Investigation of Active Transformer Winding

Geometry Using Frequency Response Analysis

By Pasindu Udagepola, N8587442Supervisor: Dr. Shawn Nielsen

BEB 801/802 Project

Topic Definition

• Current progression of higher demands for electricity, it is crucial to minimize disturbances to power utilities by implementing innovative and cost effective methods.

• In recent years, Engineers have utilised condition monitoring of electrical components in power systems to prolong their health, service time and cost cutting measures.

Topic Definition

• Power transformers are one of the most expensive and vital part of a distribution network.

• Growing demand for electricity requires power transformers to operate at higher loading levels.

• Operating at higher demands and other aspects can cause deterioration of transformer integrity due to mechanical, thermal and electrical stresses.

• A solution to maximise service time is to monitor the winding geometry and diagnose disturbances with in the transformer structure.

Topic Definition

• One of the conventional methods of monitoring transformer conditions is called the Frequency Response Analysis(FRA).

• Currently FRA is widely performed in off-service transformer monitoring.

• Offline transformer monitoring results in power outages, risk introduced to systems and higher costs.

• Motivation of the project is to investigate methods of online condition monitoring of power transformer winding using FRA.

Prior Work

Relevant Online Transformer Test Studies

• M.Bagheri(2012), Bushing Characteristics Impacts on On-line Frequency Response Analysis of Transformer Winding, IEEE Power and Energy(PE Con)

Current Online Transformer Test Studies

• Comprehensive discussion on FRA and online test setup.• Uses Capacitive Bushing as a signal injection.• Discuss about the result, challenges and uncertainties from

online test compare to the offline test.

Project Definition

What is to be done?• Currently, FRA is conduct offline to determine geometrical

changes in transformer windings.• The project is investigating methods of FRA in online transformer.• As FRA moves to online environment, the project entails of

finding a method of injecting and measuring signals on to a transformer with voltage.

• Past studies have shown that using capacitors is a viable method of online FRA by injecting low voltage signals.

Why Capacitors?

• Capacitive reactance at higher frequencies becomes low.• At higher frequencies of winding influences, transformer

results will have less effect from the capacitors.

Suitable Online Injection and Measurement Method

• Most of the power transformers have capacitive bushings to facilitate Partial Discharge testing.

• Due to low interferences from capacitors in higher frequencies, capacitive bushings can be utilized in online FRA testing.

• Online FRA would be carried out by injecting and measuring low voltage signals through the bushing test tap.

Project Report

Phase One: Investigation

What is FRA?

• A tool used in monitoring of geometrical changes in transformer by concentrating on the frequency dependent electrical response describing variation in capacitance and inductance distribution.

Winding Inductance

Series Capacitance

Shunt Capacitance

Winding Resistance

What is FRA?

• FRA is a graph that acts as a fingerprint to a transformer.

• Once transformer’s windings have been disturbed, the new response will have variance compare to the original graph enabling detection of variance.

• Impulse Frequency Response Analysis(IFRA)• Sweep Frequency Response Analysis(SFRA)

Sweep Frequency Response Analysis(SFRA)

• This project will utilise SFRA.

• Injects variable sine waves of known frequency at the input terminal and measure the response at the output terminal by sweeping through range of frequencies.

• The results are attenuation in a vector form (Magnitude and Phase)

Omicron FRANEO 800 Analyser

• Throughout the project Omicron’s analyser will be used to conduct SFRA testing.

• In simple terms, omicron analyser is a sine wave generator combined with an analyser to measure input/output voltage ratio of the transformer.

Omicron FRANEO 800 Analyser

To measure winding response:

1. Source and reference cables are connected to one end of winding.

2. Response cable is connected to the other end of winding.

3. A low-voltage sine waves are applied to one terminal of a winding and response is measured at the other end.

Note: When analysing results 50Ω coaxial cables must be incorporated to get true impedance of the response.

Method Of Interpretation

Frequency Range

Influence on Transformer

Low – 2KHz Core

2KHz – 20KHz Interaction between winding

20KHz – 1MHz Winding structure

>1MHz Measurement set-up and lead

• The International Council on Large Electric Systems CIGRE: working group A2.26 has published a comprehensive documentation on FRA

Failure Modes of Transformer

Failure Modes of Transformer

Phase Two: Design

Key Design Adaptation and Testing

Offline Test Setup:• Uses Capacitive Bushing for injection and

measurement.• Uses the Commercial FRA analyser(Omicron

FRANEO 800) as a data acquiring device.• Prior to online testing, needed a

comprehensive understanding on the effects of capacitive graded bushings.

Design and Testing

• Each component of the offline setup will have different impedance characteristics.

• The setup is in a series format.• Therefore impedance subtraction of the

bushings from the total setup results should result in transformer response.

• Prior to testing, a comprehensive understanding on capacitive bushing characteristics is required.

Capacitive Graded Bushing

• Made up of 3 capacitances.• Different Response for Injecting

and Measurement scenario.• FRA is carried out at different

points to measure each capacitance:

- C1 from point A to B- C2 from point A to C- C3 from point B to C

Capacitance values

• C1 from point A to B• C2 from point A to C• C3 from point B to C

C1,C2,C3?

