CN1141767C - Earthing protection method for small current earthing system - Google Patents
Earthing protection method for small current earthing system Download PDFInfo
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- CN1141767C CN1141767C CNB001144529A CN00114452A CN1141767C CN 1141767 C CN1141767 C CN 1141767C CN B001144529 A CNB001144529 A CN B001144529A CN 00114452 A CN00114452 A CN 00114452A CN 1141767 C CN1141767 C CN 1141767C
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Abstract
The present invention relates to a ground protection method for a small current ground system, which comprises the following steps that (1) the zero sequence voltage of a system and a negative sequence current of each feed line are monitored; the voltage of each phase and the zero sequence current of each feed line are selected and monitored; (2) if the zero sequence voltage is larger than a setting value, the system failure is judged; (3) the variable quantity of the negative sequence current of each feed line is calculated after the failure is generated; (4) when the variable quantity of the negative sequence current of a feed line is larger than an accurate operating current, the line failure can be judged respectively according to the size, the direction or the energy function of the negative sequence current. The method can enhance protection reliability and precision, and can be judged in a fragmenting mode. The present invention is easy to realize on FTU, and can satisfy the requirements of power distribution automation.
Description
Technical field
The present invention relates to relay protection of power system technology, particularly the neutral point not single-phase grounding selecting and the protection of solidly grounded system.
Background technology
Neutral point not solidly grounded system is called for short small current neutral grounding system, when single phase ground fault takes place, produces overvoltage, as untimely removing, may cause cable blast and other faults, influences the power system security and the quality of power supply.Power distribution network generally adopts the exploratory trouble-shooting circuit of operating a switch of outlet one by one at present, can not adapt to the requirement of power system development.91103633 of application numbers are called " detection method of small current neutral grounding and device " application for a patent for invention and disclose each outlet zero-sequence current AC harmonic amount size in the comparison electrical network, and harmonic content the maximum is the method for ground path.No. 94106374 patents of invention " small current system one-phase ground protection method and apparatus " principal character is that the circuit that compares the idle maximum of Zero-pharse harmonic fast is judged to be ground path.Because earth fault current is influenced by system balance degree, earth resistance etc., size and Orientation is indefinite, relation between faulty line zero-sequence current and the non-fault line zero-sequence current is influenced by system balance degree, operational mode etc. equally, come and go, relatively zero-sequence current failure line selection reliability is not high: and the Zero-pharse harmonic electric current is influenced by multiple enchancement factors such as the degree of saturation, load of transformer, adopt the ground protection of the Zero-pharse harmonic electric current difficulty of adjusting, reliability is also not high; Adopt the idle system parameters that relatively is subjected to of Zero-pharse harmonic to influence greatly, and measurement mechanism is required high, existing all methods are difficult to realize on the on-site terminal unit F TU of power distribution automation on-pole switch to the comparison of each bar outlet measuring amount, are unfavorable for using in power distribution automation.
Summary of the invention
The object of the present invention is to provide a kind of earthing protection method for small current earthing system; the main negative-sequence current variable quantity realization of passing through residual voltage and each feeder line of measurement computing system; with the reliability and the precision of raising ground protection, and be convenient on FTU, realize, satisfy the power distribution automation requirement.
Negative-sequence current variable quantity at the residual voltage of measuring computing system and each feeder line carries out under this total technical conceive of ground protection, and the present invention can realize with the following methods:
1. an earthing protection method for small current earthing system comprises the steps:
(1) residual voltage of on-line monitoring system on bus or feeder line; The negative-sequence current of each circuit of on-line monitoring on each feeder line,
(2) when residual voltage when adjusting voltage, ground connection or disconnection fault take place in decision-making system, the residual voltage sudden change is constantly for the moment takes place in fault,
(3) selecting arbitrary at each feeder line is benchmark mutually, calculates the negative-sequence current sampled value changes delta i after fault takes place
2(k), and by it calculate negative-sequence current variation delta I
2,
(4) the accurate operating current I of definition negative-sequence current variable quantity
SBe the minimum working current of big or small measure error≤30%, phase measurement error≤30 °,
When certain circuit | Δ I
2|＞| I
S|, and | Δ I
2|＞setting value
Judge this line fault, protection mechanism action, the maximum negative-sequence current variable quantity that setting value flows through this circuit during by All other routes generation metallic earthing fault is determined desirable 1.5 times safety factor.
2. an earthing protection method for small current earthing system comprises the steps:
(1) residual voltage of on-line monitoring system on bus or feeder line; The negative-sequence current of each circuit of on-line monitoring on each feeder line,
(2) when residual voltage when adjusting voltage, ground connection or disconnection fault take place in decision-making system, the residual voltage sudden change is constantly for the moment takes place in fault,
(3) selecting arbitrary at each feeder line is benchmark mutually, calculates the negative-sequence current sampled value variation delta i after fault takes place
2(k), and by it calculate negative-sequence current variation delta I
2And phase place,
(4) the accurate operating current I of definition negative-sequence current variable quantity
SBe the minimum working current of big or small measure error≤30%, phase measurement error≤30 °,
When certain circuit | Δ I
2|＞| I
S|, and this circuit negative-sequence current variable quantity phase place and the phase difference of negative-sequence current variable quantity phase place that flows to system be between 144,8 °～206.1 ° the time,
Judge this line fault, the protection mechanism action.
3. an earthing protection method for small current earthing system comprises the steps:
(1) residual voltage of on-line monitoring system on bus or feeder line; The negative-sequence current of each circuit of on-line monitoring and zero-sequence current on each feeder line,
(2) when residual voltage when adjusting voltage, ground connection or disconnection fault take place in decision-making system, the residual voltage sudden change is constantly for the moment takes place in fault,
(3) selecting arbitrary at each feeder line is benchmark mutually, calculates the negative-sequence current sampled value variation delta i after fault takes place
2(k), and by it calculate negative-sequence current variation delta I
2Same this circuit zero sequence current change quantity Δ I that calculates
0
(4) the accurate operating current I of definition negative-sequence current variable quantity
SBe the minimum working current of big or small measure error≤30%, phase measurement error≤30 °,
When certain circuit | Δ I
2|＞| I
S|, and
Judge this line fault, the protection mechanism action.
4. an earthing protection method for small current earthing system comprises the steps:
(1) residual voltage of on-line monitoring system on bus or feeder line; The negative-sequence current of each circuit of on-line monitoring and each phase voltage on each feeder line,
(2) when residual voltage when adjusting voltage, ground connection or disconnection fault take place in decision-making system, the residual voltage sudden change is constantly for the moment takes place in fault,
(3) after each feeder line is measured fault voltage minimum be benchmark mutually, calculate the negative-sequence current sampled value variation delta i after the fault generation
2(k), and by it calculate negative-sequence current variation delta I
2And phase place,
(4) the accurate operating current I of definition negative phase-sequence variable-current amount
SBe the minimum working current of big or small measure error≤30%, phase measurement error≤30 °,
When certain circuit | Δ I
2|＞| I
S|, and fault phase voltage and the phase difference that flows to feeder line negative-sequence current variable quantity be between-28 °～32.9 ° the time,
Judge this line fault, the protection mechanism action.
5. an earthing protection method for small current earthing system comprises the steps:
(1) residual voltage of on-line monitoring system on bus or feeder line; The negative-sequence current of each circuit of on-line monitoring and each phase voltage on each feeder line,
(2) when residual voltage when adjusting voltage, ground connection or disconnection fault take place in decision-making system, the residual voltage sudden change is constantly for the moment takes place in fault,
(3) after measuring fault on each feeder line voltage minimum be benchmark mutually, calculate the negative-sequence current sampled value variation delta i after the fault generation
2(k), and by it calculate negative-sequence current variation delta I
2
(4) the accurate operating current I of definition negative-sequence current variable quantity
SBe the minimum working current of big or small measure error≤30%, phase measurement error≤30 °,
When certain circuit | Δ I
2|＞| I
S|, and aforementioned (3) negative-sequence current sampled value variation delta i
2(k) the product integrated value in two cycles after fault with the minimum phase voltage sampled value of voltage is timing,
Judge this line fault, the protection mechanism action.
In the step of above-mentioned all modes (3), calculate negative-sequence current sampled value variable quantity,
After the fault, the first cycle negative-sequence current sampled value variable quantity is:
Δi
2(k)＝i
2(k)-i
2(k-T)
After the fault, the second cycle negative-sequence current sampled value variable quantity is:
Δi
2(k)＝i
2(k)-i
2(k-2T)
I wherein
2(k) be current sampled value, k is sampling instant, and T is the power frequency component cycle.By Δ i
2(k) calculate negative-sequence current variation delta I
2Or during its phase place, can adopt filtering method, fourier transform is wherein the most frequently used a kind of.
