Normas IEC para Transformadores

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1077-2618/12/$31.00©2012 IEEE
IEC standards and nonconventional
instrument transformers
T
HIS ARTICLE, COMPLETING A SERIES
of three articles on the subject [1]–[3],
presents themes related to current transform-
ers (CTs), including test results, designed and
tested from the viewpoint of the International Electrotechnical
Commission (IEC) standards and
practices outside ofNorthAmerica.
Although Ohm’s law and Max-
well’s equations are the same everywhere, how CTs are
specified and the responsibility of the different actors can
be very different. It is important for engineers to be aware
of these differences when executing applications in differ-
ent parts of the world.
Overview
Traditional CTs require a magnetic circuit to transmit
power from the high-voltage (HV)
conductors, where current is to be
measured, to the low-voltage devi-
ces using these measured values. However, magnetic circuits
can saturate, resulting in a much different waveform and value
of current in the secondary winding than in the primary wind-
ing. This often occurs in industrial applications under fault
conditions where it is common to have 50 kA flowing through
Digital Object Identifier 10.1109/MIAS.2011.943098
Date of publication: 8 November 2011
© CUTAJARC
BY PIERRE BERTRAND, MICHAEL MENDIK,
TERENCE HAZEL, & PASCAL TANTIN
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a CT having a primary current of 100 A
or even less. The severe saturation that
results, however, must not prevent the
correct operation of the protection
relay, especially when there is a fault
condition. As will be seen in this arti-
cle, the IEC standards provide no
guidelines in the correct choice of CTs
for protection functions—it is the re-
sponsibility of the protection engineer
to make this determination.
This article describes what IEC
standards say about CTs and what the
authors recommend regarding the cor-
rect selection of the CTs for protection.
Test results are given to show the cor-
rect operation even during severe CT
saturation. It also provides information
on the new technology that is being developed under IEC
standards. The new technology will provide solutions to
the CT saturation problems and will change the way pro-
tection engineers do their business in the years to come.
CT Sizing per IEC Standards
IEC Technical Committee (TC) 38 has the responsibility to
prepare the instrument transformer standards. Since 1996,
instrument transformer standards have been collected in the
IEC series 60044-X. IEC standards are in general bilingual
(English and French). This series is also published in Span-
ish. Free access to the first pages, including the scope of
IEC publications, is possible at the
IEC Web store: http://webstore.iec.ch/.
Table 1 gives a list of different parts.
Different accuracy classes are de-
fined. These accuracy classes could be
split into two main functions: mea-
surement and protection functions.
Measurement classes define high accu-
racy for a restricted variation range of
the primary current or voltage. Protec-
tion classes define the accuracy for a
large variation of the primary current
or voltage. Owing to the limitation of
the classical technology of CTs, it is not
possible, in general, to achieve both
accuracy requirements with the same
magnetic core. As a consequence, dif-
ferent cores are often needed to achieve
different types of functions. New technologies of instru-
ment transformers are able to fulfill both functions. They
are described as follows.
Measurement classes specify the amplitude error and
phase error. The manufacturer verifies these errors with a
bridge. This is a routine test. Protective classes specify
amplitude and phase error for the rated values of current
and are also tested with a bridge as a routine test.
To test the CT for higher currents, up to the rated accuracy
limit primary current, a bridge system measurement could
not be used as the duration of the test has to be less than 1 s
(maximum duration of mechanical and thermal withstand
TABLE 1. OVERVIEW OF PRESENT AND FUTURE (BETWEEN BRACKETS) IEC STANDARDS
ABOUT INSTRUMENT TRANSFORMERS.
General Requirement Standards Product Standards
61869-1 General
Requirements for
Instrument
Transformers
60044-1[61869-2] CTs
60044-2[61869-3] Inductive voltage transformers
60044-3[61869-4] Combined transformers
60044-5[61869-5] Capacitive voltage
transformers
60044-6[61869-6] CTs for transient performances
[61869-9-1] Additional
Requirements for
Electronic Instrument
Transformers and
Low-Power, Stand-Alone
Sensors
60044-7[61869-7] Electronic voltage transformers
60044-7[61869-11 Low-power, stand-alone
voltage sensors
60044-8[61869-8] Electronic CTs
[61869-9-2] Digital
Interface for Instrument
Transformers
60044-8[61869-10] Low-power, stand-alone
current sensors
[61869-12] Combines electronic
instrument transformers or
combined stand-alone
sensors
[61869-9-3] Stand-alone merging unit
USING LOW-
POWER CURRENT
SENSORS HAS A
POSITIVE IMPACT
ON THE
BEHAVIOROF THE
PROTECTION
RELAY.
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test). For this reason, IEC has defined
the concept of composite error for CTs.
Composite error takes into account am-
plitude and phase error in the same error
value. It is measured with a special setup
test circuit including a reference CT.
To cover the different types of appli-
cations, IEC has split the specifications
of CT protective classes into two parts.
The first is the IEC 60044-1 in which
the specification and tests are made in
steady-state condition, and the second
is the IEC 60044-6 dedicated to the re-
quirements for transient performance CTs.
Protective Classes in
Steady-State Conditions
Three different classes are defined in
the IEC 60044-1: the classical class P and the additional
classes PR and PX. In this naming, P means protection.
Class P is a well-known common class that has been used
for many years in classical protective applications. The use
of this class is described as follows. In addition to class P
specifications, class PR defined the remanent flux after sat-
uration has occurred. Class PR is used for specific differential
applications, e.g., for line or bus bar differential protection
of HV electrical networks. It assumes that all the dynamic
measurements permitted by the size of the core are always
available. For such behavior, the magnetic core has to be
designed with an air gap.
IEC Class PX was originally defined in British standards.
This class defines the size of the core for given applications
with other parameters as the concept of knee point. As class
PX CTs do not require an air gap, they are less expensive
than class PR, and their use is more common in industrial
networks, especially for differential protection.
The most common class P CTs are 5P and 10P in
which the numbers 5 and 10 are the highest permissible
percentage errors at the rated accuracy limit primary cur-
rent. This accuracy limit primary current is defined in
multiples of rated primary current, and the standard val-
ues used are 5, 10, 15, 20, and 30. For example, a 5P20
CT will have an accuracy of at least 5% at 20 times the
rated primary current. In addition to the winding ratio
defined as primary current/secondary current, the rated
output in volt ampere must be de-
fined. Standard values of rated output
(equivalent to the rated burden) are
2.5, 5, 10, 15, and 30 VA, with a
power factor of 0.8 inductive for all
values except for 2.5 VA where a power
factor of 1 is assumed.
By definition, a 5P20 CTwill begin
to saturate at 20 times its rated second-
ary current when loaded at its rated
burden. This is shown as point P in
Figure 1. At this point, the CT accu-
racy is �5%. At rated burden, the
rated accuracy limit factor Kn is thus
20. At the rated burden, Vn ¼
Kn(In)(Rctþ Rn), whereRn is the resist-
ance of the secondary circuit so as to
obtain the rated burden, Rct the internal
resistance of the CT secondary wiring, and Vn the point on
the saturation curve where the maximum error is equal to
5%. However, very often, the CT has a much smaller load
than its rated burden and therefore will operate much farther
from the saturation point. At this smaller burden, the real
rated accuracy limit factor Kr will be much higher than the
rated factor Kn. At this burden,Vn¼ Kr(In) (Rctþ Rr), where
Rr is the actual burden of the secondary circuit. This fact is
often used in the selection of the correct CT for the protec-
tion application. From the two equations, it is seen that
Kr=Kn ¼ (Rct þ Rn)=(Rct þ Rr).
From the above expression, the following two impor-
tant points can be observed.
n The real accuracy limit factor increases as CT
burden decreases.
n The internal resistance of the CT and the real burden
must be known, at least approximately.
Thus, to demonstrate the adequacy of the selected CT,
the protection engineer must know the impedance of the
secondary load and the internal resistance of the CTs that
he or she is using.
As mentioned earlier, the other definition often used by
relay engineers for differential protection is class PX. Class
PX CTs are defined based on a knee-point voltage such that
Ek ¼ Kn(Rct þ Rb)Isn, where Ek is the knee-point voltage,
Kn is the dimensioning factor, Rct is the resistance of sec-
ondary winding, Rb is the rated resistive burden, and Isn is
the rated secondary current.
The knee point is defined as the point where the
voltage increases only 10% for a 50% increase in the mag-
netizing current.
As the knee-point voltage of a class PX CT is very close
to the saturation voltage of a class P CT, it is perfectly possi-
ble for the equipment supplier to replace a class PX CT by a
class P CT, provided that the internal resistance of the CT is
known and the equivalence is clearly demonstrated by a cal-
culation note taking into account the burden of the CT.
Application to Protection Relays
We can roughly consider two kinds of protection functions
involved in heavy short-circuit detection: overcurrent protec-
tion and differential protection. For overcurrent protection,
the important factor to be considered is the maximum value of
the current that has to be measured accurately. When definite
1
1
10
100
0.01 0.1 1 10
P
V
o
lt
s
Amperes
A typical CT saturation curve.
PROTECTION
CLASSES DEFINE
THE ACCURACY
FOR A LARGE
VARIATIONOF
THE PRIMARY
CURRENT OR
VOLTAGE.
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time characteristic is used, this value
is the trip setting of the protection.
When time-dependent characteristic is
used, this value is 20 times the trip set-
ting since the tripping time of the pro-
tection is constant when the current is
above this value.
For relays that include a counter-
measure to overcome the saturation of
the CTs, the authors estimate that the
following rule applies: “The saturation
current of the CTs must be higher than
1.5 times the maximum value of the
current which has to be measured accu-
rately.” As a practical example, when
the overcurrent protection includes a
definite time high set below ten times
the CT-rated current, then a P15 rated
CT is suitable. Of course, this general
rule has to be confirmed by the relay
manufacturer, and it is up to the protec-
tion engineer to validate the suitability of
the CT for the selected protection relay. It can be determined
by specific simulation studies or high-current testing, as
described in this article. For other types of protection func-
tions, including differential protection, the CT sizing rule
may differ from one relay to the other. It is then a key point
to follow the CT sizing rule defined by the relay manufac-
turer. Most of the time, the CT sizing rule refers to a class
PX CT because of its inherent more precise definition.
As explained earlier, the IEC standards related to CTs
are well adapted to the specification needs of protection
relays, but it is up to the equipment supplier to validate
the choice of the CTs to be used. IEC standards do not
provide recommendations for this as is done in North
American practice.
Transient Performances of Magnetic CTs
At the end of the 1960s, short-circuit currents on inter-
connected HV networks reached higher and higher val-
ues. To avoid bad consequences of long faults, it was
decided that breakers should be opened in short time
(two or three cycles). New relays were developed using
static components, and new instruments transformers
able to insure an accurate transient response were
designed. IEC started, at that time, developing a new
standard for CTs. IEC 60044-6 (Requirements for Protective
CTs for Transient Performance) is the result of this work. Differ-
ent classes are specified and designed by TPS, TPX, TPY,
and TPZ.
For instance, TPY classes assume that the CT is able to
measure the asymmetric cycles of fault current during an
event on an HV network with no saturation of the core.
This requires an increase in the size of the core with a factor
related to the primary time constant and to design small
distributed air gaps. For a primary time constant of 120 ms,
the size of the core could be 20 times bigger. Using bigger
distributed air gaps, TPZ core is a linear CT. In case of asym-
metric cycles, these TPZ CTs give a good measurement at
industrial frequencies. The asymmetric component is fil-
tered and does not induce saturation. However, the phase
error, due to the presence of large air gaps, is higher
compared with TPY CT. For more
details on these transient CTs, it is sug-
gested to refer to this IEC standard.
Voltage Transformers
Up to now, the industrial measurements
of the voltage were made with magnetic
voltage transformers (VTs) up to 150–
170 kV. For higher voltage, capacitive
VTs (CVTs) are often used in electrical
networks. This arrangement of a capaci-
tive divider with an inductance and a
medium-voltage (MV) magnetic trans-
former is less expensive when compared
with magnetic design. CVTs are also
used with line traps for power line car-
rier data transmission.
Compared with magnetic VTs, the
CVT is a resonant circuit, and transient
performances are limited. In general, it is
not required to define transient perform-
ances for VTs, and IEC 60044-2 gives
only steady-state condition tests for protective classes. In
IEC 60044-5, transient performances are specified for CVTs.
These classes are only used inHVelectrical networks.
Behavior of CTs and Relays
During Short Circuits
The tests described in this section were recently performed
on microprocessor-based protection relays connected to
magnetic core CTs per IEC60044-1, commonly used in oil
and gas applications.
The CTs used in the testing have the following
characteristics:
n turns ratio: 50 A:1 A
n rated burden: 2.5 VA
n accuracy class: 5P
n accuracy limit: 20
n winding resistance, Rct: 0.276 X
n load resistance: 0.120 X.
The maximum short-circuit current is 50 kA. This is
typical of an industrial application where it is common to
have smaller-sized feeders supplied from the same main
switchboard used to supply the majority of the load. This
results in a worst-case scenario for the CT—low primary
current rating and very high primary current during fault
conditions. CT saturation will occur very quickly under
these conditions.
For CTsizing, it was considered that the maximum value
of the current that has to be measured accurately is 243CT-
rated current, which is the highest setting of the overcurrent
protection of the relay used for the tests. Under the condi-
tions of use of the CTs, the real accuracy limit factor Kr is
Kr=Kn ¼ (Rct þ Rn)=(Rct þ Rr) with Rn ¼ 2.5 X (rated
load resistance corresponding with the rated burden) and
Kn ¼ 20 (rated accuracy limit factor corresponding with the
rated burden). The resulting value for Kr is140 from the
above equation. Since Kr is above 1.5 times the maximum
value of the current that has to be measured accurately, the
CTcharacteristics match the needs of the overcurrent protec-
tion relay. This theoretical conclusion was then validated by
primary injection testing, simulation of CT saturation, and
THE OUTPUT OF
THE SENSING
ELECTRONICS
FEEDING A
MICROPROCESSOR-
CONTROLLED
PROTECTIVE
RELAYS CAN BE
EITHER ANALOG
OR LINEAR.
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simulation of the relay algorithm used for signal processing
of the severely distorted CTsecondary current waveform.
The tests were carried out in a high-power laboratory in
France. A three-phase, 50-kA rms current was injected
through the set of three CTs for a duration of 100 ms. The
overcurrent protection function of the microprocessor-based
relay was set to 1,200-A primary current and 50-ms definite
time (DT), typical for the short-circuit high-set protection of
feeder circuits.
Figure 2 shows the disturbance record of the protec-
tion relay. The relay input signal is sampled with a
dynamic of �40
p
2 times the rated current, and the trip-
ping occurs correctly.
Since the secondary current of the CTs was recorded, it
is possible to compare the real secondary current with the
calculated one. For simulation purpose, a MATLAB model
of the CT has been used, based on the manufacturing
characteristics of the CT (number of primary and secondary
winding turns, length and section of the magnetic core,
and magnetizing characteristic of the iron core). Figure 3
compares measured and calculated values. There is quite a
big difference between the two curves, which shows that
the air inductance of the CT (the value of the magnetizing
inductance when the core is fully saturated) is underesti-
mated in the model.
The behavior of the protection relay has been simu-
lated from the real CT output signal and also from the
simulated CT output. In both cases, the simulations re-
sulted in the correct tripping of the relay. Nevertheless,
the magnitude of the current internally calculated by the
relay is higher in the case of the real CT secondary current
compared with the simulation CT secondary current, as
shown in Figure 4. This means that the CT model is a good
basis for demonstrating that the relay operates because it
brings higher constraints than those that actually occur.
As a conclusion, the authors consider that it is perfectly
valid to use simulation models, although not perfectly
accurate, to ensure the proper operation of a protection
relay on a specific set of CTs under heavy short-circuit cur-
rents. This has been validated by the tests described earlier.
This provides the protection engineer with the tools
needed to validate the selection of CTs for his or her partic-
ular application.
Similarity Between the IEC and American
National Standards Institute Worlds
A CT/relay software analysis tool has been provided by
the IEEE Power System Relaying Committee (PSRC) to
check the compatibility between CT and relays [2]. The
calculation methods used apply correctly to CTs from the
IEC world.
The tool has been adapted by G. Swift to CTs and is
defined per IEC60044-1. This tool is more general than
the original PSRC one. The frequency can be set to 50 or
60 Hz, and the secondary current can be 1 or 5 A.
The saturation curve of the 50 A:1 A CT used for the
high-power tests is shown in Figure 1. It is described in
the PSRC saturation calculator by
n a slope S ¼ 31.5
n a saturation voltage Vs ¼ 62 V at Is ¼ 1 A.
4
3,000
2,500
2,000
1,500
1,000
500
0
0
0.
01
0.
02
0.
03
0.
04
0.
05
0.
06
0.
07
0.
08
0.
09 0.
1
P
ri
m
a
ry
 A
m
p
e
re
s
, 
R
M
S A
B
C
The magnitude of the fault current processed by the relay.
A: from CT secondary current, B: from manufacturer’s
MATLAB CT model output, and C: from PSRC saturation
calculator output. The dotted line shows the O/C threshold.
3
100
60
80
20
40
−20
−40
−60
−80
−100
0
0.
01
0.
02
0.
03
0.
04
0.
05
0.
06
0.
07
0.
08
0.
09 0.
1
0
P
ri
m
a
ry
 K
ilo
a
m
p
e
re
s
A
B
C
Accuracy of CT saturation models. A: the measured CT
secondary current, B: manufacturer’s MATLAB CT model,
and C: PSRC saturation calculator.
3,000
2,000
1,000
−1,000
−2,000
−3,000
0
0.
01
0.
02
0.
03
0.
04
0.
05
0.
06
0.
07
0.
08
0.
09 0.
1
0
P
ri
m
a
ry
 A
m
p
e
re
s
2
A disturbance record of the relay—phA current and 50-ms
DT trip signal.
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As a default value for the saturation
voltage, it is suggested to choose for Is
the rated value of the secondary current
(here, 1 A) and for Vs the result of the
above calculation:
n Vs ¼ (VAn/In) 3 Kn, where VAn
is the rated burden, In is the
rated value of the secondary cur-
rent, and Kn is the accuracy limit.
In our example, the default value for
Vs would be 50 V, leading to an under-
estimation of the secondary current.
Figure 3 compares the output of
the PSRC saturation calculator with
the recorded CT secondary current. As
explained in the “Behavior of CTs and
Relays During Short Circuits” section,
the output of the MATLAB CT model
is also shown. Both models seem to
deliver similar results. Nevertheless,
the Rogowski effect, that is the out-
put of the CT when the iron core is
fully saturated, is heavily underestimated by the PSRC
saturation calculator.
Using low-power current sensors has a positive impact
on the behavior of the protection relay. This can be noticed
in Figure 4: the magnitude of the current estimated by the
relay is much higher when processed from the true CT
secondary current than from the output of the PSRC satu-
ration calculator.
As a conclusion, we consider that
1) an accurate simulation of CT saturation is dif-
ficult to achieve and requires detailed CT manu-
facturing data
2) simplified CT simulation as provided by PSRC or
relay manufacturers systematically underestimates the
CT secondary current and can be used to prove
the good behavior of the protection relay, in both
the American National Standards Institute and the
IEC worlds.
Who Is Responsible for What?
In the IEC business world, it is common for the switch-
gear and controlgear manufacturer (hereafter called as
switchgear vendor) to select the CTs required to ensure
correct operation of the complete protection function.
The complete protection function includes the circuit
breaker, auxiliary power supply, the protection relay(s),
wiring, and the instrument transformers. The selection of
the CTs and VTs for each project should be documented
by the switchgear vendor. For some projects, the client
will require submission by the switchgear vendor of this
document for review and/or approval.
The selection of the CTs by the switchgear vendor will
be determined by the type of protection relay used. The
characteristics of the CT must meet the requirements
stated by the protection relay manufacturer for the par-
ticular application. As explained in the previous section,
the IEC standards are well adapted for the protection
relay manufacturer to express CT sizing rules. It is a com-
mon practice that the switchgear vendor also chooses the
relay since the switchgear vendor best knows how to
choose the CT that will meet the pro-
tection relay requirements for the
application and at the same time will
meet all installation constraints appli-
cable for his or her gear. It is assumed
that the switchgear vendor will use
relays from a reputable manufacturer
and will know how best to implement
them for ensuring correct operation of
the protection function as a whole (CT,
relay, and circuit breaker). When the
switchgear vendor selects all compo-
nents for use in the complete protection
function, it is clearly his responsibility
alone to ensure that all components
(relay, CTs, VTs, and powersupply)
have been chosen to ensure correct oper-
ation. Should the client require the use
of a particular component such as a relay
different than that proposed by the
switchgear vendor, then it can be
assumed that the client has validated
the selection of the relay with respect to the other compo-
nents chosen by the switchgear vendor. In such cases, the
responsibility for the correct operation of the protection
function as a whole is no longer the sole responsibility of
the switchgear vendor but split between the switchgear
vendor and the client.
In the case when the switchgear vendor wants to use
CTs outside the relay manufacturer’s specification, he or
she should ask the latter to demonstrate the proper opera-
tion of the relay that could, in specific cases, require simu-
lation of the complete protection chain (CTs and relay).
This is not an easy exercise. The switchgear manufacturer
has many constraints for the installation of CTs within the
switchgear, with size being one of the main ones. It is easy
for a relay manufacturer to specify minimum CT require-
ments to eliminate any risk of incorrect operation without
any additional calculations or simulations. It often occurs
that these CTs are overdesigned and simply will not fit
within commercially available switchgear. It is under such
circumstances that it is necessary to take additional steps to
ensure the correct match between relay and CTs.
The Future of Electronic Sensors
For many reasons, manufacturers have been looking at new
technologies for voltage and current measurements. The
development of not only electronic components, laser, digi-
tal technologies, and synthetic insulation but also digital
relays, which present a much smaller burden, give an op-
portunity for this evolution.
Current Standardization Work
IEC has developed two standards to cover this evolution:
IEC 60044-7 dedicated to electronic VTs and IEC 60044-8
dedicated to electronic CTs. Electronic instrument trans-
formers (EITs), a generic term that covers current and/or
voltage sensors, can or need not use electronic components.
As examples, a resistive divider with a low-voltage output, a
Rogowsky coil with or without passive or active integrator,
as well as optical sensors based on Faraday or Pockels effects
are considered EITs.
ACT/RELAY
SOFTWARE
ANALYSIS TOOL
HAS BEEN
PROVIDED BY THE
IEEE PSRC TO
CHECK THE
COMPATIBILITY
BETWEEN CT AND
RELAYS.
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It is possible, due to the low dynamic range of the volt-
age variations, to use an electronic amplifier with a voltage
sensor to produce an output compatible with IEC 60044-
2. This, however, is not possible for current measurements.
For this reason, only low-voltage outputs are defined in
IEC 60044-8 for current measurements. This is also in
accordance with the technologies used. Low-power CTs
(LPCTs), which include an internal shunt resistor, are able
to deliver only a low-voltage output, and the case is similar
for Rogowsky coils. It is also the case of optical sensors,
associated with primary or secondary converters to provide
the desired output.
In addition, IEC 60044-8 also standardizes a digital
output for electronic VTs and CTs. This solution was
established before the edition of IEC 61850 published by
the IEC TC 57. It was clearly a short-term solution wait-
ing for the publication of the relevant parts 9-1 and 9-2 of
IEC 61850. Close collaboration between the working
groups in charge of both TCs has insured that the voltage
and current data set of part 9-1 are compatible with IEC
60044-8. Only the protocol for the communication is dif-
ferent. The physical layer for communication could be
copper or optical fiber.
However, these two solutions were not implemented
by protective relay manufacturers. IEC 61850-9-2, which
is an Ethernet field bus solution, gives more possibilities
and was preferred by the relay community. As interoper-
ability in not insured with this alternative solution, the
user group Utility Communication Architecture (UCA),
which is officially linked with the IEC working group 10
in charge of IEC 61850 (called D Liaison in IEC word-
ing), has published a dedicated specification based on part
9-2, taking into account the interoperability aspect as
was done for part 9-1 and IEC 60044-8. This specifica-
tion is known as UCA 9-2 LE (where LE stands for light
edition). This specification is the basis for the revision of
the IEC 60044-8 by TC 38.
Optical CTs
Optical CTs have been used by several utilities in HV and
MV transmission grids since the mid 1990s in North
America and Europe. The main value for utilities was seen
in revenue metering because of the availability of 1-A current
outputs interfacing commercially available revenue meters.
Reasons for using optical CTs range from wide accuracy
specifications, nonconventional mounting possibilities, and
elimination of insulating oil. As a result of continuous
improvements in sensing technology, such as insensitivity
to any mechanical vibrations, digital, or analog outputs,
optical current transducers have made their way into
protective relaying applications.
Two distinctive technologies have emerged based on the
same physical phenomenon, the Faraday effect: the polari-
metric and interferometric principle of detection.
The Faraday effect describes the twist of the angle a of
the plane of polarized light in the presence of magnetic
induction as a¼ V 3 H 3 l, where V denotes the Verdet
constant, a material property, H is the magnetic induction,
and l is the distance of light travel. The tilt of the plane
polarization a is proportional to the applied magnetic
induction and thus the primary current.
This electromagnetic physical effect results in three major
benefits for installations in HV networks:
n linearity with respect to current, lack of magnetic
saturation
n constant sensitivity over a broad range
n lack of need for insulating oil; the information is
passed through an optical fiber from line to ground
which is in itself an insulating dielectricum.
In the case of polarimetric sensing technology, the light
is guided through a 200-lm multimode fiber and a polar-
izer into a sensing glass body that encompasses the current
carrying conductor, makes one turn, and exits the sensor
through an analyzer (see Figure 5). Pending on the tilt of
polarization, more or less light reaches a diode mounted in
the sensing electronics.
The sensing optics is mounted in a waterproof enclosure
(see Figure 6) and connected through several fibers embed-
ded in a light polymeric insulator.
The sensor head assembly is bolted on a support insula-
tor, which is designed to withstand electrodynamic and
seismic forces. Figure 7 depicts a phase of an optical CT
rated 550 kV.
6
The housing for a magnetooptical sensor can be used for
various kilovolt ratings and primary currents. The sensor
head assembly is usually completely oil or SF6 free.
National Electrical Manufacturers Association (NEMA) pads
for a desired rated current are the interface to the
substation bus.
Diode Measures
Analog Intensity of
Light ~Current
Analyzer;
Passes a
Selected
Polarization
Polarizer;
Linearly
Polarized Light
I1
L
H
5
The linearly polarized light forms one turn around a current
carrying conductor and experiences the Faraday effect;
changes in polarization translate into intensity changes
detected by a diode, thus indicating the magnitude of the
primary current I1. The optical component can be of either
fused silica (SiO2) or any other optically active material.
18
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The output of electronic sensors feeding a microprocessor-
controlled protective relay can be either analog or digital,
depending on choice of the manufacturer. As an example,
the 200-mV rated analog output is defined in IEC60044-8and IEEE PC37.92/D14 (Standards for Analog Inputs to Protec-
tive Relays from Electronic Voltage and Current Transducers).
Figure 8 shows the linearity of the outputs for various val-
ues of rated primary currents, which correspond to 200-mV
outputs. Shown is a maximum output of 8 V corresponding
to 403 . In case higher requirements on linearity are re-
quired, the nominal output can be set to 100 mV.
In industrial applications, where very high fault levels are
present, the optical CTs can be calibrated to reach almost
any desirable set points for relays. Figure 9 shows the output
of an optical CT compared with the output of a high-preci-
sion shunt mounted on the same high-current bus. The red
line shows the output of the optical CT, whereas the green
line plots the signal from the resistive shunt, and both are
perfectly superimposed. The peak of the generated short-cir-
cuit current was 38 kA. There was no injected synthetic
transient overvoltage required. The signal from the optical
CTwas within 0.5% accuracy during the whole signal dura-
tion and completely saturation free.
In the case of the example mentioned in the “Behavior
of CTs and Relays During Short Circuits” section with
50-kA fault level and feeder current of 50 A, which corre-
sponds to the rated primary current, linearity could be
achieved up to 2,000 A (403 ) or 3,150 A (633 ). In this
case, the output signal of the sensing electronics would be
200 mV at 50-A rated primary current. However, if higher
ratios are required, the output voltage can be set below
200 mV. Higher relays’ setting points allow minimizing
unwanted relay operations in the presence of transient dis-
turbances such as high inrush currents.
The benefits of optical current CTs become obvious in
grids where high fault level currents exist, such as industrial
networks, where saturation-free operation is desired. The
freedom of setting calibrations allows substation design and
relaying engineers to tune optical CTs according to their
needs. Also, when specifying the relaying performance for
optical CTs, the designers do not have to consider CTsatura-
tion effects, burden, and length of signaling cables; optical
fibers do not add any errors to the system.
Other Kinds of Low-Power Instrument Transformers
In addition to optical CTs, which are mainly used today in
HV systems, other technologies are becoming more widely
used in MV, also referring to the same standards. These are
Rogowski coils and LPCTs for current measurements and
resistive dividers for voltage measurements.
10
9
8
7
6
5
4
3
2
1
0
0
1
0
,0
0
0
2
0
,0
0
0
3
0
,0
0
0
4
0
,0
0
0
5
0
,0
0
0
6
0
,0
0
0
7
0
,0
0
0
8
0
,0
0
0
V
ARMS
500 A Rated
Primary Current
1,000 A Rated
Primary Current
2,000 A Rated
Primary Current
8
The linearity of the sensing electronics output is shown
as a function of various values of primary rated currents,
such as 500, 1,000, and 2,000 A. Shown is 403, however
633 can be reached as well leading to a higher output of
the amplifying electronics. In contrast to conventional
inductive CTs, there are no knee points that need to be
considered.
40
30
20
10
−10
−20
−30
−40
0.1 0.125 0.15 0.175 0.2
0
40
30
20
10
−10
−20
−30
−40
0
M
O
C
T
-P
 (
k
A
)
S
h
u
n
t 
3
8
 (
k
A
)
Time
9
Transient recording of an asymmetrical short-circuit current
recorded by both an optical CT (analog output) and a
resistive high-precision shunt.
7
One phase of a 550-kV rated optical CT configured for both
protective relaying and revenue metering.
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A Rogowski coil is wired around a
nonmagnetic core. It provides a voltage
output related to the derivative of the
primary current, which thus needs to
be electronically integrated before used.
It is absolutely saturation free. The
LPCT is manufactured as a standard
magnetic-core CT. Its winding is closed
by a shunt resistor, and the voltage
across the resistor is processed by the
protection relay. Because of a low resis-
tive load, it is possible to manufacture
LPCTs having simultaneously a high
saturation current and a metering class
accuracy at low current at a cost lower
than or equal to a class P CT. Figure 10
gives an example of such a CT, which
can be used with a metering class 0.5
between 100 and 1,250 A and a protection class between
5 A and 40 kA.
Resistive dividers can replace traditional VTs with a
similar accuracy, a much smaller size and lower cost, which
allow systematic installation of voltage sensors in each
cubicle. This makes the solution easier and safer. Users are
more and more interested in the advantages of these tech-
nologies. Several thousands of such low-power instrument
transformers are installed each year, with an impressive
growth rate.
