DFR- An Excellent Diagnostic Tool for Power Transformers

2008 Weidmann Annual Diagnostic Solutions Technical Conference Dr. Poorvi Patel, ABB Inc. & Mark Perkins, ABB Inc.,

4350 Semple Avenue, St. Louis Mo, 63120

Abstract

The Dielectric Frequency Response test (DFR) has been developed as an excellent diagnostic tool for transformer insulation system testing. The DFR measurement is similar to the frequently used power factor measurement, except that it is a series of power factor measurements at multiple frequencies. The advantage of doing the measurement at multiple frequencies is that it provides much more information which makes it possible to distinguish properties of both the cellulose and oil insulation separately. DFR analysis of an insulation system can give some indications of the condition of the insulation system such as a direct reading of the moisture content and detection of the presence of contamination. Since the effect of moisture and other abnormalities on the dielectric properties of the insulation system may be more pronounced at specific frequency ranges, the preferred measurement is done at a very broad frequency range, such as 0.001-1000 Hz. The dielectric properties evaluated are the real and imaginary capacitance (or permittivity) and the dissipation factor as function of frequency. Comparing measured data with data from the transformer insulation system model, humidity content and oil conductivity can be estimated. Other transformer issues may also be investigated by comparisons between the measured and modeled responses. This paper includes a general overview of DFR measuring techniques and interpretation methods and includes one case study to demonstrate the method.

Introduction The dielectric frequency response (DFR) measurement is a diagnostic test that is used to characterize the properties of an insulation system. For a mineral oil-cellulose insulation system used in transformers, the properties of the insulation system involved in this analysis include the moisture in the cellulose material, conductivity of the oil, and the presence of contaminants or other materials that affect the capacitance or dielectric loss of the system. Traditionally the moisture in the insulation is estimated by either measuring the moisture in oil or directly taking paper from the insulation for Karl Fischer titration. However, determining moisture of the insulation from the moisture content of oil has shown to be very dependent on the oil temperature and also the oil condition. Determining the moisture from pressboard or paper samples requires careful extraction and restoration so that the insulation system is not compromised, and then the samples must be correctly stored until they reach the laboratory. The insulation between the windings, which is the most important area for moisture assessment, is inaccessible unless the transformer is taken apart. Therefore, recent attention has been directed to indirect methods of determining moisture content such as measuring the effects of the moisture on electrical properties of the insulation. There are at least three types of dielectric measurements techniques:

• Return voltage measurements, sometimes also called recovery voltage – (RVM) • Polarisation and depolarisation current variation in time domain - (PDC) • Dielectric Frequency Response – (DFR)

The Return Voltage and Depolarization & Polarization methods are DC voltage tests and results are measured as function of time. The Dielectric Frequency Response is an ac test and the results are measured as function of frequency. The DFR method is well understood [1-4] and will be discussed in this report.

DFR Basics The dielectric frequency response (DFR) method is a measurement of capacitance and loss at a range of frequencies, often presented as the real and imaginary part of the complex relative permittivity εr. The well known Power factor measurement is done at 60 Hz, while the frequency range for DFR is normally between 1 mHz to 1 kHz, sometimes even higher (1 MHz) or lower (0.1 mHz). With the DFR method conduction and polarization processes, which are influenced by moisture and aging, of the insulating material are studied by measuring magnitude and phase of the current due to a sinusoidal excitation. The current could be written as follows:

[ ][ ]

)()(

)()()(

)()()(

)()()(1)(

**

*

*0

*

00

*

ωωωωωωω

ωωεωεω

ωωχωε

σωχωω

UCj

UCjCj

UjCj

UjCjI

=

′′−′=

′′−′=

��

���

�

��

�� ′′+−′+=

(1)

where ω is the angular frequency, σ the DC conductivity, χ’ and χ’’ the real and imaginary components of the complex susceptibility and C0 the geometrical capacitance. From the equation we see that the loss part consists of both polarization loss and conduction loss and that the conductivity contributes more at low frequencies than at high. From a measurement one can not distinguish the contribution from polarization loss and conductivity and more common parameters for the analysis are the complex permittivity or complex capacitance as defined above. Other common parameter for analysis of DFR spectra is the dissipation factor defined as follows:

)()(

tanωεωεδ

′′′

= (2)

The DFR spectrum is temperature dependent and will in general shift towards higher frequencies at higher temperature. The shape of the spectra is however, for most materials preserved when plotted in a log-log diagram. Based on this features a master curve could be created by shifting measurements performed at different temperatures along the frequency axis to one single curve at a chosen reference temperature [5]. A common way to describe the temperature dependence is by the Arrhenius equation, KTEae / . If the temperature dependence in the measured frequency range can be described by a single Arrhenius process, with temperature shifts proportional to

KTEae / , the activation energy Ea can be deduced. The activation energy is the main parameter describing the temperature dependence of the measured material.

Modelling of DFR measurements To model and analyze the DFR measurements of a power transformer it is essential to have the proper geometrical design of the transformer and the knowledge of the dielectric insulation system components such as the properties of oil and cellulose [2, 5- 8]. Figure 1 shows a typical winding configuration sketch. The winding configuration sketch includes the position of the windings, core and gap distances and paper/pressboard thicknesses.

Figure 1. Winding configuration of a transformer.

For a typical cylindrical core form construction, the combination of oil and cellulose in the duct is shown in (Fig. 2). The design is lumped together to an “insulation module” (Fig. 3) and we characterize the insulation structure by the relative proportion of barriers in the main duct (X) and the relative spacer coverage (Y).

Figure 2. Segment of the insulation in the main duct of a Core Form Transformer.

Figure 3. Insulation module used in the modeling.

The relative proportion of barrier in the main duct (X) and the relative spacer coverage (Y) is calculated as follows;

( )

( )formshellforwashersinerageblockspacertotalY

formcoreforducttheofwidthbarriersofthicknesstotal

Y

ducttheofwidthbarriersofthicknesstotal

X

cov=

=

=

(3)

In power transformers, X and Y can vary between 10-60% and 10-40%. Substituting the relative proportions of barriers and spacers, X and Y, in equation (4) the permittivity of the measured duct can be calculated.

( )

barrieroilbarrierspacer

duct XXY

XXY

T

εεεε

ωε+−−+

+−=

11

1, (4)

In frequency domain each material property is characterized by a complex permittivity that, in general, depends both on frequency, �, and temperature, T and moisture.

( ) ( ) ( )TiTT rrr ,,, "' ωεωεωε −= (5) The imaginary part, �“ also includes the contribution from (possible) DC conductivity. In the model the real part of the oil is constant and the imaginary part is influenced by the DC conductivity. The complex permittivity of oil is calculated by the following expression:

ωε

σεε0

iroil −= (6)

Where σ is the DC conductivity, εr is the relative permittivity of oil usually set to 2.2, and ε0 = 8,854·10-12 As/Vm. The oil-impregnated cellulose (paper and board) has a more complex dependence on frequency, temperature and moisture. The modeling is based on laboratory measurements on pressboard samples prepared to different moisture levels. From frequency shifts of the results of the measurements at different temperatures we could deduce activation energy around 0.9 eV. Furthermore for a proper modelling the geometrical capacitance between the windings should be determined. For a core form transformers the geometrical capacitance can be estimated by the cylindrical capacitance;

��

���

=

LV

HVR

Rh

Cln

200

πε (7)

Where h is the height of the windings and RHV and RLV are the radius of the high voltage and low voltage windings respectively.

Typical Response of a Normal Transformer. Figure 4 shows a typical DFR measurement along with a DFR analysis curve. Some of the inputs to the DFR analysis curve algorithm are known values, such as the geometrical properties, the temperature. Other properties, such as percent moisture in the cellulose, oil conductivity, and contaminant properties are determined by a curve fitting technique. Fortunately, since the moisture, oil conductivity and contaminant properties all affect the shape of the curve in different ways, the curve fitting technique can be used to determine all three of these unknowns.

