MV Cable Testing in Solar PV Plants: VLF, Tan Delta, and PD

AQ Electric tests and commissions solar plants across the MENA region, and on every utility-scale project the same cables carry the whole output of the plant: the medium-voltage collection cables that link the inverter stations to the substation. A failed string cable costs a few kilowatts. A failed MV feeder can take a whole section of the plant offline for days or weeks while the fault is located, excavated, and repaired. That is why MV cable testing is one of the most important — and most often under-specified — parts of solar commissioning. This article explains how MV cables in solar PV plants are tested, what each test can and cannot find, which standards apply, and how to build a test programme that catches defects before energisation and tracks cable health in operation.

What Is MV Cable Testing?

MV cable testing is the set of electrical tests used to verify the insulation and the integrity of medium-voltage cable systems — the cable itself together with its joints and terminations. In a solar PV plant, the MV network typically collects power from the inverter stations, where transformers step the inverter output up to medium voltage, and carries it to the plant substation for export.

MV cable testing is carried out at three points in a cable’s life. The IEEE 400 family of guides defines them clearly:

• Installation test — after the cable is laid but before joints and terminations are installed, to detect damage from manufacturing, transport, storage, or pulling.
• Acceptance test — after joints and terminations are installed but before energisation, to prove the complete cable system.
• Maintenance test — on cables in service, to assess condition and plan repairs before failure.

For a new solar plant, the acceptance test is the one that matters most, because it is the last chance to find workmanship defects in joints and terminations before the plant’s full output depends on them.

Why MV Cables Are a Single Point of Failure

On the DC side of a solar plant, thousands of strings work in parallel, and the loss of one string is small. The MV network works the other way: each feeder carries the output of several inverter stations, so a single cable or termination fault can disconnect a large share of the plant at once.

MV cable faults are also slow to repair. The cables are usually buried, often over long distances, and many solar plants are remote. Locating the fault, excavating, obtaining the right joint kit, making the repair, and re-testing all take time. Every day of that outage is lost generation. In one case published by Megger, a major Indian utility commissioned a condition assessment of the MV cables at two 50 MW solar plants in Karnataka after a simple cable failure had caused a long outage and significant financial losses.

MV cable testing is therefore not a box-ticking exercise. It is insurance against the most disruptive single failure a solar plant can suffer outside its substation.

Where MV Cable Systems Fail

Modern XLPE cables are manufactured and routine-tested under controlled factory conditions. The weak points are usually created in the field.

Joints and Terminations

Joints and terminations are assembled by hand on site, often in heat, dust, and wind, and every step — cutting back the semiconducting screen, positioning stress control, shrinking or fitting the components — must be done correctly. A case reported by Thorne & Derrick described MV heat-shrink terminations at a new generation facility that kept failing after energisation, even though the cable systems had passed a 0.1 Hz VLF test. The root causes included insufficient shrinkage, poorly positioned stress control relative to the screen cutback, and small voids at the insulation interface.

Connectors and Lugs

Field experience from US solar farms published by the International Association of Electrical Inspectors describes MV connector failures caused by loose terminations and mismatched aluminium and copper parts, and a 35 kV lug one size too large for the aluminium conductor it was installed on. These defects are not always visible in an insulation test; they are found through inspection, torque verification, and thermography under load.

Installation Damage and Backfill

Cables can be damaged during pulling, by sharp objects in the trench, or by poor bedding and backfill. The same IAEI field lessons describe a quarter-mile MV cable trench backfilled without special inspection of compaction, which requires verifying at least 90% dry density, and warn that any void around the cable can become a hot spot and an eventual failure.

Oversheath Damage and Water Ingress

Damage to the outer sheath may not affect the insulation immediately, but it allows moisture to reach the metallic screen and, over time, the insulation. Sheath damage is cheap to repair before energisation and expensive to find later.

The MV Cable Tests at a Glance

No single test finds every defect. A good MV cable testing programme combines tests that look at different properties of the cable system.

