CN103632029A - Method for selecting neutral point grounding mode of medium voltage distribution network - Google Patents

Abstract

The invention discloses a method for selecting a neutral point grounding mode of a medium voltage distribution network. According to the method, according to the difference of the influences of different neutral point grounding modes on the medium voltage distribution network, the reliability of the power distribution network under the different neutral point grounding modes is evaluated and compared through quantitative calculation of the line fault trip probability of the power distribution network, reliability evaluation and operating cost analysis of the power distribution network under the different neutral point grounding modes are combined, so that the optimal neutral point grounding mode of the power distribution network is selected by the way of calculating the whole life cycle cost of the power distribution network under the different neutral point grounding modes and reliability factors in a combined mode, the design and transformation engineering requirement of the medium voltage distribution network can be met, and the security and the operating maintenance efficiency of the power distribution network are improved. By the adoption of the method, the high reliability neutral point grounding mode with optimal economy can be decided on the basis that the influence of the different neutral point grounding modes on the reliability and the construction operating cost of the medium voltage distribution network is calculated.

Description

The system of selection of a kind of medium voltage distribution network neutral grounding mode

Technical field

The present invention relates to medium voltage distribution network, particularly in several neutral grounding modes of medium voltage distribution network, by assessing, determine a kind of comparatively desirable neutral grounding mode, belong to planning and design of power system field.

Background technology

The neutral ground of electric system refers to the electrical connection of neutral point and the earthing device of generator and Transformer Winding.That medium voltage distribution network neutral grounding mode mainly comprises is earth-free, through grounding through arc and resistance grounded.The system of isolated neutral is a kind of of system with non-effectively earthed neutral, in fact can be considered as the system through capacitive reactance ground connection.This electric capacity is that the coupling capacitance over the ground of all electrical equipments such as cable in electrical network, overhead transmission line, motor, transformer forms.Neutral by arc extinction coil grounding system, also referred to as resonant earthed system.Arc suppression coil is a telefault with iron core, and coil resistance is very little, and reactance is very large.Different to the degree of compensation of earthing capacitance current according to inductance in arc suppression coil, can be divided into and owe full compensation, under-compensation and three kinds of compensation ways of over-compensation.Neutral Grounding through Resistance in Electrical mode is a kind of novel medium voltage distribution network neutral grounding mode, refers in power distribution network and has at least between a neutral point and the earth and access resistance.Requirement by restriction earth-fault current size is different, and Neutral Grounding through Resistance in Electrical is generally divided into high grounding and low resistance grounding mode.

Interference, electrical safety and the earthing device etc. of the neutral grounding mode of medium voltage distribution network and the power supply reliability of system, Hyper-Voltage of Power Systems and Insulation Coordination, relay protection, communication signal system have close relationship.When breaking down, the single-phase-to-ground current of isolated neutral system is only determined by system ground capacitance, in most cases can automatically eliminate, can eliminator generally also can not cause line tripping, but now intermittent electric arc may cause arc grounding superpotential, apparatus insulated level is had relatively high expectations.Compensated distribution network, single-phase earth fault current only, for aftercurrent seldom after compensation, has obvious inhibiting effect to restriking of electric arc, can reduce high-amplitude arcing ground superpotential probability occurs.But neutral by arc extinction coil grounding system overvoltage multiple is higher, equipment manufacturing cost improves, and when suiting cable line, capacitance current changes greatly, need to promptly and accurately regulate the de-humorous degree of arc suppression coil, troublesome poeration.Adopt arc suppression coil also to make faulty line route selection difficulty greatly increase, also cannot realize to faulty line 100% accurate failure line selection at present, this also can have a strong impact on the recovery of normal power supply.Through low resistance grounding, can effectively suppress resonance overvoltage and arc grounding superpotential; power-frequency overvoltage is lower; the dielectric level of circuit and equipment requires than low through the dielectric level of grounding through arc; the Sensitivity of singlephase earth fault is high, and expense is little compared with arc suppression coil, but also because earth-fault current is large; make current potential increase higher; unfavorable to personal device security, and the rapid action excision of protection fault, can interruptedly supplying power.

Different neutral grounding modes has relative merits separately; make the selection of medium voltage distribution network neutral grounding mode need to consider the various ruuning situations (comprising normal operation and failure operation situation) of power distribution network; the requirement of power supply reliability; the impact on power-supply unit of abnormal voltage during fault and abnormal current; on the impact of communication facilities and harm; relay protection configuration requirement, insulation configuration, economic factors etc.Along with grid structure strengthens, cable ratio increases and the raising of automaticity, the neutral grounding mode of medium voltage distribution network has presented pluralistic trend.But the research of the current neutral ground problem about medium voltage distribution network mainly concentrates on the supporting safety prevention measure aspect of neutral grounding mode.Researched and analysed on a small quantity the impact of neutral grounding mode on medium voltage distribution network, but owing to lacking the lower method for quantitatively evaluating that is press-fitted electrical network economy and reliability of different grounding modes, so that the selection of medium voltage distribution network neutral grounding mode not yet forms ripe decision-making technique at present, seriously restrict planning and design of power system enforcement, hinder the lifting of medium voltage distribution network safe reliability.

Summary of the invention

For existing medium voltage distribution network neutral grounding mode, lack means ofquantity evaluation, the problem that is difficult to decision making package, the invention provides the system of selection of a kind of medium voltage distribution network neutral grounding mode, by this method can determine a kind of in economy and reliability equal comparatively desirable medium voltage distribution network neutral grounding modes.

The technical solution that the present invention realizes above-mentioned purpose is as follows:

The system of selection of a kind of medium voltage distribution network neutral grounding mode, described neutral grounding mode comprises that isolated neutral, neutral by arc extinction coil grounding, Neutral Point Through Low Resistance and neutral point are through large four kinds of earthing modes of resistance eutral grounding, it is characterized in that: carry out according to the following steps

1) the medium voltage distribution network reliability assessment of different grounding modes

1.1) calculate respectively the medium voltage distribution network line fault tripping operation probability of different grounding modes

1.1.1) the medium voltage distribution network line fault of isolated neutral tripping operation probability calculation

The tripping operation probability of cable line or overhead transmission line i fault is

P _ii=λη(1-δ)+λ(1-η)=λ-ληδ (1)

During cable line i fault, fault expansion makes the probability of regular link j tripping operation be:

P _ij=λη(1-δ)(e ₁+e ₂-e ₁e ₂) (2)

During overhead transmission line i fault, fault expansion makes the probability of regular link j tripping operation be:

P _ij=λη(1-δ)e ₁ (3)

Consider microcomputer route selection impact, first carry out microcomputer route selection, the route selection of the laggard pedestrian's work of microcomputer route selection failure, through b artificial route selection, choose faulty line i and regular link j that the probability of its tripping operation is respectively:

FP _ii=P _ii (4)

$F{P}_{\mathrm{ij}}=(1-\mathrm{\ω})(1-\frac{2}{n-1})\×(1-\frac{2}{n-2})\×...\×(1-\frac{2}{n-b+1})\×\frac{1}{n-b}{P}_{\mathrm{ii}}---\left(5\right)$

1.1.2) the medium voltage distribution network line fault of neutral by arc extinction coil grounding tripping operation probability calculation

The tripping operation probability of cable line or overhead transmission line i fault is:

P _ii=ληδ(1-α)+λη(1-δ)+λ(1-η)=λ(1-ηδα) (6)

During cable line i fault, fault expansion makes the probability of regular link j tripping operation be:

P _ij=ληδ(1-α)(e ₁+e ₂-e ₁e ₂)+λη(1-δ)(e ₁+e ₂-e ₁e ₂) (7)

During overhead transmission line i fault, fault expansion makes the probability of regular link j tripping operation be:

P _ij=ληδ(1-α)e ₁+λη(1-δ)e ₁ (8)

Consider microcomputer route selection impact, first carry out microcomputer route selection, the route selection of the laggard pedestrian's work of microcomputer route selection failure, through b artificial route selection, choose faulty line i and regular link j that the probability of its tripping operation is respectively:

FP _ii=P _ii (9)

$F{P}_{\mathrm{ij}}=(1-\mathrm{\ω})(1-\frac{2}{n-1})\×(1-\frac{2}{n-2})\×...\×(1-\frac{2}{n-b+1})\×\frac{1}{n-b}{P}_{\mathrm{ii}}---\left(10\right)$

1.1.3) the medium voltage distribution network line fault of Neutral Point Through Low Resistance tripping operation probability calculation

The tripping operation probability of cable line or overhead transmission line i fault is:

P _ii=ληδ(1-β)+λη(1-δ)(1-β)+λ(1-η)=λ(1-ηβ) (11)

When cable line or overhead transmission line i fault, fault expansion makes the probability of regular link j tripping operation be:

P _ij=ληδβ+λη(1-δ)β=ληβ (12)

1.1.4) neutral point is through the medium voltage distribution network line fault tripping operation probability calculation of high resistance ground

The tripping operation probability of cable line or overhead transmission line i fault is:

P _ii=ληδ(1-σ)1-ζ+λη(1-δ)1-ζ+λ(1-η)=ληδσ(1-ζ)+λ(1-ζη) (13)

During cable line i fault, fault expansion makes the probability of regular link j tripping operation be:

P _ij=[ληδ(1-σ)+λη(1-δ)]ζ(e ₁+e ₂-e ₁e ₂) (14)

During overhead transmission line i fault, fault expansion makes the probability of regular link j tripping operation be:

P _ij=[ληδ(1-σ)+λη(1-δ)]ζe ₁ (15)

Consider microcomputer route selection impact, first carry out microcomputer route selection, the route selection of the laggard pedestrian's work of microcomputer route selection failure, through b artificial route selection, choose faulty line i and regular link j that the probability of its tripping operation is respectively:

FP _ii=P _ii (16)

$F{P}_{\mathrm{ij}}=(1-\mathrm{\ω})(1-\frac{2}{n-1})\×(1-\frac{2}{n-2})\×...\×(1-\frac{2}{n-b+1})\×\frac{1}{n-b}{P}_{\mathrm{ii}}---\left(17\right)$

When the final tripping operation probability of arbitrary circuit i equals probability that faults itself successfully trips and other adjacent line faults in power distribution network, there is expansion and cause the probability of the final tripping operation of circuit i and the probability sum that route selection mistake causes circuit i tripping operation;

In above-mentioned formula (1)-(17), the implication of each parameter is:

λ: the probability of line failure;

η: during line fault, fault type is the probability of single-phase grounding fault;

δ: the relative probability of transient single-phase earth fault during line fault;

E ₁: the probability of circuit generation Overvoltage expansion;

E ₂: the probability of circuit generation electric arc fire failure expansion;

σ: the probability that instantaneity electric arc successfully extinguishes;

α: the probability that can successfully extinguish instantaneity electric arc during neutral by arc extinction coil grounding;

β: the probability that cannot successfully send trip signal during Neutral Point Through Low Resistance;

ζ: the probability that neutral point successfully trips when high resistance ground;

ω: the accuracy of microcomputer route selection;

N: the number of lines in system;

B: the number of times of artificial route selection;

1.2) calculate respectively the medium voltage distribution network dependability parameter under different grounding modes

For the cascade system being formed by n bar circuit, equivalent fault maintenance rate f _e, equivalent scheduled maintenance rate f _e', equivalent fault r servicing time _eand equivalent scheduled maintenance time r _e' be calculated as respectively:

$f_e=\underset{i}{\overset{n}{\mathrm{\Σ}}}{f}_i,f_e^{\′}\underset{i}{\overset{n}{\mathrm{\Σ}}}{f}_i^{\′},r_e=\underset{i}{\overset{n}{\mathrm{\Σ}}}{f}_i{r}_i/f_e,r_e^{\′}=\underset{i}{\overset{n}{\mathrm{\Σ}}}{f}_i^{\′}{r}_i^{\′}/f_e^{\′}{f}_e^{\′}---\left(18\right)$

In formula, f _iit is the fault trip rate of i bar circuit; f _i' be the scheduled maintenance rate of i bar circuit; r _iit is the breakdown maintenance time of i bar circuit; r _i' be the scheduled maintenance time of i bar circuit;

If load point is connected in series circuit end, power supply point is connected in series circuit head end, load point outage rate f _sL, average idle time r loads _sL, the load year T.T. U that stops transport _sLbe respectively:

f _SL=f _e+f _e′，r _SL=f _er _e+f _e′r _e′)/f _SL，U _SL=f _SLr _SL (19)

For double loop network, the equivalent fault maintenance rate f of parallel line _e, equivalent scheduled maintenance rate f _e', equivalent fault r servicing time _eand equivalent scheduled maintenance time r _e' be respectively:

f _e=f ₁f ₂(r ₁+r ₂) (20)

f _e′=f ₁′(f ₂r ₁′)+f ₂′(f ₁r ₂′) (21)

r _e=r ₁r ₂/(r ₁+r ₂)=(1/r ₁+1/r ₂) ^-1 (22)

$r_e^{\&prime;}=\frac{{\mathrm{<figure-callout\; id="1"\; label="f"\; filenames="DEST\_PATH\_HDA0000416593850000032.png"\; state="\{\{state\}\}">f</figure-callout>}}_1^{\&prime;}\left(f_2{\mathrm{<figure-callout\; id="1"\; label="r"\; filenames="DEST\_PATH\_HDA0000416593850000032.png"\; state="\{\{state\}\}">r</figure-callout>}}_1^{\&prime;}\right){(1/{\mathrm{<figure-callout\; id="1"\; label="r"\; filenames="DEST\_PATH\_HDA0000416593850000032.png"\; state="\{\{state\}\}">r</figure-callout>}}_1^{\&prime;}+1/r_2)}^{-1}+{\mathrm{<figure-callout\; id="2"\; label="f"\; filenames="DEST\_PATH\_HDA0000416593850000032.png"\; state="\{\{state\}\}">f</figure-callout>}}_2^{\&prime;}\left(f_1{\mathrm{<figure-callout\; id="2"\; label="r"\; filenames="DEST\_PATH\_HDA0000416593850000032.png"\; state="\{\{state\}\}">r</figure-callout>}}_2^{\&prime;}\right){(1/{\mathrm{<figure-callout\; id="2"\; label="r"\; filenames="DEST\_PATH\_HDA0000416593850000032.png"\; state="\{\{state\}\}">r</figure-callout>}}_2^{\&prime;}+1/r_1)}^{-1}}{{\mathrm{<figure-callout\; id="1"\; label="f"\; filenames="DEST\_PATH\_HDA0000416593850000032.png"\; state="\{\{state\}\}">f</figure-callout>}}_1^{\&prime;}\left(f_2{\mathrm{<figure-callout\; id="1"\; label="r"\; filenames="DEST\_PATH\_HDA0000416593850000032.png"\; state="\{\{state\}\}">r</figure-callout>}}_1^{\&prime;}\right)+{\mathrm{<figure-callout\; id="2"\; label="f"\; filenames="DEST\_PATH\_HDA0000416593850000032.png"\; state="\{\{state\}\}">f</figure-callout>}}_2^{\&prime;}\left(f_1{\mathrm{<figure-callout\; id="2"\; label="r"\; filenames="DEST\_PATH\_HDA0000416593850000032.png"\; state="\{\{state\}\}">r</figure-callout>}}_2^{\&prime;}\right)}---\left(23\right)$

