Protection Application Handbook (англ. яз.)

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EFFECTIVELY EARTHED SYSTEMS

Earth Fault Protection

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To enable use together with Distance protection giving sin-

gle-phase tripping a deﬁnite minimum time is set (normally 0.3

sec.). This ensures that a single phase tripping for heavy single

phase faults can be done by distance protection relay ﬁrst.

2.5 DIRECTIONAL COMPARISON SCHEMES.

To provide detection of high resistance earth faults in an effec-

tively earthed network it’s common to use a directional compari-

son scheme with directional earth fault relays at both ends of the

power line. The relays at the two ends are directed towards each

other and a communication between the relays, through a power

line carrier (PLC)

or a radio link, is introduced.

Principle for communication schemes

Communication can be made according to two main principles:

- permissive scheme

- blocking scheme

In a permissive scheme the Directional earth fault relays will

send a signal (CS) to the remote end at detection of a forward

fault. At reception of a signal and detection of a forward fault at

the receiving end, an instantaneous trip is given. Normally the

same situation occurs at both ends. A permissive scheme princi-

ple is shown in Figure 7.

Figure 7. A permissive overreaching scheme (POR) with directional earth

fault relays.

In a blocking scheme the directional earth fault relays are pro-

vided with a reverse locking element as a complement to the for-

ward element. The reversed element is set to be more sensitive

BL3

3I0

T0=0-150 ms

50 ms

≥1

CSEF

TR POR

Carrier send

Trip

50 ms

Forward dir

CR2

Carrier rec

Block input

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than the forward element and will, when a reverse fault is detect-

ed, send a signal (CS), to the remote end. At the remote end the

forward element is provided with a short time delay “T

” normally

set to 50-150 ms, to check if a blocking signal is received. If not,

the relay will trip. The same situation will for internal faults occur

at both line ends. The principle of a blocking scheme is shown in

Figure 8.

Figure 8. A blocking scheme with directional earth fault relays.

In most cases the Directional earth fault relays in a communica-

tion scheme also includes a communication independent

back-up tripping with a time delay. Inverse or deﬁnite time delay

can be used. Normally the inverse characteristic and the logarit-

mic inverse characteristic gives the best possibility to achieve

time selectivity

also at back-up tripping.

Single phase tripping.

When Distance protection relays with single phase tripping and

auto reclosing are used at the same line as a scheme with earth

fault relays it must be ensured that the Distance protection relays

are allowed to give their single-phase tripping ﬁrst. Earth fault re-

lays must therefor be time delayed to allow this. This is also valid

when communication schemes are used. A blocking of the earth

fault scheme at distance protection operation is often used to en-

able use of short time delays in the communicating earth fault re-

lays.

During a single phase trip an unbalance in the complete network

occurs and an earth fault currents ﬂows through the network.

BL3

3I0

T0=0-150 ms

50 ms

CSEF

TR BL

Carrier send

Trip

Forward dir

CR2

Carrier rec

Block input

3I0

Reverse dir

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These currents reaches levels up to 20% of the load current and

an unnecessary tripping from earth fault relays can therefor be

achieved. The earth fault relays are normally blocked during the

single phase auto reclose cycle.

Current reversal

A special application problem occurs together with directional

earth fault schemes communicating in a POR scheme. The prob-

lem is fault current reversal which occurs when the CB at one end

of the faulty line trips before the breaker at the other end. The fault

current changes direction in the parallel line and a timing problem

to prevent maloperation at the end with a CS signal receipt at the

original fault occurring will occur (see ﬁgure 9). A special logic ac-

cording to Figure 10 is required to prevent a unneccesary func-

tion.

Figure 9. The current reversal problem at parallel lines.

Trip 80 ms

Trip 65 ms

Forw 25-80ms

Rev 80-100ms

Rev 25-80ms

Forw 80-100ms

Trip can be

given at Cur rev

≥1

3I0

CR2

BL3

CSEF

3I0

TR POR

Carrier send

Trip

T0=0-150 ms

50 ms

70 ms

20 ms

50 ms

Forward dir

Reverse dir

Carrier rec

Block input

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Figure 10. A logic in directional earth fault relays, to prevent unnecessary

operation during current reversal.

Weak-end infeed

In special applications, a situation where the fault current infeed

from one end isn’t ensured during certain service conditions.

A special weak end infeed logic can be used together with POR

schemes. It is based on occurring zero sequence voltage and the

receipt of a carrier signal CS from the remote (strong) end. The

logic for an earth fault weak-end infeed function includes a check

of occurring U

voltage at carrier receipt and the breaker is

tripped even if no operation of DEF relay is achieved due to a to

weak source. It is also necessary to “mirror” the Carrier signal

back so the signal is sent back on receipt if the U

voltage is low,

or if the circuit breaker is open.

2.6 INRUSH CURRENT STABILIZATION

In some countries a second harmonic stabilization is required for

sensitive earth fault relays. The background to this is that the in-

rush currents occurring at transformer energizing which, in some

networks has long durations. The long durations are often

achieved in weak networks. The second harmonic stabilization

can then block the earth fault relay, during the inrush and prevent

the risk of an unnecessary operation.

It should be noted that a POR communication scheme can not

operate for inrush currents as only one end will have conditions

fulﬁlled whereas the other end has a blocking condition.

For inverse time delayed scheme a time setting is selected to

achieve selectivity to instantaneous protection. This gives a long

delay compared to normal inrush times and the inverse charac-

teristic will then match the decay of the inrush current and keep

the relay away from unwanted functions.

