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CASE STUDIES ON CT CIRCUITRY
P.K. Pattanaik
E & MR Divn. GRIDCO, BURLA, ORISSA
1. SYNOPSIS Current transformers (CT) and its circuitries play
the most vital role for the protection, control and metering of HT
and EHT lines. During the time of commissioning or modification of
the CT circuitries, mistakes in wire connections are done by the
electricians or wiring personnel. Some of the mistakes are even
overlooked by the protection engineers during the time of stability
test (Primary injection Test, Sensitivity Test etc...) due to use
of less sensitive instruments. Sometimes the testing procedures are
approximated to reduce the testing duration, which develops
problems in real practice. But after commissioning and charging of
the said circuit, these mistakes result problems in protection,
control and metering of the circuit. The following problems may
result for the wrong connection in CT circuitry.
1. False tripping of charged feeder with certain
rise of load current, before the settable limit of the over load
Relays.
2. False tripping of Transformers even for the external feeder
faults.
3. Wrong metering causes discrepancies for the operation,
control and commercial billing among the utilities.
4. Difficulties in relay co-ordination for the interconnected
feeders
5. Problems for the load assessment in any loading feeders.
6. Wrong assessment of tripping analysis.
This paper deals with the theory of analysis for easy fault
finding in the CT circuitry and suitable methods for tracing the
fault in OFF-LINE and ON-LINE system. Some practical case studies
with real time occurrences have also been described for the
supportive analysis of the faults in the circuits.
2. INTRODUCTION Current and potential transformers are the
important interfaces between the high level of power system and low
levels of protection, measurement and control circuit in terms of
current and voltage. Whenever the value of current or voltage
becomes too high, these instrument transformers are used in the
system to produce a proportional low value for a scaled down
replica to the secondary working circuit of the system. The
performance of the measuring
Transformers (CT and PT) during and following large
instantaneous changes in the input quantity are to be considered
seriously. The response of the electrical parameters of these
transformers upon the secondary circuit should be well within the
satisfactory limit for both under steady state and transient
condition. For measuring and slow speed recording application, only
the steady state accuracy is relevant, whereas for high speed
protection and other application, accuracy under transient
condition is also important.
According to the requirement of the secondary circuit the cores
in the secondary circuit of the CT are designed. These are of three
types 1. Measuring current transformers
(Metering core) 2. Protective current transformers
(Protection core) 3. Protective current transformers for
special
purpose of application (Protection core PS class)
Typical specification of a CT with these cores have been
explained in table-1
Table-1
3 FUNDAMENTALS ON CT A current transformer s used with its
primary winding connected in series with the actual line current
flow of the power system. The primary winding consists of a bar
conductor or a conductor with a very few turns and causes no
appreciable voltage drop across the winding. The secondary winding
has large number of turns, the exact number being decided by the
turns ratio of the CT. The instruments/equipments like indicating
meters, relays etc are connected on the secondary windings, which
have very low impedance circuit. So the secondary windings are
regarded as a circuit that works nearly with short circuit
condition.
Type of Core Metering Protn. Special Type
Protn. Type
OUT PUT 20 to 40 - 20 to 40 ACC. CLASS 0.5 Fs
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4. CONNECTION PRINCIPLE
Polarity and connection The primary and secondary terminals of
the CT are identified with polarity markings by the symbols like
(P1 and P2) for primary and (s1 and s2) for secondary. It is marked
with a common convention that when primary current enters the P1
terminal, secondary current leaves the s1 terminal to the load
circuit. So primary P1 terminal corresponds to the secondary of s1
terminal. It is regarded as DOT convention. Its significance is in
showing the direction of current flow relative to another current
or to a voltage as well as to aid in making the proper connection.
The same is explained in the Fig.-1
P1 P2 S1 S2
Fig-1
CT Connection related to the Ratio To obtain multi CT ratio in a
common CT, the winding s of the primary and secondary side are
controlled by different connections. These connections are of three
different types.
