In my dissertation I analysed how the induction motor contributes to the short-circuit current, when a short circuit occurs in the electric grid. In the introduction, I described the basic operation of an induction motor, the occurrence of a short circuit and the types of short circuits. When it comes to the contribution of motors to short-circuits, we are most interested in the peak short circuit current. This is the maximum instantaneous current value, after the occurrence of a fault. This is also the value to which devices in the network must be mechanically dimensioned. For this reason, it is important to consider the contribution of the motors, which influences the magnitude of the short-circuit current and consequently the magnitude of the peak short circuit current in the network and, thus the mechanical dimensioning of the equipment. Then I studied the effect of induction motors on short-circuits. When a short circuit occurs in the electric grid, the rotating magnetic field in the rotor will attempt to support the reduced voltage condition, by becoming a power source. The motor will provide additional current into the faulted electrical system. In my dessertation I focused on how to calculate the magnitude of the contribution of induction motors, according to IEC 60909. According to the standard, when a short-circuit occurs in the network, both low- and medium-voltage motors contribute to the peak short circuit current ip, the initial symmetrical current I''k, the symmetrical short circuit breaking current Ib, the unbalanced short-circuit current and the steady-state short-circuit current. The standard clarifies that the contribution of induction motors to the short-circuit current I''k in low-voltage systems can be neglected, if their contribution does not exceed 5 % of the initial short-circuit current calculated, without taking the motors into account. When calculating the initial short-circuit current, the asynchronous motors are replaced by their impedances ZM, in both the positive and negative sequence of the system. However, the method and size of zero impedance of induction motor Z(0) must be specified by the seller.
Furthermore, I also focused on the combination of all the contributions of induction motors and their connecting cables into one equivalent motor, which allows the simplification of detailed model of electrical energy system. Later, I investigated the contribution of induction motors of different power ratings to the electric grid itself. I found that the short-circuit current contributed by a medium-voltage decays slowly and lasts long, while the short-circuit current contributed by a low-voltage motor decays fast and lasts short.
I found that with the PASCAD program, it is possible to get exact course of the peak short circuit current with the equivalent model.
I then analysed how we could reduce the contribution of the induction motor to the short circuit. I found that by using a variable speed drive, we can reduce or even eliminate the contribution of induction motors to the short circuit.
This is usually worthwhile for large motors, which contribute more current to the short circuit. In addition, I described other key benefits that make the variable speed drive and variable speed drive with the AFE (Active Front End), more widely used today.
Finally, I researched the short-circuit protection of the induction motor. I found that different types of protection can be used, from simple fuses to circuit breakers and various relays, which are the most advanced today. I described in detail the protection provided by ordinary fuses and found that their choice is very important when protecting an induction motor, as they must operate in a delayed manner (delayed tripping in the event of a short circuit), to allow the induction motor to start properly.
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