Ground fault direction protection method based on inverter distributed power supply constant impedance angle control

Abstract

The invention discloses a ground fault direction protection method based on inverter distributed power supply constant impedance angle control in a power distribution network. The method comprises the following steps of: arranging a power distribution switch with functions of detecting output voltage and current components and calculating output equivalent impedance angle capacity at a grid-connected part of each IIDG (Independent Discovery Distribution Generator); after the grounding fault in the area occurs, voltage and current vectors output by any fault phase are collected at the grid connection point; under the constraint that a fault phase voltage leading current angle is a maximum sensitive angle of a directional element and low voltage ride through, reference quantities of a d axis and a q axis controlled by positive sequence current under a rotating coordinate are calculated in a control module by using a calculation formula obtained by theoretical analysis; and the reference quantity controls the equivalent impedance angle output by the IIDG at the fault phase to be the maximum sensitive angle of the directional element. Therefore, the IIDG is controlled to be a constant equivalent impedance angle during a fault, and the reliability of direction protection is further improved. Therefore, the internal fault can be accurately removed by the directional element. According to the invention, the working area of the direction element does not need to be re-formulated, so that the protection performance of the direction relay is effectively improved; the control target can be realized under different voltage drop degrees, and the control strategy meets the requirement of low voltage ride through.

Description

Technical Field

[0001] The present invention relates to the control of an inverter-type distributed power source and a direction element Background Art

[0002] With the increase in the complexity of the distribution network, the extensive use of power cables and power electronic devices, and the access of inverter-type distributed power sources (IIDGs), the fault current has increased sharply. The large-scale access of IIDGs will affect the operation stability of the distribution network if the IIDGs are simply disconnected during a fault. Therefore, generally, protection with a direction element is added on the side of the distributed photovoltaic power source, which can prevent the IIDG from continuously outputting short-circuit current when a fault occurs upstream of the IIDG and ensure the rapid and selective disconnection of the fault line. Traditional direction protection relays generally determine the power direction based on the phase angle between the voltage and current flowing through the direction element. The output characteristics of the IIDG are affected by the control strategy. In a control strategy with low-voltage ride-through capability, its output current is constrained by the voltage drop degree at the point of common coupling. Therefore, the existence of the IIDG makes the power flow direction after a distribution network fault have uncertain factors. For a distribution network containing an IIDG, its traditional direction protection element may malfunction or misoperate. Misoperation will cause an expansion of the power outage range and affect the power supply quality. To address the above problems, a ground fault direction protection method based on the constant impedance angle control of an inverter-type distributed power source is proposed. By modifying the control strategy of the inverter-type distributed power source considering low-voltage ride-through, its output constant impedance angle characteristic is realized, effectively solving the problem of non-operation or misoperation of the direction protection element affected by the IIDG output characteristics, and at the same time meeting the requirements of low-voltage ride-through. Summary of the Invention

[0003] The present invention aims to solve the problem that the direction determination of the existing direction protection element is affected by the non-constant equivalent impedance angle characteristic of the IIDG output, which may lead to its non-operation or misoperation, and at the same time can meet the requirements of low-voltage ride-through. For this purpose, the present invention proposes a ground fault direction protection method based on the constant impedance angle control of an inverter-type distributed power source.

[0004] This invention controls the equivalent impedance angle of the fault-phase output of the inverter-type distributed power source. By controlling the phase angle of the fault-phase output current to lag behind the output voltage phase angle by the maximum sensitive angle of the direction element, the problem of non-operation or misoperation of the direction element caused by its random output impedance angle characteristic can be effectively avoided, and the q-axis current reference quantity in the rotating coordinate in the positive-sequence current control is controlled within the range of low-voltage ride-through (the IIDG adopts a control strategy to suppress the output of negative-sequence current), thereby realizing low-voltage ride-through.

