All elements of the high voltage (HV) transmission grid require testing to demonstrate tolerance to such voltages to prevent failure and disruption to electricity supplies. In addition, due to the height of HV lines lightning strikes are common and cause surges of power, or ‘lightning impulses’ (LI), reaching hundreds of kV in millionths of a second. Overhead line insulators, a common grid component, are ‘puncture tested’ to determine tolerance to such excessive voltages. Testing generally involves an electricity impulse generator and a voltage divider which reduces the high-voltage signal to 10 V, or levels compatible with recording equipment. The divider itself must have adequate insulation and physical dimensions large enough to withstand the full applied voltage and be able to scale-down HVs that are transient, lasting for micro- or nanoseconds in duration, in an accurate and linear fashion.
Methods for testing and calibrating such instruments are specified by the international standard IEC 60060-2, however HV levels in Europe are now increasing to higher than currently covered by this standard. New types of voltage dividers, with greater accuracy and more sensitivity to fast-rising voltages are required, alongside validated methodologies that would allow testing comparability, independent of test location.
The EMPIR project Metrology for the electrical power industry developed a new voltage divider for puncture testing together with proof-of-performance established using newly developed calibration methodology. The design is based on ceramic disk resistors, which have low inductance, high mechanical strength and the ability to withstand high voltages. Non- linearity in results due to such things as temperature can be corrected for to give a scale factor, which is how much the input voltage is accurately reduced for recording on a data recorder, in this case of 100,000:1. The instrument can measure a maximum voltage of 600 kV for transient voltages shorter than 0.5 μs, an improvement on the 0.84 μs of existing instruments, and the full lighting impulse voltage it can withstand is 400 kV. Furthermore, the divider’s rise time, the fastest rising pulse it can measure, is a mere 2 ns (0.002 μs).
Veresence La Granja, a glass insulator manufacturer based in Spain, were one of the first to purchase the new voltage divider. The company has been making insulators for electrical networks since 1932 and believe that their experience and willingness to invest in technological developments has allowed them to offer products that have not been surpassed by any other manufacturer.
The company plan to use the new divider in their ISO 17025 accredited testing and calibration laboratory which collaborates with other independent laboratories to generate the test reports they provide with their products. La Granja consider the new divider instrument critical to this work and as HV levels continue to increase in the future, it will help improve the products that they can deliver to their clients, opening new markets in this field.
These upgraded setups for puncture tests on high-voltage line insulators will increase grid reliability, support the compatibility of testing between different test organisations and reduce the economic and social costs of disruptions caused by environmental factors.