National Standard Bushing
National Standard Bushing – GB/T 4109 Insulated Transformer Bushing for Power Equipment
National Standard Bushing: Complete Technical Guide for Power Transformer Equipment
1. Definition of National Standard Bushing
National standard bushing refers to high-voltage insulated bushings manufactured, inspected and accepted in accordance with Chinese national standards. The primary governing specification is GB/T 4109-2022 Insulated bushings for alternating voltages above 1 000 V, which replaces the outdated GB/T 4109-2008 and serves as the core technical baseline for all AC transformer bushings above 1kV. This standard is modified based on IEC 60137:2017 and adds localized technical clauses adapting to complex operating environments including high altitude, heavy industrial pollution and coastal salt fog conditions.
As a critical penetration component mounted on transformer tanks and shunt reactors, a national standard bushing enables energized conductors to pass through grounded metal enclosures while maintaining reliable insulation between live parts and earth. Most medium and high voltage national standard bushings adopt capacitance graded condenser core structure. Multiple layers of conductive grading foil are embedded inside insulation to homogenize radial electric field distribution, suppress partial discharge and corona activity, and extend long-term operational reliability.
National standard bushings are widely selected for domestic power grid construction and global overseas power projects. Many international purchasers prefer GB-compliant bushings for Asia, Africa, Latin America and Middle East power infrastructure. When required by tender documents, manufacturers can supply national standard bushings that meet both GB/T 4109 and IEC 60137 dual standards to improve cross-project compatibility.
2. Applicable National Standard Specifications
Multiple interconnected standards regulate design, production and testing of national standard bushings. The core standard system is summarized in the table below.
| Standard Code | Official English Name | Implementation Date | Main Coverage | Typical Application |
|---|---|---|---|---|
| GB/T 4109-2022 | Insulated bushings for alternating voltages above 1 000 V | Oct 1, 2022 | General terms, rated parameters, type tests, routine tests, mechanical and dielectric requirements | All AC transformer bushings, OIP, RIP, porcelain and composite types |
| GB/T 24840-2018 | Technical specification for bushings of 1000 kV AC system | Feb 1, 2019 | Extra-high voltage bushing special requirements, pollution resistance, temperature rise, partial discharge | UHV national standard transformer bushings for 1000kV transmission projects |
| GB 1094.3-2003 | Power transformers Part 3: Insulation levels, dielectric tests and external clearances in air | Jan 1, 2004 | Insulation coordination between transformer main body and matched bushings | Parameter matching for national standard bushing sets |
| DL/T 1539-2016 | Selection guide to the high-voltage bushings for power transformers and reactors | Jun 1, 2016 | Engineering selection rules, environmental adaptation and installation requirements | Procurement reference for power plants and urban substations |
3. Classification of National Standard Transformer Bushings
Classified by internal insulation medium and outer housing material, mainstream national standard bushing types are compared as follows. All performance indicators comply with GB/T 4109-2022 requirements.
| Bushing Type | Insulation Structure | Standard Voltage Range | Key GB Standard Requirements | Primary Application Scenarios |
|---|---|---|---|---|
| GB Standard OIP Bushing (Oil-Impregnated Paper) | Condenser core with kraft paper impregnated transformer oil | 12kV ~ 1200kV | Strict tightness test, limits on tan δ, moisture monitoring inside insulation | Large power transformers, conventional EHV transmission substations |
| GB Standard RIP Bushing (Resin-Impregnated Paper) | Dry-type vacuum cured epoxy resin crepe paper core | 24kV ~ 800kV | Oil-free design, arbitrary mounting angle, strict partial discharge limits | Indoor urban substations, wind farm transformers, fire-controlled zones |
| GB Standard Porcelain Housing Bushing | Porcelain sheds matched with OIP or solid insulation core | ≤550kV | Defined minimum creepage distance for different pollution classes | Medium voltage distribution transformers, industrial auxiliary transformers |
| GB Standard Composite Housing Bushing | Silicone rubber composite sheds with OIP/RIP inner core | 36kV ~ 550kV | Mandatory hydrophobicity and anti-aging verification for composite materials | Coastal salt fog areas, heavy chemical and mining pollution regions |
4. Core Technical Specifications of National Standard Bushing
All qualified national standard bushings must be clearly marked with rated parameters and pass corresponding GB standard tests. Major specification indicators are listed below.
| Parameter Item | GB/T 4109 Regulation | Common Standard Series Options |
|---|---|---|
| Rated Highest Voltage (Um) | Standard voltage sequence specified by national standard | 12kV, 24kV, 40.5kV, 72.5kV, 126kV, 252kV, 362kV, 550kV |
| Rated Continuous Current | Standard load current series for transformer winding matching | 250A, 630A, 1250A, 2000A, 3150A, 4000A, 6300A |
| Lightning Impulse Withstand Voltage (BIL) | Insulation level coordinated with GB 1094.3 | 95kV up to 1800kV peak |
| Partial Discharge Level | Mandatory routine test item | ≤5pC under specified test voltage |
| Dielectric Dissipation Factor (tan δ) | Measured at ambient temperature during factory inspection | OIP bushing ≤0.007; RIP bushing ≤0.005 |
| Pollution Class & Creepage Distance | Five pollution grades defined; minimum creepage distance mandatory | Class I, II, III, IV; extended creepage available for harsh environments |
| Allowable Mounting Angle | Differentiated limits for oil-filled and dry-type bushings | OIP: ≤30° vertical tilt; RIP: unlimited mounting angle |
| Short-Time Thermal Withstand Current | Short-circuit current withstand duration options | 2s, 4s standard configuration |
Critical Selection Notes
- Projects located at altitude above 1000m need altitude correction for external insulation and withstand voltage values according to GB requirements.
- Coastal and industrial polluted sites should select extended creepage distance composite housing national standard bushings.
Oil-free RIP national standard bushings are prioritized for densely populated urban substations to reduce fire hazards.
5. Test Classification for National Standard Bushing
GB/T 4109-2022 divides bushing tests into three categories: type tests, routine tests and special tests. Only products passing all required testing can be recognized as qualified national standard bushing.
5.1 Type Tests
Type tests are performed on prototype samples before mass production. Main test items include power frequency dry and wet withstand voltage, lightning and switching impulse withstand, temperature rise test, cantilever mechanical load test, thermal stability test and sealing tightness test for liquid-filled bushings.
5.2 Routine Tests
Every single national standard bushing must complete routine tests before delivery: dimension inspection, capacitance and tan delta measurement, partial discharge test, dry lightning impulse withstand test, test tap insulation test and tightness pressure test.
5.3 Special Tests
Special tests are implemented upon purchaser requirements, including low temperature adaptability test, seismic resistance test, composite material UV aging test and high altitude adaptation verification.
6. National Standard Bushing versus IEC Standard Bushing
Many global engineering buyers compare national standard bushing with IEC 60137 bushing. The similarities and differences are organized in the table below.
| Comparison Item | National Standard Bushing (GB/T 4109-2022) | IEC Standard Bushing (IEC 60137:2017) |
|---|---|---|
| Standard Origin | Developed based on IEC 60137 framework with localized revisions | Global baseline international standard formulated by IEC |
| Basic Technical Framework | Consistent with IEC on insulation classification, test categories and parameter series | Universal global technical baseline |
| Environmental Requirements | Stricter clauses for heavy pollution, high altitude and drastic temperature fluctuation | General global environment baseline, provides altitude correction formula |
| Oil-Filled Bushing Clauses | Additional requirements for oil moisture content and dissolved gas monitoring | Basic sealing and tightness requirements only |
| Composite Housing Requirements | Mandatory hydrophobicity and anti-pollution flashover verification | Composite material requirements set as optional clauses |
| Global Market Recognition | Widely accepted in Asia, Middle East, Africa and Latin America | Recognized in European, Southeast Asian and most international bidding projects |
7. Main Advantages of National Standard Bushing
- Mature standardized production system with complete parameter series covering medium voltage to extra-high voltage grades
- Optimized design adapting to harsh operating environments such as high altitude, salt mist and industrial pollution
- Clear standardized testing procedures, convenient for factory acceptance and on-site third-party inspection
- Optional dual-standard manufacturing complying with both GB/T 4109 and IEC 60137 to satisfy international tender rules
- Multiple structural options: OIP oil-filled, dry-type RIP, porcelain housing and composite housing to match different project budgets
Stable supply chain supporting large-scale transformer manufacturing and overseas turnkey power projects
8. Typical Application Fields
National standard bushings are widely used in various power infrastructure sectors worldwide:
- Extra-high voltage and ultra-high voltage AC transmission transformers
- Onshore and offshore wind farm step-up transformers
- Photovoltaic power station box-type distribution transformers
- Industrial power supply transformers for chemical plants, mines and steel enterprises
- Traction transformers for high-speed railway and urban rail transit power supply
- Energy storage station medium voltage transformers and shunt reactors
Urban and rural power grid upgrading distribution equipment
9. Common Operation & Maintenance Recommendations
Service life of national standard bushing is closely related to standardized maintenance. Under normal operating conditions, the design service life reaches 25–30 years.
- Carry out regular visual inspection to check external insulator contamination, sealing flange leakage and surface cracks.
- Conduct periodic on-site testing including capacitance measurement, tan delta test and partial discharge detection.
- Clean composite or porcelain sheds regularly for equipment running in heavily polluted regions to prevent surface flashover.
- Monitor operating temperature through infrared thermal imaging to detect abnormal hot spots at terminals and flanges.
Replace aging sealing gaskets during transformer overhaul to avoid moisture ingress inside the insulation core.
10. Frequently Asked Questions
Q1: Can national standard bushing directly replace IEC standard bushing?
A: Direct replacement requires verification of flange dimensions, external insulation length, electrical parameters and allowable mounting angle. Many national standard bushings support dual GB & IEC compliance to realize interchange after dimensional confirmation.
Q2: Does GB/T 4109 apply to DC converter transformer bushings?
A: GB/T 4109 mainly regulates AC national standard bushings. DC converter transformer bushings require supplementary technical specifications agreed between buyers and suppliers.
Q3: What is the difference between condenser type and non-condenser national standard bushing?
A: Condenser type bushings adopt capacitance grading foil layers for uniform electric field distribution, suitable for medium and high voltage applications above 40.5kV. Non-condenser solid bushings are generally used for low and medium voltage distribution equipment with simpler structure.
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