CN110635452B - Zero-sequence overcurrent protection method locked through resistance-capacitance ratio - Google Patents

Zero-sequence overcurrent protection method locked through resistance-capacitance ratio Download PDF

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CN110635452B
CN110635452B CN201910831524.8A CN201910831524A CN110635452B CN 110635452 B CN110635452 B CN 110635452B CN 201910831524 A CN201910831524 A CN 201910831524A CN 110635452 B CN110635452 B CN 110635452B
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resistance
zero
capacitance ratio
absolute value
capacitance
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CN110635452A (en
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董凯达
魏阳
金震
侯炜
华秀娟
冯宝成
陈建松
安振华
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NR Electric Co Ltd
NR Engineering Co Ltd
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H3/00—Emergency protective circuit arrangements for automatic disconnection directly responsive to an undesired change from normal electric working condition with or without subsequent reconnection ; integrated protection
    • H02H3/26—Emergency protective circuit arrangements for automatic disconnection directly responsive to an undesired change from normal electric working condition with or without subsequent reconnection ; integrated protection responsive to difference between voltages or between currents; responsive to phase angle between voltages or between currents
    • H02H3/32—Emergency protective circuit arrangements for automatic disconnection directly responsive to an undesired change from normal electric working condition with or without subsequent reconnection ; integrated protection responsive to difference between voltages or between currents; responsive to phase angle between voltages or between currents involving comparison of the voltage or current values at corresponding points in different conductors of a single system, e.g. of currents in go and return conductors

Abstract

The invention discloses a zero sequence overcurrent protection method locked by a resistance-capacitance ratio condition, which is suitable for a neutral point grounding system through a small resistor, and comprises the following steps of 1: acquiring zero-sequence voltage and zero-sequence current, and calculating an absolute value of a resistance-capacitance ratio in real time; step 2: comparing the absolute values of the zero-sequence current and the resistance-capacitance ratio with corresponding threshold values respectively, and judging whether the ground fault occurs or not; and step 3: and if the earth fault is judged, tripping is delayed through setting. The resistance-capacitance ratio threshold used in the method is calculated according to the neutral point grounding resistance of the system with the neutral point grounded through the small resistor and the total system grounding capacitance current level.

Description

Zero-sequence overcurrent protection method locked through resistance-capacitance ratio
Technical Field
The invention relates to a ground fault protection method for power transmission and distribution of a power system, in particular to a zero-sequence overcurrent protection method locked under a resistance-capacitance ratio condition.
Background
When the power distribution network adopts a mode that a neutral point is grounded through a small resistor, if a single-phase metallic grounding fault occurs, the fault point and the neutral point small resistor form a zero-sequence current path, so that the fault current is large, and the grounding fault can be quickly removed by using zero-sequence overcurrent protection; for a neutral point ungrounded system or a neutral point arc suppression coil system, even if a metallic grounding fault occurs, the fault current is still very small, and the fault cannot be removed by rapid action in conventional zero-sequence protection, so that the system is generally allowed to operate for 2 hours. In view of the above comparison, the mode of grounding the neutral point through the small resistor has the advantage of quickly removing the ground fault, and thus, the neutral point is increasingly applied to urban power distribution networks, for example, the popularization work of the mode of grounding the neutral point through the small resistor is being carried out in the Guangdong. In addition, due to the expansion of the scale of the power grid, the capacitance current of the system to the ground is continuously increased, so that the compensation capacity of the arc suppression coil is insufficient, and some transformer substations are gradually changed into a mode that a neutral point is grounded through a small resistor because the capacity expansion of the arc suppression coil is difficult.
However, in a 10kV system in which a neutral point is grounded through a small resistor, due to the consideration of selectivity, zero-sequence overcurrent protection generally only reflects a ground fault below a transition resistor 100 Ω at the constant value setting, and if a high-resistance ground fault above 100 Ω occurs, zero-sequence protection cannot trip, and in addition, because the open delta voltage (secondary rated value 100V) is less than 10V at this time, a threshold for sending a zero-sequence voltage alarm signal cannot be reached, the system is in a fault state that the system cannot trip and has no alarm information prompt, and the system can trip only after reaching a protection action condition after the fault develops more seriously, and even in a certain area, a situation that a lead falls into a pond to boil water without a protection action occurs.
The neutral point is generally used in the system that cable run is more through the low resistance ground connection mode, and the condition of high resistance ground connection is less, and along with promoting, the condition that relates to the overhead line has also become more, if distribution lines drops on media such as moist sand ground, dry meadow, turf, transition resistance is in 200 ~ 500 omega within ranges, will get into and can not trip also the high resistance ground connection fault state of no warning information, leaves the potential safety hazard that exists for a long time easily. Therefore, there is a need to improve the current zero sequence protection and improve the high resistance detection level of the ground fault.
