Impact of Time Synchronisation Errors on Transformer Differential Protection that uses the IEC 61850 Process Bus

Conventional transformer differential protection uses current transformers (CTs) to reduce each primary current to a suitable secondary value. If a fault occurs within the transformer, the protection initiates a trip command via a contact, opens the circuit breakers and clears the fault. With the development of the IEC 61850 process bus, the analogue measurement values can be digitised into Sampled Values (SVs) and transmitted across an Ethernet communication network. In addition, digital status information, such as indications, alarms and trip signals, can be distributed across the same network using Generic Object-Oriented Substation Event (GOOSE) messages.

An IEC 61850 based transformer differential protection intelligent electronic device (IED) compares the SVs from either, standalone Merging Units (MUs) connected to conventional CTs, or directly using Non-Conventional Instrument Transformers (NCITs). Then it matches the SV steams from each set of CTs according to their Sample Counters (SmpCnt). The protection IED then calculates the differential current and issues a trip signal if it exceeds the bias operation threshold. The correct operation of differential protection relies on the accuracy of the time synchronisation process applied to MUs or NCITs. Inadequate synchronisation accuracy will cause an angle shift of the phasors and result in an increase in differential currents, which can lead to protection mal-operations. SVs are required to have a timing accuracy of better than 1µs, and this is traditionally achieved using 1-PPS or the IRIG-B time code implemented using dedicated cabling systems. In contemporary times, the power industry is moving towards using IEEE 1588 Precision Time Protocol (PTP) to achieve the same level of timing precision without the need for separate cables. The time information is encapsulated in Ethernet messages which share the same communication network as SV and GOOSE.

This paper investigates the impact of time synchronisation errors on a transformer differential protection that uses SV process bus. The paper first uses mathematical derivations to illustrate how inadequate synchronisation affects the differential current calculations and indicates the consequences on the protection operating response during internal and external faults. A multi-vendor hardware testbed is then utilised to experimentally validate the theoretical results. The testbed simulates a real substation communication network with a redundant coupling of both station bus and process bus HSR (high-availability seamless redundancy) rings. Time deviations are created using a network impairment emulator that manipulates the PTP messages content to precisely “spoof” the MUs. The results have shown the protection scheme becomes more sensitive because of the introduced phase angle shift. The impact on the differential current is the most severe in the operating area related to the first slope of the restraint curve.