A new energy base ac transmission line distance protection method and system

Abstract

The application discloses a new energy base AC sending-out line distance protection method and system, relates to the new energy sending-out line protection technical field, and the method comprises the following steps: collecting the voltage and current of the local end and decomposing to obtain each sequence component; the opposite end inverter negative sequence current suppression strategy is used to make the opposite end negative sequence network approximate open circuit, the fault point negative sequence current is determined to be equal to the local end measured negative sequence current; the fault type is discriminated based on the zero sequence current, the fault point current is calculated according to the boundary condition and the equal relationship of the negative sequence current, the corresponding impedance element and the distance measurement equation are substituted, and the fault distance is solved by the real part and the imaginary part; the in-zone or out-zone fault is discriminated according to the comparison between the fault distance and the setting value; the application establishes the negative sequence current certainty relationship by using the inherent control strategy of the inverter, only needs the single-end electrical quantity to accurately measure the distance, fundamentally eliminates the influence of the transition resistance and the opposite end system impedance, significantly improves the reliability and the transition resistance resistance of the protection under the high resistance fault, and does not need the opposite end communication.

Description

Technical Field

[0001] This invention relates to the field of protection technology for new energy transmission lines, specifically to a distance protection method and system for AC transmission lines of new energy bases. Background Technology

[0002] Driven by the global energy transition and my country's "dual carbon" goals, large-scale new energy bases built on the abundant resources of desert, Gobi, and wasteland regions have become an important part of the construction of new power systems. The core principle of distance protection is to determine the fault location based on the measured impedance from the fault point to the protection installation point. For non-metallic faults, the measured impedance can be expressed as the sum of the positive sequence impedance of the line and the additional impedance. The additional impedance is affected by the transition resistance and the current in the systems on both sides.

[0003] When the system only has a traditional synchronous generator power supply, the short-circuit current phases of both power supplies are basically consistent when a non-metallic fault occurs on the line. The additional impedance generated by the transition resistance is approximately pure resistance, which can be eliminated by adjusting the impedance characteristics (such as polygonal impedance characteristics). Unlike traditional synchronous generators, the short-circuit current output characteristics of renewable energy inverters are highly dependent on their internal low-voltage ride-through control strategy. The control response of the inverter on the renewable energy side will cause the short-circuit current phase at the fault point to shift, and the additional impedance will no longer be resistive.

[0004] However, existing distance protection methods do not take into account the amplitude and phase abrupt changes caused by the suppression of negative sequence current by the inverter at the opposite end, resulting in serious deviations in the measured impedance during non-metallic faults, and the protection may fail to operate or operate erroneously. Summary of the Invention

[0005] To address the shortcomings of existing technologies in protecting new energy transmission lines during faults, such as difficulty in estimating additional impedance due to the combined effects of weak inverter feedback and transition resistance, measurement impedance deviation, poor tolerance to transition resistance, and susceptibility to false tripping or failure to tripping, this invention proposes a distance protection method and system for AC transmission lines in new energy bases. This novel distance protection method utilizes only single-ended electrical quantities from the conventional system side to estimate the fault point current, eliminating the need for communication and being unaffected by transition resistance, thereby improving the reliability of the protection and solving the problems existing in the prior art.

[0006] A distance protection method for AC transmission lines of new energy bases, executed by a protection device installed at the protection installation point of the AC transmission line, includes the following steps: The voltage and current at the protection installation point of the AC transmission line are collected, and the collected voltage and current are decomposed into symmetrical components to obtain each sequence component. Each sequence component includes at least negative sequence voltage, negative sequence current and zero sequence current. The protection is activated when the negative sequence voltage is higher than a set threshold. After the protection is started, the fault type is determined based on the zero-sequence current; the current at the fault point is calculated based on the boundary conditions corresponding to the fault determination result and the negative-sequence current measured at the installation location of the protection at this end; the current at the fault point is substituted into the impedance element and distance measurement equation corresponding to the fault determination result, and the real and imaginary part equations are combined to calculate the fault distance. The fault distance is used to determine whether it is an intra-zone fault. If the fault distance is less than the set value, it is determined to be an intra-zone fault, and a protection action signal is output; otherwise, it is determined to be an extra-zone fault, and the protection does not operate.

[0007] Furthermore, after the protection is activated, the fault type is determined based on the zero-sequence current; specifically, if the zero-sequence current measured at the protection installation point is 0, the fault is a phase-to-phase fault; if the zero-sequence current is measured at the protection installation point, the correlation of the fault transient voltage is used to distinguish between single-phase grounding faults and phase-to-phase grounding faults.

[0008] Furthermore, the calculation of the current at the fault point based on the boundary conditions corresponding to the fault identification result and the negative sequence current measured at the local protection installation location specifically includes: When the fault is determined to be a single-phase ground fault, the positive sequence current at the fault point should be considered. Negative sequence current Zero-sequence current Equal boundary conditions Calculate the current at the fault point ;in, When the fault is determined to be an interphase fault, the boundary condition that the currents of the two faulty phases are equal in magnitude and opposite in direction is applied. Calculate the current at the fault point ; When the fault is determined to be a phase-to-phase ground fault, the current at the fault point is calculated based on the composite sequence network diagram of a single-phase ground fault; the current at the fault point is specifically expressed as follows: Among them, the equivalent impedance of the zero-sequence network during phase-to-phase grounding faults. Negative sequence network equivalent impedance The equivalent impedance of this system ; For the negative sequence and zero sequence impedances of the AC output line; Location of the fault; , The impedance of the transformers on both sides of the line; This is the negative sequence current measured at this terminal.

[0009] Furthermore, by substituting the current at the fault point into the impedance element and distance measurement equation corresponding to the discrimination result, and simultaneously solving the real and imaginary part equations, the fault distance is calculated, specifically including: For single-phase ground faults, the corresponding first impedance element is used. and the first distance measurement equation The first ranging equation includes the fault phase voltage at this end. Local fault phase current Line zero-sequence current compensation coefficient Additional impedance Transition resistance and fault distance ; For phase-to-phase faults, the corresponding second impedance element is used. Second distance measurement equation The second ranging equation includes the voltage difference between the two faulty phases at this end. The difference in current between the two fault phases at this terminal Additional impedance Transition resistance and fault distance ; For phase-to-phase ground faults, the corresponding third impedance element is used. and the third distance measurement equation The third ranging equation includes the fault phase voltage at this end. Local fault phase current Line zero-sequence current compensation coefficient Negative sequence network equivalent impedance Equivalent impedance of zero-sequence network Additional impedance Transition resistance and fault distance .

[0010] Furthermore, the additional impedance is determined by the current at the fault point, the transition resistance, and the current measured at the protection installation point on this end.

[0011] Furthermore, each sequence component includes positive sequence voltage, negative sequence voltage, zero sequence voltage, positive sequence current, negative sequence current, and zero sequence current.

[0012] Furthermore, the method of distinguishing between single-phase ground faults and phase-to-phase ground faults using the correlation of fault transient voltages includes the following steps: Calculate the correlation coefficient between the transient voltages of each phase. If the correlation between a certain phase voltage and the voltages of the other two phases is lower than a preset threshold, it is judged as a single-phase ground fault. If the correlation between any two of the three-phase voltages is lower than a preset threshold, it is determined to be a phase-to-phase grounding fault.

[0013] This invention also includes a distance protection system for AC transmission lines of new energy bases, executed by a protection device installed at the protection installation point at the local end of the AC transmission line, comprising: The acquisition module is used to acquire the voltage and current at the protection installation point of the AC transmission line, and to perform symmetrical component decomposition on the acquired voltage and current to obtain each sequence component. Each sequence component includes at least negative sequence voltage, negative sequence current and zero sequence current. The protection is activated when the negative sequence voltage is higher than a set threshold. The calculation module is used to determine the fault type based on the zero-sequence current after the protection is started; calculate the current at the fault point according to the boundary conditions corresponding to the fault determination result and the negative-sequence current measured at the installation location of the local protection; substitute the current at the fault point into the impedance element and distance measurement equation corresponding to the fault determination result, and solve the real and imaginary part equations simultaneously to calculate the fault distance. The judgment module is used to determine whether the fault is within the zone based on the fault distance. If the fault distance is less than the set value, it is judged as an internal fault and the protection action signal is output; otherwise, it is judged as an external fault and the protection does not operate.

