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Directional Overcurrent Relaying (67) Concepts
#1

Directional Overcurrent Relaying (67) Concepts


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Abstract

Directional overcurrent relaying (67) refers to
relaying that can use the phase relationship of voltage and
current to determine direction to a fault. There are a variety of
concepts by which this task is done. This paper will review the
mainstream methods by which 67 type directional decisions are
made by protective relays. The paper focuses on how a numeric
directional relay uses the phase relationship of sequence
components such as positive sequence (V1 vs. I1), negative
sequence (V2 vs. I2), and zero sequence (V0 vs. I0) to sense fault
direction, but other concepts such as using quadrature voltage
(e.g., VAB vs IC) are included.

INTRODUCTION

In some medium voltage distribution lines and almost all high
voltage transmission lines, a fault can be in two different
directions from a relay, and it can be highly desirable for a
relay to respond differently for faults in the forward or reverse
direction. The IEEE device number used to signify a
directional element is either a 21 (impedance element, based
on Z=V/I, and having a distance to fault capability) or a 67
(directional overcurrent, generally based on the phase
relationship of V and I, with no distance to fault capability).
Some applications also might use a 32 (power element, based
on P=Re[VxI*]) for directional control, though in some
circumstances a 32 element may not be a good indication of
direction to fault. This paper will review some of the various
implementations of 67 elements as found in
electromechanical, solid state, and numeric (i.e., multifunction
programmable logic microprocessor based) relays.

CLASSICAL CONCEPTS FOR DIRECTIONAL ANALYSIS

The classic electromechanical and solid state relay, as well as
some common numeric relays, determines the direction to
fault by comparing the phase angle relationship of phase
currents to phase voltages. If only per phase watt flow (32
element) is to be considered, the basic concept would be that if
IPh is in phase with VPh-N (0°, ±90°), then power flow on that
phase is indicated as forward (or reverse, depending on one’s
perspective). However, for a phase to ground fault, the VPh-N
may collapse to 0, and I may be highly lagging, so that VPh-N x
IPh may be mostly VAR flow, and thus prevent the relay from
making a correct directional decision. To resolve the low
voltage issue, quadrature voltages (i.e., VBC vs. IA) are
commonly used. To resolve the issue that fault current is
typically highly lagging, the relay current vs.

SYMMETRICAL COMPONENTS FOR DIRECTIONAL
ANALYSIS


Many modern microprocessor relays use the angular
relationships of symmetrical component currents and voltages
and the resultant angular nature of Z1, Z2, and Z0 as calculated
from Vphase/Iphase to determine direction to fault. These three
impedances are used to create, respectively, three directional
assessments, 67POS, 67NEG, and 67ZERO, that are used in relay
logic in various ways by each manufacturer. There are
variations among manufacturers on of how one senses the
angular relationships and, in most cases since the angular
relationship is the only concern, the magnitude is not
calculated. The common concept is that in faulted conditions
there is an approximate 1800 difference of calculated Z1, Z2
and Z0 for faults in the two directions from the relay location.
This high variation in phase angle is a reliable indication of
direction to fault.

OTHER ISSUES

There is a number of subtleties involved in the forward/reverse
direction decision and element operation that will not be
covered here. One should refer to the various relay
manufacturers’ instruction manuals for details on their relays’
algorithms. Some issues that need to be understood that can
vary by manufacturer implementation:

Memory Polarization

For close-in three phase faults, the voltage at the relay may
fall to near 0. Due to the low voltage, the relay’s 67 logic
cannot be relied upon to make a correct directional analysis
decision, and in some relay configurations, if the relay cannot
determine forward or reverse, it does not trip at all. To address
this issue, numeric relay manufacturers create a memory
polarization scheme. The relay constantly is reading the
present voltage and using it to create a voltage vector (V1). If a
fault occurs that suddenly drives voltage too low to be used for
directional analysis, the relay reaches back to its memory and
projects the past voltage vector into the present. The V1
voltage vector change very slowly in the normal power
system, so a past V1 voltage vector is a good indication of the
voltage vector that would exist if a fault had not occurred, and
it is a reliable backup for directional analysis.

Close in to Fault Logic

When a breaker is closed into a three phase fault (i.e.,
grounding chains), the memory polarization scheme will not
work because there is no pre-event V1 vector for the relay to
work with. The Close In To Fault logic monitors for a breaker
close and enables a high set three phase non-directional
overcurrent sensing circuit for a short period of time. The
setting of the 50 element must be above maximum load inrush
in either direction.
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#2
This is also a special type of overcurrent relay with directional characteristics. This directional overcurrent relay uses the relay drive principle when the fault current flows in the relay in a particular direction. If the power flow is in the opposite direction, the relay will not operate. Normally, the conventional overcurrent relay (without direction) will act for the fault current in any direction. The directional overcurrent relay recognizes the direction in which the fault occurs, in relation to the location of the relay. The principle of directional protection is as follows: Consider an XY feeder, passing through station A. The circuit breaker in feeder AY is provided with a directional relay R, which will trip the CBy circuit breaker, if the fault power flow is in the AY address alone. Therefore, for faults in the AX feeder, the CBy circuit breaker does not trip unnecessarily. However, for faults in the feeder AY, the circuit breaker CBy is triggered, due to the address characteristic of the relays, configured to act in the direction AY. This type of relay is also referred to as the reverse power relay, with respect to the direction of the fault current flow (power). Reverse power flow relays with directional characteristics not only detect the flow of direction, but also measure the magnitude of the power flow. Directional Relay Connections Whenever a near or near failure occurs, the voltage becomes low and the directional relay may not develop enough torque for its operation. In order to obtain sufficient torque on all types of faults, regardless of the locations with respect to the relays, the relay connections must be modified. Each relay is energized by the current of its respective phase and voltage. One of the methods of such connections is connection 30o and another is connection 90o.
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