1. Access system principles
Considering the power quality and safe and stable operation of the low-voltage power grid, it is necessary to consider whether there are other power sources connected to the grid at the public connection point when the distributed power source is connected to the low-voltage power grid. Different choices of distributed power grid connection points may have different impacts on the stability and security protection of the power grid. The Chinese technical standard first proposes that the principle of determining the grid connection point must generally consider the common influence of distributed power generation, so as to select an appropriate grid connection point and minimize the impact of distributed power generation on the power grid.
In order to prevent the reverse current from having a greater impact on the upper-level power grid, the upper-level power grid needs to make a wide range of adjustments in relay protection settings and other aspects, the power generated by the distributed power supply should be balanced within the distribution area of the same level as much as possible. The technical standard stipulates that the total capacity of the distributed power supply should not exceed 25% of the maximum load in the power supply area of the upper level transformer in principle. On the one hand, the value of the limit value is estimated according to the load peak-valley difference in the area, and the power generated by the distributed power supply can be completely balanced in the power supply area. On the other hand, considering that distributed power generation may develop rapidly in recent years, this technical standard does not need to be updated, and 25% is an acceptable and reasonable range. The capacity mentioned here is the installed capacity of the power supply. Due to the fact that there may be special power supply structure and load characteristics, my country’s technical standards only put forward the requirements in principle. It will have a great impact on the operation of the upper-level power grid.
The power fluctuation of the distributed power source causes the voltage fluctuation at the connection point, which is closely related to the ratio of the capacity of the distributed power source to the short-circuit capacity of the access point. By specifying the ratio of the short-circuit current at the grid-connected point of the distributed power generation to the rated current of the distributed power supply, the impact of the distributed power supply on the operation of the distribution network can be reduced. Through a lot of research and analysis, the ratio of the short-circuit current at the grid-connected point of the distributed power supply to the rated current of the distributed power supply should not be lower than 10.
2. Power quality
The quality of the power generated by the distributed power source includes harmonics, voltage deviation, voltage unbalance, voltage fluctuation and flicker.
The integration of distributed power into the low-voltage power grid has a greater impact on power indicators such as voltage deviation and harmonics. Before grid connection, the distributed power source should carry out the preliminary evaluation of power quality, and the distributed power source should provide relevant technical parameters such as power supply capacity, grid connection method, and converter model required for the power quality evaluation work.
Distributed power sources should have the ability to adapt to grid operation. When the power grid is in normal operation and the power quality meets the requirements of the technical standards, the distributed power supply should be able to operate normally. The purpose of this clause is to prevent the distributed power supply from operating frequently due to the inconsistency between the range of distributed power protection settings and the range of various indicators of grid operation, which affects the normal operation of the power grid. At the same time, the distributed power supply should also meet the requirements of electromagnetic compatibility, and the electromagnetic interference generated by its equipment should not exceed the requirements of relevant equipment standards. Secondly, for the converter type power supply, the converter converts direct current into alternating current and connects it to the power grid. To prevent the direct current component from flowing into the power grid and causing harm to the electromagnetic equipment of the power grid, the direct current component should be within a certain limit. Regarding the value of the DC component, the opinions of domestic converter manufacturers have been widely solicited, and it has been determined that when the converter type distributed power supply is connected to the grid for rated operation, the DC current component fed to the grid should not exceed 0.5% of its AC rated value.
The power quality indicators in the technical standards refer to the provisions of the latest edition of the national standard on power quality. The national standard does not specify the harmonic current of 220kV voltage level, and its harmonics are implemented with reference to the 10kV voltage level standard. When the voltage fluctuation and flicker value of the grid connection point of the distributed power supply meet the requirements of GB/T12326 “Power Quality Voltage Fluctuation and Flicker”, harmonics meet GB/T14549 “Power Quality Public Grid Harmonics”, interharmonic values meet GB/T24337 “Power Quality Public Power Grid Harmonics”, three-phase voltage unbalance meets GB/ T15543 “Power Quality Three-phase Voltage Unbalance”, the distributed power supply should be able to operate normally. The quality of the power generated by the distributed power source should meet the relevant national standards in terms of harmonics, power supply deviation, voltage unbalance, voltage fluctuation and flicker. The converter type distributed power supply connected to the grid through the voltage level of 10 (6) kV ~ 35kV shall be equipped with a class A power quality online monitoring device that meets the requirements of GB/T19682 “General Requirements for Power Quality Monitoring Equipment” at the public connection point. The historical data of power quality monitoring shall be kept for at least one year.