IEEE 1246-2011 pdf download IEEE Guide for Temporary Protective Grounding Systems Used in Substations
Analytical studies indicate that when full dc offsets occur in the locations with high X/R ratios (such as close to a generating plant or a large transmission substation), the short duration (6 to 60 cycles) fusing current ratings of grounding cables calculated using Onderdonk’s equation as considered in ASTM F855 might not be conservative. The additional heating from the dc current component reduces the cable current- carrying capability. The cable symmetrical current-carrying capability for the six-cycle rating is reduced approximately 28% when the X/R ratio is changed from X/R = 40 to X/R = 0 as shown in Table 2 and Table 5, respectively.
At or near large generating plants and transmission substations, a large X/R ratio is likely because the impedance of generators and transformers contains very little resistance. Whereas in extreme cases the X/R ratio can be as high as 50, under most circumstances, the X/R ratio does not exceed 40 within the substations. Several miles away from the substation, the X/R ratio is dominated by the impedance of the line. The overall X/R ratio in such cases can be determined from the line’s X/R ratio. The typical range of X/R ratios for lines is from 2 to 20 depending on the conductor configuration.
A single, small conductor line will have a low X/R ratio, whereas a bundled large conductor line will have a higher X/R ratio. In addition to the effects on fusing current, the X/R ratio and dc offset can produce extremely high current peaks in the first few cycles relative to the rms current. Whereas the current peaks are proportional to the X/R ratio, the rate of decay is inversely proportional to the X/R ratio. The slowly decaying high current peaks, corresponding to higher X/R ratios, create the most severe electromechanical forces, which can destroy the TPG assembly long before it fails thermally. In such a case, the worker would be without protection for a longer duration before the short circuit clears. IEC 61230 requires temporary grounding (earthing) devices to withstand a peak asymmetrical current of 2.6 times the rms current value for 60 Hz systems above 1 kV.
4.3.2 Short-circuit duration including primary and backup relaying The short-circuit duration is another critical factor to consider when sizing protective grounds. The short- circuit duration is the time required to clear the short circuit by primary or backup relaying.
The short- circuit clearing time is the sum of relay and breaker operation times. Primary relaying is the first line of defense to clear a short circuit at high speed. Utilizing the primary relay short-circuit clearing time minimizes the grounding cable size; however, before relying on the primary relay operation to size the protective grounds, consider the reliability of the relays. Many circuits are protected by slower clearing fuses that can take many cycles or even seconds to interrupt the current. Backup protection is provided for possible failure in the primary protection system or for possible failure of the circuit breaker or other protective device. Remote backup and local backup are two forms of backup protection in common use on power systems. In remote backup protection, short circuits are cleared from the system, one substation away from where the short circuit has occurred. In local backup protection, short circuits are cleared locally in the same substation where the short circuit has occurred. Local backup protection will clear the short circuit from the system in less time than that provided by remote backup protection. Utilizing the backup protection, short-circuit clearing time provides a conservatively sized protective ground. If more than one relay operates to clear a short circuit on the system, the total time required for the last relay to operate determines the backup clearing time. For example, local breaker failure can add from 8 to 12 cycles to the primary clearing time. Zone 2 or remote backup relaying can add from 12 to 24 cycles to the primary clearing time. Backup protection from fuses can add seconds to the primary clearing time. Table 1 gives example ranges of clearing times for different protection schemes. Each company evaluates the primary and backup relay short-circuit clearing times on their power system and determines the short-circuit clearing time to use for sizing the protective ground.