English Original Reader for Technical Students. Power transformers: short-circuit testing, monitoring systems (Smart Grid). Александр Юрьевич Хренников
are performed in the assigned time interval in the block of calculation of voltage’s average value:
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:
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:
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.
1.4. Inductance Calculations of 167 MVA/ 500/220 kV Autotransformer
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.
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.
1.5. Smart Grid Monitoring System for Short-Circuit Testing
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.
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,