TMS-811-1/AFJMAN
32-1080
true RMS as required. Refer to TM 5-811-14 and
Transmission voltage - 69 kV
IEEE Std 242 for guidance regarding coordinated
Distribution voltage - 13.8 kV
power system protection.
(1) Oil circuit breakers. An ammeter and
Assumed utility
short circuit capacity - Infinite
switch and phase over-current relays will be used
when oil circuit breakers are specified. The meter
Assumed transformer rating - 25 MVA
and relays will be supplied by current transform-
69 kV corresponds to 350 kV BIL
ers mounted in the bushing wells of the oil circuit
breakers. IEEE Std 21 requires that potential taps
Assumed transformer impedance - 8 percent
be provided only on bushings having an insulation
(25 MVA)/(0.08 percent) - 313 MVA
class of 115 kV or above. Therefore, separately
mounted potential transformers will be specified
Use 500 MVA circuit breaker rating
when the incoming line voltage is less than 115
kV and when a potential source is required for
instruments or relays. Otherwise, potential taps on
US Army Corps of Engineers
bushings are to be specified.
Figure 4-4. Circuit Breaker Interrupting Rating
(2) Buses. The metering of station buses is not
Approximation.
required. Separate bus differential relaying provi-
sions will be specified only when protection
as to permit the installation of an additional
against bus faults is deemed to be sufficiently
incoming line and at least one additional power
important to warrant the additional expense. In-
transformer and related equipment or materials in
stead, consideration will be given to the relaying
the future with a minimum of modifications. Sta-
of buses in conjunction with any transformer
tion access roads, vehicle and personnel access
differential relaying scheme. IEEE surveys indi-
gates and other station elements should be ini-
cate an extremely low failure rate on buses, with
tially located to avoid relocation if the station is
most failures attributed to the lack of adequate
expanded in the future. Switching stations or
maintenance. This is opposed to the usual causes
conventional substations should be similarly de-
of electrical faults, such as birds, ice, lightning,
signed to allow for future modifications at a
wind, etc., or their effects.
minimum of cost. The design of modifications to
(3) Transformers. The metering of transformer
existing stations should also allow for future ex-
mains or conductors between the transformer sec-
ondary terminals and the switchgear is described
pansion to the station with a minimum of expense
below. The minimum relaying requirements are
whenever expansion is likely or possible.
noted in table 4-1. Relays and meters or instru-
4-8. Miscellaneous Station Design Criteria.
ments will be located in the metal-clad switchgear.
(4) Metal-clad switchgear. Minimum metering
a. Metering and relaying. Meters and relays will
requirements are indicated in table 4-2, and are
be limited to the types and number required to
in addition to any revenue metering or other types
comply with any requirements of the utility or the
of metering required by the utility or the Using
Using Agency, or to afford adequate protection of
Agency. Minimum relaying requirements are simi-
electrical power systems. Ranges selected will be
larly shown in table 4-3. Provisions will be made
based on the coordination study. Meters will be
Table 4-1. Minimum Relaying for Transformers
Transformer
minimum unit
ANSI
Device
capacity or other
&vice
Relay
actuation
requirement
number
function
Forced-air cooling and alarm re-
49
Winding or top oil
porting system
temperature
Tip and lock-out primary and sec-
On all units where the primary
63
Fault (sudden) pressure
ondary via 86T relay
breaker can be tripped
Primary and secondary circuit
Only where a primary circuit
Transformer differential
67
breaker tripping and lockout via
breaker is provided and unit is
86T relay
10 MVA or larger, except
where justified for smaller
units
4-9
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