ASI67FFP03E SCHNEIDER

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