CN108493985B - Identification method for out-of-limit weak link of voltage of power distribution network containing distributed power supply - Google Patents

Identification method for out-of-limit weak link of voltage of power distribution network containing distributed power supply Download PDF

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CN108493985B
CN108493985B CN201810170058.9A CN201810170058A CN108493985B CN 108493985 B CN108493985 B CN 108493985B CN 201810170058 A CN201810170058 A CN 201810170058A CN 108493985 B CN108493985 B CN 108493985B
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voltage
load
capacity
power
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CN108493985A (en
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郝丽丽
李乃双
宋朋勋
郝思鹏
刘海涛
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Nanjing Institute of Technology
Nanjing Tech University
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Nanjing Institute of Technology
Nanjing Tech University
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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
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • 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
    • H02J3/12—Circuit arrangements for ac mains or ac distribution networks for adjusting voltage in ac networks by changing a characteristic of the network load
    • 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
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H02J3/46—Controlling of the sharing of output between the generators, converters, or transformers
    • H02J3/48—Controlling the sharing of the in-phase component
    • 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
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H02J3/46—Controlling of the sharing of output between the generators, converters, or transformers
    • H02J3/50—Controlling the sharing of the out-of-phase component
    • 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
    • H02J2203/00—Indexing scheme relating to details of circuit arrangements for AC mains or AC distribution networks
    • H02J2203/20—Simulating, e g planning, reliability check, modelling or computer assisted design [CAD]

Abstract

The invention discloses a method for identifying a distribution network voltage out-of-limit weak link containing a distributed power supply, which comprises the steps of obtaining a reference voltage and a topological structure of a distribution network, a feeder line parameter and a head end voltage, a load access position and capacity, a reactive compensation device access position and capacity, and a DG access position and capacity, calculating the DG maximum access capacity of each node, identifying the voltage out-of-limit according to the difference between DG output active power of each node and the DG maximum access capacity of the node if DG is intensively grid-connected, converting into the active power of a DG at the tail end if DG is dispersedly grid-connected, and comparing with the DG maximum access capacity of the tail node to identify the voltage out-of-limit. The method can guide a power worker to quickly and accurately judge the out-of-limit reasons of the voltage, identify the weak links of the voltage and adopt a corresponding voltage regulation method, is favorable for realizing safe grid connection and voltage regulation of the distributed power supply, and has popularization prospect and practical significance.

Description

Identification method for out-of-limit weak link of voltage of power distribution network containing distributed power supply
Technical Field
The invention relates to a method for identifying a distribution network voltage out-of-limit weak link containing a distributed power supply, and belongs to the technical field of distribution network voltage protection.
Background
The voltage is an important index for measuring the normal operation of the power grid, and the voltage out-of-limit can cause part of user equipment to fail to operate normally, even endanger the system safety. As a terminal link of power distribution, a power distribution network is closely related to users, and the voltage of a distribution network feeder line is out of limit due to large-range fluctuation of user requirements. With the rapid development of global economy, energy shortage and environmental pollution problems push the energy systems around the world to be transformed to clean, intelligent and low-carbon. Distributed Generators (DG) taking wind power, photovoltaic and small hydropower as main forms are increasingly incorporated into distribution networks, and the distribution network voltage is also greatly influenced. For a power transmission system with a small impedance ratio, the output regulation of a reactive power supply can also effectively improve the voltage quality except for the voltage regulation by using an on-load tap changer, and voltage control equipment is concentrated. However, the distribution network is dense in users, various in equipment and complex in system parameters, and in many cases, the improvement of the reactive compensation of the capacitor bank on the voltage is not obvious. The output power of the distributed power supply in the distribution network can be reasonably planned and controlled, the low-voltage problem caused by long feeder lines and heavy load can be relieved, but the intermittence of the output power of the distributed power supply also restricts the capability of the distributed power supply in solving the voltage problem. The factors causing the voltage of the power distribution network to exceed the limit are numerous, and part of the factors show fluctuation, randomness and uncontrollable property. The access of an intermittent distributed power supply and the requirement of time-varying random load can amplify the randomness and the variation amplitude of the power flow of the distribution network, so that the distribution network is more prone to have light-load and heavy-load running conditions. The lower limit of the voltage of the feeder line is easy to be changed during heavy load, and the lower limit of the tail end of the feeder line is usually changed; the feeder line voltage is liable to exceed the upper limit in light load, and an extreme value that the voltage is liable to exceed the upper limit is liable to occur at the first section of the feeder line or the DG access point, so that overvoltage is also one of the main reasons for limiting the admittance capacity of the distributed power supply. At present, methods and tools for directly judging weak links and out-of-limit reasons of voltage out-of-limits easily occurring in a power distribution network are lacked, and effective distribution network planning and control references cannot be provided for avoiding the voltage out-of-limits.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provides a method for identifying a distribution network voltage out-of-limit weak link containing a distributed power supply.
