Features Integrated three-phase differential relay, overcurrent
relay and earth-fault relay
Stabilized three-phase differential relay providing
winding short-circuit and interturn fault
protection for two-winding transformers and
generator-transformer units and winding shortcircuit
protection for generators.
Earth-fault protection for transformer HV and
LV side windings according to the desired principle:
the stabilized differential current principle,
the high-impedance principle, the calculated
or measured residual current principle or
the neutral current principle
Three-stage overcurrent protection for transformers
and generators and two-stage back-up
protection for earth-fault protection
The operation characteristic of the differential
relay easily adapted for different applications
Short operate times, even with partially saturated
current transformers
Stabilization prevents unwanted operations at
faults outside the protected area and transformer
inrush currents
Blocking based on the ratio of the second harmonic
and the fundamental component of the
differential current prevents unwanted operations
at transformer inrush currents
Blocking based on the ratio of the fifth harmonic
and the basic frequency component of
the differential current prevents operation in
harmless situations of transformer overexcitation
- can be eliminated if the ratio of the fifth
harmonic and the basic frequency component
increases at high overvoltages
No interposing transformers are needed for the
protection of two-winding transformers - numerical
vector group matching on HV and LV
side
Wide CT ratio correction range - accurate correction
allowed by digital setting
Sensitive phase current and phase angle displays
facilitate the checking of measurement circuit
connection and vector group matching
Four trip and four signal relay outputs available
to the protection design engineer
Five programmable external control inputs intended
for the indication and retransmission of
alarm and trip signals of gas relays, oil temperature
sensors and other sensors of transformer
auxiliary devices
Adjustable CBFP operate time to improve reliability
of operation
Integrated disturbance recorder capable of recording
currents and digital signals - signals to
be used for triggering selectable
High immunity to electrical and electromagnetic
interference allows the relay to be used in
severe environments
High availability and system reliability due to
continuous supervision of hardware and software
4
Application The stabilized differential relay SPAD 346 C is
designed to be used to protect two-winding
transformers and generator-transformer units
against winding short-circuit, interturn fault,
earth fault and short circuit and to protect
generators against winding short-circuit and
short circuit. The relay can also be used for the
protection of a three-winding transformer provided
75% of the short circuit power is fed from
the same direction.
Description of
operation
The integrated differential relay SPAD 346 C
includes three independent relay modules: a
three-phase stabilized differential relay module
SPCD 3D53, an earth-fault relay module SPCD
2D55 and a combined overcurrent and earthfault
relay module SPCJ 4D28. The rated currents
of the relay are 1 A and 5 A. The HV and
LV side may use the same or different rated
currents.
Below a short description of the features of the
protection relay modules. The manuals for the
separate relay modules describe the modules
more in detail.
In power transformer protection differential
current is caused by CT errors, varying tap
changer positions, transformer no-load current,
transformer inrush currents, transformer
overexcitation in overvoltage and underfrequency
situations, and CT saturation at high
currents passing through the transformer. Differential
current caused by CT errors and tapchanger
position grows at the same per cent ratio
as the load current increases. In the protection of
generators the differential current is caused by
CT errors and saturation of the CTs in situations
where high currents pass through the
transformer.
High currents passing through the object to be
protected may be caused by short circuits outside
the protected area, large currents fed by the
transformer or the generator in motor start-up
The differential relay module SPCD 3D53 provides
protection for winding short-circuit and
interturn faults. The differential relay compares
the phase currents on both sides of the object to
be protected. Should the differential current of
the phase currents in one of the phases exceed
the setting of the stabilized operation characteristic
or the instantaneous protection stage of the
module, the module provides an operate signal.
Different amplitudes or phase difference of the
currents may be the reason for the differential
current.
Interposing current transformers have normally
been used in the differential protection of transformers
to obtain vector group matching and to
match the secondary currents of the main transformers.
Interposing CTs have also been used to
eliminate the zero-sequence components of the
phase currents at earth faults occurring outside
the protected area. The differential current relay
SPAD 346 C eliminates the use of interposing
transformers for the protection of two-winding
transformers as the differential relay module
allows the transformer vector group matching,
the CT ratio correction and the elimination of
the zero-sequence component of the phase currents
to be digitally implemented on the HV
and/ or the LV side.
Three-phase
stabilized
differential
relay module
SPCD 3D53
Stabilized
differential
current stage
or transformer inrush situations. Due to these
circumstances the operation of the differential
relay has been stabilized in respect of the load
current. In a stabilized differential relay the
differential current required for relay operation
is higher, the higher the load current is. The
stabilized operation characteristic of the differential
relay module and the setting range of the
characteristic is presented in the description of
the differential relay module SPCD 3D53.
The operation of the differential relay module
SPCD 3D53 is based on the fundamental frequency
components. Operation based on fundamental
frequency components is accurate and
stable: the DC component and harmonics of the
current do not cause unwanted operation of the
protection stage.
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Blocking based
on the second harmonic
of the differential
current
The blocking also prevents unwanted operation
at recovery and sympathetic magnetizing inrush.
At recovery inrush the magnetizing current
of the transformer to be protected increases
momentarily when the voltage returns to normal
after clearance of a fault outside the protected
area. Sympathetic inrush is caused by a
transformer, which runs in parallel with the
protected transformer already connected to the
network, being energized.
The connection of the power transformer against
a fault inside the protected area does not delay
the operation of the relay module, because in
such a situation the blocking based on the
second harmonic of the differential current is
prevented by a separate algorithm based on the
waveform and the rate of change of the differential
current.
Transformer magnetizing inrush currents occur
when energizing the transformer after a period
of deenergization. The inrush current may be
many times the rated current and the halving
time may be up to several seconds. To the
differential relay inrush current represents differential
current, which would cause the relay to
operate almost always when the transformer is
connected to the network. Typically, the inrush
current contains a large amount of second harmonics.
Blocking of the operation of the stabilized
stage of the relay at magnetizing inrush
current is based on the ratio of the amplitudes of
the second harmonic digitally filtered from the
differential current and the fundamental frequency
Id2f/ Id1f.
Inhibition of relay operation in situations of
overexcitation is based on the ratio of the fifth
harmonic and the fundamental component of
the differential current Id5f/ Id1f. At dangerous
levels of overvoltage which may cause damage
to the transformer, the blocking can be automatically
eliminated by a separate blocking inhibiting
setting Id5f/ Id1f> > . When required, the
blocking based on the second and fifth harmonic
of the differential current can be disabled.
In addition to the stabilized stage the differential
relay module SPCD 3D53 has a separate adjustable
instantaneous stage the operation of which
is not stabilized. The instantaneous differential
current stage operates when the fundamental
component calculated from the differential current
exceeds the set operate limit Id/ In> > or
when the instantaneous value of the differential
current exceeds the level 2.5 x Id/ In> > . The
setting range of the instantaneous stage Id/ In> >
is 5...30.
Should the stabilizing current be less than 30%
of the differential current, there is most certainly
a fault in the protected area. In such a situation
the set operate value Id/ In> > will be halved and
the blockings of the stabilized stage are automatically
prevented.
The differential relay module SPCD 3D53 is
provided with an integrated disturbance recorder
that is capable of recording six phase
currents, the internal trip and blocking signals
of the module and the control input signals.
Recording can be triggered by the rising or
falling edge of these signal. The recording length
is 38 cycles. The recording memory has the
capacity of storing one recording at a time.
Sampling frequency is 40 samples/ cycle. The
recording is downloaded by using a PC program.
The recording memory has to be reset
before a new recording is possible. |
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