Substation is an indispensable and important link in power system. It undertakes the heavy task of power conversion and power redistribution, and plays an important role in the safe and economic operation of power grid. In order to improve the safe and stable operation level of substation, reduce operation and maintenance costs, improve economic benefits and provide users with high-quality power services, substation integrated automation technology began to rise and has been widely used.

Substation integrated automation is a comprehensive automation system that applies computer technology and modern communication technology to the secondary equipment of substation (including control, signal, measurement, protection, automatic device and telecontrol device, etc.) and implements automatic monitoring, measurement, control and coordination of substation and communication with dispatching through function combination and optimization design. Realizing the comprehensive automation of substation can improve the safe and economic operation level of power grid, reduce infrastructure investment, and provide a means to promote the unattended operation of substation. The rapid development of computer technology, information technology and network technology has driven the progress of substation integrated automation technology. In recent years, with the development of digital electrical measurement system (such as photoelectric transformer or electronic transformer), intelligent electrical equipment and related communication technology, the substation integrated automation system is moving towards digitization.

1、Main functions of substation integrated automation system

The basic functions of substation integrated automation system are reflected in the functions of the following 6 subsystems:

1. Monitoring subsystem;

2. Relay protection subsystem;

3. Integrated voltage and reactive power control subsystem;

4. Low frequency load shedding control subsystem of power system;

5. Automatic switching control subsystem of standby power supply;

6. Communication subsystem.

This part is rich in content and has been described in detail in many literatures, so this paper will not elaborate.

2、Traditional substation automation system

1. System structure

At present, the structure of substation integrated automation system at home and abroad can be classified into the following three types from the design idea [1]:

(1) Centralized

Adopt computers of different grades, expand its peripheral interface circuit, collect the analog quantity, switching quantity, digital quantity and other information of the substation, conduct centralized processing and operation, and complete the functions of microcomputer monitoring, microcomputer protection and some automatic control respectively. It is characterized by high requirements for computer performance and poor scalability and maintainability. It is suitable for small and medium-sized substations.

(2) Distributed

According to the monitored object or system function of the substation, multiple CPUs work in parallel, and the data communication between CPUs is realized by network technology or serial mode. The distributed system is easy to expand and maintain, and local faults do not affect the normal operation of other modules. This mode can be installed in centralized screen or split screen.

(3) Decentralized and distributed

Each data acquisition, control unit (I / O unit) and protection unit in the Bay layer are installed locally on the switchgear or near other equipment. Each unit is independent of each other, interconnected only through the communication network, and communicates with the substation level measurement and control main unit. The functions that can be completed at the Bay layer do not depend on the communication network, such as protection function. The communication network is usually optical fiber or twisted pair, which greatly compresses the secondary equipment and secondary cable and saves the project construction investment. The installation can be dispersed in each bay, centralized panel or layered panel in the control room, or one part in the control room and the other part on the switch cabinet.

2. Existing problems

The substation integrated automation system has achieved good application results, but there are also deficiencies, which are mainly reflected in: 1. The information interaction between the primary and secondary still continues the traditional cable wiring mode, with high cost and inconvenient construction and maintenance; 2. The secondary data acquisition part repeats a lot, wasting resources; 3. Insufficient information standardization, low degree of information sharing, coexistence of multiple systems, difficult interconnection between equipment and between equipment and systems, forming an information island, and it is difficult for information to be comprehensively applied; 4 when an accident occurs, a large number of event alarm information will appear, and the lack of effective filtering mechanism will interfere with the correct judgment of the operator on duty.

3、Digital substation

Digital substation is the next stage of substation automation development. The "Eleventh Five Year" science and technology development plan of State Grid Corporation of China has clearly proposed to study digital substation and build demonstration station reference 2 during the Eleventh Five Year Plan period. At present, digital substation has been completed and put into operation, such as 110kV digital substation of Fuzhou Convention and Exhibition substation.

1. Concept of digital substation

Digital substation refers to a substation whose information collection, transmission, processing and output processes are completely digital. Its basic characteristics are equipment intelligence, communication networking, operation management automation and so on.

Digital substation has the following main features:

(1) Intelligent primary equipment

Digital output electronic transformer, intelligent switch (or traditional switch with intelligent terminal) and other intelligent primary equipment are adopted. The sampling value, state quantity, control command and other information are exchanged between primary equipment and secondary equipment by means of optical fiber transmission of digital coding information.

(2) Networking of secondary equipment

The communication network is used between secondary equipment to exchange analog quantity, switching value, control command and other information, and the control cable is cancelled.

(3) Automation of operation management system

It shall include automatic fault analysis system, equipment health monitoring system and programmed control system to improve the automation level and reduce the difficulty and workload of operation and maintenance.

2. Main technical features of digital substation

(1) Data acquisition digitization

The main sign of the digital substation is that the digital electrical measurement system (such as photoelectric transformer or electronic transformer) is used to collect electrical parameters such as current and voltage 3, which realizes the effective electrical isolation of the primary and secondary systems, increases the dynamic measurement range of electrical quantities and improves the measurement accuracy, Thus, it provides a basis for the transformation from conventional substation device redundancy to information redundancy and the application of information integration.

(2) Hierarchical and distributed system

The development of substation automation system has experienced the transformation from centralized to distributed. The second generation hierarchical distributed substation automation system mostly adopts mature network communication technology and open interconnection protocol, which can more completely record equipment information and significantly improve the response speed of the system. Physically, the structure of digital substation automation system can be divided into two categories: intelligent primary equipment and networked secondary equipment; In terms of logical structure, according to the definition of IEC61850 communication standard, it can be divided into "process layer", "Bay layer" and "station control layer". High speed network communication is adopted within and between levels.

(3) Networking of information interaction and integration of information application

Digital substation adopts low-power and digital new transformer instead of conventional transformer to directly convert high voltage and large current into digital signal. The information exchange between the equipment in the station is carried out through the high-speed network. The secondary equipment does not appear the I / O interface with repeated functions. The conventional functional device has become a logical functional module to realize the sharing of data and resources. At present, IEC61850 has been determined as the communication standard of substation automation in the world.

In addition, the digital substation integrates the information and optimizes the function of the original decentralized secondary system devices, so it can effectively avoid the problems of repeated hardware configuration, non sharing of information and high investment cost of monitoring, control, protection, fault recording, measurement and metering devices in the conventional substation.

(4) Intelligent equipment operation

The new high-voltage circuit breaker secondary system is established by using microcomputer, power electronic technology and new sensors. The intelligence of the circuit breaker system is realized by the microcomputer controlled secondary system, IED and corresponding intelligent software. The protection and control commands can reach the secondary circuit system of unconventional substation through optical fiber network, So as to realize the digital interface with the operating mechanism of the circuit breaker.

(5) Equipment maintenance state

In the digital substation, it can effectively obtain the power grid operation status data and the fault and action information of various ied devices, and realize the effective monitoring of operation and signal circuit status. There are almost no unmonitored functional units in the digital substation, and there is no blind area for the acquisition of equipment state characteristics. Equipment maintenance strategy can change from "regular maintenance" of conventional substation equipment to "condition based maintenance", so as to greatly improve the availability of the system.

(6) Measurement principle of LPCT and appearance of tester

As mentioned earlier, LPCT is actually an electromagnetic current transformer with low power output characteristics. In IEC standard, it is listed as an implementation form of electronic current transformer, which represents a development direction of electromagnetic current transformer and has a broad application prospect. Since the output of LPCT is generally directly provided to the electronic circuit, the secondary load is relatively small; Its core is generally made of high magnetic conductivity materials such as microcrystalline alloy. Under the condition of small core section (core size), it can meet the requirements of measurement accuracy.

(7) Compact system structure and standardized modeling

The digital electrical measurement system has the characteristics of small volume and light weight. It can be integrated into the intelligent switchgear system to optimize the function combination and equipment layout according to the electromechanical integration design concept of substation. In high voltage and ultra-high voltage substations, the I / O units of protection device, measurement and control device, fault recorder and other automatic devices, as a part of primary intelligent equipment, realize the process close design of IED; In medium and low voltage substations, the protection and monitoring devices can be miniaturized, compact and completely installed on the switchgear.

IEC 61850 establishes the modeling standard of power system and defines a unified and standard information model and information exchange model for substation automation system. Its significance is mainly reflected in realizing the interoperability of intelligent equipment, realizing the information sharing of substation and simplifying the maintenance, configuration and engineering implementation of the system.

3. IEC 61850 standard

IEC 61850 is a series of standards for substation communication network and system formulated by TC57 working group of International Electrotechnical Commission. It is an international standard for substation automation system based on network communication platform. It will also become a communication standard for seamless connection of power system from dispatching center to substation, in substation and distribution automation, It is also expected to become the industrial control communication standard of general network communication platform.

Compared with the traditional communication protocol system, IEC 61850 has the following outstanding characteristics in technology: 1. Using object-oriented modeling technology; 2. Use distribution and layered system; 3. Use abstract communication service interface (ACSI) and special communication service mapping SCSM technology; 4. Use MMS (manufacture message specification) technology; 5. Interoperability; 6. It has a future oriented and open architecture.

4、Conclusion

The application of substation automation system in China has achieved very remarkable results, which has played an important role in improving the safe and economic operation level of power grid. At present, with the continuous development of new technology, digital substation is rising. Compared with traditional substations, digital substations have the following advantages: reducing secondary wiring, improving measurement accuracy, improving the reliability of signal transmission, avoiding the problems of electromagnetic compatibility, transmission overvoltage and two-point grounding caused by cables, and solving the interoperability problems between equipment. Various functions of substations can share a unified information platform to avoid duplication of equipment, The level of automatic operation and management was further improved. Digital substation is the development direction of substation automation technology.