CN111969569A - Micro-grid fault protection method based on improved current phase difference - Google Patents

Abstract

The invention discloses a microgrid fault protection method based on improved current phase difference. The method utilizes the large drop of IIDG output terminal voltage and the current phase difference at both ends of the bus bar as protection criteria when the microgrid fault occurs, and overcomes the problem of microgrid bidirectional power flow and The fault current of the independent operating microgrid is small, which makes the traditional three-stage current protection may cause the problem of protection refusal due to the uncertainty of the power flow direction and the insignificant change of the current modulus value. The fault line can be accurately identified and removed, avoiding the problem that the microgrid needs to use two sets of protection devices in two operating states, reducing the protection cost, and improving the reliability and sensitivity of the microgrid protection. The protection method does not rely on the current modulus value as the starting criterion, and overcomes the problem that the fault current of the IIDG-containing microgrid is small, thus causing the protection to fail.

Description

A fault protection method for microgrid based on improved current phase difference

technical field

The invention relates to the technical field of microgrid fault determination and fault protection, and more particularly to a microgrid fault protection method based on improved current phase difference.

Background technique

At present, distributed power sources based on renewable energy such as photovoltaics and wind power generation, through the microgrid organically composed of energy storage devices, power electronic converters, loads, monitoring and protection devices, etc., can make the most effective use of renewable energy. Renewable energy is used more and more in line with the needs of energy saving and environmental protection.

Since the microgrid can either be connected to the distribution network for grid-connected operation, or disconnected from the distribution network to achieve island operation (ie, independent operation), the two operating modes have different fault characteristics and control strategies, and often Two sets of protection devices are required, and the maintenance cost is high. At the same time, the existing low-voltage distribution network fault protection method generally adopts a three-stage current protection scheme. However, due to the bidirectional power flow of the microgrid and the small fault current of independent operation, the traditional three-stage current protection is due to the uncertainty of the power flow direction and the current mode. The value does not change significantly, and the protection often refuses to move, and the problem of protection failure occurs.

Therefore, how to provide a stable, reliable, high-sensitivity, and lower-cost microgrid fault protection method is an urgent problem to be solved by those skilled in the art.

SUMMARY OF THE INVENTION

In view of this, the present invention provides a microgrid fault protection method based on improved current phase difference, the method is suitable for the microgrid containing IIDG, when the microgrid is faulty, the voltage of the output terminal of the IIDG will drop significantly and the current phase difference between the two ends of the busbar will be greatly reduced. As a protection criterion, it solves the problems of high maintenance cost and easy protection failure of the existing three-stage current protection scheme.

In order to achieve the above object, the present invention adopts the following technical solutions:

A microgrid fault protection method based on improved current phase difference, the method comprising:

Detect the current on both sides of the faulty line, and judge whether there is current on both sides;

When it is detected that there is current on both sides of the faulty line, it is detected whether the output voltage of each inverter-type distributed power supply supplying power to each line segment is reduced and whether the voltage reduction value exceeds a preset voltage drop threshold;

When it is detected that the output voltage of at least one inverter-type distributed power source decreases and the voltage reduction value exceeds a preset voltage drop threshold, determining the corresponding suspected faulty line segment according to the detected position of the inverter-type distributed power source;

Detecting the current phase difference of the busbars at both ends of the suspected faulty line section, and judging whether the current phase difference is within the preset operating current difference threshold range;

When the current phase difference is within the operating current difference threshold range, determine that the suspected faulty line section is a faulty line section, and remove the faulty line section;

When it is detected that the faulty line only has current on one side, the faulty line segment is removed through the three-stage overcurrent protection.

Considering that in the existing microgrid, the distributed power supply is mainly Inverter-Interfaced Distributed Generator (IIDG, Inverter-Interfaced Distributed Generator), and its fault current is limited within 2 times of the rated current. Based on the output voltage of the inverter-type distributed power supply and the current phase difference of the busbars at both ends of the line segment, the invention performs fault protection for the line with current on both sides, and adopts the traditional three-stage type for the line with current on one side. The overcurrent protection scheme is used for fault protection, thereby ensuring the accuracy and reliability of the protection method.

