Tan Delta Test : Purpose, Principle, Circuit, Modes and Testing

The main purpose of the tan delta test is to make sure of maintaining a secure and reliable functioning of the transformer. With the calculation of dissipation factor and capacitance values, it provides the result of insulation behavior of bushings and in windings too.

Variation in the capacitance value, for instance, it indicates partial kind of breakdowns in bushings and automated movement of windings. Insulation deprivation, aging of the equipment, enhancement in the energy levels is transformed into heat. The amount of losses in these is calculated as the dissipation factor.

With the tan delta testing method, one can easily know the dissipation factor and the capacitance values at the required level of frequencies. So, any kind of aging factor can be identified earlier and the corresponding action can be implemented.

When a pure insulator has a connection between the earth and the line, then it performs like a capacitor. In an ideal kind of insulator, as the insulating substance functions as a dielectric, which is totally pure, then the passage of current through the material holds only capacitive material. There will be no resistive element for the electric current that is flowing from the line to the earth via insulator as in the insulating component, there will be no presence of impurities. The tan delta test circuit diagram is shown as follows:

In a pure capacitive material, the capacitive current precedes the voltage level by 900. As a general, the insulating material is totally pure, and even because of the aging properties of the components, the contaminations such as moisture and dirt might get added. These contaminations create a conductive path for the current. As a result, leakage current that flows from line to earth via the insulator holds resistive elements.

Therefore, it is pointless to claim that, for a good quality of insulator, this resistive element of leakage current is correspondingly minimal. In the other aspect, the behavior of an insulator might be known by the proportion of the resistive element to that of the capacitive element. For good quality of insulator, this proportion is correspondingly less and this is termed as tanδ or tan delta. In a few cases, this is also expressed as a dissipation factor. With the below-depicted vector diagram, it can be known.

Where the x-axis represents the level of system voltage which is the resistive element of leakage current IR. As this capacitive element of leakage current IC precedes by 900, it is taken across the y-axis.

And now, the whole leakage current is given by IL(IC + IR)

And from the diagram, tanδ is (IR /IC)

tanδ =  (IR /IC)

The below process explains the method of tan delta testing in a step-by-step manner.

The requirements necessary for this test such as cable, potential transformer, bushings, current transformer, and winding on which this testing is conducted has to be initially separated from the system.

The minimal frequency level of test voltage is applied along with the equipment where the insulation to be analyzed.

At first, normal voltage levels are applied. When the tan delta values are as expected at this voltage level, then the applied voltage level is increased by 2 times as of applied voltage.

The values of the tan delta are recorded by the tan delta controller.

To the tan delta calculating component, a loss angle analyzer is connected which compares tan delta values at higher and general voltage levels and delivers accurate results.

It has to be noted that the testing procedure to be carried out at very minimal frequency levels.

It is more recommended to conduct testing at minimal frequency levels, because when the applied voltage level is more, then the capacitive reactance of the insulator device reaches very minimal, therefore the capacitive element of the current reaches more. As the resistive element is practically constant; it is based on the applied voltage level and the insulator’s conductivity value.

Whereas at increased frequency level the capacitive current, is more, and then the amplitude of the vector amount of both the capacitive and resistive elements of the current reaches very high. So, the necessary level of power for the tan delta test would become more that seems to be not acceptable. Because of this, the power constraint for dissipation factor analysis, very minimal frequency test voltage is required.

These exist mainly two approaches to analyze the situation of the insulation method at the time of tan delta testing. The first is, evaluating the past test results to know the worsening of insulation conditions because of the aging effect. Whereas the second scenario is to verify the insulation behavior directly from tanδ value. Here, there is no necessity of assessing past results with that tanδ test values.

When the insulation results are accurate, then the loss factor values are nearly similar for the entire test voltage values. But, in the case when the insulation results are not accurate, then the tanδ values get increased for a higher level of voltages. The increasing tanδ corresponds that, high resistive current element, happens in insulation. These outcomes might be matched with the outcomes of past tested insulators, to go with the appropriate decision either the equipment has to be substituted or not.

This is the way that how to test the result tan delta testing can be done.

