CN115877129B - Low-voltage distribution network neutral line disconnection comprehensive judgment method based on current and voltage - Google Patents
Low-voltage distribution network neutral line disconnection comprehensive judgment method based on current and voltage Download PDFInfo
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Abstract
The invention discloses a low-voltage distribution network neutral line disconnection comprehensive judging method based on current and voltage, which is characterized in that theoretical analysis is carried out on each electrical quantity change when a neutral line of a low-voltage distribution network containing a distributed power supply is disconnected, fault characteristics of the neutral line disconnection are clarified, and aiming at the defect that the traditional line disconnection comprehensive judging method only depends on current or offset voltage, the invention provides a line disconnection comprehensive judging method based on current criteria and voltage criteria. The method can solve the problem of misjudgment caused by traditional detection, so that the protection fixed value can be adaptively adjusted along with the asymmetry of the load, and meanwhile, the protection sensitivity when the neutral line is broken is improved.
Description
Technical Field
The invention belongs to the field of low-voltage distribution network power system protection, and relates to a comprehensive judgment method for disconnection of a neutral line of a low-voltage distribution network based on current and voltage.
Background
In a low-voltage distribution system, a power supply system widely adopts a three-phase four-wire system or a three-phase five-wire system, and a neutral line can well maintain zero potential of a neutral point. When the neutral line breaks, the neutral point potential will shift, and the load before and after the break will have different degree of overvoltage, damage the electrical equipment and even cause building fire. Therefore, it is of great importance to accurately judge whether a neutral line breaks. In addition, the distributed power supplies (Distributed Generation, DG) of China are connected in a large amount in a low-voltage distribution area, so that the traditional passive power distribution network gradually transits to a more active power distribution network, the problems of bidirectional flow of power flow, unstable DG output and the like exist, misjudgment of traditional fault identification and diagnosis is caused, and particularly misjudgment of a neutral line disconnection fault on the low-voltage side is likely to cause dangerous accidents, and therefore a neutral line disconnection judgment method of the low-voltage power distribution network containing DG needs to be studied.
The existing neutral line disconnection determination basically uses only a neutral line current, a harmonic current or a neutral point offset voltage for detection, and has certain defects. The neutral line current is used for judging, misjudgment can occur under the condition of higher unbalance of the three-phase load, and the accuracy is lower; the third harmonic change of the head end of the power distribution trunk is utilized to judge the disconnection fault of the neutral line, which is influenced by fluctuation when the load normally operates, and misjudgment and omission are easily caused; and the neutral point offset voltage is utilized for judging, and when the power distribution system operates normally and the load unbalance degree is large, fault characteristics can also appear, so that erroneous judgment is caused. In addition, since the output of the DG is not stable enough due to factors such as environment, the voltage of part of nodes may be raised or reduced, and in island operation, voltage reduction or raising may occur due to unbalance of the DG output power and the load power, so in actual engineering, linear line breakage judgment and identification are performed only by utilizing the voltage change of the neutral point at the load side, and the reliability is not high, so that obvious defects exist.
In summary, the existing neutral line disconnection judging methods have certain defects, most of the existing neutral line disconnection judging methods are single electric quantity judgment and are influenced by the change characteristics of the single electric quantity judgment, faults cannot be accurately identified, and the existing neutral line disconnection judging methods are not applicable to the traditional passive power distribution network because the neutral line disconnection characteristics are influenced by DG and adjustable topology. Therefore, the research on the comprehensive criteria of multiple electric quantities is of great significance for the neutral line disconnection faults of the low-voltage distribution network containing DG.
Disclosure of Invention
The invention provides a comprehensive judging method for the neutral line breakage of a low-voltage distribution network based on current and voltage, which can enhance the accuracy and sensitivity of identification, effectively solve the problems of misjudgment, missed judgment and the like of faults in the traditional method, enable the protection fixed value to be adaptively adjusted along with the asymmetry of loads, improve the protection sensitivity when the neutral line is broken, and be suitable for the application requirements of low-voltage active distribution networks widely accessed by current and future DGs.
In order to achieve the above purpose, the technical scheme adopted by the invention is as follows:
the comprehensive judging method for the disconnection of the neutral line of the low-voltage distribution network based on the current and the voltage is characterized by comprising the following steps of:
a, calculating neutral line current I according to a system network equation after neutral line disconnection Z With system side neutral point ground line current I N Judging the relation between the two to form a first current criterion;
b, monitoring the harmonic current at the head end of the three-phase line of the system, calculating the harmonic current mutation rate delta I, and comparing and judging with a fault set value delta to form a second current criterion;
c, introducing a voltage adaptation coefficient eta, and calculating a neutral line disconnection detection voltage criterion based on the voltage adaptation coefficient according to the relation between the load unbalance degree and the offset voltage;
and D, comprehensively judging the disconnection fault according to the current and voltage criteria obtained by calculation in the step A, the step B and the step C.
Further, step D is to comprehensively judge the broken line fault according to the current and voltage criteria calculated in step A, step B and step C, specifically:
judging according to the formula (1):
wherein I is Z Is neutral line current; i N The system side neutral point grounding wire current; delta I% is the third harmonic current mutation rate on a three-phase line, delta is a fault set value, and generally 10% -15%; u (U) oo′ Is the neutral point offset voltage when the neutral line is broken, k' is the voltage setting value coefficient, eta is the voltage adaptation coefficient, U n Is rated voltage;
if the electric quantity of the current and the voltage meets the formula (1), judging that the neutral line of the low-voltage distribution system is broken; if the neutral line is not broken, the neutral line is judged to be not broken.
Further, step A calculates the neutral line current I according to the system network equation after the neutral line is broken Z With system side neutral point ground line current I N The determining of the relation between the two, specifically includes:
s1, if the neutral line is broken at the position 1:
step (1) list node voltage equations according to the equivalent circuit after the fault:
z in 1 Indicating the system ground electrode resistance, Z 2 Representing the repeated grounding point grounding resistance at the incoming line of the user; z is Z a 、Z b 、Z c The load equivalent impedance of each phase is respectively; z is Z N Is the neutral line resistance;for the system side neutral point to ground voltage, +.>The neutral point to ground voltage at the load side; />Z i The system comprises a system phase voltage, an equivalent voltage provided by DG in each phase and an equivalent impedance of load of each phase, wherein i=a, b and c;
step (2) calculating the offset voltages U of the two neutral points on the system side and the load side according to the formula (2) oo′ System side neutral point to ground voltage
In the middle ofLoad voltage of each phase after disconnection fault;
step (3) calculating I according to formulas (3), (4) Z And I N ：
From the following componentsZero is available, and the current amplitude I of the system side neutral point grounding wire N And neutral line current amplitude I Z All equal to zero if the line split is consideredDistributing capacitance current, wherein the two currents are not completely zero, are in parallel shunt relationship, and Z 1 ＜Z N +Z 2 Then there is I N ≥I Z A relationship of approximately 0;
s2, if the neutral line is broken at the position 2:
step (1) list node voltage equation
Step (2) calculating U according to formula (6) oo′ 、And +.>
Step (3) the calculation result of the above formula is introduced into formula (5) becauseObtainable I N ＞0，I Z =0, thus there is I N ＞I Z A relation of =0.
Further, the step B calculates a harmonic current mutation rate Δi, which specifically includes:
the 3 rd harmonic mutation is detected on three phases simultaneously, and the mutation quantity delta I of the 3 rd harmonic current is obtained 3 The definition is as follows:
wherein x represents the x-th sampling data; ΔI 3 Representing the difference between the data acquisition values of the two times.
