An active power distribution network fault variation quantity impedance differential protection method

Abstract

This invention discloses a differential impedance protection method for fault changes in active distribution networks. Based on the concept of control and protection coordination, this method first switches the control strategy of the inverter-interfaced distributed generator (IIDG) to constant impedance angle control after a fault is initiated. Simultaneously, after the protection at both ends of the protected line determines the fault initiation, it acquires the fault-changing voltage and current at the protection installation location, extracts the positive-sequence components of these fault-changing quantities, and then calculates the measured impedance. Finally, based on the measured impedance at both ends of the protected line, the differential impedance and braking impedance are calculated, and protection criteria are constructed according to the relative relationship between the differential impedance and braking impedance at different fault locations, achieving fault detection and isolation. This invention can effectively identify faults in active distribution networks, has strong tolerance to transition resistance, and has low requirements for data synchronization.

Description

Technical Field

[0001] This invention belongs to the field of power system automation and active distribution network protection and control technology, and particularly relates to a fault variation impedance differential protection method applicable to active distribution networks containing inverter-type distributed power sources. Background Technology

[0002] With the large-scale integration of new power generation, grid, load, and storage systems, the traditional centralized power supply mode of large power grids is increasingly revealing its inherent defects in terms of flexibility and power supply reliability. Meanwhile, distributed power sources, represented by inverter-interfaced distributed generators (IIDGs), are widely adopted due to their advantages of high efficiency, flexibility, clean operation, and low carbon emissions, transforming traditional distribution networks into active distribution networks containing a large number of distributed power sources.

[0003] The integration of IIDG (Integrated Ionized Diode) grids transforms traditional single-ended radial distribution networks into multi-ended active distribution networks. Traditional overcurrent protection designs based on single-ended power grids are prone to losing selectivity. Furthermore, the current-limiting measures employed by IIDGs to prevent device overcurrent restrict fault current, reducing the sensitivity of traditional overcurrent protection. Clearly, the power supply structure and fault characteristics of active distribution networks incorporating IIDGs undergo significant changes, rendering traditional overcurrent protection inapplicable. To address this technical challenge in the field of relay protection for active distribution networks, existing technologies include improvements to traditional overcurrent protection and research into new methods such as differential protection and wide-area protection suitable for active distribution networks.

[0004] (1) Improvement of traditional overcurrent protection

[0005] Adaptive overcurrent protection improves the performance of traditional overcurrent protection by monitoring changes in grid voltage and current and adjusting the settings and parameters of the overcurrent protection in real time according to the grid's operating status. However, this method only passively adapts to the fault characteristics of active distribution networks containing IIDGs. While it improves the selectivity of overcurrent protection to some extent, it cannot solve the problem of decreased protection sensitivity. Existing technologies also employ the concept of control-protection coordination, actively controlling the phase angle of the IIDG equivalent impedance to make the directional element operate at its maximum sensitivity angle, thus improving the sensitivity of directional overcurrent protection. However, with the increasing penetration of distributed generation, the above-mentioned overcurrent protection improvement methods utilizing adaptive and control-protection coordination measures still have shortcomings in coordinating sensitivity and selectivity.

[0006] (2) Differential protection is used in active distribution networks

[0007] Differential protection with absolute selectivity is widely used in high-voltage transmission networks. Introducing it into active distribution networks, which also have multiple power sources, can fundamentally improve protection performance. Differential protection suitable for active distribution networks includes current differential protection and impedance differential protection. Current differential protection determines whether a fault in an active distribution network occurs within the protected line by comparing the relationship between the differential current and the restraining current. However, current differential protection requires strict synchronization of the currents at both ends of the line involved in the protection calculation, which is difficult to achieve in active distribution networks with weak communication conditions. To address this, existing technologies utilize 5G communication technology to improve the communication conditions of the distribution network or employ methods such as fault data self-synchronization (FDSS) to address the data synchronization problem of current differential protection. However, the use of 5G communication technology is too costly, while current differential protection methods based on the FDSS algorithm do not fundamentally reduce the requirement for data synchronization.

[0008] Impedance differential protection calculates the differential impedance and restraining impedance using the naturally synchronized voltage and current at the same terminal. By comparing the relationship between the differential impedance and the restraining impedance, the location of the fault inside or outside the protection zone is determined. Compared with current differential protection, it does not require strict synchronization of electrical quantities at both ends of the protected line. However, existing impedance differential protection is severely affected by transition resistance, and its sensitivity drops significantly when encountering faults with high transition resistance.

[0009] (3) Adopting wide-area protection of the entire network information

[0010] Wide-area protection is a type of protection method that utilizes information from the entire active distribution network. Existing adaptive wide-area protection methods first divide the power grid into several sub-networks or protection zones, and then calculate the fault identification vector by measuring the wide-area current to determine the fault location. Although wide-area protection methods can flexibly respond to changes in distribution network topology and operating modes, and have strong anti-interference capabilities, they require a large number of phasor measurement units and high-speed communication networks, and are not suitable for active distribution networks with weak communication conditions.

[0011] In summary, the overcurrent protection improved by using adaptive and control-protection coordination measures still has the problem of difficulty in coordinating selectivity and sensitivity. Furthermore, the existing current differential protection has high requirements for data synchronization and is not suitable for the weak communication conditions of the distribution network. Therefore, there is an urgent need to study new relay protection methods suitable for active distribution networks containing IIDG. Summary of the Invention

[0012] To overcome the aforementioned problems in the prior art, this invention discloses an active distribution network fault variation impedance differential protection method.

