# The Influence of Capacitance and Inductance Changes on Frequency Response of Transformer Windings
## 4 Examples of Industrial Measurements  
The phenomena simulated in FEM software can be found in industrial measurements. Some examples of such results are given below. They were all measured by authors in the field conditions in the end-to-end open test setup, which was already discussed and presented in Figure 1. The test equipment was a commercial device Omicron FRAnalyzer. All details of the measurement technique are requirements of the IEC standard [6]. In all examples, only one phase is presented, to show differences coming directly from introduced changes to the transformer. The first one, presented in Figure 9, shows the influence of capacitance on FR of a 250 MVA, 400/110/30 kV autotransformer. The additional capacitors were installed on the bushing of phase B, not the measured phase (phase A). The results of the measurement of phase A show that additional capacitance shifts the first resonance in the LF range (100-300 Hz), but from approximately 10 kHz, its influence disappears. These results confirm observations obtained from the model that capacitance modifications influence only LF range.  
Figure 8: FR modeling for two positions of the tap changer for 405 and 495 turns, and for 495 turns with bushing capacitance change.  
Figure 9: FR of HV phase A of a 250 MVA, 400/110/30 kV autotransformer with additional capacitances installed on HV phase B.  
A similar effect is obtained if measurement is conducted on the LV winding of phase A, with capacitors attached to the HV phase B. The capacitive influence of the active part (even though it is in the other phase and in the other voltage side) is reduced in the MF range. The first resonance shifts in a range of 100-300 Hz, while differences between curves disappear from approximately 8 kHz. It can be seen in Figure 10.  
The next case necessary to verify the modeling results is the behavior of FR when the tap changer position is shifted. The first example is measured with a 160 MVA, 220/110/15 kV autotransformer, for three positions of OLTP. The results are given in Figure 11. The FR for change values of inductance (tap position) influence not only LF range, but also MF range. Again, the industrial measurement confirmed the results obtained from the computer model. A similar effect can be observed for another unit, the 31.5 MVA, 110/30 kV transformer, presented in Figure 12, also for three positions of OLTC.  
Figure 11: FR of phase A of 160 MVA, 220/110/15 kV autotransformer for three positions of OLTC.  
Figure 10: FR of LV phase a of 250 MVA, 400/110/30 kV autotransformer with additional capacitances installed on HV phase B.  
The abovementioned results show that the influence of inductance changes can be observed in the wide frequency range, including LF. If this is the only cause of changed FR measurement, it should be easy to detect. However, if these results were to be combined with capacitance changes, such as for modeling results shown in Figures 7 and 8, the outcome would be difficult to properly interpret. Such a transformer would possibly be described as faulty, while visible changes may be the result of the wrong tap position combined with some capacitive changes on the bushing of different phases than those measured. Yet another problem is core magnetization, which influences only LF range. An example of such a measurement is given in Figure 13.  
If these changes, visible up to 1 kHz, were to be combined with capacitive influence coming from different phases, the result may be very similar to the original curve.  
Figure 12: FR of phase A of 31.5 MVA, 110/30 kV transformer for three positions of OLTC.  
Figure 13: FR of distribution transformer with different core magnetization.