Areva MiCOM P441, Technical Manual

Page: 252 / 636

P44x/EN AP/E33 Application Notes
Page 78/220 MiCOM P441/P442 & P444
2.15 Negative sequence overcurrent protection (NPS)
When applying traditional phase overcurrent protection, the overcurrent elements must be
set higher than maximum load current, thereby limiting the element’s sensitivity. Most
protection schemes also use an earth fault element operating from residual current, which
improves sensitivity for earth faults. However, certain faults may arise which can remain
undetected by such schemes.
Any unbalanced fault condition will produce negative sequence current of some magnitude.
Thus, a negative phase sequence overcurrent element can operate for both phase-to-phase
and phase to earth faults.
The following section describes how negative phase sequence overcurrent protection may
be applied in conjunction with standard overcurrent and earth fault protection in order to
alleviate some less common application difficulties.
•
Negative phase sequence overcurrent elements give greater sensitivity to resistive
phase-to-phase faults, where phase overcurrent elements may not operate.
•
In certain applications, residual current may not be detected by an earth fault relay
due to the system configuration. For example, an earth fault relay applied on the delta
side of a delta-star transformer is unable to detect earth faults on the star side.
However, negative sequence current will be present on both sides of the transformer
for any fault condition, irrespective of the transformer configuration. Therefore, an
negative phase sequence overcurrent element may be employed to provide time-
delayed back-up protection for any uncleared asymmetrical faults downstream.
•
Where rotating machines are protected by fuses, loss of a fuse produces a large
amount of negative sequence current. This is a dangerous condition for the machine
due to the heating effects of negative phase sequence current and hence an upstream
negative phase sequence overcurrent element may be applied to provide back-up
protection for dedicated motor protection relays.
•
It may be required to simply alarm for the presence of negative phase sequence
currents on the system. Operators may then investigate the cause of the unbalance.
The negative phase sequence overcurrent element has a current pick up setting ‘I2> Current
Set’, and is time delayed in operation by the adjustable timer ‘I2> Time Delay’. The user
may choose to directionalise operation of the element, for either forward or reverse fault
protection for which a suitable relay characteristic angle may be set. Alternatively, the
element may be set as non-directional.
2.15.1 Setting Guidelines
The relay menu for the negative sequence overcurrent element is shown below:
NEG SEQ O/C Default Min Max Step
I2> Status Enabled Disabled, Enabled
I2> Directional Non-Directional Non-Directional, Directional Fwd, Directional Rev
I2> VTS Non-Directionel Block, Non-Directional
I2> Current Set 0.2In 0.08In 4In 0.01In
I2> Time Delay 10s 0s 100s 0.01s
I2> Char Angle
–45
°
–95
°
+95
°
1
°