CN114243711B - Large-scale power grid voltage sag evaluation method based on change branch influence domain - Google Patents
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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
- H02J3/04—Circuit arrangements for ac mains or ac distribution networks for connecting networks of the same frequency but supplied from different sources
- H02J3/06—Controlling transfer of power between connected networks; Controlling sharing of load between connected networks
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
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- H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
- H02H7/26—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
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- 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
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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
- H02J3/12—Circuit arrangements for ac mains or ac distribution networks for adjusting voltage in ac networks by changing a characteristic of the network load
- H02J3/16—Circuit arrangements for ac mains or ac distribution networks for adjusting voltage in ac networks by changing a characteristic of the network load by adjustment of reactive power
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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
- H02J2203/00—Indexing scheme relating to details of circuit arrangements for AC mains or AC distribution networks
- H02J2203/10—Power transmission or distribution systems management focussing at grid-level, e.g. load flow analysis, node profile computation, meshed network optimisation, active network management or spinning reserve management
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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
- 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]
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- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
- Y02E40/30—Reactive power compensation
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Abstract
The invention relates to a large-scale power grid voltage sag evaluation method based on a variable branch influence domain, which comprises the following steps of: the method comprises the following steps of S1, determining a regional power grid range W to be evaluated as a whole grid, S2, carrying out whole-grid load flow calculation and generating a load flow calculation result file, S3, setting a line fault position, a fault type and fault duration in the regional power grid range W to be evaluated, S4, calculating a sag expected value and outputting the risk severity level of each bus; and S5, judging whether the power grid topological structure is changed, and when the power grid topological structure is changed, performing load flow calculation on the changed whole power grid again S6, namely keeping the severity level of each bus voltage sag risk outside the power grid range W of the area to be evaluated unchanged, and sorting the severity levels of each bus voltage sag risk inside and outside the power grid range of the area to be evaluated to obtain the severity level of each bus voltage sag risk in the whole power grid range.
Description
Technical Field
The invention relates to the field of voltage sag assessment, in particular to a large-scale power grid voltage sag assessment method based on a variable branch influence domain.
Background
In current power systems, the problem of voltage sags in terms of power quality is increasingly highlighted. Since voltage sag causes a great amount of economic loss for high-tech enterprises and industrial users, the requirement of modern loads on the quality of electric energy is more and more strict. Under the background, the method has important significance for the evaluation and treatment research of the voltage sag. In practice, the voltage sag monitoring device cannot be installed in a large number in the whole network, monitored data cannot cover the whole network, and the severity of the voltage sag risk of the whole network is difficult to be comprehensively evaluated.
Disclosure of Invention
In view of this, the present invention provides a large-scale grid voltage sag evaluation method based on a changing branch influence domain, so as to implement efficient evaluation of large-scale grid voltage sag.
In order to achieve the purpose, the invention adopts the following technical scheme:
s1, determining a regional power grid range W to be evaluated as a whole grid;
s2, carrying out load flow calculation of the whole network and generating a load flow calculation result file;
s3, setting a line fault position, a fault type and fault duration within the range W of the regional power grid to be evaluated;
s4, calculating voltage sag amplitudes of each bus under different conditions, calculating sag expected values by using the obtained voltage sag amplitudes, and outputting the risk severity grade of each bus;
step S5, judging whether the power grid topological structure is changed, when the power grid topological structure is changed, carrying out load flow calculation on the changed whole power grid again to generate a new load flow result file, carrying out steps S3 and S4 in a newly determined power grid range of the area to be evaluated, calculating and analyzing to obtain sag expected values of all buses in the area, and outputting voltage sag risk severity levels;
and S6, keeping the severity grade of each bus voltage sag risk outside the newly determined regional power grid range to be evaluated unchanged, and sorting the severity grade of each bus voltage sag risk inside the newly determined regional power grid range to be evaluated to obtain the severity grade of each bus voltage sag risk inside the whole power grid range.
Further, the step S1 specifically includes:
defining the regional power grid range W to be evaluated as the whole grid, i.e.
W＝[a 1 ,a 2 ···a i ···a n ] (1)
Wherein W is the area to be evaluatedGrid extent, a i Is the name of the node; n is the number of nodes in the whole network.
Further, the step S3 specifically includes: respectively setting single-phase short-circuit faults, two-phase grounding short-circuit faults, interphase short-circuit faults and three-phase short-circuit faults at 0%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% and 100% of each line in the power grid range W of the area to be evaluated; the fault duration is set according to different voltage grades of the line, and the line with the voltage grade of 220kV is set to be 120ms; setting a line with a voltage level of 110kV to be 300ms; setting the line with the voltage level of 35kV as 700ms; the line with a voltage level of 10kV was set to 0.5s.
Further, the calculating of the sag expectation value and the outputting of the risk severity level of each bus are as follows:
1) Calculating the sag expected value of each bus, wherein the formula is as follows:
in the formula: e (i ') is the sag expected value of the i' bus; n' is the total simulation times; u shape i′j′ Is as follows j During secondary simulation, the minimum residual voltage amplitude of the i' bus;
2) Arranging the buses from small to large according to the sag expected values of the buses;
3) The first 20% of the buses are taken as a severe grade, 20% -40% are taken as a medium grade, 40% -60% are taken as a slight grade, and 60% -100% are taken as a good grade.
Furthermore, the change of the power grid topological structure considers the connection or disconnection of a line, a main transformer and a bus, and the electrical distance between the changed node and other nodes of the power grid needs to be calculated; the change of the power grid topological structure is divided into two conditions of disconnection of an old branch and connection of a new branch: when an old branch is disconnected, selecting a node of the old branch still connected with the large power grid as a change node; and when a new branch is accessed, selecting a new access large power grid node as a change node.
Further, the electrical distance represents a coupling relationship between two nodes, and the sensitivity is selected to represent the electrical distance as follows:
the polar coordinate form of the Newton-Raphson method power flow equation of the N-node system is as follows:
wherein, P i 、Q i Respectively injecting active power and reactive power into the node i; v i Is the voltage amplitude of node i; theta.theta. ij Is the phase angle difference between the voltage vectors of the node i and the node j; g ij 、B ij Are respectively node admittance matrix elements Y ij The real part and the imaginary part of (c); j belongs to i and represents that the node j is directly connected with the node i;
the above formula is converted to obtain:
wherein, Δ P i 、ΔQ i Injecting the unbalance amount of active power and reactive power for the node i respectively; p Gi 、Q Gi Injecting active power and reactive power for a power supply at a node i; p Li 、Q Li Active power and reactive power consumed by the load at the node i;
based on the Taylor principle, the above formula is linearized to obtain a conventional trend equation as follows:
wherein, the delta P and the delta Q are unbalance matrixes of node injection active power and node injection reactive power respectively; H. n, K and L are four plates of the Jacobian matrix respectively; Δ θ, Δ V are the increments of the node voltage phase angle and voltage amplitude, respectively;
considering the relation between active power, reactive power, voltage and phase angle, obtaining the relation:
wherein the content of the first and second substances,
sensitivity to reactive injection for voltage amplitude.
Further, the newly determined power grid range of the area to be evaluated is obtained based on a threshold value α, and specifically includes:
from a in the original to-be-evaluated regional power grid range W 1 ,a 2 ···a n And screening the node to obtain the changed branch influence domain, namely the new regional power grid range to be evaluated:
W′＝[a' 1 ,a' 2 ···a' i ···a' m ] (7)
wherein, a' i The nodes are selected according to the threshold value alpha; m is the number of nodes contained in the change leg influence domain.
Compared with the prior art, the invention has the following beneficial effects:
1. in the invention, for a large-scale power grid with rapid change, on the premise of ensuring the accuracy of voltage sag evaluation, a reasonable change node influence domain is selected, and the power grid range W of a region to be evaluated is reduced;
2. the method greatly improves the high efficiency of voltage sag evaluation and strengthens the utilization of the existing historical data.
Drawings
FIG. 1 is a flow chart of the method of the present invention;
FIG. 2 is a system diagram of an IEEE30 node in one embodiment of the present invention;
fig. 3 is a diagram of a system accessed by a node 31 in an embodiment of the present invention;
FIG. 4 is an impact domain of branches 7-31 in an embodiment of the present invention;
FIG. 5 is a diagram of an 18-19 disconnect system in accordance with an embodiment of the present invention;
FIG. 6 is a graph of the impact domain of the change branches 18-19 in one embodiment of the present invention.
Detailed Description
The invention is further explained below with reference to the drawings and the embodiments.
Referring to fig. 1, the present invention provides a large power grid voltage sag evaluation method based on a changing branch influence domain, including the following steps:
s1, determining the regional power grid range W to be evaluated as a whole grid, namely:
W＝[a 1 ,a 2 ···a i ···a n ] (1)
wherein W is the regional power grid range to be evaluated of the voltage sag, a i Is the name of the node; n is the number of nodes in the whole network.
