QUESTION: For accurate radian readings, does each phase have to be individually shielded? Or, will TD work for 5kv polyethylene insulation not individually shielded but having all 3 poly insulated phases first covered by a molded rubber jacket, then shielded by a galvanized armor jacket casing the whole cable?
Let’s break it down to the basics. If the insulation of a cable is free from defects, like water trees, electrical trees, moisture and air pockets … etc., the cable approaches the properties of a perfect capacitor. It is similar to a parallel plate capacitor with the conductor and neutral being two plates separated by the insulation material.
In a perfect capacitor, the voltage and current are phase shifted 90 degrees and the current through the insulation is capacitive. If there are impurities in the insulation the resistance of the insulation decreases, resulting in an increase in resistive current through the insulation. The current and voltage will no longer be shifted 90 degrees. It will be something less than 90 degrees. The extent to which the phase shift is less than 90 degrees is indicative of the level of insulation contamination.
After reviewing your 5 kV cable issue, my suggestion would be to tie all three phases together thus forming one single conductor. Now we are back to the simple model of a conductor (the three tied conductors) and a neutral (the armor jacket) being two plates separated by the insulation material.
Unfortunately, there is not an extensive library of test values for all types of cables and accessories. Testing is typically done on a comparative basis. Keep in mind that the purpose of the test is to grade cables tested on a scale from high quality to low. The point in the testing is to help an asset owner prioritize cable replacement or injection … comparative testing will show which cables are worse than others.
While it is beneficial to have previous tests to compare to for trending purpose, it is not necessary. The very first test on your cable will render valuable information. If the cable’s insulation is perfect, the tan delta will change little as the applied voltage is increased. The capacitance and loss will be similar with 1 kV or 10 kV applied to the cable. If there is contamination changing the capacitive/resistive nature of the insulation, then the tan delta will be higher at higher test voltages.