16 th INTERNATIONAL SYMPOSIUM on POWER ELECTRONICS - Ee 2011 NOVI SAD, REPUBLIC OF SERBIA, October 26 th - 28 th , 2011 1 Abstract: One possible solution for coal mill capacity increase is analyzed and presented in the paper. It is based on application of MV variable speed drive with increased frequency of motor voltage supply in the range of 50-55Hz. In the paper the thorough analysis of the existing MV drive is presented and possibilities of mill capacity increase by speed increase are validated through experiments and calculations. Key Words: MV drive/Variable speed/Coal mill 1. INTRODUCTION In virtually all mining operations, mill systems are a critical part of the process to provide economical, reliable and energy efficient grinding. For a variety of ores, cement clinker and other materials, mills transform masses of raw material and refine it to a usable size. The physical size of a mill (in many cases spanning 15–20 meters in diameter and 30–50 meters in length) provides mechanical and electrical engineers a challenge to start the mill without affecting other processes in the operation. Industry has now seen a shift in technology to use variable speed drives to deal with the mill’s high starting torque requirement and optimized grinding speed, which results in increased production rates. Operating mills at a slightly lower speed or even a slightly higher speed than line frequency gives process engineers the advantage of the mills being optimized for the grade of material and desired throughput of the final process [1]. The problem under scope is how to increase the boiler power in thermal power plant (TPP) slightly (in range 5 to 10%). To get the target boiler power increase in order of 5 to 10% of rated power, it is necessary to increase the fuel intake and one of the possibilities for that is the coal grinding mill capacity increase. There are six coal mills supplying one boiler in Thermal power plant „Nikola Tesla – A“ configuration and question is how to slightly increase mill capacity. The coal mills under consideration are of fan type and they are started unloaded by the rule. 2. MILL DRIVE REQUIREMENTS The selection of a suitable starting and controlling method for the motor and load combination is becoming increasingly more important if the system requires variable speed control. The mechanical arrangement of the mill will determine the starting and control method that best suits the mill drive needs. This results in the application being optimized to best serve the power plant needs. The precise optimum mill RPM is determined by the forces of the material and additional mass components located in the mill. With variable speed systems, every newly commissioned drive system can be set to its optimum speed and its speed can be easily adjusted at any time to suit different process requirements. This provides the user with both the capabilities to soft start the mill (to eliminate the power system disturbances by the inrush current when starting the mill at full voltage) and still produce sufficient torque to accelerate the load to the required process speed under reduced mechanical and electrical stress. There are two main possibilities for coal mill capacity increase: the mill mechanical reconstruction by increasing mill drum size and by increasing the mill rotational speed. The first solution, by increasing the mill drum size, is the change of existing mill with bigger one, what is costly. The second solution is based on enhanced motor voltage supply by increasing frequency, what is possible by MV inverter. The main goal in this case is to supply motor with rated voltage and frequency in range between 50Hz and 55Hz, if possible [2], [3], [4], [5], [6], [7], [8], [9]. . 3. MV DRIVE ANALYSIS The double cage 6kV 800kW motor is driving coal mill via dry powder clutch, PULVIS type (Fig. 1). Fig. 1. Mill drive in thermal power plant MV VARIABLE SPEED DRIVE FOR COAL MILL CAPACITY IMPROVEMENT Žarko Janda, Aleksandar Nikolić Electrical Engineering Institute Nikola Tesla, University of Belgrade, Belgrade, Serbia Paper No. T4-2.10, pp. 1-4