The design about oversizing when constructing PV plants is contradictory for different unit subjects. On the one hand, they want to improve the DC/AC ratio to enhance the power generation, improve the project revenue, and reduce the system cost of electricity; on the other hand, they are restricted by the grid and land resources, and the contradiction of PV plants development and grid support. However, with the change of cost of various equipment in project construction, the call for increasing the DC/AC ratio is getting higher and higher, but how to design a DC/AC ratio that meets the actual project needs to be considered from several aspects.
Reasons for raising the DC/AC ratio:
First of all, excluding the influence of environmental irradiance on the output power of the module, the theoretical output of the module is reduced after the module attenuation, shading, string mismatch, line loss, etc. The input power to the inverter is reduced. Normally the system loss is about 10%-15%, if the DC/AC ratio is designed according to 1:1, the inverter, transformer, distribution cabinet and other equipment can not reach the rated power operation, resulting in the waste of AC side equipment.
Secondly, the rated output power of the module is the output power under STC standard test conditions. In actual use, due to the different irradiation in different places, the module conversion output power often does not reach the rated output power, so the actual power generated on the DC side does not reach the rated output power, so it is necessary to improve the DC/AC ratio to compensate for the power deviation caused by irradiation.
Advantages of rational configuration of the DC/AC ratio:
For the grid side, improving the DC/AC ratio can extend the full-load operation time of PV power plants, reduce the power fluctuations caused by changes in irradiation, and make the power output of PV smoother and more stable, while also relieving the peaking pressure of traditional energy sources such as hydropower and thermal power.
For users, a reasonable enhancement of the DC/AC ratio can compensate for the loss of the system on the DC side, which on the one hand improves the utilization rate of the equipment on the AC side, and on the other hand extends the full-load working time of the inverter, allowing the inverter to reach full load earlier in the morning and delaying the power drop of the inverter in the afternoon; at the same time, with a certain percentage of energy storage, it can also quickly respond to the grid’s adjustment of frequency and voltage and improve the friendliness to the grid; it can also amortize the cost on the AC side and reduce the project engineering cost.
The method of raising the DC/AC ratio:
The DC/AC ratio can be increased by compensating the loss and active over-sizing. The method of compensating the loss is to compensate the system loss during energy transmission by appropriately increasing the DC/AC ratio, so that the inverter can reach full power operation; the method of active over-sizing is to find the balance point between the increased input cost and the system power generation revenue by actively extending the full load operation time of the inverter to achieve the minimum LCOE.
In practice, upgrading the capacity distribution ratio can be done by adjusting the capacity of the equipment on the DC side and AC side. The first one is that the electric The second way is that when the capacity of the power plant is calculated according to the DC side, the capacity configuration of the AC side is reduced and the capacity of the inverter is reduced to reduce the investment to improve the capacity ratio; the third way is that the capacity of the power plant is calculated according to the AC side, but there is not enough land to install over-allocated components, so the capacity of the DC side can be determined according to the land area first, and then the best capacity ratio is selected according to different light resource conditions to reduce the AC side capacity configuration, thus improving the capacity ratio. DC side capacity, and then choose the best capacity ratio according to different light resource conditions, reduce the AC side capacity configuration, thus reducing investment costs.
Impact of overly inputed inverters :
The inverter is an important equipment for AC-DC system conversion. Improves the DC/AC ratio increase and extend the utilization rate and time of the inverter, which also brings a challenge to the performance and stability of the inverter.
1. Overvoltage capability
During active oversizing, after removing the system losses, the rated power of the inverter will still be exceeded, which will cause the inverter to run overload or will even exceed the operating range of the inverter, reach the inverter overload protection point. The inverter will adjust the operating point so that the module output deviates from the maximum operating point to ensure that the inverter output does not exceed its rated power.
2. Heat dissipasion capability
After the system is over-sizing, the full-load operation time of the inverter is extended. In the scenario where the ambient temperature is high and the heat dissipation condition is relatively poor, the heat dissipation capability of the inverter is required to be stronger, and the inverter needs to have a higher temperature tolerance and raise the threshold of temperature derating.
In order to ensure the stable operation of the inverter in different application scenarios and prolong the service life of the equipment, we need to pay attention to the adequate design and safety margin of the equipment and its components during the selection of the inverter, including the margin of AC and DC side voltage and current, and the margin of life, etc.
Design principles of the DC/AC ratio
The optimal design of the DC/AC ratio of the PV system needs to be determined after technical and economic comparisons, like geographic location, terrain conditions, solar resource conditions, module selection, installation type, layout, performance of the inverter, construction cost, losses from the PV array to the grid connected point, grid demand and other factors, after technical and economic selection.
The optimal analysis of the DC/AC ratio should be calculated using the trial algorithm, and the DC/AC ratio should be selected from low to high for multi-point calculation to reach the optimal DC/AC ratio.