IEEE C62.92.2-2017 pdf download
IEEE C62.92.2-2017 pdf download.IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems, Part II- Synchronous Generator Systems.
4.2. Limiting mechanical stress in the generator for external ground faults
Fault current limiting devices are generally required for machines manufactured in accordance with the following standards:
a) ANSIC5O.1O-1977-I-I-1
b) IEEE Std ANSI C50.12-2005 1981 (21
C) IEEE Std ANSI C50.13-2014 1977 131
Prior to the original versions of these standards, no fault current limiting device was required in the generator neutral.
Meeting the limitation ot Il±E. Std ANSI (30.12-2005 191 121 and IEEE Std ANSI C50.13-2014 19 f3. requires that at least a minimum value of impedance, either a resistance or a reactance, be installed in the neutral of all wye-connected grounded generators where the zero-sequence reactance is less than the positive-sequence subtransient reactance.
In calculating the maximum currents that can flow in the generator windings during an external fault, it is usually sufficient to consider the generator impedances alone. It can be shown that, if sufficient neutral impedance is used to make the phase-to-ground fault current less than or equal to the three 3-phase fault current with the machine isolated from the system, the winding currents for any fault will be less than or equal to the winding current for a three3-phase fault (Brown 1B51) [ii).
4.3. Limiting overvoltages on generator insulation
The class of grounding also affects generator overvoltage protection in controlling the magnitude of temporary and transient overvoltages during a ground-fault and, consequently, determines the minimum rating of surge arresters that can be employed. These considerations are common to th. grounding QLall types of apparatus. The available classes of grounding may be ranked as follows in order of increasing temporary overvoltages assuming a fault resistance that yields the highest coefficient of grounding, but neglects the effects of restrikes within a solid dielectric (Brown, Iohnson, and Stevenson [B61) [1 5):
In any discussion of transient overvoltages, it should be recognized that numerous field tests have been made in an attempt to set up and measure high-transient voltages resulting from phase-to- ground arcing faults in air. From the results of these tests, it may be concluded that the conditions for building up these high voltages in an arc in air seldom exist (Allen and Waldorf [B2j, Eaton, Peck, and Dunham [B81, and Peterson [B221) (12], [17], (24]. However, it is suspected that intermittent faults through solid insulations may produce the necessary conditions for high- transient overvoltages. On the other hand, a system may have relatively high-transient overvoltages during switching operations and if restriking occurs in the breaker. Accordingly, in considering any kind of grounding from the viewpoint of transient voltages, it is advisable to determine whether there will be switching 1ø generator voltage at present or sometime in the future.
The effect of grounding on the transient overvoltages following the occurrence and clearing of unbalanced faults at the generator terminals with and without restriking is illustrated in Figure 1, Figure 2, and Figure 3 (IEEE Committee Report [B131. Eaton, Peck, Figs I (a), I (b),and Dunham [B8l, Brown [B5J. and Peterson [B22j) 2 1 12], [17], [24]. The information presented in these figures was obtained from a Transient Network Analyzer (TNA) where clearing and restriking of the arc in a single phase-to-ground fault could be controlled to produce the maximum overvoltages. The system studied consisted of a generator model7 with adjustable neutral grounding reactance, and a circuit breaker at generator voltage. The generator was simulated by a reactance of 1.28 2 per phase, at 60 Hz, a winding capacitance of 0.35 pF per phase, and a capacitance-to-ground on the terminals of the generator of 0.20 1iF per phase. The neutral reactance was varied to give a range of ratios of Xo/Xs. Results demonstrate the importance of selecting the proper class of neutral grounding in order to limit the transient overvoltages caused by switching surges.