In formula, f ₁and f ₂it is the fault trip rate of two parallel lines; f ₁' and f ₂' be the scheduled maintenance rate of two parallel lines; r ₁and r ₂it is the breakdown maintenance time of two parallel lines; r ₁' and r ₂' be the scheduled maintenance time of two parallel lines.

For double loop network, load point outage rate f _sL, average idle time r loads _sL, the load year T.T. U that stops transport _sLbe respectively:

f _SL=f _e+f _e′，r _SL=(f _er _e+f _e′r _e′)/f _SL，U _SL=f _SLr _SL (24)

While considering route selection factor, the power off time that causes due to route selection is shorter to be disregarded, thereby the calculating of the average idle time of loading adopts and to disregard the fault trip rate of route selection factor, and the calculating of load point outage rate is still adopted to the line fault trip-out rate of taking into account route selection factor;

1.3) calculate respectively the medium voltage distribution network reliability index under different grounding modes

Adopt System average interruption frequency, Suo Xie SAIF SAIFI, user's System average interruption duration, Suo Xie SAID CAIDI and tri-indexs of scarce delivery ENS to evaluate the medium voltage distribution network reliability under different grounding modes, by following, calculate respectively,

$\mathrm{SAIFI}=\left(\underset{i}{\overset{m}{\mathrm{\&Sigma;}}}{f}_{\mathrm{SLi}}{N}_i\right)/\left(\underset{i}{\overset{m}{\mathrm{\&Sigma;}}}{N}_i\right)---\left(25\right)$

$\mathrm{CAIDI}=\left(\underset{i}{\overset{m}{\mathrm{\&Sigma;}}}{U}_{\mathrm{SLi}}{N}_i\right)/\left(\underset{i}{\overset{m}{\mathrm{\&Sigma;}}}{f}_{\mathrm{SLi}}{N}_i\right)---\left(26\right)$

$\mathrm{ENS}=\underset{i}{\overset{m}{\mathrm{\&Sigma;}}}{P}_{\mathrm{ai}}{U}_{\mathrm{SLi}}---\left(27\right)$

In formula, m is that total load is counted, f _sLifor the outage rate of load point i, N _ifor the number of users of load point i, U _sLifor the year of load point i stops transport T.T., P _aiaverage load for load point i;

2) calculate respectively medium voltage distribution network overall life cycle cost under different grounding modes

Medium voltage distribution network overall life cycle cost is calculated as follows:

U=G+X+K+F (28)

Wherein, G equipment investment cost, refers to the total investment of all support equipment facilities in the power distribution network of a certain neutral grounding mode; X is Maintenance and Repair expense, refers to the Maintenance and Repair expense that in the power distribution network 1 year of a certain neutral grounding mode, equipment failure produces; K is loss of outage cost, refers to user, cause in time period that in the power distribution network 1 year of a certain neutral grounding mode, equipment failure and malfunction elimination cause power failure the economic loss of short of electricity; F is obsolescence cost, after the power distribution network investment goods facility life cycle that refers to different neutral point modes finishes, for cleaning, destroy the expense of its required payment;

Loss of outage cost K is calculated as follows:

K=α×ENS×c (31)

In formula, a is a year penalty coefficient for power failure total losses, and c is regional electrogenesis ratio, and ENS is for lacking delivery;

3) decision-making decision condition

If the medium voltage distribution network overall life cycle cost U under certain earthing mode _Γmeet U _Γ≤ 0.9 * U _min, selecting this earthing mode is decision scheme; In formula, U _minfor the minimum overall life cycle cost under all the other neutral grounding modes except this earthing mode; Otherwise it is decision scheme that selection meets the neutral grounding mode of following relation:

SC=min{SC _i} (35)

In formula, SC _i=SAIFI _i* CAIDI _i, subscript represents i scheme.

Further, described electrogenesis refers to the ratio of the output value created in 1 year in an area and the electric weight of consumption than c,

c=GDP _i/EC (32)

In formula, GDP is the gross domestic product (GDP) that price indication is used in region then; EC is the region total electricity consumption of a year.

Further, the equipment investment cost G in medium voltage distribution network overall life cycle cost comprises equipment investment that the initial stage is fixing and the facility investment of interpolation in service, and calculating formula is:

$G=\underset{i=1}{\overset{m}{\mathrm{\&Sigma;}}}[\frac{k{(1+k)}^{t_i}}{{(1+k)}^{t_i}-1}{p}_i+a_i]---\left(29\right)$

In formula, m is the total item of investment; t _iit is the tenure of use of i item investment; K is annual rate; p _i=b _i+ e _i+ q _iit is the initial investment of i item investment; a _i=fh _i+ gh _i, be every operating cost that investment is required;

B _ifor the purchase commodity of initial investment, mainly comprise the purchase commodity of neutral earthing devices, protective relaying device, seal, automation equipment and line selection apparatus; e _ifor with the supporting engineering cost of purchase commodity; q _ifor other fees, comprise place requisition expense, migration workmen's compensation and compensation for crops; Operating facility investment refers to as ensuring personal safety safeguard procedures and the environmental protection taked and beautifies the investment needing, specifically comprises safety guide rail fh _i; The planning construction investment gh of conduit line and walkway distance etc. _i;

Maintenance and Repair expense X in medium voltage distribution network overall life cycle cost comprises maintenance cost and the regular visit maintenance cost that fault produces, and calculating formula is:

$X=\underset{i=1}{\overset{y}{\mathrm{\&Sigma;}}}{J}_i---\left(30\right)$

In formula, y is the total item of annual Maintenance and Repair task; J _ibe the expense of i item Maintenance and Repair task, comprise corresponding fee of material and labour cost;

Obsolescence cost F calculating formula in medium voltage distribution network overall life cycle cost is:

$F=\underset{i}{\overset{p}{\mathrm{\&Sigma;}}}{Q}_i---\left(33\right)$

In formula, p is total number of devices; Q _iit is the obsolescence cost of i kind equipment.

Compared with prior art, the present invention has following beneficial effect:

A kind of method that while the invention provides medium-Voltage Distribution network planning design, neutral grounding mode is selected, has made up the shortcoming that current medium voltage distribution network neutral grounding mode lacks quantitative evaluation and decision-making technique.This method has realized qualitative assessment and the comparison of distribution network reliability under different grounding modes, by calculating the overall life cycle cost of power distribution network under different grounding modes and taking into account the impact of reliability factor on cost, can decision-making go out to have the high reliability neutral grounding mode of Optimum Economic, for Distribution system design, transformation provide theoretical foundation and analysis tool.

