Abstract: Distributed photovoltaic power generation refers to the construction and operation of power generation facilities near the user's site, with the majority of operations being self-generated and self-used, with surplus electricity fed into the grid. Some projects adopt a full-grid connection model. The advantages of full-grid connection for distributed photovoltaic power generation include maximizing the power generation capacity of the system and enhancing its utilization efficiency. Developing distributed photovoltaic power generation is of great significance for optimizing the energy structure, achieving the "dual carbon" goals, promoting energy conservation and emission reduction, and realizing sustainable economic development.
Keywords: Distributed Photovoltaic; Full Grid Connection; "Dual Carbon Goals"
1. Overview
As a major global energy consumer, our country is actively advancing the "double carbon" goal with large-scale and explosive growth in the installed capacity of new energy. [1] The traditional power system is transitioning towards a high proportion of new energy power systems. The large-scale integration of distributed photovoltaics will challenge the distribution network with issues such as decreased power quality, insufficient reliability and safety in power supply, and reduced regulatory capacity. [2]
Photovoltaic power generation boasts significant energy, environmental, and economic benefits, making it one of the green energy sources. Under average sunlight conditions in our country, installing a 1-kilowatt photovoltaic power generation system can produce 1,200 kilowatt-hours of electricity in a year, reducing coal (standard coal) consumption by approximately 400 kilograms and cutting carbon dioxide emissions by about 1 ton. According to research by the World Wildlife Fund, in terms of reducing carbon dioxide, installing a 1-square-meter photovoltaic power generation system is equivalent to planting 100 square meters of trees. Currently, developing photovoltaic power generation and other renewable energy sources is one of the effective means to fundamentally solve environmental issues such as smog and acid rain.
This project features a 5,980-kilowatt distributed photovoltaic power generation system, integrated into the existing power supply system at a 10KV level. The electricity settlement principle for this project is full grid connection. [3] The project is scheduled for completion and operation by the end of March 2024. All engineering from the enterprise photovoltaic power plant to the public connection point is invested and constructed; an A-grade online power quality monitoring device is installed at the single-point connection of the enterprise photovoltaic power distribution room; and a photovoltaic power generation meter provided by the power supply company is installed at the metering meter within the enterprise power distribution room and at one of the connection points in the enterprise photovoltaic switch room.
2. System Structure
The Distributed Photovoltaic Monitoring System refers to a combination of interrelated units that, through executing specified functions, achieve a given objective. It utilizes computer technology, modern electronic technology, communication technology, and information processing techniques to recombine and optimize the functions of the secondary equipment in substation, and to monitor and measure the operation status of all equipment in the photovoltaic power station.
The system is divided into a three-tier structure: namely, the field equipment layer, the network communication layer, and the platform management layer.
On-site equipment layer: includes AM5SE-K general measurement and control device, APView500 online power quality monitoring device, IPC200 emergency frequency and voltage control device, AM5SSE-IS anti-islanding protection device, and meters, etc. These are used to collect electrical operation parameters and switch states within the substation's distribution cabinets. A DC power supply is configured at the 10kV substation to ensure a good operating environment for the on-site equipment.
The Network Communication Layer includes the ANet-2E4SM intelligent gateway. The gateway actively collects data from equipment at the field device layer, performs protocol conversion, and stores data. Collected data is then uploaded to the communication room's distributed photovoltaic monitoring system platform via the network port. Additionally, the gateway acts as a remote communication device, collecting field equipment data, encrypting it longitudinally through a switch, and then uploading it to the dispatching network.
Platform Management: Distributed Photovoltaic Monitoring System Platform, Guowang Jingzhou Power Supply Company Dispatching and Control Center Platform.
3. Solutions
The project park is powered by a 10kV supply, with one power connection point.
