power-transformer-testting-pdf

Transformer Parts and Their Possible Faults Transformer diagnostic set: see pages 6-21 Dielectric response analysis instrument: see pages 22-23 Frequency response analysis instrument: se

Diagnostic Testing Solutions for Power Transformers

testing during manufacturing

factory acceptance testing

commissioning acceptance testing

Prevention is Better than Cure - Know More About the Condition of Your Transformer

Taking the right action at the right time

> periodic testing

> testing after an event - relocation, protection trip or warning, overcurrent, overvoltage, earthquake

Keep your transformer in good condition

with testing

factors causing deterioration

mechanical impacts

>transportation event

>post fault event

>seismic activity event

Prevention is Better than Cure - Know More About the Condition of Your Transformer

replacementoperation

> periodic testing

> testing after an event - relocation, protection trip or warning, overcurrent, overvoltage, earthquake

Keep your transformer in good condition

>pumps, fans, etc

and subsequent preventive actions

transformer life expectancy

Transformer Parts and Their Possible Faults

Bushings

Partial breakdown between capacitive graded layers,

Bushing CTs Current ratio or phase error considering burden, excessive residual magnetism, non-compliance to relevant IEEE or IEC standard xInsulation materials

Aging, moisture, contamination of insulation fluids x x x

preventative autotransformer, series autotransformer or series transformer x x x

Windings

Core

Transformer Parts and Their Possible Faults

Transformer diagnostic set:

see pages 6-21

Dielectric response analysis instrument:

see pages 22-23

Frequency response analysis instrument:

see pages 24-25

Partial discharge analysis system:

Bushings

Partial breakdown between capacitive graded layers,

Bushing CTs Current ratio or phase error considering burden, excessive residual magnetism, non-compliance to relevant IEEE or IEC standard xInsulation materials

Aging, moisture, contamination of insulation fluids x x x

preventative autotransformer, series autotransformer or series transformer x x x

Windings

Core

Notes:

1) Power factor / dissipation factor measurements at 50 Hz or 60 Hz can detect high moisture contents, but have a blind spot for low moisture contents Measuring power factor / dissipation factor at

All in One: the Multi-Functional Transformer Test Set CPC 100 / CP TD1

Leakage reactance/short-circuit impedance measurement instrument

DC winding resistance measurement instrument

I

Frequency response of stray losses measurement instrument

winding strands

TRV

I

+ more substation diagnostics

> Ground impedance measurement

> Line impedance and ground factor

in wave form and frequency

Weight of units

CPC 100: 29 kg / 65 lbs

CP TD1: 26 kg / 56 lbs

All in One: the Multi-Functional Transformer Test Set CPC 100 / CP TD1

IR ICV

I

Insulation

+ more substation diagnostics

> Current transformer testing

> Voltage transformer testing

Power factor / dissipation factor measurement instrument

Transformer ratio measurement instrument

Transformer exciting current test instrument

f

coreI

Trolley

to conveniently transport:

CPC 100,

CP TD1, measurement cable, high voltage cable

Surge arresters Bushings OLTC Leads Insulation materials Windings Core

Capacitance and power factor / dissipation factor (PF / DF) measurements are performed

to investigate the condition of bushings as well

as the transformer overall insulation Aging and decomposition of the insulation, or the ingress

of water, increase the energy that is turned into heat in the insulation The level of this dissipation

is measured by the PF / DF

On surge arresters, currents and watt losses of identical units can be compared Deviations may indicate aging effects, poor contacts or open circuits between elements

Capacitance values of bushings show if there

have been breakdowns between capacitive

layers For resin bonded paper bushings, cracks

into which oil has leaked, can also change the

value of the capacitance

A rise in capacitance of more than 10 % is

normally considered to be dangerous, since it

indicates that a part of the insulation distance is

already compromised and the dielectric stress to

the remaining insulation is too high

Preventing bushings from exploding

Increased heat dissipation accelerates the

aging of the insulation If an aged insulation

can no longer withstand the electrical stress,

bushings explode

Better understanding of losses

At line frequency, conductive losses can be

represented with a parallel equivalent circuit

Polarization losses can be represented by a series

equivalent circuit consisting of an ideal capacitor

and a resistor

Increased losses may pass a test at line

frequency unnoticed, leaving the tester unaware

of an insulation in distress Measuring the DF /

PF over a broad frequency range helps to better

understand both types of losses

Measuring Capacitance & Power Factor / Dissipation Factor

Typical loss shapes in 15 - 400 Hz range

conductive lossespolarization losses

superposition of both effects

equivalent circuits *)

typical loss shapes

Damaged TR after bushing explosion

Capacitive layers in bushings

Measuring Capacitance & Power Factor / Dissipation Factor

High voltage is applied to the insulation to

be tested, i.e the bushing tip, and a low loss reference capacitor (integrated in CP TD1) is connected in parallel The currents flowing through the insulation and through the reference capacitor are measured and the time difference between their zero crossings is determined The loss angle d

is then calculated from this time difference The tangent of this angle is the dissipation factor The cosine of the angle between voltage and current is the power factor Results are compared with values given in IEEE C57.10.01 and IEC 60137, and can

be compared with a base measurement, another phase, or a sister transformer

If values deviate more than indicated by the standards, then dielectric response analysis can be performed to check for increased moisture

Chemical tests can be performed to verify the quality of the insulation fluid (DGA, dielectric breakdown strength, interfacial tension, etc.) Measuring the power factor / dissipation factor of the insulation fluid can also be done with a CPC 100 accessory, the

CP TC12 oil test cell

Capacitive layers in bushings

CA

center conductor

grounded layer and tap electrode

on flange

OIP bushing: PF / DF tip up test

*) at 50 / 60 Hz and 20 °C

Dissipation factor / power factor in % *)

OIP bushing: PF / DF variable frequency test

Measuring Capacitance & Power Factor / Dissipation Factor

Power factor / dissipation factor (PF / DF)

measurement indicates the condition of the

liquid and solid insulation within a transformer

Power and accuracy

The CPC 100 / CP TD1 can measure capacitance

and PF / DF (tan d) in laboratories, test fields

and on site

A powerful test voltage source (12 kV, 100 mA

continuous, 300 mA short-term load current)

with variable frequency (15 - 400 Hz), combined

with high accuracy measuring inputs allows fast,

effective and accurate measurements

Prepared test procedures can guide the user

through the testing process and offer a basis for

comprehensive reporting

Modular equipment

The modular equipment (CPC 100: 29 kg /

65 lbs, CP TD1: 26 kg / 56 lbs) can be easily

transported thanks to its sturdy cases, which

can also be used to place the instruments onto

them for working at a comfortable height, as

shown on page 12

For convenient transport or mobile use such as

in test fields or in substations/power plants, the

instruments can be mounted onto a trolley

The CPC 100 is used to control the test, i.e.:

> entering the voltage and frequency values

where C and cos j / tan d shall be measured

> starting and stopping the test

> supervising the measurement progress and

intermediate results

> storing results on flash disk and

USB memory stick

The CP TD1 includes

> a high voltage step-up transformer

> a reference capacitor (pressurized gas type)

> the unit to measure and compare currents in

amplitude and phase

Power factor / dissipation factor tip up results

Energize HV to measure CH + CHL | CH | CHL, then energize LV to measure CL + CHL | CL | CHL - thanks to internal switching logic with guard

LV

IN A

Measuring Capacitance & Power Factor / Dissipation Factor

Your Benefits

> perfect digitally generated sine wave test signal that is independent from power quality and line frequency

> laboratory precision for on-site use:

< 0.05 % error for capacitance Cp

> automatic tests at different voltages

> automatic tests at different frequencies: early detection of insulation stress due

to the improved sensitivity provided by measurements made in the range of

> temperature correction according

to type of insulation and relevant standard

> internal recalibration of electronic circuits of the CP TD1 with each measurement

> automatic reporting of capacitance

Cp, DF (tan d), PF (cos j), power (active, reactive, apparent), impedance (absolute value, phase, inductivity, resistance, Q)

> automatic assessment if reference values for capacitance and power factor / dissipation factor are known

> less wiring effort through two measurement inputs (IN A, IN B) that can be used to measure for example the capacitance of a bushing at the same time as the main insulation

Energize HV to measure CH + CHL | CH | CHL, then energize LV to measure CL + CHL | CL | CHL - thanks to internal switching logic with guard

Measuring Ratio & Exciting (No-Load) Current

Ratio measurement with the CPC 100

The CPC 100 measures the transformer ratio by

applying a high voltage at the HV winding of

one transformer leg In amplitude and phase, it

measures the applied voltage and the voltage at

the LV winding, as well as the exciting (no-load)

current The deviation from rated values is

displayed as a percentage

Measuring ratio per tap

The CPC 100 measures ratio and excitation

current at each tap position Each time the

user operates the tap changer, the CPC 100

automatically starts a new measurement and

measures and displays ratio, phase angle, and

for each tap, the deviation from nominal ratio is

displayed as a percentage

For automatically measuring winding resistance

and ratio of all phases and all taps, see page 16

The measurement is performed for assessing possible winding damage, such as turn-to-turn short circuits, comparing the measured ratio and magnetizing currents to specifications, factory measurement results, and/or across phases

In the factory, this measurement is performed to verify that ratio and the vector group is correct

Setup for automatically measuring ratio and resistance per tap (see page 16)

Measuring Ratio & Exciting (No-Load) Current

Setup for automatically measuring ratio and resistance per tap (see page 16)

of the mains wave form

> convenient and quick testing by automatic detection of tap changer operation as trigger for the next tap measurement

> exciting current measurement

in amplitude and phase

> variable frequency for measurements outside mains frequency for noise suppression,

if selected by the user

> accuracy and safety

> automatic reporting of measured voltage values and phase angles, measured ratio and deviation as

a percentage, exciting current in amplitude and phase

> tabular and graphical result representation for every tap

The winding ratio between primary and secondary windings is measured for each transformer leg, applying high voltage at the

HV side and measuring on the LV side The ratio

of these voltages, equalling the turns ratio, is calculated Results are compared with name-plate values and across phases