How much the high resistance ground detection level reaches is appropriate? Considering the limitation of the measurement precision of the voltage and the current of the protection device, the high-resistance grounding detection level always has a limit, compared with a neutral point non-effective grounding system, the regulation stipulates that the single-phase grounding fault current of a 10kV power grid reaches more than 10A, and a grounding mode through an arc suppression coil is adopted; when the single-phase earth fault current reaches more than 150A, the system is preferably changed into a small-resistance earth system. Therefore, for a system with a neutral point not grounded, the single-phase grounding fault current is below 10A; in contrast to the conventional arc suppression coil grounding method, an overcompensation method is generally adopted, the overcompensation degree is 5% -10%, and the single-phase grounding fault current after compensation is about 10A, so that when a small-resistance grounding system cannot remove a fault, the single-phase grounding fault current can be preferably controlled to be about 10A, for example, a system of 10kV in guangzhou, the neutral point grounding resistance is 10 Ω, and the high-resistance grounding detection capability of 10A corresponding to the high resistance is about 600 Ω.
Therefore, it is necessary to improve zero sequence protection of a power distribution network with a neutral point grounded through a small resistor, and a zero sequence overcurrent protection method is provided, so that zero sequence protection can adapt to grounding conditions of different transition resistors of a system, and particularly, the high resistance detection level of a ground fault is improved.
Disclosure of Invention
The invention aims to provide a zero-sequence overcurrent protection method locked by a resistance-capacitance ratio condition aiming at a system in which a neutral point is grounded through a small resistor, which can adapt to the grounding condition of different transition resistors of the system, and particularly improve the high-resistance detection level of a ground fault.
In order to achieve the above purpose, the solution of the invention is:
in one aspect, the present invention provides a zero sequence overcurrent protection method locked by a resistance-capacitance ratio, including:
according to the collected zero sequence voltage
Figure BDA0002190860360000031
Zero sequence current
Figure BDA0002190860360000032
Calculating absolute value of resistance-capacitance ratioRC0|；
Neutral point grounding resistor R of system grounding through small resistor according to neutral pointrSetting threshold value K of absolute value of resistance-capacitance ratio of rated phase voltage and system total earth capacitance current level0.set；
When the preset resistance-capacitance ratio locking condition is met, opening the zero sequenceAnd overcurrent protection, wherein the preset resistance-capacitance ratio locking condition is as follows: absolute value of resistance-capacitance ratio | KRC0| is greater than the threshold value K0.set。
Further, the absolute value of the resistance-capacitance ratio | KRC0The method for calculating | is as follows:
calculating zero sequence voltage
Figure BDA0002190860360000033
And zero sequence current
Figure BDA0002190860360000034
Ratio of
Figure BDA0002190860360000035
The expression is as follows:
Figure BDA0002190860360000036
in the formula, G0Representing the real part of the component produced by the resistive part, BC0An imaginary component generated to represent the capacitive part;
calculating the ratio of the real part and the imaginary part and taking the absolute value to obtain the absolute value of the resistance-capacitance ratio | KRC0The expression is as follows:
Figure BDA0002190860360000037
further, it is characterized in that the threshold value K of the absolute value of the resistance-capacitance ratio0.setThe calculation method is carried out according to the following formula:
Figure BDA0002190860360000038
in the formula, KkFor a reliability factor, Uph.nFor rated phase voltage, Σ ICIs the system total capacitance-to-ground current level.
Further, a threshold value K of absolute value of resistance-capacitance ratio0.setReliability coefficient K in calculation methodkTaking 0.6-0.9.
Further, according to the threshold value K of the absolute value of the resistance-capacitance ratio0.setCalculation formula, KkTaking a typical value of 0.8, 10kV distribution network Uph.nTake 5.77kV, RrTaking the typical value of 10 omega, sigma ICObtaining a capacitance current level value 600A containing most of 10kV power distribution network to obtain a threshold value K of an absolute value of a resistance-capacitance ratio0.setIs more than 0.77.
Further, except for the preset RC ratio locking condition, i.e. RC ratio absolute value | KRC0| is greater than the threshold value K0.setAnd also comprises a zero sequence current effective value 3I0Greater than its threshold value of 3I0.setAnd when the two conditions are met, the zero-sequence overcurrent protection is started.
After the zero sequence overcurrent protection is started, a tripping counter starts to time, and if the tripping time delay is reached, a tripping command is sent out; and if the condition is not met before the tripping delay is reached, the protection returns, and the tripping timer is cleared.