For improving the certainty of measurement of negative-sequence current, can select to adopt measurement level current transformer and hardware resistance type negative sequence filter to extract negative-sequence current.
Because faulty line negative-sequence current variable quantity is 1/3 of a residue current of ground fault in electrical, the two direction unanimity; Non-fault line negative-sequence current variable quantity is very little, approaches zero, and direction is opposite substantially with faulty line negative-sequence current variable quantity.Therefore, adopt negative-sequence current to carry out ground protection, the protection precision is only relevant with the fault earthing residual flow.When single phase ground fault residual flow during greater than the accurate operating current of 3 times of negative-sequence current variable quantities, the negative-sequence current ground protection can both precision maneuver.Thereby can be applicable to isolated neutral system, can be applicable to neutral by arc extinction coil grounding system, resistance grounded system again, and can improve the anti-transition resistance ability of ground protection.Simultaneously since the small current neutral grounding system negative sequence impedance much smaller than zero sequence impedance; in the arc extinction moment of arc grounding process; the oscillatory extinction time of negative-sequence current is much smaller than the oscillatory extinction time of zero-sequence current; therefore influenced by the arc light vibration little for the negative-sequence current ground protection, has very strong arc grounding protective capability.On the other hand, in negative-sequence current ground protection 1,3,4,5 protection schemes, only need to measure voltage, the electric current of this protected circuit, be convenient on FTU, install, realize that the segmentation of circuit is protected on the spot, satisfy the requirement of power distribution automation.
Description of drawings
The invention will be further described below in conjunction with accompanying drawing and specific implementation method.
Fig. 1 shows the negative-sequence current of normal feeder line 1 and fault feeder 4.
Fig. 2 shows the energy function waveform of normal feeder line 1 and fault feeder 4, and wherein energy function is that the product of negative-sequence current and fault phase voltage is to time integral.
Embodiment
One 35KV power distribution network is analyzed, had L1, L2, four feeder lines of L3, L4 on the bus, the feeder line parameter sees Table 1.
Table 1 feeder line parameter
| Title | Character | Length | Every equivalent ground capacity (μ F) | Alternate electric capacity (μ F) | Ratio of damping | Load (KVA) | Power factor (PF) Cos α |
| L 1 | Overhead wire | 30 | 0.15 | 0.035 | 4％ | 2000 | 0.80 |
| L 2 | Many feedback line are concentrated equivalence | 100 | 0.5 | 0.125 | 4％ | 10000 | 0.80 |
| L 3 | Cable | 30 | 1.8 | 0.72 | 3％ | 2000 | 0.80 |
| L 4 | Overhead wire | 20 | 0.1 | 0.025 | 4％ | 1000 | 0.80 |
Cross the different situations of resistance and load according to neutral grounding mode, compensativity, fault point, fault earthing mode, fault, size, the direction of having calculated negative-sequence current respectively reach the energy function based on negative-sequence current, and result of calculation and waveform are listed in table 2, table 3 and Fig. 1, Fig. 2 respectively.
Each feeder line negative-sequence current size of table 2 relatively
| Fault type | Neutral grounding mode | V | U 0(kv) | Δ3I 21(A) | Δ3I 23(A) | Δ3I 24(A) |
| A phase metallic earthing (R f=5 Europe) | Earth-free | 20.20 | 0.801 | 0.811 | 48.25 | |
| The direct ground connection of arc suppression coil | -5％ | 20.19 | 0.050 | 0.053 | 2.95 | |
| -10％ | 20.18 | 0.082 | 0.085 | 5.12 | ||
| Arc suppression coil crosstalk resistance ground connection | -5％ | 20.16 | 0.130 | 0.132 | 7.89 | |
| -10％ | 20.16 | 0.155 | 0.159 | 9.19 | ||
| A phase high resistance ground (R f=1000 Europe) | Earth-free | 7.69 | 0.307 | 0.312 | 18.41 | |
| The direct ground connection of arc suppression coil | -5％ | 18.53 | 0.041 | 0.045 | 2.71 | |
| -10％ | 18.20 | 0.081 | 0.078 | 4.61 | ||
| Arc suppression coil crosstalk resistance ground connection | -5％ | 14.60 | 0.090 | 0.095 | 5.71 | |
| -10％ | 14.21 | 0.102 | 0.106 | 6.47 |
Feeder line 4 earth faults: v is through the humorous degree of taking off of arc suppression coil earthing electric network in the table, U
0Be residual voltage, Δ I among table 2 and Fig. 1
21Be the negative-sequence current size of feeder line 1, Δ I
23Be the negative-sequence current size of feeder line 3, Δ I
24Negative-sequence current size for feeder line 4.