[0013] The specific technical solution adopted in this invention is as follows:

[0014] An active distribution network fault variation impedance differential protection method includes the following steps:

[0015] 1. Establish a data transmission system

[0016] In active power distribution networks, a data transmission system based on a peer-to-peer communication network is established, enabling adjacent protection devices to establish data transmission channels via wired physical direct connection or wireless virtual direct connection. Through this established data transmission system, protection devices at both ends of the protected line can exchange at high speed any information used for electrical quantity calculation and logical judgment, including analog sample values ​​and binary states.

[0017] 2. Obtain the three-phase voltage and three-phase current at each location.

[0018] The two ends of the protected line are designated as M-end and N-end, where M-end represents the end of the protected line closer to the distribution network, and N-end represents the end farther from the distribution network and closer to the IIDG. Three-phase voltage and three-phase current are collected at three locations: M-end, N-end, and the IIDG grid connection point.

[0019] The M-terminal protection device uses a voltage transformer installed on the busbar to collect the instantaneous three-phase voltage value u at the M-terminal. MA (t), u MB (t) and u MC (t), the instantaneous value of the three-phase current i at terminal M is collected using a current transformer installed on the protected line. MA (t), i MB (t) and i MC (t), where t represents time.

[0020] Using the same method, the N-terminal protection device collects the instantaneous value u of the three-phase voltage at the N-terminal. NA (t), u NB (t) and u NC (t) and instantaneous value of three-phase current i NA (t), i NB (t) and i NC (t).

[0021] Using the same method, the control device installed at the IIDG grid connection point collected the instantaneous three-phase voltage value u at the IIDG grid connection point. GA (t), u GB (t) and u GC (t) and instantaneous value of three-phase current i GA (t), i GB (t) and i GC (t).

[0022] Through the data transmission system established in step 1, the M-terminal protection device and the N-terminal protection device respectively send their acquired instantaneous three-phase voltage and instantaneous three-phase current values ​​to the other end, that is, the M-terminal sends u to the N-terminal. MA (t), u MB (t), u MC (t) and i MA (t), i MB (t), i MC (t), N sends u to M. NA (t), u NB (t), u NC (t) and i NA (t), i NB (t), i NC (t).

[0023] After the above direct acquisition and data transmission process, both the M-terminal protection device and the N-terminal protection device can obtain the directly acquired instantaneous values ​​of their own three-phase voltage and three-phase current, as well as the transmitted instantaneous values ​​of the opposite terminal's three-phase voltage and three-phase current. At this point, the instantaneous voltage and current values ​​obtained by the M-terminal protection device and the N-terminal protection device are identical.

[0024] In the subsequent steps 3 to 7, the M-terminal protection device and the N-terminal protection device are executed in the same way. Therefore, the M-terminal protection device is used as an example in steps 3 to 7. When the N-terminal protection device processes the data, it replaces all the electrical quantities of the local terminal mentioned in these steps with the electrical quantities of the N terminal, and replaces the electrical quantities of the opposite terminal with the electrical quantities of the M terminal.

[0025] 3. Fault start-up diagnosis

[0026] The M-terminal protection device uses the instantaneous voltage and current values ​​of its own end and the opposite end obtained in step 2 to determine whether a fault has occurred in the power grid by employing a sudden change in the starting element. Only when a fault is determined to have occurred using the instantaneous voltage and current values ​​of its own end, and simultaneously determined to have occurred using the instantaneous voltage and current values ​​of the opposite end, will the M-terminal protection device determine that a fault has been initiated and enter the fault handling process.

[0027] The fault detection and activation method of the N-terminal protection device is exactly the same as that of the M-terminal protection device.

[0028] The fault detection and activation method of the control device at the IIDG grid connection point is the same as that of the M-end protection device.

[0029] 4. Obtain fault change voltage and fault change current.

[0030] After a fault is identified as initiated in step 3, the fault handling process begins. Compared to the full fault, the fault variation is theoretically unaffected by load current, transition resistance, and IIDG operating mode. Therefore, this invention first obtains the fault variation voltage and fault variation current in the fault handling procedure for subsequent calculations.

[0031] The M-terminal protection device uses equation (1) to obtain the instantaneous value Δu of the fault change voltage at this terminal. Mφ (t), the instantaneous value Δi of the fault change current at this end is obtained using equation (2). Mφ (t).

[0032] Δu Mφ (t)=u Mφ (t)-u Mφ (t-nT) (1)

[0033] Δi Mφ (t)=i Mφ (t)-i Mφ (t-nT) (2)

[0034] In the formula, φ represents the phase, which takes values ​​of A, B, and C; T represents the fundamental period; and n is a positive integer.

[0035] The M-terminal protection device uses the instantaneous values ​​of the three-phase voltage and three-phase current at the opposite end to obtain the instantaneous value Δu of the voltage change due to the fault at the opposite end. Nφ (t) and the instantaneous value Δi of the fault change current at the opposite end. Nφ (t), when calculating, u in equations (1) and (2) Mφ (t) is replaced with u Nφ (t), i Mφ (t) is replaced with i Nφ (t).

[0036] Similar to the method used by the M-terminal protection device to obtain fault change quantities, the N-terminal protection device also obtains the instantaneous values ​​of the fault change quantities voltage and current at both the local and remote ends.

[0037] The control device at the IIDG grid connection point uses the same method to obtain the instantaneous value Δu of the fault change voltage at the IIDG grid connection point. Gφ (t), and the instantaneous value Δi of the fault change current. Gφ (t).

[0038] 5. Obtain the positive sequence components of fault change voltage and fault change current.

[0039] Compared to the negative-sequence component, which only reflects asymmetric faults, the positive-sequence component can reflect both asymmetric and symmetric faults. Therefore, the protection method constructed using the positive-sequence component in this invention more comprehensively reflects the fault type.

