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Guideline for Neutral Earthing

This guide covers the Neutral earthing of Generator, Transformer which is also called system earthing. Neutral earthing is done to protect people and equipment. Earthing will have impact on fault current and system overvoltage. Certain types of earthing reduce the earth fault current and protects the equipment like generators, motors from damage during fault. Neutral earthing also prevents over voltages in a system.

Low resistance grounding, In this method the fault current is restricted to a value which is much below the actual fault current but still sensible by the normal relays.

Picture courtesy: M S Resistances France

NGR (Neutral grounding resistors) are used in LRNG system between the neutral and earth to restrict fault current

Picture courtesy: Advanced Power Technologies (APT) (LRNG and HRNG)

High resistance neutral grounding (HRNG) is mostly used for Unit connected generators. It is also used of LV distribution and MV distribution system. Resistive current from NGR to fault, should be equal are greater than capacitive charging current from stray capacitance to fault. Normally HRNG consist of NGR and neutral grounding transformer (NGT).

 

Picture Courtesy: Pyrotech India (NGR with NGT , HRNG System)

Picture Courtesy: HEINE Resistors GmbH ( Metallic resistor and NGR with NGT )

Picture Courtesy: Meister international, LLC (NGR with NGT)

Monitoring of NGR is very important. In case of power plant and industrial application It is not mandatory to monitor NGR, but standards for mines requires monitoring of NGR. Monitoring of the resistor is done by injecting a current through the resistor and by measuring the injected current, neutral to ground voltage and the continuity.

Picture courtesy: Littelfuse, Inc

Zigzag transformer are used to derive neutral in an ungrounded system. Neutral earthing resistor along with the transformer are used to reduce the fault current level.

Picture Courtesy: Trench Austria GmbH (combination of zigzag transformer with arc suppression coils)

Resonant grounding, In this method tuned reactor in neutral in parallel with system capacitance provide a very high impedance due to resonance. The fault current is limited to a value much lower than high resistance grounding. There will be no transient over voltage as there will be no arc and re-strikes. Cost of arc suppression coil is more than resistor.

Picture Courtesy: Trench Austria GmbH

Peterson Coil or another name for resonant grounding, construction will be similar to dry type or oil type transformer or reactor. The coil consists of Iron core, winding, body with or without oil. If coil reactance is adjustable it will consist of either tapping or some other mechanism either a motor or capacitors as per the vendor design.

Picture Courtesy: Trench Austria GmbH

Tuning of arc suppression coil is done by regulators which measure the capacitive reactance of the network and adjust the inductive reactance to match the capacitive reactance. If exact resonance is achieved fault current will be very less.

Picture Courtesy: A. Eberle GmbH

In low resistance and solid earthed network, the fault current will be high enough to enable a current based relay sense the fault. Whereas in resonant earthed system it is very difficult to sense the fault with only current. Measuring the neutral to earth voltage will indicate the fault in the network but the feeder where fault has taken place cannot be determined by these voltage-based relays. Hence various advanced methods are used to determine the faulty feeder.

Picture Courtesy: Trench Austria GmbH

Ground fault neutralisers are an improvement over arc suppression coil, they consist of the variable reactors which fully compensates the capacitive current and in addition it has a residual current compensator which will completely eliminate the active part of the current thus reducing the fault current to a very low value.

Picture Courtesy: Swedish neutral

Detailed Guideline for Neutral Earthing consist of 38 sections with 70 pages. This detailed guideline gives many practical examples, calculations, illustration which will help an application engineer to actually design a plant. This full guideline can be accessed in below link.

Contents of the detailed guideline are as given below
1.0 Scope
2.0 Exclusions
3.0 Reference Documents
4.0 What is the function of Neutral earthing
5.0 Inputs required to carry out the design
6.0 What causes a fault
7.0 Coefficient of grounding and Earth fault factor
8.0 Type of grounding
9.0 Neutral treatment:
10.0 Selection of type of earthing
11.0 Different types of earthing practise used in
12.0 Low resistance neutral grounding
13.1 Advantages of resistance grounding
13.2 Low resistance earthing of MV system with VFD
13.3 Low resistance grounding with Single generators
13.4 Low resistance grounding with Single generators
13.5 Low resistance grounding with multiple generators
13.6 Effect of Third harmonics in parallel Generators on NGR design
14.0 High resistance grounding
14.1 HRNG Basics
14.2 Sequence network High resistance neutral grounding system
14.3 HRNG current limit
14.4 History and evolution of high resistance grounding
14.5 Overvoltage and insulation
14.6 Charging current contributions
14.7 Cause of stray capacitance
14.8 High resistance grounding for Generators with Unit transformer
14.9 Fault current circulation due to field discharge system
14.10 Open circuit time constant
14.11 NGR/NGT design methodology
14.14 Basic insulation level
14.15 Construction of NGR
14.16 Ingress protection
14.18 NGR time rating
14.20 Temperature coefficient of the NGR
14.21 Requirement of Neutral Grounding Transformer
14.23 Typical calculation of NGR NGT and Brief specification
14.24 Arc hazard eliminated by High resistance grounding
14.26 Oil mines earthing
14.27 High resistance grounding for MV distribution system
14.28 NGR failure and its associated problems
14.29 Monitoring of NGR with relays
14.30 Earth fault Protection for HRNG system
14.31 Locating fault in High resistance grounded system
14.32 Why HRNG were used in the beginning with only alarm
15.0 Arc energy and damage limits
16.0 Deriving neutral artificially
16.1 Deriving a neutral in Ungrounded power system
16.2 Zigzag transformer used to derive neutral in ungrounded system
16.3 Earthing transformer/ Zig zag specification
16.4 Calculation of fault current for network with zigzag transformer
16.5 Protection methods when using zig zag transformer
17.0 Resonant grounding
17.1 Different terminology for a group of similar technology
17.2 Resonant grounding system for Unit connected Generator
17.3 Calculation for resonant grounding of Unit connected generators
17.4 Resonant grounding for distribution system
17.5 Typical Arc suppression coil application SLD
17.6 Fault current of 11kV isolated network earthed with Peterson coil
17.7 Peterson Coil construction
17.8 Type of arc suppression coil
17.9 Regulator for Peterson coil
17.10 Detection of fault feeder in resonant earthed system
17.11 Voltage at the point of fault
17.12 Converting Isolated network to one with Peterson coils
17.13 System modification requirement- ASC added in solid earth system
17.14 Changing Peterson coil to Low resistance grounding
19.0 Rating of neutral grounding reactors
20.0 Advantage of ground fault current limiters over phase fault current limiters
21.0 Active earthing system or Ground fault neutralisers (GFN)
22.0 Neutral grounding resistor versus Neutral grounding reactor
23.0 Type of generator connection and related earthing system
24.0 What is hybrid system
25.0 Different types of Earthing of generator and utility system combination
26.0 LV unearthed system
27.0 Grounding of DC system
28.0 Common grounding resistor for generator earthing
29.0 Time rating of neutral grounding equipment
30.0 Voltage versus Current
31.0 Clearing ground fault within a definite time
32.0 Earthing, tripping philosophy, fault duration, time rating of neutral equipment
33.0 Types of earthing system used in generation, transmission and distribution
34.0 Conversion of one system of earthing/grounding to other system
35.0 Effect of earthing system on damage, overvoltage and relay operation
36.0 Vendor supported in preparing this guidelines
37.0 Allowable touch voltage in 11kV system
38.0 Reference

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