In another aspect, the invention provides a zero-sequence overcurrent protection system locked by a resistance-capacitance ratio condition, which is characterized by comprising a resistance-capacitance ratio absolute value calculation module, a resistance-capacitance ratio absolute value threshold value setting module and a zero-sequence overcurrent protection starting module;
the resistance-capacitance ratio absolute value calculation module is used for calculating the zero sequence voltage according to the acquired zero sequence voltage
Figure BDA0002190860360000041
Zero sequence current
Figure BDA0002190860360000042
Calculating absolute value of resistance-capacitance ratioRC0|；
The resistance-capacitance ratio absolute value threshold value setting module is used for: neutral grounding resistor R for grounding system via small resistor according to neutral pointrAnd setting a threshold value K of the absolute value of the resistance-capacitance ratio of the total system earth capacitance current level0.set；
The zero sequence overcurrent protection starting module is used for starting zero sequence overcurrent protection when a preset resistance-capacitance ratio locking condition is metThe preset resistance-capacitance ratio locking condition is as follows: absolute value of resistance-capacitance ratio | KRC0| is greater than the threshold value K0.set。
Further, the zero sequence overcurrent protection starting module is used for meeting a preset resistance-capacitance ratio locking condition and having a zero sequence current effective value of 3I0Greater than its threshold value of 3I0.setAnd then, starting zero sequence overcurrent protection, wherein the preset resistance-capacitance ratio locking condition is as follows: absolute value of resistance-capacitance ratio | KRC0| is greater than the threshold value K0.set。
The invention has the beneficial effects that:
(1) the resistance-capacitance ratio locking condition is not influenced by the polarities of PT and CT, the special zero sequence CT can be reliably used, and the smaller zero sequence current can be accurately measured, so that the reliability of high-resistance fault detection is ensured, and the level of high-resistance ground fault detection of a neutral point through a low-resistance ground system is favorably improved;
(2) the resistance-capacitance ratio locking condition is not influenced by the sizes of the transition resistance and the zero sequence voltage and is only related to the ground capacitance parameters of the system and the neutral point grounding resistance, so that a fault line and a non-fault line can be stably distinguished, and the selectivity of high-resistance grounding fault detection is ensured;
(3) the invention judges whether the preset resistance-capacitance ratio locking condition is met and the zero-sequence current effective value is 3I0Greater than its threshold value of 3I0.setWhen the high-resistance grounding fault detection circuit is used, zero-sequence overcurrent protection is started, and the problem that self capacitance current is difficult to avoid due to a too low zero-sequence overcurrent fixed value is avoided, so that the high-resistance grounding fault detection circuit is suitable for the situation of high-resistance grounding fault, meanwhile, no misoperation occurs due to the influence of a non-fault circuit on the grounding capacitance current, and the high-resistance grounding fault detection sensitivity is high;
(4) the original protection configuration is not changed, and the method is easy to implement and apply in actual engineering.
(5) The zero-sequence protection high-resistance grounding detection capability of a neutral point through a small-resistance grounding system can be improved from 100 omega to 600 omega, the sensitivity is improved to a single-phase grounding fault capable of reflecting 10A current level, and therefore the operation safety of the power distribution network is improved.
Drawings
FIG. 1 is a schematic diagram of a system main connection to a neutral point grounded via a small resistor, according to an embodiment of the present invention;
FIG. 2 is a flow chart of zero sequence overcurrent protection via RC condition locking according to an embodiment of the present invention;
fig. 3 is a schematic diagram of the protection range of zero-sequence overcurrent protection locked by the condition of the resistance-capacitance ratio according to the embodiment of the present invention.
Detailed Description
The technical scheme of the invention is explained in detail in the following with the accompanying drawings.
The main wiring diagram of the system in which the neutral point is grounded via a small resistor is shown in FIG. 1, and the grounding resistance of the neutral point is RrThe 10kV system can take a typical value of 10 omega, the line 4 in the figure 1 has single-phase earth fault, and the transition resistance is Rg. Setting three-phase voltage as
Figure BDA0002190860360000061
Figure BDA0002190860360000062
Zero sequence voltage of
Figure BDA0002190860360000063
The voltage transformer PT collects the voltage of the bus to obtain three-phase voltage
Figure BDA0002190860360000064
Zero sequence voltage obtained by PT open delta voltage and zero sequence voltage above neutral point small resistor by neglecting grounding variable impedance
Figure BDA0002190860360000065
Are consistent. In fig. 1, CT01 to CT04 are zero sequence current transformers of lines 1 to 4, respectively, and can measure zero sequence current of each line. CT0Rr is a zero sequence current transformer of the neutral point small resistance branch, which can measure the zero sequence current of the neutral point small resistance branch.
