Identification of Terminal Connection and System Function for Sensitive Frequency Response Measurement on Transformers

Abstract

Frequency response measurement on a transformer is a low voltage, off-line exercise. So, it virtually permits determination of any network/system function, by imposing any desired terminal condition for the nontested windings. The terminal conditions employed have significant influence on the achievable fault detection ability, and maximizing this ability should obviously be one of the main aims of frequency response measurements. Simply stated, this requirement translates to the ability to identify/measure as many natural frequencies as possible. However, there is a practical limitation that not all network/system functions can exhibit all natural frequencies. Hence, it is necessary to determine the most appropriate combination of terminal connection and system function for achieving this objective. Considering a two-winding transformer representation, system functions for different terminal conditions were computed. Depending on number of natural frequencies distinguishable in the amplitude frequency response of a system function, each measuring condition was ranked. Thus, it led to identification of the best configuration. Later, these findings were verified on an actual transformer. Performance of best configuration was compared with currently employed low-voltage impulse (LVI) test (used during short-circuit testing of transformers) and sweep frequency response measurement test conditions, and found to be better. In conclusion, it is believed that after adequate field verifications, the identified configuration can be declared as the preferred way of making frequency response measurements on transformers.

Theoretical considerations while applying sweep frequency response analysis method in determining the interwinding capacitance of power transformers at higher frequencies

“…The open and short‐circuited terminal connections are nothing but the condition of the secondary windings and the input voltage V(f) and neutral currents I n (f) are measured as responses to compute the respective winding transfer function TF oc and TF sc . These terminal connections and system function pair are listed in Figure 1 in the work of Satish and Saravanakumar marked as bfloat‐oc , bfloat‐sc and reported to have reasonable detection sensitivity. The present study uses the data from the currently practiced terminal connections for reference purposes.…”

“…The primary and secondary windings in the ladder network model are represented with four sections each. It is well known that, in such a circuit, the number of natural frequencies is equal to the total number of independent nodes . Being an analytic circuit, the roots (poles and zeros) would exist either as real or as a complex conjugate.…”

“…The same for the short‐circuited case would be around (six independent nodes) five complex conjugate poles and one real pole, respectively. The distribution of these poles and zeros in an s‐plane and their respective cancelation would demonstrate the importance of terminal connection and system function . Figure A‐C shows the distribution of pole‐zero location in the s‐plane .…”

Frequency Response Analysis for Phase-Shifting Transformers in Oil Field Facilities

“…In recent years, due to the advancement of powerful and customized frequency response measuring instruments, other practices such as keeping the nontested windings open and/or short-circuited with neutral floating emerged. However, there is no theoretical background to demonstrate which method is the best [7]. Sweetser and McGrail [8] propose test connections for different types of transformers, which will serve as the main reference in this paper.…”

Effect of terminal connections on SFRA results of three-winding power transformers

“…The active projects to publish a standard are the IEC [5] and the IEEE [6]. As we can see from the above standards and dates, SFRA is a new area and has been investigated by many researchers [7][8][9][10][11][12][13].…”