Further, after detecting the current phase difference of the busbars at both ends of the suspected faulty line segment, the method may further include:

It is judged whether the current phase difference is within a preset holding current difference threshold range, and when the current phase difference is within the holding current difference threshold range, it is judged that the suspected fault line segment is a non-fault line segment.

Since there is a relatively reasonable current difference threshold range when the line is in normal operation and out-of-area fault, in order to further improve the reliability of detection, the obtained current phase difference and the holding current difference threshold range and the operating current difference threshold range can be obtained respectively. For comparison, when the threshold range of the holding current difference is met, it means that the line section is running normally and no protection action is required. When the threshold range of the operating current difference is met, it means that there is a fault in the line section, and timely action is required to protect the safety of the microgrid.

Further, the calculation formula of the current phase difference is:

表示电流相位差，

和

分别为疑似故障线路段两端母线的电流值。In the formula,

represents the current phase difference,

and

are the current values of the busbars at both ends of the suspected fault line segment, respectively.

It is not difficult to find that when it is detected that there is current on both sides of the faulty line, the protection criterion adopted by the present invention can be simplified as follows:

Protection criterion 1: When a fault occurs, the voltage of the IIDG output terminal will drop significantly and exceed the preset value V _set (set to 0.2 times the normal voltage for single-phase faults, and set to 0.5 times the normal voltage for three-phase or two-phase faults , which can be adjusted according to the actual situation).

(L表示线路长度，单位km)，故电流相位差判据为：Protection criterion 2: During normal operation and out-of-area faults, the current phase difference at both ends of the busbar is 180°, and in the case of an intra-area fault, the current phase difference at both ends of the busbar is 0°. Considering the actual engineering error, in order to improve the reliability and sensitivity of the protection, the selected protection blocking angle is

(L represents the line length, in km), so the current phase difference criterion is:

表示正常运行和区外故障时母线两端电流相位差，

为区内故障时母线两端电流相位差。In the above formula,

Indicates the current phase difference at both ends of the bus during normal operation and out-of-area faults,

It is the current phase difference between the two ends of the busbar when the fault occurs in the area.

It can be seen from the above technical solutions that, compared with the prior art, the present invention provides a microgrid fault protection method based on improved current phase difference. The current phase difference is used as the protection criterion, which overcomes the fact that the two-way power flow of the microgrid and the small fault current of the independent operation of the microgrid make the traditional three-stage current protection may cause the protection to refuse to operate due to the uncertainty of the power flow direction and the insignificant change of the current modulus value. At the same time, this method can accurately identify and remove faulty lines in both operating states of the microgrid, avoiding the problem that two sets of protection devices need to be used in the two operating states of the microgrid, reducing the protection cost and improving the microgrid. Reliability and sensitivity of protection. The protection method does not rely on the current modulus value as the starting criterion, and overcomes the problem that the fault current of the IIDG-containing microgrid is small, thus causing the protection to fail.

Description of drawings

In order to explain the embodiments of the present invention or the technical solutions in the prior art more clearly, the following briefly introduces the accompanying drawings that need to be used in the description of the embodiments or the prior art. Obviously, the accompanying drawings in the following description are only It is an embodiment of the present invention. For those of ordinary skill in the art, other drawings can also be obtained according to the provided drawings without creative work.

1 is a schematic flowchart of a microgrid fault protection method based on improved current phase difference provided by the present invention;

2 is a schematic structural diagram of a microgrid in an embodiment of the present invention;

3 is a schematic diagram of an equivalent circuit of a microgrid grid-connected operation fault in an embodiment of the present invention;

FIG. 4 is a schematic diagram of an equivalent circuit of a microgrid independent operation fault in an embodiment of the present invention;

5 is a schematic diagram of a simulation waveform of the current flowing through the circuit breaker QF1 when the point f1 is faulty during the grid-connected operation of the microgrid in the embodiment of the present invention;

6 is a schematic diagram of a simulation waveform of the current flowing through the circuit breaker QF1 when the microgrid operates independently in the embodiment of the present invention when the point f1 fails;

7 is a schematic diagram of the voltage simulation waveform of the IIDG output terminal when the f1 point is faulty when the microgrid is connected to the grid according to the embodiment of the present invention;

8 is a schematic diagram of the voltage simulation waveform of the IIDG output terminal when the microgrid operates independently in the embodiment of the present invention when the f1 point fails;

FIG. 9 is a schematic diagram of the change of the current phase difference between the two ends of the fault busbars M and N at point f1 when the microgrid is connected to the grid in the embodiment of the present invention;

10 is a schematic diagram of the change of the current phase difference between the two ends of the fault busbars M and N at point f2 when the microgrid is connected to the grid in the embodiment of the present invention;

11 is a schematic diagram of the change of the current phase difference between the two ends of the fault busbars M and N at point f3 during the grid-connected operation of the microgrid in the embodiment of the present invention;

Fig. 12 is a schematic diagram of the variation of the current phase difference between the two ends of the fault busbars M and N at point f1 when the microgrid operates independently in the embodiment of the present invention;

FIG. 13 is a schematic diagram of the variation of the current phase difference between the two ends of the fault busbars M and N at point f2 when the microgrid operates independently in the embodiment of the present invention;

FIG. 14 is a schematic diagram of the variation of the current phase difference between the two ends of the fault busbars M and N at point f3 when the microgrid operates independently in the embodiment of the present invention.

Detailed ways

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

Referring to FIG. 1, an embodiment of the present invention discloses a microgrid fault protection method based on improved current phase difference, the method comprising:

S1: Detect the current on both sides of the faulty line;

S2: Determine whether there is current on both sides;

S3: When it is detected that there is current on both sides of the faulty line, it is detected whether the output voltage of each inverter-type distributed power source that supplies power to each line segment is reduced and whether the voltage reduction value exceeds the preset voltage drop threshold (that is, the protection criterion). 1);

S4: when it is detected that the output voltage of at least one inverter-type distributed power source decreases and the voltage reduction value exceeds a preset voltage drop threshold, determine the corresponding suspected fault line segment according to the detected position of the inverter-type distributed power source;

S5: Detect the current phase difference of the busbars at both ends of the suspected faulty line segment, and determine whether the current phase difference is within the preset operating current difference threshold range (that is, the second formula in the current phase difference criterion);

S6: when the current phase difference is within the operating current difference threshold range, determine that the suspected faulty line section is a faulty line section, and remove the faulty line section;

S7: When it is detected that the faulty line only has current on one side, the faulty line segment is removed through the three-stage overcurrent protection.

After detecting the current phase difference of the busbars at both ends of the suspected faulty line segment, it can also include:

S8: Determine whether the current phase difference is within a preset holding current difference threshold range, and when the current phase difference is within the holding current difference threshold range, determine that the suspected faulty line segment is a non-faulty line segment ( That is, the first formula in the current phase difference criterion).

The implementation process of the above protection method is described in detail below in conjunction with a specific microgrid structure:

As shown in Figure 2, the microgrid is connected to the distribution network through the common connection point PCC through the transformer T boosting; M, N, P, and Y are the bus bars, L1, L2, and L3 are the lines, and QF1, QF2, QF3, and QF4 , QF5, QF6 are circuit breakers (that is, protection devices), _Im , In are the fault current flowing through the circuit breaker QF1, the fault current flowing through the circuit breaker QF2, f1, f2, f3 are the fault points, Load1, _Load2 is load, IIDG1, IIDG2, IIDG3 are inverter distributed power sources, _Ig1 , _Ig2 , _Ig3 are IIDG1 output fault current, IIDG2 output fault current, IIDG3 output fault current respectively.

In this embodiment, the inverter-type distributed power supply adopts PQ control when the microgrid is connected to the grid; when the microgrid operates independently, the microgrid adopts master-slave control, wherein IIDG1 and IIDG2 controlled by PQ are slave power sources, and IIDG3 is controlled by PQ and switched to V/ f control as the main control power supply. The microgrid can be run in parallel with the distribution network, or it can be disconnected from the distribution network to achieve island operation (independent operation).

The following is an analysis of the fault current modulus characteristics:

Assuming that a three-phase short-circuit fault occurs at point f1 in Fig. 2, the equivalent circuits of grid-connected operation and independent operation are shown in Fig. 3 and Fig. 4, respectively. When the fault occurs, the PQ-controlled inverter-type distributed power supply is equivalent to being controlled by the grid-connected point voltage. The V/f control inverter distributed power source is equivalent to a voltage source controlled by the output current of the grid connection point. By disconnecting the common connection point PCC, the microgrid operates independently. In order to improve the fault ride-through of the independent operation microgrid The output fault current of the V/f control inverter distributed power supply is 3-5 times of the rated current.

Among them, ES is the system power supply, I _g1 , I _g2 , and I _g3 are the output fault current of _IIDG1 , the output fault current of IIDG2, and the output fault current of IIDG3 respectively, I _m is the fault current flowing through the circuit breaker QF1, and I _n is the fault current flowing through the circuit breaker QF1. The fault current of the circuit breaker QF2, U _g is the equivalent voltage of the V/f control inverter distributed power source IIDG3; Z _S , Z ₂ , Z ₃ , Z _M1 , Z _N1 are the system equivalent impedance, the line L2 Line impedance, line impedance of line L3, line impedance from bus M to fault point f1, and line impedance from bus N to fault point f1.

It can be seen from Figures 3 and 4 that the fault current flowing through the circuit breaker QF1 when the microgrid is connected to the grid is jointly provided by the grid and the inverter distributed power generation IIDG3; when the microgrid is running independently, the fault current flowing through the circuit breaker QF1 is only provided by Inverter distributed power supply IIDG3 is provided. At the same time, due to the current limitation of the power electronic converter, the fault current flowing through the circuit breaker QF1 during independent operation will be smaller than the fault current flowing through the circuit breaker QF1 during grid-connected operation. Therefore, it is difficult to achieve effective fault protection using the same protection scheme that only depends on the current modulus value in both operating states.

The characteristics of the current phase difference are analyzed as follows:

When the microgrid is in normal operation, the direction of the power flow is bidirectional. It can be seen from Figure 1 that it can be divided into two cases: Case 1 is that the direction of power flow flows from bus M to bus N, and case 2 is that the direction of power flow is from bus N to bus M. It is stipulated that when the current flows from the bus to the line, the current is in the positive direction, and when it flows from the line to the bus, the current is in the negative direction.

与

大小相等方向相反，相位差为180°,即：It is easy to see from Figure 1 that no matter what the direction of the power flow is when the microgrid is in normal operation, the currents at both ends of the busbars M and N are

and

The size is equal and the direction is opposite, and the phase difference is 180°, that is:

When the microgrid fails, it can be divided into the following two situations:

(1) Fault in line MN area

与

大小不定，但方向相同都是由母线流向线路，理想情况下相位相同，相位差为0°,即：When the f1 point in the line MN fails, no matter what state the microgrid is running in and the direction of the power flow, the current at both ends of the bus M and N

and

The size is indeterminate, but the direction is the same, and the flow is from the bus to the line. Ideally, the phase is the same, and the phase difference is 0°, that is:

(2) Fault outside the line MN area

与

大小相等，但方向相反，一端由母线流向线路，另一端由线路流向母线，相位差为180°,计算公式与微电网正常运行时电流相位差公式相同。When the point f2 or f3 fails, the current will flow to the fault point, no matter what state the microgrid is running in and the direction of the power flow during normal operation, the current at both ends of the bus M and N

and

The size is equal, but the direction is opposite, one end flows from the bus to the line, and the other end flows from the line to the bus, the phase difference is 180°, and the calculation formula is the same as the current phase difference formula when the microgrid is operating normally.

