CN108711850B - Method for judging safety loop closing operation of medium-voltage distribution network - Google Patents

Method for judging safety loop closing operation of medium-voltage distribution network Download PDF

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CN108711850B
CN108711850B CN201810568103.6A CN201810568103A CN108711850B CN 108711850 B CN108711850 B CN 108711850B CN 201810568103 A CN201810568103 A CN 201810568103A CN 108711850 B CN108711850 B CN 108711850B
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loop closing
current
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CN108711850A (en
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冯楠
周自强
张焰
冯煜尧
崔勇
杨永华
朱齐
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Shanghai Jiaotong University
State Grid Shanghai Electric Power Co Ltd
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State Grid Shanghai Electric Power Co Ltd
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
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Abstract

A method for judging safety loop closing operation of a medium-voltage distribution network comprises the steps of calculating an effective value of loop closing steady-state current of the head end of a feed line after loop closing, judging whether the upper limit of the effective value of the loop closing steady-state current is larger than the maximum allowable current-carrying capacity of the feed line, if not, allowing the loop closing operation, if so, continuously judging whether the out-of-limit probability of the effective value of the maximum steady-state current of the head ends of the feed lines at two sides after loop closing is smaller than 5%, if so, allowing the loop closing operation, and if not, allowing the loop closing operation. The method has the advantages of short decision time, more accurate judgment result, safety and timeliness in practical application, and is beneficial to operators to make safe and rapid decisions on the loop closing operation of the medium-voltage distribution network.

Description

Method for judging safety loop closing operation of medium-voltage distribution network
Technical Field
The invention relates to the field of safe and stable operation of a power system, in particular to a method for judging safe loop closing operation of a medium-voltage distribution network.
Background
At present, a medium-voltage distribution network in China generally adopts a power supply mode of closed-loop design and open-loop operation, and when equipment is overhauled or an emergency accident is treated, the load can be transferred without power outage through closed-loop operation of a closed contact switch, so that the power outage time is shortened, and the power supply reliability is improved. Due to the voltage difference between two sides of the fracture of the interconnection switch, the loop closing operation may generate larger transient impact current and steady-state circulating current, which causes current protection action or overload of some electrical equipment, resulting in loop closing failure and even large-scale power failure accidents. Therefore, before the loop closing operation, the operator must judge whether the loop closing operation can be performed or not and make a safety loop closing decision.
In the actual operation management of the power distribution network, at present, workers generally judge the safety loop closing operation based on production experience, and consider that the loop closing operation of the medium-voltage power distribution network can be performed when the following two conditions are met.
1. Before loop closing, the difference of the voltage amplitudes of the 10kV buses where the feeders on two sides of the loop closing point are located is not more than 10%;
2. the total load of the feeders on two sides of the loop closing point before loop closing is not more than the maximum allowable transmission capacity of the feeder on any side.
In general, the load of a feeder line in daytime is large, and loop closing is not allowed when the load is heavy in daytime as a result of loop closing decision made by adopting the method. In order to ensure the safety of the loop closing operation, the loop closing and load reversing operation is carried out at night in a light load mode. Therefore, the decision of the closing ring operation based on experience is conservative, so that some conventional maintenance operations have to be carried out at night, and great inconvenience is brought to operation and maintenance personnel. Furthermore, this decision method lacks relevant theoretical support. Analysis shows that the magnitude of the circulating current generated by the loop closing operation is related to the voltage amplitude difference and the phase angle difference of two sides of the connection switch fracture, the influence of the phase angle difference on the loop closing current is large, and the method only carries out decision making on the loop closing operation according to the voltage amplitude difference of the 10kV bus where the feeders on two sides of the loop closing point are located lacks of rigorous scientific basis. Therefore, the existing method for judging the safety loop closing based on the production experience has great limitation in theory and practice.
At present, related researches propose that a method of respectively concentrating total loads on two sides of a loop closing point at the head end and the tail end of each feeder line is adopted, the range of loop closing current is calculated, and loop closing judgment is carried out according to the range. The method has a certain theoretical basis, but the actual load distribution condition in the power distribution network is not considered, so that the loop closing decision result is conservative, and the practical value is limited.
Disclosure of Invention
The invention provides a method for judging the safe loop closing operation of a medium-voltage distribution network, which has the advantages of short decision time, more accurate judgment result, safety and timeliness in practical application, and is beneficial to operators to make safe and rapid decisions on the loop closing operation of the medium-voltage distribution network.
