New energy station sending-out line distance protection method and device

Abstract

The invention belongs to the field of power transmission line protection, and relates to a new energy station sending-out line distance protection method and device.Superconducting magnetic energy storage is connected to a direct-current bus, the fault ride-through process is divided into two stages through connection of the superconducting magnetic energy storage, and faults comprise a two-phase fault and a three-phase earth fault; and according to the fault output characteristics of the two stages, respectively listing solving equations of the line short-circuit impedance, simultaneously eliminating unknown quantities in the equations, and calculating the line short-circuit impedance. The problem that photovoltaic side distance protection is poor in anti-transition resistance capability is solved. The problem of unreliable action of distance protection can be reasonably solved, reactive power support of a photovoltaic station to a power grid during a fault period is also considered, and the low-voltage ride-through capability of the photovoltaic station is not weakened but is improved to a certain extent. Compared with other control and protection cooperation schemes, the scheme has obvious advantages in the aspects of reactive power support and protection accuracy.

Description

Technical Field

[0001] The present invention belongs to the field of transmission line protection, and in particular, to a distance protection method and device for a transmission line for a new energy power station Background Art

[0002] In recent years, new energy power generation mainly based on photovoltaic has gradually emerged, and the installed capacity of photovoltaic has shown a significant upward trend. The protection configuration of the line has also changed accordingly. As the channel connecting the photovoltaic power station and the large system, ensuring the safe and stable operation of the transmission line is of great significance. However, the access of large-capacity photovoltaic power stations has a great impact on the relay protection of the transmission line. Among them, due to the influence of the control strategy of photovoltaic inverters, the weak feed characteristics and current phase-controlled characteristics of photovoltaic power stations lead to the inability of the measured impedance of the distance protection on the photovoltaic side of the transmission line to correctly reflect the location of the fault, and the ability to resist transition resistance is greatly reduced, and the protection cannot ensure reliable operation

[0003] To solve such problems, many scholars at home and abroad have proposed various solutions. The first type of solution is to propose a new distance protection scheme according to the unique fault output characteristics of inverter-type power sources. Some literature divides the process of solving the line fault distance into the calculation of the real part and the imaginary part according to the relationship between the voltage and current at the protection installation location, but this method has many approximate calculations and the accuracy of the protection is limited. Some literature proposes an improved distance protection scheme based on time delay, which eliminates the influence of the short-circuit current on the large power source side on the small power source side by tripping the circuit breaker on the large power source side, but this scheme will greatly increase the time for the protection to cut off the fault, and the quick-acting performance is greatly reduced. Some literature proposes a new measured impedance calculation method for two-phase grounding faults, but this protection scheme requires communication. Some materials propose a distance protection scheme based on high-frequency fault components. Compared with power-frequency quantity protection, its ability to resist transition resistance has been greatly improved. However, since it is necessary to measure and extract high-frequency quantities, the hardware equipment of the protection needs to be upgraded, and the transient current of the capacitor will have a certain impact on the high-frequency components, which may affect the reliable operation of the protection

[0004] The second type of solution is to achieve reliable protection by improving the control strategy of the photovoltaic station. Some literature lists the fault loop equation according to the control strategy of the inverter, eliminates the unknown quantities such as transition resistance in the equation, and finally solves the fault distance, but this method cannot be applied to three-phase symmetrical faults. Some literature uses the control strategy of the inverter to actively adjust the output of active and reactive power, and then adjusts the phase of the output current to make the additional impedance purely resistive, but the control strategy of this scheme reduces the reactive support of the photovoltaic station to the power grid during the fault, and reduces its low voltage ride-through capability. Some literature proposes a dual current comprehensive control strategy to enable the photovoltaic station to simulate the fault characteristics of the synchronous generator, so that the protection can act correctly, but this scheme does not consider the low voltage ride-through requirements. In summary, the control strategies in the existing control and protection coordination schemes applicable to the transmission lines of photovoltaic stations are designed for the safe and reliable operation of protection, and often ignore the reactive support capability of the photovoltaic station to the power grid during the fault, which conflicts with the low voltage ride-through requirements. Summary of the invention

[0005] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a method and device for distance protection of the transmission line of a new energy station. The problem of weak resistance to transition resistance of distance protection on the photovoltaic side is solved. It can not only reasonably solve the problem of unreliable action of distance protection, but also take into account the reactive support of photovoltaic stations to the power grid during faults. Its low voltage ride-through capability is not only not weakened, but improved to a certain extent. Compared with other control and protection synergy solutions, this solution has obvious advantages in reactive support and protection accuracy.

[0006] The technical solution adopted by the present invention to solve the technical problem is:

[0007] The first aspect of the present invention provides a distance protection method for a new energy station transmission line, in which superconducting magnetic energy storage is connected to a DC bus, and the fault crossing process is divided into two stages through the access of the superconducting magnetic energy storage. The fault includes a two-phase fault and a three-phase grounding fault; according to the fault output characteristics of the two stages, the solution equations of the line short-circuit impedance are respectively written, and the equations are combined to eliminate the unknowns in the equation group, and the line short-circuit impedance is calculated.

[0008] Furthermore, in each stage, a system of equations including two equations is established, and the active and reactive outputs of the two stages are in a nonlinear relationship.

