Selection of power transformers

Selection of the required number of transformers

 Typically one or two transformers are selected in the substation. In this case, one-transformer substations are chosen:
- for power supply of electrical consumers that allow power from only one non-redundant source (electrical consumers of category III);
- for powering power consumers of any category through closed networks connected to two or more substations (or through open networks, interconnected by backup lines).
Two transformers are installed at substations that supply power consumers of I or II category and do not have a connection with other substations on the secondary voltage.
So that both transformers can reliably back up each other, they are powered from independent sources through independent lines. Due to the fact that the mutual redundancy of transformers must be equivalent, they are chosen of the same power.
The main step-down substations (GPP) of enterprises, as a rule, are built with two-transformer.
The need for more transformers is rare.
In accordance with / 1 /, / 2 / and / 6 /, single-transformer substations are recommended to be used if there are electrical receivers in the workshop that allow an interruption in power supply for the time of delivery of the "warehouse" reserve, or during redundancy carried out along low voltage lines from neighboring transformer substations, i.e. That is, they are permissible for consumers of categories III and II, as well as in the presence of a small number (up to 20%) of consumers of category I in the 380-660 V network.
It is recommended to use two-transformer substations in the following cases:
- with the predominance of consumers of category I and the presence of consumers of a special group (the latter needs a third source);
- for a concentrated shop load and detached objects of general plant purpose (compressor and pumping substations);
- for workshops with a high specific load density (above 0.5-0.7 kVA / m2).

Selection of transformer design

  By design, transformers are divided into oil, filled with synthetic fluids and dry / 3 /. The first of them have good heat dissipation from the windings and core, good dielectric impregnation of insulation, reliable protection of active parts from environmental influences, and low cost. Their disadvantage is the possibility of a fire, explosion or release of oil decomposition products in case of accidental damage to the insulation, leading to an arc short circuit (SC) inside the transformer tank, especially in case of failure or incorrect operation of the protection. Therefore, such transformers are used for outdoor installation or for installation in special transformer rooms of substations.
If transformers are to be installed inside the workshop in order to bring the TP closer to the center of electrical loads, then dry (oil-free) transformers are used for fire safety reasons. The cooling conditions of such transformers are worse than those of oil transformers, therefore, the current density in their windings is less, and the dimensions, consumption of active materials and the cost are correspondingly higher. Therefore, the choice of the type of transformer (oil or dry) is a technical and economic task.
A variety of insulation materials are used in dry-type transformers. The most reliable is cast insulation made of hardening synthetic resins and, usually two-thirds, silica powder filler.
Dry transformers with increased fire safety are manufactured by the Swedish company Mora Transformer. Their rated power ranges from 50 to 5000 kilovoltamperes (kVA). Thanks to the air-glass fiber coil insulation and ceramic support-insulating structures, the amount of combustible substances in them is reduced to 0.9 - 1.6 percent of the total weight of the transformer. Self-burning of such transformers is impossible, and the heat release during their combustion in a fire is 5-10 times less than in the case of dry transformers with cast epoxy insulation, or 40 to 80 times less than in the case of oil transformers.
The fire safety of the transformer is also ensured when using synthetic non-flammable filling fluids. Currently, new non-flammable and at the same time non-toxic liquids have been developed, for example, tetrachlorobenzyltoluene, which have not yet found widespread use.

Selection of rated voltages and method of regulating the secondary voltage of transformers

  For two-winding transformers in the passport data, the rated voltage of the winding of the higher and lower voltage is given - IHV and INN, respectively. For three-winding - respectively, the rated voltage of the high, medium and low voltage windings -
According to the method of secondary voltage regulation, transformers are divided into:
1) adjustable by switching the primary winding taps when the transformer is disconnected; such transformers are equipped with an off-circuit tap-changer (switching without excitation);
2) adjustable under load, i.e. by switching the taps of the primary winding without disconnecting the transformer; such transformers are equipped with an on-load tap-changer (on-load regulation);
In the first case, infrequent seasonal changes in the transformation ratio are possible within the range from -5 to +5 percent; usually five switching steps are used (-5; -2.5; 0; +2.5; +5 percent).
In the second case, the number of steps is larger (for example, 13 steps in the range from -9 to +9 percent, or 17 steps in the range from -12 to +12 percent, or 19 steps in the range from -16 to +16 percent). The transformer with on-load tap-changer is equipped with an external contact device for automatic step switching.
In both cases, the zero tap has a voltage corresponding to the transformer HV.
The primary voltage of the main switchgear of enterprises is maintained by the power systems so stable that usually the need to use transformers with on-load tap-changers disappears.

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