Chapter 7 Conclusion 7.1 General

Transcription

1 Chapter General The mechanical integrity of a transformer winding is challenged by several mechanisms. Many dielectric failures in transformers are direct results of reduced mechanical strength due to deformation. It is therefore necessary to detect deformation as early as possible before they lead to problems or failures. Mechanical deformations do not necessarily change the operational characteristics of the transformer. Minor deformation in the winding of a transformer indicates no significant change in the operational characteristics, but mechanical properties of the copper might be changed seriously risking rupture on next event. Less severe deformations involve insulation rupture and partial discharges which after some time normally are discovered through oil analysis or tripping the buchholz relay. Conventional condition monitoring techniques such as dissolved gas analysis (DGA), Capacitance & Power factor and leakage reactance test are unlikely to be able to detect such minor mechanical damage until it develops into a dielectric or thermal fault. So, a specialist technique is clearly required for the monitoring and assessment of mechanical condition of the transformer and to be able to detect the incipient fault in transformer. The SFRA is a powerful method for the detection and diagnosis of the defects in the active part of power transformers. It can deliver valuable information about the mechanical as well as electrical condition of core, windings, internal connections and contacts. No other single test method for the condition assessment of power transformers can deliver such a diversity of information. Therefore the SFRA is an increasingly popular test Sweep Frequency Response Analysis (SFRA) is a tool that can give an indication of core or winding movement in transformers. This is done by performing a measurement, a simple one, looking at how well a transformer winding transmits a low voltage signal that varies in frequency. Just how well a transformer does this is related to its impedance, the capacitive and inductive 176

2 elements of which are intimately related to the physical construction of the transformer. Changes in frequency response as measured by SFRA techniques may indicate a physical change inside the transformer, the cause of which then needs to be identified and investigated. SFRA has been a key tool in the decision to scrap or reenergize a transformer. To get value from an SFRA test it is necessary to make sure that the measurements are credible, which requires good test technique and simple procedures; proper training is thus essential to control the measurement process. However special care must be taken both in application of the test to acceptable standards and in interpretation of the tests results to gain value from the tests themselves. When interpreting a trace, it is important to make use of all the information present to look at the whole picture. Small variations or displacements across a large frequency range may be much more important than a large variation in one part of the frequency range. In analyzing traces, lower frequencies tend to relate to larger objects; higher frequencies relate to smaller objects. In terms of size there is a general rule of thumb that, while reviewing a trace from left to right, from 20 Hz to 2 MHz, this corresponds to the core, clamping structure and yoke, main windings, tap leads and connecting leads. The actual position of resonances in the trace depends on the size of the transformer; lower MVA transformers tend to have their resonance shifted more to the higher frequencies. However, there are always exceptions to this rule of thumb and individual traces should be inspected on their merits. 177

3 7.2 Summary of Important Findings The following conclusions have been made regarding the simulated fault conditions Winding Earth Fault Damages HV U Phase Earth Fault & LV W Phase Earth Fault We can conclude from the practically simulated earth fault damages that the nature of the SFRA traces related to an earth fault depends on the connection of the winding, i.e. winding is Delta connected or Star connected. When transformer has developed earth fault in one phase of the delta connected winding, then following points are analyzed. There is a huge difference in the starting db value of the three phase open circuit frequency responses. (In our case, it is -45 db, -62 db and -92 db respectively) There is also a huge difference in the starting db value of the three phase short-circuit frequency responses. (In our case, it is -25 db, -28 db and -75 db respectively) The open circuit and short-circuit frequency responses shows the disturbances at low frequency in the two phases even though the earth fault has been developed at one phase. When transformer has developed earth fault in one phase of star connected winding then following point is analyzed. The open circuit frequency response of that phase shows that the impedance value of it is lower in low frequency range and high in medium & high frequency range as compared to the other healthy phases. It is possible to identify Earth Fault by other conventional method like TanDelta, Exciting Currents, Insulation Resistance, TTR, etc. SFRA when used in conjunction with other diagnostic tools can provide a complete condition assessment of the transformer. 178

4 7.2.2 Short Circuited Turns or Lost Turns or Turn to Turn Short Fault Damage HV Tapping Short in V Phase When transformer has developed short circuited turns in one phase of the either winding, i.e. HV winding or LV winding then following points are analyzed. The open circuit frequency response of one phase (In our case it is V phase) of HV winding is different than the other two phases, and the first resonance is absent in the low frequency range, The open circuit frequency response of similar phase ( V phase) (as HV winding has) of LV winding is different than the other two phases, and also the first resonance is absent in the low frequency range (as HV winding has). The difference in the db value of the open circuit frequency response of the fault indicating phase with respect to other two healthy phases in the low frequency range of which winding ( HV or LV) is more, the actual lost of turns fault exist in that phase of a winding. It is possible to identify lost turns fault by other conventional method like no load excitation current, turns ratio and magnetic balance test, but in which winding HV or LV winding is a question mark. SFRA is the only test which can identify the winding and phase both of the lost turns fault if analysis is done with proper knowledge and experience Multiple Winding Fault Damages HV V Phase Tap Short + LV V Phase Earth Fault HV Winding: When the first resonance in the low frequency range is absent in the open circuit frequency response of any phase of HV or LV winding and then it merges with the other open circuit frequency responses of two phases in the medium and high frequency range, than it indicates that the short circuited turns fault has been developed in that phase of that winding. It can be found from other conventional method like, No Load Excitation Current, Turns Ratio and Magnetic Balance test also. LV Winding: When the open circuit frequency response of any phase of star connected winding shows that the impedance value of it is lower in low frequency 179

