Practical training #2 Calculation of the cross-section, type and parameters of cables according to the types, power and working conditions of electric consumers

Figure 2.2.Schemes of three-phase transformer and autotransformer.


Nominal power of the autotransformerrefers to the maximum power that it can receive from or transmit to the high-voltage network under nominal conditions:
. (2.8)
This power is also called transition power. This power refers to the maximum power that the autotransformer can transfer from the high voltage network to the medium voltage network or vice versa without a load.
The series winding of the autotransformer is designed for P-type power (Fig. 7.5, b):
, (2.9)
here - of The coefficient of preference indicating how many times smaller compared to .
In the above procedure, it can be proved that the power of the general circuit is also equal to the type power. A lower voltage circuit is also designed for rated power or less. It is expressed by nominal power as follows:
, (2.10)
here In cases where kV .
The switching scheme of a three-winding transformer and an autotransformer for the case of kV is shown in Fig. 2.2, v, For the case of kV, it is described in Fig. 2.2, b. Even in this case, the energy is wasted in just working and considered as in two-winding transformers. For three-phase transformers and autotransformers, the short-circuit losses and voltages for three phase pairs, i.e. and is given in the form of Each and refers to one of three possible experiences.
For example, and values ​​of the lower circuit is short-circuited, the middle circuit is open-circuited to the upper circuit, which ensures the nominal current flow through the lower circuit voltage is determined at a given state. In this case, as in two-winding transformers
, (2.11)
, (2.12)
, (2.13)
Equations (2.11)-(2.13) have three unknown active resistances. Solving them together, we get the expression for aging:
, (2.14)
, (2.15)
, (2.16)
here are found by the following formulas:
, (2.17)
, (2.18)
. (2.19)
Similarly are:
, (2.20)
, (2.21)
. (2.22)
Using the found values ​​of , the reactive resistances of individual pipes are determined according to the following formulas: , (2.23) , (2.24) . (2.25)


Test questions
1. What does the transmission line replacement scheme look like in general?
2. In what lines is the wasted power due to corona taken into account?
3. What is the nature of the energy wasted due to corona? How is it accounted for in the replacement scheme?