Abstract: With the rapid development of smart grids and the widespread application of new energy technologies, solar energy, as a clean energy source, has gradually become an important energy source for social production and life. Distributed photovoltaic power stations, with their advantages of low construction investment and minimal space requirements, play a crucial role in alleviating electricity supply shortages. Especially under the policy backdrop of China's promotion of energy conservation and emission reduction and green development, their momentum is particularly strong. However, these stations are often located outdoors and are more prone to various operational issues during operation. Due to their wide distribution and scattered layout, the lack of a unified operation and maintenance management system presents numerous challenges: low efficiency, high costs, difficult operations, and suboptimal economic returns. These factors severely impact the motivation of maintenance personnel, making it difficult to ensure the quality of maintenance. Establishing a comprehensive operation and maintenance management system has become a top priority.
I. Project Overview
The Hubei Jingmen Xinzhonghe Textile and other rooftop photovoltaic power generation projects (hereinafter referred to as "this project") are distributed photovoltaic power generation application demonstration projects invested and constructed in response to the national call to "optimize the energy structure and provide cleaner, more reliable energy."
The project is located adjacent to Hubei Jingmen Mingran Decoration Engineering Co., Ltd., utilizing the existing factory roof to construct a distributed photovoltaic power generation project with a total construction scale of approximately 8,340kW. The photovoltaic power generation components are situated on Xintai West Road in Dongbao District, Jingmen City, and are connected to the public power grid via a user distribution station, falling under the management scope of Jingmen Power Supply Co., Ltd.
This article first addresses the need for real-time assessment of the power station's operational status to promptly identify any anomalies; secondly, it focuses on scientifically planning operation and maintenance scheduling to enhance efficiency and quality, while also reducing operational costs.
2. Current Grid Conditions
The existing substation is a 10kV transformer substation, with seven new 10kV transformers installed, boasting capacities of 2000, 1600, 1250, 2000, 630, 1250, and 1000 kVA respectively. The DC electricity generated by the photovoltaic power generation units is converted to AC by inverters and then stepped up to 10kV by the seven new transformers. It is then connected to the 220kV substation via 10kV cables. The total installed and in-process distributed power capacity at this substation is currently 127.1 MW, leaving a remaining capacity of 52.71 MW available for connection, which meets the current demand.
The current power supply diagram is as follows:
3. Technical Solution
This project is connected to the grid through two interconnection points, with installed capacities of 5.99MW and 2.535MW (AC side), respectively, composed of 7 photovoltaic power generation units. The operational model is full grid connection. A photovoltaic power generation system is constructed on the factory building roof, with key equipment such as photovoltaic modules, inverters, and transformers sourced from domestic renowned brands. The DC power output from the photovoltaic power generation system is converted into AC power by string inverters, locally stepped up to 10KV, and then connected to the user side of the factory's 10KV incoming busbar through a switchgear, achieving grid connection. The minimum load of the substation over the past three years averages -0.15MW. Considering the N-1 condition of the main transformer, if the load of the other main transformer does not exceed 80%, the substation can accommodate a distributed photovoltaic capacity of 179.81MW. Currently, a total of 127.1MW of distributed photovoltaic capacity has been connected or is in the process of connection, leaving a remaining capacity of 52.71MW, which meets the current connection requirements. The photovoltaic monitoring system, structured at the station control level, ground level, and architecture, collects and processes information from various equipment in the equipment layer of the photovoltaic power generation system (inverters, islanding prevention protection, fault disconnection devices, power quality monitoring devices, DC screens, etc.) via communication management units or protocol converters. Processed data is then uploaded to the SCADA system and remote control devices. The data from the remote control devices is encrypted and transmitted to the Jingmen Power Supply Company's distribution dispatching via the 4G/5G wireless communication network.
3.1. Step-up Transformers and High/Low Voltage Distribution Equipment
This project is equipped with 7 three-phase AC 2000KVA dry-type transformers. The rated voltage is 10.5±2×2.5%/0.38kV, with a connection group of Dy11. The AC frequency is 50Hz, suitable for outdoor use, and the energy efficiency meets national standards.