A zero sequence overcurrent protection method locked by a resistance-capacitance ratio comprises the following steps:
the method comprises the following steps:according to the collected zero sequence voltage
Figure BDA0002190860360000066
Zero sequence current
Figure BDA0002190860360000067
Calculating absolute value of resistance-capacitance ratioRC0|；
The neutral point is earthed via a small resistor system and requires that the 10kV part of the load side transformer is delta connection, so that the sum of three-phase load currents of each line is zero, and therefore, for a non-fault line, zero sequence current is calculated
Figure BDA0002190860360000068
I.e. the sum of the capacitance-to-ground currents of the three phases:
Figure BDA0002190860360000069
in the formula: n represents the non-faulty line in fig. 1, n is 1,2, 3;
Figure BDA00021908603600000610
zero sequence current of a non-fault line; j is an imaginary unit; omega is the power frequency synchronous angular velocity; cnIs the capacitance to ground of each non-faulted line.
For a fault line, according to kirchhoff's law, zero-sequence current of all non-fault lines flows, and zero-sequence current of a neutral point small resistor flows, wherein the zero-sequence current of the neutral point small resistor is
Figure BDA0002190860360000071
Therefore, the zero-sequence current of the fault line is as shown in formula 2, wherein the negative sign is the sum of the zero-sequence currents of the non-fault line and the neutral point small resistor, which is kept to be 0:
Figure BDA0002190860360000072
in the formula:
Figure BDA0002190860360000073
zero sequence current for a faulty line, i.e. line 4 in fig. 1; the sigma C is the sum of the system capacitance to ground; c4Is the capacitance to ground of the faulty line.
Figure BDA0002190860360000074
The voltage drop will vary depending on the location of the fault point, and is greatest when a metallic short-circuit earth fault occurs at the bus, which is approximately the phase voltage.
Changing the form of equation 1 and equation 2 into a calculation
Figure BDA0002190860360000075
This expression represents that the zero-sequence currents of the fault line and the non-fault line are different in structure, and the calculation results of the non-fault line and the fault line are respectively shown in formulas 3 and 4:
Figure BDA0002190860360000076
Figure BDA0002190860360000077
both of equations 3 and 4 can be collectively expressed in the form:
Figure BDA0002190860360000078
in the formula, G0Is a real part, the meaning of which represents the component generated by the resistive part; b isC0Is an imaginary part, whose meaning represents the component generated by the capacitive part, considering the possible inverse polarity of PT and CT, G0、BC0Possibly positive or negative, for non-faulty lines, G0Theoretically 0, but actually the influence of the line profile parameter is not really 0, but the value is small.
Taking the ratio of the real part and the imaginary part in the formula 5, and taking the absolute value, which is called the absolute value of the resistance-capacitance ratio in the application of the invention, by | KRC0I represents the ratio of the resistive current to the capacitive current when a single-phase earth fault occurs in the neutral point small-resistance earth system, which is essentially represented by equation 6:
Figure BDA0002190860360000081
for a non-faulty line, since the real part is 0, the theoretical value of the absolute value of the resistance-capacitance ratio is 0.
Therefore, zero sequence voltage is collected
Figure BDA0002190860360000082
Zero sequence current
Figure BDA0002190860360000083
Calculating the ratio according to formula 5, taking the real part and the imaginary part, substituting into formula 6, and calculating to obtain the absolute value | K of the resistance-capacitance ratioRC0|。
Step two: neutral point grounding resistor R of system grounding through small resistor according to neutral pointrSetting threshold value K of absolute value of resistance-capacitance ratio of rated phase voltage and system total earth capacitance current level0.set；
For the fault line, according to equation 4, it can be obtained that the absolute value of the resistance-capacitance ratio satisfies equation 7:
Figure BDA0002190860360000085
the denominator of equation 7 is further expanded to 3 ω (Σ C-C)4) Amplifying to 3 ω Σ C, an inequality is obtained, as shown in equation 8, where the right expression of the inequality is a constant dependent on the parameters of the distribution network system, RrThe 3 omega sigma C is the neutral point small resistance value of the power distribution network and depends on the total capacitance to ground of the power distribution network.
