BECKWITH ELECTRIC M?3311A, Instruction Book

Page: 178 / 450

4–51
System Setup and Setpoints – 4
24 Volts/Hz Overexcitation

NOTE:
Two or four voltage inputs are available
for the M-3311A. The Voltage Input can
be a phase voltage input or voltage
generated from a broken delta VT con -
nection. 81O/U, 27, and 24 Functions
are only available if the voltage input is
connected to the phase voltage. If the
voltage input is connected to phase volt-
age, Function 59G will be unavailable.
Function 59G is only available if the
voltage input is connected to a broken
delta VT. If voltage input is connected to
broken delta VT, Functions 81O/U, 27,
and 24 will be unavailable.
The 24 Volts-Per-Hertz (V/Hz) function provides
over-excitation protection for the transformer. As the
volts per hertz level rises above a transformer’s limit,
leakage flux increases. The leakage flux induces
current in the transformer support structure causing
rapid localized heating.
In power plant applications, over-excitation can
occur due to sudden tripping of the generator as a
result of faults and other abnormal conditions.
In Extra High Voltage (EHV) applications, an incor -
rectly switched line can lead to over-excitation at
tapped transformers due to combined capacitance.
In transmission and distribution applications, sud-
den loss of load or improper capacitor/reactor
switching may result in overexcitation.
This function provides two Definite Operating Time
setpoints, four families of Inverse Time curves widely
used in the industry (see Appendix D, Figures D1 to
D4), and a linear reset rate programmable to match
specific cooling characteristics of the transformer.
The V/Hz function provides reliable measurements
of V/Hz for a frequency range of 10–80 Hz.
When applied for generator and unit transformer
protection, the first task in setting this relay function
is to determine the desired protective levels and
times. This can be accomplished by combining the
V/Hz limit curves of the transformer and the associ -
ated generator on one graph and simplifying the re-
sult into one curve to coordinate with the protection.
Example of Transformer limits:
• Full Load V/Hz = 1.05 PU (HV terminals)
• No Load V/Hz = 1.10 PU (HV terminals)

NOTE:
The curves must be on the same volt-
age base to be combined on one graph.
An example is shown in Figure 4-28,
Example of Capability and Protection
Curves. The manufacturer of the genera-
tor and transformer will provide these
over-excitation capability limits.
Depending on these characteristics, they can best
be matched by one of the four families of inverse
time curves, alone or in conjunction with definite
time setpoints. Coordination of capabilities and
protection is achieved when the time between the
relay operation and the capability limit is sufficient
for the breakers to open and de-energize the units.
This coordination time is read vertically between
the two curves at any given V/Hz value.
Figure 4-28, Example of Capability and Protection
Curves, illustrates a composite graph of gen-
erator limits, transformer limits, a chosen inverse
time curve, inverse time pickup, and definite time
setpoint. While inverse time curve selection may
provide more selective and sensitive protection,
a traditional two-step protection scheme may be
realized by using the two definite time functions
(24DT #1 and #2), and disabling the inverse (24IT)
element.