DIAGNOSTIC TESTING METHODS:-

For Power cables there are two (2) Diagnostic Testing Methods are Available.

1- PD = Partial Discharge2- TD = Tan Delta

Partial Discharge Activity due to the badly Terminated or jointed Cables, surface damage to the Semi-conductive tape due impurities or cavities in the Dielectric.

Dielectric Breakdown:-

XLPE and PILC Dielectric breakdown and Dielectric deterioration is generally caused by,

- Thermal breakdown is caused by the overloading or from the cables which are packed very close to each other that are unable to dissipate heat (I²R) and dielectric losses.

- Electrical Stress caused by space charges or Stress point.- Moister Ingress- Water trees in XLPE cables- Poor Earthing

TAN DELTA TEST:-

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 leads 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-dispatch 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 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.

WHAT IS “DISSIPATION FACTOR”?

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.

It is a diagnostic method of testing cables to determine the quality of the power cables insulation. This done in order to find the life expectancy and priotrize the cables ie Good, bad or dangerous (To replace the cable) etc.

In theory the Tanδ should remain constant at a fixed frequency as the voltage increases. And also the Tan Delta is independent of the length of the cable.

NOTE:- in DC Hipot testing (Sheath testing) the leakage current is dependent on the cable length and increases proportionally with the length of the cable where TD is a ratio and remain fixed and known whether the cable is XLPE or PILC.

When we apply voltage to the cable for finding the loss angle the acts as a capacitor the Ic current leads the voltage by 90⁰. So by measuring the zero crossing of the voltage and current waveform the angle delta can be determined.