[0009] Furthermore, the equation for the line short-circuit impedance under a two-phase fault is:

[0010]

[0011] Among them, R 1 , L 1 is the positive sequence resistance and inductance per unit length; R 2, L 2 is the negative sequence resistance and inductance per unit length; and The fault component current measured at protection m and protection n;

[0012] In the equation, the equivalent positive sequence impedance Z on the back side of protection m is m1 According to the voltage and current fault components at protection m, the equation group contains four unknown quantities: l MK , R f , R n1 and L n1 ;

[0013] After the fault, the superconducting magnetic energy storage completely absorbs the active power generated by the photovoltaic array within 30ms, and the inverter uses PQ control to generate an amplitude of 1.1I n The reactive power of the PV module can support the stability of the AC bus voltage to the greatest extent. At this time, the current measured at the protection m is only the reactive power generated by the PV module. Substituting it into equation (25) can obtain the equation group of the first stage. 30ms after the fault occurs, the control strategy automatically switches, and the inverter performs normal low voltage ride-through, providing reactive power support for the grid while generating a certain degree of active power. At this time, no matter what the voltage drop is, the current transmitted by the PV module to the grid is different from that in the first stage. Therefore, another different set of equations can be constructed and combined with the equation group of the first stage to solve the fault distance l. MK , that is, find the short-circuit impedance of the line.

[0014] Furthermore, by using an impedance element with quasi-quadrilateral characteristics, when a fault occurs, the line short-circuit impedance |Z 1 l MK | and set to the set value |Z set |Compare, if the line short-circuit impedance is less than the set value, the protection will work correctly; otherwise, the protection will not work. set The setting formula of | is:

[0015] |Z set |=l set |Z 1 | (26)

[0016] Where: l set is the length of the protected line; Z 1 is the impedance per unit length of the transmission line.

[0017] Furthermore, the equation of the line short-circuit impedance under a three-phase grounding fault is:

[0018]

[0019] This system of equations contains four unknowns: MK , Rf , R n1 and L n1 , and the specific solution is the same as that of the two-phase fault.

[0020] Furthermore, it also includes the criterion for the start of distance protection. The fault component voltage at the protection installation location is selected as the start criterion for distance protection. Once the fault component voltage satisfies Equation (31), the protection starts immediately.

[0021] |ΔU L | > ε ∪ |ΔU p | > ε (31)

[0022] In the formula: ΔU L is the fault component line voltage at protection m; ΔU p is the fault component phase voltage at protection m; ε is the action threshold value. To ensure a certain margin, ε is taken as 0.1 kV.

[0023] Furthermore, it also includes the discrimination of the fault direction. When the amplitude of the fault current measured at protection m satisfies Equation (32), the protection determines that the fault is in its positive direction; otherwise, it is in its negative direction.

[0024] |I m | < |I set | (32)

[0025] In the formula: I set is the setting value for fault direction discrimination, and it is taken as 2I n , where I n is the rated current output by the photovoltaic power station.

[0026] Furthermore, the specific protection scheme is as follows: when a fault occurs in the outgoing line, if the fault component voltage satisfies the start criterion, the protection starts. At this time, the amplitude of the fault current is discriminated. If the short-circuit current flowing through the protection is greater than the setting value, it indicates that the fault occurs in the reverse direction of the protection, and the protection returns; if the short-circuit current is less than the setting value, the fault occurs in the positive direction of the protection. At this time, the protection identifies the fault type and selects different methods to solve the short-circuit impedance of the line according to the fault type.

[0027] Furthermore, when calculating the short-circuit impedance of the line, the non-linear equations need to be solved iteratively. The improved target space brainstorm optimization algorithm based on knowledge learning is adopted to model the non-linear equations problem as a multi-modal optimization problem for solution.

[0028] The second aspect of the present invention is to provide a protection device for the distance protection method of the outgoing line of a new energy power station, including:

[0029] A superconducting magnetic energy storage access module is connected to the DC bus to divide the fault ride-through process into two stages through the access of the superconducting magnetic energy storage. The fault includes a two-phase fault and a three-phase grounding fault.

[0030] The line short-circuit impedance solution module lists the solution equations of the line short-circuit impedance according to the fault output characteristics of the two stages, combines them to eliminate the unknown quantities in the equation group, and calculates the line short-circuit impedance.

[0031] The advantages and positive effects of the present invention are:

[0032] (1) The present invention changes the traditional low voltage ride-through control strategy by adding superconducting magnetic energy storage to the DC busbar of the photovoltaic station, thereby solving the short-circuit impedance of the line. Under this control strategy, the voltage ride-through capability of the photovoltaic station is not only not weakened, but has been improved to a certain extent. Compared with other control and protection coordination solutions, this solution has obvious advantages in reactive power support and can meet the technical regulations for new energy grid connection in different regions.

[0033] (2) The method proposed in the present invention can accurately calculate the equivalent impedance of the opposite power grid in real time, thereby protecting it from changes in the operation mode of the power grid.

