CN106655121A - Low-impedance adaptive protection method of micro-grid bus - Google Patents

Abstract

The invention provides a low-impedance adaptive protection method of a micro-grid bus. The method comprises the following steps of monitoring bus measurement impedance modulus value and phase angle at two ends of a line; starting a voltage break variable and calculating an additional impedance angle caused by transition resistance; calculating an adaptive coefficient of the measurement impedance modulus value and correcting the bus measurement impedance modulus value; judging whether the corrected bus measurement impedance modulus value meets an impedance modulus value operation criterion or not and judging whether the bus measurement impedance phase angle meets a phase angle operation criterion or not; and if the two criteria are simultaneously met, determining an internal fault and carrying out protection operation. The influence of the additional impedance caused by the transition resistance on the bus measurement impedance modulus value is corrected, so that the low-impedance protection of the micro-grid bus has relatively good anti-transient resistance characteristics and the reliability of the protection is improved.

Description

Low-impedance self-adaptive protection method for micro-grid bus

Technical Field

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

Background

The microgrid is a regional small power network consisting of a distributed power supply (DG) and loads, and is the most effective utilization way for distributed power generation at present. The micro power supply is usually an inverter type distributed power supply (IBDG) which is grid-connected by adopting an inverter interface, and when a fault occurs inside the micro power supply, in order to protect power electronic devices from being damaged, a current limiting module of the inverter usually limits a short-circuit current provided by the IBDG within 2 times of a rated current; meanwhile, the IBDG is flexible in grid connection position, and when the IBDG is in grid connection operation, the power flow in the network can flow in two directions. Due to the characteristics, the current protection in the traditional distribution network is difficult to be directly applied to the microgrid, so a new protection method must be found to ensure the safe and stable operation of the microgrid.

For the specific fault characteristics of the micro-grid, the requirement of the micro-grid on the protection reliability cannot be met by simply using the local electrical quantity information of voltage or current. To this end, some researchers have proposed a microgrid protection scheme based in part on communication-microgrid protection based on bus low impedance (including an impedance modulus criterion and a phase angle criterion). And calculating the module value and the phase angle of the impedance of the buses at the two ends by utilizing the voltage drop of the buses at the two ends after the fault and the change of the tidal current direction, and judging the fault interval by comparing the module value and the phase angle of the impedance of the buses at the two ends before the fault. The protection method utilizes the ratio information of bus voltage and line current before and after the fault to form a protection criterion, overcomes the limitation of the current limiting module of the inverter on the magnitude of short-circuit current and the bidirectional influence of power flow in a network, simultaneously, the communication at two ends only exchanges Boolean information, has lower requirement on communication and higher protection reliability. However, after analysis and verification, when a transition resistance earth fault occurs in a region, the electrical quantity characteristic during the fault can be weakened by the large transition resistance, the reliable action of the impedance module value criterion of the impedance bus measurement is influenced, and the module value of the impedance bus measurement during the fault is too large, so that protection refusal is caused.

Disclosure of Invention

The invention aims to provide a low-impedance self-adaptive protection method for a micro-grid bus, which solves the problem that the bus low-impedance protection is rejected due to larger transition resistance when a transition resistance ground fault occurs inside the micro-grid.

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.

The invention provides a self-adaptive protection method for low impedance of a micro-grid bus, which is based on the ratio of bus measurement voltage to line current to obtain bus measurement impedance. The method is characterized in that a relation between a short-circuit grounding current and a short-circuit grounding negative sequence current phase angle is deduced by combining boundary conditions under different fault types through a symmetrical component method, and then a negative sequence equivalent network of a system is utilized to obtain a relation between a negative sequence current at a protection installation position and the short-circuit grounding negative sequence current phase angle, so that an additional impedance angle is calculated, an adaptive coefficient of a bus measured impedance modulus value is obtained, the influence of additional impedance caused by transition resistance on the bus measured impedance modulus value is corrected, the micro-grid bus low impedance protection has a good transition resistance characteristic, and the protection reliability is improved.

The invention is further described below with reference to the accompanying drawings illustrating the rate of fire.

Drawings

Fig. 1 is a circuit model diagram of a medium voltage microgrid.

Fig. 2 is a vector diagram of the m-side measured impedance.

Fig. 3 is a system negative sequence network diagram.

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

Fig. 5 is a diagram of the adaptively corrected m-side measured impedance modulus.

Fig. 6 is a schematic diagram of measured impedance angles on the m and n sides during an in-zone fault.

