Increasing Confidence in MV Cable Diagnostics

Cables & Accessories

Cable diagnostics has proven the only way to fully understand the condition of an underground medium voltage cable network. For example, application of VLF (Very Low Frequency) ramp-up diagnostics can be used for cable condition assessment. Voltage levels starting from 0.5 x Uo to maximum 1.7 x Uo (Uo being defined as phase-to-ground voltage) are sufficient for comprehensive condition assessment while avoiding risk of insulation overstress during testing.

Fig. 1: Application of VLF TD//PD and VLF FULL MWT.

Cable diagnostics with VLF TanDelta (VLF TD) and VLF Partial Discharge (VLF PD) can be conducted at the same time in a single sequence (VLF TD//PD parallel). For maintenance tests, such as often conducted after repairs and sectional cable replacements, ramp up application is followed by a 2.0 x Uo Monitored Withstand Test. For new installations, a commissioning test (also called an acceptance test) is conducted.

Fig. 2: Sequence of parallel VLF TD and VLF PD diagnostics.

To verify quality of newly laid cables, including newly installed accessories such as joints and terminations, a ramp up diagnostic sequence is followed by a 3.0 x Uo Monitored Withstand Test (MWT). It is recommended that the MWT is covering the TD loss factor measurement as well as the PD Partial Discharge measurement during the testing period (FULL MWT). Depending on cable condition, test duration can be optimized.

Fig. 3: VLF Full monitored withstand test FULL MWT.

VLF testing and diagnostics for MV cable application is defined and recommended by IEC 60502-2 – 2014 and IEEE 400.2-2013 (IEEE400.2-2013, 2013). Test voltages and the applicable diagnostic methods of VLF TanDelta Loss factor measurement and PD Partial Discharge measurement are mentioned. IEEE 400.2-2013 is the only existing field guide that describes evaluation criteria for VLF TanDelta diagnostics for different cable types.

Plan to attend the 2022 INMR WORLD CONGRESS in Berlin, where cable testing expert, from Baur in Austria, will explain how residual lifetime estimation based on the latest software allows investment planning by asset managers. His lecture will review how quality of newly laid cables, including newly installed accessories such as joints and terminations, can be verified using a ramp up diagnostic sequence followed by a monitored withstand test covering measurement of TD loss factor and PD.