Current and Voltage Transformers

4. Current Transformers- Introduction ( CT)

The current transformers are the very important aspect of the power system protection . They are used to to step down the high current in the system so as to match with the relay or meter rated currents . The rated current of the relays are normally 1A or 5A and we have the CT with secondary values 1A or 5A primary rating depend upon the rated current of the equipment to be protected.
The CTs are broadly classified in to two types - one used for the metering purposes and the other for the protection purposes . The basic difference between the metering CT and the Protection CT is the saturation point ( to be discussed ). The metering CT goes to saturation soon after the normal working current so that the high value of fault current is not passed through the metering equipment which would otherwise have damaged the metering devices due to very high fault current . On the other hand the protection CTs are meant to reproduce the fault current and it shall not saturate ideally for the fault current ,and hence the protection CTs are having higher saturation point than the metering CTs.

5.Saturation of CT

The term saturation of the CT can be defined as a condition in which the CT is no longer able to reproduce the secondary current in proportion to its primary current . The basic principle on which the CT reproduces the secondary current is based on the well known Transformer principle where current flowing through the primary winding produces a flux in the core of the transformer which is normally proportional to the current in the primary. This flux is the basic reason for the current in the Secondary winding . Now if the core is not able to reproduce a flux in proportion to the primary current , then the secondary current will not be in pro potion to the primary current and hence the measurement of the current become wrong . The phenomena of the CT core unable to produce the flux in proportion to the primary current is called saturation of CT or CT core. This is a property of the core and depend upon the material of the core used in general.More interested readers are advised to go through the numerous literature available on this topic and is not discussed in detail here . However there are number of points to be understood for the reason for the current transformer to go to saturation which includes but not limited to the following
1. With constant VA applied in the CT secondary circuit the CT goes to saturation if the primary current is increased beyond the designed value
2. With constant current in the primary and if the burden is increased beyoned the designed value , still we will find the same phenomena of CT saturation.
3. The amount of the DC component available in the primary fault current etc.

There are different methods to find out the saturation based on different standards like IEEE and IEC
As per the IEEE standard , it is the intersection of 45degree line with the magnetisation curve of the CT ( for non gapped Core ) and as per IEC it can be found out by obeserving the 50% increase in magnetising current for a 10% increase in excitation voltage.

Once the CT is saturated then it will not be able to reproduce the secondary current in proportion to its primary current and hence the relays connected to this CT circuit may maloperate indiscriminately and trips the circuit without coordination with other relays in the network or it may completely fail to operate for a fault both the condition to be avoided

Hence it is the duty of the protection design engineer to do a CT sizing calculation so that , the CT will not saturate for the worst case through fault . There are number of factors to be considered before a CT is designed for protection purposes which included the maximum symmetrical and asymmetrical short circuit current and the system X/R ratio

The symmetrical and asymmetrical short circuit current can be calculated by doing a short circuit study on the system under consideration . For small systems manual calculation of the short circuit study is possible . However for large systems the application of software are generally required , otherwise the calculation become more tedious . Etap is one of the software using which we can perform the above studies()
CT design for Relay application. In any protection , the basic requirement is that , the protection should operate for an inzone fault and it should be stable for the through faults . If the CTs are designed to meet this requirement , then the CTs kneepoint voltage requiremnt can be practically reduced . The latest numerical relays having operating time less than 10msec and hence if the time to saturation of the CT is designed in such way that it is more than the time of operation of the Relay , then the CT can be said to have designed to meet the requiremnt for the internal faults , Similarly ,the kneepoint voltage shall be calculated for the through fault currents and CT shall not saturate for this value of fault current. As discussed above the important point to be rememberd for the design of CT for protection application is the kneepoint voltage requirement for internal fault and that for external fault . In the case of internal fault ,if the CT saturates before the relay operates ,then it may lead to delayed operation of the relay or no operation at all ! Both of the conditions are not acceptable from protection point of view . The latest numerical relays are designed in such a way that , its operating time is less than the so called time to saturation of CT to avoid the problem in case of internal fault. But if there are any relays which is called upon to operate as time delayed operation such as simple over current relay , then the appreciable CT saturation can not be allowed for satisfactory opeartion of the protection However in case of a through fault , the CT satuartion issue need to be considerd more seriously as the time required for the fault clerance of the back up time need to be considerd for the design of the CT and the kneepoint voltage In case auto reclosing is used in the circuit , then the time to saturation may reduce in the sucessive shots due to the effect of remenance and hence the CT core shall be less remenance or no remenance type as in TPZ class. Time to Saturation of Current Transformer
It is the time taken for the CT to enter the saturation ( which can be find out ) region from the inception of fault . The fast operating relays time of operation are less than the time to saturation of the CT . Or in otherwords the time to saturation of the CT is designed so that , it is more than the time of operation of the relay for the correct operation of protection 6.Types of CTs for protection application