Figure BDA0002190860360000084
Determining threshold value K of resistance-capacitance ratio coefficient according to equation 80.set；
Step three: and opening zero sequence overcurrent protection when a preset resistance-capacitance ratio locking condition is met, wherein the preset resistance-capacitance ratio locking condition is as follows: absolute value of resistance-capacitance ratio | KRC0| is greater than the threshold value K0.set。
According to the formula 8, when a single-phase earth fault occurs to a neutral point through a small-resistance earth system, the fault line has the characteristic that the absolute value of the resistance-capacitance ratio is larger than a system constant, and according to the formula 3, the absolute value of the resistance-capacitance ratio of a non-fault line is about 0, and the characteristic has obvious discrimination; and in equation 8, the variable of the transition resistance is not involved, which means that equation 8 is always satisfied no matter how large the transition resistance is when the transition resistance ground fault occurs, so that the condition is applicable to the case of single-phase metallic ground and high-resistance ground; because an absolute value is taken, the calculation has no relation with whether the polarities of the CT and the PT are reversely connected, and the condition is used as a locking condition of zero-sequence overcurrent protection, so that the influence of reverse connection of the polarity of the special zero-sequence CT can be avoided.
After the problem of polarity check is solved, the special zero sequence CT can be reliably used, when the ground fault current level of 10A is required to be detected, namely the requirement of high resistance detection reaches 600 omega, the typical value of a small resistance of a neutral point of a 10kV system is 10 omega, the typical transformation ratio of the special zero sequence CT is 100/1 for example, the zero sequence current of a fault line 10A is converted into a secondary value of 100mA, the precision current of a protection device can generally reach about 20mA, and the current of 100mA can be completely and accurately measured; at this time, the corresponding zero sequence voltage (taking the open delta voltage quadratic rating of 100V as an example) is about 1.64V, and the protection device can also accurately measure the zero sequence voltage.
If the self-produced zero-sequence current of the protection three-phase current is used, the CT transformation ratio of the protection current is larger, taking typical 1000/1 as an example, the precision current of the protection device can only be guaranteed to be at the level of 0.02A, and the converted high-resistance ground fault detection level is about 300 omega, which is lower than the anti-transition resistance capability when the special zero-sequence CT is used.
Based on the above embodiment, the threshold value K of the RC ratio coefficient is calculated according to equation 80.setThe specific method comprises the following steps:
as shown in equation 10:
Figure BDA0002190860360000091
in the formula, KkTaking 0.6-0.9 as a reliable coefficient; u shapeph.nIs a rated phase voltage; sigma ICIs the system total capacitance-to-ground current level.
On the basis of the above embodiment, in order to adapt to grounding conditions of different transition resistances of a system, especially to improve a high-resistance detection level of a ground fault, a resistance-capacitance ratio locking condition is added to the conventional zero-sequence overcurrent protection to improve, and an improved zero-sequence overcurrent protection criterion is shown as a formula 9. The criterion can be used for both special zero sequence CT and protection of self-produced zero sequence current, and the transient resistance is insufficient due to the limitation of measurement precision when the protection of the self-produced zero sequence current is used.
Figure BDA0002190860360000101
In the formula, 3I0.setZero-current setting value for zero-sequence overcurrent protection; k0.setThe threshold value of the rc coefficient can be calculated according to equation 8.
Calculating K from equation 80.setIt is necessary to obtain the total capacitance-to-ground parameter of the system, and in fact, the total capacitance-to-ground of the system is directly related to the total capacitance-to-ground current level of the system, so that it can be calculated according to the total capacitance-to-ground current level of the system, as shown in equation 10.
Figure BDA0002190860360000102
In the formula, KkTaking 0.6-0.9 as a reliable coefficient; u shapeph.nFor rated phase voltage, 10kV system sampling5.77kV；ΣICIs the system total capacitance-to-ground current level.
Example 1: and (4) writing a program according to the theory, operating in a protection device, collecting zero sequence voltage and zero sequence current by the protection device, and calculating the resistance-capacitance ratio. At the ground transition resistance RgThe calculation results of the protection device are shown in table 1 when 1 Ω,80 Ω,100 Ω,300 Ω, and 600 Ω are taken. In Table 1, 3I0For zero sequence current of faulty line, | KRC0|FFor the resistance-capacitance ratio of the faulty line, | KRC0|NThe resistance-capacitance ratio of one non-fault line is shown. The results in table 1 are consistent with the theoretical derivation previously described.
