English Original Reader for Technical Students. Power transformers: short-circuit testing, monitoring systems (Smart Grid) - страница 5

During short-circuit testing of 167 MVA/ 500/220 kV autotransformer at the second short-circuit shot, i.e., experience with 100 % value of the short-circuit current 0.2 sec duration according to the test program (time set by the conditions of experience – 10 periods of current, and regulated by high voltage thyristor valves), there has been actuation of transformer gas protection.

Audit and inspection with the lifting of transformer tank and inspection of the active part of autotransformer have revealed the presence of electrical damage to the regulation winding (RW) – turn-to-turn short circuit. Figure 2 shows the real oscillograms of short-circuit current (Figure 2a), voltage (Figure 2b), estimated the average curve of inductance for 10 periods (Figure 2c), the calculated curve instantaneous inductance (Figure 2d) in the second short-circuit shot. Change of the value of the inductance of the curves in Figure 2 shows that the electric damage of RW winding happened at the 4th period and continued to develop in the remaining periods of short-circuit shot.

Calculations of inductance values show that the application of Smart Grid monitoring system and quick-working protection would submit a command to turn off the high-voltage circuit breaker in the fourth period of current that is, taking into account the work of protection and circuit breaker (at least three periods, i.e., 0.06 seconds), cease the emergency process at the 7-th period of current. Thus one could reduce the damage of RW windings and the cost of its repair at the transformer manufacturer.

a)

b)

c) and d) Figure 2. Oscillograms of short-circuit current (Figure 2a), voltage (Figure 2b), estimated the average curve of inductance for 10 periods (Figure 2c), the calculated curve instantaneous inductance (Figure 2d) in the second short-circuit shot of 167 MVA/ 500/220 kV autotransformer.

Smart Grid Monitoring System for control of parameters of the transformer when tested for withstands to short-circuit currents, part of the quick-working protection, is discussed in [by 1–3].

Figure 3. Smart Grid Monitoring System for control of transformer parameters during short-circuit testing, which is a part of the quick-working protection. 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.