3.2. Relay Protection and Safety Automatic Equipment
The main electrical equipment in this photovoltaic power station is equipped with microcomputer protection to ensure information transmission. Component protection is configured in accordance with the "Technical Code for Relay Protection and Safety Automatic Equipment" (GB14285-2006).
Line Protection
This project is a 10kV grid connection. It is recommended to configure the main outgoing switch of the photovoltaic switch station with protection measures, including directional overcurrent protection with three-stage overvoltage and directional lock, three-stage zero-sequence overcurrent protection, overloading protection, and low-frequency load shedding function. This ensures quick disconnection of the line in case of faults, preventing the expansion of the accident area.
2) Frequency and Voltage Anomaly Emergency Control Device
This photovoltaic grid-connection circuit breaker for the project requires a voltage loss tripping function, without a reclosing feature. It can achieve disconnection through low-voltage protection and frequency protection within the inverter, without configuring an independent safety automatic device.
3) Islanding Detection and Safety Automatic Equipment
Inverters equipped with anti-islanding capabilities are being utilized. These inverters must be capable of rapid detection of islanding and immediate disconnection from the grid upon detection. The anti-islanding protection scheme should be coordinated with relay protection configurations, emergency control devices for frequency and voltage anomalies, and low-voltage ride-through.
4) Power Quality Monitoring
A set of Class A online power quality monitoring devices that meet the requirements of GB/T 19862 "General Requirements for Power Quality Monitoring Equipment" has been installed at the public connection points. These devices monitor power quality parameters such as voltage, frequency, harmonics, and power factor, with the power quality monitoring data to be retained for at least one year.
3.3. System Scheduling Automation
This project involves grid connection at the 10kV voltage level, requiring the transmission of information such as active and reactive power, power factor, inverter data, electricity consumption, switchgear and circuit breaker status, power adjustment data, and power prediction data from the photovoltaic power station to the dispatching center via independent information transmission equipment. The site should be equipped with an AGC device capable of receiving and automatically executing active power control commands; it must possess primary frequency regulation capabilities; and the 10kV power generation system should report medium-term, short-term, and ultra-short-term power prediction data.
3.4. Measurement
This project employs a full grid connection operation model, with a total of 2 off-grid points and 1 gateway point, all located at the photovoltaic output sections. The project's measurement configuration principles are as follows.
At the metering point, one three-phase three-wire energy meter with an accuracy of 0.5S and bidirectional metering function is installed to measure the electricity usage between users and the power grid. A GPRS-based power consumption management system is also configured for data collection and user billing.
(2) Install C-grade electricity meters and a GPRS-based power consumption management system at the interconnection points for the collection of power generation data and remote transmission to the marketing system. The specific scale of equipment configuration will be based on the actual on-site conditions.
4. System Architecture
This photovoltaic power plant is equipped with an integrated automation system provided by Acrel Co., Ltd., featuring the Acrel-1000 distributed photovoltaic power monitoring system with functions of protection, control, communication, and measurement. It enables comprehensive automated management of the photovoltaic power generation system and switchgear station. The status signals of the inverters, high-voltage and low-voltage equipment in this project must be connected to this monitoring system.
The PV power plant monitoring system for this project consists of two parts: the station control layer and the ground layer, with an open, layered, and distributed network structure.
The monitoring system is connected to the ground level via Ethernet. The ground level is divided into different functions and systems, scattered relatively independently within inverter areas or transformer cabinets. Even in the event of a station control layer or network failure, the ground level can still independently monitor all local electrical equipment.
The station control layer consists of servers, operator stations, remote stations, and other components connected by a computer network. It provides a human-machine interface for in-station operations, enabling functions such as management and control of substation equipment, forming a comprehensive monitoring and control center. It also features interfaces for communication with remote control centers.
The stratum equipment is composed of intelligent measurement and control units, network system communication units, inverter data collection units, multifunctional electric energy meters, and other components. The main electrical equipment includes microcomputer protection, anti-islanding protection, online electric power quality monitoring devices, fault disconnection devices, multifunctional instruments, inverters, transformer substation measurement and control, etc. It directly collects and processes the original data on-site, transmits it to the station control layer's main monitoring station via the network, and simultaneously receives control operation commands from the station control layer. After validity judgment, interlock detection, and synchronization detection, it finally operates and controls the equipment.
Each photovoltaic power generation unit is equipped with a data collection device featuring wireless transmission capability. The device collects data from each photovoltaic module, inverter parameters, and data from measuring and controlling equipment and smart meters. The data is then packaged and transmitted wirelessly to the monitoring system for surveillance.
5. System Features
5.1 Distributed Photovoltaic Power Station Operation and Maintenance Management
The Acrel-1000 Distributed Photovoltaic Power Monitoring System alarm processing is divided into accident alarms and pre-alarm warnings. The former includes circuit breaker tripping and protective device action signals caused by non-operations, while the latter includes general equipment status changes, abnormal status information, analog quantity exceeding/returning to limits, various components of the computer station control system, and abnormal statuses of local units, etc.
(1) Incident Alarm
When an accident alarm occurs, the public accident alarm will immediately sound an auditory alarm. The monitoring screen indicates a change in position of the device with color changes and flashing, along with a red alarm message displayed. The printer prints the alarm message, and the data forwarding device sends the alarm information to the remote control center. The accident alarm is confirmed manually or automatically.
(2) Alarm Preview
When an alarm is triggered, its handling is similar to that of an accident alarm, with the audio and color of information provided differing from accident alarms. Information can be selectively sent to a distance.
(3) Real-time Events
The real-time event alert list displays serial numbers, occurrence times, alert content, alert levels, and confirmation statuses in separate columns. By default, alert information is displayed in descending order of occurrence time, with different colors indicating confirmed and unconfirmed signals.
(4) Historical Events
Click on the historical event icon to switch to the historical event query interface and view all system events recorded in the historical database.
(Monitoring Interface)
Monitor the current voltage, current, output power, temperature, and status of the inverter.
(6) Photovoltaic Revenue Interface
Our system supports real-time statistics on power generation and revenue from the power station, significantly reducing operation and maintenance costs.
5.2. Guangzi Brand Monitoring
With the advancement of integrated automation technology for substation, traditional light indicator plates are gradually being replaced by virtual light indicator plates in the monitoring backend. This change is primarily reflected in the following aspects:
Traditional Light Indicator Boards: Relying on hardware contacts for triggering, displaying equipment status through lights on a central signal screen. This method is widely used in substation with manned surveillance, but with the widespread adoption of unattended operation technology, its limitations are gradually becoming apparent.
Intelligent Light Signage: In unattended substation operations, the functionality of the light signage is integrated into the monitoring backend system, implemented virtually. The intelligent light signage retains the intuitiveness of traditional light signage while adding features such as information filtering, graded display, and navigation. For instance, by flashing navigation lights to indicate abnormal signals, operators can quickly confirm and address the issues.
6. Conclusion
China's distributed photovoltaic power generation is experiencing explosive growth, with its grid connection model transitioning rapidly from decentralized to scaled-up under the continuous promotion of policy incentives. This trend injects new momentum into the transformation of the energy structure, but also poses new challenges to the safe and stable operation of the power system. The integration of high-penetration distributed photovoltaics triggers a series of technical issues such as voltage fluctuations and reverse power flow, and may also impact the sustainable development of the photovoltaic industry. Notably, the scaled-up grid connection is forcing a profound transformation in power system regulation and control models. To address this, it is necessary to systematically study the key technologies of grid dispatching and operation based on the actual operation needs of the power grid and the characteristics of station management. This will enable the construction of an intelligent and high-performance energy management platform for the power grid, ultimately enhancing the multi-level coordination and balance capabilities of the new power system and improving the efficient absorption of new energy.
