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Chapter 7) 7SR158 Applications Guide
2.2
Negative Phase Sequence Overvoltage (47)
Unbalanced voltage on a 3-phase network results as a consequence of unbalanced load current causing unequal
voltage drops in network impedances. The unbalanced load current could be the result of single phase open
circuits (isolator failures, broken conductors etc.) or because of loads generating harmonics (e.g. thyristor drives).
Unbalanced voltage generated at a busbar has the knock-on
Negative Phase Sequence (NPS) protection detects phase unbalance and is widely used in protecting rotating
plant such as motors and generators. However such protection is almost universally based on detecting NPS
Current rather than Voltage. This is because the NPS impedance of motors etc. is much less than the Positive
Phase Sequence (PPS) impedance and therefore the ratio of NPS to PPS Current is much higher than the
equivalent ratio of NPS to PPS Voltage.
NPS Voltage is instead used for monitoring busbar supply quality rather than detecting system faults. The
presence of NPS Voltage is due to unbalanced load on a system. Any system voltage abnormality is important
since it will affect every motor connected to the source of supply and can result in mass failures in an industrial
plant.
The two NPS Voltage DTL elements should therefore be used as Alarms to indicate that the level of NPS has
reached abnormal levels. Remedial action can then be taken, such as introducing a Balancer network of
capacitors and inductors. Very high levels of NPS Voltage indicate incorrect phase sequence due to an incorrect
connection.
2.3
Neutral Overvoltage (59N)
The three phases of a balanced healthy system summate to zero. When a single-phase earth fault occurs the
system balance is upset and a residual/neutral voltage is produced. Neutral Over-Voltage protection, sometimes
referred to as Neutral Voltage Displacement (NVD) protection, is used to detect an earth fault where little or no
earth current flows.
This can occur where a feeder has been tripped at its HV side for an earth fault, but the circuit is still energised
from the LV side via an unearthed transformer winding. Insufficient earth current would be present to cause a trip,
but residual voltage would increase significantly; reaching up to 3-times the normal phase-earth voltage level.
If Neutral Overvoltage protection is used, it must be suitably time graded with other protections in order to prevent
unwanted tripping for external system earth faults.
Figure 2.3-1 Neutral Overvoltage Application
The neutral voltage (3V
) can be measured directly or derived from the phase-neutral voltages - see Figure 2.3-2.
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©2014 Siemens Protection Devices Limited
Chapter 7) Page 7 of 24
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