• C1 = 17.908pF • C2 = 12.501pF • C3 = 12.4916pF

Injection Simulation

• Injection Scenario

FRA Injection

FRA Measurement

Measurement Simulation

• Measurement Scenario – High Pass Filter FRA Injection

FRA Measurement

Summary Of Capacitive Graded Bushing

• Able to Inject and Measure FRA of the transformer safely through capacitive bushing.

• Injection bushing plots pure attenuation due to no load.• Measurement bushing results in a high pass filter due to C1

and the coaxial cables 50Ω resistance. • Next Step is to conduct test using the offline test setup.

Offline Test Setup

Transformer Response

Prior to conduct tests using the offline setup:• Transformer SFRA response was taken to compare with the

offline test resultsFrequency Transformer

Influences10Hz-100kHz Core Influences100kHz- 10MHz Winding

Influences10MHz- 100MHz Connections

Offline Setup Response

• For testing purposes, all the tests were done on the secondary winding by short circuiting the primary.

Offline Setup Response

• Comparison of the original transformer is shown in the plot.

Offline Setup Response • Can we get the original response through impedance

subtraction?

Interferences of the Bushings

• Injection Bushing:

Interferences of the Bushings

• Injection Bushing Equivalent Impedance:

m -0.995c 9.7272Capacitance(F) 28.4124pF

Interferences of the Bushings

• Measurement Bushing Test:

Interferences of the Bushings

• Measurement Bushing Equivalent Impedance:

• The equivalent impedance will be part resistive and part capacitive.

m -1.01757c 9.84404C avaerage 28.5(PF)R 6.43(MΩ)

Initial Attempt: Series Impedance Subtraction

• Series equivalent bushing impedances must be subtracted to extract original transformer response.

Results of Series Impedance Subtraction

• Initial attempt of impedance subtraction resulted in uncharacteristic response as oppose to the outcome initially predicted.

• High-pass filter of the measurement bushing blocks injection interferences at lower frequencies, by subtracting injection bushing results adds unwanted interference at lower frequencies.

• Subtraction or addition of large numbers can cause very large errors unless the results are very accurate.

Evaluation of Series Impedance Subtraction

• Theory behind the unsuccessful result is possibly due to attempts of subtracting large approximations causing large errors.

• Reviewing the equivalent impedance of the injection bushing, it was clear currently the complex impedances are in series causing a real and reactive part.

• To achieve true capacitance series to parallel impedance conversion is required:

True capacitance of the injection bushing.

Equivalent Series Resistance:• Leakage resistance of

the epoxy and the surface.

Evaluation of Series Impedance Subtraction

• The total parallel complex impedance did not change:

• Once converted to parallel complex impedance, the real part describing leakage properties of the injection bushing is neglected.

• Imaginary part of the injection bushing is used to as equivalent impedance describing true capacitance.

• Using the new equivalent injection impedance, second attempt of subtraction was conducted.

Second Attempt of Impedance Subtraction

• Even with true capacitance of the injection bushing, the subtraction response have made no progress due to large values.

• It is clear that the bushing combined error is very large, that the replication of original transformer response is impossible with the current setup.

Evaluation of the Impedance Subtraction

• Evaluating the offline setup results vs the transformer response:

• Comparing to original response, influences of the core cannot be replicated using capacitive graded bushings.

• However, the goal of the project was to investigate winding geometry of active transformer using FRA; core data for this project is not necessary since the focus is on windings.

• In conclusion, winding geometry of the transformer utilizing offline test setup is achievable. Granted that the attenuations have weaken compare to the original transformer response.

Fault Analysis Using the Offline Test Setup

• Offline test setup does show characteristics of the windings.• To verify the ability of fault detection using offline test setup, following

fault is introduced to the transformer.• An axial short circuit is introduced by shorting disk one and two.

Fault Detection of Isolated Transformer

• Prior to fault detection using offline setup, fault detection is conducted on just the transformer.

• Observing the results between in the frequency range of winding issues, the fault is detected easily due to variance of inductive (L) components of the shorted disks.

• Effects to (inductance)L and (capacitance)C causes changes in the resonance, which is clearly visible the above magnitude plot.

• The fault response tend to follow similar characteristics to the transformer in the higher frequencies, granted minor resonance variations. Therefore, introduced axial fault can be detected using the designed transformer.

Fault Analysis Using the Offline Test Setup

• Fault test is repeated to observe the ability to detect faults using offline test setup. • Since interference from bushing at higher frequencies are minimal, detection of

faults should be achieved. • Fault detection is possible through capacitive graded bushings since drastic

changes in frequency bands of winding influences are clearly visible.• At higher frequencies, the faulted

responses resonances have shifted illustrating detection ability of fault.

• In conclusion, with the current transformer design, fault detection through capacitive graded bushings is possible.

Reflection

• In summary, the goal of the project was to investigate a method of online condition monitoring of power transformer winding using FRA.

• Capacitive graded bushings have shown successful winding activity using an offline test setup.

• Impedance subtraction of bushing’s interferences is impossible due to large results of the equivalent bushings causing large errors.

• However, current test setup showed possibility of using capacitive graded bushing in an offline setup to detect winding faults.

What's Next?

• Next Step of the project is to conduct FRA fault detection tests using energised transformers.

• The faults of transformer available was easily detectable due to its 4 disk, 1:1 rating.

• However, in a real transformer were large amount of disks exist, thus only will have small changes to the response.

• This could lead to fault detection vulnerability of current test setup.

• Therefore there are lot of aspects still to consider prior to online FRA testing of power transformers.

THANKS!