Medium and low voltage power grid fault direction identification and setting method suitable for energy storage power station access

Abstract

The invention relates to the technical field of medium and low voltage power grid protection, in particular to a medium and low voltage power grid fault direction identification and setting method suitable for energy storage power station access. According to the technical scheme, the method comprises the steps of judging a fault type according to a voltage locking criterion, and determining whether a fault direction criterion is started or not based on whether the fault type and voltage meet a low-voltage crossing condition of the energy storage power station. According to the method, the two-phase interphase short circuit fault and the three-phase short circuit fault can be accurately distinguished by utilizing the voltage locking criterion and the positive and negative sequence voltage comprehensive judgment, the negative sequence power direction criterion is adopted for the two-phase interphase short circuit, the negative sequence impedance change range factor of the energy storage power station is considered for accurate setting when the criterion is calculated, and the accuracy of the judgment is improved. A fault point can be quickly positioned under a complex power grid working condition, and the troubleshooting time is shortened; and meanwhile, for three-phase short circuit, a criterion based on a reactive current direction and an active current amplitude is applied, and a corresponding setting value is reasonably set to accurately judge a fault direction, so that normal operation of other parts of the power grid is guaranteed.

Description

Technical Field

[0001] The present invention relates to the technical field of medium and low voltage power grid protection, and in particular to a method for identifying and setting fault directions of medium and low voltage power grids suitable for access to energy storage power stations. Background Art

[0002] In the context of large-scale access of renewable energy to the power system, energy storage systems play a key role in supporting the safe and stable operation of the power grid. After the grid-connected energy storage power station is connected to the medium and low voltage power grid, the traditional simple radiation structure of the medium and low voltage power grid is transformed into a complex structure with dual or multiple power supplies. In order to ensure that the traditional overcurrent protection does not malfunction or refuse to operate, it is necessary to add directional elements to the current protection to form directional overcurrent protection. The grid-connected energy storage power station is affected by its device current carrying capacity and control strategy, and its fault characteristics are significantly different from those of the traditional synchronous power supply, making it difficult to apply the traditional power directional elements. In addition, unlike inverter power sources such as wind power and photovoltaic power, its converter operates in rectification mode in the charging state and in inverter mode in the discharging state. The active power has the characteristics of bidirectional flow, which increases the difficulty of fault direction identification. Therefore, we propose a fault direction identification and setting method for medium and low voltage power grids connected to energy storage power stations. Summary of the invention

[0003] The purpose of the present invention is to address the problem that traditional power direction elements existing in the background technology are difficult to apply to energy storage power stations, and the active power has the characteristic of bidirectional flow, which makes it difficult to judge the fault direction of the energy storage power station. A method for identifying and setting the fault direction of a medium and low voltage power grid connected to an energy storage power station is proposed.

[0004] The technical solution of the present invention is a method for identifying and setting the fault direction of a medium and low voltage power grid connected to an energy storage power station, comprising the following steps:

[0005] Determine the fault type based on the voltage blocking criteria;

[0006] Determine whether the fault direction judgment criterion is activated based on the fault type and whether the voltage meets the low-breakthrough conditions of the energy storage power station;

[0007] The fault types include two-phase short circuit and three-phase short circuit. When the voltage lockout criterion is turned on, if the fault type is judged to be the two-phase short circuit, the negative sequence power direction criterion is started. If the fault type is judged to be the three-phase short circuit, the reactive current direction and the active current amplitude direction criterion are started.

[0008] Optionally, judging the fault type according to the voltage lockout criterion includes:

[0009] When the positive sequence voltage at the grid connection point of the energy storage power station drops higher than 0.9pu, the direction judgment criterion is locked and the fault type is not judged. When the positive sequence voltage at the grid connection point of the energy storage power station drops less than or equal to 0.9pu, the fault type is further judged based on the negative sequence voltage. The judgment method is as follows:

[0010] When the positive and negative sequence voltages at the protection installation satisfy the formula:

[0011]

[0012] It is judged as a two-phase short circuit fault, and the negative sequence power direction judgment is started. Otherwise, the negative sequence power direction judgment is started and locked. They are respectively the positive and negative sequence voltages at the protection installation location, They are respectively the positive and negative sequence voltage setting values;