7.Classification of CT for Protection

There are lots of types of CT classification based on different local and international standards . However the classification as given by IEC and IEEE ( ANSI) are of interesting and worth noting. CT classification based on IEC According to IEC the classification is based on the steady state symmetrical primary current with out considering the transient performance and the case where the steady state performance and the transient performance is taken in to consideration.
Class P is an example where the error of the CT is defined with symmetrical primary current whereas Class TPX,TPY,TPZ etc are examples in which the transient performance is the criteria for the error.

even though the performance of the class P CT is based on the steady state symmetrical fault current , it does not mean that , this CT can not be sized for the transient application with suitable oversizing . However the transient performance is more clearly defined in TPX,TPY,TPZ with the limits of error , the remenent flux available , leakage flux in the CT etc as per the standard

There are number of applications in which we need to get a non saturated waveforms to the relay within first few cycles after the fault .As the maximum effect of the transients in the CT is during the first few cycles after the fault and it dies down with time.The effect of the transient in the CT is core saturation and hence ,the CT need to be sized adequately to overcome this effect

The basic reason for the transient in the CT is the time constant of the primary ( X/R)system up to the point of fault , the point of application of the fault on the the system voltage , the remenant flux available in the CT core ,the secondary circuit time constant and the duty cycle of the CT etc

The Main difference between the TPX,TPY and TPZ Current Transformer

The remenant flux is the flux availble in the CT core once it is deenergised from the energised situation . If the remenant flux is more , then the CT will go to saturation region comparatively faster
In TPX current transformer , the remennat flux camn be as high as 80% of the maximum flux allowed in the CT and hence this is a high remenance flux CT
In TPY Current Transformer , the remenance flux allowed is very of the order of 10% and this CT is classified as low remenance CT
In TPZ Current Transformer , there is no remenance flux allowed amd hence it is called no remanance CT
The low remance flux is achieved by introducing air gap in the CT core which makes the reluctance of the core more which inturn produces more magnetising current for the CT to devolope the working flux . As the magnetising current is a direct measure of the error of the Current ransformer , the TPZ CT has maximum error among the three classification
There are specific applicational requiremnt for the three type of CTs whwre in if the CT used un circuit which uses auto reclose circuit , there are more chances for Current Transformer to go to satuartion due to the a trip followed by a close and again Trip and close operation ( Duty cycle of Current Transformer ) In this case the tendency is to go for TPY CT than a TPX CT with compromise in error . Naturally the CT selection is a compromise betwwen the error and the time to saturation and it depend on the lot of factors including the Size of the CT , the utility practice etc before the decision is made on which type of CT is best suited for a particular application
Classification based on IEEE or ANSI According to American standard ANSI , the CT is classified as class C or Class T . In brief class C is having less effect of remenance and hence its ratio can be found out by calculation whereas in case of Class T , the effect of remenance is appreciable so that ,the ratio is found out by testing the CT In this case the secondary terminal voltage is defined as the voltage that appears acros the CT secondary winding when 20 times rated current is flowing through rated burden ( defined later ) without exceeding the ratio error more than 10% Exmple C200 is the ANSI C class CT with terminal voltage equal to 200Volt when 20 times current is flowing through a standard burden connected with out exceeding the ratio error 10% . The standard burdens as per ANSI are B1, B2,B4,B8 which is equvalant to an impedance of 1 ohm , 2 ohm, 4 ohm and 8 ohm . Hence in case of C200 , the maximum burden that can be connected is 2 ohms with Ct secondary current of 5A so that the ratio error is limited to 10% at 20 times the rated current.
Transients in Current Transformer


Voltage Transformers

The main classification of the Voltage tranformer is based on its application as VT used for the metering purposes and that used for the protection purposes. The difference between the two depends on the range of accuracy specified .As per the IEC ,the metering accuracy is specified for a voltage range from 80% to 120% whereas the range of accuracy is specified from 5% to more than rated voltage incase of VT used for protection purposes.Accuracy is defined even for voltage less than 5% . to be continued