Research on Communication Center System of Power Quality Monitoring

With the development of society, power quality monitoring has attracted more and more attention from the society. The reason is not only related to the power sector, but some power quality indicators (such as harmonics, voltage fluctuations and flicker, and three-phase voltage unbalance) are often Caused by user interference, involving power generation, power supply and power consumption, related to the interests of all parties. Therefore, in order to effectively maintain the common interests of the power sector and power users, ensure the safe operation of the power grid, and purify the electrical environment, it is necessary to strengthen the management of the power quality of the power system and establish a complete power quality monitoring and analysis system to accurately monitor the power quality. Detection, evaluation and classification The development and improvement of power quality monitoring technology is related to the safety of power grid and electricity consumption.

1. Introduction to Power Quality Monitoring

Power quality is not only related to the safe and economical operation of power grid enterprises, but also affects the safe operation of users and product quality. The access of a large number of distributed energy sources (such as wind power and solar power) will further deteriorate the power quality. Continuous monitoring, analysis and evaluation of power quality information is a prerequisite for finding power quality problems and improving power quality levels. The power quality monitoring system (hereinafter referred to as "monitoring system") uses the power quality monitoring terminal (hereinafter referred to as "monitoring terminal") installed on the grid side or user side to transmit the monitoring data back to the monitoring center (monitoring master station or sub-station) through the network ), realize simultaneous monitoring of multiple locations, and publish power quality related information, which is an effective means of power quality monitoring and evaluation.

2. Goals and key points of power quality monitoring

Monitoring objectives determine monitoring equipment selection, trigger thresholds, data acquisition and storage methods, and analysis and interpretation requirements. Any power quality monitoring system should have clear monitoring objectives, including:

2.1 Describe the overall performance of the system.

2.2 Describe specific power quality problems.

2.3 Evaluate the power quality level.

2.4 Interference diagnosis and equipment maintenance.

2.5 The main points of power quality monitoring include:

2.5.1 Determining the description and analysis method of power quality.

2.5.2 Select monitoring points. Typically, measurements at a few key points can characterize the entire system. The monitoring location is usually selected at the user's power supply entrance, near the affected equipment, or at the same time at the substation and the specific user's power supply entrance.

3. Research status of power quality monitoring system

3.1 Research on the overall architecture of the power quality monitoring system

Research on architecture scheme based on a specific terminal In this type of research, the realization of a specific monitoring terminal is often the focus of discussion, and the architecture of the monitoring system is often based on the premise of adapting to this type of terminal. Due to its special requirements in sampling rate, data statistics, transmission format, etc., power quality monitoring terminals generally cannot be realized by existing terminals. In recent studies, one is to build a monitoring system for embedded devices, and the other is to build a system based on virtual instrument technology using industrial computers and host computers.

At present, regional power quality monitoring systems have been generally established in China to fully grasp the power quality conditions in various regions.   In this type of system, the number of monitoring points is large, the construction period is long, and the monitoring terminals used are often not limited to a specific type. Therefore, it is necessary to study a general monitoring system architecture scheme.

3.2 General architecture scheme

3.2.1 Architecture scheme based on CORBA. Related research uses Common Object Request Broker Architecture (CORBA) technology to realize remote access and control between the central control station and each monitor in a heterogeneous environment. CORBA technology is generally used to provide a flexible communication mechanism between different applications. When it is used to monitor data transmission, the difficulty of terminal implementation, system maintainability, and whether it can be transmitted across firewalls need further research.

3.2.2 Architecture scheme based on MAS. A multi-agent system (Multi-Agent System, MAS) is a collection of multiple Agents, where each Agent is a physical or abstract entity. The architecture based on MAS is more in line with the characteristics of the distributed structure of the power system, but there are still many practical engineering problems to be solved, such as: how to implement complex Agent programs in different types of monitoring terminals, and what standard communication and coordination are based on each Agent.

3.2.3 Architecture scheme based on dispatching system mode. This type of architecture scheme generally consists of on-site collection terminals, communication networks, hierarchical remote monitoring centers, and clients. Considering the actual situation of my country's power grid, this architecture scheme is generally adopted when building a regional monitoring system. At present, all provinces in China generally adopt a three-tier monitoring system architecture.

