Dielectric Properties in Transformers

Questions and Answers

What is primarily assessed when interpreting the results of dielectric tests in transformers and bushings?

The temperature fluctuations of transformers

The efficiency of electrical currents

The condition of the dielectric (correct)

The electrical resistance of materials

Which of the following best describes the property of permittivity in dielectric materials?

The ability of a material to conduct electricity

The ability of a material to store electrical energy

The ability of a material to dissipate heat

The ability of a material to insulate electric fields (correct)

In the context of transformers, what usually causes changes in the dielectric properties?

Insulating materials interacting with conductive materials

Direct physical contact between plates

The frequency of the applied electric fields (correct)

Temperature variations during operation (correct)

What are the main components used as insulation in oil-immersed transformers?

Mineral oils and cellulose Signup and view all the answers

What potential risks can arise from the presence of conductive molecules in dielectrics?

Dielectric breakdown and ohmic losses Signup and view all the answers

What dual role does cellulose play in transformers aside from insulation?

Provides mechanical protection and prevents conductive contact Signup and view all the answers

How do ohmic losses in dielectrics vary?

They are significantly influenced by temperature and frequency Signup and view all the answers

What is one of the primary functions of mineral oil in oil-immersed transformers?

Cool and lubricate tap-changers Signup and view all the answers

What is the primary reason for a negative dissipation factor in test measurements?

High ohmic portion of the test circuit Signup and view all the answers

How can ground loop resistance be effectively reduced?

By increasing the cross-sectional area of the conductor Signup and view all the answers

What does the term 'ground loop' refer to in the context of testing?

The pathway allowing electric potential to flow through grounded connections Signup and view all the answers

Why is it recommended to use a low-ohmic and low-inductive ground lead?

To minimize resistance and improve measurement accuracy Signup and view all the answers

What can result from connecting an additional conductor in parallel to the existing ground connector?

It minimizes the potential difference between test equipment and transformer Signup and view all the answers

What is a key indicator that the test circuit's ohmic portion is too high?

Measurement of a negative dissipation factor Signup and view all the answers

What advantage does using a conventional cable or lead provide when connected to the ground?

It is generally more cost-effective Signup and view all the answers

Which of the following is NOT considered a factor affecting test results?

The size of the transformer tank Signup and view all the answers

What represents the losses over the surface of an insulator such as a bushing shed?

Parallel connection of ohmic resistance and capacitance Signup and view all the answers

In the context of a real capacitor, what does the dissipation factor indicate?

The angle of phase difference between resistive and capacitive currents Signup and view all the answers

Which of the following correctly describes the ideal capacitive current IC̅?

It is an ideal current introduced for illustration and calculation Signup and view all the answers

When evaluating an insulator using the dissipation factor, what should not be recorded?

The current over the surface of the insulator Signup and view all the answers

What does the vector diagram for real components illustrate?

The combined effect of resistive and capacitive currents Signup and view all the answers

What type of connection is used to calculate losses within an insulator or dielectric?

Series connection of ohmic resistance and capacitance Signup and view all the answers

Which variable in the vector diagram corresponds to the actual measured test current?

The sum of ohmic IR̅ and capacitive IC̅ currents Signup and view all the answers

What is the significance of the angle δ in the vector diagram?

It represents the phase difference between voltage and current Signup and view all the answers

How does the distance of at least 10 cm to adjacent conductive parts affect the measurement?

It prevents insulation damage and incorrect measurements. Signup and view all the answers

What is a primary effect of humidity on measurement results?

It may influence the dissipation factor during testing. Signup and view all the answers

What could be a consequence of the HV cable resting on components with HV potential?

Increased risk of partial discharges and faulty measurements. Signup and view all the answers

Why is it essential to document ambient conditions during testing?

To evaluate results correctly even under unfavorable conditions. Signup and view all the answers

What is the recommended setup for connecting the HV cable to avoid measurement errors?

Maintaining sufficient distance from conductive parts. Signup and view all the answers

How does the last 50 cm of the HV cable's insulation affect testing?

It should never touch conductive parts with HV potential. Signup and view all the answers

What factor can lead to faulty measurements according to the test setup?

Inconsistent voltage applications. Signup and view all the answers

Which factor is identified as having a substantial influence on test results?

The ambient temperature during testing. Signup and view all the answers

What is the primary focus of dielectric measurements on bushings?

Aging and insulation condition between condenser layers Signup and view all the answers

Which voltage and frequency conditions are commonly used for dissipation-factor measurement on bushings?

Sine-wave 50-Hz voltage in steps of 2 kV up to 12 kV Signup and view all the answers

Why should the transformer book be requested on site when measuring bushings?

Weather conditions can make the nameplate difficult to read. Signup and view all the answers

What does the value listed as VF, tan δ, or similar in the test protocol represent?

The capacitance at rated frequency and conditions Signup and view all the answers

What complicates the assessment of bushing aging when relying solely on capacitance and dissipation factor at nominal frequency?

Water influence cannot be measured at rated frequency. Signup and view all the answers

Which of the following is a common frequency used for the diagnostic measurements on bushings?

15 Hz Signup and view all the answers

What is a potential source of error in bushing measurements when comparing laboratory results to field conditions?

Environmental factors may differ significantly from lab conditions. Signup and view all the answers

What type of measurement is crucial for determining the insulation condition between wrapping and outer insulator?

