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

Section 11
Multipurpose protection
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.
11.1.4

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).
Common base IED values for the primary current (IBase), primary voltage (VBase) and primary power
(SBase) are set in global base values for settings function GBASVAL.
GlobalBaseSel: Selects the global base value group used by the function to define IBase, VBase and
SBase. Note that this function will only use IBase .
11.1.4.1

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, but the cable negative-sequence
impedance is practically constant. It shall be noted that directional negative sequence OC element offers
174
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 IMinx (x=1 or 2 depending on step) setting to set the
minimumpickup current. Set IMinx below PickupCurr_OCx for every step to achieve ANSI
reset characteristic according to standard. If IMinx is set above PickupCurr_OCx for any step
the ANSI reset works as if current is zero when current drops below IMinx.
The function has default setting of operate values and timers according to the typical
application as a power system back up protection. These settings should be modified
according to the system back up protection strategy of individual utility. The recommended
minimum time delay setting of function should be greater than 0.05 s when used in process
bus 9-2 application.
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