Power System Protection Course- EARTH FAULT AND EARTH LEAKAGE PROTECTION

EARTH FAULT AND EARTH LEAKAGE PROTECTION
CONTENTS
Earth Fault and Earth Leakage Protection........................
SYSTEM EARTHING - GENERAL......................................................................
UNEARTHED SYSTEMS......................................................................................
EARTHED SYSTEMS............................................................................................
EARTH-FAULT CURRENTS................................................................................
RESISTANCE EARTHING...................................................................................
Example..................................................................................................................
MULTIPLE GENERATOR EARTHING..............................................................
TRANSFORMER SECONDARY EARTHING....................................................
DRILLING PACKAGES.......................................................................................


EARTH FAULT AND EARTH LEAKAGE PROTECTION

SYSTEM EARTHING - GENERAL

Offshore electrical power supply systems consist of high-voltage generators, some HV distribution, power step-down transformers and a low-voltage distribution system.   Onshore systems have all these except the generators.
If neither the HV nor the LV systems were earthed, conductors could become charged up to any voltage above earth, with risk of breakdown of their insulation to earth.   In particular, if there were a failure of insulation between the HV and LV sides of one of the transformers, high voltage could appear on the LV system, whose insulation is not designed to withstand it.
Therefore it is the practice to tie both systems to earth potential, so that neither is free to 'float away', and no part of the system can be at a higher voltage to earth than the nominal voltage of that system.   If the earthing point is properly chosen it can be even less.

UNEARTHED SYSTEMS

Consider a high-voltage generator or transformer with three output terminals R, Y and B and completely unearthed, as shown in Figure 6.1(a).  The voltage vector diagram is below, and the three line-to-line voltages VRY, VYB and VBR form a closed triangle.  The 'origin' 0, the point of zero potential, does not appear on the diagram because the voltages of the system are not related in any way to earth; they float quite freely.  The vector diagram shows only their relationship to each other, not to earth.  The above applies whether the generator or transformer is star- or delta-connected.


FIGURE 6.1  UNEARTHED SYSTEM
Suppose now a solid earth is applied, say to blue phase, as shown in Figure 6.1(b).  Then the point B of the vector diagram becomes the origin 0, which is the point of zero potential (i.e. earth).  The shape of the diagram is not altered, and the points R and Y have the same positions relative to B as before.  But since B is now at the origin, the actual voltage-to-earth of the red phase is OR, equal to the line-to-line voltage VBR, and the actual voltage-to-earth of yellow phase is OY, equal to the line-to-line voltage VYB.
Hence, in an unearthed system, the accidental earthing of one line will cause both the other lines to take up voltages to earth equal to the line voltage of the system.  This applies equally to generator-fed high-voltage and to transformer-fed low-voltage systems.
If the generator or transformer feeding the system is star-connected, the voltage-to-earth 'ON' of the neutral point N is then, as seen in Figure 6.1(c), the phase voltage (equal to line voltage divided by Ö3) above earth when one of the lines is accidentally earthed.  If the system is 4-wire (three phase and neutral), then the neutral connections of the whole system take up that voltage when a line-earth occurs.  This is sometimes referred to as 'neutral shift'.

EARTHED SYSTEMS

In order to 'anchor' the system voltage to prevent it floating free it is usual (and it is always done at onshore installations and on offshore platforms except the Drilling Packages) to tie one point of the system permanently to earth.  For the sake of symmetry the point chosen is the neutral of the supply element, generator or transformer.  To provide such a neutral point, the element must be star-connected.  Figure 6.2 shows a star-connected earthed system, which may be HV generator-fed or LV transformer-fed.
FIGURE 6.2  EARTHED SYSTEM


This arrangement has a further advantage.  At the bottom of Figure 6.2(a) is the vector diagram for such a system.  The three vectors NR, NY and NB at 120° apart represent the three phase voltages VR, VY and VB relative to the neutral point N: but as the neutral point N is earthed, it is the same as the origin 0.  The three line-to-line voltages VRy, VYB and VBR are represented by the vectors RY, YB and BR.
Since the origin 0 is at the neutral point, the voltage-to-earth of the three terminals R, Y and B can never exceed their phase voltage VR (= VY = VB), which is only 1/Ö3 of the line-to-line voltage.
Even if one line, say blue, were accidentally earthed, this would still be the case.  The situation of Figure 6.2(b) would result.  Blue phase would be completely short-circuited, since both ends B and N would be at earth potential; the phase voltage VB would disappear and B would move to 0.  But the other phase voltages VR and VY to earth - that is, the distance of the points R and Y from the origin 0 - would not be affected (unlike with the unearthed system), and they would remain at line-to-line voltage divided by Ö3, just as before the accidental earth.
Thus, whereas in an unearthed system any line can rise to full line voltage-to-earth in the event of an earth fault on a line, in an earthed system the voltage-to-earth of the lines cannot exceed system phase voltage - that is, 1/Ö3 (or 0.58) times the system line voltage.  Therefore an earthed system can use a lower level of insulation and is thus less costly than an unearthed system of the same line voltage.  For example, an unearthed 6.6kV system must be insulated throughout for the full 6.6kV to earth, whereas the corresponding earthed system need only be insulated for 3.8kV.  This is particularly significant at the higher voltages, especially those used onshore, where insulation becomes increasingly important and costly.
It should be noted that a single line-to-earth fault on an unearthed system will not cause any fault current to flow, since there is no path for it (Figure 6.1(c)).  There is nothing to operate an earth-fault relay and so to trip the circuit-breaker.  Therefore a single earth fault on an unearthed system will not shut down the system.  (Note that two simultaneous earth faults become a line-to-line short-circuit, and the ensuing overcurrent will then trip the breaker.)