In order to solve the technical problem, the invention provides a method for identifying a distribution network voltage out-of-limit weak link containing a distributed power supply, which comprises the following steps:
1) obtaining reference voltage and topological structure of power distribution network, feeder line parameters and head end voltage U0The load access position and capacity, the reactive compensation device access position and capacity and the DG access position and capacity, wherein the feeder line parameters comprise active power P transmitted on the feeder lines 1 and i1,iAnd reactive power Q1,iResistance R of the feed line 1, i1,iReactance X of the feeder 1, i1,i(ii) a Wherein, 1, i are the head and end nodes of the feeder line respectively, i is 1,2, …, n;
2) calculating the DG maximum admittance capacity of each node;
3) if the DGs are intensively grid-connected, the step 4) is carried out, and if the DGs are dispersedly grid-connected, the step 5) is carried out;
4) comparing the DG output active power of each node with the DG maximum admission capacity of the node, and when P is reachedDG,k＞PDG,kmaxWhen the grid voltage of the DG is higher, the grid voltage of the DG is higher; when P is presentDG,k＜PDG,kmaxWhen the DG output is insufficient, the lower limit of the terminal voltage is increased by connecting the front end of the feeder line under the condition that the DG output is insufficient, and the identification is finished, wherein PDG,kActive power, P, is output for DG of node kDG,kmaxMaximum admission capacity of DG for node k; for node k, k ═ 1,2, …, n on the feeder, define: the node of k belonging to (1, n/3) is the front end of the feeder line, the node of k belonging to (n/3,2n/3) is the middle end of the feeder line, and the node of k belonging to (2n/3, n) is the tail end of the feeder line;
5) the active power of all DGs which are dispersedly accessed is converted into the active power P 'of the DGs at the end of the feeder line'DG,nall；
6) Prepared from P'DG,nallDG maximum admission capacity P with the end node nDG,nmaxComparison, when P'DG,nall≤PDG,nmaxIn the process, the total DG output power of the feeder line does not cause the node voltage to exceed the upper limit, but if all the node net loads are positive, the tail end voltage is easy to exceed the lower limit, if all the node net loads are negative or zero, the voltage of each node of the line is normal, otherwise, the node voltage is easy to exceed the lower limit; when P'DG,nall＞PDG,nmaxIn this case, the DG grid connection node voltage tends to be higher.
In the step 2), the method for calculating the maximum admission capacity of each node DG during grid connection includes:
firstly, calculating the active power output by DGs on each node:
Figure BDA0001585540370000021
wherein, PDG,kActive power, Q, output for DG on node kDG,kFor reactive power, P, output by DG on node kL,jActive power, Q, for the access load on node jL,jReactive power, λ, for a load connected to node jGIs the power factor of DG, satisfies
Figure BDA0001585540370000022
γGThe ratio of the voltage drop longitudinal component caused by the DG-induced network loss to the voltage drop longitudinal component caused by the load, γ L the ratio of the voltage drop longitudinal component caused by the load-induced network loss to the voltage drop longitudinal component caused by the load, UNAt rated voltage, PL,iAnd QL,iIs, Δ U0,kThe epsilon is (-0.07,0.07), and n is the number of nodes on the feeder line;
then, take Δ U0,kSubstituting equation (14) for-0.07, the maximum allowable DG capacity P for node k is obtainedDG,kmax。
In the aforementioned step 4), when P is presentDG,k＞PDG,kmaxWhen the DG is connected to the front end of the feeder line, if the load at the tail end is too heavy or the line is too long, the lower limit of the voltage at the tail end of the line is easy to be caused; when DG is connected to the rear end of the feeder, if the front middle part of the line is overloaded, the guide is turned onSo that the terminal voltage is higher and the middle voltage is lower.
