient, and it is difficult to balance the sensitivity and reliability of fault detection. Moreover, when the T-connected optical storage and charging microgrid is involved, the selection of the braking coefficient is related to the ratio of the system terminal current to the injected current of the optical storage and charging microgrid, which further increases the difficulty of selecting the braking coefficient. The voltage amplitude difference detection technology constructs the comprehensive voltage amplitude difference using the positive sequence compensated voltages at each end of the T connection line. If this amplitude difference is not zero, it is judged as an in-zone fault. However, when the fault occurs near the T node, this amplitude difference is approximately zero and misdetected as an out-of-zone fault. Therefore, this technology has a fault detection blind area near the T node.

[0009] Compared with the fault detection method that constructs the differential principle using current quantities, the impedance quantity is calculated from voltage and current quantities and is less affected by the injected current of the optical storage and charging microgrid. The existing impedance-based differential principle constructs the fault detection criterion using the sum and difference relationships of the measured impedance or high-frequency impedance, which is more suitable for the fault detection of the T-type access lines of the optical storage and charging microgrid. However, the existing impedance differential principle is only applicable to the scenario where the optical storage and charging microgrid is connected to the bus and cannot be used for the scenario of the T-type access lines of the optical storage and charging microgrid.

[0010] In summary, the fault detection technology using electrical quantities at a single location has problems such as difficult selection of the braking coefficient and difficult setting of fixed values. The fault detection technology based on multi-terminal electrical quantity comparison, although more suitable for the T-type access lines of the optical storage and charging microgrid, has problems such as a blind area at the T node and low sensitivity. Therefore, there is an urgent need to develop a simple, reliable, blind area-free, and high-sensitivity fault detection method suitable for the T-type access lines of the optical storage and charging microgrid. Summary of the Invention

[0011] To overcome the above problems existing in the prior art, the present invention discloses a fault detection method and system for a T-shaped access line of a photovoltaic-storage-charging microgrid. Based on the measured impedances at each end of the T-shaped access line, characteristic impedance for fault branch detection, differential impedance and braking impedance for fault location judgment are constructed, and the fault detection and location of the T-shaped access line of the photovoltaic-storage-charging microgrid are completed by using the amplitude differences between the above impedances.

[0012] The specific technical solution adopted by the present invention is as follows:

[0013] A fault detection method for a T-shaped access line of a photovoltaic-storage-charging microgrid includes the following steps:

[0014] 1. Construct a communication network for the power supply system with a T-connected line

[0015] After the photovoltaic-storage-charging microgrid is connected to the distribution network line in the form of a T-connected branch line, the traditional distribution network power supply system evolves into a multi-terminal power supply system. This system includes at least the following three terminals: one terminal is the main power supply of the distribution network, denoted as the M terminal; one terminal is the concentrated area of the power supply load at the end of the distribution network, denoted as the N terminal; one terminal is the T-connected photovoltaic-storage-charging microgrid, denoted as the W terminal. The intersection point of the T-connected branch line of the photovoltaic-storage-charging microgrid and the distribution network line is called the T node. One intelligent electronic device (IED) is configured at each of the M terminal, N terminal and W terminal, and the IEDs at the three terminals are directly connected in pairs by means of wired methods such as optical fibers or wireless methods such as 5G to establish a communication network. Through the communication network directly connected in pairs, any one IED can obtain the information of the other two terminals. The transmitted information includes measured impedance values, fault start signals, protection trip signals, etc.

[0016] 2. Set parameters and thresholds offline

[0017] Before each IED is put into operation, parameters and thresholds are set offline, including the positive sequence impedance z1 and zero sequence impedance z0 per unit length of the distribution network line, the line lengths from each end to the T node, the rated voltage U of the distribution network N , the rated current I of each end N , the threshold of the starting element, etc.

[0018] 3. Fault start, fault phase selection and measured impedance calculation

[0019] The IEDs at the M terminal, N terminal and W terminal collect the instantaneous values of their respective voltages and currents, which are respectively denoted as and where represents the phase type, B or C respectively represents the electrical quantities of the three phases of ABC.

[0020] Implement fault start-up judgment. The M terminal is close to the main power supply of the distribution network. During a fault, the sudden change in current measured by the IED at the M terminal is sensitive. Therefore, the IED at the M terminal uses the sudden change in the difference of phase currents to judge whether a fault has occurred. At the N terminal in the load concentration area and the W terminal where the PV-storage-charging microgrid is connected, during a fault, the fault current shows a limited characteristic, and the sudden change in current is not sensitive, while the sudden change in voltage is sensitive. Therefore, the IEDs at the N terminal and the W terminal use the sudden change in the difference of phase voltages to judge whether a fault has occurred. Through a communication network with pairwise direct connections, after any IED at one end obtains the fault start-up signals of the other two ends, regardless of whether there is a fault start-up signal at its own end, it determines that a fault has occurred, that is, it confirms the fault start-up.

