Figure 1a–d shows the picture of test setups, in-service transformers, terminal connections, etc., adopted during DRA and FRA tests, while the terminal connection was adopted, and the parameters were measured. The excitation approach is shown in
Figure 2a–h, respectively. It is well known that the DRA tests assess the insulation and its geometry [
24,
29,
31,
32] while the FRA identifies defects in the winding and core of the transformers [
25,
26,
27,
28,
29]. The results obtained from DRA and FRA methods collectively describe the complete diagnostic status of a distribution transformer. The basic test procedure of DRA and FRA methods are the same while the test frequencies applied are different. The DRA setup involves a dielectric response analyzer (
Figure 1a) that generates sinusoidal test voltages up to 200 V with frequencies ranging from 100 µHz to 5 kHz in discrete steps. This study uses sinusoidal test voltage of 200 V in amplitude and frequencies ranging 500 µHz to 5 kHz. The frequency dependent insulation parameters such as the loss factor, permittivity, etc., are obtained. The same is compared with the model curve (using the XY model) generated using an inbuilt curve fitting algorithm [
27]. The inbuilt algorithm then fits the measured data with the model curve and calculates the moisture level and oil conductivity by referencing to database [
27]. The FRA test setup shown in
Figure 1c,d comprises of a frequency response analyzer popularly used for diagnosing the windings of a transformer. The frequency response analyzer generates spectrally pure sinusoidal signal of 1 V
rms amplitude with frequency sweep from 10 Hz to 20 MHz. Initially, a suitable terminal connection and system function pair as recommended by the current literature and IEEE/IEC standards are adopted [
12,
25,
26]. Pertinent mode of excitation, terminal connection, measurable input, and output parameters, etc., are shown in
Figure 2a–h. Following this, sinusoidal signals of 1 V
rms in amplitude with frequencies sweeping from 10 Hz to 2 MHz are injected into the terminals and the pertinent input, reference and output signals are recorded. The amplitude response is obtained from the measured data which is further analyzed to understand the diagnostic status of the transformer windings.
4.1. In-Service Transformer Units
The DRA tests on in-service transformer units employ two important analysis strategies. First is the comparative analysis of measured data with the model curve developed by an algorithm inbuilt in the device. Later, using the geometry, temperature, etc., a response that fits to the measured data is calculated and the corresponding values of oil conductivity, moisture content, etc., are estimated [
29]. The second strategy employs comparative analysis of measured data with the new or refurbished sister-unit transformers that are identical to the test object.
Table 2 shows the loss factor, capacitance, oil conductivity and estimated level of moisture ingress in the chosen set-1 and set-2 in-service distribution transformers. It can be seen from the
Table 2 that the insulation of the in-service transformers (set-1 and set-2) are in better status. The measured loss factor of all in-service units (set-1 and 2) lies within 0.002 to 0.003, which is well within the acceptable limit. The conductivity of the oil-paper insulation of set-1 distribution transformers remained within 1.3 pS to 3.7 pS, which is also within the acceptable limit. The same for the set-2 transformer manifested slightly higher values, i.e., within 6.1 pS to 7.2 pS. So, these two parameters confirm that there are no ageing or deterioration in the insulation system of set-1 and set-2 distribution transformers. However, the moisture levels of these transformers yielded different results.
Comparatively, the set-1 in-service transformers (2015 and 1994-1) emerged as ‘dry’ with a low level of moisture in its insulation. Pertinent values of moisture were estimated as 1.1% to 1.3%, respectively. At the same time, the in-service unit, identified as set-1/1994-2 emerged with a higher level of moisture in its insulation. The level of moisture is estimated to be around 3.6%, therefore marked as ‘moderately-wet’. The set-2 distribution transformers emerged as either ‘wet’ or ‘moderately wet’ as their estimated moisture levels remain within 2.0% to 3.6%. Particularly, the in-service unit identified as set-2/1983 manifested slightly higher level of moisture ingress, i.e., around 3.6%, henceforth marked as ‘wet’. The reason for such increased level of moisture might be their operating condition and location. The set-1 in-service transformer units (other than set-1/1994-2) are installed in residential areas while the set-2 transformers are installed and feeding households outside the city. Being exterior to the city limits, their constant exposure to the open weather conditions and lesser loads may be the reason for such minor deviations.
