Current Transformer Operation

EE 3092Laboratory Practice V

Instructed by: Mr.Sujeevan Name : S.P.M SudasingheIndex No. : 100523GGroup : G - 12Field : EEDate of Performance : 2013/10/02Date of Submission : 2013/10/21

CALCULATIONSTransformer Ratio CalculationsFor protection type current transformer: IS : IP = 0.6/50For measuring type current transformer: IS : IP = 0.6/50Knee point Voltage CalculationAt knee point 10% increase in voltage causes 50% percent increase in currentHence at knee point Gradient of magnetizing curve = dVdI =10 %

50 % = 0.2For protection type current transformerEquation of the magnetization curve for protection type current transformer is ( from graph 1)

V=8.3277 ln I−21.491

dVdI

=8.3277Iat knee point,

dVdI

=8.3277I

=0.2⇒I=41.64mA

∴V=8.3277 ln (41.64−21.491 )

∴ Knee point voltage V = 9.56VFor measuring type current transformerEquation of the magnetization curve for protection type current transformer is ( from graph 2)

V=1.995 ln I +9.6687

dVdI

=1.995Iat knee point,

dVdI

=1.995I

=0.2⇒I=9.975 A

∴V=1.995 ln (9.975 )+9.6687

∴ Knee point voltage V = 14.26 VBurden calculationFor protection type CT, burden ¿V s I s=0.4×0.6=¿0.24 VA

For measuring type CT, burden ¿V s I s=0.4×06=¿0.24 VADISSCUSSION1. Secondary resistance of a protective transformer is low compared to that of a measuring transformerThe protection current transformer is used to transform the higher currents in power systems to controllable value to use in protection schemes such as relay systems. If fault detected relay mechanism disconnected the supply. Usually the currents in power systems (transmission lines) is in kA range. Hence currents associated with protection type CT’s are much higher and required accuracy is low. Hence secondary resistance is low.

In measuring type transformer there is no much high current flow as in protection CT’s because small current is sufficient to get the measurement. Therefore in here need only a small current which is proportional to the high current in primary. Also measuring type requires higher accuracy than protection type CT. Therefore resistance of measuring type CT is higher than protection type CT.2. Significance of knee point voltage of the magnetizing curve

Knee-point of a current transformer is defined as the voltage at which a 10% increase in applied voltage, will increase the magnetizing current by 50%. Linear relation between V & Ie is maintained from point A & K. The point ′A′ is known as ′Ankle Point′ and point ′K′ is known as ′Knee Point′ (figure 1).

Figure 1: Magnetizing curve for typical current transformer

When the knee-point voltage of a current transformer is reached, the linear relationship between the primary and secondary currents will no longer apply. This could pose some problems in protective relay settings and coordination if the transformer is protective type one. The knee-point voltage of a protective type current transformer is critical in protective relay application because fault currents usually range from 20 to 30 times the rated currents of the current transformers. When it comes to measuring type CT’s error of the measurements increased beyond the knee point voltage due to core saturation. Hence for proper operation of both protection and measuring type current transformers operating region should lie within the linear region of magnetizing curve. That is knee point voltage should not be exceeded.

3. Selection of Current Transformer for a given application Current transformers generally use for monitoring, controlling and protection. There are various things to be considered for the selecting a current transformer for an application. Factors such as magnitude, frequency, function, sampling rate, the accuracy and effectiveness will essentially be dependent on these parameters. Aside from the possibility of compromising the transformer’s accuracy, using a current transformer above the manufacturer’s rated current specification may saturate the transformer and may cause circuit failures due to an uncontrolled rise in operating temperature. On the other hand, a current transformer that is rated much higher than the sample current might be restrictively too large and expensive for its purpose. Typically, selecting a current-transformer that is rated approximately30%above the expected maximum of the sample current is a prudent starting point. So when selecting a current transformer for a given application there are some facts that should be given special attention. Accuracy, burden and knee point voltage of a current transformer need to consider before selecting a current transformer for the application.