CN117728368A - Neutral point variable resistor grounding system - Google Patents
Neutral point variable resistor grounding system Download PDFInfo
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- CN117728368A CN117728368A CN202311848830.5A CN202311848830A CN117728368A CN 117728368 A CN117728368 A CN 117728368A CN 202311848830 A CN202311848830 A CN 202311848830A CN 117728368 A CN117728368 A CN 117728368A
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H9/00—Emergency protective circuit arrangements for limiting excess current or voltage without disconnection
- H02H9/04—Emergency protective circuit arrangements for limiting excess current or voltage without disconnection responsive to excess voltage
- H02H9/041—Emergency protective circuit arrangements for limiting excess current or voltage without disconnection responsive to excess voltage using a short-circuiting device
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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
- H02H3/02—Details
- H02H3/027—Details with automatic disconnection after a predetermined time
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
- H02H7/26—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H9/00—Emergency protective circuit arrangements for limiting excess current or voltage without disconnection
- H02H9/08—Limitation or suppression of earth fault currents, e.g. Petersen coil
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Abstract
The invention discloses a neutral point variable resistance grounding system, which overcomes the defects of non-grounding, grounding through an arc suppression coil, grounding through low resistance and grounding through high resistance, maintains the advantages of non-grounding, grounding through an arc suppression coil, grounding through low resistance and grounding through high resistance, realizes the function which can not be completely realized by any grounding device in the past, and can be applied to power systems and power supply systems of all enterprises such as petroleum, chemical industry, metallurgy, electrified railway, mine, military industry, airports, harbors, broadcast televisions and the like, and wind power, firepower, hydraulic power and photovoltaic power generation networking systems. The application mode is that the neutral point of each distribution voltage level of the transformer substation is accessed or the artificial neutral point is accessed.
Description
Technical Field
The invention relates to the field of overvoltage accident protection of power systems and industrial enterprise power supply systems, in particular to a neutral point variable resistor grounding system and an operation method thereof
Background
For a long time, the equipment insulation breakdown, the short circuit burning loss and the tripping outage accidents caused by resonance overvoltage, power frequency and high frequency arc grounding overvoltage, system neutral point potential deviation and the like of an industrial enterprise power supply system and a part of power system are not counted, and cable fire, equipment explosion or personal casualty accidents are caused when serious, so that huge direct and indirect economic losses are caused for the country and enterprises. The accident cause is closely related to the grounding mode of the neutral point of the power distribution network by comprehensive analysis, and the safe operation effect is completely different if the grounding modes are different. The neutral point grounding mode is unreasonable, so that the necessity of accidents is determined, and the method is especially unfavorable for continuous power supply of important loads. Therefore, the neutral point grounding mode of the power distribution network is a great technical principle problem related to the power supply reliability.
The current grounding mode 110KV system adopted by the domestic power distribution network is direct grounding, the 66KV system is arc suppression coil grounding, and the 35KV system, the 10KV system and the 6KV system are arc suppression coil grounding or non-grounding or low-resistance grounding or high-resistance grounding.
The advantages and disadvantages of various grounding modes are compared with the comprehensive factors such as power supply continuity, reliability, overvoltage amplitude, suppression level of resonance, suppression level of high-frequency and power-frequency arc grounding overvoltage, grounding line selection accuracy, influence on communication, step voltage and personal safety, whether multipoint fault occurrence rate occurs, cable fire and the like, and are as follows:
1. neutral point ungrounded system (neutral point grounded via arrester belongs to ungrounded system)
The advantages are that:
(1) saving the investment of the grounding device
(2) The system with smaller power frequency capacitance current and harmonic component can continuously operate for a period of time (2 hours) when single-phase grounding occurs
(3) When a single-phase earth fault occurs in the power grid, the steady-state power frequency current is small, and the transient fault such as lightning insulation flashover can be automatically cleared without tripping.
(4) The grounding current is small, and the ground potential rise is reduced. Step voltage and contact voltage are reduced; interference to the information system is reduced; impact on the low voltage network, etc. is reduced.
