Unit 4 CT & PT (6 Marks)

 Transformers used in conjunction with measuring instruments for measurement

purpose are called instrument transformer.
 The transformer used for measurement of current is called current transformer.
 Transformers used for measurement of voltage are called voltage transformer or
potential transformer

Use of Instrument Transformer:

 In power systems, currents and voltages handled are very large and therefore direct
measurement is not possible as these currents and voltages are far too large for any
meter of reasonable size and cost.
 Therefore these voltages and currents are stepped down with the help of instrument
transformers so that they can be metered with instruments of moderate sizes.

Important definitions:
1. Transformation ration(R): It is the ratio of the magnitude of the primary phasor to
secondary phasor.

𝑃𝑟𝑖𝑚𝑎𝑟𝑦 𝑤𝑖𝑛𝑑𝑖𝑛𝑔 𝑐𝑢𝑟𝑟𝑒𝑛𝑡

For C.T. 𝑅=
𝑆𝑒𝑐𝑜𝑛𝑑𝑎𝑟𝑦 𝑤𝑖𝑛𝑑𝑖𝑛𝑔 𝑐𝑢𝑟𝑟𝑒𝑛𝑡

𝑃𝑟𝑖𝑚𝑎𝑟𝑦 𝑤𝑖𝑛𝑑𝑖𝑛𝑔 𝑣𝑜𝑙𝑡𝑎𝑔𝑒

For P.T. 𝑅=
𝑆𝑒𝑐𝑜𝑛𝑑𝑎𝑟𝑦 𝑤𝑖𝑛𝑑𝑖𝑛𝑔 𝑣𝑜𝑙𝑡𝑎𝑔𝑒
2. Nominal ratio (Kn): It is the ratio of rated primary current (or voltage) to rated secondary
winding current or voltage.

𝑟𝑎𝑡𝑒𝑑 𝑝𝑟𝑖𝑚𝑎𝑟𝑦 𝑤𝑖𝑛𝑑𝑖𝑛𝑔 𝑐𝑢𝑟𝑟𝑒𝑛𝑡

For C.T. 𝐾𝑛 =
𝑟𝑎𝑡𝑒𝑑 𝑠𝑒𝑐𝑜𝑛𝑑𝑎𝑟𝑦 𝑤𝑖𝑛𝑑𝑖𝑛𝑔 𝑐𝑢𝑟𝑟𝑒𝑛𝑡

𝑟𝑎𝑡𝑒𝑑 𝑝𝑟𝑖𝑚𝑎𝑟𝑦 𝑤𝑖𝑛𝑑𝑖𝑛𝑔 𝑣𝑜𝑙𝑡𝑎𝑔𝑒

For P.T. 𝐾𝑛 =
𝑟𝑎𝑡𝑒𝑑 𝑠𝑒𝑐𝑜𝑛𝑑𝑎𝑟𝑦 𝑤𝑖𝑛𝑑𝑖𝑛𝑔 𝑣𝑜𝑙𝑡𝑎𝑔𝑒
3. Turns ratio (n):
𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑡𝑢𝑟𝑛𝑠 𝑜𝑓 𝑠𝑒𝑐𝑜𝑛𝑑𝑎𝑟𝑦 𝑤𝑖𝑛𝑑𝑖𝑛𝑔
For C.T. 𝑛=
𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑡𝑢𝑟𝑛𝑠 𝑜𝑓 𝑝𝑟𝑖𝑚𝑎𝑟𝑦 𝑤𝑖𝑛𝑑𝑖𝑛𝑔

𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑡𝑢𝑟𝑛𝑠 𝑜𝑓 𝑝𝑟𝑖𝑚𝑎𝑟𝑦 𝑤𝑖𝑛𝑑𝑖𝑛𝑔

For P.T. 𝑛=
𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑡𝑢𝑟𝑛𝑠 𝑜𝑓 𝑠𝑒𝑐𝑜𝑛𝑑𝑎𝑟𝑦 𝑤𝑖𝑛𝑑𝑖𝑛𝑔
4. Burden of an instrument transformer
 It is convenient to express load across the secondary terminals as the output in volt-
ampere at the rated secondary winding voltage.
 The rated burden is the volt ampere loading which is permissible without errors
exceeding limits of a particular class of accuracy.
Current transformer:
 A current transformer is an instrument transformer, used along with measuring or
protective devices, in which the secondary current is proportional to the primary
current (under normal conditions of operation).
Construction:
 The current transformer consists of an iron core upon which primary and secondary
windings are wound. The primary winding of the transformer is connected in series
with the line carrying current to be measured and therefore primary current is
dependent upon the load connected to the system and is not determined by the load
(burden) connected to the secondary of C.T., the secondary winding is connected to a
measuring device or a protective relay.
 The primary winding consists of few turns and therefore there is no appreciable
voltage drop across it. Secondary of C.T. has larger number of turns, determined by
the turns ratio.

 The ammeter or wattmeter current coil are directly connected across the secondary
winding terminals. The current transformer operates its secondary winding nearly
under short circuit condition. One terminal of secondary winding is earthed so as to
protect equipment and personnel in the vicinity in event of insulation breakdown of
C.T.

Working principle:
 The basic principle of the current transformer is the same as that of the power
transformer. Like the power transformer, the current transformer also contains a
primary and a secondary winding. Whenever an alternating current flows through the
primary winding, alternating magnetic flux is produced, which then induces
alternating current in the secondary winding.
  In the case of current transformers, the load impedance or “burden” is very small.
Therefore the current transformer operates under short circuit conditions. Also the
current in the secondary winding does not depend on load impedance but instead
depends on the current flowing in the primary winding.

Equivalent circuit and phasor diagram of current transformer:

Where rs-resistance of sec. Winding

xs-reactance of secondary winding
re-resistance of external burden i.e. resistance of meters, current coils etc including
leads
xe-reactance of external burden i.e. reactance of meters, current coils etc including
leads
Ep-primary induced voltage
Es-secondary induced voltage
𝟇m-working flux
rp,xp-resistance and reactance of primary winding
Ie-loss component of exciting current
Im-magnetising component of exciting current
Io-exciting current

The phasor diagram is drawn assuming that connected burden is of lagging power factor.

Errors in Current transformer:

1. Current error (Ratio error)
 In a C.T. value of the transformation ratio(R) is never equal to the turns ratio (or
nominal ration Kn)
  Secondary current is not a constant fraction of primary current but depends on
magnetising and loss component of exciting current, secondary winding load
current and its power factor.
 This introduces considerable error in the current measurements.
𝑛𝑜𝑚𝑖𝑛𝑎𝑙 𝑟𝑎𝑡𝑖𝑜 − 𝑎𝑐𝑡𝑢𝑎𝑙 𝑟𝑎𝑡𝑖𝑜
𝑃𝑒𝑟𝑐𝑒𝑛𝑡𝑎𝑔𝑒 𝑟𝑎𝑡𝑖𝑜 𝑒𝑟𝑟𝑜𝑟 = ( ) 𝑋 100
𝑎𝑐𝑡𝑢𝑎𝑙 𝑟𝑎𝑡𝑖𝑜
𝐾𝑛 −𝑅
=( ) 𝑋 100
𝑅

Multiplying the numerator and denominator by Is

𝐾𝑛 𝐼𝑠 −𝐼𝑝
% Current or ratio error= ( ) 𝑋 100
𝐼𝑝

2. Phase angle error

 The angle by which the secondary current phasor (nIs) when reversed differs in
phase from the primary current is known as phase angle of the current
transformer.
 In ideal transformer for power measurement, it is necessary that the phase
angle of secondary winding current be displaced exactly by 180° (or 0° when
secondary current is reversed) from primary current.
 But in practice, it is seen that phase difference is different from 180° by some
phase angle ‘θ’
 Phase angle error
180 𝑙 𝑐𝑜𝑠𝛿−𝐼𝑒 𝑠𝑖𝑛𝛿
Phase angle θ = ( )( 𝑚 )
𝜋 𝑛𝐼𝑠
OR
180 𝑙
θ=( ) ( 𝐼𝑚 )
𝜋 𝑝
𝛿- angle between secondary induced voltage and secondary winding current