[0013] When the voltage at the protection installation satisfies the formula:

[0014]

[0015] It is judged as a three-phase short circuit fault, and the direction criterion based on the reactive current direction and the active current amplitude is started, otherwise the reactive current direction criterion is locked, where: They are respectively the positive and negative sequence voltages at the protection installation location; They are respectively the positive and negative sequence voltage setting values;

[0016] When setting the negative sequence voltage, take the minimum negative sequence voltage that may occur at the protection installation point when avoiding two-phase short circuit. When the positive sequence voltage is set, the maximum positive sequence voltage at the protection installation location is taken when the reactive current detected by the protection is greater than 0. The formula is as follows:

[0017]

[0018] in, and is the reliability coefficient.

[0019] Optionally, after the negative sequence power direction criterion is opened, the positive criterion formula is as follows:

[0020]

[0021] If the forward criterion formula is not met, it is judged as a reverse two-phase short circuit fault, where: are the upper and lower boundaries of the action area in case of a forward fault, They are respectively the negative sequence voltage and current power frequency vectors detected by the protection, The calculation formula is as follows:

[0022]

[0023] Among them, Z down is the equivalent negative sequence impedance of the downstream of bus B, including the energy storage station, Z up is the equivalent negative-sequence impedance upstream of bus B.

[0024] Optionally, after the reactive current direction criterion is opened, the reverse fault criterion formula is as follows:

[0025]

[0026] If the above formula is met, it is judged as a reverse short circuit fault. If it is not met, the active current amplitude direction criterion is opened, where I p6.q To protect the detected reactive current, I q.set It is the setting value of reactive current amplitude.

[0027] Optionally, the formula for the active current direction amplitude criterion is as follows:

[0028]

[0029] Among them, I d.set1 It is the maximum active current flowing through the protection when avoiding three-phase short circuit at the end of the forward area of ​​the protection during setting, I d.set2 It is the minimum active current flowing through the protection when the three-phase short circuit occurs at 15% of the reverse area of ​​the protection during setting. p6.d | min , the I d.set1 ,I d.set2 The calculation formula is as follows:

[0030]

[0031] in, is the reliability coefficient.

[0032] Optionally, when the energy storage power station does not reach the reactive current limit and enters the low-through mode, the negative sequence impedance amplitude is expressed as the following formula:

[0033]

[0034] Among them, U N is the rated line voltage of the distribution network, I N is the rated current of the energy storage power station, S N is the rated capacity of the energy storage power station, and K is a constant.

[0035] Optionally, when the active current amplitude criterion is set and calculated, the fault network is solved by an iterative method to obtain the maximum boundary at which the reactive current criterion fails in the reverse direction fault. The boundary range is increased by 5% as the farthest fault position when setting the active current amplitude to solve the minimum active current in the reverse direction fault.

[0036] In summary, the present application includes at least one of the following beneficial technical effects:

[0037] 1. The present invention can accurately distinguish between two-phase short circuit and three-phase short circuit faults by combining the voltage blocking criterion with the comprehensive judgment of positive and negative sequence voltages, thus avoiding the misoperation or refusal of protection caused by misjudgment of fault type and improving the reliability of the protection device;

[0038] 2. The present invention adopts the negative-sequence power direction criterion for two-phase short circuit, and when calculating the negative-sequence power direction criterion, factors such as the negative-sequence impedance variation range of the energy storage power station are considered for accurate setting, which can reliably determine the fault direction. Under the complex topological structure and variable operating conditions of the power grid, the fault point can be quickly located, and the fault investigation time can be reduced;

[0039] 3. The present invention adopts the reactive current direction and active current amplitude judgment criteria for three-phase short circuit, and accurately judges the fault direction by reasonably setting the reactive current amplitude setting value and the active current amplitude setting value; in the case of a three-phase short circuit that may cause a greater impact on the power grid, the present invention can isolate the fault in time to ensure the normal operation of other parts of the power grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 A schematic diagram of a medium and low voltage power grid to which an energy storage power station is connected is provided in the present invention;

[0041] Figure 2 A flow chart of a method for judging a two-phase short-circuit fault and identifying its direction in the present invention is provided;

[0042] Figure 3 A flow chart of the three-phase short circuit fault judgment and direction identification method in the present invention is given. DETAILED DESCRIPTION

[0043] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only 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 ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0044] Example

[0045] The present invention proposes a method for identifying and setting the fault direction of a medium and low voltage power grid suitable for access to an energy storage power station, comprising the following steps:

[0046] First, the fault type is determined based on the voltage blocking criteria. Then, based on the fault type and whether the voltage meets the low-breakthrough conditions of the energy storage power station, the fault direction criterion is determined to be activated.

