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Section 4
1MAC050144-MB C
Protection functions
208
615 series ANSI
Technical Manual
When the voltages supplied to an operating motor become unbalanced, the
positive-sequence current remains substantially unchanged, but the negative-sequence
current flows due to the unbalance. For example, if the unbalance is caused by an open
circuit in any phase, a negative-sequence current flows and it is equal and opposite to the
previous load current in a healthy phase. The combination of positive and
negative-sequence currents produces phase currents approximately 1.7 times the previous
load in each healthy phase and zero current in the open phase.
The negative-sequence currents flow through the stator windings inducing
negative-sequence voltage in the rotor windings. This can result in a high rotor current that
damages the rotor winding. The frequency of the induced current is approximately twice
the supply frequency. Due to skin effect, the induced current with a frequency double the
supply frequency encounters high rotor resistance which leads to excessive heating even
with phase currents with value less than the rated current of the motor.
The negative-sequence impedance of induction or a synchronous motor is approximately
equal to the locked rotor impedance, which is approximately one-sixth of the normal motor
impedance, considering that the motor has a locked-rotor current of six times the rated
current. Therefore, even a three percent voltage unbalance can lead to 18 percent stator
negative sequence current in windings. The severity of this is indicated by a 30-40 percent
increase in the motor temperature due to the extra current.
4.1.9.7
Signals
Table 193: 46M Input signals
Table 194: 46M Output signals
4.1.9.8
Settings
Table 195: 46M Group settings
Name
Type
Default
Description
I2
SIGNAL
0
Negative phase sequence current
BLOCK
BOOLEAN
0=False
Block signal for activating the blocking mode
Name
Type
Description
TRIP
BOOLEAN
Trip
PICKUP
BOOLEAN
Pickup
BLK_RESTART
BOOLEAN
Overheated machine reconnection blocking
Parameter
Values (Range)
Unit
Step
Default
Description
Pickup value
0.01...0.50
xIn
0.01
0.20
Pickup value
Operating curve type
5=ANSI DT
15=IEC DT
17=Inv. Curve A
18=Inv. Curve B
5=ANSI DT
Selection of time delay curve type
Machine time Mult
5.0...100.0
0.1
5.0
Machine related time constant
Trip delay time
100...120000
ms
10
1000
Trip delay time
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Страница 29: ...1MAC050144 MB C Section 615 series ANSI 22 Technical Manual ...
Страница 389: ...Section 4 1MAC050144 MB C Protection functions 382 615 series ANSI Technical Manual ...
Страница 485: ...Section 6 1MAC050144 MB C Supervision functions 478 615 series ANSI Technical Manual ...
Страница 635: ...Section 10 1MAC050144 MB C Other functions 628 615 series ANSI Technical Manual ...
Страница 707: ...Section 15 1MAC050144 MB C Applicable standards and regulations 700 615 series ANSI Technical Manual ...
Страница 711: ...Section 16 1MAC050144 MB C Glossary 704 615 series ANSI Technical Manual ...
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