Disadvantages:
(1) the arc grounding overvoltage multiple is high, and accidents are easy to expand. The overvoltage has long duration, and the ground arc (power frequency and high frequency) of the overvoltage is repeatedly reburned throughout the whole network, so that the overvoltage and the oscillation frequency are continuously increased, and the equipment insulation is endangered. It can be expanded to multipoint faults or even cable fires in less than 0.02 seconds.
The voltage amplitude and the oscillation frequency at the time of arc reignition increase with increasing number of reignitions. As shown in figure 1 of the drawings,
ls is leakage inductance of the power transformer, co relative capacitance to ground and oscillating current of the Ios fault point.
The oscillating circuit formed by the inductance and the capacitance has the following oscillating frequency:
its overvoltage Uov =ust+ Uos
Wherein: uov is an overvoltage; uos is an oscillating voltage; ust is the final regulated voltage.
The oscillation frequency occurs when the second reburning:
amplitude of oscillating voltage:
where Uosm is the amplitude of the oscillating voltage Uos
Overvoltage device
At the time of third reburning:
i.e. when the grounded arc reburns again after half a cycle, the overvoltage continues to rise until the other or both phases of insulation breaks down, shorting out the burning device.
(2) Is easy to generate resonance overvoltage and has no inhibition measure
(3) The influence of harmonic wave is large, no inhibition measures are adopted, and the accident probability is increased
(4) The neutral point displacement voltage is higher and cannot be restrained by itself.
The grounding mode has the advantage of saving investment, but the defects are fatal, no limiting measures are taken for resonance overvoltage and arc grounding overvoltage, only the development can be realized, and the accidents of equipment damage, power failure and production stoppage are inevitable.
2. Neutral point is grounded through arc suppression coil
The advantages are that:
(1) the neutral point is grounded through the arc suppression coil, and compared with the neutral point which is not grounded, the neutral point is greatly improved, and the probability of high-amplitude overvoltage occurrence can be reduced in a mode of compensating the power frequency capacitor current to a smaller value.
(2) When the system is single-phase grounded, the energy band is grounded at a point for 2 hours.
(3) The probability of frequency division resonance overvoltage can be reduced.
Disadvantages:
(1) the insulation fit margin is low. The steady-state overvoltage of the arc suppression coil grounding system reaches 3.2PU, compared with the insulation strength of 3.5PU of equipment, the insulation fit margin is small, the fault probability is naturally higher, and if a plurality of insulation breakdown accidents of the arc suppression coil grounding system are the same;
(2) high frequency arc re-ignition cannot be suppressed.
Intermittent arc ground over-voltage comes mainly from two aspects: firstly, the industrial frequency arc is reburning; and secondly, the high-frequency arc is reburning. For a 6-10 kv system, when the capacitance current of the system to the ground power frequency is not more than 30A, continuous arc reburning is not generated generally, but the high-frequency current is different, even if the 1A current is required to generate continuous arc reburning, and is influenced by harmonic interaction, the high-frequency current can be rapidly amplified in the reburning process and even becomes a main component. Namely, the arc suppression coil can only limit the re-ignition of the power frequency arc but not limit the re-ignition of the high frequency arc, and cannot fundamentally eliminate the arc grounding overvoltage, such as arc re-ignition loss and breakdown insulation accidents which occur in the grounding systems of a plurality of conventional enterprise arc suppression coils;
(3) the suppression effect on the broken line resonance overvoltage is relatively poor, because of the process characteristics of large industrial enterprises, equipment is frequently started and stopped, resonance overvoltage with different degrees is generated each time of operation of breaking or accident tripping, and the damage to a system is large when the resonance overvoltage is serious, such as the high insulation breakdown accident rate of metallurgical enterprises with neutral points grounded through arc suppression coils and the like;
(4) the suppression effect on ferromagnetic resonance is poorer than that of resistance grounding; and ferromagnetic resonance and parametric resonance are more probable resonances;
(5) compensating tuning hysteresis, i.e. the resonant frequency exceeding 200H, being out of control in the event of sudden resonance Z at/S, the system is easy to crash or lose control.
