Every Power electrical appliances in use, undergo stress from Operating Voltage, Mechanical Vibration, Temperature, Gaseous and Solid Metallic Impurities. Under these stress, the degradation of the insulation takes place, in the Electrical equipment. This stress lead to the continuous ageing of the appliance, with regard to its Electrical Properties. Sometimes there will be an avalanche of Insulation breakdown. In the total complex network of Power Generation, Transmission, Distribution System ageing of a simple insulation may lead to a disastrous breakdown of the system, causing heavy loss. To avoid such unexpected breakdowns and for the un-interrupted service, from electrical power it is very essential to know the insulation quality of the equipment that is going to be used in the power system. And it is also essential to periodically monitor the Insulation property of equipment, that in use in this Power Network.

In Electrical AC System the Dissipation Factor Tanδ or Power Factor Cosø is considered as the indicate of quality of insulation. Hence it is very essential to measure the tanδ of dielectric material of the equipment precisely. The factory “pre-despach measurement results” are considered to be the most important as these are taken as the “reference” while determining ‘Aging Effect’. A very important factor is that, the instrument that is used for measuring the “reference” shall be precise and producing the repeatable results for many years.

Hence the tanδ of dielectric materials of High Voltage Electrical Appliances like Power Transformers, Distribution Transformers, High Voltage Bushings, Transformers, Instrument Transformers, Power Capacitors, Power cables, Oil etc is measured precisely using highly reliable methods and instruments.

The data on Dielectric Properties of Insulating material used in High Voltage Electrical Appliance gives us an idea of the Insulation Strength of the Appliances. Continuous “degradation”, “ageing” due to above mentioned stress will change these data continuously. In practice, to judge the degradation of the system in operation, the periodically recorded data are compared with a reference value, or with the value obtained by studies.

Dissipation Factor Tan Delta is the ratio of useless current in the Electrical system to a useful current in it. That means this is the ratio of Wattloss current to the Reactive current in it.
i.e tanδ = IR/ Ic
For the voltage applied V
tanδ = Watt-loss (Active) Power/Reactive Power.

Hence tan delta is considered as the measure of quality of insulation in dielectric material.
In some countries Cos ø is measured to know the Insulation Quality.

Watt loss in Power System
It is required to monitor the watt loss through the insulation of Power System, A Generator, Transformer, Cable even a Power Capacitor used in the Electric circuit of Generation, Transmission and Distribution shall have very low power loss.

Eg1. The Capacitors, that to be used for Power Factor Correction, or a Power Cable that is used in Electric Power Circuit are very essential to have very low wattloss.
i.e. 0.2/ kVAR for Power Capacitors and tan delta of 0.0005 for Power Cables. The Dissipation Factor tanδ is the direct measure of watt loss (VAcosΦ) in the total reactance of the Power Capacitor. i.e. The nominal tan delta value of 0.0002 shall be kept for Power Capacitor under use.

To ensure the correct wattloss /KVAR, the tan delta shall be measured with high precision i.e with the accuracy 0.00005 or better and this measuring system shall be producing repeatable results for many years.

Eg2. All over the world for years together the data on Dielectric Material have been recorded and monitored. Normally the Insulating Oil contributes major part as the dielectric material, in a Electrical Power Appliance. Hence it is very essential to precisely know the different Electrical and Chemical properties of Insulating Oil.

To know the electrical properties of the Insulating Oil, the tan delta test and to know the chemical impurities the Dissolved Gas Analysis (DGA) of the oil is conducted. These two tests are considers as is the perfect yard stick for Failure Analysis and also for ‘Aging Effect’ of the Power Equipment under use.

For fresh insulating Oil the Dissipation Factor is noted using very precise instrument. In this case the Dissipation Factor is as less as 0.0001 and this measurement shall be with accuracy of ± 0.00005 (0.005%) of reading (as per IEC 250).