Development of Automatic Transfer Device for Tie Line between Substation Areas in View of Carrier Communication

Wang Shu,Zhou Dakai,Wang Mingling,Jin Chunhua,Meng Yanping

(State Grid Anshan Electric Power Supply Company,Anshan,Liaoning Province 114000)

Abstract:In consideration of the low reliability of some 220kV substations with weak system connection,and even the problem of substation stoppage,this paper develops an automatic transfer device for tie line between substation areas based on carrier technology.After an adjacent substation performs real-time signal transmission through carrier device,a substation is powered by the adjacent substation when it loses pressure and stops completely,so that the two substations are spared each other to ensure power supply reliability.

Keywords:substation;automatic transfer device;tie line;carrier technology

I.Introduction

As people's demand for electricity continues to increase and power grid rapidly develops, the requirements for power quality are also more stringent, making power supply reliability an important indicator for power supply assessment.All along,reliable and uninterrupted power supply of the power system is the core work of the power sector.However,due to the limitation of the development funds of the power grid,it is impossible to solve this problem fundamentally only from the improvement of the power grid structure.Therefore,it is necessary to develop an automatic transfer device for tie line among some 220kV substations with relatively weak system connection to ensure safe,stable and reliable operation of the power grid.The device realizes the signal transmission between the two substations in a reasonable communication manner.If any of the stations loses pressure and stops completely, the power can be supplied by another adjacent substation,thereby effectively preventing the whole station from being lost due to the line trip,ensuring uninterrupted power supply to the system.

The new type of automatic transfer device for tie line can overcome the shortcomings of the existing technologies and provide carrier communication between 220kV substations with weak system connection,and remotely control and data of two substations through carrier devices collecting and realizing signal transmission between substation areas,with a view to achieving mutual backup and reducing accidents such as substation stop.When two 220kV substations are mutually standby,the backup power supply can use the 66kV system tie line.

II.Carrier Technology

With the development of modern communication technology,the power line carrier communication has been greatly improved.The use of power line carrier communication for remote control and data acquisition has become a new development direction of power carrier technology, which has a good development prospect and a vast market.At the same time,power line carrier communication technology has the advantages of simple networking,low cost and easy implementation,and it has attracted more and more attention[1].

In recent years,technologies mainly including GPRS communication,optical fiber communication,TCP/IP Ethernet communication, carrier communication and have certain advantages and disadvantages.For example,wireless communication is susceptible to electromagnetic interference;the distance of Ethernet transmission is short;and optical fiber communication cost is relatively high[2],to a certain extent,and it cannot meet the requirements of on-site communication.The power line carrier technology is the only wired communication method in the power system that does not require line investment.It uses the power line as a signal transmission carrier,and transmitted the needed data into a carrier modulated analog or digital signal through a power line carrier communication machine.And it is a signal transmission method in which an analog or digital signal is coupled into a power line and then transmitted at a high speed through the line coupling device.Power line carrier communication utilizes existing power lines to transmit data without additional transmission lines,and does not occupy existing communication frequency resources[3].It has advantages in reliability, anti-interference,real-time,stability,etc.In this paper,the power carrier mode is used for signal communication between substations.

III.Automatic transfer device for Tie Line

The automatic transfer device (referred to as automatic bus transfer)is an automatic control device that helps standby power or the backup device or other normal working power automatically put into operation when they are disconnected by a power system failure or other reason and allows the user to quickly restore power.The device can quickly and reliably put into standby power supply and shorten the power outage time[4].It is an important measure to ensure continuous and reliable power supply of the power system,and is one of the basic functions of the substation integrated automation system.

The automatic transfer device of standby power has been widely used in the substation contact line.This type of device has both a local standby and remote automatic bus transfer function.At present,the domestic automatic transfer device is mainly used in substations of 110kV and below.There are a small number of automatic bus transfer operation at 220kV voltage level,which are roughly divided into the following three types according to their functions:(1)automatic bus transfer; (2)automatic transfer for transformer; (3)automatic transfer for 110kV line.The above three types of automatic transfer, the action principle is relatively simple[5].

The automatic transfer device for the 66kV tie line backup power supply installed in a substation is an important means to prevent a complete stop and ensure continuous and reliable power supply to the user.When the 220kV system loses power,the 66kV tie line is started by the automatic transfer device,and all or part of the load is transferred to the other power supply through the 66kV tie line.Apart as the traditional automatic transfer device,the two outlets of automatic transfer for tie line can be located in two different substations.It is necessary to send the data of the two outlets and the corresponding bus to one place, and send the trip command to different substations.Therefore,the development of automatic transfer device for tie line needs to consider the choice of the automatic transfer communication methods.

IV.New Type of Automatic Transfer Device for Tie Line

(i)Carrier Communication

The carrier communication system is composed of an automatic transfer device,a communication interface, a carrier device and a communication module,as shown in Figure 1.

Figure 1 Carrier Communication System Network

Wherein the communication module is connected to the automatic transfer device with the carrier device through the communication interface,and then it will link to the corresponding 220kV substation.Each of two carrier devices communicates with any one of the existing power lines.

The carrier device includes three parts:dominant carrier, subcarrier and terminal.It completes remote control and data acquisition of two corresponding 220kV substations,and masters all voltage and current in the substations as well as real-time position information of transformers,disconnect switch and circuit breakers.The dominant carrier is connected to the subcarrier,and subcarrier is connected to the terminal.The dominant carrier is connected to the 220kV substation control system through the optical fiber communication network,and uses the full duplex communication mode to set all the carriers and the subcarriers in the same carrier channel have independent signal transmission channels, so that the same carrier can receive multiple signals at the same time to realize phase-to-phase communication.The communication interface is a serial data cable interface or a network data cable interface.

The automatic transfer system for tie line of two substations is shown in Figure 2.The two substations of A and B are connected by a carrier communication system.After the communication interface and module are installed on the automatic transfer device,the carrier device is connected.When the substation A loses power,the signal is transmitted to the station B via the carrier device,and the station B determines whether it has the conditions for supplying power to the station A,and realizes mutual standby.Substation B can directly supply power to station A through the automatic transfer device and also can cut off the power supply to the substation B after the unimportant load is cut to ensure the uninterrupted power supply of the important load provided by the two substations and improve the reliability of power supply.

Figure 2 Carrier Communication Device System of Automatic Transfer for Tie Line of Two Substations

The carrier device is located at the access layer of the communication system in the distribution network.The uplink data channel can be connected to the optical fiber communication system through the carrier, and the downlink data channel can be connected to the data port of the distribution network terminal to establish a set of communication carrier network of medium-voltage power lines.The dominant carrier and the subcarrier are divided into a network data cable interface and a serial data cable interface according to actual needs.

The carrier device adopts full-duplex communication mode,and the frequency resources are reasonably divided in the same carrier channel to improve the transmission capability of the carrier data.At the same time,the frequency reuse mode of the channel is used so that all carrier machines in the same carrier channel have independent transmission channels and has the capability of receiving multiple signals at the same time,realizing frequency division multiplexing carrier communication and effectively solving the carrier signal queuing problem in the transmission process.

(ii)Characteristic Analysis

The device developed in this paperhas the following characteristics: (1)for a certain reason,when a substation loses pressure and stops completely, it can be powered by the adjacent substation,and it is truly mutually standby.(2)The application of the device can reduce the number of accidents such as the full stop of the substation on the one hand,and improve the reliability of the power supply for the weak power supply area of the power grid on the other hand.(3)It makes full use of the power carrier technology, complete the data transmission with the power line, and load the high-frequency signal carrying the real-time information onto the power line,which can save a lot of social costs and operating costs,and has significant economic benefits.(4)Before the standby substation is put into operation,it can automatically control and reasonably remove part of the load according to the real-time operation mode and system load of the substation to ensure the important load supply of the two stations.

V.Conclusion

The development of this device has further improved the series of automatic transfer devices for spare power in China and the reliability of power supply in weak areas of the power grid,achieving remarkable economic and social benefits and solving the problem of low reliability of power supply with great promotion value.