CN114154287A - Current-carrying type selection method for direct-buried cable of photovoltaic system - Google Patents
Current-carrying type selection method for direct-buried cable of photovoltaic system Download PDFInfo
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- 238000010187 selection method Methods 0.000 title claims abstract description 9
- 230000000737 periodic effect Effects 0.000 claims abstract description 28
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- 239000002689 soil Substances 0.000 claims description 14
- 238000004088 simulation Methods 0.000 claims description 12
- 239000004020 conductor Substances 0.000 claims description 9
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- 229920003020 cross-linked polyethylene Polymers 0.000 description 1
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Abstract
The invention relates to a current-carrying type selection method for a direct-buried cable of a photovoltaic system. According to the invention, based on the periodic load characteristic of the photovoltaic system, a daily load period curve flowing through a cable in the photovoltaic system is fitted according to the hourly data of Pvsyst. And then calculating a cable cycle load factor through software (CYMCAP or the like) based on the confirmed daily load cycle curve and the actual laying environment and configuration of the directly-buried cable, and confirming a cable current peak value under the cycle load, thereby providing a calculation method suitable for the type selection of the directly-buried cable of the photovoltaic system. The invention also provides a table look-up method for the load factor M value of the 35kV direct-buried cable, confirms the minimum value of M, is convenient for practical engineering application, and effectively reduces the cable section so as to reduce the cable cost.
Description
Technical Field
The invention belongs to the field of photovoltaic system design, and particularly relates to a photovoltaic system direct-buried cable current-carrying capacity type selection method and application.
Background
With the large-scale development of the photovoltaic power generation industry in China, how to effectively reduce the total construction cost of a photovoltaic power station becomes a great focus in the new energy industry. As an important component of a photovoltaic power generation system, the cost of the cable accounts for a large proportion of the overall cost of a photovoltaic project, so that the cable model is optimized and selected during design, the safety standard is met, and the construction cost of the project is greatly reduced.
In power design, the cable model is usually selected based on analytic calculation of the current-carrying capacity of the cable. As a common cable model selection in photovoltaic power generation systems, the long-term allowable maximum operating temperature of a crosslinked polyethylene power cable is 90 ℃. If the temperature of the cable conductor exceeds 90 ℃, the insulation of the cable is accelerated to age, and the service life of the cable is shortened; if the temperature of the cable conductor is too low than 90 ℃, the conveying capacity of the cable is not fully exerted.
The IEC 60287 (corresponding to the national standard JB 10181) standard established by the International Electrotechnical Commission (IEC) provides a method for calculating the rated current-carrying capacity of a constant-load cable with a load factor of 1.0. Considering the periodic load characteristic of the photovoltaic system, namely the photovoltaic system only operates in the daytime and does not operate at night, the periodicity of the operation stage is relatively fixed, the transmission capability of the cable cannot be fully utilized based on the long-term current-carrying capacity calculation result with the load factor of 1.0, and the economy is reduced. The IEC 60853 standard gives a calculation method of the periodic load flow coefficient (M) taking into account the external heat capacity of the cable machine. In a periodic load power system, the current carrying capacity of the selected cable can meet the maximum load current in a daily period, namely the rated current carrying capacity of the cable multiplied by a periodic negative load flow coefficient (M) is larger than the allowable peak current value in a daily (24h) period.
The photovoltaic power generation system is used as a typical periodic load, and although the calculation of the cable load factor under a periodic curve is determined by related specifications, a calculation formula in the specifications is abstract and complex, and cannot be widely applied to actual engineering. For the photovoltaic system direct-buried cable, the periodic negative load flow coefficient of the cable is related to the soil thermal resistance coefficient, the environment temperature, the loop number, the loop spacing, the laying depth and the like, the influence factors are multiple, and the calculation is difficult.
Therefore, it is necessary to research and invent a method convenient for engineering application, optimize the cable section of the photovoltaic system, and improve the engineering economy.
Disclosure of Invention
The invention aims to provide a current-carrying type selection method for a direct-buried cable of a photovoltaic system. The invention provides a simulation calculation method for simplifying and popularizing the cable periodic load factor M value of the periodic load curve, and provides a table look-up method for the periodic load factor M of the 35kV direct-buried cable based on a simulation result, so that the application of actual engineering is facilitated.
