Features of operation of the grid connected photovoltaic power station with a capacity of 10 kW

Photovoltaic power station (PPS) operating function (without redundancy) with a nominal capacity of 10 kW connected to the low voltage electrical network established by "Zhejiang Chint Electrics Co Ltd" (PRC) under Tashkent conditions is defined. Operation parameters and characteristics of the PPS and parameters at the output of the network inverter are given. The deviations of voltage of each phase from the standard nominal voltage at the point of electric network transmission are studied. The analysis of the results of the evaluation of the power generation of the PPS for the conditions of clear weather and clear cloudiness was carried out. According to the monitoring data for the winter period is 2211,5 kW·h. The problems of PPS connected to the low-voltage network, connected to the loss of electric power with the account of influence of external factors and reliability of stable voltage and frequency in a permissible range are revealed.


Introduction
Energy shortages can be partially filled in many countries, including Uzbekistan, by using renewable sources of energy which are environmentally sustainable and affordable to consumers. In this regard it is advisable to use solar energy to produce electricity on an industrial scale, considering the substantial potential of solar energy in Uzbekistan. This is verified by the development and prospective photovoltaic solar station (PPS) programs, both abroad and in Uzbekistan [1][2][3].
The advancement of solar energy in Uzbekistan will undoubtedly contribute to addressing the country's energy security, social problems, environmental stability and improving the quality of life of the population. Unfortunately, the share of electricity generated in Uzbekistan using solar energy does not exceed 1 percent. The main obstacle to the wide introduction of solar energy is the lack of incentive measures for the population and industrial consumers, typical for China, Germany, Japan, Spain and the United States, and as a result, high capital investments, which fully fall on the shoulders of the consumer. Another significant problem is the lack of enterprises for serial production of photovoltaic converters, panels and component equipment based on modern technologies [4].

Method
The development of technical profiles for solar photovoltaic stations based on higher education institutions creates opportunity for students and researchers to collect experimental data on the functioning of stations, evaluating potential technological solutions.
Using analytical measurement methods with the aid of programs, monitoring and control of photovoltaic battery (PB) development parameters, elimination of photovoltaic characteristics, their study according to meteorological parameters.

Results and discussion
A 10 kW power plant was constructed at the Heliopolygon of the Power Engineering Faculty of Tashkent State Technical University on the initiative of "Zhejiang Chint Electrics Co Ltd" and small enterprise "Chint Distribution Uzbekistan" (Fig. 1), and a scientific and educational laboratory "Inspection of electrical devices and creative control of their control and measuring mode" was developed. PPS consists of 2 parallel groups of PVBs electrically connected to each other, each containing 20 batteries connected in series. PPS is placed at an angle of 460 in the south direction. However, it is also worth considering the seasonal change in the angle of incidence of solar radiation. For each latitude, in which the region is located, there is an optimal angle of inclination of the PVB installation. The PVB rises above the Earth's level to create space for free air circulation. The distance between the horizontal plane and the PVB is greater than ≥100 cm.
The article addresses issues relating to optimum PPS configuration and coordination of components and equipment with electrical low voltage network.
The distance between the two rows when mounting the PPS is of great importance for a PPS which is not fitted with a tracking system and which maintains a static location throughout the year. The greater the distance between the rows, the less likely it is that the first row will be shaded above the horizon at a low altitude of the Sun, thereby ensuring reliable electricity generation and transmission to the grid. At the same time, an increase in this distance leads to an increase in the area occupied by the PPS; therefore, finding the maximum and minimum values of this distance contributes to an increase in the average annual value of the PPS efficiency.
Calculations have been made to find the optimal distance between the rows of PVB.
where α is the angle of the Sun's height above the horizon; β is the angle of inclination of the PVB to the horizon; d is the optimal distance between the PVB rows; L is the length of the PVB; S 1 is the projection of the PVB length; S 2 is the projection of the line of the solar radiation angle of incidence. ( ) (2) were -is the northern latitude of the terrain; -the declination angle of the Sun for a given day.
Let the raw data be: If ; ; , we have still conducted experimental studies to measure the length of the PVB shadow on June 22 nd and December 22 nd 2019 at 12:22 hours. The values of the minimum and maximum distance between the rows of PVB with a fixed location on the support structure are given in Table  1. Therefore, Table 1 shows that the value of the optimal distance between the PVB rows, measured and calculated using the formula (1), differ slightly ~2-2,5%. According to the obtained results, it can be concluded that the proposed measurement technique and calculation of the optimal distance provide high accuracy.
It is possible to draw a conclusion that at stationary placing of PVB, when an angle of slope is equal to value of northern latitude of the terrain, it is necessary to consider distance between rows of PVB. If at designing not to consider optimum distance between rows of PVB, then increase of distance form shading of the bottom sites of the following rows, accordingly leads to decrease of efficiency of PVB and the occupied superfluous area of the Earth. In the first case, the maximum power point of the shaded PPS is shifted to the negative voltage zone and opens a bypass diode, which is connected in parallel to each battery to prevent the shift of the operating point of the battery in the energy consumption zone and, as a consequence, failure of the PPS.
In addition, a majority of power grids are unable to cope with the excessive supply of electricity generated by a photovoltaic network. The generation of electricity from renewable energy sources is intermittent. Therefore, integration of PPS into the power system without damage to the quality of electric power is a more complicated task [6][7][8][9].
Electricity quality means the degree to which the voltage and frequency in the grid correspond to their normalized values. Decrease in power quality is defined as any power problem that manifests itself in deviations in voltage, current and/or frequency that result in failure and/or malfunction of end user equipment. Indicators of electric power quality related to slow changes of electric power supply voltage are negative δU     According to the monitoring data (Fig.4) (Fig.5) that with a decrease in ambient temperature, the load consumed increases in the local electrical network, respectively, resulting in a significant decrease in the voltage deviation of each step from day time. At the point of the low voltage network, sharp fluctuations of positive and negative voltage differences will lead to a change of the normalized frequency value of 50 ± 0,2 Hz. The interval between ( ) is -6,5÷-1,5 percent, according to the results. The indication (Fig.6) on 26.12.2019 on the electric power generation by the grid PPS was ~7 kW·h.

Conclusion
The following factors may pose problems in the operation of a solar photovoltaic plant connected to the low voltage network: -The conversion efficiency of solar cells is still low and operating conditions differ considerably from standard ones (AM 1.5, 1000 W/m 2 solar flux density, 25 0 C solar cell temperature) [12][13][14]; -Lack of PPS solar tracking system. Usually, high-power PPS s is not equipped with PB orientation mechanisms taking into account seasonal variations; -Extreme temperature conditions leading to a decrease in the power and efficiency of photovoltaic systems up to 40 percent in the summer months of the year [15]; -Relatively high atmospheric pollution by solid and gaseous particles, which leads to a decrease in the power of the PPS, as well as ice, frost and PVB [16,17]; -Losses at conversion of direct current into variable. The efficiency of network inverters is not stable and depends on the power of solar energy flow. At low density of solar radiation flow the network inverter may switch off; -The quality of electrical energy generated by power plants based on PPS should meet the requirements of GOST 32144-2013; -Not accurate design at installation of PPS can lead to formation of partial shading between PPS rows, which cause a decrease in efficiency and some cases of PPS failure.