GE L90, Инструкция по эксплуатации

Страница: 231 / 900

CHAPTER 5: SETTINGS OVERVIEW
L90 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL 5-7
5
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 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 L90, 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 L90; 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
added to form the net current into the protected device. The internal grouping of current and voltage signals forms an
internal 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, 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).