Читать книгу English Original Reader for Technical Students. Power transformers: short-circuit testing, monitoring systems (Smart Grid) - Александр Юрьевич Хренников - Страница 6

The algorithm of IMS’s work is the following. The continuous control of the winding’s state of the controlled power transformer is ensured by a constant monitoring of deviation of inductance value from the base value of inductance, which is obtained from the block of the base inductance.

During the work of the three-phase controlled power transformer (T) for the three-phase resistive load (Load) the value of primary voltage U1 is obtained by measuring converters primary voltage (high-voltage transformers TV1).

Signal from the converters was input to the entrance of the block which brings the primary voltage to the second one. In this block the value of the primary voltage, which is corrected to the second, is calculated:

Signal from the converters was entered to the entrance of the block of bringing the primary voltage to the second. In this block the value of the primary voltage, which is corrected to the second, is calculated:

(1.1)

where: Kt is the known given value of the transformation ratio of the power transformer.

Signals from the measuring converters of second voltage (voltage transformers TV2) and signals from the output of the previous block are input to the entrance.

In the block of calculation of voltage difference, which is corrected to the second side, we determine

,(1.2)

where: U2 is the value of second voltage, measured by converters TV2.

The calculations are performed in the assigned time interval in the block of calculation of voltage’s average value:

(1.3)

where: uj is the difference between corrected to the second side voltages on the transformer;

t1 and t2 are the temporary boundaries of the partition interval.

In the block of calculation of the current derivation the increase of the current in the assigned time interval is calculated:

(1.4)

Here ij is the value of current in the secondary winding of the controlled transformer, measured by current converters (current transformers CT).

In the block of calculation of inductance, the instantaneous value of inductance is determined in the assigned time interval:

(1.5)

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:

(1.6)

Further using the expressions below

(1.7)

we obtain .

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

(1.8)

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:

(1.9)

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:

(1.10)

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

(1.11)

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.