Summary: With the rapid development of smart grids and the widespread application of new energy technologies, solar energy, as a clean energy source, has gradually become an important energy source for social production and life. Distributed photovoltaic power stations, thanks to their low construction investment and small spatial requirements, play a significant role in alleviating power supply tension. Especially under the policy background of China's promotion of energy conservation and emission reduction and green development, their momentum is particularly robust. However, due to their long-term outdoor operation, these stations are more prone to various fault issues. Their widespread and scattered distribution, along with the lack of a unified operation and maintenance management system, poses numerous challenges to maintenance work: low efficiency, high costs, difficult operations, and suboptimal economic returns. These factors severely impact the enthusiasm of maintenance personnel, making it difficult to ensure the quality of maintenance. Establishing a comprehensive operation and maintenance management mechanism has become a top priority.
I. Project Overview
The Hubei Jingmen Xinzhonghe Textile and other rooftop photovoltaic power generation projects (hereinafter referred to as "this project") are demonstration projects for distributed photovoltaic power generation applications, constructed in response to the national call to "optimize the energy structure and provide cleaner and more reliable energy."
The project is located adjacent to Hubei Jingmen Mingran Decoration Engineering Co., Ltd., utilizing the existing factory roof to construct a distributed photovoltaic power generation project with a total construction scale of approximately 8340kW. The photovoltaic power generation components are situated on Xintai West Road, Dongbao District, Jingmen City, and are connected to the public power grid through the user distribution station, falling under the management scope of Jingmen Power Supply Company.
This article first addresses the need for real-time assessment of the power station's operational status to promptly identify anomalies; secondly, it focuses on scientifically planning maintenance and operation scheduling to enhance efficiency and quality while reducing operational costs.
2. Current grid situation
The existing user substation is a 10kV transformer substation. Seven new 10kV transformers have been installed, with capacities of 2000, 1600, 1250, 2000, 630, 1250, and 1000 kVA. The DC power generated by the photovoltaic power generation units is converted to AC by inverters and then stepped up to 10kV by the seven new transformers. It is then connected to the 220kV substation via 10kV cables. The total installed and in-process distributed power capacity of this substation is currently 127.1 MW, leaving a remaining capacity of 52.71 MW that can be connected. This meets the current connection requirements.
The power supply diagram is as follows:
3. Technical Solutions
This project is connected to the grid through two grid connection points, with installed capacities of 5.99MW and 2.535MW (AC side), respectively, consisting of 7 photovoltaic power generation units. The operational model is for full grid connection. A photovoltaic power generation system is built on the factory building roof, with key equipment such as photovoltaic modules, inverters, and transformers utilizing renowned domestic products. The DC electricity generated by the photovoltaic power generation system is converted into AC by string inverters and then stepped up to 10KV locally. It is then connected to the user side of the factory's 10KV incoming busbar through a switchgear, achieving grid connection. The minimum load average of the substation in the past three years is -0.15MW. Considering the N-1 condition of the main transformer, if the load on the other main transformer does not exceed 80%, the substation can accommodate a distributed photovoltaic capacity of 179.81MW. Currently, the substation has connected and in-process distributed photovoltaic capacity totaling 127.1MW, leaving a remaining capacity of 52.71MW, which meets the current connection requirements. The photovoltaic monitoring system, structured at the station control, ground, and architecture levels, collects and processes information from various equipment in the equipment layer of the photovoltaic power generation system (inverters, islanding prevention, fault disconnection devices, power quality monitoring devices, DC screens, etc.) via communication management units or protocol converters. Processed data is then uploaded to the SCADA system and remote control devices. The data from the remote control devices is encrypted vertically and transmitted to the Jingmen Power Supply Company's distribution dispatching via the 4G/5G wireless communication network.
3.1. Step-up Transformers and High/Low Voltage Distribution Equipment
This project is equipped with 7 three-phase AC 2000KVA dry-type transformers. The rated voltage is 10.5±2×2.5%/0.38kV, with a connection group of Dy11. The AC frequency is 50Hz, suitable for outdoor use, and the energy efficiency meets national standard requirements.