[0005] A grounding fault direction protection method based on constant impedance angle control of an inverter-type distributed power source according to an embodiment of the present invention. The method includes: equipping a distribution switch with the functions of detecting output voltage and current components and calculating the output equivalent impedance angle at the grid connection point of each IIDG. After a grounding fault occurs in the area, the voltage and current phasors output in any fault phase are collected at the grid connection point; and under the constraints that the angle of the fault phase voltage leading the current is the maximum sensitive angle of the direction element and low voltage ride-through, the reference quantities of the d-axis and q-axis for positive sequence current control in the rotating coordinate are calculated in the control module by using the calculation formula obtained through theoretical analysis. The reference quantities will control the equivalent impedance angle output by the IIDG in the fault phase to be the maximum sensitive angle of the direction element. Since the load equivalent impedance of the feeder downstream of the IIDG is much larger than the impedance from the grid-connected bus to the fault grounding point, most of the fault current flows to the fault point, so the influence of the downstream load of the IIDG on the impedance angle measured by the direction element on the power supply side of the grid-connected bus can be ignored. Thus, the internal fault can be accurately removed by the direction element.

[0006] The grounding fault direction protection method based on constant impedance angle of an inverter-type distributed power source according to an embodiment of the present invention has at least the following beneficial effects: There is no need to redefine the working area of the direction element, effectively improving the protection performance of the direction relay; the control target can be achieved under different voltage drop degrees, and the control strategy meets the requirements of low voltage ride-through.

[0007] The additional aspects and advantages of the present invention will be partially given in the following description, partially become obvious from the following description, or be understood through the practice of the present invention. Description of the Drawings

[0008] The above and / or additional aspects and advantages of the present invention will become obvious and easy to understand from the description of the embodiments in conjunction with the following drawings, where:

[0009] Figure 1 It is a schematic flow chart of the method of the embodiment of the present invention

[0010] Figure 2 It is a low voltage ride-through graph

[0011] Figure 3 It is an equivalent model of the distribution system

[0012] Figure 4 It is a vector diagram of voltage and current components

[0013] Figure 5 It is the line parameters adopted in the simulation experiment of the method of the embodiment of the present invention

[0014] Figure 6 It is the fault phase voltage and current phase angles output by the IIDG

[0015] Figure 7 The equivalent impedance angle of the IIDG output

[0016] Figure 8 Schematic diagrams of control results for different fault locations and fault types Specific implementation method

[0018] According to Figure 1 The schematic diagram of steps, the method of the embodiment of the present invention is as follows. First, a distribution switch with the functions of detecting output voltage and current components and calculating the output equivalent impedance angle needs to be equipped at the grid connection point of each IIDG. After an in-zone ground fault occurs, the output voltage and current of the IIDG are collected for control. Under the condition that the equivalent impedance angle of the fault phase output is the maximum sensitive angle of the directional element and meets the requirements of low voltage ride-through, the reference quantities of the d-axis and q-axis of the positive sequence current control are calculated. According to Figure 2 The low voltage ride-through diagram, when the voltage dip is within 10% (including 10%), there is no requirement for the output reactive current during low voltage ride-through at this time. Under the constraints of the maximum sensitive angle of the output voltage and current angle of the fault phase and the output current limit, the reference quantities of the d-axis and q-axis of the positive sequence current control are calculated; when the voltage dip is in the range of 10% to 80% (including 80%), the reference quantity of the q-axis is determined according to the requirement of the output reactive power during low voltage ride-through, and then on this basis, combined with the constraint that the output voltage and current angle is the maximum sensitive angle, the reference quantity of the d-axis is calculated; when the voltage dip is greater than 80%, according to the requirements of low voltage ride-through, more reactive current must be output to support the grid voltage, and at the same time, the output current is also limited. Therefore, the adjustment range of the reference quantities of the d-axis and q-axis becomes smaller, and it is difficult to control the output voltage and current angle. At this time, just meet the conditions of low voltage ride-through and current limit. Because the equivalent impedance of the load on the downstream feeder of the IIDG is much larger than the impedance from the grid-connected bus to the fault grounding point, most of the fault current flows to the fault point. Therefore, the influence of the downstream load of the IIDG on the impedance angle measured by the directional element on the power supply side of the grid-connected bus can be ignored. Thus, the measured power direction angle of the directional element is the maximum sensitive angle, solving the influence of the non-constant impedance characteristic of the IIDG on the directional element.

