Alternating-current micro-grid directional protection method suitable for high-proportion new energy and stored energy access

Abstract

The invention provides an AC micro-grid direction protection method suitable for high-proportion new energy and energy storage access, and the method comprises the following steps: S1, obtaining a fault voltage and a fault current, and carrying out S2 if the fault voltage and the fault current meet protection starting conditions; s2, protection is started, then three-phase fault current at a system power outlet is collected, and power outlet negative-sequence current corresponding to the three-phase fault current is calculated; s3, determining a fault type by using the negative sequence current at the outlet of the power supply, if the fault type is a two-phase short circuit, performing S4, and if the fault type is a three-phase short circuit, performing S5; s4, judging the fault direction based on negative sequence current amplitude direction protection; s5, judging a fault direction by adopting direction protection based on transient voltage and current polarity comparison; according to the method, the direction element based on the negative sequence current amplitude and the direction element based on voltage and current transient state quantity polarity comparison are matched, and reliable identification of the fault direction under different types of inter-phase faults can be met.

Description

Technical Field

[0001] The present invention relates to the technical field of AC microgrid relay protection, and in particular to a directional protection method for an AC microgrid with a high proportion of new energy and energy storage access. Background Art

[0002] Distributed power sources, new loads and energy storage can be connected to urban microgrids in various forms according to local conditions to make full use of local renewable energy and replace and reduce fossil energy consumption to the maximum extent. Therefore, microgrids with full use of flexible and controllable sources, loads and storage as the core will become an important part of the new urban power grid in the future.

[0003] However, in the context of new power system construction, microgrids will have the following main features: 1. At the power supply level, the power supply characteristics of microgrids will gradually change from being dominated by synchronous power supply to being dominated by distributed photovoltaic and other new energy sources.

[0004] Second, at the network level, the microgrid structure will gradually evolve into a new form of highly power-electronic, flexible and interconnected microgrids based on the existing AC microgrids. At the same time, the high proportion of distributed photovoltaic, energy storage and other power electronic equipment in different forms and decentralized access further highlights the characteristics of microgrids with multiple points and wide coverage.

[0005] 3. At the load level, under the background of the development of new power systems, new power electronic loads such as electric vehicles, urban rail transit, and data centers have grown explosively. At the same time, the energy storage system installed at the end user can significantly improve the reliability of power supply through power storage and release, and its access scale and proportion will also increase significantly.

[0006] However, the access of high proportion of new energy and energy storage not only makes the microgrid topology and operation mode more complex, but also fundamentally changes the fault characteristics. While putting forward new requirements for relay protection performance, it also poses severe challenges to the existing fault analysis methods and protection principles. The traditional fault analysis method based on single power radial topology and the staged overcurrent protection principle are difficult to apply, and the volatility of the protection action range and the complexity of step-by-step setting coordination are greatly increased. In particular, with the continuous increase in the proportion and capacity of distributed new energy and energy storage access, fault handling measures such as low-voltage decoupling sacrifice the fault ride-through capability of distributed power sources, making it difficult to meet the power supply reliability requirements after high proportion and multi-point access of distributed photovoltaic power distribution networks. In view of the overcurrent protection mismatch problem caused by high proportion of new energy and energy storage access, the influence of downstream photovoltaic power generation during faults can be eliminated by configuring directional protection, but the traditional directional protection cannot be reliably operated due to the destruction of transient linear conditions of faults. With the vigorous development of new energy power generation technology in my country, the scale of grid connection of new energy power sources and energy storage equipment has gradually expanded. However, power electronic equipment such as new energy power sources present highly nonlinear characteristics, which makes it difficult to apply the traditional protection principle based on power frequency steady-state quantity. Therefore, in view of this situation, it is necessary to study the line directional protection scheme for microgrids with a high proportion of new energy and energy storage access. Summary of the invention

[0007] The present invention solves the problem that the applicability of traditional directional protection is greatly affected by the access capacity of new energy and energy storage for microgrids with a high proportion of new energy and energy storage access. A directional protection method suitable for AC microgrids with a high proportion of new energy and energy storage access is proposed. The method uses a directional element based on the negative sequence current amplitude and a directional element based on the comparison of the polarity of voltage and current transient quantities to meet the reliable identification of fault directions under different types of phase-to-phase faults.

[0008] In order to achieve the above objectives, the following technical solutions are proposed: The directional protection method for AC microgrids with high proportion of new energy and energy storage access includes the following steps: S1, obtain the fault voltage and fault current, if the fault voltage and fault current meet the protection start condition, proceed to S2; S2, protection starts, then collects the three-phase fault current at the system power outlet, and calculates the negative sequence current at the power outlet corresponding to the three-phase fault current; S3, determine the fault type using the negative sequence current at the power outlet. If the fault type is a two-phase short circuit, proceed to S4; if the fault type is a three-phase short circuit, proceed to S5; S4, using protection based on negative sequence current amplitude direction to determine fault direction; S5, uses directional protection based on transient voltage and current polarity comparison to determine the fault direction.