The project is powered for general factory use, supplied according to level 4 load requirements, with a single busbar connection method for the 10kV system. The project adopts the "full grid connection" model, utilizing the existing power point as the photovoltaic high-voltage connection point to the grid. The connection point is equipped with a collection line cabinet, transformer cabinet, SVG cabinet, PT cabinet, metering cabinet, and grid connection outgoing line cabinet. The newly added photovoltaic system is configured with an automation system, which collects and uploads grid connection information in real-time to the local dispatching center's DMS system. The photovoltaic power generation inverter's power voltage is stepped up to 10kV by an indoor step-up transformer and then connected through high-voltage cables to the newly added 10kV photovoltaic high-voltage cabinet, merging with the original 10kV municipal power high-voltage cabinet.
3.1. Summary of the Proposal
This project is a 5.98MWp distributed photovoltaic power generation project, adopting the "all上网" model. In response to the user's distribution system management requirements, comprehensive monitoring and protection of the 10kV switchgear, photovoltaic inverters, AC/DC systems, and more are necessary to promptly detect and address faults, ensuring the reliable operation of the distribution system.
Acrel-1000DP distributed photovoltaic monitoring system has been installed in the photovoltaic power distribution room, which collects real-time data from various photovoltaic power distribution rooms, including relay protection devices, power quality, safety automatic equipment, and photovoltaic inverters, through communication management machines and network switches. This enables the power monitoring and automated management of the entire factory's power supply and distribution system.
A set of DC power systems (with battery backup) and UPS power supplies are installed in each substation, providing a stable and reliable power source for the important equipment such as circuit breakers, secondary equipment, and monitoring hosts in the entire photovoltaic power station.
In each photovoltaic distribution room, a metering energy meter and a grid-connected energy meter are configured in the photovoltaic metering cabinet for billing compensation of photovoltaic power generation.
This project employs a wired method, transmitting data through photovoltaic vertical encryption to the communication room of Jingzhou Power Supply Company and connecting to the main substation. It features a remote communication screen, which is equipped with 1 comprehensive network gateway, 1 switch, 1 GPS clock, 1 forward isolation device, 1 modular intelligent gateway, and three 485 expansion modules. Additionally, there is 1 emergency frequency and voltage control device.
3.2. Technical Solution
3.2.1. Relay Protection and Safety Automation Device Requirements
The configuration of distributed power relay protection and safety automatic equipment complies with the relevant relay protection technical regulations, operation procedures, and accident prevention measures. The set values of the equipment should be coordinated with the relay protection and safety automatic equipment of the power grid to prevent false operation or failure to operate of the relay protection and safety automatic equipment, ensuring the safety of personnel, equipment, and the power grid. For distributed power sources connected at 10kV, the protection and safety automatic equipment configuration should also meet the requirements of "Technical Specification for Protection of Distributed Power Sources Involving the Power Grid" (Q/GDW11198).
Line Protection and Control Device: In the event of a short-circuit fault in the photovoltaic power station's power line, the line protection operates quickly, instantly tripping the corresponding substation circuit breaker to meet the requirement for rapid and reliable fault isolation during a full-line fault. To ensure power supply reliability and minimize the scope of power outages, one set of directional overcurrent protection is installed in each of the high-voltage distribution room's photovoltaic grid connection cabinets, the 1# photovoltaic collection line cabinet, and the 2# photovoltaic collection line cabinet.
2. Capacitor Protection Device: A set of capacitor protection devices has been installed in the 10kV SVG cabinet of the high-voltage distribution room, which provides under-voltage, over-voltage, zero-sequence voltage, and unbalanced voltage protection, as well as abnormal emergency control functions, to trip the capacitor circuit breaker.
3. Emergency Frequency and Voltage Control Device: Achieves load reduction control for substation low frequency and low voltage, with the capability to measure the voltage and frequency of two busbars or two interconnection lines, serving as a basis for discrimination.
4. Public Measurement and Control Device: Suitable for the incoming lines of medium-voltage distribution systems and outgoing lines of large-capacity main transformers, it achieves remote measurement of electrical parameters, remote signaling of switch states, and alarm signals.
