Applying Sweep Frequency Response Analysis and Leakage Reactance to Determine Mechanical Faults in a Power Transformer

Presented By:
Charles Sweetser
OMICRON Electronics Corp. USA
TechCon 2019


Sweep Frequency Response Analysis (SFRA) and Leakage Reactance (LR) testing provide essential information needed to determine the “mechanical” condition of transformer assets. Fault events or shipping impacts are the main causes of transformer winding damage. Winding movement and/or deformation may cause changes in the leakage channels and associated passive RLC elements. Both Sweep Frequency Response Analysis (SFRA) and Leakage Reactance (LR) can be used in conjunction to identify and confirm changes in the leakage channels and associated passive RLC elements:

These changes are used to identify the following mechanical failure modes: 

  • Radial Deformation 
  • Axial Deformation
  • Bulk Winding Movement

This paper focuses on how the Sweep Frequency Response Analysis (SFRA) and Leakage  Reactance (LR) tests can be applied to power transformers. The audience will be provided with an understanding, application, and analysis of these tests, supported by specially selected case studies validating the value that these diagnostic tests bring to testing, and finally, assessing power transformers. 


The primary goal when performing diagnostic tests on power transformers is to ensure safe operation and accomplish life extension. Failure modes are often categorized in three common descriptions: dielectric, thermal, and mechanical failure modes. Understanding the mechanical condition of the power transformer can help predict an impending failure mode. This paper focuses on mechanical failure modes, and how Sweep Frequency Response Analysis and Leakage Reactance tests can be used for this condition. We will investigate test procedure, test preparation, and expected results for these two important tests. For the purpose of this paper, we will focus on a delta-wye power transformer (Dyn1); this will simplify our discussion.

We introduce and focus on the following “mechanical” tests:

  1. Sweep Frequency Response Analysis (SFRA) 
  2. Leakage Reactance (LR)

The test plan, procedure, and analysis recommendations found in this paper are based on the contents of:  

  • IEEE C57.149-2012, “IEEE Guide for the Application and Interpretation of  Frequency Response Analysis for Oil-Immersed Transformers”. 
  • IEEE C57.152-2013, “IEEE Guide for Diagnostic Field Testing of Fluid-Filled Power  Transformers, Regulators, and Reactors”.

Diagnostic Testing – Mechanical Movement 

1. Sweep Frequency Response Analysis (SFRA) 

Sweep Frequency Response Analysis (SFRA) is a diagnostic tool used to assess the mechanical and electrical integrity of power transformers. The SFRA test consists of measuring the transfer function (Vout/Vin) of a power transformer winding over a wide sweep of frequencies from 20 Hz to 2 MHZ. The equivalent circuit of a transformer winding includes the coil resistance and inductance as well as capacitances between the turns and the other windings, and between the winding, the tank wall, and the core. Winding movement and/or deformation will cause changes in these passive RLC elements, thus changing the frequency response of the transformer winding. Deviations in the SFRA Measurements can be used to identify the following  mechanical failure modes:  

  • Radial Deformation (faults) 
  • Axial Deformation (faults)  
  • Bulk Winding Movement (transportation) 

It can also identify electrical problems such as:  

  • Broken or Loose Connections  
  • Shorted Turns (Compromised Insulation) 

Test Preparation: 

  1. Ensure that the transformer tank and core are solidly grounded, also include both the test instrument and power source ground to this point. We will refer to this point as the  “GROUND” node. 
  2. Completely isolate the transformer terminals; remove external connections, such as  cables, from H1, H2, H3, X1, X2, X3, and X0. 
  3. Confirm that the bushing flanges are clean and acceptable. They are being used as a  ground reference for the SFRA measurement.

Test Procedure: 

Based on the IEEE C57.149 guide [1], 9 tests are recommended for the Dyn1 configuration. The  9 tests are shown in Table 1

Table 1 - SFRA Test Plan

Expected Results:

For a Dyn1 transformer, as shown in Figure 1, the expected results are as follows: 

Figure 1 – Typical SFRA Results (Dyn1)

Three groups of traces emerge from a typical SFRA measurement. These three groups are known as HV Open Circuit, LV Open Circuit, and HV-LV Short Circuit responses. It should be noted for reference that the HV-LV Short Circuit responses correspond with the same configuration used by the Leakage Reactance test.

Experience has shown that winding movement caused by a high current fault in a 3-phase core form transformer is generally associated with the LV winding in the form of radial deformation.  So, diagnostic focus will be given to the LV Open Circuit and HV-LV Short Circuit responses.  IEEE C57.149 Guide for SFRA Testing [1] documents both Open Circuit and HV-LV Short  Circuit responses. Figure 2, shown below, illustrates what happens to the measurements during a high-current radial deformation event.

Figure 2 – IEEE C57.149 - Radial Movement

If the transformer is not a two-winding unit, then the test plan will vary. Shown below are additional test plan architectures that include three-winding and auto-transformer configurations.

  • 2 Winding (H, X)
    • 3-H OC 
    • 3-X OC
    • 3-HX SC
  • 3 Winding (H, X, Y)
    • 3-H OC 
    • 3-X OC  
    • 3-Y OC 
    • 3-HX SC 
    • 3-HY SC
  • Auto Transformer (Series, Common, Tert)
    • 3-H Series OC 
    • 3-X Common OC 
    • 3-Y Tert OC  
    • 3-HX SC 
    • 3-HY SC 

2. Leakage Reactance

The field Leakage Reactance test is an AC (60Hz) short-circuit impedance test, which is performed to detect mechanical winding movement and/or deformation within a power transformer. There are two methods for performing Leakage Reactance tests, as follows: 

  1. Three-Phase (3-Phase) Equivalent Test 
  2. Per-Phase Test

The Leakage Reactance measurement directly corresponds to the leakage flux. Leakage flux is flux that does not link all the turns of the winding. It is normal that some of the flux escapes.  This leakage flux also helps create impedance that is used to limit short circuit current. Leakage flux creates reactive magnetic energy that behaves like an inductor in series in the primary and secondary circuits. This impedance can be easily measured, analyzed, and trended. This simple model is shown in Figure 3. Winding movement changes the reluctance of the leakage flux path,  resulting in a change in the expected leakage reactance measurement.

Figure 3 - Leakage Reactance Circuit Model

When performing the Three Phase (3-Phase) Equivalent Test, the secondary is short-circuited and the neutral connection, if present, is not included. Please note that single-phase units can also be tested. Shown below, in Figure 4, are the equations that are used to calculate the per-unit Leakage Reactance impedance.

Figure 4 - Leakage Reactance Equations

Figure 5, as an example, puts the 3-Phase equivalent equation to use. The base power, base voltage, and individual phase impedance measurements are applied.

Figure 5 - Leakage Reactance Typical Results

Test Preparation:

  1. Ensure that the transformer tank and core are solidly grounded, also include both the test instrument and power source ground to this point. We will refer to this point as the  “GROUND” node. 
  2. Completely isolate the transformer terminals; remove external connections and buswork  from H1, H2, H3, X1, X2, X3, and X0. 
  3. Isolate H1, H2, and H3, making sure that they are not connected together. 
  4. Document temperatures and humidity. 
  5. Supply #4 solid bare copper conductor and C Clamps/Vice Grips/Channel Nuts.
  6. Solidly short X1, X2, and X3, do NOT include X0; ground X0. 
  7. Identify impedance, base power, and base voltage from nameplate. 
  8. Verify that the DETC and OLTC are in the nominal rated tap position. If not, the 3-phase  equivalent measurement will not be comparable to nameplate.

