About Ac Sources - GE D90 Plus Instruction Manual

POWER SYSTEM

About AC sources

176
Frequency Tracking
Range: Disabled, Enabled
Default: Enabled
This setting should only be programmed to "Disabled" in very unusual circumstances;
consult GE Grid Solutions for special variable-frequency applications.
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The D90 may be used on systems with breaker-and-a-half or ring bus configurations. In
these applications, each of the two three-phase sets of individual phase currents (one
associated with each breaker) can be used as an input to a breaker failure element. The
sum of both breaker phase currents and 3I_0 residual currents may be required for the
circuit relaying and metering functions. For a three-winding transformer application, it
may be necessary to calculate watts and vars for each of the three windings, using voltage
from different sets of VTs. These requirements can be satisfied with a single D90
equipped with sufficient CT and VT input channels by selecting the appropriate parameter
measurement. A mechanism is provided to specify the AC parameter (or group of
parameters) used as the input to the protection and control comparators and some
metering elements.
Selection of the measurement parameters is partially performed by the design of a
measuring element or protection/control comparator by identifying the type of parameter
(fundamental frequency phasor, harmonic phasor, symmetrical component, total
waveform RMS magnitude, phase-phase or phase-ground voltage, etc.) to measure. The
user completes the process by selecting the instrument transformer input channels to use
and some of the parameters calculated from these channels. The input parameters
available include the summation of currents from multiple input channels. For the summed
phase, 3I_0, and ground currents, currents from CTs with different ratios are adjusted to a
single ratio before summation.
A mechanism called a source configures the routing of CT and VT input channels to
measurement sub-systems. Sources, in the context of the UR
logical grouping of current and voltage signals such that one source contains all the
signals required to measure the load or fault in a particular power apparatus. A given
source may contain all or some of the following signals: three-phase currents, single-phase
ground current, three-phase voltages, and an auxiliary voltage from a single VT for
checking for synchronism.
To illustrate the concept of sources, as applied to current inputs only, consider the breaker-
and-a-half scheme below. In this application, the current flow is indicated by the arrows.
Some current flows through the upper bus bar to some other location or power
equipment, and some current flows into transformer winding 1. The current into the line is
the phasor sum (or difference) of the currents in CT1 and CT2 (whether the sum or
difference is used depends on the relative polarity of the CT connections). The protection
elements require access to the net current for line protection, but some elements may
need access to the individual currents from CT1 and CT2.
Figure 155: Breaker-and-a-half scheme
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D90 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 7: PROTECTION
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-series relays, refer to the