1. Ratio by primary control 2. Ratio by secondary control 3.
Ratio by both side control
Ratio by primary control By the connection of available primary
windings in different fashions like all in series or all in
parallel or combination of series and parallel, the ratios of the
CT are changed. The detail connections are shown in Fig-2. The
connection sequences are described inTable-2 P1 C2 C1 C4 C3 C6
C5 P2 S1 S2
Fig-2
Table-2 Connection Sequence Primary Secondary
Current Ratio
(C1 + C2), (C3 + C4) (C5 + C6)
S1 S2 Lowest Ratio ( CTR1 )
(P1 + C1), (C2 + C3+ C4 + C5) (P2 + C6)
S1 S2 Middle Ratio ( 2 CTR1 )
(P1 + C1 + C3+ C5) (P2 + C2 + C4+ C6)
S1 S2 Highest Ratio ( 4CTR1 )
Ratio by Secondary control By the use of tapping terminals on
the secondary side of the windings, different CT ratio can be
obtained. By this principle primary connection remains fixed and
ratios are controlled on secondary terminals. The connection
diagram is shown in the fig-3. The connection sequences are
described inTable-2
S1 S2 S3 S4 Fig-3 Table-3 Connection Sequence Primary
Secondary
Current Ratio
P1-P2 S1 S2 Lowest Ratio ( CTR1 )
P1-P2 S1 S3 Middle Ratio ( 2 CTR1 )
P1-P2 S1 S4 Highest Ratio ( 4CTR1 )
Ratio by Both control Both primary and secondary winding can be
connected with the tapping terminals to obtain different CT Ratio
from a common CT. By this type of control, the connection stability
and flexibility increases and ratios are obtained as per the
suitability. Now-a-days maximum CTs are designed with CTR control
from both primary and secondary control. The detail connections are
shown in Fig-4. The connection sequences are described inTable-4 P
1 C2 C1 P2
S1 S2 S3 Fig-4
R
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Table-4 Connection Sequence Primary Secondary
Current Ratio
(C1 + C2) S1 S2 Lowest Ratio ( CTR1 )
(P1 + C1), (P2 + C2) S1 S2 Middle Ratio ( 2 CTR1 )
(C1 + C2) S1 S3 Middle Ratio ( 2 CTR1 )
(P1 + C1), (P2 + C2) S1 S3 Highest Ratio ( 4CTR1 )
Concept of CT circuit connection
Star connection circuit ( Y- connection ) CT secondary windings
are connected either in star (Y) or in delta () connection as per
the requirement of the circuit. For star connection, the currents
on each phase (IR, IY, and IB) are related vectorialy and expressed
in sequence components are as follows. IR= IR1+ IR2+ IR0 IY= IY1+
IY2+ IY0 = a
2 IR1+ aIR2+ IR0
IB= IB1+ IB2+ IB0 = a IR1+ a
2 IR2+ IR0
So, IR+ IY+ IB = 3 IR0 = 3 IY0 = 3 IB0 Where 1, 2, 0 designate
+ve,-Ve and zero sequence components. a, a
2 are the operators,
Note: - Current vectors are in +ve sequence only with Current on
R phase is taken as reference.
IR Vectorial expression IR = I , angle( 0
0 )
IY = I, angle (-1200)
IB = I, angle (1200)
IR+ IY+ IB= IN
IB IY
Delta connection circuit ( - connection ) For delta connection
of the CT circuit, the pattern of connection can be made by two
possible ways. In one type of connection secondary S1 of one phase
is connected to the secondary S2 of next phase in regular sequence
(R, Y, and B). In other type of connection secondary S1 of one
phase is connected to the secondary S2 of next phase in opposite
sequence (R, B, and Y). The connections of the windings are shown
in fig-5 and 6.
R phase S1 S2 Y phase B phase I1 I2 I3 Fig-5 IR ( IB-IR) (
IR-IY) IB ( IY-IB) IY Fig-5 A R phase S2 S1 Y phase B phase I11 I21
I31 Fig-6 IR
( IR-IB) (IY-IR) IB ( IB-IY) IY Fig-6-A
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Considering vector analysis principle with R phase as reference
and balance load connection, the following results are expressed.
For Fig 5 & 5-A IR = I , angle( 0
0 ) , IY = I, angle (-120
0)
IB = I, angle (1200), So, IR+ IY+ IB= IN = 0
I1 = ( IR-IY) = 3 I, angle (30
0 ),
I2 = ( IY-IB) = 3 I, angle ( -900 ),
I3 = ( IB-IR) = 3 I, angle ( 1500 ),
For Fig 6 & 6-A IR = I, angle ( 0
0 ) , IY = I, angle (-120
0)
IB = I, angle (1200), So, IR+ IY+ IB= IN = 0
I11 = ( IY-IR) = 3 I , angle (-150
0 ),
I21 = ( IB-IY) = 3 I , angle( 900 ),
I31 = ( IR-IB) = 3 I , angle( -300 ),
5. CASE STUDIES ON CT CIRCUITRY The application of the theories
and fundamental principles as described in various paragraphs will
be now discussed in different case studies to analyze the faults in
the CT circuitries. In each situation/problem, the behavior of
currents in the circuit are measured and analyzed by drawing the
basic vector diagram. From the theoretical analysis, the faulty
connections are traced and rectifications are done easily.
Case Study No 1
Situation/problem Tripping of one 132 KV feeder on E/F relay was
observed at a 220/132 KV Grid Sub-station, during peak load
condition for the rise of load current above a particular load.
Steps attempted During off-peak load condition, the currents on
the secondary circuits, used for back-up relays were measured by
means of clamp-on ammeter. The results were obtained as follows in
the table-5.
Table-5 Phase /
Wire No. Current in m Amp.
Ref. from Vector
Diagram
Remarks
R ph. C11 61 OA O.K Y ph. C31 62 OB O.K B ph. C51 60 OC O.K
Neutral. C71 120 BD=2OB Doubt? C11+ C31 105 AB=3OA Doubt? C31+
C51 104 BC=3OC Doubt? C11+ C51 62 CD=OA O.K
Conclusion of the readings From the readings of Back-up core as
described in the table, it got concluded that Y phase
secondary CT terminals have been altered and connected in the
circuit. The same can be confirmed form the vector analysis.(Ref.
Fig- 7 A )
C11 C31
C51 C71 Fig-7 A D O C B Fig-7 A
Case Study No 2.
Situation/problem In one 132/33 KV S/S the indicating
instruments (Wattmeter, ammeter etc) on 132 KV Incomer feeder were
recording erroneous readings.
Steps attempted. During loading condition of the said feeder the
currents were measured in the metering core circuit by means of
clamp-on ammeter. The results were obtained as follows in the
table-6.
Table-6 Phase / Wire No.
Current in m Amp.
Ref. from Vector
Diagram
Remarks
R ph. D11 0 Doubt? Y ph. D31 80 OC O.K B ph. D51 82 OE O.K
Neutral D71 81 OD=OC Doubt? D11+ D31 83 OC O.K D31+ D51 81 OD
O.K D11+ D51 82 OE O.K D71+ D11 81 OD Doubt? D71+ D31 140 OG=3OC
Doubt? D71+ D51 142 OH=3OE Doubt?
Conclusion of the readings From the readings of metering core as
described in the table, it got concluded that R phase secondary CT
terminals might have been shorted.
R
Y
B
E
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So the detail physical connections of the R phase were checked.
But no such short circuiting of the R phase was found. Instead of
short circuiting, mixing of R phase winding was observed with other
core of same phase as like shown in fig-8. The analysis was
confirmed by drawing the vector diagram (Ref 8-A).
Another core D11 Metering Core D31 D51 D71 Fig- 8 A F B O E C H
D G Fig- 8-A
Case Study No 3.
Situation/problem One 33 KV feeder was tripping on E/F relay
frequently for the load current more than approximately 45 Amp.
Line CTR = 200/1, Setting of E/F PSM= 0.1 Steps attempted Load
current was restricted to 30 ampere for measurement of secondary
current in the Back up core. The currents are measured and
tabulated as in Table-9
Table-9 Phase /
Wire No. Current in
m Amp. Remarks
R ph. C11 184 Doubt? Y ph. C31 151 O.K B ph. C51 153 O.K E/F C71
38 Doubt?
C11+ C31 172 Doubt? C31+ C51 152 O.K
C11+ C51 176 Doubt?
Conclusion of the readings From the readings of Back-up core as
described in the table, it got concluded that R phase CT is
associated with wrong CTR, may be due to saturation of core or
wrong primary connection.
The detail secondary circuit of R phase was checked and found
O.K. But on physical verification it was found with the carbonized
opening of one link on Primary side of the CT (Ref. Fig 10) P1 C2
Carbonized opening Resulting CTR error. C1 C4 Fig- 10 C3 C6 C5 P2
5.3.4. Detail analysis The carbonized opening between the link C6
and P2, due to loose contact and sparking, has developed an
erroneous CTR. For correct connection of links, the CTR is 200/1
with equivalent resistance (R ohm). But opening of one link as
shown in fig-10, has resulted the rise of equivalent resistance
(1.5 R ohm) Since I1
2R1 = I2
2R2
200
2 x R = I2
2 1.5R and I2 = 163.3
So, new ratio becomes 163.3/1 instead of 200/1. The CT link was
replaced by a new one.
Case Study No 4.
Situation/problem One newly commissioned 220/132 KV Auto
transformer was tripping in REF relay for the external fault on any
132 outgoing feeder. HT CTR = 300/1, LT CTR=600/1.
Steps attempted During load condition of the transformer, the
currents on various windings were measured. The values were
obtained as mentioned in Table 10. Table-10
Phase / Wire No.
Current in m Amp.
Remarks
HT R ph. C11 450 O.K HT Y ph. C31 454 O.K
HT B ph. C51 455 O.K HT E/F C71 18 O.K REF circuit 21 Doubt?
Sec. NCT 12 O.K
Detail analysis
E
Y
B
R
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Net REF current = Residual current Secondary NCT Current
Residual current = (Secondary 220 KV current ~ Secondary 220 KV
current)
Residual current = (18~18 x 220/132) = 12 mAmp So, Net REF
current = (12 12) mAmp For additive REF current = (12 + 12) = 24 m
Amp. For subtractive REF current = (12 - 12) = 0 m Amp
As measurement value comes 21 mA, so suspecting the reverse
connection of NCT secondary terminals, the polarity was changed.
But the problem was not solved, So the detail circuit was checked
and found with unequal CTR for HT and LT. The residual path of the
REF was connected from the Aux. CT connection of differential
core.
Rectification and modification 1. The differential core was
separated from
the REF circuit. 2. Separate core was used for REF circuit
and
same CTR was used for all the windings (HT CTR = 300/1, LT CTR =
300/1, NCTR = 300/1). For connection Refer Fig. no. 11
LT CTR = 300/1
HT CTR= 300/1 NCTR= 300/1 REF RELAY Fig. 11
Case Study No 5
5.5.1 Situation/problem During stability test (Load balancing
test) of a 132/11 KV transformer, the following currents were
obtained in differential circuit. Auxiliary CTs are used on both
side of the CT secondary winding.
HT side LT side Operating coil Phase Currents in m Ampere
R phase 27 23 12
Y phase 27 0 27 B phase 27 23 12
5.5.2 Steps attempted From the readings as obtained it got
concluded that 1. HT side CT connections are correct in nature.
2. LT side CT connections are expected with faulty connection. Y
phase terminals might have been shorted with other connection.
Physically the LT CT secondary side was checked and found with a
wrong connection link of both S1 terminal of Y phase and B phase on
the primary side of Auxiliary CT as like shown in Fig. 12.
