Transformer failures can be very expensive such as the cost of repairing the transformer, the cost of energy not delivered because of transformer unavailability, and the possible additional cost on spreading damage to adjacent equipment or of a significant power system blackout.
Transformers failures are also dangerous. Internal and external faults and other abnormal operating conditions, such as overload, overexcitation, overvoltage and mechanical stress due to loading on transformers. The transformer protection schemes should disconnect the protected device before extensive damage occurs and/or power system. In addition to protection elements, transformer protection relays should also provide thermal and through-fault monitoring functions. The electrical protection of a transformer includes differential protection, negative sequence percentage differential protection, combined differential and restricted earth protection and the backup overcurrent protection schemes and the mechanical protection of a transformer includes Buchholz relay, pressure protection and thermal protection.
In 1941, author  proposed a new type of relays using the principle of harmonic restraint, which can distinguish between the magnetizing-inrush current and the internal fault current by their difference in the waveform shape. Authors  proposed a new digital algorithm to detect winding faults in single-phase and three-phase transformers. This algorithm is suitable to measure winding currents. Various operating conditions were simulated to test the algorithm. In 1997,  presented a new method to discriminate internal fault current from inrush current. To avoid unnecessary trip caused by the magnetizing inrush current, the second harmonic component is used to block the differential relays on the power transformers.
IEC 61850 standard has developed two types of models based on peer-to-peer communication, which are Sampled Values (SV) and Generic Object-Oriented Substation Event (GOOSE). IEC 61850 has magnificent features such as high priority, great flexible and reliable mechanism for the fast transmission events of the substation (trips commands, alarms or indications) . Authors  used the Phase-Locked Loops (PLL) technology to propose a novel current differential protection scheme. The scheme solves the problem of delay-non-determinism, and it also establishes a research platform to show the delay-non-determinism problem using the OPNET Modeller. The proposed method analyses the transferring time delay of sampled values from Non-Conventional Instrument Transformer (NCIT) to the differential protection IED using the IEC 61850 standard network. In this research work, Phase-Locked Loops (PLL) technology is used to implement the time synchronisation between Merging Unit (MU) and IEDs.
In 2009, authors  presented a new protection scheme based on IEC 61850–7-2 GOOSE communication. The proposed scheme uses analogue measurements (current and voltage) and digital signals between differential (87) and distance (21) protection functions. As soon as a location of the faulted equipment is identified, it is isolated by the proposed new protection scheme in a coordinated manner entirely by sending a trip signal to both local and remote breakers. Using the IEC 61850 standard technology, every single IED of the proposed scheme also detects the faulted adjacent line, and it sends a trip signal remotely to the corresponding breaker.
The impact of network traffic and sampling synchronisation error on the performance of the transformer differential protection based on IEC 61850 standard was investigated and compared to conventional hardwired connections by authors . To test the transformer differential protection performance caused by synchronisation error between Merging Units (MU), a testbed of precision time protocol (PTP) clocks, protection IED, merging units, and Ethernet switches to be investigated is developed. IEC 61850 GOOSE communication is used to trip Circuit Breakers (CB), report the tap changer position and transduce differential current measurements.
Authors , provides five different methods, namely, (1) Harmonic Restraint (HR), (2) conventional Waveform Identification (WI), (3) Fuzzy logic, (4) Wavelet analysis, and (5) Artificial Neural Network (ANN) to discriminate between magnetizing inrush and internal fault conditions. Harmonic Restraint is a typical way of restraining a trip. It is assumed that magnetizing-inrush contains a high level of second harmonic currents which is the HR principle of operation. The HR method uses the magnitude of the second harmonic in the differential current and compares it to the magnitude of the fundamental frequency component. The tripping of the differential element is blocked as soon as the ratio exceeds the set threshold.
The HR method  is based on the fact that the second harmonic (sometimes fifth) component of the magnetizing inrush current is considerably larger than a typical fault current. In traditional substations, overcurrent relays are used as a backup protection with time delay higher than that for the main protection. However the main drawback to realize on backup protection is due to time delay and loss of selectivity which may cause serious impact on the power system.
In smart substation, Intelligent Electronic Devices (IEDs) compliance with IEC61850 standard are deployed to obtain sampling data and state information of other IEDs through IEC61850 communication network which lead to improve the protection performance of the backup protection.
In this research work, transformer current differential protection using SEL487E and its backup overcurrent protection using SEL751A, both IED’s support IEC61850–7-4 logical nodes to share their data and communicate their Trip/block signals to the other IEDs on the network. This improves the protection speed and reliability of the backup overcurrent protection scheme using IEC61850 GOOSE communication in comparison with traditional hardwired signals with longer time delay setting. Therefore the research work reported in  implemented IEC61850 standard-based differential protection scheme which sent harmonic blocking signal to backup overcurrent relay during inrush current conditions to improve the speed and reliability of the backup protection.
Even-numbered harmonics (second or fourth) provide security during energization, while fifth-harmonic blocking provides security for overexcitation conditions. The research work presented in this manuscript consider second harmonic blocking method to provide security during energization (inrush current conditions) of both main differential and backup overcurrent protection for transformers. Overexcitation conditions are not part of this research work, therefore higher harmonic order such as fifth harmonics onwards not considered in this research work.
The authors  proposed a flux-restrained differential current for power transformer protection. In 1990,  presented an algorithm, based on HR, using discrete hartley transform. Different advanced digital filtering algorithms such as Kalman filtering , Fourier-based method , etc., are used in HR differential protection schemes. However, the HR-based method sometimes fails to prevent false tripping due to high second harmonic components during internal faults. Low second-harmonic components which are generated during magnetizing inrush of the transformers having modern core material. Therefore, the techniques based on detection of the second/fifth harmonic component may not be a proper method to discriminate between the inrush and fault condition on power transformers.
Differential protection and backup overcurrent protection are the most proficient internal faults protection for the power transformer. The differential protection uses differential currents, which result from the difference between HV side and LV side currents and overcurrent uses only primary current. Energization of transformers causes inrush current passing through the transformer coils . The inrush current is a harmonic rich current including slowly decaying DC component  because of transformer core saturation which leads to overcurrent relay mal-operation.
The differential transformer protection uses a conventional technique based on the second harmonic restraint; however, overcurrent relay does not have in-built computation for harmonic function in order to discriminate between an internal fault and inrush currents. This leads to the trip of the elements of the backup overcurrent relay due to transformer magnetizing inrush current conditions which affect the power systems stability. Therefore, the research project was motivated to develop a new method to prevent the malfunction of the backup overcurrent relay due to the transformer magnetizing-inrush current conditions.
The digital protection algorithms that were proposed in the past for protecting power transformers have been focused on using transformer differential protection. Most of the research work reviewed is concentrating on solving the differentiation between inrush current and internal faults on the main transformer protection relay and failed to focus on backup overcurrent. Therefore, this research work is focused on IEC 61850 standard-based harmonic blocking method to prevent the backup overcurrent relay from tripping during inrush current conditions. This paper provides the test bench implementation for the harmonic blocking scheme using both hardwired DC signal and IEC 61850 standard-based GOOSE message. The developed algorithm for the harmonic blocking scheme is presented. Two case studies are studied, one for the malfunction of the SEL-751A IED due to TMIC and another one to prevent the tripping of the SEL-751A IED due to TMIC using the harmonic blocking scheme.