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GRT100
Time-overcurrent Protection
The overcurrent protection can be applied to two- or
three-windings for phase-to-phase faults, and neutral
points of the transformer for phase to earth faults on
the high- and low-voltage side to provide backup
protection. The inverse time overcurrent elements
conform to either of three IEC standard characteristics
(Standard inverse, Very inverse, and Extremely
inverse) or a Long-time inverse characteristic. The
characteristics of each element are shown in Figure
3.
The high-set overcurrent element provides an instantaneous
or definite time overcurrent protection.
Figure 3. Characteristics of inverse time delayed
overcurrent element
Thermal Overload Protection
(alarming and/or tripping)
The characteristics are exponential functions according
to IEC 60255-8 standard and take into account the
2
R losses due to the particular operational current
I
and the simultaneous cooling due to the coolant. In
this way the tripping time during an overload condition
takes the pre-overload into consideration. An alarm
can be set to operate before reaching the tripping
condition.
Thermal image:
2
I
Trip: t =τln
2
I
(I
Alarm: t =τln
where
t: Operating time
τ: Thermal time constant
k: Constant
I:
Relay current
I
: Basic current
B
Ip: Specified load current before the overload
occurs
T
: Time for alarm
A
I
Trip
Alarm
K ⋅ I B
0
Figure 4. Characteristic of thermal overload element
Frequency Protection
GRT100 has two frequency elements, which provide
the following schemes:
Underfrequency protection
Overfrequency protection
4
2
- Ip
2
- (k I
)
B
2
2
- Ip
) (1-T
/τ)
A
2
2
I
- (k I
)
B
t
- Quick Links:
- Current Differential Protection
- Relay Setting
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