Figure 4. Typical DFR curve for a normal transformer

Influence of Moisture and Oil Conductivity on the DFR. Figure 5 shows the influence of moisture and oil conductivity on the dielectric response curves from a new transformer (green curves) with clean oil, a normal service aged transformer (blue) with normal oil conductivity and a wet transformer with high oil conductivity. It is shown that the higher the moisture content and oil conductivity is in the insulation, the dielectric losses response is shifted in the vertical upward direction and the capacitance increases at lower frequencies [8]. The moisture predominately affects the magnitude of the capacitance and

dielectric losses response at the higher frequency region between 0.1 and 1000 Hz and at the low frequency region between 0.01 and 0.0001 Hz.

0.5%, 1E-13 S/m1.5%, 1E-12 S/m

3.0%, 1E-11 S/m

0.5%, 1E-13 S/m1.5%, 1E-12 S/m

3.0%, 1E-11 S/m

0.5%, 1E-13 S/m0.5%, 1E-13 S/m1.5%, 1E-12 S/m1.5%, 1E-12 S/m

3.0%, 1E-11 S/m3.0%, 1E-11 S/m

Figure 5. Effect of different moisture and oil conductivity levels on the DFR response.

Influence of Temperature on the DFR. Figure 6 shows the influence of temperature on the dielectric response curves of a power transformer. It is shown that the dielectric losses are highly influenced by the temperature.

10 ºC30 ºC50 ºC70 ºC

10 ºC30 ºC50 ºC70 ºC

Figure 6. Effect of temperature on the DFR response.

Influence of Geometry on the DFR. The %X and %Y also affects the DFR response curves. It is shown in Figure 7 how the different combinations of %X and %Y influences the dielectric losses curves. Note that the curve for %X = 50% and Y = 10% is not similar as %X = 10% and %Y = 50%.

10-3

10-2

10-1

100

101

102

103

10-12

10-11

10-10

10-9

10-8

10-7

Frequency [Hz]

Cap

acita

nce

& L

oss +

x

+

x

+

x

+

x

RE 10/10IM 10/10RE 10/50IM 10/50RE 50/10IM 50/10RE 50/50IM 50/50

Figure 7. Effect of different geometrical parameters on the DFR response. The notation in

the legend is such that "RE 10/10" means (real) capacitive part, X=10% and Y=10% etc

To get a good estimation of the moisture content, or any other abnormalities of the insulation, it is important to have accurate temperature measurements of the oil during the DFR measurements. It is also important to have the design information of the transformer, to calculate accurate values of the geometrical parameters %X and %Y. An error in the temperature measurement can affect the analysis of the moisture in the insulation. For example, if the top oil temperature is used instead of the average temperature, then the moisture estimate and oil conductivity estimate will both be underestimated to compensate for the incorrect temperature as shown in Figure 8

Figure 8. Effect of error in the temperature value used on the analysis of moisture. Errors in the geometrical properties of the insulation system can also affect the accuracy of the results. For example, if the relative amount of cellulose material in the duct is overestimated, this can lead to an underestimation in the amount of moisture as shown in Figure 9. Finally, the presence of contamination in the insulation structure affects the shape of the loss curve. Failure to recognize the presence of contamination, and attempting to fit a contaminated insulation measurement set with a normal, uncontaminated response curve can lead to gross errors or implausible results.

Figure 9. Effect of error in geometrical properties on the moisture and oil conductivity.

DFR- Measurements The DFR test is performed by applying a varying frequency sinusoidal low voltage to the insulation system under test and measuring the applied voltage, current and phase angle to determine the specimen capacitance and loss factor over the frequency range of interest. The applied voltage for the DFR test is usually much lower than the power factor test due to requirements on the power supply size to supply the test current at the highest frequencies. DFR on power transformer is normally performed at frequency range between 1 mHz and 1 kHz, however measurements may be done at lower or higher frequencies. The connections for the test

are the same as those used for the standard power factor and capacitance measurement. As such, the elements in the transformer that are tested generally include the insulation between isolated winding sections and between the windings and ground. The measurements between windings to ground generally include the insulation between the windings and the core or other grounded parts of the transformer, the bushings, and the insulation of tap changers, reactors or other accessories connected to line potential. What is not included is the insulation within the individual winding sections such as the paper around the winding conductors except the part facing the main duct or the insulation between the layers or disks of the windings. DFR is an off line diagnostic test. Prior to starting the measurements the transformer should preferably be completely disconnected from the station power connections, i.e. all connections to the bushings should be dismantled, and the station connections should be properly grounded.

