Voltage / frequency anti-islanding protection method and device based on negative sequence fault component acceleration

Abstract

The invention discloses a voltage / frequency anti-islanding protection method based on negative sequence fault component acceleration. The method comprises the following steps: monitoring voltage and frequency at a distributed power supply grid-connected point in real time; when the voltage or frequency at the distributed power supply grid-connected point exceeds a preset threshold value, detecting whether line protection acts or not and judging whether the fault type is an inter-phase fault or not; after a line protection action is performed and an inter-phase fault is confirmed, calculating a grid-connected point negative sequence voltage component UDG (2) in a fault transition period; the negative sequence voltage component UDG (2) is compared with a preset setting value UDGset (2), if UDG (2) is larger than UDGset (2), it is judged that the distributed power supply is located in a fault line, and an off-network acceleration action is executed; if UDG (2) is smaller than or equal to UDGset (2), off-network is executed according to the voltage / frequency anti-islanding protection criterion. According to the method, the negative sequence voltage amplitude difference of the fault line and the healthy line at the grid-connected point is combined with the voltage / frequency anti-islanding protection criterion, so that the anti-islanding protection of the distributed power supply on the fault line can be accelerated when the phase-to-phase fault occurs.

Description

Technical Field

[0001] This invention belongs to the field of distribution network anti-islanding protection. Background Technology

[0002] The construction of a new power system based on new energy sources presents entirely new challenges to the protection and control of distribution networks, among which research on islanding protection for distributed generation is particularly crucial. When a distribution line fails, the distributed generation (DG) on the faulty line needs to achieve rapid fault disconnection, while the DG on the non-faulty line should have low voltage ride-through (LVRT) capability to avoid disconnection from the grid. Regarding voltage / frequency anti-islanding protection methods, some methods meet the requirements of the current national standard GB / T 33593-2017 "Technical Requirements for Distributed Power Generation Grid Connection", which stipulates that voltage / frequency anti-islanding protection devices must complete their operation within 2 seconds when the voltage is lower than the set value. These methods are prone to causing distributed generation (DG) on adjacent lines to disconnect from the grid successively when the fault occurs on the current line, exacerbating the power imbalance in the system and threatening the stability of the power grid. Some methods meet the requirements of the international standard IEEE 1547 for voltage / frequency anti-islanding protection devices, which can reach several hundred seconds in some scenarios. These methods are prone to causing slow subsequent fault handling such as reclosing. Most existing methods have problems such as long disconnection time of distributed generation on faulty lines, insufficient frequency sensitivity, and false disconnection of distributed generation on non-faulty lines, which cannot meet the requirements of the distribution network for fault response speed and selectivity. Summary of the Invention

[0003] The purpose of this invention is to provide a voltage / frequency anti-islanding protection method based on negative sequence fault component acceleration, which can realize rapid islanding detection after a fault on the line and can meet the requirements of rapid disconnection of DG when a fault occurs and maintaining ride-through operation when voltage / frequency is abnormal.

[0004] This invention utilizes the difference in negative sequence voltage amplitude between the faulty line and the healthy line at the grid connection point, and combines it with the voltage / frequency anti-islanding protection criterion, which can accelerate the anti-islanding protection action of the distributed power source on the faulty line during phase-to-phase faults.

[0005] The technical solution adopted in this invention is: a voltage / frequency anti-islanding protection method based on negative sequence fault component acceleration, comprising:

[0006] Real-time monitoring of voltage and frequency at the grid connection point of distributed power sources;

[0007] When the voltage or frequency at the grid connection point of the distributed power source exceeds the preset threshold, it is necessary to detect whether the line protection operates and determine whether the fault type is a phase-to-phase fault.

[0008] After the line protection operates and a phase-to-phase fault is confirmed, calculate the negative sequence voltage component U at the grid connection point during the fault transition period T. DG(2) ;

[0009] The negative sequence voltage component U DG(2) With preset setting value U DGset(2) Comparison, if U DG(2) >U DGset(2) If the distributed power source is located on a faulty line, an accelerated disconnection action will be performed.

