Master student Tsyplakova D.D., Professor Ushakov I.V., Ph.D. Dyakov I.A.

Tambov State Technical University, Russia

THE METHOD OF COMPUTING SHORT-CIRCUIT CURRENTS

Short-circuit (SC) is any unintended normal operating conditions, circuit between phases, and in systems with earthed neutral (or four) as the closure of one or more phases on the ground (or neutral).

To select the devices and conductors, to determine the effect on the load-bearing structures in the calculation of short circuit currents based on the following provisions: all the sources that feed the point under consideration, working with rated load; synchronous machines have automatic voltage regulators and high-speed devices forcing excitation; fault occurs at the time point when the fault current has a maximum value; electromotive force all power supplies are in phase; rated voltage of each step taken by 5% above the nominal voltage.

Take into account the impact on the short-circuit currents connected to the network synchronous compensators, synchronous and asynchronous motors. Effect of induction motors to short-circuit currents are not taken into account when:

a) unit capacity of 100 kW electric motors, electric motors if the fault location remote from one stage of transformation;

b) any power when they are remote from the fault position by two or more stages of transformation, or if the current from them can come to a place of RS only through those elements through which the main circuit current from the network, which have significant resistance (line transformers, etc. .d.).

The calculation scheme for determining the fault current is a diagram in one-line version, which introduced generators and motors, affecting short-circuit current, as well as elements of the system power supply (line, transformer), connecting power sources to the short circuit.

By design scheme constitute an equivalent circuit in which the transformer is replaced by the electrical connection (Figure 1). Elements of the power supply system (PSS), connecting power sources to the short circuit is introduced into the equivalent circuit impedances, and power sources - resistance and EMF. Resistance and EMF equivalent circuit must be brought to the same voltage level.

The calculation is usually carried out in basic terms: basic full power S_б; the base voltage of the first level U_б1.

Then the the base current of the first level: .

The basic voltage and current of the second stage:

The basic voltage and current of the third level:

, .

Resistance element equivalent circuit SES at the following stages of calculation determined by baseline value.

Find the resistance of the main elements of the equivalent circuit of the power supply system (the system of overhead lines, cable lines, transformers, asynchronous motors):

System: resistance of the system referred to the power system S_б:

x_сmin =_сmin, x_сmax =_сmax.

Electromotive force system:

Air line: the reactance of the overhead line: , where x₀ - specific reactance of the overhead line, Ohm km; L - length of lines, km. Resistance of overhead lines: _.

Transformer: reactance transformer load tap changer:, , where U_к – Voltage short circuit%; S_{тр н} – nominal capacity of the transformer, MBA; U_н – ated voltage of the transformer, kV.

Resistance without transformer tap changer: where DР_к – short-circuit power, MW. Impedance transformer: , .

Cable line: reactance: , where X_КЛ – reactance cable line, Ohm; active resistance: , where R_КЛ – active resistance cable line, Ohm. Impedance cabling КЛ:

Calculation is carried out for the current three-phase short-circuit (SC), as This mode is heaviest.

Short-circuit current is given by:, where Е_экв – equivalent electromotive force; Z_экв – the equivalent resistance circuit faults.

Short-circuit current is given by: , where Е_экв – equivalent electromotive force; Z_экв – the equivalent resistance circuit faults.

After finding the fault current to the basic values of the value transferred to the named unit. For example, we find the fault current at the point K1. Design scheme for the calculation of short-circuit current is as follows (Figure 2).

For further calculation we transform design scheme into an equivalent circuit diagram in basic values (Figure 3).

We find the equivalent electromotive force:

Equivalent resistance:

Short-circuit current:

Short-circuit current in the named values:

The calculation of short circuit currents for the remaining points of the circuit to a high voltage runs similarly.