Negative Sequence Directional Overcurrent Protection (67_2) - GE Multilin 850 Instruction Manual

PROTECTION
Negative Sequence
Directional
Overcurrent
Protection (67_2)
4–172
The 850 relay is equipped with the Negative Sequence Directional Overcurrent protection
element. The element provides both forward and reverse fault direction indications
through its output operands Neg Seq Dir OC FWD and Neg Seq Dir OC REV, respectively.
The output operand is asserted if the magnitude of the operating current is above a Pickup
level (overcurrent unit) and the fault direction is seen as forward or reverse, respectively
(directional unit). The overcurrent unit of the element essentially responds to the
magnitude of a fundamental frequency phasor of the negative sequence phase current. A
positive sequence restraint is applied for better performance: a small user-programmable
portion of the positive sequence current magnitude is subtracted from the negative
sequence current magnitude when forming the element operating quantity:
Iop = |I_2| - K * |I_1|
The positive sequence restraint allows for more sensitive settings by counterbalancing
spurious negative sequence currents resulting from:
• system unbalances under heavy load conditions
• current transformer (CT) transformation errors
• fault inception and switch-off transients.
The operating quantity depends on the way the test currents are injected into the relay.
For single phase injection:
Iop = 1/3 * (1 – K) × Iinjected for I_2 mode
The directional unit uses the negative sequence current (I_2) and negative-sequence
voltage (V_2).
The following tables define the Negative Sequence Directional Overcurrent element.
Table 4-35: Negative Sequence Directional Overcurrent Unit
Mode
Negative-Sequence
Table 4-36: Negative Sequence Directional Unit
Direction
Forward
Reverse
Forward
Reverse
The negative sequence voltage must be greater than 0.02 x VT to be validated for use as a
polarizing signal. If the polarizing signal is not validated, neither forward nor reverse
indication is given. The figure below explains the usage of the voltage polarized directional
unit of the element by showing the phase angle comparator characteristics for a phase A
to ground fault, with settings of:
ECA = 75° (element characteristic angle = centerline of operating characteristic)
FWD LA = 80° (forward limit angle = ± the angular limit with the ECA for operation)
REV LA = 80° (reverse limit angle = ± the angular limit with the ECA for operation)
The element incorporates a current reversal logic: if the reverse direction is indicated for at
least 1.25 of a power system cycle, the prospective forward indication is delayed by 1.5 of
a power system cycle. The element is designed to emulate an electromechanical
directional device. Larger operating and polarizing signals result in faster directional
discrimination bringing more security to the element operation.
Operating Current
Iop = |I_2| - K x |I_1|
Compared Phasors
-V_2
-V_2
-V_2
-V_2
850 FEEDER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 4: SETPOINTS
I_2 x 1∠ECA
-(I_2 x 1∠ECA)
I_2 x 1∠ECA
-(I_2 x 1∠ECA)