IEEE C62.92-1989 pdf free.IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems Grounding of Synchronous Generator Systems.
IEEE C62.92 model was such that the circuit breakers on the terminals of the generator were to force current zero sufficiently to give an arc extinction voltage equal to system lineto ground voltage. For these conditions, FIg 1, (a) and (b), shows transient line-to-neutral voltages that were obtained on the various phases of the generator and across the neutral reactor with inductance grounding. Figure 1(a) gives the results with no restrike: Fig 1(b) gives the results with one restrike followed by a clearing at the next current zero. For the arc extinction voltage assumed, Fig 1(b) indicates that the ratio of X0/X1 should not exceed 311 the transient voltages are to be limited to less than 254) percent of normal line-to- neutral crest voltage. However, this voltage is still less than 75 percent of the manufacturer’s generator high potential test voltage (II, (8J. Each case should be studied using specific characteristics and appropriate modeling techniques.
Figure 2 gives peak transient voltages for highresistance grounding. The voltage is plotted against the ratio of the 3-phase capacitive reactance to ground and the effective neutral resistance of the circuit, X1R (see ANSl IEEE C62.92- 1987, Figs 1 and 2 (6J). If this ratio is kept to I or greater, the peak voltage can be limited to about 26() percent of normal peak line-to-neutral voltage, which is also less than 75 percent o( the generator test voltage. This curve applies for any number of re-strikes for ratios greater than 1 because each oscillation is damped out and a buildup in transient voltage is prevented.
Figure 2 can also be used to indicate the magni tude of possible transient voltages on ungrounded machines. The ca.se to be compared is where the ratio of the 3 phase capacitive reactance to ground (X) and thr neutral resistance (Re) of the circuit is less thai, the 0.1 lower limit of Fig 2. Thus, transient voltages of 4 to 5 times normal line-to-ground voltage crest may be reached If breaker restriking occurs on the ungrounded system.
Temporary overvoltages on a generator can also be caused by a ground fault on the high voltage side of the main power step up trans former. Such an occurrence impresses a neutral displacement voltage on the generator grounding equipment. The generator neutral grounding in conjunction with the transformer high to low side capacitive coupling forms a voltage divider circuit for the zero-sequence voltage impressed upon the transformer high voltage winding (23). Consider ation must be given to the generator grounding impedance and associated protective features to avoid temporary overvoltages that can damage the insulating systems or cause undesirable generator ground relay operations. The k)wer the generator system zero-sequence impedance, the lower will be the impressed neutral displacement voltage. Therefore, this occurrence is a particular consideration for resonant grounded generator systems.
The user of this guide should be aware that there is a degree of uncertainty as to the impuLse strength of the generator insulation as compared to that of oil insulated apparatus of the same volt age because of the different types of insulation systems and general construction. Because of this uncertainty, care should be taken in selecting both the class of grounding and the ratings of surge protective equipment.
ProvidIng a Means of Generator System Ground-Fault Protection. The grounding class chosen for a generator has a significant impact on the sensitivity and speed ofgroundfault relaying for the generator and other apparatus connected to the generator voltage system. In general, ungrounded, highresistance, and resonant- grounded systems allow for the most sensitive groundfault detection. In systems where genera tors are bussed together at generator voltage or where feeders are taken out at the generator voltage. relaying requirements may dictate a ground ing class other than one which would provide maximum sensitivity for generator stator ground faults.IEEE C62.92 pdf download.