(6) The neutral point displacement voltage of the power grid is high, the system safety is not facilitated, although national standard technical specifications provide limiting conditions for arc suppression coils, accidents in actual operation still occur, such as unbalanced three-phase voltage, high phase insulation is damaged, and faults of a motor, a generator, a capacitor and the like are caused;
(7) when a single-phase grounding occurs to the system, the generated 5-time zero sequence harmonic current cannot be limited and can only be developed at will;
(8) when the harmonic wave of the power grid reaches a certain component, neutral point voltage U when the arc suppression coil grounding system generates single-phase metal grounding O Naturally also contains harmonic components, for example, phase A is grounded, as shown in FIG. 2, i.e
U 0 ＝Ua＝U 1 +U 2 ++…+Un+…(1-1)
The corresponding ground current inevitably contains a harmonic component, and the nth harmonic current content Icn depends on the harmonic voltage content Un. Neglecting L 0 Is approximated as:
Icn≈n×Un (1-2)。
taking the 5 th harmonic as an example, when the 5 th harmonic voltage content Un is 5.0%, the corresponding 5 th harmonic current content is Ic5 ≡5×5% = 25%, and it is seen that the harmonic voltage with a lower content will produce a higher harmonic current content of n times. When the capacitance of the system to the ground is large or the harmonic frequency is high, the impedance L of the system is caused 0 The existence of the grounding harmonic current can cause a more obvious capacitance rising and amplifying phenomenon and even tends to be resonant.
Calculated according to the formula (1-2), the 5-harmonic current content Ic is calculated at 3.5% of the 5-harmonic voltage 5 Theoretically, the capacitance should be 5×3.5% =17.5%, and the measured value is about 20% when the capacitance to ground is 30 μf or less, and L is ignored 0 The theoretical value of the influence is relatively close, but as the capacitance to ground is increased, although the system background harmonic content is not increased, the harmonic is measuredThe wave current content has obvious amplifying trend and L is ignored 0 The theoretical value of the influence deviates more and more. It can be seen that the system harmonic impedance L cannot be ignored when the capacitance to ground is large or the number of harmonics is high 0 Amplifying the harmonics.
In contrast, the choke coil exhibits high impedance at a high-frequency harmonic voltage, and the corresponding n-th harmonic current content ILn is iln= (1/n) Un (1-3), and even at a 5-th harmonic voltage of un=5%, the 5-th harmonic current content ILn in the choke coil compensation current is only 1%, and the compensation effect on the harmonic ground current is negligible. Theory and actual measurement show that the arc suppression coil has good compensation effect on the power frequency grounding current, but basically has no compensation effect on the harmonic part in the grounding current, when the power grid obviously has harmonic waves, the grounding residual current mainly has the harmonic waves, the grounding residual current is extremely easy to exceed the standard limit value of 10A, and when the grounding capacitance is large and the harmonic pollution is serious, the residual current can even exceed 30A.
Therefore, the limit value of the ground residual current of the DL/T1057-2007 standard is less than or equal to 10A, the limit value of the residual current of the product of a common manufacturer is less than or equal to 5A, and the measurement error of the capacitance current is less than or equal to 2%. In practice, this is not possible in the presence of harmonics in the system, and may be severely overstocked.
The working principle of the arc suppression coil grounding system is resonance grounding, the resonance point is close to the power frequency of 50Hz, and the working principle of the arc suppression coil grounding system determines the uncompensated capability to high-frequency harmonic current. And therefore is significantly less adaptable to systems with harmonics.
(9) The system load is not suitable for capacity increase, namely, the arc suppression coil also needs capacity increase, and spare part cost and the like are additionally increased.
3. Neutral point is grounded through low resistance
The advantages are that:
(1) the arc light grounding overvoltage can be 2.5PU, and the insulation fit margin is good.
(2) When the line is permanently grounded, the line selection is accurate, and the grounding fault line can be tripped immediately.
Disadvantages:
(1) poor power supply continuity, immediate trip of single-phase grounding, adverse to the power supply continuity of important loads;
(2) due to the artificial current increase, the instantaneous ground which is automatically eliminated immediately is easily evolved into a tripping power failure. If the time is required to be delayed, the burning loss with larger fault points occurs;
(3) the resonance is not completely restrained, and the low resistance is a secondary damping resistance or an ineffective damping resistance.
