Introduction To Ac Sources - GE C60 Instruction Manual

OVERVIEW CHAPTER 5: SETTINGS

5.2.2 Introduction to AC sources

5.2.2.1 Background
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 can 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.
The basic idea of an AC source is to select a point on the power system where the voltages and currents are of interest. To
illustrate the concept of sources, as applied to current inputs only, consider the breaker-and-a-half scheme that follows.
(The breaker-and-a-half scheme is used for illustrative purposes and is available on select UR products.) 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 can need access to the individual currents from CT1 and CT2.
Figure 5-1: Breaker-and-a-half scheme
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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 can 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
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 are to be added to form the net current into the protected device.
The internal grouping of current and voltage signals forms an AC source. This source can be given a specific name through
the settings and becomes available to protection and metering elements in the UR platform. Individual names can be
given to each source to help identify them for later use. For example, in the scheme shown in the preceding figure, the user
configures one source to be the sum of CT1 and CT2 and can name this source as "Wdg1 I."
5-6 C60 BREAKER PROTECTION SYSTEM – INSTRUCTION MANUAL