In order to achieve the above object, the present invention provides a method for determining a safety loop closing operation for a medium voltage distribution network, comprising the steps of: calculating the loop closing steady-state current effective value of the head end of the feed line after loop closing, judging whether the upper limit of the loop closing steady-state current effective value is larger than the maximum allowable current-carrying capacity of the feed line, if not, allowing the loop closing operation, if so, continuously judging whether the out-of-limit probability of the maximum steady-state current effective values of the head ends of the feed lines at two sides after loop closing is smaller than 5%, if so, allowing the loop closing operation, and if not, not allowing the loop closing operation.
The method for judging whether the upper limit of the loop closing steady-state current effective value is larger than the maximum allowable current-carrying capacity of the feeder line or not comprises the following steps:
1. performing state estimation on the high-voltage distribution network by adopting a weighted least square criterion to obtain the voltage amplitude and phase angle of 10kV buses at the head ends of the two closed-loop feeders;
2. calculating the range of the closed loop steady-state circulation effective value;
closed loop steady-state circulation
Figure BDA0001685048930000021
Expressed as:
Figure BDA0001685048930000022
wherein the content of the first and second substances,
Figure BDA0001685048930000023
in order to communicate the voltage difference across the break of the switch,
Figure BDA0001685048930000024
and
Figure BDA0001685048930000025
respectively, the voltage phasors, Z, on both sides of the break of the interconnection switchΣIs the total impedance of the loop closing network;
taking a 10kV bus node at the head end of the closed loop feeder line as a reference node, respectively regarding the total load of the feeder line as being concentrated at the head end and the tail end of the line, and performing two times of deterministic load flow calculation and closed loop stable circulation calculation to obtain a closed loop stable circulation effective value IcA range of (d);
3. calculating the maximum steady-state current effective value;
is provided with
Figure BDA0001685048930000026
If the effective value of the steady-state current at the head end of the feed line before loop closing is I, the effective value of the steady-state current at the head end of the feed line after loop closing is I' and meets the following requirements:
Figure BDA0001685048930000027
the maximum steady-state current effective value of the head end of the feed line after loop closing is I + Ic；
4. The maximum steady-state current effective value I + IcMaximum allowable current carrying capacity I with feeder linemaxAnd comparing, if the maximum steady-state current effective value is less than the maximum allowable current-carrying capacity of the feeder line, judging that loop closing is allowed, and if the maximum steady-state current effective value is greater than or equal to the maximum allowable current-carrying capacity of the feeder line, continuously judging whether the out-of-limit probability of the maximum steady-state current effective values of the head ends of the feeder lines at two sides after loop closing is less than 5%.
The method for judging whether the out-of-limit probability of the maximum steady-state current effective value of the head ends of the feed lines at the two sides after loop closing is less than 5 percent comprises the following steps:
1. solving each-order semi-invariant of each input variable in the season and time period of the loop closing time based on historical load data;
selecting the ratio of the active load and the reactive load of each load point to the active power and the reactive power of the head end of the line as an input variable kPConstructed for finding the input variable kPThe sample set S of each order of semi-invariant is divided into a plurality of sub-sample sets according to seasons and time periods, and an input variable k is obtained through analysis of a certain sub-sample setPAt the moment of closing the ringEach order of semi-invariants within seasons and time periods;
if the sub-sample set corresponding to the loop closing time has N discrete historical data, which is expressed as { kP1,kP2,kP3,…,kPN}；
Calculating the origin moment alpha of each orderv：
Figure BDA0001685048930000031
Calculating the semi-invariant gamma of each order according to the relation between the semi-invariant and the origin momentv：
Figure BDA0001685048930000032
Wherein the content of the first and second substances,
Figure BDA0001685048930000033
the number of different combinations of j elements from v elements;
2. calculating loop closing current at a reference operation point according to topology information, equipment parameters and real-time operation data of a power grid before loop closing to obtain a loop closing current expected value and a conversion matrix;
and K is an input variable of the probability load flow calculation and represents the ratio of the active power and the reactive power of each node to the active power and the reactive power of the head end of the line, W represents the injection power of each node in the system, W is AK, wherein A is a diagonal matrix formed by the active power and the reactive power of the head ends of two feeders, and X is theta1,V1,θ2,V2,…,θn,Vn]TIs a system state variable;
the system power flow equation is expressed as:
W＝f(X) (5)
let I1And I2The initial current effective values of the head ends of the two side feeder lines before the loop closing are respectively represented by formula (1) and formula (2), and the maximum steady state current effective value I possibly appearing at the head ends of the two side feeder lines after the loop closing is represented by formula (1) and formula (2)1tAnd I2tAre respectively represented as：
I1t＝I1+Ic＝g1(X) (6)
I2t＝I2+Ic＝g2(X) (7)
Set variable Z ═ I1t,I2t]TFor the loop closing current variable, the loop closing current equation is expressed in the form of a matrix as follows:
Z＝g(X) (8)
the input variable K is a random variable, denoted K0+ Δ K, where K0The expected value of the random variable K is a system reference operating point;
similarly, the system state variable X is represented as X0+ Δ X, injected power W at each node is denoted as W0+ Δ W, closed-loop current variable Z being denoted as Z0+ΔZ；
And performing Taylor series expansion on the system power flow equation and the closed loop current equation and omitting high-order terms to obtain a linear relation between delta Z and delta K:
Figure BDA0001685048930000041
wherein the content of the first and second substances,
Figure BDA0001685048930000042
at a reference operating point K0Where (W at this time)0＝AK0) Performing deterministic load flow calculation according to the formula (5) to obtain a system state variable X0And Jacobian matrix J0(ii) a Then at X0Calculating the loop closing current according to the formula (8) to obtain a loop closing current variable Z0And a coefficient matrix G0(ii) a Finally, a conversion matrix T is obtained0；
3. Solving each-order semi-invariant of closed-loop current variables based on a probability power flow theory;
Figure BDA0001685048930000043
wherein, Δ Z(v)Is a v-order semi-invariant of closed-loop current variables, Δ K(v)A v-order semi-invariant representing an input variable,
Figure BDA0001685048930000044
represents T0A coefficient matrix formed by v powers of all the elements in the array;
4. calculating the cumulative probability distribution function and the out-of-limit probability of the maximum steady-state current effective values of the head ends of the feeder lines at the two sides after loop closing;
and (3) solving the cumulative probability distribution of the closed-loop current by adopting a Cornish-Fisher series according to each-order semi-invariant of the closed-loop current variable:
let the cumulative distribution function of a loop current variable Z be F (Z), and the standard normal distribution function be phi (Z), and the alpha quantiles of F (Z) and phi (Z) are respectively expressed as Z (alpha) and
Figure BDA0001685048930000051
i.e. z (α) ═ F-1(α)，
Figure BDA0001685048930000052
Z (. alpha.) and
Figure BDA0001685048930000053
the following relationship is satisfied:
Figure BDA0001685048930000054
wherein, gvNormalizing the semi-invariants for the v-order of the closed-loop current variable Z, i.e. if the v-order semi-invariants of the random variable Z is gammavWith a standard deviation of σ, then
Figure BDA0001685048930000055
The cumulative probability distribution function can be obtained from the semi-invariant of each order of the closed loop current according to the formula (11);
the loop closing current variable Z comprises the maximum steady state current effective value I of the head ends of the feeder lines at two sides after loop closing1tAnd I2tIs provided with I1tAnd I2tRespectively, is F1(x)、F2(x) To close the maximum allowable current-carrying capacity I of the feeder lines on both sides of the loop pointmax,1And Imax,2As reference value, then I1tAnd I2tIs out of limit probability P1And P2Respectively as follows:
P1＝P(I1t≥Imax,1)＝1-F1(Imax,1) (12)
P2＝P(I2t≥Imax,2)＝1-F2(Imax,2) (13)
5. according to the out-of-limit probability P1And P2If P is greater than P, the second loop closing judgment is performed1And P2If the loop closing rate is less than 5%, the loop closing is judged to be allowed, otherwise, the loop closing is judged not to be allowed, and the whole decision process is finished.
The invention has the following beneficial effects:
1. whether the loop closing operation can be executed or not is judged by calculating the steady-state current possibly generated by the loop closing operation and comparing the steady-state current with the maximum allowable current-carrying capacity of a line, and the problem of insufficient theoretical performance of the existing method is solved.
2. The safety and timeliness of a loop closing operation decision in actual production are comprehensively considered, two loop closing judgments are carried out before the decision, the decision can be made quickly in a larger safety margin preferentially, some timeliness is sacrificed to carry out more accurate decision in a smaller safety margin, and the problem that the existing decision making method is insufficient in practicability is solved.
3. After the first loop closing judgment, the load distribution condition on the loop closing feeder line is considered, the loads of all load points are used as random variables, the out-of-limit probability of the loop closing current is obtained based on the probability load flow theory, and the second loop closing judgment is carried out, so that the problem that the decision result of the existing method is conservative is solved.
Drawings
Fig. 1 is a flowchart of a method for determining a safety loop closing operation for a medium-voltage distribution network according to the present invention.
Fig. 2 is a schematic diagram of a closed loop operation of a medium voltage distribution network.
Fig. 3 is a flowchart of a determination method for performing a safety loop closing operation on the medium-voltage distribution network in the embodiment.
Detailed Description
The preferred embodiment of the present invention will be described in detail below with reference to fig. 1 to 3.
As shown in fig. 1, the present invention provides a method for determining a safety loop closing operation for a medium voltage distribution network, comprising the following steps:
step S1, calculating a loop closing steady-state current effective value of the head end of the feed line after loop closing, judging whether the upper limit of the loop closing steady-state current effective value is larger than the maximum allowable current-carrying capacity of the feed line, if so, performing step S2, and if not, allowing loop closing operation;
and step S2, judging whether the out-of-limit probability of the maximum steady-state current effective value of the head ends of the feed lines at the two sides after loop closing is less than 5%, if so, allowing the loop closing operation, and if not, disallowing the loop closing operation.
As shown in fig. 2, the system grid operates in a ring configuration, with the distribution network operating open loop at voltage levels of 110kV and below. And Q1 and Q2 are outlet circuit breakers of feeders on two sides of the loop closing point respectively, and the two 10kV feeders are connected through a connection switch Q3.
Figure BDA0001685048930000061
And
Figure BDA0001685048930000062
respectively the voltage phasor of the 10kV bus where the feeder lines on the two sides are located,
Figure BDA0001685048930000063
and
Figure BDA0001685048930000064
respectively the voltage phasors at both sides of the break of the tie switch.
As shown in fig. 3, in an embodiment of the present invention, a method for determining a safety loop closing operation for a medium voltage distribution network includes the steps of:
step I, calculating a closed loop steady-state current effective value of the head end of the feed line after closed loop, judging whether the upper limit of the closed loop steady-state current effective value is larger than the maximum allowable current-carrying capacity of the feed line, if so, performing step II, and if not, allowing closed loop operation;
and II, judging whether the out-of-limit probability of the maximum steady-state current effective value of the head ends of the feeder lines at the two sides after loop closing is less than 5%, if so, allowing the loop closing operation, and if not, not allowing the loop closing operation.
Further, step I specifically includes the following steps:
i-1, estimating the state of a high-voltage distribution network to obtain the voltage amplitude and phase angle information of a 10kV bus at the head ends of feeders on two sides of a closed loop point;
when the closed-loop steady-state current of the voltage distribution network is calculated, a 10kV bus node where a closed-loop feeder is located is generally selected as a reference node, so that the voltage amplitude and the phase angle of 10kV buses on two sides of a tie switch are required to be known. At present, a measurement system of a medium-voltage distribution network can acquire a voltage amplitude, but phase angle information of the voltage amplitude cannot be acquired. Aiming at the problem, on the basis of acquiring topological information, equipment parameters and real-time operation data of a power transmission network and a high-voltage distribution network in a closed loop, performing state estimation on the high-voltage distribution network by adopting a Weighted Least square criterion (WLS) to obtain a voltage amplitude and a phase angle of a 10kV bus at the head end of the closed loop feeder line, and correcting some bad measurement data;
step I-2, taking a 10kV bus node at the head end of the closed-loop feeder line as a reference node, respectively regarding the total load of the feeder line as being concentrated at the head end and the tail end of the circuit, and performing two times of deterministic load flow calculation and closed-loop steady-state circulating current calculation to obtain the range of the closed-loop steady-state circulating current effective value;
the real-time data of the voltage, the current and the power factor of the head ends of the feeders at two sides before the loop closing can be obtained through the distribution network information acquisition system, and then the active power and the reactive power of the head ends of the feeders can be determined, however, as shown in fig. 2, because the medium-voltage distribution network has more load nodes and is limited by the configuration of an actual measurement system, the real-time load data of each load point before the loop closing is generally difficult to obtain, (voltage at two sides of a contact switch fracture), (the real-time load data of the contact switch are limited by the configuration of an actual measurement system), (the voltage of two sides of the contact switch fracture), (the real-time load data of the contact switch are limited by the configuration of the real-time measurement system, and the real-time load data of the contact switch
Figure BDA0001685048930000071
And
Figure BDA0001685048930000072
) Is difficult to obtain through deterministic load flow calculation, and further difficult to calculate closed loop steady-state circulation IcThe exact value of (d);
firstly, when the load is respectively concentrated at the head end and the tail end of the feeder line at the two sides of the closed-loop switch, the voltage difference at the two sides of the fracture of the interconnection switch
Figure BDA0001685048930000073
The maximum value and the minimum value of the load are reached, and the load can be obtained by deterministic load flow calculation under the following two conditions
Figure BDA0001685048930000074
The range of (A):
1. the load of a feeder line on the left side of the loop closing switch is concentrated at the head end of the feeder line, and the load of a feeder line on the right side of the loop closing switch is concentrated at the tail end of the feeder line;
2. the load of a feeder line on the left side of the loop closing switch is concentrated at the tail end of the feeder line, and the load of a feeder line on the right side of the loop closing switch is concentrated at the head end of the feeder line;
closed loop steady-state circulation
Figure BDA0001685048930000075
Can be expressed as:
Figure BDA0001685048930000076
wherein the content of the first and second substances,
Figure BDA0001685048930000077
for interconnecting the voltage difference across the break of the switch, ZΣIs the total impedance of the loop closing network;
calculating the closed loop steady-state circulation effective value I by the formula (1)cThe range of (1).
Step I-3, calculating the maximum steady-state current effective value;
is provided with
Figure BDA0001685048930000078
If the effective value of the steady-state current is I, the steady-state current I' at the head end of the feed line before loop closing meets the following requirements:
Figure BDA0001685048930000079
therefore, the maximum steady-state current effective value which can appear at the head end of the loop-closing feedback line is I + Ic；
Step I-4, comparing the maximum steady-state current effective value with the maximum allowable current-carrying capacity of the feeder line, if the maximum steady-state current effective value is smaller than the maximum allowable current-carrying capacity of the feeder line, judging that loop closing is allowed, and finishing the decision process; if the maximum steady-state current effective value is larger than or equal to the maximum allowable current-carrying capacity of the feeder line, further judgment needs to be carried out in the step II, because the step only considers the most unfavorable condition of the safety loop closing and neglects the actual load distribution condition when calculating the maximum steady-state current effective value, the safety margin of the calculation result is larger, and at the moment, if the judgment of not allowing the loop closing is carried out, the decision is more conservative;
the above loop closing judgment generally does not consider the transient process of the loop closing operation for the following reasons:
the delay time setting value of the current II section protection at the head end of the feeder line is generally not less than 0.2s, and the attenuation time constant of the non-periodic component of the loop closing transient current is generally less than 0.2s, namely the loop closing transient process is basically attenuated within the current II section protection time limit, and the loop closing impact current only influences the current I section protection (instantaneous current quick-break protection). The safe loop closing conditions are as follows: after loop closing, the effective value of the steady-state current does not exceed the maximum allowable current-carrying capacity of the feeder line, and the transient process of loop closing does not cause current protection action. Setting the current I section protection setting value of the line as Iset,IMaximum allowable current carrying capacity of feeder line is ImaxAccording to the safe loop closing condition, when the formula (3) and the formula (4) are simultaneously met, the safe loop closing can be realized;
I+Ic＜Imax (3)
I+kIc＜Iset,I (4)
in practice, the current I section protection setting value generally meets Iset,I＞kImaxIf the condition of expression (3) is satisfied, there are:
I+kIc＜k(I+Ic)＜kImax＜Iset,I (5)
that is, the condition of the formula (4) can be satisfied;
therefore, in practice, when the steady-state current effective value after loop closing does not exceed the maximum allowable current-carrying capacity of the feeder, the loop closing transient process does not influence the safety of the loop closing operation.
Further, in the step II, the probability distribution of the load values of each load point on the feeder line is considered, and since the real-time load of each load point on the feeder line is generally difficult to obtain, the load value of each load point is regarded as a random variable, and the ratio of the active load and the reactive load of each load point to the active power and the reactive power of the head end of the line is selected as an input variable to perform the probabilistic load flow calculation.
The step II comprises the following steps:
II-1, solving each-order semi-invariant of each input variable in the season and time period of the loop closing time based on historical load data;
setting an input variable kPIs the ratio of the active load of a certain load point to the active power of the head end of the feeder line, the annual discrete measured data of the input variable is obtained by sorting from the load point and the annual 96-point daily load historical data of the outgoing line, and the annual discrete measured data is constructed and used for solving the input variable kPThe sample set S of each order of semi-invariant;
considering that the load may exhibit different characteristics in different seasons and different time periods each day, the sample set S is divided into 12(4 × 3 ═ 12) sub-sample sets (S) by seasons (spring, summer, fall, and winter) and time periods (00:00-08:00, 08:00-18:00, 18:00-24:00)1，S2，…，S12)；
By analyzing a certain subsample set, the input variable k is obtainedPSemi-invariants of each step in seasons and time periods of loop closing time;
if it is combinedThe subset of samples corresponding to the ring time has N discrete historical data, denoted as { k }P1,kP2,kP3,…,kPN}；
First, the origin moment alpha of each order is calculatedv：
Figure BDA0001685048930000091
Then, the relationship between the semi-invariant and the origin moment is used to calculate the semi-invariant gamma of each orderv：
Figure BDA0001685048930000092
Wherein the content of the first and second substances,
Figure BDA0001685048930000093
the number of different combinations of j elements from v elements;
II-2, calculating the closed loop current at a reference operation point according to topology information, equipment parameters and real-time operation data of the power grid before closed loop network to obtain a closed loop current expected value and a conversion matrix;
and K is an input variable of the probability load flow calculation and represents the ratio of the active power and the reactive power of each node to the active power and the reactive power of the head end of the line, W represents the injection power of each node in the system, W is AK, wherein A is a diagonal matrix formed by the active power and the reactive power of the head ends of two feeders, and X is theta1,V1,θ2,V2,…,θn,Vn]TIs a system state variable;
the system power flow equation is expressed as:
W＝f(X) (8)
let I1And I2The initial current effective values of the head ends of the two side feeder lines before the loop closing are respectively represented by formula (1) and formula (2), and the maximum steady state current effective value I possibly appearing at the head ends of the two side feeder lines after the loop closing is represented by formula (1) and formula (2)1tAnd I2tRespectively expressed as:
I1t＝I1+Ic＝g1(X) (9)
I2t＝I2+Ic＝g2(X) (10)
set variable Z ═ I1t,I2t]TFor the loop closing current variable, the loop closing current equation is expressed in the form of a matrix as follows:
Z＝g(X) (11)
the input variable K is a random variable, denoted K0+ Δ K, where K0The expected value of the random variable K is a system reference operating point;
similarly, the system state variable X is represented as X0+ Δ X, injected power W at each node is denoted as W0+ Δ W, closed-loop current variable Z being denoted as Z0+ΔZ；
And performing Taylor series expansion on the system power flow equation and the closed loop current equation and omitting high-order terms to obtain a linear relation between delta Z and delta K:
Figure BDA0001685048930000101
wherein the content of the first and second substances,
Figure BDA0001685048930000102
at a reference operating point K0Where (W at this time)0＝AK0) Performing deterministic load flow calculation according to the formula (8) to obtain a system state variable X0And Jacobian matrix J0(ii) a Then at X0Calculating the loop closing current according to the formula (11) to obtain a loop closing current variable Z0And a coefficient matrix G0(ii) a Finally, a conversion matrix T is obtained0；
II-3, solving each-order semi-invariant of the closed-loop current variable based on the probability load flow theory;
Figure BDA0001685048930000103
wherein, Δ Z(v)Is a loop closing current variableSemi-invariant of order v,. DELTA.K(v)A v-order semi-invariant representing an input variable,
Figure BDA0001685048930000104
represents T0A coefficient matrix formed by v powers of all the elements in the array;
II-4, calculating the cumulative probability distribution function and the out-of-limit probability of the maximum steady-state current effective values of the head ends of the feeder lines at the two sides after loop closing;
and (3) solving the cumulative probability distribution of the closed-loop current by adopting a Cornish-Fisher series according to each-order semi-invariant of the closed-loop current variable:
let the cumulative distribution function of a loop current variable Z be F (Z), and the standard normal distribution function be phi (Z), and the alpha quantiles of F (Z) and phi (Z) are respectively expressed as Z (alpha) and
Figure BDA0001685048930000105
i.e. z (α) ═ F-1(α)，
Figure BDA0001685048930000111
Z (. alpha.) and
Figure BDA0001685048930000112
the following relationship is satisfied:
Figure BDA0001685048930000113
wherein, gvNormalizing the semi-invariants for the v-order of the closed-loop current variable Z, i.e. if the v-order semi-invariants of the random variable Z is gammavWith a standard deviation of σ, then
Figure BDA0001685048930000114
The cumulative probability distribution function can be obtained from the semi-invariant of each order of the closed loop current according to the formula (14);
the loop closing current variable Z comprises the maximum steady state current effective value I of the head ends of the feeder lines at two sides after loop closing1tAnd I2tIs provided with I1tAnd I2tIs accumulated inDistribution functions are respectively F1(x)、F2(x) To close the maximum allowable current-carrying capacity I of the feeder lines on both sides of the loop pointmax,1And Imax,2As reference value, then I1tAnd I2tIs out of limit probability P1And P2Respectively as follows:
P1＝P(I1t≥Imax,1)＝1-F1(Imax,1) (15)
P2＝P(I2t≥Imax,2)＝1-F2(Imax,2) (16)
step II-5, according to the out-of-limit probability P1And P2If P is greater than P, the second loop closing judgment is performed1And P2If the loop closing rate is less than 5%, the loop closing is judged to be allowed, otherwise, the loop closing is judged not to be allowed, and the whole decision process is finished.
