Introduction To Ac Sources - GE L90 Instruction Manual

5 SETTINGS
• PICKUP DELAY setting: This setting sets a time-delay-on-pickup, or on-delay, for the duration between the pickup
and operate output states.
• RESET DELAY setting: This setting is used to set a time-delay-on-dropout, or off-delay, for the duration between the
Operate output state and the return to logic 0 after the input transits outside the defined pickup range.
• BLOCK setting: The default output operand state of all comparators is a logic 0 or "flag not set". The comparator
remains in this default state until a logic 1 is asserted at the RUN input, allowing the test to be performed. If the RUN
input changes to logic 0 at any time, the comparator returns to the default state. The RUN input is used to supervise
the comparator. The BLOCK input is used as one of the inputs to RUN control.
• TARGET setting: This setting is used to define the operation of an element target message. When set to Disabled, no
target message or illumination of a faceplate LED indicator is issued upon operation of the element. When set to Self-
Reset, the target message and LED indication follow the Operate state of the element, and self-resets once the oper-
ate element condition clears. When set to Latched, the target message and LED indication will remain visible after the
element output returns to logic 0 - until a RESET command is received by the relay.
• EVENTS setting: This setting is used to control whether the Pickup, Dropout or Operate states are recorded by the
event recorder. When set to Disabled, element pickup, dropout or operate are not recorded as events. When set to
Enabled, events are created for:
(Element) PKP (pickup)
(Element) DPO (dropout)
(Element) OP (operate)
The DPO event is created when the measure and decide comparator output transits from the pickup state (logic 1) to
the dropout state (logic 0). This could happen when the element is in the operate state if the reset delay time is not '0'.
a) BACKGROUND
The L90 may be used on systems with breaker-and-a-half or ring bus configurations.
In these applications, each of the two three-phase sets of individual phase currents (one associated with each breaker) can
be used as an input to a breaker failure element. The sum of both breaker phase currents and 3I_0 residual currents may
be required for the circuit relaying and metering functions. Two separate synchrocheck elements can be programmed to
check synchronization between two different buses VT and the line VT. These requirements can be satisfied with a single
L90, equipped with sufficient CT and VT input channels, by selecting proper parameter to measure. A mechanism is pro-
vided to specify the AC parameter (or group of parameters) used as the input to protection/control comparators and some
metering elements. Selection of the measured parameter(s) is partially performed by the design of a measuring element or
protection/control comparator by identifying the measured parameter type (fundamental frequency phasor, harmonic pha-
sor, symmetrical component, total waveform RMS magnitude, phase-phase or phase-ground voltage, etc.). The user com-
pletes the process by selecting the instrument transformer input channels to use and some parameters calculated from
these channels. The input parameters available include the summation of currents from 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 L90 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 may
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 below. 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 pha-
sor sum (or difference) of the currents in CT1 and CT2, depending on the current distribution on the upper bus.
GE Multilin L90 Line Current Differential System
5.1 OVERVIEW

5.1.3 INTRODUCTION TO AC SOURCES

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