SFRA

'SFRA' Sweep Frequency Response Analysis (SFRA)

What Is SFRA?

Sweep Frequency Response Analysis (SFRA) is a powerful and sensitive method to evaluate the mechanical integrity of core, windings and clamping structures within power transformers by measuring their electrical transfer functions over a wide frequency range. The SFRA principle (sweep frequency response analysis) – a worldwide proven method for measurements in frequency domain. The SFRA is a comparative method, i.e. an evaluation of the transformer condition is done by comparing an actual set of SFRA results to reference results. Three methods are commonly used to assess the measured traces:

• Time-based (Current SFRA results will be compared to previous results of the same unit)

• Type-based (SFRA of one transformer will be compared to a type-equal one)

• phase comparison (SFRA results of one phase will be compared to the results of the other phases of the same transformer)

File:SFRA.JPEG

SFRA

Type of connection for SFRA

Some of the common types of test connections, which are found to be very sensitive to different type of faults in a transformer, employed in SFRA measurements are listed below.

a) End-to-end (open) measurement

b) End-to-end (short-circuit) measurement

c) Capacitive inter-winding measurement

d) Inductive inter-winding measurement

How does SFRA work?

Transformers consist of multiple complex networks of capacitances and resistors that can generate a unique signature when tested at discreet frequencies and plotted as a curve. The distance between conductors of the transformer forms a capacitance. Any movement of the conductors or windings will change this capacitance. This capacitance being a part of complex L(inductance), R(Resistance) and C(Capacitance) network, any change in this capacitance will be reflected in the curve or signature.

The SFRA test is carried out to obtain initial signature of the transformer sweep frequency response by injecting various discreet frequencies, which is then used as reference for future comparisons. Any fault giving rise to change in winding movement etc. will result in change in capacitance or inductance thereby affecting the measured curves.

Tests are carried out periodically or during major external events like short circuits and results compared against the initial signature to test for any problems. The basic functionality of SFRA supports following measuring modes. Voltage transfer function Uo/Ui (f) SFRA test reveals if the transformers mechanical or electrical integrity has been compromised.

What Problems Can Be Detected?

• The FRAnalyzer can detect problems such as: winding deformation – axial & radial, like hoop buckling, tilting,spiraling

• displacements between high and low voltage windings

• partial winding collapse

• shorted or open turns

• faulty grounding of core or screens

• core movement

• broken clamping structures

• problematic internal connections

Utilizations of SFRA

• To obtain initial signature of healthy transformer for future comparisons.

• Periodic checks as part of regular maintenance.

• Immediately after a major external event like short circuit.

• Transportation or relocation of transformer.

• Earthquakes.

• Pre-commissioning check.

SFRA Kit in market

• Dobel : M5200 Sweep Frequency Response Analyzer

• Omicron: VE000660 SFRA analyzer complete set

• SIVA Instrument :Sweep Frequency Response Analyzer

Reference

External links

http://imghost1.indiamart.com/data2/KF/OY/MY-557030/sfra.pdf

http://npsc2010.uceou.edu/papers/7023.pdf

http://www.doble.com/products/m5200_sfra.html

http://www.omicron.at/en/products/pro/primary-testing-diagnosis/franalyzer/

http://www.omicron.at/fileadmin/user_upload/files/pdf/en/FRAnalyzer_Brochure_ENU.pdf

http://www.omicron.at/fileadmin/user_upload/files/pdf/en/FRAnalyzer_Brochure_ENU.pdf

--Ameyaket (talk) 16:15, 6 January 2012 (UTC)Ameya S. Ket