Δi% is calculated from formula (10):
further, step C introduces a voltage adaptation coefficient eta, calculates a neutral line disconnection detection voltage criterion based on the voltage adaptation coefficient according to the relation between the load unbalance degree and the offset voltage, and specifically includes:
setting the same load power factor of each phase, maximizing the load power of the c phase, and defining the load unbalance degree of the a phase and the b phase as follows:
step (2) taking the above formula (12) into formulas (3) and (7), respectively, and calculating U oo′ 、Relationship with three-phase load imbalance:
from equations (13) and (14), it is known that unbalanced current is generated by unbalanced load of three phases, and flows through the neutral line, and the neutral point on the load side is grounded due to neutral line impedanceAs the load unbalance degree increases, the magnitude U of the offset voltage of the neutral point when the neutral line is disconnected oo′ The degree zeta of asymmetry of the three-phase load is determined and is not influenced by other factors;
step (3) defines ζ=max { ζ according to the formula in step (1) a ，ζ b The magnitude of the zeta value corresponds to the asymmetry degree of the three-phase load, the greater the zeta value is, the more the three-phase load is asymmetric, the voltage adaptation coefficient eta is introduced, the traditional neutral point offset voltage criterion is improved, and the value of the voltage adaptation coefficient eta mainly reflects the neutral line current I Z The ratio to the load voltage is as follows:
the neutral line breakage detection voltage criterion based on the voltage adaptation coefficient in the step (4) is as follows:
further, the tuning principle of k' is as follows: ensuring that the neutral line is protected from misoperation during normal operation, (k' +eta) U n Setting the setting value of (2) according to the maximum value of neutral point offset voltage which is generated when the neutral line is avoided from normal operation, and when ζ is more than or equal to 0.1, ηU is calculated n The braking effect is obvious, so the setting of the k' fixed value can be satisfied when zeta is less than 0.1 and the protection is not wrong.
Further, the k' constant value is selected to be 0.08-0.10.
Compared with the prior art, the invention has the following beneficial effects:
according to the invention, through analyzing each electric quantity under the condition of neutral line disconnection of a low-voltage active power distribution network containing DG, summarizing fault characteristics of neutral line disconnection, combining current and voltage change characteristics under the fault condition, a neutral line disconnection fault current and voltage comprehensive detection and identification method suitable for the DG-containing active low-voltage power distribution network is provided, a current criterion is formed by utilizing the change of neutral line current, system side neutral point grounding line current and line monitoring third harmonic current, a voltage adaptation coefficient is introduced, a protection criterion of neutral point offset voltage is improved, a voltage criterion is formed, and finally whether the neutral line disconnection fault occurs is judged jointly through the current criterion and the voltage criterion. The method effectively avoids the limitation of single current or voltage criterion in complex network and complex fault state, can carry out complementary judgment according to the current and voltage criterion at the same time, and improves the adaptability of the traditional current and voltage criterion, so that the traditional current and voltage criterion can not be influenced by load change in normal operation state, and the judgment accuracy is increased.
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In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings that are needed in the embodiments of the present invention will be briefly described below, and it is obvious that the drawings described below are only some embodiments of the present invention, and other drawings may be obtained according to these drawings without inventive effort for a person skilled in the art.
FIG. 1 is an equivalent circuit diagram of a low voltage active power distribution network system;
FIG. 2 is a flow chart of the method for comprehensively judging the disconnection of the neutral line of the low-voltage distribution network based on current and voltage;
FIG. 3 is a graph of line neutral current variation provided by an embodiment of the present invention;
FIG. 4 is a graph of current variation at load imbalance provided by an embodiment of the present invention;
fig. 5 is a diagram showing a 3 rd harmonic current change when a neutral line is broken according to an embodiment of the present invention.
Detailed Description
The present invention will be described in further detail with reference to the following examples in order to make the objects, technical solutions and advantages of the present invention more apparent. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the invention.
Aiming at the problems existing in the prior art, the invention provides a comprehensive judging method for the disconnection of a neutral line of a low-voltage distribution network based on current and voltage, and the invention is described in detail below with reference to the accompanying drawings.
An equivalent circuit model of the low-voltage active distribution network is shown in fig. 1. The DGs exist between the system side and the load side, are comprehensively equivalent to be connected into a voltage source grid, and are directly grounded through small resistors. The low-voltage side of the system is provided with a three-phase uncontrolled rectifier, an A-phase controlled rectifier with adjustable power, a B, C-phase uncontrolled rectifier with the same power, and three balanced linear load power adjustable circuits with nonlinear loads are constructed.