In the aforementioned step 4), when P is presentDG,k＜PDG,kmaxWhen the DG is connected to the middle end of the feeder line, if the load of the tail end is too heavy, the voltage of the tail end of the line is easy to lower, and when the DG is connected to the tail end of the feeder line, the voltage of a node before a DG access point is easy to lower.
The converted active power P 'in the step 5)'DG,nallThe calculation method comprises the following steps:
Figure BDA0001585540370000031
wherein eta isiIs a conversion coefficient, ηi＝PDG,nmax/PDG,imax，PDG,iThe active power output by DG at node i, i ═ 1,2, …, n.
If the voltages of all the nodes can not be qualified in the output allowable range of the reactive power compensation device and the DG, the voltage out-of-limit reason is that the newly increased load is too heavy, the load which is allowed to be additionally increased by each node is calculated, and then the load node which is easy to have the voltage out-of-limit is identified by using the difference between the newly increased load capacity of each node and the maximum allowable load capacity of each node;
the allowed additional load capacity of each node is:
Figure BDA0001585540370000032
wherein, PL,kNew load capacity, λ, for node kL,kFor newly increasing the load power factor, the following requirements are met:
Figure BDA0001585540370000033
QL,knewly adding load reactive power for the node k;
the maximum admissible load capacity for node k is: taking Delta U0,kIs 0.07- (U)k-min(Ui) Carry it into formula (17) to calculate the maximum admissible load capacity P of node kL,kmaxWherein, UiThe voltage value of the node i is represented by k which is not less than i and not more than n.
The invention achieves the following beneficial effects:
aiming at the characteristic that the voltage of the power distribution network has a plurality of influence factors, the voltage out-of-limit identification method is provided according to the acquired reference voltage and topology structure of the power distribution network, the feeder line parameter and head end voltage, the load access position and capacity, the reactive compensation device access position and capacity and the distributed power supply access position and capacity, so that an electric power worker can be guided to quickly and accurately judge the voltage out-of-limit reason, identify the voltage weak link and adopt a corresponding voltage regulation method, the safe grid connection and voltage regulation of the distributed power supply can be realized, and the method has popularization prospect and practical significance.
Drawings
FIG. 1 is a circuit diagram of voltage drop mechanism analysis according to the present invention;
FIG. 2 is a multi-node voltage drop circuit of the present invention;
FIG. 3 is a flow chart of the method of the present invention.
Detailed Description
The invention is further described below. The following examples are only for illustrating the technical solutions of the present invention more clearly, and the protection scope of the present invention is not limited thereby.
The invention discloses a method for identifying a distribution network voltage out-of-limit weak link containing a distributed power supply, and the specific flow is shown in figure 3, and the method comprises the following steps:
1) the method comprises the steps of obtaining reference voltage and a topological structure of the power distribution network, feeder line parameters and head end voltage, load access position and capacity, reactive power compensation device access position and capacity and Distributed Generation (DG) access position and capacity.
2) And analyzing a mechanism of voltage drop of the distribution network and reasons influencing voltage out-of-limit according to the topological structure of the distribution network.
Referring to fig. 1, the end voltage on the feeder line caused by the load alone is expressed as follows:
Figure BDA0001585540370000041
wherein, U0For the voltage at the head end of the feed line, U1For the low-side voltage of the transformer, UiIs the voltage value of node i; k is the transformation ratio of the on-load tap changer; p1,i,Q1,iThe active power and the reactive power transmitted on a feeder line 1, i (1, i are respectively the head node and the tail node of the feeder line, i is 1,2, …, n); r1,iIs the resistance, X, of the feed line 1, i1,iIs the reactance of the feeder 1, i.