Test circuits for UST, GST and GSTg measurements In performing power frequency power factor tests, several configurations of the test equipment are used in order to assess different segments of the insulation system. These are generally defined as the UST (Ungrounded Specimen Test), GSTg (Grounded Specimen Test with guard) and an optional configuration GST (Grounded Specimen Test). Since the GST test is redundant, it is usually omitted for expediency.

Test Connection for Two winding transformer Figure 10 shows a schematic equivalent circuit of the insulation of a two winding transformer.

Hi

Lo

Ground

Hi

Lo

Ground

Figure 10. Two winding transformer example of connection for CHL and CH measurements.

For two winding transformers there are three recommended measurement set-ups given in Table 1.

Table 1: Test configurations for two winding transformers

CLGST-GuardTankHighLowGST3

CHGST-GuardTankLowHighGST2

CHLUSTTankLowHighUST1

MeasureConfigurationGround

(Black)

Lo

(Blue)

Hi

(Red)

ModeNo

CLGST-GuardTankHighLowGST3

CHGST-GuardTankLowHighGST2

CHLUSTTankLowHighUST1

MeasureConfigurationGround

(Black)

Lo

(Blue)

Hi

(Red)

ModeNo

All bushings of each winding should be connected together. Contamination on bushing surfaces or moisture can affect the test results, so the bushings should be clean and dry before performing the test.

Test Connection for- Three winding transformer Figure 11 shows a schematic equivalent circuit of the insulation of a three winding transformer.

Figure 11. Three winding transformer.

Table 2 gives a list of the most common measurement set-ups for a three winding transformer.

Table 2. List of Measurement set-ups for 3 winding transformers

DFR- Case Study

Influence of bad shield contact A one phase, two winding generator step up transformer at a hydro electric plant in northern part of US was producing combustible gasses such as hydrogen & acetylene, Figure 13. Tests such as capacitance and tanδ measurements of windings and bushings were taken, as well as, winding and core insulation resistance, winding resistance, excitation current, turn ratio- all showed normal results with no indication of what could be the cause for the increased gas production. An internal inspection was performed and no abnormalities were found. Based on high gas production the owner was preparing to ship the unit for repair. However, as a last effort before shipping, ABB was contracted to perform, - DFR to see if any abnormal response could be detected and possible cause identified.

Combustible Gases

0

10

20

30

40

50

60

70

80

90

07-aug-06 09-aug-06 11-aug-06 13-aug-06 15-aug-06 17-aug-06 19-aug-06 21-aug-06 23-aug-06

Date

(ppm

)

0

50

100

150

200

250

Tota

l Co

mb

. Gas

(ppm

)

Hydrogen (H2)

Methane (CH4)

Acetylene (C2H2)

Ethane (C2H6)

Ethylene (C2H4)

Total Combustible Gas

Figure 13. DGA results

The transformer had the winding configuration shown in Figure14. For the DFR measurements three configurations were measured CHL, CH and CL. CHL measures the main duct insulation between the HV and XV windings. CL measures the XV bushings and the insulation between XV winding and core. CH measures the HV bushings, and insulation on the end of HV winding to the core and the winding/leads to tank/ground.

XV

XV

HV

HV

Cor

e

XV

XV

HV

HV

HV

HV

Cor

e

Figure 14. Winding configuration.

Results from the test were analyzed with help of a software, which also allows analyzing non-moisture related measurement data and follows a patented process for identifying abnormalities in power transformers [9]. The measured (red marks) and the modeled (blue line) data for CH configuration are shown in Figure 15. The measurements of CL and CHL were normal, but the test on CH showed the abnormal response seen in Figure 15. The analysis of this response indicated a likely defect in the HV shield for the H1 bushing.