[0040] The M-terminal protection device uses the instantaneous sequence component extraction method to calculate the instantaneous value Δu of the positive sequence component of the fault change voltage at this terminal. M1 (t) and the instantaneous value Δi of the positive sequence component of the fault change current. M1 (t), and calculate the instantaneous value Δu of the positive-sequence component of the voltage change at the opposite end of the fault. N1 (t), Instantaneous value Δi of the positive sequence component of the fault change current N1 (t).

[0041] The N-terminal protection device uses the same method to obtain the positive sequence components of the fault change voltage and current at its own and opposite ends.

[0042] The control device at the IIDG grid connection point also uses the same sequence component extraction method to calculate the instantaneous value Δu of the positive sequence component of the fault change voltage. G1 (t), instantaneous value Δi of the positive sequence component of the fault change current G1 (t).

[0043] Then, the M-terminal and N-terminal protection devices and the IIDG grid connection point control device use a phasor extraction algorithm to calculate the effective value and phase of the phasor corresponding to the positive sequence components of each of the above-mentioned fault change quantities. The above-mentioned phasors include the positive sequence phasor of the fault change quantity voltage at the M-terminal. The positive-sequence phasor of the fault change current at the M-terminal The positive-sequence phasor of the N-terminal fault change voltage The positive-sequence phasor of the fault change current at the N-terminal terminal. IIDG grid connection point fault change voltage positive sequence phasor Positive sequence phasor of fault change current at IIDG grid connection point

[0044] 6. Implement constant phase angle control for IIDG.

[0045] After the control device at the IIDG grid connection point detects a fault, it uses a smooth switching method to switch the constant power control strategy or constant frequency and constant voltage control strategy before the fault to the constant phase angle control strategy after the fault, and implements constant phase angle control of the IIDG until the fault is cleared.

[0046] After the control strategy is switched, the reference value of the inner current loop of the IIDG dual closed-loop control is calculated according to the following steps.

[0047] (1) Locate the instantaneous three-phase voltage values ​​at the grid connection point before the fault start-up from the data buffer of the control device, and extract its positive sequence component u using the instantaneous sequence component extraction method. G|0| (t), and then use the phasor extraction algorithm to calculate the corresponding phasor. After a fault start-up, the control device extracts the positive sequence component u using the instantaneous value of the three-phase voltage at the grid connection point. G1 (t), and then use the phasor extraction algorithm to calculate the corresponding phasor.

[0048] (2) Use equation (3) to calculate the voltage drop degree k.

[0049]

[0050] Calculate using equation (4) Lag The angle δ.

[0051] δ=θ U,G|0| -θ U,G1 (4)

[0052] (3) Calculate the reference value of the q-axis current in the inner current loop according to equation (5). and d-axis current reference value

[0053]

[0054] In the formula, The phase angle control target for IIDG should be close to the phase angle of the equivalent impedance of the distribution network, i.e. θ ΔU,G1 The positive-sequence component of the IIDG grid connection point fault change voltage obtained in step 5. The phase; β∈[1.2,2.0], I N This indicates the rated current of the IIDG.

[0055] 7. Calculate the measured impedance at terminals M and N.

[0056] Based on the positive-sequence components of the fault change voltage and current obtained in step 5 at terminal M. And the positive sequence components of the voltage and current changes during faults at the N-terminal. The M-terminal protection device calculates the measured impedance Z at this terminal using equations (6) and (7), respectively. CM and the measured impedance Z at the other end CN .

[0057]

[0058]

[0059] Z CM and Z CN These respectively reflect the equivalent impedance of the back-side equivalent power supply of the M-terminal and N-terminal protection devices.

[0060] The method for calculating the measured impedance of the N-terminal protection device is the same as that of the M-terminal protection device, which is to calculate the measured impedance Z at this terminal. CN Also calculate the measured impedance Z at the other end. CM .

[0061] 8. Calculate the differential impedance and braking impedance using the impedance measurements at both ends.

[0062] The M-terminal protection device calculates the measured impedance Z at this terminal. CM and the measured impedance Z at the other end CN Then, the differential impedance Z is calculated using equation (8). dif The braking impedance Z is calculated using equation (9). res .

[0063] Z dif =Z CM -Z CN =|Z dif |∠θ d (8)

[0064] Z res =Z CM +Z CN =|Z res |∠θ r (9)

[0065] The results of measuring impedance, differential impedance, and braking impedance for faults at different locations are shown in Table 1.

[0066] Table 1. Results of measured impedance, differential impedance, and braking impedance for faults at different locations.

[0067] Fault location <![CDATA[Z CM ]]> <![CDATA[Z CN ]]> <![CDATA[Z dif ]]> <![CDATA[Z res ]]> <![CDATA[Z dif With Z res Relationship Within the district <![CDATA[Z S ]]> <![CDATA[Z DG ]]> <![CDATA[Z S -WITH DG ]]> <![CDATA[Z S +Z DG ]]> <![CDATA[|Z dif |<|Z res |]]> Outside the M-end area <![CDATA[Z S ]]> <![CDATA[-Z S -WITH L1 ]]> <![CDATA[2Z S +Z L1 ]]> <![CDATA[-Z L1 ]]> <![CDATA[|Z dif |>|Z res |]]> Outside the N-end area <![CDATA[-Z DG -WITH L1 ]]> <![CDATA[Z DG ]]> <![CDATA[-2Z DG -WITH L1 ]]> <![CDATA[-Z L1 ]]> <![CDATA[|Z dif |>|Z res |]]>

[0068] Z in Table 1 S The positive sequence impedance of the equivalent system of the distribution network behind the M-terminal protection installation location is obtained by performing Thevenin equivalence on the distribution network; Z DG The positive sequence equivalent impedance of the power grid containing the IIDG section on the back side of the N-terminal protection installation location; Z L1 This is the positive sequence impedance of the protected line.