4. Neutral point is grounded through high damping resistor
The advantages are that:
(1) various resonance overvoltage of the system is effectively limited;
after the neutral point is connected with the resistor, the neutral point is as shown in fig. 3: the total pressure drop of the loop will become DeltaU', which can be written as
Namely: i, UL and UC are determined by R, and do not tend to infinity, and play a role in damping of resistance, so that resonance condition is destroyed, and resonance does not occur any more.
(2) The intermittent arc grounding overvoltage amplitude is effectively limited to be less than 2.5 times of phase voltage, and when a single-phase grounding happens to the system, accidents are not expanded, and the cable fire accident caused by sudden multipoint faults is prevented.
(3) Reducing neutral point displacement voltage of a power grid;
(4) when single-phase earth fault occurs, the system can be operated for 2 hours with one point of earth;
(5) the relay protection mode of the system is not changed;
(6) because the resonance overvoltage is eliminated, the amplitude of the arc connection overvoltage is reduced, which is equivalent to improving the insulation level of the system equipment, prolonging the service life of the system equipment and improving the power supply reliability;
(7) the resistor is suitable for the conditions of capacity expansion and large-range change of capacitance current to the ground after the system, and the resistor does not need to be adjusted; the equipment is simple, reliable and long in service life.
Disadvantages:
(1) the selectivity of the grounding line selection is relatively poor;
(2) the resistor has high quality requirement standard, and the disposable investment is slightly higher than that of other grounding devices;
5. intelligent dynamic power distribution network neutral point grounding method and complete equipment
CN103208788A discloses a method and complete equipment for grounding neutral points of intelligent dynamic power distribution network, and the advantages are:
(1) the problem of arc suppression coil grounding system ground connection route selection is difficult is solved. The method realizes the grounding route selection by connecting the damping resistor in series with the low-value resistor and the dynamic conversion in the arc suppression coil loop, and trips the grounding fault line.
When the power grid normally operates, the arc suppression coil is grounded, the damping resistor and the low-value resistor are in a shutdown state, when single-phase grounding occurs, instantaneous grounding is filtered out by short delay, when the power grid is permanently grounded, the damping resistor and the low-value resistor are put into operation, line selection action conditions are provided for a zero sequence protection system of a transformer substation, a grounding fault line is tripped by short delay, and then the resistor is taken out of operation. The method solves the problem of difficult line selection under the mode that the neutral point passes through the arc suppression coil to be grounded, and is a great progress in the technology.
(2) Can filter out instantaneous grounding
Disadvantages:
(1) the power supply continuity of important loads is not facilitated;
the permanent grounding of the power grid is not beneficial to the power supply continuity of important loads through short delay tripping, and the sudden power failure of the important loads is easy to cause, so that large loss is caused;
(2) the suppression effect on resonance overvoltage is relatively poor;
the method is an improvement on the basis of the arc suppression coil, and when the power grid runs daily, the main body of the grounding device is still the arc suppression coil, and most of defects of the arc suppression coil cannot be overcome. (the defects of the arc suppression coil are described), the effect of the damping resistor with higher resonance suppression effect in daily operation is poor, and the main source of insulation breakdown of system equipment is from resonance. If the method is used, the arc suppression coil is out of operation in a short time when the power grid is permanently grounded and the resistor is put into operation.
(3) Insulation fit margin is low
The overvoltage amplitude of the neutral point through the arc suppression coil grounding system is 3.2PU, and compared with the overvoltage amplitude of a resistor grounding mode, the insulation fit margin is relatively low. The method and the device only have the advantages that the overvoltage amplitude is low, the insulation fit margin is high in a short time after the permanent grounding is converted into the small-resistance grounding mode, and most of the time when the permanent grounding does not occur comprises the step of filtering out the state that the insulation fit margin is low during the instant grounding.
The summary of incomplete statistics and investigation of the comparison of the operation conditions of the various grounding modes of the neutral point of the power grid is shown in table 1:
table 1 comparison table for various different earthing modes of neutral point of electric network
Disclosure of Invention
The invention overcomes the defects of the modes of ungrounded neutral point, arc suppression coil grounding, low-resistance grounding and high-resistance grounding, maintains the advantages of ungrounded neutral point, arc suppression coil grounding, low-resistance grounding and high-resistance grounding, realizes the function which can not be completely realized by any grounding device in the past, and can be applied to power supply systems of all enterprises such as electric power systems, petroleum, chemical industry, metallurgy, electrified railways, mines, military industry, airports, ports, broadcast televisions and the like, and wind power, thermal power, hydraulic power and photovoltaic power generation networking systems. The application mode is that the neutral point of each distribution voltage level of the transformer substation is accessed or the artificial neutral point is accessed.
The technical scheme adopted by the invention for achieving the purpose is as follows: the neutral point variable resistor grounding device comprises a first resistor (1), a second resistor (2), a grounding transformer (3), a digital measurement and control device (4), a first current transformer (5), a second current transformer (6) and a circuit breaker (7), wherein the grounding transformer (3) is connected with the first resistor (1) and is connected with the second resistor (2) through the circuit breaker (7); the digital measurement and control device (4) is respectively connected with the first current transformer (5) and the operating mechanism of the circuit breaker (7); the first resistor is connected with the first current transformer through a magnetic circuit, and the second resistor is connected with the second current transformer through a magnetic circuit; the first current transformer and the second current transformer are arranged on a partition board in the box body; the breaker is installed on the bottom plate of the case. The line can be accurately selected during grounding, the secondary load trips immediately during grounding, the important load trips after acting on signals or long delay, the accident loss and accident risk are reduced, the neutral point variable resistor grounding device, during the filtering transient grounding period in the normal operation and grounding initial stage of the connected power grid, the circuit breaker (7) is in a switching-off state, the second resistor (2) and the second current mutual inductor (6) are always in a shutdown state.
Further, the first resistor (1) is a high damping resistor, and the application range of the first resistor to capacitance current is IR: ic is less than or equal to 1:5.67; the second resistor (2) is a low-value resistor, and the resistance value is selected based on the principle of meeting the zero sequence protection selectivity and reducing the burning loss; the circuit breaker (7) is a high voltage vacuum circuit breaker.
Further, the digital measurement and control device (4) is an acquisition, retrieval, judgment and control center. Comprises a singlechip, a chip, a liquid crystal, a static relay, a communication module, a patch electric element, software and the like.
When the device is connected to the power grid, the power grid is always under the protection of a high damping resistor grounding mode in the whole process of normal operation and grounding fault treatment, namely resonance overvoltage, power frequency and high-frequency arc grounding overvoltage and neutral point displacement voltage are always under a suppressed state during normal operation and grounding. On the premise that the single-phase permanent grounding occurs to the power grid system accessed by the step (1), flexible selection conditions can be provided for zero sequence protection of the transformer substation, the continuous operation time of the important load and the secondary load is differentiated, and the power supply continuity of the important load is maintained. The circuit can accurately select the line, so that a grounded secondary load circuit trips in time, and a grounded important load circuit continuously runs with the ground, thereby being convenient for switching power sources or preparing for stopping production and preventing major accidents such as major equipment damage or personal injury or explosion and economic loss caused by sudden power failure of the important load. (2) The instantaneous ground is automatically filtered, and the instantaneous ground which is automatically cleared immediately is prevented from evolving into a tripping power failure. (3) The burning loss of the fault point can be reduced, and the high-frequency current does not enter the fault point.
And the connected power grid is normally operated and judged, and the transient grounding and permanent grounding fault judgment, dynamic switching and other fault treatment whole process periods are filtered. The first resistor (1) and the first current transformer (5) are always in an operation state, and the grounding transformer (3) digital control device (4) is used for effectively damping system resonance and inhibiting resonance overvoltage, power frequency and high-frequency arc grounding overvoltage and neutral displacement voltage; when the power grid is in single-phase grounding, the digital measurement and control device (4) collects grounding fault current through the first current transformer (5), judges fault properties, automatically filters out the faults if the faults are in instantaneous grounding, sends out instructions to drive the circuit breaker (7) to switch on if the faults are in permanent grounding, inputs the faults into the second resistor (2) to accurately select lines of the zero sequence protection system of the transformer substation, realizes accurate selection and tripping of the permanent grounding fault lines to isolate the fault lines from the system, and then drives the circuit breaker (7) to delay switching-off through the digital measurement and control device (4). And if the power grid is in single-phase grounding again, repeating the searching judgment and the action process.