[0014] This invention provides a distance protection method for AC transmission lines in new energy bases, which has the following beneficial effects: This invention utilizes the inherent negative sequence current suppression control strategy of the opposite-end renewable energy inverter to make the opposite-end negative sequence network in the composite sequence network approximately open-circuited, establishing a deterministic relationship between the negative sequence current at the fault point and the negative sequence current measured at the local end. Thus, only single-end electrical quantities are needed to accurately estimate the fault point current and construct the distance measurement equation. By simultaneously solving the real and imaginary equations, the fault distance and transition resistance are decoupled, fundamentally eliminating the influence of the opposite-end system impedance and transition resistance on the distance measurement accuracy. This enables the protection to reliably and accurately determine the fault within the zone under various fault types and high-resistance grounding conditions, without relying on the communication data of the opposite end, significantly improving the reliability and transition resistance resistance of the distance protection for renewable energy transmission lines. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of a composite sequence network for a single-phase grounding fault in an embodiment of the present invention; Figure 2 This is a composite sequence network diagram of phase-to-phase faults in an embodiment of the present invention; Figure 3 This is a fault composite sequence network diagram in an embodiment of the present invention; Figure 4 This is a flowchart of the distance protection method for AC transmission lines of new energy bases in an embodiment of the present invention; Figure 5 This is a test system topology diagram in an embodiment of the present invention. Detailed Implementation

[0016] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0017] This invention proposes a distance protection method for AC transmission lines of new energy bases. This method utilizes a negative sequence current suppression control strategy implemented after an inverter fault. During the fault, the negative sequence network on the new energy side of the composite sequence network can be approximated as an open circuit. The fault point current is estimated using only the single-ended electrical quantities on the conventional system side to construct a new type of distance protection. It does not require communication, is not affected by transition resistance, and improves the reliability of the protection.

[0018] like Figure 4 As shown, the method specifically includes the following steps: S1. Collect the voltage and current at the protection installation point of the AC transmission line. When the start-up criteria are met, start the protection.

[0019] Specifically, if the voltage of any phase is lower than 0.9 pu or the negative sequence voltage is higher than 0.01 pu at the protection installation point (m point) of the AC transmission line, the starting element will activate.

[0020] S2. Using the symmetrical component method to convert the three-phase voltage and three-phase current According to equation (1), it can be decomposed into sequence components, which are positive sequence voltages. Negative sequence voltage Zero-sequence voltage Positive sequence current Negative sequence current Zero-sequence current .

[0021] (1) (2) S3. Typically, inverters on the renewable energy side employ a control strategy that suppresses negative-sequence current, allowing the renewable energy system on the opposite side to be approximated as an open circuit in the negative-sequence network. Therefore, the negative-sequence current at the fault point... Provided by this system, it can be represented as: (3).

[0022] S4. After the protection is started, if the zero-sequence current measured at the protection installation point is 0, the fault is a phase-to-phase fault; if the zero-sequence current is measured at the protection installation point, the correlation of the fault transient voltage is used to distinguish between single-phase grounding fault and phase-to-phase grounding fault.

[0023] The correlation of transient voltages during a fault can be used to distinguish between single-phase grounding faults and phase-to-phase grounding faults. Specifically, this involves calculating the correlation coefficient between the transient voltages of each phase. If the correlation between a voltage of a certain phase and the voltages of the other two phases is lower than a preset threshold, it is determined to be a single-phase grounding fault. If the correlation between any two of the three phase voltages is lower than a preset threshold, it is determined to be a phase-to-phase grounding fault.

