IEEE or the Institute of Electrical and Electronics Engineers is the largest organization covering more than 160 countries. Its scientific efforts focus on teaching and the advancement of technological advances in electrical, electronic, and computer engineering. IEEE-developed standards are extensively adopted and acknowledged around the world. The experience and knowledge of such a respected organization can assist the design engineer in developing a thorough understanding of the daily activities to be completed, identifying solutions to complicated item problems, and generally benefiting their own professional development.
The IEEE Std 80-2000 IEEE Guide for Safety in AC Substation Grounding outlines the guidelines for constructing grounding arrangements for substations with a voltage greater than 1 kV in networks with an effectively grounded neutral. Let's consider some aspects of this standard in comparison with the corresponding Russian documents: EIC and FGC (Federal Grid Company) STO 56947007- 29.130.15.114-2012 “GROUNDING DEVICES DESIGN GUIDELINES FOR THE SUBSTATIONS WITH THE VOLTAGE OF 6-750 kV”. It will be interesting to analyze the fundamental approach to design, to note the similarities and differences.
Differences of Russian regulatory documents
The rules of OAO FGC UES STO include requirements identical to paragraphs 1.7.88-1.7.95 of the EIC, but with a few variations and additions. Unlike the EIC, the FGC STO standard requires that the limit values of both parameters be followed; it does not permit designing by grounding resistance or touch voltage. Since not every substation belongs to FGC, the engineer can use one of two approaches when designing according to EIC. Making such a decision is risky, thus it's important to consider the repercussions. Such a choice is fraught with danger; it is worth paying attention to its consequences
Requirements for employee safety
Human safety in emergency mode is given particular consideration in IEEE Std 80-2000, where the amount, duration, and frequency of current passing through the human body are crucial factors. The need for building a substation in such a way that the amount of current going through the body is less than the fibrillation threshold is described. However, due to the protection reaction time, it is impossible to protect a person from currents that cause pain and significant injury; the sole objective is to keep currents from exceeding the threshold of fibrillation.
The resistance of a short-circuit circuit between two points is assessed in Section 7.3 of IEEE Std 80-2000: between the point of contact with the metal structure and the foof, as well as between both feet. Next, the corresponding permissible touch voltages Etouch = Ib(Rb + 1.5r) and step Estep = Ib(Rb + 6.0r) are determined, where Ib is the current flowing through the human body, Rb is the body resistance, which is calculated to be 1000 ohms, and p is the soil's equivalent resistivity (which accounts for the coating and the lower soil layer). It is evident from this that the permissible step voltage is far greater than the touch voltage, making it the primary parameter in substation grounding design.
Not all of the international standard's parameters are considered in the Russian standard. It is thought that obtaining a normalized grounding resistance is the only way to guarantee safety. According to paragraph 1.7.88 of the EIC, the grounding arrangement must be used in accordance with the specifications for either the touch voltage or the resistance. The constructive implementation, which is governed by articles 1.7.92–1.7.93, must be followed in both situations.
This gives the designer a choice. If it was chosen to design solely based on touch voltage, then GOST 12.1.038 must be used, according to paragraph 1.7.91. The maximum allowable touch voltage in this document is regulated in Table 3, which also represents the exposure time resulting from the relay protection and fault shutdown automation working times. The resistance of the grounding arrangement in such a case shall be calculated from the permissible voltage according to 1.7.89, which in most cases is 10 kV, and from the short-circuit current.
The permissible voltage in relation to the zero potential zone, as well as the touch voltage, are no less important than providing an earthing resistance of 0.5 Ohm, but its calculation is considerably more complicated than the simple calculation of the grounding arrangement resistance. Special software must be used to determine the potentials distributed over the grounding grid in the substation area under the influence of short-circuit current.
There is no guarantee that all designers would select a design method that meets the touch voltage requirements and carefully design the grounding arrangement, particularly in workplaces near hardware. Although approaching the design only based on earthing resistance is not technically incorrect, a grounding arrangement that complies with it is unlikely to be regarded as safe, particularly for personnel. The inexperience of the designer, who has no knowledge of touch voltage design, will be a contributing element. This will make it impossible to determine the proper design of ground terminals at workplaces.
In contrast to IEEE Std 80-2000, Russian standards simply include the term "touch voltage" and limit values, with little explanation of the detrimental consequences of electric shock on humans. There is no "incentive" to design for contact voltage if one is unaware of the potentially lethal outcomes.
The IEEE Std 80-2000 publication mentions the idea of building a substation with high earthing resistance, which would be safer than a substation with low resistance. It mentions that the location of the grounding electrodes, local soil characteristics, and other factors that can lead to excessive potential on the earth's surface may cause the grounding arrangement's failure to withstand the short-circuit current for which relay protection devices are designed, leading to dangerous consequences for personnel. Taking all of the foregoing into account, it is possible to infer that the designer should first pay attention to the design of the grounding arrangement, which will allow him to ensure the required pitch and touch voltages before achieving the grounding resistance norm.
Conclusion
Russian regulations, if properly followed, make it possible to design a substation with a reliable and safe grounding arrangement. The international standard provides theoretical information that a skilled designer can use in their work. If the tacit use of IEEE Std 80-2000 rules does not contradict Russian regulations and the designer is experienced enough to use the helpful information from the international standard mentioned in the article, the substation grounding arrangement project will only benefit.
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