Intelligence Development Trend of PV Power Plant

With China's return to the market, the development of China's photovoltaic (PV) industry is accelerated. According to statistics of the National Energy Administration, in 2015, China's new PV installed capacity is 15.13 GW and the total PV installed capacity is 43.18 GW, surpassing Germany and ranking at the first place among global PV installed capacity.

As the Internet infrastructure is being improved and related analysis technologies are becoming mature, the "Internet plus" plan has entered a rapid development period and continues to obtain political support. To improve the share of renewable energy, promote clean and efficient use of fossil energy, and accelerate energy market opening and industry upgrading, the National Development and Reform Commission, National Energy Administration, and Ministry of Industry and Information Technology jointly issued the Guidance on Promoting "Internet Plus" Smart Energy Development (hereinafter referred to as "the Guidance") recently.

The Guidance focuses on the following aspects: promote renewable energy production intelligence, encourage the construction of smart wind farms, smart PV power plants, and Internet-based smart cloud platforms, to achieve intelligent renewable energy production; promote the joint development of centralized and distributed energy storage, and promote the deployment of energy storage power plants of an appropriate scale at the centralized new energy plant bases, to achieve collaborative and optimized operations among the energy storage systems, new energy systems, and grids; accelerate the energy consumption intelligence, strengthen the management on the electric power demanding side, popularize intelligent energy consumption monitoring and diagnosis technologies, accelerate the construction of energy management centers of industrial enterprises, and construct Internet-based information service platforms. The Guidance indicates the official debut of "Internet plus", which was proposed in 2015, in the energy industry. Of the same origin as the "Internet +" trend, the "Smart PV" concept once again attracts public attention.

"Smart PV" is an innovation crossing the PV power generation and information technology fields. The innovative product of "Smart PV" is the smart PV power plant. The whole process of the PV power plant deployment, from the construction to O&M, is optimized and innovated. Digital information technologies, Internet technologies, and PV technologies are converged to optimize initial investment, lower O&M costs, improve the electric energy yield, and increase the return on investment (ROI). The smart PV power plant is applicable to various complex scenarios such as large-scale surface power plants, power plants in mountainous areas, Photovoltaic Agriculture, and Photovoltaic Fishery.

1、 Definition of the PV Power Plant

Intelligence means a reduction of manual intervention and implementation of fully automated operation. An intelligent system can automatically discover, diagnose, and rectify faults, thereby improving the electric energy yield, lowering O&M costs, and increasing system revenues.

The intelligence process consists of the following phases: automation, informatization, and intelligence.

Automation means reducing on-site manual operations. The system of the power plant is equipped with anti-damageable component and is maintenance- free. Therefore, no on-site expert diagnosis or manual repair is required. Informatization refers to high-precision intelligent detection on strings, high- speed, high-reliability, and low-cost data transmission, as well as high- reliability storage and monitoring of background data. Intelligence achieves

Intelligence means a reduction of manual intervention and implementation of fully automated operation. An intelligent system can automatically discover, diagnose, and rectify faults, thereby improving the electric energy yield, lowering O&M costs, and increasing system revenues.

The intelligence process consists of the following phases: automation, informatization, and intelligence.

Automation means reducing on-site manual operations. The system of the power plant is equipped with anti-damageable component and is maintenance- free. Therefore, no on-site expert diagnosis or manual repair is required. Informatization refers to high-precision intelligent detection on strings, high- speed, high-reliability, and low-cost data transmission, as well as high- reliability storage and monitoring of background data. Intelligence achieves proactive problem discovery and O&M suggestion provision based on big data analysis. based on the remote mobile O&M system, experts can remotely provide guidance that can be reused for different sites; based on the expert O&M system, preventive maintenance can be implemented; based on the automatic reporting system, data at different layers can be automatically reported.

1.1 Automation

The traditional automation solutions face the following issues:

Generally, devices are deployed in brick houses or inside containers. The device size is large. Therefore, heavy lifting equipment is required for installation, resulting in high installation costs.

A large number of fuses are used in a solar junction box (SJB) for overcurrent protection. The string protection using fuses requires a large amount of preventive maintenance work. With the long-term operation, fuses are aged, resulting in fuse failures. According to statistics, the DC fuse failure rate of a power plant is significantly increased starting from the fourth year of use.

