DETECTION OF INTER TURN FAULT IN TRANSFORMER WINDINGS BY PARAMETERS OF THEIR TRANSITION PROCESSES

: The proposed method of detection the inter turn fault of transformer windings relates to the area of defectoscopy and allows detecting inter turn faults in a wide range of damaged (closed) turns. Power and instrument transformers with iron-core are widely used in power networks. As the insulation ages or is damaged, the wires between various transformer sections short circuits occur, which inevitably leads to a complete damage of the transformer. Short-circuited part of the transformer forms an additional winding, the outputs of which are short-circuited. The transition process of current increasing when DC voltage is connected to the transformer outputs occurs in diverse ways in undamaged (cut-off) winding section and in the damaged (short-circuited) section. The current growth rate in the undamaged section of the winding is determined by high magnetization inductance. The inductance of the short-circuited part of the winding is much less, so, the current growth rate in the short-circuited part is significantly greater than the current growth rate in the undamaged part of the winding. The article presents observations from computer models and real measurements of the substation auxiliary power transformer, which show the possibility of determining the presence of a turn fault in regards to the transition process parameters, the rate of current increase and decay in the transformer winding. The device aimed to find the inter turn faults in the transformer windings, working according to the proposed method will be quite simple and have a high sensitivity.


Introduction
One of the most frequently-occurring transformer defects is the inter turn fault, when some of the transformer winding turns is short-circuited [1]. Usually, the presence of inter turn faults is detected from the parameters of the transformer normal operation at industrial frequency of 50 Hz [2][3][4][5][6]. Increasing the frequency range during measurements of transformer parameters can increase the sensitivity of detection methods of inter turn faults [7,8].
In case of inter turn faults, two parts of a winding are formed: the undamaged part (A in Fig. 1), which plays the role of the primary transformer winding, and short-circuited one (B in Fig.  1), which works as a secondary transformer winding.
To analyze the operation of the transformer winding with an inter turn fault, consider the Tshaped model [9] (Fig. 1), where the active resistance of the A turns is Rа; scattering inductance of the A turns is La; active resistance of B turns is Rb; scattering inductance of the B turns is Lb; inductance of saturation is L'o (which is less than the initial inductance Lo due to short-circuiting of B turns); E is the EMF connected to the winding. (All values are given for side A).

Materials and methods
To illustrate the operating principle of method for detection the inter turn fault of transformed windings, the Matlab/Simulink software was used to build the model (Figure 2).   Fig. 3), consequently, the rate of magnetization current change is small (Fig. 4).
When a turn short circuit occurs (transformer secondary turns bc are closed), the model has two magnetization inductances: the primary circuit BC and the short circuit of bc turns. Moreover, the number of turns in the primary winding is much larger than the number of turns in the turn faults (as it can be seen in Fig. 3, from Winding 1 parameters V1 and Winding 2 parameters V2). Therefore, the magnetization inductance of the primary BC circuit is much greater than the magnetization inductance of the inter turn short circuit bc. This is reflected in the rate of increase and decrease of the magnetization current (Fig. 5): first (up to a time of the order of 0.1 s), a rapid increase in current occurs in the inter turn short circuit bc. After current saturation in inter turn short circuit (as shown in Fig. 6), only the slow growth of the magnetization current component in the primary winding BC remains (at times from about 0.2 to 0.5 s). After the primary winding BC closes, a similar process occurs: a rapid decrease in the inter turn short circuit current (Fig. 6) (approximately in the range from 0.5 to 0.7 s), then a slow decrease in the current in the primary BC circuit (see Fig. 5).

Results
From the presented model of inter turn failures in transformers (Figs. 2 -6), it can be seen that, based on the large difference in the rates of current change in the primary winding BC and in the inter turn short circuit bc, it becomes possible to create a device for detecting coil faults in transformer windings [10], using which the search for coil damage is significantly facilitated.
To verify and test the proposed methodology, transients were measured using an auxiliary transformer TSKS-40/145/10-UZ of Askold Electrotechnical Plant LLC (TU 3411-002-65653492-2010, the transformer idle run current is not more than 12%, power is 38 kVA, number of phases is 3, frequency is 50 Hz, degree of protection is IP 00, rated voltage of HV winding is 10 kV, rated voltage of HV winding is 0.4 kV, rated current of HV winding is 2.19 A, rated current of LV winding is 54.9 A , short circuit voltage is 1.48%, insulation class F, circuit and group connection is Y/Yn-O).
To record transients, the digital oscilloscope 6501 was used; the current in the transformer windings was measured by recording the voltage drop across the active resistance, connected in series with the measured transformer winding. Initially, a constant voltage of 8 V (through an active resistance of 47 Ohms) was applied to the primary HV winding of the transformer (BC pins). The current through the winding was measured by the voltage drop at a resistance of 10 Ohms, while the behavior of the transient process (with open - Fig. 7 and with closed -Fig. 8 turns of the secondary LV transformer windings) was similar to the picture observed in the Matlab/Simulink program (see Figs. 4, 5).
There are several ways for increasing the sensitivity of the proposed method: 1. By increasing DC voltage supplied to the transformer windings, while reducing the measurement time interval (to maintain the same current in the transformer winding).
2. By applying a constant voltage (measuring transient) in the windings of the lower voltage (LV).
3. By obtaining the reference measurements of the transient process in all windings of the undamaged (without winding short circuits) transformer. Further, during periodic measurements, the current transient measurements in the transformer windings should be compared with the saved reference measurements. Further DC voltage of 8 V (through an active resistance of 10 Ohms) was applied to the secondary LV transformer winding (bc pins), the current through the winding was measured by the voltage drop at a resistance of 1 Ohm. A clear difference between the transient process in the absence (Fig. 9) and in the presence (Fig. 10) of inter turn fault (additionally wound one shortcircuited turn on the B phase core) was observed.

Conclusions
Thus, in the proposed method for detection the inter turn faults of transformed windings, the transients are measured in wide ranges of times, which allows tuning away from transformer magnetization currents, by measuring the transient in the fault itself, namely in the short-circuited turns of the transformer winding. This solves the task of detecting the inter turn faults in the transformer windings in a wide range of damaged (closed) turns.
The author expresses his gratitude to Badretdinov Nail (head of the laboratory of the educational and research center "Electric Power Engineering" of KSPEU) and Zapechelnyuk Eduard (head of the laboratory of the department "Relay protection and automation" of KSPEU) for assistance in measurements.