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

Detailed Description

A low-impedance self-adaptive protection method for a micro-grid bus comprises the following steps:

firstly, calculating the m and n side measured impedances of buses at two ends of a circuit in a microgrid. Taking the case that one line of the microgrid has a transition resistance ground fault as an example, the m-side impedance is calculated. The expression of the m-side measured impedance is shown as formula (1):

in the formula:the m-side bus phase voltage is shown,the phase current flowing on the line m side is shown. According to the formula, the m-side bus voltage is measured by using the protection measurement and control deviceAnd m side line currentMeasuring impedance Z_mCan be calculated, therefore Z_mIs a quantity which can be directly solved, so that the module value | Z of the bus measuring impedance can be easily solved_mSum phase angleSimilarly, the n-side bus measurement impedance can also be directly obtained by following the method of m-side bus measurement impedance.

And secondly, solving a bus measurement impedance modulus value adaptive coefficient during the fault period. Taking the example of solving the adaptive coefficient of the measured impedance at the m-side, the n-side can be similarly derived. Bus measuring voltageMay be represented by formula (2):

in the formula:representing the voltage of a line fault point to ground;representing the zero sequence current flowing through the m side of the line; z₁Line positive sequence impedance expressed as fault point to m-side protection installation; z₀And representing the zero sequence impedance of the line from the fault point to the protection installation at the side m. Then Z_mIs represented by the following formula (3):

in the formula: z_tWhen the resistance is a metallic grounding fault, the measured impedance of the m side is delta Z, and the measured additional impedance caused by the transition resistance is delta Z;grounding current for fault points; r_gIs the transition resistance. From the formula (3), the transition resistance R_gThe bus impedance measurement can generate an additional impedance, resulting in an increased impedance modulus. From the impedance vector relationship of equation (3), equation (4) can be derived:

in the formula:measuring the phase angle of the impedance for the m-side;a phase angle for the additional impedance (referred to as an additional impedance angle);the impedance angle is measured for the m-side at the time of the metallic failure. The self-adaptive coefficient k of the m-side bus measured impedance modulus value can be obtained by the formula (4)_zmAs shown in formula (5):

due to the fact thatCan be directly obtained by the protection measurement and control device, so that the additional impedance angle in fault can be correctly estimatedThat is to sayBy adaptive coefficient k_zmAnd compensating the influence of the transition resistance on the bus measured impedance modulus criterion.

Third, calculating an additional impedance angle for different fault typesFrom the expression of the impedance Δ Z added in the formula (3), it can be seen thatSolving the formula is shown in equation (6):

in actual operation, short-circuit grounding currentThe phase angle of the short-circuit grounding current and the short-circuit grounding negative sequence current cannot be directly measured, so the core idea of the invention is to deduce the relation between the short-circuit grounding current and the short-circuit grounding negative sequence current phase angle according to different fault types and by combining the boundary conditions of faults and a symmetric component method, and then deduce the relation between the negative sequence current at the protection installation position and the short-circuit grounding negative sequence current phase angle by using a negative sequence equivalent network of the system, so as to calculate the additional impedance angle. Taking the example of calculating the additional impedance angle of the m-side measured impedance as follows, the additional impedance angle of the n-side measured impedance can be calculated by the same method:

when the single-phase earth fault occurs, the fault line,the calculation formula is shown in formula (7):

when the two-phase ground fault occurs, the fault state of the two-phase ground fault occurs,the calculation formula is shown in formula (8):

when the three-phase earth fault occurs, the fault can be detected,the calculation formula is shown in formula (9):

and fourthly, correcting the bus measured impedance amplitude value during the fault by using the self-adaptive coefficient, judging a modulus criterion and a phase angle criterion of the bus measured impedance, and determining a fault interval. The self-adaptive compensation coefficient k can be obtained through the second step and the third step_zm、k_znThe corrected bus measurement impedance modulus value | Z can be obtained by the formula (4)_{m is from}|、|Z_{n is from}If the protection measuring and control device is used, the measured impedance change angle of the buses at the two ends of the line before and after the fault can be obtained

The bus measurement impedance modulus criterion is as shown in formula (10):

|Z_{m is from}|≤|Z_set|&|Z_{n is from}|≤|Z_set| (10)

In the formula: i Z_{m is from}I is the m-side measured impedance module value after self-adaptive compensation; i Z_{n is from}I is the n-side measured impedance module value after self-adaptive compensation; i Z_setAnd | is a low impedance threshold value, and can be set according to the actual condition of the system.

The phase angle criterion of the bus measured impedance is shown as the formula (11):

in the formula: to measure the impedance angle for the m-side after the fault,measuring an impedance angle for the m side before the fault; measuring an impedance angle for the n-side after the fault;measuring an impedance angle for the n sides before the fault; θ is a margin angle, and is usually ± 30 °.

If the two criteria are met simultaneously, the fault in the area is judged, and the protection action and the breaker trip are carried out.