Accompanying drawing explanation

The decision process of Fig. 1-medium voltage distribution network neutral grounding mode.

The medium voltage distribution network line fault tripping operation probability model of Fig. 2-isolated neutral.

The medium voltage distribution network line fault tripping operation probability model of Fig. 3-neutral by arc extinction coil grounding.

The medium voltage distribution network line fault tripping operation probability model of Fig. 4-Neutral Point Through Low Resistance.

Fig. 5-neutral point is through the medium voltage distribution network line fault tripping operation probability model of large resistance eutral grounding.

Fig. 6-RBTS Bus2 system schematic.

Embodiment

The present invention utilizes the comprehensive selection that relatively realizes medium voltage distribution network neutral grounding mode of overall life cycle cost and reliability index, in medium voltage distribution network overall life cycle cost, take into account the reliability factor of medium voltage distribution network under different grounding modes, can decision-making go out to have the high reliability neutral grounding mode of Optimum Economic, can make up medium voltage distribution network neutral grounding mode can not quantitative evaluation and the problem of decision-making, can meet that medium voltage distribution network is newly-built, the engineering demand of improvement and design, improve electric network security and operation maintenance efficiency.

Below in conjunction with the drawings and specific embodiments, the present invention is described in further detail.

Fig. 1 is the aggregate decision flow process of medium voltage distribution network neutral grounding mode of the present invention.Concrete steps are as follows:

1) the medium voltage distribution network reliability assessment of different grounding modes

1.1) medium voltage distribution network line fault tripping operation probability calculation

1.1.1) medium voltage distribution network of isolated neutral

The medium voltage distribution network line fault tripping operation probability model of isolated neutral is shown in Fig. 2, and the tripping operation probability of cable line or overhead transmission line i fault is

P _ii=λη(1-δ)+λ(1-η)=λ-ληδ (1)

During cable line i fault, fault expansion makes the probability of regular link j tripping operation be:

P _ij=λη(1-δ)(e ₁+e ₂-e ₁e ₂) (2)

During overhead transmission line i fault, fault expansion makes the probability of regular link j tripping operation be:

P _ij=λη(1-δ)e ₁ (3)

Consider the impact of microcomputer route selection, first carry out microcomputer route selection, the route selection of the laggard pedestrian's work of microcomputer route selection failure.Through b artificial route selection, choose faulty line i and regular link j that the probability of its tripping operation is respectively:

FP _ii=P _ii (4)

$F{P}_{\mathrm{ij}}=(1-\mathrm{\&omega;})(1-\frac{2}{n-1})\&times;(1-\frac{2}{n-2})\&times;...\&times;(1-\frac{2}{n-b+1})\&times;\frac{1}{n-b}{P}_{\mathrm{ii}}---\left(5\right)$

1.1.2) medium voltage distribution network of neutral by arc extinction coil grounding

The medium voltage distribution network line fault tripping operation probability model of neutral by arc extinction coil grounding is shown in Fig. 3, and the tripping operation probability of cable line or overhead transmission line i fault is:

P _ii=ληδ(1-α)+λη(1-δ)+λ(1-η)=λ(1-ηδα) (6)

During cable line i fault, fault expansion makes the probability of regular link j tripping operation be:

P _ij=ληδ(1-α)(e ₁+e ₂-e ₁e ₂)+λη(1-δ)(e ₁+e ₂-e ₁e ₂) (7)

During overhead transmission line i fault, fault expansion makes the probability of regular link j tripping operation be:

P _ij=ληδ(1-α)e ₁+λη(1-δ)e ₁ (8)

Consider the impact of microcomputer route selection, first carry out microcomputer route selection, the route selection of the laggard pedestrian's work of microcomputer route selection failure.Through b artificial route selection, choose faulty line i and regular link j that the probability of its tripping operation is respectively:

FP _ii=P _ii (9)

$F{P}_{\mathrm{ij}}=(1-\mathrm{\&omega;})(1-\frac{2}{n-1})\&times;(1-\frac{2}{n-2})\&times;...\&times;(1-\frac{2}{n-b+1})\&times;\frac{1}{n-b}{P}_{\mathrm{ii}}---\left(10\right)$

1.1.3) the medium voltage distribution network line fault of Neutral Point Through Low Resistance tripping operation probability model

The medium voltage distribution network line fault tripping operation probability model of Neutral Point Through Low Resistance is shown in Fig. 4, and the tripping operation probability of cable line or overhead transmission line i fault is:

P _ii=ληδ(1-β)+λη(1-δ)(1-β)+λ(1-η)=λ(1-ηβ) (11)

Cable line or overhead transmission line i fault make the probability of regular link j tripping operation be:

P _ij=ληδβ+λη(1-δ)β=ληβ (12)

1.1.4) neutral point is through the medium voltage distribution network of high resistance ground

Neutral point is shown in Fig. 5 through the medium voltage distribution network line fault tripping operation probability model of high resistance ground, and the tripping operation probability of cable line or overhead transmission line i fault is:

P _ii=ληδ(1-σ)1-ζ+λη(1-δ)1-ζ+λ(1-η)=ληδσ(1-ζ)+λ(1-ζη) (13)

Cable line i fault makes the probability of regular link j tripping operation be:

P _ij=[ληδ(1-σ)+λη(1-δ)]ζ(e ₁+e ₂-e ₁e ₂) (14)

Overhead transmission line i fault makes the probability of regular link j tripping operation be:

P _ij=[ληδ(1-σ)+λη(1-δ)]ζe ₁ (15)

Consider the impact of microcomputer route selection, first carry out microcomputer route selection, the route selection of the laggard pedestrian's work of microcomputer route selection failure.Through b artificial route selection, choose faulty line i and regular link j that the probability of its tripping operation is respectively:

FP _ii=P _ii (16)

$F{P}_{\mathrm{ij}}=(1-\mathrm{\&omega;})(1-\frac{2}{n-1})\&times;(1-\frac{2}{n-2})\&times;...\&times;(1-\frac{2}{n-b+1})\&times;\frac{1}{n-b}{P}_{\mathrm{ii}}---\left(17\right)$

To sum up, the final trip-out rate P of circuit i _iwhile equaling probability that faults itself successfully trips and other adjacent line faults, there is the probability that probability that expansion causes the final tripping operation of circuit i and route selection mistake cause circuit i to trip.

