This is very reliable and sensitive method or tool for condition monitoring of the physical condition of transformer windings. windings. The winding of transformer may be subjected to mechanical stresses during transportation, transportati on, heavy short circuit faults, transient switching impulses and lightening impulses etc. These mechanical stresses may cause displacement of transformer windings from their position and may also cause deformation of these windings. Wi Windings ndings collapse in extreme cases, such physical defects eventually lead to insulation failure or dielectric faults in the windings. Sweep Frequency esponse !nalysis Test or in short SF! Test can detect efficiently, displacement of transformer core, deformation deformati on and displaceme displacement nt of winding, faulty core grounds, collapse of partial winding, bro"en or loosen clamp connections, short circuited turns, open winding conditions etc.
The principle of SF! is quite simple. !s all the electrical equipments theoretically have some resistance,, inductance resistance inductance and and some capacitance capacitance values values hence each of them can be considered as a complex #$ circuit. circuit .
The term %theoretically% means some equipment may have very low or &ero resistance compared to their inductance and capacitance values again, some equipments may have very low or &ero inductance compared to their resistance and capacitance and again some equipments may have very low or &ero capacitance compared to their resistance and inductance but theoretically all of them can be considered as #$ circuit although may ma y be ' (, or # ' ( or $ ' (. )ut in most cases the resistance, resistance, inductance and capacitance of an equipment have non &ero values. *ence most of the electrical equipments can be considered as #$ circuit hence they response to the sweep frequencies and produce an unique signature. !s in a transformer each winding turn is separated from other by paper insulation which acts as dielectric and windings themselves have inductan ce and resistance, a transformer can be considered as a complicated distributed networ" of resistance, inductance, and capacitance or in other words a transformer is a complicated #$ circuit.
)ecause of that each winding of a transformer exhibits a particular frequency response. +n Sweep Frequency esponse !nalysis a sinusoidal voltage i is applied to one end of a winding and output voltage o is measured at the other end of the winding. -ther windings are "ept open. !s the winding is itself an distributed #$ circuit it will behave li"e #$ filter and gives different output voltages at different frequencies. That means if we go on increasing the frequency of the input signal without changing its voltage level we will get different output voltages at different frequencies depending upon the #$ nature of the winding. +f we plot these output voltages against the corresponding frequencies we will get a particular patter for a particular winding. )ut after transportation, heavy short circuit faults, transient switching impulses and lightening impulses etc, if we do same Sweep Frequency esponse !nalysis test and superimpose the present signature with the earlier patterns and observe some deviation between these tow graphs, we can asses that there is mechanical displacement and deformation occurred in the winding.
+n addition to that, SF! test also helps us to compare between physical condition of the same winding of different phases at the same tap position. +t also co mpares different transformers of the same design. !nalysis #ow frequency response / Winding behaves as a simple # circuit formed by series inductance and resistance of the winding 0!t low frequencies capacitance acts as almost open circuit/ 1/ !t low frequencies winding inductance is determined by the magnetic circuit of the transformer core. *igh frequency response 2/ !t high frequencies winding beh aves as #$ circuits 3/ Winding exhibits many resonant points 4/ Frequency response is more sensitive to winding movement.
Signal applied across transformer terminals
Conditions
HV Red phase to Neutral
LV Red Yellow Blue phases are open
HV Yellow phase to Neutral
LV Red Yellow Blue phases are open
HV Blue phase to Neutral
LV Red Yellow Blue phases are open
HV Red phase to Neutral
LV Red Yellow Blue phases are shorted
HV Yellow phase to Neutral
LV Red Yellow Blue phases are shorted
HV Blue phase to Neutral
LV Red Yellow Blue phases are shorted
LV Red to Yellow phase
HV Red Yellow Blue phases and LV Blue phase are open
LV Yellow to Blue phase
HV Red Yellow Blue phases and LV Red
phase are open LV Blue to Red phase
HV Red Yellow Blue phases and LV Yellow phase are open