[0034] (3) The method proposed in the present invention can accurately calculate the line short-circuit impedance based on local quantity information, and eliminate the influence of transition resistance on distance protection through control and protection synergy. In addition, under different fault types, the distance protection can operate reliably and accurately. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 Send out line diagrams to photovoltaic stations;

[0036] Figure 2 This is the fault network diagram of phase A;

[0037] Figure 3 This is the zero-sequence component network diagram of phase A fault;

[0038] Figure 4 This is the structural model diagram of the photovoltaic station;

[0039] Figure 5 is the inverter control strategy diagram;

[0040] Figure 6 This is a typical topological diagram of SMES;

[0041] Figure 7 It is the overall control strategy diagram of SMES;

[0042] Figure 8 It is the BC two-phase fault network diagram;

[0043] Fig. 9It is the sequence network diagram of BC two-phase fault components;

[0044] Fig.10 It is the three-phase fault network diagram;

[0045] Fig.11 It is the sequence network diagram of three-phase fault components;

[0046] Fig.12 It is the protection flow chart;

[0047] Fig.13 It is the simulation result diagram when the fault occurs at 20 km;

[0048] Fig.14 It is the operation characteristic diagram when the fault occurs at 20 km;

[0049] Fig.15 It is the simulation result diagram when the transition resistance is 30 Ω;

[0050] Fig.16 It is the operation characteristic diagram when the transition resistance is 30 Ω. Specific implementation manners

[0051] The present invention will be further described in detail below through specific embodiments. The following embodiments are only descriptive and not restrictive, and the protection scope of the present invention cannot be limited thereby.

[0052] The present invention analyzes the fault network diagram and fault component sequence network diagram of the outgoing line under different fault conditions, derives the solution equations of the line short-circuit impedance, changes its fault control strategy by adding superconducting magnetic energy storage (SMES) to the DC bus of the photovoltaic power station, actively adjusts the output characteristics after the photovoltaic fault, eliminates multiple unknowns in the equations, and then accurately calculates the line short-circuit impedance. The proposed control and protection coordination scheme not only does not weaken the low-voltage ride-through ability of the photovoltaic, but also improves the voltage support ability of the photovoltaic to the power grid during faults. Compared with other control and protection coordination schemes, this scheme has certain advantages in terms of reactive power support and protection accuracy.

[0053] 1 Adaptability analysis of distance protection for outgoing line of photovoltaic power station

[0054] The outgoing line of the photovoltaic power station is as Figure 1 shown. A short-circuit fault occurs at point K in the outgoing line MN. At this time, the relationship between the voltage measured at protection m and the current is as shown in Equation (1):

[0055]

[0056] According to the relationship between voltage and current, the measured impedance Z at protection point m can be obtained m , as shown in Equation (2):

[0057]

[0058] where Z MK is the impedance from protection point m to the short - circuit point K, R 0 is the transition resistance at the short - circuit point, is the current on the large - power - grid side measured at protection point n, is the short - circuit point current.

[0059] 2 Photovoltaic power station control strategy based on superconducting magnetic energy storage

[0060] The reason why the distance protection of the outgoing line of the photovoltaic power station cannot operate correctly is that there is an uncertain phase difference in the fault currents at both ends, resulting in a large error between the actual impedance measured at the protection point and the line short - circuit impedance (the line impedance from the protection installation point to the short - circuit point). If the line short - circuit impedance can be directly solved by cooperating with the control strategy of the photovoltaic power station, the influence of the transition resistance can be eliminated and the adaptability of the distance protection can be improved.

[0061] 2.1 Solution of line short - circuit impedance

[0062] Taking the single - phase - to - ground fault as an example, when a phase - A - to - ground fault occurs on the outgoing line, its fault network is as Figure 2 shown. According to the fault network, the relationship between the phase - A voltage and current measured at protection point m can be obtained, as shown in Equation (3):

[0063]

[0064] where R 1 and L 1 are the positive - sequence resistance and inductance per unit length of the outgoing line, l MK is the distance from protection point m to the short - circuit point k, is the zero - sequence current at protection point m, ω is the power angular frequency, k R and k L are the zero - sequence current resistance compensation coefficient and zero - sequence current inductance compensation coefficient respectively, and their expressions are k R =(R 0 -R 1 ) / 3R 1 and k L =(L 0 -L 1 ) / 3L 1 .

[0065] There are many unknowns in the equation (3), so the fault distance cannot be solved. In order to simplify (3), a fault zero-sequence component network is constructed, such as Figure 3 As shown, the equation established by the zero-sequence fault component network can be expressed as:

[0066]

[0067] Among them, Z m0 and Z n0 are the back-side equivalent zero-sequence impedances of busbar M and busbar N, respectively, which can be calculated from the zero-sequence voltage and zero-sequence current measured at protection points m and n; MN is the total length of the transmission line MN. At this time, the short-circuit current at the short-circuit point K It can be expressed by the zero-sequence current at both ends of the line:

[0068]

[0069] In order to eliminate the influence of zero-sequence current on the short-circuit current, equation (4) and equation (5) are combined to eliminate As shown below:

[0070]

[0071] Substituting equation (6) into equation (3), we can simplify it to obtain a nonlinear equation, as shown in equation (7).