[0024] Taking a single-phase ground fault as an example, the three-phase circuit is decomposed into positive, negative, and zero-sequence networks. Based on the boundary conditions at the fault point, the three-sequence currents are equal, and they are connected in series at the ports, as shown in the diagram. Figure 1 The composite sequence network diagram shown is as follows. Figure 1 middle, , , These are the positive-sequence, negative-sequence, and zero-sequence currents provided by the new energy system side (W side). Let M be the system's equivalent positive-sequence electromotive force. The equivalent positive-sequence, negative-sequence, and zero-sequence impedances of the transformer on the W side of the system are given. The equivalent positive-sequence, negative-sequence, and zero-sequence impedances of the transformer on the M side of the system are given. The overall equivalent negative sequence impedance on the M side of the system, This represents the positive-sequence, negative-sequence, and zero-sequence impedance of the line at the W-side port from the fault point. This indicates the positive-sequence, negative-sequence, and zero-sequence impedances of the line at the M-side port from the fault point.

[0025] The boundary conditions show that the sequence currents at the fault point are equal. The current at the fault point can be obtained. for: (4) Phase-to-phase fault, zero-sequence current is 0, zero-sequence network does not exist, positive and negative sequence currents sum to 0, positive and negative sequence networks are connected in parallel, composite sequence network diagram is as follows. Figure 2 As shown, by boundary conditions The current at the fault point can be obtained as follows: : (5) In the case of a phase-to-phase fault, the zero-sequence network, through a transition resistor, is connected in parallel with the positive-sequence and negative-sequence networks to form a composite sequence network, as shown in the diagram below. Figure 3 As shown, considering the transformer wiring method, the equivalent impedance of the zero-sequence network during a phase-to-phase grounding fault is... Negative sequence network equivalent impedance As shown in equations (5) and (6) respectively.

[0026] (6) (7) In the formula: For the negative sequence and zero sequence impedances of the AC output line, Location of the fault. This is the equivalent impedance of the system on this side; , The impedance of the transformers on both sides of the line.

[0027] Therefore, the current at the fault point can be obtained based on the composite sequence network. The relationship between the negative sequence current measured by the protection is shown in equation (7).

[0028] (8) In the formula: This is the transition resistance.

[0029] S5. Combining the fault currents obtained in step 4 for each fault type, taking phase A ground fault, phase BC inter-fault, and phase BC inter-ground fault as examples, the corresponding impedance elements and ranging equations are shown in Table 1. This is the positive sequence impedance of the AC output line. For additional impedance, This refers to the zero-sequence current at the fault point. This is the zero-sequence current compensation coefficient for the line. , These are the voltage and current phasors for the faulty phase at this end.

[0030] Table 1. Impedance elements and ranging equations corresponding to fault types. The fault distance can be obtained by writing out the equations for the real and imaginary parts respectively. and transition resistance Thus, the fault location and transition resistance of the new energy transmission line are obtained. If the calculated fault location... satisfy If the fault is within the zone, it is determined to be a fault within the zone; otherwise, it is a fault outside the zone.

[0031] This invention utilizes the common fault control strategy of "negative sequence current suppression" in the counterpart renewable energy inverter. In the composite sequence network, the negative sequence network on the renewable energy side is approximately open-circuited, establishing a deterministic relationship between the negative sequence current at the fault point and the negative sequence current measured at the local end. Using boundary conditions, the measured negative sequence current is used to estimate the current at the fault point, and this is substituted into the traditional impedance measurement equations to construct a set of distance measurement equations. Mathematically, by solving the real and imaginary parts of the equations, the true fault distance and transition resistance are completely decoupled even without communication data from the counterpart end, theoretically ensuring the feasibility of the protection method.

[0032] To verify the correctness of the proposed novel distance protection scheme, taking the AC transmission line of the Shagohuang New Energy Base as an example, a simulation was established based on the electromagnetic transient simulation software PSCAD / EMTDC. Figure 5 The simulation model shown has a fault set at point f on the AC line, with protection installed on side M and side N being the new energy source side. This indicates the transformer impedance on this side. The transformer impedance of the opposite new energy system is shown in Table 2. The main parameters of the system are shown in Table 2. The rated frequency of the system is 50Hz, and the line model adopts the Π model.

