In the design of photovoltaic system, in addition to the design of battery capacity and the size of solar cell array, we should also consider how to select appropriate system equipment, that is, how to select solar cell array, battery, controller, cable, junction box, module support that meet the needs of the system. The selection of various equipment needs to comprehensively consider the actual situation of the system location, the scale of the system, customer requirements and other factors.
Since solar photovoltaic lighting devices are usually independent power supply devices, the power distribution system is relatively simple. The distribution line of the solar lighting system is used to realize the transmission of solar energy between the components of the system, so the distribution line is required to achieve efficient, reliable and safe transmission. The requirements for power distribution of solar lighting system are as follows.
① High efficiency: under the condition of rated power transmission, the voltage loss of DC line is required to be small enough, usually controlled below 5%.
② Reliable: the line connection between various components is reliable, which usually requires common distribution boards to be connected, and disconnection points are set for maintenance.
③ Safety: the distribution line is required to be equipped with a short-circuit device, and ensure that the system has good grounding and lightning protection measures to ensure the safety of the distribution line.
The power distribution design of solar lighting system, the selection of wire type, installation, erection, inspection and maintenance all have an important impact on solar lighting. In order to ensure the operation and protection of transmission and distribution lines, distribution boxes are usually set at the terminals of the system.
1.1 distribution box
Generally, the solar lighting system is independent, so the use of distribution box can basically meet the requirements of the line. Generally, the distribution box is required to have distribution switch board, protective component board, indicating equipment and other parts.
① Distribution switch board: install air switch, disconnector, two position disconnector and AC contactor. Solar charging controller can also be installed on this panel.
② Protection component board: install fuses, overcurrent protectors, undervoltage protectors, leakage protectors and fault alarms.
③ Indicating equipment: signal indicating lamps, voltmeters, ammeters, frequency meters, electricity meters and other equipment can be installed according to actual needs.
In the distribution box, different switch switching is adopted for different load lines. Usually, the solar controller will install the set program to realize automatic switching. For example, if the illumination of the day is insufficient and the storage capacity of the battery is insufficient, the controller can output half power; When the sunshine is sufficient and the storage capacity of the battery is sufficient, the load can be output with full power.
The distribution box is set at the appropriate position of the solar lighting system to minimize the length of the transmission line to reduce the voltage loss of the line. When setting the distribution box, we should also consider the factors such as convenient maintenance and safe use. Choosing a reasonable location of the distribution box can not only save electric energy, but also ensure the reliability of the system.
The design of distribution box shall fully consider the line safety, and the specific requirements are as follows.
① Prevent line overload and short circuit. The power supply system needs to pay attention to the problems of load overload and electrical short circuit. In order to prevent the line current from exceeding the maximum safe current of the conductor, in addition to designing the conductor according to the maximum safe current carrying capacity of the conductor section specified in the national electrical standards, overcurrent protection measures should also be taken on the line. The commonly used over-current and short-circuit protection measures are air switches or fuses. Air switches are not suitable for use in DC lines, but mainly in AC lines. Because if the contact of the air switch is used in the DC line, the arc will often be burned due to overcurrent, while in the AC line, if the breaking capacity of the switch contact is sufficient, the line can be cut off, and the current design makes the contact not burned. The advantage of the air switch is that the circuit will trip after overcurrent, and it can be reused as long as it is simply reset. After overcurrent in the fuse, the circuit will be cut off by burning the fuse or fuse blade. The fuse needs to be replaced for reuse. If the air switch is not identified and can be used in the DC system, the fuse or DC circuit breaker with arc extinguishing function should be used for overcurrent protection.
② Prevent electric leakage and low voltage of the line. In the distribution system with leakage protector and undervoltage relay installed in the distribution box, when the line leakage or voltage is too low, the leakage protector and undervoltage relay will work, thus cutting off the circuit. Only after the system fault is solved can the system work again.
③ Power interlock control. If there are photovoltaic, wind and municipal power generation systems in the system, there should be a power interlock device in the distribution box to prevent short circuit accidents between different power supplies.
Generally, the power distribution system design of the independent solar lighting system can be relatively simple, and most devices can be omitted according to the needs and functions of the controller. However, in the solar energy system with complementary scenery and municipal power, the power distribution function of each line should be fully designed, and safety measures should be designed for each line.
1.2 conductor
In the installation of electrical distribution equipment, we often encounter the problem of wire selection, and the correct selection of wire is a very important work. If the cross-sectional area of wire is selected to be small, the electrical load is large, which is easy to cause electrical fire; If the cross-sectional area is selected to be large, the cost will be high and the materials will be wasted. The following factors shall be considered in the selection of conductors.
(1) Rated voltage
The rated voltage depends on the phase to phase voltage of the cable system, the type of system, and the troubleshooting time of protective equipment. In ungrounded system, the cable will run for a long time in case of single-phase grounding fault. However, the voltage gradient between lines will be generated between the insulation of the other two ungrounded conductors, requiring the insulation layer to be thicker. It is impossible to apply all line to line voltage for a long time between two faultless phases; If the protection equipment can remove the fault within 1min, the cable with 100% rated voltage can be selected in this grounding system. For Ungrounded systems, cables with 133% rated voltage shall be selected when the 1min clearing time specified for 100% rated voltage level cannot be met, but the fault section can be cleared within 1h. When the time for clearing the grounding fault section is quite long, the insulation with 173% rated voltage level shall be selected.
(2) Wire selection
When selecting the conductor specification, we should consider the requirements of the national electrical code, the thermal effect of load current, mutual heating effect, loss caused by electromagnetic induction, dielectric loss, indicators related to load current, emergency overload indicators, voltage drop limits and fault current indicators.
(3) Load current index
The current carrying capacity table lists the minimum specifications of wires required. However, the selection of cables in engineering is often conservative, taking into account the growth of load, voltage drop, heating of short-circuit current and other factors.
(4) Emergency overload index
The normal load limit of insulated wires and cables is based on practical experience and represents the aging rate of cables. This aging rate is expected to make the effective life of the cable last 20-30 years. When the normal daily load temperature rises by 8 ℃, the average failure rate will be doubled and the insulation life of the cable will be shortened by half. It is an extraordinary measure for the cable to operate continuously under the condition of exceeding the maximum rated temperature or rated current carrying capacity. The temperature rise is directly proportional to the conductor loss, and the loss increases with the square of the current. A large voltage drop may cause unexpected danger to the continuity of equipment and power supply. The cable shall operate under the maximum emergency overload temperature for no more than 100h per year, and such 100h overload period shall not exceed 5 times within the cable life. All kinds of insulated cables have overload coefficient of short-time overload. The emergency or overload current value of this insulated cable can be obtained by multiplying the overload coefficient by the nominal rated current value of the cable.
(5) Voltage drop index
If the cross section of the power supply line is not large enough, excessive voltage drop will occur in the circuit. The voltage drop is proportional to the line length. Considering the normal startup and operation of the motor, lighting equipment and other loads with large impulse current, the code for acceptance of construction quality of electrical engineering in buildings (GB50303-2002) stipulates that the steady-state voltage drop of power, electric heating or lighting feeders should not exceed 3%, and the total voltage drop, including feeders and branches, should not exceed 5%. In case of short circuit, the temperature of the conductor rises rapidly. However, due to the thermal characteristics of cable insulation, sheath and coating materials, the cooling process of the conductor is slow after the short circuit is eliminated. Failure to pay attention to the thermal stability of the cable will cause permanent damage to the cable insulation due to the deterioration of the insulating material, which may be accompanied by smoke and combustible gases. If there is enough heat, these gases will ignite and cause a serious fire. Even if the degree is not so serious, the insulation or sheath of the cable may expand, resulting in voids, which may lead to failure. This is particularly serious for high-voltage cables. In addition to thermal stress, thermal expansion also generates mechanical stress in the cable. Due to rapid heating, these stresses may cause unwanted cable movement. However, the new cable strengthens the binding and sheath, which significantly reduces the impact of this stress. When selecting and using cables within the predetermined temperature range, it is generally unnecessary to pay attention to their mechanical properties, unless the cables are very old or lead coated. In case of short circuit or large impulse current, the single core cable will bear the mutual repulsion or attraction between cables. In order to prevent the damage of cables caused by this movement, the cables laid on the cable support or cable tray should be fixed.
The model content of the cable includes its use category, insulation material, conductor material, armor protective layer, etc. The number of cores, cross-section, working voltage and length are also indicated behind the cable model. The meaning of cable model is shown in Table 1.2, and the meaning of cable outer sheath code is shown in table 1.3.
The selection of conductor section shall be carried out according to the following requirements.
① The cable core temperature under the maximum working current shall not exceed the allowable value determined according to the service life of the cable.
② The thermal effect produced by the action time of the maximum short-circuit current shall meet the thermal stability conditions.
③ The voltage drop of the connecting circuit under the action of the maximum working current shall not exceed the allowable value of the circuit.
④ Economic section should be selected for long-distance high current circuit, which can be based on the principle of “minimum annual cost”.
⑤ The section of aluminum core cable should not be less than 4mm2.
The selection of conductor section shall meet the requirements of mechanical strength, working current and allowable voltage drop at the same time. Among them, the requirement that the conductor bear the lowest mechanical strength refers to the self weight of the conductor, wind, snow, ice, etc. without breaking the wire; The conductor shall be able to meet the needs of the load to pass through the maximum current of normal operation for a long time; The voltage drop on the conductor shall not exceed the specified allowable voltage drop. Generally, the voltage drop of the public power grid shall not exceed 5% of the rated voltage. Improper selection of power cable core section will affect reliable operation, shorten service life, endanger safety and bring economic losses, which can not be ignored. The continuous working temperature of cable core is related to the heat-resistant life of cable insulation. Generally, the service life is 30-40 years, and the allowable working temperature is determined according to the characteristics of different insulating materials. When the working temperature is higher than the allowable value, the corresponding service life is shortened. For example, if the working temperature of cross-linked polyethylene is increased by about 8 ℃ compared with the allowable value, the corresponding ampacity is increased by 7%, and the service life is reduced by half. The continuous working temperature of the cable core also affects the reliability of the conductor connection of the cable core, which needs to be determined by considering the actual possible conductor connection process conditions of the project.
The thermal effect produced by the short-circuit current acting on the cable core meets the transient physical performance that does not affect the insulation of the cable, maintains the normal use, enables the conductor connection containing the cable joint to work reliably, and does not endanger the normal operation of the cable structure under the action of the electrodynamic force for the split phase turnkey cable, which is collectively referred to as meeting the thermal stability conditions. Otherwise, the oil paper insulation lead package will be cracked, the insulation paper will be burnt, the cable core will be ejected, and the cable end will smoke and other faults will occur.
The evaluation method of the principle of “minimum annual cost expenditure” is based on the “Interim Regulations on economic analysis of electric power engineering” issued by the former Ministry of water and electricity 1982 Dian Ji Zi No. 44 document. The expression of annual cost expenditure B recommended in this document is as follows:
Where, Z is investment; N is the annual operating cost.
The coefficient is based on taking the economic service life of 25 years and the number of construction years as one year. The reason for limiting the use of small cross-section of aluminum core is that it is easy to be damaged and broken when it is less than 4~6mm2 in past engineering practice. For high-voltage single core cables above 35kV and the use mode of cables that cause additional heating and poor heat dissipation, it is generally appropriate to directly determine the allowable ampacity by calculation or testing.
The influence of ambient temperature on the conductor is relatively large, which should be fully considered in the solar lighting system. When the skin temperature of the cable is about 50 ℃, the moisture near the cable will gradually migrate and become dry, resulting in an increase in thermal resistance, and a vicious cycle of cable core operating temperature exceeding the rated value will occur, affecting the acceleration of cable insulation aging, resulting in insulation breakdown accidents. When the cable route passes through sections with different heat dissipation conditions, the working temperature of the cable core in each section under the same cable core section may be different.
In the solar lighting system, the conductor section can be initially selected according to the safe carrying capacity of the conductor. The conductor must be able to withstand the temperature rise caused by the long-term passage of load current, and the insulation of the conductor cannot be damaged due to overheating. The maximum current allowed by the conductor to pass for a long time is called the safe current carrying capacity of the section. The safe current carrying capacity of the same conductor section is different under different laying conditions. For example, the safe current carrying capacity of conductors with the same cross-section laid in the overhead line is larger than that laid in the pipe. The pipes used for crossing pipelines and the number of pipelines all affect the safe carrying capacity. Table 1.4 shows the safe current carrying capacity of plastic insulated wire. Table 1.5 shows the safe current carrying capacity of rubber insulated wire. The calculation method is based on the actual current of the load, and then check the safe ampacity meter according to the current to get the conductor section. According to the length, sectional area and maximum current value of the branch, the resistance and voltage drop of the conductor are calculated. If the voltage drop value meets the required range, the conductor section can be used. If the voltage drop is too high, the cross section of the conductor should be re selected.
In addition, when selecting conductors, it should also be noted that the minimum cross-section takes into account the difference in environmental temperature, the difference in soil thermal resistance coefficient during direct burial laying, the influence of multiple cables in parallel, and the influence of sunlight without sunshade during outdoor overhead laying. Due to the large difference in implementation, it can be adjusted as appropriate. For cables with operating temperature greater than 70 ℃, factors shall be considered when calculating the continuous allowable ampacity. If the cable is directly buried in dry or wet soil, the soil thermal resistance coefficient should be less than 2.0 ℃ except for the implementation of soil replacement treatment to avoid water migration m/W。 In the low-voltage distribution network, the power loss is very amazing. This paper considers the selection of conductor section in the distribution network from the perspective of loss reduction and energy saving. Under the principle of economy and rationality, appropriately increase the conductor section to reduce the power loss, so as to achieve the purpose of more supply and less loss.
1.3 grounding and lightning protection device
In order to use electricity safely and prevent electric shock accidents, the solar lighting system should adopt grounding devices. Grounding can be divided into working grounding, protective grounding and lightning protection grounding.
① Working grounding: the neutral point of low-voltage power grid shall be directly grounded, and the system grounding resistance shall be less than 10 Ω.
② Protective grounding: in order to prevent equipment leakage from endangering personal and equipment safety, all exposed conductive parts of electrical equipment shall be grounded through grounding wires, and the grounding resistance shall meet the safety action requirements of leakage protector.
③ Lightning protection and grounding: in order to prevent electrical equipment from being struck by lightning, lightning rods, lightning wires, lightning arresters and other lightning protection devices shall be grounded, and the grounding resistance shall be less than 10 Ω.
④ Neutral connection: connect the metal shell and metal frame of electrical equipment with the neutral grounding wire.
The grounding wire shall be insulated wire, and the whole wire shall be used, and there shall be no joint in the middle. The shortest distance from the grounding wire of lightning arrester to the grounding wire shall be selected. See table 1.6 to table 1.9 for the selection of protective wire, equipotential bonding wire, grounding wire and grounding body.
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