GE T35 Instruction Manual page 208

OVERVIEW CHAPTER 5: SETTINGS
Figure 5-1: 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 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
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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."
Once the sources have been configured, the user has them available as selections for the choice of input signal for the
protection elements and as metered quantities.
5.2.2.2 CT/VT module configuration
CT and voltage transformer (VT) input channels are contained in CT/VT modules. The type of input channel can be phase/
neutral/other voltage, phase/ground current, or sensitive ground current. The CT/VT modules calculate total waveform
RMS levels, fundamental frequency phasors, symmetrical components, and harmonics for voltage or current, as allowed
by the hardware in each channel. These modules can calculate other parameters as directed by the CPU module.
A CT/VT module contains up to eight input channels, numbered 1 through 8. The channel numbering corresponds to the
module terminal numbering 1 through 8 and is arranged as follows. Channels 1, 2, 3, and 4 are always provided as a group,
hereafter called a "bank," and all four are either current or voltage, as are channels 5, 6, 7, and 8. Channels 1, 2, 3 and 5, 6,
7 are arranged as phase A, B, and C respectively. Channels 4 and 8 are either another current or voltage.
5-6 T35 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL