Memory Polarization - GE D90 Plus Instruction Manual

Plus
THE D90 DISTANCE ELEMENTS
Table 15-18: Non-directional quadrilateral ground distance functions
Characteristic
Forward reactance
Reverse reactance
Right blinder
Left blinder
Fault type

Memory polarization

604
Comparator inputs
Input 1 Input 2
I × Z – V j × I_0 × e
I_0 × Z –j × I_0 × e
D
I × Z – V I × Z
R
I × Z – V I × Z
L
I_0 I_2
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
Plus
expires, the D90 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 538: Dynamic shift of the memory-polarized mho characteristic
The same desirable effect of memory polarization applies to the directional comparator of
the quadrilateral characteristic.
Limit angle
jΘ jΘ
or j × I_2 × e Comparator limit
jΘ jΘ
or –j × I_2 × e Comparator limit
90°
R
90°
L
50° (removed during open pole conditions or
when 3I_0 > overcurrent supervision and
I_2 < cutoff)
PLUS
D90 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 15: THEORY OF OPERATION