[0069] The N-terminal protection device also uses the same method as Equations (8) and (9) to independently calculate the differential impedance and braking impedance.

[0070] 9. Fault detection and isolation

[0071] Whether the M-end protection device continuously judges whether equation (10) is valid.

[0072] |Z dif |<|Z res | (10)

[0073] Equation (10) is the constructed protection criterion, which is maintained for T hours after the fault starts. d If equation (10) is satisfied continuously within a time period, it is determined to be an intra-zone fault. The M-end protection device sends a trip signal to its own end, i.e., the M-end circuit breaker, to achieve fault isolation; otherwise, it is identified as an extra-zone fault and no trip signal is sent to its own circuit breaker.

[0074] Similarly, if the N-terminal protection device is activated due to a fault, it will continue to operate for T... d If equation (10) is satisfied continuously within a time period, it is determined to be a fault within the zone, and a trip signal is sent to the circuit breaker at the local end, i.e., the N-end, to achieve fault isolation.

[0075] The beneficial effects of this invention include:

[0076] (1) A protection criterion was constructed using the measured impedances at both ends of the protected line to achieve fault identification. The constructed protection criterion is only related to the equivalent impedances of the power supplies at both ends and the line impedance, and the setting is simple. Based on the concept of control and protection coordination, the protection performance during faults in the zone can be further improved by controlling the phase angle of the equivalent impedance of IIDG.

[0077] (2) Compared with current differential protection, this invention uses the naturally synchronized voltage and current at the same terminal to calculate the measurement impedance, thus reducing the data synchronization requirements. Under the condition of data asynchrony, the protection only delays operation when there is a fault within the zone, and reliably does not operate when there is a fault outside the zone.

[0078] (3) The present invention improves the resistance to transition resistance by utilizing the fault variation amount, and the protection performance is not affected by the penetration rate of distributed power sources and the line length. Attached Figure Description

[0079] Figure 1 This is a diagram of a 10kV active distribution network.

[0080] Figure 2 Here is a flowchart of the fault detection and fault isolation process for the M-terminal protection device;

[0081] Figure 3 The waveform diagrams for faults within the area are as follows: (a) Three-phase voltage and three-phase current at terminal M; (b) Voltage and current changes due to fault at terminal M; (c) Positive sequence components of voltage and current changes due to fault at terminal M; (d) Measured impedances at terminals M and N; and (e) Differential impedance and braking impedance.

[0082] Figure 4 The waveform diagrams for faults outside the zone are shown in Figure 1. (a) Measured impedances at M and N terminals, and (b) Differential impedance and braking impedance. Detailed Implementation

[0083] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings and examples, but this is not intended to limit the scope of protection of the present invention. All technical solutions obtained by equivalent substitution or equivalent transformation are within the scope of protection of the present invention.

[0084] In such Figure 1 In the 10kV active distribution network shown, line L1 is the protected line, and busbars B1 and B3 correspond to the two ends of the protected line L1, namely the M end and the N end, respectively. The length of line L1 is 5km. f1, f2, and f3 are the fault points within the protected line L1, corresponding to positions at 5%, 50%, and 95% of the distance from busbar B1, respectively; f4, f5, and f6 are the fault points outside the protected area on the back side of the M end, the fault points outside the protected area on the back side of the N end, and the fault points outside the protected area on the adjacent line L2, respectively. The process of the M-end protection device completing fault detection and fault isolation is as follows: Figure 2 As shown, the fault detection and fault isolation process of the N-terminal protection device is similar; it only requires... Figure 2 The acquisition end is changed to the N end, and the receiving end is changed to the M end.

[0085] Example 1:

[0086] Assume a phase-C ground fault occurs at point f2, with a fault time of t0 = 1.0s and a duration of 0.1s. This is considered an in-zone fault for the protected line L1.

[0087] 1. Establish a data transmission system

[0088] In an active power distribution network, a data transmission system based on a peer-to-peer communication network is established. Adjacent protection devices transmit data via direct fiber optic connections, employing an Ethernet link layer direct mapping communication protocol based on the IEC 61850 standard. The SV (Sampled Value) and GOOSE (Generic Object Oriented Substation Event) protocols, conforming to the IEC 61850 standard, are used to transmit analog instantaneous values ​​and binary status information, respectively. Specifically, the analog instantaneous values ​​transmitted between adjacent protection devices are sequentially composed of SV frames, including phase A voltage, phase B voltage, phase C voltage, phase A current, phase B current, and phase C current. Binary status information is sequentially composed of GOOSE frames, including fault initiation status, fault identification results within the zone, tripping commands, and standby status. The transmission frequency of SV frames is consistent with the sampling frequency; the transmission frequency of GOOSE frames is densely transmitted when there is a status change, and a heartbeat message is sent every 5 seconds when there is no status change. The transmission mechanism complies with the IEC 61850 standard.

[0089] 2. Obtain the three-phase voltage and three-phase current at each location.

[0090] M-end protection device utilizes the installation on Figure 1 The voltage transformer at busbar B1 collects the instantaneous voltage value u. MA (t), u MB (t) and u MC (t), utilizing the device installed in Figure 1 The current transformer at R1 collects the instantaneous current value i. MA (t), i MB (t) and i MC (t). Figure 3 (a) shows the instantaneous waveforms of the three-phase voltage and the three-phase current at terminal M.

[0091] N-terminal protection device utilizes the installation on Figure 1 The voltage transformer at busbar B3 collects the instantaneous voltage value u. NA (t), u NB (t) and u NC (t), utilizing the device installed in Figure 1 The current transformer at R2 collects the instantaneous current value i. NA (t), i NB (t) and i NC (t).

