**3. Operation improvement and stability**

In virtually all coal preparation operations, mill systems are a critical part of the process to provide economical, reliable and energy‐efficient grinding. Operating mills at a slightly lower speed or even a slightly higher speed than line frequency give process engineers the advan‐ tage of the mills being optimized for the grade of material and desired throughput of the final process [8]. To get the target boiler power increase in order for 5–10% of rated power, it is nec‐ essary to increase the fuel intake and one of the possibilities for that is the coal grinding mill capacity increase [9]. Proposed solution in Ref. [9] is based on enhanced motor voltage supply by increasing frequency, what is possible by medium voltage (MV) inverter. The main goal is to supply motor with rated voltage and frequency in range between 50 and 55 Hz to obtain increase of plant power for 10% by increasing grinding mill capacity. Additional benefits are reduced mechanical stress during start‐up and the additional possibility of mill slow running for inspection purposes.

In order to improve power plant stability while operating close to its capability limits, as a requirement of a deregulated electricity market, one solution could be to optimally coordinate the synchronous generators' reactive power outputs in order to maintain the total reactive power delivered by a steam power plant (SPP) or the voltage at a steam power plant high volt‐ age (HV) busbar [10]. In such way, it is possible to aggregate the multimachine power plant into single virtual generator, thus enabling more sophisticated zonal voltage control across power transmission network.
