636
D90
PLUS
LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
THE D90
Plus
DISTANCE ELEMENTS
CHAPTER 15: THEORY OF OPERATION
Table 15-18: Non-directional quadrilateral ground distance functions
Memory polarization
All distance functions use memory polarization. The positive-sequence voltage, either
memorized or actual, is used as a polarizing signal. The memory is established when the
positive-sequence voltage remains above 80% of its nominal value for five power system
cycles. The memory voltage is a two-cycle old voltage.
Once established, the memory is applied for the user-specified time interval. The memory
timer is started when the voltage drops below 80% of nominal or when the user-
programmable condition is asserted to force memory polarization. After the memory
expires, the D90
Plus
checks the magnitude of the actual positive-sequence voltage. If it is
greater than 10% of nominal, the actual voltage is used; if it is less than 10% of nominal,
the memory voltage continues to be used.
A provision is added to force self-polarization from any user-programmable condition.
The memory-polarized mho has an extra directional integrity built-in as illustrated below.
The self-polarized mho characteristic is shifted in the reverse direction for a forward fault
by an amount proportional to the source impedance, and in the forward direction for a
reverse fault.
Figure 548: Dynamic shift of the memory-polarized mho characteristic
The same desirable effect of memory polarization applies to the directional comparator of
the quadrilateral characteristic.
Characteristic
Comparator inputs
Limit angle
Input 1
Input 2
Forward reactance
I
×
Z
–
V
j
×
I
_0 × e
j
Θ
or
j
×
I
_2 × e
j
Θ
Comparator limit
Reverse reactance
I
_0 ×
Z
D
–
j
×
I
_0 × e
j
Θ
or –
j
×
I
_2 × e
j
Θ
Comparator limit
Right blinder
I
×
Z
R
–
V
I
×
Z
R
90°
Left blinder
I
×
Z
L
–
V
I
×
Z
L
90°
Fault type
I
_0
I
_2
50° (removed during open pole conditions or
when 3I_0 > overcurrent supervision and
I_2 < cutoff)
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