Dielectric measurement Signup and view all the answers

What is the primary purpose of a shield around a conductor in the context of interference mitigation?

To absorb external electric or magnetic fields and reduce their strength within the conductor. Signup and view all the answers

When connecting the shield potential to ground, which of the following statements is most accurate?

Connecting one or both ends to ground depends on the nature of the interfering signals. Signup and view all the answers

In the expression $H_i \ll H_O$, what does this indicate about the magnetic field strengths?

The external magnetic field strength is significantly greater than the internal magnetic field strength. Signup and view all the answers

What role does capacitive influence play in conductors located near each other?

It adds current to adjacent conductors, leading to potential voltage disturbances. Signup and view all the answers

Which of the following statements regarding the use of a cable with one conductor and two shields is correct?

It is employed to mitigate both capacitive and inductive interferences. Signup and view all the answers

What phenomenon could cause a negative dissipation factor to be measured in transformers?

Inductive or electrical couplings Signup and view all the answers

Which of the following factors is NOT correctly associated with electromagnetic interferences in dielectric measurements?

Short circuits in windings Signup and view all the answers

What is the purpose of switch matrices in test equipment for measuring dielectric properties?

To redirect and manage measurement inputs and outputs Signup and view all the answers

In terms of measurement setup, which condition can potentially distort dielectric test results?

Poor grounding connections Signup and view all the answers

What is a crucial aspect when measuring the capacitive current in transformers?

Utilizing a guard to bypass surface currents Signup and view all the answers

What might be an underlying issue if a dielectric material shows a negative dissipation factor?

Resonances from unfavorable connections Signup and view all the answers

Which statement best explains why dielectric materials cannot produce a negative dissipation factor?

Dielectric materials do not emit electrons Signup and view all the answers

When assessing dielectric properties in test measurements, what aspect is often ignored but crucial for accuracy?

Ambient conditions during testing Signup and view all the answers

What is the maximum allowable touch voltage for personal protection during device grounding?

50 AC voltage Signup and view all the answers

How do surface currents behave in relation to frequency?

They depend on both frequency and moisture conditions. Signup and view all the answers

What is one of the main roles of the dielectric in a capacitor?

To minimize losses and prevent charge carrier penetration. Signup and view all the answers

What is the role of the guard in the insulation measurement setup?

To bypass currents around the measuring unit. Signup and view all the answers

How does the presence of conductive molecules impact the dielectric properties?

They can result in changes and potential faults in the dielectric. Signup and view all the answers

What type of currents can be bypassed around the measuring unit in an insulation testing circuit?

Both capacitive and resistive currents Signup and view all the answers

What primary factor must be considered when conducting tests on dielectrics in transformers?

Both temperature and frequency during testing. Signup and view all the answers

Which of the following statements about the point of ground connection during transformer testing is correct?

It needs to be identical with the transformer ground point. Signup and view all the answers

What consequence can moisture on the insulation fins of a bushing have on test graphs?

It may cause changes depending on the test voltage frequency. Signup and view all the answers

What is the primary consequence of having a high permittivity in insulating materials?

It allows for better insulation against electric fields. Signup and view all the answers

What dual role does mineral oil play in oil-immersed transformers?

Cooling and lubrication of mechanical parts. Signup and view all the answers

Why is it essential for transformer test equipment to include safety plugs?

To protect against faults and ensure personal safety. Signup and view all the answers

In the context of currents in insulation arrangements, which types of currents are emphasized?

Both ohmic and capacitive currents Signup and view all the answers

Which is a reason why temperature testing is crucial when evaluating dielectrics?

It directly affects dielectric losses and material integrity. Signup and view all the answers

What type of materials are commonly used for insulation in oil-immersed transformers?

Mineral oils, cellulose, and pressboard. Signup and view all the answers

Why is it essential to perform tests at different frequencies when assessing dielectrics?

It helps identify the influence of disturbing molecules. Signup and view all the answers

What occurs when negative dissipation factors are measured by the test equipment?

The ohmic portion of the test circuit is too high. Signup and view all the answers

How can the ground loop resistance be improved?

By increasing the cross-sectional area of the conductor. Signup and view all the answers

Which component is essential to connect for effective ground loop mitigation?

The ground point of the transformer and the test equipment. Signup and view all the answers

What is NOT a characteristic of a low-ohmic ground lead?

It has a high inductive reactance. Signup and view all the answers

What connects the transformer tank and test equipment housing in a ground loop setup?

Conductive grounding cables. Signup and view all the answers

Why is measuring the ground loop resistance critical in test results?

It influences the accuracy of the test measurements. Signup and view all the answers

What is a consequence of having a high total resistance in the ground loop?

Overestimation of insulation quality. Signup and view all the answers

What happens when the cross-sectional area of the grounding conductor is not increased?

Ground loop resistance will remain high. Signup and view all the answers

What is the primary function of a guard in measurement technology?

To bypass currents around the measuring unit. Signup and view all the answers

How should the second shield of a three-conductor cable be connected to ground potential?

Connected on both sides. Signup and view all the answers

What type of current develops when voltage is applied to insulated conductive materials?

Capacitive current. Signup and view all the answers

What does the use of a guard belt in bushing measurements primarily aim to achieve?

To improve the accuracy of the test signal. Signup and view all the answers

What indicates the potential current flow in the real insulator due to surface contamination?