5 range and high in medium & high frequency range as compared to the other healthy phases than it indicates that the earth fault has been developed in that phase of star connected winding. It is possible to identify Earth Fault by other conventional method like, Tan Delta, Exciting Currents, Insulation Resistance, and TTR also. Based on the above SFRA data analysis it is concluded that the transformer has developed multiple winding fault, i.e. short circuited turns fault (i.e. Tap short fault) in V phase of HV winding and earth fault in V phase of LV winding. SFRA is able to identify both the faults very clearly which is not possible to achieve by only single test by any other method. It is the experience and interpretation of SFRA results plays a major role in identifying such multiple faults in transformer HV & LV Winding Short Fault Damage - HV & LV V Phase Short Fault Based on the SFRA data analysis it is concluded that when the transformer has developed HV and LV V phase short fault then following points are analyzed related to HV & LV windings. HV winding: The measured open circuit frequency response and the measured short circuit frequency response of HV winding shows that, U and V phase have problem due to V phase of HV delta connected winding is getting shorted to the V phase of LV star connected winding. LV winding: The measured open circuit frequency response of LV winding shows that at low frequency, U & V phase have problems where as at higher frequency, only V phase has problem due to V phase of HV winding is getting shorted to the V phase of LV winding Neutral & Phase of LV Star winding Short Fault Damage LV N & U Short Fault Based on the SFRA data analysis, it is concluded that when the U phase of LV winding is getting shorted with the neutral, i.e. when the transformer has developed short-circuited winding fault in U phase of LV winding. (LV N & U Short) then following points are analyzed. 180

6 Due to star connected LV N & U Short fault, the open circuit frequency response of U phase of delta connected HV winding is looks as the short circuit frequency response of it. When the winding of one phase is short circuited with the neutral, then the impedance value of open circuit frequency of that winding (In our case it is U phase) is 0 db throughout the whole frequency range Phase to Phase Short Fault Damage in LV Star Winding LV U & V Short Fault When the transformer has developed two windings of U & V phase short circuited fault in LV winding. then following points are analyzed.. Due to two phases U & V of LV star winding are short circuited, its effect can be seen in the open circuit frequency responses of U & V phase of HV delta winding. It happens because of impedance of LV winding has been shifted into the HV winding. The open circuit frequency responses of U phase (2U 2N) and V phase (2V 2N) of LV winding have the same shape and similar, but are different than the open circuit frequency response of W phase (2W 2N) which appears to be normal. The impedance value of open circuit frequency response of U phase & V phase of LV winding is same and is lower than the impedance value of W phase throughout the whole frequency range. It can happen if the SFRA signal is getting two windings short circuited path Winding Open Circuit and Short Circuit Fault Damages HV W phase open fault and HV U and V short fault damages. We can conclude from the practically simulated HV W Phase open fault & HV U & V Short fault damages that when transformer has developed open circuited winding fault in HV delta connected winding and when transformer has developed short circuited winding fault in HV delta connected winding then its SFRA responses consist following characteristics. 181

7 When any phase of delta connected HV winding is open circuited then following points are analyzed. The open circuit frequency responses of three phase of HV winding are similar and having general shape as delta connected winding has. The short circuit frequency response of faulty phase (open circuited phase) has higher starting db value than the starting db value of other two healthy phases and then short circuit frequency responses of all the three phases of HV windings have similar shape from medium frequency to higher frequency (in our case it is from 30 KHz to 2 MHz frequency) The open circuit frequency response of similar (faulty phase of HV winding) phase of LV star connected winding is different than the open circuit frequency responses of other two phases in only low frequency ranges (in our case it is from 1 KHZ to 10 KHZ frequency). When any phase of delta connected HV winding is short circuited then following points are analyzed. The starting db value of faulty phase (short circuited phase) of open circuit frequency response of HV delta connected winding is 0 db and it remains 0 db up to high frequency (in our case it is 200 KHZ frequency) and then increases up to few db (in our case it is -7 db) at 2 MHz frequency. The short circuit frequency response of faulty phase (short circuited phase) is similar as its open circuit frequency response. The starting db value of open circuit frequency response of similar (faulty phase of HV winding) phase of LV star connected winding is 0 db and it remains 0 db up to low frequency (in our case it is 2 KHz frequency) and then it is merged with the open circuit frequency responses of other two phases at medium lower frequency (in our case it is 25 KHZ frequency). 182

