CN106655121B - A kind of micro-capacitance sensor bus Low ESR adaptive guard method - Google Patents

Abstract

The invention provides a low-impedance self-adaptive protection method for a busbar of a microgrid, comprising the following steps: monitoring the busbars at both ends of the line to measure the impedance modulo value and phase angle; starting the voltage sudden change, calculating the additional impedance angle caused by the transition resistance; calculating and measuring the impedance modulo value adaptive coefficient, modify the bus measured impedance modulus value; judge whether the corrected bus measured impedance modulus value meets the impedance modulus value action criterion, and judge whether the bus measured impedance phase angle meets the phase angle action criterion; if the two criteria are at the same time If it is satisfied, it is judged as a fault in the area, and the protection acts. The invention corrects the influence of the additional impedance caused by the transition resistance on the measured impedance modulus value of the busbar, so that the low-impedance protection of the microgrid busbar has better anti-transition resistance characteristics and improves the reliability of the protection.

Description

A low-impedance adaptive protection method for microgrid busbars

technical field

The invention relates to a power system relay protection technology, in particular to a low-impedance self-adaptive protection method for a micro-grid bus.

Background technique

Microgrid is a regional small power network composed of distributed generation (DG) and loads, and is currently the most effective way to utilize distributed generation. The micro-power supply is usually an inverter-type distributed power supply (IBDG) connected to the grid using the inverter interface. When a fault occurs inside the micro-grid, in order to protect the power electronic devices from damage, the current limiting module of the inverter usually integrates the IBDG. The short-circuit current provided is limited to 2 times the rated current; at the same time, the IBDG grid connection position is flexible, and the power flow in the grid will flow in both directions when the grid is connected. These characteristics make it difficult for the current protection in the traditional distribution network to be directly applied to the microgrid. Therefore, a new protection method must be sought to ensure the safe and stable operation of the microgrid.

In view of the above-mentioned unique fault characteristics of the microgrid, simply using the local electrical quantity information such as voltage or current cannot meet the requirements of the protection reliability of the microgrid. To this end, some scholars have proposed a partial communication-based microgrid protection scheme--microgrid protection based on low bus impedance (including impedance modulus criterion and phase angle criterion). Using the drop of the bus voltage at both ends and the change of the direction of the power flow after the fault, the modulus value and phase angle of the bus impedance at both ends are calculated, and the fault interval is determined by comparing with the modulus value and phase angle of the bus impedance at both ends before the fault. This protection method uses the ratio information of the bus voltage and line current before and after the fault to form a protection criterion, which overcomes the limitation of the inverter current limiting module on the short-circuit current size and the influence of the two-way power flow in the network. At the same time, the communication between the two ends only exchanges Boolean information, lower communication requirements and higher protection reliability. However, after analysis and verification, when a grounding fault occurs in the area through a transition resistance, the larger transition resistance will weaken the electrical characteristics of the fault, and affect the reliable action of the impedance bus measurement impedance modulus criterion, making the bus measurement impedance during the fault. The modulus value is too large, causing the protection to refuse to move.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a low-impedance self-adaptive protection method for the busbar of the microgrid, which solves the problem that the busbar low-impedance protection refuses to operate due to the large transitional resistance when a grounding fault occurs in the microgrid.

A low-impedance self-adaptive protection method for a microgrid bus is characterized by comprising the following steps:

Monitor the busbars at both ends of the line to measure the impedance modulus and phase angle;

The voltage sudden change is started, and the additional impedance angle caused by the transition resistance is calculated;

Calculate the adaptive coefficient of the measured impedance modulo value, and correct the busbar measured impedance modulo value;

Judging whether the corrected bus measured impedance modulus value meets the impedance modulus value action criterion, and judges whether the bus measured impedance phase angle meets the phase angle action criterion;

If the two criteria are satisfied at the same time, it is judged as a fault in the area, and the protection acts.

The invention proposes a low-impedance self-adaptive protection method for a busbar of a microgrid. The method obtains the busbar measurement impedance based on the ratio of the busbar measurement voltage to the line current. Through the symmetrical component method, combined with the boundary conditions under different fault types, the relationship between the short-circuit grounding current and the short-circuit grounding negative-sequence current phase angle is derived, and then the negative-sequence equivalent network of the system is used to obtain the protection installation. The relationship between the phase angle of the grounding negative sequence current, so as to calculate the additional impedance angle and obtain the adaptive coefficient of the bus measured impedance modulus value, correct the influence of the additional impedance caused by the transition resistance on the bus measured impedance modulus value, and make the microgrid bus low impedance. The protection has better anti-transition resistance characteristics, which improves the reliability of the protection.

The present invention will be further described below in conjunction with the accompanying drawings.

Description of drawings

Figure 1 is a circuit model diagram of a medium-voltage microgrid.

Figure 2 is the m-side measured impedance vector diagram.

Figure 3 is a negative sequence network diagram of the system.

Fig. 4 is the m-side measured impedance modulus graph without adaptive correction.

Figure 5 is an adaptively corrected m-side measured impedance modulo diagram.

FIG. 6 is a schematic diagram of the measured impedance angles on the m and n sides during an intra-area fault.

Figure 7 is a flow chart of the low impedance adaptive protection of the microgrid bus.

Detailed ways

A low-impedance adaptive protection method for a microgrid bus includes the following steps:

The first step is to calculate the measured impedances of the m and n sides of the busbars at both ends of the line in the microgrid. Taking the ground fault of a line in the microgrid as an example, the m-side measurement impedance is calculated. The m-side measurement impedance expression is shown in formula (1):

where: Indicates the bus phase voltage on the m side, Indicates the phase current flowing on the m side of the line. It can be seen from the above formula that as long as the protection measurement and control device is used to measure the busbar voltage on the m side with m-side line current The measured impedance Z _m can be calculated, so Z _m is a quantity that can be directly obtained, then it is not difficult to find the modulus value |Z _m | and the phase angle of the measured impedance of the bus In the same way, the measured impedance of the busbar on the n-side can also be directly obtained by imitating the method of measuring the impedance of the busbar on the m-side.

The second step is to solve the adaptive coefficient of the measured impedance modulus of the bus during the fault. Taking the solution of the m-side measurement impedance adaptation coefficient as an example, the n-side can be derived similarly. Bus measurement voltage It can be represented by formula (2):

where: Indicates the line fault point-to-ground voltage; Represents the zero-sequence current flowing through the m-side of the line; Z ₁ represents the line positive sequence impedance from the fault point to the m-side protection installation; Z ₀ represents the line zero-sequence impedance from the fault point to the m-side protection installation. Then the specific expression of Z _m is shown in formula (3):

In the formula: Z _t is the measurement impedance of the m side when the metallic ground fault occurs, and ΔZ is the additional measurement impedance caused by the transition resistance; is the grounding current at the fault point; R _g is the transition resistance. It can be seen from formula (3) that the transition resistance R _g will cause an additional impedance to be generated in the bus measurement impedance, resulting in an increase in the impedance modulus value. Equation (4) can be derived from the impedance vector relationship of Equation (3):

where: is the phase angle of the measured impedance on the m side; is the phase angle of the additional impedance (referred to as the additional impedance angle); The impedance angle is measured for the m-side at metallic faults. From equation (4), it can be concluded that the adaptive coefficient k _zm of the m-side busbar measurement impedance modulus value is shown in equation (5):

because can be directly obtained through the protection measurement and control device, therefore, as long as the additional impedance angle when the fault is correctly estimated The influence of the transition resistance on the criterion of the impedance modulo value of the busbar measurement can be compensated by the adaptive coefficient k _zm .

The third step is to calculate the additional impedance angle for different fault types From the additional impedance ΔZ expression in formula (3), it can be known that The solution formula is shown in formula (6):

In actual operation, short-circuit ground current Therefore, the core idea of the patent of the present invention is to deduce the relationship between the short-circuit grounding current and the short-circuit grounding negative sequence current phase angle according to different fault types, combined with the boundary conditions of the fault and the symmetrical component method, and then use the system The relationship between the negative sequence current at the protection installation and the phase angle of the short-circuit grounding negative sequence current is derived, and the additional impedance angle is calculated. Taking the calculation of the additional impedance angle of the measured impedance on the m side as an example, the additional impedance angle of the measured impedance on the n side can be calculated in the same way:

When a single-phase ground fault occurs, The calculation formula is shown in formula (7):

In the event of a two-phase ground fault, The calculation formula is shown in formula (8):

When a three-phase ground fault occurs, The calculation formula is shown in formula (9):

The fourth step is to use the adaptive coefficient to correct the magnitude of the measured impedance of the bus when the fault occurs, and to judge the modulus criterion and phase angle criterion of the measured impedance of the bus to determine the fault interval. Through steps 2 and 3, the adaptive compensation coefficients k _zm and k _zn can be obtained, and the modified busbar measurement impedance modulo values |Z _{m from} | and |Z _{n from} | can be obtained by entering formula (4). The device can obtain the measured impedance change angle of the busbars at both ends of the line before and after the fault

The criterion for the modulus value of the measured impedance of the bus is shown in formula (10):

|Z _{m from} |≤|Z _set |&|Z _{n from} |≤|Z _set | (10)

In the formula: |Z _mself | is the measured impedance modulus value of the m side after adaptive compensation; |Z _nself | is the measured impedance modulus value of the n side after adaptive compensation; |Z _set | is the low impedance threshold , can be adjusted according to the actual situation of the system.

The criterion for the phase angle of bus measurement impedance is shown in formula (11):

where: measure the impedance angle for the m side after the fault, Measure the impedance angle for the m side before the fault; Measure the impedance angle for the n-side after the fault; Measure the impedance angle for the n-side before the fault; θ is the margin angle, generally ±30°.

If the above two criteria are met at the same time, it is judged as a fault in the area, the protection action and the circuit breaker trips.

Example

Taking a line in a 10kV medium-voltage microgrid that occurs at 0.2s after stable operation, the ground fault occurs through the transition resistance in the area as an example, and the transition resistance R _g = 50Ω to specifically illustrate the method. The implementation steps are as follows:

The first step is to use the protection measurement and control device to calculate the measurement impedance of the m and n sides of the busbars at both ends of the line. A line model of a 10kv medium-voltage microgrid is shown in Figure 1. The measured voltage of the m-side busbar can be measured by using the protection measurement and control device. Line current flowing through the m-side busbar n-side busbar measurement voltage Line current flowing through the n-side bus Using the formula (1), the measured impedances of the m and n sides can be calculated as follows:

In the formula: |Z _m | is the measured impedance modulus value of the m-side busbar; is the measured impedance phase angle of the m-side busbar; |Z _n | is the measured impedance modulus value of the n-side busbar; Measure the impedance phase angle for the n-side bus. In the experiment, Z _m =143.13∠41.84° and Z _n =142.17∠-137.8° under the normal operation state of the system.

The second step is to solve the adaptive coefficient of the measured impedance modulus value of the bus during the fault period. Taking the calculation of the adaptive coefficient of the measured impedance modulus value of the m side as an example, the n side can be derived similarly. Bus measurement voltage It can be represented by formula (2):

where: Indicates the line fault point-to-ground voltage; Represents the zero-sequence current flowing through the m-side of the line; Z ₁ represents the line positive sequence impedance from the fault point to the m-side protection installation; Z ₀ represents the line zero-sequence impedance from the fault point to the m-side protection installation. Taking Z ₀ =1.5Z ₁ in the experiment, the specific expression of Z _m is shown in formula (3):

In the formula: Z _t is the measurement impedance of the m side when the metallic ground fault occurs, and ΔZ is the additional measurement impedance caused by the transition resistance; is the grounding current at the fault point; R _g is the transition resistance. The impedance vector diagram can be drawn from the formula (3), as shown in Figure 2, and the relational formula (4) can be obtained according to the triangle angle formula:

where: is the phase angle of the additional impedance (referred to as the additional impedance angle); |Z _t | is the measured impedance modulus value of the m side when the metallic ground fault occurs; The impedance angle is measured for the m-side at metallic faults. From the formula (4), the adaptive coefficient of the m-side measurement impedance modulus value can be obtained as shown in the formula (5):

In the formula: k _zm is the self-adaptive coefficient of the m-side measurement impedance modulus value. From the formula of Z _t in formula (3), it can be known that The calculation expression of is shown in formula (6):

where: is the line impedance angle, which belongs to the inherent parameters of the line. It can be seen from equation (6) that as long as the current waveform and zero-sequence current waveform of the line m side are collected by the protection measurement and control device, it can be calculated. value of . Therefore, there is only an additional impedance angle in Eq. (5) is an unknown quantity, as long as it is estimated correctly Then the adaptive coefficient k _zm of the m-side measured impedance modulus can be obtained.

The third step is to calculate the additional impedance angle for different fault types From the additional impedance ΔZ expression in formula (3), it can be known that The solution formula is shown in formula (7):

In actual operation, short-circuit ground current Therefore, the core idea of the patent of the present invention is to deduce the relationship between the short-circuit grounding current and the short-circuit grounding negative sequence current phase angle according to different fault types, combined with the boundary conditions of the fault and the symmetrical component method, and then use the system The relationship between the negative sequence current at the protection installation and the phase angle of the short-circuit grounding negative sequence current is derived, and the additional impedance angle is calculated.