Considering the actual situation of management methods, construction costs, and communication network architecture, some regional monitoring systems also adopt a two-tier structure scheme. The main difference between it and the three-tier structure is that no monitoring sub-station is set up. The Italian power quality monitoring system adopts a two-tier structure; the open monitoring system of the Jiangxi section of the Zhejiang-Jiangxi electrified railway temporarily adopts a two-tier distributed structure, but reserves a data exchange interface for the transition to a three-tier structure; the Shanghai power quality monitoring system adopts a Two-tier architecture, and multiple communication front-end processors are used in the master station to share the communication pressure.

From the perspective of development trend, the three-tier structure can support access to more monitoring points, has high scalability, and can meet the demand for massive monitoring data.

3.2.4 Architecture scheme of open monitoring system. Related studies apply the open system idea to the monitoring system, and build an open unified platform through the abstraction of the system's functional interface and the standardization of communication to meet the needs of integration and interaction of different types of monitoring terminals and systems.

The advantages of open architecture and compatibility can make the monitoring system develop smoothly and have a long life cycle. But most of the current research is only theoretical discussion, and there are not many studies on the actual implementation.

4. Research on communication system for power quality monitoring

4.1 Research on Transmission Media

Networking is an inevitable trend in the development of monitoring systems. In the currently commonly used two-tier or three-tier architecture, the communication network is responsible for transmitting the data collected by each monitoring terminal to the monitoring center, and issuing commands from the monitoring center to each terminal, which is the key to the normal operation of the monitoring system.

The early networked power quality monitoring remote communication network generally uses the public telephone network, and the transmission rate is low, which is only suitable for building a small monitoring system. With the completion of the domestic electric power data communication dedicated network, the current monitoring system generally uses the Ethernet network based on the TCP/IP protocol to transmit data. Distribution line carrier communication has not been widely used.

The wired network has disadvantages such as limited coverage, large investment in construction and maintenance, and long cycle. Due to the problems of security, reliability and transmission speed, wireless GPRS has limited application range. Wireless network can be used as a supplement to wired network, and plays an important role in some specific occasions as well as backup network.

4.2 Research on communication standards

At present, most monitoring systems transmit monitoring data directly on high-speed Ethernet without using complex compression schemes. Therefore, communication protocols and data format standards are the focus of research. Existing research mainly includes the following two aspects:

4.2.1 Research on communication standards based on PQDIF. The Power Quality Data Interchange Format (PQDIF) proposed in the IEEE P1159.3 standard is completely independent of the monitoring terminal software and hardware, and is widely used as a standard format for monitoring data exchange. The embedded monitoring terminal directly generates PQDIF files and transmits them to the monitoring center, which can avoid maintaining multiple interface conversion programs in the center that convert the private communication protocol of each terminal manufacturer into PQDIF, which is conducive to reducing the burden on the monitoring center and improving system reliability.

4.2.2 Research on communication standards based on IEC61850. The second edition of IEC61850 "Communication Network and System for Public Utility Automation" has officially written the supplementary draft content of the power quality appendix into the standard. Detection and analysis of logical node classes (Q-), etc. Early research adopted the method of setting up a data concentrator in each substation to realize the conversion of non-standard protocols of various manufacturers into IEC61850 services. The current plan is: the power quality monitoring IED is directly modeled based on IEC61850, and implements the IEC61850 server; the monitoring center implements the IEC61850 client, and performs operations such as data reading and configuration downloading on the monitoring terminal that conforms to the IEC61850 standard.

5. Research on power quality monitoring center system

5.1 Research on Software Architecture Scheme of Monitoring Center

In the currently commonly used two-tier or three-tier architecture, the monitoring center system is the core of the entire monitoring system, which needs to realize connection with monitoring terminals, monitoring data analysis and processing, massive monitoring data management, high-level analysis applications and information visualization Publishing and other functions. At present, the software system architecture of the monitoring center generally adopts the C/S mode or the B/S mode. The former is like the Turkish National Power Quality Monitoring System, and the latter is like the Brazilian National Power Quality Monitoring System. An open network structure is the future development trend. The B/S mode can reduce the cost of system maintenance and upgrading, and can be used as the main form of use by ordinary users in the monitoring system. At the same time, some functional modules in the monitoring system involve frequent operations of the database, and the supervision and management work requires high security and high reliability, so the C/S mode is more suitable. Therefore, the scheme of "data management C/S mode, information publishing B/S mode" can be adopted.

