29-04-2014, 04:27 PM
Sweep Frequency Response Analysis Transformer Applications
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Executive Summary
This paper presents technical details regarding Sweep Frequency Response Analysis
(SFRA) and the role it plays in transformer test and maintenance. SFRA is an electrical
test that provides information relating to transformer mechanical integrity.
Details of the SFRA test method are given alongside practical results and case studies.
For reasons of range, resolution and repeatability, the SFRA technique is shown to be
superior to other frequency response techniques, providing results that may be used in
key decisions by transformer engineers and asset managers.
The Doble M5100 is a robust and field proven instrument, which combines simple
measurement techniques with powerful technical support from Doble Engineering.
What is SFRA?
Sweep Frequency Response Analysis (SFRA) is a tool that can give an indication of core
or winding movement in transformers. This is done by performing a measurement, albeit
a simple one, looking at how well a transformer winding transmits a low voltage signal
that varies in frequency. Just how well a transformer does this is related to its impedance,
the capacitive and inductive elements of which are intimately related to the physical
construction of the transformer. Changes in frequency response as measured by SFRA
techniques may indicate a physical change inside the transformer, the cause of which then
needs to be identified and investigated.
Figure 1 gives an example where SFRA has diagnosed a shorted turn in a generator step
up transformer. The SFRA results for each phase of the transformer are plotted as dB
responses against frequency. In this case, the response of one phase is clearly very
different from the other two, and the form of difference indicates a shorted turn in this
case. It is important to get good resolution in results such as this to give clear and
unambiguous traces at low frequencies
Power Transformers and Mechanical Integrity
Power transformers are specified to withstand the mechanical forces arising from both
shipping and subsequent in-service events, such as faults and lightning. Transportation
damage can occur if the clamping and restraints are inadequate; such damage may lead to
core and winding movement. The most severe in-service forces arise from system faults,
and may be either axial or radial in nature. If the forces are excessive, radial buckling or
axial deformation can occur. With a core form design the principal forces are radially
directed, whereas in a shell form unit they are axially directed, and this difference is
likely to influence the types of damage found.
Once a transformer has been damaged, even if only slightly, the ability to withstand
further incidents or short circuits is reduced. There is clearly a need to effectively identify
such damage. A visual inspection is costly and does not always produce the desired
results or conclusion. During a field inspection, the oil has to be drained and confined
space entry rules apply. Since so little of the winding is visible, often little damage is seen
other than displaced support blocks. Often, a complete tear down is required to identify
the problem. An alternative method is to implement field-diagnostic techniques that are
capable of detecting damage, such as SFRA.
Which measurements are made?
The most useful SFRA measurement is the response of individual winding sections of the
transformer at different frequencies. This allows problems to be associated with
individual winding sections, rather than on a phase or winding generally. The SFRA does
this in a simple way by injecting a signal of known frequency into one end of the winding
and measuring the response at the other. By sweeping through the frequency range of
interest, from just above DC to several MHz, it is possible to make accurate, repeatable
and reliable measurements. SFRA measurements are independent of the lead arrangement
and of the measuring device up through the frequencies of interest. For tests on a large
power transformer 20m leads are needed which give reliable and repeatable results up to
the MHz range.
Is it an easy measurement to make?
SFRA is a very easy measurement to perform. The transformer should be prepared as it
would for a standard Doble power factor and capacitance test. The SFRA test requires a
3-lead approach, with the leads providing signal, reference and test. This approach means
that the signal put into the test winding is measured to provide a reference which is then
compared with the signal which emerges at the far end of the winding and is measured by
the test lead. The three lead approach reduces the effect of the test set on the test results –
making the measurement robust, repeatable and reliable.
What are the bands and sub-bands shown in some results?
The development of FRA was centered, in the late 1980’s, on use of a Hewlett-Packard
Network Analyzer. This had a restriction of only allowing 400 points per trace, which
meant poor resolution at lower frequencies on a scan of 2 MHz. To account for this,
several scans were done, the so called ‘sub-bands’ – 2 kHz, 20 kHz, 200 kHz, 2 MHz and
10 MHz. These sub-bands allowed analysis in detail across the frequency range. With a
set step of 5 Hz in the 2 kHz band, the resolution never bettered 2.5%. The 10 MHz
results were shown to be repeatable in a laboratory setting, but for field practicality
results above 2 MHz are rarely used.