GE D90 Plus Instruction Manual page 189

CHAPTER 7: PROTECTION
PLUS
D90 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
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 158: Breaker-and-a-half scheme
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 D90
have been included for all the current signals to be brought to the D90
grouping, ratio correction and summation are applied internally via configuration settings.
A major advantage of using internal summation is that the individual currents are
available to the protection device; for example, as additional information to calculate a
restraint current, or to allow the provision of additional protection features that operate on
the individual currents such as breaker failure.
Given the flexibility of this approach, it becomes necessary to add configuration settings to
the platform to allow the user to select which sets of CT inputs will be added to form the
net current into the protected device.
POWER SYSTEM
Plus
-series relays, refer to the
Plus
, provisions
Plus
device where
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