Application With Capacitor Cone Units; Derived Nvd Voltage; Negative Phase Sequence Overvoltage (47); Under/Over Frequency (81) - Siemens 7SR10 Argus Series Manual

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 manufacturers 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.
2.16

Under/Over Frequency (81)

During normal system operation the frequency will continuously vary over a relatively small range due to the
changing generation/load balance. Excessive frequency variation may occur for:
Loss of generating capacity, or loss of mains supply (underfrequency): If the governors and other
regulating equipment cannot respond to correct the balance, a sustained underfrequency condition may
lead to a system collapse.
Loss of load – excess generation (overfrequency): The generator speeds will increase causing a
proportional frequency rise. This may be unacceptable to industrial loads, for example, where the
running speeds of synchronous motors will be affected.
In the situation where the system frequency is falling rapidly it is common practise to disconnect non-essential
loads until the generation-load balance can be restored.
underfrequency is implemented. Underfrequency relays are usually installed on the transformer incomers of
distribution or industrial substations as this provides a convenient position from which to monitor the busbar
frequency. Loads are disconnected from the busbar (shed) in stages until the frequency stabilises and returns to
an acceptable level.
The relay has four under/over frequency elements.
An example scheme may have the first load shedding stage set just below the nominal frequency, e.g. between
49.0 - 49.5Hz. A time delay element would be associated with this to allow for transient dips in frequency and to
provide a time for the system regulating equipment to respond. If the first load shedding stage disconnects
sufficient plant the frequency will stabilise and perhaps return to nominal. If, however, this is not sufficient then a
second load shedding stage, set at a lower frequency, will shed further loads until the overload is relieved. This
process will continue until all stages have operated. In the event of the load shedding being unsuccessful, a final
stage of underfrequency protection should be provided to totally isolate all loads before plant is damaged, e.g.
due to overfluxing.
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7SR10, 7SR11, 7SR12 Applications Guide
Usually, automatic load shedding, based on
Chapter 7 Page 26 of 41