What is it about?
The neutral point of secondary circuits of Instrument transformers e.g., Capacitive Voltage Transformer and Current Transformer (CVT & CT) must be earthed, IEEE emphasizes that the secondary of instrument transformer must be grounded to protect the connected equipment and to ensure safety of working personnel. if the secondary neutral point of CVT has multiple earthings, the output voltage is not a replication of primary voltage during fault. This leads to mal-operation or non-operation of protection relays. As no discrepancy in secondary voltages is sensed by relays during normal operation, hence multiple earthings remain unnoticed and thereby compromising the integrity of relay operation. This paper, based on field data and vectorial mathematical analytics establishes the causes of distortions in the CVT secondary voltages due to multiple earthing on its neutral, causing maloperation of protection/relay. Study conclusively brings out correlation between CVT secondary magnitude, phase angle and harmonic generation and suggests formulation of a suitable algorithm/logic in Intelligent electronics device (IEDs)/relay for automatic identification of problem.
Featured Image
Photo by Ernest Brillo on Unsplash
Why is it important?
With the increase in complexity of interconnecting lines/transformers, the requirement of system reliability and availability for an integrated grid is of utmost importance. Availability of the network also has financial implications on transmission utilities. Hence accurate and reliable behaviour of protection system is highly desirable. Accurate fault location identified by the distance relays is also important for quick restoration of transmission lines, which contribute in improving system availability. Instrument transformers, i.e., CT & CVT are very important to the protection systems. To understand the influence of multiple earthing of CVT secondary in extra-high voltage (EHV) transmission lines, actual incidents have been analyzed to find the characteristics of voltage waveform on multiple earthing. The findings, such as presence of specific harmonics, correlation of change in voltage magnitude and phase angle during fault will be very useful in understanding the phenomenon and implementation of suitable configuration in IEDs for automatic identification of multiple earthing
Perspectives
Based upon the investigation of distance relay operation, DR and the field experience gained from implementation of corrective measures, it can be concluded that there exists a pattern in the behaviour of voltage seen by the relay during faults having multiple earthing in CVT secondary. This is summarized as below: 1. Voltage increases in the faulty (UL1) phase & healthy phase (UL2) along with a drop-in magnitude in the 3rd phase (UL3). 2. Presence of different harmonics, predominantly 3rd harmonics (20 to 35 %) in healthy phase Voltage (UL3) with drop-in its magnitude. 3. The tendency of voltage vectors to align with each other during the fault. 4. Drifting in voltage magnitude/phase angle and presence of harmonics depends upon the severity of the fault These finding can help protection Engineer to easily recognize the problem which may arise due to faulty cable (ageing/mechanical failures) or commissioning error. Relay manufacture can formulate/introduce a suitable logic in relay configuration which may generate an alarm to identify such problems. This will enable the Site Engineer to rectify the problem related to multiple neutral earthing thereby avoiding maloperation of protection relays thus preventing the outage of Transmission Lines and disturbance in the POWER SYSTEM Network. CVT manufacturers can also be requested to write a caution message for removing extra earthing before final commissioning to sensitize the Commissioning Engineer to avoid such errors.
Mr Yogeshwar Singh Rana
Powergrid Corporation of India Ltd
Read the Original
This page is a summary of: A case study on Capacitive Voltage Transformer (CVT) behaviour on multiple earthing of CVT secondary circuit in a 400 kV Transmission line., December 2021, ACM (Association for Computing Machinery),
DOI: 10.1145/3508297.3508305.
You can read the full text:
Contributors
The following have contributed to this page
