Electrical measurement & measuring instruments [emmi (nee-302) -unit-2]

ELECTRICAL MEASUREMENT & MEASURING INSTRUMENTS (NEE-302)
Md Irshad Ahmad
irshad.ahmad@jit.edu.in
Electrical Engineering Department
LECTURE
on
UNIT –II
Instrument Transformers

CONTENTS
CT and PT their errors
Applications of CT and PT in the
extension of instrument range
Introduction to measurement of speed
frequency
Introduction to measurement of power
factor

What is an Instrument Transformer ?:
 It is a transformer that is used in conjunction with any
measuring instrument (i.e., Ammeter, Voltmeter,
Wattmeter, Watt-hour-meter, …etc.)or protective
equipment (i.e., Relays).
 It utilizes the current-transformation and voltage
transformation properties to measure high ac current and
voltage.

Types of instrument transformers :
These instrument transformers are of two types:-
1. Current transformers
2. Potential transformers

Applications of Instrument Transformers:
 For measurement of high ac current, it is usual to use low range
ac ammeter with suitable shunt.
 For measurement of high ac voltage, low range ac voltmeters are
used with high resistances connected in series.
 For measurement of very high ac current and voltage, we cannot
use these methods. Instead, we use specially constructed HV
instrument transformers to insulate the high voltage circuit from
the measuring circuit in order to protect the measuring
instruments from burning.

 Application of Instrumentation Transformers In dc circuits
for current and voltage measurement, we use low range
dc ammeters and voltmeters with rectifiers connected in
their secondary circuits.
 The vast primary application of Instrument Transformers
is for the protection and control of power system and
power equipment of high and very high ratings.
 The working of these instrument transformers are similar
to those of the ordinary transformers.

Basic Construction and Magnetic Circuit of Instrument Transformers

Polarity of Instrument Transformers

Common connections of instrument transformers

What is current Transformer (CT)?:
 A current transformer is a transformer, which produces in its secondary winding
low current, which is proportional to the high current flowing in its primary
winding.
 The secondary current is usually much smaller in magnitude than the primary
current.
 The design of CT depends on which type of instrument is connected to its
secondary winding. Measuring instrument OR Protective instrument.
-Measuring instrument CT is expected to give accurate results up to a
maximum of 125% of its normal full-load rated current.
-Protective instrument CT is expected to be accurate for up to 20 times of its
normal full-load rated current (about 2000% of its full-load rated current!!..??).
 Based on the type of equipment for which the Ct is used for, its saturation point
will vary. At the same time it is expected to be linear in the entire working range.
Current Transformers

Construction of C.T.:
 C.T. has a primary coil of one or more turns made of
thick wire connected in series with the line whose
current is to be measured.
 The secondary consists of a large number of turns
made of fine wire and is connected across an
ammeter or a relay’s terminals.

Construction Types of Current
Transformers
Window-type Bar-type

Function of CT:
 The principal function of a CT is to produce a proportional
current at a level of magnitude, which is suitable for the
operation of low-range measuring or protective devices such
as indicating or recording instruments and relays.
 The primary and secondary currents are expressed as a ratio
such as 100/5 or 1000/5 .
 With a 100/5 ratio CT, 100A flowing in the primary winding will
result in 5A flowing in the secondary winding, provided that
the correct rated burden is connected to the secondary
winding.

“Class” of a CT:
 The extent to which the actual secondary current
magnitude differs from the calculated value, expected by
the virtue of the CT ratio, is defined as the accuracy “Class”
of the CT.
 The greater the number used to define the class, the
greater the permissible “current error” [the deviation in
the actual secondary current from the calculated value].

Specifications of CT:
CTs should be specified as follow:
RATIO : Input / output current ratio
VA: Total burden (rating) including pilot wires.
Common burden ratings are 2.5, 5, 10, 15 and 30 VA. For
example:
 Moving iron ammeter is 1-2 VA
 Moving coil rectifier ammeter is 1-2.5 VA
 Electro-dynamic instrument is 2.5-5 VA
 Maximum demand ammeter is 3-6 VA
 Recording ammeter or transducer is 1-2.5 VA

CLASS : The accuracy required for the operation
DIMENSIONS: Maximum & minimum limits. For example:
 0.1 or 0.2 for precision measurements.
 0.5 for high grade kilowatt hour meters and commercial
grade kilowatt hour meters.
 3 for general industrial measurements.
 3 or 5 for approximate measurements.
BURDEN (OHMIC): (Depending on pilot lead length)

Current Transformers Stepping:
 These are used with low range ammeters to measure current
in high voltage alternating circuits where it is not practical to
connect instrument and meters directly to lines.
 They are step-up transformers (voltage ratio) because when
we step-up the voltage the current decreases.
 The current is a step-down in a known ratio called the
current ratio .

Connections of Current
Transformers
Single-Ratio
CT
Multi-Ratio CT

Multi-ratio CT:
 As indicated in the previous Figure, current transformers having a center
tapped secondary are referred to as a dual ratio CT.
 Dual ratio CT are used in applications where it is necessary to have
available two ratios of primary to secondary current from the same
secondary winding of the CT.
 This may be accomplished by adding a tap in the secondary winding to
get a second ratio.
 The ratio obtained by the tap is usually one-half the ratio obtained by the
full secondary winding.
 A schematic example is previously shown with 200 amperes flowing in
the primary, a connection X2 – X3 will produce 5 amperes out of the
secondary. As the load grows to 400 amperes, the secondary circuit will
be reconnected to X1 – X3 to still produce 5 amperes in the secondary
circuit.

Working (Measurement):
 If a current transformer has primary to secondary current ratio of 100:5
then it steps up the voltage 20 times and step down the current 1/20 times
of its actual value.
 If we know the current ratio and the reading of an a.c. ammeter, the
primary current can be calculated as:
Primary Current = CT ratio × ammeter reading

Why CT secondary should never be open ?:
 Ammeter resistance is very low ,the current transformer normally works as a
short-circuited instrument.
 If for any reason the ammeter is taken out of secondary winding then the
secondary winding must be short-circuited with the help of a short-circuit
switch.
 If this is not done, then a high m.m.f. (Ampere-turns IT) will set up a high
flux in the magnetic core and it will produce excessive core loss which
produce heat and high voltage across the secondary terminals .
 The high voltage can damage any electronic components in secondary side.
 Hence the secondary of any current transformer should never be left open.

Construction of the DOUGHNUT Type C.T. :
The most common type of C.T. construction is the “DOUGHNUT” type. It
is constructed of an iron toroid, which forms the core of the transformer,
and is wound with many secondary turns.
Secondary Winding Primary Conductor
Iron Core

 The `doughnut' fits over the primary conductor, which constitutes one primary turn.
If the toroid is wound with 240 secondary turns, then the ratio of the C.T. is 240 : 1
or 1200 : 5A
 The continuous rating of the secondary winding is normally 5 AMPS in North
America, and 1 AMP or 0.5 AMP in many other parts of the world.
 This type of `doughnut' C.T. is most commonly used in circuit breakers and power
transformers. The C.T. fits into the bushing, and the porcelain bushing fits through
the centre of the `doughnut'.
 Up to four C.T.'s of this type can be installed around each bushing of an oil circuit
breaker. This arrangement is shown in a following diagram.

Substation Class Circuit Breakers' CTs
Polyester Taped Bushing CT on
Outdoor Circuit Breaker

Primary Current
(100 amps)
Secondary Current
(5 amps)
Primary Current
Secondary Current
Transformer Ratio = _____________________
100
5
___ = 100:5 or 20:1
CT Turns-ratio (TR)

Direction of
Primary Current
Direction of
Secondary Current
H1
X1
P1
IEEE
IEC
Primary
Polarity
Marks
IEEE
IECS1
Secondary
Polarity
Marks
Polarity
Primary current into “polarity” forces
Secondary current out of “polarity”

Direction of
Secondary Current
Direction of
Primary Current
H1
X1
P1
IEEE
IEC
Primary
Polarity
Marks
IEEE
IECS1
Secondary
Polarity
Marks
Primary current into “non-polarity” forces
Secondary current out of “non-polarity”
Polarity

CT Metering Accuracy
Since actual secondary current Rated secondary current
=
The difference in % is known as the
“Accuracy” or “Class”
of the CT
Then:

Burden
Load connected to CT secondary
Includes devices & connecting leads
Expressed in ohms
Standard values = B0.1, B0.2, B0.5, B0.9,
B1.8
E0.04, E0.2

CT accurate 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
RL = Resistance of leads in ohms (Total loop distance)
ZB = Device impedance in ohms

Factors Affecting Degree and Time to Saturation
1. High DC offset
2. High fault current Magnitude (symmetrical current)
Example:
•100 to 5 CT@20 times= 2000 amps.
•Also @20,000 amps we have 200 times CT
3. Low CT Turns Ratios
4. High secondary burden
5. Low CT Accuracy (Class)
6. High reminance flux
• Can occur if current interrupted when core is saturated
• If DC flows in windings during testing
• Need a voltage above 60% of knee point to reduce the Reminance to less than 10% of
saturation flux density.

Tips for Avoiding CT Saturation
1. Use higher ratio CTs
2. Use separate set of high ratio CTs for high fault current
tripping
3. Reduce secondary burden by:
-Selecting low burden relays & meters
-Distributing single phase burdens among phases
-Increasing the size of secondary leads
-Reducing the length of secondary leads
-Using “step down” auxiliary CTs

Avoiding CT Saturation
with Linear Operation
(Knee limit of the Curve)

CT Actual Connections
Typical window CT

Outdoor Type BO7 for Retrofit
Power Transformers’ CT Connections
Slip over current transformer for installation over
exterior of outdoor bushing

Substation Class Circuit Breakers’ CT’s
Outdoor BO7 - Replaces BCT’s
in shielded aluminum housing
Ground Shield

Generators CT Connections
Board Mounted Generator CT

What is a Potential Transformer (PT) or (VT)?:
 A PT or sometimes called VT is a step-down transformer
having many primary turns but few secondary turns.
 In a step-down transformer the voltage decreases and the
current increases, thus voltage can be easily measured by
using a low-range voltmeter instrument.
 The voltage is stepped-down in a known ratio called the
voltage ratio.
Potential Transformers

Construction and working of P.T.:
Construction
 A potential transformer has many primary winding turns but few number of secondary
winding turns that makes it a step-down transformer.
 A Voltmeter is connected to the secondary winding is usually a voltmeter of 150 V.
Working (Measurement):
 Primary terminals are connected in parallel across the line to which the voltage is to be
measured.
 The voltmeter reading gives the transformed value of the voltage across the secondary
terminals.
 The deflection of the voltmeter when divided by the transformed ratio gives the actual
voltage across the primary winding as:
The Line voltage = deflection / transformation-Ratio
Where transformation ratio = V2/V1

Precaution for P.T.:
 Since the secondary of a p.t. is connected to relays, their ratings
are usually 40 to 100 Watts.
 For safety purpose the secondary should be completely
insulated from the high voltage primary and should be in
addition grounded.

Types of P.T. :
Some types of p.t. are:
 Shell type
 Dry type
 Oil type
Rating Type
1. Below 5000 v Shell type
2. 5000-13800 v Dry type and oil type
3. Above 13800 v only oil type

Electrical measurement & measuring instruments [emmi (nee-302) -unit-2]

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