[0009] Microgrids generally adopt an ungrounded neutral point mode. When the system is in an asymmetric operating state, there are negative-sequence voltages and negative-sequence currents. The double-frequency components acting on the inverter control link will affect the control stability and the life of the capacitor. Therefore, the grid-connected inverter adopts a control strategy to suppress the negative-sequence current. Therefore, for inverter-type power supply access lines, when an asymmetric fault occurs, the direction of the phase-to-phase short-circuit fault can be identified by detecting the negative-sequence current. The present invention fully considers the fault characteristics of different fault types, and can accurately reflect the fault direction using only single-end quantity information, and can reliably identify the fault direction of the distribution line. In addition, the introduction of directional protection can increase the access capacity of new energy power supplies and energy storage equipment, thereby coping with the severe challenges brought by the nonlinear response characteristics of faults of power electronic equipment such as high-proportion distributed photovoltaics, energy storage, and flexible interconnection devices to the protection action performance.

[0010] Preferably, the protection start condition in S1 is that the fault voltage change rate modulus or the fault current change rate modulus is greater than a set protection start threshold value.

[0011] Preferably, the fault voltage change rate modulus is |du / dt|, where du is the fault voltage change value within time t; the fault current change rate modulus is |di / dt|, where di is the fault voltage change value within time t.

[0012] Preferably, the S3 determines the fault type by using the negative sequence current at the power outlet and specifically includes the following steps: Determine whether the negative sequence current at the power outlet is greater than the negative sequence current setting value at the power outlet. If so, determine that the fault type is a two-phase short circuit. If not, determine whether the negative sequence current at the power outlet is less than the negative sequence current setting value at the power outlet. If so, determine that the fault type is a three-phase short circuit.

[0013] Preferably, the S4 specifically comprises the following steps: Collect the three-phase fault current at the protection point, calculate the negative-sequence current at the protection point corresponding to the three-phase fault current, and determine whether the negative-sequence current at the protection point is greater than the set value of the negative-sequence current at the protection point. If so, determine that the fault type is a forward direction fault; if not, determine that the fault type is a reverse direction fault.

[0014] Preferably, the S5 specifically comprises the following steps: S51, collecting three-phase fault voltage data and three-phase fault current data at the protection location; S52, performing wavelet transformation on the voltage and current data of each phase, and obtaining the first extreme value of the three-phase voltage detail coefficient and the first extreme value of the three-phase current detail coefficient; S53, obtaining a first extreme value of a voltage detail coefficient and a first extreme value of a current detail coefficient corresponding to a phase with the largest first extreme value of a three-phase current detail coefficient; S54, determining the fault type by using the product of the first extreme value of the voltage detail coefficient and the first extreme value of the current detail coefficient.

[0015] Preferably, the S54 specifically includes the following steps: Determine whether the product of the first extreme value of the voltage detail coefficient and the first extreme value of the current detail coefficient is less than 0. If so, determine that the fault type is a forward direction fault; if not, determine that the fault type is a reverse direction fault.

[0016] The present invention has the following beneficial effects: the present invention fully considers the fault characteristics of different fault types, and can accurately reflect the fault direction by only using single-end quantity information, and can reliably identify the fault direction of the distribution line. In addition, the introduction of directional protection can increase the access capacity of new energy power supplies and energy storage equipment, thereby coping with the severe challenges brought by the nonlinear response characteristics of faults of power electronic equipment such as high-proportion distributed photovoltaics, energy storage, and flexible interconnection devices to the protection action performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a schematic diagram of an AC microgrid accessed by new energy sources of the present invention; Figure 2 It is the positive and negative fault additional network of the AC microgrid accessed by the new energy of the present invention; Figure 3 It is a flow chart of the method of the present invention. DETAILED DESCRIPTION

[0018] Example: In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of the present invention.

[0019] Microgrids generally use an ungrounded neutral point. When the system is in an asymmetric operating state, there are negative-sequence voltages and negative-sequence currents. Acting on the inverter control link will produce frequency-doubled components, affecting control stability and capacitor life. Therefore, the grid-connected inverter adopts a control strategy to suppress negative-sequence currents. Therefore, for inverter-type power supply access lines, when an asymmetric fault occurs, the direction of the phase-to-phase short-circuit fault can be identified by detecting the negative-sequence current. Figure 1 The schematic diagram of photovoltaic access to AC distribution network is shown in the figure, in which: DG is the photovoltaic power source, B1-B4 are busbars 1, 2, 3 and the line protection at the DG outlet. Figure 1 Take the protection of B4 as an example, refer to Figure 2When an asymmetric fault occurs in the positive direction of the protection, the fault current provided by the system side power supply measured by protection B4 will have negative sequence current; when an asymmetric fault occurs in the back side of the protection, the fault current provided by the inverter power supply measured by protection B4 basically does not have negative sequence current. Therefore, the protection criterion of the asymmetric fault direction element is proposed as shown in the following formula: Where: is the negative sequence current flowing through the protection, It is the setting value of the protection criterion based on the negative sequence current amplitude direction protection.

