BECKWITH ELECTRIC M-3425A, Instruction Book

Page: 196 / 482

4–65
System Setup and Setpoints – 4
40 Loss of Field
The Loss-of-Field function (40) provides protection
for a partial or complete loss of field. A variety of
possible settings make the M-3425A Generator
Protection Relay very flexible when applied to
loss-of-field protection. Ranges and increments are
presented in Figure 4-46.
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. Each element has two
time delay settings. The second time delay (delay
with VC) is applicable with voltage control, and the
timer only starts if the positive sequence voltage
is below the voltage control setting. The function
with voltage control and without voltage control
can be programmed to send to two different output
contacts, if desired. The delay with voltage control
may be enabled on each element but the voltage
level setting is common. The voltage control allows
for faster tripping when low voltage may be caused
by the VAr intake by the machine with loss of
excitation. A common directional unit is provided to
block the relay operation during slightly underexcited
conditions (since approach #1 with negative offset
is inherently directional, the directional element is
not required). The directional unit’s angle setting
(Q
D
) can be set from 0° to 20°.
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-3425A relay. Both approaches
require knowledge of the reactances and other
parameters of the generator. They are described
in Figure 4-44, Loss of Field (40) — Protective
Approach I and Figure 4-45, Loss of Field (40) —
Protective Approach II.
Positive sequence impedance measurements are
used for the loss of field functions. All impedance
settings are secondary relay quantities and can be
derived from the following formula:
Z
SEC
= Z
PRI
x (R
C
÷ R
V
)
where Z
SEC
= secondary reflected impedance,
Z
PRI
= primary impedance
,
R
C
= current transformer
ratio, and R
V
= voltage transformer ratio.
The first approach
is shown in Figure 4-44, Loss
of Field (40) — Protective Approach I. Here, both
of the offset mho elements (#1 and #2) are set
with an offset of –X
l
d
÷2, where X
l
d
is the (saturated)
direct axis transient reactance 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 provides fast protection.
The diameter of the larger circle (#2) is set equal
to X
d
, where X
d
is the (unsaturated) 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 time delay with voltage control is typically set
shorter than the other time delay.
The second approach is shown in Figure 4-45,
Loss of Field (40) – Protective Approach II. In this
approach, one of the mho elements is set with an
offset of –X
l
d
÷ 2, a diameter of 1.1 X
d
-(X
l
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.
The following table provides suggested time settings
when time delay with VC is used in addition to
standard time delay.
Typical setting is 13° (0.974 power factor). This
setting is common to both element #1 and #2.
Approach #1 can also be used for Zone #1, and
approach #2 for Zone #2, where better coordination
with AVR limiters, machine capability limits, and
steady state stability limits can be obtained.