ON-LINE PROTECTION, OPERATION AND LOADING OF POWER TRANSFORMERS

Abstract

Power and distribution transformer forms an important link between generation transmission and distribution of power. It is essential that a transformer gives a very stable, reliab le and efficient performance during normal service. Failure of power/distribution transformer may completely disrupt the supply of area fed from it. It causes the loss of revenue, loss of industrial production and harassment to the public at large. At the same time it is a costly equipment at the sub-station. It requires proper and fast (a) protection, (b) load-monitoring and control with over-load time-limit control, (c) monitoring of life saved/consumed in a day, total consumed life etc., for efficient operation and maintenance such that maximum benefit can be derived from its heavy investment. 8085A microprocessor based equipment for the work listed above (i.e. (a), (b) and (c)) is developed by the author using 16 channel ADC module, analog sequence current filter, hot-spot temperature measurement circuit, peak detectors, amplifiers, band pass filters and buffer circuits. The scheme is also supervised using personal computer (on-line) with compatible ADC and I/O cards. The response of the developed scheme implemented there upon is faster than other computer methods. PC also provides backup to microprocessor based scheme, if the later fails on any account. VI The prediction of switching transient in transformer has received considerable attention due to advent of EHV, UHV and HVDC transmission, and due to introduction of comparatively high working flux density with cold-rolled grain oriented silicon steel as transformer core material. A practical method for analysing the transient behaviour of a transformer by taking into account the nonlinearity of the magnetic material by method of electronic proto-type modelling of core material B-H curve is developed. The required scaling can also be included in the model. The mathematical model has been developed for a current transformer (CT) to predict the transient performance during swi tch-in of power transformer, with residual flux (Inrush). The results of the analysis are verified using the data of 50-100-200/1, 40VA, 10 P 20, 132 KV and 300/1, 120VA, 5P 15, 220KV CTs, the secondary current response both for fault and inrush have been computed with different values of CT time constants, and relation of negative peak, decaying rate towards zero, angle of lag at zero crossing (At) are established for inrush. The relations so established are verified practically for different residual flux (varying DC voltage) and different time constants (for different CTs) using the electronic proto-type model prepared, for both single phase and 3-phase transformers. The satisfactory agreement exists between the computed and experi mental results. vii From the computed and experimental check-up of inrush wave observed on CT secondary it is concluded that the inrush current wave shall have unique low current value and low rate of change of current which occurs within the first 45 degree of a cycle after zero crossing (positive-rise of rectified wave form) . The condition enumerated above is not possible with (a) load current, (b) fault current, and (c) fault current with maximum possible DC offset, where i and di/dt are considerably higher compared to inrush. Utilizing the above facts, New one cycle static blocking scheme for inrush in differential relay is developed. This static scheme can be used with any of the static or electromech anical differential relay to avoid maloperation due to inrush. A new one cycle microprocessor based differential relay for power transformer is developed, this relay is also based on wave-shape approach. In addition to normal operation the devel oped relay shall also take care of maloperation due to (a) heavy through fault, (b) inrush, (c) one of the CT saturating with external fault, (d) internal short circuits and (e) internal short circuit with CT saturation. A further protection scheme is developed as back-up to the differential relay discussed above using analog sequence current filter. The developed scheme also gives the type of fault from the limiting values of positive, negative and zero sequence com ponents of the currents, with the discrimination of external/int ernal. The response of the developed scheme is between 2 and 3 cycles. ' viii It is well known that the negative and zero sequence curr ent are low (ideally zero) for balanced load, this fact is utilized in the scheme enumerated in the previous paragraph for priority based load-balance control in normal working condition. It is further shown that this philosophy of load-balance control, extented to medium and large sized alternators wherein negative sequence component due to severe unbalance causes over heating. The substation (power transformers) is serving the load demand of a geographically defined area. When load demand of the area increases substation modification to satisfy the load demand is must. Th.e substation engineer has to evaluate and recommend either the existing facilities (transformers) to be over loaded or extra facilities be added before the load growth. Power transformers are designed to carry continuous load equal to the name plate rating. The deterioration of its insul ation is a function of time and temperature. It has been shown that it is possible to meet the short-time peak load requirements even beyond name plate rating from the existing transformers, without consuming more than the normal life of these transformers (Indian Standards 6600-1972). However none of the electricity boards are availing the full benefit of short-time over-loading permitted by the IS 6600-1972 due to fear of damaging costly tra nsformers, in the absence of automatic, accurate and reliable monitoring scheme, for temperature and extra life consumed/saved per day and to compute over-load time-limit for the given over load without consuming extra life of costly equipment. ix Here, a low cost automatic hot spot temperature measure ment and over-load time-limit computation and control scheme, on the basis of total consumed life and consumed/saved life a day, is developed. Which is simple in structure, accurate and reliable. A new strategy based on the criterion, "remaining life of the day is more than the life to be consumed if transfor mer is loaded at name plate rating for the remaining time of the day? has been implemented on the scheme. Thus the transformer is utilized very efficiently and optimally. With the permitted over-loading by strategy discussed the copper loss cost increases. Since energy cost is going up every month, the efficient and optimal utilization of power tra nsformer may not be economical above certain over-load limit. Hence the discussed scheme is further extended to compute and display the additional life cost, copper loss cost and off feeder (i.e., load shedding) cost of last 30 days at every 24 Hrs. The selection of load priority along with economy is left to the operator. It is economical to over-load the transformer with priority based load shedding up to certain limit till the addi tional Qoss) cost due to over-load is less than the predetermined "LIMIT" The 'LIMIT His decided on the basis of cost of no-load loss, copper loss, maintenance repair along with revenue to be collected by the addition of a predecided capacity. If additi onal (loss) cost Is greater than LIMIT the recommendation is displayed. Thus the scheme developed helps planners in (a) choosing appropriate ratings of power transformers, (b) deferring or reducing future augumentation, (c) helping to avoid switching off the feeders in most cases during peak load when load exceeds transformer name plate rating and (d) saving in additional core losses during lean periods. The protection, fault-type detection, load balance control, hot spot measurement, computation of saved/consumed life in a day, total consumed life, time for which over load Is permitted and capacity addition for power transformer are carried out by the same microprocessor module continuously at required approp riate time intervals. The operation of microprocessor based module developed for fast protection, fault-type detection, load-balance control etc. is also supervised by the PC by monitoring the differential relay output, filtered sequence current components and line curr ents. The PC also provides the supervision on substation working by monitoring of loading, protection and status of the equipments. All the slow protection of substation are carried out by PC.