[0092] The control device at the IIDG2 grid connection point uses voltage and current transformers installed at the grid connection point to collect data. Figure 1 Instantaneous value of three-phase voltage u at R10 GA (t), u GB (t) and u GC (t), and the instantaneous value of the three-phase current i GA (t), i GB (t) and i GC (t).

[0093] Through the data transmission system established in step 1, the M-end protection device receives the u sent by the peer protection device. NA (t), u NB (t), u NC (t), i NA (t), i NB (t) and i NC (t); The N-end protection device receives u sent by the peer protection device. MA (t), u MB (t), u MC (t), i MA (t), i MB (t) and i MC (t).

[0094] After the above direct acquisition and data transmission process, both the M-terminal protection device and the N-terminal protection device can obtain the directly acquired instantaneous values ​​of their own three-phase voltage and three-phase current, as well as the transmitted instantaneous values ​​of the opposite terminal's three-phase voltage and three-phase current. At this point, the instantaneous voltage and current values ​​obtained by the M-terminal protection device and the N-terminal protection device are identical.

[0095] In the subsequent steps 3 to 7, the M-terminal protection device and the N-terminal protection device are executed in the same way. Therefore, the M-terminal protection device is used as an example in steps 3 to 7. When the N-terminal protection device processes the data, it replaces all the electrical quantities of the local terminal mentioned in these steps with the electrical quantities of the N terminal, and replaces the electrical quantities of the opposite terminal with the electrical quantities of the M terminal.

[0096] 3. Fault start-up diagnosis

[0097] Based on the instantaneous voltage and current values ​​obtained in step 2, the M-terminal protection device performs fault start judgment. It uses the phase current mutation start method to determine whether a fault has occurred for the three-phase current at the M-terminal and the phase voltage mutation start method to determine whether a fault has occurred for the three-phase voltage at the N-terminal. Only when both the M-terminal phase current and the N-terminal phase voltage indicate a fault, the M-terminal protection device determines that a fault has occurred.

[0098] The process of the phase current sudden change start-up method adopted by the M-terminal protection device is as follows.

[0099] First, use equation (11) to calculate the phase current mutation |Δi| corresponding to the kth instantaneous value at this end. φ (k)|.

[0100] |Δi φ (k)|=|i φ (k)-i φ (kN T )|-|i φ (kN T )-i φ (k-2N T (11)

[0101] In the formula, i φ (kN T ) and i φ (k-2N T ) respectively represent the (kN)th phase current T ) and the (k-2N) T ) instantaneous values; N T N represents the number of sampling points per wave, which is taken as N in this embodiment. T =24.

[0102] The instantaneous value at t = 0s is denoted as k = 1, and the sampling frequency is 1200Hz. Therefore, the instantaneous value at fault occurrence time t = 1.0s is k = 1200. At k = 1202, k = 1203, and k = 1204, the sudden changes calculated by the M-terminal protection device using the C-phase current at this terminal are |Δi|, respectively. C (1202)|=0.102kA、|Δi C (1203)|=0.160kA and |Δi C (1204)|=0.225kA. At this time, the three consecutive instantaneous values ​​satisfy equation (12).

[0103] |Δi φ (k)|>I set (12)

[0104] In the formula, I set The phase current start-up threshold value is I, which is taken as I in this embodiment. set =0.1kA.

[0105] Therefore, when k = 1204, the M-terminal protection device uses the instantaneous value of the phase current at this terminal to determine that a fault has occurred in the power grid.

[0106] Similar to the judgment process of the phase current surge initiation method described above, the M-terminal protection device uses the phase voltage surge initiation method based on the instantaneous value of the phase voltage at the opposite end to determine whether a fault has occurred. Calculations show that at k=1201, k=1202, and k=1203, the phase voltage surge initiation calculated by the M-terminal protection device using the phase C voltage at the opposite end are |Δu C (1201)|=5.455kV、|Δu C (1202)|=6.678kV and |Δu C (1203)|=7.725kV. At this time, the three consecutive instantaneous values ​​satisfy equation (13).

[0107] |Δu φ (k)|>U set (13)

[0108] In the formula, U set This is the phase voltage start-up threshold value. In this embodiment, it is set to U. set =0.05kV.

[0109] Therefore, when k = 1203, the M-terminal protection device also determines that a fault has occurred in the power grid by using the instantaneous value of the phase voltage at the opposite end.

[0110] Based on the above judgment results, when k=1204, the M-terminal protection device has determined that a fault has occurred in the power grid using both the phase current at this end and the phase voltage at the opposite end. At this time, the M-terminal protection device determines that the fault has been activated and enters the fault handling process.

[0111] The fault activation process of the N-terminal protection device is the same as that of the M-terminal protection device: it uses the instantaneous value of the M-terminal phase current and the phase current mutation to determine whether a fault has occurred, and it also uses the instantaneous value of the N-terminal phase voltage and the phase voltage mutation to determine whether a fault has occurred. Consistent with the determination result of the M-terminal protection device, the N-terminal protection device also determines fault activation when k=1204.

[0112] The IIDG2 grid connection point control device also uses its instantaneous grid connection point voltage and phase voltage start-up method to determine fault start-up, which is the grid connection point voltage. Figure 1 The voltage of the intermediate bus B3. In this embodiment, this voltage is exactly the same as the N-terminal voltage used by the M-terminal protection device and the N-terminal protection device in the fault start judgment. Therefore, the time when the IIDG2 grid connection point control device judges the fault start is the same as the time when the fault start is judged by the N-terminal phase voltage, that is, when k=1203, the IIDG2 grid connection point control device judges the fault start.

