Basler BE1-87B busbar differential setting test

Complete guide for setting calculation and on-site testing of Basler BE1-87B high impedance bus differential relay

In power plants and substations, the busbar is the core confluence node of the power system. Once a fault occurs, if it cannot be quickly cut off, it will lead to widespread power outages and equipment damage. Bus differential protection is the main protection for bus faults, and its action speed, sensitivity, and reliability are directly related to system safety. The BE1-87B from Basler Electric is a high impedance, solid-state relay designed specifically for bus differential protection, featuring outstanding features such as fast action speed (up to<5.5ms), flexible setting, and built-in CT circuit monitoring. Starting from engineering practice, this article systematically analyzes the protection principle, selection points, installation and wiring, parameter setting calculation (including maximum voltage for external faults and minimum sensitivity for internal faults), and on-site testing methods of the relay, providing a technical reference for relay protection engineers that can be directly grounded.

1. Principle of high impedance differential protection and action logic of BE1-87B

BE1-87B belongs to the high impedance differential relay, and its core idea is to achieve selectivity by utilizing the difference in current and voltage flowing through the relay when faults occur inside and outside the busbar area. When there is a normal or out of zone fault, the secondary currents of each CT are basically cancelled out in the differential circuit, and the voltage at both ends of the relay is extremely low; When there is a fault in the area, all CTs supply current to the fault point, and a high voltage is generated at both ends of the relay, triggering the internal SCR to conduct and output a trip.

The relay adopts a dual criterion of voltage and current internally:

Voltage component: detects the effective value of the power frequency voltage at both ends of the differential circuit (set range 50~400V, step size 50V). When the instantaneous peak voltage reaches 2.83 times the set value (corresponding to the full offset waveform), the SCR is triggered to conduct.

Current element: After the SCR is turned on, the current flowing through the interior of the relay must be greater than the set current threshold (0.25~2.5A, step size 0.25A) in order to finally output the trip signal.

This "voltage start+current hold" design effectively prevents misoperation caused by CT saturation during external faults, while improving the reliability of internal faults.

2. Interpretation of Model Code and Key Selection Points

The model code of BE1-87B defines options such as primary/secondary rating, phase number, timing characteristics, power supply, chassis, etc. Taking S5AA1YNONOF as an example:

S: Single phase current detection (also available in three-phase M-type).

5: 5A current input range (standard CT secondary 5A).

A: The front panel has a CT test button channel.

A1: Equipped with a 20ms delay (for use with high-speed fuses), and A2 (2ms delay for lightning protection interference).

Y: Control power supply 48/125Vdc or 110Vac (wide range), and also Z (125/250Vdc or 110/230Vac).

The N series indicates no special options.

F: Semi embedded S1 chassis (single-phase); Three phase is M1 or 19 inch rack.

Selection precautions:

Busbar protection usually requires three phases, so choose a three-phase model (starting with M).

If there are feeders (such as capacitor banks) protected by high-speed fuses in the busbar branch, A1 delay (20ms) should be selected to avoid relay misoperation when the fuse cuts off the fault.

If there is often transient interference such as lightning strikes near the busbar, A2 (2ms delay) can be selected to improve anti-interference performance.

The power supply selection needs to match the DC/AC voltage inside the station, with Y-shaped coverage of the most common 48/125Vdc and 110Vac.

3. Installation wiring and grounding specifications

BE1-87B adopts S1 (single-phase) or M1 (three-phase) drawer type chassis, supporting semi embedded, protruding, and 19 inch rack installation. Please refer to the manual for installation dimensions and hole drawings.

Key wiring points:

Current circuit: All CT secondary windings are connected in parallel to the differential junction point at the same end, and each phase is connected to the corresponding input terminal of the relay (single-phase: terminals 5 and 7; three-phase: A phase 5 and 7, B phase 3 and 7, C phase 1 and 7). The CT circuit must use shielded twisted pair or twisted cable to reduce induced interference.

Locking contact (86): After tripping, the relay input should be short circuited (terminals 5-6, 3-4, 1-2) through the normally open contact of the external locking relay (86) to protect the internal SCR from long-term high current impact. The action time of the latch relay should be less than 1 cycle (16ms).

CT test source: If a CT diagnostic test source (P/N 9282300014) is selected, its output is connected to terminals 7 (common) and 10 (test) for regular testing of CT circuit integrity.