Current Transformer f2
-
Upload
mohammedsaadanihassani -
Category
Documents
-
view
114 -
download
23
Transcript of Current Transformer f2
Current TransformerBy:
تاج / يحيى مهندسالدين
APPLICATIONCurrent transformers (CT,s) are instrument
transformers that are used to supply a reduced value of current to protective relays , meters and other instruments.
CT,s provide isolation from the high voltage primary , permit grounding of the secondary windings for safety , and step down the magnitude of the measured current to a value that can be safely handled by the instruments
RatioThe most common CT secondary full load current is
1A or 5A.
Example a 1000/1 A CT will produce 1A of secondary
current when 1000 A flows through the primary. As the primary current changes the secondary
current will vary accordingly.
CT ratio are expressed as a ratio of rated primary current to the rated secondary current .
POLARITY
All CT,s are subtractive polarity .
On subtractive polarity transformers the H1 primary lead and the X1 secondary lead will be on the same side of the transformer.
Polarity refers to the instantaneous direction of the primary current with respect to the secondary current and is determined by the way the transformer leads are brought out of the case
CT POLARITY
P1
S1
Terminals Marking
general rules
The terminal markings shall identify: the primary and secondary windings; the winding sections, if any; the relative polarities of windings and winding sections; the intermediate tapings, if any
Graphic symbols of current transformers
The main tasks of instrument transformer are:
• Insulate secondary circuits from the primary.
• To transform current, or voltages, from a high value to a value easy to handle for relays and instruments.
• permit the use of standard current ratings for secondary equipment.
1
2
2
1
N
N
I
I
CT Ring Type
CT
Summation CT
CT equivalent circuit
Excitation Curve
Non saturated - zone
intermediate - zone
TEST RESULTVolt (V)Current (mA)Volt (V)Current (mA)
8.70.959527.7
11.31.260328.8
15.11.462532.3
21.51.964535.8
30.22.465037.6
51.73.468043.7
103.365.669045.9
215.349.271053.3
26710.773060.6
301.4811.875096.7
387.614.5770150.5
430.6816785204.3
473.7518.3800287.0
516.820.6805379.8
56023.8820573.4
59527.78401111.0
60328.88602150
SECONDARY EXCITING CURRENTMAG CURVE
1
10
100
1000
0.1 1.0 10.0 100.0 1000.0 10000.0
MAG CURVE
A C.T consists essentially of an iron core with two windings. One winding is connected in the circuit whose current is to be measured.
The flow of current in the primary winding produces an alternating flux in the core and this flux induces an e.m.f in the secondary winding which results in the flow of secondary current when this winding is connected to an external closed circuit .
The magnetic effect of the secondary current , in accordance with fundamental principles , is in opposition to that of the primary and the value of the secondary current automatically adjust itself to such a value , that the resultant magnetic effect of the primary and secondary currents , produce a flux required to induce the e.m.f. necessary to drive the secondary current through the impedance of the secondary.
TERMS & SPECIFICATIONS
Thermal continuous current rating The thermal continuous current rating (r.m.s.value in operates) 1.2 times , or in
extended-range current transformers 1.2 or 2.0 times , the rated current.
Thermal short –time current IthIth is the value of current quoted on the
rating plate with a duration of 1 sec. whose heating effect the current transformer can withstand without damage with the secondary winding short circuited (r.m.s in KA)
Ith =
Ik =
(50/f) 0.05 (t Ik
Un* 3
Ssc
ExampleMVA SC = 5000 MVA
V = 380 KV
I k =
I k = 7.597 KA
Ith =
I dyn = 2.5 Ith
380* 3
5000
)0.05(50/60(1 Ik
I dyn is the h ighest amplitude of current whose mechanical effects the CT can withstand , with the secondary winding short circuited , without damage (peak value in KA)
I dyn = 2.5 … 3 I th
Dynamic current rating I dyn
Burden = The impedance of the secondary circuit in ohms and power factor. The burden is usually expressed as the apparent power (S) in volt-amperes absorbed at a specified power-factor at the rated secondary current.
Burden
RB
LB
BURDEN=
VA / I²
EXTERNAL BURDEN
To protect instrument and meters from high fault currents the metering cores must be saturated 10-40 times the rated current depending of the type of burden.
The instrument security factor “Fs”
sib
in FPP
PPn *
The main characteristics of protection CT cores are:
• Lower accuracy than for measuring transformer .
• High saturation voltage.
• Little , or no turn correction at all.
5P and 10P The error is then 5 and 10 at the specified ALF and at rated burden.
The Accuracy Limit Factor indicates the over current as a multiple times the rated current , up to which the rated accuracy (5P or 10P) is fulfilled (with the rated burden connected).
ALFPp
PPn
ib
in *
No. of primary turns = 1 turn
No. of secondary turns = N turn
Ip = N * Is
Ideal transformer for (Is) to flow through R there must be some potential Es = The E.M.F
Es = Is * R
Es is produced by an alternating flux in the core.
dt
dEs
Flux required to produce Es
BsCTss zIRIE **
AB*Where
B = Flux density in the coreA = cross-sectional area of core
NAfBEk ****44.4
LCTBss zzzIE Requiredsk EE
Equ. 1
Equ. 2
CT 2000/5 , Rs =0.31, Imax =40 KA , MaX Flux density =1.6 Tesla
Find maximum secondary burden permissible if no saturation is to occur.
Solution
N=2000/5 = 400Turns
Is max = 40000/400 = 100Amps
From Equ.1
Vk = 4.44*1.6*20*60*(400/10000) = 340 Volt
Max burden = 340/100 = 3.4 ohmsMax connected burden = 3.4 - 0.31 = 3.09
CT ratio are selected to match the maximum load current requirements.i.e. the maximum design load current should not exceed the CT rated current.
The CT ratio should be large enough so that the CT secondary current does not exceed 20 times rated current under the maximum symmetrical primary fault current.
It is customary to place CT,s on both sides of the breaker. So that the protection zones will overlap.
The protection Engineer can determine which side of the breaker is best for CT location .
All possibilities of fault position should be considered .
The overlap should occur across a C.B, so the C.B lies in both zones for this arrangement it is necessary to install C.Ts on both sides of the C.B.
C.T,s mounted on both sides of breaker no unprotected region
No region un protected
Current transformers mounted on C.B side only of breaker fault shown not cleared by bus bar protection.
Current transformers mounted on bus bar side only of breaker fault shown not cleared circuit protection.
C,B will open by line protection but fault will last.
PROTECTION IEC standard
RATIO 2000/5 APOWER 20 VACLASS 5P20
MEASURING IEC standard
RATIO 2000/5 APOWER 20 VACLASS 0.5SF5
CT Class X
THE FOLLOWING INFRMATION IS REQUIREDTurns RatioKnee Point VoltageMaximum Excitation Current Secondary Circuit Resistance
TPX, TPY AND TPZ Current Transformers
CTs of class P, models were developed for CTs of class TPX (closed-core), TPY and TPZ (nonclosed-core). All models are based on known rated values of the CTs. This is an advantage of the presented method, because no additional measurements of the parameters of the CTs are needed.
TPXHigh remanence type CTThe high remanence type has no limit for the remanence
flux. This CT has a magnetic core without any air gap and a remanence flux might remain for almost infinite time. In this type of transformers the remanence flux can be up to 70-80% of the saturation flux.
Typical examples of high remanence type CT are class P, TPS, TPX according to IEC,class P, X according to BS (British Standard) and non gapped class C, K according to
ANSI/IEEE.
TPY
Low remanence type CTThe low remanence type has a specified
limit for the remanence flux. This CT is made with a small air gap to reduce the remanence flux to a level that does not exceed 10% of the saturation flux.
The small air gap has only very limited influence on the other properties of the CT. Class TPY according to IEC is a low remanence type CT.
TPZNon remanence type CTThe non remanence type CT has practically
negligible level of remanence flux. This type of CT has relatively big air gaps in order to reduce the remanence flux to practically zero level. At the same time, these air gaps minimize the influence of the DC-component from the primary fault current.
The air gaps will also reduce the measuring accuracy in the non-saturated region of operation. Class TPZ according to IEC is a non remanence type CT.
As a matter of safety, the secondary
circuits of a current transformer should never be opened under load, because these would then be no secondary mmf to oppose the primary mmf, and all the primary current would become exciting current and thus might induce a very high voltage in the secondary.
General As a matter of safety, the secondary circuits
of a current transformer should never be opened under load, because these would then be no secondary mmf to oppose the primary mmf, and all the primary current would become exciting current and thus might induce a very high voltage in the secondary.
f
Ip
Ie Ze
XpRp e Rs
Sec
g
h
c
d
Pri
Is
EQUIVALENT DIAGRAM
Ve = EXCITATION VOLTAGE Vef Ie = CURRENT
Ze = IMPEDANCEVt = TERMINAL VOLTAGE Vgh
KNEE POINT OR EFFECTIVE POINT OF SATURATION
ANSI/IEEE: as the intersection of the curve with a 45° tangent line
IEC defines the knee point as the intersection of straight lines extended from non saturated and saturated parts of the excitation curve.
IEC knee is higher than ANSI - ANSI more conservative.
ANSI/IEEE KNEE POINT
Exc
itatio
n V
olts
Kne
e P
oint
Vol
ts
45° LINE
IEC KNEE POINT
ANSI/IEE KNEE POINT
EX: READ THE KNEE POINT VOLTAGE
RATIO CONSIDERATIONS
CURRENT SHOULD NOT EXCEED CONNECTED WIRING AND RELAY RATINGS AT MAXIMUM LOAD. NOTE DELTA CONNECTD CT’s PRODUCE CURRENTS IN CABLES AND RELAYS THAT ARE 1.732 TIMES THE SECONDARY CURRENTS
RATIO CONSIDERATIONS
SELECT RATIO TO BE GREATER THAN THE MAXIMUM DESIGN CURRENT RATINGS OF THE ASSOCIATED BREAKERS AND TRANSFORMERS.
RATIO CONSIDERATIONS
RATIOS SHOULD NOT BE SO HIGH AS TO REDUCE RELAY SENSITIVITY, TAKING INTO ACCOUNT AVAILABLE RANGES.
RATIO CONSIDERATIONS
THE MAXIMUM SECONDARY CURRENT SHOULD NOT EXCEED 20 TIMES RATED CURRENT. (100 A FOR 5A RATED SECONDARY)
RATIO CONSIDERATIONS
HIGHEST CT RATIO PERMISSIBLE SHOULD BE USED TO MINIMIZE WIRING BURDEN AND TO OBTAIN THE HIGHEST CT CAPABILITY AND PERFORMANCE.
RATIO CONSIDERATIONS
FULL WINDING OF MULTI-RATIO CT’s SHOULD BE SELECTED WHENEVER POSSIBLE TO AVOID LOWERING OF THE EFFECTIVE ACCURACY CLASS.
The core should be demagnetized as the final test before the equipment is put in service. Using the Saturation test circuit, apply enough voltage to the secondary of the CT to saturate the core and produce a secondary current of 3-5 amps. Slowly reduce the voltage to zero before turning off the variac.
Core Demagnetizing
TESTING
SaturationThe saturation point is reached when there is a rise in the test current but not the voltage.
Burden
TESTING
PolarityThis test checks the polarity of the CT
RatioInsulation test
SATURATIONAbnormal high primary currentHigh secondary burdenCombination of the above two factors will result in the creation of high flux density in the current transformer iron core.
When this density reaches or exceeds the design limit of the core , saturation results.
SATURATION
The accuracy of the CT becomes very poor.
The output wave form distorted.The result secondary current lower in magnitude.
The greatest dangerous is loss of protective device coordination
SATURATION
list of CT problems usually found at site:
• Shorted CT secondaries• Open-circuited CT secondaries• Miswired CTs• CTs that had not been wired• CTs installed backwards• Incorrect CTs• Defective CTs• CTs with incorrect ratios or on the wrong taps
THANKSتحياتى : مع
الدين / تاج يحيى م