Figure 3a–d shows the insulation parameters (i.e., loss factor, capacitance, resistance, and impedance) of in-service distribution transformers grouped under set-1 category. It becomes clear from these figures that the insulation parameters of in-service transformers identified as set-1/2015 and set-1/1994-1 are within the acceptable level as their pertinent traces match each other. Pertinent insulation parameters such as loss factor (
Figure 3a), capacitance (
Figure 3b), resistance (
Figure 3c), and impedance (
Figure 3d) remain in-line to each other. Such observations are not surprising as these in-service units, grouped in set-1 category are either new (set-1/2015) or newly refurbished (set-1/1994-1). However, the transformer identified as set-1/1994-2 emerged with mild deviations in its respective insulation parameters. The loss factor of this set-1/1994-2 unit emerged with slight deviations throughout the frequency span. These deviations in the loss factor (
Figure 3a), capacitance (
Figure 3b) and impedance (
Figure 3c) appeared more dominant at very lower frequencies (i.e., say from 100 mHz to µHz) indicating moisture ingress in the insulation. Such deviations in the resistance (
Figure 3b) appeared predominantly from few 100 Hz to µHz, respectively. These deviations in insulation parameters collectively indicate that this transformer (set-1/1994-2) relatively has higher moisture ingress in insulation. This finding is in consonance with observations (moisture 3.6%) from
Table 2.
Figure 4a–d shows the insulation parameters measured from the in-service distribution transformers grouped under set-2 category. As opposed to the set-1 units, the insulation parameters of the set-2 distribution transformer manifested notable deviations in the response function. The loss factor of all the three units shown in
Figure 4a manifested deviations throughout the frequency span. In particular, the deviations in the mid-frequency range (i.e., within 100 mHz to 100 Hz) of transformer unit set-2/2002 indicate a slightly higher level of oil conductivity. The same observation can also be made from
Table 2. Nevertheless, the percentage of change in the oil conductivity is relatively smaller, and can therefore be deemed an acceptable level. At the same time, the deviations in the lower frequencies of the loss factor (
Figure 4a), capacitance (
Figure 4b), and resistance (
Figure 4c) of set-2/1983 in-service unit indicates moisture ingress in the insulation, thereby positioning it in ‘wet’ state. This is expected as the in-service transformer unit identified as set-2/1983, is relatively older than the others and had been operating in the outer-skirts of residential area.
Figure 5a–i shows the open and short-circuit responses of set-1 in-service distribution transformers. At first glance, it becomes clear from these figures that the in-service units identified as set-1/2015 and set-1/1994-1 manifested no significant deviations in their respective open and short-circuit response functions. The high voltage open-circuit (HVOC) response of phases A-C (
Figure 5a), B-A (
Figure 5d), and C-B (
Figure 5g) exhibits an exact match between each other. Similarly, the high voltage short-circuits (HVSC) response of phases A-C (
Figure 5b), B-A (
Figure 5e), and C-B (
Figure 5h) also exhibit an exact match between each other. Naturally, there are no significant changes observable in the low voltage open-circuit (LVOC) response of all the three phases viz., a-n (
Figure 5c), b-n (
Figure 5f) and c-n (
Figure 5i). So, the outcome of the FRA tests altogether confirms that the windings of all the in-service units identified as set-1/2015 and set-1/1994-1 are intact without any major deviations from its diagnostic condition. However, the open and short-circuit response of the other in-service unit identified as set-1/1994-2 manifested a small deviation in its amplitude response function. These changes are insignificant and follows the pattern of other phases. It hence can be concluded that this transformer unit is also in good condition.
Further FRA tests (
Figure 6a–i) on set-2 in-service distribution transformers revealed similar results.