EARTH-FAULT CURRENTS

A solidly earthed system is shown in Figure 6.3(a), where an earth fault has appeared on blue phase.  It is clear that a short-circuit current will flow between blue phase terminal and the neutral point via the earth link.  This short-circuits blue phase and produces a fault current limited only by the impedance of the generator phase winding; such a current could initially be many times the normal designed full-load current of the generator and, if allowed to continue, could permanently damage it by overheating the winding insulation or by mechani­cal strain.  A further hazard is the situation at the fault point itself.  The fault is most likely to take the form of an arcing earth, and the fierce short-circuit current could cause intense local heating by the arc at the fault point, with risk to personnel and likelihood of fire.


This might be regarded as a disadvantage of an earthed system.  It would not occur on an unearthed machine with a single fault, as there would be no return path for the fault current (see Figure 6.1(c)).  A compromise is therefore made, especially in high-voltage systems, whereby the voltage-limiting effect of an earthed system is retained, but the earth-fault currents which result are reduced to a level which is not damaging to the generator, at least for short periods, and which also limits the energy released by the arc at the fault point.
FIGURE 6.3  NEUTRAL EARTHING

RESISTANCE EARTHING

This is achieved by the method shown in Figure 6.3(b).  Instead of earthing the generator neutral point solidly, it is earthed through a heavy-duty, short-time rated resistance of low ohmic value.  It can be seen from the figure that, in the event of a line earth, the short-circuit current in the earthed phase is now limited not only by the impedance of the machine's winding, but also by the earthing resistance.  Since a generator's impedance is almost wholly reactive (X), it adds vectorially to the earthing resistance (R) to limit the current to produce an impedance (Z) to the fault current, as shown on the right of the figure.
If the value of the resistance is correctly chosen, the earth-fault current can be limited so as not to exceed the normal full-load current of the generator, or indeed it may be chosen to limit it to half, or even a quarter or less, of the full-load current.  Clearly it is desirable to limit it as much as possible, but sufficient fault current must be allowed to remain, even with less-serious earth faults, to actuate the protective gear.  In the vector diagram of Figure 6.3(b) the reactance vector X is combined with the resistance vector R to give the total impedance Z which limits the current.  If the current had to be limited yet further, the resis­tance would have to be increased as shown dotted in the figure.


With the 15MW generator sets on some installations, the resistance value chosen was 10 ohms which, with a typical generator reactance value of 0.63 ohm, gives an earth-fault current of about 400A, one-quarter of full load.  On other sets the fraction may be a little more.  Usually the resistance value is so much greater than the generator's reactance that the latter may be neglected - i.e.  Z = R.
The earthing resistance itself must carry the limited fault current until the generator is tripped, so it must be heavy duty.  It is usually arranged to be short-rated for 15 or 30 seconds only.  It is customarily used with an isolating link or switch; this must always be opened when megger testing the generator windings for insulation resistance.

Example

An 18MVA, 6.6kV star-connected generator is provided with a neutral earthing resistor.  What must be the value of this resistor if it is to limit the earth-fault current of one phase to one-half of the full-load current?  (The reactance of the generator winding may be neglected.)
First calculate the full-load current IF






18000
Ö3x6.6
 

KVA
Ö3kV
 

 

IF =                      =                         = 1575A
Fault current (IE) is to be limited to half this:


1575
2
 
 

                        IE =                      = 788A        ...         (i)
In Figure 6.3(b) the fault current IE is given from the fault circuit whose emf is the phase voltage VP where VP = VL/Ö3= 3.81kV.  Ignoring the generator's phase reactance X, the fault current is given by Ohm's Law:


VP
R
 
 

IE =





VP
IE
 


3.81 x 103
788
 

 

or    R =          =                                from (i) above
 = 4.84 ohms

MULTIPLE GENERATOR EARTHING

Ideally a system consisting of several generators in parallel should only be earthed at one point, in order to prevent harmonic currents circulating between the generators through their neutral points and so increasing their loading.  This means that, if more than one set is running, only one link should be closed.  In practice, however, most generators are now designed to accept such currents, and links are left closed in all machines (see Figure 6.4).  There are exceptions; sometimes the links are monitored by a logic system which gives an alarm at the control board if the correct ones have not been closed or opened as necessary.




FIGURE 6.4  MULTIPLE EARTHING OF GENERATORS

TRANSFORMER SECONDARY EARTHING

Whereas resistance-earthing of HV generators is common throughout all platforms, the secondary neutral points of all transformers are solidly earthed, either at the transformer itself or more usually through the neutral busbar of the LV switchboard which it supplies (see Figure 6.5).  There is then no resistor to limit the earth-fault current.
FIGURE 6.5  TRANSFORMER EARTHING
The reason for this is two-fold.  First, the secondary (high-current) windings of power transformers are, in any case, robust and can withstand better than an HV generator the full earth-fault currents allowed by a solid earth.  Secondly, the amount of energy available to be released at the fault by an arcing earth is much less downstream of a transformer than on the HV side, where it comes straight from the terminals of the generators.  This factor is therefore of less importance.


DRILLING PACKAGES

The power systems of platform drilling packages are usually separate from those of the platform itself; they consist of diesel-driven 600V generators which are delta-connected.  As such they cannot be earthed, and drilling systems are consequently run unearthed.  At 600V system voltage, any voltage rise on the healthy lines due to an earth fault on one line is not large enough to be significant.

Drillers prefer the use of unearthed systems since, as explained earlier, a single earth fault will not cause the supply to trip.  Loss of power while a drill is in the hole can be serious, leading possibly to the total loss of the equipment.
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