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Saturation Analysis of Current Transformer

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 International Journal of Research and Review
www.gkpublication.in E-ISSN: 2349-9788; P-ISSN: 2454-2237

Original Research Article

Saturation Analysis of Current Transformer

Riyas Ahamed M1, VimalrajM2, Arjuna Rao S3
1
Department of Electrical and Electronics Engineering, C. Abdul Hakeem College of Engineering and Technology,
Vellore, Tamil Nadu, India
2
Department of Electrical and Electronics Engineering, TPEVR Govt. Polytechnic College, Vellore, Tamil Nadu,
India
3
Short Circuit Laboratory, CPRI, Bangalore, Karnataka, India
Corresponding Author: Riyas Ahamed M

Received: 11/05/2015 Revised: 09/06/2015 Accepted: 12/06/2015

ABSTRACT

Current Transformers (CT) are the key device in power system protection. Generally current transformers
are designed such a way that, its secondary current is substantially proportional to the primary current
under normal operating condition. It is very important to analyze the behavior of a current transformer for
its applied primary current, which may contain large magnitude alternating current (AC) component, or
small amount of direct current (DC) component that will result in saturation of CT. The study about CT
saturation is on more demand due to the intrusion of DC offset currents in to the protection system. In
paper different factors which lead CT to saturate are analyzed.

Keywords: Current transformers, Hysteresis and saturation.

I. INTRODUCTION faithfully reproduce the actual current

Current transformer is one of the waveform. A simplified equivalent circuit of
essential parts in electric power systems. All current transformer is shown in Fig.1.An ideal
types of protection and control devices needs current transformer should perform these two
current transformers to perform measurement tasks over the wide range of currents. In
of current at ground level. It is well known practical CT’s having limitations, it consists
that current and voltage profiles may carry of the major components: laminated steel
several transient features about the nature of core, secondary winding around the core and
fault. Some of these features are very high insulating material. When current travels
frequency components. To perfectly transform through a cable/bus duct which is current
these transients the electromagnetic current carrying device, develops magnetic field at
transformers sometime are found incapable right angles to the flow of current. As the
due to their magnetic saturation. [1] current magnitude varies, strength of the
An ideal current transformer would magnetic field changes for all operating
proportionally scale down the value of the conditions. [2-4] As learned in transformer
power system current to a useable known theory, [5] when a magnetic field cuts a wire, it
value. Secondly the scale-down output should will induce a current to flow in that wire. By

International Journal of Research & Review (www.gkpublikation.in) 337

Vol.2; Issue: 6; June 2015
using the strength of the magnetic field and the rated current. But protection and PS class
knowing the turns ratio we can obtain a value CT’s can be operated for a wide range of rated
of current that is useable for meters, relays current. [6] When the secondary induced EMF
and other current sensing device. In order to passes the level of knee point voltage (V K)
scale a value of high current flowing in a shown in fig. 2, the core goes for saturation.
conductor, the engineer needs to introduce a Measurement class CT’s can’t operate beyond
specific number of uniformly distributed turns the knee point, but the protection and PS class
of wire around the core to scale down the CT’s can operate around the knee point
system current. This will ensure that the depends on the class of CT.
output current is always proportional to the Abnormal high magnitude primary
current flowing in the conductor. In the past, current and high secondary burden these two
there were two main values of secondary results high flux density in the current
current typically used in measuring current. In transformer core. When the flux density
the United States, engineers typically use a 5- reaches the designed limit of the core it results
amp output and other countries have adopted saturation. [7] Fig.3 shows the result of
a 1-amp output. In the advent of saturation, the CT accuracy becomes poor, the
microprocessor meters and relays, the industry secondary side output waveform becomes
is seeing the 5-amp or 1-amp secondary being distorted; also the magnitude of secondary
replaced with a mA secondary. Typically, wave reduces due to core impedance.
devices with mA output are called “current
sensors,” as opposed to current transformers.

