GE L30 Instruction Manual page 184

OVERVIEW CHAPTER 5: SETTINGS
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 L30 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 voltages from a single-phase VT for checking for synchronism.
To illustrate the concept of Sources, as applied to current inputs only, consider the breaker-and-a-half scheme shown.
Some protection elements, like breaker failure, require individual CT current as an input. Other elements, like distance,
require the sum of both current as an input. The line differential function requires the CT currents to be processed
individually to cope with a possible CT saturation of one CT during an external fault on the upper bus. The current into
protected line is the phasor sum (or difference) of the currents in CT1 and CT2, depending on the current distribution on the
upper bus.
Figure 5-1: Breaker-and-a-half scheme
5
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 L30, 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 the L30; 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 are
5-6 L30 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL