THREE STEPS for

diagnostic testing of bushings

By Sanket Bolar, Ankit Porwal

Tan Delta (PF/DF) Testing

Narrowband Dielectric
Frequency Response

Dielectric Frequency
Response Measurements

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Introduction

Today, condenser bushings are used everywhere in applications

exceeding 25 kV. Based on the materials used in the insulation
system, condenser bushings can be classified into – oil
impregnated paper (OIP), resin impregnated paper (RIP), resin
bonded paper (RBP) and resin impregnated synthetic (RIS)
bushings. Among these technologies, OIP is perhaps the most
widely used.

Bushing failure is, alongside tap-changers, one of the main

causes of transformer failure. Hence, bushing insulation
health needs to be monitored effectively during its service
life to ensure bad bushings are replaced in a timely manner.
Periodic testing of capacitance and tan delta (power factor or
dissipation factor) at line frequency (50 or 60 Hz) has been done
on bushings for close to a century now. In recent times, the
use of dielectric frequency response, an advanced application
of power factor testing, has become an increasingly popular
and effective method for bushing diagnostics.

Field Experience
Condition assessment of HV Bushings

Step 1:
Line – frequency tan delta (PF/DF) Analysiss
Despite the advancement in the last decade in the field, tan delta (PF/DF) Bushing
Nameplate % Measured %
testing technology, most end-users continue testing at only one single power factor power factor

frequency value, line-frequency 50 or 60 Hz. In this example, Out of X1 0.23 0.47

X2 0.23 0.27
three OIP HV bushings are tested at line-frequency ONLY, an increase in X3 0.24 1.40
the power factordielectric losses was observed on two bushings: X1 and
Table 1. Nameplate and measured
X3, as illustrated in Table 1. pf values for three bushings

A. According to IEEE C57.152, the line frequency power factor values for X1 and X3 at 20°C exceed acceptable
limits.

B. According to CIGRE TB 445, the power factors of the bushing X1 and X2 come within limits, while only
bushing X3 comes outside acceptable limits.

C. Based only on the line frequency power factor values, we may say that the bushings X1 and X3 are not in good
condition, and the bushing X2 may not require further investigation.

D. Hence, we move onto step 2 and run a narrowband DFR test to evaluate dielectric losses factor at frequencies
between 1 and 500 Hz.

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Step 3
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2 Three steps for diagnostic testing of bushings

Step 2:
Narrowband Dielectric Frequency Response (NB DFR) Analysis

NBDFR test was carried out on all three bushings from 1 The temperature corrected power factor values
Hz to 500 Hz and the results are presented in Figure 1. obtained at different frequencies are shown in Table 2.

Bushing %PF at 60 Hz %PF at 15 Hz %PF at 1 Hz

X1 0.454 0.514 1.32

X2 0.271 0.278 0.660
X3 1.14 2.76 11.9
Table 2. Temperature corrected %pf values obtained
for three bushings at different frequencies

As per the studies carried out by Megger based on over

20 years of expertise in DFR, the limits shown in Table
3 have been proposed for analysis of %pf measured at
the frequency of 1 Hz.

Bushing
%PF at 20°C and 1Hz
condition
Figure 1. NB DFR measurements on all three bushings As new 0.2 – 0.5
Good 0.5 – 0.75
Aged 0.15 – 0.2 and 0.75 – 1.25
Investigate <0.15 and >1.25

Table 3. Proposed limits for %pf at 20°C and 1 Hz

A. X1 bushing shows very linear response between 60Hz and 15 Hz, both frequencies may be considered in the
field as acceptable at 20°C. The 1 Hz % PF value exceeds Megger’s experience for a good condition bushing
(<1%) and it calls for further investigation
B. X2 bushing shows a linear response between 60 and 15 Hz. At 1 Hz the %PF value is below 1% and it is
considered as a good bushing. Further investigation not required.
C. X3 bushing shows high LF PF value with high losses at 15Hz and clearly defined as a bad bushing when
tested at 1 Hz.

Upon completion of Step 2, X1 and X3 indicatesshow insulation-related problems, but thethe cause of higher
losses is yet to be determined by a definitive analysis tool dielectric frequency response measurement in the range
from 10mHz to 1kHz

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Step 3
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Step 3:
Dielectric Frequency Response (DFR) Analysis

The dielectric response of all three bushings is obtained

for definitive analysis and to confirm the assessment
in steps 1 and 2. For HV bushings, the typical range is
from 10 mHz and up to 1 kHz as presented in Figure
2. The results that were obtained from the full DFR
analysis are shown in Table 4. The limits for %mc and
conductivity of the oil proposed by Megger are shown
in Table 5.

