Partial Discharge Testing for Photovoltaic Modules: Standards, Methods, and Technical Requirements

Partial discharge (PD) testing for photovoltaic modules is a critical safety evaluation that verifies insulation integrity under high electrical stress and confirms a module's maximum system voltage rating in accordance with IEC 61730-2. By detecting internal voids, delamination, and encapsulation defects before they escalate into dielectric breakdown, ground faults, or fire hazards, PD testing safeguards long-term reliability in both 1000 V and 1500 V solar installations. This article explains the principles, standards, sample requirements, test procedures, and acceptance criteria for photovoltaic module partial discharge detection.

Technical Overview

Partial discharge is a localized dielectric breakdown that occurs within or on the surface of an insulating material when the local electric field exceeds the breakdown strength of that region, without completely bridging the electrodes. In photovoltaic modules, PD typically originates in micro-voids within the encapsulant, at delaminated interfaces between cells and encapsulation material, along contaminated glass or backsheet surfaces, at cell edges, and near poorly insulated junction box terminations.

Although a single discharge event is microscopic, sustained partial discharge progressively degrades insulation through ionic bombardment, chemical attack, and localized heating. Over time this leads to electrical treeing, surface tracking, encapsulant carbonization, cell corrosion, and eventually dielectric failure, ground faults, or arc ignition. Because modern utility-scale plants operate at DC system voltages of 1000 V or 1500 V, the margin between operating stress and insulation capability has narrowed, making PD behavior a decisive safety parameter.

The partial discharge test defined in IEC 61730-2 (Photovoltaic module safety qualification – Part 2: Requirements for testing) is used to verify that a module does not exhibit sustained partial discharge at its rated maximum system voltage. Measurement instrumentation and circuit configuration follow IEC 60270 (High-voltage test techniques – Partial discharge measurements), while insulation coordination principles derive from IEC 60664-1. In North America, the harmonized standard UL 61730 adopts the same test approach, and the Chinese national standard GB/T 20047.2 mirrors the IEC requirements.

Testing Samples

Crystalline Silicon Modules

Monocrystalline and multicrystalline silicon modules are the most common test articles. Samples are typically complete, production-representative modules with frames, junction boxes, cables, and connectors installed, since PD behavior depends on the full insulation system including edge seals and potting compounds.

Thin-Film Modules

Thin-film technologies such as cadmium telluride, CIGS, and amorphous silicon present different insulation geometries because the active semiconductor is deposited directly on the superstrate or substrate. Special attention is paid to laser-scribed isolation scribes and intrinsic buffer layers, which can be sites of PD activity.

Glass-Glass and Bifacial Modules

Double-glass laminates, including bifacial designs, must be evaluated for PD along all edges and through the encapsulant sandwich. The absence of a polymer backsheet changes the electric field distribution and requires careful assessment of edge sealing quality.

Flexible and Building-Integrated Modules

Flexible laminates and building-integrated photovoltaic products use thinner dielectric layers and often have reduced creepage and clearance distances, so PD verification at the intended system voltage is especially important for these categories.

Sample Preparation and Conditioning

Test specimens are usually conditioned at standard laboratory conditions of 23 °C ± 5 °C with relative humidity below 75% prior to testing. Module frames are grounded, bypass diodes are rendered non-conducting or bypassed as specified, and terminals are prepared for high-voltage connection. The ambient electromagnetic background must be low enough that noise does not mask the discharge signals being measured.

Testing Items

The partial discharge evaluation of a photovoltaic module comprises the following measurement items:

• Partial discharge inception voltage (PDIV) – the lowest applied voltage at which sustained partial discharge of a specified apparent charge magnitude is first observed.
• Partial discharge extinction voltage (PDEV) – the highest voltage at which partial discharge ceases when the applied voltage is reduced from above the inception level.
• Apparent discharge magnitude – the measured apparent charge, expressed in picocoulombs (pC), determined by direct calibration of the test circuit.
• Maximum system voltage verification – confirmation that the extinction voltage exceeds the module's rated maximum system voltage, such as 1000 V or 1500 V.
• Background noise verification – demonstration that the measuring system noise floor is sufficiently below the specified discharge magnitude criterion.

Testing Methods

Test Circuit and Instrumentation

The measurement circuit is configured in accordance with IEC 60270 and consists of a low-distortion high-voltage AC source operating at power frequency (50 Hz or 60 Hz), a coupling capacitor, a coupling device with a quadrupole measuring impedance, and a calibrated partial discharge detector. Before testing, the complete measuring system is calibrated by injecting a known charge across the module terminals using a calibrator, establishing the scale factor for apparent charge readings in pC.