As can be seen from equation (1), the voltage at the head end of the feeder, the impedance and the transmission power all cause voltage out-of-limit, and the specific voltage regulation method that may be adopted is as follows:
2a) adopting a generator to regulate the voltage: for a small power network directly supplying power without boosting, when the power supply radius is not long and the network loss is not large, the voltage quality can be improved by changing the voltage at the end of the generator; for a power supply system controlled by multistage voltage, the improvement effect of the voltage at the tail end of a feeder line is not great by adopting the voltage regulation of a generator, reactive redistribution on the feeder line is easily caused under the condition of insufficient system reactive power, and contradiction is generated with the economic distribution of reactive power.
2b) Controlling the number of the switched capacitor banks to regulate voltage: when the load needs heavy reactive power and the feeder line has reactive power shortage, the reactive power can be compensated by switching a capacitor bank, such as Q shown in figure 1C,iThe reactive compensation mode comprises centralized compensation in a transformer substation, line decentralized reactive compensation and on-site reactive compensation of electric equipment.
The expression of the reactive compensation improvement tail end voltage is as follows:
Figure BDA0001585540370000042
in the formula, QC,iAnd switching the reactive power of the capacitor bank for the node i.
2c) Reducing the impedance of the feed line may reduce power loss on the transmission path and thereby increase the feed line termination voltage. The distribution network feeder is improved, the power supply radius is shortened, and the section area of the lead is increased.
2d) The reactance on the transmission network feeder is far greater than the resistance, and the sensitivity of the reactive compensation device to improving the voltage at the end of the feeder is greater, and when the resistance of the distribution network feeder is greater than the reactance, the sensitivity of the active power transmitted on the feeder to voltage drop is greater, see fig. 1, and the DG compensation is expressed as:
Figure BDA0001585540370000043
in the formula, PDG,iActive power, Q, output for DG on node iDG,iIs the reactive power output by DG on node i.
3) Referring to fig. 2, the feeder flow is calculated by a push-back substitution method considering the influence of the network loss on the voltage droop longitudinal component:
Figure BDA0001585540370000051
Figure BDA0001585540370000052
Figure BDA0001585540370000053
Figure BDA0001585540370000054
the following formulae (4) to (7) can be used:
Figure BDA0001585540370000055
in formulae (4) to (7), Δ Pk-1,kIs the active loss, Delta Q, of the feeder line k-1, kk-1,kThe reactive loss of a feeder line k-1, k; p'kActive power, Q ″, flowing into for node k "kReactive power flowing in for node k; p'kActive Power, Q 'flowing out for node k'kReactive power flowing out for node k; pL,iActive power, Q, for the access load on node iL,iThe reactive power of a load is accessed to the node i; rk-1,kIs the resistance of the feed line k-1, k, Xk-1,kIs the reactance of the feeder line k-1, k; u shapekIs the present voltage value, Δ U, of node k0,kFor the voltage drop vertical component of feed line 0, k, feed line node k is 1,2, …, n. In the context of figure 2, it is shown,
Figure BDA0001585540370000056
the apparent power flowing in for node k.
Figure BDA0001585540370000057
The apparent power flowing out for node k.
Considering that the voltage drop of a normally operating power distribution system is not large generally, the voltage of each node of a feeder line is rated by a rated voltage UNIt is shown that the longitudinal component of the voltage drop caused by the network loss on the feed line 0, k is recorded as
Figure BDA0001585540370000058
The longitudinal component of the voltage drop caused by the load is recorded as Δ UL,0,kThen the voltage droop component of the feed line 0, k can be expressed as:
Figure BDA0001585540370000059
wherein the content of the first and second substances,
Figure BDA0001585540370000061
considering that in the formula (10), Δ Pm,m+1,ΔQm,m+1Relative to Δ PL,m,ΔQL,mSmall, negligible in the calculation, equation (10) can be further simplified as:
Figure BDA0001585540370000062
the ratio of the voltage drop longitudinal component caused by the network loss to the voltage drop longitudinal component caused by the load is defined as a voltage drop ratio coefficient:
Figure BDA0001585540370000063
according to the data analysis of the practical system, the gamma is about 0.02-0.06 under the normal condition.
