Demand for electricity generation is increasing day by day, and power stations must be able to meet these demands. There are several ways to generate power, such as thermal power plants. There are many variables in a thermal power plant boiler unit (steam unit), but the boiler drum level is one of the most important variables that has a very complex dynamics and it is necessary that the control system can keep it in a safe range. In this paper, two time-varying transfer functions are considered for the drum level (output) to water and steam (inputs). In the presence of parametric uncertainties in the model, the three controllers, Sliding Mode Controller (SMC), Model Reference Adaptive Control (MRAC) and PID Controller are compared to track the desired level of the drum-boiler to different inputs. The results show that the SMC has relatively better tracking results, but the control signal in MRAC is more optimal than the other two controllers and is more suitable for practical applications. پرونده مقاله