English Original Reader for Technical Students. Power transformers: short-circuit testing, monitoring systems (Smart Grid) - страница 4

where: uaverage is the average value of voltage,

dij/dt is the value of current derivation.

Expression (1.5) can be obtained from Ohm's law for the magnetic circuit:

Further using the expressions below

we obtain

Disregarding the second term (assuming L = const), and assuming the linear characteristic of the medium, we have

which is analogous to (1.5).

In the block of bringing the value of inductance to the nominal frequency the instantaneous value of inductance corrected to the nominal frequency is calculated:

where: fmeas is the value of the frequency (Hz) measured by frequency converter,

fnom. is the nominal value of the frequency.

Lj meas. is the instantaneous value of inductance.

In the following block the average value of inductance during each period is calculated:

In the block of calculation of deviation, Laverage value during the period is compared with the base L0 value, and their difference is calculated:

where: L average is the average value of inductance during the period;

L0 is the base value of transformer inductance, determined from the preliminary experiment.

When winding deformations begin or in the case of winding turn-to-turn internal short-circuit, the value of inductance L tends to increase, or to decrease from one time period to the next period in case of irreversible destruction of the controlled power transformer windings.

Then, the signal from the control block enters the protection block (rapid digital protection), where a signal to switch off is formed in the high-voltage circuit breaker (B). After that Information-measuring system and the connected with it protection block stop the process of winding destruction [by 1–4].

This Smart Grid monitoring system is a perspective direction of diagnostics under operating voltage.

Basing on the foregoing, the algorithm (1.1–1.11) was developed and now constitutes a special program for calculating instantaneous inductance L during electrodynamic test of the power transformer by short-circuit currents. The program allows determining the average value of the inductance during each period. This value is more significant in the case of damage to the coil (primary deformation, coil circuit). To evaluate the potential effectiveness of the monitoring system, the calculations of instantaneous and average values of inductance (L) were performed for the case of accidental and non-regime (ANR) due to damage during short-circuit testing at Power Testing Laboratory (Togliatti) of 167 MVA/ 500/220 kV autotransformer.