Protection Application Handbook (англ. яз.)

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DIFFERENT TYPES OF SYSTEM EARTHING

System Earthing

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3.5 HIGH RESISTIVE EARTHING

High resistive earthing concerns the cases where the factor

“3R

≤X

”, where “X

” is the total capacitance of the network

per phase against earth, is obtained by using the biggest possi-

ble resistance R

to earth.

If the capacitive current in a network at a fully developed earth

fault is less than 30 A it’s possible, up to voltages of 25 kV, to pro-

vide the system with a neutral-point resistance for currents of at

least the same size as the capacitive current of the network.

At transient earth faults the voltage in the network stays within

reasonable limits (see ﬁgure 3) and the current to earth increas-

es with only 50% compared to the unearthed system. The neu-

tral-point resistance can, if desired, relatively easy be installed to

withstand the phase voltage. Therefore it’s not necessary to have

a tripping earth fault relay at locations where it would create ma-

jor disadvantages for the operation. This can e. g. be the case for

industrial plants where tripping can cause a big disturbance for

the service.

When high resistive earthing is mentioned, basically in American

literature, an earthing through an one-phase transformer loaded

with a resistor secondary is intended. This is electrically equiva-

lent to a resistor directly connected between the neutral-point

and earth. See ﬁgure 5.

Figure 5. Two equivalent methods for high resistance earthing.

DIFFERENT TYPES OF SYSTEM EARTHING

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3.6 RESONANCE EARTHING (EARTHING WITH PE-

TERSÉN-COIL)

For resonance earthing an inductance calibrated to the capaci-

tance of the network at rated frequency is chosen. This leads to

a small resulting operating frequency earth fault current and it is

only caused by the current due to insulation leakage and corona

effect. An arc in the fault point can therefore easily be extin-

guished since current and voltage are in phase and the current is

small. Observe that a strike through a solid material like paper,

PVC, cambric or rubber isn’t self-healing why they don’t beneﬁt

from the resonance earthing.

The resonance coil should be calibrated to the network for all

connection alternatives. Therefore the setting must be changed

every time parts of the network are connected or disconnected.

There however exists equipment, e. g. in Sweden, Germany and

Austria, that will do this automatically.

At a resonance earthing it is often difﬁcult to obtain selective

earth fault relays. Therefore the resonance coil is connected in

parallel with a suitable resistor giving a current of 5 to 50 A at full

zero-point voltage, i. e. a solid fault. The resistor should be

equipped with a breaker for connecting and disconnecting at

earth faults. The breaker should be used for the In-Out automatic

and for the thermal release of the resistor, if the fault is not auto-

matically cleared by the protection relays, as these normally are

not designed to allow continues connecting.

The theories about resonance earthing are very old and were de-

veloped at the time when only overhead transmission lines exist-

ed. In cable networks, high harmonic currents are generated

through the fault point. Even if the network is exactly calibrated

it’s possible that currents up to several hundred amperes ap-

pears. This of course leads to difﬁculties for an earth fault to self

extinguish.

DIFFERENT TYPES OF SYSTEM EARTHING

System Earthing

for worldwide cooperation

BA THS / BU Transmission Systems and Substations

LEC Support Programme

Resonance earthing can also be a way to fulﬁl the requirements

concerning the standards for maximum voltage in earthed parts.

Through resonance earthing a smaller earth leakage current is

obtained and consequently a higher earthing grid resistance can

be accepted.

In an unearthed or voltage transformer earthed system the risks

for overvoltages at transient faults are not considered.

Generally it is recommendable to avoid unearthed systems. In

the British standards this is explicitly stated.

TO OBTAIN A NEUTRAL-POINT

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4. TO OBTAIN A NEUTRAL-POINT

In a three phase network the neutral-point often is available in a

“Y”

connected

transformer

or in a generator neutral. This should

then be utilized at system earthing.

If the transformer is connected “Y0/D” or “Y-0/Y-0” with delta

equalizing winding, the “Y-winding” can be utilized even for direct

earthing. An “Y/Y” connected transformer without such an equal-

izing winding shall not be directly earthed but can under some cir-

cumstances be utilized for high resistance or resonance earthing.

4.1 Y0/Y-CONNECTED TRANSFORMER

If one side of a “Y/Y” connected transformer is earthed, theoreti-

cally there wouldn’t ﬂow any current through the earth connection

since it is impossible to create a magnetic balance in the wind-

ings for this kind of currents. However, the ﬂow is closed through

leakage ﬁelds from yoke to yoke through insulation material and

plates. This can create a local heating that will damage the trans-

former.

If the transformer has a magnetic return conductor as e. g. a ﬁve

leg transformer or three one phase units the earth current leads

to a ﬂow in the return conductor.

The zero sequence impedance for an “Y0/Y” transformer is al-

ways high but varies, depending on the design, from 3 to 40 times

the short circuit current impedance.

If you are aware of the high zero sequence impedance and if the

transformer construction accepts the heating problems a “Y0/Y”

connected direct earthed transformer limiting the earth fault cur-

rent to about the transformers rated current can be utilized in the

power system.

4.2 Z/0-CONNECTED EARTHING TRANSFORMER

In the “Z/0” connected transformer, see ﬁgure 7, a magnetic bal-

ance for zero sequence current leading to a low zero sequence

TO OBTAIN A NEUTRAL-POINT

System Earthing

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LEC Support Programme

impedance is created. The neutral point of the transformer can ei-

ther be connected directly to earth or through another neutral

point apparatus such as a resistor or reactor. In the ﬁrst case the

thermal consequences must be cleared in advance as the

“Z/0”.transformer often has limited thermal capability. The earth-

ing “Z/0” transformer is thus often speciﬁed for 10 or 30 sec ther-

mal rating.