[0072]

[0073] Because Z n0 =R n0 +jωL n0 The nonlinear equation contains four unknown quantities, namely l MK , R f , R n0 and L n0 Therefore, we want to solve the distance l from the short-circuit point to the protection m MK , that is, to solve the short-circuit impedance of the line, a set of equations containing four equations needs to be constructed. The nonlinear equation consists of vectors containing real and imaginary parts. By decomposing the real and imaginary parts, two unrelated equations can be obtained. According to the above analysis, if two more unrelated equations can be constructed, the fault distance can be directly calculated, that is, the short-circuit impedance of the line can be solved.

[0074] Most of the existing methods for constructing multiple equations utilize the flexibility of the inverter control strategy to obtain uncorrelated equations by adjusting the positive-sequence current of the inverter power supply. However, it is difficult to take into account the reactive power support of the photovoltaic power station to the network during a fault, and the low-voltage ride-through ability of the photovoltaic after a fault is reduced to solve the protection problem, which conflicts with the new energy grid connection regulations. The present invention proposes to add a superconducting magnetic energy storage on the DC side of the photovoltaic power station, and on the basis of improving the low-voltage ride-through ability of the photovoltaic, construct four uncorrelated equations to reasonably solve the problem of misoperation or refusal to operate of the distance protection of the outgoing line.

[0075] 2.2 Photovoltaic power station low-voltage ride-through strategy based on superconducting magnetic energy storage

[0076] The present invention proposes to improve the low-voltage ride-through ability of the photovoltaic by stage-adjusting the active and reactive power output by the inverter through adding a superconducting magnetic energy storage on the DC side of the photovoltaic power station without changing the maximum power tracking operation mode of the photovoltaic. During a fault, the superconducting magnetic energy storage is used to balance the output active power, while ensuring the stability of the DC bus voltage, increasing the reactive power output of the inverter, improving the support ability of the photovoltaic power station to the grid voltage during a fault, and at the same time solving the problem that the short-circuit impedance of the line cannot be calculated.

[0077] 2.2.1 Photovoltaic power station structure model

[0078] The photovoltaic power station generally has a two-stage structure. The first-stage structure is a Boost boost circuit, through which the photovoltaic realizes maximum power tracking; the second-stage structure is a DC-AC inverter, through which the photovoltaic realizes AC grid connection through the PQ control strategy; a bidirectional DC-DC connection is adopted between the superconducting magnetic energy storage and the DC bus. The main structure model is as Figure 4 shown.

[0079] The DC filter capacitor between the Boost circuit and the inverter decouples the two stages and buffers the energy change between the two stages. The reason for choosing the superconducting magnetic energy storage to improve the low-voltage ride-through ability of the photovoltaic during the fault transient period instead of the battery is that the SMES has advantages such as fast response speed and high-power output, and can quickly respond to the requirements of the control strategy during the transient period, reducing the protection action delay time.

[0080] 2.2.2 Boost circuit control strategy

[0081] The output power of a photovoltaic (PV) system varies with the voltage, and there is a unique extreme point that maximizes its output power. However, during the operation of a PV power station, it does not necessarily operate at the maximum power point, resulting in a significant reduction in the output efficiency of the PV system. When the light and temperature conditions change, the maximum power point of the PV system also changes. Therefore, a PV power station needs to have the ability of maximum power point tracking (MPPT) during operation. In a two-stage grid-connected PV power station system, a Boost circuit is usually used to implement the MPPT function.

[0082] Currently, the commonly used MPPT algorithms mainly include: incremental conductance method, perturb and observe method, voltage feedback method, etc. In this invention, the widely studied perturb and observe method is adopted to implement MPPT.

[0083] 2.2.3 DC-AC Inverter Low-Voltage Ride-Through Control Strategy

[0084] When a voltage dip occurs on the grid side, the superconducting magnetic energy storage system can enable the PV array to continue operating in the maximum power tracking control mode, improve the power generation efficiency of the PV array, quickly balance the power fluctuation problem of the DC bus, suppress the overvoltage on the DC side, achieve constant voltage control of the DC bus, help the PV grid-connected system achieve controllable and adjustable operation, and improve its reliability.

[0085] The PV DC-AC inverter is a typical two-level voltage source type system structure. Based on its system structure, a model is established. After Park transformation, its mathematical model is:

[0086]

[0087] Among them, e d , e q are the input voltages on the d and q axes, i d , i q are the input currents on the d and q axes, S d , S q are the switching functions on the d and q axes.

[0088] At the initial stage of the fault, the grid-connected point voltage drops significantly, and the system voltage is extremely unstable with large fluctuations. At this time, the SMES completely absorbs the active power generated by the photovoltaic array, and uses PQ control to increase the reactive power output of the photovoltaic inverter. At this time, the photovoltaic power station only outputs reactive power, and the active power output to the system is 0, supporting the stability of the AC bus voltage to the greatest extent; in the middle stage of the fault (after 30 ms), the bus voltage has been relatively stable. To prevent the SMES from reaching the state of charge (SOC) limit quickly and to give full play to the role of the photovoltaic array itself to provide a certain amount of active power for the system load, at this time, the photovoltaic inverter performs normal low-voltage ride-through according to the degree of grid-connected point voltage drop, and the active power output to the system increases while the active power absorbed by the SMES decreases. During the fault, the unbalanced power absorbed by the energy storage system is the active power output of the photovoltaic array itself minus the active power output of the inverter. This strategy can not only well protect the SMES from overcharging and over-discharging, but also does not change the maximum power operation mode of the photovoltaic array, and has strong economy.

[0089] During normal operation, the control strategy of the inverter does not change, and the double closed-loop PQ control strategy combining the power outer loop and the current inner loop is still adopted. After the fault, the active power generated by the photovoltaic array is no longer all sent out. At this time, it is of little significance to control the power outer loop. Therefore, the inverter control strategy is simplified and the single current closed-loop control is directly adopted, as Figure 5 shown. The reference current of the q-axis is as follows:

[0090] (1) Initial stage of the fault (0 - 30 ms)

[0091]

[0092] (2) Middle and late stages of the fault (after 30 ms)

[0093]

[0094] where, U N and I N are the rated voltage and rated current of the power grid; is the reference current value of the q-axis; U is the grid-connected point voltage of the photovoltaic. To prevent the inverter from being damaged due to overcurrent, its output current during the fault is at most 1.1 times the rated current. Then the reference value of the active current is:

[0095]

[0096] 2.2.4 SMES charge and discharge control strategy

[0097] When there is excess energy on the DC bus side, the SMES absorbs the excess power. Conversely, it needs to generate a certain amount of active power to support the stability of the DC bus voltage. Therefore, the SMES is connected to the DC bus through a bi-directional DC-DC circuit, and its typical topology is as Figure 6 shown.

[0098] Assume that the duty cycles of the two switching tubes are both D. Then the terminal voltage of the superconducting magnet is:

[0099] U sc =(2D - 1)U dc (12)

[0100] Substitute the current i sc flowing through the magnet into Equation (12), and the power of the magnet can be obtained as:

[0101] P sc =U sc i sc =(2D - 1)U dc i sc (13)

[0102] According to the above analysis, during the fault, the unbalanced power P SMES absorbed by the SMES is the active power P PV output by the photovoltaic array itself minus the output P grid of the inverter, as shown in Equation (14).

[0103] P SMES =P PV -P grid (14)

[0104] Due to the existence of the shunt capacitor on the DC bus, the active power P sc actually allocated to the superconducting magnet is:

[0105] P sc =P SMES -P cap (15)

[0106]

[0107] Among them, P cap is the active power absorbed by the shunt capacitor on the DC bus, and W cap is the energy stored in the capacitor. According to Equation (16), the active power of the shunt capacitor is determined by the DC bus voltage (the C of the capacitor is a constant value). Therefore, the control of its active power output can be equivalent to the control of the DC bus voltage.

[0108] In order to control the duty cycle, an expression for the duty cycle needs to be obtained. Combining Equation (13) and Equation (15) gives:

[0109] P sc =U sc i sc =(2D - 1)U dc i sc =P SMES -P cap (17)

[0110] According to the above formula, the duty cycle is calculated as:

[0111]

[0112] In summary, the overall control of SMES is as Figure 7 shown.

[0113] 3 Transmission line distance protection scheme based on superconducting magnetic energy storage

[0114] Based on the above analysis, the present invention proposes a transmission line distance protection scheme based on superconducting magnetic energy storage, which reasonably solves the problem that the transmission line is greatly affected by the transition resistance. First, by connecting superconducting magnetic energy storage to the DC bus and adopting a two-stage control strategy to divide the FRT period, the low voltage ride-through ability of the photovoltaic is improved. In each stage, a system of equations containing two equations can be established, and at any voltage drop level, the active and reactive power outputs of the two stages must not be linearly related, that is, the systems of equations constructed by the two stages are not related. By combining the systems of equations of the two stages, the short-circuit impedance of the line is calculated to eliminate the influence of the transition resistance.

[0115] 3.1 Solution scheme for short-circuit impedance of the line under two-phase faults

[0116] When a BC two-phase short-circuit fault occurs at point K of the transmission line, its fault network is as Figure 8 shown. According to the relationship between the short-circuit point K and the voltages and currents at both ends of the line, the following system of equations can be constructed:

[0117]

[0118] According to the short-circuit boundary conditions, the short-circuit current at point B can be expressed by the positive-sequence and negative-sequence currents of phase A:

[0119]

[0120] In order to further simplify the short-circuit current at point B , a fault component sequence network diagram is built, as Fig. 9 shown.

[0121] Among them, R 1 , L 1 are the positive-sequence resistance and inductance per unit length; R 2, L 2 is the negative sequence resistance and inductance per unit length; and are the fault component currents measured at protection m and protection n. Since the PV power station suppresses the negative sequence current during an asymmetric fault and its output negative sequence current is 0, the negative sequence network side in the figure operates in an open circuit state. According to the analysis of the fault component sequence network diagram, the relationship between the positive sequence fault component currents on both sides is:

[0122]

[0123] Expressing the current at the B-phase short-circuit point using the fault components at both ends As shown in Equation (22):

[0124]

[0125] By combining Equation (21) and Equation (22) and eliminating the that cannot be measured at protection m, we can obtain:

[0126]

[0127] Substituting Equation (23) into the system of equations (19), we can obtain:

[0128]

[0129] Therefore, the equation for solving the line short-circuit impedance is:

[0130]

[0131] In the equation, the equivalent positive sequence impedance Z m1 on the back side of protection m can be directly calculated from the voltage and current fault components at protection m, while the equivalent positive sequence impedance Z n1 composed of R n1 and L n1 on the opposite side cannot be directly calculated from the local quantities. Therefore, there are four unknowns in this system of equations: l MK , R f , R n1 and L n1 .