5. Island Protection Device: Proposed for scenarios where distributed power sources may continue to operate and supply power to the grid lines after a power outage in the power grid. Island operation poses risks to the safety of both grid maintenance personnel and users, as it disrupts the normal reclosing of the grid. Additionally, during island operation, the voltage and frequency in the grid are uncontrolled, which can cause damage to distribution equipment and user devices. The island protection device should ensure that the power source is promptly disconnected from the grid during line faults, guaranteeing that the reclosing operates correctly.
6. The photovoltaic power station should be equipped with the function of alarm and protection for faults and abnormal operating conditions.
7. Photovoltaic power stations should support the dispatching authority in implementing the "Four Remotes" (remote measurement, remote signaling, remote control, and remote adjustment) application functions.
8. Grid Restoration and Reconnection:光伏发电系统 shall not reconnect to the grid after being disconnected due to grid disturbances until the grid voltage and frequency return to normal operating ranges. Once the grid voltage and frequency are restored to normal, distributed power sources reconnecting via the 10kV voltage level must obtain permission from the grid dispatching authority.
9. System relay protection should utilize secondary windings of dedicated current transformers and voltage transformers. The current transformer should ideally have an accuracy class of 5P or 10P, while the voltage transformer should ideally have an accuracy class of 0.5 or 3P.
10. Direct current (DC) power screens (with batteries) and UPS power supplies are required to be installed within the photovoltaic power station to power new equipment such as protective devices, measurement and control devices, and online power quality monitoring devices.
3.2.2. Scheduling Automation Requirements
The photovoltaic power station, once operational, will be dispatched by the municipal dispatch center and managed by the municipal power supply company. Therefore, it is necessary to establish communication channels and data transmission pathways for scheduling and remote control information between the photovoltaic power station and the municipal dispatch center.
The 10kV photovoltaic power plant requires a distribution automation terminal monitoring system with bidirectional communication capability with the grid dispatching agency. It should be capable of remote monitoring and control functions, able to receive and execute instructions for remote control, paralleling, shutdown, and power generation output from the dispatching end. The system should have group control and remote operation features. Information collection and processing of the photovoltaic power plant should be completed by the monitoring system, with long-distance transmission functions that comply with relevant standard communication protocols. The distribution automation terminal monitoring system of the photovoltaic power plant collects real-time grid connection operation information, including the main circuit breaker status, switch status at the connection point (with remote control capability), voltage and current at the connection point, active and reactive power of the photovoltaic power generation system, photovoltaic power generation output, frequency, etc., and uploads this information to the city power supply company's distribution automation system. When the dispatching end has control requirements for the active power and reactive voltage of distributed power sources, the local monitoring system should be able to receive and execute control commands from the upper-level dispatching master station system.
Stationary timing method: When connecting 10kV distributed power, the timing function should be achievable, which can adopt Beidou timing, GPS timing, or network timing methods.
3.2.3. Electricity metering requirements
Pursuant to the provisions of Q/GDW10347-2016 "General Design Specification for Electric Energy Measuring Equipment," this project requires the installation of boundary metering electric energy meters and grid-connected electric energy meters for billing and compensation of photovoltaic power generation.
3.2.4. Online Power Quality Monitoring Requirements
In accordance with the requirements of Q/GDW10651—2015 "Technical Guidelines for Electric Power Quality Assessment"; inverters of power supply types connected to the grid through voltage levels of 10(6)kV to 35kV should be equipped with an A-grade electric power quality online monitoring device that meets the requirements of GB/T19862 at the public connection point. This is to comply with the communication specifications for the power quality monitoring sub-station of the Fujian Power Grid. The monitoring of electric power quality parameters such as voltage, frequency, harmonics, and power factor should be conducted, and the power quality monitoring data should be retained for at least one year.
Photovoltaic power stations must comply with the requirements of the "Guiding Opinions on High-Quality Service for the Connection of Medium and Low-Voltage Distributed Power Sources to the Power Grid" issued by Hubei Electric Power Company. When distributed power sources connected to the distribution network cause the power quality at the public connection point to fail to meet the relevant requirements, the operator and management entity shall take measures to improve the power quality within the specified time. If no remedial measures are taken or if the power quality still fails to meet the requirements after measures are taken, the power grid operation and management department shall adopt the control measure of disconnecting the distributed power source until the power quality meets the requirements, at which point it can be reconnected to the grid.
