Next generation of Line and Cable fault Locator for HVDC transmissions
HVDC is often used for long transmission lines or long submarine cables creating a need for some kind of line fault locator for HVDC lines. The commonly used method in AC lines, to measure the impedance during the fault, does not work for HVDC lines. Therefore, ever since the mid 1970's, arrangements using the travelling wave principle with time tagging of the incoming waves has been employed. This requires accurately synchronized clocks in the two stations and up till the late 1980's this was accomplished by using dedicated high speed communication channels for clock synchronization. In 1990 the first line fault locator based on local clock synchronization using time synchronization from GPS satellites was introduced in the Quebec New England multiterminal HVDC link. For cable transmissions the need for on line fault location has been considered less important, as there are no temporary cable faults, so off line methods have in many cases been considered sufficient. When using the travelling wave principle on cables there is also the problem that the wave front arriving at the far end can be highly attenuated, which has so far made it difficult, or even impossible, to detect incoming waves with the high accuracy needed to apply the travelling wave type of fault locators. Now with the availability of high speed analog to digital converters, fast Digital Signal Processors (DSP) and large Field Programmable Gate Arrays (FPGA) it has been possible to introduce what can be referred to as the third generation on line fault locators. Here the incoming wave is not just detected and time tagged, but also digitally recorded with very high sample rate and with each sample accurately time tagged. This makes it feasible to apply the two sided travelling wave fault locators also to long HVDC cable installations. This new type of line fault locator has been installed and tested in the Pacific Intertie HVDC link, in two sections of the Quebec New England Multiterminal HVDC system and in the Baltic Cable, which is a 250 km submarine cable combined with a 12 km overhead line. This report presents the main characteristics of this new on line fault locator system and some staged fault measurement results.