[0132] After the fault, within 30 ms, the superconducting magnetic energy storage completely absorbs the active power generated by the PV array, and the inverter uses PQ control to generate a magnitude of 1.1I nThe reactive power of the photovoltaic power plant can support the stability of the AC bus voltage to the greatest extent. At this time, the current measured at the protection point m is only the reactive power generated by the photovoltaic power plant. Substituting it into equation (25) can obtain the equation group of the first stage. 30ms after the fault occurs, the control strategy automatically switches, and the inverter performs normal low voltage ride-through, providing reactive power support for the grid while generating a certain degree of active power. At this time, no matter what the voltage drop is, the current transmitted by the photovoltaic power plant to the grid is different from that in the first stage. Therefore, another different set of equations can be constructed and combined with the equation group of the first stage to solve the fault distance l MK , that is, the short-circuit impedance of the line is obtained. The present invention uses an impedance element with quasi-quadrilateral characteristics. When a fault occurs, the short-circuit impedance of the line |Z 1 l MK | and set to the set value |Z set |When the line short-circuit impedance is less than the set value, the protection will work correctly; otherwise, the protection will not work. |Z set The setting formula of | is:

[0133] |Z set |=l set |Z 1 (26)

[0134] Where: l set is the length of the protected line; Z 1 It is the impedance per unit length of the transmission line. By directly judging the size of the short-circuit impedance of the line, the fault identification capability of the existing distance protection is improved, and the possibility of false protection is effectively reduced.

[0135] 3.2 Solution for line short-circuit impedance under three-phase grounding fault

[0136] When a three-phase ground short circuit fault occurs at point K of the transmission line, the fault network is as follows: Fig.10 As shown. Since the three-phase fault is a symmetrical fault, there are no negative-sequence and zero-sequence components in the fault network, so the voltage and current relationship of the transmission line is directly represented by the positive-sequence component:

[0137]

[0138] Similarly, in order to short-circuit current Further simplify, build the fault component sequence network diagram, such as Fig.11 shown.

[0139] According to the fault component sequence network diagram analysis, the positive sequence fault component current relationship on both sides is the same as that when the two phases are short-circuited, as shown in the following formula:

[0140]

[0141] Using the fault components at both ends to represent the current at the B-phase short-circuit point As shown in Equation (29):

[0142]

[0143] Substituting Equation (29) into Equation (27), we can obtain:

[0144]

[0145] Similar to the case of two-phase short circuit, this system of equations contains four unknowns: l MK , R f , R n1 and L n1 . For the specific solution process, refer to the case of two-phase short circuit. The analysis process of two-phase short circuit to ground is similar to that of three-phase short circuit, which will not be elaborated here.

[0146] 3.3 Distance protection starting criterion

[0147] During normal operation, the fault component voltage of the outgoing line is almost zero; when a fault occurs, the voltage at the grid connection point of the PV power station drops rapidly, and the fault component voltage increases rapidly. Therefore, the fault component voltage at the protection installation location is selected as the starting criterion for distance protection. Once the fault component voltage satisfies Equation (31), the protection will start immediately.

[0148] |ΔU L | > ε ∪ |ΔU p | > ε (31)

[0149] In the formula: ΔU L is the fault component line voltage at protection m; ΔU p is the fault component phase voltage at protection m; ε is the operating threshold value. To ensure a certain margin, ε is taken as 0.1 kV, and in actual engineering, the value of ε can be adjusted according to the transformer error, protection reliability requirements, and voltage level, etc.

[0150] 3.4 Fault direction discrimination

[0151] Since the PV power station adopts an inverter current limiting control strategy, when a fault occurs in the outgoing line, the fault current flowing through protection m is provided by the PV power station, and its amplitude is much smaller than the fault current provided by the large power grid; when a short-circuit fault occurs inside the PV power station, the fault current flowing through protection m is provided by the large power grid, and its amplitude is much larger than the fault current flowing through during the outgoing line fault. Based on this fault characteristic, the fault direction is discriminated. When the amplitude of the fault current measured at protection m satisfies Equation (32), the protection determines that the fault is in its positive direction, otherwise it is in its negative direction.

[0152] |I m | < |I set| (32)

[0153] Where: I set is the fault direction determination setting value, which is taken as 2I in the present invention. n , where I n is the rated current output by the photovoltaic station. In actual projects, I set to adjust the value.

[0154] 3.6 Distance protection scheme

[0155] When a fault occurs on the transmission line, the fault component voltage meets the starting criteria and the protection starts. At this time, the fault current amplitude is judged. If the short-circuit current flowing through the protection is greater than the setting value, it means that the fault occurs in the reverse direction of the protection and the protection returns; if the short-circuit current is less than the setting value, the fault occurs in the positive direction of the protection. At this time, the protection identifies the fault type and selects different methods for solving the line short-circuit impedance according to the fault type.