As shown in fig. 2, the method for comprehensively determining the disconnection of the neutral line of the low-voltage distribution network based on the current and the voltage provided by the embodiment of the invention specifically includes:
a, calculating neutral line current I according to a system network equation after neutral line disconnection Z With system side neutral point ground line current I N Judging the relation between the two to form a first current criterion;
b, monitoring the harmonic current at the head end of the three-phase line of the system, calculating the harmonic current mutation rate delta I, and comparing and judging with a fault set value delta to form a second current criterion;
c, introducing a voltage adaptation coefficient eta, and calculating a neutral line disconnection detection voltage criterion based on the voltage adaptation coefficient according to the relation between the load unbalance degree and the offset voltage;
and D, comprehensively judging the disconnection fault according to the current and voltage criteria obtained by calculation in the step A, the step B and the step C.
Specifically, step D is determined according to formula (1):
wherein, the neutral line current is; the system side neutral point grounding wire current; the third harmonic current mutation rate on the three-phase line is a fault set value, and generally 10% -15%; the neutral point offset voltage when the neutral line is broken is a voltage setting value coefficient, a voltage adaptation coefficient and a rated voltage. If the electric quantity of the current and the voltage meets the formula (1), judging that the neutral line of the low-voltage distribution system is broken; if the neutral line is not broken, the neutral line is judged to be not broken.
Step A, calculating neutral line current I according to a system network equation after neutral line disconnection Z With system side neutral point ground line current I N The determining of the relation between the two, specifically includes:
s1, if the neutral line is broken at the position 1.
Step (1): and listing a node voltage equation according to the equivalent circuit after the fault.
Wherein Z1 represents the system-side grounding electrode resistance, and Z2 represents the repeated grounding point grounding resistance at the user incoming line position; za, zb and Zc are the load equivalent impedance of each phase respectively; ZN is neutral line resistance; the system side neutral point voltage to ground is the load side neutral point voltage to ground; and (i=a, b, c) are the system phase voltages, the equivalent voltages provided by DG in each phase, and the equivalent impedances of the load in each phase, respectively.
Step (2): the offset voltages of the two neutral points on the system side and the load side and the voltage to ground of the neutral point on the system side are calculated according to the formula (2).
Wherein the formula is the load voltage of each phase after the occurrence of the disconnection fault.
Step (3): calculating I according to formulas (3), (4) Z And I N ：
From the following componentsThe system side neutral point ground line current amplitude IN and the neutral line current amplitude IZ are equal to zero, which is zero. If the distributed capacitance current of the line is considered, the two currents are not completely zero and are IN parallel shunt relationship, and Z1 is less than ZN+Z2, the relationship that IN is more than or equal to IZ is approximately equal to 0 exists.
S2, if the neutral line is broken at the position 2.
Step (1) list node voltage equation
Step (2) calculating U according to formula (6) oo′ 、And +.>
Step (3): the calculation result of the above formula is taken into formula (5), and since > 0, IN > 0, iz=0 can be obtained, there is a relationship of IN > iz=0. This result is consistent with the result obtained in S1, so the discriminant is still valid at different break locations of the neutral line.
And B, calculating the harmonic current mutation rate delta I, which specifically comprises the following steps:
when the neutral line is normally connected, 3 rd harmonic current generated by the nonlinear device flows to the system side through the neutral line because the load impedance is far greater than the system impedance; when the neutral line breaks, harmonic current generated by the nonlinear load cannot flow back to the feeder line head end, but flows to the load impedance, and only 3 rd harmonic current before the fault position exists at the line head end. Therefore, for the loop head end, 3 harmonic mutation can be detected simultaneously on three phases, and the mutation amount of 3 harmonic current is defined as:
wherein I3 represents a third harmonic current, x represents an xth sampling data, and x represents a difference between the front and rear data acquisition values.
From equation (10) it can be calculated as:
if the load after the fault point is unbalanced, the 3 rd harmonic current can be detected at the head end of the three-phase line after the fault, and the current suddenly changes at the same time, but the neutral point potential is offset at the moment. It should be noted that when the threshold value of the harmonic current mutation rate is selected, the value of the harmonic current mutation rate cannot be selected to be too large because the harmonic current mutation rate is smaller when a line break fault occurs at the end of the line, and if the setting value is selected to be too large, the situation of missed judgment is caused.
Step C, introducing a voltage adaptation coefficient eta, and calculating a neutral line disconnection detection voltage criterion based on the voltage adaptation coefficient according to the relation between the load unbalance degree and the offset voltage, wherein the step C specifically comprises the following steps:
step (1): and setting the load power factors of all phases to be the same, and maximizing the load power of the c-phase. Defining the load unbalance degree of the a phase and the b phase as follows:
step (2): the above equation (12) is taken into equations (3) and (7), respectively, and the relationship between the three-phase load unbalance degree and the calculation is calculated:
from equations (13) and (14), it is known that unbalanced current is generated by the unbalanced load of three phases, and the neutral point-to-ground voltage on the load side increases as the degree of unbalanced load increases due to the neutral line impedance. When the neutral line is disconnected, the magnitude of the offset voltage of the neutral point is determined by the asymmetry degree zeta of the three-phase load and is not influenced by other factors.
Step (3): from the formula in step (1), ζ=max { ζa, ζb }, the magnitude of ζ value corresponds to the degree of asymmetry of the three-phase load, and the larger ζ value indicates the more asymmetry of the three-phase load. Introducing a voltage adaptation coefficient, improving the traditional neutral point offset voltage criterion, wherein the numerical value of the voltage adaptation coefficient mainly reflects the ratio of neutral line current IZ to load voltage, and the numerical value is represented by the following formula:
step (4): the neutral break-before-wire detection voltage criterion based on the voltage adaptation coefficient is as follows:
after the voltage adaptation coefficient is introduced, compared with the existing protection criterion using neutral point offset voltage, the method has 2 advantages: (1) Under the normal operation condition, the voltage adaptation coefficient can be increased along with the increase of the load asymmetry, so that the neutral line is ensured to be protected reliably and not to act when being normal. (2) Analysis of the voltage detection criterion (16) shows that when a neutral line disconnection fault occurs, the voltage adaptation coefficient=0, and the protection criterion is equivalent. The braking action ensures that the neutral line is protected from misoperation when normal, and the setting value can be lowered. Therefore, the sensitivity of the protection scheme after the voltage adaptation coefficient is introduced is obviously better than that of the traditional protection.
The k' setting principle is as follows: the condition that misoperation does not occur in the normal operation of the neutral line is guaranteed, the setting value setting needs to be set according to the maximum value of the neutral point offset voltage which is avoided when the neutral line is in normal operation, and at the moment, the Un braking effect is obvious, so that the setting value setting only meets the condition that misoperation does not occur in the protection. From the above analysis, it can be seen that: the neutral point offset voltage value is much less than 10% of the rated voltage when the neutral line is operating normally. The normal voltage range of the power load is comprehensively considered to be +/-10% of the rated value, so that the fixed value is selected to be 0.08-0.10, and the protection sensitivity is obviously improved.
The three discriminants represented by equation (1) can be categorized into two points:
(1) A current criterion. According to the method, the 3 rd harmonic mutation rate is easily obtained according to three-phase line monitoring data, and if the reduction of the third harmonic is observed to be |delta I% | 15% on the three phases of the head end of the main line, the neutral line disconnection fault is indicated to occur with high probability. And a larger |Δi% | indicates that the fault location is closer to the head end. If |Δi% | <10%, it can be determined that disconnection failure has not occurred. IN order to further ensure the accuracy of the neutral line breakage detection and identification, the second step of judgment is performed through the change of the relation between the neutral line current IZ and the neutral line grounding current IN, so that the interference to the breakage identification caused by harmonic mutation due to load fluctuation and other types of faults is avoided.
(2) Because DG is connected, the traditional voltage offset criterion is invalid, the load unbalance degree not only affects the result obtained by the current criterion, but also affects the voltage criterion, and the false judgment of broken lines is easy to cause.
The current criterion and the voltage criterion are effectively combined to form complementation, and the neutral line disconnection faults are judged together, so that a current-voltage comprehensive criterion is formed, and the current-voltage comprehensive criterion is shown as a formula (1).
The technical scheme of the invention is further described below in connection with simulation experiments.
Examples:
the low-voltage system simulation model shown in figure 1 is constructed by Matlab/Simulink simulation tools, and the effectiveness of the provided comprehensive neutral line breakage identification method is verified. Wherein the distributed power source DG is controlled by PQ, the grounding resistance of a neutral line at the power source side is 4 omega, the grounding resistance of a user incoming line terminal is 5 omega, the phase conductor is a single-core copper cable, and the sectional area is 240mm 2 The resistance per unit length was 0.0911 Ω/km. A. The load power factors of the B, C three phases are all 0.85, and the C-phase load is set as a rated load, and the rated load impedance value is Z=0.35+j0.22. A. The load impedance of the B-phase varies with the asymmetry of the load. The neutral line was disconnected after the system was set to run for 0.5 s.
After neutral line break fault, the discriminant of current and voltage to be calculated can be known by the formula (1), and the specific contents are as follows:
s101, since the disconnection fault occurs at the position 1 or the position 2 and has no influence on the discriminant of the neutral line current, the invention takes the disconnection at the position 1 as an example to carry out simulation analysis and calculation. As shown in FIG. 3, the neutral line current change curve is obtained by simulation, and the load unbalance degree ζ is calculated according to the formula (12), wherein the load unbalance degree ζ is respectively a ＝0.3、ζ b =0.3 and ζ a ＝0.6、ζ b I when=0.6 Z And I N The current profile is shown in fig. 4. At 0.6s, ζ can be calculated from formula (3) a ＝0.3、ζ b Neutral point offset voltage U when=0.3 oo′ =23.85V, system side neutral point to ground voltageThen according to (5), the system side neutral point connection can be calculatedGround current amplitude I N =9.85a and neutral current amplitude I Z =0.59a; in zeta a ＝0.6、ζ b When=0.6, U can be calculated as well oo′ ＝55.04V、/>I N =16.34A and I Z =0.23a. As can be seen from the calculation result, after the neutral line is broken for 0.5s, the load has I under any load compensation balance degree N ＞I Z The neutral line current criterion theory part is consistent with the simulation result.
S102, monitoring 3 rd harmonic current of the sudden change of the head end of the three-phase line after the neutral line breaks down, wherein the 3 rd harmonic current is shown in fig. 5. When the three-phase load after disconnection is in a balanced state, namely zeta a ＝0、ζ b =0, the harmonic current variation curve is shown in fig. 5 (a); zeta when three-phase load is in unbalanced state a ＝0.3、ζ b For example, =0.3, the harmonic current variation curve is shown in fig. 5 (b). And then, according to the formula (11), the 3-order harmonic current mutation rates delta I% = 9.5% and 13.7% are calculated respectively, so that the harmonic current at the head end of the three-phase line after the neutral line is disconnected can be changed, and the harmonic current mutation rates delta I% under different load unbalance degrees are different. Finally, judging 9.5 percent less than delta and 13.7 percent E delta according to the criterion delta I percent more than or equal to delta in the formula (1), and judging that the criterion is invalid when the three-phase load is balanced according to a judging result, wherein the disconnection fault can be judged when the three-phase load is unbalanced.
S103, calculating the neutral point offset voltage U after normal operation and line breakage according to formulas (13) and (14) oo′ The adaptive coefficient η is calculated according to equation (15) using the neutral line current, the load voltage, and the a-phase and B-phase load unbalance, and the calculation results are shown in tables 1 and 2.
TABLE 1 Voltage and Voltage adaptation coefficient in neutral Normal State
Table 2 data of voltage after neutral line break
As can be seen from Table 1, when the neutral line is in a normal operation state and the load asymmetry is greater than 0.1, U is in the criterion oo′ ≥(k′+η)U n The voltage adaptation coefficient eta of the neutral line is increased along with the increase of the asymmetry zeta of the load, so that the neutral line has remarkable braking effect, and the neutral line is ensured to be reliably protected from action when being normal; as can be seen from Table 2, after neutral line breakage fault, criterion U is used oo′ ≥(k′+η)U n I.e. when the voltage is adapted to the neutral line break protection criterion, no current flows through the neutral line Z =0, when A, B phase load asymmetry ζ a >0.3、ζ b >And 0.3, meeting the criterion and protecting the operation.
S104, because the criterion in S101 is easily affected by fault current, misjudgment and missed judgment can occur under the condition of complex network equation; the judgment is easily affected by the load unbalance degree only by the criteria in S102, the degree of dependence on the fault set value delta is higher, and the accuracy and reliability of the disconnection fault judgment are lower; the effective discrimination cannot be performed only by the criterion in S103 when the unbalance is smaller than 0.3, and the recognition range is still limited. The invention effectively combines the three criteria in S101, S102 and S103 to form complementation, and jointly judges neutral line disconnection faults, and utilizes the current criterion to make up for the limitation of the identification range of the improved voltage criterion, so that accurate disconnection identification can be realized under any unbalanced load, protection misoperation caused by misjudgment of single current or voltage criterion identification is avoided, voltage change is obvious due to DG access, correction judgment can be provided for the current criterion by utilizing the improved voltage criterion, and the judgment result is more accurate and effective.
In conclusion, compared with the traditional judging method and the single electric quantity judging method (voltage or current), the comprehensive judging method provided by the invention has the advantages of strong applicability, high accuracy, coordination with protection and improvement of sensitivity, so the method has practical significance.
Claims (5)
1. The comprehensive judging method for the disconnection of the neutral line of the low-voltage distribution network based on the current and the voltage is characterized by comprising the following steps of:
a, calculating neutral line current I according to a system network equation after neutral line disconnection Z With system side neutral point ground line current I N Judging the relation between the two to form a first current criterion;
b, monitoring the harmonic current at the head end of the three-phase line of the system, calculating the harmonic current mutation rate delta I, and comparing and judging with a fault set value delta to form a second current criterion;
c, introducing a voltage adaptation coefficient eta, and calculating a neutral line disconnection detection voltage criterion based on the voltage adaptation coefficient according to the relation between the load unbalance degree and the offset voltage;
d, comprehensively judging the disconnection fault according to the current and voltage criteria obtained by calculation in the step A, the step B and the step C;
step A, calculating neutral line current I according to a system network equation after neutral line disconnection Z With system side neutral point ground line current I N The determining of the relation between the two, specifically includes:
s1, if the neutral line is broken at the position 1:
step (1) list node voltage equations according to the equivalent circuit after the fault:
z in 1 Indicating the system ground electrode resistance, Z 2 Representing the repeated grounding point grounding resistance at the incoming line of the user; z is Z a 、Z b 、Z c The load equivalent impedance of each phase is respectively; z is Z N Is the neutral line resistance;for the system side neutral point to ground voltage, +.>The neutral point to ground voltage at the load side; />Z i The system comprises a system phase voltage, an equivalent voltage provided by DG in each phase and an equivalent impedance of load of each phase, wherein i=a, b and c;
step (2) calculating the offset voltages U of the two neutral points on the system side and the load side according to the formula (2) oo′ System side neutral point to ground voltage
In the middle ofLoad voltage of each phase after disconnection fault;
step (3) calculating I according to formulas (3), (4) Z And I N ：