4) According to the voltage allowable limit value of each node, when the load of each node is fixed, under the condition of considering the network loss, the voltage drop longitudinal component of a feeder line 0 containing a DG access distribution network, k is as follows:
Figure BDA0001585540370000064
wherein, PDG,jFor DG at node j (j ═ 1,2, …, n), the active power, γ, is outputLThe ratio of the voltage drop longitudinal component caused by the network loss caused by the load to the voltage drop longitudinal component caused by the load; γ G is the ratio of the voltage droop due to the grid loss caused by DG alone to the voltage droop due to the load.
4a) When only one node k is connected to the DG, the DG at the node k outputs an active power expression according to equation (13):
Figure BDA0001585540370000065
in the formula, PDG,kActive power, Q, output for DG on node kDG,kThe reactive power output by DG on the node k; lambda [ alpha ]GIs the power factor of DG, then
Figure BDA0001585540370000066
According to the operating specification, usually Δ U0,k∈(-0.07,0.07)。
4b) When there are multiple nodes accessing DG at the same time, referring to FIG. 2, n nodes are all connectedEntering DG, defining DG power conversion coefficient etakIt can be calculated from the following formula:
ηk＝PDG,nmax/PDG,kmax (15)
wherein, PDG,nmaxAnd PDG,kmaxThe maximum admissible capacity allowed by DG at the feeder end node n and any node k within the admissible range of feeder voltage drop is respectively. The smaller the value of η k, the smaller the sensitivity of DG output power at node k to voltage drop, and the larger the DG capacity that these nodes can access without causing the upper limit of voltage constraint.
When Δ U0,kWhen-0.07, the DG maximum allowable capacity P of the node n is calculated according to equation (14)DG,nmax。
DG active power injected into any node on a feeder line passes through etakThe active power of the DG converted and aggregated to the feeder end node n is as follows:
Figure BDA0001585540370000071
and comparing the total output capacity of the actual DG obtained by the conversion of the formula (16) to judge whether the whole feeder line has the possibility of exceeding the upper limit, if P'DG,nall≤PDG,nmaxIn time, the total DG output power of the feeder line cannot cause the node voltage to exceed the upper limit; p'DG,nall＞PDG,nmaxThe DG grid-connected point voltage tends to be higher.
5) If all node voltages can not be qualified within the allowable range of the reactive power compensation device and the DG output, the reason of voltage out-of-limit may be that the load is accessed remotely or the feeder is operated heavily, so that the load is a PQ node model, and under the condition of considering network loss and under the condition that the feeder has the load, the load capacity allowed to be additionally increased by each load node is as follows:
Figure BDA0001585540370000072
in the formula, PL,kAdding load active power, lambda, to node kL,kIn order to increase the load power factor, i.e.,
Figure BDA0001585540370000073
in order to ensure that the voltage of the newly added load node and the voltage of the node behind the newly added load node are not out of limit, the delta U is adopted0,kHas a maximum value of 0.07- (U)k-min(Ui) I is more than or equal to k and less than or equal to n, the lowest node voltage just reaches the critical lowest value at the moment, and the newly added load capacity obtained by calculation is the maximum admissible load capacity which ensures that the voltage is not out of limit and is marked as PL,kmaxAnd further, the difference between the actual load capacity and the maximum allowable load capacity is used to identify the load node which is easy to generate voltage out-of-limit.
