Dielectric Spectroscopy in Time and Frequency

Domain

Term Paper PresentationEE – 5232

High Voltage & Insulation Engineering

Presented by:Girish GuptaRoll no. 15082023

Power SystemM. Tech

Vivek KumarRoll no. 15082020

Power SystemM. Tech

Presented to:Dr. J C Pandey,Assistant Professor,

Department of Electrical Engineering,

IIT-BHU

CONTENTSNeed for Dielectric SpectroscopyWhy Dielectric SpectroscopyDielectric ResponseWhat is Dielectric SpectroscopyFrequency Domain SpectroscopyTime Domain SpectroscopyResults from SpectroscopyPrecautions

Most dangerous breakdowns are caused by the aging effects of HV insulation systems used within HV components.

Traditionally to avoid any damage to the equipments Time Based Maintenance have been used which is costly in nature.

Now there is a move towards Condition Based Maintenance which reduces the maintenance cost and increases the life of the equipments.

For CBM, the actual conditions of equipments must be known.

For knowing the actual conditions, Dielectric Spectroscopy is used as a powerful tool.

1. NEED FOR DIELECTRIC SPECTROSCOPY

Dielectric properties are dependent on many factors, e.g. on frequency, time, temperature, chemical composition of an individual dielectric, or on the structure of an insulation system composed of different dielectrics.

Most of the above factors are measured and analyzed using standardized test but they are performed at power frequency only.

The above quantities at single frequency is insufficient to find changes is dielectric properties of materials.

So Dielectric Spectroscopy is used.

2. WHY DIELECTRIC SPECTROSCOPY

Every kind of insulation material consists, at an atomic level, of negative and positive charges balancing each other.

When a material is exposed to an electric field the positive and negative charges become oriented thus forming different kinds of dipoles even on atomic scales.

These dipoles leads to Dipole Moment which can be written as

P = α E where α = Polarizability E = applied Electric Field P = Dipole Moment / Polarizability

3. DIELECTRIC RESPONSE

Different types of Polarization are :- a. Electron polarization - the displacement of

nuclear and electrons in the atom under the influence of external electric field. It is effective up to optical frequencies and is very fast.

b. Atomic polarization - the displacement of atoms or atom groups in the molecule under the influence of external electric field. It can be polarized up to Infra Red frequencies.

c. Dipolar polarization – materials containing molecules with permanent dipole moments with orientations statistically distributed due to the action of thermal energy. It follow frequency up to MHz or GHz.d. Interfacial polarization – is effective in insulating materials composed of different dielectric materials. It occurs in Power frequency and below.

In summary, the dielectric polarization is the result of a relative shift of positive and negative charges in a material.

During all of these processes, the electric field is not able to force the charges to escape from the material, which would cause inherent electric conduction in Dielectric or Insulator.

For insulators Polarization P and electric field E is related as

P = χ ε E where χ = susceptibility ( relating to all process) ε = permittivity of free space.

Now usual sense says that when applied Electric field should be made zero, Polarization should also become zero i.e. Depolarization should happen.

But in reality that do not happen. There is a delay in the whole depolarization process which is known as relaxation time.

So longer the relaxation time, lesser the quality of Insulator.

Similarly there is polarization time also for a material.

Dielectric Spectroscopy picks the above said points for the analysis.

Also as all dielectric quantities depends on temperature also, it is also taken into account during the process.

Dielectric spectroscopy measures the dielectric properties of a medium as a function of frequency or time.

It is also known as Impedance Spectroscopy or Electrochemical Impedance Spectroscopy(EIS).

It is based on the interaction of an external field with the electric dipole moment of the sample, often expressed by permittivity.

This technique measures the impedance of a system over a range of frequencies, and so the frequency response of the system, including the energy storage and dissipation properties, is revealed.

4. WHAT IS DIELECTRIC SPECTROSCOPY

Often, data obtained by EIS is expressed graphically in a Bode plot or a Nyquist plot. It can also be expressed as logarithmic function of frequency also.

It is of two types:- Frequency Domain and Time Domain.

It is also an experimental method of characterizing electrochemical systems.

It includes the measurement of capacitance and dissipation factor (tan delta) over a frequency range of 0.1 mHz to 1 kHz.

This technique can also be seen as the extension of the measurement of the dissipation factor at the power frequency.

Also here Frequency response analyzers is used in measuring dielectric permittivity's in the frequency range 10-2 - 106 Hz.

5. FREQUENCY DOMAIN SPECTROSCOPY

An a.c. voltage V1 is applied to the sample, and then a resistor R, or alternatively a current-to-voltage converter for low frequencies, converts the sample current Is, into a voltage V2 .

Taking an example of Transformer bushing, A sinusoidal signal is applied to the high voltage bushing and current is measured through the low voltage terminal.

If the applied voltage is an alternating signal at a frequency w, then the measured capacitance is a complex quantity and whose real and imaginary parts correspond directly to the real and imaginary components of the complex permittivity.

1 2 1 2

2s

s

V V V VZ RI V

Here the sample is represented by an equivalent parallel plate capacitor.

Here A is the plate area of the capacitance, C(w) is the permittivity and w is the distance between two plates. C’(w) corresponds to the ordinary capacitance, while the imaginary component C’’(w) represents the dielectric loss component.

Now the dissipation factor from above eq. is calculated as

This factor is plotted as the function of frequencies and the plotted over the Nyquist plot.

On the basis of the readings taken, the graph is plotted which can be seen here. Here is the plotting of dissipation factor w.r.t. frequency for the samples.

On the basis of the readings taken, the graph is plotted which can be seen here. Here is the plotting of capacitancew.r.t. frequency for the samples.

Here the measurement of polarization and depolarization currents (PDC) following a dc voltage step is done.

It is made sure that voltage source free from any ripple and noise is taken to measure the above currents with sufficient accuracy.

The procedure consists in applying a dc charging voltage of certain magnitude to the test object for a long time (e.g., 10,000 s).

As each type of polarization have different time periods from short to large, that’s why such a large time is taken.

5. TIME DOMAIN SPECTROSCOPY

As P = χ ε E

and all the polarization are time dependent, the susceptibility in the above eq. is treated as a function of time to do the analysis. So for doing the analysis, a step voltage is

applied to the sample with the following arrangement:-

Here DC supply upto 1000 volts can be used and current is in picoamperes measured by electrometers.

In the above, Ipol is measured until it decays down to zero or takes a steady state value of low magnitude for considerable larger time.

Now after the constant value of polarization current, sample is short circuited to measure the depolarization current.

The same procedure is applied for the depolarization current also so as to measure the time taken by the sample to depolarize completely.

The waveforms for the voltages and current is shown as below: -

The time period upto Tc and T is used to characterize the dielectric properties of the material.

Here the larger the value of the currents, the larger the conductivity of the material so more worse it is.

Many a times when the dielectric spectroscopy have been used, certain conclusions have been found out about the samples which are:-polarisation and depolarisation currents

increase with temperature increase.Both currents also increase with the moisture

content around the sample.temperature, ageing and water content caused

a higher increase of dissipation factor and capacitance at lower frequencies.

5. RESULTS FROM SPECTROSCOPY

The equipment in operation must be removed from service before performing measurements using this technique.

Dielectric measurements require constant insulation temperatures during application for accurateness, as the polarization phenomena are temperature dependent. So temperature must be kept constant during the test.

The charging time period should be large enough to complete all the polarization and depolarization technicques.

Moisture content around sample should be controlled and measured for getting precise data.

6. PRECAUTIONS

Electromagnetic devices should be avoided at all cost as it can disrupt the devices used.