[0047] Among them, the fault types include two-phase short circuit and three-phase short circuit. When the fault type is judged to be two-phase short circuit and the voltage meets the low-breakthrough conditions of the energy storage power station, the negative sequence power direction criterion is started. When the fault type is judged to be three-phase short circuit and the voltage meets the low-breakthrough conditions of the energy storage power station, the reactive current direction and active current amplitude criterion are started.

[0048] like Figure 1 and Figure 2 As shown, the two-phase short-circuit fault judgment and direction identification method includes the following steps:

[0049] Protection start-up criterion and data preprocessing: First, the phase current difference mutation start-up criterion is performed. The system detects the phase current difference mutation Δi(k) and compares it with the set threshold Δi set For comparison, if Δi(k)>Δi set , the voltage and current are sampled, and the sampled data are preprocessed. The data preprocessing first performs a 20ms delay operation. Since the full-wave Fourier algorithm time window is 20ms, and the data window within the first 20ms after the fault includes the voltage and current information before the fault, the protection is delayed for 20ms after starting. After the delay, the voltage and current are calculated using the full-wave Fourier algorithm to obtain the power frequency voltage and current values. Then, the symmetrical component method is used to extract the positive sequence component and the negative sequence component from the calculated power frequency voltage and current. After the positive and negative sequence components are extracted, the voltage blocking criterion is performed;

[0050] Fault type judgment: After the voltage lockout criterion is opened, the positive and negative sequence voltages at the protection installation are compared with the set values. When the positive and negative sequence voltages at the protection installation meet the following formula:

[0051]

[0052] The fault type is judged to be a two-phase short circuit fault, and the negative sequence power direction judgment criterion is started. Otherwise, the process ends, wherein: They are respectively the positive and negative sequence voltages at the protection installation location, They are respectively the positive and negative sequence voltage setting values;

[0053] Direction judgment: The direction judgment adopts the negative sequence power direction judgment criterion, and compares the phase difference between the negative sequence voltage and negative sequence current measured by the protection with the setting range. The positive judgment formula is as follows:

[0054]

[0055] If it meets the forward criterion formula, it is judged as a forward two-phase short-circuit fault, otherwise it is a reverse two-phase short-circuit fault, where: are the upper and lower boundaries of the action area in case of a forward fault, are the negative sequence voltage and current power frequency vectors detected by the protection, among which, The calculation formula is as follows:

[0056]

[0057]

[0058] Among them, Z down is the equivalent negative sequence impedance of the downstream of bus B, including the energy storage station, Z up is the equivalent negative-sequence impedance upstream of bus B, the subscript max represents the maximum value, min represents the minimum value, and arg represents the phase angle.

[0059] When performing constant value setting for the above two-phase interphase short-circuit fault judgment and direction identification method, since the equivalent negative-sequence impedance of the energy storage power station is artificially constructed through converter control, its impedance angle is 90° and the impedance amplitude is not constant. The size of the negative-sequence impedance is related to its installed capacity and fault conditions. Therefore, it is necessary to determine its specific change boundary. The method is as follows:

[0060] A composite sequence network is made for the two-phase short circuit. In the positive sequence network, the energy storage power station is equivalent to a current source controlled by the positive sequence voltage of the grid connection point, and in the negative sequence network, it is equivalent to a current source controlled by the negative sequence voltage of the grid connection point.

