CN117713191A - Distributed photovoltaic access method suitable for medium-low voltage distribution network - Google Patents
Distributed photovoltaic access method suitable for medium-low voltage distribution network Download PDFInfo
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Abstract
The invention discloses a distributed photovoltaic access method suitable for a medium-low voltage distribution network, which comprises the steps of firstly, evaluating the distributed photovoltaic access bearing capacity of a transformer with 66-220 kilovolts, a line with 66-220 kilovolts and a bus with 10-220 kilovolts based on a distributed power supply access power grid bearing capacity evaluation guideline; then, based on the distributed photovoltaic access bearing capacity of the 10 kilovolt bus, calculating the photovoltaic bearing capacity of the 10 kilovolt line one by taking voltage deviation, voltage fluctuation and thermal stability of the 10 kilovolt line as constraint conditions; finally, the photovoltaic load carrying capacity of the bay is calculated based on the voltage limit and the capacity limit. The invention has the advantages of clear calculation flow, convenient use and accurate calculation.
Description
Technical Field
The invention relates to the technical field of distributed photovoltaic, in particular to a distributed photovoltaic access method suitable for a medium-low voltage distribution network.
Background
Along with the rapid development of social economy, the increasing severity of energy problems, the rapid increase of load demands and the gradual increase of the requirements of users on power supply reliability, the traditional power generation mode has the defects of long construction period, poor load tracking capability, serious environmental pollution and the like, and is difficult to meet the requirements of current users on high-quality and reliable electric energy. The distributed photovoltaic is a novel renewable energy source and has the advantages of strong load tracking capability, flexible power generation mode, low construction cost, small environmental pollution, high power supply reliability and the like. The novel renewable energy source is adopted as the supplement of the traditional power generation, is beneficial to the comprehensive utilization of the energy source, and is a necessary choice for realizing the sustainable development of economy and society in China.
However, with the rapid development of distributed photovoltaic, the power grid is gradually developed from a 'passive network' to an 'active network', and the operation and maintenance management of the power grid is more complex. During the large distribution photovoltaic period, local area off-grid tide becomes light, even the system voltage is increased, even out of limit, and the holiday period is particularly prominent.
Disclosure of Invention
Aiming at the problems, the invention provides a method for measuring and calculating the distributed photovoltaic bearing capacity of a medium-low voltage distribution network by taking voltage deviation, voltage fluctuation and thermal stability as constraint conditions, which is used for measuring and calculating the photovoltaic bearing capacity of a certain 10 kilovolt line or a certain area and guiding the distributed photovoltaic to be orderly accessed.
In order to achieve the above object, the present invention provides a distributed photovoltaic access method suitable for a medium-low voltage distribution network, which includes:
s1: calculating the bearing capacity of each level of distributed photovoltaic access power grid of the 10-220 kilovolt bus, which comprises the following specific steps:
s101: defining an area to be evaluated and an evaluation object, wherein the area to be evaluated is divided into power supply areas of a single 220-kilovolt transformer, and the evaluation object comprises all transformers with 66-220 kilovolt grades, 66-220 kilovolt-grade lines and 10-220 kilovolt-grade buses in the power supply area;
s102: if the total output of the distributed photovoltaic in the area to be evaluated is larger than the power load, judging that: reversely transmitting power to 220 kilovolts and more power grids by using distributed photovoltaic, wherein the grade of the distributed photovoltaic bearing capacity of each voltage level in the region to be evaluated is red;
s103: counting the current situation value and the harmonic actual measurement value of each bus short-circuit current and voltage deviation of the current voltage class according to the voltage class from high to low, checking by referring to each limit value, and if the checking is not passed, setting the distributed photovoltaic bearing capacity class of the voltage class and the below to-be-evaluated area to be red;
s104: under the normal operation mode of the area to be evaluated, performing thermal stability evaluation and determining the reverse load rate lambda (t) of the current voltage level transformer and the current line and the newly increased distributed photovoltaic capacity P m The calculation formula is as follows:
wherein P is D (t) is the output of the distributed photovoltaic at the moment t in the power supply range of the transformer or the line; p (P) C (t) outputting power at the moment t of other power sources except the distributed photovoltaic; p (P) L (t) is the electricity load at time t; p (P) Net (t) is the power of the transformer or the line to get off the network at the moment t, and the time resolution of t is usually 15min; s is S e Is of variable pressureActual operating limits, k, of the device or line r As a margin coefficient, k r ≤1；
Maximum value lambda of reverse load rate in statistical evaluation period max If lambda is max More than 80%, the voltage class and the distributed photovoltaic bearing capacity class of the region to be evaluated below are red;
s105: calculating and checking short-circuit current and voltage deviation according to the newly-increased distributed photovoltaic capacity and a distributed power supply access power grid bearing capacity evaluation guideline;
wherein, the short circuit current check formula is:
I xz ＜I m
wherein I is XZ I is short-circuit current of system bus m For the allowed short-circuit current limit, the minimum value of the opening current limit of the corresponding circuit breaker on all equipment connected with the bus and the feed-out line is selected;
the voltage deviation check is to calculate the maximum positive and negative voltage deviations of the area after the newly added distributed power is connected according to the capacity of the distributed power to be checked and the national standard requirement, and the maximum positive and negative voltage deviations are respectively expressed as delta U H And δU L ；
In which Q max Maximum reactive power positive and negative values, U, calculated for the required values of grid-connected point power factors of different types of distributed power supplies according to national standards N R is the rated voltage of the bus in the area L 、X L For the resistance and reactance components of the power grid impedance, the power grid resistance components can be ignored in the high-voltage power grid, and the voltage deviation is checked according to the following formula:
ΔU H ＞δU H and DeltaU L ＜δ U L
S106: if the checking is not passed in the step S105, gradually reducing the capacity of the newly increased distributed photovoltaic, repeating the step S105 until the checking is passed, wherein the capacity passing the checking is the distributed photovoltaic bearing capacity of the current voltage level of the area to be evaluated;
s107: comparing the measurement result with the previous measurement result of the voltage class according to the topological connection relation after the measurement of the current voltage class to-be-evaluated area is completed, taking a smaller value between the measurement result and the previous measurement result of the voltage class as the current evaluation result, gradually reducing the voltage class, and repeating S103-106 until the measurement of all the voltage classes of the to-be-evaluated area is completed;
s108: summarizing measurement results of all voltage classes, listing the distributed photovoltaic bearing capacity margin of each voltage class bus, and drawing a distributed photovoltaic bearing capacity result graph of the region to be evaluated according to the power grid topology;
s2: with the voltage deviation and the voltage fluctuation of the 10 kilovolt line as constraints, calculating the photovoltaic bearing capacity of the single 10 kilovolt line, comprising the following specific steps:
s201: calculating node voltage deviation and voltage fluctuation;
s202: solving the node voltage deviation and voltage fluctuation pair l according to the national standard limit value k The photovoltaic carrying capacity under the condition that the first derivative is equal to 0 is calculated by the following formula:
s3: with thermal stability as constraint, calculating the photovoltaic carrying capacity of a single 10 kilovolt line, which comprises the following specific steps:
s301: the reverse load rate lambda is calculated by using the wire transmission limit value, the line load condition and the distributed power supply output characteristic factor, and the calculation formula is as follows:
λ＝(P D -P L )/S e *100％
wherein P is D For distributed power supply output, P L For simultaneous equal-utility electrical loads, i.e. load subtracting other power supply forces than distributed power supply, S e The reverse load rate lambda can not exceed 80% at maximum for the actual operation limit value of the transformer or the circuit;
s302: calculating newly increased distributed power supply capacity P of 10KV line m The calculation formula is as follows:
P m ＝(1-λ max )*S e *k r
wherein k is r Taking 0.8 for the operation margin coefficient of the equipment;
s4: the accessible capacity calculated by taking the voltage deviation and the voltage fluctuation of the 10 kilovolt line as constraint conditions is compared with the accessible capacity calculated by taking the thermal stability of the 10 kilovolt line as constraint conditions, and the accessible capacity with the smaller value is the photovoltaic carrying capacity of the single 10 kilovolt line;