According to the phase difference changes of the currents at both ends of the busbars M and N when the above-mentioned microgrid is in normal operation and faults occur at different positions, the analysis conclusions are shown in Table 1 below:

Table 1 Change of current phase difference at both ends of bus M and N

It can be seen from Table 1 that only when the fault occurs in the MN area, the phase difference between the currents at both ends of the bus M and N is 0°. In normal operation and faults outside the MN area, the phase difference between the currents at both ends of the bus M and N is 180°.

The following is an analysis of the characteristics of the output terminal voltage of the inverter-type distributed power supply under the two operating states of microgrid grid-connected operation and independent operation:

(1) Grid-connected operation

When the microgrid is connected to the grid, no matter which point f1, f2, and f3 in Figure 1 fails, due to the lack of support from the large power grid after the fault and the limited capacity of the inverter distributed power supply, the inverter distributed distributed power supply on the faulty line side. The output voltage of the power supply will drop significantly, namely:

分别为故障后以及故障前故障线路侧逆变型分布式电源输出端的电压。in,

are the voltages at the output terminals of the inverter distributed power supply on the line side of the fault after the fault and before the fault, respectively.

(2) Independent operation

When the independently running microgrid fails, due to the current limiting effect of the power electronic converter in the inverter distributed power supply, the voltage at the output end of the inverter distributed power supply will drop significantly, which is different from the grid-connected operation. The situation is the same.

Therefore, when the microgrid fails, the output voltage of the inverter-type distributed power supply on the line side of the grid-connected operation fault will drop sharply, and the output terminal voltage of the inverter-type distributed power supply on the same feeder or adjacent feeder will drop sharply during independent operation. . Therefore, the suspected fault line can be selected by monitoring the voltage drop of the output terminal of the inverter distributed power supply, that is, the low-voltage protection can be used as the starting protection.

The following is a specific example to verify the authenticity of the characteristic analysis results of the current phase difference and the characteristic analysis results of the output voltage of the inverter distributed power supply:

S_Ld2＝0.25MV·A，

将线路的中点设置为故障点，系统稳定运行后的0.4s时发生三相金属性短路故障,故障时间为0.1s，f1点故障时流过保护QF1的并网运行和独立运行电流仿真波形图分别如图5和图6所示。According to Figure 1 to build a 10kV micro-grid system, the rated capacities of inverter distributed power sources IIDG1, IIDG2, and IIDG3 are 0.3WM, 0.6WM, and 0.9WM, respectively. Short, the capacitance to ground can be ignored, the transmission line uses an equivalent PI circuit, the line length L1=3km, L2=L3=1km, the positive sequence impedance per unit length Z ₁ =(0.6+j0.86)Ω/km, Z ₁ =Z ₂ , Z ₀ =1.5Z ₁ (wherein Z ₁ , Z ₂ , Z ₀ represent the positive-sequence, negative-sequence, and zero-sequence impedances of unit length, respectively, the parameters set during simulation are multiplied by the length of the corresponding line, The impedance of the corresponding line can be obtained); the end loads are: S _Ld1 = 0.6MV·A,

S _Ld2 =0.25MV·A,

The midpoint of the line is set as the fault point, the three-phase metallic short-circuit fault occurs 0.4s after the system runs stably, the fault time is 0.1s, and the simulation waveform of grid-connected operation and independent operation current flowing through the protection QF1 at point f1 Figures are shown in Figure 5 and Figure 6, respectively.

It can be seen from Figures 5 and 6 that the fault current flowing through the circuit breaker QF1 when the microgrid operates independently is smaller than the fault current flowing through the circuit breaker QF1 during grid-connected operation, which is consistent with the characteristic analysis results of the current phase difference.

