An introduction of GB Chinese National Transformer bushing
Chinese National Transformer Bushing Standards: Complete Technical Guide
Table of Contents
- Overview of Chinese National Transformer Bushing Standards
- Core GB Standards for Transformer Bushings
- Classification of Transformer Bushings under GB/T 4109-2022
- Key Technical Performance Requirements (Reference Table)
- Mandatory Test Items Specified by GB/T 4109-2022
- GB/T 4109-2022 vs IEC 60137: Main Differences (Comparison Table)
- GB Standard Requirements for Epoxy Resin Transformer Bushings
- Application Scenarios of GB Compliant Transformer Bushings
- Selection Guidelines for GB Standard Transformer Bushings
- Industry Trends for GB Standard Transformer Bushings in China
- Frequently Asked Questions
Conclusion
1. Overview of Chinese National Transformer Bushing Standards
Transformer bushings are critical insulating components that enable energized conductors to pass through transformer grounded tanks while maintaining reliable dielectric isolation. All transformer bushings manufactured and deployed within China’s power grid must comply with unified Chinese national standards (GB standards). The primary governing document is GB/T 4109-2022 Insulated bushings for alternating voltages above 1 000 V, which replaced GB/T 4109-2008 and is modified based on international standard IEC 60137:2017.
Chinese national standards define unified classification systems, electrical thresholds, mechanical indicators, testing sequences and marking rules for oil-filled transformer bushings, porcelain bushings, and solid cast epoxy resin bushings. Compared with international standards, GB specifications adjust voltage series, pollution grade requirements and thermal test conditions to match domestic grid operating environments, altitude conditions and pollution distribution across China.
As Chinese transformer manufacturers expand global exports, many suppliers produce dual-certified bushings that satisfy both GB national standards and IEC standards. Epoxy resin transformer bushings, as a mainstream dry-type insulation solution, need to fulfill strict GB requirements on partial discharge, thermal cycling and surface creepage distance to be matched with dry-type transformers and compact distribution transformers.
2. Core GB Standards for Transformer Bushings
Multiple coordinated national standards form the complete technical framework for transformer bushings in China:
- GB/T 4109-2022: Insulated bushings for alternating voltages above 1 000 V. Core standard covering general definitions, classification, type tests and routine tests for all transformer bushings, reactor bushings and switchgear bushings.
- GB/T 1094.3-2017: Power transformers – Insulation levels, dielectric tests and external clearances. Defines matching insulation coordination between transformers and supporting bushings.
- DL/T 1539-2016: Guideline for selection of high voltage bushings for power transformers and reactors. Industry recommended standard for power grid procurement.
- GB/T 4585: Artificial pollution test of high voltage insulators for AC systems, regulating pollution withstand performance for outdoor transformer bushings.
- GB/T 1408 series: Test methods for solid electrical insulating materials, widely adopted for epoxy resin bushing material verification.
GB/T 4109-2022 applies to AC systems with frequency range from 15 Hz to 60 Hz and equipment maximum voltage above 1000 V. The standard covers independently supplied bushings; integrated bushings built into complete transformers shall undergo combined testing together with the transformer equipment.
3. Classification of Transformer Bushings under GB/T 4109-2022
According to insulation medium and structural design, GB/T 4109-2022 divides transformer bushings into the following mainstream categories:
- Oil-impregnated paper capacitive bushing: Traditional solution for high voltage and UHV oil-immersed power transformers
- Porcelain oil-filled bushing: Widely used for medium voltage distribution transformers
- Solid cast epoxy resin bushing: Dry-type insulation, no internal oil, suitable for dry-type transformers, indoor substations and new energy equipment
- Gas-insulated bushing: Designed for SF6 sealed transformer and GIS combined equipment
- Compound-filled bushing: Adopts solid insulating compound instead of liquid oil
By installation environment, GB standards separate products into indoor type, outdoor type and indoor-outdoor through type. Outdoor transformer bushings require extended creepage sheds and enhanced anti-pollution performance to meet PD II, PD III or PD IV pollution degrees defined in national specifications.
4. Key Technical Performance Requirements (Reference Table)
All data are general GB standard reference values; precise parameters depend on specific voltage class and product type test reports.
| Parameter Item | GB Standard General Requirements | Remarks |
|---|
| Applicable Frequency | 15 Hz ~ 60 Hz | Covers 50 Hz Chinese domestic grid |
| Rated Voltage Series | 12kV, 24kV, 36kV, 72.5kV, 126kV etc. | Matches GB/T 311.1 voltage series |
| Rated Current Range | 630A, 1250A, 2000A, 3150A, 4000A | Customized ratings available |
| Partial Discharge Level | ≤10 pC at 1.5×Um for capacitive and epoxy resin bushings | Critical index for long service life |
| Operating Ambient Temperature | -40°C ~ +40°C | Special low-temperature versions can be negotiated |
| Maximum Allowable Altitude | ≤1000 m above sea level; correction required above 1000 m | GB standard altitude correction rule differs slightly from IEC |
| Pollution Degree Classification | PD I, PD II, PD III, PD IV | Coastal and industrial zones require PD III / PD IV |
| Flammability Requirement (Epoxy Type) | UL94 V-0 recommended by GB industry guidelines | Not mandatory baseline but widely adopted |
| Dielectric Loss Factor tanδ | ≤0.007 for new epoxy resin bushing | Measured under room temperature routine test |
5. Mandatory Test Items Specified by GB/T 4109-2022
GB/T 4109-2022 clearly distinguishes routine tests (tests on every finished unit) and type tests (batch certification tests):
Routine Tests for Each Transformer Bushing
- Visual inspection and dimensional verification
- Capacitance and dielectric loss factor measurement
- Power frequency dry withstand voltage test
- Partial discharge measurement
- Test tap insulation test (for capacitive bushings)
Type Test Series
- Lightning impulse withstand voltage test (BIL)
- Switching impulse withstand voltage test (for high voltage grades)
- Long-duration power frequency withstand voltage test
- Temperature rise test under rated current
- Short-time thermal current withstand test
- Cantilever mechanical load test
- Thermal cycling test
- Artificial pollution withstand test (outdoor models)
- Sealing tightness test (oil-filled bushing types)
All test records must be retained, and type test certificates are required before GB compliant bushings can be supplied to major power grid projects.
6. GB/T 4109-2022 vs IEC 60137: Main Differences (Comparison Table)
| Comparison Item | GB/T 4109-2022 | IEC 60137:2017 |
|---|
| Standard Origin | Modified version based on IEC 60137 (MOD) | Original international standard |
| Standard Voltage Series | Aligned with GB domestic grid voltage grades | Global universal voltage series |
| Altitude Correction Rules | Independent correction coefficient for external insulation | Adopts unified IEC altitude correction formula |
| Pollution Test Reference | GB/T 4585 artificial pollution standard | IEC 60507 pollution test standard |
| Thermal Test Environment Settings | Adjusted for typical Chinese climate conditions | Global average climate baseline |
| Preferred Partial Discharge Threshold | Strict ≤10 pC for medium voltage epoxy bushing | Allows wider negotiated range between buyer and supplier |
| Application Priority | Mandatory reference for domestic Chinese power grid bidding | Primary standard for most overseas power projects |
| Export Adaptation | Can be designed to meet dual GB & IEC requirements | Directly accepted by most Asian, European, African buyers |
7. GB Standard Requirements for Epoxy Resin Transformer Bushings
Epoxy resin transformer bushings are fast-growing dry-type products under GB/T 4109-2022. Since no oil or paper insulation is used, these components attract widespread adoption for cast resin dry-type transformers.
Under GB specifications, epoxy resin bushings must eliminate internal casting voids to control partial discharge. Outdoor cycloaliphatic epoxy formulations need to resist UV aging and salt mist erosion. The standard requires uniform electric field design to avoid surface tracking and flashover under humid and contaminated operating conditions.
Key GB focus points for epoxy resin transformer bushings:
8. Application Scenarios of GB Compliant Transformer Bushings
Transformer bushings complying with GB/T 4109-2022 serve all segments of China’s power system:
- Oil-immersed distribution transformers and power transformers for urban and rural power grids
- Cast resin dry-type transformers for data centers, underground substations and commercial buildings
- New energy station transformers for photovoltaic power plants and onshore wind farms
- Railway traction transformers and industrial high-voltage power equipment
- Energy storage step-up transformers with frequent load fluctuation
Many Chinese transformer exporters develop GB & IEC dual-standard bushings to simplify product management for both domestic and international orders.
9. Selection Guidelines for GB Standard Transformer Bushings
Design engineers and purchasers should follow these core principles according to GB industry guidelines:
- Confirm equipment maximum voltage, rated continuous current and short-circuit withstand requirements
- Define installation type: indoor, outdoor or indoor-outdoor through structure
- Confirm site pollution degree, operating altitude and ambient temperature range
- Select insulation type: oil-filled capacitive bushing, porcelain bushing or epoxy resin bushing
- Verify required test levels including lightning impulse voltage and partial discharge limits
- Check flange mounting dimensions, conductor connection interface and transformer sealing matching
Request formal GB standard type test reports before mass procurement
10. Industry Trends for GB Standard Transformer Bushings in China
Driven by new power system construction and energy transition, several clear trends emerge for GB standard transformer bushings:
First, dry-type epoxy resin transformer bushings gain larger market share. Policies promoting fire-safe, leakage-free power equipment accelerate replacement of traditional oil-type bushings in densely populated regions.
Second, stricter partial discharge and long-term aging requirements are gradually added into grid procurement specifications, pushing manufacturers to upgrade vacuum casting and APG molding technology.
Third, more bushing suppliers carry out dual-standard certification (GB plus IEC) to support transformer export growth.
Fourth, intelligent monitoring accessories are integrated into new-generation GB standard bushings, supporting online partial discharge and temperature monitoring for smart transformers.
11. Frequently Asked Questions
Q1: Can GB/T 4109 transformer bushings be used for overseas projects?
A: GB/T 4109-2022 is modified from IEC 60137. Many manufacturers produce dual-standard bushings. Buyers need to verify voltage levels, pollution test standards and altitude correction rules to match local grid requirements.
Q2: What is the biggest difference between epoxy resin bushing and oil-filled bushing under GB standards?
A: Epoxy resin bushings belong to dry solid insulation without oil leakage risk. GB imposes stricter mechanical impact and thermal cycling tests on epoxy products, while oil-filled bushings focus more on sealing performance and dielectric loss stability.
Q3: Is GB/T 4109-2022 compulsory legislation?
A: GB/T means recommended national standard. However, most power grid tenders in China require all transformer bushings to comply fully with GB/T 4109-2022 as a bidding condition.
Q4: How to handle installation above 1000 m altitude according to GB requirements?
A: External insulation such as sheds needs creepage distance upgrade. Manufacturers should calculate altitude correction factor following GB guidelines and adjust bushing external insulation design.
12. Conclusion
Chinese national transformer bushing standards represented by GB/T 4109-2022 form a complete, mature technical system for medium and high voltage transformer insulation components. The standard balances alignment with international IEC norms and adaptation to domestic power grid operation conditions.
With the rapid development of dry-type transformers, new energy power projects and Chinese transformer export business, GB compliant epoxy resin transformer bushings will maintain continuous market expansion. Proper selection based on GB technical specifications, standardized type test verification and matching between bushings and transformer main equipment can effectively reduce insulation failure risks and extend the whole lifecycle of power distribution assets.
