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.

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

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

Checking the Polarity of Current Transformers

The Polarity of current transformers is extremely important. Just like a battery, a current transformer too has a polarity. The polarity determines the direction of the secondary current in relation to the primary current.

Wrong connection of the current transformers can cause false operation of the protection relays. Hence, it is vital to ensure that the current transformers are connected with the correct polarity.

The figure shows a setup to test the polarity of a current transformers.

A DC source is connected with the positive terminal to P1 and the negative terminal to P2. An analog voltmeter is connected to the secondary terminal of the CT. The positive terminal of the meter is connected to terminal S1 of the CT while the negative is connected to terminal S2.

A contact is momentarily made through the switch. The contact is made for a second and broken. This is important as continuous contact can short-circuit the battery. The momentary make-break contact causes a deflection in the analog multimeter in the positive direction, if the polarity is correct.

If the deflection is negative, it indicates that the polarity of the current transformer is reversed. The terminals S1 and S2 need to be reversed and the test can be carried out.

Reverse Power Relay

The Reverse power relay is used to protect a synchronous generator, running in parallel, from motoring. Motoring occurs due to the failure of the prime mover such as a turbine or an engine driving a generator that is connected to the grid. The generator which is running at the synchronous speed will continue to run at the same speed. However, the power required to keep the generator running along with the prime mover will be drawn from the mains. Hence, power flows in the reverse direction i.e. bus to generator. This condition is called reverse power.

Reverse power operation may cause damage to the prime mover. Hence, reverse power protection is a vital part of the generator protection scheme.

The reverse power relay operates by measuring the active component of the load current, I x cos φ. When the generator is supplying power, the I x cos φ is positive, in a reverse power situation it turns negative. If the negative value exceeds the set point of the relay, the relay trips the generator breaker after the preset time delay.

The typical setting for reverse power is 4% in case of turbines and 8% in case of diesel engines. The time delay can be set from 2 to 20 seconds.

The Vector Surge Relay

The vector surge relay is used to decouple synchronous generators from the grid utility in case of grid failure.

Synchronous generators are generally operated in parallel with the grid utility. This ensures greater reliability and enables the generator to export power to the grid. In this condition, there is a chance, of a momentary interruption of the grid supply which may result for a few milliseconds. Such temporary interruptions can be caused to mal-operation of the circuit breakers on the grid transformer side.

For a synchronous generator, running in parallel with the grid utility, such a temporary interruption and restoration of the supply can be dangerous. As the restoration of the supply can be asynchronous i.e. the generator and the grid are now not in a synchronised condition. The can lead to the consequences of wrong synchronization such as damage to the generator or the prime mover.

The vector surge relay prevents this condition by decoupling the generator from the grid as soon as the grid supply fails. This is an extremely fast acting relay with an operating time of less than 300ms from relay operation to breaker opening.

Principle:

The vector surge relay functions by monitoring the rate of change of the rotor displacement angle of the generator. During parallel operation there is an angular difference between the terminal phase voltage (Up) and the internal synchronous voltage of the generator (Ui). This is due to the fact that the generator rotor is magnetically coupled to the generator stator and is forced to rotate at the grid frequency. The angle between the vector of the mains voltage Up and synchronous electro-motive force is known as the rotor displacement angle.

This angle is constantly varying and is dependent on the torque produced by the generator rotor. In the case of the grid failure, there is sudden change in the rotor displacement angle.

This causes a surge in the generator voltage shown in the figure. The relay works by monitoring the time taken between the zero-crossings in the waveform. Under normal operation, the time interval between two consecutive zero-crossings is almost constant. During the grid failure, the vector surge which occurs causes a delay in the zero-crossing. This delay is detected by a highly sensitive timer inside the relay and the relay operates.

The relays are usually set to operate for a change in the rotor displacement angle of 0 to 20 degrees