Test Procedure:

Six tests are to be performed; 3 (3-Phase Equivalent) and 3 (Per-Phase). A four-wire Kelvin connection will be applied. An AC test current should be injected to establish a 30 -100 VAC  drop across the primary winding. It is recommended to start with 1.0 A of current injection.  With 1.0 A of current injection, the voltage developed across the primary winding should be verified that the voltage is in the 30 -100 VAC range at 60 Hz. If not, adjust the current injection proportionately to obtain a voltage drop within 30 -100 VAC range at 60 Hz. For some sources in combination with some transformers, it may not be possible to achieve a voltage drop of 30 – 100 VAC. Always optimize the source and meters if possible. 

Table 2 and Table 3, shown below, provide the connections for both the 3-Phase Equivalent tests and Per-Phase tests, respectively. 

Table 2 and Table 3, shown below, provide the connections for both the 3-Phase Equivalent tests and Per-Phase tests, respectively.

Expected Results: 

The purpose of the 3-Phase equivalent test is to produce a test result to compare to the factory short-circuit impedance percentage value (Z% nameplate), which can be found on the transformer nameplate. A deviation greater than ±3% of the reported value should be investigated [2].  

If one or more of the Per-Phase measurements is dissimilar from the others, a mechanical failure may exist within the transformer, which should then trigger further investigation. We recommend that the measured impedance (Ω) values of the three Per-Phase measurements compare to within ±3% of the average of the three (Ω) values.

Case Studies 

Leakage Reactance and SFRA I: 

This case study is an example of winding deformation identified by both the Leakage Reactance and SFRA tests. The transformer experienced a fault and acetylene gas was produced. After confirming the gas, Leakage Reactance and SFRA tests were performed. Figure 6, Figure 7, and Figure 8 present the DGA, Leakage Reactance, and SFRA tests, respectively. 

Figure 6 - DGA Results
Figure 7 - Leakage Reactance Results
Figure 8 - SFRA Results LV Open Circuit Tests

The DGA clearly indicated a substantial event, while both the SFRA and Leakage Reactance results exhibit an anomaly on Phase B. Winding deformation is expected. Upon internally inspecting the unit, it was clear that there was obvious winding deformation on the Phase B LV winding. This is shown in Figure 9.

Figure 9 - Observed Winding Movement LV Winding Phase B

Leakage Reactance and SFRA II:

The transformer in this case study is rated at 50 MVA 90.2kV/34.5kV. The unit is configured as a Dyn1. It tripped from service and oil appeared to be leaking from the LV DETC.

The standard test protocol was applied, which included SFRA and Leakage Reactance.

The Power Factor tests would not run on the low side at 10 kV, so the voltage was lowered to 7 kV. A Power Factor of almost 50% was obtained for the CL insulation; this is clearly unacceptable. The Power Factor results are shown in Figure 10.

Figure 10 - LV Power Factor Results

The Leakage Reactance results indicated an unexpectedly high impedance on Phase B. This is typical of an open circuit. The DC Winding Resistance test also confirmed an open circuit. However, the other phases, A and C, produced the expected 40 mΩ. The Leakage Reactance Results are shown in Figure 11.

Figure 11 - Leakage Reactance Results

The SFRA results indicate a high impedance fault on Phase B of the LV Winding. Both the LV Open Circuit and HV-LV Short test provide evidence. Figure 12 and Figure 13 illustrate this failure mode on Phase B.

Figure 12 - SFRA LV Open Circuit Results
Figure 13 - SFRA HV-LV Short Circuit Results


  • SFRA and Leakage Reactance testing can provide useful and in-depth information regarding the condition of the power transformer. Mechanical incipient failure modes can not only be identified but also located. The problem winding and phase are often identified.
  • Proper procedures should be followed to ensure useful results. The test data is only as good as the technician performing the tests. Other information (test data) such as power factor, TTR, DC Winding Resistance, Exciting Currents, and DGA should be used in conjunction with SFRA and Leakage Reactance in making diagnostic decisions.


[1] IEEE C57.149-2012, “IEEE Guide for the Application and Interpretation of Frequency Response Analysis for Oil-Immersed Transformers”.

[2] IEEE C57.152-2013, “IEEE Guide for Diagnostic Field Testing of Fluid-Filled Power Transformers, Regulators, and Reactors”.

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