BECKWITH ELECTRIC M-3520, Instruction Book

Page: 74 / 217

2–41
Application - 2
78 Out-of-Step
The Out-of-Step function (78) is used to protect the
generator from out-of-step or pole slip conditions.
Out-of-Step conditions can occur when faults are
being cleared on the distribution system, and the DG
accelerates as mechanical input exceeds electrical
demand and synchronizing power is lost. This function
uses one set of blinders, along with a supervisory
MHO element. Ranges and increments are presented
in Figure 2-30.
The pickup area is restricted to the shaded area in
Figure 2-29, Out-of-Step Relay Characteristics,
defined by the inner region of the MHO circle, the
region to the right of the blinder A and the region to
the left of blinder B. For operation of the blinder
scheme, the operating point (positive sequence
impedance) must originate outside either blinder A or
B, and swing through the pickup area for a time
greater than or equal to the time delay setting and
progress to the opposite blinder from where the
swing had originated. When this scenario happens,
the tripping logic is complete. The output contact will
remain closed for the amount of time set by the seal-
in timer delay.
Consider, for example, Figure 2-29. If the out-of-step
swing progresses to impedance Z
0
(t
0
), the MHO
element and the blinder A element will both pick up.
As the swing proceeds and crosses blinder B at
Z
1
(t
1
), blinder B will pick up. When the swing reaches
Z
2
(t
2
), blinder A will drop out. If TRIP ON MHO EXIT
option is disabled and the timer has expired
(t
2
–t
1
>time delay), then the trip logic is complete. If
the TRIP ON MHO EXIT option is enabled and the
timer has expired, then for the trip to occur the swing
must progress and cross the MHO circle at Z
3
(t
3
)
where the MHO element drops out. Note the timer is
active only in the pickup region (shaded area). If the
TRIP ON MHO EXIT option is enabled, a more
favorable tripping angle is achieved, which reduces
the breaker tripping duty. The relay can also be set
with a Pole Slip Counter. The relay will operate when
the number of pole slips are greater than the setting,
provided the Pole Slip Reset Time was not expired.
Typically, the Pole Slip Counter is set to 1, in which
case the Pole Slip Reset Time is not applicable.
A B
Z
3
(t
3
)
Z
2
(t
2
)
Z
1
(t
1
)
Z
0
(t
0
)
Figure 2-29 Out-of-Step Relay Characteristics