The document discusses various protection schemes and current transformer design requirements for protection. It describes high impedance and low impedance differential protection schemes. High impedance schemes are used for busbars, generators, and transformers, requiring CTs with identical ratios, magnetizing curves, and resistance. Low impedance schemes are used for busbars and lines, allowing some ratio mismatch corrected via biasing. CT requirements depend on the protection application and aim to prevent saturation during faults. Distance protection requires the highest knee point voltage due to zone 1 reach point faults containing DC components.

2. FLOW OF PRESENTATION
PROTECTION
SCHEMES
CURRENT
TRANSFORMER
CT DESIGN
REQUIREMENTS FOR
VARIOUS PROTECTION

3. PROTECTION SCHEMES

4. PROTECTION SCHEMES
 Over Current Protection
 Earth Fault 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
 C T s 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 biascharacteristics.
 The higher the slope, the larger is the tolerance of the relay to errorsand
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
Through fault detection.

11. HIGH & LOW IMPEDANCE DIFF. PROTECTION
High impd. diff. Low impd. Diff.
Application • Bus bars, • Bus bars
• Generator windings and • EHV lines
• Y-connected or auto transformer
windings
CT Ratio Matching CT ratio to avoid spill current
during healthy state
CTs can have different ratios
CT saturation Knee point voltage is of concern. Saturation can be tolerated, hence Vk is not
voltage of much concern.
Routing of CT All CT connections are looped in the yard CT wires directly to the relay
connection and single cable taken to the relay
CT ckt. supervision Detected by using a 3 phase rectifier relay A current operated auxiliary relay is used to
to effect the summation of the bus wire detect any unbalance sec current for
voltages and short the pilot wire from the supervision of the CT ckts. Current setting
affected phase 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
is an
which
Current
Transformer
instrument
transformer
transforms current
from one level to
another level.
1000/1A,
 e . g .
200/5A Terminal Box
P
P
S
S
Insulator
Secondary winding
Primary winding
Core
CB
Bus Feeder

14. CURRENT TRANSFORMER
CONNECTION TYPE

15. CTS – WINDINGS &
CORES
 C T s
have

1
or more primary
windings (with 1 or more
taps), and

1
or more secondary
different
windings on
cores.
• Types of CTcores
•
•
•
Measuring cores
Protection cores
Protection cores forspecial

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 voltage high peak voltage
Fine turns ratio
adjustment
not possible when
primary rating is
low
Always possible

17. SATURATION FACTOR
•Ips/Ipn iscalled
•
•
Instrument Security Factor (FS) for the measuring CTs, and
Accuracy Limit Factor (ALF) for the protective CTs.
•These two saturation factors are practically thesame,
•FS or ALF = (Vsat/Vrated)*Inom.

18. CT - KNEE POINT VOLTAGE
 CT excitationcurve
 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 excitingcurrent
 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 becomesnon-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)
Accuracyclass
(5P, 10P, 15P)
Knee Point Voltage
(Vk)
Burden (15,20,30VA) ALF (5, 10, 15, 20, 25,
30)
CT Secondarywinding
resistance (RCT)
corrected to75OC
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,
Vk > 200V, RCT < 2.0
ohms, Ie < 30mA at
Vk/4

21. Applicatio
n
IEC 60044-1 IEC 60044-6 IEEE C57.13 / ANSI
Metering 0.1,0.2,0.3,
0.5,1,3,5
0.3, 0.6, 1.2
(burden @ p.f. 0.9)
Protection 5P, 10P, 15P C100, T100,
C200, T200,
C400, T400,
C800, T800
(burden@ p.f. 0.5)
Protection PX
special
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 gap Remanance Application
TPS No High upto 85% high impedance circulating
current protection
TPX No High upto 85% line protection.
TPY small Low <10% line protection with auto-
reclose.
TPZ Large 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
CT is furnished with excitation
characteristics which can be used
to “Calculate” the CT
C performance.
same as C rating but the knee-
point voltage must be at least 70%
of the secondary terminal voltage
K rating.
the ratio error must be determined
T by ‘Test’.
ANSI
Volt at Burden
100A (ohm)
C100 100 1
C200 200 2
C400 400 4
C800 800 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)
Two-way
1Φ to
ground
faults
3Φ faults One-way
 A C saturation
 To avoid saturation, the
CT shall developadequate
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 leadresistance
(Ohms)
Rb =CT secondary connectedburden
(Ohms)

26. CT SATURATION
 D C 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
 C T shall have enough capacity t
o
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
• C T SATURATION CAN BE AVOIDED
•  B Y INCREASING THE CT RATIO (THEREBY
REDUCING ACTUALSECONDARY CURRENT
DURING FAULT TO LESS THAN 100A)
•  B Y REDUCING THE SECONDARY CONNECTED
BURDEN
 BY REDUCING THE CONNECTED RELAYBURDEN,
 REDUCING THE LEAD RESISTANCE (BY EITHER



reducing the distance between the relay to the CT,
multiple parallel runs of CT leads,
thicker wire sizeetc.)
 M o s t of the faults are ground faults which
tend t
o
have lesser DC offset and associated
saturation issues. The ground faults tend to have
more resistance (lower X/R ratio)

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

32. HIGH IMPEDANCE
DIFF.
Protection
setting VR >K x If x (RL + RCT ) (V
olts)
 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)
 RCT=CT secondary windingresistance
 Rlead= lead resistance of the farthest CT in parallelgroup
 If= Maximum through fault current up to which relay should remain stable (referred to CTsecondary)
 Biased Differential scheme
Vk≥ K.2.IR.(Rct+2.Rl)
 IR=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 ofthe
fault current)
 If= Maximum CT secondary current for fault at zone1 reachpoint
 Zrelay= Relay ohmicburden

34. If
LIMITED
BY
Transformer Maximum through fault Z1%
current
Busbar Maximum through fault switchgear breaking capacity
current
Generator Maximum through fault
current
Xd”
Motor Maximum starting current 6x load current for DOL
Motors
Shunt reactors Maximum charging current X
Short feeders Maximum through fault for fault at busbar
current