Partial Discharge (PD) is an important precursor to insulation deterioration in high-voltage electrical equipment. The sensitivity of modern PD measurement has reached the picocoulomb (pC) level, while external electromagnetic interference (EMI) can easily overwhelm the weak discharge pulses. Therefore, the partial discharge shielding chamber (Faraday cage) has become an indispensable environmental guarantee for the factory test, commissioning test and type test of high-voltage power equipment. This article systematically elaborates on the role and technical principles of the partial discharge shielding chamber in the partial discharge tests of typical equipment such as transformers, GIS, cables and accessories, and also summarizes the international (IEC/IEEE), national (GB) and industry (DL/T) standard and regulation systems that are compatible with it, and provides the key engineering points for construction, calibration and acceptance.
I. Partial Discharge Test and Electromagnetic Environment Challenges
Partial discharge refers to the local breakdown that occurs in the insulation inside, on the surface or at the edge of the electrode in the concentrated electric field of high-voltage equipment. It is both a sign of insulation aging and a factor that further deteriorates the insulation. Conducting partial discharge measurements on equipment such as transformers, GIS, cables, bushings, transformers and other devices is the core means to assess the insulation integrity.
The typical characteristics of partial discharge signals are extremely small amplitudes (from several pC to several thousand pC), steep rising edges (nanosecond level), and short durations. With 1 pC = 10⁻¹² C, this is an almost imperceptible charge event. In contrast, there are a large number of electromagnetic interference of the same frequency band in laboratories and industrial areas:
Radio broadcasting, walkie-talkies, mobile phones and their base stations radiation;
Nearby corona of high-voltage equipment, operation overvoltage of disconnect switches;
Harmonics and spikes generated by power electronic devices such as frequency converters and switching power supplies;
Pulse interference conducted through the power grid (requiring suppression at the power supply end).
When the background noise is at the same level or even higher than the true partial discharge pulses, the measurement system will be unable to distinguish the authenticity of the discharge, resulting in misjudgment, missed detection or false alarms. This is the fundamental reason for the existence of the partial discharge shielding chamber – to provide a “electromagnetic silence” controlled environment for the detection of pC-level weak signals.
II. Technical Principles and Key Indicators of the Partial Discharge Shielding Chamber
The partial discharge shielding chamber is essentially a Faraday cage with high electromagnetic shielding efficiency: assembled with high conductivity metals (copper plates / copper nets, aluminum plates, often made of copper + steel composite), a six-sided equipotential closed conductor shell is formed, combined with single-point grounding, power filtering and cutoff waveguide ventilation, to significantly attenuate external space radiation and conduction interference. The shielding mechanism is reflection loss R + absorption loss A + multiple reflection correction B, and the total shielding efficiency calculation formula is SE (dB) = R + A + B.
2.1 Two Core Indicators
Background noise level (the most critical) High-performance indicators: <1 ~ 2 pC
Shielding efficiency SE 14 kHz–1 MHz frequency band > 60 dB; 1 MHz–1 GHz frequency band > 90 dB
Background noise refers to the total sum of all stray signals picked up by the measurement system when no test voltage is applied and the test sample does not discharge. It directly determines the minimum discharge quantity that can be detected. The general requirement of IEC 60270 is that the background noise of the test circuit (including the environment) should be lower than the specified partial discharge allowable limit. The industry commonly accepted criterion is to be at least 6 dB lower (i.e., the amplitude should not exceed half of the limit); for strict limits such as 10 pC (such as cast dry-type transformers), the noise must be lower than 5 pC, which poses very high requirements for the shielding chamber.
Shielding efficiency SE represents the attenuation ability of the shielding chamber to electromagnetic waves, with units in dB, and needs to be measured in full frequency bands at different positions and different frequency bands according to the standard, not relying solely on a single value. For example, 1 GHz at 100 dB means that the field strength at this frequency has been attenuated to one ten-thousandth of its original value.
2.2 Necessary Supporting Systems
Power purification: Configure power filters + dual shielded isolation transformers to suppress conductive interference from the power grid, such as harmonics and spike pulses;
Grounding: Build a low-resistance grounding system, with the grounding resistance typically < 0.5 Ω, using a single-point grounding method to avoid ground circulation;
Environmental control: Conduct temperature and humidity regulation to control the temperature at 20 ℃ ± 5 ℃ and relative humidity < 80%, ensuring consistent test conditions and comparable test data;
Safety interlock: High-voltage tests involve electric shock risks, so safety facilities such as emergency shutdown, door interlock, sound and light alarm, and video monitoring must be configured.