The traditional solutions adopt external fans for cooling. However, the protection class of fans is only IP54/55, which is low. In addition, external fans have various issues such as large noise, poor reliability, and high replacement and maintenance costs. once a fan is faulty, the cooling capability of an inverter is greatly weakened. The inverter equipment room requires periodically cleaning, because accumulated dust or salt spray can cause serious device failures. Experts need to visit the site for troubleshooting. Frequent and lengthy O&M is required, and once a fault occurs, the losses incurred can be huge.

By removing damageable components, integrating complex functions, and removing components and operations that require maintenance, the smart PV power plant has a simple overall architecture. Generally, the smart PV solution makes the power plant construction easier by simplifying the design and purchase links, shortening the construction period, and simplifying the relationship between spare parts. The smart PV solution can improve the system reliability and electric energy yield.

1.2 Informatization

The traditional PV solutions are not satisfying in terms of data communication.

Traditional solutions have large data monitoring granularity, low detection precision, and low transmission reliability. No string monitoring is available for centralized or distributed inverters as well as DC SJBs. The detection precision is only 3%. only current detection is available for the centralized inverters.

The signal transmission reliability of the entire communication network is low, because various components in the traditional power plant array adopt the RS485 protocol for communication, including the DC SJB, DC cabinet, inverter, and box-type substation. Because the devices are provided by different manufacturers and have design differences, a ground potential difference is generated after they are connected, resulting in interface module damages. In addition, the system tests cannot be performed for pre-combined components.

The traditional solutions also have a poor environmental adaptability. Traditional RS485 cables can be easily damaged. When the RS485 cables are arranged together with power cables, they suffer interference. In special deployment scenarios such as wet environment, frozen soil, and arable land,the RS485 cables are prone to damages, resulting in communication interruption. In addition, an optical ring network is generally deployed for intra–power plant transmission. It is difficult to locate a fault when the optical network is faulty, resulting in difficulties in fault identification and rectification.

The smart PV solution supports high-precision intelligent string detection. Advanced power line communication (PLC) and 4G wireless communication technologies are used for intra-array transmission and intra-power station communication, respectively. High- precision Hall sensors are used for string detection. By means of high-frequency differential compensation algorithm and high-precision factory calibration, the detection precision reaches 0.5%. The smart PV solution can accurately monitor two-dimensional information of the string voltage and current. The monitoring precision is more than 6 times that of the intelligent SJB. In addition, the smart PV solution can monitor the string status in real time, automatically generate alarms for anomalies, and accurately locate string faults.

After the PLC technology is introduced to replace the RS485 solution, the transmission rate is greatly increased from 9.6 bit/s (a maximum transmission rate supported by RS485 cables is only 19.2 kbit/s) to 200 kbit/s. In terms of construction, the PLC technology uses AC cables as channels without additional cable arrangement, saving costs on communication cables and engineering by 0.01 RMB/W. However, engineering of the traditional RS485 solution is complex and requires ditches for burying cables. By using AC cables as channels, the PLC solution has a high reliability. When a component is damaged, you only need to replace the faulty component, which provides excellent maintainability.

However, the traditional RS485 solution is prone to incorrect cable connections. If an intermediate link is interrupted, partial network communication is affected. If a cable is interrupted, ditching is required for cable replacement. Therefore, the PLC technology can both save communication cable costs and improve the reliability.

The smart PV solution uses the 4G wireless communication technology for intra-power station communication and to construct an intelligent management network. This advanced technology has various advantages: With respect to the function,the coverage of a single station can reach 80 square kilometers, the transmission delay is smaller than 50 ms, and smooth expansion of the power plant is supported. With respect to the engineering and O&M, optical fibers and ditching operation are not required, and therefore troubleshooting and maintenance become simple. With respect to the management, mobile Internet, smart PV terminals, drones, and remote expert support work in a collaborative manner to ensure efficient O&M.

1.3 Intelligence

The traditional PV solution does not involve the transformation of the top-level intelligence, while the smart PV solution pioneers the intelligence solution. In this intelligence plan, a set of globalized automated O&M system is established, to implement intelligence, and an integrated cloud platform is constructed, to provide a basis for energy Internet-oriented applications. The intelligence is embodied in the following aspects:

A. Actively mine low-efficiency components by means of big data analysis to achieve preventive maintenance.

Proactively optimize the operation state of the power plants for power plant owners, especially those with a large number of PV power plants, taking both maintenance and operations of the power plants into consideration, for example, comprehensively analyzing the line loss of all power plants in terms of components, cables, inverters, box-type substations, and step-up section based on the 5-point 4-section PR analysis method, and identifying power plants and sections that has low efficiency and requires optimization through horizontal and vertical analysis.

