Basler Electric 87T Transformer Differential Relay

The Basler Electric 87T is a solid‑state high‑impedance differential relay specifically designed for the protection of power transformers. It compares the currents entering and leaving the transformer windings; any difference indicates an internal fault (phase‑to‑phase, phase‑to‑ground, or inter‑turn). The relay trips rapidly to minimize damage and isolate the transformer from the system. Its high‑impedance principle provides inherent stability for external faults and CT saturation conditions.

Operating Principle

The relay uses a voltage‑operated high‑impedance circuit. CTs on each side of the transformer are connected in parallel with a stabilizing resistor. Under normal or through‑fault conditions, the CT secondary currents are balanced, resulting in negligible voltage across the relay. During an internal fault, the imbalance produces a voltage exceeding the pickup threshold, causing the relay to operate. The pickup setting is adjustable via tap selection and a potentiometer.

Key Features

The 87T relay offers adjustable pickup settings (typically 0.1‑2.0 A secondary), a built‑in test circuit for functional verification, and a target indicator with manual reset. It includes two independent output contacts (Form C) for tripping and alarm. The relay features a wide operating temperature range and is immune to DC offset and harmonics. A harmonic restraint feature (optional) prevents tripping on magnetizing inrush.

Technical Specifications

Input CT secondary: 1 A or 5 A. Frequency: 50/60 Hz. Pickup range: 0.1‑2.0 A (adjustable). Operating time:<30 ms at 5x pickup. Output contacts: 2 SPDT, rated 5 A at 250 VAC. Power supply: 24‑250 VDC or 120‑240 VAC. Stabilizing resistor: built‑in or external. Operating temperature: –30°C to +65°C. Enclosure: metal, panel‑mount.

Applications

This relay is used on power transformers of all ratings, from distribution to large generation step‑up units. It provides primary protection for winding faults and can be applied to two‑winding or three‑winding transformers. It is often coordinated with backup protection such as overcurrent or distance relays. The 87T can also be used for reactor protection.

Installation and Commissioning

Mount the relay in a standard panel cutout. Connect the CT leads from both sides of the transformer, ensuring correct polarity and grounding. Commissioning involves setting the pickup tap, adjusting the stabilizing resistor, and performing secondary injection tests to verify trip levels and timing. The relay includes a test pushbutton for quick functional checks.

Reliability and Standards

Compliant with IEEE C37.91 and IEC 60255, the 87T is built with high‑quality solid‑state components and conformal‑coated boards for environmental resilience. It has a proven track record in critical applications. Periodic testing is recommended to maintain accuracy.

Conclusion

The Basler Electric 87T transformer differential relay is a fast, secure, and stable protection solution for transformer internal faults. Its high‑impedance design and adjustable settings make it a preferred choice for protection engineers worldwide.