Wrong Connection Link S1 LT Side R Transformer Y Winding B S1
Diff. RELAY. S1 S1 S1 Aux. CT HT CT Secondary
Fig. 12 5.5.3 Analysis of Current flow. For the above wrong
connection, the current flow in different circuits are explained
below (Fig-13)
x x/2 x/2 3x 1.5x
3x 3x 1.5x
IR0 Fig-13
5.5.4 Vector Analysis of Current flow. The current on operating
coils of the differential relay are obtained due to the combination
of currents from both HT and LT side secondary values. The values
are analyzed in the table 11
Table-11 Phase HT side LT side Operating coil R phase BA =3x
( 300 )
AC=1.5x ( 180
0 )
BC= DB/2 ( - 90
0 )
Y phase DB =3x ( - 90
0 )
0 DB =3x ( - 90
0 )
B phase AD =3x ( 150
0 )
DE=1.5x ( 180
0 )
EA= DB/2 ( - 90
0 )
R
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E A O C D B
Fig- 14
5.5.5 Remarks on Stability Test for Transformer. The transformer
secondary circuits contain both star and delta connections. So
behavior of current flow and analysis of the same becomes difficult
for delta connection. So for easy study of the current behavior,
single phasing supply connection should be used instead of three
phase connection.
6. FAULT FINDING STUDY FOR STAR CONNECTED CT CIRCUITORY
Sl Current in the CT secondary Expected Faults
1 R=Y=B= x Amp, N= 0 Amp No Fault in the circuits R=Y=B= x Amp,
& N= 2x Amp ANY ONE OF THE PHASE CT POLARITY
REVERSED
i. If (R+Y ) = ( Y + B ) = 3x, & ( B + R )= x Y PHASE
REVERSED ii. If (R+Y ) = ( B + R ) = 3x &( Y + B ) = x R PHASE
REVERSED
2
iii. If (Y+B ) = ( B + R ) = 3x &( R +Y ) = x B PHASE
REVERSED I. If R = 0 Amp, & Y = B= N = x Amp Then Check for all
other R phase CT secondary cores, if values obtained in same
pattern, then
R PHASE PRIMARY SIDE OPEN
II. Similarly for Y phase and B Phase also. CORRESPONDING PHASE
PRIMARY SIDE OPEN III. If R = 0 Amp, & Y = B= N = x Amp For
only in One core, Then
R PHASE SECONDARY IS SHORTED ( OR ) R PHASE IS MIXED WITH OTHER
CORES ( OR ) WITH USE OF AUX. CT, ANY ONE OF THE SIDE MIGHT BE
SHORTED.
3
IV. .Similarly for Y phase and B Phase also. CORRESPONDING PHASE
4 R=Y=B= x Amp, N= 3x Amp All phases have been connected to one CT
only
instead of different cores as 1st, 2nd
3rd cores etc. As R phase cores and Y phase cores and B phase
cores. OR Primary Side has been connected from a Single Source
I. R=Y= x/2 Amp, B= x Amp, N= 0 Amp R & Y phases of CT
Secondary similar polarities have been shorted.
II. Y=B= x/2 Amp, R= x Amp, N= 0 Amp Y & B phases of CT
Secondary similar polarities have been shorted.
5
III. B=R= x/2 Amp, Y= x Amp, N= 0 Amp B & R phases of CT
Secondary similar polarities have been shorted.
I. R= x Amp. , R=B= 0 Amp. N=x Amp. Y & B phases of CT
Secondary have been shorted. II. Y= x Amp., Y=B= 0 Amp. N=x Amp. B
& R phases of CT Secondary have been shorted.
6
III. B= x Amp., R=Y= 0 Amp., N=x Amp R & Y phases of CT
Secondary have been shorted. 7 R=Y=B=N=0Amp All the 3 CTs are
shorted. 8 If the values are resulted other than the
above readings as described. 1. CTR may be different. 2. Wrong
primary link connection. 3. Phase angle problem. 4.CT saturation
problems
7. CONCLUSION The study of CT circuitry depends upon the
fundamental approach of the vector analysis of the currents from
three phase supply source. Combination of the vector analysis and
study of the current value measurement in different circuits
provides the conclusive idea regarding the faults in the system.
The various case studies as described above are the physical
examples of the practical
occurrences. Moreover the table described with the faulty
finding study for star connected circuitry becomes quite helpful to
trace the possible faults directly, without the study of vector
analysis and fundamental approaches.
