Protection Application Handbook (англ. яз.)
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INTRODUCTION
Setting of Protection
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1. INTRODUCTION
The main function of the protection system is to isolate the mini-
mum possible part of the power system at a fault or an abnormal
condition. This is achieved by selecting a protection system suit-
able for the plant and object protected, and to set the protection
relays to suitable values. Suitable protection relays with incorrect
setting can cause severe disturbances of power supply and/or
unnecessary damage of equipment.
Faults occurring in the power system should always be detected
by two different protection relays to allow for a single failure. (Fus-
es are “secure break points” and do thus not require back-up pro-
tection). The protection relays are normally designated Main and
Back-up protection.
According to IEC it is the MAIN PROTECTION that is normally
expected to take the initiative in cases of a fault in the protected
zone. The BACK-UP PROTECTION is provided to act as a sub-
stitute for the main protection in case of failure or inability of this
to perform its intended function.
The protection system shall, as mentioned before, be able to iso-
late minimum possible part of the system. We can then introduce
the term SELECTIVITY.
SELECTIVE PROTECTION is a protection which determines
that the fault is within its zone and isolates that zone only. Selec-
tive protection relays can be ABSOLUTE SELECTIVE or RELA-
TIVE SELECTIVE.
ABSOLUTE SELECTIVITY is when a protection responds only to
faults within its own zone.
Examples of ABSOLUTE SELECTIVE protection relays are:
Differential protection relays (Transformer-, Pilot wire-, Busbar
differential protection)
Buchholz and Transformer Temperature devices
Tank and frame leakage protection
Distance protection first- and accelerated- zone, where operating
area is decided by impedance setting protection relays for indi-
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vidual objects such as Reactor protection, Motor protection,
Shunt capacitor protection
As these protection relays only operates for faults within a certain
zone they are not required to be shown in the selectivity plan. Still
it can be advantageous to show them in the selectivity plan to get
the relation to other protection relays.
RELATIVE SELECTIVITY is when the selectivity is obtained by
grading the settings (i.e. time or current) of the protection relays
of several zones, all of which may respond to a given fault. These
protection relays can be either time or current selective or a mix-
ture of both.
Examples of RELATIVE SELECTIVE protection relays are
- Distance protection back-up zones
- Overcurrent protection
- Earth-current protection
- Overload protection
-Voltage protection
For these protection relays selectivity plans are made as they op-
erate for faults in a big part of the power system and disconnect
the faulty part only by grading of current, voltage and time setting.
Most types of faults initiate several protection relays in above
groups. For example, short circuits with earth connection can ini-
tiate overcurrent protection, earth current protection, overload
protection, differential protection, negative sequence protection
and undervoltage protection dependent on fault location, type of
earthing and the protection system provided.
SELECTIVITY CHARACTERISTIC or SELECTIVITY PLAN.
Diagram or table showing the operating times and corresponding
actuating quantities or fault positions for the selective protection
relays of a network.
The purpose of the SELECTIVITY PLAN is to coordinate the re-
lay settings so that:
- faulty equipment is tripped as fast as possible
- the least possible disturbance is obtained for healthy equipment
- a back-up protection is obtained if a main protection or its break-
er fails to trip.
The selectivity plan is performed in similar way for short circuit
and earth-fault protection relays. In a solidly earthed system an
intercheck between earth-fault and short circuit protection's oper-
ating times must be made as short circuit protection relays often
INTRODUCTION
Setting of Protection
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will operate for earth-faults as well. It is then checked that a short
circuit protection, measuring phase component of fault current,
will not have shorter operating time than the earth-fault protection
at the same location, for an earth-fault.
of the protection relays of several zones, all of which may re-
spond to a given fault. These protection relays can be either time
or current selective or a mixture of both.
Examples of RELATIVE SELECTIVE protection relays are
- Distance protection back-up zones
- Overcurrent protection
- Earth-current protection
- Overload protection
-Voltage protection
For these protection relays selectivity plans are made as they op-
erate for faults in a big part of the power system and disconnect
the faulty part only by grading of current, voltage and time setting.
Most types of faults initiate several protection relays in above
groups. For example, short circuits with earth connection can ini-
tiate overcurrent protection, earth current protection, overload
protection, differential protection, negative sequence protection
and undervoltage protection dependent on fault location, type of
earthing and the protection system provided.
SELECTIVITY CHARACTERISTIC or SELECTIVITY PLAN.
Diagram or table showing the operating times and corresponding
actuating quantities or fault positions for the selective protection
relays of a network.
The purpose of the SELECTIVITY PLAN is to coordinate the re-
lay settings so that:
- faulty equipment is tripped as fast as possible
- the least possible disturbance is obtained for healthy equipment
- a back-up protection is obtained if a main protection or its break-
er fails to trip.
The selectivity plan is performed in a similar way for short circuit
and earth-fault protection relays. In a solidly earthed system an
intercheck between earth-fault and short circuit protection relays
operating times must be made as short circuit protection relays
often will operate for earth-faults as well. It is then checked that a
short circuit protection, measuring phase component of fault cur-
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rent, will not have shorter operating time than the earth-fault pro-
tection at the same location, for an earth-fault.
2. PROTECTION RELAYS SETTING
Absolute selective protection relays
Distance protection
Distance protection relays are set according to the setting in-
struction for the specific protection. Below follows just a brief in-
formation of the general principle of setting Distance protection
relays.
Distance protection relays use the impedance characteristic of
the transmission line to decide the distance to the fault. Due to in-
accuracies in instrument transformers, relays, line data etc. the
protection is normally set to cover about 80% of the line on the
first instantaneous zone. Faults up to 80% from each line end is
thus instantaneously tripped by the close end relay. The central
60% of the line will be instantaneously tripped from both line
ends.
