Let us learn in this topic about the overview of power plant control system. At the end of this topic, you will be able to learn about the overview and concept of control system, the evolution of control system and automated control system, the types of automated control system. Let's first understand the basics of control and it's requirement. What is control? Control is the process of altering the performance parameters of the system to get desired output. Why control is required? The system does not behave optimally on its own, so control is required to regulate various parameters of the system to improve efficiency, quality, safety, and overall performance of the plant. Let us understand the concept of control systems with a simple analogy. Control system operates very much like a human body. We perceive or sense our surroundings using our sensory organs, like eyes, ears, nose, tongue, and skin. These sensory organs continuously send signals to the brain through the complex nervous system. The brain processes the input signal and sends command signals to hands, legs, and other parts of the body to act. Similarly in power plants, various sensors like transmitters, switches, analysis, etc, that resemble our sensory organs, transmit real-time plant parameters continuously to the controller that resembles our brain through a network of cables that resembles our nervous system. The controller processes these input signals as per the predefined logic and responds by sending output signals to actuators and indicators that resemble our hands and legs acting as final control elements. Thus, through this analogy, we have understood the concept of control systems. Let us now look at the evolution of the control system. In older day, when automated controls were not developed, the operator used to control the plant manually from the field. Let us take an example of the temperature control loop in process. In this process, we can see a typical boiler used to heat feed water to generate the steam of desired quality. The temperature of the steam can be controlled by varying the fuel flow to the boiler furnace. In the manual control system, an operator is modulating the valve by observing the temperature gauge to maintain steam temperature. As we can see, the operator is facing difficulty in reading the temperature gauge at a distance and modulating the valve to achieve the desired temperature. In a supercritical power plant, hundreds of critical control loops are required to be controlled in a coordinated manner simultaneously for efficient operation of the power plant. Just think, how many operators will be required in the plant? What if operator is not able to reach the control element on a timely manner? What if operator cannot control it accurately? If not, the process will become unstable and furnace temperature may shoot up, leading to a blast in the furnace. With the evolution and advancement of electronics, sophisticated automated control systems are implemented in power plants. In the earlier example of the temperature control loop, the temperature gauge is replaced by a temperature transmitter, which continuously transmits real-time temperature value, which is the process value, PV to the electronic controller. The controller will generate an error signal between the process value PV and set point, SP as per the stored program and generate the output signal. This output signal will be used to modulate the control valve to maintain desired steam temperature. Starting with the small microprocessor based control system. Nowadays, the control systems are so advanced with artificial intelligence built-in such that complete power plant can be efficiently controlled automatically from even a mobile or PC at the remote location and that too with the minimum number of operators. In the modern power plant, different types of automated control systems are used as per system requirements. Relay-based control system. It is a very old type of system and is rarely used in power plant. Sometimes it is used for sump pump system. Microprocessor based control system is typically used for the main fire alarm system of the plant considering a cost-effective solution due to simple control requirements and less input or output. PLC, programmable logic controller. PLCs are used for mechanical systems where a dedicated control system is required, and interface with main power plant control system is minimal. For example, DM water generation system, electrostatic precipitator system, etc. DCS, distributed control system. DCS is used to control main power plant systems like feed water systems, condensate systems, heater systems, boilers systems, etc. SCADA, supervisory control and data acquisition. It is used for monitoring and control of switch yard, indoor substation, and other process plants. Let us summarize what you have learned. The control system analogy to humans. Need for the automated control system from manual control system. Classification of control system.