The first resistor (1) can be operated continuously for 2 hours in the case of a single phase of the network. The resistor thereof satisfies the condition of operating at a high temperature for 2 hours. The change of the resistance value is changed into the change under the grounding mode of the uninterrupted high-damping resistor, namely the first resistor (1) is always in an operation state under any working condition.
The ground fault has light burning loss and short duration, and the running time of the second resistor (low-value resistor) is less than or equal to 1 second.
The beneficial effects of the invention are as follows: the defects of non-grounding, grounding through an arc suppression coil, grounding through a low resistance and grounding through a high resistance are overcome, the advantages of the non-grounding, grounding through the arc suppression coil, grounding through the low resistance and grounding through the high resistance are maintained, the functions that any one set of grounding device cannot be completely realized in the past are realized, the application range of the high resistance to the capacitance current is widened, the resonance overvoltage can be effectively restrained, the solitary grounding overvoltage is effectively restrained to below 2.5PU, the neutral point displacement voltage is effectively restrained, the instantaneous grounding is timely filtered, the instantaneous grounding tripping and the power failure are prevented, and the fault point burning loss is reduced; the line can be accurately selected when the line is permanently grounded, the secondary load can act on immediate tripping, so that the sound phase insulation breakdown short circuit is prevented, the system voltage is instantaneously reduced, and the large-scale production stopping of the whole system production line equipment due to voltage loss tripping occurs; the important load can act on the signal or trip after a long delay time of 2 hours, so that the important load is prevented from suddenly cutting off power to cause great loss or personal injury to production and operation; the system provides power supply continuity and reliability technical guarantee for a power system and an industrial enterprise power supply system and special important loads, so that accidents of the whole system including single-phase grounding faults are greatly reduced, accident loss is reduced, and economic benefits and social benefits are improved.
Drawings
Fig. 1 is an equivalent wiring diagram of a neutral ungrounded system.
Fig. 2 is an equivalent ground diagram of the arc suppression coil grounding system in the harmonic background.
Fig. 3 is a non-linear resonant tank with a resistor present.
Fig. 4 is a main wiring diagram of the neutral point variable resistor grounding device.
Fig. 5 is a schematic block diagram of a control flow of the neutral point variable resistor grounding device.
Description of the embodiments
The invention is further illustrated by the following examples.
As shown in the figure, the neutral point variable resistance grounding system comprises a first resistor 1, a second resistor 2, a grounding transformer 3, a digital measurement and control device 4, a first current transformer 5, a second current transformer 6, a breaker 7, a box and other accessory elements; wherein: the grounding transformer 3 is connected with the first resistor 1 and is connected with the second resistor 2 through the breaker 7; the digital control device 4 is respectively connected with the operating mechanisms of the first current transformer 5 and the circuit breaker 7; the first resistor 1 is connected with the first current mutual inductor 5 through a magnetic circuit, and the second resistor 2 is connected with the second current transformer 6 through a magnetic circuit.
The system operation method comprises the following steps: the first resistor 1 is always in an operating state under any working condition; when the power grid is in normal operation, the circuit breaker 7 is in a switching-off state, and the second resistor 2 is in a shutdown state; when the system is in single-phase grounding, the digital measurement and control device 4 collects current through the first current transformer 5, judges grounding property after short delay, automatically filters out the current if the current is in instantaneous grounding, and prevents instantaneous grounding which is cleared automatically at once from evolving into tripping power-off; if the circuit breaker 7 is permanently grounded, the circuit breaker 7 is driven to be closed, then the time delay T is opened, a line selection action condition is provided for a zero sequence protection system LXn of the transformer substation, the grounded secondary load acts on immediate tripping, the grounded important load acts on signal alarming, the important load is enabled to continuously run with grounding, and the ground fault point is isolated or the time delay is long for 2 hours after the important load is switched. And if the power grid is in single-phase grounding again, repeating the searching judgment and the action process.