10-3

10-2

10-1

100

101

102

103

10-12

10-11

10-10

10-9

10-8

Frequency [Hz]

Cap

acita

nce

& L

oss

<--re-1 <--re-1

<--im-1 <--im-1

+

x

<-re-EHVguardLV <-re-EHVguardLV

<-im-EHVguardLV <-im-EHVguardLV

o

x

C' modelC'' modelC' measC'' meas

Figure 15. DFR results for CH case.

The results of the completed tests indicated an abnormality in the HV to ground path. Based on this information, internal inspection of the transformer was done focusing on the bushing shield.

Figure 16. Inspection of shielding tube.

The inspection of the shielding tube showed that the sleeve (also called union coupling) that connects the vertical tube with the horizontal Y tubes of the HV winding was loose and did not provide a proper contact, Figure 16. There had been arcing at the sleeve and also between the cable inside the shielding tube and the tube. The transformer could be repaired on site and did not have to be returned to the factory, lowering repair cost considerably.

After the repair DFR measurements were repeated, Figure 17. The results of the CH measurement were normal indicating that the problem had been corrected and the transformer could be returned to service.

10-3

10-2

10-1

100

101

102

103

10-12

10-11

10-10

10-9

10-8

Frequency [Hz]

Cap

acita

nce

& L

oss

<--re-1 <--re-1

<--im-1 <--im-1

+

x

<-re-eh1guardlv <-re-eh1guardlv

<-im-eh1guardlv <-im-eh1guardlv

o

x

C' modelC'' modelC' measC'' meas

Figure 17. DFR results for CH case after repair. Normal insulation model fitted to

measurement.

REFERENCES 1. Zaengl, W.S. "Dielectric spectroscopy in time and frequency domain for HV power

equipment. I. Theoretical considerations" Electrical Insulation Magazine, IEEE Volume 19, Issue 5, Sep-Oct 2003 Page(s):5 - 19

2. G. Frimpong, M. Perkins, A. Fazlagic, U. Gafvert “Estimation of Moisture in Cellulose and Oil Quality of Transformer Insulation Using Dielectric Response Measurements,” 2001 Conference of Doble Clients

3. U. Gafvert, G. Frimpong, J. Fuhr, “Modeling of Dielectric Measurements on Power Transformers”, Paper 15-103 CIGRE Session, Paris 1998

4. M. Perkins, A. Fazlagic, G. Frimpong, “Dielectric Frequency Response Measurement as a Tool for Troubleshooting Insulation Power Factor Problems,” Proceedings of the 2002 IEEE International Symposium on Electrical Insulation, Boston, Massachusetts

5. A. K. Jonscher: ”Dielectric Relaxation in Solids”, Chelsea, Dielectric Press, 1983. 6. U. Gafvert, “Condition Assessment of Insulation System-Analysis of Dielectric Response

Methods”, Proc. 1996 Nordic insulation Symposium (Nord-IS 96), Bergen Norway, June, p.1, 1996.

7. S.M. Gubanski, P. Boss, G. Csepes, V.D. Houhanessian, J. Filippini, P. Guuinic, U. Gäfvert, V. Karius, J. Lapworth, G. Urbani, P. Werelius and W.S. Zaengl - "Dielectric Response Methods for Diagnostics of Power Transformers”, Electra, No. 202, 2002, pp 23-34; also in CIGRE Technical Brochure, No. 254, Paris 2004.

8. G. Frimpong, U. Gafvert and J. Fuhr, “Measurements and Modeling of Dielectric Response of Composite Oil/Paper Insulation”. Proc. 5th Int. Conference on Properties and Applications of dielectric Materials, Seoul, Korea, May 25-30, 1997, p. 86.

9. United States Patent no. US 6,870,374 B2 - “Process for Identifying Abnormalities in Power Transformers”, inventors: Mark D. Perkins; Asim Fazlagic, March 22, 2005.

10. Blennow, J.; Ekanayake, C.; Walczak, K.; Garcia, B.; Gubanski, S.M. “Field experiences with measurements of dielectric response in frequency domain for power transformer diagnostics”, , IEEE Transactions on Power Delivery, Volume 21, Issue 2, April 2006 Page(s):681 - 688