Fig. 2 Secondary excitation curve representing knee-point

Fig. 1 Simplified equivalent circuit of CT

CT saturation is a term used to

describe the state where a CT is no longer able
to reproduce an output current in proportional
to its primary current or as per its ratio. [6] The
basic reason for CT saturation is due to
number of reasons like large primary current,
small DC current, high burden at the Fig. 3 CT current waveforms due to saturation
secondary and open circuit in the secondary.
CT saturation depends on number of factors:
II. CONCEPTS OF CURRENT  Physical CT characteristics (size,
TRANSFORMER SATURATION rating, winding resistance, saturation
The saturation is the concept related to voltage)
property of magnetic materials used for the  Connected CT secondary burden
current transformer core. In usual the metering (wires with relay/meter/protective
CT’s core saturates for smaller multiples of equipment’s)

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Vol.2; Issue: 6; June 2015
  Primary current magnitude, DC offset If the saturation level is reached the
(system X/R) magnetizing current will strongly increase.
 Residual flux in the core. The CT needs all current for magnetizing and
no current is left for the secondary side. The
2.1 Saturation due to large magnitude of secondary current will be strongly distorted
current (AC saturation) and the error will be considerable. Fig. 4
Large magnitude of primary current shows secondary currents for some examples
(symmetrical in nature) during fault condition of saturation due to large primary AC current
can cause the saturation to the CT core. [8] and fig. 5 shows an example of CT saturation
Current with higher magnitude of the rated with DC offset. [1,3-4] The current is
primary current (about 20 times of the rated reproduced correctly up to the level where the
value) can cause saturation. This level can be core starts to saturate but during the periods
adjusted by choosing the knee point voltage of the core is saturated the secondary current
the core. Fig. 4 shows the core saturation due practically disappears. Saturation will cause
to larger magnitude of primary current. both amplitude and phase errors. The
2.2 Saturation due to DC offset fundamental current component amplitude
Presence of DC offset in fault current will be reduced and the phase will be
is the causes of saturation in current advanced.
transformer core. [8] This will be more Remanence
pronounced if the X/R ratio of the system up The DC component decays with a time
to the fault location is considerably high. Fig. constant L1/R1 as mentioned. The flux level in
5 shows the CT core saturation due to DC the core will not go to zero till the DC
offset. component dies-out. The steady state flux
variation will not reduce the flux to zero; if
any other fault occurs the flux excursion will
start at “remanence flux” (residual flux).
Remanence or remaining flux in the CT core
influences the time to saturation. [9] When
there is no DC offset the remanence will only
affect the first half cycle of the current
waveform. If the fault current has a DC offset,
the remanence will just impact the first
moment when the CT will saturate and the
Fig. 4 CT core saturation due to large primary current
time to saturation can be decreased. The
saturated secondary current has the same
characteristics as for DC saturation without
any remanence.

III. REMEDIES TO OVERCOME

SATURATION
The event of transient leads the current
transformer core to partial or severe
saturation. The effect of saturation are can be
reduced, also in some cases we can overcome
Fig. 5 CT core saturation due to DC offset it; by taking corresponding remedial actions.

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Vol.2; Issue: 6; June 2015
The remedies which can overcome the It is possible to calculate the time to
saturation are follows. saturate for any CT given the set of saturation
3.1 CT oversizing factor and residual voltage, remanence level, details of connected
flux burden etc. Fig. 6 shows the saturation time
Current transformer core saturation curve of CT once the time to saturation is
can be avoided on DC components by over known a quick check against the time of
sizing the core. The core can be designed as operation of the protective relay would
the DC flux is oversized at corresponding flux indicate whether the application would
density. function properly with respect to the CT
In normal, the flux (∅) carries both AC and characteristics. Special care is needed when
DC components high speed auto reclose is concerned since the
remanence magnetism and the CT secondary
∅𝐦𝐚𝐱 = ∅𝐦𝐚𝐱 𝐦𝐚𝐱
𝐀𝐂 + ∅𝐃𝐂 transient effects are the maximum when a
reclose is attempted with a permanent fault on
∅max
AC +∅DC
max
the line.
The factor is called as oversizing
∅max
AC
factor.
∅max
The core oversizing factor = + ∅DC
max ;
AC
ωL line
= 1 + ωt ; =1+ R line

