NATIONAL POWER TRAINING INSTITUTE OF NIGERIA

INSTRUMENT TRANSFORMER

 Basic Principles Of Instrument transformers

 Importance of Instrument transformers in protection

 Types of Instrument transformers

 Instrument Transformers Why is it Needed?

 Reduce the voltage or current into values that

other equipment can handle.
 Isolate the equipments from the power network
(i.e. we need more insulation)
 To provide possibilities of a standardization of
instruments.
Protection System Analogy

Brain
Relay

Eyes, Ears, Nose &

Skin CTs, VTs

Hands & Legs

Circuit
Breakers

3/4/2013 7:29:24 PM 3
Protection System Analogy

Fault in the Power

System

Sensed by Instrument
Transformers &
communicated to
Relay
Relay Issues Trip
Command To
Breaker
Breaker Trips &
Clears
3/4/2013 7:29:24 PM Fault 4
 Instrument Transformer Accuracy

• Measuring CTs are required to be very accurate

over normal working range of current while protective CTs
accuracy is not important because they are meant to
saturate at maximum fault current.
• For metering purpose, it is desired that the CT should be
very accurate as the metered data are used for tariff
purpose.
 Factors Affecting Instrument Transformer
Accuracy
1. Design – This is as result of inherent error.
2. Circuit condition – These are conditions such as voltage
level, current level and system frequency.
3. Burden – This is the impedances of the total load
connected to the CT or VT secondary circuit at rated
secondary current. It is usually expressed in VA or Ω
 Burden instrument transformer consists of :
1. Secondary winding of the instrument transformer
2. Interconnecting lead
3. Relay and/other equipment
 The connected burden should be less than rated burden.
 Types of Current Transformer
• Current transformers are broadly classified into two
based on:
1. Construction – This is further classified into four (4)
types namely: Wound type, Bar type, Bushing type and
Window type.

2. Application - This is further classified into two (2)

types namely: Metering CT and Protection CT .
 Definition of Terms in a Current
Transformer
• Primary rated current – This is specified by the
manufacturer according to some international standards
either ANSI, BS or IEC. The standard values of primary
rated currents are: 5, 10, 15, 20, 30, 60, 75, 50, 100, 150,
200, 300, 400, 600, 800, 1000, 1500, 2000, 3000, 4000
and above.
• Secondary rated current - This is specified by the
manufacturer according to some international standards
either ANSI, BS or IEC. The standard values of secondary
rated currents as per BS 3938 are 5A, 2A and 1A and as
per IEC; 5A or 1A.
 Cont’d

• Choice of CT secondary rated current – Depends on:

1. Location of the CT and
2. Connected cable or lead burden.
• 5A secondary current– This is preferred for an indoor
installation where lead burden is insignificant and
insulation is a serious issue. E.g. switchgear cubicles with
closely located relays.
• 1A secondary current - This is preferred for an outdoor
installation where lead burden are high and insulation is
insignificant.
 Cont’d

• Current transformer Ratio – This simply the ratio of primary rated

current to the secondary rated current. Example: 600/1, 200/5 etc.
• Short time rated thermal Current (Ith) - This is the rms value of the
primary current which the C.T. will withstand for one second
without suffering any internal damage with the secondary core
being shorted.
Ith​= Isc  * √[t + 0.05 x 50] KA rms.
Where     f 
​Ith - Rated short time thermal current for 1 sec.
Isc - Short circuit current at C.T. location in KA rms
t - short circuit duration in sec.
f - Rated system frequency.
For system frequency of 50 Hertz
Ith​= ​Isc √[t + 0.05] KA rms.
 Cont’d
• Dynamic rated current - This is the peak value of the
primary current which the C.T. will withstand without
suffering any internal damage with the secondary core
being shorted.
I dyn​=​2.5 Ith

• Accuracy class – The accuracy class specified by BS & IEC

standard are X, 5P, 10P, 0.2, 0.5, 1, etc. Class X, 5P, 10P is
for protection while Class 0.2, 0.5, 1 is for measurement.
• Accuracy Limit Factors (ALF) – This is the ratio of maximum
fault current to the rated primary current. The standardized
accuracy limit factors are 5, 10, 15, 20 and 30.
 Cont’d
• Instrument Security Factor (ISF) – This is the safety factor
or value which determines the safety of the connected
instrument (meter) when a fault current flows through the
primary.
• Instrument security factor is important only for metering
CT.
• The standardized ISF values according to IEC standards are
3, 5, 7 and 10.
• The best practice is to keep the ISF value of a CT low in
order to protect the connected instrument and ensure
high accuracy.