6) In specific implementation, referring to fig. 3, the maximum admission capacity of DG grid connection can be calculated in step 4), and power workers are instructed to judge whether the node voltage exceeds the upper limit according to whether the actual access capacity of DG meets the maximum admission capacity of DG, and when DG is grid-connected in a centralized manner, when P is the maximum admission capacity of DGDG,k＞PDG,kmaxWhen the voltage at the DG grid-connected part is higher than the upper limit, the following situations also exist according to the position of the DG connected to the feeder: when the DG is connected to the front end of the feeder line, if the load of the tail end is heavy or the line is too long, the voltage of the tail end of the line is easy to exceed the lower limit; when P is presentDG,k＜PDG,kmaxWhen the DG output is insufficient, the lower limit of the terminal voltage is caused by accessing the front end of the feeder line, and according to the position of the DG accessed to the feeder line, the specific out-of-limit condition is as follows: when a DG is connected to the front end of a feeder line, if a line is long and the load of the tail end is heavy, the lower limit of the voltage of the tail end of the line is easy to cause, when the DG is connected to the middle end of the feeder line, the lower limit of the voltage of the tail end of the line is easy to cause, and when the DG is connected to the tail end of the feeder line, the lower limit of the voltage of a node in front of the DG. For node i, i ═ 1,2, …, n on the feeder, as defined in the present invention: i belongs to (1, n/3) as the front end of the feeder line, i belongs to (n/3,2n/3) as the middle end of the feeder line, i belongs to (2n/3, n) as the feeder lineThe wire ends.
When DGs are dispersedly accessed to the power grid, the active output of all DGs dispersedly accessed is converted into the active power P 'of the DGs at the end of the feeder line by the formula (16)'DG,nallThen DG maximum admissible capacity P with the end node nDG,nmaxComparing and judging the voltage out-of-limit condition so as to identify weak DG grid-connected nodes, specifically: when P'DG,nall≤PDG,nmaxIn time, the total DG output power of the feeder line does not cause the node voltage to exceed the upper limit, but if all node net loads are positive, the terminal voltage may exceed the lower limit, if all node net loads are negative or zero, the voltage of each node of the line is normal, otherwise, the node voltage may exceed the lower limit. When P'DG,nall＞PDG,nmaxIn time, the DG grid connection node voltage tends to be higher.
And finally, in the output allowable range of the reactive power compensation device and the DG, calculating the maximum load capacity allowed to be increased by each node according to the step 5), comparing the uniform distribution of the load and the difference of terminal voltage drops when the load is concentrated on the front end, the middle end and the rear end, and guiding a power worker to identify the weak load node which is easy to generate the lower limit of voltage according to the actual load of the newly added node.
The above description is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, several modifications and variations can be made without departing from the technical principle of the present invention, and these modifications and variations should also be regarded as the protection scope of the present invention.

Claims (6)

1. The identification method of the out-of-limit weak link of the voltage of the power distribution network containing the distributed power supply is characterized by comprising the following steps of:
1) obtaining reference voltage and topological structure of power distribution network, feeder line parameters and head end voltage U0The load access position and capacity, the reactive compensation device access position and capacity and the DG access position and capacity, wherein the feeder line parameters comprise active power P transmitted on the feeder lines 1 and i1,iAnd reactive power Q1,iResistance R of the feed line 1, i1,iReactance X of the feeder 1, i1,i(ii) a Wherein 1, i are respectively a feeder lineI ═ 1,2, …, n;
2) calculating the DG maximum admittance capacity of each node;
3) if the DGs are intensively grid-connected, the step 4) is carried out, and if the DGs are dispersedly grid-connected, the step 5) is carried out;
4) comparing the DG output active power of each node with the DG maximum admission capacity of the node, and when P is reachedDG,k＞PDG,kmaxWhen the grid voltage of the DG is higher, the grid voltage of the DG is higher; when P is presentDG,k＜PDG,kmaxWhen the DG output is insufficient, the lower limit of the terminal voltage is increased by connecting the front end of the feeder line under the condition that the DG output is insufficient, and the identification is finished, wherein PDG,kActive power, P, is output for DG of node kDG,kmaxMaximum admission capacity of DG for node k; for node k, k ═ 1,2, …, n on the feeder, define: the node of k belonging to (1, n/3) is the front end of the feeder line, the node of k belonging to (n/3,2n/3) is the middle end of the feeder line, and the node of k belonging to (2n/3, n) is the tail end of the feeder line;
5) the active power of all DGs which are dispersedly accessed is converted into the active power P 'of the DGs at the end of the feeder line'DG,nall；
6) Prepared from P'DG,nallDG maximum admission capacity P with the end node nDG,nmaxComparison, when P'DG,nall≤PDG,nmaxIn the process, the total DG output power of the feeder line does not cause the node voltage to exceed the upper limit, but if all the node net loads are positive, the tail end voltage is easy to exceed the lower limit, if all the node net loads are negative or zero, the voltage of each node of the line is normal, otherwise, the node voltage is easy to exceed the lower limit; when P'DG,nall＞PDG,nmaxIn this case, the DG grid connection node voltage tends to be higher.