The distributed power sources interconnected through a 10KV voltage level should possess both low voltage and high voltage ride-through capabilities. The test curves for low and high voltage ride-through should meet the current technical specification requirements.
Interconnected distributed power sources with voltage levels of 10(6)kV to 35kV should provide operational characteristic test reports to the local power supply company within six months after grid connection. The test results must comply with the requirements of Q/GDW10651—2015 "Technical Guidelines for Power Quality Assessment." The testing points for distributed power sources connecting to the grid are at the connection points, and should be conducted by units or departments with the corresponding qualifications. The testing plan should be reported and filed with the local power supply company's control center prior to testing.
Following the commissioning of new (expanded) harmonic source users, the Marketing Department of the local power supply company organized tests on the users' harmonics. In cases where the assessment indicates non-exceedance but actual measurements show exceedance, the Marketing Department of the local power supply company shall issue a harmonic rectification notice to the user and promptly implement harmonic control measures.
4. System Features
4.1. Real-time monitoring
The Acrel-1000DP Distributed Photovoltaic Monitoring System features a user-friendly human-machine interface, displaying the operation status of distribution lines in the form of a primary distribution diagram. It provides real-time monitoring of electrical parameters such as voltage, current, power, and power factor for each circuit. It dynamically supervises the switching states of circuit breakers, disconnect switches, and earth switches, as well as relevant fault and alarm signals. Additionally, it allows for the design of an overall interface, enabling users to view corresponding photovoltaic components or high-voltage sections for each substation.
2.4. Power Quality Monitoring
In the power quality monitoring graph, you can directly view total effective value of current and voltage, voltage fluctuations, total voltage distortion, forward and reverse active energy, active and reactive power, and other power quality information. These details enable you to monitor the quality of on-site power and promptly implement response strategies.
3.4. Network Topology Diagram
The system supports real-time monitoring of the communication status of all devices connected to the system, providing a complete display of the entire system's network structure. It allows for online diagnosis of device communication status, and can automatically display faulty devices or components, along with their malfunctioning parts, on the interface in the event of network anomalies.
4.4. Curve Query
Users can directly view various electrical parameter curves on the curve query interface, including three-phase current, three-phase voltage, active power, reactive power, and power factor curves.
4.5. Direct Current Screen LED Sign
Users can directly view various electrical parameter curves on the curve query interface, including three-phase current, three-phase voltage, active power, reactive power, and power factor curves.
5. Conclusion
In the context of the "dual carbon" initiative, with the widespread construction of distributed new energy sources, the voltage issues inevitably arise upon the high-penetration distributed photovoltaic (PV) systems connecting to the distribution network. Therefore, a secure and reliable distributed PV monitoring and control solution is essential during the process of promoting distributed PV grid connection. This solution will assist users and the grid in facilitating the orderly and high-proportioned grid connection of distributed PV, strengthen unified control over distributed PV, promote coordinated operation between distributed PV and the large-scale grid, and establish a new scheduling management system for distributed new energy sources that features data transparency, convenient regulation, and energy interaction.
References
Han Xiaoqing, Li Tingjun, Zhang Dongxia, et al. New-Type Power System Planning Issues and Key Technologies under the Double Carbon Goals[J]. High Voltage Technology, 2021, 47(9): 3036-3046.
Chen Guoping, Li Mingjie, Lu Zongxiang, et al. Research on Technological Bottlenecks in the Development of New Energy [J]. Proceedings of the CSEE, 2018, 38(7): 1893-1904, 2205.
Tian Youjia, Dai Bin, Guo Gang, et al. A Review of Distributed Photovoltaic Grid Connection Methods and Data Collection and Control Techniques[J]. 2023, 4(3)
Author Introduction:
Li Xuewei, female, currently employed at Ankeai Electrical Co., Ltd., specializes in research on integrated automation solutions for substation and distributed photovoltaic monitoring solutions. Mobile: 17821733155 (WeChat number same)
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