Conditions for the use of autotransformers


   With a correctly calculated ratio of primary and secondary voltages autotransformer has significant advantages over a transformer with the same voltage ratio and the same power. The autotransformer has lower mass, dimensions, no-load and load losses, magnetizing current and short-circuit resistance. As you know, the linear dimensions of the transformer are proportional to its power to the power of 0.25 (S0.25), and the volume and mass - to the power of 0.75 (S0.75), all other things being equal. Thus, the lower the typical power (transformer power, determined mainly by the size of the magnetic circuit, i.e., its mass) in comparison with the throughput (the total power transmitted by the autotransformer from the primary network to the secondary), the smaller the dimensions, weight and losses of the autotransformer. So with a typical power that is half the throughput, the mass, losses and no-load current of the autotransformer will be 10% less than that of a transformer of the same throughput power. By reducing losses, the efficiency is increased. Reducing the short-circuit resistance reduces the voltage drop during autotransformer operation.
                               силовой автотрансформатор
  The reduced weight and dimensions of the autotransformer create more favorable conditions for its delivery to the installation site. If it is necessary to transform a very high power, for example, when connecting two very powerful power systems, only an autotransformer can be manufactured within the transport restrictions in terms of weight and overall dimensions, that is, in one transport unit 


  The presence of a galvanic connection of the windings in the autotransformer results in certain disadvantages. As a rule, the windings of the autotransformer are connected in a star with a grounded neutral. Other connections are theoretically possible, but are associated with certain inconveniences and therefore are rarely used. The neutral grounding mode of both systems must be the same: solid ground or resistance grounding. In this case, the resistance value must be such that unacceptable voltages do not arise at the medium voltage inputs of healthy phases during a ground fault of one phase in a high voltage system. This danger increases as the voltage difference between the two systems increases.
  For the same reason, autotransformers are not used in systems with a grounded neutral. High potentials of lightning overvoltages at the idle input of the autotransformer when an overvoltage wave is applied to another input makes it necessary to install arresters on the inputs, which cannot be switched off when the line connected to this input is disconnected. The series winding of an autotransformer and its longitudinal insulation can be subject to very severe lightning effects when the voltage values of the two systems are close. However, in practice, such combinations of voltages do not exist. The regulating winding, when regulated in a high voltage or medium voltage line, is subjected to all the influences normalized for the line input. Sometimes it is difficult to ensure the dielectric strength of the insulation of the regulating winding and the switching device, especially for extra-high voltage (class 525 kV and above). 
  The short-circuit resistance of the autotransformer is relatively low, which is the reason for the more severe effects of short-circuit currents. Special measures have to be taken to increase the short-circuit resistance. Ensuring strength in single-phase faults requires special attention. The presence of a low voltage winding (tertiary winding) requires its dynamic strength, for example, by increasing the zero sequence resistance (neutral or delta resistance).
  Conditions for the use of autotransformers

Compared with conventional transformers the same parameters autotransformers are smaller, but require certain conditions that limit their use in power systems. Disregarding special applications where no alternative is available, autotransformers should be selected after detailed consideration of all operating conditions. In general, the decision to use autotransformers can be made under the following conditions:

  • system with a grounded neutral;
  • the system has a limited short-circuit power:
  • favorable situation with overvoltage;
  • transformation ratio close to unity (0.5-2);
  • balanced load.

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Total comments: 2

  • Николай
    By Nikolay Added on August 15, 2018 at 00:36

    How much will a 6kVA 110V autotransformer cost?

    To answer
  • leader
    By leader Added on August 15, 2018 at 00:40

    Answered the mail

    To answer

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