Application to Industrial Networks
Low-power, stand-alone current sensors, voltage sensors,
and combined ones give definite advantages when applied
to industrial networks. They are
n no or reduced saturation effects, which remove any
concern about CT sizing
n large range of rated currents covered by only one
sensor, making standardization, storage, and short
lead times possible
n good accuracy: the same sensor can be used for
both protection and metering
n better safety: no dangerous voltage on secondaries
n lower manufacturing cost.
Many customers now have a very positive experience
with low-power sensors from different manufacturers,
which proves that the technology is mature and well
adapted to distribution networks.
As this technology is now supported
by efficient standardization work, we
believe it will be possible in a near
future to mix sensors from one manu-
facturer with protection relays from
another one.
Conclusions
CTs have the same electrical character-
istics and are subjected to the same
constraints no matter how they are
specified. It is important, however, to
know how to specify them correctly to
be sure that the selected CTs will oper-
ate correctly with the protection relays
to which they are connected. The IEC
standards provide the information nec-
essary for correctly specifying CTs in an
unambiguous manner. They do not, however, provide any
guidance as the characteristics that are required for particu-
lar protection applications. This is the responsibility of the
protection engineer. It is thus very important for protection
engineers to know how to select the correct CTcharacteris-
tics and afterward to specify the CT accordingly. This arti-
cle provides many of the guidelines required for ensuring
that the complete protection function operates satisfacto-
rily under all fault conditions.
In the near future, EITs will provide accurate, saturation-
free signals more appropriate to protection relays and meters
than traditional ones. This will open new possibilities, such
as the merging together of metering, power monitoring,
power quality, and protection. This is actively supported by
the IEC standardization committees to guarantee compati-
bility between instrument transformers and protection re-
lays and make the life of protection engineers easier.
References
[1] R. Coss�e, D. Dunn, and R. Speiwak, “CT saturation calculations—Are
they applicable in the modern world?—Part I: The question,” in Proc.
IEEE Petroleum and Chemical Industry Conf. (PCIC) Rec., 2005.
[2] R. Coss�e, D. Dunn, R. Speiwak, S. Zocholl, T. Hazel, and D. Rollay,
“CT saturation calculations—Are they applicable in the modern world?—
Part II: Proposed responsibilities,” in Proc. IEEE Petroleum and Chemical
Industry Conf. (PCIC) Rec., 2007.
[3] R. Coss�e, D. Dunn, R. Speiwak, and J. Bowen, “CT saturation
calculations—Are they applicable in the modern world?—Part III:
Low-ratio, high-current CT/microprocessor relay comparisons at a high-
current testing laboratory,” in Proc. IEEE Petroleum and Chemical Industry
Conf. (PCIC) Rec., 2008.
[4] T. Hazel, J. Tastet, N. Quillon, and B. Lusson, “Implementingbackup
protection using microprocessor based protection relays,” in Proc. IEEE
Petroleum and Chemical Industry Conf. (PCIC) Rec., 2001, pp. 53–62.
Pierre Bertrand (pierre.bertrand@fr.schneider-electric.com) is
with Schneider Electric in Grenoble, France. Michael Mendik
is with ABB Inc. in Mount Pleasant, Pennsylvania. Terence
Hazel is with Schneider Electric in Grenoble, France. Pascal
Tantin is with EDF R&D in Paris, France. Mendik is a
Member of the IEEE. Hazel is a Senior Member of the IEEE.
This article first appeared as “CT Saturation Calculations—
Are They Applicable in the Modern World?—Part IV: CT
Sizing as per IEC Standards and the Benefits of Nonconven-
tional Instrument Transformers” at the 2009 Petroleum and
Chemical Industry Conference.
5
1.5
0.75
0.5
E
rr
o
r 
(%
)
Module
5 
A
20
 A
10
0 
A
1 
kA
1.
25
 k
A
10
 k
A
40
 k
A
IP
10
An example of LPCT delivering metering-class accuracy
and high saturation current for protection.
THE SELECTIONOF
THE CTS AND VTS
FOR EACH
PROJECT SHOULD
BE DOCUMENTED
BY THE
SWITCHGEAR
VENDOR.
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