Protection Application Handbook (англ. яз.)

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TO CHOOSE SYSTEM’S EARTHING POINT

System Earthing

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LEC Support Programme

Figure 11. Resistance earthing, of the generator neutral-point, with a neutral

busbar and individual neutral-point breakers.

Figure 12. Resonance earthing, of transformer neutral-point, with a neutral

busbar and individual disconnectors.

Due to the third harmonic problem only one of the breakers in ﬁg-

ure 12 resp. 13 should be closed at a time.

When one of the generators is taken out of service it’s important

that the corresponding neutral point breaker (or disconnector) is

opened. This since the neutral busbar will be current carrying at

an earth fault and achieve the phase voltage to earth.

6.3 COMMON EARTHING THROUGH A EARTHING

TRANSFORMER ON THE BUSBAR.

An effective and often cheap way to make sure that the system

always is correctly earthed is to connect any earthing transform-

er, according to section 4, to the busbar. See ﬁgure 13 which

shows the same network as in ﬁgure 10 but with an alternative

method of earthing.

TO CHOOSE SYSTEM’S EARTHING POINT

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Figure 13. Earthing, with a Z/O-connected transformer on the busbar.

G G

Z-0

PRACTICE OF EARTHING

System Earthing

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7. PRACTICE OF EARTHING

The practice of earthing differs very much from country to coun-

try. It is however possible to distinguish countries as Germany,

Netherlands and Sweden etc., where the main direction has

been to protect the telephone networks and people. It is also pos-

sible to distinguish countries as USA, Canada and England,

where the power network protection has been considered ﬁrst.

The ﬁrst mentioned countries has focused on limiting the earth

faults currents to low values, while the latter countries has ac-

cepted the higher earth faults currents to prevent overvoltages in

the power system and simplify fault clearance.

A summation of the practices in different countries would unfor-

tunately be very extensive, especially as the networks differs

even within the countries. However, a simpliﬁed summary follows

below.

7.1 VOLTAGES OVER 100 KV

At high voltages there is an economic advantage in earthing the

network directly (effectively). By doing so transformers and insu-

lators etc. can be built with a lower test voltage at neutral and a

graded insulation, which gives considerable cost savings.

In most countries it’s normal with a direct earthing at voltages

over 100 kV. In e. g. Germany, Netherlands and Norway, however

there are 130kV networks with resonance earthing.

7.2 VOLTAGES BETWEEN 25 AND 100 KV

USA Most parts of the country are directly earthed but resistance

and reactance earthing occur.

ENGLAND Most parts of the country are resistance earthed in the

neutral-point of the power source. The resistance gives an earth

PRACTICE OF EARTHING

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fault current of the same size as the rated current in the trans-

former. Some 33 kV networks can however be resonance

earthed.

GERMANY, SWITZERLAND, AUSTRIA, NETHERLANDS, BELGIUM,

SPAIN, IRELAND, NORWAY, DENMARK, SWEDEN, JAPAN.

Uses reso-

nance earthing.

FRANCE, SOUTH AFRICA.

Most parts of the countries are resistance earthed (reactance

earthing occur). France is investigating the possibilities of a

change over into resonance earthing (with transient measuring

earth fault protection).

AUSTRALIA Uses direct earthing and resonance earthing. Some

33 kV networks can however be resistance earthed.

NEW ZEALAND Uses direct earthing

INDIA, MALAYSIA Resonance earthing is the most common earth-

ing but also resistance earthing occurs. And in India also direct

earthing can occur.

7.3 VOLTAGES BETWEEN 1 AND 25 KV FOR DISTRI-

BUTION WITH OR WITHOUT DIRECT CONNECTED

GENERATORS

In most countries varying types of earthing can occur but the res-

onance earthing is the most common. However unearthed net-

work as well as high resistance earthings can occur.

Resistance earthing is most common in USA and England.

Direct earthing is most common in Australia and Canada but can

also occur in USA and Finland.

The earthing in Sweden, is mainly decided by §73 in Kommer-

skollegi standards, which says it’s practically impossible to use di-

rect, or reactance earthing concerning these voltage levels. This

is due to high requirements on detection of fault resistances at

PRACTICE OF EARTHING

System Earthing

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earth faults. Fault resistance values of 3000 resp. 5000 Ω are

mentioned depending of type of feeder.

7.4 GENERATOR NETWORK

Generator networks, are the type of limited networks that consist

of one or several generators connected to a primary transformer,

but without direct connection to the distribution lines.

These limited networks are almost always high resistance

earthed. However at new constructions unearthed networks rare-

ly occurs.

7.5 VOLTAGES UNDER 1 KV

These networks are normally direct earthed. In industries with

pure motor networks, unearthed or high resistance earthed net-

works are mainly used.

A special type of earthing, is used by the Swedish state power

board in their “unearthed” motor network. The high resistance

network has been replaced with a voltage dependent resistor. At

earth faults a small current is created through the resistor and the

network can be considered as unearthed. At ﬂash-over from the

network, at the primary side of the transformer (normally a high

resistance earthed 10 kV network) the resistance in the resistor

will be so low that the overvoltage in the low voltage network will

be limited to 2 kV. See ﬁgure 15.

< 2 kV

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Figure 14. Earthing with a voltage dependent resistor, to limit the overvolt-

age, at a break-down of a high voltage network.

