By Anzar Hasan
There have been some major B.I losses involving electrical power cables both overhead and underground (buried and that run in conduits). The testing and maintenance of cables is often not addressed during risk assessment surveys. Due to limitation of space for this article, the contents focus on non-destructive electrical field-testing of cables rather than types and specifications of different type of cables.
The field-testing of cables is important to determine the operating condition of the cable.
The field testing can be divided into two broad categories: Destructive and Non-Destructive. Both of these categories of tests can be conducted as part of on-reel testing, installation testing, acceptance testing and/or maintenance testing. Most owners would not consider the destructive type testing for cables in service.
There are various procedures for field testing the cables utilizing the non destructive method of testing depending on type of cables, run length of cables, voltage, current, age, etc. and what information is to be required for instant analysis or historical data. Like any other electrical testing procedures, based on the trending of results the interval of testing should be established that could range from minimum of annual to maximum of every three years. Below are listed some commonly employed methods of cable testing with brief description on the method of testing.
1. Partial Discharge Testing (PD): The PD test is performed on the cable insulation at voltages below operating voltage, operating voltage, one and one-half times the cable rated voltage, and two times the cable-rated voltage using an alternating voltage (ac) source. Partial discharges are small electric sparks or discharges in the order of Pico-coulombs that occur in the cable insulation, splices, terminations or on the surfaces of terminations. The discharges do not completely bridge the insulation, but emit measurable pulses of various frequencies from about 100 kHz to 400 kHz.
This test detects the location, severity and repetition rate of the PD that occurs at that test voltage. Based on the results of, the electrical integrity of the cables can be established for timely replacement if required.
2. Power factor/Dissipation Factor/Tan Delta Testing (Tan δ): The Power Factor or Tan Delta Test is performed on the cable insulation at a voltage about one-fourth of rated cable voltage, operating voltage, and about one and one-half times the rated cable voltage. Power factor equals watts divided by volt-amperes (IR/IT) and tan delta equals watts divided by reactive power (IR/IC). Good insulation has a low power factor or tan delta and very little increase of power factor or tan delta with increased voltage applied. This test does not locate a bad spot, but it does measure the overall quality of the entire cable insulation.
3. Time Domain Reflectometry Testing (TDR) testing: This test by itself does not evaluate the insulation of shielded power cables. When low voltage pulses are used (≤600V), the test can be considered a non-destructive diagnostic test.
The test uses pulse reflection to measure the distance to changes of characteristic impedance in the cable. In theory, a completely uniform cable that is properly terminated will exhibit no characteristic.
The TDR sends a low-voltage pulse in the order of a few nanoseconds duration between the cable conductor and neutral down the cable, which reflects back from the other end or any open or partially open location in the cable or neutral. Increases of impedance between the cable conductors cause a positive reflection and decreases of impedance cause a negative reflection. Splices cause a positive and a negative reflection, because as a pulse enters a splice, the conductors separate causing an increase of impedance and as the pulse leaves the splice the conductors become closer together again causing a decrease of impedance. Broken strands cause an increase of impedance and a positive reflection. The more broken strands that exist at a point, the larger the reflection becomes.
The height of the anomaly determines the severity of the neutral damage. If less than 25% of the strands are broken, the anomaly is too small to be recognized. If 25% to 50% of the strands are broken, the anomaly is smaller than a splice. If 50% to 75% of the strands are broken, the anomaly is larger than a splice but is smaller than the cable end reflection. If 75% to 100% of the strands are broken, the anomaly is larger than the cable end reflection. The TDR calculates the distances from the end where the test is being applied to each anomaly and to the cable end.
4. Insulation Resistance Test (Megger testing): The megger test determines the total insulation resistance of each cable. The resistance measurement is used to determine if the circuit will operate without excessive leakage current through the insulation when energized. Measured values can be impacted by certain external factors (temperature, moisture, etc.), which may result in questionable readings, even when evaluated on a satisfactory length of cable.
The megger test is performed by applying an elevated DC voltage to the conductor and measuring the current flow to a ground reference. With the known voltage and measured current, an insulation resistance value can be calculated.
This is a time‑resistance method of testing the insulation resistance and voltage is applied to the equipment under test over a period of time, usually ten minutes. The ratio of the ten-minute reading to the one-minute reading is called the polarization index (P.I), and is used as a guide to the condition of insulation.
It is important to recognize that megger testing non-shielded cables may produce marginal results due to the inherent lack of a completely encompassing and uniform ground plane over the dielectric of the cable.
Note: If anyone requires reference standards or further details on testing of cables, please drop me an email at email@example.com.