Surface current. Signup and view all the answers

What should be neglected according to the measuring principle described?

Losses in the dielectric. Signup and view all the answers

What is the effect of credible grounding for associated shields in a three-conductor cable?

It mitigates test signal distortion. Signup and view all the answers

What does connecting the shield to ground potential help achieve in terms of test signal integrity?

Reduces the impact of external electric fields. Signup and view all the answers

What is the primary benefit of measuring bushings at 12 kV compared to lower voltages?

It improves detection of contact problems at the test tap. Signup and view all the answers

What component is crucial for ensuring the insulation resistance meets the minimum requirement during measurements?

Insulating sleeves and washers Signup and view all the answers

What is a common misconception about the vent screw in bushing measurements?

It is often confused with the voltage tap. Signup and view all the answers

In the context of bushings, why are aluminum layers utilized?

To improve the homogenization of the electric field. Signup and view all the answers

What must be removed from the bushing setup before performing capacitance measurements?

Potential lead Signup and view all the answers

Which of the following statements regarding insulating gaskets used in bushing testing is correct?

They should separate transformer and bushing flanges electrically. Signup and view all the answers

What should be ensured when using insulating materials in bushing designs for high voltages?

The materials must be non-conductive. Signup and view all the answers

What crucial aspect must not be overlooked when assessing bushings at high voltages?

The moisture level in the insulation material. Signup and view all the answers

What primarily accounts for the weakening of dielectric properties in transformers?

Aging, moisture, gases, and particles Signup and view all the answers

What happens to the dissipation factor when a dielectric is ideal?

It becomes zero Signup and view all the answers

Which measurement setup accurately represents an ideal insulator in terms of current flow?

Current is 90 degrees behind voltage Signup and view all the answers

What is typically tested in transformers to assess their insulation quality?

Water content and electric conductivity Signup and view all the answers

In the context of the dissipation factor, what does the term 'ohmic resistance' signify for an ideal insulator?

It is zero Signup and view all the answers

What electrical condition indicates the presence of losses in a dielectric?

A nonzero dissipation factor Signup and view all the answers

What is the significance of reducing the ground loop resistance in a test circuit?

It ensures accurate measurements by stabilizing the potential. Signup and view all the answers

What condition is implied by the relationship $Z̅ = R ∢ 0° + X ∢ − 90 °$ in relation to a dielectric?

Capacitive reactance dominates over resistance Signup and view all the answers

Which factor primarily contributes to the occurrence of negative dissipation factors in measurements?

An overly high ohmic portion of the test circuit. Signup and view all the answers

What is generally accepted as a characteristic of the ideal current in dielectric materials?

It is purely capacitive with no losses Signup and view all the answers

How does increasing the cross-sectional area of the conductor between test equipment and the transformer affect measurements?

It reduces ground loop resistance significantly. Signup and view all the answers

What does the term 'ground loop' specifically refer to in the context of testing procedures?

The part of the circuit involving potential connections and ground cables. Signup and view all the answers

What is an effective method to ensure the accuracy of dielectric testing results?

Maintaining a consistent distance between conductive parts. Signup and view all the answers

What effect does using a conventional cable have when it is connected to the ground in the test setup?

It may introduce additional inductive losses. Signup and view all the answers

Why might a double connection between the transformer and test equipment be beneficial?

It reduces the likelihood of negative dissipation factors. Signup and view all the answers

Which of the following options is often misrepresented regarding the effects of ground loop resistance?

It affects only the dielectric constant. Signup and view all the answers

What is the minimum temperature allowable for performing measurements on bushings?

5 °C Signup and view all the answers

Which temperature should be recorded to correspond with the highest oil temperature in the transformer?

Flange temperature at bushing base Signup and view all the answers

What is the effect of humidity on the test tap at the bushing flange?

It modifies the measurement. Signup and view all the answers

Why must dirt and humidity be removed from bushing surfaces before measurements?

To avoid false readings and inaccuracies. Signup and view all the answers

What is the primary component used for the return measurement in measuring capacitance C̅1?

The test tap on the bushing flange Signup and view all the answers

Which of the following must be done to maintain the integrity of the measurements?

Dry the test tap with a blow-dryer. Signup and view all the answers

What issue can arise from leaving textile fibers or oil films on bushing surfaces?

They lead to reduced measurement accuracy. Signup and view all the answers

Which of the following describes capacitance C̅2 in relation to the transformer?

It depends on the specific transformer and previous measurements. Signup and view all the answers

A capacitor becomes ineffective when it is connected to a voltage source at any frequency.

False Signup and view all the answers

Short-circuiting a capacitor prevents it from self-discharging.

False Signup and view all the answers

Connecting both sides of a capacitor-like arrangement to ground can prevent unintentional charging during DC measurements.

True Signup and view all the answers

Capacitive coupling can only occur between components that have the same electrical potential.

False Signup and view all the answers

Grounding the secondary winding of a transformer changes the effective capacitance present.

True Signup and view all the answers

Immediately switching off a voltage source will not result in self-discharge of a capacitor.

False Signup and view all the answers

Complex test objects like transformers often require connections to ground to manage unwanted capacitance effects.

True Signup and view all the answers

The presence of balanced potentials in a capacitor-like arrangement results in electron movement.

False Signup and view all the answers

A negative dissipation factor indicates that a dielectric is functioning as a current source.