Insulation Resistance Testing

Insulation resistance testing applies a DC voltage between each conductor and earth (and between phases) and measures the resulting resistance, usually in gigaohms for a healthy MV cable. It is quick, low risk, and the natural first step in MV cable testing, because it reveals gross faults — a damaged section, a wet termination, a missed earth — before higher-stress tests are applied.

Time-based ratios such as the polarisation index (PI) and the dielectric absorption ratio (DAR) add information by showing how the resistance develops over the test period. Results are compared between phases and between similar circuits: a phase that reads markedly lower than its neighbours deserves investigation even if it clears an absolute minimum. In the Megger case from Karnataka, insulation resistance and DAR differed between phases to earth on the circuit that later showed abnormal tan delta results.

Insulation resistance testing is a screening tool, not a proof of quality. A cable can show excellent insulation resistance and still contain a defect in a termination that will fail under AC stress. That is why it is followed by AC-based tests.

Oversheath Integrity Test

The oversheath test checks that the outer protective sheath of the cable was not damaged during laying. Under IEC 60229, a DC voltage of 4 kV per millimetre of specified sheath thickness, up to a maximum of 10 kV, is applied for one minute between the metallic screen or sheath and the surrounding earth, to demonstrate the integrity of the oversheath after laying.

The test applies only to cables with an oversheath designed and tested for this purpose, so the cable specification should be checked first. On long collection runs, it is one of the most cost-effective tests available: sheath damage found before energisation can be repaired with a sleeve in hours, while damage found after years of moisture ingress may mean replacing a section.

VLF Withstand Testing

VLF testing of MV cables applies an AC voltage at very low frequency — typically 0.1 Hz — at a level above the normal operating voltage, for a defined period. If a significant defect exists, the elevated stress drives it to failure during the test, under controlled conditions, rather than in service. The cable either withstands the test or it does not.

Why Very Low Frequency?

Long MV cables have large capacitance. Testing them at the 50 or 60 Hz power frequency would require a large, heavy test source. Lowering the frequency to 0.1 Hz reduces the power the source must deliver in proportion, which makes portable test sets practical for field use while still stressing the insulation with AC rather than DC. For the instruments themselves and how they differ, AQ Electric’s sister company EE Electronics explains the equipment side in its article on VLF cable testing.

Test Voltages and Duration

IEEE 400.2 sets VLF test voltages by cable rating for installation, acceptance, and maintenance tests. For a 35 kV-class cable, the sinusoidal VLF values in IEEE 400.2-2013 are 39 kV rms for installation, 44 kV rms for acceptance, and 33 kV rms for maintenance, phase to ground, with test durations of 30 to 60 minutes. IEC 60502-2, which covers extruded cables from 6 to 30 kV, allows several after-installation AC test options by agreement between purchaser and contractor, including a 0.1 Hz test at 3 U₀ for 15 minutes. The project specification should state which standard and which values apply, and the cable and accessory manufacturers should confirm them.

What a Pass Does and Does Not Mean

A VLF withstand pass means the cable system did not fail at the test voltage within the test time. It does not prove the absence of defects. Published work summarised in the technical literature reports on-test failure rates of 0.2% to 4% for 30-minute tests at IEEE 400.2 levels — evidence that the test does find real defects — but the termination failures described by Thorne & Derrick show that some workmanship defects can survive a VLF test and fail later in service. For high-consequence circuits, a diagnostic test adds the information that a go / no-go test cannot.

Why Not DC Hipot?

DC high-potential (hipot) testing was the traditional method for testing MV cables, and it is still requested in some older specifications. For modern polymer-insulated cables, it is no longer considered effective. DC withstand testing does not reproduce the AC stress distribution the cable sees in service, so it is ineffective at finding many defects in XLPE and EPR insulation, and it can shorten the remaining life of cables with aged polymer insulation. VLF testing was developed to replace it.

Where a specification calls for DC hipot on XLPE collection cables, it is worth raising the point with the owner’s engineer and the cable manufacturer before testing begins; AQ Electric explains that role in its article on the owner’s engineer for solar PV projects. Updating the specification to a VLF-based method is usually straightforward and gives more meaningful results.

