Bevel-helical gear unit B4 backstop is attached in such a way that the direct B4-FH19A

Flender/Flender Gear Units/Bevel-helical gear unit B4

with an offset equal to half of the machines direct -axis transient reactance (Xd / . Time delay = 5 cycles. Zone 2 diameter is set for 1.0 times the machines direct axis synchronous reactance (Xd/ with an offset equal to half of the machines direct -axis transient reactance (Xd / . Time delay = 3 cycles. Both Zone 1 and 2 are supe rvised by loss of potential element to ensure the element only operates when operating voltage is present. The settings utilized are within the IEEE C3.1 -2 standard guidelines. 5.5 Negative Sequence Current -4 Negative Sequence Overcurrent protection, or unbalance current protection, is provided to protect the generator against unbalanced currents. Unbalanced currents can be caused due to unbalanced loads, open phase conductors, or unbalanced system faults. If the generator is subjected to prolo nged unbalance currents, the generator rotor can overheat causing severe damage. Negative Sequence protection is provided by the Alstom ABB Combiflex protective relay package as well as the SEL -3G protective relay. The pickup value of the element is se at 1% which matches the machine rating. The permissible 2T is equal to 4, which is provided by the manufacturer. The SEL -3G and Combiflex relays are set with matching settings. The Negative Sequence Element conforms to IEEE C3.1 -2 standard gu idelines. 5.6 Under/Overvoltage - 2/5 Overvoltage conditions beyond the machine design rating can cause insulation failure and subsequent short circuit damage. Undervoltage conditions can cause the machine to overheat. Under/Overvoltage protection is provided by the Alstom ABB Combiflex protective relay package as well as the SEL -3G protective relay. The Undervoltage element is set to operate at 8% of the generator rating at 3 second time delay. The Overvoltage element is set to operate at 1% of the generator rating at 3 second time delay. The Under/Overvoltage Elements conforms to IEEE C3.1 -2 standard