False Signup and view all the answers

Causes for negative dissipation factors in transformers can be attributed to inductive couplings or electromagnetic interferences.

True Signup and view all the answers

Switch matrices in test equipment are used to restrict the measurement inputs to only one channel.

False Signup and view all the answers

The primary purpose of a guard in testing is to ensure the bushing’s surface currents are bypassed around the measuring unit.

True Signup and view all the answers

Electromagnetic interferences have no impact on the dissipation factor measurements in electronic components.

False Signup and view all the answers

A positive value for the dissipation factor is expected under normal circumstances in dielectrics.

True Signup and view all the answers

Ground loop impedances can influence the behavior of a dielectric during testing.

True Signup and view all the answers

All test equipment must utilize a constant measurement input throughout the testing process.

False Signup and view all the answers

A guard belt made of flexible, semi-conductive material is used for bushing measurements.

True Signup and view all the answers

In measurement technology, the term 'guard' only refers to the physical guard used around electrical components.

False Signup and view all the answers

Connecting one shield of a three-conductor cable to ground potential on both sides is required for optimal interference protection.

False Signup and view all the answers

Capacitive current does not develop when voltage is applied to insulated conductive materials.

False Signup and view all the answers

The test current in a dielectric measurement is primarily the sum of surface current and magnetic current.

False Signup and view all the answers

The insulation in a three-conductor cable can absorb moisture, impacting the performance during tests.

True Signup and view all the answers

The connections on the test equipment and terminal box for guarding are referred to as guard shields.

False Signup and view all the answers

When evaluating dielectric properties, the capacitive current is not influenced by dirt particles on the insulation surface.

False Signup and view all the answers

The dissipation factor of a newly installed bushing is typically higher than the lab-measured dissipation factor.

False Signup and view all the answers

Moisture can affect the measurement of capacitive properties in paper and oil insulation.

True Signup and view all the answers

RBP bushings allow moisture penetration due to their design.

False Signup and view all the answers

The capacitance and dissipation factor values of a bushing typically remain unchanged regardless of conditions.

False Signup and view all the answers

Aging can lead to a rise in the dissipation factor of a bushing back to its nominal value.

True Signup and view all the answers

The loss of condenser layers in a bushing will lead to a decrease in measured capacitance.

False Signup and view all the answers

Bushing technology differences have a critical impact on measurement results.

False Signup and view all the answers

The dissipation factor may be considered acceptable if it is measured at or below nominal values under field conditions.

True Signup and view all the answers

The dissipation factor is defined as the ratio of resistive current to capacitive current in a capacitor.

True Signup and view all the answers

In a two-winding transformer, the shorted capacitances of the secondary and grounded winding are crucial for assessing its performance.

True Signup and view all the answers

Negative dissipation factors are commonly achieved through the use of resistive materials.

False Signup and view all the answers

Moisture has no influence on the dielectric frequency response of insulation materials.

False Signup and view all the answers

Guarding techniques can help reduce the influence of stray capacitance during measurements.

True Signup and view all the answers

The measuring principle of dielectric frequency response is unrelated to the test setup configuration.

False Signup and view all the answers

Vibro-acoustic measurements are primarily concerned with the mechanical vibrations of transformers.

True Signup and view all the answers

Capacitance measurements on bushings are inconsequential to their operational reliability.

False Signup and view all the answers

An increase in the temperature of the insulation material generally enhances its dielectric strength.

False Signup and view all the answers

The influence of frequency on the dissipation factor is negligible within transformer diagnostics.

False Signup and view all the answers

Hot collars are used in the measurement process to isolate sections of the insulation system.

True Signup and view all the answers

Ground loop resistance plays no role in the accuracy of dielectric measurements.

False Signup and view all the answers

The dielectric frequency response can provide valuable insights into the moisture content of insulating materials.

True Signup and view all the answers

The evaluation of the capacitive current is auxiliary to the main analysis of transformer diagnostics.

False Signup and view all the answers

The presence of conductive molecules in dielectric materials enhances their insulating properties.

False Signup and view all the answers

Components used in bushing diagnostics play a minor role in the overall measurement process.

False Signup and view all the answers

Capacitive influences in conductors do not contribute to disturbing voltages.

False Signup and view all the answers

According to the Faraday principle, conductive surfaces can block electric or magnetic fields.

True Signup and view all the answers

Inductive interference is not present in substations or industrial complexes.

False Signup and view all the answers

In the expression $H_i gtr H_O$, the magnetic field strength inside the conductor is much smaller than outside.

True Signup and view all the answers

Two shields are unnecessary when using a cable with one conductor in industrial applications.

False Signup and view all the answers

Currents bypassed around the measuring unit via the guard can only be ohmic.

False Signup and view all the answers

Surface currents depend on frequency, which affects the ohmic portion of loss current.

True Signup and view all the answers

Device grounding should ensure that touch voltage exceeds 50 AC volts for safety.

False Signup and view all the answers

In transformer tests, the ground point of the ground cable must be different from the transformer ground point.

False Signup and view all the answers

Moisture on insulation fins could lead to measurement changes that are independent of the test voltage frequency.

False Signup and view all the answers

Parasitic currents in a device should be directed to ground for component protection.

True Signup and view all the answers

The guard principle is applicable only to simple geometrical arrangements and not to transformers.