S2, carrying out load flow calculation of the whole network and generating a load flow calculation result file;
s3, setting a line fault position, a fault type and fault duration within the range W of the regional power grid to be evaluated;
in the present embodiment, preferably, a single-phase short-circuit fault, a two-phase ground short-circuit fault, an inter-phase short-circuit fault, and a three-phase short-circuit fault are respectively set at 0%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, and 100% of each line of the regional power grid range W to be evaluated. The fault duration time needs to be set according to different voltage grades of the line, and the line with the voltage grade of 220kV is set to be 120ms; setting a line with a voltage level of 110kV to be 300ms; setting the line with the voltage level of 35kV as 700ms; the line with a voltage level of 10kV was set to 0.5s.
S4, calculating voltage sag amplitudes of each bus under different conditions, calculating sag expected values by using the obtained voltage sag amplitudes, and outputting the risk severity grade of each bus;
preferably, the risk severity level is calculated as follows:
1) Calculating the sag expected value of each bus, wherein the formula is as follows:
in the formula: e (i ') is the sag expected value of the i' bus; n' is the total simulation times; u shape i′j′ The minimum residual voltage amplitude of the i 'bus is obtained in the j' th simulation;
2) Arranging the buses from small to large according to the sag expected values of the buses;
3) The first 20% of the buses are taken as a severe grade, 20% -40% are taken as a medium grade, 40% -60% are taken as a slight grade, and 60% -100% are taken as a good grade.
Step S5, judging whether the power grid topological structure is changed, when the power grid topological structure is changed, carrying out load flow calculation on the changed whole power grid again to generate a new load flow result file, carrying out steps S3 and S4 in a newly determined power grid range of the area to be evaluated, calculating and analyzing to obtain sag expected values of all buses in the area, and outputting voltage sag risk severity levels;
the change of the power grid topological structure is divided into two conditions of disconnection of an old branch and connection of a new branch: when an old branch is disconnected, selecting a node of the old branch still connected with a large power grid as a change node (1 or 2 nodes can be selected); and when a new branch is accessed, selecting a new accessed large power grid node as a change node.
The electrical distance represents the coupling relationship between two nodes, but the strength of the two-point coupling is not clear. Therefore, it is necessary to express the coupling relationship between two variables respectively belonging to two nodes, and the sensitivity is a value obtained on the basis of linearizing the coupling relationship between the two variables, so that the sensitivity is selected to express the electrical distance.
The polar coordinate form of the Newton-Raphson method power flow equation of the N-node system is as follows:
wherein, P i 、Q i Respectively injecting active power and reactive power into the node i; v i Is the voltage amplitude of node i; theta.theta. ij Is the phase angle difference between the voltage vectors at node i and node j; g ij 、B ij Are respectively node admittance matrix elements Y ij The real part and the imaginary part of (c); j e i indicates that node j and node i are directly connected.
The above formula is converted to obtain:
wherein, Δ P i 、ΔQ i Injecting unbalance amounts of active power and reactive power into the node i respectively; p Gi 、Q Gi Injecting active power and reactive power for a power supply at a node i; p is Li 、Q Li Active and reactive power consumed by the load at node i.
Based on the Taylor principle, the above formula is linearized to obtain a conventional power flow equation as follows:
wherein, the delta P and the delta Q are unbalance matrixes of node injection active power and node injection reactive power respectively; H. n, K and L are four plates of the Jacobian matrix respectively; Δ θ, Δ V are increments of the node voltage phase angle and voltage magnitude, respectively.
Considering the relationship between active and reactive power and the voltage and phase angle, the relationship can be obtained as follows:
wherein the content of the first and second substances,
sensitivity to reactive injection for voltage amplitude.
(7) In practical engineering application, the influence domain of the variable branch is screened by setting a certain threshold value alpha. The larger the sensitivity is, the smaller the electrical distance between two nodes is, and the stronger the node coupling relation is. Therefore for nodes with sensitivity less than or equal to the threshold αI.e. by
The influence of the change of the power grid topological structure on the branch voltage sag evaluation can be considered to be negligible; for nodes with sensitivity greater than the threshold α
The node a can be considered to be influenced by the evaluation result of the voltage sag of the node due to the change of the topological structure of the power grid i Classified as a change leg impact domain. By the method, a in the original to-be-evaluated regional power grid range W can be selected 1 ,a 2 ···a n And screening the node to obtain the influence domain of the changed branch, namely the new power grid range of the area to be evaluated:
W′＝[a' 1 ,a' 2 ···a' i ···a' m ] (7)
wherein, a' i The nodes are selected according to the threshold value alpha; m is the number of nodes contained in the change leg influence domain. At this time, the new regional power grid range W to be evaluated is greatly reduced compared with the whole power grid.
