Current Transformer

CONSTRUCTION

      

                    BAR TYPE                                                      Wound Type

BASIC FORMULAE

Ek                                  Ek   =   444  x  B A f N volts  

 

                    Where :

                                    Ek   =  secondary induced volts (rms value, known as the knee-point

                                                voltage)

                                    N   =   number of secondary turns

                                    f     =   system frequency in hertz

                                    A    =   net core cross-sectional area in square meters.

Example

            Assume that a bar primary type 2000/5A (CROSS core) current transformer having a core csa area of 20 square cm's is available with a secondary resistance of 031 ohm.  The maximum current up to which the transformer must maintain its current ratio is 40,000 amperes.  It is required to determine the maximum secondary burden permissible if core saturation is to be avoided.  Assume that the current transformer core will start to saturate at 16 tesla.

            From the data given :

                                    N   =   2000/5   =   400 turns

                                    f     =   50 Hz

                        Secondary current (Is) with a primary current of 40,000A is given by :

                                    Knee point voltage Ek is given as follows :

                                    

                                           =   284 volts

                        Maximum burden permissible (including ct secondary resistance and lead burden) is equal to 284 / 100                  =  284 ohms

                  Consequently, the connected burden including that of the pilots can be as high as 2.84 - 031 = 253 ohms for negligible saturation in the core.  Thus it may be seen that the secondary burden and the maximum available fault current are two important criteria in determining the performance of a given current transformer.

KNEE-POINT

The knee-point of the excitation characteristic is defined as the point at which a 10% increase in secondary voltage produces a 50%  increase in exciting current.  It may, therefore, be regarded as practical limit beyond which a specified current ratio may be maintained. 

                  The current transformer magnetisation curve, is usually expressed in terms of Kv and Ki which when multiplied by the flux density in teslas and ampere-turns per cm respectively gives corresponding volts and amperes :

                        Es   =   444     f  B  A  N  volts.

                  A current transformer may operate satisfactorily :

a)            At a high primary current where the connected secondary burden is low

b)            At a lower primary current where the secondary burden is high.

 LIMITS OF ERRORS IN CTs

Class 0.1 to 1.0: The Current Error and phase displacement Error at the rated frequency shall not exceed the values given below when the secondary burden is any value from 25% to 100% to the rated burden.

Limits of %  error at % of rated Current					Phase displacement in minutes at % of r.ct
Class	10	20	100	120	10	20	100	120
0.1	±0.25	±0.20	±0.10	±0.1	±10	±8	±5	±5
0.2	±0.50	±0.35	±0.20	±0.2	±20	±15	±10	±10
0.5	±1.00	±0.75	±0.75	±0.5	±60	±45	±30	±30
1.0	±2.00	±1.50	±1.50	±1.0	±120	±90	±60	±60

Class 3&5	50%	100%
3	±3	±3
5	±5	±5

Application	Standards
	IS	BS
Precession Metering	0.1or 0.2	BL  BL
Comml. or Indl.metering	0.5 or 1.0	AM  BM  CM
Ammeters, power meter	1.0 or 3.0	CD
Relays	5P10 or 5P20	STU
Selective protection	PS

                      

  Composite Error for Protection ISS 2705 Part.III

Accuracy Class	Current error at rated prim. current	Phase displacement at rated prim current +Min.	Composite error at rated prim. current +
5 P	1	60	5
10 P	3	-	10
15 P	5	-	15

Improve Ampere-Turn

1.  By improving the quality of the magnetic material

            Cold rolled grain oriented silicon steel (C.R.O.S.S.) has a magnetisation characteristic with a knee point at 1.6 tesla.

            Nickel steel (Proprietary name Mumetal) has a knee point of 0.7 tesla.

2.  By decreasing the mean magnetic path of the core.

3.  By reducing the flux density in the core.

RATIO ERROR (CURRENT ERROR)

The ratio error is defined as the error in the secondary current due to the incorrect ratio and is expressed as a percentage, by the expression :

            

            Where :

Kn                    is the nominal ratio  (rated primarycurrent/ratedsecondarycurrent)

                        Is         is the actual secondary current

                        Ip         is the actual primary current

            The ratio is considered positive when the actual secondary current of the transformer is larger than the rated current

PHASE ANGLE ERROR

               The phase angle error is the angle by which the secondary current vector, when reversed, differs in phase from the primary current.  This angle is considered as positive if the reversed secondary current vector leads the primary current.

                On very low burden power factors the phase angle error may be negative.  Only on rare occasions is it necessary to determine the phase error of a CT used for relaying.  These occasions occur when very low circuit settings ie 1% - 5% of rated current, are used in relays which are directionalised by voltage transformer inputs.  For example, sensitive reverse power relays may require taking phase error into account in order to ensure correct directional operation where very low power factor primary currents are involved eg 1% - 3% power when certain types of prime movers are being motored eg, steam turbines and hydro sets.

C.T POLARITY TEST

           

            Each current transformer should be individually tested to verify that the polarity markings on the primary and secondary windings are correct. The following figure shows the test unit for this.

The ammemeter ‘A’ is a robust, moving coil, permanent magnet centre zero type instrument. A low voltage battery is used to energise the primary windings through a single pole push button. On closing the push-button, with above C.T ammeter markings, the ammeter should give a positive flick, indicating correct polarity of the C.T

PRIMARY INJECTION TEST

            This test is carried out to ensure the C.T ratio of current transformers. If this test is carried out after C.T secondary wiring is completed it ensures not only the correct ratio of C.Ts but also the correctness of the entire C.T secondary wiring comprising protection and metering portions. The testing equipment consists of a loading (injection) transformer, controlled by a variable transformer to get the required current on the primary side of the C.T under test.

            For carrying out the ratio test on C.Ts, the following circuit is made use of.

Current is passed through the primary windings of the standard C.T and C.T under test. The ratio of the C.T can be determined by comparing the currents in ammeters A1 and A2.

TRANSFORMER CT’s SECONDARY CONNECTIONS

BUSBAR CT’s CONNECTIONS

GENERATOR CT’s CONNECTIONS

LINE CT’s SECONDARY CONNECTIONS