## Tan Delta testing - Principle and Method

Tan Delta is a a diagnostic test conducted on the insulation of cables and windings. It is used to measure the deterioration in the cable. It also gives an idea of the aging process in the cable and enables us to predict the remaining life of the cable. It is alternatively known as the loss angle test or the dissipation factor test.

Principle

The Tan Delta test works on the principle that any insulation in its pure state acts as a capacitor. The test involves applying a very low frequency AC voltage. The voltage is generally double the rated voltage of the cable or winding.

A low frequency causes a higher value of capacitive reactance which leads to lesser power requirement during the test. Besides, the currents will be limited enabling easier measurement.

In a pure capacitor, the current is ahead of the voltage by 90 degrees. The insulation, in a pure condition, will behave similarly. However, if the insulation has deteriorated due to the entry of dirt and moisture. The current which flows through the insulation will also have a resistive component.

This will cause the angle of the current to be less than 90 degrees. This difference in the angle is known as the loss angle. The tangent of the angle which is Ir/Ic (opposite/adjacent) gives us an indication of the condition of the insulation. A higher value for the loss angle indicates a high degree of contamination of the insulation.

Method of Testing

The cable or winding whose insulation is to be tested is first disconnected and isolated. The test voltage is applied from the Very Low Frequency power source and the Tan delta controller takes the measurements. The test voltage is increased in steps upto the rated voltage of the cable. The readings are plotted in a graph against the applied voltage and the trend is studied. A healthy insulation would produce a straight line.

The test should be continued only if the graph is a straight line. A rising trend would indicate weak insulation which may fail if the test voltage is increased beyond the rated voltage of the cable.

Interpretation of the test data

There are not standard formulae or benchmarks to ascertain the success of a tan delta test. The health of the insulation which is measured is obtained by observing the nature of the trend which is plotted. A steady, straight trend would indicate a healthy insulation, while a rising trend would indicate an insulation that has been contaminated with water and other impurities.

Insulation Resistance

Polarization Index

Earth Resistance Measurement

## Earth Resistance Measurement

Measurement of Earth Resistance is a vital part of the maintenance of any electric installation. The function of a sound earthing system is to ensure that all electric equipment are connected to the ground potential. Hence, a well-maintained earthing system ensures the proper functioning of protection systems, absorbs electrical noise and provides safety to operating personnel. The earth resistance is measured using an earth meggar.

“Fall of Potential” Method:
The Earth resistance is measured using the “Fall of Potential” Method. The method works by injecting a constant current between two spikes which are inserted into the ground and measuring the voltage at points between them (as shown in the figure)

The “Fall of Potential” Method is a three terminal test. The electrode whose earth resistance is to be measured is disconnected from the system or earthing grid. The earth meggar has a current terminal, a voltage terminal and a common terminal. The common terminal is connected to the electrode,

Equal lines of potential

When an electrode is inserted into the ground and current flows through it, the potential around the electrode takes the form of concentric circles of equal potential.

It is essential that the equal lines of the common terminal and the current terminal do not overlap. Therefore distance between the ground electrode to be tested and the current terminal is vital. The distance should be sufficient so that the equal lines of potential of the common terminal and the current terminal do not overlap.

Method of Measurement:
The readings are taken at points close to the ground electrode and then gradually away from it. The resistance readings in ohms are plotted against the distance in a graph. The graph should take the form as below. At around 62% of the distance between the ground electrode and the current terminal, the graph levels off. This reading is taken as the value of the earth resistance. This point should be outside the equipotential zones of both the current terminal and the ground electrode

Insulation Resistance

Tan Delta Testing

Polarization Index

## Insulation Resistance Measurement

Insulation Resistance Measurement is an important check in the maintenance of electrical equipment such as motors, transformers. It is estimated that nearly 80% of all maintenance activities in the industry is related to checking the insulation of machines. It is therefore vital that the engineer has a fair idea of the principle behind the measurement of Insulation Resistance and the methods used. Insulation resistance is measured using a meggar.

In the normal operation of machinery, the insulation is subjected to moisture, oil, dust, electrostatic stress due to machine operation and a host of other elements. Hence, insulation ages and deteriorates. It is vital that the health of the insulation be monitored continually to avoid sudden, catastrophic failure of machines.