Voltage Application Procedure

The test voltage is applied between the shorted module output leads and the grounded frame (or grounded mounting surface for frameless modules). The voltage is raised gradually from a level below the expected inception voltage until sustained partial discharge exceeding the specified magnitude criterion is detected; this value is recorded as the PDIV. The voltage is then decreased until discharges fall below the criterion and cease; this value is recorded as the PDEV. Voltage is applied only for the minimum time necessary to make valid readings to avoid artificially aging the specimen.

Acceptance Criteria

The core requirement is that the partial discharge extinction voltage must be higher than the rated maximum system voltage of the module. A module intended for 1500 V systems must therefore demonstrate a PDEV exceeding 1500 V, ensuring that even if discharges are transiently triggered by switching surges, they will self-extinguish once the voltage returns to the normal operating level. The apparent charge criterion and the measurement magnitude threshold are defined by the applicable standard, and background noise must remain well below this level throughout the test.

AC Versus DC Considerations

Although modules operate on DC in the field, the standardized test uses power-frequency AC voltage. This convention follows insulation coordination practice in IEC 60664-1, provides reproducible and comparable results with well-established instruments, and represents a conservative screening of insulation quality. Research-level DC PD measurement can be performed as a supplementary diagnostic, but certification relies on the AC method.

Applications

Partial discharge detection serves multiple purposes across the photovoltaic value chain:

• Safety certification – demonstrating compliance with IEC 61730-2, UL 61730, EN 61730-2, and GB/T 20047.2 as part of module safety qualification for market access.
• Maximum system voltage rating – establishing the basis for rating a module for 600 V, 1000 V, or 1500 V plant architectures, including 1500 V utility-scale designs.
• Design and development verification – comparing encapsulants, edge seal geometries, backsheet materials, and junction box insulation systems during product development.
• Aging and durability studies – retesting PD behavior after damp heat, thermal cycling, or humidity-freeze exposure to quantify insulation degradation over simulated service life.
• Failure analysis and root cause investigation – locating active discharge sites in field-returned modules or in production lots exhibiting delamination or insulation anomalies.
• Quality control – screening pilot production samples to confirm that manufacturing processes consistently deliver discharge-free insulation at the intended system voltage.

Accredited third-party laboratories routinely perform these tests as part of IEC 61730 certification programs, providing manufacturers, project developers, lenders, and insurers with independent evidence of insulation safety.

FAQ

Why is an AC voltage used if photovoltaic modules operate on DC?

The AC method is used because it is reproducible, supported by mature IEC 60270 instrumentation, and recognized in insulation coordination standards such as IEC 60664-1. Requiring the PD extinction voltage to exceed the rated DC system voltage provides a conservative assessment of the insulation system's ability to suppress discharge activity in service.

What does it mean if a module fails the partial discharge test?

A failure indicates that discharges either persist below the rated system voltage or extinguish only at voltages at or below that rating. Sustained PD under normal operation will progressively degrade the encapsulant and insulation, increasing the risk of ground faults, insulation resistance failures, and in extreme cases fire. Corrective actions typically involve encapsulant selection, lamination process improvement, or edge insulation redesign.

Where do discharges usually originate in a module?

Common discharge sites include voids and bubbles in the encapsulant, delaminated regions between cells and glass or backsheet, contaminated cell edges, sharp solder joints, junction box potting defects, and regions with insufficient creepage distance between live parts and the grounded frame.

How is partial discharge testing different from PID testing?

Partial discharge testing evaluates the intrinsic dielectric capability of the module insulation system at elevated voltage, whereas potential-induced degradation (PID) testing, covered by IEC TS 62804, assesses long-term performance loss caused by leakage currents under system voltage stress in humid conditions. The two phenomena are related to system voltage but represent distinct failure mechanisms and test protocols.

Can a module exhibit partial discharge and still pass certification?

Yes. Discharges occurring above the rated system voltage do not by themselves constitute a failure, provided the extinction voltage remains above that rating. The key criterion is that any discharge activity self-extinguishes before the voltage falls to the maximum level at which the module will operate.

How often should partial discharge testing be performed?

PD testing is required for initial safety certification and must be repeated whenever the insulation system changes materially — for example, a new encapsulant, backsheet, glass thickness, edge design, or junction box — or when the module is re-rated for a higher system voltage. Many manufacturers also incorporate periodic PD checks into ongoing quality surveillance programs.