IEEE 1246:2011 pdf free download – IEEE Guide for Temporary Protective Grounding Systems Used in Substations
Analytical studies indicatc that when full dc offsets occur in the locations with high XIR ratios (such asclose to a generating plant or a large transmission substation), the short duration(6 to 60 cycles) fusingcurrent ratings of grounding cables calculated using Onderdonk’s equation as considered in ASTM F855might 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 reducedapproximately 28% when the XiR ratio is changed from xXR=40 to XIR = 0 as shown in Table 2 andTable 5, respectively.
At or near large generating plants and transmission substations,a large X/R ratio is likely because theimpedance of generators and transformers contains very little resistance. Whereas in extreme cases the X/Rratio can be as high as 50，under most circumstances，the X/R ratio does not exceed 40 within thesubstations. Several miles away from the substation, the XIR ratio is dominated by the impedance of theline. The overall X/R ratio in such cases can be determined from the line’s X/R ratio. The typical range ofX/R ratios for lines is from 2 to 20 depending on the conductor configuration. A single,simall conductorline will have a low X/R ratio, whereas a bundled large conductor line will have a higher X/R ratio.
ln addition to the effects on fusing current, the X/R ratio and dc offset can produce extremely high currentpeaks in the first few cycles relative to the rms current. Whereas the current peaks are proportional to theX/R ratio， the rate of decay is inversely proportional to the X/R ratio. The slowly decaying high currentpeaks，corresponding to higher X/R ratios,create the most severe electromechanical forces,which candestroy the TPG assembly long before it fails thermally. In such a case, the worker would be withoutprotection 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 Hzsystems 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 ofdefense to clear a short circuit at high speed.Utilizing the primary relay short-circuit clearing timeminimizes the grounding cable size; however,before relying on the primary relay operation to size theprotective grounds,consider the reliability of the relays. Many circuits are protected by slower clearingfuses 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 ofthe circuit breaker or other protective device.Remote backup and local backup are two forms of backupprotection in common use on power systems. In remote backup protection, short circuits are cleared fromthe system, one substation away from where the short circuit has occurred. In local backup protection, shortcircuits are cleared locally in the same substation where the short circuit has occurred. Local backupprotection will clear the short circuit from the system in less time than that provided by remote backupprotection. Utilizing the backup protection，short-circuit clearing time provides a conservatively sizedprotective ground. If more than one relay operates to clear a short circuit on the system, the total timerequired for the last relay to operate determines the backup clearing time.For example, local breaker failurecan add from 8 to 12 cycles to the primary clearing time.Zone 2 or remote backup relaying can add from12 to 24 cycles to the primary clearing time. Backup protection from fuses can add seconds to the primaryclearing time. Table i gives example ranges of clearing times for different protection schemes. Eachcompany evaluates the primary and backup relay short-circuit clearing times on their power system anddetermines the short-circuit clearing time to use for sizing the protective ground.