The document discusses current transformers (CTs) and voltage transformers (VTs), which are used to transform high voltages and currents in power systems to safer levels for metering and protection purposes. It provides information on selecting appropriate CT ratios and classes for metering or protection applications. It also covers VT connections, types including electromagnetic and capacitive VTs, and accuracy considerations for both instrument transformers.

1. Selection of
Current Transformers

2. Protection System Analogy
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Brain Relay
Eyes, Ears, Nose & Skin
CTs, VTs
Hands & Legs Circuit
Breakers

3. Protection System Analogy
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Fault in the Power System
Sensed by Instrument
Transformers &
communicated to Relay
Relay Issues Trip
Command To Breaker
Breaker Trips & Clears
Fault

4. Current Transformer Secondary Rating
• switchgear cubicles with closely located relays)
• Preferred where primary current ratings are very high
• Comparatively low peak voltage when secondary
gets open
• Fine turns ratio adjustment is not possible when primary rating
is low
– 1A Secondary
• Preferred when CTs are outdoor and lead burden are high
• Comparatively high peak voltage when secondary is open
• Fine turns ratio adjustment possible
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• Choice of CT secondary rating
– 5A Secondary
• Preferred where lead burden is insignificant (e.g. indoor

5. Current Transformer Accuracy
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• Measuring CTs are required to be accurate
over normal working range of current, while
protective CTs required to maintain the accuracy up
to several times of the rated current
• Metering if we want to measure current for metering
purpose, we desire that
➢whatever
accurate as the metered data may be used for tariff
current we measure, that should be very
purpose
•Accuracy Class
A designation assigned to a current transformer,
the errors of which remains within specified limits
under prescribed condition of use

6. Classification of Current Transformer
• Metering Class CTs
1. class : High precision testing
2. class : Laboratory class
0.5 class : industrial metering
1.0 class : First grade indicating wattmeter
3.0 & 5.0 class : For general use/WTI
• Protection Class CTs
–5P
, 10P
, 15P
–PS class
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7. Measuring Current Transformer
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• Designation of Metering CTs
Metering CTs are specified in terms of –
Ratio, Accuracy class, Burden (VA
(Instrument Security Factor)
Example: 2000/1, Class 0.2, 20VA, ISF – 5
rating), ISF
• Standard Error Class – 0.1, 0.2, 0.5, 1.0, 3.0, 5.0
•The errorsare specified between 5‐120%
of rated current and 25‐100% of rated burden
connected
•Higher errors are permitted at lower currents

8. Current Transformer Accuracy Limits
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Metering Cores
• IEC 60044‐1 Limits of error for accuracy Class of metering cores
Class 5% of
rated I
20% of
rated I
100% of
rated I
120% of
rated I
0.2 0.75 0.35 0.2 0.2
0.5 1.5 0.75 0.5 0.5

9. Current Transformer Accuracy Limits
• IEC60044‐1 has laid down standards on this
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10. Instrument Security factor (ISF)
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• The instruments connected to the secondary of a CT should
be protected from getting damaged during primary fault
condition, when primary current is many times higher than the
rated value, the core should get saturated
• For this purposes, Instrument Security Factor (ISF) for
Metering CTs has been defined
• The CT cores should be such that it saturates at its
instrument security factor (ISF) for safeguarding the instrument
from getting damaged under fault current condition
• ISF is defined as the ratio of rated instrument
security primary current to rated primary current
• ISF is expressed as 3,5,7 or 10 (it shall be chosen
as small as possible)

11. Protection Current Transformer
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• Protection Class
• During fault condition, value of primary current may
be 10 to 20 times the rated primary current
• Here, main requirement is ability of CT to faithfully
transform the primary current during fault condition
• At such high level of primary current, if CT is
not properly designed, it may saturate and relay
will receive very less current and, therefore,
would not make right decision

12. Protection Current Transformer
• Designation of Protection CTs
• Protection CT are specified in terms of –
• Ratio, Accuracy class, Burden (VA rating), ALF
(Accuracy Limit Factor)
• Example: 200/1, 5P20, 10VA
• Standard Error Class/ALF/VA rating
• – Error Class 5P, 10P, 15P
• – ALF 5, 10, 15, 20, 25, 30
• – VA rating 5, 10, 15, 30
• The errors are specified at rated current and
ALF times rated current with rated burden
connected 3/4/2013 7:29:24 PM
40

13. Protection Current Transformer
.

14. Current Transformer Accuracy Limits
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Protection Cores
• BS 3938:1973 Limits of error for accuracy Class 5P and 10P
Accuracy Current Error Phase displacement Composite Error
Class at rated error at rated at rated accuracy
Primary Primary Current limit (ALF)
Current Primary Current
5P ±1% ±60 min ±1.8
centiradians
±5%
10P ±3% - - ±10%

15. Accuracy Limiting Factor (ALF)
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• Unlike measuring CTs, which are required to be accurate
over the normal working range of currents, protective CTs are
usually required to maintain their ratio up to several times
the rated primary current
• At some value of primary current above the rated value,
core commence to saturate, resulting in increase in
secondary current error
• Protection Class CTs cores should not get saturated below
its Accuracy Limiting Factor (ALF) up to which the primary
current should be faithfully transformed to the secondary
side, maintaining the specified accuracy
• ALF is defined as the ratio of the rated accuracy limit
primary current to the rated primary current

16. Protection Current Transformer
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• For a given CT, VA and ALF are inversely related, for example, if
connected burden is less than rated then ALF would increase
• Applications of this CT are Over current
relay, Inverse relay, earth fault protection, Phase
fault protection etc.
• While selecting 5P10 class CT for IDMT O/C or Earth fault relays
– CT should have optimum ALF/VA rating, so that they do not
saturate up to at least 20 times current rating (either by
selecting low burden relays or by selecting a ratio
of appropriate high value)
– Over rated CTs having high VA rating and ALF may
produce high secondary currents during severe faults (in
excess of 20 times setting) that may cause thermal
stressing of relay current coils and eventual failures

17. Protection Current Transformer
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• Designation of Protection CTs for special applications
For protection like circulating current differential, restricted
earth fault etc. where balanced of current/turns is required
between associated CTs with close tolerance
Special class Protection CT of are specified in terms of –
1) Ratio
2) Accuracy class
3) Knee Point Voltage (Vk)
4) CT Secondary winding resistance (RCT) corrected to75OC
5) Excitation current (Ie) usually at Knee Point Voltage or a
stated percentage thereof
Example ‐ 200/1, PS Class, Vk > 200V, RCT < 2.0 ohms, Ie < 30mA at Vk/4
• The turn ratio error are limited to +0.25% which helps
in maintaining balance between the protection system
during maximum through fault condition

18. VoltageTransformers

19. What is Voltage Transformer
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• Voltage Transformer is an
instrument transformer which
transforms voltage from one level to
another level such as
330KV/√3:110V/√3 (VT ratio)
i.e. transforms voltage from the
level of 330KV/√3 into voltage of
110V/√3 level
• Direct measurement of high voltage (in
the tune
possible
of 3.3kV or more) is not
as devices used for
measurement of voltage are
not designed to handle such high
level of voltage

20. Why Voltage Transformer is Required
•System has two basic requirements
➢metering of energy sourced or
consumed
➢protection of the electrical system from
faults and disturbances
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21. Why Voltage Transformer is Required
•Faults can be of many kinds, some faults such
as O/C can be detected solely on current
measurement, but current does not provide
discretion about nature and location of the
fault
•Therefore, when voltage is also measured
along with current during faults, we can in a way
compute power or impedance of system along
with its direction
•Moreover O/V, U/V, O/F, U/F and over
fluxing protections are also configured from VTs
•Voltage signal also used for synchronizing,
Disturbance recorders and event logs
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22. How Voltage Transformer is connected
•VT has a primary and one or more
secondary windings
•Metering and Protection devices are
connected to the secondaries of the VT
•In voltage operation or shunt mode, the
primary winding is connected in parallel
with the power system to transform the phase
voltage to usually 63.5 volts suitable for the meter
or relay
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23. Voltage Transformer Theory
• For a transformer in no load the following is valid
Voltage transformation is proportional to the ratio of primary
ndar
E1

N1
E 2 N 2
• An ideal voltage transformer is a transformer under no‐load
conditions where the load current is zero and the voltage drop is
only caused by the magnetizing current and is thus negligible

24. Voltage Transformer Theory
• Ratio error, which is defined as the difference in
magnitude of the primary and secondary voltage
expressed as percentage of primary voltage
p
100
V .K V
Voltage(Ratio) Error  s n
V p
Kn= Rated
transformation ratio Vp = Actual primary voltage Vs =
Actual secondary voltage
• Phase Angle error
is the difference between the reversed
secondary and the primary voltage vectors

25. Voltage Factor
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• Voltage factor determines the maximum operating voltage for voltage
transformers expressed in per unit of rated voltage, which in turn
dependent on the system and voltage transformer earthing conditions
• VTs used in non‐effectively earthed system have high voltage factor since in
the event of an earthed fault in one of the phases, the healthy phase
voltage may rise to phase to phase value
Voltage
Factor VF
Duration Earthing conditions
V.T. primary
winding
System
1.2 Continuous Non‐earthed Effectively or non‐effectively earthed
1.5 30 s Earthed Effectively earthed
1.9 30 s Earthed Non‐effectively earthed with
automatic E/F tripping
1.9 8 h Earthed Isolated neutral or resonant earthed
without automatic E/F tripping

26. Protection of Voltage Transformer
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• Protection of EVT from accidental overloads and short
circuit across its secondary terminal is achieved by incorporating
fuses or MCB in secondary circuit located near to
transformer as possible
• Normal secondary current is not more than 5A and short circuit
current in the range of 100A, simple fuses can be employed
• Short circuit on secondary winding gives only a few amperes in
primary winding and is not sufficient to rupture a high voltage
fuse at primary side (HRC fuses on primary side up to 66kV)
• Hence high voltage fuse on primary side do not protect
transformer, they protect only network in case of any short
circuit on the primary side
• CVT invariably solidly connected to the system so that there is
no primary protection

27. Voltage Transformer Accuracy
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• As stated for CT, we need it for
➢Meterin
g
voltage measurement, energy, power
measurement
➢Protection for distance protection, O/V,
U/V, O/F and U/F protections, field failure,
over‐fluxing etc
•For metering VTs we need high accuracy in the
voltage measurement during stable conditions
i.e. 80% to 120% of nominal system voltage
with burdens from 25% to 100% of rated
burden at power factor of 0.8 lagging
•Combination of magnitude and phase error
depends on the power factor of the burden

28. Voltage Transformer Accuracy
• IEC 60044‐2 and 60044‐5 defines this as
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80

29. Voltage Transformer Accuracy
• For Protection VTs we need faithfulness of
voltage measurement in the higher range of
voltage such as from value as low as 2% of
nominal voltage to the rated voltage multiplied
by rated voltage factors such as 1.2, 1.5,
1.9 with burden of 25% to 100% of rated
burden at 0.8 pf lagging
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30. Voltage Transformer Accuracy
• IEC 60044‐2 and 60044‐5 defines this as
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31. Voltage Transformer Connections
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• There are three types of connections
– V‐V connection
– Star/Star connection
– Star/Open delta connection
• V‐V connection
– Used for measurement and for those protections which
do not require phase to neutral voltage input (2 VTs are used)
– Primary of VTs is connected in V (one VT primary across R‐Y
phase and other across Y‐B phase) with identical
V connection for the secondary
– In this connection zero sequence voltage can not be
produced

32. Voltage Transformer Connections
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• Star/Star connection
– Either 3 separate single phase VTs
or a single 3 phase, 3 limb VT is
used
– Both primary and secondaries
are connected in star with
both star neutrals solidly
grounded
– Each primary phase limb is
thus connected between
phase to earth of the supply
circuit and replicate similar
phase to earth voltage on the
secondary

33. Voltage Transformer Connections
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• Star/Open Delta connection
– Primary windings are connected in
star with star neutral solidly
grounded and the secondaries
are connected in series to
form an open delta connection
– This type of connection is called
residual connection and require
either 3 single phase VTs or
a single 3 phase 5 limb VT
– This residual connection is used for
polarising directional earth
fault relays or for earth fault
detection in non‐effectively
grounded or isolated neutral
system

34. Types of Voltage Transformer
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• Types of Voltage Transformer (VT)
• Electromagnetic Voltage Transformer (EVT)
• Capacitive Voltage Transformer (CVT)
M
P
P
M
P
P
INDUCTIVE VOLTAGE
TRANSFORMER
CAPACITIVE VOLTAGE
TRANSFORMER

35. Types of Voltage Transformer
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• Electromagnetic Voltage Transformers similar to a small power
transformer and differs only in details of design that
control ratio accuracy over the specified range of output,
cooling (output not more than 200‐300 VA), insulation
(designed for system impulse voltage level) and mechanical
aspects
• At high system voltages the cost of conventional potential
transformer is high, due to prohibitive cost of insulation,
hence, at 132 kV and higher voltages, CVT may be more
economical than EVT particularly when the high voltage
capacitors can serve also for carrier current coupling
(PLCC), but may be inferior in transient performance
• Capacitors allow the injection of high frequency signals onto
the power line conductor to provide end‐to‐end
communications between substations for distance relays,
telemetry/supervisory and voice communication

36. Capacitive Voltage Transformer
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Definition
A CVT is a voltage transformer comprising of capacitor
divider unit and an electromagnetic unit so
designed and interconnected that the secondary
voltage of the electromagnetic unit is substantially
proportional to and in phase with the primary voltage
applied to the capacitor divider unit (IEC 186)
What does a CVT do?
•Inputs to measuring and protection devices
•Galvanic isolation
Main Parts of a CVT
• Capacitor Part ‐ Capacitor Stack, Insulator
- PT, HV Choke, FR circuit
• Electromagnetic Unit

37. Why HV Choke is required
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• L is variable
inductive choke used
for phase angle C1
error correction
•It is tuned to
resonate with C
(=C1+C2) at nominal
power frequency
C2
Up
L
Us
R
Wound PT
• Wound PT is used to increase the available output power, for
a given maximum error limit and C1

38. Equivalent Circuit Diagram of CVT
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• Leq is the sum of choke
inductance and leakage
inductance of the U1
I1
C1
wound PT
•Magnetizing
inductance of the PT is
neglected
•It can be seen that the
choice of a suitable
value of L tends to
reduce the phase angle
error
Up
Leq I
+ UL -
Us
C2 R
U2
I2
Wound
PT

39. • A practical CVT consists of capacitance, tuning inductance and
wound PT which is having exciting impedance of non‐linear
characteristics
• Whenever a capacitor and non‐linear inductor are connected in
series, there is a danger of non‐linear energy interchanges
at sub‐harmonic frequencies and causes sustained oscillation
and consequently large overvoltage in the circuit
• Such oscillations are less likely to occur when the losses in the
circuit are high, hence resistive load is increased in CVT (it also
impair the transient response)
• To avoid Ferro‐resonance the operating flux of iron parts is kept
at 1/2 to 1/3rd of the saturation flux density, which
prevents high exciting currents during circuit transients
• Alternately a special provision for damping the oscillations
is provided
Ferro-resonance
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98

40. Capacitive Voltage Transformer

41. Coupling Capacitor
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• In Power Line Carrier Communication (PLCC), Coupling
Capacitor (CC) is used as coupling device between
power line and carrier accessories to allow
high frequency (40‐500KHz.) carrier signals
into/out of carrier accessories (Line Matching Unit
(LMU) etc.)
• Some times, the capacitor part in CVT is used as CC in
PLCC
• When CVT is used as CC the terminal HF will be
connected to carrier accessories (carrier coupling unit)
instead of grounding it
100

42. Power Line Carrier (PLC)
equipment
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C1
Wave Trap
>500KHZ NOISE PICKUP
<30KHZ-HARMONIC
LIGHTENING,CORONA
C3 L3
L1
C4
C2
Carrier
oscillator Matching
Transformer
Coupling capacitor
VT
L2
Transmitter
and receiver
fa = 30kHz to 500
kHz