[0019] For Figure 3 Schematic diagram, when an AB-phase ground fault occurs at L1 on the distribution line, the specific implementation steps of the method of the embodiment of the present invention are as follows:

[0020] 1. First, a distribution switch CB9 with the functions of detecting output voltage and current components and calculating the output equivalent impedance angle needs to be equipped at the grid connection point of the IIDG.

[0021] 2. Figure 2 Indicates the low voltage ride-through requirements, and it outputs reactive power current i according to the degree of voltage dip q , and the formula is as follows:

[0022]

[0023] where k is the ratio of the positive-sequence voltage amplitude output by the IIDG to the rated voltage, and I N is the rated current. After a fault occurs in the zone, the fault phases A and B are selected according to the existing phase selection strategy, and phase A is taken as the analysis object. Measure the output voltage and current of the fault phase of the IIDG in CB9, and then calculate the reference positive-sequence currents of the d-axis and q-axis in the control strategy according to the Figure 4 vector diagram shown. Since the grid-side connection of the inverter grid-connected transformer is in Δ connection, the current measured by CB9 has no zero-sequence current and also has no negative-sequence current output under the control strategy of suppressing negative-sequence current output. The positive-sequence current is the output current.

[0024] When k > 0.9, because there is no requirement for the output of reactive current during low-voltage ride-through, the reference value of the q-axis of the positive-sequence current of phase A can be selected arbitrarily to meet the control requirements, and The values of and depend on the angle θ between the amplitude of the positive-sequence current of phase A and Taking the maximum sensitive angle of the direction element as 60° as an example, when the angle between the output voltage and the output current Figure 4 of phase A of the IIDG is controlled at 60°, according to it can be known that the angle θ can be determined by using the phase angle and the phase angle of the positive-sequence voltage output of phase A The difference β, and is calculated by formula (2). Since the output current is limited after the fault, so we can take

[0025] θ = 60° - β (2)

[0026] Thus, the values of the d-axis and q-axis of the positive-sequence current are

[0027]

[0028] When 0.2 ≤ k < 0.9, in order to meet the requirements of low-voltage ride-through, it must be satisfied that Calculate in this way. First calculate AB in ΔOAB, from which AC can be obtained, and then CB can be obtained. Calculate OC in ΔOAC, that is First, ∠OB, ∠COA, and ∠BOA can be obtained through the vector diagram.

[0029]

[0030] Applying the sine theorem in ΔOAB again, we can obtain

[0031]

[0032] Then we can get

[0033]

[0034] Thus, we can obtain

[0035] Similarly, OC can be calculated in ΔCOA, as shown in Equation (7)

[0036]

[0037] Therefore can be calculated, as shown in Equation (8)

[0038]

[0039] Finally, when 0.2 ≤ k < 0.9 and are

[0040]

[0041] When k < 0.2, from Figure 2 it can be seen that the voltage drops below 20%, and it will be required that the IIDG be disconnected from the distribution network. However, if it fails to be disconnected, it will be required to output reactive current Meanwhile it will be limited by the output current. Therefore and The values of are not controllable, and the control of the output equivalent impedance angle cannot be carried out. From this, it can be obtained that when k < 0.2 and are

[0042]

[0043] By formulating the above control strategy, the influence of the non-constant equivalent output impedance angle of the IIDG on the directional element is avoided.

[0044] Technical verification simulation analysis: Build a 10 kV active distribution network simulation model in matlab as Figure 3 The distribution switch CB9 with the functions of detecting the output voltage and current components and calculating the output equivalent impedance angle and the CB4 with a directional relay are installed at the connection point of the IIDG to the grid Figure 5The shown are the sequence parameters of the distribution network lines. The lengths of each line segment are L1 = 12 km, L2 = 8.1 km, L3 = 5.1 km, L4 = 8 km, L5 = 11 km, and L6 = 7.9 km respectively. Assume that the system starts the simulation time t = 0 s, and an AB-phase ground fault occurs at F1 at t = 0.25 s.

