Stability and sensitivity of differential protection (87) have been a topic of continuing interest for protection and control (P&C) engineers over the years. The misoperation of 87 function due to CT saturation, incorrect vector matching, incorrect current ratio and zero sequence current compensation are the usual challenges in a differential scheme. In differential protection schemes, such as transformer differential (87T) and low impedance bus differential (87B), loss of one of the contributing CTs may result in misoperation, especially under system normal condition or out-of-zone fault conditions.
Several mitigation techniques have been developed over the years to improve the stability of differential protection without compromising the sensitivity of protection to clear a fault within the protected zone. These mitigation techniques are equally valid in a digital substation. However, there is a new element to be considered to ensure the stability of 87 function in a digital substation. In the digital substation
architecture, the analog current measurements from the current transformers (CT) are digitized using the substation merging units (SMU). The sampled values (SV) from the SMU are fed to the 87-protection algorithm for monitoring and protecting the equipment within the zone of protection. Furthermore, the SV streams are time synchronized to a precision clock with microsecond accuracy per IEEE 1588 PTP
standard. One of the major benefits of IEC 61850 standards based digital substation design is the ability to continuously monitor the health and availability of the SMU, protection functions and the redundant communication network. Any component failure is immediately detected, and alarms are generated to proactively notify the operator.
How should the 87-function behave in the event of bad quality of an SV data stream or in the event of a loss of time synchronization should be a matter of careful consideration during P&C scheme design. In a digital substation differential protection scheme, the loss of any contributing MU causes the same outcome as losing a contributing CT. Therefore, adequate mitigation and back up protection scheme need to be included in the P&C design. This paper discusses the strategies implemented in a real world 345kV/34.5kV digital substation P&C scheme design to provide adequate protection to the power transformer and the substation busbar.