Basler Electric BE1-67 DIRECTIONAL OVERCURRENT RELAY

Description

BE1-67 Phase Directional Overcurrent Relays are designed for the protection of transmission and

distribution lines where the direction as well as the magnitude of the fault current (or power flow) is to be

considered in the tripping decision.

BE1-67 relays are directionally controlled, microprocessor based, time overcurrent relays. The directional

element is polarized by the phase-to-phase quadrature voltage of the power system. That is, the

directional element monitoring the phase A current uses the voltage between phases B and C to

determine the direction of current (or power) flow into the fault. Then, if enough current flows in the

tripping direction of the relay, the relay will pickup, time out, and trip. The angle of maximum sensitivity for

the relay is also adjustable to allow the directional characteristic to be matched to the line and system

conditions. Figure 1-1 illustrates the operation of the directional element and defines the terms that are

used in the following discussion.

Figure 1-1a shows the connections to the sensing circuits for a single phase BE1-67. Figure 1-1b

illustrates the phasor quantities monitored by the relay for a unity power factor condition, and for a single

phase fault. Figure 1-1c shows the protected line on an R-X diagram. The angle alpha in Figure 1-1b and

Figure 1-1c are the characteristic angle settings for the relay.

The directional characteristic of the relay is adjustable to allow the relay to be sensitive for phase faults

and to maximize sensitivity at the characteristic angle representing typical power factor.

Twelve standard time-current characteristic curves are available to aid in the coordination of this relay

with other protective devices in the system. These include seven characteristic curves that are standard in

North America and five that are compatible with British or IEC standards requirements. Any of the curves

may be switch selected to suit requirements at the time of installation.

If the supply to the protected portion of the system is constant, and if the magnitude of the fault current is

determined primarily by the location of the fault on the line, the selection of a more inverse time

characteristic is more desirable to provide selective coordination with adjacent line protection. However, if

the capacity of the supply varies significantly over a period (such as a day), a less inverse time or even

the definite time characteristic, may be preferred to provide smoother coordination.

Limited Region of Operation

A limited region-of-operation option is available to provide additional protection against false tripping on

mutually coupled lines. Faults on adjacent lines that share the same poles, towers, or right-of-way may

induce currents on the protected line which appear as fault currents in the tripping direction. The limited

region of operation mode provides discrimination between faults on the protected line and faults on the

adjacent line. To order this option, specify option 3-5 or 3-6.

One consideration in applying a phase directional overcurrent relay is the definition of trip direction. For

most applications, the setting of the relay directional element is based upon the impedance characteristics

of a given circuit. This angle is then used as the maximum torque angle and any current flowing in the

half-plane defined by this angle is considered to be in the trip direction.

However, there are at least two situations where the half-plane trip region is not adequate. They are when

load current approaches the fault current and when leading current flows in the non-trip direction above

the relay pickup setting.

Load Current Approaches Fault Current

Pickup settings on a phase overcurrent device are normally set below the expected fault current levels on

that line by some margin. Consequently, it is possible for load current to approach (or exceed) the pickup

setting on the relay. This could lead to an undesirable trip for an acceptable load condition.

Weak Infeed Condition

During a period of abnormally low system voltage, leading power factor current above relay pickup can

flow in the non-trip direction of a line. Probable current sources are outlying capacitor banks. This could

cause the current to be sensed as lagging current flowing in the trip direction and leading to an

undesirable trip. (A condition often referred to as weak infeed because the lower voltage system - where

load is present - attempts to correct the undervoltage condition on the higher voltage system.)

Application

Without the ability to act on the direction of current flow, it is difficult to coordinate the settings of time

overcurrent relays on lines that interconnect a series of substations. Without this capability, either

undesired tripping of adjacent lines may occur or a fault may go undetected because of the high settings

required by non-directional relays.

With directional time overcurrent relays, the settings and time delays can be decreased and the undesired

tripping eliminated. Figure 1-5 illustrates the use of directional overcurrent relays on a group of

interconnected distribution substations fed from a common source. In this example, non-directional

overcurrent relays (51) are used to protect the lines leaving the supply bus because there is only one

source of fault current. However, the breakers at the load buses (C, D, E, and F) are protected by

directional time overcurrent relays (67) to prevent overtripping in the event of a fault. This will remove the

faulted line and retain service to the connected loads.

In the case where two sources of power can supply fault current, as shown in Figure 1-6. directional

overcurrent relays will need to be applied to each end of the protected lines to prevent undesired tripping.