The photovoltaic modules in the distributed generation system or photovoltaic power station are connected in series and parallel to the combiner box, and then connected to the DC power distribution cabinet through the combiner box. The photovoltaic array with a capacity of 1MWp in the photovoltaic power station covers an area of about 25 acres. The distance between the photovoltaic modules and the combiner box, between the combiner box and the inverter is large, and the amount of DC cables is large. The loss is relatively high. On the other hand, the distances from different modules to the inverter are different, and the circuit resistance and output current of each PV module circuit are different. When the PV inverter operates in the maximum power tracking mode (MPPT), only A working voltage will inevitably cause the actual operating point of many module strings to deviate from the optimal operating point, resulting in the failure of photovoltaic modules to output at the maximum power, resulting in output power loss.
To increase the power generation of photovoltaic power stations, we can start from improving the temperature coefficient of photovoltaic modules, optimizing the inclination angle of photovoltaic modules, improving inverter efficiency, reducing equipment failure rates, and optimizing the layout of photovoltaic power stations. Among them, reducing the line loss and improving the MPPT tracking effect is one of the most worthy of attention in the process of photovoltaic optimization design. The designers of photovoltaic power stations are also constantly optimizing the wiring scheme. For example, the cables between the primary combiner box and the DC distribution cabinet are selected according to the distance, so that the total voltage drop from each string of components to the inverter is close to Consistent to ensure that the inverter tracks the maximum power point of the module. However, this study only mentioned the optimal design of the secondary bus cables. In fact, in the photovoltaic array, the primary bus cables from the photovoltaic modules to the combiner box are far more than the secondary bus cables, and the distance difference is also larger.
Aiming at the characteristics of long distance and large amount of cables in photovoltaic power generation systems, such as photovoltaic module strings, combiner boxes, inverters, etc., through analysis of line loss and line voltage drop, on the basis of satisfying economy, this book proposes a The differentiated configuration technology of DC bus cables with large distance difference adopts first-level bus cables and second-level bus cables with different cross-sections according to the distance, which improves the energy utilization rate of the photovoltaic power generation system and increases the output power of the photovoltaic power generation system.
- Conventional method of cable selection
The connection diagram of the photovoltaic power generation system is shown in Figure 1. Several photovoltaic module strings are connected in parallel to the same combiner box through the first-level collector cables, and several combiner boxes are connected in parallel to the DC power distribution cabinet through the second-level collector cables. The DC distribution cabinet is connected to the inverter through a very short DC cable. Although the photovoltaic power generation system using thin film photovoltaic modules has three-level bus cables, the analysis method is the same as that of the photovoltaic power generation system using crystalline silicon photovoltaic modules. Therefore, this book only analyzes the photovoltaic power generation system composed of crystalline silicon photovoltaic modules.
In Figure 1, one inverter is connected to N combiner boxes, and each combiner box is connected to M photovoltaic module strings. Define the resistance on the secondary cable connecting the DC distribution cabinet and the nth combiner box as Rbn (n=1, 2…, N), and the resistance between the nth combiner box and the connected mth PV module string. The resistance on the first-level bus cable is Rnm (m=1, 2…, M), the inlet voltage of the DC power distribution cabinet is Ux, and the terminal voltage of the combiner box is U. , the PV module string outlet voltage is Um, the current on the secondary cable connected to the nth combiner box is Iw, and the current on the primary return cable connected to the mth PV module string is lm. In the DC bus cable design of photovoltaic power stations and distributed photovoltaic power generation systems, the loss caused by the cable configuration is not only the line loss, but also the power loss when the photovoltaic modules are not working at the maximum power point. For inverters with only one maximum power tracking (MPPT) input, the distance from the PV module string to the inverter is different, the cable length is different, and the line resistance is different, resulting in different line voltage drops, so there must be some PV modules that cannot be Work at the MPPT power point, thereby losing part of the output power. Under the circumstance that it is difficult to increase the conversion efficiency of photovoltaic modules by 0.1%, by optimizing the configuration of the DC bus cable, the line loss can be reduced, the output voltage consistency of the photovoltaic module string can be improved, and the output power of the photovoltaic modules can be improved. Economics has strong practical significance.
The selection of DC bus cables generally requires the line voltage drop to be less than 2% and the power loss to be less than 2%. Combined with the layout characteristics of photovoltaic power stations and distributed power generation systems and the selection method of economical current cross-section in the national standard, the DC bus is initially selected. Specifications of the cable. The main line loss is the DC line loss between the PV module string and the DC power distribution cabinet. The voltage loss Unm of the mth PV module string of the nth combiner box is calculated as:
Taking a combiner box and the connected first-level bus cables, second-level bus cables, and photovoltaic module strings as a photovoltaic power generation module, it can be seen from the formula (Figure 2) that the voltage drop is determined by the first-level bus cable voltage drop and the second-level bus cable drop. The voltage drop of the bus cable consists of two parts.
The more the number of photovoltaic modules connected to the combiner box, the greater the voltage drop on the secondary DC link cable. Therefore, when selecting the DC link cable, it is necessary to take the photovoltaic power generation module as the analysis object, and according to the photovoltaic power generation module connected to the combiner box in the actual project. The number of module strings to determine the line voltage drop of each PV module string.
Similarly, when calculating the DC line loss, the photovoltaic power generation module should also be taken as the analysis object, and the line loss Pa of the photovoltaic power generation module should be determined according to the number of photovoltaic module strings connected to the combiner box in the actual project.
is (B. R) + Ra (]lm)
In the actual project, according to the above-mentioned line voltage drop and line loss indicators, combined with the economical cable section selection method, the primary selection of DC bus cable specifications is realized. At present, the photovoltaic power station is equipped with cables. The first-level bus cable adopts a cable with a minimum cross-section of 4mm², and the second-level bus cable adopts a minimum cable with a cross-section of 50mm².
- Differentiated configuration of DC bus cables
From Figure 1 and Figure 2, it can be seen that when the output current of the photovoltaic modules is not much different, reducing the line loss requires reducing the line resistance, while improving the consistency of the output voltage of the photovoltaic module string requires connecting the same The difference between the line voltage drops on the PV module string branches of the inverter is as small as possible. However, considering the economy, it is not possible to use large-section cables for all DC bus cables. Therefore, under the condition of satisfying the basic voltage drop and line loss, configure the cables according to the distance between the PV module string and the combiner box, the combiner box and the inverter. When it is far away, use a bus cable with a larger cross-section. This differentiated configuration technology of DC bus cables with large distance difference not only meets the requirements of economy, but also appropriately reduces line loss and improves the consistency of line voltage drop, thereby improving the consistency of output voltage of photovoltaic module strings.
Taking the first phase project of a photovoltaic power station in Golmud, Qinghai as an example, the DC power distribution cabinet of each inverter is connected to a total of 9 combiner boxes, and each combiner box is connected to 12~13 photovoltaic strings, which are connected to the same combiner box The length and resistance value of the primary bus cable are shown in Table 7-1, and the length and resistance value of the secondary bus cable connected to the DC power distribution cabinet are shown in Table 7-2. Convergence cable, scheme 2 means using different specifications of the converging cable according to the length of the cable. According to the second scheme, the main steps to select the first-level bus cable are as follows:
1) Considering the cost and the convenience of procurement, it is difficult to realize that each photovoltaic string and combiner box use cables with different cross-sections, so the first-level collector cables use 3 to 4 specifications. According to the cable length, the 5 cables with the shortest length are grouped into group A, the cables with the same cross-section are used, the slightly longer 4 cables and the longest 4 cables are grouped into groups B and C in turn.
2) Group A selects the cable with the smallest cross-section (4mm² specification) as the first-level bus cable according to the requirements of voltage drop and line loss, and group B uses the first-level bus cable with a section larger than 4mm² (usually 6mm² specification), and so on. . Larger cross-section cables are also available, but are less economical.
3) Fine-tuning the cable section. No. 9 in group B uses a 6mm² cross-section cable with a resistance value of 0.59Q, which makes the resistance difference of the 13 first-level bus cables reach 0.57Q, while the resistance value of the 10mm² cross-section cable is 0.34Ω, and the resistance value of the 13 first-level bus cables is 0.34Ω. 0.5Q, so the No. 9 first-class bus cable adopts the cable of the same specification as the C group.
4) The configuration method of the secondary bus cable is similar to that of the primary bus cable.