When it comes to tan delta test, there are essentially three modes of power factor testing. Those are

GST Guard – This calculates the amount of current leakage to the ground. This method eliminates the current leakage through red or blue leads. Whereas in UST, the ground is termed to be guard because grounded edges are not calculated. When the UST method is applied on the device, then the current measurement is only through blue or red leads. The current flow through ground lead gets automatically bypassed to the AC source and thus excluded from the calculation.

UST Mode – This is employed for the calculation of insulation in between ungrounded leads of the equipment. Here the individual portion of isolation has to be separated and analyze it having no other insulation connected to it.

GST Mode – In this final mode of operation, both the leakage pathways are calculated by the test apparatus. The current, capacitance values, UST, and GST guards, loss in watts need to be equal to the GST test parameters. This provides the entire behavior of the test.

When the summing value of GST Guard and UST is not equal to the GST parameters, then it can be known that there is some crashing in the test set, or might the test terminal are not correctly designed.

On the whole, this is a detailed explanation of the Tan Delta Test. Here, in this article, we are completely aware of what is a tan delta test, its principle, purpose of it, methods, and testing technique. Also know about what are LV to earth test, HV to earth test, and LV-HV tan delta testing methodologies.

A pure insulator when is connected across line and earth behaves as a capacitor. In an ideal insulator, as the insulating material which acts as dielectric too, is 100 % pure, the electric current passing through the insulator, only have a capacitive component. There is no resistive component of the current, flowing from line to earth through the insulator as in ideal insulating material, there is zero per cent impurity.

In a pure capacitor, the capacitive electric current leads the applied voltage by 90o.

In practice, the insulator cannot be made 100% pure. Also due to the aging of insulators, the impurities like dirt and moisture enter into them. These impurities provide the conductive path to the current. Consequently, an electric leakage current flowing from line to earth through the insulator has a resistive component.

Hence, it is needless to say that, for a good insulator, this resistive component of the electric leakage current is quite low. In another way, the healthiness of an electrical insulator can be determined by the ratio of the resistive component to the capacitive component. For a good insulator, this ratio would be quite low. This ratio is commonly known as tanδ or tan delta.

Sometimes it is also referred to as the dissipation factor.

Thus, tan δ = IR/ IC

NB: This δ angle is known as the loss angle.

The main purpose of the tan delta test is to make sure of maintaining a secure and reliable functioning of the transformer. The calculation of dissipation factor and capacitance values provides the result of insulation behavior of bushings and in windings too.

Variation in the capacitance value, for instance, indicates partial kind of breakdowns in bushings and automated movement of windings. Insulation deprivation, aging of the equipment, enhancement in the energy levels is transformed into heat. The amount of losses in these is calculated as the dissipation factor.

With the tan delta testing method, one can easily know the dissipation factor and the capacitance values at the required level of frequencies. So, any kind of aging factor can be identified earlier and the corresponding action can be implemented.

As we know that “Transformers” plays a very crucial part in power, so, first of all, will discuss Tan Delta Testing in Transformers.

The below process explains the method of tan delta testing in a step-by-step manner

The requirements necessary for this test such as cable, potential transformer, bushings, current transformer, and winding on which this testing is conducted has to be initially separated from the system.

The minimal frequency level of test voltage is applied along with the equipment where the insulation to be analyzed.

At first, normal voltage levels are applied. When the tan delta values are as expected at this voltage level, then the applied voltage level is increased by 2 times as of applied voltage.

The values of the tan delta are recorded by the tan delta controller.

To the tan delta calculating component, a loss angle analyzer is connected which compares tan delta values at higher and general voltage levels and delivers accurate results.

It has to be noted that the testing procedure to be carried out at very minimal frequency levels.

It is more recommended to conduct testing at minimal frequency levels, because when the applied voltage level is more, then the capacitive reactance of the insulator device reaches very minimal, therefore the capacitive element of the current reaches more. As the resistive element is practically constant; it is based on the applied voltage level and the insulator’s conductivity value.

Whereas at increased frequency level the capacitive current, is more, and then the amplitude of the vector amount of both the capacitive and resistive elements of the current reaches very high. So, the necessary level of power for the tan delta test would become more that seems to be not acceptable. Because of this, the power constraint for dissipation factor analysis, very minimal frequency test voltage is required.

When it comes to the tan delta test, there are essentially three modes of power factor testing. Those are

GST Guard – This calculates the amount of current leakage to the ground. This method eliminates the current leakage through red or blue leads. Whereas in UST, the ground is termed to be guard because grounded edges are not calculated. When the UST method is applied to the device, then the current measurement is only through blue or red leads. The current flow through the ground lead gets automatically bypassed to the AC source and thus excluded from the calculation.

