Be prepared to make an emergency shutdown when starting the engine, turbine, or other type of prime mover, to protect against runaway or overspeed with possible personal injury, loss of life, or property damage.
This 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, a
negative phase sequence overcurrent element may be employed to provide timedelayed back-up protection for any un-cleared asymmetrical fault.
− Where fuses protect motors on rotating machines, a blown fuse produces a large
amount of negative sequence current. This is a dangerous condition for the motor due
to the heating effects of negative phase sequence current at double frequency. A
negative phase sequence overcurrent element may be applied to provide efficient
back-up protection for dedicated motor protection relays.
− It may also be required to simply set an alarm at the presence of negative phase
sequence currents on 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>,
I2>>, I2>>>, and are time delayed in operation by the adjustable timers tI2>, tI2>>, tI2>>>.
2.9.1 I2 Thresholds Setting Guidelines
This protection element includes three thresholds.
The first threshold can be set as DT or IDMT trip delay time.
The curves are the same as for the [50/51], [50N/51N] protection.
The current pick-up threshold (settable in the menu PROTECTION G1/[46] Neg. Seg. OC)
must be set higher than the normal negative phase sequence current due to the normal load
unbalance that is always present on the system. This can be set at the commissioning stage,
making use of the relay measurement function to display the standing negative phase
sequence current, and apply a setting at least 20% above.
Where the negative phase sequence element is required to operate for specific un-cleared
asymmetric faults, a precise threshold setting has to be based on an individual fault analysis
for that particular system due to the complexities involved. However, to ensure operation of
the protection element, the current pick-up setting must be set approximately 20% below the
lowest calculated negative phase sequence fault current for a specific remote fault condition.
The correct setting of the time delay is vital for this protection element. It should be also seen
that this element is applied primarily to provide back-up protection to other protective devices
or to provide an alarm. Therefore, it would be associated with a long delay time.
It must be ensured that the time delay is set longer than the operating time of any other
protective device (at minimum fault level) on the system, which may respond to unbalanced
faults, such as:
− Phase overcurrent protection elements
− Earth fault protection elements
− Broken conductor protection elements
− Negative phase sequence influenced thermal protection elements