8 7.2.8 Multiple Short Circuit Winding Fault Damages HV U & V and LV V & W Short Fault When the transformer has developed multiple short circuited winding fault, i.e. in HV winding U & V phase short and in LV winding V and W phase short, then following points are analyzed. HV Winding: The open circuit frequency response and short circuit frequency response of HV U phase is same and having 0 db value up to 200 khz frequency. It indicates that the winding of HV U phase (1U 1V) is getting short circuited. The open circuit frequency responses and short circuit frequency responses of HV V & W phase are same and they have starting db value of -22 db in both the cases. It indicates that the two windings of V & W phases of LV winding are short circuited. LV Winding: The starting db values of the open circuit frequency responses of all the three phases of LV winding are 0 db and it remains 0 up to 2 khz frequency for U phase response and remains 0 up to 4 khz frequency for V and W phase responses. The impedance value of V & W phases is lower than the impedance value of U phase from 2 khz to 2 MHz frequency. Above can happen if the V & W phases of LV winding are short circuited and U phase of HV winding is short circuited Winding & Core Short Fault Damages LV V & Core Short and HV V & Core Short fault damages We can conclude from the above practically simulated core & LV V phase winding short fault and core & HV V phase winding short fault damages that the nature of the SFRA traces related to core & winding short fault is different for the Star connected winding and Delta connected winding. When transformer has developed core & winding short fault at one phase of star connected winding then following point is analyzed. 183

9 The open circuit frequency response of the phase which is shorted with the core and the open circuit frequency responses of the other two healthy phases are similar and having general shape up to high frequency (in our case it is 100 KHz frequency) and then after that frequency, the open circuit frequency response of that shorted phase has different magnitudes of inductive and capacitive resonances than the open circuit frequency responses of the other two phases. When transformer has developed core & winding short fault at one phase of delta connected winding then following points are analyzed. The open circuit and short circuit frequency responses of the two phases have the same and lesser impedance value with respect to open circuit and short circuit frequency response of the third phase between medium frequency to high frequency range. (in our case it is from 30 KHz to 2 MHz frequency) Due to delta connection, the short circuit occurs at one phase of the winding and the core; its effect can be seen in the other phase also Core and Frame Short Fault Damage Practically simulated core and frame short fault on the transformer could not be identified by when the data analysis is performed on three phase comparison basis. When SFRA data analysis of Core & Frame Short Fault has been done (for the Dyn11 transformer) on the basis of the comparisons can be made against the baselines and previous data then following points are analyzed. The open circuit frequency response of U phase, V phase and W phase of HV winding for the core & frame short fault condition is having lesser impedance value (In our case it is of 2.5 db, 5.0 db and 7.5 db respectively) than the open circuit frequency response of U phase, V phase and W phase of HV winding for the healthy condition of transformer at lower frequency (In our case it is 10 khz frequency). The measured short circuit frequency response of U phase and V phase of HV winding for the core & frame short fault condition is having higher impedance value (In our case it is of 9 db and 4 db respectively) than the measured short circuit frequency response of U phase and V phase of HV winding for the 184

10 healthy condition of transformer where as the measured short circuit frequency response of W phase of HV winding for the core & frame short fault condition is having lesser impedance value (in our case it is 5 db). than the measured short circuit frequency response of W phase of HV winding for the healthy condition of transformer at medium frequency (In our case it is 28 khz frequency) The measured open circuit frequency response of U phase, V phase and W phase of LV winding for the core & frame short fault condition is different than the measured open circuit frequency response of U phase, V phase and W phase of LV winding for the healthy condition of the transformer at higher frequency (In our case it is after 300 khz frequency) Coil and Core Assembly Displacement (CCA displaced by 30 mm approx.) Practically simulated Coil and Core Assembly displacement (CCA displacement) for 30 mm (approx.) displacement fault on the transformer could not be identified by when the data analysis is performed on three phase comparison basis and when the data analysis is performed on the basis of the comparisons can be made against the baselines and previous data also. 7.3 Future Scope of Research Work Consequent to investigations carried out in this thesis, the following aspects are being suggested as future research work to be carried out. The first important step towards improving FRA interpretation is to ensure that the measurement is of good quality, as presented in chapter 4 of this thesis.. When the quality of the measurement is confirmed and the frequency range for interpretation assessed, the visual inspection of the variations between the test and the reference results is the actual approach for FRA interpretation. The experimental work provided in this thesis shows how various faults can be detected by FRA. 185

11 The following aspects are being suggested to improve FRA interpretation as future research work. Compare the sensitivity of the different FRA test types i.e. end-to-end (open and short circuit), capacitive inter-winding, and inductive inter-winding to detect different transformer failure modes. Assess the possibility of detecting faults by comparing the results obtained using both directions of end-to-end tests when no reference result is available. Transformer modeling based on geometrical parameters as a means to support the interpretation and derive a fundamental understanding of the FRA resonances, possibly also to generate a kind of database collecting the effects of different winding damage on the FRA spectra. 186