And S6, keeping the severity grade of each bus voltage sag risk outside the power grid range W of the area to be evaluated unchanged. And sorting the bus voltage sag risk severity grades inside and outside the power grid range of the area to be evaluated to obtain the bus voltage sag risk severity grades in the whole power grid range.
Example 1:
in this embodiment, as shown in fig. 2, the IEEE30 node system model includes 6 infinite power supplies and 6 transformers.
(1) Determining that the area range of the power grid to be evaluated is a 30-node system, namely:
W＝[1,2···30]
(2) And carrying out the whole network load flow calculation and generating a load flow calculation result file.
(3) And setting the line fault position, the fault type and the fault duration time within the range of the power grid area to be evaluated, and performing simulation.
(4) And obtaining the voltage sag amplitudes of the buses under different conditions through the calculation process. Using the obtained voltage sag amplitude value according to a formula
And calculating the sag expected value. And sequencing the sag expected values of all nodes in the power grid range W of the area to be evaluated to obtain the risk severity level corresponding to each node.
(5) The change of the power grid topological structure is divided into two conditions of disconnection of an old branch and connection of a new branch:
1) Access of new tributaries
When a new node 31 is connected to the IEEE30 node system, as shown in fig. 3, the topology of the power grid changes. And determining the branch 7-31 as a change branch, and the node 31 as a change node, and performing load flow calculation analysis on the whole network again. The sensitivity between each node and the change node 31 is calculated according to equations (3) to (6).
And screening out the influence domain of the change branch 7-31 through a set transfer coefficient threshold value alpha, and taking the influence domain as a new regional power grid range W to be evaluated. Suppose that the influence domains of the screened change branches 7-31, as shown in fig. 4, include nodes: 2,4,5,6,7,8,9,28, 31. The regional power grid range to be evaluated at this time is W = [2,4,5,6,7,8,9,28,31].
2) Disconnection of old branch
When the transformers on the branches 18-19 are shut down, the branches 18-19 are disconnected, as shown in fig. 5, and the grid topology changes. And determining the branch 18-19 as a changed branch, and performing load flow calculation analysis on the whole network again, wherein the 18 node and the 19 node are changed nodes. The sensitivity between each node and the change node 18 and the change node 19 is calculated according to equations (3) to (6).
And screening out the influence domain of the change branches 18-19 through a set transfer coefficient threshold value alpha, and taking the influence domain as a new regional power grid range W to be evaluated. Suppose that the influence domains of the screened change branches 18-19, as shown in fig. 6, contain nodes: 10,12,14,15,18,19,20,21,22,23, 24. The regional power grid range W to be evaluated at this time is W = [10,12,14,15,18,19,20,21,22,23,24].
(6) And (5) repeating the steps (3) and (4) to obtain a new severity level of the voltage sag risk of each node in the power grid range W of the area to be evaluated.
(7) And keeping the severity grade of each bus voltage sag risk outside the power grid range W of the area to be evaluated unchanged. And sorting the bus voltage sag risk severity grades inside and outside the power grid range of the area to be evaluated to obtain the bus voltage sag risk severity grades in the whole power grid range.
The above description is only a preferred embodiment of the present invention, and all the equivalent changes and modifications made according to the claims of the present invention should be covered by the present invention.