UST Mode – This is employed for the calculation of insulation in between ungrounded leads of the equipment. Here the individual portion of isolation has to be separated and analyze it having no other insulation connected to it.

GST Mode – In this final mode of operation, both the leakage pathways are calculated by the test apparatus. The current, capacitance values, UST, and GST guards, loss in watts need to be equal to the GST test parameters. This provides the entire behavior of the test.

When the summing value of GST Guard and UST is not equal to the GST parameters, then it can be known that there is some crashing in the test set, or might the test terminal are not correctly designed.

On the whole, this is a detailed explanation of the Tan Delta Test. Here, in this article, we are completely aware of what is a tan delta test, its principle, its purpose, its methods, and its testing technique.

We perform tan delta test to define the state of insulation of the asset or to find out how far we are from the insulation faults. Since insulation has a very non-linear characteristic, a very small increase in resistive current can suddenly make the insulation break. It is important to say that high value (if not too high) is not necessarily a sign that we need to change the asset. Often, we just need to follow the trend and operate if the change is too big.

IEEE defines electrical insulation as “material or a combination of suitable non-conducting materials that provide electrical isolation of two parts at different voltages”.

The most common technique used to assess the insulation of primary equipment like a transformer, rotating machine, cables, circuit breaker, is an AC test.

AC test is used to check the aging of the insulation and possible mechanical deformation. 

What kind of electrical insulation is used today?

Solid: Cellulose (Paper or Pressboard), cast resin, porcelain

Liquid: mineral oils, silicone oils, synthetic compounds

Gaseous: Sulphur Hexafluoride (SF6), air and vacuum, nitrogen

A pure insulator acts like a capacitor, when it is connected across line and earth. In an ideal insulation system, as the insulating material which acts as dielectric too, is 100 % pure, the electric current passing through the insulator, only have capacitive component. There is no resistive component of the current, flowing from line to earth through insulator as in ideal insulating material. In a pure capacitor, the capacitive electric current leads the applied voltage by 90o. Practically, the insulator cannot be manufactured 100% pure and also the dirt and moisture provide the conductive path to the current. Hence, an electric leakage current flowing from line to earth through the insulator has a resistive component.

A pure insulator acts like a, when it is connected across line and earth. In an ideal insulation system, as the insulating material which acts as dielectric too, is 100 % pure, thepassing through the, only have capacitive component. There is no resistive component of the current, flowing from line to earth through insulator as in ideal insulating material. In a pure capacitor, the capacitive electric current leads the appliedby 90. Practically, the insulator cannot be manufactured 100% pure and also the dirt and moisture provide the conductive path to the current. Hence, an electric leakage current flowing from line to earth through the insulator has a resistive component.

For good insulator, this resistive component of electric leakage current must be low. So, the health condition of an electrical insulator can be determined by the ratio of the resistive component to the capacitive component. This ratio is known as “tan delta”, sometimes also defined “dissipation factor”.

In the vector scheme: x-axis represents the voltage system and the resistive component of leakage current, IR.

As the capacitive component of leakage electric current IC leads system voltage by 90o, it will be drawn along the y-axis.

Total leakage electric current IL(Ic + IR) makes an angle δ with y-axis.

The ratio, IR to IC is nothing but tanδ or tan delta.

First, the normal voltage is applied. If the value of tan delta looks good enough, the voltage is raised to 1.5 to 2 times of normal voltage, of the asset. A that point, the testing unit takes measurement of tan delta values and compares the tan delta values at normal voltage and higher voltages and analyze the results.

During the test, it is essential to apply test voltage at a very low frequency, in order to keep the power requirement for this dissipation factor test. The frequency range for tan delta test is generally from 0.1 to 0.01 Hz depending upon size and nature of insulation.

There are two ways of reading the condition of an insulation system during a tan delta test.

The first one, is to compare the results of previous tests, to determine the deterioration of the insulation due to aging effects. While, the second one, is to determine the condition of insulation from the value of tan delta, directly. If the insulation is perfect, the loss factor will be approximately the same for all range of test voltages. But if the insulation is not enough, the value of tan delta increases in the higher range of test voltage.