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.

Power supply: The power supply is connected to terminals 15 and 16, with no polarity.

Grounding: The casing must be separately grounded to the ground grid with a copper wire of not less than 12 AWG; It is recommended that each device has independent wiring.

Insulation test warning: Before conducting the withstand voltage test, the connecting plug must be unplugged, otherwise high voltage may damage the internal semiconductor devices.

4. Setting calculation (core steps)

The purpose of setting BE1-87B is to ensure reliable non operation in case of faults outside the zone and reliable operation in case of minimum faults within the zone. The setting is divided into three steps: voltage setting, current setting, and sensitivity verification.

4.1 Voltage setting (based on the maximum voltage of external faults)

When there is a fault outside the area, the fault line CT may saturate, causing its secondary impedance to drop to the winding resistance. The current of other non fault CTs will all flow through the fault CT circuit, generating voltage at both ends of the differential circuit. The voltage must be less than the relay voltage setting value.

Calculation formula:

VDIFF=1.25×(RS+P⋅RL)×IFNV DIFF=1.25×(R S +P⋅R L)× NI FVDIFFV 

DIFF: Minimum required voltage setting value (taking the next step greater than the calculated value, with a step size of 50V).

RSR S: Fault CT secondary winding DC resistance+lead resistance (converted based on maximum operating temperature).

RLR L: one-way cable resistance from differential junction to fault CT (also calculated based on maximum temperature).

PP: Phase number coefficient, set to 1 for three-phase short circuits and 2 for single-phase grounding (because the current passes through a two core cable circuit during single-phase).

IFI F: Maximum out of zone fault current (primary symmetrical effective value), usually taken as the maximum breaking current of the circuit breaker or the maximum short-circuit current of the system.

NN: CT ratio (e.g. 1200/5, then N=240).

1.25 is the reliability coefficient.

Simplified method (compliance): directly take the maximum breaking current of all circuit breakers as

IFI F， Take the cable resistance of the farthest CT as RLR L and set P=2. Calculate once to obtain the maximum value

VDIF FV DIFF， Then take a higher level. This method is simple and does not require recalculating with system changes.

Accurate method: Calculate the three-phase and single-phase faults at each feeder end separately, take the corresponding IFI F and RLR L, calculate them separately, take the maximum value, and then take the higher level. This method may result in lower setting values and improve sensitivity.

Example: A 1200/5 CT, RS=0.524 Ω R S=0.524 Ω, farthest CT cable one-way resistance 0.493 Ω, maximum short-circuit current 12500A, N=240. then

VDIFF=1.25×(0.524+2×0.493)×12500/240≈98.3V 

V DIFF=1.25 × (0.524+2 × 0.493) × 12500/240 ≈ 98.3V, select the next level of 100V.

4.2 Current setting

The current setting needs to consider the following factors:

Avoiding CT circuit induced noise: Due to the high impedance of the differential circuit, external faults may induce voltage, but the induced current is extremely small. To prevent misoperation, the current setting is generally not less than 0.5A.

Combined with CT testing function: When using a CT testing source, the test current under an unhealthy CT circuit (short circuit) is approximately

Vtest/100V 

Test/100 (100 Ω series resistance) is usually around 0.3A, so the current setting should be greater than this value, such as 0.5A.

If there is a lightning arrester, the current setting must be set to 2.5A (highest level) to avoid the surge current during the operation of the lightning arrester.

If high sensitivity is desired, 0.25A or 0.5A can be set when there is no lightning arrester and the system is impedance grounded.

4.3 Minimum internal fault sensitivity verification

It is necessary to ensure that the relay can reliably operate in the event of minimal internal faults on the busbar (such as single-phase grounding). Sensitivity verification requires the peak characteristics corrected by the excitation characteristic curve of CT.

Steps:

Find the inflection point voltage based on the conventional excitation curve (effective value) of CT

EsE s and inflection point current IeI e.

Calculate points on double logarithmic coordinates

V=7Es

V=7E s，I=5Ie

I=5I e， Draw a straight line with a slope of 1/2 across this point (voltage corresponds to two ten times the current for every ten times the current), and form a corrected peak curve at the intersection with the extension of the lower straight line of the original curve.

Calculate the voltage required for the operation of relay voltage components

VS=22×VDIFF

V S=22 × V DIFF (in this case, VS=2.83 × 100=283VV S=2.83 × 100=283V).

Read the excitation current Ie'E '(approximately 0.05A in this case) corresponding to the voltage from the correction curve.

The current of the relay itself at the critical point of action is IR=(2 × VDIFF)/5000 Ω=0.04AI

R=(2 × V DIFF)/5000 Ω=0.04A (internal impedance of the relay is approximately 5000 Ω).

The total operating current (secondary value) is the sum of the excitation currents of each CT plus the relay current. If there are n CTs with the same characteristics, then

Iminsec=n × Ie '+IRI min_dec=n × Ie+IR, multiplied by the transformation ratio N to obtain the minimum operating current once.

Example (5 CTs, each 0.05A):

Iminsec=5 × 0.05+0.04=0.29AI min_dec=5 × 0.05+0.04=0.29A, with a primary value of 0.29 × 240 ≈ 70A. When the current is set to 0.5A, the corresponding primary current is 0.5 × 240=120A, and the larger one is taken, so the minimum operating current is 120A once. If the minimum fault current of the system is much greater than this, then the sensitivity is satisfied.

5. On site testing and verification

After new installation or maintenance, the relay's various functions should be tested according to the following steps.

5.1 Power Status and Basic Functions

Disconnect the power supply and ensure that terminals 11-12 (normally closed power status) are conducting.

Connect the rated power supply, the Power LED lights up, and 11-12 should be disconnected (indicating that the power supply is normal).

Disconnect the power supply, 11-12 should restore conductivity (fail safe design).

5.2 Alarm Voltage (CT OV) Test

Assuming Pickup Voltage=50V and Alarm Voltage=10% (i.e. 5V).

Apply 4.5V (90% set) for 10 seconds, and the alarm should not activate (LED not on, 13-14 not closed).

Apply 5.5V (110% setting), and the alarm action should be triggered after a few seconds (LED on, 13-14 closed). After removal, the LED will turn off (with a delay of about 1 second).

Repeatedly verify other alarm levels (20%~80%).

5.3 Voltage Pick up (SCR Trigger) Test

Set Pickup Voltage to 50V and Pickup Current to 0.25A.

Connect the voltage source to the input terminal and configure it to automatically disconnect when tripped.

When the voltage is slowly increased to about 100V (2 × 50V), the SCR should conduct, the output contacts 17-18 should close, and the Trip LED should light up. Error ± 5%.

Press Reset to reset the LED.

Assuming Pickup Voltage=100V, repeat and operate at 200V ± 4%.

Three phase models need to be tested phase by phase.

5.4 Current Pick up Test

Set the Pickup Current to 0.25A and adjust the voltage setting to 50V (to avoid voltage misoperation, but actual current testing can be triggered by short circuiting the input or applying sufficient voltage).

When the current source slowly increases from 0 to 0.25A ± 5%, the output contact closes and the Trip LED lights up.

Test each gear (0.5, 0.75,..., 2.5A) in sequence and record the action values.

5.5 Delay Test (if used)

If it is A1 (20ms delay), set the current to 3 times the set value (such as 0.75A), and measure the time from current application to contact closure, which should be within about 24.4ms (actual delay+inherent time).

A2 (2ms delay) is approximately 6.4ms.

If no delay is required, place the jumper in position 1-2 (without delay), with an action time of<7ms (1.5 times) or<5.5ms (6 times).

5.6 CT Circuit Integrity Test (Using CT Test Button)

According to the previous tuning example, assuming V_DIFF=150V and Alarm=10% (15V), calculate the required test voltage (usually using 30V or 60V range).

Connect the CT diagnostic test source to terminals 7 and 10, press the CT Test button for about 1 minute. If there is no short circuit in the CT circuit, the CT OV LED should light up and the Alarm contact should close; If the circuit is short circuited, there will be no response.

Attention: The test current should be less than the current setting value to ensure that there is no false tripping during testing. For example, when setting 0.5A, the unhealthy current tested is about 0.3A, and the safety margin is sufficient.

6. CT testing source application and security considerations

The CT test source consists of an isolation transformer and a 100 Ω series resistor, which is used to detect whether there is a short circuit in the CT circuit without exiting the protection. When the CT Test button is pressed, the test voltage (30V or 60V) is applied to the differential circuit through a resistor. If the impedance of the circuit is normal (CT parallel impedance is several hundred ohms), the voltage at the relay terminal after resistor division reaches the alarm threshold, indicating that the circuit is intact; If there is a short circuit somewhere, almost all the voltage drops across the resistor, the relay terminal voltage is insufficient, and the alarm does not operate.

Safety design: The test voltage is much lower than the SCR triggering voltage (2 × V_DIFF is usually ≥ 100V), and even if SCR is accidentally triggered, the relay will not trip due to the test current being less than the current setting value (such as 0.5A), so the testing process does not affect the safety of protection.

7. Maintenance and Storage Suggestions

BE1-87B is a solid-state relay that only requires regular (recommended annual) functional testing for daily maintenance. If stored for a long time, the internal electrolytic capacitor should be powered on for 30 minutes every year to maintain its performance. When abnormalities occur, contact the manufacturer for repair and do not dismantle or repair on site.