IEEE Std C57.13.3 pdf free download

IEEE Std C57.13.3 pdf free download.Grounding of Instrument Transformer Secondary Circuits and Cases.
It is necessary to ground secondary circuits of instrument transformers for protecting the protection, monitoring, and control equipment that are connected to them and for the safety of personnel who might come into contact with them. The primary circuits of instrument transformers arc at high voltages; in many cases, they are in the range of hundreds of kilovolts. There is a (stray) capacitance between the high-voltage circuit and the secondary circuit of the instrument transformer. If a secondary circuit is not grounded, there will be a capacitance between the secondary circuit and the ground. The capacitances between the high- voltage circuit and the secondary circuit of the instrument transformer, and the capacitance between the secondary circuit and ground, are now connected in series and act like a voltage divider. A charge could accumulate on the secondary winding of the instrument transformer. Depending on the relative values of the two capacitances, the electrostatic potential of the secondary circuit with respect to ground could be substantial. This potential rise of the secondary circuit could damage the equipment connected to it; one of the failure modes could be insulation failure in the protection and control devices. The high electrostatic potential of the secondary circuit of the instrument transformer would be a hazard for personnel who might come into contact with the circuit. Because of these reasons, grounding has received attention in many standards, recommended practices. and guides. While these standards provide sufficient guidance, the reader is advised to make certain that the grounding regulations provided in the National Electrical Code’ (NECR) (NFPA 70) and the National Electrical Safety Code’ (NESCR) (Accredited Standards Committee C-2) are always adhered to. Article 90.2 (A) of the NEC lists the installations that are covered by the Code, and Article 90.2 (B) lists the installations that arc not covered by the Code. If the secondary circuit of an instrument transformer is grounded, the capacitance between the secondary circuit and ground is short- circuited and electrostatic potential does not build up on the secondary circuit.
Similarly, there is a (stray) capacitance between the primary circuit and the case of a device used in the power system environment. If the case is not grounded, there is a (stray) capacitance between the case and the ground. These two capacitances form a series circuit that acts as a voltage divider. In this case also, the electrostatic potential of the case could be substantial. This would be a hazard fbr personnel who might come in contact with the case. The grounding of the case short-circuits the capacitance between the case and the ground and keeps the case at ground potential—eliminating the hazard. As stated in the previous paragraph, the reader must adhere to the requirements of the NEC and the NESC.
The issue of grounding instrument transformers requires insight into the operation of power systems during normal working conditions and during faults between one or more phases to ground. In these cases, parts of the fault currents how in the ground path and raise the voltage of the grounding grids in generating stations and substations. When this happens, different locations in the generating stations and substations are at different potentials. These difTerences are taken into consideration when choosing the manner in which the secondary circuits of the CTs and VTs are connected. This clause examines the relevant issues on the grounding needs and practices.
Specific issues addressed in this clause include grounding at a single location on a secondary circuit, location of the ground. marking of ground connections, and the minimum size of conductor used for grounding the secondary circuits.IEEE Std C57.13.3 pdf download.