[0010] If U DG(2) ≤U DGset(2) If the voltage / frequency anti-islanding protection criterion is met, the grid disconnection will be performed.

[0011] The preset threshold is set based on the IEEE 1547-2018 standard, including voltage deviation from the rated value or frequency deviation from the rated value.

[0012] The voltage deviation from the rated value specifically refers to:

[0013] or (5)

[0014] In the formula, pu represents the voltage at the grid connection point of the distributed power source, where pu is a per-unit value.

[0015] The frequency deviation from the rated value specifically refers to:

[0016] Hz or Hz (6)

[0017] In the formula, The frequency at the grid connection point of the distributed power source.

[0018] The basis for determining whether a fault is a phase-to-phase fault is the presence of a negative sequence voltage component.

[0019] The fault transition period T=[t trans , t resp ]; where t trans For the dynamic response delay time of the distributed power inverter, t resp To control the full response time of the strategy.

[0020] The calculation of the negative sequence voltage component U at the grid connection point DG(2) The method is as follows:

[0021] Based on the composite sequence network diagram of the distribution network, the node voltage equation (4) is written, which, together with the mathematical model equation (5) of the distributed power source, forms a set of equations to be solved. The iterative method is used to solve the equations, and the final convergence criterion is shown in equation (6).

[0022] (4)

[0023] (5)

[0024] (6)

[0025] In the formula: Y is the node admittance matrix of the distribution network; U (1) The node positive sequence voltage vector is composed of the positive sequence voltages of each node; I s I PV.f These are the system power injection current vector and the photovoltaic output current vector, respectively; U̇ i(1) Let be the positive-sequence voltage phasor of node i; the superscript k represents the iteration number; i is the node number; ε is the set convergence threshold; the voltages and line currents of each node in the distribution network are obtained, and based on this, the phase-to-phase fault boundary conditions are introduced. In the negative-sequence network, the negative-sequence voltage component U at the grid connection point is separated and calculated using the symmetric component method. DG(2) .

[0026] The preset setting value U DGset(2) The system is dynamically adjusted based on the location of the fault, specifically as follows:

[0027] (8)

[0028] In the formula, U 出口 U is the negative sequence voltage at the grid connection point when a fault occurs at the output of this line. 末端 U is the negative sequence voltage at the grid connection point when a fault occurs at the end of this line. 相邻线路出口 This refers to the negative sequence voltage at the grid connection point when an adjacent line experiences an outlet fault; K rel This is the reliability coefficient.

[0029] The accelerated disconnection time is the sum of the longest operating time of the line protection (0.6s) and the preset time margin (0.1s) (0.7s).

[0030] The voltage / frequency anti-islanding protection criteria include voltage anomaly criteria and frequency anomaly criteria (both based on the IEEE 1547-2018 standard).

[0031] The voltage anomaly criterion is as follows:

[0032] When the voltage at the grid connection point of the distributed power source The network will be disconnected at the corresponding time when the following conditions are met:

[0033] (9)

[0034] The frequency anomaly criterion is:

[0035] When the frequency at the grid connection point of the distributed power source The network will be disconnected at the corresponding time when the following conditions are met:

[0036] (10).

[0037] Another aspect of the present invention provides a voltage / frequency anti-islanding protection device based on negative sequence fault component acceleration, comprising:

[0038] The voltage and frequency monitoring module is used to monitor the voltage and frequency at the grid connection point of distributed power sources in real time.

[0039] The line protection action and fault judgment module is used to detect whether the line protection has been activated and to determine whether the fault type is a phase-to-phase fault when the voltage or frequency at the grid connection point of the distributed power source exceeds a preset threshold.

[0040] The negative sequence voltage component calculation module is used to calculate the negative sequence voltage component U at the grid connection point during the fault transition period T after the line protection has been activated and a phase-to-phase fault has been confirmed. DG(2) ;

[0041] The accelerated grid disconnection judgment and execution module is used to determine the negative sequence voltage component U. DG(2) With preset setting value U DGset(2) Comparison, if U DG(2) >U DGset(2) If the distributed power source is located on a faulty line, an accelerated disconnection action will be performed.