Figure 6a–i shows the open and short circuit responses of set-2 in-service transformers adopted in this study. The open and short-circuit responses of units set-2/2015 and set-2/2002 manifested no notable deviations in their open and short-circuit response function. All their traces of HVOC and HVSC response of phases A-C (
Figure 6a,b), B-A (
Figure 6d,e) and C-B (
Figure 6g,h) and their LVOC responses of phases a-n (
Figure 6c), b-n (
Figure 6f) and c-n (
Figure 6i) match each other without any deviations. So, it can be confirmed that the windings of these in-service units (set-2/2015, set-2/2002) are in good condition. The in-service unit identified as set-2/1983 manifested mild deviations in their respective HVOC, HVSC, and LVOC response function. In all, the deviations appear more prominent at the low voltage phases, i.e., a-n, b-n, and c-n, respectively. As the deviations are quite smaller in magnitude and the respective pattern followed is similar to the other phases, it can be once again concluded that this in-service unit is in good condition.
Thus, it becomes clear from these experiments that there are no significant changes in the respective open and short circuit response functions. This confirms the fact that the transformers grouped in this category are in good condition and can be further employed in the distribution network. With this information, the defective distribution transformers grouped under set-3 category are subjected to diagnostic investigation.
4.2. Defective Distribution Transformer Units
Two distribution transformers (
Table 1) that had several interruptions during its service time are selected and subjected to diagnostic tests. The first defective transformer (set-3/1994) grouped under set-3 category, was in-service at an industrial site and was tripped out at least by three intervals due to elevated temperature. During each interruption, an oil sample was extracted, and the gas dissolved in oil is estimated. After the third interruption, the transformer suffered a failure causing the oil to vent, forcing it to go offline. After gathering this information, the dielectric, mechanical and thermal integrity of this defective transformer (set-3/1994) is studied and the results that reveal interesting information are alone discussed. The second defective transformer (set-3/1998) was feeding a residential area in the outskirts of the city.
Figure 7a,b shows the picture of set-3/1998, the second defective transformer that has winding failure initiated by the localized hot spot temperature and arcing phenomena. The oil system and thermal indicators of this transformer (set-3/1998) were initially normal, however the relay has tripped repeatedly at peak loads. Eventually, this transformer suffered an untimely failure, so it was removed from service and subjected to tests. After these tests, this transformer was opened to visually inspect the failure locations.
Figure 7a,b pictorially describes the failure locations in the windings.
Table 3 lists the values of gases detected in the oil sample and the possible reasons for their development. The oil samples extracted during three interruptions are mentioned in the
Table 3 as sampling intervals 1, 2, and actual. The actual being the present case and the intervals 1 and 2 are data from previous samples. It can be observed from
Table 3 that the gases hydrogen (H
2), carbon monoxide (CO), and carbon-dioxide (CO
2) have gradually evolved to a higher value. In all, the H
2 evolved from 14 ppm to 150 ppm, indicating minor interior discharge activities. The CO increased from 57 to 1000, indicating carbonization problems. The CO
2 gas that increased from 4979 ppm to a higher value of 12,000 ppm indicated possible moisture content in the transformer. In addition, the presence and increase of methane (CH
4) (up to 100 ppm) dissolved in the gas indicated interior sparking problems. Additionally, the ethane (C
2H
6) and ethylene (C
2H
4) gases were identified, which increased to higher values indicating localized heating in the transformer. So, acetylene (C
2H
2) gas evolved indicating winding hot spot.
In all, the presence of H
2, CO does not pose a serious threat as their values when compared with the reference data are well within the acceptable limit. The level of CO
2 when compared with the reference values indicated minor moisture content, which is also not a serious threat. At the same time, the level of gases such as methane, C
2H
6, and C
2H
4 indicates sparking, localized heating and hot spot problems, which is a serious issue. This might be the reason for the thermal failure of this distribution transformer. Following this, the insulation condition of these defective transformers is studied.
Table 4 shows the values of insulation parameters of defective transformers (set-3/1994, set-2/1998) measured using the DRA method. It can be observed from
Table 4 that the status of the oil of both the defective transformers are in acceptable condition. However, a noticeable level of moisture has ingress into the insulation of both the defective transformers. The transformer with thermal fault emerged with 4.6% of moisture while the same for the winding fault was close to 4.5%, respectively. So, to make a detailed analysis, the measured insulation parameters are compared with a sister-unit transformer.
