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Hilkar Neutral Grounding Resistors
Neutral Grounding Resistors (NGRs) are used to limit the fault current for safety of
equipment and personnel in industrial systems.
In solid grounding, the system is directly grounded and the fault current is limited only by
the soil resistance. The fault current can be very high and can damage the transformers,
generators, motors, wiring and other equipments in the system. The NGRs, which are inserted
between neutral and ground, increase the net resistance in the event of an earth fault and limit
the current to a safe level.
Advantages of NGRs include:
¡ Reduce the single phase fault currents for securing
each equipment in MV electrical networks
¡ Reduce the transient overvoltages which can occur
during an earth fault, and be monitored and used to
activate the earth fault relay
¡ Increase protection of generators, transformers and
related equipments
¡ Reduce operation/maintenance costs,
¡ Increase safety
¡ Provide simple, reliable, selective means of
protection
¡ Allow the use of equipment, and in particular cables
with lower insulation levels than for an insulated
neutral scenario The fault current value should
¡ Reduce the step voltage be limited to a value that can be
safely handled by the machine or
transformer. It also needs to be high
enough to be sensed by the earth
Generator Transformer
fault protection relays. If the NGR
If
resistance value is too high, the fault
current will be very low and will not
be able to activate the earth fault
N Current
protection relay during earth fault
Transformer
conditions.
If In a three phase star connection,
Neutral the capacitances are formed with the
Grounding
ground. In the event of an earth fault,
Resistor
these capacitances may get charged
with the line voltage and may cause
G
transient overvoltages. The NGR
should have a value that permits a
let-through current which enables
the capacitances to discharge.
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Hilkar Neutral Grounding Resistors
The resistances are also categorized by the time that they can withstand the fault current.
Typical durations are 5-10 seconds. The extended time rating resistors are used in systems where
the reliability of the system is critical, e.g. petroleum industries, mines, etc. In these situations,
high resistances are used for withstanding long periods of earth fault. When an earth fault occurs
in one phase, an alarm will be generated. However, the system continues to run until the next
scheduled shutdown.
Hilkar Neutral Earthing Resistors are designed to absorb a large amount of energy without
exceeding temperature limitations defined in IEEE 32. Hilkar NGRs can be used for indoor and
outdoor, and the neutral point is connected with a porcelain bushing or with a high voltage
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(XLPE) cable, typically (minimum cross-section = 70mm copper or 95mm aluminium) from the
bottom, top or side. The most common protection degree preferred for NGRs is IP 23 as it allows
the resistor elements to cool easier and they can be used both at sea shores and deserts because
the resistor elements are completely stainless steel and do not get affected by extreme
environmental conditions. NGRs are shipped with maintanence and installation guidelines. In
these guidelines, recommended relay settings for each NGR are stated as well. Hilkar provides
complete technical assistance in order to meet your specifications or site conditions.
ZN YNdyn (open) ZNyn
10 kV
CT
CT
CT 51N R 51N
R 51N R
400 V 400 V
Examples of providing artificial neutral point if there is no provision
to reach the neutral point of the source or connection is delta
(a) (b)
51N CT 51N CT
R R
Examples of grounding the neutral
point (a) by resistor (b) by resistor
over a transformator
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Hilkar Neutral Grounding Resistors
General Features
¡ Stainless-steel resistor elements
¡ Current transformer included (EN 61869-2)
¡ Bolted resistor element connections instead of welded connections providing assemble of
spare parts on site immediately
¡ Painted on demand
¡ Typically, 2 mm hot dip galvanized steel enclosure
¡ High thermal capacity to absorb high currents
¡ High altitude ratings
¡ Custom made lifting eyes providing secure lifting
¡ Rugged shock-resistant construction
¡ Solid top cover sloped to prevent water
accumulation
¡ Corrosion resistant stainless steel nameplate
Technical Specification
¡ Operation Voltage : Up to 110 kV line to line systems
¡ Rated Current : Up to 5000 A
o
¡ Ambient Temperature : Up to 55 C (For higher ambient temperature, consult the factory.)
¡ Resistance Alloy : Stainless-steel (CrNi or CrAl)
¡ Protection Degree : IP 23 (outdoor) and others on demand
¡ Standards : IEEE 32, IEC 60137, IEC 60273, EN ISO 1461, EN 10346, EN 12 ISO 129 44, IEC
60071, IEC 60060, IEC 60529
NGR Options
¡ Elevated support stands are provided for ground clearance and safety
¡ Specially designed units for hazardous and extreme locations (Ex-proof, ATEX certified)
¡ Stainless steel or aluminium enclosures on demand
¡ Installation of voltage transformers
¡ Installation of protection relays
¡ Porcelain entrance bushings can be mounted on top or on the side of enclosure
¡ Grounding transformer installation
¡ Motorized or manual single pole disconnector switches, load break switches, vacuum
contactors, circuit breakers, surge arresters and heaters in the Neutral Grounding Resistors
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Hilkar Neutral Grounding Resistors
Routine Tests
¡ Measurement of insulation resistance between
enclosure and resistor
¡ Measurement of DC resistance POWER RESISTORS APPLICATIONS (R)
RC Filter
¡ Insulation test of resistor blocks Dynamic
Neutral
M Braking
Grounding
¡ Thickness measurement of galvanization C
and/or paint R
R R
Type Tests
¡ Temperature rise test
M
¡ Protection degree test R ˜ R L R L C
¡ Impulse voltage test (1.2 / 50 µs)
R C
M
Special Tests (on demand)
Motor Starting Harmonic Filter Test Loads
¡ Measurement of AC resistance
¡ Seismic test
¡ Insulation resistance test (Megger)
Selection Details
¡ System Voltage (kV)
¡ Line to Neutral Voltage (kV)
¡ Desired Current Rating (A)
¡ Desired Resistance Value (ohm)
¡ Maximum Time ON (seconds) and
duty cycle
¡ Bushing Entry or Cable Entry
¡ Current Transformer Ratings (if
applicable)
¡ Disconnector Switch (if applicable)
¡ Special Options (if applicable)
110/Ö3 kV, 400 A, 10 s NGR
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