[0156] When calculating the short-circuit impedance of the line, the nonlinear equations need to be solved iteratively. The accuracy of the iterative algorithm is the prerequisite for ensuring the correct action of the protection. Therefore, the present invention adopts the improved target space brainstorming optimization (LBSOOS) algorithm based on knowledge learning to model the nonlinear equations problem as a multi-modal optimization problem for solution.

[0157] The protection action criterion is as mentioned above. The setting value of the present invention is Take 0.9l MN , Section II setting value Take 1.2l MN . Protection process such as Fig.12 shown.

[0158] 4 Simulation Verification

[0159] In order to verify the proposed method, a PSCAD / EMTDC electromagnetic transient simulation platform was built. Figure 1 The simulation model shown.

[0160] The simulation verification is carried out for short-circuit faults under different conditions, and the full-wave Fourier algorithm is used to extract the data with a sampling rate of 2kHz. In order to ensure the rapidity of protection, only the first 30ms data of the second stage are selected to construct the equation group, which is combined with the steady-state data within 0-30ms after the fault to solve the line short-circuit impedance. In order to reduce the error and make the calculation result more accurate, the present invention corresponds the sampling points within 0-30ms after the fault to the sampling points within 30-60ms to form an equation group, and the line short-circuit impedance obtained by solving each equation group is averaged to obtain the final result, and finally the fault identification inside and outside the distance protection section I and section II is performed.

[0161] 4.2 Verification of protection schemes at different fault locations

[0162] Simulate the short circuit at different fault locations. Fig.13 The simulation results are given for a fault that occurs 20 km from the photovoltaic station grid connection point. Assuming that a fault occurs on the transmission line at 1 s (R f =20Ω), the protection starts, and at 1.03s the protection starts to calculate the line short-circuit impedance. From the simulation results, it can be seen that the line fault distance calculated by the protection at this time has a small fluctuation around the actual value, but the error is not large, and taking the average value can further reduce the error. Due to the transient process within 5ms at the beginning of the fault, the voltage fluctuates irregularly and the fluctuation amplitude is large, so the error of the calculated value within 5ms after 1.03s is relatively large, which is consistent with the simulation results.

[0163] The average of the line fault distances obtained by solving each set of equations is multiplied by the line unit impedance to obtain a measured impedance of 2.857+j7.756Ω, which is located in the quasi-quadrilateral action area. Fig.14 As shown, protection correct action.

[0164] 4.3 Verification of protection scheme under different transition resistances

[0165] The simulation calculation is carried out for the scenario where the fault occurs at 80km at the end of the line through different transition resistances. Fig.15 It shows that a phase A ground fault occurs on the transmission line, and R f =30Ω, the calculation results of the proposed method, Fig.16 The operation of the quasi-quadrilateral relay is shown. At this time, the calculated line fault distance is 80.89 km, the measured impedance is 11.325+j30.738Ω, and the protection can still operate correctly.

[0166] The second aspect of the present invention is to provide a protection device for a distance protection method of a new energy station transmission line, comprising:

[0167] A superconducting magnetic energy storage access module is connected to the DC bus to divide the fault ride-through process into two stages through the access of the superconducting magnetic energy storage. The fault includes a two-phase fault and a three-phase grounding fault.

[0168] The line short-circuit impedance solution module lists the solution equations of the line short-circuit impedance according to the fault output characteristics of the two stages, combines them to eliminate the unknown quantities in the equation group, and calculates the line short-circuit impedance.

[0169] Those skilled in the art should understand that the embodiments of the present application can be provided as methods, devices, or computer program products. Therefore, the present application can take the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware aspects. Moreover, the present application can take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) that contain computer-usable program code.

[0170] The present application is described with reference to the flowcharts and / or block diagrams of methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each flow and / or block in the flowchart and / or block diagram can be implemented by computer program instructions, as well as the combination of flows and / or blocks in the flowchart and / or block diagram. These computer program instructions can be provided to the processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing devices to generate a machine, such that the instructions executed by the processor of the computer or other programmable data processing devices generate a device for implementing the functions specified in Figure 1 one or more of the processes or multiple processes and / or blocks Figure 1 one or more of the blocks or multiple blocks.

[0171] These computer program instructions can also be stored in a computer-readable memory that can direct a computer or other programmable data processing device to work in a specific manner, such that the instructions stored in the computer-readable memory generate a manufactured article including an instruction device that implements the functions specified in Figure 1 one or more of the processes or multiple processes and / or blocks Figure 1 one or more of the blocks or multiple blocks.

[0172] These computer program instructions can also be loaded onto a computer or other programmable data processing device, such that a series of operation steps are executed on the computer or other programmable device to generate a computer-implemented process, so that the instructions executed on the computer or other programmable device provide steps for implementing the functions specified in Figure 1 one or more of the processes or multiple processes and / or blocks Figure 1 one or more of the blocks or multiple blocks.

[0173] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the inventive concept, several modifications and improvements can be made, and these all belong to the protection scope of the present invention.

