7SR210 & 7SR220 Applications Guide
Page 28 of 48 ©2018 Siemens Protection Devices Limited
2.14.1 Application with Capacitor Cone Units
Capacitor cones provide a cost effective method of deriving residual voltage. The wide range of capacitor cone
component values used by different manufacturer’s means that the relay cannot be connected directly to the
cones.
The external adaptor unit contains parallel switched capacitors that enable a wide range of values to be selected
using a DIL switch and hence the Capacitor Cone output can be scaled to the standard relay input range.
2.14.2 Derived NVD Voltage
Alternatively NVD voltage can be derived from the three phase to neutral voltages, this setting is available within
the relay. Note with this method the NVD protection may mal-operate during a VT Fail condition.
2.15 Negative Phase Sequence Overvoltage (47)
Negative Phase Sequence (NPS) protection detects phase unbalances 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.
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