Taking a 10KV system as an example: assuming that the capacitance current to ground of the 10KV system is 40A,2, 3, 5, 7, 11 and 13 times of harmonic components exceeds the standard by less than or equal to 300 percent: first resistor 606.24 Ω/10 A.+ -. 2%, according to IR: ic is less than or equal to 1:5.67 principle; the second resistor has a melting point of 1600 ℃ of 30.31 omega/200A + -3%; a circuit breaker 250A; ground transformer capacity selection is referred to IEEE C62.92.3-1993 power system neutral grounding guidelines; the first current transformer and the second current transformer are matched according to the currents of the first resistor and the second resistor; the digital measurement and control device meets the functions of acquisition, retrieval, judgment and driving (1) the functions of meeting the requirements of instantaneous grounding and automatic filtering, (2) when the digital measurement and control device is permanently grounded (when the first current transformer has current and reaches a starting value), the digital measurement and control device drives the circuit breaker 7 to close, and then the digital measurement and control device delays to open, and (3) if single-phase grounding occurs again, the acquisition, retrieval, judgment and action process is repeated. The above embodiments can meet the description and actual demands and effects.
The foregoing is only a preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art, who is within the scope of the present invention, should make equivalent substitutions or modifications according to the technical solution and the concept of the present invention, and should be covered by the scope of the present invention.
Claims (5)
1. The neutral point variable resistor grounding system is characterized by mainly comprising a first resistor (1), a second resistor (2), a grounding transformer (3), a digital measurement and control device (4), a first current transformer (5), a second current transformer (6) and a circuit breaker (7), wherein the grounding transformer (3) is connected with the first resistor (1) and is connected with the second resistor (2) through the circuit breaker (7); the digital control device (4) is respectively connected with the operating mechanisms of the first current transformer (5) and the circuit breaker (7); the first resistor (1) is connected with the first current transformer (5) through a magnetic circuit, the second resistor (2) is connected with the second current transformer (6) through a magnetic circuit, the first resistor (1) is a high damping resistor, the second resistor (2) is a low value resistor, and the selection of the resistance value of the low value resistor is based on the principles of meeting zero sequence protection selectivity and reducing burning loss; the first resistor (1) is always in an operating state under any working condition; when the power grid is in normal operation, the circuit breaker (7) is in a switching-off state, and the second resistor (2) is in a shutdown state; when a single-phase grounding occurs in the system, the digital measurement and control device (4) collects current through the first current transformer (5) and judges grounding property after short delay, and if the current is in instantaneous grounding, the current is automatically filtered, so that instantaneous grounding which is cleared automatically is prevented from being evolved into tripping and power-off; if the load is permanently grounded, the circuit breaker (7) is driven to close, then the circuit breaker is delayed to break, a line selection action condition is provided for the zero sequence protection system LXn of the transformer substation, the operation time of the important load and the operation time of the secondary load can be distinguished, and the continuous power supply of the important load can be kept.
2. The neutral point variable resistor grounding system of claim 1, characterized in that its range of applicability to capacitive currents is IR: ic is less than or equal to 1:5.67.
3. the neutral point variable resistor grounding system according to claim 1, characterized in that the circuit breaker (7) is a high voltage vacuum circuit breaker.
4. The neutral point variable resistor grounding system according to claim 1, characterized in that the digital measurement and control device (4) comprises a single chip microcomputer, a chip, a liquid crystal, a static relay, a communication module, a patch electric element and software.
5. A neutral point variable resistor grounding system according to claim 3, characterized in that the delay opening time T of the circuit breaker (7) is set to t.ltoreq.1s.
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| CN206452106U (en) * | 2016-11-14 | 2017-08-29 | 国家电网公司 | Neutral point high resistant damping resistor |
| CN106655120A (en) * | 2016-11-30 | 2017-05-10 | 孙嘉宁 | Intelligent ground protection method and system for neutral point of power distribution network |
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