III. Typical Applications in High Voltage Electrical Equipment Partial Discharge Tests
3.1 Power Transformers (Factory / Commissioning)
Transformer partial discharge tests are usually conducted in combination with induced voltage tests with PD measurement (IVPD): Under a frequency multiplier (e.g. 100/150 Hz), increase the test specimen voltage to 1.5・Um/√3 or above, and simultaneously monitor the partial discharge. The test must be completed in a shielded room, isolating external corona and power frequency magnetic fields. According to GB 1094.3 / IEC 60076‑3, the common acceptance limits for apparent discharge quantity are as follows:
For liquid impregnated, Um ≤ 72.5 kV: 250 pC (50 pC when required by the contract);
For liquid impregnated, Um > 72.5 kV: 100 pC;
For cast (IEC 60076‑11): 10 pC (the limit is the strictest for solid insulation that cannot self-heal).
3.2 GIS (Gas Insulated Metal Enclosed Switchgear)
The internal partial discharge signals of GIS are weak and the external accessibility is poor. Factory and on-site partial discharge detection generally uses an electrical method (IEC 60270) + UHF (Ultra High Frequency) method for combined detection. According to GB 7674 / IEC 62271‑203, the factory partial discharge test has strict process and judgment requirements, and the shielded room can provide low-noise background for UHF and electrical method measurements. The background control effect of GIS partial discharge affects the identification ability of free metal particles and poor shielding defects.
3.3 Power Cables and Accessories
Cross-linked polyethylene (XLPE) cables and terminals, intermediate joints require partial discharge tests at the factory and commissioning stages, based on standards such as GB/T 12706 / IEC 60502‑2. The interface pressure and sealing defects of cable accessories mostly manifest as partial discharge, and the low background environment in the shielded room is an important prerequisite for ensuring the credibility of the accessory test results.
3.4 Other Capacitive Equipment
Poles (GB/T 4109), current transformers / voltage transformers (GB/T 20840 / IEC 61869), coupling capacitors, lightning arresters, etc., their partial discharge tests, especially on-site tests, follow DL/T 417 requirements, and also require a shielding environment or equivalent anti-interference measures. For this type of capacitive insulation structure equipment, the partial discharge quantity is a key acceptance indicator.
The shielded room is at the core in laboratory and factory test environments; while on-site tests are limited by conditions and the equipment cannot be moved, most adopt UHF, ultrasonic, HFCT, etc. local anti-interference detection technologies, but in principle, they still need to meet the equivalent requirements of “low background, calibrable, comparable”.
IV. Relevant Test Standards and Procedures System
The construction, installation, calibration and acceptance of the partial discharge shielding room need to follow a complete standard system covering four major aspects: electromagnetic shielding effectiveness, partial discharge measurement methods, equipment product acceptance, and on-site/preventive tests.
The international standard for partial discharge measurement is IEC 60270:2000 “High Voltage Test Technology – Partial Discharge Measurement”, which is a guiding method standard. It stipulates the measurement circuit, calibration procedures, and anti-interference requirements, clearly stating that the background noise should be much lower than the specified limit. The industry generally requires a difference of at least 6 dB. The corresponding core standard in China is GB/T 7354-2018 “High Voltage Test Technology – Partial Discharge Measurement”. This standard adopts the IEC 60270:2000.
The acceptance of the shielding room’s shielding effectiveness follows GB/T 12190-2021 “Measurement Methods for Shielding Room’s Shielding Effectiveness”, applicable to the frequency range of 9 kHz – 18 GHz, and can be extended to 50 Hz / 100 GHz, etc., equivalent to IEEE Std 299. IEEE Std 299 / 299.1 is an authoritative international shielding effectiveness test standard, covering the frequency band of 300 MHz – 50 GHz.
The general standard for high voltage tests is GB/T 16927.1-2011 “High Voltage Test Technology – Part 1: General Definitions and Test Requirements”, equivalent to IEC 60060-1, which sets general constraints for high voltage test environment and safety.
Various equipment product standards specify the partial discharge test clauses and discharge limits for corresponding equipment, including the power transformer standard GB 1094.3 / IEC 60076-3, the GIS equipment standard GB 7674 / IEC 62271-203, the cable product standard GB/T 12706 / IEC 60502-2, the insulation bushing standard GB/T 4109, and the transformer standard GB/T 20840 / IEC 61869.
The on-site measurement follows DL/T 417-2019 “Guidelines for On-site Partial Discharge Measurement of Power Equipment”, replacing the old version DL/T 417-2006, which standardizes the measurement methods, calibration processes, interference suppression means and allowable indicators for various capacitive equipment in the laboratory and on-site. The交接 test refers to GB 50150-2016 “Standard for Electrical Installation Engineering – Electrical Equipment Commissioning Test”； the periodic insulation assessment of operating equipment refers to DL/T 596 “Procedures for Preventive Tests of Electrical Equipment”. For non-electrical partial discharge detection， the guidelines for UHF and ultrasonic detection technologies are IEC 62478， DL/T 363， and DL/T 1250， which can complement the electrical detection methods in the shielding room.
