Introduction To Ac Sources - GE L90 Instruction Manual

5 SETTINGS
a) BACKGROUND
The L90 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 fail-
ure element. The sum of both breaker phase currents and 3I_0 residual currents may be required for the circuit relaying
and metering functions. Two separate synchrocheck elements can be programmed to check synchronization between two
different buses VT and the line VT. These requirements can be satisfied with a single L90, equipped with sufficient CT and
VT input channels, by selecting proper parameter to measure. A mechanism is provided to specify the AC parameter (or
group of parameters) used as the input to protection/control comparators and some metering elements. Selection of the
measured parameter(s) is partially performed by the design of a measuring element or protection/control comparator by
identifying the measured parameter type (fundamental frequency phasor, harmonic phasor, symmetrical component, total
waveform RMS magnitude, phase-phase or phase-ground voltage, etc.). The user completes the process by selecting the
instrument transformer input channels to use and some parameters calculated from these channels. The input parameters
available include the summation of currents from multiple input channels. For the summed currents of phase, 3I_0, and
ground current, current 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 L90 series relays, refer to the 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 voltages from a single-phase 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. Some
protection elements, like breaker failure, require individual CT current as an input. Other elements, like distance, require the
sum of both current as an input. The line differential function requires the CT currents to be processed individually to cope
with a possible CT saturation of one CT during an external fault on the upper bus. The current into protected line is the pha-
sor sum (or difference) of the currents in CT1 and CT2, depending on the current distribution on the upper bus.
In conventional analog or electronic relays, the sum of the currents is obtained from an appropriate external connection of
all CTs through which any portion of the current for the element being protected could flow. Auxiliary CTs are required to
perform ratio matching if the ratios of the primary CTs to be summed are not identical. In the L90 relay, provisions have
been included for all the current signals to be brought to the device where grouping, CT ratio correction, and summation are
applied internally via configuration settings. Up to 4 currents can be brought into L90 relay; current summation and CT ratio
matching is performed internally. A major advantage of internal summation is that individual currents are available to the
protection device (for example, as additional information to apply a restraint current properly, or to allow the provision of
GE Multilin
CB1
CT1
VT1
1PH
VT3
Figure 5–1: BREAKER-AND-A-HALF SCHEME
L90 Line Differential Relay

5.1.3 INTRODUCTION TO AC SOURCES

CB2
CT2
VT2
1PH
PROTECTED
LINE
831783A1.CDR
5.1 OVERVIEW
5
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