3. Composite error
 RMS value of the difference under steady state conditions, integrated over one
cycle between
(a) Instantaneous value of primary current
(b) Product of rated transformation ratio and instantaneous value of
secondary current.
 It is generally expressed as percentage of the RMS value of primary unit.
100 1 𝑇 2
Composite error = ( 𝐼 ) (√𝑇 ∫0 (𝑘𝑛 𝑖𝑠 − 𝑖𝑝 ) 𝑑𝑡)
𝑝

Where
𝐼𝑝 − Primary current
𝑖𝑝 − instantaneous value of primary current
𝑖𝑠 − instantaneous value of secondary current
𝑇 - duration of cycles in seconds
kn - rated transformer ratio
 5. Burden of CT
 It is defined as volt ampere (VA) of connected load across the terminals of
secondary winding of C.T.
 The burden is usually expressed as the apparent power in volt absorbed at a
specific power factor and at rated secondary current.
 Thus the total VA burden of the CT can be calculated by adding the VA burden
of the followings.
1. The VA burden of the measuring equipment like protection relay and
measuring instruments
2. The VA burden of the leads connected in between the current transformer
and the relay/measuring meter
3. The secondary resistance of the current transformer

Ratings and their selection

1. Rated primary current
 The value of primary current of C.T. which is specified by the manufacturer

2. Secondary current
 The value of secondary current is marked on rating plate
 for transformers intended for delta connected groups these are divided by √3.

3. Rated output
 It is the value of apparent power (in VA at specified p.f.) which C.T. is intended to
supply to secondary circuit at rated secondary current with rated burden
connected to it.

4. Rated short time current

 It is defined as RMS value of AC component of current which the C.T. can carry for
rated time without damage due to thermal or electromagnetic stresses.

5. Rated accuracy class

 The class assigned to the C.T. with specified limits of ratio error and phase angle
error.

6. Insulation level of C.T.

 Refers to withstand able value of power, frequency, voltage and impulse withstand
voltage.

Types of Current transformer based on construction:

The current transformer may be classified as
i. Wound type: A current transformer having a primary winding of more than one full turn
wound on core.
ii. Bar type: A current transformer in which primary winding consists of a bar of suitable
size and material forming an integral part of transformer.
Bar type C.T. (refer fig.):

 In bar type C.T. the core and secondary are the same as in a ring type transformer
but fully insulated bar conductor consisting the single turn primary winding is now
integral part of C.T.
 Insulation on primary may be Bakelized paper tube or resin mounted directly on bar.

Wound type C.T. (refer Fig.):

 In low voltage wound type C.T. secondary winding is wound on a Bakelite former or
bobbin and the heavy primary conductor is either wound directly on top of
secondary winding, suitable insulation being first applied over secondary winding.
 Or sometimes primary is wound entirely separately, taped with suitable insulating
material and then assembled with secondary winding on core.

Effect of secondary open circuit of a C.T.

 C.T. are always used with the secondary winding circuit closed through ammeters,
wattmeter current coils or relay coils.
 The secondary circuit of C.T. should never be opened while its primary is being
energized. Failure to observe these precaution may lead to serious consequences
both to the operating personnel and to the transformer.

Reason:
 As we know in case of power transformer current flowing in primary is an reflection
of secondary current. Whereas in C.T. current to be measured is passed through
primary which is no way controlled or determined by secondary winding circuit.
  Under normal working condition both primary and secondary windings produce mmf
which oppose each other. Secondary mmf is slightly less than primary mmf. The small
difference in mmf called resultant mmf is required for maintaining flux in the core and
to supply iron losses. Since mmf is small, resultant flux and hence secondary emf
induced is very small.
 But if secondary winding is open circuited when primary is energised, the opposing
secondary mmf becomes zero. The resultant mmf equals primary mmf which is very
large.This large mmf produces large flux which in turn induces high voltage in
secondary. This high voltage is dangerous to insulation and to the person who has
opened the circuit.
 Also the eddy current and hysteresis losses would be high and due to this transformer
may be overheated and completely damaged.

Difference between metering and protective C.T.

 There are 2 classes of current transformer depending on operational requirements
1) instrument transformer(metering C.T.) and 2) protective transformer

Instrument transformer:
 Instrument C.T. are used with ammeter, wattmeter, kWH meters, kVA meter for
reducing the line current to a low value (eg. 1 or 5A)
 Instrument transformer is required to measure even small fractions of the rated
primary current with adequate accuracy.
 Some extensions beyond rated current is also necessary to take consideration of the
normal system overloads.
 Instrument transformer is therefore required to maintain the accuracy class with 5
(or 10%) to 125% of rated current and small measuring errors within the range.
 It is required that beyond say 125 or 150% or rated current it is desirable that the
core saturates so that in event of faults when heavy currents flow in the primary of
the C.T.’s the connected instruments, line ammeter, wattmeter integrated kWh
meter etc. are not burnt out.

Protective transformer
 They are used for overcurrent protection, earth fault protection, differential
protection, impedance protection.
 Unlike instrument transformer it is not the function of the protective transformer to
maintain great precision over the normal operating range. Rather it should maintain
the current error and phase difference within reasonable limits in the fault current
range.
 It is necessary that the protective transformer does not saturate up the value of the
current necessary to operate the relays.


Information to be supplied for inquiring or tendering of C.T.s

1) System voltage, type of supply and earthing conditions (eg. 66kV system, 3 phase
solidly earthed)
2) Insulation level
 3) Frequency
4) Rated transformation ratio in terms of rated primary and secondary current
e.g. 100/5A
100-200/5A
50/5-5A
5) Rated output
6) Class of accuracy
metering-0.2s,0.2,0.5s,0.5,1
protection-PX/PS,5p/10p
7)Accuracy limit factor for current transformer protective purpose.
8)For protective current transformer class 5P
i) Rated primary current
ii) Nominal turn ratio
iii) Knee point emf
iv) Maximum secondary winding resistance
v) Limit (s) on exciting current
9) Rated continuous thermal current
10) Short time current and its duration
11) Climatic conditions. E.g. Max and min ambient temperature, humidity, exposure to
steam vapour.
12) Installation conditions such as indoor or outdoor altitude above sea level, vibrations,
limiting dimensions, etc.

Clamp on ammeter
 A device that is used to measure current in an efficient, convenient, and safe manner
without disconnecting the circuit is known as clamp meter.
 A current transformer with a single conductor is used in combination with a bridge
rectifier and a dc milli-ammeter to produce this very useful service meter.

Working:
 When the core is clipped onto a cable, the cable under test acts as the single turn
primary winding of a transformer and secondary is connected to D.C. ammeter.
Primary has a single turn (the conductor through which the current to be
measured is flowing), but secondary has many turns. So it acts as a step up
transformer.
  Due to ac current flowing through the conductor (primary), there is an induced
ac current flowing in the secondary. The bridge rectifier converts this ac current
into a direct current (dc) and applies it to the dc ammeter. The deflection of the
dc ammeter is proportional to the rectified secondary current.
 The meter scale can be calibrated to measure the current through the conductor
directly.
 As shown in Fig. above a trigger switch is used to split the core of the transformer
and hence core can be clamped around a live conductor for the current
measurement. With change in shunt resistance by changing the position of the
current range selector switch, various ranges of D.C. milli-ammeter can be
obtained
 This arrangement avoids the necessity of breaking the current in order to insert a
current measuring device in series with the circuit to measure the value of current.
 By changing shunt resistance milli ammeter circuit ranges from 0-5A to 0-600A can
be obtained.

Voltage/potential transformer
• Potential transformer is a voltage step-down transformer which reduces the voltage
of a high voltage circuit to a lower level for the purpose of measurement. These are
connected across or parallel to the line which is to be monitored.
• The primary winding consists of a large number of turns which is connected across
the high voltage side or the line in which measurements have to be taken or to be
protected. The secondary winding has lesser number of turns which is connected to
the voltmeters, or potential coils of wattmeter and energy meters, relays and other
control devices. These can be single phase or three phase potential transformers.
• Irrespective of the primary voltage rating, these are designed to have the secondary
output voltage of 110 V.

 Classification of voltage transformer

The voltage transformer are classified as
1. Magnetic type
2. Capacitive type

1) Magnetic type voltage transformer

 Magnetic type voltage transformer works on the same principle as power
transformers.
  But design of VT is different as compared to power transformer because of different
requirement in two cases. The load transmitted through a voltage transformer is
quite limited depending on purposes generally around a few hundred VA.
 The main objective in design of voltage transformer is to minimise ratio and phase
angle errors, in order to introduce minimum possible errors in measured values.
 The errors are due to
1. Voltage drop in primary winding caused by exciting current.
2. Voltage drop in both winding caused by load current.
 Since load current is for a given burden is fixed the drop caused by it can be reduced
by reducing resistance and reactance of transformer.
 This is done by using relatively few terms and a large section of both iron and
copper.
 In lower voltage group of VT’s active part is contained in steel housing and primary
terminal is brought out through a bushing
 For higher system voltage above 66kV active part is placed in porcelain housing

2) Capacitive potential transformer

 At voltage above 100kV the conventional electromagnetic type of potential

transformer becomes expensive owing to insulation requirements. A less expensive
alternative is capacitive voltage transformer.
 This consist of a capacitance potential divider used in conjunction with a
conventional auxiliary transformer.
 The capacitance potential divider steps down the voltage to be measured (say to
about 10kV)
 The capacitance divider output is further stepped down by the auxiliary transformer
to a desired voltage (say 110V).
 The auxiliary transformer consists of an inductance L to compensate the voltage
increase on the capacitance voltage divider. Value of L adjusted to
(1⁄[𝜔2 (𝐶1 + 𝐶2 )] )
 Thus overall transformation ratio is product of divider and transformer ratio.

 In practice however the compensation is not complete because of losses in

inductances and also due to small frequency changes that may appear.
 The performance of capacitor voltage transformer is slightly inferior to
electromagnetic type P.T.
Errors in potential transformer:
1) Ratio(voltage) error
 If there is a difference between the ideal voltage and actual voltage, then the
voltage error occurs.
or
 The actual ratio of transformation varies with operating conditions and
introduces error in secondary voltage which may be defined as

2) Phase angle error

 In an ideal voltage transformer there should not be any phase difference
between primary winding voltage and secondary winding voltage reversed.
 However in actual transformer there exist a phase difference between
primary and secondary voltage reversed.
 The phase angle is taken positive when secondary winding voltage reversed
leads primary winding voltage.
 The phase angle is taken negative when secondary winding voltage reversed
lags primary winding voltage.

Information to be supplied with the enquiry or the purchase order:

1. Transformation ration
2. System voltage, type of supply and earthing conditions
3. Rated Burden
4. Insulation Level
5. Rated Voltage Factor
6. Accuracy Class
The standard accuracy classes for measuring voltage transformers shall be 0.1, 0.2, 0.5, 1.0
and 3.
0.5 means ±0.5 percent voltage error and ±20” phase displacement.
The standard accuracy classes for protective voltage transformers are “3P” and “6P”.
Here 3P means ±3 percent voltage error and ±120” phase displacement.
7. Rated Frequency
8. Service conditions such as indoor or outdoor, altitude above sea level, ambient
temperature, maximum and minimum humidity, exposures to steam or vapour fumes,
explosive gases, excessive dust.
9. Limiting dimensions if any
Difference between C.T. and P.T. :

Current Transformer Potential Transformer

Transform the current from Transform the voltage from high value to the low value.
high value to the low value
Connected in series with the Connected in parallel with the instrument.
instrument
Primary circuit has lesser Primary has more number of turns compared to
number of turns compared secondary circuit
to secondary circuit
Range: 5A or 1A Range: 110V

Types: Bar type and wound Types: Electromagnetic type and capacitor voltage
type transformer

Low impedance High impedance

The secondary of CT is The secondary of PT is practically open circuit.
almost short circuit.

Primary winding carries full Primary winding has full line voltage impressed on its
line current terminals
Primary current is Primary current is dependent on secondary burden
independent of secondary
burden.