172 D90
PLUS
LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
PROTECTION OVERVIEW CHAPTER 7: PROTECTION
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
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 a 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
is 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 that 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, outlined as follows:
•
Function settings — These settings program the element to operate when selected as
“Enabled.” The factory default is “Disabled.” Once “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 can be provided to define the range of the
measured parameters that 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
supervise the comparator. The BLOCK input is used as one of the inputs to RUN
control.
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