[0113] 4. Obtain fault change voltage and fault change current.

[0114] After the fault is detected in step 3, the M-terminal protection device uses equation (14) to extract the instantaneous value Δu of the fault change voltage at this terminal. Mφ (t), the instantaneous value Δi of the fault change current at this end is extracted using equation (15). Mφ (t).

[0115] Δu Mφ (t)=u Mφ (t)-u Mφ (t-nT) (14)

[0116] Δi Mφ (t)=i Mφ (t)-i Mφ (t-nT) (15)

[0117] In this embodiment, T = 20ms and n = 5 are used to obtain the instantaneous value Δu of the three-phase fault change voltage at terminal M. Mφ (t) Corresponding waveform and instantaneous value Δi of three-phase fault change current. Mφ The waveform corresponding to (t) is as follows: Figure 3 As shown in (b).

[0118] Using the same method as equations (14) and (15), the M-terminal protection device calculates the instantaneous value Δu of the three-phase fault voltage change at the opposite end. Nφ (k) and the instantaneous value Δi of the three-phase fault change current at the opposite end. Nφ (k).

[0119] Using the same method as equations (14) and (15), the control device at the IIDG2 grid connection point calculates the instantaneous value Δu of the three-phase fault change voltage at the IIDG2 grid connection point. Gφ (t) and the instantaneous value Δi of the three-phase fault change current. Gφ (t).

[0120] 5. Obtain the positive sequence components of fault change voltage and fault change current.

[0121] The instantaneous symmetrical component method is selected, utilizing the instantaneous voltage value Δu of the fault change at terminal M obtained in step 4. Mφ (t), extract the instantaneous value Δu of the positive sequence component. M1 (t), the specific process is as follows.

[0122] (1) The instantaneous value Δu of the fault change voltage at terminal M is expressed by equation (16). Mφ (t).

[0123]

[0124] In the formula, U MA U MB and U MC These represent the effective values ​​of the three-phase fault voltage changes at terminal M; ω represents the fundamental angular frequency. and These represent the initial phases of the voltage changes during a three-phase fault.

[0125] (2) Construct Δu in equation (16) MA (t), Δu MB (t), Δu MC The rotating phasor corresponding to (t) is shown in equation (17).

[0126]

[0127] In the formula, and Δu MA (t), Δu MB (t) and Δu MC (t) represents the rotating phasor; j represents the imaginary part.

[0128] (3) Based on the rotating phasor of equation (17), the instantaneous value Δu of the positive sequence component of the fault change voltage at terminal M is calculated using equation (18). M1 (t), the calculation result is as follows Figure 3 As shown in (c).

[0129]

[0130] In the formula, and They represent taking and The imaginary part; and They represent taking and The real part.

[0131] Using the same method, the M-end protection device also extracted, such as Figure 3 (c) shows the instantaneous value Δi of the positive-sequence component of the fault change current at terminal M. M1 (t), extract the instantaneous value Δu of the positive sequence component of the fault change voltage at the opposite end, i.e., the N-terminal. N1 (t) and the instantaneous value Δi of the positive sequence component of the fault change current. N1 (t).

[0132] Using the same method as the M-terminal protection device, the N-terminal protection device extracts the positive sequence components of the fault change voltage and current at its own end (N-terminal) and the opposite end (M-terminal).

[0133] Using the same method as the M-terminal protection device, the control device at the IIDG2 grid connection point extracts the instantaneous value Δu of the positive sequence component of the fault change voltage at the IIDG2 grid connection point. G1 (t), Instantaneous value Δi of the positive sequence component of the fault change current G1 (t).

[0134] Then, the M-terminal and N-terminal protection devices and the IIDG2 grid connection point control device all use the half-cycle Fourier algorithm to calculate the effective value and phase of the positive sequence component of the above-mentioned fault change, including and

[0135] 6. Implement constant phase angle control for IIDG.

[0136] In step 3, after the control device of the IIDG2 grid connection point detects the fault start, it uses the smooth switching method shown in equation (19) to switch the constant power control strategy before the fault to the constant phase angle control strategy after the fault, and implements constant phase angle control of IIDG.

[0137]

[0138]

[0139] In the formula, and These are the actual given inner loop q-axis reference current and d-axis reference current, respectively. and These are the q-axis reference current and d-axis reference current of the inner current loop calculated from the outer voltage loop in the constant power control strategy. and These are the q-axis and d-axis reference currents of the inner current loop calculated in the constant phase angle control strategy; k1 and k2 represent the calculated... and The weights; t1 is the time when the control device of the IIDG2 grid connection point determines the fault start, and t2 is the set switching completion time.

[0140] In this embodiment, through step 3, the control device of the IIDG2 grid connection point has determined that the fault has started when k = 1203, and the corresponding fault start time is t1 = (k × T) / N. T = (1203 × 0.02) / 24 = 1.0025s. Set t2 = 1.005s. Calculated according to equation (19) based on the values ​​of t1 and t2. and It stabilized at t3 = 1.02 s. and Therefore, the calculation process at t = 1.02s is used as an example to illustrate the process. and The calculation steps.

[0141] (1) After the fault is started, the IIDG2 control device locates the instantaneous value of the three-phase voltage at the grid connection point at t = t3 - n × T = 1.02 - 5 × 0.02 = 0.92s before the fault starts from the data buffer, and extracts its positive sequence component u using the instantaneous sequence component extraction method. G|0| (t), and then use the phasor extraction algorithm to calculate the corresponding phasor. At t = 1.02s, the control device extracts the positive sequence component u using the instantaneous value of the three-phase voltage at the grid connection point. G1 (t), and then use the phasor extraction algorithm to calculate the corresponding phasor.

[0142] (2) Calculate the voltage drop degree k using equation (21).

[0143]

[0144] Calculate using equation (22) Lag The angle δ.

[0145] δ=θ U,G|0| -θ U,G1 =6.46° (22)

[0146] (3) Calculate the reference value of the q-axis current in the inner current loop according to equation (23). and d-axis current reference value

[0147]

[0148] In this embodiment, the IIDG phase angle control target is taken as Let β = 1.5. Then, according to equation (23), the inner loop q-axis reference current is... d-axis reference current is

[0149] 7. Calculate the measured impedance at terminals M and N.

[0150] Based on the positive-sequence components of the fault change voltage and current obtained in step 5 at terminal M. And the positive sequence components of the voltage and current changes during faults at the N-terminal. The M-terminal protection device calculates the measured impedance Z at this terminal using equations (24) and (25), respectively. CM and the measured impedance Z at the other end CN Then measure the impedance Z. CM and Z CN The calculation results are as follows Figure 3 As shown in (d).

[0151]

[0152]

[0153] The method for calculating the measured impedance of the N-terminal protection device is the same as that of the M-terminal protection device. The measured impedance Z at this terminal (N-terminal) is calculated accordingly. CN Also calculate the measured impedance Z at the opposite end, i.e., the M end. CM Z calculated by the N-terminal protection device CM and Z CN and Figure 3 The results shown in (d) are consistent.

[0154] 8. Calculate the differential impedance and braking impedance using the impedance measurements at both ends.

[0155] The M-terminal protection device calculates the measured impedance Z at this terminal. CM and the measured impedance Z at the other end CN Then, the differential impedance Z is calculated using equations (26) and (27). dif and braking impedance Z res The calculation result Z dif and Z res The magnitude and phase angle are as follows Figure 3 As shown in (e).

[0156] Z dif =Z CM -Z CN =|Z dif |∠θ d (26)

[0157] Z res =Z CM +Z CN =|Z res |∠θ r (27)

[0158] The N-terminal protection device also uses the same method to calculate the differential impedance and braking impedance, and the calculation results are consistent with... Figure 3 (e) Consistent.

[0159] 9. Fault detection and isolation

[0160] M-end protection device utilizes Figure 3 The differential impedance amplitude |Z obtained in (e) dif |and braking impedance amplitude|Z res |, according to the fault detection criterion shown in Equation (28), determine whether the fault point is located in the protected line L1.

[0161] |Z dif |<|Z res | (28)

[0162] according to Figure 3 The calculation results in (e) show that within the time period of 20ms to 25ms after the fault occurs, i.e., 1.020s to 1.025s, |Z dif |and|Z res | Relative Relationship Continuous T d =5ms satisfies equation (28), then at t=1.025s, the M-end protection device determines that it is a fault within the zone and sends a trip signal to the local end, i.e. the M-end circuit breaker, to achieve fault isolation.

[0163] The N-terminal protection device also detected continuous T at 1.025s. d =5ms satisfies the criterion of equation (28), so a trip signal is sent to the circuit breaker at the local end, i.e., the N-end, to achieve fault isolation.

[0164] Example 2:

[0165] Assume an AC phase-to-ground fault occurs at point f5, with a fault time of t0 = 1.0s and a duration of 0.1s. For the protected line L1, this is an external fault outside the N-terminal zone.

[0166] This embodiment is similar to the execution process of steps 1 to 9 in embodiment 1. Therefore, this embodiment only describes the differences between the processing process of each step in embodiment 1 and that in embodiment 1.

[0167] Step 7: The M-terminal protection device utilizes the positive-sequence components of the fault change voltage and current at this terminal. Calculate the measured impedance Z at this end. CMAnd utilize the positive sequence components of the voltage and current changes during faults at the opposite end. Calculate the measured impedance Z at the opposite end CN The calculated measured impedance Z CM and Z CN like Figure 4 As shown in (a).

[0168] Similar to the method used by the M-terminal protection device to calculate the measured impedance, the N-terminal protection device also independently calculates its own measured impedance Z. CN and the measured impedance Z at the other end CM Z calculated by the N-terminal protection device CM and Z CN and Figure 4 The results shown in (a) are consistent.

[0169] Step 8, the M-terminal protection device calculates the measured impedance Z. CM and Z CN Then, the differential impedance Z is calculated using equation (29). dif The braking impedance Z is calculated using equation (30). res The calculated result Z dif and Z res The magnitude and phase angle are as follows Figure 4 As shown in (b).

[0170] Z dif =Z CM -Z CN =|Z dif |∠θ d (29)

[0171] Z res =Z CM +Z CN =|Z res |∠θ r (30)

[0172] The differential impedance Z of the N-terminal protection device is calculated using the same method. dif and braking impedance Z res Z dif and Z res The calculation results and Figure 4 (b) Consistent.

[0173] Step 9, according to Figure 4 The differential impedance amplitude |Z obtained in (b) dif |and braking impedance amplitude|Z res The M-terminal protection device uses the fault detection criterion shown in equation (31) to determine whether the fault point is located in the protected line L1.

[0174] |Z dif |<|Zres | (31)

[0175] according to Figure 4 (b) The calculation results show that from the occurrence of the fault to its termination, |Z dif |and|Z res The relative relationship between | and | remains constant. dif |>|Z res |, does not satisfy equation (31). In this embodiment, the protection device at the M end determines that it is an external fault and does not send a trip signal to the circuit breaker at the M end.

[0176] The N-terminal protection device also maintains |Z throughout the period from the occurrence of the fault to the end of the fault. dif |>|Z res | does not satisfy equation (31). Therefore, the N-terminal protection device also determines it to be an external fault and does not send a trip signal to the N-terminal circuit breaker.

