Protection Application Handbook (англ. яз.)
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RELAY PROTECTION
Transformer Protection
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Figure 4. Location of buchholz, top oil thermometer and winding tempera-
ture indication
Gas detector relay (Buchholz)
At a fault in an oil immersed transformer the arc will cause the oil
to decompose and gas will be released. The gas will pass
through the pipe between the main tank and the conservator and
can be detected by a gas detector relay.
The gas detector have an alarm unit collecting the gas, and one
unit for tripping responding to the high flow of gas at a serious in-
ternal fault.
The collected gas can be analyzed and give information about
what caused the gas.
It should be noted that the trip signal from the Buchholz some-
times can be very short at a serious internal fault due to that the
relay will be destroyed (blown away). Receiving of this signal in
the relay system therefore must have a seal-in feature to ensure
that a sufficient length of the tripping signal is given to the CB and
to indication relays.
TRIPALARM
TOP OIL
THERMOMETER
BUCHHOLZ
COMPENSATED
THERMOMETER
TOP OIL DEVICE
ROOM
DRYER
EXPANSION
ROOM
PROTECTION BLOCK DIAGRAM
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Temperature monitoring
A too high temperature in a transformer can be caused by over-
loading or by problems in the cooling equipment. Overfluxing can
also cause a temperature raise.
Oil immersed transformers are supervised with thermometers.
These are included in the power transformer standard equip-
ment. There are two types to choose between, oil-, or winding
temperature measuring devices and both are normally supplied
on transformers bigger than a few MVA.
Both types are overloading sensors for the transformer. There is
normally one alarm and one trip level at each type of measuring
devices.
4. PROTECTION BLOCK DIAGRAM
The protection system for a typical HV/MV power transformer is
shown in figure 5. It should be noted the mix between protection
for internal transformer faults and the back-up protection provid-
ed for external faults mainly in the low voltage system.
PROTECTION BLOCK DIAGRAM
Transformer Protection
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Figure 5. A typical protection system for a HV/MV power transformer.
PROTECTION BLOCK DIAGRAM
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The protection system for a typical EHV/HV power transformer is
shown in figure 6. For such big system transformers a sub-divid-
ed protection system is normally required and provided. Further
back-up protection are often of impedance type to allow setting
matching the line protection on the outgoing lines.
Figure 6. A typical protection system for a EHV/HV system power
transformer.
NEED FOR SHUNT REACTORS
Reactor Protection
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1. NEED FOR SHUNT REACTORS
Shunt reactors are used in high voltage systems to compensate
for the capacitive generation of long overhead lines or extended
cable networks.
The reasons for using shunt reactors are mainly two. The first
reason is to limit the overvoltages and the second reason is to
limit the transfer of reactive power in the network.
If the reactive power transfer is minimized i. e. the reactive power
is balanced in the different part of the networks, a higher level of
active power can be transferred in the network.
Reactors to limit overvoltages are most needed in weak power
systems, i.e. when network short-circuit power is relatively low.
Voltage increase in a system due to the capacitive generation is:
where “
Q
c
” is the capacitive input of reactive power to the network
and “S
sh.c
” is the short circuit power of the network.
With increasing short circuit power of the network the voltage in-
crease will be lower and the need of compensation to limit over-
voltages will be less accentuated.
Reactors to achieve reactive power balance in the different part
of the network are most needed in heavy loaded networks where
new lines cannot be built because of environmental reasons.
Reactors for this purpose mostly are thyristor controlled in order
to adapt fast to the reactive power required.
Especially in industrial areas with arc furnaces the reactive power
demand is fluctuating between each half cycle. In such applica-
tions there are usually combinations of thyristor controlled reac-
tors (TCR) and thyristor switched capacitor banks (TSC). These
together makes it possible to both absorb, and generate reactive
power according to the momentary demand.
∆U%()
Q
C
100×
S
sh
˙
c
------------------------
=
CONNECTIONS IN THE SUBSTATION
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Four leg reactors also can be used for extinction of the secondary
arc at single-phase reclosing in long transmission lines. Since
there always is a capacitive coupling between phases, this ca-
pacitance will give a current keeping the arc burning, a second-
ary arc. By adding one single-phase reactor in the neutral the
secondary arc can be extinguished and the single-phase auto-re-
closing successful.
2. CONNECTIONS IN THE SUBSTATION
The reactors can be connected to the busbar, a transformer ter-
tiary winding or directly to the line, with or without a circuit-break-
er (see figure 1).
Figure 1. Different locations of shunt reactors.
Tertiary connected reactors will have the lowest cost but extra
losses will be obtained in the transformer. The rated voltage of
the reactor must be taken into account as well as the large volt-
age drop at the power transformers.
A voltage drop in the tertiary, equal to the power transformer im-
pedance, will be obtained when the reactor is connected. Lets
assume that the transformer have an impedance voltage of 15%
and a rated voltage in the tertiary of 20 kV. Also assume that the
SYNCHRONOUS SWITCHING OF SHUNT REACTOR CIR-
Reactor Protection
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auxiliary power to the station is taken from the tertiary through a
20/0.4 kV transformer.
When a reactor with rated power equal to the rated power of the
tertiary winding, is connected, the voltage in the tertiary will de-
crease to 0.85
x20 = 17 kV. This will also mean that the auxiliary
power voltage will decrease with 15% and some kind of voltage
stabilization regulation is therefore necessary to keep the 0.4 kV
voltage level within required limits.
If the reactors are to be used for secondary arc extinction at sin-
gle-pole reclosing, they must be connected directly to the line.
This also have to be done if the reactors are supposed to keep
down the voltage at the open end of a long transmission line.
Busbar connected reactors can be used for voltage regulation
and reactive power balance.
Line Reactors can either be connected continuously together
with its line, or be connected to the network at low load condi-
tions.
3. SYNCHRONOUS SWITCHING OF SHUNT
REACTOR CIRCUIT-BREAKERS
Synchronized closing, and opening of shunt reactors CB:s, is
possible by using the Switchsync relay. This will give the closing
impulse at the correct instance to achieve pole closing at maxi-
THYRISTOR (PHASE-ANGLE) CONTROLLED REACTORS
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mum phase voltages. Closing at voltage maximum gives the min-
imum possible disturbances in the network (see fig. 2).
Figure 2. Synchronized closing of Shunt Reactor Circuit Breakers.
Shunt reactors CB:s should be closed at voltage maximum. CB:s
with Switchsync relay to control the closing instance don’t require
any pre-insertion resistors, which mean a less mechanical com-
plex CB, and of course, a less expensive CB.
Switchsync can also be equipped with an adaptive control which
automatically will adjust to changes in the CB:s operating time.
4. THYRISTOR (PHASE-ANGLE) CONTROLLED
REACTORS (TCR)
In order to get a continuous control of the reactive power and to
damp power swings in the network, thyristor controlled reactors
Man Close
Synch
close
t
CB closing time
+3.33 ms (50 Hz)
+6.66 ms (50 Hz)
L1
Closing impulse
is sent.
L2
L3
THYRISTOR (PHASE-ANGLE) CONTROLLED REACTORS
Reactor Protection
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can be installed. These can respond within only half a cycle delay
to the required amount of reactive power needed.
The current is controlled in such a way, that the firing pulses to
the thyristors are delayed so that they wont disturb the natural
zero crossings of the current. This regulating can be done contin-
uously and reactive power can therefor be steplessly adjusted to
the required value (see fig. 3).
Figure 3. Voltage and current wave form and thyristor firing control.
This control is practically transient free but causes harmonics,
that have to be taken care of by harmonic filters. The harmonics
generated, are
“n x f±1”, where “n” is an integer. The harmonics of
the lowest order are the largest and the level decrease with high-
er orders. Filters therefore normally only are needed for 5’th, and
7’th harmonics.
TCR reactors are always both connected to the network through
a power transformer, and delta connected (see fig. 4). The volt-
age level is chosen to get a current suitable for the rated currents
of the thyristors selected.
U
I
r
Firing pulses
SINGLE, AND THREE-PHASE REACTORS
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Figure 4. Thyristor controlled reactors (TCR).
5. SINGLE, AND THREE-PHASE REACTORS
Three-phase reactors are manufactured for system voltages up
to 400 kV. At higher voltages the reactors usually are of sin-
gle-phase type and more or less necessary for the operation of
the system. Using single-phase units makes it easier with spares,
as only one single phase unit have to be kept in the substation.
Sizes can either be standardized by the customer or individually
optimized for each case.
6. RELAY PROTECTION FOR DIRECT
EARTHED SR:S
A differential relay, of high impedance type should be used as
main protection. CT:s should be specified at both the phase and
the neutral side of each phase and a three phase protection
should be used as a three phase protection gives a higher sensi-
tivity for internal faults.
5TH 7TH