In the cases where it is necessary to limit the fault current then the current limiting element must be inserted in the neutral and earth. One of the ways of achieving this is the use of resistance earthing where one or more resistances are connected between neutral and earth. The resistor may be either of wire or water column resistances for voltages of 6.6. KV and above. Metallic resistors do not change with time and requires little maintenance. But owing to its inductive nature they disadvantages with overhead lines exposed to lighting as impulses or the travelling waves are subjected to positive reflection and cause stress on insulation resulting in its breakdown. Liquid resistors are free from these disadvantages and have simple and robust construction.
As shown in the Fig. 1(a) let the earth fault occurs on phase B. The corresponding phasor diagram is shown in the Fig. 1(b). The capacitive currents and flow through the health lines. The fault current not only depends on the zero sequence impedance of the source but also on the resistance in the earth circuit. This fault current can be resolved into two components one inphase with the fault phase voltage and other lagging the faulty phase voltage by . This lagging component of current is in phase opposition to capacitive current and it changes with change in value of earthing resistance. Thus the value of this resistance is designed in such a way that during fault on any phase, a current equal to full load current of largest alternator or transformer flows in earth resistance which will keep the overvoltages within limits. With fault current lagging component equal to capacitive current the system operation is similar to solidly earth system and no transients occur due to arcing ground.
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| Fig. 1 |
With high value of earthing resistance and low value of reactive current than the capacitive current then system conditions approach to that of ungrounded system with chances of transient over voltages to occur. The line to earth voltage of the healthy phases at the time of fault is little more than line to earth voltage of the solidly grounded system operating under similar conditions. The duration of this voltage can be reduced by using suitable protective switchgears to avoid any harmful effect that may be caused.
The value of resistance to be inserted in earth circuit is given by,
R = VL/√3.I
where VL = Line to Line Voltage
I = Full load current of largest alternator or transformer
The advantages of this system are as follows,
1) The discriminative of type of switchgears may be used for protection.
2) The hazards of arcing grounds are minimzed.
3) The influence of neighbouring communication circuits is minimized due to lower value of fault current flowing through earth as compared to that in case of solidly grounded system.
The disadvantages of this method are given below,
1) As the neutral is shifted during earth faults, the equipments are to be selected for greater voltages.
2) The system is expensive than the solidly grounded system.
3) There is energy loss in neutral grounding resistors for dissipation of fault energy.
This method is normally adopted in system with voltages from 2.2. KV and 33 KV with a power source capacity more than 5000 KVA.
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