CT design aspects - Nageswar-6

1. M.Nageswar Rao,
Sr.Manager (Engg.)
NESCL, Noida
Date: 16.08.13

2. Presentation Layout
PROTECTION
SCHEMES
CURRENT
TRANSFORMER
CT DESIGN
REQUIREMENTS
FOR VARIOUS
PROTECTION

4. Protection schemes
Over current protection
Unit Protection
Differential protection
REF protection
Line differential (Pilot wire)
Distance Protection

5. Diff. protection
Monitors an area limited by CTs which measure
incoming & outgoing currents
Types
High impedance
Low impedance (Biased diff.)

6. High impedance Diff. protection
Scheme used for
Bus bars,
generator windings and
Y-connected or auto
transformer windings.
CTs must be selected with
Same ratio
Same magnetizing curve
(same Vkmin & same Ie at
Vk/2)
Same Rctmax.

7. High impedance Diff. protection
High impd. Busbar diff.
REF protection of T/f

8. High impedance Diff. protection
Line or cable diff. protection with pilot wires

9. Low impedance Diff. protection
For double bus bar protection
Used for
busbar diff. protection
EHV lines
CTs can have different ratios
Bias is used to correct small
ratio mismatch
Larger ratios can be matched
using Aux. CTs

10. Low impedance Diff.
– slope characteristics
Have operating characteristics with pickup increasing with higher through
fault currents. This is defined by a slope of the bias characteristics.
The higher the slope, the larger is the tolerance of the relay to errors and
CT saturation.
Modern numerical relays using special saturation detectors or special
through fault detectors.
Automatic slope adjustment is achieved with the help of modern
numerical relays using special saturation detectors or special through fault
detectors.
low slope is maintained (sensitive differential protection) when
When there is no saturation or
when no through fault is detected,
High slope is maintained (for good stability) when,
severe saturation or
through fault detection.

11. High & Low impedance diff. protection
High impd. diff. Low impd. Diff.
Application •Bus bars,
•Generator windings and
•Y-connected or auto transformer
windings
•Bus bars
•EHV lines
CT Ratio Matching CT ratio to avoid spill current
during healthy state
CTs can have different ratios
CT saturation
voltage
Knee point voltage is of concern. Saturation can be tolerated, hence Vk is not
of much concern.
Routing of CT
connection
All CT connections are looped in the yard
and single cable taken to the relay
CT wires directly to the relay
CT ckt. supervisionDetected by using a 3 phase rectifier relay
to effect the summation of the bus wire
voltages and short the pilot wire from the
affected phase
A current operated auxiliary relay is used to
detect any unbalance sec current for
supervision of the CT ckts. Current setting
of the supvn relay must be less than that of
main diff relay
Cost & space req.Less cost & space. Very costly and space consuming, as it
requires large no. of modules & matching
CTs.

13. Current Transformer
Current
Transformer is an
instrument
transformer which
transforms current
from one level to
another level.
e.g. 1000/1A,
200/5A
Terminal Box
PP
S
S
Insulator
Secondary winding
Primary winding
Core
CB
Bus Feeder

15. CTs – windings & cores
CTs have
1 or more primary
windings (with 1 or more
taps), and
1 or more secondary
windings on different
cores.
•Types of CT cores
•Measuring cores
•Protection cores
•Protection cores for special
applications

16. CT secondary current rating
5A Secondary 1A Secondary
Applications 1.Indoor
switchgear
cubicles
2.Higher primary
current ratings.
Outdoor
When secondary
gets open
low peak voltagehigh peak voltage
Fine turns ratio
adjustment
not possible when
primary rating is
low
Always possible

17. Saturation factor
•Ips/Ipn is called
•Instrument Security Factor (FS) for the measuring CTs, and
•Accuracy Limit Factor (ALF) for the protective CTs.
•These two saturation factors are practically the same,
•FS or ALF = (Vsat/Vrated)*Inom.

18. CT - Knee Point Voltage
CT excitation curve
is the magnetizing characteristic (plot between secondary applied voltage and the
corresponding magnetizing current)
Knee point voltage
Corresponds to the point on excitation curve beyond which an increase of 10% in exciting e.m.f.
produces an increase of 50% in the exciting current
is defined as the point on the excitation curve where the tangent is at 45 degree to the abscissa.
represents the point beyond which the CT becomes non-linear.

20. Metering class Protection class Protection special
class
Application Measuring Protection Unit Protection
CT Selection Ratio Ratio Ratio
Accuracy class
(0.1,0.2,0.3, 0.5,1,3,5)
Accuracy class
(5P, 10P, 15P)
Knee Point Voltage
(V
k
)
Burden (15,20,30VA)ALF (5, 10, 15, 20, 25,
30)
CT Secondary winding
resistance (R
CT
)
corrected to75
O
C
ISF (3.5.7) Burden (15,20,30VA)Ie (Excitation current)
at Vk or a stated % of
Vk.
CT Selection example:e.g.: 2000/1, Class 0.2,
20VA, ISF – 5
e.g. : 5P20, 40VA, ALF-
5
e.g. : 200/1, PS Class,
V
k
> 200V, R
CT
< 2.0
ohms, I
e
< 30mA at
V
k
/4

21. Applicatio
n
IEC 60044-1 IEC 60044-6 IEEE C57.13 / ANSI
Metering0.1,0.2,0.3,
0.5,1,3,5
0.3, 0.6, 1.2
(burden @ p.f. 0.9)
Protection5P, 10P, 15P C100, T100,
C200, T200,
C400, T400,
C800, T800
(burden@ p.f. 0.5)
Protection
special
PX TPS, TPX,
TPY, TPZ

22. CT - Remanance
Remanance flux is the value of flux, that would remain in
the core, 3 mins after interruption of exciting current of
sufficient magnitude to induce the saturation flux.