s5: taking the accessible photovoltaic capacity of a 10 kilovolt bus as constraint, and the distributed photovoltaic access total capacity of a 10 kilovolt line under the same bus is less than or equal to the accessible photovoltaic capacity of the 10 kilovolt bus;
s6: and calculating the distributed photovoltaic bearing capacity of the station area under the condition of capacity limitation, wherein the calculation formula of the accessible capacity of the single station area is as follows:
P m station area ＝λ×P e +P l
Wherein lambda is the reverse load rate, the maximum reverse load rate is 80%, P e For rated capacity of station area, P l Load for the area;
the sum of the accessible capacities of a plurality of areas is smaller than the photovoltaic bearing capacity P of all 10kV lines m all The checking formula is as follows:
∑P m station area <P m all
Under the preferred mode, the distributed photovoltaic bearing capacity of the platform area is checked under the limiting conditions that the voltage deviation is not out of limit and the voltage deviation of the photovoltaic grid-connected point is not out of limit;
the voltage deviation of the platform area is not checked out of limit as follows: setting the upper limit L of the outlet voltage of the platform area, wherein the value range of the L is not more than 7% -15% relative to the rated voltage, checking the outlet voltage of each platform area on the line during the expected output period of the photovoltaic before the distributed photovoltaic network connection, if 50% of the outlet voltage of the platform area on the line is more than L, the line is not suitable to be connected with a distributed power supply continuously, and if the outlet voltage of a certain platform area is more than L, the platform area is not suitable to be connected with the distributed power supply continuously;
the grid-connected point voltage deviation non-out-of-limit verification is as follows: checking the voltage of the user in the same area during the period of the predicted power of the photovoltaic before the distributed power supply is connected with the network, if 50% of the voltage of the user exceeds 242V, the user is not suitable to be connected with the distributed power supply continuously in the area, and if the voltage of the user exceeds 242V, the user is not suitable to be connected with the distributed power supply.
Preferably, the calculation formula of the node voltage deviation is:
the calculation formula of the node voltage fluctuation is as follows:
the beneficial effects of the invention are as follows: the invention has the advantages of clear calculation flow, convenient use and accurate calculation.
Drawings
FIG. 1 is a flow chart of a distributed photovoltaic access method suitable for a medium-low voltage distribution network;
FIG. 2 is a schematic representation of a power distribution network incorporating distributed photovoltaics according to the present invention;
fig. 3 is a graph of a typical distribution curve and distribution function of a distributed photovoltaic of the present invention.
Detailed Description
Specific examples:
the invention provides a distributed photovoltaic access method suitable for a medium-low voltage distribution network. Firstly, based on DL/T2041-2019 'distributed power supply access power grid bearing capacity evaluation guideline', the distributed photovoltaic access bearing capacity of a 66-220 KV transformer, a 66-220 KV line and a 10-220 KV bus is evaluated. Then, based on the distributed photovoltaic access bearing capacity of the 10 kilovolt bus, the photovoltaic bearing capacity of the 10 kilovolt line is calculated one by taking voltage deviation, voltage fluctuation and thermal stability of the 10 kilovolt line as constraint conditions. Finally, the photovoltaic carrying capacity of the bay is calculated based on voltage and capacity constraints, including 10kv line capacity constraints and bay own capacity constraints.
1. Calculation of carrying capacity of 10-220 KV bus each-level distributed photovoltaic access power grid
Step 1: and (5) defining the range of the regional power grid to be evaluated, and drawing a topological graph of the regional power grid to be evaluated. In general, the evaluation range is divided into a power supply area of a single 220 kv transformer, and the evaluation object includes all transformers of 66 kv to 220 kv, lines of 66 kv and buses of 10kv to 220 kv in the area.
Step 2: basic parameters such as power grid equipment parameters, operation data, power supply characteristics and the like are collected, whether the distributed photovoltaic to 220 kilovolts and more power grid reverse power transmission occurs in the region to be evaluated is judged, namely whether the total distributed photovoltaic output of the region is larger than the power consumption load or not, and if the total distributed photovoltaic output is reverse power transmission, the distributed photovoltaic bearing capacity grade of each voltage level in the region to be evaluated is red.
