MV-509 · Phase 3 Site, T&C and operations

MV Testing & Commissioning Lab

Pick a test, choose the condition of the equipment, and run it. Readings build up live like the real instrument, and the lab interprets them the way a commissioning engineer would. Then work through the commissioning checklist, which saves in your browser.

In short

Insulation resistance and the polarisation index (PI = R_{10 min} / R_{1 min}) show moisture and contamination. A PI below 1 is a warning sign, and above 2 is generally good.
VLF tan delta at 0.5, 1.0 and 1.5 U₀ grades XLPE cable condition. IEEE 400.2 looks at the mean value, stability and tip-up.
A turns ratio test should match the nameplate within about 0.5% on every tap. Winding resistance phases should match each other within about 2%.
Secondary injection proves relay pickup and trip times against the setting curve before the relay is trusted with a live circuit.
Test results are only valid with calibrated instruments, the approved procedure and the utility witness where required.

By Eng. Mannan Maqsood, Electrical EngineerPart of the MV Hub

Virtual Test Bench 2D Animated

Choose the test and the equipment condition, then press Run. In Practitioner and Expert mode you can type in your own site readings instead of simulating.

Interpretation

Commissioning ITP Checklist Saved

A typical inspection and test plan for an 11 kV consumer substation. Ticks are saved in this browser. Use Copy to paste the status into a report.

What tests are done on an MV installation before energising?

Visual and mechanical: nameplates against the approved submittals, torque marks, clearances, earthing connections, interlocks, gas pressure gauges.
Cables: insulation resistance of the core and the oversheath, a withstand test (often VLF at about 3 U₀, or as the utility specifies), optional tan delta and partial discharge diagnostics, and phasing end to end.
Transformers: insulation resistance, turns ratio on every tap, winding resistance, vector group check, oil tests for oil-filled units, and protection devices (temperature, gas or pressure relays).
Switchgear: insulation resistance, contact resistance, timing on breakers, interlock function, VPIS checks.
Protection: CT ratio, polarity and magnetisation checks, secondary injection of every relay, and trip tests to the breaker.
Earthing: earth resistance and continuity, and for larger sites, touch and step measurements.

PractitionerRecord the instrument serial and calibration date, temperature and humidity, and the test voltage and duration for every result. Insulation resistance changes a lot with temperature, so correct readings to a reference temperature (usually 20 °C) before comparing them.

How do you read VLF tan delta results?

Tan delta measures the losses in the insulation. Healthy XLPE has very low, stable tan delta that barely rises with voltage. Water trees make tan delta rise with voltage (high tip-up). Contaminated or wet joints and terminations make readings unstable. IEEE 400.2 combines the mean at U₀, the standard deviation (stability) and the difference between 1.5 U₀ and 0.5 U₀ (tip-up) into three classes: no action, further study, and action required.

ExpertTan delta is an average over the whole cable length. A single bad joint on a long route can be hidden by many metres of healthy cable, so add partial discharge testing with location where joints are a concern.

Regional notes

GCC

Utilities usually witness MV cable tests and pre-energisation checks and have their own test formats and voltages. High site temperatures affect insulation readings, so record the temperature and correct.

Australia and New Zealand

Network operators specify cable test methods and voltages, with VLF and tan delta widely used. AS/NZS 3017 and AS/NZS 3019 cover verification of LV installations.

Asia

Utilities such as TNB and SP Group publish test and commissioning requirements and witness the tests before energisation.

Test yourself

0 / 8

8 questions. Pick an answer to see the explanation. Practise about 50 more MV-509 questions in the MV Question Bank.

Frequently asked questions

What is a good polarisation index?

As a general guide, PI below 1.0 is dangerous, 1.0 to 2.0 is questionable, 2.0 to 4.0 is good and above 4.0 is very good. Modern XLPE cables can show low PI with very high IR values, so look at both.

What VLF test voltage is used for 11 kV cables?

Many specifications use about 3 U0 at 0.1 Hz for 15 to 60 minutes for new cables, which is around 19 kV rms for 6.35/11 kV cables. Always use the voltage and duration in the utility or project specification.

What are the IEEE 400.2 tan delta criteria for XLPE?

For PE-based insulation, IEEE 400.2-2013 classes results as no action needed when the stability is below 0.1 x 10^-3, the tip-up below 5 x 10^-3 and the mean at U0 below 4 x 10^-3. Higher values move to further study or action required. Check the current edition.

What does a turns ratio test find?

Shorted turns, open windings, wrong tap connections and wrong vector group. The measured ratio should be within about 0.5% of the nameplate on every tap.

Related MEPVirtual labs

MV-502 MV Cables & Accessories Lab MV-504 Distribution Transformer Lab MV-507 MV Protection & Relay Coordination E-108 Meter Lab

Standards referenced

IEEE 400.2 VLF field testing of cables
IEC 60502-2 Tests after installation
IEC 60076-1 Transformer routine tests
IEC 60255-151 Overcurrent relays
ANSI/NETA ATS Acceptance testing