Areva MiCom P120, Техническое руководство

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Application Guide P12x/EN AP/A96
MiCOM P120/P121/P122/P123
Page 45/80
20. NEGATIVE SEQUENCE OVERCURRENT PROTECTION (P122 & P123
ONLY)
In traditional phase overcurrent protection schemes, overcurrent thresholds must be set
above maximum load current levels. This limits the sensibility of the relay. Most protection
schemes also use an earth fault element using residual current, which improves sensitivity
for earth faults. However, it can happen that some faults occur and stay undetected by such
schemes.
Any unbalanced fault condition will produce negative sequence current. Thus, a negative
phase sequence overcurrent element can detect both phase-to-phase and phase to earth
faults.
This section describes how negative phase sequence overcurrent protection may be applied
in conjunction with standard overcurrent and earth fault protection in order to solve some
problems of application.
•
Negative phase sequence overcurrent elements is more sensitive to resistive phase-
to-phase faults, whereas phase overcurrent elements may not operate.
•
In some applications, an earth fault relay may not be able to detect a residual current
because of the configuration of the network. For example, an earth fault relay
connected 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 in any fault condition, independently of the transformer configuration.
Therefore, negative phase sequence overcurrent element may be used to provide
time-delayed back-up protection for any uncleared asymmetrical faults.
•
Where fuses are used to protect motors on rotating machines, a blown fuse produces
a large amount of negative sequence current. This is a dangerous condition for the
machine because negative phase sequence current generates overheating. Then, a
negative phase sequence overcurrent element may be used to back-up motor
protection relays.
•
It may also be required to trigger an alarm to announce the presence of negative
phase sequence currents in the system. Operators are then prompted to investigate
the cause of the unbalance.
The negative phase sequence overcurrent elements have a current pick up settings I2> and
I2>>, and can be time-delayed using configurable timers tI2> and tI2>>.
20.1 I2> and I2>> Setting Guidelines
I2> and I2>> thresholds can be set under the PROTECTION G1 (2)/[46] Neg Seg 0C menu.
The current pick-up threshold I2> must be set to a value that is higher than the normal
negative phase sequence current because of the normal unbalance conditions on the
network. This can be done practically during the commissioning, using the
MEASUREMENTS menu of the relay to display the negative phase sequence current value.
Then, this value has to be increased by 20%.
Where negative phase sequence element is used to clear particular cases of uncleared
asymmetric faults, the threshold setting have to be calculated based on a fault analysis of
that particular system, due to the complexities involved. However, to ensure that the
protection element will operate, the current pick-up value has to be set to approximately 20%
below the lowest calculated negative phase sequence fault current for a specific remote
fault.