Figure 8a–d shows the insulation parameters of the thermally defective (set-3/1994) distribution transformer. It becomes clear from these figures that the insulation condition of the thermally defective transformer remains in acceptable condition. The loss factor of the thermally defective transformer within the frequency span of 10
−2 Hz to 10
2 Hz appears to remain in-line. The loss factor beyond 100 Hz appears higher than its sister-unit counterpart. Similar observations can be made at frequencies below 10
−2 Hz. The reason for the increase in loss factor can be attributed to the moisture ingress in the insulation. The other insulation parameters such as capacitance (
Figure 8b) and impedance (
Figure 8d), manifests a close match with the sister-unit data throughout the frequency. However, the resistance of the thermally defective seems to be slightly higher than the sister-unit data at very higher frequencies. This confirms the fact that the insulation system of the thermally defective transformer is intact, but suffers moisture problems. This finding is in consonance with the observation made from the
Table 4.
Figure 9a–d shows the insulation parameters measured from the set-3/1998 winding-defective transformer. It becomes clear from these figures that the condition of the insulation of the winding-defective transformer unit (set-3/1998) remains compromised. The values of the loss factor shown in
Figure 9b are relatively higher, indicating a drastic change in the condition of the oil-paper insulation. Additionally, the changes of the loss factor in the mid-frequency span, i.e., between 10
−2 Hz and 10
2 Hz indicate deviations in the condition of oil conductivity, which is in consonance with the model-curve algorithmic analysis (refer to
Table 4).
So, these observations collectively confirm that the integrity of the insulation arrangement of the winding-defective transformer remains compromised. This is reasonable, as it can be observed from
Figure 9a,b that the winding failure was so severe that it has disturbed the insulation arrangement at this location. The reason for such a failure is due to the lack of timely diagnostic support and inadequate maintenance strategy. Understanding this, detailed FRA tests are conducted on the set-3/1998 winding-defective transformer.
Figure 10a–c shows phase-to-phase comparison of open and short-circuit responses of set-3/1998 winding-defective transformer. It becomes clear from these figures that the windings in the outer phases A-C and C-B (
Figure 10a,b) of high voltage and a-n, b-n, and c-n in the low voltage (
Figure 10c) have no significant defect. Pertinent magnitude function match each other, indicating that there is no change in impedance. At the same time, the impedance of the winding in the middle phase has drastically changed. These changes appear more prominent at the lower and mid frequencies of open and short-circuit response functions (
Figure 10a,b) indicating stronger displacement between the winding-core and individual turns, respectively. These changes altogether confirm that the winding in the middle phase (L
2 or phase B-A of the high voltage winding) has suffered severe damage. This observation matches with the findings made during visual inspection (
Figure 10a,b) of the winding defective (set-3/1998) defective transformer. In order to confirm this, the open and short-circuit responses of the faulty unit is compared with the sister-unit transformer.
Figure 11a–i shows the magnitude function of the open and short-circuit responses of the defective transformer. It becomes clear from these figures that the winding in middle phase B-A (refer
Figure 11d,e) has severe damage. Pertinent open and short-circuit impedance manifest stronger deviations against the sister-unit measurements. Additionally, the corresponding damages are so severe that they have induced or displaced the outer phase windings (phase A-C, C-B) that are in proximity. These induced damages in the outer phase windings appear as deviations in the magnitude responses shown in
Figure 11a, b, g, and h, respectively. At the same time, it appears from
Figure 11c,f,i that the low voltage windings are free from any defects as there are no changes appeared in its magnitude response. The same is ensured during visual inspection of the faulty transformer.
Hence, it appears from this onsite study that it is essential to employ diagnostic test methods on in-service distribution transformers to monitor their operating condition and to ensure their lifetime and reliability. The major problems faced by the distribution transformers are caused by moisture ingress, intermittent loads, short-circuit, etc., which significantly affects either the insulation and/or the winding-core arrangement. In all, the current maintenance strategies exercised on the distribution transformers does not reveal the diagnostic status in timely manner, causing the failure events to be catastrophic. Naturally, the modern diagnostic methods such as DGA, DRA, and FRA tests adequately describe the integrity of the distribution transformers and help in exercising preventive measures to evade occurrence of an incipient fault condition. Such attempts are desirable as they improve the asset management.
Thus, the experiments on in-service and defective transformers revealed their diagnostic status clearly and therefore their corresponding data can be further accumulated with respect to time and can be used for resolving their lifetime and reliability.