## 2 PV System Monitoring  
In actual fact, the basic effective monitoring of PV modules requires data on current, voltage, power, temperature and irradiance, etc. It is, however, technically feasible to install sensors for each PV module, but this would inevitably increase the costs of the PV system. Moreso, they are not applicable to large PV systems and raise the question of how to transfer large amounts of data efficiently and quickly to the standlogger. By utilizing fewer sensors or simpler devices to measure a variety of parameters and developing accurate mathematical models to calculate the necessary parameters for performance evaluation, the cost-effectiveness and intelligence of PV system performance monitoring can be improved. For these reasons, this study uses the intelligent commercial polycrystalline PV modules which integrate the digital sensors in the junction boxes in Figure 1 and are made by a manufacturer [21]. The digital sensors delivery string current at MPP, voltage at MPP and temperature in the junction box for each PV module. They have lower costs than the conventional current-voltage sensors, temperature sensors (PT100/PT1000), and are independent of module type and junction box. The sensors can be mounted inside or outside the junction box and are also suitable for other module types and junction boxes. Figure 1: Intelligent PV module. The specification of the used PV modules (AEG AS-P603) [21] under standard test conditions (STC) is shown in Table 1. Each module consists of 60 solar cells connected in series with three substrings connecting to three bypass diodes. Then, they are arranged in many rows of PV arrays to be a large PV system which comprises 63 modules various interconnected to each other based on the series-parallel topology structure. This PV system is mounted on the rooftop of the Bielefeld University of Applied Sciences (Campus Minden) and is made up of three different subsystems, as shown in Figure 2. The installed capacities of the individual PV subsystem ranges from 1 kWp to 5.8 kWp. All subsystems are ground mounted with a fixed tilt of 30\({}^{\circ}\) or 10\({}^{\circ}\). The common three phase grid-connected PV inverters (one inverters with 10 kW and one inverter with 15 kW) [22] are used for the three subsystems. Each inverter connected the subsystems has three maximum power trackers. Meanwhile, the RT1 smart rooftop monitoring system [23], including a tilted irradiance sensor (range: 0-2000 W/m\({}^{2}\)) and a temperature sensor PT100 (range: \(-\)20 \({}^{\circ}\)C\(\sim\)100 \({}^{\circ}\)C) connected to the backsheet of PV module, was applied to this system, as shown in Figure 3. Furthermore, a professional wireless weather station was connected to a local WIFI, uploading data to a professional weather database with a resolution of 5 min, which provides constant and reliable measured values for air temperature, horizontal solar radiation, wind speed, wind direction, humidity, rainfall and atmospheric pressure. Thus, a powerful sensors network including the sensors in junction box, meteorological sensors, titled irradiance sensor and backsheet temperature sensor was formed in DC side, as shown in Figure 4. All data from single PV module are collected by the sensors in junction box and transferred via powerline to the string reader and further to the gateway via RS485, which does not require the configuration of more sensor cables. Then, the data continues to be passed over the ethernet network from the Gateway and Raspberry Pi to the local database in a monitoring system. The sampled data with the time interval of 2 min are from 00:00 to 24:00 on each day. All data are saved in a MySQL database. Additionally, the data contains the ID information for system, string, inverter and sensor locations. Up to this point, a smart PV system including the components mentioned above that can automatically measure individual module current-voltage at MPP, temperature in the junction box, weather data, solar radiation and store the data into the local database is created.