Lecture
The rms value of the periodic component of the fundamental-frequency current at any instant during the interval from the onset of the short circuit to the steady-state condition is determined by the expression
, (4.8)
where 
- are arbitrary values of the periodic components along the d and q axes of the synchronous machine.
From
,
where 
- are, respectively, the periodic components of the steady-state, transient, and subtransient short-circuit currents.
The current decay time constants are determined by the expressions
Time constant of the field winding with the stator winding closed and the damper winding open:
The values of the parameters entering the expression for the time constants are calculated using the formulas:
The function 
),
where 
– time constant of the excitation system,
.
The total phase short-circuit current at an arbitrary instant consists of a periodic component, an aperiodic component, and a double-frequency component caused by rotor asymmetry and the presence of the aperiodic component of the stator current .
For example, the current of phase A:
(4.8)
where 
– negative-sequence reactance.
Expression (4.8) shows that the exact calculation of short-circuit currents at an arbitrary instant in complex power supply systems containing, besides the power sources, heavy loads, is quite difficult.
+Taking this circumstance into account, as well as the lack of exact data and the variety of possible power-system operating conditions, approximate calculation methods are used in practice to determine short-circuit currents at an arbitrary instant.
Comments