Earthing neutral point in MV network 10(20)kV.

[mbrele]

In general our network is 10(20)kV isolated system
(that means secondary winding transformer 110/10 kV
or 35/10 kV is isolated) . But utility company now
in some cases had request that in the new project
neutral point have to earthed through resitor.
How to find rules, or standards to take more
knowledge about when and wich cases MV network will
be earthed neutral point throught resistor, Petersen
coil,or isolated.

 

[RalphChristie]

Check the IEEE Green Book:

http://standards.ieee.org/colorbooks/sampler/Greenbook.pdf

From the J&P Transformer Book: (Twelfth Edition - Martin Heathcote)
(This Book have some good info regarding neutral earthing)

The subject of neutral earthing is a complex one and, whenever discussed by electrical engineers, views are varied and discussion lenghthy. Practices vary in different countries, and even within different utilities in the same country.

Guiding principles in relations to earthing in the UK are determined by statue, in the form of the Electricity Supply Regulations of 1988. Part II of the regulations says that:
•Every electrical system rated at greater than 50V shall be connected to earth.
•How that earth connection is to be made differs between high-voltage and low-voltage systems.


There are different methods of earthing a trsf:
Solid earthing, resistance earthing, reactance earthing, Arc Suppression Coil (Petersen Coil) and earthing through a zig-zag trsf. Every method has its own advantages and disadvantages and based on that a earthing-method is chosen.

Solid
Advantages
•Neutral held effectively at earth potential.
•Phase-to-ground faults of same magnitude as phase-to-phase faults, no need for special sensitive relays.
•Cost of current limiting device is eliminated.
•Size and cost of trsf is reduced by grading insulation towards neutral point.
Disadvantages
•As most faults are phase-to-ground, severe shocks are more considerable than with resistance earthing.
•Third harmonics tend to circulate between neutrals.
•High degree of damage at fault point and possible damage to feeder equipment.
•High Step and touch potentials.
Resistance
Mainly used below 33kV, value is such as to limit earth fault current to between 1 and 2 times full load rating of trsf. Alternatively to twice normal rating of largest feeder, whichever is the greater.
Advantages
•Limits electrical and mechanical stress on system during earth faults, but at same time current is sufficient to operate normal protective equipment.
•Lower degree of damage at fault point and usually no damage to feeder equipment.
•Reduced Step and touch potentials.
Disadvantages
•Full line-to-line insulation required between phase and earth.
•Higher initial cost.
Reactance
Value of reactance are approximately the same as used for resistance earthing.
To achieve same value as the resistor, the design of reactor is smaller and thus cheaper.
Arc Suppression Coil (Petersen Coil)
Tuneble reactor is connected between N and E
•Value of reactance is chosen such that reactance current neutralises capacitance current. Current at the fault is therfore theoretically 0 and unable to maintain the arc, hence the name.
•Virtually fully insulated system, so current available to operate protective equipment is so small as to be negligible. To offset this, the faulty section can be left in service indefinetely without damage to the system as most faults are earth faults of transient nature, the initial arc at fault point is extinguised and does not re-strike
•Sensitive wattmetrical relays are used to detect permanent earth faults
Zig-zag trsf
Provides an earth point for a delta system and combines the virtues of resistance and reactance earthing.




Some other Standards and Books regarding an earthing system:
•BS7430:1991 Code of practice for earthing.
•EA Technical Specification 41-24:1992 Guidelines for the design, testing and main earthing systems in substations.
•The book "Earthing Practice" published by the Copper Development Association (Trevor Charlton)

Regards
Ralph

 

[RRaghunath]

IEEE Red book includes discussion on power system earthing.

I see now a days it is universally accepted that MV systems (3kv to below 33kV) shall be earthed through neutral grounding resistors, limiting earth fault currents between 300 and 600A.