[0033] Table 2 Main parameters of the system model Single-phase grounding fault, phase-to-phase fault, and phase-to-phase grounding fault were set at different fault locations on the AC transmission line of the Shagohuang New Energy Base. The transition resistances were set to 5Ω, 50Ω, and 100Ω, respectively. Table 3 shows the operation when the protection principle proposed in this invention is used during the fault.

[0034] Table 3 Protection Action Status under Various Fault Conditions As shown in Table 3, the protection system operates reliably under different fault locations, varying transition resistances, and different fault types. Even under a high-resistance fault of 100Ω, the protection system reliably identifies it as an in-zone fault and operates correctly. This fully demonstrates that the single-ended protection method proposed in this invention, based on the negative sequence current suppression strategy of the counterpart renewable energy inverter, has extremely high ranging accuracy and is largely unaffected by transition resistance, making it safe and reliable for application in large-scale renewable energy transmission lines.

[0035] The main content of this invention is a novel distance protection system-side protection for the AC transmission line and renewable energy collection line of the Shagohuang renewable energy base, utilizing a negative sequence current suppression strategy of the inverter at the opposite end. Unlike traditional protection, the protection method provided by this invention uses a negative sequence current suppression control strategy activated after an inverter fault. During the fault, the negative sequence network on the renewable energy side of the composite sequence network can be approximated as an open circuit. The novel distance protection is constructed by estimating the fault point current using only the single-ended electrical quantities on the conventional system side. It requires no communication, is unaffected by transition resistance, and improves the reliability of the protection.

[0036] Based on the same inventive concept, this invention also proposes a distance protection system for AC transmission lines of new energy bases, which is executed by a protection device installed at the protection installation point of the AC transmission line, comprising: The acquisition module is used to acquire the voltage and current at the protection installation point of the AC transmission line, and to perform symmetrical component decomposition on the acquired voltage and current to obtain each sequence component. Each sequence component includes at least negative sequence voltage, negative sequence current and zero sequence current. The protection is activated when the negative sequence voltage is higher than the set threshold.

[0037] The calculation module is used to determine the fault type based on the zero-sequence current after the protection is started; based on the boundary conditions corresponding to the fault determination result and the negative-sequence current measured at the protection installation point, the current at the fault point is calculated; the current at the fault point is substituted into the impedance element and distance measurement equation corresponding to the fault determination result, and the real and imaginary equations are solved simultaneously to calculate the fault distance.

[0038] The judgment module is used to determine whether the fault is within the zone based on the fault distance. If the fault distance is less than the set value, it is judged as an internal fault and the protection action signal is output; otherwise, it is judged as an external fault and the protection does not operate.

[0039] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for distance protection of AC transmission lines in a new energy base, characterized in that, The protection device installed at the protection installation point on the AC transmission line performs the following steps: The voltage and current at the protection installation point of the AC transmission line are collected, and the collected voltage and current are decomposed into symmetrical components to obtain each sequence component. Each sequence component includes at least negative sequence voltage, negative sequence current and zero sequence current. The protection is activated when the negative sequence voltage is higher than a set threshold. After the protection is started, the fault type is determined based on the zero-sequence current; the current at the fault point is calculated based on the boundary conditions corresponding to the fault determination result and the negative-sequence current measured at the installation location of the protection at this end; the current at the fault point is substituted into the impedance element and distance measurement equation corresponding to the fault determination result, and the real and imaginary part equations are combined to calculate the fault distance. The fault distance is used to determine whether it is an intra-zone fault. If the fault distance is less than the set value, it is determined to be an intra-zone fault, and a protection action signal is output; otherwise, it is determined to be an extra-zone fault, and the protection does not operate.

2. The distance protection method for AC transmission lines of a new energy base according to claim 1, characterized in that, After the protection is activated, the fault type is determined based on the zero-sequence current. Specifically, if the zero-sequence current measured at the protection installation point is 0, the fault is a phase-to-phase fault. If the zero-sequence current is measured at the protection installation point, the correlation of the fault transient voltage is used to distinguish between single-phase grounding faults and phase-to-phase grounding faults.

