Negative Phase Sequence Overcurrent (46Nps); Undercurrent (37); Thermal Overload (49) - Siemens Argus 7SR21 Technical Manual

7SR210 & 7SR220 Applications Guide

2.10 Negative Phase Sequence Overcurrent (46NPS)

The presence of Negative Phase Sequence (NPS) current indicates an unbalance in the phase currents, either
due to a fault or unbalanced load.
NPS current presents a major problem for 3-phase rotating plant. It produces a reaction magnetic field which
rotates in the opposite direction, and at twice the frequency, to the main field created by the DC excitation system.
This induces double-frequency currents into the rotor which cause very large eddy currents in the rotor body. The
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resulting heating of the rotor can be severe and is proportional to (I ) t.
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Generators and Motors are designed, manufactured and tested to be capable of withstanding unbalanced current
for specified limits. Their withstand is specified in two parts; continuous capability based on a figure of I , and
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short time capability based on a constant, K, where K = (I ) t. NPS overcurrent protection is therefore configured
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to match these two plant characteristics.

2.11 Undercurrent (37)

Undercurrent elements are used in control logic schemes such as Auto-Changeover Schemes, Auto-Switching
Interlock and Loss of Load. They are used to indicate that current has ceased to flow or that a low load situation
exists. For this reason simple Definite Time Lag (DTL) elements may be used.
For example, once it has been determined that fault current has been broken – the CB is open and no current
flows – an auto-isolation sequence may safely be initiated.

2.12 Thermal Overload (49)

The element uses measured 3-phase current to estimate the real-time Thermal State, θ, of cables or
transformers. The Thermal State is based on both past and present current levels.
θ = 0% for unheated equipment, and θ = 100% for maximum thermal withstand of equipment or the Trip
threshold.
Figure 2.12-1 Thermal Overload Heating and Cooling Characteristic
For given current level, the Thermal State will ramp up over time until Thermal Equilibrium is reached when
Heating Effects of Current = Thermal Losses.
The heating / cooling curve is primarily dependent upon the Thermal Time Constant. This must be matched
I
against that quoted for the item of plant being protected. Similarly the current tripping threshold, is related to the
q
thermal withstand of the plant.
Thermal Overload is a slow acting protection, detecting faults or system conditions too small to pick-up fast acting
protections such as Phase Overcurrent. An Alarm is provided for θ at or above a set % of capacity to indicate that
a potential trip condition exists and that the system should be scrutinised for abnormalities.
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