Engineer's Notes: HV testing and commissioning
Test to prove, not to tick boxes
Commissioning tests prove that equipment arrived undamaged, was installed correctly and is safe to energise. Each result needs a clear acceptance criterion written in the ITP before the test, the instrument serial and calibration date, the temperature, and the names of the tester and witness.
Cables: VLF has replaced DC
For XLPE cables, a VLF withstand at around 3 × U0 for 15 minutes is the usual after-installation test. VLF tan delta adds a health check: low, stable values with little tip-up mean a good cable. A sheath test confirms the outer jacket survived pulling and backfill.
Transformers: ratio, group and resistance
The turns ratio test on every tap, a vector group check, and winding resistance per phase catch most factory, transport and tap-changer faults. Oil breakdown voltage confirms the oil is dry. Compare everything with the factory test report.
Protection: test the whole chain
Secondary injection proves the relay settings and curves. A trip test proves the breaker actually opens. CT polarity, ratio and knee point make sure the relay sees the right current during a fault.
Frequently asked questions
What tests are done on a new 11 kV cable before energising?
Typical after-installation tests are insulation resistance of each core, an outer sheath test, a withstand test (usually VLF at about 3 x U0 for 15 minutes, or as the project and utility specify), and on important circuits VLF tan delta and partial discharge diagnostics. Phasing and screen continuity are also checked.
Why is DC hipot no longer used on XLPE cables?
High-voltage DC testing can leave space charges trapped in XLPE insulation, which can lead to failures after the cable goes back into service, and it is poor at finding defects that matter under AC. VLF (very low frequency, usually 0.1 Hz) AC testing is now the normal choice.
How is VLF tan delta assessed?
IEEE 400.2 uses three numbers for PE and XLPE cables: the stability of tan delta at U0 (standard deviation), the tip-up between 1.5 U0 and 0.5 U0, and the mean tan delta at U0. Each is graded as no action, further study, or action required, and the worst grade decides the condition.
What is the tolerance on a transformer turns ratio test?
IEC 60076-1 allows the lower of plus or minus 0.5 % of the declared ratio, or one tenth of the short-circuit impedance in percent, on the principal tapping. Each tap is checked on every phase.
What is the knee point of a CT?
The knee point is the voltage on the excitation curve where a 10 % increase in voltage causes a 50 % increase in exciting current (IEC 61869-2). Above it the CT saturates, so protection CTs need a knee point comfortably above the voltage needed during a fault.
Topics: ITP · insulation resistance · polarization index · sheath test · VLF withstand · VLF tan delta IEEE 400.2 · TDR · partial discharge · TTR · vector group test · winding resistance · magnetising current · oil BDV IEC 60156 · contact resistance · CB timing · CT knee point · secondary injection · energisation
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