[0021] After confirming the fault start-up, implement fault phase selection. The IED at the M terminal uses the sudden change in the difference of phase currents for phase selection, and the IEDs at the N terminal and the W terminal use the sudden change in the difference of phase voltages for phase selection. The phase selection results are divided into two categories: single-phase faults and inter-phase faults, and the phase selection results are respectively recorded as the single-phase fault phase φ and the inter-phase fault phase φφ. φ represents the fault phase, and φ = A, B, or C represents the three phases of A, B, and C respectively.

[0022] According to the fault phase, calculate the measured impedance. Taking the IED at the M terminal as an example, if the phase selection result is a single-phase fault, calculate the single-phase impedance of the fault phase as the measured impedance at the M terminal:

[0023]

[0024] In the formula, and are the fault phase current and the fault phase voltage respectively; is the zero-sequence current; K is the zero-sequence compensation coefficient.

[0025] If the phase selection result is an inter-phase fault, calculate the inter-phase impedance of the fault phase as the measured impedance at the M terminal:

[0026]

[0027] In the formula, and are the inter-phase voltage and the inter-phase current of the fault respectively.

[0028] Similarly, the IEDs at the N terminal and the W terminal obtain the measured impedance Z N 、Z W .

[0029] 4. Each IED at one end obtains the measured impedances of the IEDs at the other two ends

[0030] After step 3, the IEDs at the M terminal, the N terminal, and the W terminal respectively obtain their own measured impedances Z M 、Z N and Z W. The IEDs at each end use the communication network established in Step 1 to send the measured impedance of their own end to the IEDs at the other two ends. Specifically, the IED at the M end sends the measured impedance Z to the IEDs at the N end and the W end respectively. M ; The IED at the N end sends the measured impedance Z to the IEDs at the M end and the W end respectively. N ; The IED at the W end sends the measured impedance Z to the IEDs at the M end and the N end respectively. W . The impedance is converted into real numbers in the form of real part, imaginary part or amplitude, phase angle and transmitted in the form of data frames on the communication link. The IEDs at the M end, N end and W end can all obtain the measured impedance of their own end calculated directly and the measured impedances from the IEDs at the other two ends.

[0031] 5. Fault branch judgment process

[0032] Use the measured impedances Z M , Z N and Z W calculated by the IEDs at the M end, N end and W end to construct the characteristic impedances Z′ M , Z′ N and Z′ W as follows:

[0033] Z′ M =|Z M -Z L1 | (3)

[0034] Z′ N =|Z N -Z L2 | (4)

[0035] Z′ W =|Z W -Z L3 | (5)

[0036] Among them, Z L1 , Z L2 and Z L3 are the line impedances from the T node to the M end, N end and W end respectively.

[0037] When there is a fault within the protection area of each branch of the optical storage charging microgrid T-type access line, the expressions of the characteristic impedances Z′ M , Z′ N and Z′ W are derived respectively and summarized in Table 1. In the expressions of Table 1, and The fault phase currents measured by the IEDs at the M terminal, N terminal, and W terminal respectively; α1 is the ratio of the distance from the fault point on the M branch to the T node to the length of the line MT from the M terminal to the T node, α1 ∈ [0, 1]; α2 is the ratio of the distance from the fault point on the N branch to the T node to the length of the line NT from the N terminal to the T node, α2 ∈ [0, 1]; α3 is the ratio of the distance from the fault point on the W branch to the T node to the length of the line WT from the W terminal to the T node, α3 ∈ [0, 1].

[0038] After analysis, when there is a fault outside the protection zone of each branch of the optical storage charging microgrid T-type access line, the characteristic impedances Z′ M , Z′ N and Z′ W also satisfy the relationship in the last column of Table 1.

[0039] Table 1 Characteristic impedances Z′ M , Z′ N and Z′ W

[0040]

[0041] According to the rules in Table 1, determine the fault branch according to the following process. If Z′ W > k1Z′ N , it is judged as a fault in the NT branch; if Z′ W < k2Z′ N , then it is judged as a fault in the WT branch. If the relationships between Z′ N and Z′ W do not satisfy the above two judgment conditions, then judge whether Z′ W ≤ k3Z′ M is satisfied. If it is satisfied, it is judged as a fault at the T node; if it is not satisfied, it is judged as a fault in the MT branch.

[0042] The coefficients k1, k2, and k3 in the judgment conditions are determined according to the following conditions:

[0043] k1 = 1 + (ε Z + δ Z ) (6)

[0044] k2 = 1 - (ε Z + δ Z ) (7)

[0045] k3 = ε Z + δ Z (8)

[0046] In the formula, ε Z is the error limit in the impedance calculation process; δ Z is the margin.