X1
=1+
R1

In order to design CT which will always

reproduce current accurately it is necessary to
𝐗
𝟏+ 𝟏
𝐑𝟏
increase the CT size by the term, . Fig.6 Saturation time curve for CT
𝟏−𝛙
Where 𝛙 is the per unit maximum flux
3.3 Impact of Secondary burden
remaining in the core after removal of the Increase in secondary winding circuit
primary current. In practice high amount of burden means an increase in volt ampere
over sizing is not possible. rating. [10] This necessitates an increase in
Residual flux secondary winding induced voltage. Increase
During faults, if the remanence-flux in secondary winding induced voltage can be
direction is opposite to that of the DC current done by increased flux & flux density. Fig. 7
component the CT will produce correct shows that the secondary current is purely
secondary waveform. On the contrary, if the sinusoidal for burden of 0.3 ohms. If the same
remanence flux and the DC transient occur in CT is connected to a burden of 100 ohms the
the same direction, a more distorted waveform secondary current wave gets distorted.
than usual occurs. Various methods are used
3.4 Impact of X/R ratio
to reduce the effects of remanence. [9] Mainly speaks about the parameters on
 Using different grades of steel for the primary side of CT, X/R is responsible for the
core decaying DC component. DC component
 Gapped core produces constant magnetic flux contributes
 Biased core CTs. to CT saturation. Since X/R ratio cannot be
3.2 Time to saturate CT core altered for the power protection purpose it can
International Journal of Research & Review (www.gkpublikation.in) 340
Vol.2; Issue: 6; June 2015
only be suggested that the described clears that more X/R ratio makes the distorted
phenomenon should be carefully taken under secondary current waveform even at smaller
consideration when choosing a CT. In fig. 8, magnitude.

Fig. 7 Secondary current wave with various burden values

Fig. 8 Impact of X/R ratio over CT saturation

IV. CONCLUSION 2. Davarpanah M, Sanaye-Pasand M,

The concept, causes and remedies of Iravani R. “A saturation suppression
saturation makes it clear that: Consequences approach for the current transformer -
of CT saturation are the secondary current Part I: fundamental concepts and
will not faithfully replicate the primary design”, IEEE Transactions on Power
Delivery, Vol. 28, No. 3, pp.1928-1935,
current, secondary current is clipped and this 2013.
clipping of CT current leads to blinding of 3. Rafajdus P, Bracinik P, Hrabovcova V,
the protection system. The saturation can be Saitz J, Kankula L. “Current transformer
caused due to large primary AC currents or analysis under transient conditions”,
smaller magnitude DC offset. The remedies XIX International Conference on
to overcome saturation clears that, it Electrical Machines - ICEM 2010,
requires more care while choosing the Rome, 2010.
designing parameters (includes burden). 4. Pandey R. P, Patel R. N. “An efficient
detection algorithm for ct saturation
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Power Delivery, Vol. 25, No.3, pp.1340- Engineering) from Ganadipathy
1347, 2010. Tulsi’s Jain Engg College,
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BIOGRAPHIES His areas of Interests areas are Short Circuit Studies,
Failure Analysis of Instrument Transformers & LV
Mr. M. Riyas Ahamed received switchgear.
his Diploma (EEE) from TPEVR
Govt. Polytechnic, Vellore (TN),
India in 2007 and B.E (Electrical
& Electronics Engineering) from
CAHCET, Vellore (TN), India in
2010. He completed his M.E in
Power Systems Engg. in 2014
From Kingston Engineering College, Vellore (TN),

How to cite this article: Ahamed MR, Vimalraj M, Rao SA. Saturation analysis of current transformer.
Int J Res Rev. 2015; 2(6):337-342.

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