[0177] The above description represents only preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. An active power distribution network fault change quantity impedance differential protection method, characterized in that, Includes the following steps: Step (1): Establish a data transmission system based on a peer-to-peer communication network in the active power distribution network; Step (2): The two ends of the protected line are respectively labeled as M end and N end, where M end represents the end of the protected line that is closer to the distribution network and N end represents the end that is farther away from the distribution network and closer to the IIDG; Through direct acquisition and the data transmission system established in step (1), the M-terminal protection device obtains the instantaneous value of the three-phase voltage at this terminal. Instantaneous values ​​of three-phase current and the voltage at the opposite end, i.e., the N-terminal. Current ,in Indicates time, subscript Indicates separation; The N-terminal protection device also receives data from this terminal. , and the other end, i.e., the M end , ; The IIDG grid-connected point control device acquires the IIDG grid-connected point voltage via direct data acquisition. and current ; Step (3): Based on the voltage and current obtained in step (2), the M-terminal and N-terminal protection devices and the control device of the IIDG grid connection point respectively use the sudden change starting element to determine whether the grid has a fault. If it is determined to be a fault start, then proceed to the fault handling process after step (4). If there is no fault, continue to detect whether the fault has started. Step (4): After the fault is determined to be started in step (3), the M-terminal protection device calculates the fault change voltage at this terminal using equations (1) and (2) respectively. and fault change current : (1) (2) In the formula, For the fundamental frequency period, It is a positive integer; Using the same method, the voltage change due to fault at the other end was calculated. and fault change current ; The N-terminal protection device also independently calculates the change in fault amount at its own terminal, i.e. , and the change in fault at the other end, i.e. and ; The control device of the IIDG point of interconnection calculates the fault variation voltage of the IIDG point of interconnection in the same way and the fault variation current ; Step (5): Based on the instantaneous values ​​of each fault change voltage and current obtained in step (4), the positive sequence phasors of the fault change voltage and current at the M-terminal and N-terminal protection devices are extracted using the instantaneous sequence component extraction method and phasor extraction algorithm. and And the positive-sequence phasors of the fault changes at the N-terminal voltage and current. and ; The control device at the IIDG grid connection point also employs the same sequence component extraction method and phasor extraction algorithm to extract the positive sequence phasors of the fault-related voltage and current at the IIDG grid connection point. and ; Step (6): After the IIDG grid connection point detects the fault start in step (3), the control device uses a smooth switching method to switch the control strategy to a constant phase angle control strategy, calculates the reference value of the inner current loop in the IIDG dual closed-loop control and implements constant phase angle control until the fault is cleared. Step (7): Based on the positive sequence components of the fault change voltage and current obtained in step (5), the M-terminal protection device calculates the measured impedance at this terminal using equations (3) and (4), respectively. and the impedance measured at the other end : (3) (4) and These respectively reflect the equivalent impedance of the back-side equivalent power supply of the M-terminal and N-terminal protection devices; The N-terminal protection device calculates its local measured impedance using the same method as equations (3) and (4). and the impedance measured at the other end ; Step (8): The M-terminal protection device calculates the differential impedance using Equation (5) and Equation (6) , the braking impedance : (5) (6) The N-terminal protection device calculates the differential impedance independently using the same method as for the formulas (5) and (6) and the braking impedance ; Step (9): Based on the differential impedance amplitude obtained in step (8) and braking impedance amplitude The M-terminal protection device uses the protection criterion shown in equation (7) to detect whether an intra-zone fault has occurred. If it is determined to be an intra-zone fault, it sends a trip signal to the circuit breaker to achieve fault isolation. (7) Similarly, the N-terminal protection device also uses equation (7) to detect whether an intra-zone fault has occurred. If it is determined to be an intra-zone fault, it sends a trip signal to the circuit breaker to achieve fault isolation.

2. The active distribution network fault change impedance differential protection method according to claim 1, characterized in that, The specific method for determining fault start in step (3) is as follows: the protection device determines fault start only when the fault is determined by the instantaneous value of the current at the M terminal and the fault is also determined by the instantaneous value of the voltage at the N terminal.

3. The method of claim 1, wherein the method is characterized by, The steps for calculating the inner current reference value of the IIDG dual closed-loop control in step (6) are as follows: (1) Locate the instantaneous three-phase voltage values ​​at the grid connection point before the fault starts from the data buffer of the control device, and extract the positive sequence components using the instantaneous sequence component extraction method. Then, the corresponding phasors are calculated using the phasor extraction algorithm. After a fault is initiated, the control device extracts the positive sequence component using the instantaneous value of the three-phase voltage at the grid connection point. Then, the corresponding phasors are calculated using the phasor extraction algorithm. ; (2) Calculate the voltage drop using equation (8) : (8) Calculate using equation (9) Lag Angle : (9) (3) Calculate the current in the inner loop according to equation (10). Shaft current reference value and Shaft current reference value : (10) In the formula, For IIDG phase angle control target, ; The positive sequence component of the IIDG grid connection point fault change obtained in step (5) The phase; , , This indicates the rated current of the IIDG.

4. The active distribution network fault change impedance differential protection method according to claim 1, characterized in that, Step (9) requires that after the fault is started, the differential impedance amplitude... and braking impedance amplitude The relative relationship continues If equation (7) is satisfied continuously within a time period, it is determined to be an intra-zone fault; the M-terminal and N-terminal protection devices each send a trip signal to their respective circuit breakers to achieve fault isolation; otherwise, it is identified as an extra-zone fault and no trip signal is sent to the local circuit breaker.

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