GE L90 Instruction Manual page 136

5.1 OVERVIEW
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
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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 four 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
additional 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. The internal grouping of current and voltage signals forms an inter-
nal source. This source can be assigned a specific name and becomes available to protection and metering elements in the
relay. Individual names can be given to each source to identify them for later use. For example, in the scheme shown
above, three different sources are be configured as inputs for separate elements:
• Source 1: CT1 current, for the breaker failure 1 element and first current source for the line differential element.
• Source 2: CT2 current, for breaker failure 2 element and second current source for the line differential element.
• Source 3: the sum of the CT1 and CT2 currents for the distance function.
In addition, two separate synchrocheck elements can be programmed to check synchronization between line voltage and
two different bus voltages (SRC3–SRC1 and SRC3–SRC2).
b) CT/VT MODULE CONFIGURATION
CT and 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 may 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.
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Figure 5–1: BREAKER-AND-A-HALF SCHEME
L90 Line Current Differential System
5 SETTINGS
GE Multilin