Basler Electric BE1-87G Variable Percentage Differential Relay

 Basler Electric BE1-87G Variable Percentage Differential Relay

selective, high-speed, differential protection for generators, motors, and shunt reactors.

Differential relaying selectivity is based on the ability of a relay to distinguish between an internal fault

(within the protected zone) and an external fault. Under normal operating conditions, the current flowing

into the protected zone equals the current flowing output of the protected zone with a net operating

current equal to zero. Internal faults upset this balance and result in a difference between the input and

output currents. External faults have relatively little effect on the balance because the protected zone

input current still equals the output current. Therefore, by comparing the currents on both sides of the

protected element or zone and detecting when these currents are not equal, a differential relay acts to

isolate the element or zone from the system with unsurpassed effectiveness.

BE1-87G relays typically trip a lockout (86) relay which in turn trips the generator breaker and, when

present, the field and/or neutral breakers.

are recommended for the following specific applications:

• Generators:

o Any terminal voltage and a rating of 1.000 kVA and above

o Any kVA rating and a terminal voltage of 5 kV and above

o A terminal voltage of 2.200 V or higher and a rating of more than 500 kVA

• Motors rated at 1.500 horsepower and above

• As primary protection on shunt reactors for transmission lines

• Generator ground differential

Differential relaying is the most selective form of fault protection which may be applied to the individual

elements or zones of ac power systems. Various types of differential relays and relaying systems have

evolved to take advantage of the differential principle.

Variable Restraint Characteristic

At high current levels, the inevitable difference in the saturation characteristics between current

transformers indicates a need for a compensating decrease in relay sensitivity. The design of the

BE1-87G provides a restraint factor that is proportional to input current when the restraining current (IR) is

greater than nominal (5 Aac for sensing input range 1 or 1 Aac for sensing input range 2). The BE1-87G

compares the protected zone sensed input and output currents. The lesser of the two sensed current

levels becomes the restraining current. The difference between the two sensed currents (the operating

current) is compared to a reference established by the sensitivity setting, and adjusted by an amount

proportional to the restraining current. This makes the BE1-87G more sensitive to low current internal

faults, and less sensitive to external faults with high levels of through current.

When the restraining current is at nominal (5 Aac for sensing input range 1 or 1 Aac for sensing input

range 2) or less, the relay trips if the differential current exceeds the relay setting (IS). But when the

restraining current is greater than nominal, the overall sensitivity is a combination of the front panel

setting and the restraint factor.

Design Highlights

Some of the many advantages of the BE1-87G relay are summarized below:

• Seven sensitivity levels on each of the two sensing input ranges. The seven levels allow

compensation for CT mismatch and provide the flexibility and adaptability necessary for many special

applications such as split-winding generator protection.

• Stabilizing reactor. Minimizes dissimilar performance of system CTs. Reactor can be located on the

back of the relay or remotely from the BE1-87G for flexibility of system installation.

• Variable restraint. The variable restraint characteristic allows increased sensitivity to low-current,

internal faults while providing increased security against high levels of through-current caused by

external faults.

• Single- or three-phase availability. Either configuration is available in the Basler Electric S1 draw-out

case.

• High-speed operation. The BE1-87G operates in 30 milliseconds for fault levels of 10 times the

sensitivity setting. This high-speed operation minimizes potential damage to the protected equipment.

Response characteristics for sensing input ranges 1 and 2 are shown in the Testing section.

Model and Style Number

The model number BE1-87G designates the relay as a Basler Electric, Class 100. Variable Percentage

Differential Relay.

The electrical characteristics and operational features of the BE1-87G relay are defined by a combination

of letters and numbers which constitutes the relay style number. The style number describes the features

and options of the BE1-87G relay and appears on the front panel, draw-out cradle, and inside the case

assembly.

Style Number Chart

The BE1-87G style number chart is illustrated in Figure 1-3.

Style Number Example

As an example, if the style number of a BE1-87G were G1EA1JA0C0F, the relay would have the

following characteristics and features: