Neutral Directional Overcurrent Protection - GE 845 Instruction Manual

PROTECTION
Neutral Directional
Overcurrent
Protection
4–176
The 845 Neutral Directional Overcurrent protection element provides both forward and
reverse fault direction indications: the Ntrl Dir OC FWD and Ntrl Dir OC REV, respectively.
The output operands are 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 responds to the magnitude of a fundamental frequency phasor of
the neutral current calculated from the phase currents. There are separate Pickup settings
for the forward-looking and reverse-looking functions. The element applies a positive-
sequence restraint for better performance; a small user-programmable portion of the
positive-sequence current magnitude is subtracted from the zero sequence current
magnitude when forming the operating quantity.
Iop = 3 * (|I_0| - K * |I_1|)
The positive-sequence restraint allows for more sensitive settings by counterbalancing
spurious zero-sequence currents resulting from:
• system unbalances under heavy load conditions
• current transformer (CT) transformation errors of during double-line and three-phase
faults
• switch-off transients during double-line and three-phase faults.
The positive-sequence restraint must be considered when testing for Pickup accuracy and
response time (multiple of Pickup). The operating quantity depends on the way the test
currents are injected into the relay (single-phase injection: Iop = (1 – K) × Iinjected ; three-
phase pure zero-sequence injection: Iop = 3 × Iinjected).
The positive-sequence restraint is removed for low currents. If the positive-sequence
current is below 0.8 x CT, the restraint is removed by changing the constant K to zero. This
facilitates better response to high-resistance faults when the unbalance is very small and
there is no danger of excessive CT errors as the current is low.
The directional unit uses the zero-sequence current (I_0) for fault direction discrimination
and may be programmed to use either zero-sequence voltage ("Calculated V0" or
"Measured VX"), ground current (Ig), or both for polarizing. The following tables define the
neutral directional overcurrent element.
Where:
V_0 = 1/3 * (Vag + Vbg + Vcg) = zero sequence voltage
I_0 = 1/3 * In = 1/3 * (Ia + Ib + Ic) = zero sequence current
ECA = element characteristic angle
In = neutral current
When POLARIZING VOLTAGE is set to "Measured VX," one-third of this voltage is used in
place of V_0. The following figure explains the usage of the voltage polarized directional
unit of the element by showing the voltage-polarized phase angle comparator
characteristics for a phase A to ground fault, with:
ECA = 90° (element characteristic angle = centerline of operating characteristic)
FWD LA = 80° (forward limit angle = the ± angular limit with the ECA for operation
845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 4: SETPOINTS