The exciting current is the corresponding current flowing in the HV winding if the LV winding is open Results are compared with a reference measurement, or a measurement performed on

a sister transformer; in three phase transformers, the two outer phases can also be compared

With the turns-ratio test, shorted turns can

be detected If a problem is suspected from

a DGA, a dissipation factor test, or a relay trip, a turns-ratio test can be performed to rule out / verify if turns are shorted

If the exciting current test shows deviations, and DC winding resistance and ratio test do not show errors, then the cause may be a core failure or unsymmetrical residual flux

CPC 100 TRRatio test card

Exciting current [mA] per tap

Winding resistance per tap

Measuring DC Winding Resistance and OLTC

Measuring resistance with the CPC 100

The CPC 100 injects DC current into the

winding, measures current and voltage and then

calculates and displays the resistance When the

resistance value is stable, the CPC 100 makes the

final measurement, and reduces the test current

to zero to discharge the energy saved in the

winding When it is safe to remove test leads,

the CPC 100 illuminates its green safety light

Tapped windings and OLTC

In semi-automatic mode, the CPC 100 measures

the resistance of each subsequent tap position

Each time the user operates the OLTC, the

CPC 100 waits until the values stabilize, and

then measures and displays the winding

resistance at this tap position When all taps

have been measured, the CPC 100 discharges

the inductive energy stored in the winding and

indicates when this process is completed For

automatically measuring static and dynamic

winding resistance and ratio of all phases and

all taps, see page 16

Dynamic resistance measurement

The OLTC has to switch from one tap position

to another without interrupting the load

current When switching the tap changer during

winding resistance measurement, the DC current

temporarily decreases This current decrease

should be measured and compared across

Winding resistance measurements are performed for assessing possible winding damage It is also used to check the On-Load Tap Changer (OLTC) - to know when to clean

or replace OLTC contacts, or to know when

to replace or refurbish the OLTC itself, which has a shorter life span than the active part of the transformer

In the factory, this measurement is performed to calculate the I2R component of conductor losses and to calculate winding temperature at the end

of a temperature test

Table in CPC 100 TRTapCheck test card

Switching process

Measuring DC Winding Resistance and OLTC

Slope per tap

Ripple per tap

Your Benefits

> convenient and quick testing by using OLTC operation as a trigger for the next tap measurement

> additional condition assessment of the individual OLTC taps through dynamic resistance measurement, recorded

as a part of “classical” resistance measurement, without extra effort

> high accuracy and safe testing through the use of a 4-wire connection The CPC 100 visually indicates when it

is safe to remove test leads, even

if its power supply is interrupted during testing If the test leads are removed or interrupted accidentally, the test current will flow through the voltage path, preventing dangerous overvoltages If the CP SA1 accessory

is in use during such an accidental interruption of test leads, damage to the CPC 100 will be prevented

> automatically created report showing the test duration, the resistance value at measurement and reference temperature, etc

> tabular and graphical results are produced for every tap for easy visual comparison

To measure the winding resistance, the winding under test must first be loaded with energy (E=1/2*L*I2) until the inductance of the winding is saturated Then the resistance can be determined by measuring DC current and DC voltage For tapped windings, this should be done for every tap position, hence testing the OLTC and the winding together

Results should be compared to a reference measurement, across phases, or with a sister transformer In order to compare measurements, the resistance values have to be re-calculated,

to reflect different temperatures during the measurements

Results should not differ more than 1 % compared to the reference measurement Differences between phases are usually less than 2 - 3 %

Transformer turns ratio or frequency response analysis can be used to confirm contact problems In both cases, hot spots

in the transformer will result in a DGA indicating increased heat However, gas signatures are not unique and thus do not allow for the identification of the root cause

Burn-off at a diverter switch

Switching process

Automatically Measuring Ratio & Winding Resistance of All Taps and All Phases

Using the CP SB1 accessory, the CPC 100 can

automatically

> measure ratio and the exciting current of all

of the taps and all phases

> confirm the vector group

> measure static and dynamic winding

resistance of all of the taps and all phases

This accessory helps to save a lot of time as

wiring is only necessary once With the same

cabling, both ratio and resistance measurements

can be performed

Through the CP SB1, the CPC 100 is connected

to all phases of a transformer The up and down

command inputs of the OLTC are also connected

and controlled by the CPC 100 and the CP SB1

Ratio measurement

The CPC 100 only requires the user to enter

ratio and the vector group to measure the

ratio and the exciting current for each tap of

each phase automatically For each tap, results

are compared to the specified ratio and the

deviations are displayed

Winding resistance measurement

With the CP SB1, the CPC 100 injects DC current

into each tap of each winding The CPC 100

then waits for the current to stabilize and

measures the resistance value, as well as the

data describing the switching process (dynamic

resistance measurement)

The tap changer is then operated automatically

until the measurement on one transformer

phase is finished Between measuring the

different phases, the energy stored in the

windings is quickly discharged When the

windings are fully discharged, the CPC 100 /

CP SB1 automatically switches to the next phase

At the end of the measurement, the last winding

is discharged and the operator is notified visually

that it is safe to remove the wiring

LV