ABB Relion 670 Series Applications Manual page 248

Section 10
Multipurpose protection
Higher quantities of machine current and voltage (3 to 4 per unit current and 50% to 70% rated voltage)
can be expected if the generator is connected 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.
10.1.4 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 value.
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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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