Step 3: the evaluation is performed in terms of voltage class from high to low stratification. Based on the collected system data, equipment parameters and operation data, the current situation value and the harmonic actual measurement value of the short-circuit current and the voltage deviation of each bus of the current level are counted, and checking is carried out by referring to each limit value, if the checking is not passed, the voltage level and the distributed photovoltaic bearing capacity level of the regional power grid below are red.
Step 4: under the normal operation mode of the regional power grid to be evaluated, the method is as followsEquation P m ＝min((1-λ(t))×S e )×k r And carrying out thermal stability evaluation to determine the reverse load rate of the current-level transformer and the current-level circuit and the newly-increased distributed photovoltaic capacity. Maximum value lambda of reverse load rate in statistical evaluation period max If lambda is max And if the voltage level is more than 80%, the distributed photovoltaic bearing capacity level of the regional power grid below the voltage level is red.
Step 5: and calculating and checking short-circuit current and voltage deviation according to the newly-increased distributed photovoltaic capacity obtained by Step 4, the distributed power supply access power grid bearing capacity evaluation guideline of DL/T2041-2019 and GB/T33593.
Step 6: if Step5 check is not passed, the capacity of the newly increased distributed photovoltaic is gradually reduced, step5 is repeated until check is passed, and the capacity passing check is the distributed photovoltaic bearing capacity of the current level of the power grid to be evaluated.
Step 7: after the calculation of the current voltage level power grid is completed, comparing the calculation result with the last voltage level according to the topological connection relation, and taking a smaller value between the calculation result and the last voltage level as a current level evaluation result. Then gradually reducing the voltage level, and repeating Step 3-6 until the measurement and calculation of all the voltage levels of the power grid to be evaluated are completed.
Step 8: summarizing measuring and calculating results of all levels, listing distributed photovoltaic bearing capacity margin of all levels of buses, and finally drawing a distributed photovoltaic bearing capacity result graph of the area according to the power grid topology.
2. Photovoltaic load-carrying capacity calculation for 10 kilovolt line
And calculating the photovoltaic bearing capacity of a single 10 kilovolt line by taking voltage deviation, voltage fluctuation and thermal stability of the 10 kilovolt line as constraint conditions. And the photovoltaic capacity of the 10 kilovolt bus which can be connected with is taken as constraint to guide the total capacity of the distributed photovoltaic connection under the same bus section.
1. Calculating the photovoltaic bearing capacity of a single 10 kilovolt line by taking voltage deviation and voltage fluctuation of the 10 kilovolt line as constraints
The invention provides a concept of extremely accessible capacity of a distributed photovoltaic power supply, and calculates the safe access capacity of the 10 kilovolt distributed photovoltaic power supply by combining 6 typical distribution scenes.
(1) Voltage deviation and voltage fluctuation calculation
An analytical model of a power distribution network containing distributed photovoltaic power sources is shown in fig. 2. In order to avoid loss of generality, n nodes are provided, each node is connected with a load and a photovoltaic power supply, and if a certain node does not have a photovoltaic power supply or a load, the corresponding power is set to be zero. In the figure, the node 0 represents a distribution bus, R 0 +jX 0 Represents the system impedance of the main power supply side, R k +jX k Equivalent impedance representing the kth segment of the feed line, P L.k +jQ L.k Represents the firstLoad power of k nodes, P PV.k Representing the photovoltaic power at the kth node. Let rated capacity of k node distribution transformer be S NT.k The reactive loss amplitude of the composite material occupies S NT.k Is a ratio of alpha k The load power factor is phi, and the load active power occupies S NT.k Is beta in ratio of k 。
The voltage deviation calculation model is as follows:
the voltage fluctuation calculation model is as follows:
(2) Typical distribution function
Because of the diversity of the distribution situation of the distributed photovoltaic power supply, for the convenience of analysis and without losing generality, 6 typical distribution scenes are set for analysis according to the distribution photovoltaic capacity, and the method comprises the following steps: the tail ends are concentrated, distributed in an increasing way, distributed uniformly, distributed in a decreasing way, distributed in a small way at the two ends in the middle and distributed in a large way at the two ends in the middle.
The 6 typical profiles and distribution functions are shown in figure 3.