TABLE 1 RC ratio calculation results in line 4 fault
Figure BDA0002190860360000111
Based on the theoretical analysis and the calculation result of the embodiment 1, the invention provides a zero-sequence overcurrent protection method locked by a resistance-capacitance ratio condition, the flow is shown in fig. 2, and the method mainly comprises the following three steps:
step 1: collecting zero sequence voltage
Figure BDA0002190860360000112
Zero sequence current
Figure BDA0002190860360000113
Calculating absolute value of resistance-capacitance ratioRC0The absolute value calculation method of the resistance-capacitance ratio is as follows:
first, the zero sequence voltage and zero sequence current are used to calculate according to equation 5
Figure BDA0002190860360000114
Namely the following formula:
Figure BDA0002190860360000115
in the formula, G0Is a real part, the meaning of which represents the component generated by the resistive part; b isC0Is the imaginary part, whose meaning represents the component produced by the capacitive part;
then, the proportion of the real part and the imaginary part is calculated according to the formula 6, and the absolute value is taken to obtain the absolute value | K of the resistance-capacitance ratioRC0I, i.e.:
Figure BDA0002190860360000116
step 2: the ground fault is judged according to the formula 9, namely, the zero sequence current effective value is 3I0Absolute value of resistance-capacitance ratio | KRC0Respectively comparing with respective threshold values, and determining that the ground fault occurs when the following 2 conditions are met, and starting zero-sequence overcurrent protection:
Figure BDA0002190860360000117
in the formula, 3I0.setIs a zero sequence current threshold value; k0.setIs a threshold value of the absolute value of the resistance-capacitance ratio;
and step 3: after the protection is started, a trip counter starts to time, and if the trip delay is reached, a trip command is sent out; and if the condition is not met in the step 3 before the tripping delay is reached, the protection is returned, and the tripping timer is reset.
In step 1 of the zero-sequence overcurrent protection method locked under the condition of the resistance-capacitance ratio, the absolute value threshold value K of the resistance-capacitance ratio0.setCalculated according to equation 10, i.e. by means of the neutral grounding resistance R of the low-resistance grounding systemrAnd system total capacitance-to-ground current level calculation:
Figure BDA0002190860360000121
in the formula, KkTaking 0.6-0.9 as a reliable coefficient; u shapeph.nThe voltage is rated phase, and 5.77kV is taken as a 10kV system; sigma ICIs the system total capacitance-to-ground current level.
The action region of the zero sequence overcurrent protection method locked by the resistance-capacitance ratio condition is shown in fig. 3.
In the context of figure 3, it is shown,
Figure BDA0002190860360000125
for rotating phasors, to
Figure BDA0002190860360000122
Calculating angle for reference, horizontal axis
Figure BDA0002190860360000123
Representing purely resistive zero-sequence current, vertical axis
Figure BDA0002190860360000124
Representing a purely capacitive zero-sequence current. The shaded portion A, B is an active area and the blank area C is a non-active area. The angle θ in FIG. 3 is used to determine the boundary of the motion region, and K0.setThe relationship of (A) is shown in formula 11.
θ＝tan-1 K0.set (11)
As can be seen from equation 11 and fig. 3, the larger the capacitance current level is, the larger the protection range is, and if a value that can contain most of the capacitance current levels of the substation is adopted, a general absolute value of the resistance-capacitance ratio can be obtained, for example, the capacitance current level of 600A is taken as a typical value, and K obtained by calculation is calculated0.setCan contain most 10kV systems, and 600A is substituted into the formula 9, KkTaking the typical value of 0.8, equation 12 is obtained. The requirements of most 10kV systems can be met by adopting the formula 12, and the threshold value of the resistance-capacitance ratio coefficient does not need to be specially calculated. If some special 10kV systems are encountered with particularly large capacitance-to-ground current levels, exceeding 600A, the calculation can be performed by equation 10.
K0.set＞0.77 (12)
Considering that when the same-phase metallic grounding faults of different lines exist simultaneously, two metallic grounding fault points form a path, the fault characteristics similar to the direct grounding of a neutral point can appear at the moment, the phase relation of zero-sequence current and zero-sequence voltage does not accord with the grounding fault characteristics of the neutral point in a mode of grounding through a small resistor any more, but at the moment, the current is very large, the fault is cut off rapidly, and in order to prevent the fault locking under the condition of the resistance-capacitance ratio, the zero-sequence overcurrent I section or the zero-sequence overcurrent rapid disconnection is not locked under the condition of the resistance-capacitance ratio. The resistance-capacitance ratio locking condition is suitable for only the zero sequence overcurrent II section and the zero sequence overcurrent III section. The zero sequence overcurrent II section is used as a backup of the zero sequence overcurrent I section or the zero sequence overcurrent quick break, and needs to be matched with the adjacent section, if the fixed value needs to be set to be lower, a resistance-capacitance ratio locking condition needs to be put into, and therefore the limitation of the maximum ground capacitance current of the circuit can be avoided. The zero-sequence overcurrent III section is used as backup protection, fixed values of a zero-sequence overcurrent I section and a zero-sequence overcurrent II section are generally required to be avoided, and detection of the high-resistance grounding fault is also required to be executed by the zero-sequence overcurrent III section, so that the threshold setting is low, the maximum grounding capacitance current of the circuit is probably not avoided, and therefore resistance-capacitance ratio locking conditions are required to be put into use.
