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M-3410A Instruction Book
3–24
40
Loss of Field (Generator Protection Only)
The Loss-of-Field function (40) provides protection
for a partial or complete loss of field. A variety of
possible settings make the M-3410A Intertie/
Generator Protection Relay very flexible when
applied to loss-of-field protection.
The loss-of-field function is implemented with two
offset mho elements, an undervoltage element,
and a directional element. The setting for each mho
element, diameter, offset, and time delay, are
adjusted individually. Voltage control may be enabled
on each element, and the voltage control level
setting is common. When voltage control is enabled,
the measured positive sequence voltage must be
less than the voltage control setting for the loss-of-
field function to operate. The common directional
unit is provided to block the relay operation during
slightly underexcited conditions (since approach
#1, Figure 3-29, with negative offset is inherently
directional, the directional element is not required).
The directional unit’s zero sensitivity (torque) line
is placed at –13° from the R axis.
The settings of the offset mho elements should be
such that the relay detects the loss-of-field condition
for any loading while not mis-operating during power
swings and fault conditions. Two approaches are
widely used in the industry, both of which are
supported by the M-3410A relay. Both approaches
require knowledge of the reactances and other
parameters of the generator. They are illustrated in
Figure 3-29, Loss-of-Field (40) – Protective Approach
1, and Figure 3-30, Loss-of-Field (40) – Protective
Approach 2.
The impedance can be set in PU quantities. The
PU impedance is based on the nominal voltage and
nominal current setting.
The first approach
is shown in Figure 3-29. Here,
both of the offset mho elements (#1 and #2) are set
with an offset of -X
’
d
/2, where X
’
d
is the direct axis
transient reactance (unsaturated) of the generator.
The diameter of the smaller circle (#1) is set at 1.0
PU impedance on the machine base. This mho
element detects loss-of-field from full load to about
30% load. A small time delay (5–10 cycles) provides
fast protection.
The diameter of the larger circle (#2) is set equal to
X
d
, where X
d
is the direct axis synchronous
reactance of the machine. This mho element can
detect a loss-of-field condition from almost no load
to full load. A time delay of 30 to 60 Cycles (#2)
should be used in order to prevent possible incorrect
operation on stable swings.
The second approach
is shown in Figure 3-30. In
this approach, one of the mho elements is set with
an offset of -X
’
d
/2, a diameter of 1.1 X
d
-(X
’
d
/2), and
a time delay of 10 to 30 Cycles. The second
element is set to coordinate with the generator
minimum excitation limit and steady-state stability
limit.
In order to obtain proper coordination, the offset of
this element must be adjusted to be positive.
Typically, the offset is set equal to the unit
transformer reactance (X
T
). The diameter is
approximately equal to (1.1 X
d
/ X
T
). A time delay of
30 to 60 Cycles would prevent mis-operation on
stable swings.
Although the voltage control is common to both
zones, either one can be enabled or disabled and is
typically set at 80% to 90% of the nominal voltage.
The voltage control should be applied after careful
study of the system since, depending on the
stiffness of the system, the voltage may not be
reduced enough to operate the undervoltage element
during loss-of-field conditions.
Содержание Syncrocloser M-3410A
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