In summary, the present invention provides a method for determining a safety loop closing operation for a medium-voltage distribution network, the method includes two determinations: judging whether loop closing can be carried out or not according to the calculation result of the maximum loop closing steady-state current effective value for the first time, wherein the required time is short because deterministic load flow calculation is carried out only twice; and judging whether the loop can be closed according to the out-of-limit probability of the maximum steady-state current effective value of the head ends of the feeder lines at the two sides after the loop is closed for the second time, so that the judgment result is more accurate. Therefore, the judgment method not only has theoretical support, but also gives consideration to safety and timeliness in practical application. The method is applied to actual production, and is helpful for operators to make safe and rapid decisions on the loop closing operation of the medium-voltage distribution network.
While the present invention has been described in detail with reference to the preferred embodiments, it should be understood that the above description should not be taken as limiting the invention. Various modifications and alterations to this invention will become apparent to those skilled in the art upon reading the foregoing description. Accordingly, the scope of the invention should be determined from the following claims.

Claims (2)

1. A method for determining safe loop closing operation of a medium-voltage distribution network is characterized by comprising the following steps: calculating a closed loop steady-state current effective value of the head end of the feed line after closed loop, judging whether the upper limit of the closed loop steady-state current effective value is larger than the maximum allowable current-carrying capacity of the feed line, if not, allowing closed loop operation, if so, continuously judging whether the out-of-limit probability of the maximum steady-state current effective values of the head ends of the feed lines at two sides after closed loop is smaller than 5%, if so, allowing closed loop operation, and if not, not allowing closed loop operation;
the method for judging whether the upper limit of the loop closing steady-state current effective value is larger than the maximum allowable current-carrying capacity of the feeder line or not comprises the following steps:
1. performing state estimation on the high-voltage distribution network by adopting a weighted least square criterion to obtain the voltage amplitude and phase angle of 10kV buses at the head ends of the two closed-loop feeders;
2. calculating the range of the closed loop steady-state circulation effective value;
closed loop steady-state circulation
Figure FDA0003316405090000011
Expressed as:
Figure FDA0003316405090000012
wherein the content of the first and second substances,
Figure FDA0003316405090000013
in order to communicate the voltage difference across the break of the switch,
Figure FDA0003316405090000014
and
Figure FDA0003316405090000015
respectively, the voltage phasors, Z, on both sides of the break of the interconnection switchΣIs the total impedance of the loop closing network;
taking a 10kV bus node at the head end of the loop closing feeder line as a reference node, respectively regarding the total load of the feeder line as being concentrated at the head end and the tail end of the line, and performing twice deterministic load flow calculation and loop closing stable circulation calculation to obtain loop closing stable stateEffective value of state circulation IcA range of (d);
3. calculating the maximum steady-state current effective value;
is provided with
Figure FDA0003316405090000016
If the effective value of the steady-state current at the head end of the feed line before loop closing is I, the effective value of the steady-state current at the head end of the feed line after loop closing is I' and meets the following requirements:
Figure FDA0003316405090000017
the maximum steady-state current effective value of the head end of the feed line after loop closing is I + Ic；
4. The maximum steady-state current effective value I + IcMaximum allowable current carrying capacity I with feeder linemaxAnd comparing, if the maximum steady-state current effective value is less than the maximum allowable current-carrying capacity of the feeder line, judging that loop closing is allowed, and if the maximum steady-state current effective value is greater than or equal to the maximum allowable current-carrying capacity of the feeder line, continuously judging whether the out-of-limit probability of the maximum steady-state current effective values of the head ends of the feeder lines at two sides after loop closing is less than 5%.
2. The method for determining a safe loop closing operation for a medium voltage distribution network according to claim 1, wherein said method for determining whether the out-of-limit probability of the maximum steady-state current effective value at the head ends of the feeder lines at both sides after the loop closing is less than 5% comprises the steps of:
1. solving each-order semi-invariant of each input variable in the season and time period of the loop closing time based on historical load data;
selecting the ratio of the active load and the reactive load of each load point to the active power and the reactive power of the head end of the line as an input variable kPConstructed for finding the input variable kPThe sample set S of each order of semi-invariant is divided into a plurality of sub-sample sets according to seasons and time periods, and an input variable k is obtained through analysis of a certain sub-sample setPSemi-invariants of each step in seasons and time periods of loop closing time;
if the sub-sample set corresponding to the loop closing time has N discrete historical data, which is expressed as { kP1,kP2,kP3,…,kPN}；
Calculating the origin moment alpha of each orderv：
Figure FDA0003316405090000021
Calculating the semi-invariant gamma of each order according to the relation between the semi-invariant and the origin momentv：
Figure FDA0003316405090000022
Wherein the content of the first and second substances,
Figure FDA0003316405090000023
the number of different combinations of j elements from v elements;
2. calculating loop closing current at a reference operation point according to topology information, equipment parameters and real-time operation data of a power grid before loop closing to obtain a loop closing current expected value and a conversion matrix;
and K is an input variable of the probability load flow calculation and represents the ratio of the active power and the reactive power of each node to the active power and the reactive power of the head end of the line, W represents the injection power of each node in the system, W is AK, wherein A is a diagonal matrix formed by the active power and the reactive power of the head ends of two feeders, and X is theta1,V1,θ2,V2,…,θn,Vn]TIs a system state variable;
the system power flow equation is expressed as:
W＝f(X) (5)
let I1And I2The initial current effective values of the head ends of the two side feeder lines before the loop closing are respectively represented by formula (1) and formula (2), and the maximum steady state current effective value I possibly appearing at the head ends of the two side feeder lines after the loop closing is represented by formula (1) and formula (2)1tAnd I2tRespectively expressed as:
I1t＝I1+Ic＝g1(X) (6)
I2t＝I2+Ic＝g2(X) (7)
set variable Z ═ I1t,I2t]TFor the loop closing current variable, the loop closing current equation is expressed in the form of a matrix as follows:
Z＝g(X) (8)
the input variable K is a random variable, denoted K0+ Δ K, where K0The expected value of the random variable K is a system reference operating point;
similarly, the system state variable X is represented as X0+ Δ X, injected power W at each node is denoted as W0+ Δ W, closed-loop current variable Z being denoted as Z0+ΔZ；
And performing Taylor series expansion on the system power flow equation and the closed loop current equation and omitting high-order terms to obtain a linear relation between delta Z and delta K:
Figure FDA0003316405090000031
wherein the content of the first and second substances,
Figure FDA0003316405090000032
at a reference operating point K0W is0＝AK0Performing deterministic load flow calculation according to the formula (5) to obtain a system state variable X0And Jacobian matrix J0(ii) a Then at X0Calculating the loop closing current according to the formula (8) to obtain a loop closing current variable Z0And a coefficient matrix G0(ii) a Finally, a conversion matrix T is obtained0；
3. Solving each-order semi-invariant of closed-loop current variables based on a probability power flow theory;
Figure FDA0003316405090000033
wherein, Δ Z(v)Is a v-order semi-invariant of closed-loop current variables, Δ K(v)A v-order semi-invariant representing an input variable,
Figure FDA0003316405090000034
represents T0A coefficient matrix formed by v powers of all the elements in the array;
4. calculating the cumulative probability distribution function and the out-of-limit probability of the maximum steady-state current effective values of the head ends of the feeder lines at the two sides after loop closing;
and (3) solving the cumulative probability distribution of the closed-loop current by adopting a Cornish-Fisher series according to each-order semi-invariant of the closed-loop current variable:
let the cumulative distribution function of a loop current variable Z be F (Z), and the standard normal distribution function be phi (Z), and the alpha quantiles of F (Z) and phi (Z) are respectively expressed as Z (alpha) and
Figure FDA0003316405090000041
i.e. z (α) ═ F-1(α)，
Figure FDA0003316405090000042
Z (. alpha.) and
Figure FDA0003316405090000043
the following relationship is satisfied:
Figure FDA0003316405090000044
wherein, gvNormalizing the semi-invariants for the v-order of the closed-loop current variable Z, i.e. if the v-order semi-invariants of the random variable Z is gammavWith a standard deviation of σ, then
Figure FDA0003316405090000045
The cumulative probability distribution function can be obtained from the semi-invariant of each order of the closed loop current according to the formula (11);
the loop closing current variable Z comprises the maximum steady state current effective value I of the head ends of the feeder lines at two sides after loop closing1tAnd I2tIs provided with I1tAnd I2tRespectively, is F1(x)、F2(x) To close the maximum allowable current-carrying capacity I of the feeder lines on both sides of the loop pointmax,1And Imax,2As reference value, then I1tAnd I2tIs out of limit probability P1And P2Respectively as follows:
P1＝P(I1t≥Imax,1)＝1-F1(Imax,1) (12)
P2＝P(I2t≥Imax,2)＝1-F2(Imax,2) (13)
5. according to the out-of-limit probability P1And P2If P is greater than P, the second loop closing judgment is performed1And P2If the loop closing rate is less than 5%, the loop closing is judged to be allowed, otherwise, the loop closing is judged not to be allowed, and the whole decision process is finished.
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