(a) and (b) are neutral by arc extinction coil grounding among Fig. 1, and taking off humorous degree is-10%, and fault resstance is 5 Ω; (c), (d) be neutral by arc extinction coil grounding, taking off humorous degree is 0 (full compensation), arc grounding, fault resstance are 5 Ω.
From table 2, Fig. 1 obviously finds out: no matter which kind of earthing mode neutral point adopts, during single phase ground fault, fault feeder negative-sequence current variable quantity is opposite substantially with the phase place of non-fault feeder negative-sequence current variable quantity, the ratio of size remains unchanged, approximately differ 60 times, ratio approximates the ratio of load negative sequence impedance and power supply negative sequence impedance.Therefore, adopt the negative-sequence current route selection to have higher accuracy.
Table 3 is negative-sequence current size ground protection, employing method 1 and a method 3.The maximum negative-sequence current variable quantity that the negative-sequence current setting value of using in the method 1 flows through this circuit during by All other routes generation metallic earthing fault is determined desirable 1.5 times safety factor.Use Δ I in the method 3
04Zero-sequence current variable quantity size for feeder line after the fault 4.
Table 3 negative-sequence current size ground protection
| Fault type | Neutral grounding mode | The negative-sequence current setting value Δ 3I of guard method 1 feeder line 4 SET (A) | L3 earth fault | L 4Ground connection is put barrier | ||||||
| Protect expanding method 1 | Guard method 3 | Guard method 1 | Guard method 3 | |||||||
| Δ3I 24(A) | L 4Action is not | Δ3I 04(A) | The L4 action is not | Δ3I 04(A) | L 1Action is not | Δ3I 04(A) | L 4Action is not | |||
| A phase metallic earthing (R 1=5 Europe) | Earth-free | 0.599 | 0.399 | Not | 1.903 | Not | 48.25 | Action | 46.43 | Action |
| High resistance ground (400 Europe) | 0.870 | 0.580 | Not | 1.879 | Not | 70.21 | Action | 68.95 | Action | |
| The direct ground connection V=-10% of extinguisher | 0.063 | 0.042 | Not | 1.903 | Not | 5.12 | Action | 6.94 | Action | |
| Arc suppression coil and grounding through resistance (1000 Europe) V=0 | 0.270 | 0.180 | Not | 1.894 | Not | 21.77 | Action | 21.86 | Action | |
| A phase high resistance ground (R 1=1000 Europe | Earth-free | 0.599 | 0.152 | Not | 0.725 | Not | 18.41 | Action | 17.69 | Action |
| High resistance ground (400 Europe) | 0.870 | 0.136 | Not | 0.442 | Not | 16.51 | Action | 16.21 | Action | |
| The direct ground connection v=-10 of extinguisher % | 0.063 | 0.038 | Not | 1.715 | Not | 4.61 | Action | 6.26 | Action | |
| Arc suppression coil and grounding through resistance (1000 Europe) V=0 | 0.270 | 0.087 | Not | 0.914 | Not | 10.51 | Action | 10.55 | Action |
In table 3, other feeder lines, as feeder line L
3During earth fault, feeder line L
4Ground protection based on the negative-sequence current size is failure to actuate, and only works as L
4During faults itself, the protection action.
Table 4 is a negative-sequence current direction ground protection, has adopted preceding method 2 and method 4.
