P12x/EN T/Eb6 - Schneider - error

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P12x/EN AP/Eb6Page 44/78Application GuideMiCOM P120/P121/P122/P12320. NEGATIVE SEQUENCE OVERCURRENT PROTECTION (P122 & P123ONLY)In traditional phase overcurrent protection schemes, overcurrent thresholds must be setabove maximum load current levels. This limits the sensibility of the relay. Most protectionschemes also use an earth fault element using residual current, which improves sensitivityfor earth faults. However, it can happen that some faults occur and stay undetected by suchschemes.Any unbalanced fault condition will produce negative sequence current. Thus, a negativephase sequence overcurrent element can detect both phase-to-phase and phase to earthfaults.This section describes how negative phase sequence overcurrent protection may be appliedin conjunction with standard overcurrent and earth fault protection in order to solve someproblems of application.• Negative phase sequence overcurrent elements is more sensitive to resistive phaseto-phasefaults, whereas phase overcurrent elements may not operate.• In some applications, an earth fault relay may not be able to detect a residual currentbecause of the configuration of the network. For example, an earth fault relayconnected on the delta side of a delta-star transformer is unable to detect earth faultson the star side. However, negative sequence current will be present on both sides ofthe transformer in any fault condition, independently of the transformer configuration.Therefore, negative phase sequence overcurrent element may be used to providetime-delayed back-up protection for any uncleared asymmetrical faults.• Where fuses are used to protect motors on rotating machines, a blown fuse producesa large amount of negative sequence current. This is a dangerous condition for themachine because negative phase sequence current generates overheating. Then, anegative phase sequence overcurrent element may be used to back-up motorprotection relays.• It may also be required to trigger an alarm to announce the presence of negativephase sequence currents in the system. Operators are then prompted to investigatethe cause of the unbalance.The negative phase sequence overcurrent elements have a current pick up settings I2> andI2>>, and can be time-delayed using configurable timers tI2> and tI2>>.20.1 I2> and I2>> Setting GuidelinesI2> 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 normalnegative phase sequence current because of the normal unbalance conditions on thenetwork. This can be done practically during the commissioning, using theMEASUREMENTS 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 unclearedasymmetric faults, the threshold setting have to be calculated based on a fault analysis ofthat particular system, due to the complexities involved. However, to ensure that theprotection 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 remotefault.
Application GuideP12x/EN AP/Eb6MiCOM P120/P121/P122/P123 Page 45/78It is essential to set correctly the time delay associated to this function. It should also benoted that this element is used primarily as a back-up protection to other protective devicesor to provide an alarm. Therefore, this function is usually set with a long time delay.Care must be made to ensure that the time delay is set above the operating time of anyother protection device (at minimum fault level) present on the system and that may react tounbalanced faults, such as:• Phase overcurrent elements• Earth fault elements• Broken conductor elements• Negative phase sequence influenced thermal protection elementstI2> and tI2>> timers associated to I2 threshold can be set under the menu PROTECTION7G1(2)/[46] Neg Seg OC.
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