(3) Analysis of voltage problems caused by distributed photovoltaic power access
In order to obtain the extreme capacity limit of the distributed photovoltaic power supply which can be accessed under 6 corresponding typical scenes, a scientific decision basis is provided for the safe access of the distributed photovoltaic power supply, and the load is 0, namely P L Analysis of the extreme case of =0 can result in the case of such extreme case, in the 6 typical distribution cases, the feeder line distance from the busbar caused by the distributed photovoltaic power supply is l k Voltage deviation DeltaU at PV(lk) % and voltage fluctuation d pv(lk) % is calculated as follows.
Voltage deviation DeltaU under the distribution of a, b, c and d in the table PV(lk) % and voltage fluctuation d pv(lk) % vs l k Deriving and orderingIt can be seen that: for the a, b, c, d distribution in the table above, forAll hold at lk=l.
Whereas for the e, f distribution it was analyzed as an increasing function over the corresponding interval and by comparison the maximum value was taken at the end of the interval lk=l, i.e. it was shown that the voltage deviation and voltage ripple caused by the distributed photovoltaic power supply was always the greatest at the end of the feeder. By substituting lk=l into the above table, the maximum voltage deviation and the voltage fluctuation caused by the distributed photovoltaic power supply of various distributions in the extreme case can be obtained.
(4) Extremely accessible capacity limit analysis
In order to meet the requirement that the voltage deviation of the power grid of the distributed photovoltaic power supply does not exceed the upper limit of the allowable voltage and does not exceed the voltage fluctuation limit value caused by the distributed photovoltaic power supply after the distributed photovoltaic power supply is connected to the power grid under the extreme condition of 0 load, the capacity constraint condition of the distributed photovoltaic power supply under the condition of meeting two indexes of the voltage deviation and the voltage fluctuation under the condition of 6 typical distribution conditions is given, wherein DeltaU PV, mark% Represents the national standard limit value of voltage deviation and voltage fluctuation delta d pv. mark% The calculation results of the national standard limit value representing the voltage fluctuation are shown in the following table.
(5) Access distributed photovoltaic limit capacity calculation
The power supply radius of the urban power distribution network is considered as l=5 km, and the power supply radius of the rural power distribution network is considered as l=15 km. A large number of actual observations indicate that: for a photovoltaic power supply, the amplitude of its output power variation due to weather or the like is generally not more than half of its maximum output power, i.e. λ=2. Voltage deviation and voltageThe fluctuation is according to the national power quality standard limit, namely DeltaU PV (photovoltaic) mark Taking-0.07 and +0.07 respectively, Δd pv. label Taking 0.03. Based on the data, the extreme capacity limits of the distributed photovoltaic power supplies of the urban power distribution network and the rural power distribution network are respectively obtained.
From the above calculations, the following conclusions are drawn:
1) In the extreme case of a load of 0, the voltage fluctuations and voltage deviations generated at the feeder ends are the most severe, regardless of the distribution of the distributed photovoltaic power supply access. 2) The extremely accessible capacity limit of the distributed photovoltaic power supply is more conservative and stricter than the actual situation under the condition that the voltage deviation and the voltage fluctuation caused by the distributed photovoltaic power supply meet the requirements under the extreme condition that the load is 0. 3) The extremely accessible capacity limit of the distributed photovoltaic power supply of the rural power distribution network is obviously lower than that of the urban power distribution network, and the rural power distribution network is reflected to have stricter capacity requirements on the access of the distributed photovoltaic power supply. 4) The larger the power supply radius or the smaller the cross-sectional area of the lead, the smaller the limit of the photovoltaic extreme accessible capacity allowed to be accessed, the maximum photovoltaic extreme accessible capacity allowed to be accessed is obtained under the condition that distributed photovoltaic is distributed in a descending mode along the feeder line, the minimum photovoltaic extreme accessible capacity allowed to be accessed is obtained under the condition that the tail end of the distributed photovoltaic concentrated feeder line is accessed, and the limit of the extreme accessible capacity allowed to be accessed by the cable line is larger than that of the overhead line under the condition of the same cross-sectional area of the lead.