Principle of Insulation Resistance Measurement

The method used to measure insulation resistance is based on Ohm’s law. A high voltage is applied across the resistance; the current that flows through the insulation is measured. The ratio of voltage and current gives the resistance. The value of the insulation resistance is usually in the order of mega ohms

Instruments used in Measurement

The instrument used to measure Insulation Resistance is known as the Megger. It is similar in principle to the ohmmeter except for the fact that a higher voltage is used. The typical meggars have a test voltage of 500V, 2500V or 5000V. The Meggar has a high internal resistance hence, there it is safe to use despite the high voltage generated. The meggar has 3 terminals. Line, Earth and Guard.

The test voltage appears on the “Line” Terminal. This terminal is connected to the winding whose insulation needs to be checked. The “Earth” Terminal is connected to the ground. The “Guard” Terminal is connected to the surface of the insulation to measure the surface currents which tends to flow along the surface of the insulation.

Method of Insulation Resistance Measurement

The winding to be tested should first be isolated. The other windings of the machine which are not being tested should be connected to the ground. The voltage is applied to the winding and the reading is taken after about 60 seconds. The reading is noted. After the test is over, the winding needs to be “discharged”. This is because the insulation acts as a dielectric forming a capacitor between the winding and the earth. This can store charge and can deliver a shock if not discharged. Discharging can be done by connecting to the ground.

What should be the value of the Insulation Resistance?

The Insulation Resistance thus measured is usually in the order of mega ohms. A general rule of the thumb is that the minimum value should be greater than 1 mega ohm for every 1kV rating of the machine. Thus, for a machine rated for 11kV, the minimum acceptable value would be 11 mega ohms. Temperature has a direct impact on the value of the Insulation Resistance. The Insulation Resistance decreases with increase in temperature. Thus the values should be normalized for a standard temperature.

That is, a value measured at 20 deg. C cannot be compared with a value measured at 30 deg. C. The value at 30 deg. C needs to be corrected. A general rule of thumb is that the insulation resistance decreases by a factor of 2 for every 10 degree rise in temperature.

Hence, the value taken at 30 deg. C needs to be multiplied by a 2 to get a value corrected to 20 degrees.

How do we ensure a good value of IR?

The Insulation Resistance of a machine depends chiefly on the dryness of the windings. The entry of moisture into the windings lowers the Insulation Resistance. The ingress of moisture can be prevented by ensuring that the windings are kept dry. Special heaters known as anti-condensation heaters are provided in machines to keep them dry. It must be ensured that these heaters are kept on.

How do we improve the Insulation Resistance value?

If machines are found with low Insulation Resistance values below the permissible limits, heating the windings by connecting lamps around them is an effective method of driving moisture from the windings. If no improvement is seen even after heating, other reasons such as insulation wear or deterioration can be suspected.

Other parameters related to the health of the Insulation are the Polarization Index(PI), tan delta, hipot test, step test, etc

Tan Delta Testing

Polarization Index

Earth Resistance Measurement

## Core Balance Current Transformers

The CBCT or the Core Balance Current Transformer is a current transformer is used for earth fault protection in grounded three phase systems. It is also known as the zero-sequence current transformer. The CBCT is a current transformer through all the three phases are made to pass as in the diagram. Thus the magnetic fluxes caused by the three phase currents cancel each other.

The net resultant flux being zero does not induce any current in the secondary of the transformer. Thus the secondary current of the core balance current transformerwhen all the three phases are healthy is zero.

When an earth fault occurs in one of the phases, the zero-sequence fault current which flows is not cancelled by the flux of the other two phases and hence induces a current in the secondary.

The core balance current transformer can be connected to an earth fault relay which can be used to generate the tripping signal.

## Polarization Index

Polarization Index is an indicator which gives an idea of the cleanliness of the windings.

It is a ratio of the Insulation Resistance Measured for 10 minutes to the insulation resistance value measured after 1 minute. Since it is a ratio; it does not have any units.

The Polarization Index should be above 2.0 to be permissible. Machines having PI below 2.0 cannot be operated.

The Polarization Index test works on the principle that impurities in a winding act as charge carriers and are responsible for the leakage current which flows when the insulation is tested.  These impurities can be polarized over a period of time.  By measuring the rate of polarization, we can determine the amount of impurities in the winding and the cleanliness of the winding.

The Polarization Index does not have any significant relation with temperature upto 50 deg. C. However, the Polarization Index test should not be conducted at a temperature beyond 50 deg. C