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LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
PROTECTION OVERVIEW
CHAPTER 7: PROTECTION
above the setting and sets an operate flag when the input current has been at a level
above the pickup setting for the time specified by the time-current curve settings. All
comparators use analog parameter actual values as the input.
Protection elements are arranged into two classes,
grouped
and
control
. Each element
classed as a grouped element is provided with six alternate sets of settings, in setting
groups numbered 1 through 6. The performance of a grouped element is defined by the
setting group that is active at a given time. The performance of a control element is
independent of the selected active setting group.
The main characteristics of an element are shown on the element logic diagram. This
includes the inputs, settings, fixed logic, and the output operands generated.
Some settings for current and voltage elements are specified in per-unit (pu) calculated
quantities. Per-unit quantities are calculated as follows.
Eq. 1
For current elements, the base quantity is the nominal secondary or primary current of the
CT. Where the current source is the sum of two CTs with different ratios, the base quantity
will be the common secondary or primary current to which the sum is scaled (that is,
normalized to the larger of the two rated CT inputs). For example, if CT1 is 300 / 5 A and
CT2 is 100 / 5 A, then CT2 is scaled to the CT1 ratio for summation. In this case, the base
quantity is 5 A secondary or 300 A primary.
For voltage elements the base quantity is the nominal primary voltage of the protected
system which corresponds (based on VT ratio and connection) to secondary VT voltage
applied to the device. For example, on a system with a 13.8 kV nominal primary voltage
and with 14400:120 V delta-connected VTs, the secondary nominal voltage (1 pu) is:
Eq. 2
For wye-connected VTs, the secondary nominal voltage (1 pu) is:
Eq. 3
Many settings are common to most elements and are discussed below:
•
Function
settings: These settings program the element to be operational when
selected as “Enabled”. The factory default is “Disabled”. Once programmed to
“Enabled”, any element associated with the function becomes active and all options
become available.
•
Name
settings: These settings are used to uniquely identify an element.
•
Source
settings: These settings select the parameter or set of parameters to be
monitored.
•
Pickup
settings: For simple elements, these settings are used to program the level of
the measured parameter above or below which the pickup state is established. In
more complex elements, a set of settings may be provided to define the range of the
measured parameters which will cause the element to pickup.
•
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
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