2. Calculating the photovoltaic bearing capacity of a single 10 kilovolt line by taking thermal stability as constraint
The thermal stability evaluation of the 10 kilovolt line is to calculate the reverse load rate lambda according to the factors such as the wire transmission limit value, the line load condition, the distributed power supply output characteristic and the like. Reverse load factor lambda calculation formula:
wherein PD is distributed power supply output, PL is equal-effect power load at the same time, namely load subtracting and removing other power supply outputs except the distributed power supply. Se is the actual operating limit of the transformer or line. The reverse load rate lambda cannot exceed 80% at maximum.
The calculation method of the newly added distributed power supply capacity Pm of the 10 kilovolt line comprises the following steps:
P m ＝(1-λ max )×S e ×k r
kr is a plant operating margin factor, typically taken to be 0.8.
3. And comparing the accessible capacity calculated by taking the voltage deviation and the voltage fluctuation of the 10 kilovolt line as constraint conditions with the accessible capacity calculated by taking the thermal stability of the 10 kilovolt line as constraint conditions, and obtaining the photovoltaic carrying capacity of the single 10 kilovolt line if the accessible capacity is smaller.
4. The photovoltaic capacity of the 10 kilovolt bus which can be connected is taken as constraint, and the total photovoltaic connection capacity of the 10 kilovolt line distribution type under the same bus section is less than or equal to the photovoltaic capacity of the 10 kilovolt bus.
3. Measuring and calculating photovoltaic bearing capacity of transformer area
1. Voltage limitation-based calculation of district distributed photovoltaic bearing capacity
For low-voltage distributed photovoltaics with larger capacity and positioned at the tail end of power supply, when a low-voltage circuit is directly connected, circuit power flow is reversed during photovoltaic output, voltage rise near a grid-connected point can be caused, and power supply quality of other peripheral users is adversely affected.
(1) Power grid current situation assessment before distributed photovoltaic access
a) The voltage deviation of the platform area is not out of limit
Considering the safe and stable operation of a distribution network and the requirement of the national energy bureau on the "easy-to-connect" of distributed photovoltaic, starting from the actual operation condition of a large-connection power grid, setting the upper limit L of the outlet voltage of a platform area, wherein the value range of the L is not more than 7% -15% relative to the rated voltage, checking the outlet voltage of each platform area on a line during the expected output period of photovoltaic before the distributed photovoltaic connection, if 50% of the outlet voltage of the platform area on the line exceeds L, the line is not easy to be connected with a distributed power supply continuously, and if the outlet voltage of a certain platform area exceeds L, the platform area is not easy to be connected with the distributed power supply continuously.
b) Grid-connected point voltage deviation is not out of limit
The GB/T40586 grid-connected power grid-related protection technical requirement specifies that the steady-state voltage of a distributed power grid-connected point should normally operate when the steady-state voltage is 0.9-1.1 times of the nominal voltage. In order to avoid the voltage out-of-limit of the user side caused by the distributed photovoltaic access, the voltage of the user under the same platform area is checked before the power is estimated by the photovoltaic before the distributed power supply is connected to the network, and if 50% of the voltage of the user exceeds 242V, the voltage of the user under the platform area is not suitable to be connected to the distributed photovoltaic continuously. If a certain user voltage exceeds 242V, the user is not suitable to access the distributed power supply.
(2) Evaluation of expected voltage rise after distributed photovoltaic access
And under the condition that the grid-connected point voltage is calculated to exceed the limit value, namely U grid-connected point-U grid=22V, the photovoltaic maximum output constraint is not caused by the voltage drop of the grid-connected point voltage to the grid-connected point outlet voltage under the condition that the grid-connected point outlet voltage is calculated to be the rated value.
According to a typical scene that the power supply radius of the low-voltage line is not more than 500m and the wire diameter of the main line is not less than 50mm < 2 >, the openable capacity of the low-voltage outlet wire of one circuit is calculated to be 40kW, and the requirements of different circuit parameters on the limitation of the photovoltaic output can be referred to the following table.
Voltage-limitation-based distributed photovoltaic grid-connected capacity limitation reference
2. Capacity-constraint-based calculation of district distributed photovoltaic bearing capacity
(1) A single zone may access capacity. The calculation method comprises the following steps: the single station area accessible capacity minus the station area load divided by the distribution transformer rated capacity, the calculated reverse load rate should not exceed 80%.
