Abstract: In order to simplify the complexity of power system automation networking, this article develops a new type of serial port server and proposes a detailed networking scheme based on this device. The device adopts a fully digital design, effectively avoiding the complexity and port security issues of traditional PCM networking methods. This scheme not only effectively simplifies the complexity of power automation system networking, but also improves the reliability and quality of automated data transmission.

Power automation systems based on the 101 protocol typically use traditional PCM devices for networking, and utilize the 2/4-line EM interface of PCM devices to provide low-speed asynchronous data channels. The data from RTU devices needs to be modulated by an external modem before entering the 2/4-line EM interface. Correspondingly, a modem is also required at the data receiving end to demodulate and restore the data. This networking method has three issues:

Firstly, data needs to be modulated and demodulated before transmission. Due to limited audio channel bandwidth, VME7865RC V7865-23003 can only provide a maximum data transmission rate of 2.4kb/s under existing modulation methods, and due to external interference, the data error rate is relatively high.

Secondly, PCM equipment and automation equipment are managed by different departments and installed in their respective computer rooms. A large number of audio cables need to be laid between the communication room and the automation room. Long lines not only bring unforeseeable interference, but also increase the difficulty of daily line maintenance and fault diagnosis.

Thirdly, the multi-channel low-speed data output by the modem needs to be converted into IP packets by a regular multi serial port server for processing by the SCADA system. Not only does it require a large number of modems, but also a large number of port connections between the modems and the multi serial port server, which is costly and poses a risk of poor port connections.

To address the aforementioned issues, this article proposes a new automated data networking solution and develops a new type of serial port server for its implementation. By adopting this scheme, not only can PCM, modems, IOLAN and other equipment be saved, but also the reliability of communication can be greatly improved due to the elimination of many intermediate conversion links.

1、 Description of the new networking solution

On the basis of implementing functions, the simpler the system, the higher its reliability, and the binary full digital transmission method is also more stable than analog systems. Based on the above two principles, this article proposes a new automated data networking scheme, as shown in the following block diagram:

 

Taking the networking of one central station and two substations as an example in the figure, the VME7865RC V7865-23003 central station main station equipment is interconnected with the substation terminal equipment through the E1 circuit of the SDH network. Assuming that each substation needs to access 4 channels of low-speed data, occupying 1-4 time slots of their respective E1 circuits. The 2 E1 signals are transmitted to the central station through the SDH network. In order to merge the 1-4 time slots already used in the 2 E1 signals into one E1 circuit, the central station needs to be equipped with a time slot scheduling device (DDN device). The E1 circuit, which has been merged through time slots, is directly connected from the communication room to the new serial port server in the automation room. Upward direction:

The new serial port server extracts the effective time slots from the E1 signal and packages them into a standard IP packet to send to the front-end processor, which contains the corresponding time slot number. Downward direction: The new serial port server receives IP packets sent by the front-end machine and inserts the data in the IP packet into the corresponding E1 time slot according to the slot number in the IP packet. A low-speed data insertion E1 signal 64K time slot conversion device needs to be added to the substation end, and for substations with existing PCM installations, the existing RS232/RS422 plug-in board can be used.

The old networking solution based on PCM equipment is shown in the following figure:

 

In the old plan, the RTU information at the substation end first needs to go through a modem and be connected to the 2/4 line EM interface of the PCM equipment. After being transmitted through the SDH network to the PCM equipment at the central station, it is restored to the 2/4 EM interface. After passing through the distribution frame, it extends from the communication room to the automation room and is restored to low-speed data by a modem. Several low-speed data paths are connected to the IOLAN equipment, which is converted into IP packets and sent to the front-end machine.

Through the schematic analysis of the new and old schemes, it can be seen that, under the same function, the new scheme not only saves a lot of equipment investment (including PCM equipment, modems, IOLAN devices), but also simplifies the wiring management of the line, realizes end-to-end digitization of data, and effectively improves the quality and reliability of data transmission.

2、 The schematic diagram of a new type of serial port server

In the new networking solution, a new type of serial port server is applied. Unlike previous IOLAN devices, this server can directly convert the data carried by each time slot in the E1 signal to IP packets. In the upstream direction: the device can extract the effective data carried in the E1 time slot and convert it into corresponding IP packets to send to the front-end machine; In the downlink direction, the device can receive IP packets sent by the front-end processor and insert the data in the IP packet into the corresponding E1 circuit in the time slot.

The following diagram is the schematic diagram of the new serial port server:

 

Upward direction: The E1 signal input to the E1 interface is first decoded by the HDB3 decoding module, and the decoded NRZ code is sent to the E1 unframed circuit. The E1 unframed circuit decodes the effective data of 30 time slots according to the frame structure of G.704. The effective data of each time slot is sent to their respective IP packaging circuits to convert it into Ethernet packets that comply with the IEEE802.3 standard, and sent to the front-end machine.

Downward direction: Each IP packet receiving circuit receives IP packets that meet its own time slot port transmitted from the front-end machine, extracts valid data after unpacking, and inserts the data into the corresponding time slot of the ST bus. The E1 framing circuit converts the ST bus containing data from each time slot into a standard NRZ stream according to the recommendation of G.704, and forms a standard HDB3 signal through the HDB3 encoding circuit.

3、 Protection of E1 interface

The application environment of the power system is special and requires high reliability. In the networking scheme proposed in this article, the E1 interface cable needs to extend from the communication room to the automation room, with a long transmission distance, and is exposed to complex electromagnetic environments, making it susceptible to surge impacts or even lightning strikes. To ensure the safety and reliability of the interface, it is necessary to strengthen the design of lightning protection and surge protection.