5.2 Research on Data Management Strategy

Due to the small number of monitoring points and the small amount of data at the beginning, most of the monitoring system data is stored in a fixed-structure data table, which has great limitations and limited storage capacity. The Turkey National Power Quality Monitoring System adopts the scalable power quality database architecture based on the metadata model, designs a detailed conceptual model of the database structure, counts the data volume of each data table, and conducts in-depth research on the database design in the field of power quality for the first time.

5.3 Research on Information Release Forms

Early monitoring systems usually only use data combined with simple graphs or reports to display analysis results, and their performance is poor. Existing monitoring systems generally combine power quality information with geographical distribution information of power grids or transmission lines to directly reflect the indicators of various places and display them on a large screen.

6. Summary of research status of power quality monitoring

With the development of network communication technology and information technology, networking, informatization and standardization have become the general requirements of the power quality monitoring system. At present, the two-tier or three-tier architecture with reference to the scheduling automation mode has become the actual construction plan of the regional monitoring system; the optical fiber Ethernet is the main method, and the wireless communication network is the auxiliary method, which can meet the requirements of the current network to transmit massive monitoring data; IEC61850 as the power The only international standard for seamless communication in the field of system automation, it is an inevitable trend to apply it to the field of power quality monitoring, but PQDIF, as the only data format for power quality at present, will still play an important role; There is less research on the release form.

7. Problems existing in the existing power quality monitoring system for power quality monitoring

7.1 Lack of perfect and unified communication standards

The communication standard is the only protocol for the monitoring terminal and the central system to identify each other, but there is no generally accepted communication standard for power quality monitoring in the international context. The domestic existing systems mainly adopt PQDIF or IEC61850, both of which have imperfections in the application process.

As a data format standard, PQDIF does not specify the communication form used for monitoring data transmission; in practical applications, due to the lack of national standards, there are differences in the PQDIF specifications of each province and the PQDIF files generated by each terminal, which hinders its promotion and application; Since its transmission and analysis are time-consuming, it cannot meet the needs of real-time monitoring.

In the application of IEC61850, there are not many cases of putting it into the actual monitoring system. Most studies only discuss the distribution and modeling of power quality monitoring IED functions based on IEC61850 on the three functional layers in the substation, and less involve the communication modeling of the monitoring center. However, in practical applications, power quality monitoring is generally not carried out in a single substation, and monitoring terminals are not connected to the substation automation system. Instead, several terminals in a certain area form a remote monitoring system, which essentially belongs to the extended application category of IEC61850.

7.2 Lack of interaction and reuse among related systems

In order to solve the power quality problem, many different related systems have been put into application in the power supply enterprises. However, each system is a tightly coupled business application software with serious heterogeneity, and there are two specific problems in the following aspects:

7.2.1 Each application is an isolated and closed system, which does not have the capability of information sharing and service interaction with other heterogeneous systems, such as power marketing management system, condition maintenance management system, etc.

7.2.2 During the development process of the new system, the mature modules in the original system cannot be used and need to be rebuilt, which is inefficient and expensive. Some common functional modules, such as harmonic index evaluation module and impedance calculation module, are developed and maintained separately, resulting in bloated system and waste of resources.

7.3 Lack of effective management and utilization of massive monitoring data

In addition to basic voltage, current, frequency, and power, power quality monitoring data also includes harmonics, interharmonics, three-phase unbalance, flicker, and transient events; in addition to real-time data, it also includes maximum and minimum values , average value and 95% probability value and other statistical data; it is one of the most comprehensive and complete operating data in the power system. At the same time, due to the uniqueness of its information, it is very important for analyzing related problems such as system disturbances.

At present, there are few domestic and foreign literatures on the management strategy of massive monitoring data. The analysis and application of monitoring data are often relatively simple, and there is no in-depth utilization of monitoring data. The imported software PQView commonly used at present cannot fully meet the needs of domestic power quality management. First of all, the secondary development needs to call PQView.dll to obtain data, the interface form is fixed, and the limitations are large; second, the database structure and debugging functions are not open, once a problem occurs, it is difficult to find the cause; finally, the currently released version cannot support IEC61850 standard.