[0047] ε ZDetermined according to the following conditions:

[0048]

[0049] Where ε i is the error limit of the current transformer; ε u is the error limit of the voltage transformer.

[0050] The IEDs at the M terminal, N terminal, and W terminal each calculate Z′ M , Z′ N and Z′ W , independently judge the faulty branch, and send their respective judgment results to the IEDs at the other two terminals through the communication network. If the judgment results of the IEDs at each terminal are different, determine the final faulty branch according to the following process: If the judgment results of any two of the three IEDs are consistent or the results of the three IEDs are all consistent, then trust this result; if the judgment results of the three IEDs are all different, then the judgment result of this time is invalid, delay for T d time and then continue to judge until there is a situation where the judgment results of any two of the three IEDs are consistent or the results of the three IEDs are all consistent. After obtaining a clear result of the faulty branch, enter the next process of judging the fault location. If the judgment result is always invalid during the fault duration, then do not enter the subsequent process and wait for the fault to be detected and isolated by other devices.

[0051] 6. Fault Location Judgment Process

[0052] After clearly judging the faulty branch, use the three-terminal measured impedances Z M , Z N and Z W to calculate the differential impedance Z dif and the restraint impedance Z res , and continuously judge whether the formula (10) is satisfied.

[0053] |Z dif | < |Z res | (10)

[0054] If the criterion is satisfied continuously for T s time, it is judged as an internal fault; otherwise, it is judged as an external fault.

[0055] The differential impedance Z dif and the restraint impedance Z res are determined respectively according to the different faulty branches in the following manner.

[0056] When it is determined that the fault occurs in the MT branch, determine Z dif and Z res according to formula (11):

[0057]

[0058] When it is determined that the fault occurs in the NT branch, Z is determined according to Equation (12). dif and Z res :

[0059]

[0060] When it is determined that the fault occurs in the WT branch, Z is determined according to Equation (13). dif and Z res :

[0061]

[0062] After it is determined that the fault occurs in the MT branch, NT branch or WT branch, the fault location judgment process is executed by the IEDs at each end respectively. After the fault location is judged, the fault isolation process is entered.

[0063] 7. Fault Isolation

[0064] The fault branch is determined through Step 5, and after the fault location is determined by the IED of the fault branch through Step 6, if it is an in-zone fault, the IED of the fault branch sends a tripping command to the local circuit breaker, and sends the tripping signal to the other two ends through the communication network. The IEDs of the other two ends send tripping commands to their respective circuit breakers, thus completing the fault isolation. After all three circuit breakers trip, when the IEDs at each end detect that there is no current on their own sides, they withdraw the tripping command, completing the entire process of fault handling. When it is determined to be an out-of-zone fault after the processes of Step 5 and Step 6, the IEDs at each end do not send tripping commands and wait for the fault to be isolated by other equipment.

[0065] The beneficial effects of the present invention include:

[0066] (1) The present invention constructs the characteristic impedance for detecting the fault branch, the differential impedance and the braking impedance for judging the fault location by using the three-terminal measured impedance of the T-type access line and the line impedance. Through the comprehensive judgment of the above impedance characteristics, the fault point location is accurately determined, eliminating the fault detection blind area near the T node.

[0067] (2) The present invention uses the impedance characteristics to complete the fault detection function. Compared with the current-based fault detection technology, it has the advantages of being not affected by the operation mode of the multi-terminal power supply system, simple threshold setting, and no need to select the braking coefficient, and is more suitable for the T-connected line of the optical storage and charging microgrid with multi-directional fault current flow.

[0068] (3) The present invention first determines the fault branch, and then constructs the fault location judgment criterion by respectively selecting different differential impedances and braking impedances according to the fault branch, so as to achieve the effect that the braking impedance is much larger than the differential impedance when the in-zone faults occur in different branches, thereby enabling the detection method of the present invention to have a high sensitivity when the faults occur in each branch. Description of the Drawings

[0069] Figure 1 It is a schematic structural diagram of the T - type access line of the optical storage charging micro - grid;

[0070] Figure 2 It is an implementation flowchart of a fault detection method for the T - type access line of the optical storage charging micro - grid;

[0071] Figure 3 It is a schematic diagram of the communication scheme;

[0072] Figure 4 It is a diagram of voltage, current waveforms and calculation results of each impedance when a fault occurs in the MT branch within the area;

[0073] Figure 5 It is a diagram of calculation results of each impedance when a fault occurs in the NT branch within the area;

[0074] Figure 6 It is a diagram of calculation results of each impedance when a fault occurs outside the area in the WT branch. Detailed Implementation Manner

[0075] The following further elaborates on the detailed implementation manner of the present invention in conjunction with the drawings and embodiments, but it does not limit the protection scope of the present invention. Any technical solutions obtained by means of equivalent replacement or equivalent transformation are within the protection scope of the present invention.