Table 4 negative-sequence current direction ground protection
| Fault type | Neutral grounding mode | L 3Fault | L 4Earth fault | ||||||
| Guard method 2 | Guard method 4 | Guard method 2 | Guard method 4 | ||||||
| α(°) | L 4Action is not | β(°) | L 4Action is not | α(°) | L 4Action is not | β(°) | L 4Action is not | ||
| A phase metallic earthing (R 1=5 Europe) | Earth-free | -22.5 | Not | 168 | Not | 175.4 | Action | 2.5 | Action |
| High resistance ground | -22.4 | Not | 165 | Not | 175.6 | Action | 2.3 | Action | |
| The direct ground connection of extinguisher | -22.5 | Not | 160 | Not | 174.8 | Action | 2.5 | Action | |
| Arc suppression coil and grounding through resistance | -22.6 | Not | 167 | Not | 175.8 | Action | 2.9 | Action | |
| A phase high resistance ground (R 1=1000 Europe) | Earth-free | -22.5 | Not | 168 | Not | 175.7 | Action | 2.8 | Action |
| High resistance ground | 22.4 | Not | 163 | Not | 175.9 | Action | 2.4 | Action | |
| The direct ground connection of extinguisher | -22.3 | Not | 167 | Not | 176.1 | Action | 2.0 | Action | |
| Arc suppression coil and grounding through resistance | -22.4 | Not | 164 | Not | 175.8 | Action | 2.7 | Action | |
α is for comparing the phase difference of electric current (flowing to the negative-sequence current variable quantity of power supply) and polarization current (flowing to feeder line negative-sequence current variable quantity); β is the phase difference of comparative voltage (fault phase voltage) and polarization current (flowing to feeder line negative-sequence current variable quantity).
As seen from Table 4, adopt the negative-sequence current direction protection of said method 2,4, irrelevant with neutral grounding mode and fault resstance, protection accuracy height.
Fig. 2 is the energy function waveform of normal feeder line 1 and fault feeder 4, wherein
(a) neutral by arc extinction coil grounding, taking off humorous degree is-10%, arc grounding, fault resstance are 100 Ω;
(b) neutral by arc extinction coil grounding, taking off humorous degree is-10%, fault resstance is 1k Ω;
(c) neutral by arc extinction coil grounding, taking off humorous degree is-10%, fault resstance is 20k Ω;
(d) neutral earth-free, fault resstance is 20k Ω.
The product of getting negative-sequence current and fault phase voltage is an energy function to the integration of time.As can be seen from Figure 2: the energy function of fault feeder is dull after the fault rises, and the energy function of non-fault feeder is zero substantially.During arc grounding, non-fault is charged mutually, there is a transition process in fault phase discharge fault feeder energy function; During high resistance earthing fault, the fault feeder energy function steadily rises.Fault earthing mode and earth resistance are little to the fault line detection influence based on energy function, and when high resistance earthing fault and arcing ground fault, transient state energy ground protection (said method 5) still has higher accuracy.
Claims (5)
1. an earthing protection method for small current earthing system comprises the steps:
(1) residual voltage of on-line monitoring system on bus or feeder line; The negative-sequence current of each circuit of on-line monitoring on each feeder line,
(2) when residual voltage when adjusting voltage, ground connection or disconnection fault take place in decision-making system, the residual voltage sudden change is constantly for the moment takes place in fault,
(3) selecting arbitrary at each feeder line is benchmark mutually, calculates the negative-sequence current sampled value changes delta i after fault takes place
2(k), and by it calculate negative-sequence current variation delta I
2,
(4) the accurate operating current I of definition negative-sequence current variable quantity
SBe the minimum working current of big or small measure error≤30%, phase measurement error≤30 °,
When certain circuit | Δ I
2|＞| I
S|, and | Δ I
2|＞setting value
Judge this line fault, protection mechanism action, the maximum negative-sequence current variable quantity that setting value flows through this circuit during by All other routes generation metallic earthing fault is determined desirable 1.5 times safety factor.
2. an earthing protection method for small current earthing system comprises the steps:
(1) residual voltage of on-line monitoring system on bus or feeder line; The negative-sequence current of each circuit of on-line monitoring on each feeder line,
(2) when residual voltage when adjusting voltage, ground connection or disconnection fault take place in decision-making system, the residual voltage sudden change is constantly for the moment takes place in fault,
(3) selecting arbitrary at each feeder line is benchmark mutually, calculates the negative-sequence current sampled value variation delta i after fault takes place
2(k), and by it calculate negative-sequence current variation delta I
2And phase place,
(4) the accurate operating current I of definition negative-sequence current variable quantity
SBe the minimum working current of big or small measure error≤30%, phase measurement error≤30 °,
When certain circuit | Δ I
2|＞| I
S|, and this circuit negative-sequence current variable quantity phase place and the phase difference of negative-sequence current variable quantity phase place that flows to system be between 144.8 °～206.1 ° the time,
Judge this line fault, the protection mechanism action.
