Schweitzer Engineering Laboratories SEL-311A, Instruction Manual

Page: 41 / 476

Date Code 20011205 Distance and Overcurrent Elements 3-1
SEL-311A Instruction Manual
SECTION 3: DISTANCE AND OVERCURRENT ELEMENTS
D
ISTANCE
E
LEMENTS
Phase Distance Elements
The SEL-311A Relay has two independent zones of mho phase distance protection. All zones are
independently set. Zone 1 and 2 are fixed to operate in the forward direction only. The phase
distance elements use positive-sequence voltage polarization for security and to create an
expanded mho characteristic. The phase distance elements operate on phase-to-phase, phase-to-
phase to ground, and three-phase faults.
Compensator distance elements are included for distance relaying through wye-delta transformer
banks and for users who desire a different operating principle for backup relaying. Compensator
distance phase-elements implemented in the SEL-311A Relay detect phase-to-phase, phase-to-
phase-to-ground and three-phase faults.
Operating Principles of Phase Distance Elements
A digital relay mho-element tests the angle between a line drop-compensated voltage and a
polarizing (reference) voltage using the following concepts:
Sampled currents and voltages are represented in the relay as vectors by using the most recent
sample as the real vector component and the sample taken one quarter cycle earlier as the
imaginary vector component. See Figures 12.3 and 12.4 in
Chapter 12: Standard Event Reports
and SER for a description of this process.
·
If vector V
1
= |V
1
|
Ð
θ
1
and vector V
2
= |V
2
|
Ð
θ
2
, then
V
1
• (V
2
conjugate) = V
1
• V
2
* = [ |V
1
| • |V
2
| ]
Ð
( θ
1
- θ
2
)
The angle between vector quantities V
1
• V
2
* is the test angle of the mho element.
·
Test for V
1
• V
2
* balance point at θ
1
- θ
2
= 0 degrees by calculating sin ( θ
1
- θ
2
). In a
digital relay, this is done by examining the sign (+ or -) of the imaginary component
of V
1
• V
2
*, written Im (V
1
• V
2
*).
·
Test for V
1
• V
2
* balance point at θ
1
- θ
2
= 90 degrees by calculating
cos ( θ
1
- θ
2
). In a digital relay, this is done by examining the sign (+ or -) of the real
component of V
1
• V
2
*, written Re (V
1
• V
2
*).
Table 3.1 shows the different calculations used for the positive-sequence polarized mho elements
and compensator-distance mho elements. Notice that the positive-sequence polarized mho
element equation is the solution of Equation 1 for the quantity “|Z|”, which represents the relay
reach at the balance point. This equation is in the form of a line drop-compensated voltage and a
polarizing (reference) voltage.
( )
[ ]
mem V V I Z
*
Re 0
· - · =
Equation 1