Selecting the Master Station in SCADA systems

The master unit (MTU) is one of the most important components in a SCADA control system; therefore, it is important to understand the functions this equipment performs within the system and the selection criteria for it.

The MTU master unit sends information to each RTU using a common communication medium and protocol. Both the master station and the RTU essentially have the same communication equipment. The difference lies in the fact that only the MTU master station can initiate the communication. This communication is initiated by a program within the MTU, which can be triggered by a manual instruction from the operator or by the MTU's own program. 99% messages are initiated automatically by the MTU as part of its control routines.

Many MTUs use proprietary protocols depending on the equipment vendor. This protocol must be compatible with all stations connected to the network. This type of technology is known as a closed architecture, which is characterized by requiring the use of equipment from only one vendor.

Fig. 1

Example of a communication module for a master station:

When working with open technology systems, it's assumed that you can work with different vendors during the system design phase. This is primarily because a standard is shared among the vendors.

SETTING UP A PROCESS SCREEN

To view accurate status information from remote stations, all necessary field devices must be connected to the RTUs. This allows the MTU to obtain this information.

Fig. 2

The following is an example of a simple process for controlling a fuel supply line.

Fig. 3

The RTU 1 monitors the pump's status, indicating whether it is on or off. It can also control the pump's on/off state. Similarly, it can open and close the valve and identify its status. Additionally, it displays the total flow through the pipeline and alerts for low pressure in the line.

RTU 2 has only one valve control block. It controls the opening and closing of the valve and monitors its status. As with RTU 1, it also monitors the minimum pressure in the process line.

The RTU3 has the same function as the RTU1, except that there is no pump to control.

A simple system is in place for monitoring the SCADA system. The goal is to prevent leaks in the process feed line. Detection can be performed by measuring the difference between the inlet and outlet totalizer readings. Additionally, continuous monitoring of pressure loss in the line is performed in case of a fuel leak. When either of these conditions occurs, an alarm signal is triggered in the master station's data table, prompting direct action on the process. In this case, a signal is sent to sequentially close the process valves. If confirmation that the valves have closed is not received, the MTU resends the closure signal. If the problem persists, an alarm signal is sent to the operator to take direct action on the process.

In case of failure, the MTU should automatically re-establish communication with the RTUs. If communication cannot be re-established, a routine can be activated to stop the process. This means the RTUs will close their valves and cut off the flow.

The process status is visualized from a monitoring station consisting of a PC with monitoring software that allows the status of the equipment to be viewed graphically.

Fig. 4

The following are the codes that are sent to the RTU1 for process control.

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