NGR is employed in AC distribution networks to limit the fault current which would flow from the transformer or generator neutral star point in the event of an earth fault in the systems. It is used when the neutral of supply transformer is accessible and its own impedance is not enough to limit fault current. The rating of the protection relays within the required time.
For Grounding Neutral of Transformer or Generator, Resistors upto 33 KV are offered for fault of short duration like 10 sec., 30 sec., 60 sec., continuous etc. Material of resistor is normally Stainless Steel, FeCrAl, Cast Iron, Copper Nickel or Nichrome.
Temperature rise is normally 375oC/760oC or as required by customer.
Features of Neutral Grounding Resistor
The metallic NGRs are designed for specific resistance values and voltages specially for shorter time ratings. The Metallic NGRs have the following advantages over a liquid type:
Their resistance value is precise and determined at the manufacturing stage remaining constant throughout their life.
Their temperature Time constant is shorter allowing a resistor of shorter time rating to be used.
They are easily to install on site.
They require virtually no maintenance and have a longer useful life.
No anti frost heating is required nor anti condensation heaters generally.
Because of problem resulting from the evaporation of gel and electrolyte the liquid resistor require constant maintenance.
Invariably the type of cooling will be natural air-cooling.
Neutral Grounding / Earthing Resistor panels up to 33 KV.
Neutral Grounding / Earthing Resistors are employed in AC Distribution Networks to limit the Restricted Earth Current to a safe value which would flow from the Transformer or Generator star point in the event of an earth fault in the system without exceeding specified temperature limits. In case of short time rating, dissipation is negligible & the temperature rise for the resistor element depends on its capacity to store the energy generated in proportion to the mass & specific heat of the material used. Theses NGR are connected between the neutral point of transformer / generator & the earth.
In no Earth fault condition continious current of 5 % to 10 % of full load current may pass through the resistor due to unbalance in system.
In Earth fault condition the Restricted earth fault current passes through the Metallic Resistor protecting the system till automatic protection takes place by absorbing the earth pressure, protecting the winding etc.
When the current flows continuously for long duration, the temperature of resistor element increases and then stabilizes at a value for which energy supplied is entirely dissipated to the atmosphere. Hence these resistors are designed for allowing maximum heat dissipation
Important Definitions & Construction:
Rated Voltage : For neutral grounding / earthing resistor rated voltage means line- to neutral voltage i.e. System Voltage divided by root3 i.e. 1.732.
Rated Current : Means Initial current that will flow through resistor when it is cold. Normally Full load current value is same as Rated current.
Time Rating : It is length of time for which NGR must tolerate rated voltage.
Short Time Rating : Normally it is 10 sec. or 30 sec. or 60 sec. depending on design parameters of protection system. IS-3043 recommends 30 sec. rating.
Continuous Rating : It is normally 10 % of full load current for healthy system neutral grounding resistor to be designed for continuous rating of 5 % to 10 % of full load current (if required).
Material of Resistance Element : We offer resistor elements made from any stainless steel or FeCrAl or Cu-Ni or Ni-Cr or any other resistance material as per requirement. Normally formed stainless steel element are best and economical for NGRS.
Temperature Rise Allowed: Normally Temperature rise is restricted to 375 deg.c. above ambient temperature as per BS-587. It can be 760 deg.c. as per IEEE-32. It can be offered of any other temperature rise as per customer requirement.
Construction: NGR include resistor assembly, plus required insulators, internal connections and hardware etc.
Cabling / Termination : Suitable arrangement and clearance is provided for termination of cable, flats or bus duct as per requirement with undrilled removable gland plate.
Group of Resistance elements are mounted together to form a Bank. Banks are then connected in series or parallel to provide the current and ohmic value required.
Resistance banks are bolted on angles welded to the angle iron structure, facilitating easy drawout facility when required .
All Resistance Bank to Bank interconnections will be done by solid stainless steel / copper.
Insulation between resistance element and supporting tie rod is provided by a mica tube.
The tie rod is insulated from the supporting structure by 3 Groove Insulators.
A sloped bottom sheet is welded to the angle iron structure and drain plug is provided at suitable place.
In case of continuous current ratings for better heat dissipation, perforated sheet / jali with welded GI wire mesh is provided at the bottom.
Foundation channels welded or bolted to the angle iron structure are provided on two sides with arrangement to fix on foundation by bolts at site.
Indoor enclosures do not have sloping or overhang features of the canopy. The bottom welded sheet is also not sloped.
Gland plate for bottom cable entry in terminal box is provided if applicable. In case of bus duct connections, a flange of specified dimensions is provided for connection to transformer.
Terminals are brought through Porcelain bushings of appropriate voltage class or brought down as per requirement of customer.
In case of higher voltage, the banks are placed on stand off insulators housed inside the enclosures, or the enclosures is also insulated from ground by using stand off insulator if asked by customer
The enclosures designed to give degree of protection required as per customer's requirement for outdoor application DOP-IP-33.
Degree of Protection is IP-33 unless otherwise specified however any degree of protection IP-00 to IP-55 can be offered.
Canopy is sloped to prevent accumulation of water. Overhang is provided to cover terminal box.
Forged Eye bolts on Lifting Angles ensure easy handling.
Basic construction of enclosure is an angle iron welded structure which provided rigidity.
Removable bolted side covers of suitable thickness mild steel (M.S) are louvered for ventilation. GI wire mesh is welded from inside to assure required degree of protection. No louvers are given for IP-55 enclosures.
Hinged doors are provided whenever any instruments like relays or meters are to be flush mounted.
The enclosures are painted with synthetic enamel paint of specified shade. Other option are epoxy paint or Aluminum paint. Pretreatment which includes degreasing rust / scale removing, phosphating as per applicable standards is done followed by 2 coats of finishing paint of required shade. Ovendrying or stoving is done on request.
(A) ROUTINE TESTS: - FOLLOWING TESTS WILL BE CONDUCTED.
Visual and Dimensional.
Insulation resistance value measurement by using 500V / 1000V megger before and after dielectric test.
Resistance value measurement with ± 10 % tolerance.
H.V. Test of twice the rated voltage +1000 volts for voltage class below 660 volts or 2.25 times of rated voltage +2000 volts for voltages above 1100 volts.
(B) TYPE TESTS :-
Temperature rise test i.e., Heat run test if required will be conducted by passing equivalent / rated current for rated / equivalent time keeping IT Constant through a part of Resistor.
INFORMATION TO BE FURNISHED TO US:
For Neutral Grounding Resistor:
Material of Resistor
Duty rating 10 sec.,30 sec.,60 sec., & continuous etc.
If details against cannot be given then inform details of transformer such as KVA rating, full load current, Primary & Secondary Voltage.
System voltage / operating voltage.
Type of Enclosure and material for enclosure ( protection class as per IS- 12063)
Location: Indoor or outdoor.
Temperature rise allowed over an ambient temperature.
Whether space heater, current transformer , isolating switch on load / off load, manually / motorised, or any other accessories are required. Details of atmospheric conditions at site and height above sea level.
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NATIONAL RESISTORS An ISO 9001:2008 Certified Company
Gat No. 1559, Shelarwasti, Dehu-Alandi Road,Chikhli - 411062, Maharashtra, India