False Signup and view all the answers

The dissipation factor tan δ equals zero for ideal capacitors due to no losses in the dielectric.

True Signup and view all the answers

Transformer test equipment is typically not equipped with safety plugs.

False Signup and view all the answers

In a real insulator, the impedance is composed only of the resistive component.

False Signup and view all the answers

The active current in a real insulator represents the losses incurred by the insulator.

True Signup and view all the answers

The angle between test current and capacitive current is 90° for ideal insulators and dielectrics.

False Signup and view all the answers

Surface currents can contribute to losses in a real insulator, impacting its performance.

True Signup and view all the answers

In the formula for impedance, the reactive component is represented as $X ∢ 90°$.

False Signup and view all the answers

An ideal insulator allows for a test current that is distinctively different from capacitive current.

False Signup and view all the answers

For an ideal dielectric, the dissipation factor equals the ratio of actual current to capacitive current.

False Signup and view all the answers

The insulation condition between the single condenser layers does not significantly affect the operational safety of transformer bushings.

False Signup and view all the answers

Dissipation-factor measurement on bushings typically utilizes a sine-wave voltage of 60 Hz for diagnostic evaluations.

False Signup and view all the answers

Capacitance and dissipation factor measurements alone are significant indicators of bushing aging.

False Signup and view all the answers

The environmental conditions present during laboratory testing of bushings are identical to those in field conditions.

False Signup and view all the answers

Testing bushing insulation must always take place under a controlled temperature of 25 °C to ensure accuracy.

False Signup and view all the answers

The transformer book is essential for accurately documenting measurements taken on bushings.

True Signup and view all the answers

Voltage and frequency conditions for dissipation-factor measurements are flexible and do not need to be predefined.

False Signup and view all the answers

Weather conditions can influence the legibility of bushing nameplates, complicating the assessment of bushing measurements.

True Signup and view all the answers

The course of graphs regarding measurements can indicate an incorrect test setup if they are voltage-dependent.

True Signup and view all the answers

A bushing of the neutral conductor typically experiences higher temperatures due to carrying current.

False Signup and view all the answers

Measuring negative values for dissipation factors is an indicator of correct testing conditions.

False Signup and view all the answers

Cleaning insulation fins and the guard is unnecessary for obtaining accurate measurements.

False Signup and view all the answers

The comparison between phases is one of the factors used to check the plausibility of measured values.

True Signup and view all the answers

If capacitance values are low in neutral conductor bushings, this is typically due to higher operating voltages.

False Signup and view all the answers

The exterior appearance of a bushing can reliably indicate the technology used within it.

False Signup and view all the answers

The three factors used to check plausibility include value range, deviations from manufacturer tests, and the course of graphs.

True Signup and view all the answers

A positive dissipation factor indicates that electrons can flow from the dielectric.

False Signup and view all the answers

Negative dissipation factors in transformers can be caused by electromagnetic interferences.

True Signup and view all the answers

A switch matrix in test equipment only serves to remove measurement inputs.

False Signup and view all the answers

Ground loop impedances can contribute to measuring inaccuracies in dielectric tests.

True Signup and view all the answers

A real insulator emits electrons when a negative dissipation factor is measured.

False Signup and view all the answers

The measurement of capacitive current in transformers is typically conducted on multiple channels simultaneously.

False Signup and view all the answers

Testing methods UST and GST can utilize switch matrices for enhanced measurement accuracy.

True Signup and view all the answers

Visualizing the vector diagram for real components helps indicate the actual measured test current.

True Signup and view all the answers

The losses over the surface of an insulator are presented as a series connection of ohmic resistance and capacitance.

False Signup and view all the answers

The ideal capacitive current IC̅ flows physically in a circuit during testing.

False Signup and view all the answers

The equation $Uq ≂ R + XC$ represents the relationship between voltage, resistance, and capacitance in a real insulator.

False Signup and view all the answers

UST measurement is preferred over GST measurement due to its stability and insensitivity to disturbances.

True Signup and view all the answers

The vector diagram applies only for ideal components in electrical systems.

False Signup and view all the answers

All materials are either conductive or non-conductive based solely on their color.

False Signup and view all the answers

Current over the surface should be recorded when evaluating an insulator using the dissipation factor.

False Signup and view all the answers

Dissipation factor can indicate the efficiency of an insulating material.

True Signup and view all the answers

In GST measurements, it is possible to measure both Input A and Input B simultaneously while grounding the potential.

False Signup and view all the answers

The presence of strong electromagnetic stress in substations is a major concern during transformer testing.

True Signup and view all the answers

For simplicity, the analysis of losses within the insulator is included in the compendium.

False Signup and view all the answers

Free electrons in conductive materials align when an electric field is applied, creating an electric potential across structures.

True Signup and view all the answers

The equation $Im = IC + IR$ depicts the relationship between capacitive and resistive currents.

True Signup and view all the answers

The maximum number of channels that can be measured concurrently in grounded test circuits is two.

False Signup and view all the answers

Capacitive and inductive couplings do not affect measurement accuracy in transformers.

False Signup and view all the answers

GST measurements always involve bypassing the measuring unit for both inputs.

False Signup and view all the answers

Induction voltage in adjacent conductors is solely influenced by DC power lines.

False Signup and view all the answers

A conductive surface can effectively block electric and magnetic fields, according to the Faraday principle.

True Signup and view all the answers

Capacitive influences in conductors do not add current in adjacent conductors.

False Signup and view all the answers

Connecting the shield potential to ground can discharge the voltages and currents that develop in the shield.

True Signup and view all the answers

The expression $H_i eq H_O$ denotes that the magnetic field strength inside a conductor is typically greater than outside.

False Signup and view all the answers

The dissipation factor of a bushing increases immediately after installation in a transformer.

False Signup and view all the answers

Moisture has no effect on paper and oil insulation inside bushings.

False Signup and view all the answers

Changes in capacitance and dissipation factor typically indicate specific aging processes or faults.

True Signup and view all the answers

RIS and RIF bushings allow moisture to penetrate their insulation.

False Signup and view all the answers

The design and manufacturing process of bushings do not influence capacitance and dissipation factor.

False Signup and view all the answers

Over time, even a 15-year-old bushing with dissipation factor values near the nominal value may indicate aging.

True Signup and view all the answers

Crumbling of the insulating material can lead to drastic changes in the capacitance of a bushing.

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Loss of condenser layers decreases the measured capacitance in bushings.

False Signup and view all the answers

Study Notes

Dielectric Properties and Insulating Materials

Transformers require effective insulation to separate components at different electrical potentials.
Insulating materials penetrate electric fields and are referred to as dielectrics, characterized by permittivity.
Permittivity reflects the ability of a material to insulate electric fields, essential for the performance of devices like capacitors.

Losses and Polarization

Dielectric losses can occur if conductive or polarizable molecules are present, affecting insulation quality.
Losses depend on frequency and temperature, necessitating temperature monitoring during field tests.
Most tested transformers use oil-immersed insulation, commonly made from mineral oils, cellulose, and pressboard.

Dissipation Factor and Insulation Testing

The dissipation factor quantifies losses, viewed as a combination of ohmic resistance and capacitance.
In assessing insulation, surface current should not be measured; the focus is on the internal dielectric properties.
Ideal capacitive current is introduced for illustrative purposes in calculations.

Ground Loop Resistance

The dissipation factor of a dielectric must always be positive; negative values indicate excessive resistance in the test circuit's ohmic part.
Ground loop resistance can be decreased by increasing the conductor's cross-sectional area, enhancing test accuracy.

Measurement Techniques for Bushings

Bushing insulation is crucial for transformer safety; dielectric measurements focus on insulation between condenser layers.
Dissipation factor measurements employ various sine-wave voltages and frequencies, typically starting from 2 kV to as high as 12 kV.
Capacitance measurements are also essential; values are recorded both in laboratory conditions and on-site for monitoring aging effects.

Impact of Environmental Factors

Weather conditions can inhibit readability of nameplates; it's crucial to document ambient conditions during measurements.
Maintaining distance between bushings and adjacent conductive parts is essential to avoid insulation failures.
External factors like humidity and improper cable connections can significantly influence measurement accuracy; avoiding partial discharges is critical.

Test Setup Considerations

The high voltage (HV) cable setup should maintain adequate distance from HV potential components to prevent measurement errors and potential damage.
Employing suitable support during measurements can facilitate proper testing conditions and enhance safety protocols on-site.

Conclusion

Understanding dielectric properties and the detailed process of testing insulating materials is vital for maintaining operational integrity in transformers.

Dielectric Properties and Insulation

Transformers require effective insulation between components with different electrical potentials.
Electrically insulating materials are termed dielectrics, whose capability to insulate is referred to as permittivity.
A dielectrical capacitor demonstrates insulation by allowing electric fields to pass without significant loss, while preventing charge carrier intrusion.
Polarization occurs when conductive molecules within a dielectric alter its properties, potentially leading to ohmic losses or breakdowns.
Dielectric losses are influenced by frequency and temperature, necessitating constant monitoring during field tests.

Insulating Materials in Transformers

Most tested transformers are oil-immersed, commonly using mineral oils, cellulose, and pressboard for insulation.
Cellulose and pressboard also provide mechanical protection against direct contact between conductive parts.

Dissipation Factor Considerations

The dissipation factor of a dielectric material should always be positive, indicating energy loss.
Negative dissipation factors can be reported due to inductive couplings, electromagnetic interferences, or poor grounding, not from the dielectrics themselves.
It is essential to identify and mitigate interferences to obtain accurate measurements.

Testing Methods and Equipment

Advanced test equipment can utilize switch matrices to selectively measure capacitive currents and guard against surface currents.
Environmental factors like AC power lines and adjacent conductors can induce interference, affecting measurement accuracy.
Using conductive shielding like metal foil can block electric or magnetic interferences.

Grounding Techniques

Proper grounding is crucial to reduce touch voltage and ensure personal safety during transformer testing.
Ground loops must be minimized, ensuring the ground connections of test equipment match the transformer’s ground point.
Negative dissipation factor readings may be linked to high ohmic components in the test circuit; reduce ground loop resistance for more reliable results.

Improving Measurement Accuracy

Increasing the cross-sectional area of ground conductors improves ground loop resistance and measurement accuracy.
Utilizing low-ohmic, low-inductive leads for grounding connections enhances test conditions and results.

Key Points on Frequency and Surface Currents

Surface currents vary with frequency, affecting the efficacy of dielectric materials.
Moisture presence on insulation can lead to altered measurement graphs, further reinforcing the need to consider frequency effects during tests.

Dielectrics in Transformers

Aging, moisture, gases, and particulates significantly weaken dielectric properties in transformers.
Routine testing for water content and electric conductivity is essential for insulation integrity in transformers and bushings.

Dissipation Factor

The dissipation factor indicates the quality and aging of an insulator or dielectric.
Defined as a comparison of the resistive power loss versus the capacitive power supply.
An ideal dielectric insulates conductors without losses, meaning the current is purely capacitive.

Ideal Dielectric Characteristics

In an ideal situation, the impedance of an insulator is purely capacitive with no ohmic resistance.
For a perfect dielectric, the dissipation factor is zero; this indicates no energy is lost as heat.

Grounding and Shielding in Measurements

Essential to reduce interference in signal measurements by connecting one shield to ground on one side and the outer shield on both sides.
Guarding refers to the use of current bypassing techniques to ensure accurate measurement without error from external influences.

Resistance and Measurement Principles

Dissipation factors for real insulations are always positive; negative factors indicate high ohmic resistance in test circuits, requiring ground loop resistance reduction.
Enhancing ground loop resistance improves overall measurement accuracy, often achieved by increasing conductor cross-sectional area.

Bushing Testing Procedures

Bushings should be tested for moisture effects using the Frequency Domain Spectroscopy (FDS) method, especially at voltages over 110 kV.
Insulation resistance should be at least 10 kΩ for transformer and bushing isolation during testing.

Influence of Temperature on Measurements

Optimal measurement temperature for bushings is around 20 °C; below 5 °C can lead to inaccurate dissipation factor results.
Record temperatures at the bushing base, bushing head, and ambient temperature both before and after measurements.

Importance of Cleaning and Preparation

Surface cleaning is crucial to avoid errors in measurements; dirt or moisture on bushings can interfere with test results.
Recommended cleaning agents should align with manufacturer guidelines, especially for silicon bushings.

Capacitance Measurement

Capacitance C̅1 is measured by connecting test equipment to the bushing bolt, while C̅2 is measured applying a maximum of 500 V at nominal frequency.
Ground loops are critical for return measurements, as capacitance is affected by the transformer's configuration and should be compared against previous data.

Introduction to Transformer Diagnostics

Contains foundational concepts regarding transformer measurements, building on theories from Part I.
Provides insights into dielectric properties, dissipation factors, and measurement techniques.

Dielectric Properties and Dissipation Factor

Dissipation factor indicates the efficiency of an insulating material.
Comprised of definitions and influencing factors, including temperature and frequency.
Ideal vs. real dielectrics: differences in behavior and typical values related to transformers and bushings.

Measurement Techniques

Discusses methods like UST and GST relevant for assessing transformer and bushing capacitance.
Emphasizes importance of correct device grounding to minimize measurement errors.

Polarization and Interference

Explains polarization types in dielectric materials and the time constants involved.
Identifies potential interferences that may affect diagnostic signals during measurements.

Bushing Diagnostics

Detailed evaluation of bushing components and their performance during testing.
Specifies common moisture-related issues and their impact on dielectric frequency response.

Magnetizing Current

Defines magnetizing current and its role in maintaining transformer efficiency.
Evaluates measuring principles and factors influencing its performance.

Vibro-Acoustic Measurement

Describes measuring principles and techniques for assessing transformer sound and vibration.
Highlights signal-to-noise ratios and the significance of documentation for interpretation of results.

Frequency Response Analysis

Outlines basics of frequency response testing for transformers.
Distinguishes methodologies to evaluate amplitude and phase shifts.

Practical Applications and Attachments

Offers templates for temperature documentation and factors influencing performance evaluations.
Provides insight into actual measurements and corrective actions for improving diagnostics.

Tables and Figures

Includes numerous figures and tables illustrating key concepts, measurement setups, and derived relationships in transformer diagnostics.

Conclusion

Reinforces the importance of various diagnostic techniques in maintaining transformer reliability and performance.### Capacitor Behavior in Voltage Systems
A capacitor's load varies with the frequency of the voltage source applied.
Short-circuiting a capacitor renders it ineffective and can lead to self-discharge when the voltage source is turned off.
Charge time is critical to consider, especially in direct current applications, for efficiency and safety.
Ground connection of capacitive arrangements in complex test objects, like transformers, helps avoid unwanted charging and discharging.

Implications of Capacitive Coupling

Circulating currents arise from adjacent components having different electrical potentials, leading to an electric field.
Equalizing the potential on both sides of a capacitive setup prevents electron movement, balancing the system.
Unintended charging from capacitive coupling can be rectified by grounding both sides of the setup during DC measurements, particularly with high-frequency AC voltages.

Transformer Measurements

Connecting ground potential to the secondary winding of a transformer influences capacitance values and measurements.
Necessary to disconnect any ground potential connections from transformers when measuring capacitance, especially for industrial transformer capacitor banks.

Negative Dissipation Factor

Normally, dissipation factors are positive; however, negative values have been occasionally recorded during transformer and bushing measurements.
Negative dissipation factors do not signify a current source; causes might include inductive or electrical couplings, electromagnetic interferences, or resonance from ground loop impedances.
Eliminating these causes often involves straightforward adjustments.