When the microgrid is connected to the grid and runs independently, when the f1 point fails, the voltage simulation waveforms of the output terminal of the inverter-type distributed power supply are shown in Figure 7 and Figure 8. It can be seen from Figure 7 and Figure 8 that the output voltage of the inverter distributed power supply will drop significantly when the microgrid fails, which is consistent with the characteristic analysis results of the output voltage of the inverter distributed power supply.

Based on the above analysis and verification results, the protection criteria of the microgrid can be obtained as follows:

Protection criterion 1: When a fault occurs, the output terminal voltage of the inverter distributed power supply will drop significantly and exceed the preset value V _set (set to 0.2 times the normal voltage for single-phase faults, and set to three-phase or two-phase faults. 0.5 times the normal voltage, which can be adjusted according to the actual situation).

(L表示线路长度，单位km)，故电流相位差判据为：Protection criterion 2: During normal operation and out-of-area faults, the phase difference between the currents at both ends of bus M and N is 180°, and in case of intra-area faults, it is 0°. Considering the actual engineering error, in order to improve the reliability and sensitivity of the protection, the selected protection blocking angle is

(L represents the line length, in km), so the current phase difference criterion is:

和

为母线M、N两端电流

与

的相位差，具体地，

表示正常运行和区外故障时母线两端电流相位差，

为区内故障时母线两端电流相位差。

and

is the current at both ends of bus M and N

and

The phase difference of , specifically,

Indicates the current phase difference at both ends of the bus during normal operation and out-of-area faults,

It is the current phase difference between the two ends of the busbar when the fault occurs in the area.

Regardless of whether the microgrid is connected to the grid or operates independently, the output voltage of the inverter distributed power supply on the faulty line side will drop sharply during a fault, which effectively solves the problem of the short-circuit current of the inverter distributed power supply after the fault due to the current limiting of the power electronic devices. The problem that the change is not obvious can be used as the starting criterion for protection; the current phase difference does not depend on the modulus value of the fault current, which avoids the problem of small fault current when the microgrid operates independently, and can correctly distinguish the internal and external faults.

When the microgrid operates normally or fails, there will be a single power supply. Then, only one-sided current may flow on the line during the fault, making the above protection scheme in this embodiment invalid. In order to fully protect the microgrid, when the line is detected When there is only one-sided current, the overcurrent protection can be used to cut off the faulty line.

In conjunction with accompanying drawing 1, the whole protection scheme process flow is briefly described as follows:

时，则判定该线路为故障线路，输出跳闸信号给断路器，切除故障线路；(1) Check whether there is current on both sides of the line. If there is current on both sides, execute the above protection criterion 1. When the protection criterion 1 is satisfied, select the suspected fault line (when a fault occurs, the fault line side The voltage at the output terminal of IIDG will drop significantly, so the suspected fault line can be selected by monitoring the voltage drop at the output terminal of the IIDG), and the current phase difference detection element (current transformer beside the circuit breaker) is started at the same time. If the current phase difference criterion is satisfied middle

When the fault occurs, it is determined that the line is a fault line, and a trip signal is output to the circuit breaker to remove the fault line;

(2) If only one side has current, cut off the fault line through overcurrent protection. The overcurrent protection is the traditional three-stage current protection. When a fault occurs, the fault current will increase, and the fault line will be cut off through the change of the current amplitude.

利用PSCAD/EMTDC仿真软件分别对A相发生接地短路，B、C两相短路，B、C两相接地短路以及A、B、C三相短路4种故障类型进行仿真，以验证基于电流相位差的微电网保护方法的有效性和可靠性。在电力系统中常发生单相接地短路故障，本实施例仅以A相发生金属性接地短路为例进行仿真与分析，微电网并网运行f1点故障、f2点故障、f3点故障时母线M、N两端的电流相位差变化分别如图9、图10和图11所示，微电网独立运行f1点故障、f2点故障、f3点故障时母线M、N两端的电流相位差变化分别如图12、图13和图14所示。The above protection method is simulated below. The parameter settings in the simulation process are the same as those in the above example of verifying the authenticity of the analysis results. It is assumed that f1 in Figure 1 is the fault point in the interval MN, and f2 is the same feeder outside the interval MN. The fault point, f3 is the fault point of the adjacent feeder outside the interval MN, the fault point is set as the midpoint of the line, the fault occurs 0.4s after the system runs stably, the duration is 0.1s, L1=3km, so the blocking angle