[0061] when Figure 1 When an interphase short circuit fault occurs on the system line AB shown in the figure, under the same transition resistance, the distance between the fault position and busbar B is proportional to the positive sequence voltage of busbar B and inversely proportional to the negative sequence voltage of busbar B. At this time, the energy storage power station needs to generate more positive sequence active power and absorb more negative sequence reactive power, and it is less difficult to reach the reactive current limit value, so that the negative sequence impedance of the energy storage power station is greater than the value of the negative sequence impedance amplitude in the following formula;

[0062] In addition, the change of system impedance affects the voltage of the energy storage power station grid connection point during the fault period, and then affects its negative sequence impedance. From the above analysis, it can be concluded that when the system is in the minimum operation mode, the negative sequence impedance of the energy storage power station is the largest when a metallic two-phase short circuit fault occurs at the forward export. The maximum value of the negative sequence impedance of the energy storage power station can be obtained by solving the fault network at this time by the iterative method. When the reactive current limit is not reached, the negative sequence impedance amplitude of the energy storage power station is expressed as the following formula:

[0063]

[0064] Among them, U N is the rated line voltage of the distribution network, I N is the rated current of the energy storage power station, S N is the rated capacity of the energy storage power station, K is a constant and K=2.

[0065] like Figure 1 and Figure 3 As shown, the three-phase short circuit fault judgment and direction identification method includes the following steps:

[0066] Protection start-up criteria and data preprocessing: The protection start-up criteria and data preprocessing for three-phase short-circuit faults are the same as those for two-phase short-circuit faults, and will not be described in detail.

[0067] Voltage blocking criterion: After the voltage blocking criterion is opened, the positive and negative sequence voltages at the protection installation are compared with the set values ​​to determine whether the positive and negative sequence voltages at the protection installation satisfy the following formula:

[0068]

[0069] in, They are respectively the positive and negative sequence voltages at the protection installation location, They are respectively the positive and negative sequence voltage setting values;

[0070] When the positive and negative sequence voltages at the protection installation satisfy the above formula, it is judged as a three-phase short circuit fault. When the fault type is judged to be a three-phase short circuit and the voltage drops below 0.9pu, the direction judgment based on the reactive current direction and the active current amplitude is started, otherwise, the process ends;

[0071] Direction judgment: Direction judgment adopts the direction judgment based on the reactive current direction and the active current amplitude. First, enter the reactive current direction judgment and detect the amplitude I of the reactive current flowing through the protection. p6.q Does it satisfy the following formula:

[0072]

[0073] When I p6.q After satisfying the above formula, the active current amplitude criterion is entered to determine the active current amplitude I p6.d Does it satisfy the following formula:

[0074]

[0075] If I p6.d When the above formula is satisfied, it is judged as a forward three-phase short circuit fault, otherwise, it is judged as a reverse three-phase short circuit fault;

[0076] Among them, I q.setis the reactive current amplitude setting value, I d.set1 It is the maximum active current flowing through the protection when avoiding three-phase short circuit at the end of the forward area of ​​the protection during setting, I d.set2 It is the minimum active current flowing through the protection when the three-phase short circuit occurs at 15% of the reverse area of ​​the protection during setting. p6.d | min

[0077] The above three-phase short-circuit fault judgment and direction identification method is as follows when performing active current amplitude setting:

[0078] First, determine the interval where the reactive current criterion fails. Figure 1 Under the system and parameters shown in the figure, the farthest position where the reactive current criterion fails when the line BC three-phase fault occurs is calculated by iteration method. The calculation result is about 10% of the line. Therefore, considering a certain margin, the minimum active current flowing through protection 6 when a fault occurs at 15% of the BC line is taken as |I p6.d | min , the maximum active current flowing through protection 6 when line BC is faulty and the energy storage is in charging state is taken as |I p6.d | max .

[0079] The above specific embodiments are only several optional embodiments of the present invention. Based on the technical solutions of the present invention and the relevant inspirations of the above embodiments, those skilled in the art can make various alternative improvements and combinations to the above specific embodiments.

Claims

1. A method for identifying and setting fault direction of medium and low voltage power grids connected to energy storage power stations, characterized in that: The following steps are involved: Determine the fault type based on the voltage blocking criteria; Determine whether the fault direction criterion is activated based on the fault type and whether the voltage meets the low-breakthrough conditions of the energy storage power station; The fault types include two-phase short circuit and three-phase short circuit. When the voltage lockout criterion is turned on, if the fault type is judged to be the two-phase short circuit, the negative sequence power direction criterion is started. If the fault type is judged to be the three-phase short circuit, the reactive current direction and the active current amplitude direction criterion are started.