Claims

1. A distance protection method for transmission lines of new energy stations. It is characterized in that Superconducting magnetic energy storage is connected to the DC bus. The fault ride-through process is divided into two stages through the connection of superconducting magnetic energy storage. The fault includes a two-phase fault and a three-phase grounding fault. According to the fault output characteristics of the two stages, the solution equations of the line short-circuit impedance are listed respectively, and the unknown quantities in the equation group are eliminated by combining them to calculate the line short-circuit impedance.

2. The distance protection method for the transmission line of the new energy station according to claim 1, It is characterized in that In each stage, a system of equations consisting of two equations is established, and the active and reactive outputs of the two stages are nonlinearly related.

3. The distance protection method for the transmission line of the new energy station according to claim 2, It is characterized in that The equation for the line short-circuit impedance under a two-phase fault is: Among them, R 1 , L 1 are the positive-sequence resistance and inductance per unit length; R 2 , L 2 are the negative-sequence resistance and inductance per unit length; and are the fault component currents measured at protection m and protection n; In the equation, the equivalent positive-sequence impedance Z protecting the dorsal side of m m1 is directly obtained from the voltage and current fault components at protection m. This system of equations contains four unknowns: l MK , R f , R n1 , and L n1 ; After the fault, the superconducting magnetic energy storage completely absorbs the active power generated by the photovoltaic array within 30ms, and the inverter uses PQ control to generate an amplitude of 1.1I n The reactive power of the PV module can support the stability of the AC bus voltage to the greatest extent. At this time, the current measured at the protection m is only the reactive power generated by the PV module. Substituting it into equation (25) can obtain the equation group of the first stage. 30ms after the fault occurs, the control strategy automatically switches, and the inverter performs normal low voltage ride-through, providing reactive power support for the grid while generating a certain degree of active power. At this time, no matter what the voltage drop is, the current transmitted by the PV module to the grid is different from that in the first stage. Therefore, another different set of equations can be constructed and combined with the equation group of the first stage to solve the fault distance l. MK , that is, find the short-circuit impedance of the line.

4. The distance protection method for the transmission line of the new energy station according to claim 3, It is characterized in that An impedance element with a quasi-quadrilateral characteristic is adopted. When a fault occurs, the short-circuit impedance |Z 1 l MK | of the line is obtained and compared with the setting value |Z set |. If the short-circuit impedance of the line is less than the setting value, the protection operates correctly; otherwise, the protection does not operate. The setting formula of |Z set | is as follows: |Z set | = l set |Z 1 | (26) where: l set is the length of the protected line; Z 1 is the impedance per unit length of the outgoing line.

5. The distance protection method for the transmission line of the new energy station according to claim 4, It is characterized in that The equation of line short-circuit impedance under three-phase grounding fault is: The system of equations contains four unknowns: l MK , R f , R n1 and L n1 , and the specific solution is the same as that for a two-phase fault.

6. The distance protection method for the transmission line of the new energy station according to claim 1, It is characterized in that It also includes the criterion for the start of distance protection. The fault component voltage at the protection installation is selected as the start criterion for the distance protection. Once the fault component voltage satisfies formula (31), the protection is immediately started. |ΔU L |>ε ∪ |ΔU p |>ε (31) where: ΔU L is the fault component line voltage at protection point m; ΔU p is the fault component phase voltage at protection point m; ε is the operating threshold value. To ensure a certain margin, ε is taken as 0.1 kV.

7. The distance protection method for the transmission line of the new energy station according to claim 1, It is characterized in that It also includes the determination of the fault direction. When the fault current amplitude measured at protection point m satisfies equation (32), the protection determines that the fault is in its positive direction, otherwise it is in its reverse direction. |I m |<|I set | (32) Where: I set is the setting value for fault direction discrimination, and it is taken as 2I n , where I n is the rated current output by the PV power station.

8. The distance protection method for the transmission line of the new energy station according to claim 1, It is characterized in that The specific protection scheme is: when a fault occurs on the transmission line, the fault component voltage meets the starting criterion and the protection starts. At this time, the magnitude of the fault current is judged. If the short-circuit current flowing through the protection is greater than the setting value, it means that the fault occurs in the reverse direction of the protection and the protection returns; if the short-circuit current is less than the setting value, the fault occurs in the positive direction of the protection. At this time, the protection identifies the fault type and selects different methods for solving the line short-circuit impedance according to the fault type.

9. The distance protection method for the transmission line of the new energy station according to claim 1, It is characterized in that When calculating the short-circuit impedance of the line, the nonlinear equations need to be solved iteratively. An improved target space brainstorming optimization algorithm based on knowledge learning is used to model the nonlinear equations problem as a multimodal optimization problem for solution.

10. A protection device for the distance protection method for the transmission line of a new energy station according to claim 1, It is characterized in that include: A superconducting magnetic energy storage access module is connected to the DC bus to divide the fault ride-through process into two stages through the access of the superconducting magnetic energy storage. The fault includes a two-phase fault and a three-phase grounding fault. The line short-circuit impedance solution module lists the solution equations of the line short-circuit impedance according to the fault output characteristics of the two stages, combines them to eliminate the unknown quantities in the equation group, and calculates the line short-circuit impedance.

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