2. The method for identifying the out-of-limit weak link of the voltage of the power distribution network with the distributed power supply according to claim 1, wherein in the step 2), the method for calculating the maximum access capacity of the DG grid-connection of each node comprises the following steps:
firstly, calculating the active power output by DGs on each node:
Figure FDA0002726075030000011
wherein, PDG,kActive power, Q, output for DG on node kDG,kFor reactive power, P, output by DG on node kL,jActive power, Q, for the access load on node jL,jReactive power, λ, for a load connected to node jGIs the power factor of DG, satisfies
Figure FDA0002726075030000012
γGGamma is the ratio of the longitudinal component of the voltage drop due to the grid loss caused by DG alone to the longitudinal component of the voltage drop caused by the loadLThe ratio of the longitudinal component of the voltage drop caused by the network loss caused by the load alone to the longitudinal component of the voltage drop caused by the load, UNFor rated voltage, Δ U0,kThe epsilon is (-0.07,0.07), and n is the number of nodes on the feeder line;
then, take Δ U0,kSubstituting equation (14) for-0.07, the maximum allowable DG capacity P for node k is obtainedDG,kmax。
3. The method for identifying the out-of-limit weak link of the voltage of the power distribution network with the distributed power supply according to claim 2, wherein in the step 4), when P is reachedDG,k＞PDG,kmaxWhen the DG is connected to the front end of the feeder line, if the load at the tail end is too heavy or the line is too long, the lower limit of the voltage at the tail end of the line is easy to be caused; when the DG is connected to the rear end of the feeder, if the front middle part of the line is overloaded, the end voltage is caused to exceed the upper limit, and the middle end voltage is caused to exceed the lower limit.
4. The method for identifying the out-of-limit weak link of the voltage of the power distribution network with the distributed power supply according to claim 2, wherein in the step 4), when P is reachedDG,k＜PDG,kmaxWhen DG is connected to the front end of a feeder line, if the line is too long and the load at the tail end is too heavy, the lower limit of the voltage at the tail end of the line is easily caused, when DG is connected to the middle end of the feeder line, if the load at the tail end is too heavy, the lower limit of the voltage at the tail end of the line is easily caused, and when DG is connected to the tail end of the feeder line, the DG access pointThe lower the previous node voltage.
5. The method for identifying the out-of-limit weak link of the voltage of the power distribution network with the distributed power supplies according to claim 2, wherein in the step 5), the converted active power P'DG,nallThe calculation method comprises the following steps:
Figure FDA0002726075030000021
wherein eta isiIs a conversion coefficient, ηi＝PDG,nmax/PDG,imax，PDG,iThe active power output by DG at node i, i ═ 1,2, …, n.
6. The method for identifying the voltage out-of-limit weak link of the power distribution network with the distributed power supply as claimed in claim 2, wherein in the output allowable range of the reactive power compensation device and the DG, if the voltages of all nodes cannot be qualified, the voltage out-of-limit reason is that newly increased loads are too heavy, the calculation of each node allows additional load increase, and then the difference between the newly increased load capacity of each node and the maximum allowable load capacity of each node is used for identifying the load node which is easy to have voltage out-of-limit;
the allowed additional load capacity of each node is:
Figure FDA0002726075030000022
wherein, PL,kNew load capacity, λ, for node kL,kFor newly increasing the load power factor, the following requirements are met:
Figure FDA0002726075030000023
QL,knewly adding load reactive power for the node k;
the maximum admissible load capacity for node k is: taking Delta U0,kIs 0.07- (U)k-min(Ui) Carry it over to the calculation of formula (17)Obtaining the maximum admissible load capacity P of the node kL,kmaxWherein, UiThe voltage value of the node i is represented by k which is not less than i and not more than n.
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