English Original Reader for Technical Students. Power transformers: short-circuit testing, monitoring systems (Smart Grid) - страница 6

Control of the average value of inductance Laverage for the period during the test allows fixing moment of the beginning of the emergency regime and reducing the scale of the accident if the tested transformer is timely disconnected. The Monitoring System provides a more accurate measurement of inductance and increases the reliability of the power transformer in case of dangerous deformations.

Quick-working protection prevents accidental destruction of the test object and increases the crash safety of the test (Figure 3) [by 1–5].

In Figure 3 the following details of the equipment are shown: 1-power supply (network), 2-safety high-voltage circuit breaker, 3-test transformer, 4-synchronous short-circuiter, 5–7-capacitive voltage dividers, 8–9, the control block, 9 – voltage transformer, 10–12-current-measurement shunts, 14–22-the functional blocks of the inductance average value’s calculation of the deviation from the original value, 23-testing transformer in the secondary winding short-circuit mode [1–5].

Consider the work of the monitoring system in Figure 3 with an example of the 400 MVA/220 kV transformer testing. Current and voltage oscillograms at the second short-circuit shot on the phase «C» of the 400 MVA/220 kV transformer are shown in Figure 4.

Figure 4. Current oscillogram (1) and voltage oscillogram (2) in the second short-circuit shot on the phase «C» of the 400 MVA/220 kV transformer.

Current oscillogram analysis shows that the value of aperiodical (shock) component of short-circuit current at the beginning of the short-circuit shot amounted to 12.8 kA, and through 10 periods after attenuation of aperiodical (shock) component at the end of the shot, then periodic component is only 10.2 kA.

The calculated curve derivative from current and calculated inductance Ls-c curve of 400 MVA/220 kV transformer in the short-circuit shot on the phase «C» with 100 % of the value of the aperiodical (shock) short-circuit current are shown in Figure 5. Deviation of Ls-c amounted to +1.3 % in the short-circuit shot.

Figure 5. The calculated curve in the short-circuit shot on the phase «C» with 100 % of the value of the aperiodical (shock) short-circuit current of 400 MVA/220 kV transformer: a) derivative from current; b) calculated inductance Ls-c curve.