3.2. Relay Protection and Safety Automatic Equipment
The main electrical equipment within this photovoltaic power station utilizes micro-computer protection to ensure information transmission. Component protection is configured in accordance with the "Technical Code for Relay Protection and Safety Automatic Equipment" (GB14285-2006).
Line Protection
This project is a 10kV grid connection, and it is recommended to configure the photovoltaic switch station's main outgoing line switch with overcurrent protection, including three-phase overcurrent protection that can be reset after overvoltage and directional locking, three-phase zero-sequence overcurrent protection, overload protection, and low-frequency underload reduction function. This ensures quick disconnection of the line in the event of a fault, preventing the expansion of the accident area.
2) Frequency and Voltage Anomaly Emergency Control Device
This photovoltaic grid-tie circuit breaker project requires a voltage loss trip function, without reclosing. It can achieve disconnection through low-voltage protection and frequency protection within the inverter, without installing an independent safety automatic device.
Island Detection and Safety Automatic Equipment
Our inverters are equipped with anti-islanding capabilities, which must include the ability to rapidly detect islanding and immediately disconnect from the grid upon detection. The anti-islanding protection scheme should be coordinated with relay protection configurations, emergency control devices for frequency and voltage anomalies, and low-voltage ride-through.
4) Power Quality Monitoring
A set of A-class on-line power quality monitoring devices, meeting the requirements of GB/T 19862 "General Requirements for Power Quality Monitoring Equipment," has been installed at a public connection point. These devices monitor power quality parameters including voltage, frequency, harmonics, and power factor, with the power quality monitoring data to be retained for at least one year.
3.3. System Scheduling Automation
This project involves grid connection at the 10kV voltage level, requiring the transmission of information such as active and reactive power, power factor, inverter data, electricity consumption, switch and knife switch status, power adjustment data, and power prediction data from the photovoltaic power station to the dispatching center via independent information transmission equipment. The site should be equipped with an AGC device capable of receiving and automatically executing active power control commands; it should also possess primary frequency regulation capabilities. The 10kV power generation system should report medium-term, short-term, and ultra-short-term power prediction data.
March 4th - Measurement
This project employs a full-grid operation model, with a total of 2 off-grid sites and 1 gateway site, all located at the photovoltaic output points. The principles for the metering configuration of this project are as follows.
(1) A three-phase three-wire electric energy meter with a precision of 0.5S and bidirectional measurement capability has been installed at the gateway measurement point to measure the electricity usage between the users and the power grid. A GPRS-based on-site power management system has also been configured for data collection and user electricity billing.
(2) Install C-grade electricity meters and a GPRS-based power consumption management system at the interconnection points for the collection of power generation data and remote transmission to the marketing system. The specific scale of equipment configuration will be based on the actual on-site conditions.
4. System Architecture
This photovoltaic power plant is equipped with a comprehensive automation system provided by Acrel Co., Ltd., featuring the Acrel-1000 distributed photovoltaic power monitoring system, which includes protection, control, communication, and measurement functions. It enables full-function comprehensive automation management for the photovoltaic power generation system and switchgear stations. The status signals of inverters, high-voltage and low-voltage equipment in this project must be connected to this monitoring system.
This photovoltaic power station monitoring system consists of two parts: the station control layer and the ground layer, with an open, layered, and distributed network structure.
The monitoring system is connected to the ground level through Ethernet. The ground level is divided according to different functions and systems, scattered in an independently relative manner within inverter areas or transformer boxes. Even in the event of station control layer or network failure, the ground level can still independently complete the monitoring of various electrical equipment on site.
The station control layer consists of servers, operator stations, remote stations, and other networked computing devices, providing an HMI (Human-Machine Interface) for in-station operations and enabling management and control of substation equipment. It forms a comprehensive monitoring and control center for the entire station and is equipped with interfaces for communication with remote control centers.