[0042] Voltage / frequency anti-islanding protection execution module, used in U DG(2) ≤U DGset(2) At that time, the grid disconnection shall be performed according to the voltage / frequency anti-islanding protection criterion.

[0043] The beneficial effects of this invention are as follows: (1) This invention proposes to use the difference in amplitude of negative sequence voltage at the DG grid connection point on faulty and healthy lines to identify DG located on faulty lines. (2) The method proposed in this invention operates effectively at different distributed power supply access locations and during phase-to-phase faults. In some scenarios, the action time of the method is reduced by 20.3s compared with the action time of voltage / frequency anti-islanding protection in international standards. (3) This invention proposes an anti-islanding protection method based on the acceleration of negative sequence fault components, which enables DG on faulty lines to be de-grid accelerated, while DG on non-faulty lines maintains low voltage ride-through. Attached Figure Description

[0044] Figure 1 A schematic diagram of the time period during which a distributed power source outputs negative sequence current.

[0045] Figure 2 This is a schematic diagram of the negative sequence current output by the DG under the negative sequence suppression strategy.

[0046] Figure 3 This is a usage scenario diagram of the present invention and an actual scenario diagram in the embodiments: a two-feeder power distribution system topology with distributed power source access.

[0047] Figure 4 This is a composite sequence network diagram for two-phase short-circuit faults in a two-feeder multi-DG distribution network.

[0048] Figure 5 To protect the action logic diagram.

[0049] Figure 6 This is a flowchart of a voltage / frequency anti-islanding protection method based on negative sequence fault component acceleration. Detailed Implementation

[0050] This embodiment provides an anti-islanding protection method based on accelerated negative-sequence fault component, applied to distribution networks with distributed generation (DG) access. Addressing the issue of long operating times in existing voltage / frequency anti-islanding protection due to fault ride-through considerations, this invention first reviews the setting configuration schemes of voltage / frequency anti-islanding protection in existing standards and analyzes the fault component characteristics at the anti-islanding protection installation location during phase-to-phase faults in distribution lines. Based on this, the difference in amplitude of the negative-sequence voltage at the DG connection point between the faulty and healthy lines is used to identify DGs located on the faulty line, and this is combined with voltage / frequency anti-islanding protection commonly used in actual field applications. This achieves rapid disconnection of DGs on faulty lines, effectively avoiding the problem of slow reclosing and other subsequent fault handling due to long anti-islanding protection operating times, and can simultaneously meet the requirements of rapid DG disconnection during fault occurrence and maintaining ride-through operation during voltage / frequency anomalies. Specifically, it includes the following:

[0051] 1. Fault component analysis;

[0052] Both grid disturbances and distribution line faults can cause changes in voltage or frequency parameters at the DG (Distributed Generation) connection point. However, during grid disturbances, there are generally no fault components on the lines; while during distribution line faults, the anti-islanding protection device monitoring point can detect the fault components. Utilizing these differences, fault components can be used to distinguish between grid disturbances and distribution line faults. When a distribution line fault occurs, the anti-islanding protection action time for voltage / frequency can be reduced.

[0053] Although the positive sequence fault component has the ability to characterize all fault types, it is difficult to distinguish between overload and high resistance faults in power system protection. It is easily affected by load fluctuations, which can lead to misjudgment. Its reliance on system symmetry makes it poorly adaptable in asymmetrical operation or dynamic power grids. When the inverter of the distributed generation is connected to the 10kV distribution network through a transformer, the transformer adopts a star-connected ungrounded or delta winding connection on the distributed generation side. Therefore, there is no zero-sequence path on the distributed generation side of the transformer, and the distributed generation does not output zero-sequence current.

[0054] To prevent grid instability caused by the immediate disconnection of the distributed generation (DG) during a fault, the DG needs to have low-voltage ride-through capability. When a system fault occurs, the positive-sequence power outer loop is disconnected, and the reference value for the inner current loop is obtained from the low-voltage ride-through formula in equation (1), k. u This indicates the degree of voltage drop in the positive sequence at the DG grid connection point. The current reference value in negative sequence control is the same as above. If we assume that the current value output by the DG quickly tracks the corresponding reference value, then during a fault, the DG is equivalent to a voltage-controlled current source in the positive sequence circuit and an open circuit in the negative sequence circuit.