(2) The access capacity of a plurality of areas is calculated by taking the 10 kilovolt line bearing capacity as constraint.
The foregoing is only a preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art, who is within the scope of the present invention, should be covered by the protection scope of the present invention by making equivalents and modifications to the technical solution and the inventive concept thereof.
Claims (3)
1. The distributed photovoltaic access method suitable for the medium-low voltage distribution network is characterized by comprising the following steps of:
s1: calculating the bearing capacity of each level of distributed photovoltaic access power grid of the 10-220 kilovolt bus, which comprises the following specific steps:
s101: defining an area to be evaluated and an evaluation object, wherein the area to be evaluated is divided into power supply areas of a single 220-kilovolt transformer, and the evaluation object comprises all transformers with 66-220 kilovolt grades, 66-220 kilovolt-grade lines and 10-220 kilovolt-grade buses in the power supply area;
s102: if the total output of the distributed photovoltaic in the area to be evaluated is larger than the power load, judging that: reversely transmitting power to 220 kilovolts and more power grids by using distributed photovoltaic, wherein the grade of the distributed photovoltaic bearing capacity of each voltage level in the region to be evaluated is red;
s103: counting the current situation value and the harmonic actual measurement value of each bus short-circuit current and voltage deviation of the current voltage class according to the voltage class from high to low, checking by referring to each limit value, and if the checking is not passed, setting the distributed photovoltaic bearing capacity class of the voltage class and the below to-be-evaluated area to be red;
s104: under the normal operation mode of the area to be evaluated, performing thermal stability evaluation and determining the reverse load rate lambda (t) of the current voltage level transformer and the current line and the newly increased distributed photovoltaic capacity P m The calculation formula is as follows:
wherein P is D (t) distributed photovoltaic t-time in the range of power supply to transformers or linesThe carved output force; p (P) C (t) outputting power at the moment t of other power sources except the distributed photovoltaic; p (P) L (t) is the electricity load at time t; p (P) Net (t) is the power of the transformer or the line to get off the network at the moment t, and the time resolution of t is usually 15min; s is S e For actual operating limits, k, of transformers or lines r As a margin coefficient, k r ≤1；
Maximum value lambda of reverse load rate in statistical evaluation period max If lambda is max More than 80%, the voltage class and the distributed photovoltaic bearing capacity class of the region to be evaluated below are red;
s105: calculating and checking short-circuit current and voltage deviation according to the newly-increased distributed photovoltaic capacity and a distributed power supply access power grid bearing capacity evaluation guideline;
wherein, the short circuit current check formula is:
I xz ＜I m
wherein I is XZ I is short-circuit current of system bus m For the allowed short-circuit current limit, the minimum value of the opening current limit of the corresponding circuit breaker on all equipment connected with the bus and the feed-out line is selected;
the voltage deviation check is to calculate the maximum positive and negative voltage deviations of the area after the newly added distributed power is connected according to the capacity of the distributed power to be checked and the national standard requirement, and the maximum positive and negative voltage deviations are respectively expressed as delta U H And δU L ；
In which Q max Maximum reactive power positive and negative values, U, calculated for the required values of grid-connected point power factors of different types of distributed power supplies according to national standards N R is the rated voltage of the bus in the area L 、X L For the resistance and reactance components of the power grid impedance, the power grid resistance components can be ignored in the high-voltage power grid, and the voltage deviation is checked according to the following formula:
ΔU H >δU H and DeltaU L <δU L
S106: if the checking is not passed in the step S105, gradually reducing the capacity of the newly increased distributed photovoltaic, repeating the step S105 until the checking is passed, wherein the capacity passing the checking is the distributed photovoltaic bearing capacity of the current voltage level of the area to be evaluated;
s107: comparing the measurement result with the previous measurement result of the voltage class according to the topological connection relation after the measurement of the current voltage class to-be-evaluated area is completed, taking a smaller value between the measurement result and the previous measurement result of the voltage class as the current evaluation result, gradually reducing the voltage class, and repeating S103-106 until the measurement of all the voltage classes of the to-be-evaluated area is completed;