In above-mentioned formula, the implication of each parameter is:

λ: the probability of line failure;

η: during line fault, fault type is the probability of single-phase grounding fault;

δ: the relative probability of transient single-phase earth fault during line fault;

E ₁: the probability of circuit generation Overvoltage expansion;

E ₂: the probability of circuit generation electric arc fire failure expansion;

σ: the probability that instantaneity electric arc successfully extinguishes;

α: the probability that can successfully extinguish instantaneity electric arc during neutral by arc extinction coil grounding;

β: the probability that cannot successfully send trip signal during Neutral Point Through Low Resistance;

ζ: the probability that neutral point successfully trips when high resistance ground;

ω: the accuracy of microcomputer route selection;

N: the number of lines in system;

B: the number of times of artificial route selection;

This method has successfully been set up isolated neutral, neutral by arc extinction coil grounding, Neutral Point Through Low Resistance and the neutral point medium voltage distribution network line fault tripping operation probability model in large resistance eutral grounding situation, taken into account the impact that single-phase earth fault line selection factor centering is press-fitted electric network reliability, considered that under different grounding modes, fault expands the impact that centering is press-fitted electric network reliability;

1.2) calculate respectively the medium voltage distribution network dependability parameter under different grounding modes

For the cascade system being formed by n bar circuit, equivalent fault maintenance rate f _e, equivalent scheduled maintenance rate f _e', equivalent fault r servicing time _eand equivalent scheduled maintenance time r _e' be calculated as respectively:

$f_e=\underset{i}{\overset{n}{\mathrm{\&Sigma;}}}{f}_i,f_e^{\&prime;}\underset{i}{\overset{n}{\mathrm{\&Sigma;}}}{f}_i^{\&prime;},r_e=\underset{i}{\overset{n}{\mathrm{\&Sigma;}}}{f}_i{r}_i/f_e,r_e^{\&prime;}=\underset{i}{\overset{n}{\mathrm{\&Sigma;}}}{f}_i^{\&prime;}{r}_i^{\&prime;}/f_e^{\&prime;}{f}_e^{\&prime;}---\left(18\right)$

In formula, f _iit is the fault trip rate of i bar circuit; f _i' be the scheduled maintenance rate of i bar circuit; r _iit is the breakdown maintenance time of i bar circuit; r _i' be the scheduled maintenance time of i bar circuit;

If load point is connected in series circuit end, power supply point is connected in series circuit head end, load point outage rate f _sL, average idle time r loads _sL, the load year T.T. U that stops transport _sLbe respectively:

f _SL=f _e+f _e′，r _SL=(f _er _e+f _e′r _e′)/f _SL，U _SL=f _SLr _SL (19)

For double loop network, the equivalent fault maintenance rate f of parallel line _e, equivalent scheduled maintenance rate f _e', equivalent fault r servicing time _eand equivalent scheduled maintenance time r _e' be respectively:

f _e=f ₁f ₂(r ₁+r ₂) (20)

f _e′=f ₁′(f ₂r ₁′)+f ₂′(f ₁r ₂′) (21)

r _e=r ₁r ₂/(r ₁+r ₂)=(1/r ₁+1/r ₂) ^-1 (22)

$r_e^{\&prime;}=\frac{{\mathrm{<figure-callout\; id="1"\; label="f"\; filenames="DEST\_PATH\_HDA0000416593850000032.png"\; state="\{\{state\}\}">f</figure-callout>}}_1^{\&prime;}\left(f_2{\mathrm{<figure-callout\; id="1"\; label="r"\; filenames="DEST\_PATH\_HDA0000416593850000032.png"\; state="\{\{state\}\}">r</figure-callout>}}_1^{\&prime;}\right){(1/{\mathrm{<figure-callout\; id="1"\; label="r"\; filenames="DEST\_PATH\_HDA0000416593850000032.png"\; state="\{\{state\}\}">r</figure-callout>}}_1^{\&prime;}+1/r_2)}^{-1}+{\mathrm{<figure-callout\; id="2"\; label="f"\; filenames="DEST\_PATH\_HDA0000416593850000032.png"\; state="\{\{state\}\}">f</figure-callout>}}_2^{\&prime;}\left(f_1{\mathrm{<figure-callout\; id="2"\; label="r"\; filenames="DEST\_PATH\_HDA0000416593850000032.png"\; state="\{\{state\}\}">r</figure-callout>}}_2^{\&prime;}\right){(1/{\mathrm{<figure-callout\; id="2"\; label="r"\; filenames="DEST\_PATH\_HDA0000416593850000032.png"\; state="\{\{state\}\}">r</figure-callout>}}_2^{\&prime;}+1/r_1)}^{-1}}{{\mathrm{<figure-callout\; id="1"\; label="f"\; filenames="DEST\_PATH\_HDA0000416593850000032.png"\; state="\{\{state\}\}">f</figure-callout>}}_1^{\&prime;}\left(f_2{\mathrm{<figure-callout\; id="1"\; label="r"\; filenames="DEST\_PATH\_HDA0000416593850000032.png"\; state="\{\{state\}\}">r</figure-callout>}}_1^{\&prime;}\right)+{\mathrm{<figure-callout\; id="2"\; label="f"\; filenames="DEST\_PATH\_HDA0000416593850000032.png"\; state="\{\{state\}\}">f</figure-callout>}}_2^{\&prime;}\left(f_1{\mathrm{<figure-callout\; id="2"\; label="r"\; filenames="DEST\_PATH\_HDA0000416593850000032.png"\; state="\{\{state\}\}">r</figure-callout>}}_2^{\&prime;}\right)}---\left(23\right)$

In formula, f ₁and f ₂it is the fault trip rate of two parallel lines; f ₁' and f ₂' be the scheduled maintenance rate of two parallel lines; r ₁and r ₂it is the breakdown maintenance time of two parallel lines; r ₁' and r ₂' be the scheduled maintenance time of two parallel lines.

For double loop network, load point outage rate f _sL, average idle time r loads _sL, the load year T.T. U that stops transport _sLbe respectively:

f _SL=f _e+f _e′，r _SL=(f _er _e+f _e′r _e′)/f _SL，U _SL=f _SLr _SL (24)

While considering route selection factor, the power off time that causes due to route selection is shorter to be disregarded, thereby the calculating of the average idle time of loading adopts and to disregard the fault trip rate of route selection factor, and the calculating of load point outage rate is still adopted to the line fault trip-out rate of taking into account route selection factor;

1.3) the medium voltage distribution network reliability index of different grounding modes is calculated

Adopt System average interruption frequency, Suo Xie SAIF SAIFI, user's System average interruption duration, Suo Xie SAID CAIDI and tri-indexs of scarce delivery ENS, be calculated as respectively:

$\mathrm{SAIFI}=\left(\underset{i}{\overset{m}{\mathrm{\&Sigma;}}}{f}_{\mathrm{SLi}}{N}_i\right)/\left(\underset{i}{\overset{m}{\mathrm{\&Sigma;}}}{N}_i\right)---\left(25\right)$

$\mathrm{CAIDI}=\left(\underset{i}{\overset{m}{\mathrm{\&Sigma;}}}{U}_{\mathrm{SLi}}{N}_i\right)/\left(\underset{i}{\overset{m}{\mathrm{\&Sigma;}}}{f}_{\mathrm{SLi}}{N}_i\right)---\left(26\right)$

$\mathrm{ENS}=\underset{i}{\overset{m}{\mathrm{\&Sigma;}}}{P}_{\mathrm{ai}}{U}_{\mathrm{SLi}}---\left(27\right)$

In formula, m is that total load is counted, f _sLifor the outage rate of load point i, N _ifor the number of users of load point i, U _sLifor the year of load point i stops transport T.T., P _aiaverage load for load point i.

The present invention adopts System average interruption frequency, Suo Xie SAIF SAIFI, user's System average interruption duration, Suo Xie SAID CAIDI and tri-indexs of scarce delivery ENS to evaluate the reliability evaluation of medium voltage distribution network under different grounding modes, except the impact of different grounding modes fault trip rate, consider the scheduled maintenance time under different grounding modes, the impact on distribution network reliability of breakdown maintenance time and scheduled maintenance time simultaneously;

2) the medium voltage distribution network overall life cycle cost of different grounding modes

According to reliability evaluation, calculate the medium voltage distribution network overall life cycle cost of different grounding modes:

U=G+X+K+F (28)

Wherein, the total investment of all support equipment facilities in the power distribution network that G is a certain neutral grounding mode, the Maintenance and Repair expense that in power distribution network that X is a certain neutral grounding mode 1 year, equipment failure produces; K causes the economic loss of short of electricity to user in the time period that in the power distribution network 1 year of a certain neutral grounding mode, equipment failure and malfunction elimination cause power failure; After obsolescence cost F refers to that the power distribution network investment goods facility life cycle of different neutral point modes finishes, for clearing up, destroy the expense of its required payment.

1. equipment investment cost G

Equipment investment cost comprises equipment investment that the initial stage is fixing and the facility investment of interpolation in service:

$G=\underset{i=1}{\overset{m}{\mathrm{\&Sigma;}}}[\frac{k{(1+k)}^{t_i}}{{(1+k)}^{t_i}-1}{p}_i+a_i]---\left(29\right)$

In formula, m is the total item of investment; t _iit is the tenure of use of i item investment; K is annual rate; p _i=b _i+ e _i+ q _iit is the initial investment of i item investment; a _i=fh _i+ gh _ifor every operating cost that investment is required.

For different neutral ground schemes, initial investment mainly comprises the purchase commodity b of neutral earthing devices, protective relaying device, seal, automation equipment and line selection apparatus etc. _i, also comprise supporting with it engineering cost e _iwith other fees q _i, as place requisition takes, moves workmen's compensation, compensation for crops etc.Operating facility investment refers to as ensuring personal safety safeguard procedures and the environmental protection taked and beautifies the investment needing, specifically comprises safety guide rail fh _i, conduit line and walkway distance etc. planning construction investment gh _i.

2. Maintenance and Repair expense X

Maintenance and Repair expense comprises the maintenance cost that fault produces, and regular visit maintenance cost:

$X=\underset{i=1}{\overset{y}{\mathrm{\&Sigma;}}}{J}_i---\left(30\right)$

In formula, y is the total item of annual Maintenance and Repair task; J _ibe the expense of i item Maintenance and Repair task, comprise corresponding fee of material and labour cost.

3. loss of outage cost K

According to the reliability index ENS under neutral grounding mode, and be multiplied by and analyze regional electrogenesis than c, can calculate loss of outage cost and be: a is a year penalty coefficient for power failure total losses,

K=α×ENS×c (31)

Electrogenesis is than the ratio that refers to the output value (representing with money-form) that created in a year in an area and the electric weight consuming,

c=GDP _i/EC _i (32)

In formula, GDP _ifor using then the gross domestic product (GDP) of price indication in region; EC _ifor the region total electricity consumption of a year.

4. obsolescence cost F

$F=\underset{i}{\overset{p}{\mathrm{\&Sigma;}}}{Q}_i---\left(33\right)$

In formula, p is total number of devices; Q _iit is the obsolescence cost of i kind equipment.

3) medium voltage distribution network neutral grounding mode decision-making

For arbitrary neutral grounding mode, if the medium voltage distribution network overall life cycle cost U under this earthing mode _Γmeet the following conditions, selecting this earthing mode is decision scheme:

U _Γ≤0.9×U _min (34)

In formula, U _minfor the minimum overall life cycle cost under all the other neutral grounding modes except this earthing mode.

If there is not the earthing mode that meets formula (34), the neutral grounding mode of selecting to meet following relation is decision scheme:

SC=min{SC _i} (35)

In formula, SC _i=SAIFI _i* CAIDI _i, subscript represents i scheme.

Be the decision-making of neutral grounding mode to take the absolute optimum of economy be target, when the difference of the medium voltage distribution network overall life cycle cost of different earthing modes is larger, select neutral grounding mode that overall life cycle cost is minimum as decision scheme; At overall life cycle cost hour, the economic factors that cost difference causes is non-key factor, selects the neutral ground mode with optimal reliability as decision scheme.

This method has considered that different grounding modes centering is press-fitted the difference of electric network influencing, utilize the quantitative calculating of distribution network failure trip-out rate to realize assessment and the comparison of distribution network reliability under different grounding modes, the reliability evaluation of power distribution network under different grounding modes can be combined with Construction and operation cost analysis, by calculating the overall life cycle cost of power distribution network under different grounding modes combined reliability because usually realizing the optimal selection of neutral grounding mode.

According to the decision process of the neutral grounding mode of medium voltage distribution network shown in Fig. 1, in conjunction with a specific embodiment, the present invention is described in further detail below.

Fig. 6 is embodiment RBTS Bus2 system, the 33/11kV intermediate distribution system being comprised of 2 main transformers, 4 feeder lines, 36 circuits, 22 load point and substation transformer.Circuit is pure cable, and parameter is: f=0.04, f '=0.04, r=8, r '=3, η=8, δ=0.55, α=0.9, β=0.01, ζ=0.2, e ₁=e ₂=0.1, m=2, ω=0.5.Line length and load parameter are as shown in Table 1 and Table 2.

Table 1 line length

Table 2 load parameter

(1) evaluating reliability of distribution network of different grounding modes

The reliability index of network shown in calculating chart 6 under different neutral ground schemes, result is as shown in table 3.

Reliability index under table 3 different grounding modes

(2) the power distribution network overall life cycle cost of different grounding modes

1) input cost

According to data such as the specified > > of < < power engineering construction budget, the every input cost that obtains different neutral ground schemes is as shown in table 4.

Equipment investment cost under table 4 different grounding modes

Equipment investment cost under continued 4 different grounding modes

If n tenure of use of each equipment is 10 years, annual rate k=5%, the input cost calculating under different grounding modes is as shown in table 5.

The input cost of the different neutral ground schemes of table 5

2) maintenance recondition expense

According to statistics, obtain the year maintenance cost of different neutral ground schemes, as shown in table 6.

The year maintenance cost of the different neutral ground schemes of table 6

3) loss of outage cost

System disappearance electric weight ENS according under different grounding modes, gets electrogenesis than c=5.27 unit/kWh, and reliability penalty coefficient α=1.5, show that the scarce delivery under different grounding modes is as shown in table 7.

The loss of outage cost of the different neutral ground schemes of table 7

4) loss of outage cost

Under different grounding modes, the cost of every input after discarded, as shown in table 8.

The obsolescence cost of the different neutral ground schemes of table 8

To sum up, calculate the overall life cycle cost of network shown in Fig. 6 under different grounding modes, as shown in table 9.

Total financial cost of the different neutral ground schemes of table 9

(3) neutral grounding in distribution power network decision-making

According to table 9, life cycle cost during Neutral Point Through Low Resistance is close with overall life cycle cost when earth-free, for network shown in Fig. 6, does not have the economy earthing mode of absolute optimum.

Reliability index SC while calculating Neutral Point Through Low Resistance _rreliability index SC during with isolated neutral _nas follows:

SC _r=1.7336，SC _n=2.7261

SC _rsignificantly be less than SC _n, showing in the situation that overall life cycle cost is close, reliability during Neutral Point Through Low Resistance is apparently higher than the situation of isolated neutral.So, select Neutral Point Through Low Resistance as the neutral grounding mode of network shown in Fig. 6.