23. CT
Class
Air gapRemanance Application
TPSNo High upto 85%high impedance circulating
current protection
TPXNo High upto 85%line protection.
TPYsmall Low <10% line protection with auto-
reclose.
TPZLarge Negligible 0% special applications such as
differential protection of
large generators

24. CT specification – ANSI (IEEE Std C57.13- 1993)
CT classes as per ANSI
 
C
CT is furnished with excitation 
characteristics which can be used 
to “Calculate” the CT 
performance.
K
same as C rating but the knee-
point voltage must be at least 70% 
of the secondary terminal voltage 
rating.
T
the ratio error must be determined 
by ‘Test’.
ANSI
Volt at
100A
Burden
(ohm)
C100100 1
C200200 2
C400400 4
C800800 8
•The standard current transformer secondary winding is rated at 5A as per        
ANSI standards. (20times of 5A is max. recommended CT secondary current).

25. CT Saturation
In case of  Rl (lead 
resistance)
1Φ to 
ground 
faults
Two-way
3Φ faults One-way
AC saturation
To avoid saturation, the
CT shall develop adequate
voltage such that
Vx > If (Rct+Rl+Rb)
where,
If = Fault current on CT secondary (Amps)
Rct = CT Secondary resistance (Ohms)
Rl = CT Secondary total lead resistance
(Ohms)
Rb =CT secondary connected burden
(Ohms)

26. CT Saturation
DC saturation
Decaying dc
current introduces
during a fault.

27. CT Saturation - Excursion of flux waveform Φ
Is well within the saturation
limits with AC current waveforms
shoots past the saturation limits
quickly with DC transients

28. CT Saturation
CT shall have enough capacity to
develop the following voltage
not to saturate at all for a
combination of AC and DC
transient.
Vx > If (1+X/R) (Rct+Rl+Rb)
Saturation due to DC transient
distorts the AC waveform
output as well

29. CT saturation – how to avoid
CT saturation can be avoided
By increasing the CT ratio (thereby reducing actual
secondary current during fault to less than 100A)
By reducing the secondary connected burden
by reducing the connected relay burden,
reducing the lead resistance (by either
reducing the distance between the relay to the CT,
multiple parallel runs of CT leads,
thicker wire size etc.)
Most of the faults are ground faults which tend to
have lesser DC offset and associated saturation
issues. The ground faults tend to have more
resistance (lower X/R ratio)

31. ProtectionCurrent
demand
Operati
ng time
Transient
saturation
AC
saturation
Remarks
Time OC 20-30 In NO YES
High-set
Phase or
Ground OC
1 cycleYES YES high speed of
Operation is to be
ensured.
Distance
Protection
1.5 In YES NO Saturation is accepted
after the operation of the
Zone-1 operation.
Differential
protection
(Biased)
YES Saturation voltage is of
concern
Differential
protection
(High
impedance)
1 cycle knee point voltage
rather is of concern

32. High Impedance Diff.
Protection
setting VR >K x If x (RL + RCT ) (Volts)
If = Secondary Fault current (Amps)
RL = CT secondary lead resistance (Ohms)
RCT = CT secondary resistance (Ohms)
K = Margin Factor (=1 for full saturation)

33. CT requirements
-for various Protection applications
High Impedance Differential scheme
Vk≥2.If.(Rct+2.Rl)
R
CT
= CT secondary winding resistance
R
lead
= lead resistance of the farthest CT in parallel group
I
f
= Maximum through fault current up to which relay should remain stable (referred to CT secondary)
Biased Differential scheme
Vk≥ K.2.IR.(Rct+2.Rl)
I
R
= Relay rated current
K = Constant specified by the manufacturer usually based on conjunction test (the constant is usually
chosen to ensure positive operation of highest differential unit on severe internal fault with extreme
CT saturation)
Distance Protection scheme
Vk≥ (1+X/R).If.(Rr+Rct+n.Rl)
X/R = Primary system reactance/resistance ratio (to account for the DC component of the
fault current)
I
f
= Maximum CT secondary current for fault at zone1 reach point
Z
relay = Relay ohmic burden
R
CT
= CT secondary winding resistance; nR
lead
= Lead resistance

34. If limited by
TransformerMaximum through fault
current
Z1%
Busbar Maximum through fault
current
switchgear breaking capacity
Generator Maximum through fault
current
X
d
”
Motor Maximum starting current 6x load current for DOL
Motors
Shunt reactorsMaximum charging current X
Short feedersMaximum through fault
current
for fault at busbar