Tan Delta Testing

Tan delta, or dissipation factor, measures the ratio of resistive (loss) current to capacitive current in the insulation. Perfect insulation would behave like a pure capacitor with very low losses; moisture, water trees, contamination, and poor joints all increase the losses. Tan delta testing is usually performed with a VLF source at 0.1 Hz, with measurements taken at several voltage steps, typically from about 0.5 U₀ up to 1.5 or 2 U₀.

IEEE 400.2 assesses tan delta results using three criteria: the magnitude of tan delta, the change in tan delta between voltage steps (the “tip-up”), and a hierarchical evaluation of the insulation system. Comparing the three phases of the same circuit is especially useful. In the Megger case from Karnataka, 20 circuits of three-year-old 33 kV XLPE cable were assessed; 19 were found in good condition, while one phase of one circuit showed a large tip-up and dielectric losses at 1.5 U₀ roughly 20 to 40 times higher than the other two phases, and was classified as requiring action under IEEE 400.2-2013.

Tan delta has limitations. It gives an average picture of the whole circuit, so a local defect on a long cable can be diluted, and some defects produce no measurable losses at all. It tells the engineer that something is wrong, but not exactly where.

Partial Discharge Testing

Partial discharges are small electrical sparks inside voids or along interfaces in the insulation system that do not bridge the full insulation. They are the typical signature of the workmanship defects found in joints and terminations, and over time they erode the insulation until it fails.

Off-line PD testing energises the cable from a test source and measures discharge activity, typically recording the voltage at which discharges start (inception) and stop (extinction), their magnitude in picocoulombs, and their location along the cable using time-domain reflectometry. IEEE 400.3 describes field PD assessment of shielded power cables, and IEC 60270 provides the measurement basis.

PD testing is the most powerful tool for finding local defects in new accessories. In the termination case reported by Thorne & Derrick, an off-line 50/60 Hz PD test at 1.5 U₀ with 5 pC sensitivity identified the defective termination that a VLF withstand test had passed. In the Karnataka case, the engineers recommended a PD test rather than a further withstand test for the suspect phase, because a withstand test could not guarantee a controlled breakdown of the local defect. PD results are best interpreted by experienced specialists, and trends across repeated measurements carry more weight than a single reading.

A Practical Test Sequence for Solar Plants

How should MV cable testing fit into solar commissioning? A practical sequence for a new solar PV plant looks like this:

1. Factory stage — review the cable manufacturer’s routine test certificates for each drum, and witness factory tests where the project requires it. AQ Electric delivers factory and arrival checks through its FAT and SAT quality assurance services.
2. Delivery and storage — inspect drums for damage, check sealed cable ends, and verify storage conditions.
3. Installation inspection — witness pulling, bending radius, trench bedding, and backfill compaction.
4. After laying — insulation resistance and oversheath tests on each length, before joints and terminations are made.
5. Accessory workmanship — inspect jointing and termination work at hold points, with installer qualification checked and torque verified.
6. Acceptance testing — insulation resistance, continuity and phasing, and VLF withstand on the complete cable system; add tan delta and PD on critical or long feeders, or where workmanship is in doubt.
7. Energisation — energise in a controlled sequence, then perform thermography of terminations and switchgear under load.
8. Baseline records — hand over all results as the reference for future maintenance tests.

MV cable testing is one strand of a wider programme that includes DC string tests, inverter commissioning, and performance tests. AQ Electric explains how the strands fit together in its article on the solar PV commissioning process. The MV cables also connect directly to the plant’s transformers and switchgear, which are tested in parallel; see AQ Electric’s article on solar power plant transformers.

MV Cable Testing in Operation

MV cable testing does not end at commissioning. In operation, the goal shifts from proving the cable to tracking its condition.