3. The distance protection method for AC transmission lines of a new energy base according to claim 2, characterized in that, The calculation of the fault point current based on the boundary conditions corresponding to the fault identification result and the negative sequence current measured at the local protection installation location specifically includes: When the fault is determined to be a single-phase ground fault, the positive sequence current at the fault point should be considered. Negative sequence current Zero-sequence current Equal boundary conditions Calculate the current at the fault point ;in, When the fault is determined to be an interphase fault, the boundary condition that the currents of the two faulty phases are equal in magnitude and opposite in direction is applied. Calculate the current at the fault point ; When the fault is determined to be a phase-to-phase ground fault, the current at the fault point is calculated based on the composite sequence network diagram of a single-phase ground fault; the current at the fault point is specifically expressed as follows: Among them, the equivalent impedance of the zero-sequence network during phase-to-phase grounding faults. Negative sequence network equivalent impedance The equivalent impedance of this system ; For the negative sequence and zero sequence impedances of the AC output line; Location of the fault; , The impedance of the transformers on both sides of the line; This is the negative sequence current measured at this terminal.

4. The distance protection method for AC transmission lines of a new energy base according to claim 3, characterized in that, Substituting the current at the fault point into the impedance element and distance measurement equation corresponding to the discrimination result, and simultaneously solving the real and imaginary part equations, the fault distance is calculated, specifically including: For single-phase ground faults, the corresponding first impedance element is used. and the first distance measurement equation The first ranging equation includes the fault phase voltage at this end. Local fault phase current Line zero-sequence current compensation coefficient Additional impedance Transition resistance and fault distance ; For phase-to-phase faults, the corresponding second impedance element is used. Second distance measurement equation The second ranging equation includes the voltage difference between the two faulty phases at this end. The difference in current between the two fault phases at this terminal Additional impedance Transition resistance and fault distance ; For phase-to-phase ground faults, the corresponding third impedance element is used. and the third distance measurement equation The third ranging equation includes the fault phase voltage at this end. Local fault phase current Line zero-sequence current compensation coefficient Negative sequence network equivalent impedance Equivalent impedance of zero-sequence network Additional impedance Transition resistance and fault distance .

5. A distance protection method for AC transmission lines of a new energy base according to claim 4, characterized in that, The additional impedance is determined by the current at the fault point, the transition resistance, and the current measured at the protection installation point on this end.

6. The distance protection method for AC transmission lines of a new energy base according to claim 1, characterized in that, The sequence components include positive sequence voltage, negative sequence voltage, zero sequence voltage, positive sequence current, negative sequence current, and zero sequence current.

7. A distance protection method for AC transmission lines of a new energy base according to claim 2, characterized in that, The method of distinguishing between single-phase ground faults and phase-to-phase ground faults using the correlation of fault transient voltages includes the following steps: Calculate the correlation coefficient between the transient voltages of each phase. If the correlation between a certain phase voltage and the voltages of the other two phases is lower than a preset threshold, it is judged as a single-phase ground fault. If the correlation between any two of the three-phase voltages is lower than a preset threshold, it is determined to be a phase-to-phase grounding fault.

8. A distance protection system for AC transmission lines in a new energy base, characterized in that, Performed by the protection device installed at the protection installation point of the AC transmission line, including: The acquisition module is used to acquire the voltage and current at the protection installation point of the AC transmission line, and to perform symmetrical component decomposition on the acquired voltage and current to obtain each sequence component. Each sequence component includes at least negative sequence voltage, negative sequence current and zero sequence current. The protection is activated when the negative sequence voltage is higher than a set threshold. The calculation module is used to determine the fault type based on the zero-sequence current after the protection is started; calculate the current at the fault point according to the boundary conditions corresponding to the fault determination result and the negative-sequence current measured at the installation location of the local protection; substitute the current at the fault point into the impedance element and distance measurement equation corresponding to the fault determination result, and solve the real and imaginary part equations simultaneously to calculate the fault distance. The judgment module is used to determine whether the fault is within the zone based on the fault distance. If the fault distance is less than the set value, it is judged as an internal fault and the protection action signal is output; otherwise, it is judged as an external fault and the protection does not operate.

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