Corona discharge characteristics of two of LPI’s “Guardian” air terminal families were assessed at the LCOE HV LAB using a new HV DC generator in November 2022. These tests were carried out because the corona performance of air terminals is thought to be instrumental in their response to approaching lightning strikes, as explained below.
In the mid-eighteenth century, Benjamin Franklin and others started experimenting with lightning and this marked the beginning of “lightning protection” by means of a simple “Franklin rod” conductor. These rods were geometrically sharp so that they could produce a “corona” or “point discharge” in thunderstorm electric fields, with the intention of capturing the downward lightning strike. Since that time, many different lightning conductors have been developed, tested, and used.

Corona discharge produces of a space charge “volume” or “cloud” above the air terminal. International research has shown conclusively that space charge accumulation around the tip of an air terminal has a detrimental effect on its ability to initiate and sustain an upward leader. Since a continuous, uninhibited upward leader is essential for reliable interception of the downward lightning leader and the presence of space charge above the air terminal makes lightning capture less reliable, it is important to measure the corona characteristics of air terminals. These measurements are made under both dry and wet conditions and for both polarities.

According to technical specification provided by LPI, the dependence of corona DC current on HVDC electric field was measured first on a standard Franklin rod and then on a variety of LPI’s corona-minimising Guardian air terminals.

LPI Guardian CAT device during corona testing under wet conditions.

Each air terminal was placed on the floor centrally under a metallic overhead electrode 4 m above the ground of the testing area. HVDC test voltage was applied on the electrode from 25 kV up to 400 kV and the current to ground from the air terminal was measured by using a sensitive DC current meter.
The measurements were performed with both positive and negative DC voltages under dry and wet conditions. The latter was implemented with “artificial rain” on the test objects according to the requirements, flow rate and water conductivity of the standard IEC 60060-1.

Overhead Electrode and Franklin Rod during HVDC corona measurement.

The corona DC measurements were performed by LCOE with a new HVDC generator. This generator was designed and manufactured by the LCOE team, and it is currently being prepared for carrying out tests and calibrations up to a voltage level of 600 kVDC. During 2023, the system will be expanded to reach a maximum voltage level of 1200 kVDC.

More information about this publication www.lcoe-hv.com
Tomás García tgarcia@ffii.es
Paula Cid paula.cid@ffii.es