Finally explanation is, above embodiment is only unrestricted in order to technical scheme of the present invention to be described, although the present invention is had been described in detail with reference to preferred embodiment, those of ordinary skill in the art is to be understood that, can modify or be equal to replacement technical scheme of the present invention, and not departing from aim and the scope of technical solution of the present invention, it all should be encompassed in the middle of claim scope of the present invention.

Claims (3)

1. medium voltage distribution network neutral grounding mode system of selection, described neutral grounding mode comprises that isolated neutral, neutral by arc extinction coil grounding, Neutral Point Through Low Resistance and neutral point are through large four kinds of earthing modes of resistance eutral grounding, it is characterized in that: carry out according to the following steps

1) the medium voltage distribution network reliability assessment of different grounding modes

1.1) calculate respectively the medium voltage distribution network line fault tripping operation probability of different grounding modes

1.1.1) the medium voltage distribution network line fault of isolated neutral tripping operation probability calculation

The tripping operation probability of cable line or overhead transmission line i fault is

P _ii=λη(1-δ)+λ(1-η)=λ-ληδ (1)

During cable line i fault, fault expansion makes the probability of regular link j tripping operation be:

P _ij=λη(1-δ)(e ₁+e ₂-e ₁e ₂) (2)

During overhead transmission line i fault, fault expansion makes the probability of regular link j tripping operation be:

P _ij=λη(1-δ)e ₁ (3)

Consider microcomputer route selection impact, first carry out microcomputer route selection, the route selection of the laggard pedestrian's work of microcomputer route selection failure, through b artificial route selection, choose faulty line i and regular link j that the probability of its tripping operation is respectively:

FP _ii=P _ii (4)

$F{P}_{\mathrm{ij}}=(1-\mathrm{\&omega;})(1-\frac{2}{n-1})\&times;(1-\frac{2}{n-2})\&times;...\&times;(1-\frac{2}{n-b+1})\&times;\frac{1}{n-b}{P}_{\mathrm{ii}}---\left(5\right)$

1.1.2) the medium voltage distribution network line fault of neutral by arc extinction coil grounding tripping operation probability calculation

The tripping operation probability of cable line or overhead transmission line i fault is:

P _ii=ληδ(1-α)+λη(1-δ)+λ(1-η)=λ(1-ηδα) (6)

During cable line i fault, fault expansion makes the probability of regular link j tripping operation be:

P _ij=ληδ(1-α)(e ₁+e ₂-e ₁e ₂)+λη(1-δ)(e ₁+e ₂-e ₁e ₂) (7)

During overhead transmission line i fault, fault expansion makes the probability of regular link j tripping operation be:

P _ij=ληδ(1-α)e ₁+λη(1-δ)e ₁ (8)

Consider microcomputer route selection impact, first carry out microcomputer route selection, the route selection of the laggard pedestrian's work of microcomputer route selection failure, through b artificial route selection, choose faulty line i and regular link j that the probability of its tripping operation is respectively:

FP _ii=P _ii (9)

$F{P}_{\mathrm{ij}}=(1-\mathrm{\&omega;})(1-\frac{2}{n-1})\&times;(1-\frac{2}{n-2})\&times;...\&times;(1-\frac{2}{n-b+1})\&times;\frac{1}{n-b}{P}_{\mathrm{ii}}---\left(10\right)$

1.1.3) the medium voltage distribution network line fault of Neutral Point Through Low Resistance tripping operation probability calculation

The tripping operation probability of cable line or overhead transmission line i fault is:

P _ii=ληδ(1-β)+λη(1-δ)(1-β)+λ(1-η)=λ(1-ηβ) (11)

When cable line or overhead transmission line i fault, fault expansion makes the probability of regular link j tripping operation be:

P _ij=ληδβ+λη(1-δ)β=ληβ (12)

1.1.4) neutral point is through the medium voltage distribution network line fault tripping operation probability calculation of high resistance ground

The tripping operation probability of cable line or overhead transmission line i fault is:

P _ii=ληδ(1-σ)1-ζ+λη(1-δ)1-ζ+λ(1-η)=ληδσ(1-ζ)+λ(1-ζη) (13)

During cable line i fault, fault expansion makes the probability of regular link j tripping operation be:

P _ij=[ληδ(1-σ)+λη(1-δ)]ζ(e ₁+e ₂-e ₁e ₂) (14)

During overhead transmission line i fault, fault expansion makes the probability of regular link j tripping operation be:

P _ij=[ληδ(1-σ)+λη(1-δ)]ζe ₁ (15)

Consider microcomputer route selection impact, first carry out microcomputer route selection, the route selection of the laggard pedestrian's work of microcomputer route selection failure, through b artificial route selection, choose faulty line i and regular link j that the probability of its tripping operation is respectively:

FP _ii=P _ii (16)

$F{P}_{\mathrm{ij}}=(1-\mathrm{\&omega;})(1-\frac{2}{n-1})\&times;(1-\frac{2}{n-2})\&times;...\&times;(1-\frac{2}{n-b+1})\&times;\frac{1}{n-b}{P}_{\mathrm{ii}}---\left(17\right)$

When the final tripping operation probability of arbitrary circuit i equals probability that faults itself successfully trips and other adjacent line faults in power distribution network, there is expansion and cause the probability of the final tripping operation of circuit i and the probability sum that route selection mistake causes circuit i tripping operation;

In above-mentioned formula (1)-(17), the implication of each parameter is:

λ: the probability of line failure;

η: during line fault, fault type is the probability of single-phase grounding fault;

δ: the relative probability of transient single-phase earth fault during line fault;

E ₁: the probability of circuit generation Overvoltage expansion;

E ₂: the probability of circuit generation electric arc fire failure expansion;

σ: the probability that instantaneity electric arc successfully extinguishes;

α: the probability that can successfully extinguish instantaneity electric arc during neutral by arc extinction coil grounding;

β: the probability that cannot successfully send trip signal during Neutral Point Through Low Resistance;

ζ: the probability that neutral point successfully trips when high resistance ground;

ω: the accuracy of microcomputer route selection;

N: the number of lines in system;

B: the number of times of artificial route selection;

1.2) calculate respectively the medium voltage distribution network dependability parameter under different grounding modes

For the cascade system being formed by n bar circuit, equivalent fault maintenance rate f _e, equivalent scheduled maintenance rate f _e', equivalent fault r servicing time _eand equivalent scheduled maintenance time r _e' be calculated as respectively:

$f_e=\underset{i}{\overset{n}{\mathrm{\&Sigma;}}}{f}_i,f_e^{\&prime;}\underset{i}{\overset{n}{\mathrm{\&Sigma;}}}{f}_i^{\&prime;},r_e=\underset{i}{\overset{n}{\mathrm{\&Sigma;}}}{f}_i{r}_i/f_e,r_e^{\&prime;}=\underset{i}{\overset{n}{\mathrm{\&Sigma;}}}{f}_i^{\&prime;}{r}_i^{\&prime;}/f_e^{\&prime;}{f}_e^{\&prime;}---\left(18\right)$

In formula, f _iit is the fault trip rate of i bar circuit; f _i' be the scheduled maintenance rate of i bar circuit; r _iit is the breakdown maintenance time of i bar circuit; r _i' be the scheduled maintenance time of i bar circuit;

