Protection Application Handbook (англ. яз.)

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Figure 12. Communication with remote end, in Permissive schemes or

Blocking schemes. CS from Underreaching zone or Overreaching zone.

Blocking schemes

a blocking signal “CS”, is sent to the remote

end when the fault is detected to be reverse. Tripping is achieved

when the acceleration signal “CS” is not received within a time of

and if the local relay has detected a forward fault as well. A

time margin T

of 20-40ms to check if the signal is received is al-

ways needed in a blocking scheme.

The accelerated tripping after “T

” are from “Z

” or “Z

”.

If different types, or manufacturer, of Distance protection relays

are used at the two line ends blocking schemes should be used

only after checking that the relays will operate together. A relay

Carrier send CS = Z< Forward, under or overreach

Trip = Z< Forward Z1, Z2 or Z3 & Carrier received

Permissive system

Carrier send CS = Z< Reverse zone

_____________

Trip = Z< Forward Z1, Z2 or Z3 & Carrier received &T0

Blocking system

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where a blocking signal is sent for “start” but not “forward” should,

as a general principle, never be used together with a relay with a

true reverse directional element sending the blocking signal.

The using of Permissive schemes or Blocking schemes in the

system depends on the preference for Security or Dependability.

A Blocking scheme will be Dependable, i. e. it will operate for an

internal fault also with a failing communication link, but it has a

lower security as it can maloperate for an external fault due to a

failing communication link. The Permissive scheme has the op-

posite behavior. The advantage with the blocking scheme is that

communication signals are sent on healthy lines whereas on per-

missive scheme the communication signals are sent over a faulty

line.

Reasons for incorrect impedance measurement

To enable a correct impedance measurement the measured volt-

age must be a function of only the locally measured current “I

”

and the impedance at the fault. This is naturally not always the

case in double-end infeed and meshed transmission networks.

REMOTE FAULTS

If a fault occurs on an outgoing line in the re-

mote substation where the own line will feed fault current “I

”,

see ﬁg 13, the other lines in the remote station will also contribute

with the fault currents “I

” and “I

”. The measured impedance at

the local station will then be as in the ﬁgure and the measured im-

pedance at the fault will seem much higher than the “true” imped-

ance to the fault. The relays will thus get an apparent underreach.

This means that, in practice the possibility to get a “remote

back-up” in a transmission network is limited. A local back-up

must therefor normally be provided.

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Figure 13. The Distance protection underreach at a remote fault.

HIGH RESISTIVE LINE FAULTS At high resistive line faults on a

transmission line with double-end infeed a similar situation will

occur. In normal service a load current “I

” ﬂows through the line.

The current level is expressed:

If1

If2

If3 If1+If2+If3

If1

Measured voltage

Um=If1*ZL+(If1+If2+If3)*ZF

Measured impedance

Zm=Um/Im

[]

Load

[Ω]

–

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

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“U

” and “U

” must have a phase difference to allow the current

to ﬂow. This, if the voltage amplitude is the same, which is a “nor-

mal case” in the transmission network. When a fault occurs, the

currents

“I

” and “I

”, are lagging “E

” respectively “E

”, and

have therefor also a phase difference compared to each other.

The resistance “R” at the fault will be seen as a resistance plus a

reactance reduction (at export) and a reactance addition (at im-

port).

Figure 14. Measuring error at a high resistive earth fault in a line with

double side infeed.

The measuring error will be:

At the two line ends (see ﬁg), “Θ” is the phase angle difference

between “A” and “B” stations. The exporting end will thus over-

reach i. e. a fault at the line end can be seen as an internal fault,

–

-------

Θsin

–

-------

Θsin

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and the import end will underreach i. e. a fault will seem further

off than it is in reality.

If the power direction always is the same a compensation can be

made when setting the relays. The earth fault loop setting must

especially be considered as the high resistive faults normally only

occur at earth faults only.

PARALLEL POWER LINES When parallel lines at the same tower

are used there will be a mutual impedance, normally only of in-

terest when an earth fault occurs. Normal values of the mutual

impedance at earth faults for transmission line towers are “Z

”

“0,5-0,6xZ0”.

The mutual impedance will for earth faults mean over- respective-

ly underreach for the two line ends. The over- or underreach is

dependent on the fault position and the zero sequence sources

at the two line ends. The level of overreach resp underreach in

the measurement is in the range up to 16%.

Different situations will occur when:

- Parallel lines are in service.

- Parallel lines are out of service and earthed.

- Parallel lines are out of service (ﬂoating).

Compensation have to be done for the different cases when set-

ting the relay earth fault reach. The “K

” factor is adjusted to

achieve a suitable setting. Normally the worst case is selected

and the “K

” is set to prevent overreaching, and thus unneces-

sary operation. The overreach is caused by the parallel line out of

service and earthed at both ends and the factor K

should be set

as:

– X

–

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

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When the parallel line is out of service but ungrounded the reach

will be reduced with a factor k

. This factor should be checked

and the overlapping of zone 1 from both end at underreaching

schemes should be veriﬁed.

The setting of K

factor for zones 2 and up should always be as

the normal setting with an extended factor of 1,25 to cover a

worst case which occurs when no fault current is fed in at the re-

mote terminal.

Note that the change of reach for parallel lines with compensated

is only valid for single phase faults. For multi phase faults the

reaches are not inﬂuenced.

Figure 15 shows the principle of the mutual impedance.

Figure 15. Inﬂuence of mutual impedance, at parallel lines, at the same

tower.

+()3X

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

Z0M

I 0

Z0M

Z0-Z0M

1/ 1

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Special functions

Some special functions are of interest with Distance protection

relays and should be mentioned.

SWITCH ONTO FAULT (SOTF) At energizing a power line onto a for-

gotten earthing, portable or ﬁxed, no measuring voltage will be

available and the directional measuring can thus for three phase

faults not operate correctly. A special SOTF function is thus pro-

vided in Distance protection relays. Different principles can be

utilized, from an one phase current/one low voltage measure-

ment to an undirectional impedance measuring as per ﬁgure 16.

The SOTF function is connected for some second/s only when

energizing. The criteria that there is a SOTF condition can either

be taken from the manual closing signal (called DC SOTF) acti-

vating an input in the relay, or can be detected internally by the

relay (AC SOTF) where a no voltage-no current condition for a

certain time is taken as conﬁrmation that the line is dead.

Figure 16. Switch Onto Fault function ensures fast tripping when energizing

a line onto a forgotten earthing.

U=0 V

R[ ]

X [ ]

R [ ]

X [ ]

[Ω]

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POWER SWING BLOCKING (PSB) FUNCTION

Power swings can occur at double end in-feed networks. A Power

swing can be started by a sudden load change, due to e. g. man-

ual switching, or at a fault somewhere in the network.

Close to the centre of the power swing low voltages and thus low

impedances will occur. A Distance protection relay can thus op-

erate at a power swing and this is in most countries not accepted.

A Power Swing Blocking device is available for all schemes. The

most used principle used is to “measure the speed of the imped-

ance locus”. This is done by two impedance circles, or rectan-

gles, and a measuring of the time between passing the outer and

the inner line. Normally the time used is 35-40ms.

Exceeded time will make an alarm telling that a Power swing has

occurred. A blocking of the Distance protection zones will then be

made. Normal power swings in networks have a swing cycle of

0.5-10 seconds.

Figure 17. Power Swing Blocking function. Two rectangles can be provided

and the time between passing the outer and inner rectangle is measured.

STUB PROTECTION FUNCTION In one and a half and ring busbar

arrangements the voltage transformer will be located outside the

R [ ]

Load

X [ ]

Power swing

locus

[Ω]

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line disconnector but the bays can be in full service with the

breaker closed even if the line disconnector is opened e. g. due

to make maintenance on the power line.

A fault in the line bay section, will not be possible to detect by the

distance protection. It’s also a risk of incorrect directional mea-

surements due to induced voltages or back-feed of voltage from

the remote end. The Distance protection directional impedance

measuring is blocked and a so called Stub protection is intro-

duced. The Stub protection is a simple current relay activated

only when line disconnector is open.

Figure 18. Stub protection function protects the line exit when the line

disconnector is open.

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3.4 APPLICATION PROBLEMS

A number of interesting application problems occurs in a trans-

mission line. Among them should be mentioned: Current reversal

In the quite common “parallel line applications” current reversal

can occur. The principal problem is shown in ﬁgure 17 and an ex-

ample of the logic used to ensure correct operation is shown in

ﬁgure 18

Figure 19. Fault current reversal occurring, with parallel lines, at the same

tower.

Figure 20. Fault current reversal logic for overreaching scheme in RELZ

100/REL 511/521./531 “ZM2” is measuring zone 2, “ZM3R” is Zone 3 re-

verse and “CR” is carrier receive.

When a fault occurs at a line, on a parallel connection, the fault

will always be cleared from one line end ﬁrst. When the ﬁrst

breaker opens, the fault current in the parallel line will have a

change of direction and if nothing is done in the logic for commu-

nication, maloperation can occur on the parallel healthy line. It

should be noted that the problem occurs for overreaching

schemes only.

80ms

110ms

rev

forw

ZM2

ZM3R

Carrier

send

Trip