Figure 6. Z/0-connected earthing transformer.

DISTRIBUTION OF EARTH FAULT CURRENT

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5. DISTRIBUTION OF EARTH FAULT CURRENT

In calculating earth fault currents it’s advantageous, but not nec-

essary, to use symmetrical components to check the ﬂow of the

earth fault currents.

A very simple way of checking, where only peripherally the theory

of symmetrical components is used, could be utilized. According

to this theory a current

“3I

” (three times the zero sequence cur-

rent), ﬂows through the neutral-point. This can be used in letting

the fault be represented by three “current arrows” and then exam-

ine how these can be distributed in the system maintaining the

magnetic balance. Possible positive and negative sequence cur-

rents are then not considered. Some examples of these calcula-

tions are shown in the text that follows but ﬁrst an explanation of

the “zero sequence impedance”.

5.1 ZERO SEQUENCE IMPEDANCE

In the previous discussion the name “zero sequence impedance”

has been used at several occasions. The easiest way to under-

stand this term is to indicate how to measure it in practice.

The measurement of zero sequence impedance is done by short

circuiting the three phases with preserved earthing of the net-

work. Each phase has then a z

ero sequence impedance which is

three times

the impedance measured between the three phases

and earth.

5.2 CURRENT DISTRIBUTION AT AN EARTH FAULT

WITH A Z/O EARTHING TRANSFORMER.

Whether the network has been earthed in the neutral-point, or

with a Z/O connected transformer on the generator’s line termi-

nals, the result of the earthing will be the same. This is illustrated

in ﬁgures 8 and 9.

DISTRIBUTION OF EARTH FAULT CURRENT

System Earthing

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Figure 7. The earth fault current distribution, with a resistance in the gener-

ator neutral-point.

Figure 8. The earth fault current distribution, with a neutral-point resistance,

connected to a Z/O-connected earthing transformer.

The ﬁgures above shows that the earthings are equal from the

networks point of view.

In the ﬁrst case a zero sequence current goes through the gen-

erator.

In the second case the current does not contain any zero se-

quence component since the sum of the currents in the three

phases is zero.

The generator current contains only one positive and one nega-

tive sequence component.

5.3 CURRENT DISTRIBUTION AT AN EARTH FAULT

IN A TRANSMISSION NETWORK.

To show that the earth fault current not necessarily need to come

from the same direction as the power an earth fault, in a directly

Figure7. Figure 8.

DISTRIBUTION OF EARTH FAULT CURRENT

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earthed transmission network, where the receiving transformer is

earthed but unloaded is chosen, see ﬁgure 10.

Figure 9. Current distribution in a transmission network.

It is possible to see in the ﬁgure, that neither the generator nor the

transformer “T1” has a zero sequence current since the sum of

the currents in all phases is zero. It is however easy to realize that

a negative sequence current exist. An earth current protection

measuring negative sequence current at “T2:s” HV line terminal,

would thus operate despite the fact that “T2” is unloaded.

T1 T2

TO CHOOSE SYSTEM’S EARTHING POINT

System Earthing

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6. TO CHOOSE SYSTEM’S EARTHING POINT

Normally in directly earthed and effectively earthed systems ev-

ery available neutral point is earthed. Deviations from this occurs

when the transformers neutral points are left unearthed. This is

done to limit the maximum earth fault current which can arise to

reasonable values. In these cases the neutral-point is equipped

with surge arresters. This is only acceptable if the network at all

operation modes still can be considered as effectively earthed

(X0≤3X1).

For the other earthing methods it is somewhat more complicated.

For example the wish to keep the earth fault current more or less

constant, at the same time as the network always must be

earthed independently of the operation mode, gives contradic-

tions and difﬁcult choices.

6.1 INDIVIDUAL EARTHING IN EVERY NEUTRAL

POINT OF THE POWER SOURCES

Figure 10. Earthing individual neutral-point with impedances.

When there only are a few generators or transformers in a sta-

tion, individual neutral-point impedances are often used. Hereby

the neutral-point connection is ﬁxed without intervening connec-

G G

TO CHOOSE SYSTEM’S EARTHING POINT

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tion devices. When just two power sources are used individual

neutral-point impedances are preferred to a common earthing

impedance. When several power sources are used the earth fault

current is increased every time a power source is connected and

can reach unwanted values. At resistance earthing every resistor

must be dimensioned for a current high enough to satisfy the op-

eration of the relay equipment, when this is working alone. Con-

sequently the total earth fault current at several aggregates,

becomes several times the value required for a satisfying relay

function. A disconnector is thus often provided to enable discon-

nection of resistors when system is in parallel service condition.

The method with individual earthing is normally utilized at resis-

tance and reactance earthing, but it can also be utilized at high

resistive earthing. For resonance earthing the method is very un-

suitable.

Together with generators or motors, multiple earthing can be un-

suitable due to the danger of circulating third harmonic currents.

6.2 COMMON EARTHING THROUGH A NEUTRAL

BUSBAR.

When there are more than two generators, or transformers, in a

station it can be preferable to use just one neutral-point appara-

tus. The neutral-point of every power source is then connected

through a coupling device, breaker or disconnector, to a common

neutral busbar which is earthed through a resistor or a reactor.

This arrangement keeps the earth fault current at optimal size,

since it never has to be higher than what is needed to avoid ov-

ervoltages or to give a safe relay protection operation. There is al-

ways the same earth fault current independent of the service

condition.

Two different connections with neutral-point busbars are shown

in ﬁgures 12 and 13.