The stratum equipment is composed of intelligent measurement and control units, network system communication units, inverter data collection units, multi-functional electric energy meters, and other components. The main electrical equipment includes microcomputer protection, anti-islanding protection, on-line electric power quality monitoring devices, fault disconnection devices, multi-functional instruments, inverters, transformer substation measurement and control, etc. It directly collects and processes the original data on-site, transmits it to the station control layer's main monitoring station via the network, and simultaneously receives control and operation commands from the station control layer. After validity judgment, interlock detection, and synchronization detection, it finally operates and controls the equipment.
Each photovoltaic power generation unit is equipped with a data collection device featuring wireless transmission capability, which collects data from each photovoltaic module, inverter parameters, measuring and controlling devices, and smart metering instruments. The data is then packaged and transmitted wirelessly to the monitoring system for surveillance.
5. System Features
5.1 Distributed Photovoltaic Power Station Operation and Maintenance Management
The Acrel-1000 Distributed Photovoltaic Power Monitoring System's alarm handling is divided into accident alarms and pre-alarm warnings. The former includes circuit breaker trips and protective device action signals caused by non-operation, while the latter includes general equipment status changes, abnormal status information, analog value over-limit/recovery, various components of the computer control system, and abnormal statuses of local units, etc.
(1) Incident Alert
When an accident alarm occurs, the public accident alarm will immediately sound an auditory alarm. The monitoring screen will indicate the device's movement with color changes and flashing, while displaying a red alarm message. The printer will print the alarm message, and the data forwarding device will send the alarm information to the remote control center. Accidents are confirmed through manual or automatic means.
(2) Pre-alarm Alert
When an alarm is triggered, its handling is similar to that of an accident alarm, with the sound and color of information provided differing from the accident alarm. Information can be selectively sent to a distance.
(3) Live Events
The real-time event alert list is displayed with columns for serial number, occurrence time, alert content, alert level, and confirmation status. Alert information is default sorted in descending order by occurrence time, with different colors distinguishing confirmed and unconfirmed signals.
(4) Historical Events
Click the historical event icon to switch to the historical event query interface and view all system events recorded in the historical database.
(Monitoring Interface)
Monitor current voltage, current, output power, temperature, and current status of inverters.
(6) Photovoltaic Yield Interface
Our system supports real-time statistics of power plant generation and revenue, significantly reducing operational and maintenance costs.
5.2. Guangzi Monitoring
With the advancement of substation integrated automation technology, traditional light indicator plates are gradually being replaced by virtual light indicator plates in the monitoring back-end. This shift is primarily reflected in the following aspects:
Traditional Light Signage: Relying on hardware contacts to trigger, it displays equipment status through lights on a central signal screen. This method is widely used in substations with staff on duty, but with the prevalence of unattended operation technology, its limitations are gradually becoming apparent.
Intelligent Light Indicator Plates: In unattended substation operations, the function of the light indicator plates is integrated into the monitoring backend system, implemented virtually. The intelligent light indicator plates retain the intuitiveness of traditional plates while adding features such as information filtering, graded display, and navigation. For instance, with navigation lights flashing to indicate abnormal signals, operators can quickly confirm and address the issues.
6. Conclusion
China's distributed photovoltaic power generation is experiencing explosive growth, rapidly transitioning from decentralized to large-scale grid connections, driven by policy incentives. This momentum injects new vitality into the transformation of the energy structure but also poses new challenges to the safe and stable operation of the power system. The integration of high-penetration distributed photovoltaics triggers a series of technical issues such as voltage fluctuations and reverse power flow, potentially impacting the sustainable development of the photovoltaic industry. Notably, large-scale grid connections are forcing a deep transformation in power system control models. To address this, it's crucial to systematically research key technologies for grid dispatch and operation based on the actual operation needs of the power grid and the characteristics of station management. This will enable the construction of an intelligent and high-performance energy management platform for the power grid, ultimately enhancing the multi-level coordination and balance capabilities of the new power system and improving the efficient utilization of renewable energy.