[0076] The structure of the T - type access line of the optical storage charging micro - grid is as Figure 1 shown. Figure 1 The T - type line between the buses M, N, and W in the figure is the fault detection area, that is, the protected area. Faults within this area are identified as in - area faults, and faults outside this area are identified as out - of - area faults. The M end is the access end of the system power supply, the N end is the access end of the load - concentrated area, the W end is the access end of the optical storage charging micro - grid, and the T point is the access point where the optical storage charging micro - grid accesses the distribution network. f1, f2, and f3 are the fault points on the lines MT, NT, and WT within the protected area respectively, and f4 is the fault point on the back side of the bus W outside the protected area. Figure 2 It is the flowchart for the present invention to complete fault detection. Figure 3 It is a schematic diagram of the communication scheme.

[0077] Example 1:

[0078] Assume that a single - phase ground fault occurs at f1 at t0 = 0.4 s, and the fault duration is 0.1 s. For the protected area, it is a single - phase in - area fault in the M branch.

[0079] 1. Construct a communication network for the power supply system with a T - connected line

[0080] After the photovoltaic-storage-charging microgrid is connected to the distribution network line in the form of a T-connected branch line, the traditional distribution network power supply system evolves into a multi-terminal power supply system. The three terminals of this system are denoted as the M terminal, the N terminal, and the W terminal respectively. One intelligent electronic device is configured at each of the three terminals, and the three IEDs at the three terminals are directly connected pairwise by optical fibers to establish a communication network. Through the pairwise-connected communication network, the binary information transmitted between the three IEDs forms different types of communication frames in sequence. Information such as the measured impedance value, fault start signal, and protection trip signal after encoding forms a GOOSE (Generic Object Oriented Substation Event) frame in sequence. The sending frequency of the GOOSE frame is densely sent when there is a status change, and a heartbeat message is sent every 5 seconds when there is no status change. The sending mechanism follows the provisions of the IEC61850 standard.

[0081] 2. Offline Setting of Parameters and Thresholds

[0082] Before each IED is put into operation, parameters and thresholds are set offline, including the positive sequence impedance z1=(0.17 + j0.34)Ω / km and zero sequence impedance z0=(0.42 + j1.51)Ω / km per unit length of the distribution network line, and the line lengths L MT =4km, L NT =5km, and L WT =5km from the busbars of the M terminal, N terminal, and W terminal to the T node, the rated voltage U N =10kV of the distribution network, the rated current I N =0.98kA on the main power supply side of the distribution network, the starting threshold I set =k i I N =0.2×0.98kA = 0.196kA for the sudden change of phase current difference, and the starting threshold U set =k u U N =0.005×10kV = 0.05kV for the sudden change of phase voltage difference. The above parameters and thresholds are respectively input into the IEDs at the three terminals of the M terminal, N terminal, and W.

[0083] 3. Fault Starting, Fault Phase Selection, and Measurement Impedance Calculation

[0084] The IED at the M terminal uses the voltage transformer and current transformer installed at the Figure 1 M terminal to collect the three-phase voltages u MA , u MB , u MC and the three-phase currents i MA , i MB , i MC . Similarly, the IED at the N terminal collects the three-phase voltages u NA , u NB , uNC and three-phase current i NA 、i NB 、i NC ; The W-terminal IED collects three-phase voltages u WA 、u WB 、u WC and three-phase current i WA 、i WB 、i WC , and the waveform diagram is shown in Figure 4 (a).

[0085] Perform fault start judgment. The M-terminal IED uses the sudden change of the phase current difference to judge whether there is a fault. After the fault occurs, the sudden change of the phase current difference Δi(k) is collected at time t k as shown in Equation (14).

[0086] Δi(k) = |i(k) - i(k - N)| - |i(k - N) - i(k - 2N)| (14)

[0087] In the formula, i(k), i(k - N), and i(k - 2N) respectively represent the sampling values of the phase current between t k time, t k the previous cycle, and the previous 2 cycles; N is the number of sampling points per cycle, and N = 40 is taken.

[0088] In this embodiment, the AB-phase current difference is selected, and the sampling value serial number corresponding to the fault occurrence time t = 0.4 s is k = 8000. At k = 8100, k = 8101, and k = 8102, the sudden changes of the AB-phase current difference are |Δi(8100)| = 0.430 kA, |Δi(8101)| = 0.415 kA, and |Δi(8102)| = 0.481 kA respectively.

[0089] Substitute Δi(k) into the phase current difference sudden change start criterion shown in Equation (15).

[0090] |Δi(k)| > I set = 0.196 kA (15)

[0091] The start criteria shown in Equation (15) are satisfied by the sampling values of 3 consecutive points. At k = 8102, the M-terminal IED determines that a fault has occurred.

