Basler BE1-87G Generator Differential Relay
The Basler BE1-87G is a high‑impedance differential relay specifically designed for the protection of large AC generators against stator winding phase‑to‑phase and phase‑to‑ground faults. Its fast operating time and sensitive setting range make it a cornerstone of generator protection schemes in utility and industrial power plants.
Operating Principle
The relay operates on the high‑impedance differential principle, where a stabilising resistor and voltage‑dependent resistor are used to create a high‑impedance path for fault current. Under normal conditions, the currents entering and leaving the generator stator are balanced, resulting in zero differential current. During an internal fault, the imbalance produces a voltage across the relay that exceeds the pickup threshold, triggering a trip signal. The high‑impedance design ensures that CT saturation during external faults does not cause maloperation.
Key Features
High‑impedance design immune to CT saturation during external faults.
Adjustable pickup current from 0.2 to 20 A in discrete steps.
Fixed or adjustable time delay for coordination.
Built‑in transient voltage suppression.
LED indicators for trip and power.
Compact drawout case for easy maintenance.
Technical Specifications
The BE1-87G operates from a nominal DC supply of 24/48/125/250 VDC. The pickup setting range is 0.2–20 A AC (rms) at 50/60 Hz. The relay has a nominal burden of 0.1 VA at rated current. Operating time is typically 1.5 cycles (25 ms) at 20 times pickup. The stabilising resistor value can be selected to match CT secondary ratings, and the relay includes a built‑in voltmeter test jack for verification.
Application Guidelines
This relay is used in conjunction with current transformers (CTs) connected in a circulating‑current scheme. It is suitable for generators of any size, from small industrial units to large utility turbines. The relay provides primary protection for stator windings and backup protection for system faults when combined with other relays.
Setting and Calibration
Set the pickup current based on the generator rated current and CT ratio. The minimum pickup should be above the maximum unbalance current expected from CT errors during external faults. The time delay, if used, is set to coordinate with downstream protection devices. Calibration is performed using the test jack and a variable AC current source to verify the trip threshold.
Installation and Wiring
Mount the relay in a standard 19‑inch rack or panel. Connect the CT secondaries in a differential circuit, ensuring proper polarity. The stabilising resistor is connected in series with the relay operating coil. All wiring should be short and twisted to minimise loop area and reduce induced noise. Ground the CT neutrals at a single point as per plant practice.
Testing and Commissioning
Before commissioning, perform a secondary injection test to confirm the pickup and timing settings. Use a test set to inject differential current and observe the trip output. Also check the continuity of the CT circuits and the resistance of the stabilising resistor. Record all test results for future reference.
Maintenance
Periodically inspect the relay for dust, corrosion, and loose connections. Verify the LED indicators and test jack functionality. Re‑calibrate the pickup settings if the system CTs or generator parameters change. The relay requires minimal maintenance due to its solid‑state design.
Comparison with Other Protection Methods
Compared to percentage differential relays, the high‑impedance BE1-87G offers simpler setting calculation and better stability against CT saturation. It also provides faster clearing times for severe faults, reducing damage to the generator and improving system stability.
Conclusion
The Basler BE1-87G Generator Differential Relay is a proven, reliable component for stator fault protection. Its high‑impedance principle, adjustable settings, and rugged construction ensure dependable operation in the most critical power generation environments.