Another unique advantage of the DFR test is the Figure 2. DFR measurements on all three bushings X1, X2, X3
ability to convert the dielectric response in the
frequency domain to the dielctric response in the
temperature domain and provide the “INDIVIDUAL” Bushing
%MC or Conductivity of the oil at
contami- nation 25°C (pS/m)
thermal behavior of PF, DF or tan delta for a specific HV
X1 2.5 0.185
bushing based on its UNIQUE dielectric condition and
X2 1.9 0.05
not any average table. X3 4.8 0.034

Table 4. The results that were obtained from the full DFR analysis

Bushing %MC or Conductivity of the

condition contamination oil (pS/m)
As new 0.15 – 0.5 0.001 – 0.37
Good 0.5 – 1 0.37 – 3.7
Aged 1 – 2.5 3.7 – 37
Investigate >2.5 >37

Table 5. %mc estimation and conductivity of

Figure 3. Temperature dependence curves for all three bushings the oil obtained from full DFR analysis

A. X1 bushing DFR shows very good quality of liquid insulation but average condition of the solid insulation
inside that bushing. Suggested yearly tan delta (PF / DF) test at line frequency and 1 Hz results to be properly
corrected to 20°C using the Individual Temperature correction (ITC) algorithm).
B. X2 bushing DFR shows a very good condition of liquid insulation and aged condition of the solid insulation.
Suggested tan delta (PF / DF) test at line frequency and 1 Hz in two more years or after a system fault.
Results to be properly corrected to 20°C using the Individual Temperature correction (ITC) algorithm).
C. X3 bushing DFR confirms the results from line-frequency and 1 Hz tan delta (PF/DF) measurements. High
contamination of solid insulation suggests removal and replacement of existing bushing for safe operation of
the transformer.

Overall diagnostics
Summary of step 1 to Step 3 diagnostics.
LFPF as per LFPF as per %PF at %PF at Conductivity of the
Bushing %LFPF %mc FINAL DECISION
IEEEC57.152 Cigré TB 445 1Hz 10Hz oil (pS/m) at 25°C
X1 0.454 Bad Good 1.32 Aged 2.5 0.185 Aged solid insulation
X2 0.271 Good Good 0.660 As new 1.9 0.058 Good – retest in 2 years
X3 1.14 Bad Bad 11.9 Investigate 4.8 0.034 Investigate

6. Final assessment based on all measurements

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4 Three steps for diagnostic testing of bushings

Conclusion
 The traditional line frequency tan delta (power factor or dissipation factor) testing, covered in Step 1
shwos its limitations

n State of the art technology now tests the HV bushings at three different frequencies: 505, 60
(or 50) and 1 Hz.

n Having 1 Hz as part of the fundamental tan delta (PF or DF) testing allows for much better
interpretation of the condition of the insulation

 NB DFR is Step 2, where one can see graphically the deviation of the dielectric response in that specific
frequency range from 1 to 505 Hz. Specially important to identify connection and contamination issues.

 Step 3 finally covers a complete and definitive analysis using DFR technology. Megger’s 25 years of expe-
rience using DFR technology in the field and the research carried out in thousands of OIP bushings allows
recommendation of suggested limits for frequencies beyond line-frequency

 If the assessment of a HV bushing following these 3 steps shows “investigate”, that means that you should
contact your Megger local specialist and or your manufacturer to discuss your results before you put that
bushing back in service.

References
1. W. A2.34, “Guide for transformer maintenance,” Cigre, 2011
2. IEEE C57.19.01-2017 - IEEE Standard for Performance Characteristics and Dimensions for Power Transformer
and Reactor Bushings
3. IEC 60137:2017 - Insulated bushings for alternating voltages above 1 000 V
4. IEEE C57.152-2013 - IEEE Guide for Diagnostic Field Testing of Fluid-Filled Power Transformers, Regulators,
and Reactors
5. IEEE C57.19.100-2012 - IEEE Guide for Application of Power Apparatus Bushings
6. IEEE C57.12.90-2015 - IEEE Standard Test Code for Liquid- Immersed Distribution, Power, and Regulating
Transformers
7. Dr. P. Werelius, Dr. D. Robalino, J. Cheng and M. Ohlen, “Dielectric Frequency Response Measurements and
Dissipation Factor Temperature Dependence,” 2012
8. Dr. D. Robalino, “Accurate Temperature Correction of Dissipation Factor Data for Oil- Impregnated Paper
Insulation Bushings: Field Experience,” 2011

Ankit Porwal,
Sanket Bolar, is Megger’s APAC
MSc is a Substation responsible for the
Applications Engineer power transformer
at Megger Americas. segment in South Asia.

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