[0055] (1)

[0056] In the formula, I N U N S and S represent the rated current, rated voltage, and rated capacity of DG, respectively. The positive sequence current components of DG on the d-axis and q-axis are i. d(1) with i q(1) I max This is the maximum current that the DG can output, typically taken as 1.2 I. N .

[0057] The above control strategy aims to make the negative sequence current of the DG output zero, but due to the slow actual dynamic response of the inverter in the distributed power source, the current decays from transient to steady state (t... trans =10ms) until the control policy fully responds (t resp The transition period T = [t = 20ms) is 20ms. trans , t resp Within this range, the distributed power source will continue to output negative sequence current, such as... Figure 1 , Figure 2 As shown.

[0058] 2. Negative sequence characteristics of faults at the grid connection point of distributed power sources;

[0059] Although during the transition period after the fault, T=[t trans , t resp Within the grid, the DG continuously outputs negative-sequence current. However, due to the limitation on the three-phase current imbalance requirement in the "Technical Requirements for Grid-Connected Photovoltaic Inverters," and the effect of the control strategy, its output negative-sequence current is small and can be approximated as zero. Therefore, the influence of the DG in the negative-sequence network is not considered; it is only equivalent to a positive-sequence voltage-controlled current source in the positive-sequence network.

[0060] Currently, there are more and more scenarios where multiple distributed power sources are connected. Further analysis is conducted based on the above analysis method. Figure 3It is a distribution network with multiple distributed power sources connected to two feeders. Feeders 1 and 2 are connected to DG1, DG2, and DG3 at busbars B, C, and E, respectively. To prevent unplanned islanding, anti-islanding protection elements must be installed on the circuit breakers CB2, CB3, CB4, and CB6 between DG and the system.

[0061] Composite sequence networks of two-feeder distribution systems with distributed power sources, such as Figure 4 As shown.

[0062] according to Figure 4 The node voltage equations (2) are written out, which together with the mathematical model of the distributed generation (3) form a set of equations to be solved. Since the mathematical model of the distributed generation is nonlinear, iterative methods are used to solve it. The final convergence criterion is shown in equation (4).

[0063] (2)

[0064] (3)

[0065] (4)

[0066] In the formula: Y is the node admittance matrix of the distribution network; U (1) The node positive sequence voltage vector is composed of the positive sequence voltages of each node; I s I PV.f These are the system power injection current vector and the photovoltaic output current vector, respectively; U̇ i(1) Let k be the positive-sequence voltage phasor of node i; the superscript k represents the iteration number; i is the node number; and ε is the set convergence threshold.

[0067] Seeking Figure 4 By taking the voltages and line currents of each node and substituting boundary conditions, the negative sequence voltages and currents of each node are finally obtained in the negative sequence network.

[0068] 3. Negative sequence characteristics at the installation location of anti-islanding protection:

[0069] Since the negative-sequence equivalent circuit does not include a distributed generation (DG), the distribution characteristics of negative-sequence current and voltage are the same as those of a traditional single-source radial distribution network: utilizing the characteristic that the negative-sequence voltage is highest at the fault point and decreases with distance from the fault point. When a two-phase short-circuit fault occurs, negative-sequence voltage exists at all DG anti-islanding protection installation points. When the distance between the DG on the faulty line and the fault point is less than the distance between the DG on the non-faulty line and the fault point, the negative-sequence voltage at the grid connection point of the former is greater than that of the latter.

[0070] The above analysis shows that the negative sequence voltage amplitude based on the grid connection point can distinguish whether a DG (Distributed Gauge) is located in a faulty or non-faulty line area. However, as the fault point gets closer to the line outlet, the contradiction between sensitivity and reliability becomes increasingly prominent, posing a serious challenge to protection judgment.