The following derivation process provides an example for each type of failure:

1) Single-phase ground fault

When a ground fault occurs in phase A of the line, the boundary conditions of the short-circuit point are shown in equation (8):

where: is the short-circuit point A phase-to-ground voltage; They are the short-circuit point-to-ground currents of phases B and C, respectively. Using the symmetric component method, formula (8) can be written in the form of order components as shown in formula (9):

where: are the positive-sequence, negative-sequence and zero-sequence voltages of the short-circuit point to ground, respectively; are the positive-sequence, negative-sequence and zero-sequence currents from the short-circuit point to the ground, respectively; It is the short-circuit current of the faulty phase short-circuit point to ground. Combining formula (7) and formula (9), it can be concluded that when phase A is grounded The expression is shown in (10):

where: is the phase A current of the m-side line. The negative sequence network of the system is shown in Figure 3, and the following relationship can be drawn from Figure 3:

In the formula: C _m2 is the negative sequence current distribution coefficient, which can be taken as a real number to simplify the calculation; is the negative sequence current on the m side of the line. Combining equations (10) and (11), we can get The final expression is as formula (12):

That is, the phase angle of the negative sequence current at the protection installation is used to calculate the phase angle of the short-circuit grounding negative sequence current, and the phase angle of the negative sequence current at the protection installation can be measured by the protection measurement and control device. Therefore, the additional angle of the impedance be calculated. The fault type obtained in the experiment Theoretical Additional Impedance Angle It can be seen that the additional impedance angle calculated by this method is more accurate. When a single-phase ground fault occurs in the other two phases, the calculation formula of the additional impedance angle is as follows:

2) Two-phase ground fault

When a ground fault occurs in two phases of line BC, the boundary conditions of the short-circuit point are shown in equation (14):

where: is the short-circuit point-to-ground voltage of phase B; It is the voltage of the C-phase short-circuit point to ground. Using the symmetric component method, equation (14) can be written in the form of order components:

The composite sequence network diagram of BC two-phase grounding fault is shown in Figure 4, which can be obtained from the composite sequence network diagram The expression is as follows:

In the formula: x _1∑ , x _2∑ , x _0∑ are the total reactance of the positive sequence, negative sequence and zero sequence of the system, respectively. By formula (16), the expression of the three-phase short-circuit grounding current at the short-circuit point can be written as follows:

In the formula: a=e ^j120° ; a ² =e ^-j120° ; Usually, in order to simplify the calculation, take x _0∑ =kx _2∑ , and k is a real number. Combining Equation (7) and Equation (17), the additional impedance angles of B and C phases can be obtained as follows:

obtained in the experiment theory The results obtained by this algorithm are close to the theoretical real results. AB ground fault and CA ground fault, additional impedance angle The calculation expression is as follows:

3) Three-phase ground fault

The three-phase ground fault is a symmetrical fault, and the system has no negative sequence and zero sequence network, the additional impedance The expression is as follows:

To sum up, the additional impedance angle under each fault type obtained in the third step Bringing in Equation (5), the corresponding impedance replication adaptive coefficient can be obtained. The adaptive coefficients of the m-side measurement amplitude obtained under each fault type in the experiment are summarized as follows:

The fourth step is to use the adaptive coefficient to correct the magnitude of the measured impedance of the bus when the fault occurs, and to judge the modulus criterion and phase angle criterion of the measured impedance of the bus to determine the fault interval.

The criterion for measuring the impedance modulus value is as follows:

|Z _m |≤|Z _set |&|Z _n |≤|Z _set | (22)

In the formula: |Z _m | is the measured impedance modulus value of the m side; |Z _n | is the measured impedance modulus value of the n side; |Z _set | is the impedance action threshold value, which can be set according to the actual situation. Measured impedance |Z _set |=35.75Ω.

The criterion for measuring the impedance phase angle is as follows:

or

where: measure the impedance angle for the m side after the fault, Measure the impedance angle for the m side before the fault; Measure the impedance angle for the n-side after the fault; Measure the impedance angle for the n-side before the fault; θ is the margin angle, generally ±30°.

The experiment takes the m-side as an example to illustrate the effectiveness of the algorithm proposed in the present invention, and the n-side has the same principle. Transition resistance R _g = 50Ω, A phase grounding, BC grounding, and ABC grounding faults occur respectively in 0.2s. When the adaptive coefficient of the measured impedance modulus value is not introduced, the m-side measured impedance modulus value waveform is shown in Figure 4. It can be seen that the transition resistance generates additional impedance, which causes the measured impedance modulo value to be too large, and the impedance modulo value criterion refuses to move. After introducing the adaptive coefficient of the measured impedance modulus value, the m-side measured impedance modulus value waveform is shown in Figure 5. It can be seen that the method proposed in the present invention can well eliminate the influence of the additional impedance caused by the transition resistance, so that the protection has Good resistance to transition resistance, improving the reliability of protection. The phase angle waveforms of the measured impedances on the m and n sides are shown in Figure 6. The phase angle criterion enables the protection to operate reliably when there is a fault within the zone. When there is a fault outside the zone, the protection does not malfunction, which ensures the selectivity of the protection. Figure 7 Flowchart of the protection algorithm.

