Grid-connected equipment selection
(1) General principles. The selection of engineering equipment for distributed power supply access to the user’s distribution network should follow the following principles:
① Distributed power supply access system engineering should select equipment whose parameters and performance meet the safe and reliable operation of power grid and distributed power supply.
② The grounding method of the distributed power supply should be consistent with the grounding method on the distribution network side, and should meet the requirements of personal equipment safety and protection cooperation. The neutral point of the synchronous generator directly connected to the grid with a voltage level of 10kV and above should be grounded through the arrester.
③ When the access capacity of the distributed power source of the converter type exceeds 25% of the rated capacity of the distribution transformer in this area, the main fuse switch on the low-voltage side of the distribution transformer should be transformed into a low-voltage main switch, and an anti-islanding device should be installed at the low-voltage busbar of the distribution transformer; the low-voltage main switch and the anti-islanding device shall have an operation blocking function. When there is communication between the busbars, the tie switch shall also have an operation blocking function with the anti-islanding device.
(2) Main wiring selection. The electrical main wiring method of distributed power booster station or output summary point should be determined through technical and economic analysis and comparison according to the planned capacity of the distributed power source, the phased construction situation, the power supply range, the local load situation, the access voltage level and the number of outgoing circuits. The following main wiring methods can be used:
① 220V: Use unit or single bus wiring.
② 380V: Use unit or single bus wiring.
③ 10kV: Use line-transformer group or single-bus connection.
④ 35kV: Use line-transformer group or single-bus connection.
⑤ The distribution station area connected with distributed power sources shall not establish low-voltage connection with other station areas (except for the connection between the distribution room and the low-voltage busbar of the box-type transformer).
(3) Electrical equipment parameters. The parameters of electrical equipment used for distributed power supply access to the distribution network project shall meet the following requirements.
① Distributed power step-up transformer. Parameters should include the number of units, rated voltage, capacity, impedance, voltage regulation method, voltage regulation range, connection group, tap and neutral grounding method. The transformer capacity can be selected according to the actual situation.
② Distributed power transmission line. The selection of the cross-section of the wire of the distributed power supply sending line should follow the following principles:
a. The selection of the wire cross section of the distributed power transmission line should be selected according to the required transmission capacity and grid-connected voltage level, and factors such as the power generation efficiency of the distributed power supply should be considered.
b. When connecting to the public power grid, suitable conductors should be selected according to the planning and construction of the local distribution network.
③ circuit breaker. The selection of circuit breakers for distributed power access system engineering should follow the following principles.
a.380/220V: Distributed power grid connection points should be equipped with circuit breakers that are easy to operate, have obvious breaking indications, and have the ability to break fault currents. The circuit breaker can be selected from miniature, molded case or universal circuit breakers. The breaking capacity of the equipment should be selected according to the short-circuit current level, and a certain margin should be reserved. It should have the ability to reverse the connection between the power supply and the load. Among them, the grid connection point of the distributed power source of the converter type should be equipped with a low-voltage grid connection special switch, and the special switch should have the functions of voltage loss tripping and low voltage blocking closing.
b.35/10kV: Distributed power grid connection points should be installed with easy-to-operate, lockable, with obvious breaking points, grounding conditions, and breaking fault current.
c. When the common connection point of the distributed power supply is a load switch, it should be transformed into a circuit breaker; and the breaking capacity of the equipment should be selected according to the short-circuit current level, leaving a certain margin.
The reactive power configuration of the distributed power access system engineering design should meet the following requirements:
(1) The reactive power and voltage regulation capability of the distributed power supply should meet the relevant regulations of Q/GDW212-2008 “Technical Principles of Reactive Power Compensation Devices for Power Systems” and GB/T29319-2012 “Technical Regulations for Connecting Photovoltaic Power Generation Systems to Distribution Networks”, and should be compared through technical and economic comparisons, and reasonable reactive power compensation measures should be proposed, including the capacity, type and installation location of reactive power compensation devices.
(2) The calculation of the reactive power compensation capacity of the distributed power system should be based on factors such as the power factor of the converter, the reactive power loss of the collecting line, the transformer and the sending line.
(3) For the synchronous motor type distributed generation system, the reactive power calculation can be omitted.
(4) The type, capacity and installation position of the reactive power compensation device configured in the distributed power generation system shall be determined in combination with the actual connection of the distributed power generation system, and the dynamic reactive power compensation device shall be installed if necessary.
The power factor of the grid connection point in the engineering design of the distributed power supply access system should meet the following requirements:
(1) 380V voltage level: Through the 380V grid-connected distributed generation system, the power factor at the grid-connected point should be above 0.95.
(2) 35/10kV voltage level: the power factor of the distributed photovoltaic power generation system connected to the user system and self-generated and used (including the surplus on the Internet) should be above 0.95; the power factor of the distributed power generation using synchronous motors connected to the grid should be above 0.95; the power factor should be between 1 and 0.95 when using induction motors and grid-connected distributed power sources other than photovoltaics.