s108: summarizing measurement results of all voltage classes, listing the distributed photovoltaic bearing capacity margin of each voltage class bus, and drawing a distributed photovoltaic bearing capacity result graph of the region to be evaluated according to the power grid topology;
s2: with the voltage deviation and the voltage fluctuation of the 10 kilovolt line as constraints, calculating the photovoltaic bearing capacity of the single 10 kilovolt line, comprising the following specific steps:
s201: calculating node voltage deviation and voltage fluctuation;
s202: solving the node voltage deviation and voltage fluctuation pair l according to the national standard limit value k The photovoltaic carrying capacity under the condition that the first derivative is equal to 0 is calculated by the following formula:
s3: with thermal stability as constraint, calculating the photovoltaic carrying capacity of a single 10 kilovolt line, which comprises the following specific steps:
s301: the reverse load rate lambda is calculated by using the wire transmission limit value, the line load condition and the distributed power supply output characteristic factor, and the calculation formula is as follows:
λ＝(P D -P L )/S e *100％
wherein P is D For distributed power supply output, P L For simultaneous equal-utility electric loads, i.e. load-reducing removal of other power sources than distributed power sourcesForce S e The reverse load rate lambda can not exceed 80% at maximum for the actual operation limit value of the transformer or the circuit;
s302: calculating newly increased distributed power supply capacity P of 10KV line m The calculation formula is as follows:
P m ＝(1-λ max )*S e *k r
wherein k is r Taking 0.8 for the operation margin coefficient of the equipment;
s4: the accessible capacity calculated by taking the voltage deviation and the voltage fluctuation of the 10 kilovolt line as constraint conditions is compared with the accessible capacity calculated by taking the thermal stability of the 10 kilovolt line as constraint conditions, and the accessible capacity with the smaller value is the photovoltaic carrying capacity of the single 10 kilovolt line;
s5: taking the accessible photovoltaic capacity of a 10 kilovolt bus as constraint, and the distributed photovoltaic access total capacity of a 10 kilovolt line under the same bus is less than or equal to the accessible photovoltaic capacity of the 10 kilovolt bus;
s6: and calculating the distributed photovoltaic bearing capacity of the station area under the condition of capacity limitation, wherein the calculation formula of the accessible capacity of the single station area is as follows:
P m station area ＝λ×P e +P l
Wherein lambda is the reverse load rate, the maximum reverse load rate is 80%, P e For rated capacity of station area, P l Load for the area;
the sum of the accessible capacities of a plurality of areas is smaller than the photovoltaic bearing capacity P of all 10kV lines m all The checking formula is as follows:
∑P m station area <P m is all.
2. The distributed photovoltaic access method suitable for the medium-low voltage distribution network according to claim 1, wherein the distributed photovoltaic bearing capacity of the platform area is checked under the limiting conditions that the voltage deviation is not out of limit and the voltage deviation of the photovoltaic grid-connected point is not out of limit;
the voltage deviation of the platform area is not checked out of limit as follows: setting the upper limit L of the outlet voltage of the platform area, wherein the value range of the L is not more than 7% -15% relative to the rated voltage, checking the outlet voltage of each platform area on the line during the expected output period of the photovoltaic before the distributed photovoltaic network connection, if 50% of the outlet voltage of the platform area on the line is more than L, the line is not suitable to be connected with a distributed power supply continuously, and if the outlet voltage of a certain platform area is more than L, the platform area is not suitable to be connected with the distributed power supply continuously;
the grid-connected point voltage deviation non-out-of-limit verification is as follows: checking the voltage of the user in the same area during the period of the predicted power of the photovoltaic before the distributed power supply is connected with the network, if 50% of the voltage of the user exceeds 242V, the user is not suitable to be connected with the distributed power supply continuously in the area, and if the voltage of the user exceeds 242V, the user is not suitable to be connected with the distributed power supply.
3. The distributed photovoltaic access method applicable to the medium-low voltage distribution network according to claim 1, wherein the calculation formula of the node voltage deviation is as follows:
the calculation formula of the node voltage fluctuation is as follows:
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