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Settings
The table below shows the setting elements necessary for the thermal overload protection and their
setting ranges.
Element
THM
THMIP
TTHM
THMA
[THMEN]
[THMAEN]
Note: THMIP sets a minimum level of previous load current to be used by the thermal element,
and is typically used when testing the element. For the majority of applications, THMIP
should be set to its default value of zero, in which case the previous load current, Ip, is
calculated internally by the thermal model, providing memory of conditions occurring
before an overload.
2.6 Negative Sequence Overcurrent Protection
The negative sequence overcurrent protection (NPS) is used to detect asymmetrical faults
(phase-to-phase and phase-to-earth faults) with high sensitivity in conjunction with phase
overcurrent protection and residual overcurrent protection. It also used to detect load unbalance
conditions. This function is NOT for models 820 and 821.
Phase overcurrent protection is forced to be set to lower sensitivity when the load current is large
but NPS sensitivity is not affected by magnitude of the load current, except in the case of erroneous
negative sequence current due to the unbalanced configuration of the distribution lines.
For some earth faults, only a limited amount of zero sequence current is fed while the negative
sequence current is comparatively larger. This is probable when the fault occurs at the remote end
with a small reverse zero sequence impedance and most of the zero sequence current flows to the
remote end.
In these cases, NSP backs up the phase overcurrent and residual overcurrent protection. The NPS
also protects the rotor of a rotating machine from over-heating by detecting a load unbalance.
Unbalanced voltage supply to a rotating machine due to a phase loss can lead to increases in the
negative sequence current and in machine over-heating.
Two independent negative sequence overcurrent elements are provided for tripping and alarming.
The elements are programmable for instantaneous or definite time delayed operation.
The tripping and alarming outputs can be blocked by scheme switches or a binary input signal.
Scheme Logic
Figure 2.6.1 shows the scheme logic of the NSP. Two negative sequence overcurrent elements
NPS1 and NPS2 with independent thresholds output trip signal NPS1 TRIP and alarm signal
NPS2 ALARM through delayed pick-up timers TNPS1 and TNPS2.
ICD is the inrush current detector ICD, which detects second harmonic inrush current during
Range
Step
0.50 – 10.0 A
0.01 A
0.0 – 5.0 A
0.01 A
0.5 - 500.0 min
0.1 min
50 – 99 %
1 %
Off / On
Off / On
38
Default
Remarks
1.00 A
Thermal overload setting.
(THM = I
: allowable overload current)
AOL
0.00 A
Prior load setting.
10.0 min
Thermal time constant
80 %
Thermal alarm setting.
(Percentage of THM setting.)
Off
Thermal OL enable
Off
Thermal alarm enable
6 F 2 T 0 1 7 2
- Quick Links:
- Inverse Time Overcurrent Protection
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