[0092] The N-terminal and W-terminal IEDs use the sudden change of the phase voltage difference as the starting element. The AB-phase voltage difference is used, and the current quantity in Equation (14) is replaced by the voltage quantity. The starting criterion shown in Equation (15) is replaced by |Δu(k)| < U set. The sudden change amounts of phase voltage differences calculated by the N-terminal IED are |Δu(8100)| = 1.713 kV, |Δu(8101)| = 1.459 kV, and |Δu(8102)| = 1.220 kV respectively. When k = 8102, three consecutive sampling values satisfy |Δu(k)| < U set , and the N-terminal IED determines that a fault has occurred.

[0093] . The sudden change amounts of phase voltage differences calculated by the W-terminal IED are |Δu(8100)| = 1.654 kV, |Δu(8101)| = 1.454 kV, and |Δu(8102)| = 1.213 kV respectively. When k = 8102, three consecutive sampling values satisfy |Δu(k)| < U set , and the W-terminal IED determines that a fault has occurred.

[0094] It can be seen that when k = 8102, the IEDs at the M-terminal, N-terminal, and W-terminal all judge that a fault has occurred.

[0095] After confirming the fault startup, a phase current difference sudden change phasor selection element is used to identify the fault phase. Taking the calculation process of the M-terminal IED as an example to illustrate the phasor selection steps. Calculate the sudden change phasor of the phase current difference starting from k = 8102,

[0096]

[0097] where, is the phase current difference after the fault; is the phase current difference before the fault.

[0098] The identification process of the phase current difference sudden change phasor selection element is as follows:

[0099] (1) When the magnitude of any one sudden change amount of current difference is much smaller than the magnitudes of the other two sudden change amounts of current difference, it is judged that a single-phase grounding fault has occurred. The phases commonly involved in the two larger sudden change amounts of current difference are the fault phases.

[0100] (2) When the absolute values of the three sudden change amounts of phase current differences are equal, it is judged that a three-phase short-circuit fault has occurred.

[0101] (3) When the absolute values of the three sudden change amounts of phase current differences do not meet the conditions of single-phase grounding fault and three-phase short-circuit fault, it is judged that an interphase fault has occurred, and the two phases involved in the largest sudden change amount of phase current difference are the fault phases.

[0102] According to the data calculated by formulas (16), (17), and (18), is much smaller than and Meeting the above criterion (1), it is judged as a ground fault on phase A.

[0103] Calculate the measured impedance according to the fault phase. First, calculate the zero-sequence current compensation coefficient K.

[0104]

[0105] Then use Equation (20) to calculate the relative-to-ground impedance Z M .

[0106]

[0107] Similarly, the impedances Z N , Z W are obtained at the N terminal and the W terminal by using Equation (21) and Equation (22) respectively.

[0108]

[0109] Z M , Z N , Z W The calculation results of are as shown in Figure 4 (b).

[0110] 4. Each terminal IED obtains the measured impedances of the IEDs at the other two terminals

[0111] After step 3, the IEDs at the M terminal, N terminal, and W terminal respectively obtain their own measured impedances Z M , Z N and Z W . Each terminal IED uses the communication network established in step 1 to send the measured impedance of its own terminal to the IEDs at the other two terminals. Specifically, the M-terminal IED sends the measured impedance Z M to the N-terminal IED and the W-terminal IED respectively; the N-terminal IED sends the measured impedance Z N to the M-terminal IED and the W-terminal IED respectively; the W-terminal IED sends the measured impedance Z W to the M-terminal IED and the N-terminal IED respectively. The impedances are converted into real numbers in the form of real part, imaginary part or amplitude, phase angle and transmitted in the form of data frames on the communication link. The IEDs at the M terminal, N terminal, and W terminal can all obtain the measured impedance of their own terminal calculated directly and the measured impedances sent from the IEDs at the other two terminals.

[0112] 5. Fault branch judgment process

[0113] Use the measured impedances Z M , Z N and Z W calculated by the IEDs at the M terminal, N terminal, and W terminal to construct the characteristic impedance for detecting the fault branch:

[0114] Z′ M = |Z M - ZL1 | (23)

[0115] Z′ N =|Z N -Z L2 | (24)

[0116] Z′ W =|Z W -Z L3 | (25)

[0117] The calculation results of the characteristic impedance are as Figure 4 (b) shows.

[0118] In this embodiment, the error limit of the current transformer is taken as ε i =5%, and the error limit ε u of the voltage transformer is 5%. Then the error limit ε Z of the impedance calculation process is determined according to the following conditions:

[0119]

[0120] The margin is taken as δ Z =3%, then the coefficients k1, k2 and k3 are determined according to the following conditions:

[0121] k1 = 1+(ε Z +δ Z ) = 1+(7% + 3%) = 1.1 (27)

[0122] k2 = 1-(ε Z +δ Z ) = 1-(7% + 3%) = 0.9 (28)

[0123] k3 = ε Z +δ Z =7% + 3% = 0.1 (29)

[0124] From Figure 4 the calculation results shown in (b), it can be seen that 21 ms after the fault occurs, that is, when t = 0.421 s, the impedance calculation results are stable at Z′ N =2.43 Ω and Z′ W =2.43 Ω, and the relationship between Z′ N and Z′ W does not satisfy the judgment condition Z′ W >1.1Z′ N for the NT branch fault, nor does it satisfy the judgment condition Z′ W <0.9Z′ N for the WT branch fault, nor does it satisfy the judgment condition Z′ W ≤0.1Z′ M, thus it is determined that there is a fault in the MT branch.

[0125] The IEDs at the M terminal, N terminal, and W terminal all detected Z′ at t = 0.421 s. N and Z′ W The relationship between them satisfies the MT branch fault condition, so the IEDs at all 3 terminals are determined to have faults in the MT branch. The IEDs at each terminal send the judgment results to the other 2 terminals through the communication network. The judgment results of the IEDs at the 3 terminals are consistent, and the result of the MT branch fault is taken as reliable, and the fault location judgment process is entered.

[0126] 6. Fault Location Judgment Process

[0127] After step 5, it has been clearly determined that the fault occurred in the MT branch, then the differential impedance Z is calculated according to Equation (30). dif and the restraining impedance Z res .

[0128]

[0129] The calculation results of the differential impedance Z dif and the restraining impedance Z res are as shown in Figure 4 (b).

[0130] According to Figure 4 (b)'s calculation results, during the time period from 21 ms to 26 ms after the fault occurred, that is, from 0.421 s to 0.426 s, the relationship between Z dif and Z res continuously satisfies the criterion Equation (31) for T s = 5 ms.

[0131] |Z dif | < |Z res | (31)

[0132] Then at t = 0.426 s, the M-terminal IED determines that it is a fault within the zone.

[0133] 7. Fault Isolation

[0134] Through step 5, it has been determined that there is a fault in the MT branch, and through step 6, it is determined by the M-terminal IED that it is a fault within the MT branch zone. Then the M-terminal IED sends a tripping command to the local circuit breaker and sends the tripping signal to the N terminal and W terminal through the communication network. The IEDs at the N terminal and W terminal respectively send tripping commands to their own circuit breakers, thus completing the fault isolation. After all 3 circuit breakers trip, the IEDs at each terminal withdraw the tripping command when they detect no current on their own side, completing the entire process of fault handling.

[0135] Embodiment 2:

[0136] Assume that at the moment of t0 = 0.4 s, an AB-phase ground fault occurs at f2, and the fault duration is 0.1 s. For the protected area, it is a non-single-phase fault within the N-terminal area.

[0137] The implementation process of Steps 1 to 7 in Embodiment 2 is similar to that in Embodiment 1. Therefore, in this embodiment, only the differences in the processing procedures of each step in Embodiment 1 will be elaborated.

[0138] Step 3: When k = 8025, the IEDs at the M-terminal, N-terminal, and W-terminal all judge that a fault has occurred. After confirming the fault startup, the phase current difference sudden change phase selection element is used to identify the fault phase. Taking the calculation process of the IED at the M-terminal as an example, the phase selection steps are described as follows. The sudden change phase vectors of the phase current differences with k = 8025 as the starting moment are respectively and

[0139] According to the recognition process of the phase current difference sudden change phase selection element, the three phase current difference sudden changes do not satisfy the first criterion and the second criterion, and is the maximum phase current difference sudden change, so it is judged as an AB-phase interphase fault.

[0140] According to the phase selection result, the interphase impedance of the fault phase is calculated using Equation (32) as the measured impedance at the M-terminal:

[0141]

[0142] Similarly, the impedances Z N , Z W are obtained at the N-terminal and W-terminal using Equation (33) and Equation (34) respectively.

[0143]

[0144] Z M , Z N , Z W The calculation results are as shown in Figure 5 .

[0145] Step 5: The IEDs at the M-terminal, N-terminal, and W-terminal all use the obtained measured impedance values Z M , Z N , Z W to calculate the characteristic impedance Z′ M , Z′ N , Z′ W of the detected fault branch. The calculation results are as shown in Figure 5 .

[0146] From Figure 5 the shown calculation results, it can be seen that at 22 ms after the fault occurs, that is, at t = 0.422 s, the impedance calculation result is detected to be stable at Z′N = 0.95Ω and Z' W = 15.17Ω, Z' N The relationship between Z' W and Z' satisfies the judgment condition for the NT branch fault, Z' W > 1.1Z' N , so it is determined that there is a fault in the NT branch.

[0147] The IEDs at the M end, N end, and W end all detect Z' at t = 0.422s N and Z' W The relationship between them satisfies the NT branch fault condition, so the IEDs at the 3 ends are all judged to have faults in the NT branch. The IEDs at each end send the judgment results to the other 2 ends through the communication network. If the judgment results of the 3-end IEDs are consistent, the result of the NT branch fault is taken as true, and the fault location judgment process is entered.

[0148] Step 6, after determining that the fault occurs in the NT branch, select formula (35) to calculate the differential impedance Z dif and the braking impedance Z res :

[0149]

[0150] The differential impedance Z dif and the braking impedance Z res The calculation results are as Figure 5 shown.

[0151] According to Figure 5 the calculation results, within the time period of 22ms - 27ms after the fault occurs, that is, 0.422s - 0.427s, continuous T s = 5ms satisfies the criterion formula (36):

[0152] |Z dif | < |Z res | (36)

[0153] Then at t = 0.427s, the N-end IED determines that it is a fault within the zone.

[0154] Embodiment 3:

[0155] Suppose that at the moment t0 = 0.4s, a BC-phase interphase fault occurs at f3, and the fault duration is 0.1s. For the protected area, it is an interphase fault outside the W end area.

[0156] The execution processes of steps 1 to 7 in Embodiment 3 are similar to those in Embodiment 1, and the phase selection results are the same as those in Embodiment 2, both being interphase faults. Therefore, in Embodiment 3, only the differences in the processing processes and judgment results of each step in Embodiment 1 and Embodiment 2 are described.

[0157] Step 3: IEDs at the M terminal, N terminal, and W terminal all use the phase-to-phase voltage and the phase-to-phase current to calculate the measured impedance Z M , Z N , Z W . The calculation results are as Figure 6 shown

[0158] Step 5: IEDs at the M terminal, N terminal, and W terminal each calculate the characteristic impedance Z′ M , Z′ N , Z′ W of the detected fault branch. The calculation results are as Figure 6 shown

[0159] From Figure 6 the calculation results shown, it can be seen that 27 ms after the fault occurs, that is, when t = 0.427 s, the impedance calculation results are stable at Z′ N = 66.80 Ω and Z′ W = 4.33 Ω. The relationship between Z′ N and Z′ W satisfies the judgment condition for a WT branch fault: Z′ W < 0.9Z′ N . Therefore, it is determined that the WT branch is faulty

[0160] IEDs at the M terminal, N terminal, and W terminal all detect that the relationship between Z′ N and Z′ W satisfies the WT branch fault condition at t = 0.420 s. Therefore, the IEDs at all 3 terminals are judged to have a WT branch fault. The IEDs at each terminal send the judgment results to the other 2 terminals through the communication network. Since the judgment results of the 3-terminal IEDs are consistent, the result of the WT branch fault is taken and the fault location judgment process is entered

[0161] Step 6: After determining that the fault occurs in the WT branch, select formula (37) to calculate the differential impedance Z dif and the braking impedance Z res .

[0162]

[0163] The calculation results of the differential impedance Z dif and the braking impedance Z res are as Figure 6 shown

[0164] According to Figure 6 the calculation results, it can be seen that from the occurrence to the end of the fault, the relative relationship between Z dif and Z res does not satisfy the criterion formula (38):

[0165] |Z dif |<|Z res | (38)

[0166] Then the IED at the W terminal determines it as an external fault.

[0167] In step 7, since it has been determined as an external fault after the processes of step 5 and step 6, the IEDs at each terminal do not issue trip commands and wait for the fault to be isolated by other devices.

[0168] As shown above, only the preferred embodiments of the present invention are presented, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims.

Claims

1. A fault detection method and system for a T-type access line of a photovoltaic storage and charging microgrid, characterized in that: The following steps are involved: Step 1: The power supply system with T-connected lines includes three terminals, namely M, N and W. Each terminal is equipped with an intelligent electronic device, referred to as IED. The three-terminal IEDs are directly connected to each other to establish a communication network. Step 2: Before the IEDs at each end are put into operation, the line parameters of the T-type access power supply system and the judgment thresholds for fault detection are set offline; Step 3: The IED at the M end collects the voltage and current at the local end, implements the fault start judgment, and after confirming the fault start, implements the fault phase selection. If the phase selection result is a single-phase fault, the single-phase impedance of the fault phase is calculated as the M-end measurement impedance Z M If the phase selection result is a phase-to-phase fault, calculate the phase-to-phase impedance of the faulty phase as the measurement impedance Z at the M end. M ; The N-end IED and the W-end IED use the same method to obtain their respective measured impedance Z N and Z W ; Step 4: After the three IEDs obtain their respective measured impedances, each IED sends the measured impedance of the local end to the other two IEDs using the communication network established in step 1, so that the three IEDs can obtain the directly calculated measured impedance of the local end and the measured impedance from the other two IEDs; Step 5: Use the measured impedance of the 3-terminal IED to construct the characteristic impedance Z′ of the fault branch M , Z′ N and Z′ W , determine the faulty branch according to the following process: If Z′ W >k1Z′ N , it is judged as NT branch fault; if Z′ W <k2Z′ N , then it is judged as WT branch fault; if Z′ N and Z′ W If the relationship does not satisfy the above two judgment conditions, then continue to judge whether Z′ is satisfied. W ≤k3Z′ M If it is satisfied, it is judged as a T node failure. If it is not satisfied, it is judged as an MT branch failure. Among them, k1, k2 and k3 are all judgment thresholds; Step 6: According to the different fault branches identified, each IED uses the three-terminal measured impedance to calculate the differential impedance Z dif and braking impedance Z res , and continue to judge whether the criterion of formula (1) is satisfied: |Z dif |<|Z res | (1) If continuous T s If all the criteria are met within the time, it is judged as an internal fault of the branch; otherwise, it is judged as an external fault of the branch. Step 7: After determining the fault location, if it is a fault within the T-connected line area, the IED sends a trip command to the circuit breaker to complete fault isolation; if it is a fault outside the T-connected line area, the IED does not send a trip command and waits for other devices to isolate the fault.