Example 2: the zero sequence overcurrent protection method locked by the resistance-capacitance ratio condition is programmed and operated in a protection device, the protection device operates in a 10kV voltage level neutral point small resistance grounding system, and the protection device collects zero sequence voltage and zero sequence current to judge single-phase grounding faults, particularly high resistance grounding faults. The grounding transformer is positioned on a 10kV bus, and the small resistance of a neutral point is 10 omega. The line parameters are zero sequence resistance 1.35 omega/km, zero sequence inductive reactance 1.104 omega/km, zero sequence capacitive reactance 0.1555M omega/km, and the line lengths are 45km, 35km, 50km and 65km respectively. PT transformation ratio is 10kV/100V, and zero sequence CT transformation ratio is 100/1. The fixed value of the protection device is 0.06A through resistance-capacitance ratio locking zero sequence overcurrent protection. The operation of the line 4 in the case of a single-phase earth fault is shown in table 2.
Table 2 action results of protection device in case of line 4 fault
Figure BDA0002190860360000131
3I in Table 20、U0Secondary values, | K, calculated for actual acquisition in the deviceRC0I is a calculated value in the fault line protection device, and K is a calculated value in the non-fault line protection deviceRC0Calculated value of | is close to 0 and is not in the tableAre listed in (a). And (3) changing the fault point to other lines, simulating the condition that the line 4 is a non-fault line, wherein the resistance-capacitance ratio condition can be correctly locked, and the zero sequence protection is reliable and motionless. Through embodiment 2, it is shown that the resistance-capacitance ratio blocking condition can reliably distinguish a faulty line from a non-faulty line, and can reliably act when a high-resistance ground fault occurs, so that the improved zero-sequence overcurrent protection can reach a high-resistance ground detection capability of a 600 Ω level, and the sensitivity is improved to a single-phase ground fault that can reflect a 10A current level.
In conclusion, the zero-sequence overcurrent protection method based on the resistance-capacitance ratio conditional locking is not influenced by the polarities of PT and CT, and the special zero-sequence CT is used for accurately measuring the smaller zero-sequence current, so that the reliability of high-resistance fault detection is ensured; the resistance-capacitance ratio locking condition is not influenced by the sizes of the transition resistance and the zero sequence voltage and is only related to the earth capacitance parameter and the neutral point earth resistance of the system, so that a fault line and a non-fault line can be stably distinguished, and the selectivity of protection is ensured; the problem that the zero sequence overcurrent fixed value is too low to avoid self capacitance current is avoided, and the sensitivity is high; the method has the advantages that the original protection configuration is not changed, the implementation is easy, the high resistance earth fault detection capability of zero sequence protection can be improved, the 100 omega level of the traditional zero sequence overcurrent protection is improved to the 600 omega level, the sensitivity is improved to the single-phase earth fault capable of reflecting the 10A current level, and therefore the operation safety of the power distribution network with the neutral point grounded through the small resistor is improved.
As will be appreciated by one skilled in the art, embodiments of the present application may be provided as a method, system, or computer program product. Accordingly, the present application may take the form of an entirely hardware embodiment, an entirely software embodiment or an embodiment combining software and hardware aspects. Furthermore, the present application may take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, and the like) having computer-usable program code embodied therein.
The present application is described with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the application. It will be understood that each flow and/or block of the flow diagrams and/or block diagrams, and combinations of flows and/or blocks in the flow diagrams and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.
These computer program instructions may also be stored in a computer-readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including instruction means which implement the function specified in the flowchart flow or flows and/or block diagram block or blocks.
These computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.
While the present invention has been described with reference to the embodiments shown in the drawings, the present invention is not limited to the embodiments, which are illustrative and not restrictive, and it will be apparent to those skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope of the invention as defined in the appended claims.

