Figure 7 demonstrates two approaches in using grounding transformers. Then, like fuses, they protect personnel and equipment from damage. This lowers the prospective groundfault current to a predetermined value. University of pennsylvania design standards revision may 2017 grounding and bonding 260526 2. High resistance grounding and low resistance grounding. Neutral grounding resistor systems can be inserted between the neutral and ground in a power system to provide ground fault protection through resistance. The neutral grounding is an important aspect of power system design because the performance of the system regarding short circuits, stability, protection, etc. Download neutral ground resistor schematic free files. Perform ground fault protection system func tional testing for distribution equipment having ground fault protection and for any generator system. The low resistance ground system can have from 25 to several thousand amperes a depending upon the value of the resistance. Hansen system grounding of medium voltage cable grids in. Low voltage high resistance grounding system basics, c.
A er you have installed your grounding system, baseline recommends that you measure the ground resistance at all grounding points including the twowire and the enclosure ground. Ngr series resistance grounding protection relays from. In case of a delta connected system, the neutral point can be derived through grounding transformers. In resistance grounded systems the transformer neutral is connected to ground through a resistance. This is an improvement from low resistance and solidly grounded systems, which do not use ngrs and therefore have a ground fault flash hazard that can cause substantial pointoffault damage. Ar01 grounding and earthing of distributed control systems and power electronic systems chapter 2 power quality requirements the commercial ac supply usually could be threephase or singlephase, 5060 hz. Substation grounding resistance is the resistance in ohms between the substation neutral and earth ground zero potential reference. Neutral grounding transformer post glover resistors. Substation grounding resistance is the resistance in ohms between the substation neutral and earth ground zero. White paper neutral grounding resistors for limiting fault. The fundamental purpose of a neutral grounding resistor ngr is to limit ground fault currents to safe levels so that all the electrical equipment in a power system is protected. A system grounded throu gh a resistance, the value of which can be such as to provide either a low or highresistance ground system. The grounding resistance is sized so that the magnitude of ground current during a line to ground. An option to item 3 is to high resistance ground both the generators and the external sources with the bus being low resistance grounded via a grounding transformer supplied through a circuit breaker.
Low resistance systems use a neutral ground resistor to absorb 200 to 1200 amps of ground fault current. If the resistance of the element is more than 15% off from the nameplate value, the. The nfpa and ieee recommend a ground resistance value of 5 ohms or less while the nec has stated to make sure that system impedance to ground. The proper grounding of a substation is essential and very important for the following two reasons. Hrg systems allow for continuous operation by inserting a resistor between the neutral and ground.
A widely utilised approach to managing fault currents is the installation of neutral earthing resistors ners. The value of the resistor is selected to limit the current, ir, to a magnitude equal to or slightly greater than the total capacitance charging current, 3 ico. High resistance grounding is typically limited to fault. For effectivelygrounded systems this ratio is usually at least 60% 2. The method of neutral lowresistance grounding will be discussed by applying a stepbystep calculation procedure to an actual case, in order to properly size the. The zero sequence current of fault ground is2 0 0 12 0 0 3 33 p. Grounding the neutral of electrical systems through low resistance grounding resistors. In a resistancegrounded system, the neutral of the transformer or generator is connected to ground through a resistor. Neutralgrounding resistors ngrs are used to ground power systems by inserting a resistor between the system neutral and ground. Grounding can be done any time, any place, anywhere and no one has to know.
Ground resistance depends on grounding electrode selection, soil resistivity, soil contact, and other factors. Grounding system theory and practice ced engineering. Perform tests using clampon digital ground resistance tester with 2% accuracy or better. Ground resistance ground resistance is the measurement of the conduc ng connec on between the grounding system and earth. Low resistance grounding is normally used on medium voltage to high voltage systems to. What is the main reason of using ngr in a power transformer. Ideally, a ground should be zero ohms of resistance, but.
High resistance grounding of the neutral limits the ground fault current to a very low level typically from 1 to10 amps and this is achieved by connecting a current limiting resistor between the neutral of the transformer secondary and the earth ground. The ngr system is designed for use with littelfuse neutral grounding resistor monitors for complete system grounding. This paper describes ngr design, its function in grounding. Line to ground voltage monitoring on ungrounded and. The littelfuse neutral grounding resistor system is a neutral grounding resistor ngr, current transformer, and sensing resistor installed in a nema 3r enclosure used to high resistance ground transformers and generators. Ground resistance is a measurement of the conducting connection between a grounding system and earth. High resistance neutral grounding resistor mingrui pdf. The system will alarm but not trip in the case of a lineto ground. Ngr neutral grounding resistor is nothing but a high resistance grounding which in context of electrical power system means the grounding of the system neutral. Neutral grounding in neutral grounding system, the neutral of the system or rotating system or transformer is connected to the ground.
Ners, sometimes called neutral grounding resistors, are used in an ac distribution networks to limit transient overvoltages that flow through the neutral point of a transformer or generator to a safe. There are many earth resistance measurement methods available, depending on the type of neutral system. An application case article pdf available in ieee transactions on industry applications. Generally speaking, there are two types of resistors used to tie an electrical systems neutral to ground. A properly designed ngr system provides benefits over both ungrounded and solidly grounded. Unfortunately, there is not one standard ground resistance threshold recognized by all certifying agencies. Neutral grounding resistor calculation info megaresistors. These devices are then able to quickly clear the fault, usually within a few seconds. Before going to the reason of using ngr one should first know what is ngr.
Damage comes from two factors, how long the fault lasts and how large the fault is. A neutral grounding resistor ngr is an effective means for limiting fault current in industrial power distribution systems. Low resistance grounding of the neutral limits the ground fault current to a high level typically 50 amps or more in order to operate the protective fault clearing relays and current transformers. A connection to ground from the neutral point or points of a circuit, transformer, motor, generator, or system. Data can be logged to a pc file at userdefined time. The value of the resistance connected from the neutral to the ground of a resistance grounded system.
A 200 a max ground current limit would be more appropriate for a mv system e. Ieee std 1422007 revision of ieee std 1421991 ieee. Earthing grounding of power electronic and distributed. Pdf grounding the neutral of electrical systems through low. There are two methods to accomplish this system of grounding, namely the low resistance grounding and high resistance grounding. Neutral grounding transformers asco neutral grounding transformers provide high resistance grounding for medium voltage generators and transformers. Studying area neutral resistance grounding influence into the soil depth, soil resistivity, capacitance and other factors on the step voltage, step voltage. Resistance grounding in an electrical system is defined as the use of a resistor as a grounding path between the neutral point of transformer or generator and the earth ground. A high resistance grounded system uses a neutral grounding resistor ngr with a low letthrough current to limit the ground fault current. Resistance grounding systems can be classified as low or high resistance systems. This option is effective only if adequate high resistance grounding. The substation grounding system is an essential part of the every electrical system. The research on grounding protection for 110kv resistance.
Principles of electrical grounding john pfeiffer, p. Inserting our neutral ground ing resistor be tween. Low resistance grounded neutral low resistance grounding of the neutral limits the ground fault current to a high level typically 50 amps or more in order to operate protective fault clearing relays and current transformers. In a lv system, a high resistance grounding scheme is employed with a resistor typically sized to limit ground. Low resistance grounding is typically used in systems with voltages exceeding volts linetoline. Methods of neutral earthing depending on application and network, there are several methods of neutral earthing including. Highresistance grounding highresistance grounding employs a neutral resistor of high ohmic value. Pdf grounding the neutral of electrical systems through. Within resistance grounded systems two different methods are wellknown. Low resistance grounded neutral low resistance grounding of the neutral limits the ground fault current to a high level typically 50 amps or more in order to operate protective fault. High or low resistance grounding high resistance grounding is used.
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