Claims (6)

1. A low-impedance self-adaptive protection method for a micro-grid bus is characterized by comprising the following steps:

monitoring the impedance module value and the phase angle of the buses at two ends of the line;

starting a voltage abrupt change, and calculating an additional impedance angle caused by transition resistance;

calculating a self-adaptive coefficient of the measured impedance module value, and correcting the measured impedance module value of the bus;

judging whether the corrected bus measured impedance modulus value meets an impedance modulus value action criterion or not, and judging whether a bus measured impedance phase angle meets a phase angle action criterion or not;

if the two criteria are met simultaneously, the fault in the area is judged, and the action is protected.

2. The method of claim 1, wherein monitoring the mode and phase angles of the measured impedance of the bus at the two ends of the line comprises:

collecting bus m-terminal voltagePassing m-terminal line currentBus n-terminal voltageCurrent flowing through n-terminal circuitCalculating the measured impedance modulus value and the phase angle of the bus at two ends by the following formula (1):

3. the method of claim 1, wherein the calculating additional impedance angle due to transition resistanceAdditional impedance angle including m-terminal measured impedanceAnd an additional impedance angle of the n-terminal measured impedanceIn which the impedance is measured at the m-terminalAdding impedance angleFor example, the additional impedance angle of the n-terminal measured impedanceThe corresponding parameters are adjusted, and then the corresponding parameters are adjusted,

wherein,Z_0mfor line zero sequence impedance, Z, at the point of fault to m-terminal protection installation_1mFor line positive sequence impedance at the fault point to the m-terminal protection installation, is a zero sequence current flowing through the m end of the line,in order to flow the phase current on the line m side,in order to flow the a-phase current on the line m side,in order to flow the B-phase current on the line m side,the C-phase current flows through the line m side.

4. The method according to claim 1, wherein the calculating the adaptive coefficients of the measured impedance modulus values includes adaptive coefficients of the impedance modulus values of an m-terminal and an n-terminal, wherein the n-terminal adjusts the corresponding parameters by taking the m-terminal as an example, and the specific process is as follows:

step 31, obtaining the m-end bus measurement voltageAnd n-terminal bus measurement voltageThe method for obtaining the measured voltage of the buses at the two ends is the same, taking the m end as an example, the n end adjusts the corresponding parameters

Wherein,Z_0mfor line zero sequence impedance, Z, at the point of fault to m-terminal protection installation_1mThe line positive sequence impedance at the installation site is protected for the m end of the fault point, is a zero sequence current flowing through the m end of the line,for a line fault point to ground voltage,phase current flowing through the line m side;

step 32, obtaining the m-end bus measurement impedance module value Z_m＝Z_t+ Δ Z, wherein Z_tAt m ends in case of metallic earth faultMeasuring impedance, and Delta Z is the additional impedance caused by the transition resistance, grounding current for fault points, R_gIs a transition resistance;

step 33, obtainingWhereinMeasuring the phase angle of the impedance for the m terminal;the phase angle of the additional impedance;measuring an impedance angle for the m-terminal at the time of the metallic failure;

step 34, obtaining the module value self-adaptive coefficient of the m-end bus measurement impedance

5. The method of claim 4, wherein the modified bus bar measured impedance modulus value is

|Z_{m is from}|＝k_zm·|Z_m|

|Z_{n is from}|＝k_zn·|Z_n|。

6. The method of claim 1,

the bus measurement impedance modulus criterion is as follows:

|Z_{m is from}|≤|Z_set|&|Z_{n is from}|≤|Z_set|

Wherein, | Z_{m is from}I is the m-end measured impedance module value after self-adaptive compensation; i Z_{n is from}L is the n-terminal measured impedance module value after adaptive compensation,

|Z_seti is a low impedance threshold value which takes the value of the system rated impedance of 0-1;

the bus measurement impedance phase angle criterion is as follows:

or

Wherein, the impedance angle is measured for the m-terminal after the fault,to measure the impedance angle for the fault front end side,

for measuring the impedance angle for the n-end side after the fault,the impedance angle is measured for the n-terminal before the fault,

theta is a margin angle.