Claims (8)

1. A zero sequence overcurrent protection method locked by a resistance-capacitance ratio condition is characterized in that,
according to the collected zero sequence voltage
Figure FDA0003097376710000011
Zero sequence current
Figure FDA0003097376710000012
Calculating absolute value of resistance-capacitance ratioRC0|；
Neutral point grounding resistor R of system grounding through small resistor according to neutral pointrSetting threshold value K of absolute value of resistance-capacitance ratio of rated phase voltage and system total earth capacitance current level0.set；
And opening zero sequence overcurrent protection when a preset resistance-capacitance ratio locking condition is met, wherein the preset resistance-capacitance ratio locking condition is as follows: absolute value of resistance-capacitance ratio | KRC0| is greater than the threshold value K0.set，
Absolute value of resistance-capacitance ratio | KRC0The method for calculating | is as follows:
calculating zero sequence voltage
Figure FDA0003097376710000013
And zero sequence current
Figure FDA0003097376710000014
Ratio of
Figure FDA0003097376710000015
The expression is as follows:
Figure FDA0003097376710000016
in the formula, G0Representing the real part of the component produced by the resistive part, BC0An imaginary component generated to represent the capacitive part;
calculating the ratio of the real part and the imaginary part and taking the absolute value to obtain the absolute value of the resistance-capacitance ratio | KRC0The expression is as follows:
Figure FDA0003097376710000017
2. the zero-sequence over-current protection method locked by the RC condition as claimed in claim 1, wherein the RC absolute value threshold K is set0.setThe calculation method is carried out according to the following formula:
Figure FDA0003097376710000018
in the formula, KkFor a reliability factor, Uph.nFor rated phase voltage, Σ ICIs the system total capacitance-to-ground current level.
3. The zero-sequence over-current protection method locked through the resistance-capacitance ratio condition as claimed in claim 2, wherein the reliability coefficient K iskTaking 0.6-0.9.
4. The zero-sequence over-current protection method through RC condition locking according to claim 2, wherein K iskTaking a typical value of 0.8, 10kV distribution network Uph.nTake 5.77kV, RrTaking a typical value of 10 omega, the capacitance current level Σ ICTaking 600A to obtain a threshold value K of the absolute value of the resistance-capacitance ratio0.setIs more than 0.77.
5. The zero-sequence over-current protection method locked by the RC ratio condition according to any one of claims 1 to 4, wherein the RC ratio locking condition is the absolute value of RC ratio | K except for the predetermined RC ratio locking conditionRC0| is greater than the threshold value K0.setAnd also comprises a zero sequence current effective value 3I0Greater than its threshold value of 3I0.setAnd when the two conditions are met, the zero-sequence overcurrent protection is started.
6. The zero-sequence overcurrent protection method locked through the resistance-capacitance ratio condition according to claim 5, wherein after the zero-sequence overcurrent protection is started, a trip counter starts to time, and if the trip delay is reached, a trip command is sent out; and if the condition is not met before the tripping delay is reached, the protection returns, and the tripping timer is cleared.
7. A zero sequence overcurrent protection system locked by a resistance-capacitance ratio condition is characterized by comprising a resistance-capacitance ratio absolute value calculation module, a resistance-capacitance ratio absolute value threshold value setting module and a zero sequence overcurrent protection starting module;
the resistance-capacitance ratio absolute value calculation module is used for calculating the zero sequence voltage according to the acquired zero sequence voltage
Figure FDA0003097376710000021
Zero sequence current
Figure FDA0003097376710000022
Calculating absolute value of resistance-capacitance ratioRC0|；
The resistance-capacitance ratio absolute value threshold value setting module is used for: neutral grounding resistor R for grounding system via small resistor according to neutral pointrSetting threshold value K of absolute value of resistance-capacitance ratio of rated phase voltage and system total earth capacitance current level0.set；
The zero sequence overcurrent protection starting module is used for starting zero sequence overcurrent protection when a preset resistance-capacitance ratio locking condition is met, and the preset resistance-capacitance ratio locking condition is as follows: absolute value of resistance-capacitance ratio | KRC0| is greater than the threshold value K0.set，
The resistance-capacitance ratio absolute value calculation module calculates the resistance-capacitance ratio absolute value | K by adopting the following methodRC0|：
Calculating zero sequence voltage
Figure FDA0003097376710000031
And zero sequence current
Figure FDA0003097376710000032
Ratio of
Figure FDA0003097376710000033
The expression is as follows:
Figure FDA0003097376710000034
in the formula, G0Representing the real part of the component produced by the resistive part, BC0An imaginary component generated to represent the capacitive part;
calculating the ratio of the real part and the imaginary part and taking the absolute value to obtain the absolute value of the resistance-capacitance ratio | KRC0The expression is as follows:
Figure FDA0003097376710000035
8. the rc condition-locked zero-sequence overcurrent protection system according to claim 7, wherein the zero-sequence overcurrent protection starting module is configured to start the zero-sequence overcurrent protection when a preset rc condition is satisfied and the zero-sequence current has an effective value of 3I0Greater than its threshold value of 3I0.setAnd starting zero sequence overcurrent protection, wherein the preset resistance-capacitance ratio locking condition is as follows: absolute value of resistance-capacitance ratio | KRC0| is greater than the threshold value K0.set。
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