7.4 Unable to support friendly interaction with electricity users

Both the power supply and the user attach great importance to the huge economic loss caused by the power quality problem, but the two parties often have differences on the cause and responsibility of the power quality problem. At present, the assessment of the power quality of the power grid by the power supply department and the requirements of the power users for power quality are independent of each other. The power quality monitoring data inside the power supply department is not open to users, resulting in information asymmetry and opacity.

The existing power quality monitoring system is generally located in the internal information network of the power system, and power users generally cannot access it; the software architecture does not support the use of power users; due to information security and network coverage, it cannot access the user side that is not in the power private network Monitoring terminal.

8. Development trend of power quality monitoring system

8.1 Power quality information platform in smart grid

The intelligence of the power grid is reflected in the ability to comprehensively and timely grasp the operation information of the power grid and make a fast and optimal response. Therefore, an accurate, fast, open, and shared information platform is the foundation of the smart grid, and it is also the biggest difference between the smart grid and the traditional grid. At present, the actual needs and development trends of power quality management have gone far beyond the traditional power quality monitoring system that only monitors various power quality indicators. Therefore, it is necessary to construct a complete power quality information platform (hereinafter referred to as "information platform") ) is very important, and its future may become an important platform to keep pace with SCADA, PMU and AMI.

The information platform should include power quality monitoring subsystem, data management and analysis subsystem, and information release subsystem. Among them, power quality monitoring is the basis and premise for realizing other functions of the information platform. The power quality information platform in the future smart grid will have the following development directions.

8.2 Deepening utilization of power quality monitoring data

It is an important function of the future information platform to use massive power quality monitoring data to provide support for the actual production and operation of the power system. At present, the in-depth application of monitoring data is mostly limited to the scope of power quality problems, such as disturbance identification and classification. In the future, combined monitoring data for dynamic load modeling, distribution network fault prediction, fault location, state monitoring and evaluation of capacitor banks and circuit breakers, and load monitoring will make better use of monitoring data.

8.3 Compatible and open information platform

A compatible and open platform is conducive to the effective use of monitoring data. To achieve compatibility and openness, standardization is the premise. The corresponding information platform standards should be studied according to the existing communication protocols and system interaction standards in the power system, combined with the particularity of power quality. At present, it has become a consensus to adopt PQDIF and IEC61850 standards when monitoring terminal access platforms, but there is still no mature solution for information interaction and service calls between different systems of the information platform and between the platform and other systems.

None of the existing monitoring systems model the power grid, and only save the ledger information of a single monitoring point. In the future, with the development of high-level applications, harmonic power flow calculation, harmonic state estimation, and voltage sag analysis will all need models of actual power grids that carry real-time monitoring data. From the perspective of future development trend, adopting IEC61970 as the interaction standard between systems should be an inevitable choice. How to model power quality-related systems based on CIM, which packages in CIM are needed, and which types need to be expanded are the focus of future research.

8.4 Architecture of information platform based on cloud computing

The development of the field of smart grid and power quality puts forward three basic requirements for the information platform:

8.4.1 It is required to establish a unified information platform with compatible terminal access, transparent information sharing, integrated standard specifications, and support for synergy and interoperability of power quality information management services.

8.4.2 It is required to reliably store and optimize management of massive data, fully tap the potential value of information, and improve the level of intelligent analysis and auxiliary decision support for power quality.

8.4.3 The information platform is required to have a sound release system to avoid information asymmetry between power suppliers and consumers, and to support friendly interaction between power suppliers and consumers.

As an emerging computing model, cloud computing has the advantages of high reliability, huge data processing capacity, flexibility and scalability, and high equipment utilization. It provides a new solution for building a power quality information platform that meets the above requirements. Computing has the characteristics that can meet the realization goal of the information platform.

Since the 1980s, power quality monitoring systems have undergone tremendous development. In the smart grid, the power quality information platform will integrate the monitoring system and other related systems to become a data support platform that is crucial to the smart grid. With its development, the existing system architecture, communication protocol, data access form, information release and system management mode can no longer meet the needs. Therefore, there will be many key technologies to be studied. At present, the power industry already has the software and hardware conditions for cloud computing practice, which will become an optional solution for the future power quality information platform.