[0071] 4. Triggering Criteria:

[0072] International standards stipulate that distributed generation anti-islanding protection should be activated when the voltage or frequency at the grid connection point deviates from the rated value by a certain amount. Based on the voltage / frequency protection requirements in the IEEE 1547-2018 standard, anti-islanding protection will be triggered when grid parameters exceed the following thresholds:

[0073] or (5)

[0074] In the formula, pu is the per-unit value and U is the grid-connected voltage of DG.

[0075] The frequency protection setting value in the IEEE 1547-2018 standard is based on a 60Hz system. When converted to a 50Hz system, the corresponding criterion for starting the frequency protection is shown in equation (6), where f is the system frequency.

[0076] Hz or Hz (6)

[0077] 5. Criterion for accelerating actions of negative-order components:

[0078] Based on the above analysis of fault components under two-phase short-circuit faults, the amplitude of the negative-sequence current output by the distributed generation (DG) is limited by its own capacity and control strategy constraints, and its value can be approximated as zero. Therefore, when constructing the negative-sequence component acceleration criterion, this invention uses the negative-sequence voltage at the grid connection point of the distributed generation as a characteristic parameter for DG identification on the faulty line.

[0079] The farther the distributed generation (DG) grid connection point is from the fault point, the smaller the negative sequence voltage. That is, when the distance between the fault point and the DG grid connection point on the faulty line is closer than the distance between the DG grid connection point on the non-faulty line, the negative sequence voltage characteristic parameter at the location of the distributed generation anti-islanding protection device has line identification capability and can be used to distinguish between faults on this line and faults on adjacent lines.

[0080] (7)

[0081] In the formula, U DGi(2) For DG i The negative sequence voltage at the grid connection point, U DGiset(2) For DG i The tuning value of the acceleration criterion for negative sequence components.

[0082] The criterion for accelerating the negative sequence component should ensure that the DG on the faulty line operates correctly, while the DG on the non-faulty line does not malfunction. Based on the aforementioned analysis of the fault component distribution characteristics, when the electrical distance to the fault point and the system imbalance meet the extreme value conditions, the DG grid connection point will exhibit the maximum negative sequence voltage amplitude. Therefore, the negative sequence voltage setting value of the DG grid connection point can be set according to the larger of the three extreme cases: the fault occurring at the outlet of an adjacent line, the outlet of this line, and the end of the line.

[0083] by Figure 3 Taking the negative sequence component acceleration action criterion setting value at the DG1 anti-islanding protection installation location as an example, its discrimination principle is shown in equation (8). Considering factors such as short-circuit current calculation error, non-periodic component, and necessary margin, a reliability coefficient K is introduced. rel =1.1.

[0084] (8)

[0085] In the formula, U DG1(2)set U is the tuning value for the acceleration criterion of the negative sequence component of DG1. DG1(2)1出口 U is the negative sequence voltage at the grid connection point of DG1 when a fault occurs at the output of line 1; DG1(2)1末端 U is the negative sequence voltage at the grid connection point of DG1 when a fault occurs at the end of line 1; DG1(2)2出口 This is the negative sequence voltage at the grid connection point of DG1 when a fault occurs at the outlet of line 2.

[0086] Similarly, set the acceleration action criterion tuning values ​​for the negative sequence components of other DGs.

[0087] 6. Voltage / frequency anti-islanding protection operation criteria;

[0088] During islanding, the imbalance between distributed generation (DG) and load power will cause changes in electrical quantities at the grid connection point. Faults will cause voltage dips, and the operation of system-side circuit breakers will further trigger frequency and voltage changes. Therefore, voltage-frequency anti-islanding protection is used to control the disconnection of DG from the grid.

[0089] According to the requirements for voltage islanding protection in the IEEE 1547-2018 standard, when an abnormal voltage is detected at the grid connection point, the voltage islanding protection should respond according to the time specified in the standard.

