**4. Discussion**

Because of the time-varying motor torque value as well as the speed-dependent ventube load torque, the starting RMS current and the starting time analysis have to be done using numerical simulation, based on the parameters for the double-cage induction motor equivalent circuit. Because of the specific structure of mine electrical power network, there is a need to analyze the motor starting conditions at different values of network voltage below the rated value. Overcurrent protections with Standard Inverse, Very Inverse and Extremely Inverse characteristics consistent with Reference [4] do not allow for starting of the drive with a high moment of inertia, especially in conditions of low power voltage. Whereas, protections with Long Inverse [4] or Invertim [16] characteristics allow for the proper starting of the motor, but barely protect the motor in case of failed starting and in case of relatively slight overloads—due to the long operating time.

It would be beneficial to introduce protection relays with characteristics that are intermediate between Standard and Extremely Inverse, as well as Long Inverse and Invertim. This paper presented the proposed parameters describing such a characteristic, designed on the basis of a numerical simulation study. These new characteristics can be easily implemented in modern digital protection relays widely used in mining power networks as they need no new equation (and therefore no significant changes in relay algorithm) but only small changes in coefficient values. Nowadays, microprocessor-based protection relays are widely used even in low-voltage mining electrical equipment, as they conveniently integrate many different protective functions (like short-circuit protection, overload protection, ground-fault protection, insulation monitoring, temperature monitoring) in one compact device. Flexible user-defined characteristics are hard to implement in electromechanical protective relays, but their role in modern industrial power networks is diminishing [23].