In order to achieve the purpose, the invention adopts the following technical scheme:
a current-carrying type selection method for a photovoltaic system direct-buried cable is characterized by comprising the following steps:
(1) fitting a daily load cycle curve of a cable flowing through the photovoltaic system according to the hourly data of the Pvsyst;
(2) based on the confirmed daily load cycle curve and the actual laying environment and configuration of the directly buried cable, calculating the steady-state current-carrying capacity Ir of the cable and the periodic load factor M of the cable through software (CYMCAP or the like), and confirming the current peak value I of the cable under the periodic loadp＝Ir×M；
When the simulation condition is not met, the 35kV direct-buried cable confirms the minimum value of the periodic load factor of the cable according to a table look-up method and confirms the current peak value of the cable under the periodic load;
and carrying current and type selection of the directly-buried cable of the photovoltaic system is carried out according to the current peak value of the cable.
The daily load cycle curve is fitted to a sine wave according to the hourly data of Pvsyst. Considering that most photovoltaic projects have a power generation time less than 12 hours, the calculation time can be considered as T-12 hours.
The actual laying environment parameters of the buried cable comprise: soil resistivity, ambient temperature, number of loops, loop spacing, laying depth, cable specifications, conductor materials and the like; the sensitivity to cable cycle loading factor is ranked as follows: the soil temperature or conductor material (basically without influence) < cable section < circuit distance or laying depth < soil thermal resistivity.
The table look-up method is based on different full-time of daily load cycle curves of the photovoltaic system, sensitivity analysis of the load factor M to each environment and laying factors is carried out, main influence factors, namely soil thermal resistance coefficients and loop quantity are considered, and the rest influence factors are considered according to the minimum value of the M value to list a load factor M look-up table. The minimum M value is based on the above sensitivity analysis, and the relationship curve of the non-sensitivity factors, i.e. the cable laying depth, the cable section and the loop spacing factor, and the load factor M and each parameter under a specific laying condition is shown in fig. 2. From the curve trend of fig. 2, the following conclusions can be drawn:
for the cable laying depth, the load factor increases along with the increase of the cable depth, according to the requirements of electric power engineering cable design specification GB 50217-2018, the depth from the cable sheath to the ground is not less than 0.7M, the minimum distance from the cable center point to the ground is considered as 0.8M in consideration of the outer diameter of the cable and the practical engineering application, and therefore the value of M is minimum under the laying depth of 0.8M.
For the cable section, the load factor increases along with the increase of the cable section, the thermal stability requirement of the cable in practical engineering projects is considered,the minimum cross section of the selected cable is usually 70mm2, so that the M value is 70mm in the cross section2The lower is the smallest.
For the loop spacing, the load factor is reduced along with the increase of the cable spacing, according to the power engineering cable design specification GB 50217 and 2018, appendix 4, the correction coefficient is mainly aimed at the cable clear distance of 100-300 mm and the actual engineering land-saving principle, the maximum center distance of the cable is usually controlled within 400mm (the cable outer diameter is considered as 100 mm), so the value of M is the minimum when the laying center distance is 400 mm.
The table of the table lookup method is as follows:
TABLE 1 daily load curve is chord wave, and infinite conditions
TABLE 2 daily load curve is sine wave, 2 hours of limited delivery
TABLE 3 Sun load curve is chord wave, 4 hours of limited emission
TABLE 4 daily load curve is sine wave, limited to 6 hours
TABLE 5 daily load curve Square wave, 12 hours full hair
From the above description of the minimum M value, the applicable ranges specifying the above table are as follows:
a. the daily cycle load curve calculated is a sine wave and a square wave, and the cycle is 12 hours;
b. the values in the table are based on 3X70 mm2The table is suitable for the cable section not less than 70mm2Three-core and single-core cables are laid in a triangle;
c. the numerical values in the table are based on the simulation result that the cable laying center distance is 400mm, and the table is suitable for the laying condition that the cable spacing is not more than 400 mm;
d. the numerical value in the table is 0.8m based on the cable laying depth, and the table is suitable for the condition that the buried depth is not less than 0.8 m;
e. when the number of laid loops is more than 4, considering according to 4 loops;
f. the table is suitable for a 35kV direct-buried cable system;
g. for the case where the thermal resistivity is between the data given in the table, interpolation can be used to obtain the value of M.
Drawings
FIG. 1 is a typical daily cycle load curve. Wherein, (a) is square wave, i.e. m is 12 hours and full hair; (b) is a sine wave, wherein the hair-limiting time is 0 hour; (c) is a sine wave, wherein the hair-limiting time is 2 hours; (d) is a sine wave, wherein the hair-limiting time is 4 hours; (e) is a sine wave, wherein the hair-limiting time is 6 hours;
FIG. 2 is a relationship curve between a periodic load factor M and laying parameters, wherein the environmental parameters include thermal resistance coefficient (0.8-2.0 K.m/W), number of parallel laying loops (1-4 loops) in the same trench, cable laying depth (0.2-2M), cable specification (75-400 mm2), and parallel loop spacing (center distance, 200-500 mm).
Detailed Description
In order to more specifically describe the present invention, the following detailed description of the embodiments of the present invention is provided with reference to the accompanying drawings. It should be understood that the described embodiments are only a few embodiments of the present invention, and not all embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
Example 1 simulation method
In the embodiment, the minimum load factor under a typical daily load curve is confirmed by a simulation method, the current-carrying capacity is finally obtained, and the corresponding model selection is performed on the cable.
The photovoltaic system direct-buried cable current-carrying capacity type selection method based on the periodic load curve comprises the following steps:
(1) and fitting a daily load cycle curve of the cable flowing through the photovoltaic system according to the hourly data of the Pvsyst.
A typical load cycle curve is a sine wave, and the number of hours to be issued can be confirmed from Pvsyst simulation data, and the daily cycle load curve function can be approximately confirmed according to equations (1) to (3).
(2) Based on the confirmed daily load cycle curve and the actual laying environment and configuration of the directly buried cable, calculating the steady-state current-carrying capacity Ir (IEC 60287 method) of the cable and the periodic load factor M of the cable through software (CYMCAP or the like), and confirming the current peak value I of the cable under the periodic loadp＝IrAnd (4) x M. And carrying capacity selection of the direct-buried cable of the photovoltaic system is carried out according to the current peak value of the cable.
The load curve of the invention can be a sine wave as a typical daily cycle load curve, and for different trigger time, the daily load curve can be expressed as:
wherein m is the number of hours of hair growth, T is 12 hours, and K is a correction coefficient under the hair growth limit, and is expressed as follows:
when the daily load curve cannot be determined, an extreme daily load curve can be in a square wave form and can be represented as:
the daily load curves for the different limit hours and square waves are shown in figure 1.
The actual laying environment parameters of the buried cable comprise: soil resistivity, ambient temperature, number of loops, loop spacing, laying depth, cable specifications, conductor materials, and the like. The sensitivity simulation analysis to the cable cycle load factor is based on the following environmental conditions:
the soil environment temperature: 30 ℃;
thermal resistivity of soil: 1.2 K.m/W;
number of parallel loops: 4, a loop;
the circuit interval: 400mm (center-to-center distance);
laying depth: 0.8 m;
cable model: YJLV22-3X 7026/35 kV;
and (3) analyzing the influence of the periodic load factor M and each parameter respectively aiming at the above conditions by adopting a control variable method, wherein the influence is specifically shown as a curve in fig. 2. The cyclic load factor is related to the respective environmental conditions as follows:
(a) the cycle load factor M increases with the increase of the thermal resistance coefficient, and when the thermal resistance varies between 0.8 and 2K.m/W, the difference of the M values is about 0.16.
(b) The periodic load factor M increases with the number of loops, and when the number of loops is between 1 and 4, the difference of M is about 0.16.
(c) The period load factor M increases along with the increase of the laying depth, and when the laying depth is changed between 0.8M and 2M, the difference value of the M values is about 0.09.
(d) The cyclic load factor M increases with increasing cable gauge when the cable is 70mm from three cores2～400mm2The difference in the values of M when varied was about 0.05.
(e) The periodic load factor M decreases with increasing loop pitch, and the difference in M is about 0.1 when the cable pitch varies between 200 and 500.
(f) The periodic load factor M of the single-core cable is slightly higher than that of a three-core cable with the same specification.
(g) The ambient temperature and the conductor type have substantially no effect on the value of the cyclic load factor M and are negligible.
For the above analysis, the sensitivity of each factor is ranked as follows: the soil temperature or conductor material (basically without influence) < cable section < circuit distance or laying depth < circuit quantity < soil thermal resistivity.
Example 2, the table look-up method,
the table look-up method is based on the results of the simulation method, and when the thermal resistance is changed between 0.8 and 2K.m/W, the loop number is changed between 1 and 4 loops, the laying depth is changed between 0.8 and 2m, and the cable is 70mm from three cores2～400mm2When the cable spacing is changed between 200 and 500, only the influence of the loop number and the thermal resistance coefficient on the M value is considered, the operation steps are simplified, the practical engineering application is facilitated, and the cable section is effectively reduced, so that the cable cost is reduced.
Taking a 35kV photovoltaic system as an example, the photovoltaic system 35kV direct-buried cable carrying capacity type selection method based on the periodic load curve comprises the following steps:
(1) similar to the simulation method, the daily cycle load limit hours are confirmed first.
(2) And according to the limited hours, the thermal resistance coefficient and the loop number of the direct-buried cable, and the minimum cycle load factor M in the lookup tables 1-5 is corresponded. When the number of hours of day-cycle load full can not be confirmed, table 5 can be used to query the minimum cycle load factor M under the square wave.
(3) And calculating the long-term steady-state current-carrying capacity Ir of the cable according to IEC 60287.
(4) Calculating the peak value of the current of the cable under the curve of the corresponding daily load cycle, i.e. Ip＝Ir×M。
And carrying capacity selection of the direct-buried cable of the photovoltaic system is carried out according to the current peak value of the cable.
TABLE 1 daily load curve is chord wave, and infinite conditions
TABLE 2 daily load curve is sine wave, 2 hours of limited delivery
TABLE 3 Sun load curve is chord wave, 4 hours of limited emission
TABLE 4 daily load curve is sine wave, limited to 6 hours
TABLE 5 daily load curve Square wave, 12 hours full hair
Claims (5)
1. A current-carrying type selection method for a photovoltaic system direct-buried cable is characterized by comprising the following steps:
(1) fitting a daily load cycle curve of a cable flowing through the photovoltaic system according to the hourly data of the Pvsyst;
(2) based on the confirmed daily load cycle curve and the actual laying environment and configuration of the directly buried cable, calculating the steady-state current-carrying capacity Ir of the cable and the periodic load factor M of the cable, and confirming the current peak value I of the cable under the periodic loadp＝Ir×M；
When the simulation condition is not met, the 35kV direct-buried cable confirms the minimum value of the periodic load factor of the cable according to a table look-up method and confirms the current peak value of the cable under the periodic load;
and carrying current and type selection of the directly-buried cable of the photovoltaic system is carried out according to the current peak value of the cable.
2. The method of claim 1, wherein the daily duty cycle is characterized by a sinusoidal waveform.
3. The method according to claim 1, wherein the actual laying environment parameters of the buried cable comprise: soil resistivity, ambient temperature, number of loops, loop spacing, laying depth, cable specification and conductor material; the sensitivity to cable cycle loading factor is ranked as follows: the soil temperature or conductor material (basically without influence) < cable section < circuit distance or laying depth < soil thermal resistivity.
4. The method of claim 1, wherein the lookup table is based on different full-time of daily load cycle curves of the photovoltaic system, and mainly takes into account the thermal resistivity of soil, the influence of the number of installations in the same trench, and the remaining influence factors are taken into minimum consideration according to the M value.
5. The method of claim 1, wherein the table of the table lookup is:
TABLE 1 daily load curve is chord wave, and infinite conditions
TABLE 2 daily load curve is sine wave, 2 hours of limited delivery
TABLE 3 Sun load curve is chord wave, 4 hours of limited emission
TABLE 4 daily load curve is sine wave, limited to 6 hours
TABLE 5 daily load curve Square wave, 12 hours full hair
Wherein,
a. the daily cycle load curve calculated is a sine wave and a square wave, and the cycle is 12 hours;
b. the values in the table are based on 3X70 mm2The table is suitable for the cable section not less than 70mm2Three-core and single-core cables are laid in a triangle;
c. the numerical values in the table are based on the simulation result that the cable laying center distance is 400mm, and the table is suitable for the laying condition that the cable spacing is not more than 400 mm;
d. the numerical value in the table is 0.8m based on the cable laying depth, and the table is suitable for the condition that the buried depth is not less than 0.8 m;
e. when the number of laid loops is more than 4, considering according to 4 loops;
f. the table is suitable for a 35kV direct-buried cable system;
g. for the case where the thermal resistivity is between the data given in the table, interpolation can be used to obtain the value of M.
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| CN115859701A (en) * | 2023-03-03 | 2023-03-28 | 天津有容蒂康通讯技术有限公司 | Cable detection data-based incremental analysis method and system |
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| CN115859701A (en) * | 2023-03-03 | 2023-03-28 | 天津有容蒂康通讯技术有限公司 | Cable detection data-based incremental analysis method and system |
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