Test Methods: UST and GST

Testing devices may incorporate switch matrices to manage multiple inputs and outputs, allowing selective measurements while bypassing unwanted currents.
An example includes measuring capacitive currents on one channel while diverting surface currents on another using guards.

Guarding Practices

"Guard" denotes protecting the measurement signal by rerouting currents around the measuring unit.
Guard mechanisms, such as semi-conductive shells for bushing measurements, enhance measurement accuracy by preventing interference.

Influence of Insulation on Measurements

Capacitive current develops in parallel insulated conductive materials, whereas surface currents arise from dirt or moisture on the insulator.
In real-world conditions, aging increases the dissipation factor, approaching nominal values, which could imply deterioration.

Bushing Technology and Capacitance Measurements

Capacitance and dissipation factors are influenced by internal insulation materials and the number of condenser layers.
Moisture penetration into insulation (e.g., paper and oil) can alter capacitance and dissipation, while resin-impregnated materials resist moisture intrusion.
Changes in dissipation factor or capacitance often signal aging, dirt accumulation, or faults, particularly in RBP bushings with increased limits due to design features.

Ideal Capacitors and Insulators

Ideal capacitors have a zero dissipation factor (tan δ = 0) due to the absence of losses; test current equals capacitive current.
For real insulators, losses occur from factors like polarization and surface currents, affecting the impedance calculation of the insulator.

Impedance of Real Insulators

Impedance can be expressed as Z̅ = R ∢ 0° + 1/(2πfC) ∢ -90°; active current signifies insulator losses.
Inductive and capacitive influences from AC power or overhead lines lead to voltage disturbances in adjacent conductors.

Shielding and Grounding

Conductive surfaces, like metal foil or shield mesh, block electric or magnetic fields, reducing internal magnetic field strength (Hi ≪ HO).
To discharge voltage and current in shields, the shield potential is often connected to ground, enhancing safety against interference.

Guard Principle

A guard connected to the insulator’s surface helps bypass unwanted currents around measuring units, maintaining measurement integrity.
Surface currents impacted by frequency variations can alter measurement outcomes, necessitating careful analysis of insulation conditions.

Device Grounding

Device grounding ensures electrical components are safely connected to ground potential, limiting touch voltage to a maximum of 50 AC for safety.
Fault currents and parasitic currents are directed to ground to protect devices and maintain operational integrity.

Insulation in High Voltage Bushings

Insulation quality between condenser layers is critical for the safety of transformers and bushings; dielectric measurements focus on aging and insulation conditions.
Measurements using a range of voltage and frequencies (up to 12 kV and various Hz values) help diagnose bushing conditions.

Measurement Evaluation

Evaluate measured values against specified standards, including value ranges and manufacturer tests at rated conditions.
Identify potential issues through visual inspection and repeat measurements to ensure accuracy, especially in cases of discrepancies.

Frequency Influence

The impact of external environments on high voltage bushings can render laboratory conditions different from in-situ conditions.
Moisture and environmental factors can change graphs, necessitating continuous monitoring for accurate assessment over time.

Substitute Diagram and Vector Diagram

The substitute diagram illustrates an insulator's electrical behavior, depicting current components: ohmic resistance (IR) and capacitive current (IC).
The vector diagram represents real components, demonstrating the relationship between the measured test current and the ideal capacitive current.
Capacitive current (IC̅) is theoretical and does not flow but is necessary for calculations; the actual measured current is the sum of IR̅ and IC̅.

Losses in Insulators

Losses in insulators are represented as a parallel connection of ohmic resistance and capacitance on the surface.
Losses within the insulator or dielectric are modeled as a series connection.
Evaluating an insulator using the dissipation factor excludes surface current measurements to enhance accuracy.

Negative Dissipation Factor

Typically, dissipation factors for insulators are positive; negative values suggest measuring errors due to inductive/electrical couplings or electromagnetic interferences.
Negative dissipation factors indicate installation or measurement anomalies rather than physical properties of the dielectric.

Test Methods: UST and GST

UST (Universal Substation Testing) uses switch matrices for measuring capacitance selectively while bypassing surface currents.
GST (Grounded Test Circuits) allows concurrent measurement of multiple grounded channels but is more influenced by environmental electromagnetic stress.
UST measurements are generally preferred due to their stability against external interferences.

Polarization in Conductive Materials

All materials contain free electrons, categorizing them into conductive and non-conductive based on electron availability.
Applying voltage aligns free electrons, creating an electric field, which can be disrupted by nearby electrical/magnetic fields.

Shielding and Grounding

Conductors can shield against external electric/magnetic fields to prevent interference using materials like metal foil or mesh.
Connecting shield potential to ground discharges any voltages and currents built up in the shield, enhancing measurement accuracy.

Dissipation Factor Changes Over Time

The dissipation factor may decrease after a bushing is installed due to environmental influences but might increase again with aging, indicating potential degradation.
Measured values near or below nominal values in aged bushings may still be acceptable.

Measurement Influences

Environmental factors, moisture level, and material properties influence capacitance and dissipation factor.
Changes in these values can signal aging processes, dirt accumulation, or faults within the insulating material.

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Description

This quiz focuses on the interpretation of dielectric properties related to transformers and bushings, particularly the dissipation factor of insulating materials. Understanding these properties is crucial for assessing the condition and performance of electrical insulation systems. The subject matter will be explored in detail through various methods and their correlations.