Claims (4)
1. A large-scale power grid voltage sag evaluation method based on a variable branch influence domain is characterized by comprising the following steps:
s1, setting a power grid range W of a region to be evaluated as a whole power grid;
s2, carrying out whole-network load flow calculation and generating a load flow calculation result file;
s3, setting a line fault position, a fault type and fault duration within the range W of the regional power grid to be evaluated;
s4, calculating voltage sag amplitudes of each bus under different conditions, calculating sag expected values of each bus by using the obtained voltage sag amplitudes, and outputting the severity levels of the voltage sag risks;
s5, judging whether a power grid topological structure is changed, when the power grid topological structure is changed, carrying out load flow calculation on the changed whole power grid again to generate a new load flow calculation result file, carrying out S3 and S4 in a newly determined power grid range of the area to be evaluated, calculating and analyzing to obtain sag expected values of all buses in the area, and outputting a voltage sag risk severity level;
s6, keeping the severity grade of each bus voltage sag risk outside the newly determined regional power grid range to be evaluated unchanged, and sorting the severity grade of each bus voltage sag risk inside the newly determined regional power grid range to be evaluated to obtain the severity grade of each bus voltage sag risk inside the whole power grid range;
the change of the power grid topological structure considers the connection or disconnection of a line, a main transformer and a bus, and the electrical distance between a change node and other nodes of the power grid needs to be calculated; the change of the power grid topological structure is divided into two conditions of disconnection of an old branch and connection of a new branch: when an old branch is disconnected, selecting a node of the old branch still connected with the large power grid as a change node; when a new branch is accessed, selecting a new access large power grid node as a change node;
the electrical distance represents the coupling relationship between two nodes, and the sensitivity is selected to represent the electrical distance as follows:
the polar coordinate form of the Newton Raphson method power flow equation of the N-node system is as follows:
wherein, P i 、Q i Respectively injecting active power and reactive power into the node i; v i Is the voltage amplitude, V, of node i j Is the voltage amplitude of node j; theta.theta. ij Is the phase angle difference between the voltage vectors of the node i and the node j; g ij 、B ij Are respectively node admittance matrix elements Y ij The real and imaginary parts of (c); j belongs to i and represents that the node j is directly connected with the node i;
the above formula is converted to obtain:
wherein, Δ P i 、ΔQ i Injecting the unbalance amount of active power and reactive power for the node i respectively; p is Gi 、Q Gi Injecting active power and reactive power for a power supply at a node i; p Li 、Q Li Active power and none consumed for load at node iWork power;
based on the Taylor principle, the above formula is linearized to obtain a conventional trend equation as follows:
wherein, the delta P and the delta Q are unbalance matrixes of node injection active power and node injection reactive power respectively; H. n, K and L are four plates of the Jacobian matrix respectively; Δ θ, Δ V are the increments of the node voltage phase angle and voltage amplitude, respectively;
the relation between active power, reactive power, voltage and phase angle is considered, and the relation is obtained as follows:
wherein the content of the first and second substances,
sensitivity to reactive injection for voltage amplitude;
for nodes with sensitivity greater than threshold alpha
Considering that the voltage sag evaluation result of the node is influenced by the change of the power grid topology structure, the node a is connected i Classifying into a change branch impact domain;
the newly determined area power grid range to be evaluated is obtained based on a threshold value alpha, and specifically comprises the following steps:
a in the range W of regional power grid to be evaluated 1 ,a 2 …a n And screening the node to obtain the influence domain of the changed branch, namely the new power grid range of the area to be evaluated:
W′＝[a 1 ',a' 2 …a i '…a' m ] (7)
wherein, a i ' is a node screened according to a threshold value alpha; m being included in the influence domain of the varying branchesThe number of nodes.
2. The method for evaluating voltage sag of a large-scale power grid based on a varying branch influence domain according to claim 1, wherein the step S1 specifically comprises:
defining the regional power grid range W to be evaluated as the whole grid, i.e.
W＝[a 1 ,a 2 …a i …a n ] (1)
Wherein W is the regional power grid range to be evaluated, a i Is the name of the node; n is the number of nodes in the whole network.
3. The method for evaluating voltage sag of a large-scale power grid based on a varying branch influence domain according to claim 1, wherein the step S3 specifically comprises: setting single-phase short-circuit faults, two-phase grounding short-circuit faults, interphase short-circuit faults and three-phase short-circuit faults at 0%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% and 100% of each line in the power grid range W of the area to be evaluated; the fault duration is set according to different voltage levels of the line, and the line with the voltage level of 220kV is set to be 120ms; setting the line with the voltage level of 110kV as 300ms; setting the line with the voltage level of 35kV as 700ms; the line voltage level 10kV was set to 0.5s.
4. The large-scale grid voltage sag evaluation method based on the changing branch influence domain according to claim 1, wherein the sag expectation value is calculated and the risk severity level of each bus is output, specifically as follows:
1) And calculating the sag expected value of each bus, wherein the formula is as follows:
in the formula: e (i ') is the sag expected value of the i' bus; n' is the total simulation times; u shape i′j′ Is a first j′ During secondary simulation, the minimum residual voltage amplitude of the i' bus;
2) Arranging the temporary drop expectation values of all buses from small to large;
3) The first 20% of the buses are taken as a severe grade, 20% -40% are taken as a medium grade, 40% -60% are taken as a slight grade, and 60% -100% are taken as a good grade.
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| CN112345853A (en) * | 2020-10-30 | 2021-02-09 | 广东电网有限责任公司广州供电局 | Method for evaluating improvement degree of voltage sag characteristic of transformer substation bus by fault current limiter |
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