Resistance earthed systems

A system where at least one of the neutral points is connected to earth via a resistor is called a resistance earthed system.

To improve the earth fault detection in a power system a resistance can be connected between a transformer neutral point and the station earthing system. In order to facilitate high-impedance earth fault detection in systems with weak capacitive connection to earth, the difference between high-impedance earth fault currents and voltages, and those during normal operation must be increased. One way to increase the margin between high-impedance earth fault currents and currents due to normal operation unbalances is to connect a neutral point resistance to the neutral points of some of the transformers in the system.

56

Figure: 4.5. Earth fault in a network with a resistance earthed neutral

Source: System Earthing by Anna Guldbrand Dept. of Industrial Electrical Engineering and Automation Lund University.

Figure 7 shows the corresponding Thevenin equivalent.

Figure: 4.6. Thevenin equivalent of a network with a resistance earthed neutral

Source: System Earthing by Anna Guldbrand Dept. of Industrial Electrical Engineering and Automation Lund University.

Figure 4.7 Sequence network equivalent of an earth fault in a resistance earthed system.

Source: System Earthing by Anna Guldbrand Dept. of Industrial Electrical Engineering and Automation Lund University.

In a system with very weak capacitive connection to earth the reactance of the earth capacitance will be large compared to the neutral point resistance. The neutral point resistance, instead as for the isolated systems the capacitive connection to earth, will therefore determine the maximum earth fault current. Equation 4 gives the earth fault current in case of a solid earth fault.

ef R C

3

o e

E

I

I

I

j wC E

R









In overhead line systems with weak capacitive connection to earth, the capacitive shunt reactance is very large compare to the parallel neutral point resistance and the maximum earth fault current is therefore determined almost exclusively by the neutral point resistance:

1

R

ef C e R

E

I



_{…… 4} Fault detection

58

ef e f

E

I

R

R





…… 5

As mention in previous section, systems with a very weak capacitive connection to earth are normally resistance earthed. To make the difference between fault currents of isolated and resistance earthed systems visible, earth fault current calculations have been carried out using Matlab. The resulting current as functions of fault resistance is shown in Figure 4.8. It is an obvious difference in earth fault current for fault resistances around 5 k ohm, while the difference in fault current for very high impedance faults, and hence unsymmetrical condition during normal operation, is small. If the systems capacitive earth fault current instead is 2 A there will hardly be any difference in earth fault current of isolated and resistance earthed systems for fault resistances above a couple of thousands ohm.

Figure: 4.8. Fault current as function of fault resistance, for an isolated system Ic and resistance earthed system Ir.

Source: Lehtonen, M. & Hakola, T.‖Neutral earthing and power system protection‖, ISBN 952- 90-7913-3, ABB Transmit Oy, Vaasa 1996

As in the case of a fault in an isolated system, the fault current gives rise to a neutral displacement voltage across the system’s impedance to earth. In the case of a resistance earthed system the impedance to earth is the neutral point resistance in parallel to the phase

to earth capacitances. Equation 6 gives the neutral displacement voltage which in case of a solid earth fault equals the pre-fault phase to earth voltage of the faulted phase.





2 2 0 1 3 ef n e I U wC R    _     ……. 6

4.2.3.1 Importance of Resistance Grounding Systems

Resistance grounding has been used in three-phase industrial applications for many years and it resolves many of the problems associated with solidly grounded and ungrounded systems. Resistance Grounding Systems limits the phase-to-ground fault currents.

Grounding Resistors are generally connected between ground and neutral of transformers, generators and grounding transformers to limit maximum fault current as per Ohms Law to a value which will not damage the equipment in the power system and allow sufficient flow of fault current to detect and operate Earth protective relays to clear the fault. Although it is possible to limit fault currents with high resistance Neutral grounding Resistors, earth short circuit currents can be extremely reduced.

As a result of this fact, protection devices may not sense the fault. Therefore, it is the most common application to limit single phase fault currents with low resistance Neutral Grounding Resistors to approximately rated current of transformer and / or generator.

In addition, limiting fault currents to predetermined maximum values permits the designer to selectively coordinate the operation of protective devices, which minimizes system disruption and allows for quick location of the fault.

The main reasons for limiting the phase to ground fault current by resistance grounding are:

 To reduce burning and melting effects in faulted electrical equipment like switchgear, transformers, cables, and rotating machines.

 To reduce mechanical stresses in circuits/Equipments carrying fault currents.

 To reduce electrical-shock hazards to personnel caused by stray ground fault.

 To reduce the arc blast or flash hazard.

 To reduce the momentary line-voltage dip.

 To secure control of the transient over-voltages while at the same time.

 To improve the detection of the earth fault in a power system.

60

1. Low resistance Grounding

In document Research and Thesis on Earthing System in LV Networks (Page 55-60)