[0090] (9)

[0091] The reference frequency of the system in the IEEE 1547-2018 standard is 60Hz. For ease of comparison, this invention converts it to 50Hz. When an abnormal frequency is detected at the grid connection point, the frequency anti-islanding protection should respond according to equation (10):

[0092] (10)

[0093] 7. Time setpoint coordination;

[0094] For three-stage current protection, instantaneous overcurrent protection operates immediately, while time-limited instantaneous overcurrent protection operates with a time step longer than the instantaneous overcurrent protection of the downstream line. Overcurrent protection operates with a time step longer than the overcurrent protection of the downstream line, thus ensuring higher protection sensitivity and reliability. The time step difference of relay protection devices should be reasonably determined based on factors such as the action return time, circuit breaker opening time, and timing errors. A time step difference of 0.3s is recommended for protection coordination. During a fault, the longest protection operation time for the circuit breaker on the line is 0.6s. A 0.1s time margin is reserved on this basis to avoid possible fluctuations in the circuit breaker protection operation time and to prevent overlap with the 0.6s operation interval of the line protection. Therefore, the detection time for the accelerated disconnection of the DG network can be set to 0.7s.

[0095] 8. Overall flow of protection logic and methods

[0096] The protection logic of the voltage / frequency anti-islanding protection method based on negative sequence fault component acceleration adopts a multi-level collaborative criterion and timing coordination mechanism, such as... Figure 5 As shown, the negative sequence component acceleration action criterion serves as an acceleration channel, accelerating tripping only when the line protection operates and the start-up criterion conditions are met. This ensures the selectivity of the protection while avoiding the risk of false tripping.

[0097] The workflow of the voltage / frequency anti-islanding protection method based on negative sequence fault component acceleration is as follows: Figure 6 As shown. The voltage and frequency at the grid connection point of the distributed power source are monitored in real time. When the voltage / frequency exceeds the protection setting value of the start-up criterion (Equation (5), (6)), after the line protection operates and is judged to be a phase-to-phase fault, if the negative sequence voltage exceeds the setting value in the negative sequence component acceleration action criterion (Equation (7)), it is determined that this distributed power source is on the fault line, and the DG is accelerated to be disconnected. If the negative sequence voltage at the grid connection point does not meet the acceleration action condition, the action is performed according to the voltage / frequency anti-islanding protection criterion (Equation (9), (10)).

[0098] To verify the performance of the algorithm proposed in this invention, the method was implemented using PSCAD and tested. (Reference) Figure 3The actual scenario diagram shows that the capacities of DG1, DG2, and DG3 on the distribution line are 5.9MW, 5.4MW, and 1.4MW, respectively. The lengths of line sections AB, BC, CD, AE, and EF are 1km, 1km, 5km, 1km, and 3km, respectively. The equivalent positive and negative sequence impedance per unit length of the overhead line is 0.132 + j0.357, the transition resistance is 0.01Ω, the load capacities are 4MVA, 2MVA, and 2MVA, and the load power factor is 0.85. The islanding protection sampling frequency is 1.2kHz. DG adopts a PQ control strategy and has negative sequence suppression and low voltage ride-through control strategies.

[0099] Under the above parameters, the tuning value of the acceleration criterion for the negative sequence components of DG1 and DG2 is U. DG1(2)set =1.788kV, U DG2(2)set =1.779kV.

[0100] To fully verify the effectiveness of the proposed method in the actual operation environment of the power grid, three typical scenarios were simulated:

[0101] (1) Scenario 1: Negative sequence current output by distributed power source at different times

[0102] Taking a two-phase short-circuit fault f1 occurring 0.5km from bus A on line AB as an example, the negative sequence current at each point was measured at 15ms (within the transition period T) and 50ms (when the control strategy fully responds). The measurement data are shown in Table 1.

[0103] Table 1. Negative sequence currents and voltages of each line after a two-phase short-circuit fault f1 in Scenario 1.

[0104]

[0105] Table 3, I CB1 I f I DG1 I DG2 and I DG3 The negative sequence currents at the line outlet CB1, the fault point, and DG1, DG2, and DG3 were respectively measured. The relevant values ​​verified that during a two-phase short-circuit fault in a 10kV distribution line, the DGs inject a negative sequence current component into the grid during the transition period T, but its value is extremely small. After the DG control strategy fully responds, the distributed generation does not output negative sequence current.