From the following componentsZero is available, and the current amplitude I of the system side neutral point grounding wire N And neutral line current amplitude I Z All equal to zero, if the distributed capacitance current of the line is considered, the current of the two is not completely zero, and is in parallel shunt relation, and Z 1 ＜Z N +Z 2 Then there is I N ≥I Z A relationship of approximately 0;
s2, if the neutral line is broken at the position 2:
step (1) list node voltage equation
Step (2) calculating U according to formula (6) oo′ 、And +.>
Step (3) the calculation result of the above formula is introduced into formula (5) becauseObtainable I N ＞0，I Z =0, thus there is I N ＞I Z A relation of =0;
step C introduces a voltage adaptation coefficient eta, calculates a neutral line disconnection detection voltage criterion based on the voltage adaptation coefficient according to the relation between the load unbalance degree and the offset voltage, and specifically comprises the following steps:
setting the same load power factor of each phase, maximizing the load power of the c phase, and defining the load unbalance degree of the a phase and the b phase as follows:
step (2) taking the above formula (12) into formulas (3) and (7), respectively, and calculating U oo′ 、Relationship with three-phase load imbalance:
from equations (13) and (14), it is known that unbalanced current is generated by unbalanced load of three phases, and flows through the neutral line, and the neutral point on the load side is grounded due to neutral line impedanceAs the load unbalance degree increases, the magnitude U of the offset voltage of the neutral point when the neutral line is disconnected oo′ The degree zeta of asymmetry of the three-phase load is determined and is not influenced by other factors;
step (3) defines ζ=max { ζ according to the formula in step (1) a ，ζ b The magnitude of the zeta value corresponds to the asymmetry degree of the three-phase load, the greater the zeta value is, the more the three-phase load is asymmetric, the voltage adaptation coefficient eta is introduced, the traditional neutral point offset voltage criterion is improved, and the value of the voltage adaptation coefficient eta mainly reflects the neutral line current I Z The ratio to the load voltage is as follows:
the neutral line breakage detection voltage criterion based on the voltage adaptation coefficient in the step (4) is as follows:
U oo′ ≥(k′+η)U n (16)。
2. the method for comprehensively judging the disconnection of the neutral line of the low-voltage distribution network based on the current and the voltage according to claim 1 is characterized by comprising the following steps of: and D, comprehensively judging the disconnection fault according to the current and voltage criteria calculated in the steps A, B and C, wherein the method specifically comprises the following steps:
judging according to the formula (1):
wherein I is Z Is neutral line current; i N The system side neutral point grounding wire current; delta I% is the third harmonic current mutation rate on a three-phase line, delta is a fault set value, and generally 10% -15%; u (U) oo′ Is the neutral point offset voltage when the neutral line is broken, k' is the voltage setting value coefficient, eta is the voltage adaptation coefficient, U n Is rated voltage;
if the electric quantity of the current and the voltage meets the formula (1), judging that the neutral line of the low-voltage distribution system is broken; if the neutral line is not broken, the neutral line is judged to be not broken.
3. The method for comprehensively judging the disconnection of the neutral line of the low-voltage distribution network based on the current and the voltage according to claim 1 is characterized in that the step B is used for calculating the harmonic current mutation rate delta I and specifically comprises the following steps:
the 3 rd harmonic mutation is detected on three phases simultaneously, and the mutation quantity delta I of the 3 rd harmonic current is obtained 3 The definition is as follows:
ΔI 3 ＝I 3,x+1 -I 3,x (10)
wherein x represents the x-th sampling data; ΔI 3 Representing the difference between the data acquisition values of the front and rear data;
Δi% is calculated from formula (10):
4. the method according to claim 1, characterized in that: the tuning principle of k' is as follows: ensuring that the neutral line is protected from misoperation during normal operation, (k' +eta) U n Setting the setting value of (2) according to the maximum value of neutral point offset voltage which is generated when the neutral line is avoided from normal operation, and when ζ is more than or equal to 0.1, ηU is calculated n The braking effect is obvious, so the setting of the k' fixed value can be satisfied when zeta is less than 0.1 and the protection is not wrong.
5. The method according to claim 4, wherein: the fixed value of k' is selected to be 0.08-0.10.
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