[0020] When a three-phase short circuit fault occurs in the system, there is no negative sequence quantity in the microgrid, so the above-mentioned directional protection based on the negative sequence current amplitude is no longer applicable. Considering that the three-phase short circuit fault current amplitude is large, and the voltage and current transient quantity amplitudes are large, which can meet the accuracy requirements of the measurement device, the solution proposed in this patent uses the voltage and current transient quantity polarity comparison method to identify the direction of the three-phase short circuit fault, and the protection criterion is shown as follows: Where: d 1_u.first Indicates the first extreme value of voltage detail coefficient; d 1_i.first Indicates the first extreme value of the current detail coefficient.

[0021] On this basis, this embodiment proposes an AC microgrid directional protection method suitable for high-proportion new energy and energy storage access, referring to Figure 3 , including the following steps: S1, obtain the fault voltage and fault current. If the fault voltage and fault current meet the protection start condition, proceed to S2. The protection start condition in S1 is that the fault voltage change rate modulus or the fault current change rate modulus is greater than the set protection start threshold. The fault voltage change rate modulus is |du / dt|, where du is the fault voltage change value within t time; the fault current change rate modulus is |di / dt|, where di is the fault voltage change value within t time.

[0022] In this embodiment, the specific process of S1 is to measure the voltage and current at the protection point. If |du / dt|>Δ1 or |di / dt|>Δ2 is satisfied, the protection is started, |du / dt| represents the fault voltage change rate modulus, |di / dt| represents the fault current change rate modulus, Δ1 represents the protection start threshold value based on the voltage change rate modulus, Δ2 represents the protection start threshold value based on the current change rate modulus, and t represents the time; S2, protection starts, then collects the three-phase fault current at the system power outlet, and calculates the negative sequence current at the power outlet corresponding to the three-phase fault current; The specific process of S2 in this embodiment is to collect the three-phase fault current at the power outlet of the system: Calculate the negative sequence current corresponding to the three-phase short-circuit current The negative sequence current is calculated as follows: S3, using the negative sequence current at the power outlet to determine the fault type, specifically includes the following steps: Determine whether the negative sequence current at the power outlet is greater than the set value of the negative sequence current at the power outlet. If so, determine that the fault type is a two-phase short circuit. If not, determine whether the negative sequence current at the power outlet is less than the set value of the negative sequence current at the power outlet. If so, determine that the fault type is a three-phase short circuit. If the fault type is a two-phase short circuit, proceed to S4. If the fault type is a three-phase short circuit, proceed to S5. The specific process of S3 in this embodiment is to determine the fault type by using the negative sequence current at the system power supply: If the two phases are short-circuited, execute S4; If it is a three-phase short circuit, execute S5. Set the negative sequence current value at the power outlet; S4, using the negative sequence current amplitude direction protection to determine the fault direction, specifically including the following steps: Collect the three-phase fault current at the protection point, calculate the negative-sequence current at the protection point corresponding to the three-phase fault current, and determine whether the negative-sequence current at the protection point is greater than the set value of the negative-sequence current at the protection point. If so, determine that the fault type is a forward direction fault; if not, determine that the fault type is a reverse direction fault.

[0023] The specific process of S4 in this embodiment is: Collect three-phase fault current at the protection point Calculate the negative sequence current at the protection point corresponding to the three-phase short-circuit current The negative sequence current calculation at the protection point is shown in the following formula: like If it is positive, it is a fault in the positive direction; otherwise, it is a fault in the negative direction.

[0024] S5, using directional protection based on transient voltage and current polarity comparison to determine the fault direction, specifically including the following steps: S51, collecting three-phase fault voltage data and three-phase fault current data at the protection location; S52, performing wavelet transformation on the voltage and current data of each phase, and obtaining the first extreme value of the three-phase voltage detail coefficient and the first extreme value of the three-phase current detail coefficient; S53, obtaining a first extreme value of a voltage detail coefficient and a first extreme value of a current detail coefficient corresponding to a phase with the largest first extreme value of a three-phase current detail coefficient; S54, determining the fault type by multiplying the first extreme value of the voltage detail coefficient and the first extreme value of the current detail coefficient, specifically includes the following steps: Determine whether the product of the first extreme value of the voltage detail coefficient and the first extreme value of the current detail coefficient is less than 0. If so, determine that the fault type is a forward direction fault; if not, determine that the fault type is a reverse direction fault.

