Resistance Earthing Vs Reactance Earthing

Or how about capacitance earthing? Capacitance earthing is ideal where a 1/4-wavelength vertical antenna is coupled to a 75Ω feeder coaxial cable to the terminals of a radio transceiver.

It is important to keep in mind that the earth fault current limitation counter acts the earth fault detection, no matter what earthing method is chosen. The earthing design shall limit the earth fault current and enable fault detection while keeping the associated costs as low as possible. The cost associated to the earthing of isolated systems is relatively low. When possible, power systems are therefore often isolated.It is not suitable isolate the neutral points of the systems if the system's capacitive connection to earth is either very weak or quite strong. If the capacitive connection to earth is very weak neutral point resistors are connected to the system. If the capacitive connection to earth on the other hand is quite strong it is necessary to connect neutral point reactors.

Pl. refer:

http://www.iea.lth.se/publications/Reports/LTH-IEA-7216.pdf

http://www.eon-uk.com/distribution/CiCdocs/01%20Technical%20Documents/CN%20Combined/Earthing%20Manual/E2%20Earthing%20Manual%20Earth%20Guidance%20Notes.pdf

Vinu_Answers Sure_Answers

RESISTANCE EARTHING: THE NEUTRAL POINTS OF TRANSFORMER OR GENERATOR ARE GROUNDED THROUGH IMPEDANCE, THE PRINCIPAL ELEMENT OF WHICH IS RESISTANCE. THIS METHOD IS USED WHEN THE EARTH FAULT CURRENT WOULD BE TOO LARGE IF NOT RESTRICTED.

Here, a resistor is connected intentionally between the neutral and earth. This is to limit the earth fault current. The reasons to limit the earth fault current are:

In rotating electrical machines like motors and generators, if the earth fault current is high, as in the case of solid earthing, the core damage would be high. To limit the damage to the core, machine manufacturers allow only a limited ground fault current. This is given in the form of a core damage curve. A typical value would be 25A for 1 second. This value is used as a guide in selecting NGR and setting stator earth fault relays in generator protection. Winding damage in rotating electrical machines is not of serious concern. The repairs to winding damages can be done by the local re-winder. But, in case of core damage, repairs cannot be carried out at site. The machine has to be sent back to the manufacturer's works for repairs thus resulting in prolonged periods of loss of production.

Since rotating electrical machines are not present in voltage levels from 22kV onwards, these systems are usually solidly grounded. At EHV level too, solid grounding is universally adopted for two reasons:

- Cost of insulation at EHV level is significant.

- Primary protection in such systems would clear the fault within 5 cycles.

If rotating machines are present in 3.3kV, 6.6kV & 11kV levels, the systems are grounded through resistor or reactor to limit the earth fault current. If rotating machines are not present at these voltage levels, then these systems can be solidly grounded.

In case of LV Systems, though rotating electrical machines are present, the system is solidly grounded to confirm to IE Rules. [Rule 61 (1) e]. Since LV System is also handled by general public, for safety reasons, solid grounding is mandated. Sufficient ground fault current is allowed to flow so that protective devices can operate and clear the faults at the earliest. Of course, the core damage at the point of fault in rotating machines will be high.

Since a large number of rotating machines are present at the LV level, it may be worth considering resistance grounded system, even at this level to limit the earth fault current. The LV Bus can be segregated into those supplying rotating machines (this should be a resistance grounded system) and those supplying static loads like lighting and heaters (with solid grounding).

Other reasons for going in for resistance grounding systems are:

• Reduce burning and melting in electrical equipment (which are caused by the high fault currents in a solidly earthed system).

• Reduce mechanical stresses (F ∞ i²) compared to solidly earthed systems.

• Reduce re-striking/arcing faults when compared to unearthed systems.

Reactance Grounding: A reactor is intentionally connected between the neutral point and earth. For system voltages greater than 36kV, it is preferable to use a reactor rather than a resistor because of the difficulties arising from heat emission in the event of a fault in a resistance earthed system.

Here, an inductive reactance limits the earth fault current and still keeps the over voltages within limits. However, protection must be provided to automatically clear the first fault.

To reduce switching surges and to allow simple detection, the current must be much higher than the total capacitive current of the power system. In distribution systems, higher values are used (300 to 1000A), since they are easier to detect.

Thanks a lot.

sir

our consultant for 220/33 kv grid power supply suggesting for resistive earthing of 33 kv side neutral over soild earthing please suggest which is best practice now as on today

If there are no rotating machines present at the voltage level, then there is no need for going for Resistance Earthing. Solid Earthing would turn out to be economical, as it would greatly reduce the cost of insulation of the equipment in the network.