CURRENT TRANSFORMERS

S.D.Kurhekar

Ex. Chief Engineer, MSEDCL

CURENT TRANSFORMERS

• C.T. transforms higher current in to a more

manageable secondary current.

• This transformation is made possible by

copper coils wound around an iron core.

• Ratio between primary & secondary

current is determined by the ratio between

number of primary & secondary turns.

• Bar/ ring type CTs don’t have physical

primary winding. Hence those are

considered to have one primary turn.

When current flows through primary

winding, the following actions occur

• The iron core inside the CT is magnetized.

• The magnetic field in the iron core induces

voltage in the secondary coils.

•If the secondary circuit is closed, a current flows

through the secondary circuit in proportion to the

CT ratio.

• The current transformation requires a small amount

of energy to magnetize the iron core that creates

small energy losses such as eddy currents & heat

caused by current flowing through the windings.

Hence Is is not a perfect representation of Ip.

Types of CTs Four typical types

• Window type,

• Bushing

• Bar type

• Wound CTs.

Window CTs/Ring type CTs –

• Most commonly used. No primary winding & are installed around a primary conductor.

• Electric field created by current flowing through the conductor interacts with the CT core to transform the current to the appropriate secondary output. Primary conductor needs to be disconnected when installing solid window CTs.

Types of CTs

Bushing CTs

Bushing CTs •These CTs are specially constructed to

fit around a bushing. Usually they can’t

be accessed & their name plates can be

found on the transformer or circuit

breaker cabinets.- eg. CTs in Bulk oil

Circuit Breakers

Types of CTs

Bar type CTs

•These CTs operate on the same principle of window/

ring type CTs but have a permanent bar installed as a

primary conductor.

Wound CTs:-

• These CTs have one primary & one secondary winding

like a normal transformer. These CTs are rare & used

at very low ratios & currents – typically in CT

secondary Circuits to compensate for low currents, to

match different CT ratios in summing applications or

to isolate different CT circuits

Types of CTs

• Low Primary Impedance, Primary Connected in Series

• Construction –

• 1) Core – Rectangular / Shell type / Ring form

a) Rectangular – L shaped Laminations Primary wound on secondary therefore Ample space for insulation.

b) Shell form – Windings placed on Central Limb, therefore Good Protection. But this form is difficult to build.

c) Ring form – Most commonly used. Primary is a Single bar when Ip is large.

Robust Construction –Joint less core, therefore low reluctance

CONSTRUCTION OF CTs

PRINCIPLE OF CTs • Principle – Primary in series. Sec. Connected to

metering or relay

• Burden means VA Burden of Relay coil + Sec winding impedance + lead Burden (I2 R)

• I Primary has Two components –

• i) I sec which is transformed in inverse ratio of

Turns Ratio.

• ii) Exciting current – which supplies eddy current

• & hysterisis losses & magnetizes the core. This is

not transformed therefore this is a cause for error

in C.T.

• A1T1 = A2T2 + A.T. required to magnetize the core.

CONSTRUCTION OF CTs • 2) Windings – Should be enclosed together to

reduce leakage reactance as this increases

Ratio error. Large forces created during short

ckts, therefore Bar primary CTs are most

suitable from this point of view.

• 3) Insulation – Windings are seperately

wound & insulated by tape/varnish below 7

KV level. For 7 KV & above, CTs are oil

immersed or compound filled (cooling is

poor).

Core material for CTs

• 1) Measuring CTs – High Permeability

core material with low saturation levels is

suitable e.g. (Nickel-Iron alloys)

• 2) Prorective CTs – This basically requires

High Saturation level, since they offer

advantage of high KPV. Therefore here

grain oriented steel core is used.

Specifications of CT 1) Rated Trf ratio – 100/5A, 500/1A

2) Rated Burden VA – 2.5 VA, 5, 7.5, 10, 15, 30 VA

3) Accuracy class – Should match the duty required. Else large size CTS. Errors increase with under loading.

30/5P 10 means – 30 VA, AC-5, P-Prot, ALF-10

4) No. of cores - 3 or 5 core CTs

5) Knee Point Voltage –

6) Accuracy Limit Factor(ALF)/ Saturation factor – 5,10,15,20,30

= Highest Ip at which CT maintains compo.Error accuracy

Rated IP

7) Over current factor – or short time current e.g. 13.1 KA/1 sec.

Specifications of CTs

• 1) Rated Transformation Ratio

• 2)

• 3)

• Where Ip – RMS Value of Iprim.

Kn – Rated Transformation ratio.

ip,is - Instantaneous Values of Primary & Sec. Currents.

T - duration of one cycle .

This error is important for Protective CTs. During faults, high currents pass through CT Primary winding, the core Saturates and may not maintain wave shape exactly Sinusoidal & may be deformed. Hence the difference (Kn.is-ip) is integrated over one complete cycle.

Composite Error Ec= 100

Ip

1

T 0

f

(Kn X is-ip)2.dt

4) Phase displacement Error - Difference

in phase angles of Ip and reversed Is

vectors.

5) Exciting current - RMS Value of

current taken by sec. Winding of CT,

when an A.C Voltage is applied to its

terminals.

Specifications of CTs

6) Knee point voltage

It is defined as that Sinusoidal voltage of rated freq.applied to Secondary terminals (all other windings being open circuited) which when increased by 10% causes the exiting current to increase by 50 %, Beyond knee point the CT is said to have entered into Saturation. Thereafter the secondary Currents may not be proportional to Primary Currents & may depart from sine wave.

Specifications of CTs

KNEE POINT VOLTAGE(KPV)

• Defined as that Sinusoidal Voltage at

rated frequency applied to sec.

terminals of CT, which when

increased by 10%, causes the exciting

current to increase by 50%.

• Beyond knee point – CT is said to

have entered in Saturation.

CT Saturation Curve Tester •Apply voltage ( O-1000V AC adjustable) to secondary

winding of CT under test.

•Sec. Excitation voltage gradually increased from zero by

using an adjustable auto transformer while incremental

voltage & current readings are taken.

A step of 10 ma is a good first step.

•Excitation voltage & corresponding currents are plotted.

This is then compared with the manufacturers published

curves.

•A deviation from these curves indicates- either a primary

one turn short circuit due to improper mounting or a

shorted secondary turns.

Magnetising Characteristic of C. T.

• Magnetising Characteristic of a CT is a plot

between the secondary applied voltage and the

corresponding magnetising current taken by the C.

T. as shown in the fig. below

VK

Saturation

Knee Point

Linear Region

Ankle Point

Exciting Voltage

Exciting Current

7) Rated output-

The output of a CT consists of individual

burdens of instruments, meters/relays and

of connecting leads. The performance of

C.T depends upon its actual connected

burden. Hence the rated output of the C.T.

Should be chosen as near to (but not less

than) the actual output at which it has to

operate.

Specifications of CTs

Accuracy class of CT • A classification assigned to a CT, the

errors (current ratio error in case of measuring CTs and the composite errors in case of protective CTs) of which lie within the specified limits as per IS.

• Measuring CTs - Std. Accuracy class- 0.1,0.2,0.5,1,3,5

• Protective CTs- 5P,10P,15P

• CTs for special protections- PS Class

Accuracy Limit Factor (ALF) • It is the ratio of highest value of Ip, at which the CT

will comply with the appropriate limits of composite error under the prescribed conditions,to the rated Ip.

• Standard ALFs are- 5,10,15,20, & 30.

• To have an economical CT of a given accuracy class, the product of VA burden and ALF chosen should be nearly 150

• Over Current Factor (OCF) for a Specified time

• OCF of CT – Let CTR = 50/5, 11 KV ckt. Br. Capa. = 150 MVA.

• Therefore I rms of Ckt. Br. = 150,000 /11 x \/3 = 7900 A

• OCF = 7900/50 = 158 for 0.5 sec.

Burden

•The CT burden is the amount of

impendence (A.C. resistance)

connected to the CT secondary & is

usually rated in ohms.The manufacture

only guarantees CT accuracy up to a

maximum burden rating, & CT

performance with degrade if secondary

burden is larger than rated.

General VA burdens of diff.

Instruments -

• Metering – VA burdens

• Iron Ammeter – 1VA, Induction Am.– 6VA, Induction Watt meter 1.5 VA

• Watt-hr meter – 1.2 VA, p.f. meter – 1VA

• Protective – VA burdens –

• Solenoid Relay – 15 VA, Induction Relay – 6 to 10 VA, OCB Trip coil – 35 VA.

• VA burden X ALF = 150 =constant

CHOICE OF CT SECONDARY

CURRENT

• Choice of CT sec – 5A or 1A.

• Relay Burden – 10 VA, Lead Resistance = 0.1ohm,

• CT Sec. current – 5 A

• Therefore total VA burden – 10 + I²R = = 10 + (5 x

5 x 0.1) = 12.5 VA.

• If I sec. Of CT = 1A, then total VA burden = 10

+(1) x 0.1 = 10.1VA.

Choice of CT secondary Rating

• 5 A secondary CT

• Preferred where lead burden is insignificant (e.g. used in indoor switchgear cubicles with closely located Relays or where primary ratings are very high e.g. 10000A/5A).

• Comparatively low peak voltage when secondary gets opened.

• Fine Turns Ratio adjustment is not possible when primary rating is low particularly for Bar primary CTs (e.g. 25/5A).

/1 A secondary CT

• Preferred where lead burden is1 high e.g. CTs are located out door.

• Comparatively high peak voltage when secondary gets opened.

• Fine Turns Ratio adjustment is possible.

Choice of CT secondary Rating

Ratio Changing Arrangements in CTs

On sec. Side --

S1S2 = 20Turns – Turns Ratio 20/1,

CT Ratio – 100/5A

S1S3 = 40 Turns– Turns Ratio = 40/1

CT Ratio = 200/5

On primary side--

Series Conn. – C1 to C2 –

Lower Ratio e.g. 100/5

Parallel Conn. – P1 to C2 & C1 to P2

Higher Ratio} 200/5

Testing of CT

1) Polarity Test • DC ammeter Positive Connected to S1 &

Negative to S2

• Battery Cell (2 cells) -+ve to P1, Negative to P2 through a

switch.

2) Ratio Test • Current injected by Prim. Injection Kit or loading Trf &

Prim & sec. currents are measured.

3) IR Test • Prim to Earth, Sec to Earth , Prim to Sec

Measuring CTs (IS 2705 – 1992 Part II)

Specified in terms of

• Accuracy class – 1 (Std. Classes are – 0.1, 0.2,0.5,1,3,5

• VA Rating - 15

• Higher errors are permitted at lower currents e.g. class 1 CT will have ratio error of +- 1% for 100-120% of rated I

• (+/- )1.5% for 20% of rated I

• (+/- )3% for 50% of rated I

Specifications of Measuring CTs

Specifications of Protection CTs

•Protection class CTs are not as accurate as

metering class CTs but are designed to

operate over a wide range of current. This

wider range is necessary to allow the

protection relay to operate at different fault

levels.

•Protection class CTs are typically rated to

operate accurately up to 10 or 20 times the

CT rating.

• (IS 2705 – 1992 Part III)

• Specified in terms of Accuracy class – 5 P, 10 P, 15P

• ALF – 5, 10, 15, 20, 30.

• VA Rating VA – 5, 10, 15, 30 VA

• Errors are specified at rated I and ALF times rated I with rated burden connected.

• Illustration – 5P10/15VA Prot. CT will have composite error of +- 5% at 10 times rated I, and ratio error of +-1% at rated I with rated burden of 15VA connected.

• VA x ALF = constant = 150

Specifications of Protection CTs

Protective CTs for Special applications (PS)

• Special Application CTs are used where balance is required between associated CTs, e.g. Distance, Diff, REF, B/B diff

• They are specified in terms of --

• a) Knee Point voltage (Vk)

• b) Imagnetisting at Knee Point voltage or % there of

• c) CT sec. winding Resistance (RCT)

• Turns Ratio errors limited to (+/-) 0.25%, which helps in maintaining the balance of protective systems during maximum through fault conditions

• Illustration- For PS class CT

• Ratio – 100/1 A

• Vk >/ 100 V

• Im </ 30 A

• Winding RCT </ 100 hm

• Open circuiting of CT sec, When Ip is flowing?

• Fuse to be used in CT Sec. Circuit ?

Comparison of CT & PT Current Transformers Potential Transformers

CT Primary is connected in series with the load or

Line

PT primary is connected across the line i.e. between

Phase & Earth

Should have very low Primary winding Impedance.

Bar Primary CTs with one turn are popular

Primary of PT should have high Impedance

Primary no of turns are low. Large no of Secondary

winding turns.

Primary has large no of turns and secondary has less

no of turns.

Primary current depends on the load connected.

Secondary current depends on primary current

Primary current depends on secondary current. Both

currents are comparatively low. Secondary voltage

depends upon Primary voltage.

Primary has two terminals one Incoming and other out

going.

Primary one terminal is connected to line or Bus and

second terminal is connected to earth.

CT secondary should never get open circuited when

Primary current is flowing. Otherwise excessively

high voltages will appear across secondary open

circuited terminals and may damage CT secondary

insulation

PT secondary should not be short circuited. Other wise

PT may get damaged due to excessive heating.

Fuse should never be provided in CT Secondary

Circuit

Fuse Should be used PT Secondary Side to protect the

PT from damage due to excessive heat produced due

to short circuit currents.

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