4.2 Logical Relationship between Standards
The key to understanding this system is to distinguish the division of labor between “method standards” and “limit standards”：
IEC 60270 / GB/T 7354 stipulates the measurement methods， calibration procedures and background noise control indicators， which are the direct basis for the design of the shielding room；
GB/T 12190 / IEEE 299 stipulates the detection methods for the shielding room’s shielding effectiveness， serving as the basis for the acceptance of the shielding room；
The various equipment product standards clearly specify the allowable partial discharge quantities for each type of equipment， indirectly imposing constraints on the test environment and background noise of the shielding room by citing the measurement method standards；
DL/T 417， GB 50150， DL/T 596 are used to guide the implementation and result determination of tests in the on-site and operation stages.
V. Key Points of Construction， Calibration and Acceptance (Project Implementation)
Background noise verification: The background signal of the measurement system is collected in the absence of test samples and without applying test voltage; according to the spirit of IEC 60270, the background noise should not exceed half of the specified limit for the tested equipment, that is, it should be 6 dB lower than the limit. In a high-precision test scenario, the target for background noise is less than 1-2 pC.
Shielding effectiveness measurement: According to GB/T 12190-2021, in the frequency range of 9 kHz – 18 GHz, the shielding effectiveness SE is measured in different regions and frequency bands to confirm that it meets the industry-standard indicators: 14 kHz – 1 MHz > 60 dB, 1 MHz – 1 GHz > 90 dB.
Measurement system calibration: Before each partial discharge test, a standard calibration pulse is injected according to IEC 60270 requirements. Generally, a 5 – 50 pC pulse signal is selected to establish the pC/mV dimensional conversion relationship; test reports without calibration data have no engineering reference value.
Anti-interference closed-loop control: Space radiation interference is suppressed by Faraday cages and cutoff waveguide windows; conductive interference is handled by power filters combined with isolation transformers; the grounding system achieves single-point grounding and the grounding resistance is less than 0.5 Ω. The shielding door, wall penetration openings, cable entrances and exits are the main leakage positions of shielding effectiveness, and need to use finger springs, conductive pads and EMI filters to achieve a complete electromagnetic closed loop.
Safety compliance: It meets the electrical safety, building and fire protection regulations of high-voltage test laboratories; safety devices such as interlock, emergency shutdown, sound and light alarms shall not be deleted.
Failure of the shielding room is mostly not due to the insufficient performance of the shielding material, but due to electromagnetic leakage at details such as gaps, shielding doors, wiring holes, ventilation openings and grounding treatment. After the completion of the shielding room construction, full-band shielding effectiveness measurement must be carried out according to GB/T 12190, and the background noise level must be rechecked. It is not allowed to directly complete the acceptance based on the material nominal parameters.
The partial discharge shielding room is the “infrastructure” for high-voltage power equipment partial discharge tests. It relies on the principle of Faraday cage to build a low-background, calibrated and comparable test environment for pC-level weak discharge signals, directly determining the credibility of partial discharge measurement results for transformers, GIS, cables and various capacitive equipment. The construction and acceptance of the shielding room require the core of GB/T 7354 partial discharge measurement method and GB/T 12190 shielding effectiveness measurement standards, while following the partial discharge limit clauses in various equipment product standards, as well as the requirements of on-site and preventive tests-related procedures. Only by implementing key measures such as continuous sealing of the shell, single-point grounding, and port filtering, can the shielding room become a reliable technical foundation for evaluating the insulation status of equipment.
Appendix: List of main reference standards
IEC 60270: 2000 “High-voltage test techniques – Partial discharge measurements”
GB/T 7354-2018 “High voltage test technology – Partial discharge measurement” (adopted from IEC 60270: 2000)
GB/T 12190-2021 “Measurement methods for shielding effectiveness of electromagnetic shielding rooms” (equivalent to IEEE Std 299)
IEEE Std 299 / 299.1 “Standard for Measuring the Effectiveness of Electromagnetic Shielding Enclosures”
GB/T 16927.1-2011 “High Voltage Test Technology – Part 1: General Definitions and Test Requirements” (equivalent to IEC 60060-1)
GB 1094.3 / IEC 60076-3 “Power Transformers – Insulation Levels, Dielectric Tests and External Insulation Air Gaps”
GB 7674 / IEC 62271-203 “Rated Voltage 72.5 kV and Above Gas-Insulated Metal-Enclosed Switchgear”
GB/T 12706 / IEC 60502-2 “Sheathed Insulated Power Cables and Accessories”
GB/T 4109 “Insulating Bushings with AC Voltage Higher than 1000 V”; GB/T 20840 / IEC 61869 “Transformers”
DL/T 417-2019 “Guidelines for On-Site Measurement of Partial Discharge in Power Equipment”
GB 50150-2016 “Electrical Installation”
Post time: Aug-18-2026