2. According to claim 1, a method for identifying and setting the fault direction of a medium and low voltage power grid suitable for access to an energy storage power station is characterized in that: Determining the fault type according to the voltage lockout criterion includes: When the positive sequence voltage at the grid connection point of the energy storage power station drops higher than 0.9pu, the direction judgment criterion is locked and the fault type is not judged. When the positive sequence voltage at the grid connection point of the energy storage power station drops less than or equal to 0.9pu, the fault type is further judged based on the negative sequence voltage. The judgment method is as follows: When the positive and negative sequence voltages at the protection installation satisfy the formula: It is judged as a two-phase short circuit fault, and the negative sequence power direction judgment is started. Otherwise, the negative sequence power direction judgment is started and locked. They are respectively the positive and negative sequence voltages at the protection installation location, They are respectively the positive and negative sequence voltage setting values; When the voltage at the protection installation satisfies the formula: It is judged as a three-phase short circuit fault, and the direction criterion based on the reactive current direction and the active current amplitude is started, otherwise the reactive current direction criterion is locked, where: They are respectively the positive and negative sequence voltages at the protection installation location; They are respectively the positive and negative sequence voltage setting values; When setting the negative sequence voltage, take the minimum negative sequence voltage that may occur at the protection installation point when avoiding two-phase short circuit. When the positive sequence voltage is set, the maximum positive sequence voltage at the protection installation location is taken when the reactive current detected by the protection is greater than 0. The formula is as follows: in, and is the reliability coefficient.

3. According to claim 1, a method for identifying and setting fault direction of a medium and low voltage power grid suitable for access to an energy storage power station is characterized in that: After the negative sequence power direction criterion is opened, the positive criterion formula is expressed as follows: If the forward criterion formula is not met, it is judged as a reverse two-phase short circuit fault, where: are the upper and lower boundaries of the action area in case of a forward fault, They are respectively the negative sequence voltage and current power frequency vectors detected by the protection, The calculation formula is as follows: Among them, Z down is the equivalent negative sequence impedance of the downstream of bus B, including the energy storage station, Z up is the equivalent negative-sequence impedance upstream of bus B.

4. According to claim 3, a method for identifying and setting fault directions of medium and low voltage power grids suitable for access to energy storage power stations is characterized in that: After the reactive current direction criterion is opened, the reverse fault criterion formula is expressed as follows: If the above formula is met, it is judged as a reverse short circuit fault. If it is not met, the active current amplitude direction criterion is opened, where I p6.q To protect the detected reactive current, I q.set It is the setting value of reactive current amplitude.

5. According to claim 1, a method for identifying and setting fault direction of a medium and low voltage power grid suitable for access to an energy storage power station is characterized in that: The active current direction amplitude criterion formula is expressed as follows: Among them, I d.set1 It is the maximum active current flowing through the protection when avoiding three-phase short circuit at the end of the protection forward area during setting, I d.set2 It is the minimum active current flowing through the protection when the three-phase short circuit occurs at 15% of the reverse area of ​​the protection during setting. p6.d | min , the I d.set1 ,I d.set2 The calculation formula is as follows: in, is the reliability coefficient.

6. A method for identifying and setting fault directions of medium and low voltage power grids suitable for access to energy storage power stations according to claim 1, characterized in that: When the energy storage power station does not reach the reactive current limit and enters the low-through mode, the negative sequence impedance amplitude is expressed as follows: Among them, U N is the rated line voltage of the distribution network, I N is the rated current of the energy storage power station, S N is the rated capacity of the energy storage power station, and K is a constant.

7. A method for identifying and setting fault directions of medium and low voltage power grids suitable for access to energy storage power stations according to claim 1, characterized in that: When the active current amplitude criterion is set and calculated, the fault network is solved by an iterative method to obtain the maximum boundary of the reactive current criterion failing in the reverse direction fault, and the minimum active current value in the reverse direction fault is solved by increasing the boundary range by 5% as the farthest fault position when setting the active current amplitude.

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