Current Transformer
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Transcript of Current Transformer
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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