Figure 1. A Distance protection with impedance characteristic for a power
line.
On higher voltages where fast fault clearance is required from
stability point of view faults on the last 20% of the line are tripped
instantaneously by aid of communication with remote end.
The two sides can communicate through:
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- Radio link
- Power line carrier (PLC)
- Pilot wires
- Optical fibre
Figure 2. The principle for Distance protection relays communication.
PLC is up to now by far the most common communication link but
during the 90’th the optical communication has started to be-
come popular due to the possibility to send big amounts of infor-
mation between the stations.
The two distance protection relays mainly communicate in two
different modes:
- Tripping (permissive) mode
- Blocking mode
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In tripping mode instantaneous tripping is achieved by tripping
the circuit breaker as soon as acceleration signal is received from
remote end.
In blocking mode the accelerated step trips after a short time de-
lay Tko to have time to check that no blocking signal is received.
When protection relays operate in blocking mode reverse looking
elements are added at both ends to send blocking signal for re-
verse faults. Sometimes the blocking signal is sent for a criteria
-Fault (from undirectional element) but not forward. This can
mean that short unnecessary blocking pulses are sent for forward
faults due to differences in operating time for the undirectional
and the directional element. This can cause problems for relays
with true directional element reverse.
Line impedance
.
Line positive sequence reactance is normally of size 0.32-0.4
ohms/km at 50 Hz for HV - MV lines. Line zero sequence reac-
tance is approximately 3 times the positive sequence value. Line
positive sequence impedance angle is normally 70-85° and zero
sequence value slightly lower.
The impedance to be covered by Distance protection is the line
impedance up to the fault plus an additional arc resistance which
is approximately achieved from the following formula:
R
arc
= 28700* (a+2*vt) / I
1.4
Ω (Warrington Volume 2)
The arc voltage has a square wave form and is of size 1–2.5
kV/m. This can also be used to estimate an arc resistance as:
Rarc = 1 - 2.5 • a/ I
where
a is arc length (m)
v is wind speed (m/s)
t is arcing time (s)
I is fault current (A)
Both formulas give approximate, similar, values.
Arc voltage as function of current according to tests at ABB. Dif-
ferent designations are tests at different occasions.
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Figure 3. Distribution of arc voltage and current at many tests in ASEA.
Typical values for “a” (arc length) are:
72,5 kV 4.0 m
145 kV 5.0 m
245 kV 7.2 m
400 kV 12 m
T
ower foot resistance.
In addition to the arc resistance a tower foot resistance exists.
The tower foot resistance depends on the soil resistivity, the ef-
fectiveness of the tower earthing and possible available top lines
between the towers.
The primary impedance of the line is calculated to the secondary
side of instrument transformers by use of the formula: Z
p
= U
p
/I
p
Z
s
= U
s
/I
s
Z
s
= Z
p
* (U
s
*I
p
) /(I
p
*U
p
)
where
U
p
/U
s
and I
p
/I
s
is the voltage respectively the current transformer
ratio.
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Distance protection principle.
The required reactance and resistance reach is achieved by, in
principle, two different characteristics available. These character-
istics are “quadrilateral” and “mho” characteristics. It must be re-
membered that R
arc
is a loop resistance whereas the distance
protection relays measures the phase value.
When load is transferred on a power line with generation at both
ends, an angle difference will occur between the two sources.
This will give a current through the fault resistance which is not in
phase with the current measured at the two line ends and will
thus partly be measured as an additional reactance by the dis-
tance protection.
The situation for a fault at far line end is indicated in below figure.
Figure 4. A fault at a remote station will due to infeed from other objects be
seen as much further off than it really is.
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Figure 5. The Distance protection export end will overreach when a fault re-
sistance is added in a double end infeed system. The import end will under-
reach.
The importing end will measure a higher reactance and thus un-
derreach and the exporting end will measure a lower reactance
and overreach. This must be compensated for, when a high resis-
tive cover is required, with a decreased zone 1 E/F setting at ex-
porting end.
For power lines in parallel on the same towers an additional prob-
lem occurs due to mutual coupling between the two lines. The
mutual reactance can for an earth-fault be approximately 0.5 •
X0L whereas for a phase fault the value is just a few percent of
XlL. The mutual coupling can for earth-faults cause an overreach
of normally 5-15% and need to be compensated for in setting of
distance protection relays' earth-fault reach.
The second, third and possibly fourth zones are set with imped-
ance and time to be selective to distance protection relays first,
second and third zones respectively in a remote station and to
the protection for other objects, e.g. transformers in remote sta-
tion.
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At setting the back-up zones consideration need to be given the
infeed of fault currents from other lines in the remote station and
the apparent increase of measured impedance. Distance protec-
tion in station A will measure a higher voltage, and thus a higher
impedance U/I than only current would have caused. This means
that a remote back-up function is mostly difficult to achieve in a
HV meshed system and local back-up is thus necessary.
Differential protection relays
Differential protection relays are used for busbars, transformers
and feeders to give instantaneous primary protection of the ob-
ject. Differential protection relays measure the difference in cur-
rent and are thus absolute selective protection as they will only
respond to faults within a certain zone.
Examples of differential protection relays are:
- Busbar differential protection
- Transformer differential protection
- Transformer restricted earth-fault protection
- Optical line differential protection
- Pilot wire differential protection
These relays are set according to setting instructions for respec-
tive relay. Below follows just a brief information of the principles
for the respective protection relay type.
Busbar differential protection RADSS and REB 103.
RADSS and REB 103 are medium impedance percentage re-
straint relays which are normally set to the required value at de-
livery.