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

Practical training #2


Calculation of the cross-section, type and parameters of cables according to the types, power and working conditions of electric consumers.
A two-winding transformer(Fig. 2.1, a) switching scheme is represented in G-shaped form (Fig. 2.1, b).





Figure 2.1. Schemes of a two-winding transformer.
Longitudinal section of the exchange scheme has active and reactive resistances of the transformer. These resistances are equal to the sum of the active and reactive resistances of the primary and secondary windings of the transformer, respectively. In such a scheme, the resistance of the secondary winding is brought to the primary winding without the presence of a transformation, that is, an ideal transformer. If the network connected to the transformer is considered together, and the network is not brought to the same level of voltage, then an ideal transformer is installed in the switching circuit of the transformer.
Transverse branch (magnetization branch) in the switching scheme , consisting of active and reactive conductances. Active conduction is the magnetizing current in the steel core of the transformer represents wasted active power. The reactive conductivity is determined by the mutual induction magnetic flux in the windings of the transformer.
When calculating power networks with a voltage of 220 kV and below, transformers are represented by simplified switching schemes (Fig. 2.1,v). In this scheme, instead of the magnetizing station, the power dissipated in the transformer steel or in idle mode is considered as an additional load.
The following parameters (catalogue data) are known for each transformer: - nominal capacity of the transformer, MVA; - nominal voltages of upper and lower pipes, kV; - active waste in idle mode, kW; %- pure working current, from %; - short circuit loss, kW; %- short circuit voltage, % from From this data, all the parameters of the switching circuit (resistances and conductances), as well as their losses, can be found.
The conductors of the magnetizing station are found using the results of a purely operational experiment. In this case, the transformer only dissipates power equal to the state of pure operation:
. (2.1)
Based on this, the conductors are found according to the following expressions:
, (2.2)
, (2.3)
The magnetizing current in the transformer has a very small active component:
,
here - reactive founder of .
From above
. (2.4)
Transformers and resistances are found using the results of short-circuit experiments. In this experiment, the secondary winding of the transformer is short-circuited and the primary winding is supplied with a voltage such that the rated currents flow in both windings. This voltage is short circuit voltage is held as In case of short circuit is very small compared to
, (2.5) and (2.6)
In modern high-power transformers and . From short circuit experience
. and . (2.7)
Three-phase transformers and autotransformers.In most cases, the substation has three nominal voltages – high , medium and lower voltages are required. Two two-phase transformers can be used for this. However, it is more economical to use one three-winding transformer or three-winding autotransformer than two two-winding transformers. The windings of a three-winding transformer are in mutual magnetic connection (Fig. 2.2, a). The connection diagram of autotransformer coils is shown in Fig. 2.2, b. The lower coil is in magnetic connection with the other two coils, and the series and common coils are in electrical and magnetic coupling with each other (P and O in Fig. 2.2, b) . In a row along the chulgam , and along the general winding current flows.