If load point is connected in series circuit end, power supply point is connected in series circuit head end, load point outage rate f _sL, average idle time r loads _sL, the load year T.T. U that stops transport _sLbe respectively:

f _SL=f _e+f _e′，r _SL=(f _er _e+f _e′r _e′)/f _SL，U _SL=f _SLr _SL (19)

For double loop network, the equivalent fault maintenance rate f of parallel line _e, equivalent scheduled maintenance rate f _e', equivalent fault r servicing time _eand equivalent scheduled maintenance time r _e' be respectively:

f _e=f ₁f ₂(r ₁+r ₂) (20)

f _e′=f ₁′(f ₂r ₁′+f ₂′(f ₁r ₂′) (21)

r _e=r ₁r ₂/(r ₁+r ₂)=(1/r ₁+1/r ₂) ^-1 (22)

$r_e^{\&prime;}=\frac{f_1^{\&prime;}\left(f_2{r}_1^{\&prime;}\right){(1/r_1^{\&prime;}+1/r_2)}^{-1}+f_2^{\&prime;}\left(f_1{r}_2^{\&prime;}\right){(1/r_2^{\&prime;}+1/r_1)}^{-1}}{f_1^{\&prime;}\left(f_2{r}_1^{\&prime;}\right)+f_2^{\&prime;}\left(f_1{r}_2^{\&prime;}\right)}---\left(23\right)$

In formula, f ₁and f ₂it is the fault trip rate of two parallel lines; f ₁' and f ₂' be the scheduled maintenance rate of two parallel lines; r ₁and r ₂it is the breakdown maintenance time of two parallel lines; r ₁' and r ₂' be the scheduled maintenance time of two parallel lines.

For double loop network, load point outage rate f _sL, average idle time r loads _sL, the load year T.T. U that stops transport _sLbe respectively:

f _SL=f _e+f _e′，r _SL=(f _er _e+f _e′r _e′)/f _SL，U _SL=f _SLr _SL (24)

While considering route selection factor, the power off time that causes due to route selection is shorter to be disregarded, thereby the calculating of the average idle time of loading adopts and to disregard the fault trip rate of route selection factor, and the calculating of load point outage rate is still adopted to the line fault trip-out rate of taking into account route selection factor;

1.3) calculate respectively the medium voltage distribution network reliability index under different grounding modes

Adopt System average interruption frequency, Suo Xie SAIF SAIFI, user's System average interruption duration, Suo Xie SAID CAIDI and tri-indexs of scarce delivery ENS to evaluate the medium voltage distribution network reliability under different grounding modes, by following, calculate respectively,

$\mathrm{SAIFI}=\left(\underset{i}{\overset{m}{\mathrm{\&Sigma;}}}{f}_{\mathrm{SLi}}{N}_i\right)/\left(\underset{i}{\overset{m}{\mathrm{\&Sigma;}}}{N}_i\right)---\left(25\right)$

$\mathrm{CAIDI}=\left(\underset{i}{\overset{m}{\mathrm{\&Sigma;}}}{U}_{\mathrm{SLi}}{N}_i\right)/\left(\underset{i}{\overset{m}{\mathrm{\&Sigma;}}}{f}_{\mathrm{SLi}}{N}_i\right)---\left(26\right)$

$\mathrm{ENS}=\underset{i}{\overset{m}{\mathrm{\&Sigma;}}}{P}_{\mathrm{ai}}{U}_{\mathrm{SLi}}---\left(27\right)$

In formula, m is that total load is counted, F _sLifor the outage rate of load point i, N _ifor the number of users of load point i, U _sLifor the year of load point i stops transport T.T., P _aiaverage load for load point i;

2) calculate respectively medium voltage distribution network overall life cycle cost under different grounding modes

Medium voltage distribution network overall life cycle cost is calculated as follows:

U=G+X+K+F (28)

Wherein, G equipment investment cost, refers to the total investment of all support equipment facilities in the power distribution network of a certain neutral grounding mode; X is Maintenance and Repair expense, refers to the Maintenance and Repair expense that in the power distribution network 1 year of a certain neutral grounding mode, equipment failure produces; K is loss of outage cost, refers to user, cause in time period that in the power distribution network 1 year of a certain neutral grounding mode, equipment failure and malfunction elimination cause power failure the economic loss of short of electricity; F is obsolescence cost, after the power distribution network investment goods facility life cycle that refers to different neutral point modes finishes, for cleaning, destroy the expense of its required payment;

Loss of outage cost K is calculated as follows:

K=α×ENS×c (31)

In formula, a is a year penalty coefficient for power failure total losses, and c is regional electrogenesis ratio, and ENS is for lacking delivery;

3) decision-making decision condition

If the medium voltage distribution network overall life cycle cost U under certain earthing mode _Γmeet U _Γ≤ 0.9 * U _min, selecting this earthing mode is decision scheme; In formula, U _minfor the minimum overall life cycle cost under all the other neutral grounding modes except this earthing mode; Otherwise it is decision scheme that selection meets the neutral grounding mode of following relation:

SC=min{SC _i} (35)

In formula, SC _i=SAIFI _i* SAIFI _i* CAIDI _i, subscript represents i scheme.

2. medium voltage distribution network neutral grounding mode according to claim 1 system of selection, is characterized in that: described electrogenesis refers to the ratio of the output value created in 1 year in an area and the electric weight of consumption than c,

c=GDP _i/EC (32)

In formula, GDP is the gross domestic product (GDP) that price indication is used in region then; EC is the region total electricity consumption of a year.

3. medium voltage distribution network neutral grounding mode according to claim 1 system of selection, it is characterized in that: the equipment investment cost G in medium voltage distribution network overall life cycle cost comprises equipment investment that the initial stage is fixing and the facility investment of interpolation in service, and calculating formula is:

$G=\underset{i=1}{\overset{m}{\mathrm{\&Sigma;}}}[\frac{k{(1+k)}^{t_i}}{{(1+k)}^{t_i}-1}{p}_i+a_i]---\left(29\right)$

In formula, m is the total item of investment; t _iit is the tenure of use of i item investment; K is annual rate; p _i=b _i+ e _i+ q _iit is the initial investment of i item investment; a _i=fh _i+ gh _ifor every operating cost that investment is required;

B _ifor the purchase commodity of initial investment, mainly comprise the purchase commodity of neutral earthing devices, protective relaying device, seal, automation equipment and line selection apparatus; e _ifor with the supporting engineering cost of purchase commodity; q _ifor other fees, comprise place requisition expense, migration workmen's compensation and compensation for crops; Operating facility investment refers to as ensuring personal safety safeguard procedures and the environmental protection taked and beautifies the investment needing, specifically comprises safety guide rail fh _i; The planning construction investment gh of conduit line and walkway distance etc. _i;

Maintenance and Repair expense X in medium voltage distribution network overall life cycle cost comprises maintenance cost and the regular visit maintenance cost that fault produces, and calculating formula is:

$X=\underset{i=1}{\overset{y}{\mathrm{\&Sigma;}}}{J}_i---\left(30\right)$

In formula, y is the total item of annual Maintenance and Repair task; J _ibe the expense of i item Maintenance and Repair task, comprise corresponding fee of material and labour cost;

Obsolescence cost F calculating formula in medium voltage distribution network overall life cycle cost is:

$F=\underset{i}{\overset{p}{\mathrm{\&Sigma;}}}{Q}_i---\left(33\right)$

In formula, p is total number of devices; Q _iit is the obsolescence cost of i kind equipment.

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