Instrument safety current = Rated prim. Current * ISF value

 Case Study 1
• Given a CT of 600/5A, what is the instrument safety
current of the CT if the ISF value is 5 and what happens to
the metering core if a current of 10kA flows through its
primary.
 CT Burden Calculation

ZT = RCT + RL + ZB

ZT = Total burden in ohms (vector summation of

resistance and inductance components)

RCT = CT secondary resistance in ohms @75 deg C (DCR)

RL = Resistance of leads in ohms (Total loop distance)

ZB = Device impedance in ohms

 Case Study 2

100:5 C.T. Secondary Winding Resistance (DCR) = .062 ohm

Resistance of Cable from C.T. to Relay and back = .1 ohms
Resistance of Relay Coil = .02 ohms

.062 .02

.1

If we have a fault of 2,000 amps and the C.T. ratio is

100:5. What is the knee point voltage?
 Case Study 3

The resistance of the C.T. secondary circuit, or C.T. burden is:

C.T. Secondary Winding Resistance = 1Ω
Resistance of Cable from C.T. to Relay and back = 2Ω
Resistance of Relay Coil = 2Ω
If the fault current is 12,000 Amps, and the C.T. ratio is
1200:5A. For the C.T. to operate with good accuracy, without
saturating before the maximum fault current, the knee point
voltage will be what?
 Types of CT Test

1. Polarity test

2. Excitation test

3. Insulation Test

4. Continuity Test

5. Ratio Test
CT Characteristics Curve Cont’d
CT Typical Excitation Curve
Voltage Transformers
 What is Voltage Transformer
• 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 is not possible as
devices used for measurement of voltage are not
designed to handle such high level of voltage
Why Voltage Transformer is Required

• Power system has two

basic requirements for VT:
1. Metering of energy
sourced or consumed.
2. Protection of the electrical
system from faults and
disturbances
 Cont’d
• 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, the power or impedance of the
system along with its direction can be computed.
• Moreover O/V, U/V, O/F, U/F and over fluxing
protections are also configured from VTs.
• Voltage signal is also used for synchronizing, Disturbance
recorders and event logs.
How Voltage Transformer is connected
• VT has a primary and one or more secondary
windings
• Metering and Protection devices are connected to the
secondary core 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.
 Voltage Factor
• Voltage factor determines the maximum operating
voltage for a VT expressed in per unit of rated
voltage which depends on the system and VT’s earthing
conditions.
• VTs used in non‐effectively earthed system have high
voltage factor since in the event of an earth fault in
one of the phases, the healthy phase voltage may rise
to phase to phase value.
Voltage Duration Earthing conditions
Factor VF V.T. System
primary
winding
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 8h Earthed Isolated neutral or resonant earthed
without automatic E/F tripping
 Protection of Voltage Transformer
• Protection of EVT from accidental overloads and short
circuit across its secondary terminal is achieved by
incorporating fuses or MCB in the secondary circuit of
the marshaling kiosk located near to the VT as
possible.
• Normal secondary current is not more than 5A but short
circuit current is in the range of 100A, therefore fuses can
be employed.
• High voltage fuse on primary side do not protect
transformer, they protect only network in case of any
short circuit on the primary side.
 Voltage Transformer Accuracy
• Metering: 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, high accuracy are required 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 .
 Voltage Transformer Connection
• There are three types of connections
1. V‐V connection
2. Star/Star connection
3. 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 cannot
be produced.
 Cont’d
• Star/Star connection
‒ Either 3 separate single phase
VTs or a single 3 phase, 3 limb VT
is used.
‒ Both primary and secondary 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.
 Cont’d
• Star/Open Delta connection
– Primary windings are connected in
star with star neutral solidly
grounded and the secondary 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
polarizing directional earth fault
relays or for earth fault detection in
non‐effectively grounded or
isolated neutral system.
 Types of Voltage Transformer
• Types of Voltage Transformer (VT)
1. Electromagnetic Voltage Transformer (EVT)
2. Capacitive Voltage Transformer (CVT)

M
P M

P P
P

INDUCTIVE VOLTAGE CAPACITIVE

TRANSFORMER VOLTAGE
TRANSFORMER
 Cont’d
• 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
• 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.
Capacitive Voltage Transformer
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