How to Select the Right Elbow Surge Arrester for Pad-Mounted Transformers

Recently, industry experts issued a detailed technical note addressing the safe operation and overvoltage protection of combined transformers. The document elaborates on the protection configuration for the high-voltage side of combined transformers and strictly regulates the selection of key parameters for surge arresters, specifically noting that imported arresters must meet the specific requirements of the Chinese power grid.

Terminal Protection and Full-Range Protection Configuration

The high-voltage side of combined transformers structurally adopts a protection method consisting of “load switch + current-limiting fuse + expulsion fuse.” Among these, the current-limiting fuse is mainly used for large-scale short-circuit fault protection, while the expulsion fuse is used for small-scale overcurrent protection. To prevent the destruction caused by through overvoltages from faults on the low-voltage side, the expert panel recommends using two groups of surge arresters connected in series for full-range voltage protection. This solution is scientific, economical, and convenient to operate, effectively addressing the risks faced by terminal-type transformers during lightning overvoltages and system switching overvoltages.

Strict Selection of Core Arrester Parameters

The document explicitly lists four core indicators for arrester selection:

Rated Voltage (Un​):​ To ensure safety, the rated voltage of the arrester must satisfy the formula Un​⩾KU=1.1KUm kV. Here, K is taken as 1.3, and Um​ is the maximum system operating voltage.

Continuous Operating Voltage (Uc​):​ The RMS value of the power frequency voltage that can be continuously applied between the arrester terminals, generally requiring Uc​⩾0.8Um.

Nominal Discharge Current (In​):​ A standardized discharge current peak used to classify arrester levels; the document recommends that distribution arresters typically select 5kA.

Residual Voltage (Ures​) and V-A Characteristics:​ Residual voltage refers to the maximum voltage peak across the arrester terminals when the discharge current passes through it. To prevent the arrester from prematurely conducting when power frequency overvoltage exceeds Ucl​, its resistance should drop as low as possible. Furthermore, the ratio of the residual voltage under the nominal discharge current to the lightning impulse protective level must not exceed 1.15.

Square-Wave Current Capacity and Comprehensive Selection Recommendations

Regarding the 2ms square-wave current capacity for distribution arresters with a nominal discharge current level of 5kA, the document emphasizes its importance. Although the standard specifies a 75A current carrying capacity—which is much larger than what overhead lines require—when selecting arresters for combined transformers, the specific value must still be determined based on system parameters, cable types, and regional lightning density.

Synthesizing the requirements of the latest national standard GB11032-2000, experts advise that when selecting arresters suitable for domestic grid requirements, the following hard indicators should be met:

1.Power Frequency Reference Voltage​ should be greater than 15kV.

2.Continuous Operating Voltage​ greater than 12kV.

3.Nominal Discharge Current​ greater than 5kA.

4.2ms Square-Wave Current Capacity​ not less than 50kA.

5.Lightning Impulse Residual Voltage​ not greater than 50kV.

Finally, the experts pointed out that selecting excessively high ratings will reduce the service life of the arrester, while selecting too low will fail to provide protection. Therefore, when using imported arresters, it is essential to ensure their parameters fully comply with the insulation coordination requirements of the domestic power grid system to avoid potential safety hazards caused by improper use.