Negative-sequence differential protection - principles, sensitivity, and security

Abstract

This paper explains the principles of negativesequence differential (87Q) protection, its basis for excellent sensitivity and speed, and the need for securing it with external fault detectors to deal with the saturation of current transformers. The paper reviews applications of 87Q elements to lines and transformers. It explains why 87Q elements cannot be used for turn-to-turn fault protection in shunt reactors and stators of generators and motors. The paper presents applications of negative-sequence directional elements for turnto-turn fault protection in reactors and stators. Finally, it derives two new protection principles based on negative sequence for generator turn-to-turn fault protection. 978-1-4799-8722-1/15/$31.00

Negative Sequence-Based Schemes for Power System Protection - Review and Challenges

“…This negative sequence filter is embedded with transformer's protection scheme and is hardly affected by harmonic distortion under the DC bias of the protected transformer [16]. Reference [14] also showed that using negative sequence current based differential overrides the long delay time and unwanted operation under external faults whilst increasing the sensitivity to internal faults. Reference [14] indicated that embedded negative elements are able to detect faults across up to 10 % of transformer winding turns, while the traditional relay can detect faults only for 2% of the transformer windings.…”

“…The other advantage of the negative sequence current components over zero sequence current component is that mutually coupled parallel line currents do not influence the measurement and only the three-phase currents are used as inputs (the neutral current is not needed) [5]. Reference [14] highlighted the advantage of having a line differential protection with a negative element, where the sensitivity of the differential relay increases due to removing the load effect from the differential relay constraint signals. In addition it is less affected by the line charging current.…”

Fault Detection in Transmission Lines —A Novel Voltage-Based Scheme for Differential Protection

“…The conventional current-based differential relay operates perfectly to some extent compared to other protection schemes and works even better when considering sequence components as described in [7], [20], [21]. However, the tendency of differential relays to fail in the real world during heavy fault events is high.…”

Transmission lines differential protection based on an alternative incremental complex power alpha plane

“…Last but not least, aiming to overcome the capacitive charging current effect on CDP algorithms, it is necessary the use of not only current but also voltage measurements at each line end [5, 8], likewise PDP algorithms do. Additionally, since modern numerical relays provide several protection and monitoring functions, voltage measurements are usually available in the relay.…”

“…However, these elements tend to be much less stable than phase elements when external faults lead current transformers (CTs) to saturate. Furthermore, sequence elements depend on additional functions to provide faulted phase selection [8]. The capacitive charging current effect of long transmission lines on CDP performance is thoroughly evaluated in [9], revealing that the use of not only current but also voltage measurements is necessary to overcome this drawback.…”