PRACTICE OF EARTHING

System Earthing

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7.6 CHOICE OF EARTHING METHOD

It’s rarely possible to change an already established earthing

practice. At new deliveries or extensions it’s therefore necessary

to adapt to the already existing practice. Depending on the qual-

ities considered important, the earthing that will give the most ad-

vantages and the lowest total cost solution, regarding the

methods described in the following section is chosen.

7.7 COMPARISON OF DIFFERENT EARTHING PRIN-

CIPLES

A comparison between different system earthing principles, can,

based on the different parts according to table 1, be done. In the

table resistance earthing is divided into low resistance and high

resistance earthing depending on the effects regarding the dam-

ages from the current.

Tabell 1:

Type of

earthing/

Feature

Solidly/

Effectively

Reactance

Low

resistance

Resonance Unearthed

High

Resistance

Damage of

equipment

--- -- - ++ +++ ++

Damage of

property

-- -- - ++ +++ ++

Person inju-

ries

--- --- --- + + -

Arcing faults +++ +++ +++ ++ ++ ---

Overvoltage +++ ++ ++ + + ---

Ferro reso-

nance

+++ +++ +++ +++ +++ ---

Req on

earthing grid

--- -- - + ++ ++

Apparatus

insulation

+++ - - - - ---

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DAMAGES ON EQUIPMENT Damages on the electrical equipment

are normally pure quadrilateral dependent on the current and the

fault time. Since “I

t” measures the energy, a lower current is a

big advantage in limiting the damages. This is of major impor-

tance when rotating machines are included in the network.

DAMAGES ON PROPERTY Damages on property are depending on

the current but normally in steps since damages on equipment

(for example the telephone network) occur, when its durability is

surpassed. The risk for ﬁre due to an earth fault, follows the “I

t”

relation. Note that for higher voltages there are such big distanc-

es to earth and between phases that a good protection exist in

the physical construction. On distribution level the risk for damag-

es on property is however of greater importance.

INJURIES ON PERSONS As mentioned earlier, some countries have

chosen to keep the current low to protect people and property.

Other countries have considered it as more important to optimize

the system and ensure simple and fast fault clearance.

The risk for injuries on persons can be divided into occuring step

and touch voltages at primary faults of the power system.

Figure 16 shows the risk for injuries of people in an electrical ac-

cident and how contact and step voltages arises. The injuries

consists partly of the risk for heart stop due to heart oscillation

which occurs at 30 mA and the burn injuries in the skin layers at

which the contact was made. The type of burn injuries are of

course completely dependent on the current level.

Selective

protection

++ +++ +++ + - -

Tabell 1:

Type of

earthing/

Feature

Solidly/

Effectively

Reactance

Low

resistance

Resonance Unearthed

High

Resistance

PRACTICE OF EARTHING

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Figure 15. How the step and touch voltages, arises in a direct earthed net-

work.

TRANSIENT EARTH FAULTS These consist of increased voltage at

healthy phases during a reignition of the earth fault current after

a zero crossing. A connection of the network to earth limits the

overvoltages according to the previous discussion (See ﬁgure 3).

FERRO RESONANCE (See section 2.5). A system earthing limits

the risk of ferro resonance in the voltage transformers. The un-

earthed system, or systems who risks to be unearthed under cer-

tain operation circumstances, have a big disadvantage regarding

the risk of ferro resonance.

REQUIREMENTS ON THE EARTHING GRID A limit set to the earth

fault current reduces the requirements on the earthing grid since

these are constructed to limit arisen step, and touch voltages,

which in their turn are totally dependent of the fault current.

Step voltage

Touch

voltage

DESIGN OF EARTH FAULT PROTECTION RELAYS

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SELECTIVE EARTH FAULT PROTECTION With the relay technique of

today selective earth fault protection relays can be used in most

networks (independent of system earthing). Directional protec-

tion relays, with sensitivities of a few Ampere primary operates on

the transient and detects the earth fault even with high fault resis-

tance. Modern Protection relays which measures fundamental or

high frequency can sucessfully be used at earth fault protection.

Historically the using of selective earth faults was an important

reason to choose a high earth fault current. In directly earthed

transmission network that generates earth fault current from

many points, the using of selective earth fault protection will be

more complicated and require direction, inverse time character-

istics and calculations on current distribution in all possible fault

situations, to select settings which will give selectivity.

APPARATUSES INSULATION TO EARTH In an effectively (direct)

earthed network a graded insulation can be utilized. This gives

cost savings of 10-15% for voltages of type 130 kV. Regarding

the apparatus prices at higher voltage levels a fully insulated ze-

ro-point and the requirements on the insulation becomes impor-

tant. At voltages above 130-170 kV, nothing else but direct

earthing is used.

8. DESIGN OF EARTH FAULT PROTECTION

RELAYS

An earth fault protection relay should be constructed to interact

well with the system earthing method chosen for the network.

The system earthing shouldn’t be negatively affected by the re-

quirements on the earth fault protection relays. In some cases,

the system earthing can be chosen to make relays work more

safely.

A few of these cases will be penetrated below and Earth fault pro-

tection is further discussed in a separate section.

8.1 PARALLEL RESISTORS FOR THE RESONANCE