[0025] The specific process of S5 in this embodiment is: Collect three-phase fault voltage data and three-phase fault current data at the protection point; Mallat wavelet transform is performed on the voltage and current data of each phase, and the first extreme value d of the three-phase voltage detail coefficient is obtained. 1_u.first =[d 1_ua.first d 1_ub.first d 1_uc.first ], the first extreme value of the three-phase current detail coefficient d 1_i.first =[d 1_ia.first d 1_ib.first d 1_ic.first ]; Compare 1_ia.first ,d 1_ib.first and d 1_ic.first The size between them is selected, and the phase x with the largest detail coefficient is selected, and the first extreme value d of the voltage and current detail coefficient of this phase is used. 1_ux.first and d 1_ix.first Determine the fault direction; If d 1_ux.first ·d 1_ix.first If it is less than 0, it is a positive direction fault, otherwise it is a reverse direction fault.

[0026] The present invention fully considers the fault characteristics of different fault types, and can accurately reflect the fault direction by only using single-end quantity information, and can reliably identify the fault direction of the distribution line. In addition, the introduction of directional protection can increase the access capacity of new energy power supplies and energy storage equipment, thereby coping with the severe challenges brought by the nonlinear response characteristics of faults of power electronic equipment such as high-proportion distributed photovoltaics, energy storage, and flexible interconnection devices to the protection action performance.

Claims

1. A directional protection method for AC microgrids with a high proportion of new energy and energy storage access, characterized by: The following steps are involved: S1, obtain the fault voltage and fault current, if the fault voltage and fault current meet the protection start condition, proceed to S2; S2, protection starts, then collects the three-phase fault current at the system power outlet, and calculates the negative sequence current at the power outlet corresponding to the three-phase fault current; S3, determine the fault type using the negative sequence current at the power outlet. If the fault type is a two-phase short circuit, proceed to S4; if the fault type is a three-phase short circuit, proceed to S5; S4, using protection based on negative sequence current amplitude direction to determine fault direction; S5, uses directional protection based on transient voltage and current polarity comparison to determine the fault direction.

2. The AC microgrid directional protection method suitable for high-proportion new energy and energy storage access according to claim 1 is characterized in that: The protection start condition in S1 is that the fault voltage change rate modulus or the fault current change rate modulus is greater than a set protection start threshold value.

3. The AC microgrid directional protection method applicable to high proportion of new energy and energy storage access according to claim 2 is characterized in that: The fault voltage change rate modulus is |du / dt|, where du is the fault voltage change value within time t; the fault current change rate modulus is |di / dt|, where di is the fault voltage change value within time t.

4. The AC microgrid directional protection method applicable to high proportion of new energy and energy storage access according to claim 1, 2 or 3, characterized in that: The S3 uses the negative sequence current at the power outlet to determine the fault type, which specifically includes the following steps: Determine whether the negative sequence current at the power outlet is greater than the negative sequence current setting value at the power outlet. If so, determine that the fault type is a two-phase short circuit. If not, determine whether the negative sequence current at the power outlet is less than the negative sequence current setting value at the power outlet. If so, determine that the fault type is a three-phase short circuit.

5. The AC microgrid directional protection method applicable to high proportion of new energy and energy storage access according to claim 1, 2 or 3, characterized in that: The S4 specifically comprises the following steps: Collect the three-phase fault current at the protection point, calculate the negative-sequence current at the protection point corresponding to the three-phase fault current, and determine whether the negative-sequence current at the protection point is greater than the set value of the negative-sequence current at the protection point. If so, determine that the fault type is a forward direction fault; if not, determine that the fault type is a reverse direction fault.

6. The AC microgrid directional protection method applicable to high proportion of new energy and energy storage access according to claim 1, 2 or 3, characterized in that: The S5 specifically includes the following steps: S51, collecting three-phase fault voltage data and three-phase fault current data at the protection location; S52, performing wavelet transformation on the voltage and current data of each phase, and obtaining the first extreme value of the three-phase voltage detail coefficient and the first extreme value of the three-phase current detail coefficient; S53, obtaining a first extreme value of a voltage detail coefficient and a first extreme value of a current detail coefficient corresponding to a phase with the largest first extreme value of a three-phase current detail coefficient; S54, determining the fault type by using the product of the first extreme value of the voltage detail coefficient and the first extreme value of the current detail coefficient.

7. The AC microgrid directional protection method applicable to high proportion of new energy and energy storage access according to claim 6 is characterized in that: The S54 specifically includes the following steps: Determine whether the product of the first extreme value of the voltage detail coefficient and the first extreme value of the current detail coefficient is less than 0. If so, determine that the fault type is a forward direction fault; if not, determine that the fault type is a reverse direction fault.

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