IEEE C57.149-2012 pdf free download.IEEE Guide for the Application and Interpretation of Frequency Response Analysis for Oil-Immersed Transformers.
Impulse voltage method: In the impulse voltage method, also referred to as LVI (Low Voltage Impulse) method, for making Frequency Response Analysis (FRA) measurements, the wide range of required frequencies is generated via one or more voltage impulses injected into one terminal. If more than one impulse is used, the wave shapes are very similar so as to provide a more uniform test result. Inductive inter-winding measurement: Performed over a wide range of frequencies between two electrically isolated windings that each has one end of the winding referenced to ground. Voltage is injected into one end of a winding, the input, and the response, the output, is measured at another winding. Measurement ground: The reference connection for the Frequency Response Analysis (FRA) measurement is typically the ground connection between the source/reference measurement cables and the measuring cables. These ground connections are generally made at each bushing flange. Mechanical movement: Detecting mechanical movement damage to transformer windings is one of the main interests of Frequency Response Analysis (FRA) test measurement. Mechanical movement refers to the actual movement of transformer parts (coils, core, leads, or accessories) with respect to each other or to ground in such a manner as to change the internal inductances or capacitances of the test specimen. This may be caused by seismic or shipping forces or by in-service conditions such as through-faults, load currents, mechanical breakdown of components, or failures. Minor deviation: A change in amplitude, phase angle, or frequency displacement that is considered to be within the normal deviation for a test configuration. Noise and interference: These are unwanted disturbances that may be superimposed upon a useful (desired) signal. Open-circuit measurement: The open-circuit measurement is performed over a wide range of frequencies where voltage is injected into one end of a winding, the input, and the response, the output, is measured at the other end of the winding. Open-circuit measurements are made on a winding with all other windings complete and floating. Phase angle displacement: The difference between the phase angle of a previous Frequency Response Analysis (FRA) “fingerprint” measurement (e.g., baseline measurement at the factory, at an earlier date in the substation or before a short-circuit test) and a new measurement (e.g., after transformer relocation, after suspected damage or after short-circuit test). The difference can also be between phase angle measurements on two different phases of the same transformer or between a transformer and a duplicate or near-duplicate transformer. Short-circuit measurement: Performed over a wide range of frequencies where voltage is injected into one end of a winding, the input, and the response, the output, is measured at the other end of the winding. Short-Circuit measurements are made on a winding with one or more windings shorted. Significant deviation: A change in amplitude, phase angle, or frequency displacement that is considered to be outside the normal deviation for a test configuration. A significant deviation may warrant further investigation or be considered as diagnostic evidence of change in the internal configuration of a transformer. Square pulse method: In the square pulse method for making Frequency Response Analysis (FRA) measurements, the wide range of required frequencies is generated via square pulses injected into one terminal. The square pulse shapes are different so as to provide a more uniform spectral density for calculating the results.
3. FRA measurement overview The FRA measurement provides diagnostic information, in the form of a transfer function, related to the RLC network of the specimen under test. The RLC network is integrally related to the physical geometry and construction of the test specimen. Physical changes within the test specimen alter the RLC network, and in turn can alter the transfer function. The transfer function behavior can reveal a wide range of mechanical or electrical changes in the test specimen. Different transformer failure modes can have different effects on the network admittances, may alter the transfer function. It is also possible that a particular failure mode may have no recognizable effect on the transfer function at all. FRA can often detect gross transformer defects, as can other electrical tests. However, because of the sensitivity of the test, a primary benefit of FRA is the potential for detection of defects in the mechanical or electrical integrity of the transformer that are not apparent with other electrical tests.