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2.10
Unbalanced Load (Negative Sequence) Protection (ANSI 46)
General
2.10.1 Functional Description
Unbalanced Load
Determination
Warning stage
Thermal
Characteristic
Limitation
Cool Down
7UM62 Manual
C53000-G1176-C149-3
Unbalanced load protection detects unbalanced loads. The negative sequence
currents associated with unbalanced loads create reverse fields in three-phase
induction machines, which act on the rotor at double frequency. Eddy currents are
induced at the rotor surface, and local overheating of the rotor end zones and the slot
wedges begins to take place. Another effect of unbalanced loads is an overheating of
the damper winding. In addition, it may be used to detect interruptions, faults, and
polarity problems with current transformers. It is particularly useful in detecting phase-
to-ground, phase-to-phase, and double phase-to-ground faults with magnitudes lower
than the maximum load current.
The unbalanced load protection feature of the 7UM62 relay uses filtering to calculate
the symmetrical components from the phase currents. It evaluates the negative-phase
sequence system, the negative phase-sequence current I
sequence current exceeds a set threshold value, the trip timer starts. A trip command
is transmitted as soon as this trip time has expired.
If the value of the continuously permissible, negative phase-sequence current I2> is
exceeded, a warning message "I2> Warn" is transmitted after a selectable time T
WARN (see Figure 2-21).
The machine manufacturers indicate the permissible unbalanced load by means of the
following formula:
where t
K
-------------
t
=
zul
I
2
æ ö
2
---- -
è ø
I
N
The asymmetry factor depends on the machine and represents the time in seconds
during which the generator can be loaded with a 100 % unbalanced load. This factor
is typically in a range between 5 s and 30 s.
The heating up of the object to be protected is calculated in the relay as soon as the
I2> permissible unbalanced load is exceeded. The current-time-area is calculated
constantly to ensure a correct consideration of the changing load. The thermal
characteristic is tripped as soon as the current-time-area ((I
K asymmetry factor.
The model of the heating of the object to be protected is limited to a 200 % trip
temperature overrange.
A settable cool-down time starts as soon as the value falls below the constantly
permissible unbalanced load I2>. The parameter T COOL DOWN is defined as the time
required by the thermal image to cool down from 100 % to 0 %. The cool-down time
depends on the construction type of the generator, and especially on the damper
winding. Preloading is taken into consideration when unbalanced loading occurs
during the cool-down period. The protective relay will thus trip in a shorter time.
Unbalanced Load (Negative Sequence) Protection (ANSI 46)
=maximum permissible application time of the
perm
negative-sequence current I
K
=Asymmetry factor (machine constant)
I
/I
=Unbal. load (ratio neg. phase-sequ. I
2
N
. If the negative phase-
2
2
nom. cur. I
)
2
N
⋅ t) has reached the
2
/I
)
2
N
65
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