IEEE C62.92.5-2020 pdf free download – IEEE Guide for the Applicaton of Neutral Grounding in Electrical Utlity Systems—Part V: Transmission Systems and Subtransmission Systems.
The above examples from these studies show the effects of system parameters (including system grounding) on the TOVs and transient voltages of a simplifed transmission system. The results indicate that on an effectively grounded system [where X 0 /X 1 ≤ 3 and R 0 /X 1 < 1 (IEEE Std C62.92.1™)], the transient voltages due to ground faults should be less than two times the normal line-to-neutral voltage, since X C0 /X 1 is usually well above 10. 4.3 Control of ground fault currents Transmission systems are normally grounded by grounded-wye primary (high voltage) generator step up transformer banks with delta-connected tertiary or delta-connected secondary windings. In some systems, the zero-sequence impedance is relatively low, and it might be desirable to increase this impedance by installing neutral reactors or operating with some transformer bank neutrals ungrounded.
Neutral reactors are not usually installed in autotransformer neutrals. However, if it is determined that one is required, the equivalent zero- sequence impedance diagram for an autotransformer with a neutral reactor and with a delta-connected tertiary is shown in Figure B.2. The neutral of an autotransformer bank should never be operated ungrounded; to do so would allow the zero-sequence voltage from the high-voltage system to be applied directly to the low-voltage system without transformation. If the zero-sequence impedance of a transformer system is too low, the following undesirable effects can result:
— The short-circuit duty on power circuit breakers will be greater for ground faults than for three-phase faults (X 0 < X 1 ), and it can exceed the breaker rating.
— The station ground potential can rise excessively with respect to remote ground during ground faults because of a high-magnitude fault current.
— The system stability margins can be excessively reduced when considering failure of circuit breakers to interrupt fault current during ground faults.
— During a ground fault, the positive-sequence voltage is reduced and there is a negative-sequence voltage. This effect is greater for lower X 0 /X 1 ratios. These voltage changes are undesirable because they can produce excessive torques on three-phase machines. In a few situations, low-resistance neutral grounding resistors have been used on generator transformer neutrals to provide the damping of torques during ground faults. These systems, however, are still effectively grounded.
— The zero-sequence voltage during a ground fault is lower for the low values of X 0 /X 1 . This voltage produces operating and polarizing currents in the ground relays at remote terminals and might not be suffcient for positive operation. These problems have to be studied by the system engineer and an acceptable value of zero-sequence impedance selected to balance the advantages of smaller fault currents against the increase in fault overvoltages in the system. If neutral reactors or resistors are installed in the transformer neutral, voltage during ground faults should be calculated to verify that the transformer neutral and the reactor or resistor are not overstressed.
That is, the neutral-to-ground voltage resulting during a fault should not exceed the rated voltage of the transformer neutral. When a neutral reactor or resistor is used, a surge arrester is usually needed to protect the transformer neutral from voltages due to switching surges from the system or from lightning surges. Lightning striking the station can cause the station ground potential to rise above the potential of the transformer neutral. An arrester with the same voltage rating as the transformer neutral has been frequently used. See IEEE Std C62.22 for specifc guidance in applying this arrester.IEEE C62.92.5 pdf download.