Introduction To Ac Sources - GE D60 Instruction Manual

5 SETTINGS
a) BACKGROUND
The D60 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. For a three-winding transformer application, it may be required to calculate watts and vars for each
of three windings, using voltage from different sets of VTs. These requirements can be satisfied with a single UR, equipped
with sufficient CT and VT input channels, by selecting the 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 parameter(s) to measure 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 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 UR 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
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 flows as shown 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 Winding 1 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 same considerations apply to transformer Winding 2. The protection elements require access to the net
current for transformer protection, but some elements may need access to the individual currents from CT1 and CT2.
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 UR series of relays, provisions
have been included for all the current signals to be brought to the UR device where grouping, ratio correction and summa-
tion 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.
GE Multilin
CT1
Through Current
WDG 1
UR
Platform
WDG 2
CT3
Figure 5–1: BREAKER-AND-A-HALF SCHEME
D60 Line Distance Relay

5.1.3 INTRODUCTION TO AC SOURCES

CT2
Power
Transformer
CT4
827791A2.CDR
5.1 OVERVIEW
5
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