Application of Fieldbus in Dispatching Automation

1. System composition
The dispatching automation system based on fieldbus is mainly composed of plant-side system, dispatching-side system and communication system.

(1) Plant-side system

The monitoring and control system at the plant and station end (power plant, substation end) consists of a front-end intelligent unit, an industrial computer, a network communication management adapter card, a dedicated modem and corresponding software.

1) Front-end intelligent unit. On the one hand, the front-end intelligent unit completes the measurement of electrical parameters such as line current (A phase, B phase and C phase), voltage (A phase, B phase and C phase voltage; AB phase voltage; BC phase voltage; CA phase voltage), three-phase/single-phase active power, three-phase/single-phase reactive power, three-phase/single-phase positive active energy, three-phase/single-phase negative active energy, three-phase/single-phase positive reactive energy, three-phase/single-phase negative reactive energy, three-phase/single-phase power factor and frequency; on the other hand, it also completes the detection and control of each switch state. The front-end intelligent unit generally adopts a 16-bit microprocessor system, which mainly includes acquisition control logic, analog signal processing, switch input and output processing, communication processing, system monitoring and protection, real-time clock and power supply. The front-end intelligent unit can work reliably in harsh electromagnetic environments and can be installed in outdoor terminal boxes.

2) Industrial computers. Industrial computers are used to display the main electrical connections of the power plant and its operating conditions, record various data, display various curves and charts, alarm and print abnormalities or accidents, and forward and receive information. Generally, only one industrial computer is needed in a substation to complete all the above tasks. In a power plant, the number of industrial computers can be increased according to the needs of the monitoring points.

3) Network communication management adapter card. The network communication management adapter card is inserted into the expansion slot of the industrial computer, and realizes high-speed data exchange between the front-end intelligent unit and the industrial computer through the field bus, and realizes the management of the network.

4) Modem. Modem is a special modem for the power system, which completes the data communication between the power plant, substation and dispatching end.

(2) Dispatching system

The dispatching end monitors, controls and manages the entire system. The dispatching end system consists of an industrial computer, a modulation and demodulation device, a multi-channel communication interface device, and a large-screen display. The modem receives the modulation signal from the carrier machine, and the command issued by the dispatching end can be sent to the carrier channel after being modulated by the modem, and then sent to the plant end. The multi-channel communication interface completes the data communication between the multi-channel digital signal and the industrial computer at the plant end. The large-screen display is used to display the power network diagram, power grid parameters and load curves, etc.

(3) Digital Communications

The data exchange between the plant and the dispatching end is transmitted through data channels. At present, the power system has carrier channels, microwave channels, and optical fiber channels. For carrier channels and microwave channels, it is necessary to use a dedicated modem for the power system at both ends of the channel. The modems at both ends of the same channel are preferably the same model from the same manufacturer.

2. System Features

The real-time monitoring and control system of power plants and substations based on fieldbus and the dispatching automation system formed by it can avoid the use of a large number of cables to transmit the secondary side electrical parameters of PT (Potential Transformer) and CT (Current Transformer) to the central control room, and reduce the error caused by line loss, thus improving the measurement accuracy. In addition, the central control room can be eliminated to achieve unmanned operation, which not only saves floor space and infrastructure investment, but also frees the operating personnel from the fatigue of day and night duty. Compared with the traditional plant and station monitoring and control system, the real-time monitoring and control system of power plants and substations based on fieldbus has the following characteristics:

1) The front-end intelligent unit (or field intelligent device) can be installed in the CT or PT terminal box to process the collected data on-site and then transmit it digitally (via the field bus).

2) The network is composed of two twisted pair cables with low failure rate and high communication baud rate.

3) It solves the problem of voltage drop between CT and PT and the central control room in the traditional monitoring system, and has high measurement accuracy.

4) Strong anti-interference ability and can operate under harsh outdoor conditions.

5) Simple structure, small size, easy installation and maintenance.

Based on the above characteristics of the system, it can be predicted that the monitoring and surveillance system based on fieldbus will be widely used in power plants and substations of the power system, and it will constitute a new generation of dispatching automation system together with data channels and dispatching-end monitoring systems.