n OUTLINE n EXECUTIVE SUMMARY Transformer Oil Monitoring A pulsating sensor has been developed for monitoring transformer oil degradation. The in-house developed device of new kind has been tested at CPRI, Bangaluru with a large number of service oil samples from different transformers, and compar ed wit h the conventional tec hni que s for oil cha rac ter isa tio n. The dev ice qua lif ied for its use as the rapid scr eening inst rument by the main tena nce pers onne l. Sl ow buil d up of oxidation pr oducts as a result of che mic al rea ctions wit h oxygen and moi sture fro m environment under the preva iling serv ice conditions and physical ingress of polar molecules such as water cause the die lec tri c per meabilit y of the ser vic e oil in any transformer to increase with time. A pulsating sensor has bee n dev elo ped for monito ring tra nsformer oil deg radati on. The sen sor is bas ica lly cap abl e of detect ing minor shift in dielectric permeability of the oil during service. A laboratory made uniformly spaced multiple number of disc type stainless steel electrode assembly, immersed in the mineral oil of interest, serves as the timing capacitor of an oscillator circuit which is driven by a dc supply. Wi th al l other factors rema ining unchanged, the digital pulse frequency at the output is governed by the dielectric permeability of the oil that acts as thedielectri c med ium in the cap aci torassemb ly. The pul se fre quency cha nges sensit ive ly wit h the change in die lectri c permeability and, hence, the reduction in measured pulse frequency with respect to the fr esh oil is a me asur e of di el ectric degradat ion of the used oil durin g service. The mea sur ement set up is shown schemati cal ly in Figure 1. Freque ncy can be measured by any of the commercia lly available hand-held freq uenc y mete r. However, for a detailed study, the frequency data as a functi on of time canalso be recorded thr oug ha PC usi ng a labor atorydeveloped inter face hard ware and software. The di gi tal pulse frequency data asa func ti on of ti me for fresh oil as well as for a slightly degraded oil using a typical device is shown in Figure 2. Decrease in pulse fre que ncy for the use d oil cor res ponds to inc rea se in the diele ctric permea bilit y by about 1.6 %. Thus, precise fre que ncy dat a per mit s mon itoring of ver y min ute shi fts in dielectric permeabi lity, hence enabli ng earl y detection of onset and progress of degradation in the qua lit y of ser vic e oil . Und er an MoU bet wee n IGCAR and Cen tra l Power Research Institute (CPRI), the devices made in IGCAR were test ed at CPRI al ong wi th the conventi onal standard techniques for degradation assessment. Two hundred oi l samp les fr om power tr ansf or mers of diffe rent ratings were colle cted and eval uate d by the D ie le ctric L ab or at or y of C PR I for the sta nd ard characteristics viz., interfacial tension, neutralization value, d i el e ct r ic dissipation factor, r e si s ti v ity, breakdown vol tag e and wat er content fol lowing the pres cribe d proce dures. The digit al pulse freq uenc ies were also recorded by CPRI on each sample using the pulsa ting sensor base d oil moni torin g devi ces. Thus , seve n para mete rs were deter mine d exper imentally on ever y oil sample. It was observed that the pulse frequency, as measured by the IGCAR devices, reduced by about seven percent for ser vic e oil s nea ring rej ect ion compar ed to tha t for the fresh oil. As the devices permit precise measurement of frequency shifts less than 0.2%, onset of degradation is det ect abl e at an ear ly sta ge. Fig 1 : Schematic representation of the oil monitoring device Fig. 2 : Pulse fre quen cies recorded as a function of time for fresh oil and a sampleof ser vice oil 214 TECHNOLOGY-23