[0045] Figure 6 and Figure 7 show the voltage and current phase angles of the faulty phase A output by the IIDG after the fault occurs. It can be seen that the output phase angle is controlled at 60°; Figure 8 indicate the simulation results under different fault points and different fault types. The simulation results show that when 0.2 ≤ k < 0.9 and k > 0.9, the error between the actual impedance angle measured at CB4 and the control target is small, achieving the expected control effect and avoiding the influence on the performance of the directional element; when k < 0.2, due to the limitation of the output current, the d-axis and q-axis components of the positive-sequence current cannot be arbitrarily controlled, resulting in the failure to meet the control target. From the above, it can be seen that the adoption of the present invention improves the action reliability and sensitivity of the directional element in the active power grid.

[0046] The embodiments of the present invention have been described in detail above in conjunction with the accompanying drawings. However, the present invention is not limited to the above embodiments. Various changes can be made without departing from the spirit of the present invention within the scope of knowledge possessed by those of ordinary skill in the art.

Claims

1. A ground fault directional protection method based on constant impedance angle control of inverter-type distributed power supply, characterized in that: The following steps are involved: Step 1 First, each IIDG grid connection point needs to be equipped with a distribution switch with the function of detecting output voltage and current components and calculating the output equivalent impedance angle; Step 2 Measure the voltage, current and sequence component of any fault phase and calculate the phase difference between the positive sequence voltage and the actual voltage of the fault phase and the voltage drop degree k Step 3 After the ground fault occurs in the area, the reference quantities of the positive sequence current control d-axis and q-axis in the rotating coordinate are calculated when the equivalent impedance angle of the fault phase output is the maximum sensitivity angle of the directional element and the requirements of low voltage ride-through are met. According to the requirements of low voltage ride-through, the voltage drop is within 10% (including 10%). At this time, low voltage ride-through has no requirements for output reactive current. The reference values ​​of the d-axis and q-axis of the positive sequence current control are calculated under the constraints that the output voltage and current angle of the fault phase is the maximum sensitive angle and the output current limit; when the voltage drop is within the range of 10% to 80% (including 80%), the reference value of the q-axis is determined according to the requirements of low voltage ride-through output reactive power, and on this basis, the reference value of the d-axis is calculated in combination with the constraint that the output voltage and current angle is the maximum sensitive angle; when the voltage drop is greater than 80%, according to the requirements of low voltage ride-through, more reactive current must be output to support the grid voltage, and the output current is also limited. Therefore, the adjustment range of the d-axis and q-axis reference values ​​becomes smaller, and it is difficult to achieve the control of the output voltage and current angle. At this time, it is sufficient to meet the conditions of low voltage ride-through and current limit.

2. A ground fault directional protection method based on constant impedance angle control of inverter-type distributed power supply according to claim 1, characterized in that: It is necessary to equip each IIDG with a distribution switch capable of detecting output voltage and current components and calculating the output equivalent impedance angle at the grid connection point to calculate the phase difference between the positive sequence voltage and the actual voltage of the fault phase at the IIDG grid connection point during a fault, as well as the voltage drop degree k.

3. The ground fault directional protection method based on constant impedance angle control of inverter-type distributed power supply according to claim 1, characterized in that: There is no need to redefine the working area of the directional element. Only the control strategy of the IIDG needs to be modified. At the same time, the requirements for low voltage ride through of the IIDG can also be met. The methods to meet the requirements for low voltage ride through include: for different degrees of voltage dips, different reactive current outputs are determined. When k > 0.9, there is no requirement for reactive current output during low voltage ride through, and the reactive current output can be controlled under the constraint that the phase angle of the voltage and current in the fault phase is the maximum sensitive angle. When 0.2 < k < 0.9, at this time, the reference value of the reactive current needs to satisfy Under this constraint, the reference value of the active current output is determined by the phase angle of the voltage and current in the fault phase being the maximum sensitive angle. When k < 0.2, it is required that the IIDG be disconnected from the distribution network. However, if it fails to be disconnected, it is required to output the reference value of the reactive current At the same time, the reference value of the active current output will be limited by the output current, so that the output current cannot be controlled. At this time, the conditions for low voltage ride through and current limitation are satisfied. By implementing the above control strategy, the influence of the IIDG output on the directional element can be avoided, and its operation reliability can be improved.

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