The voltage level connected to the distribution network
The grid connection point of the distributed power supply refers to the connection point between the distributed power supply and the power grid, and the power grid may be a public power grid or a user power grid.
The legend of the grid connection point is shown in Figure 1. The user grid is connected to the public grid through the public connection point C. In the user’s internal power grid, there are two distributed power sources, which are connected to the user’s power grid through grid connection points A and B, and grid connection points A and B are not public connection points. There are distributed power sources directly connected to the public grid at point D, which is both a grid-connected point and a public connection point.
For a single grid-connected point, the connected voltage level should be determined after comprehensive comparison and selection in accordance with the principles of safety, flexibility, and economy, according to the capacity of distributed power sources, power generation characteristics, current carrying capacity of the conductor, the acceptability of upper-level transformers and lines, and the distribution network in the user’s area.
When more than one power source is incorporated at the common connection point, their influence should be considered as a whole. In principle, the total capacity of distributed power sources should not exceed 25% of the maximum load in the power supply area of the upper-level transformer. The ratio of the short-circuit current of the distributed power grid connection point to the rated current of the distributed power source should not be less than 10.
Connection point selection
The selection of the distributed power grid connection point is mainly determined by the capacity and voltage level of the connected distributed power supply and the surrounding grid conditions. The principle of determining the grid connection point is that the power supply can effectively transmit power after the power supply is integrated into the grid and can ensure the safe and stable operation of the grid.
There are also differences for different types of distributed power sources. The following specifically discusses the principle of selecting the grid connection point of various distributed power sources.
(1) Wind power generation system. Different from ordinary forms of power generation, wind power generation capacity generally ranges from a few megawatts to hundreds of megawatts. Due to the large intermittent fluctuations of wind power output, direct access to the distribution network line will have a greater impact on the quality of power supply for distribution users. Therefore, it is recommended to adopt the dedicated line access mode and directly connect to the 10kV bus of a nearby 110kV substation through a 10kV dedicated line.
(2) Photovoltaic power generation system. Photovoltaic power generation systems with a capacity of several megawatts to several tens of megawatts and hundreds of megawatts cover a large area and require sufficient sunshine time. Therefore, it is generally chosen to be in the suburbs with sufficient sunlight and sufficient land area. Photovoltaic grid-connected systems smaller than this capacity but greater than 10kW are generally installed on the roofs of large enterprises, large building groups, or built small photovoltaic power stations. In order to avoid excessive neutral line output current, they are generally connected to a low-voltage three-phase power grid. The ones below 10kW are generally small household photovoltaic power generation systems, which are usually installed on the roofs of houses and offices, and can be directly connected to the single-phase power grid on the low-voltage power distribution side.
(3) Micro gas turbines and fuel cells. Because of its small power generation capacity, generally below 50kW, the supply load is generally consumed locally:
① Micro gas turbines and fuel cells for power generation systems with a capacity greater than 10kW are generally installed in important load areas of large enterprises and large buildings to provide reliable energy supply;
② For micro gas turbines and fuel cells with a capacity of less than 10kW, they are generally used in households. They are usually installed in open areas of houses and offices for timely treatment and heat dissipation, and they are directly connected to the low-voltage power distribution side 230V single-phase power grid.