Ever since we hosted a Webinar on Travelling Wave Fault Locators (TWFL) during the COVID-19 lockdown, we have been inundated with many queries from the Webinar attendees who wanted to know more about TWFL and it’s applicability in their respective systems.
In this post, we have compiled 10 frequently asked questions (FAQs) about TWFL. But before we begin, let’s quickly understand what is a Travelling Wave and how it helps in locating the faults to ensure unhindered power operations by ensuring minimum downtime.
Travelling Waves
‘Travelling Wave’ is a temporary surge of current generated at the source of the fault and moves along either ends of the Transmission Line. These transient waves are created mainly due to switching operations such as breaker operations, short circuits, transient faults, lightning strikes, etc.
Though travelling waves are short bursts that usually last only for a few microseconds, they travel at the speed of light and can reach any distance without suffering any significant attenuation. Identifying travelling waves thus becomes a reliable fault detection method in power system operations.
Travelling Wave Fault Location System
While many methodologies like the Impedance method, Offline Pulse Echo method, etc. exist to locate faults in Transmission and Distribution Lines, TWFL has emerged as one of the most reliable and effective methods to identify and locate faults in the power lines to minimise the downtime while keeping the analysis simple for the user.
SCOPE marketed Kehui XC-100E is a pioneer in TWFL systems and has proven itself as a groundbreaking solution with over 2500 installations worldwide.
FAQs on TWFL
1. Is Impedance method an online or offline method? Also, what are your thoughts about the Pulse Echo method?
A. The Impedance method is an online method primarily used in Distance Protection Relays however there are limitations to the impedance method in accurately locating the faults in Transmission Lines. The Pulse Echo method is another method for fault location; however, it is an offline method that can be used only after taking a complete shutdown on the line which increases the downtime.
2. Can a TWFL system be used for an 11kV Transmission Line?
A. TWFL systems can identify and localise faults on Transmission Lines irrespective of the voltage level and the type of the Transmission Line (such as HVDC, T-Branch Lines, Series Compensated Lines, Overhead and Cable-mixed Lines, etc.). Although, TWFL is independent of voltage level and line configuration, it is rarely used for short Transmission Lines typical of 11kV lines due to the high costs for TWFL system.
3. Can a Travelling Wave enter a Substation’s earthing grid? If yes, can it affect the Auxiliary equipment?
A. Travelling Waves are transients that travel to the instrument transformers via the transmission lines. These traveling wave can be detected using a clamp-on CT placed around the secondary connections of the instrument transformers in control room or KIOSK. Travelling waves are reflection of the fault currents and do not affect the Auxiliary equipment.
4. What is the accuracy of a TWFL system?
A. The accuracy of a TWFL system varies from manufacturer to manufacturer. Advanced TWFL systems like SCOPE marketed Kehui XC-100E delivers an industry-best accuracy of ±150 meters irrespective of the line length, line parameters, fault resistance, CVT transients, conductor and tower geometry.
5. Are separate CTs and PTs required for a TWFL? Or can existing CTs and PTs be used?
A. TWFL is a non-destructive method of identification and localisation of faults, which means that, existing systems are not disturbed by it, and the input to the Data Acquisition Unit (DAU) can be from CT, CVT or PT as per the requirement. Hence, separate CTs and PTs are not required; the existing ones are used.
6. Given that the time difference for time of arrival of travelling waves may be as short as in nanoseconds, how are they recorded in such a short span of time?
A. Travelling Waves are recorded using high-speed data acquisition, analysis and processing system. SCOPE marketed Kehui XC-100E TWFL system facilitates a sampling rate up to 36MHz. This high frequency is sufficient to capture the Travelling Waves at all 8 channels of XC-100E simultaneously.
7. Can a single TWFL unit suffice for double circuit lines?
A. Yes. A single Data Acquisition Unit (DAU) of advanced TWFL systems like SCOPE’s Kehui XC-100E can seamlessly identify faults up to 8 Transmission Lines irrespective of the line configuration.
8. Does the DAU need input of CT only or does it need the input of both CT and PT?
A. The input can be selected and changed as per the requirement. Generally, CT input serves the purpose but in HVDC Lines the input is taken from the CVT. Each of the 8 channels can be configured as per the requirement.
9. Should the DAUs be placed at terminal towers or at circuit breakers? What about the breaker status?
A. Since Travelling Waves are short lived and may attenuate if the distance between the DAU and the sensing point is more than 20 meters, the DAUs are placed in the Control Panel or Kiosk. However, the breaker status can be taken using the DI inputs of the DAUs.
10. In the double-ended method, how do the DAUs communicate with each other?
A. DAUs do not communicate with each other as this puts the reliability of the system at risk given that a failure in any single DAU can fail the whole TWFL system. Instead, each DAU communicates independently with the Master Station that is set up at the user’s premises thus maintaining the reliability of the system. The data from the DAUs at different Substations is transmitted to the Master Station using the available communication such as TCP-IP, P2P or Dial-up. The system also supports communication media such as PLCC or FO or GPRS.
Summing Up
Precisely locating faults depends on a trifecta of equipment, subject matter expertise and on-field experience. To learn more about SCOPE marketed Kehui XC-100E TWFL system, please click here or write to us at marketing@scopetnm.com.
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