Perform discrete analysis for all strings and devices by means of big data analysis. Identify abnormal strings or devices for which no alarm is generated. For example, for a hot streak, the system may not generate any alarm, but the efficiency of this string is lower than that of other strings. Therefore, you can locate this string by means of discrete analysis, to perform preventive maintenance. This is an example of active operation.

In addition, you can also evaluate devices by performing device comparison analysis and measuring long-term device efficiency and faults. The evaluation results can be used as references for future device selection and solution design.

B. Remote Maintenance, Unattended, or Few-person Attended

The power plants do not require any on-duty personnel on site. Experts implement monitoring and analysis at the HQ in a centralized manner. When a fault occurs, the system actively sends alarms and rectification suggestions to the on-duty personnel. The on-duty personnel can rapidly handle the issues and close the loop by following the system instructions. In case of complex issues, the system can provide comprehensive information about the field status, including videos, voice, and data, to the cloud data center in real time. The experts at the data center can remotely provide guidance. In this way, the safety of on-site personnel is assured, the fault rectification process is normalized, and the rapid close-loop is achieved.

C. Accurate Fault Locating, Reduced Diagnosis Error Rate, and Improved O&M Efficiency

In most centralized power plants, it is hard to detect string faults, and the loss of electric energy yield cannot be compensated. based on the high- precision string-level detection of the smart PV controller, the system can detect faults in a timely manner. By analyzing databases, the system can accurately locate a specific faulty device and propose handling suggestions based on preset measures and O&M experience. Therefore, the O&M personnel can visit the site with a clear purpose and address all the issues upon one visit without requiring repeated verification and device preparation.

2、Composition of the Smart PV Power Plant

The intelligence of PV power plants is divided into three layers: bottom layer (hardware intelligence, including intelligence of PV components, inverters, and power distribution devices), intermediate layer (intelligence of production monitoring and management and optimal power generation control), and top layer (major region decision-making).

A. Bottom layer: This layer contains hardware devices in the PV field and is equipped with an intelligent PV controller. The controller can precisely monitor data of each PV battery string and independently detect each channel of input voltage and current. In this way, the detection precision is improved, laying a basis for accurate fault locating and O&M efficiency improvement. The bottom layer has more MPPTs so that it can implement fine-grained energy management. High-precision sensing devices are used, to ensure high data precision and improve the electric energy yield and maintainability of the power plant system.

B. Intermediate layer: This layer contains the smart PV power plant O&M system that basically includes the smart PV monitoring system and smart PV production management system. With the interworking between systems, the information management system can interwork with each substation. According to the integrated design principle, an integrated computer monitoring system is deployed on the unified communication platform to monitor the running status of each device of the power plant.

C. Top layer: This layer contains a group HQ or regional centralized O&M center. The O&M center can manage power plants in a centralized manner, thereby improving the power plant management and O&M efficiency, increasing the electric energy yield, and reducing the management costs. based on the cloud computing platform, the O&M center supports the data access capabilities for managing tens of gigawatts and hundreds of power plants. The O&M center supports data storage for 25 years of data of hundreds of TB;

provides complete rights control and authentication mechanisms to ensure data security; and supports the access of diversified power plants and expansion with new power plants. The O&M center can manage multiple power plants geographically distributed in various areas throughout the country as local logical power plants; analyze the implementation status of annual or monthly power generation plans of each power plant as well as the O&M investment, to help group executives to make decisions; and summarize production data of multiple power plants, perform converged analysis, and generate a set of inter-power station KPIs for operations evaluation and health status evaluation, so as to identify shortcomings and provide optimization suggestions.

3、Technical Features and Customer Values

Compared with traditional power plants, the smart PV power plant has the following advantages:

A. The smart PV power plant adopts advanced technologies such as multi-channel MPPTs and multi-modal MPPTs, which effectively reduces the loss caused by component attenuation, shadow sheltering, inconsistent engineering and installation, inconsistent terrains, and DC voltage drop. The system performance ratio (PR) reaches 83% or higher. Compared with that of the traditional PV solution, the average electric energy yield of the smart PV power plant is improved by 5% or higher, and the internal rate of return (IRR) is improved by 3% or higher.

B. The smart PV power plant supports 25-year reliable and maintenance-free running. The intelligent PV controller is at the IP65 protection class and supports isolation between internal and external environments, so that components are running in a stable environment, and impacts caused by external environments such as temperature, wind, and salt spray on the component lifetime are reduced; the system is not equipped with any damageable components, or any components that need to be replaced periodically such as fuses and fans, thereby achieving maintenance-free; all of the components and system adopt a 25-year reliability design and lifetime simulation, and also pass strict verification tests. Therefore, system components do not need to be replaced within the entire lifecycle and can reliably and economically operate.

The actual usage of the installed capacity of the PV power plant is high. Compared with that of the traditional solution, the annual mean number of failures of the smart PV power plant is decreased by 30%. The probability of system failures that affect the electric energy yield is only one-tenth that of the traditional solution, and maintenance costs after the warranty period is only one-fifth that of the traditional solution. Essentially, the traditional PV power plant is a serial system that contains DC SJBs, DC power distribution cabinets, equipment room cooling devices, auxiliary power supply devices, and inverters. If any component is faulty, a part of or the entire PV system suffers power generation losses. The traditional PV system also requires professional maintenance, resulting in a long maintenance period and high costs. However, the smart PV power plant has a simple structure and is essentially a parallel system. The failure of a single inverter does not affect the operation of other devices. Due to its small size, light weight, availability of on-site spare parts of the integrated system, and easy-to-maintain feature, the system reliability of the smart PV power plant is greatly improved.

C. The string-level intelligent monitoring and multi-channel MPPT technologies ensure a visible, credible, manageable, and controllable power plant. The intelligent PV controller can independently detect voltage and current of each string. The detection precision is 10 times higher than that of the traditional intelligent SJB solution, and lays groundwork for accurately locating string failures and improving the O&M efficiency. By adopting the multi-channel MPPT technology and reducing impacts caused by sheltering, dust, and string mismatch, the electric energy yield of the smart PV power plants at the flat terrain is improved by 5% or higher; by reducing the impacts caused by different orientations and shadow sheltering, the electric energy yield of the smart PV power plants on the roofs or mountains is improved by 8% to 10%. By cooperating with the tracking system and integrating tracking control with the controller, the smart PV power plant can independently trace a support and increase the electric energy yield. The intelligent controller and trace support can achieve optimal performance when working together.

D. The smart PV power plant supports upgrade and evolution. When components have a technological progress or the running environment is changed, the intelligent PV controller software can be used to remotely upgrade the system online. With the backward compatibility design, the system can utilize the latest technological achievements by means of algorithm upgrade without replacing online devices, thereby maximizing the reuse rate of existing devices.

E. The intelligent and active grid-adaption technologies adopted by the smart PV power plant achieve grid-friendliness. By taking advantages such as high-speed processing capability high sampling and control frequencies, and control algorithms of the intelligent controller, the smart PV power plant can positively adapt to changes of grids to better implement multi-system parallel control, achieve better grid-connected harmonics quality, better satisfy grid access requirements, and improve the capability to adapt to harsh grid environments.

F. By reducing the DC power transmission distance, the smart PV power plant proactively achieves safe power transmission. Safety transmission and protection of DC power are important and difficult. based on a no-DC junction design, the smart PV power plant implements remote-distance power transmission by outputting DC current from the strings to inverters. Then, the inverters convert the DC current to AC current. In this way, the smart PV power plant actively avoids the safety and protection issues caused by DC power transmission and reduces potential safety risks caused by DC arc discharge. In this way, the power plant is safer.

Component power attenuation caused by the PID effect greatly affects the ROI. By using an intelligent controller to automatically detect the component electric potential, the smart PV power plant can actively adjust the system working voltage, so that the system can have positive grounding voltage without grounding the negative electrode of the solar panels, thereby effectively eliminating the PID effect. Because the negative electrode of the solar panels is not grounded and the residual current monitoring circuit inside the inverter can monitor the leakage current, the system can detect leakage current higher than 30 mA.

"User experience" is the focus of the Internet industry and is the only way for PV enterprise development. As China's PV industry becomes more mature, application scenarios of PV power plants gradually become diversified. In such a case, improving the electric energy yield and ensuring safety running of power plants become the basic requirements. PV power plant intelligence not only can provide suitable solutions for power plants deployed in different regions and different scenarios, but also can reduce O&M costs and increase revenues. The smart PV power plant plays an important role in China's intelligent energy industry system and also becomes a new trend of PV development.

FR:Shine Magazine