The only time when a stabilization is necessary is when very sen-

sitive deﬁnite time delayed relays are used. In such cases the in-

rush can cross the corner with minimum current before the time

elapses and an unwanted function can occur.

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3. REACTANCE EARTHED SYSTEMS

The earth fault protection in a reactance earthed system is made

with a simple design as the generation of fault current comes

from the source side of the network only. The earthing is made at

the feeding transformer or at the busbar through a Z-0 earthing

transformer.

3.1 Z-0 EARTHING TRANSFORMER.

The earthing transformer is selected to give an earth fault current

with a well deﬁned level. Normally the selected current is 750 to

1500A. The Z-0 (zig zag) transformer principle is shown in Figure

11.

Figure 11. The principle of a Zig-zag transformer providing earth fault cur-

rent in a reactance earthed system.

3I0

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The reactance is selected to give the fault current. It must be re-

member that the zero sequence reactance in Ω/phase is three

times the reactance calculated by the formula:

respectively

This means that, for a 20kV system with a 1000A earthing the Z-0

transformer shall be selected with a zero sequence reactance.

A Z-0 transformer can often be provided with an auxiliary power

supply winding with a 400V secondary. This is possible up to 800

-1000kVA and the protection system have to be checked so that

the proper fault protection of the low voltage side of the auxiliary

transformer is achieved. One solution is to use delta connected

short circuit protection at the HV side (Z-0 winding).

Fuses and breakers placed at the HV side of a Z-0 transformer

should be avoided as the network earthing then can be discon-

nected and the risk of earth fault trips disappears. It will also in-

volve a risk of arcing faults and of ferro resonance in the voltage

transformers.

3.2 FAULT RESISTANCE AND FAULT CURRENT

LEVELS

In a reactance earthed system the current still is quite high. The

fault resistance will therefor decrease and a beginning fault will,

rather quickly, develop into a fault situation.

The fault current level will be quite independent of the fault posi-

tion. If a reactance earthing with 1000 A is used in a 20 kV sys-

tem, the apparent neutral reactance will be 12 Ω. This means that

a reactance to the fault, through a line of 12 Ω, will make the fault

3I0 i. e. I

1000A

--------------------==

------------------ Z

-------

1000 11.6 Select 35Ω phase⁄⇒==

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current level half (12 Ω will be the reactance of a 30k m line in a

50 Hz system).

If a fault resistance is introduced the resistance will add vectorial

to the reactance and the current will change slowly with the fault

resistance.

3.3 NEUTRAL POINT VOLTAGES.

The neutral point voltage is calculated in the same way as the

source impedance (i.e. Z

(1/3Z0)xI

). This means that for a fully

developed earth fault full neutral displacement occurs and a fault

current of 20% e.g. 200 A in a 1000 A earthed system will be

20% of “U

”.

3.4 RESTRICTED EARTH FAULT PROTECTION.

A Restricted earth fault protection REF can not be justiﬁed in a

reactance earthed system in the same way as in a directly (effec-

tively) earthed system. This is because the fault current is much

lower and by that also the occurring damage which is dependent

of the “I

t” condition.

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However in many countries REF protection relays are speciﬁed

also in reactance earthed systems the connection of a REF relay

is shown in Figure 12.

Figure 12. A Restricted earth fault relay in a reactance earthed system with

a Z-0 transformer.

The application is fully possible but it must be ensured that a high

sensitivity is achieved. Operation values of 5-10% of the maxi-

mum earth fault current generated is required.

When the operating voltage of the REF is calculated the check of

required operating value must be done by ﬁrst checking the earth

fault loops for phase CT:s and neutral CT:s. However, as the

maximum through fault current is limited by the reactance a

check of the occurring loop at phase faults must be performed.

The unbalanced voltage is caused by one phase CT which is sat-

urated and the other is not. This can be caused by, for example,

the DC components at fault current which is not equal between

the phases. This check will often set the required operating volt-

age as the fault current at phase faults is much higher. It is advan-

tageous to summate the three CT:s as close to the current

transformers as possible. This can sometimes be difﬁcult be-

cause of the transformer differential relay and its interposing CT’s

are located in the same circuit.

To Diff

protection

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3.5 EARTH FAULT PROTECTION.

The earth fault protection in a reactance earthed system is nor-

mally provided with time delayed simple and undirectional earth

fault current measuring relays.

Time grading of the protection, furthest out in the system to the

protection in the neutral of the zig-zag transformer is used. The

time delay can be with normal inverse, or deﬁnite time delay. A

solution with inverse time delayed relay is shown in ﬁgure 13.

Figure 13. An earth fault protection system for a reactance earthed network

where selectivity is achieved by inverse time delayed relays.

3I0

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4. HIGH RESISTANCE EARTHED SYSTEMS

4.1 PRINCIPLE

The high resistance earthed networks are earthed at the source

side of the network only. Normally the earthing is restricted to one

point only and an arrangement is provided to allow varying con-

nection possibilities of the earthing point due to the service con-

ditions (see ﬁgure 14).

Figure 14. A high resistance earthed network and a selection of service

conditions with disconnectors.

4.2 FAULT RESISTANCE AND FAULT CURRENT

LEVELS.

The earthing resistor is mostly set to give an earth fault current of

5-15A. The trend to decrease the current level gives lower re-

quirement of earthing grid and higher neutral point voltages at

high resistive earth faults.

Open

A disconnector arr

allows separated

service. When BS is

closed only one

resistor is connected