- System reliability model and index system
According to the reliability state division of the system, a six-state reliability model can be obtained. Its state transition is shown in Figure 1. The reliability index system of the system is divided into: time index, output state index and overall system index. In order to reflect the system failure, the overall system indicators are divided into failure indicators and operational indicators.
- Power supply reliability
According to the different operation modes of distributed power sources such as photovoltaic systems, their impact on the reliability of the distribution network is also different. If the distributed power source is used as a backup power supply for the distribution network, its access can improve the reliability of the system; if the distributed power source is connected to the system power supply, poor control may reduce the reliability of the system, and vice versa Then reliability can be improved. Since the reliability of the distributed power supply itself is an important factor affecting the reliability of the system power supply during grid-connected operation, the distributed power supply has problems such as instability and low reliability, which is far from the reliability of the traditional power distribution system. Therefore, a separate distributed power supply is generally not adopted.
- Power Quality Requirements
The photovoltaic system is connected to the grid through the inverter, which is easy to generate harmonics and three-phase current imbalance; the randomness of the output power can easily cause grid voltage fluctuation and flicker. The building photovoltaic is directly connected to the grid on the user side, and the power quality may directly affect the user. Safety of electrical equipment.
①Causes voltage fluctuations. The traditional distribution network is generally radial, and the voltage gradually decreases along the direction of the lock line. After the photovoltaic power source is connected, the transmission power on the feeder decreases, which may cause the voltage at each load node along the feeder to be raised. Even if the voltage deviation exceeds the standard, how much the voltage is raised is related to the location and capacity of the photovoltaic system. The power generation of the photovoltaic system changes with the solar irradiance, which may cause voltage fluctuations and flicker in the distribution line. Load changes superimposed together, will cause greater voltage fluctuations and flicker.
②Injecting spectral waves and DC external quantities, since the energy conversion of the photovoltaic system is interspersed and unstable, and the photovoltaic system is connected to the grid through the power electronic inverter, harmonic current and DC current components will be injected into the grid.
- Security issues caused by silos
The islanding effect may cause the following hazards or adverse effects: endanger the personal safety of maintenance personnel and users of the distribution network; affect the quality of power supply for users connected to the islanded area (rate and voltage are out of the normal operating range), and damage electrical equipment; The protection devices inside the island cannot be coordinated: after the power supply of the grid is restored, the phases will be out of synchronization; the asynchronous reclosing of the island grid and the main grid will cause operating overvoltage; the single-phase distributed generation system will cause the two-phase load of the system to lack phase power supply.
- Site selection
The site selection of distributed photovoltaic power stations should be comprehensively considered according to the national mid- and long-term development plan for renewable energy, regional natural conditions, solar energy resources, transportation, access to the power grid, regional economic development plans, and other facilities. In the site selection work, we should proceed from the overall situation and correctly handle the relationship with neighboring agriculture, forestry, animal husbandry, fishery, industrial and mining enterprises, urban planning, national defense facilities and people’s lives.
- System lightning protection device design
In addition to the lightning rods, a general grounding grid is planned to be set up throughout the project. Based on the principle of “one-point grounding”, the grounding of photovoltaic modules and brackets, the shells of various high and low voltage electrical equipment, the grounding side of each lightning protection module, and the roof lightning protection belt In order to meet the requirements of the microcomputer monitoring and protection system for the grounding resistance, the total grounding resistance of the whole site except for the lightning rod grounding should meet the requirements of the regulations not greater than 1Ω. In order to ensure equipment and personal safety, and meet the requirements of contact potential and step voltage; the lightning rod grounding system should be set up separately, and the underground distance from other grounding systems should not be less than 3m, and the grounding resistance should not be greater than 10Ω. If the grounding resistance does not meet the requirements, It can be treated by deep burying in aquifer or adding resistance reducing agent.
Read more: How many types of PV grid-connected inverters are there?