2. A method and system for fault detection of a T-type access line of a photovoltaic storage and charging microgrid according to claim 1, characterized in that: The parameters and thresholds set offline in step 2 include the positive-sequence impedance z1 and zero-sequence impedance z0 per unit length of the distribution network line, the line length from each end to the T node, and the rated voltage U N , rated current at each end I N , starting element threshold, etc.

3. A method and system for fault detection of a T-type access line of a photovoltaic storage and charging microgrid according to claim 1, characterized in that: The logic of determining the fault start in step 3 is: through the two-to-two direct communication network, after the IED at either end obtains the fault start signal from the other two ends, regardless of whether there is a fault start signal at the local end, it is determined that a fault has occurred and the fault start is confirmed.

4. A method and system for fault detection of a T-type access line of a photovoltaic storage and charging microgrid according to claim 1, characterized in that: The construction method of characteristic impedance in step 5 is: WITH' M =|From M -WITH L1 | (2) WITH' N =|From N -WITH L2 | (3) WITH' W =|From W -WITH L3 | (4) Among them, Z L1 , Z L2 and Z L3 They are the line impedances from the T node to the M, N and W terminals respectively.

5. A method and system for fault detection of a T-type access line of a photovoltaic storage and charging microgrid according to claim 1, characterized in that: The method for determining the thresholds k1, k2 and k3 in step 5 is: k1=1+(e Z +d Z ) (5) k2=1-(e Z +d Z ) (6) k3=e Z +d Z (7) In the formula, ε Z is the error limit of the impedance calculation process, δ Z is the margin; ε Z Determine according to the following conditions: In the formula, ε i is the error limit of the current transformer, ε u is the error limit of the voltage transformer.

6. A method and system for fault detection of a T-type access line of a photovoltaic storage and charging microgrid according to claim 1, characterized in that: Differential impedance Z in step 6 dif and braking impedance Z res The method for determining is: When it is determined that the fault occurs in the MT branch, it is determined according to formula (9): When it is determined that the fault occurs in the NT branch, it is determined according to formula (10): When it is determined that the fault occurs in the WT branch, it is determined according to formula (11).

7. A method and system for fault detection of a T-type access line of a photovoltaic storage and charging microgrid according to claim 1, characterized in that: The tripping logic for achieving fault isolation in step 7 is as follows: if the fault is within the T-connected line area, the faulty branch IED sends a tripping command to the circuit breaker at the local end, and sends the tripping signal to the other two ends through the communication network, and the IEDs at the other two ends send tripping commands to their respective circuit breakers to achieve fault isolation; after all the circuit breakers at the three ends trip, the IED at each end withdraws the tripping command when it detects that there is no current on this side; if the fault is outside the T-connected line area, the IED at each end does not send a tripping command, waiting for other equipment to isolate the fault.

8. A fault detection system for a T-type access line of a photovoltaic storage and charging microgrid based on the fault detection method according to any one of claims 1 to 7, comprising a collection module, a fault start module, a phase selection module, an impedance calculation module and a judgment module, characterized in that: The acquisition module is used to collect the bus voltage and line current at each end of the T-type access line of the photovoltaic storage and charging microgrid; A fault start module, in which a sudden change start element is set to determine whether a fault occurs based on the collected voltage and current; Phase selection module, in which a sudden change phase selection element is set to select the fault phase according to the collected voltage and current; An impedance calculation module, which calculates the measured impedance of each end of the T-type access line according to the determined fault phase, and calculates the characteristic impedance used to detect the fault branch; A judgment module judges the fault branch according to the amplitude relationship of the characteristic impedance, determines the calculation method of the differential impedance and the braking impedance according to the fault branch, and then determines the fault location according to the amplitude relationship of the differential impedance and the braking impedance.

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