Abstract: With the rapid growth of distributed photovoltaic installed capacity, the problem of grid backflow is becoming increasingly prominent. When the photovoltaic power generation exceeds the local load power, excess electricity will flow into the grid, forming a reverse flow. This not only leads to an increase in voltage in the power grid system, affecting the quality of power supply, but may also damage equipment on the user side and photovoltaic power generation side, threatening the safe and stable operation of the power grid system. In addition, the "Management Measures for the Development and Construction of Distributed Photovoltaic Power Generation" issued by the state proposes that large-scale industrial and commercial distributed photovoltaics should generally choose the mode of all self use. According to this policy, the demand for anti backflow in large-scale industrial and commercial distributed photovoltaic projects has emerged.

Keywords: distributed photovoltaics; Spontaneous self use; Master slave anti backflow protection; GOOSE Communication

1 Project Overview

The distributed photovoltaic project of Fengcheng Qujiang Coal Development Company is located in Yichun City, Jiangxi Province, with an installed capacity of 5.8MW. It adopts a spontaneous self use and surplus electricity not connected to the grid mode.

The distributed photovoltaic project of Fengcheng Qujiang Coal Development Company is a 35/10kV project. The 35kV side adopts a dual power supply single busbar segmented system, with one incoming line from the State Grid Xian Curve incoming line and the other incoming line from the user's own power plant switch curve incoming line. This project has set up one 10kV grid connection point, which is equipped with three photovoltaic boost box transformers. The distance between the fairy curve incoming line and the grid connection point is 100-200m, and the distance between the electric curve incoming line and the grid connection point is 6 kilometers. For the connected photovoltaics, the project requires self use and no surplus electricity to be connected to the grid. The backflow detection point is set at the 35kV Xian curve incoming line of Qujiang substation and the Xian line incoming line of the power plant substation. Based on the above basic conditions and the original construction drawing design, the following is developed to achieve photovoltaic grid connection without grid connection through anti backflow master-slave machines.

2 Project Requirements

Measure the power at the two incoming lines of the fairy curve and the electrical curve. When the power of the grid is too low, gradually trip the low-voltage side switches of three photovoltaic boost box transformers; When the power of the grid is gradually restored, manually close the low-voltage side switches of the three photovoltaic boost box transformers.

3 Product Solutions

This project adopts the AM5SE-PV series master-slave anti backflow protection according to user requirements, and achieves anti backflow rigid tripping through GOOSE communication scheme. The detailed scheme is as follows.

Figure 1 Product Configuration of Fengcheng Qujiang Coal Project

Figure 1 shows the product configuration diagram of this project. One anti backflow host, 1 # AM5SE-PVM-GOOSE, is installed on the incoming line of the Xian power line in the power plant substation to collect the incoming current and voltage, and calculate the power Ppcc1; Install one anti backflow host 2 # AM5SE-PVM-GOOSE on the 35kV Xian curve incoming line of Qujiang Substation to collect incoming current and voltage, and calculate power Ppc2; Install one anti backflow slave AM5SE-PVS-GOOSE on the low voltage side of the booster box transformer for opening and closing the low voltage side switch. A local area network is formed between the anti backflow host and the anti backflow slave through an optical port switch via the fiber optic network of the mobile company's base station.

The rigid anti backflow protection strategy is as follows:

1) Power plant substation Xian wire incoming:

Ø 当Ppcc1

Ø 当Ppcc1

Ø 当Ppcc1

2) Qujiang Substation Xian Curve Incoming Line:

Ø 当Ppcc2

Ø 当Ppcc2

Ø 当Ppcc2

4 Project Topology Scheme

1) Scheme topology diagram

Figure 2 Topology diagram of project proposal

2) Anti backflow protection

Ø Product pictures

AM5SE-PVM-GOOSE

AM5SE-PVS-GOOSE

3) Project List

5 on-site installation pictures

The anti backflow host and slave of this project are installed using wall mounted boxes. The on-site installation pictures are as follows.

Figure 2 Photovoltaic Grid Connection Point of Qujiang Substation

Figure 3: Incoming Line of Xian Electric Line in Power Plant Substation

6 Product association plan

In addition to the GOOSE communication scheme mentioned above, the AM5SE-PV series also supports fiber optic direct jump scheme, mainly divided into AM5SE-PVM host, AM5SE-PVS slave, and AM5SE-PVS2 slave. The host and slave are connected through fiber optic connection, and can choose multimode or single-mode fiber optic. Multimode fiber optic is suitable for mains incoming lines and grid connection points with a distance of 100m-1.5km, and single-mode fiber optic is suitable for mains incoming lines and grid connection points with a distance of 1.5km-20km. The AM5SE-PVS slave is suitable for single mains incoming lines, while the AM5SE-PVS2 is suitable for multi mains incoming lines. The following are the application scenarios for single mains incoming lines and dual mains incoming lines:

Single line incoming fiber optic direct jump anti backflow

Dual power incoming line fiber optic direct jump anti backflow

7 Conclusion

The anti backflow master-slave machine introduced in this article can gradually disconnect the photovoltaic boost box transformer during high photovoltaic power generation or heavy load shedding, achieving photovoltaic grid connection without grid connection. At the same time, GOOSE communication scheme and fiber optic direct jump scheme can ensure the collection of current and voltage signals and accurate operation of anti backflow function when the incoming and connecting points of the mains power are far away, which is helpful for the implementation of spontaneous self use and residual power not connected to the grid mode in subsequent large-scale industrial and commercial photovoltaic projects.

Application scenario: When a distributed system is disconnected from the power grid, an "island" that can still carry electricity will appear, which poses a certain degree of danger.

The detection methods for islanding effect are divided into active and passive methods. Passive detection is to determine whether the power grid is disconnected by real-time monitoring of parameters such as voltage, frequency, phase, and harmonics at the grid connected port of the inverter; Active inspection

Measurement refers to actively injecting small disturbances into the power grid and determining whether the power grid exists by detecting feedback signals. This device adopts passive monitoring method and has overvoltage protection, low-voltage protection, high-frequency protection, low-frequency protection, voltage slip protection, and frequency slip protection. Users can input them according to their needs.

AM5SE-PVMandAM5SE-PVS2All support anti islanding protection function.

Application scenario: When the distributed system is disconnected from the power grid, a device that can still load electricity will appear

The phenomenon of 'isolated island' poses certain risks