[0106] (2) Protection operation status at different fault locations

[0107] Let α be the percentage of the distance from the fault point to point A relative to the AB line, β be the percentage of the distance from the fault point to point B relative to the BC line, and γ be the percentage of the distance from the fault point to point C relative to the CD line. Measure the correlation quantities at 15ms (within the transition period T). When the fault location coefficient is 56%≤α≤100% or 0%≤β≤12%, the negative sequence voltage at the grid connection point of DG1 continuously exceeds the setting threshold of its criterion. In this case, the negative sequence component acceleration action criterion can effectively trigger the DG1 fast disconnection protection. When the fault location coefficient is 54%≤α≤100%, 0%≤β≤100%, or 0%≤γ≤7%, the negative sequence voltage at the grid connection point of DG2 continuously exceeds the setting threshold of its criterion. In this case, the negative sequence component acceleration action criterion can effectively trigger the DG2 fast disconnection protection. When the fault location coefficient is 0%≤α≤54% or 7%≤γ≤100%, the amplitude of the negative sequence voltage at the grid connection point of both DG1 and DG2 is lower than their corresponding setting threshold, causing the proposed negative sequence component acceleration action criterion to completely fail, and the fast disconnection of DG on the faulty line cannot be achieved. This area is the insensitive area of ​​this method. Regardless of the above situations, DG3 on the healthy line does not disconnect.

[0108] (3) Scenario 3: Protection action status under different fault types

[0109] Table 2 Comparison of the performance of the method and voltage protection operation under different fault types in Scenario 3

[0110]

[0111] Table 3 Comparison of the performance of method and frequency protection actions under different fault types in Scenario 3

[0112]

[0113] When a phase-to-phase fault occurs in a distribution line, the analysis of Tables 2 and 3 shows that the voltage amplitude at all distributed power source monitoring points exceeds the activation threshold. Among them, the negative sequence voltage amplitudes of DG1 and DG2 on the faulty line significantly exceed the setting value of the negative sequence component acceleration action criterion. Therefore, they can quickly disconnect from the grid after the line protection action time limit has elapsed, which is 20.3 seconds faster than the voltage anti-islanding protection action time. This can solve the problem of DG on the faulty line failing to operate due to small frequency changes in some cases. The negative sequence voltage of DG3 on the non-faulty line does not reach the action threshold due to the attenuation effect of electrical distance. It needs to maintain grid-connected operation and provide dynamic reactive power support, which meets the system voltage stability requirements.

[0114] The experimental data above demonstrates that, compared to the IEEE 1547-2018 standard, this protection scheme results in a shorter disconnection time for the DG on the faulty line, while also preventing the DG on the non-faulty line from disconnecting briefly.

Claims

1. A voltage / frequency anti-islanding protection method based on negative sequence fault component acceleration, characterized in that: include: Real-time monitoring of voltage and frequency at the grid connection point of distributed power sources; When the voltage or frequency at the grid connection point of the distributed power source exceeds the preset threshold, it is necessary to detect whether the line protection operates and determine whether the fault type is a phase-to-phase fault. After the line protection operates and a phase-to-phase fault is confirmed, calculate the negative sequence voltage component U at the grid connection point during the fault transition period T. DG(2) ; The negative sequence voltage component U DG(2) With preset setting value U DGset(2) Comparison, if U DG(2) >U DGset(2) If the distributed power source is located on a faulty line, an accelerated disconnection action will be performed. If U DG(2) ≤U DGset(2) If the voltage / frequency anti-islanding protection criterion is met, the grid disconnection will be performed.

2. The voltage / frequency anti-islanding protection method based on negative sequence fault component acceleration according to claim 1, characterized in that: The preset thresholds include voltage deviation from the rated value or frequency deviation from the rated value.

3. The voltage / frequency anti-islanding protection method based on negative sequence fault component acceleration according to claim 2, characterized in that: The voltage deviation from the rated value specifically refers to: or (5); In the formula, pu represents the voltage at the grid connection point of the distributed power source, where pu is a per-unit value. The frequency deviation from the rated value specifically refers to: Hz or Hz (6); In the formula, The frequency at the grid connection point of the distributed power source.