Examples

Taking a grounding fault of one line in a 10kV medium-voltage microgrid in a 0.2s moment after stable operation through a transition resistor as an example, the transition resistor R_gThe method is specifically described as 50 Ω, and the implementation steps are as follows:

firstly, calculating the m and n side measurement impedance of buses at two ends of a line by using a protection measurement and control device. A line model of a 10kv medium-voltage micro-grid is shown in figure 1, and m-side bus measurement voltage can be measured by using a protection measurement and control deviceLine current flowing through m-side busn-side bus measurement voltageLine current flowing through n-side busThe m and n side measured impedances can be calculated by using the formula (1) as follows:

in the formula: i Z_mI is the impedance module value measured by the m-side bus;measuring an impedance phase angle for the m-side bus; i Z_nI is an impedance module value measured by the n-side bus;impedance phase angle is measured for the n-side bus. Z in normal running state of system in experiment_m＝143.13∠41.84°，Z_n＝142.17∠-137.8°。

And secondly, solving the bus measurement impedance modulus value adaptive coefficient during the fault period, taking the solution of the m-side measurement impedance modulus value adaptive coefficient as an example, and the n-side can be similarly deduced. Bus measuring voltageMay be represented by formula (2):

in the formula:representing the voltage of a line fault point to ground;representing the zero sequence current flowing through the m side of the line; z₁Line positive sequence impedance expressed as fault point to m-side protection installation; z₀And representing the zero sequence impedance of the line from the fault point to the protection installation at the side m. In the experiment, get Z₀＝1.5Z₁Then Z is_mIs represented by the following formula (3):

in the formula: z_tWhen the resistance is a metallic grounding fault, the measured impedance of the m side is delta Z, and the measured additional impedance caused by the transition resistance is delta Z;grounding current for fault points; r_gIs the transition resistance. An impedance vector diagram can be obtained by the equation (3), as shown in fig. 2, and the relationship (4) can be obtained according to a triangle corner formula:

in the formula:a phase angle for the additional impedance (referred to as an additional impedance angle); i Z_tI is the measured impedance module value of the m side when the metallic grounding fault occurs;the impedance angle is measured for the m-side at the time of the metallic failure. The adaptive coefficient of the m-side measured impedance modulus can be obtained by the formula (4) and is shown in the formula (5):

in the formula: k is a radical of_zmAnd measuring the adaptive coefficient of the impedance modulus for the m side. By Z in formula (3)_tCan learn aboutIs represented by equation (6):

in the formula:the impedance angle of the line is the inherent parameter of the line, and is taken out in the experimentAs can be seen from the formula (6), the m-side current waveform and the zero-sequence current waveform of the line can be calculated by only collecting the m-side current waveform and the zero-sequence current waveform of the line through the protection measurement and control deviceThe value of (c). Therefore, only the additional impedance angle in equation (5)Is an unknown quantity, provided that it is estimated correctlyThe adaptive coefficient k of the m-side measured impedance modulus value can be obtained_zm。

Third, calculating an additional impedance angle for different fault typesExpressed by the additional impedance deltaz in equation (3),it can be known thatSolving the formula is shown in equation (7):

in actual operation, short-circuit grounding currentThe phase angle of the short-circuit grounding current and the short-circuit grounding negative sequence current cannot be directly measured, so the core idea of the invention is to deduce the relation between the short-circuit grounding current and the short-circuit grounding negative sequence current phase angle according to different fault types and by combining the boundary conditions of faults and a symmetric component method, and then deduce the relation between the negative sequence current at the protection installation position and the short-circuit grounding negative sequence current phase angle by using a negative sequence equivalent network of the system, so as to calculate the additional impedance angle.

The following derivation lists one example description for each type of fault:

1) single phase earth fault

When the phase A of the line has a ground fault, the boundary condition of the short-circuit point is shown as the formula (8):

in the formula:phase point A is the voltage to ground of the short circuit point A;b, C-phase short-circuit point-to-ground currents respectively. Writing formula (8) into a sequence component by using a symmetric component method, wherein the form of the sequence component is shown as formula (9):

in the formula:positive sequence voltage, negative sequence voltage and zero sequence voltage of the short circuit point to the ground respectively;positive sequence current, negative sequence current and zero sequence current of the short circuit point to the ground respectively;the fault phase short-circuit point is short-circuited to ground. When the A phase is grounded, as shown by the combination of the formula (7) and the formula (9)Is shown in (10):

in the formula:phase a current for the m-side line. The system negative sequence network is shown in fig. 3, and the following relation can be obtained from fig. 3:

in the formula: c_m2The negative sequence current is distributed with coefficients, the calculation is simplified, and the real number can be taken;is the line m side negative sequence current. The combination of formula (10) and formula (11) givesThe final expression is as in formula (12):

namely, the phase angle of the short circuit grounding negative sequence current is calculated by utilizing the phase angle of the negative sequence current at the protection installation position, and the negative sequence current phase angle at the protection installation position can be measured by a protection measurement and control device, so that an impedance additional angleIs calculated. Determined by the type of fault in the experimentTheoretical additional impedance angleTherefore, the additional impedance angle calculated by the method is high in precision. When single-phase earth faults occur in other two phases, the calculation formula of the additional impedance angle is as follows:

2) two-phase earth fault

When two phases of the line BC have a ground fault, the boundary condition of the short-circuit point is as shown in equation (14):

in the formula:grounding the B-phase short circuit point;and grounding the C-phase short circuit point. Using the symmetric component method, equation (14) can be written in the form of an order component:

the composite sequence network diagram of BC two-phase ground fault is shown in FIG. 4, and can be obtained from the composite sequence network diagramThe expression is as follows:

in the formula: x is the number of_1∑、x_2∑、x_0∑Respectively positive sequence, negative sequence and zero sequence total reactance of the system. The expression of the three-phase short-circuit grounding current of the short-circuit point can be written by the formula (16) as follows:

in the formula: a ═ e^j120°；a²＝e^-j120°(ii) a Usually to simplify the calculation, take x_0∑＝kx_2∑And k is a real number. The B, C-added impedance angle can be obtained by combining the equations (7) and (17)The following were used:

obtained in experimentsTheory of the inventionThe result obtained by the algorithm is similar to the theoretical real result. Additional impedance angle at AB ground fault and CA ground faultThe calculation expression is as follows:

3) three-phase earth fault

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

in summary, the additional impedance angle of each fault type obtained in the third stepAnd (5) carrying out the operation, so that the corresponding impedance copy adaptive coefficient can be obtained. The adaptive coefficients of the m-side measured amplitude values obtained under each fault type in the experiment are summarized as follows:

and fourthly, correcting the bus measured impedance amplitude value during the fault by using the self-adaptive coefficient, judging a modulus criterion and a phase angle criterion of the bus measured impedance, and determining a fault interval.

The criterion for measuring the impedance modulus is as follows:

|Z_m|≤|Z_set|&|Z_n|≤|Z_set| (22)

in the formula: i Z_mI is the m-side measured impedance module value; i Z_nI is an impedance module value measured by the n side; i Z_setI is an impedance action threshold value, can be set according to actual conditions, and the measured impedance Z of 0.25 times of normal operation is taken in the experiment_set|＝35.75Ω。

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

or

In the formula: to measure the impedance angle for the m-side after the fault,measuring an impedance angle for the m side before the fault; measuring an impedance angle for the n-side after the fault;measuring an impedance angle for the n sides before the fault; θ is a margin angle, and is usually ± 30 °.

Experiment takes m side as an example to illustrate the inventionThe effectiveness of the algorithm provided by the invention is the same as the n-side principle. Transition resistance R_gWhen the adaptive coefficient of the measured impedance module value is not introduced, the waveform of the measured impedance module value at the m side is as shown in fig. 4, so that the transition resistance generates additional impedance, the measured impedance module value is larger, and the impedance module value criterion fails. After introducing the adaptive coefficient of the measured impedance modulus, the waveform of the measured impedance modulus at the m side is shown in figure 5, and therefore, the method provided by the invention can well eliminate the influence of additional impedance caused by the transition resistance, so that the protection has good capability of resisting the transition resistance, and the reliability of the protection is improved. The waveform of phase angle of the measured impedance at m and n sides is shown in figure 6, the phase angle criterion ensures that the protection reliably acts when the fault occurs in the area, the protection does not act mistakenly when the fault occurs outside the area, and the protection selectivity is ensured, and figure 7 is a protection algorithm flow chart.

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 impedanceAdditional impedance angle in which the impedance is measured at the m-terminalFor 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_1mThe line positive sequence impedance at the installation site is protected for the m end of the fault point, is zero sequence current flowing through the m end of the line.

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, and the m end and the n end are used for adjusting corresponding parameters

${\stackrel{\·}{U}}_m={\stackrel{\·}{U}}_k+({\stackrel{\·}{I}}_m+3k_{zm}{\stackrel{\·}{I}}_0)Z_{1m}$

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, zero sequence current flowing through the m end of the line;

step 32, obtaining the m-end bus measurement impedance module value Z_m＝Z_t+ Δ Z, wherein Z_tIs the measured impedance at the m-terminal at the time of a metallic ground fault, az is the measured 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_setl is a low impedance threshold value, which takes on the valueA 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, and theta is the margin angle.