Use PSCAD/EMTDC simulation software to simulate four types of fault types: A-phase short-circuit, B and C two-phase short-circuit, B and C two-phase ground short-circuit, and A, B, C three-phase short-circuit to verify the current phase Effectiveness and reliability of poor microgrid protection methods. Single-phase-to-ground short-circuit faults often occur in the power system. In this embodiment, only the metal-to-ground short-circuit of phase A is used as an example for simulation and analysis. When the microgrid is connected to the grid, the busbar M, The changes of the current phase difference at both ends of N are shown in Figure 9, Figure 10 and Figure 11, respectively. The changes of the current phase difference at both ends of the bus M and N when the microgrid operates independently at point f1, point f2, and point f3 are shown in Figure 12. , Figure 13 and Figure 14.

The specific data involved in the above current phase difference change are shown in Table 2:

Table 2 Microgrid single-phase ground fault simulation results

时，微电网母线M、N两端的保护才动作。It can be seen from Table 2 that no matter whether the microgrid is connected to the grid or operates independently, only when the current phase difference between the busbars M and N meets the

The protection at both ends of the busbars M and N of the microgrid will only act.

The various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments, and the same and similar parts between the various embodiments can be referred to each other. For the device disclosed in the embodiment, since it corresponds to the method disclosed in the embodiment, the description is relatively simple, and the relevant part can be referred to the description of the method.

The above description of the disclosed embodiments enables any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims (5)

1. A microgrid fault protection method based on improved current phase difference is characterized by comprising the following steps:

detecting currents on two sides of a fault line, and judging whether the currents exist on the two sides of the fault line;

when the current is detected on both sides of a fault line, detecting whether the output voltage of each inverter type distributed power supply supplying power for each line section is reduced and whether the voltage reduction value exceeds a preset voltage drop threshold value;

when the output voltage of at least one inverter type distributed power supply is detected to be reduced and the voltage reduction value exceeds a preset voltage drop threshold value, determining a corresponding suspected fault line section according to the detected position of the inverter type distributed power supply;

detecting the current phase difference of buses at two ends of a suspected fault line section, and judging whether the current phase difference is within a preset action current phase difference threshold range;

when the current phase difference is within the range of the action current phase difference threshold, judging that the suspected fault line section is a fault line section, and cutting off the fault line section;

and when the fault line is detected to have current on only one side, the fault line section is cut off through three-section type overcurrent protection.

2. The microgrid fault protection method based on an improved current phase difference as claimed in claim 1, characterized in that after detecting the current phase difference of the buses at the two ends of the suspected fault line section, the method further comprises:

and judging whether the current phase difference is within a preset holding current phase difference threshold range, and when the current phase difference is within the holding current phase difference threshold range, judging that the suspected fault line section is a non-fault line section.

3. The microgrid fault protection method based on an improved current phase difference as claimed in claim 1 or 2, characterized in that the calculation formula of the current phase difference is as follows:

in the formula,

the phase difference of the currents is represented,

and

and the current values of the buses at the two ends of the suspected fault line section are respectively.

4. The microgrid fault protection method based on improved current phase difference as claimed in claim 2, characterized in that the holding current phase difference threshold range is

To

Wherein the protection locking angle

The calculation formula of (2) is as follows:

in the formula, L represents a line length in km.

5. The microgrid fault protection method based on improved current phase difference as claimed in claim 1, characterized in that the action current phase difference threshold range is 0 ° to

Wherein the protection locking angle

The calculation formula of (2) is as follows:

in the formula, L represents a line length in km.

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