4. The voltage / frequency anti-islanding protection method based on negative sequence fault component acceleration according to claim 1, characterized in that: The basis for determining whether a fault is a phase-to-phase fault is the presence of a negative sequence voltage component.

5. The voltage / frequency anti-islanding protection method based on negative sequence fault component acceleration according to claim 1, characterized in that: The fault transition period T=[t trans , t resp ]; where t trans For the dynamic response delay time of the distributed power inverter, t resp To control the full response time of the strategy.

6. The voltage / frequency anti-islanding protection method based on negative sequence fault component acceleration according to claim 1, characterized in that: The calculation of the negative sequence voltage component U at the grid connection point DG(2) The method is as follows: Based on the composite sequence network diagram of the distribution network, the node voltage equation (4) is written, which, together with the mathematical model equation (5) of the distributed power source, forms a set of equations to be solved. The iterative method is used to solve the equations, and the final convergence criterion is shown in equation (6). (4)； (5)； (6)； In the formula: Y is the node admittance matrix of the distribution network; U (1) The node positive sequence voltage vector is composed of the positive sequence voltages of each node; I s I PV.f These are the system power injection current vector and the photovoltaic output current vector, respectively; U̇ i(1) Let k be the positive-sequence voltage phasor of node i; the superscript k represents the iteration number. i represents the node number; ε represents the set convergence threshold; the voltage and line current of each node in the distribution network are obtained, and based on this, the phase-to-phase fault boundary conditions are introduced. In the negative sequence network, the negative sequence voltage component U at the grid connection point is separated and calculated using the symmetrical component method. DG(2) .

7. The voltage / frequency anti-islanding protection method based on negative sequence fault component acceleration according to claim 1, characterized in that: The preset setting value U DGset(2) The system is dynamically adjusted based on the location of the fault, specifically as follows: （8）； In the formula, U 出口 U is the negative sequence voltage at the grid connection point when a fault occurs at the output of this line. 末端 U is the negative sequence voltage at the grid connection point when a fault occurs at the end of this line. 相邻线路出口 This refers to the negative sequence voltage at the grid connection point when an adjacent line experiences an outlet fault; K rel This is the reliability coefficient.

8. The voltage / frequency anti-islanding protection method based on negative sequence fault component acceleration according to claim 1, characterized in that: The accelerated disconnection time is the sum of the longest operating time of the line protection and the preset time margin.

9. The voltage / frequency anti-islanding protection method based on negative sequence fault component acceleration according to claim 1, characterized in that: The voltage / frequency anti-islanding protection criteria include voltage anomaly criteria and frequency anomaly criteria; The voltage anomaly criterion is as follows: When the voltage at the grid connection point of the distributed power source The network will be disconnected at the corresponding time when the following conditions are met: (9)； The frequency anomaly criterion is: When the frequency at the grid connection point of the distributed power source The network will be disconnected at the corresponding time when the following conditions are met: (10)。 10. A voltage / frequency anti-islanding protection device based on negative sequence fault component acceleration, characterized in that: include: The voltage and frequency monitoring module is used to monitor the voltage and frequency at the grid connection point of distributed power sources in real time. The line protection action and fault judgment module is used to detect whether the line protection has been activated and to determine whether the fault type is a phase-to-phase fault when the voltage or frequency at the grid connection point of the distributed power source exceeds a preset threshold. The negative sequence voltage component calculation module is used to calculate the negative sequence voltage component U at the grid connection point during the fault transition period T after the line protection has been activated and a phase-to-phase fault has been confirmed. DG(2) ; The accelerated grid disconnection judgment and execution module is used to determine the negative sequence voltage component U. DG(2) With preset setting value U DGset(2) Comparison, if U DG(2) >U DGset(2) If the distributed power source is located on a faulty line, an accelerated disconnection action will be performed. Voltage / frequency anti-islanding protection execution module, used in U DG(2) ≤U DGset(2) At that time, the grid disconnection shall be performed according to the voltage / frequency anti-islanding protection criterion.

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