• Thermography of terminations and switchgear — the simplest periodic check, performed under load, finds loose or overheating connections before they fail; AQ Electric explains the technique in its article on infrared thermography for solar PV. Substation and termination thermography can also be done from the air; AQ Electric covers this in its article on drone inspection for solar PV plants.
• Condition assessment with tan delta and PD — on critical feeders, periodic diagnostic tests compared with the commissioning baseline reveal deterioration trends.
• Maintenance withstand testing — at reduced voltage levels defined for in-service cables, after repairs or when condition data suggests a problem.
• Fault location — when a fault occurs, systematic pre-location and pinpointing shorten the outage.

These activities belong inside the plant’s maintenance plan. AQ Electric explains how a structured programme protects yield in its article on solar PV plant operation and maintenance. For acquisitions, the condition of the MV network and its test history are part of the asset review described in AQ Electric’s article on solar PV technical due diligence.

What About DC Cables?

Insulation resistance testing in solar plants is not limited to MV cables. On the DC side, IEC 62446-1 requires insulation resistance testing of the PV array circuits as part of its Category 1 test sequence, alongside continuity, polarity, open-circuit voltage, and current tests. The test voltage and acceptance values are set by the standard according to system voltage, and the readings are affected by moisture and temperature, so conditions should always be recorded.

DC cable insulation faults are a common cause of earth faults and inverter trips in solar plants, and they often trace back to installation damage or water ingress at connectors. AQ Electric explains the DC test sequence in its IEC 62446-1 article.

MV Cable Testing in the Gulf and MENA

MV cable testing in the region follows the same international standards as elsewhere, but local conditions affect both cable performance and the testing itself.

Heat and Soil Conditions

High ambient and ground temperatures reduce the current-carrying capacity of buried cables, and dry desert soils can have a high thermal resistivity. Bedding and backfill quality therefore matter even more, because voids and poor compaction create local hot spots. Installation inspection should give as much attention to the trench as to the cable.

Workmanship in Harsh Conditions

Joints and terminations made in heat, wind, and blowing sand are more exposed to contamination and errors. Shelters, controlled working windows, installer qualification, and hold-point inspection reduce the risk, and they make diagnostic testing more valuable where conditions were difficult.

Long Collection Runs

Large regional plants have long MV feeders. The test sets must be sized for the capacitance of the longest circuits, and long cables make local defects harder to see in averaged measurements such as tan delta, which is another reason to consider PD testing on critical feeders.

Grid and Utility Requirements

Grid operators and utilities in the region set their own requirements for the plant’s interface equipment and test records before energisation. AQ Electric’s experience of those requirements is covered in its articles on solar PV testing services in Jordan, solar testing services in Egypt, and solar PV testing services in Saudi Arabia.

How AQ Electric Tests MV Cable Systems

AQ Electric for Power Solutions, founded in Jordan in 2019 and expanded to Egypt in 2021, includes MV cable testing within its testing and commissioning scope for solar PV plants across the MENA region. Its electrical testing services cover DC and AC insulation testing, transformer and switchgear testing, earth and equipotential bonding continuity testing, earth fault identification and troubleshooting, thermal imaging of medium-voltage stations, termination kits, and switchgear cabinets, and review of commissioning plans and procedures. Full details are on AQ Electric’s solar PV testing and commissioning services page.

AQ Electric’s engineers bring field experience from utility-scale projects including the 230 MWp Komombo plant in Aswan, Egypt, delivered with China Energy Engineering Group (CEEC), and the 120 MWp Wadi Ad Dawasir plant in Saudi Arabia, delivered with SEPCO – Power China. Where the owner needs independent verification of a contractor’s cable testing, AQ Electric can witness and review the work through its third-party inspection services, and it supports MV network condition monitoring in operation through its solar O&M services.

AQ Electric operates under an ISO 9001, ISO 14001, and ISO 45001-certified management system from offices in Amman, Jordan and New Cairo, Egypt, with more than 1.0 GW of PV project experience. When selecting any testing provider for MV work, the criteria in AQ Electric’s article on how to choose a solar testing and commissioning partner apply in full. The project portfolio is on AQ Electric’s projects page.

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