Setting Guidelines; Directional Negative Sequence Overcurrent Protection - ABB Relion 670 Series Applications Manual

Section 10
Multipurpose protection
to a strong system. Lower current and voltage values (1 to 2 per unit current and 20% to 40% rated
voltage) are representative of weaker systems.
Since a generator behaves similarly to an induction motor, high currents will develop in the rotor
during the period it is accelerating. Although the rotor may be thermally damaged from excessive
high currents, the time to damage will be on the order of a few seconds. Of more critical concern,
however, is the bearing, which can be damaged in a fraction of a second due to low oil pressure.
Therefore, it is essential that high speed tripping is provided. This tripping should be almost
instantaneous (< 100 ms).
There is a risk that the current into the generator at inadvertent energization will be limited so
that the "normal" overcurrent or underimpedance protection will not detect the dangerous
situation. The delay of these protection functions might be too long. The reverse power protection
might detect the situation but the operation time of this protection is normally too long.
For big and important machines, fast protection against inadvertent energizing should, therefore,
be included in the protective scheme.
The protection against inadvertent energization can be made by a combination of undervoltage,
overvoltage and overcurrent protection functions. The undervoltage function will, with a delay for
example 10 s, detect the situation when the generator is not connected to the grid (standstill) and
activate the overcurrent function. The overvoltage function will detect the situation when the
generator is taken into operation and will disable the overcurrent function. The overcurrent
function will have a pick-up value about 50% of the rated current of the generator. The trip delay
will be about 50 ms.
10.1.3

Setting guidelines

The parameters for the general current and voltage protection function (CVGAPC) are set via the
local HMI or Protection and Control Manager (PCM600).
GlobalBaseSel : Selects the global base value group used by the function to define ( IBase ), ( VBase )
SBase ).
and (
10.1.3.1

Directional negative sequence overcurrent protection

Directional negative sequence overcurrent protection is typically used as sensitive ground-fault
protection of power lines where incorrect zero sequence polarization may result from mutual
induction between two or more parallel lines. Additionally, it can be used in applications on
underground cables where zero-sequence impedance depends on the fault current return paths,
230
When inverse time overcurrent characteristic is selected, the trip time of the stage
will be the sum of the inverse time delay and the set definite time delay. Thus, if
only the inverse time delay is required, it is important to set the definite time delay
for that stage to zero.
The overcurrent steps has a
minimumpickup current. Set
ANSI reset characteristic according to standard. If
PickupCurr_OCx for any step the ANSI reset works as if current is zero when current
IMinx .
drops below
IMinx ( x =1 or 2 depending on step) setting to set the
IMinx below PickupCurr_OCx for every step to achieve
IMinx is set above
1MRK 511 358-UUS A
IP15009-1 v2
GUID-F7AA2194-4D1C-4475-8853-C7D064912614 v4
M13088-237 v4
M13088-3 v5
Application manual