3. an earthing protection method for small current earthing system comprises the steps:
(1) residual voltage of on-line monitoring system on bus or feeder line; The negative-sequence current of each circuit of on-line monitoring and zero-sequence current on each feeder line,
(2) when residual voltage when adjusting voltage, ground connection or disconnection fault take place in decision-making system, the residual voltage sudden change is constantly for the moment takes place in fault,
(3) selecting arbitrary at each feeder line is benchmark mutually, calculates the negative-sequence current sampled value variation delta i after fault takes place
2(k), and by its calculating negative-sequence current become 1 change amount Δ I
2Same this circuit zero sequence current change quantity Δ I that calculates
0,
(4) the accurate operating current I of definition negative-sequence current variable quantity
SBe the minimum working current of big or small measure error≤30%, phase measurement error≤30 °,
When certain circuit | Δ I
2|＞| I
S|, and
Judge this line fault, the protection mechanism action.
4. an earthing protection method for small current earthing system comprises the steps:
(1) residual voltage of on-line monitoring system on bus or feeder line; The negative-sequence current of each circuit of on-line monitoring and each phase voltage on each feeder line,
(2) when residual voltage when adjusting voltage, ground connection or disconnection fault take place in decision-making system, the residual voltage sudden change is constantly for the moment takes place in fault,
(3) after each feeder line is measured fault voltage minimum be benchmark mutually, calculate the negative-sequence current sampled value variation delta i after the fault generation
2(k), and by it calculate negative-sequence current variation delta I
2And phase place,
(4) the accurate operating current I of definition negative-sequence current variable quantity
SBe the minimum working current of big or small measure error≤30%, phase measurement error≤30 °,
When certain circuit | Δ I
2|＞| I
S|, and fault phase voltage and the phase difference that flows to feeder line negative-sequence current variable quantity be between-28 °～32.9 ° the time,
Judge this line fault, the protection mechanism action.
5. an earthing protection method for small current earthing system comprises the steps:
(1) residual voltage of on-line monitoring system on bus or feeder line; The negative-sequence current of each circuit of on-line monitoring and each phase voltage on each feeder line,
(2) when residual voltage when adjusting voltage, ground connection or disconnection fault take place in decision-making system, the residual voltage sudden change is constantly for the moment takes place in fault,
(3) after measuring fault on each feeder line voltage minimum be benchmark mutually, calculate the negative-sequence current sampled value variation delta i after the fault generation
2(k), and by it calculate negative-sequence current variation delta I
2
(4) the accurate operating current I of definition negative-sequence current variable quantity
SBe the minimum working current of big or small measure error≤30%, phase measurement error≤30 °,
When certain circuit | Δ I
2|＞| I
S|, and aforementioned (3) negative-sequence current sampled value variation delta i
2(k) the product integrated value in two cycles after fault with the minimum phase voltage sampled value of voltage is timing,
Judge this line fault, the protection mechanism action.
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|---|---|---|---|
| CNB001144529A CN1141767C (en) | 2000-03-30 | 2000-03-30 | Earthing protection method for small current earthing system |
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| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CNB001144529A CN1141767C (en) | 2000-03-30 | 2000-03-30 | Earthing protection method for small current earthing system |
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| CN1141767C true CN1141767C (en) | 2004-03-10 |
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| CN111337855B (en) * | 2020-03-18 | 2022-06-21 | 贵州电网有限责任公司 | Active power distribution network disconnection fault protection method based on negative sequence current ratio |
| CN112904233B (en) * | 2021-01-27 | 2023-05-02 | 国网江苏省电力有限公司苏州供电分公司 | Ground fault positioning method and device based on ground wire current reactive component |
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2000
- 2000-03-30 CN CNB001144529A patent/CN1141767C/en not_active Expired - Fee Related
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN100418282C (en) * | 2005-01-20 | 2008-09-10 | 长沙理工大学 | Single-phase gruonded fault protecting method for distributing net |
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