Differential Protection Relay in Transformers

 Differential Protection Relay (87T) – Brief Overview

Purpose:

• Detects internal transformer faults (winding-to-winding, winding-to-earth, turn-to-turn).
• Provides fast and selective protection, minimizing damage and downtime.
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Working Principle:

• Based on Kirchhoff’s Current Law: Sum of currents entering and leaving the transformer winding = 0 under normal conditions.
• CTs on primary and secondary sides feed the relay.
• If sum ≠ 0 → Internal fault → Relay trips the breaker.

Figure – Working Concept:

    HV Side CT ---------\

                        \        

                         Differential Relay 87T ----> Trip CB

                        /

    LV Side CT ---------/

Key Features:

• High-speed operation (~10–50 ms)
• Stability during inrush and external faults
• Percentage bias to avoid false trips

Applications:

• Transmission & distribution power transformers
• Generator step-up transformers
• Critical industrial transformers

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Suppliers & Examples

Company	Relay Series / Model Example	Key Features	Typical Use
Siemens	7UT8, 7SD82	High-speed, multifunctional, IEC61850 ready	Transmission/Distribution
ABB	RET670, REF615	Integrated transformer protection, digital	All voltage levels
Schneider Electric	Sepam S82	Modular, configurable, user-friendly	Distribution transformers
GE Grid Solutions	Multilin 87T, L90	Digital, communication-enabled, event logging	Substations & industrial
SEL (Schweitzer)	SEL-587, SEL-387	Flexible logic, wide CT ratio support	Transmission & distribution
NR Electric	NR-87T series	Reliable, cost-effective	Medium voltage transformers
C&S Electric / Orbis	Orbis Differential Relay	Compact, industrial use	Distribution networks

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Figure – Typical Connection Layout in a Transformer:

       HV Winding          LV Winding

       -----               -----

      |     |             |     |

      CT1   CT2           CT3   CT4

       |     |             |     |

       \_____|_____________|_____/

                |

        Differential Relay 87T

                |

             Trip Signal

                |

             Circuit Breaker

• CT1 & CT2: High voltage side CTs
• CT3 & CT4: Low voltage side CTs
• 87T Relay: Measures differential current and trips CB on internal fault

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