Application and Practice of High-Precision Solar Resource Monitoring Technology in Photovoltaic Power Plant Area

Solar resource monitoring and evaluation is the foundation of informatization of photovoltaic power station. In 2016, China began to bring in high-precision solar resource monitoring technology which provides reliable basic data for the photovoltaic informatization development. This paper systematically sorts out design basis, monitoring elements and system architecture of high-precision monitoring station, and analyzes operation effect of high-precision solar resource monitoring station from performance of solar radiation meter and prediction results of luminous power, which provide important data support for analysis of power generation efficiency of photovoltaic module, prediction of power generation of power station, evaluation of operation effect of power station, etc.

Compared with traditional solar resource monitoring station, the high-precision solar resource monitoring station has higher requirements for the precision of observation data and the stability of equipment. The equipment shall meet the following requirements: 1) technical requirements of BSRN, WMO standards, ISO 9060 and IEC 60904 international standards, and the requirement of working normally all day [5]; 2) since most of the solar resource monitoring stations are set in the field, the whole set of equipment and installation technology shall meet the requirement of lightning protection, and requirement of protecting from freezing, strong wind and other extreme weather conditions happened once in 30 years [6]; 3) the monitoring station shall be equipped with a power supply system due to the unstable field power supply (battery pack with continuous power supply of more than 48 hours).
The national advanced technology photovoltaic demonstration base in Datong mining subsidence area is a construction project aiming to improve energy structure of Shanxi Province, improve ecological environment quality of Datong subsidence area, promote technological progress and industrial development of photovoltaic power generation, and promote economic structure transformation of Datong City. It was built in 2015. The planned installation capacity is 1000 MWp, which is divided into 13 individual projects, including 7 "Top Runner" program projects, with the scale of a single individual project being 100,000 kW; 6 "Top Runner Program + New Technology and New Model Demonstration" projects, with the scale of a single project being 50,000 kW. Taking Datong Top Runner Base as an example, this paper analyzes technical design basis, monitoring elements, system composition and effect evaluation of high-precision solar resource monitoring station, which provides technical support for construction of China's photovoltaic power plant area and analysis of photovoltaic power generation data.

Design Basis and Monitoring Elements
The construction of solar resource monitoring station shall meet requirements in industries like energy, water conservancy, meteorology, environmental protection, big data, etc., which cover more than 10 national standards, industrial technical requirements and policies in solar energy resource assessment, real-time monitoring, resource level and measurement, precipitation monitoring, ground meteorological monitoring, big data application, photovoltaic power station construction, etc., such as Assessment Method for Solar Energy Resources

Location of Monitoring Station
Monitoring station shall be located in an area with relatively good solar energy resource, relatively gentle terrain and no major engineering geological problems.
The site selection requirements [8] [9] are as follows: 3) The specific installation position of solar radiation sensor shall ensure that there are no or less shelter buildings from east by north 37.5˚ to west by north

Solar Resource Monitoring System
The solar

System Composition
The solar resource monitoring system is mainly composed of two parts: solar energy resource monitoring station and data monitoring platform. Its system architecture is shown as Figure 1. The self-reporting mode is adopted for the working mechanism of the system, which automatically collects, transmits and stores solar radiation and related weather information in intervals of 1 min, 5 min, 10 min, 60 min and 1 day. According to the characteristics of solar resource information collection, and in combination with the current situation of the development of communication transmission technologies, the solar resource measurement system adopts GPRS communication mode, which sends the monitored data to the data monitoring center in real time.

Analysis on the Performance of Solar Radiometer
The core to evaluate the operation effect of solar resource monitoring station is  Table 2.
From below table, the difference between above 4 types of radiometers is small in the aspect of measurement scope and sensitivity, but is significant in the aspect of precision and response time. Especially, the difference in the performance index of operating temperature range is relatively great.
Taking Datong photovoltaic generation leading base as the example, the out-    used in unattended field environment with complex weather, have extremely strong low temperature resistance and high wind resisting coefficient, and can be self-heated to prevent it from freezing in Datong's gelid outdoor environment in winter. Meanwhile, for comparative analysis, 1 Jinzhou Sunshine Technology PC-2-T2 photovoltaic power station environment monitoring system is equipped.
Since the base was put into operation in 2016, the equipment of Netherlands Kipp & Zonen has been operated stably with the operation assurance rate at 100% and there is no failure to operate due to the equipment itself not being able to adapt to the environment. The equipment's response time in winter is lower than 4 seconds. The equipment from Jinzhou Sunshine Technology is unable to operate normally sometimes under Datong's extremely cold condition in winter.

Analysis of Solar Output Power Prediction Result
As the State Grid has continuously improved the accuracy of photovoltaic output power prediction for the past few years [10], the provincial grid companies have successively formulated the standard for punishment on the solar output power prediction, where the solar output power short-term prediction shall be higher than 85% and the accuracy rate of ultra-short-term prediction shall be no lower than 90%. shows that the accuracy rate of short-term prediction of solar output power is 86.5%, while the accuracy rate of ultra-short-term prediction of solar output power is 91%. It is indicated that the prediction of solar output power by the data from high-accuracy monitoring station corresponds to the actual power generation trend.

Conclusions
Taking Datong leading base as the example, by sorting the high-precision solar resource monitoring station technologies systematically, and evaluating the operation effect of solar resource monitoring station, the author drew the following conclusions: 1) As an important component of PV power station, the high-precision solar resource monitoring station plays a prominent role in analyzing the generation efficiency of photovoltaic modules, predicting the generating capacity of power station, evaluating the operation effect of power station, etc.
2) The solar resource monitoring station has high requirement on the precision of observation data and the stability of equipment. The equipment is required to meet multiple industry requirements such as energy, water conservancy, weather, environmental protection, big data, etc. Moreover, high performance of a sunshine radiometer is required.
3) Solar resource monitoring data based on precision is the foundation to evaluate the incident photon-to-electron conversion efficiency of PV power sta-