Current Transformers

-

  Did we want to really concerned about the current transformers ? Its just an instrument transformer
Yeah!,  that's make it different in protective relaying
Lets dig deep

                         

  In the most protective relays the input quantity is current and these current are supplied by current transformers. located on or near to the protected equipment. The main functions of CT are

1. To isolate the power system circuit from protection circuit

2.Reduction of magnitude to a level accessed by relays and meters

 When considering Constructional features, these are similar to normal transformers consisting a magnetically coupled primary and secondary windings, wound on a common iron core and the primary is connected in series with  the network and the ratio could be found from basics
ie,   Ampere-turns in primary=Ampere-turns in secondary
ie, low no. of turns in primary and large no. of turns in secondary to get low output currents.

                            
 For constructional simplicity and compactness manufacturers combines several cores in one package referring to special protection, metering, protection etc., where the accuracy class and accuracy limit factor are different.
                        
                                             single core and double core ct diagram


These different cores are specified in the secondary as 1,2,3 indicating first,second and third respectively. With composition of secondary terminals it may seem as 1S1-1S2, 2S1-2S2 etc.
ie, the first digit refers the core number while the last digit indicates the terminal number. Dont get confused with when there are tapping(s) in the secondary winding ie, like this 1S1-1S2-1S3, 2S1-2S2-2S3 ,etc.The tapping(s) are provided to introduce a new CT ratio, usually the half of the main winding.

  The fact that CT is basically a step up transformer has to be keep in mind to avoid the danger of high voltage from its secondary terminals and also don't let the secondary open circuited(To avoid this we have to use ring sockets instead of open headed/plug type in the path from secondary to relay terminals).
   
    A CT is specified on the basis of burden, accuracy class in conjunction with its purpose of application, and accuracy limit factor(ALF, a limit beyond that the accuracy is not guaranteed).
 Burden refers to the ability of CT to supply power to external connected load at a specified power factor and commonly used are 10 VA & 15VA.



 A typical CT  nameplate
 The accuracy class for metering applications are 0.2,0.5,1 or 3 ie, the metering CT has to be accurate from 5% to 120% of the rated primary current ,at 25% and 100% 0f the rated burden at the specified power factor.In the case of protection core accuracy class of 5P,10P are usually observed .
  While coming to ALF its the value of primary current upto which the CT complies with composite error requirements.this is typically 5, 10 or 15 times the rated.



Comments

Post a Comment

Popular posts from this blog

Electrical Substations

-
Image
     Electrical Substations An electrical substation forms an integral part of the power system. It is placed at the both ends of the long distant transmission line... but why??!! Of-course to transform voltage!  A substation at the beginning End or say generation side normally used to boost up the voltage to higher magnitude suitable for transmitting the generated power to long distant load with minimum losses.( Uses a step up transformer )!! Yes the other type is at the other end of the long distant transmission line nearer to load centre... here the high voltage is stepped down to a relevant magnitude which may be used for distribution purpose or another voltage level transmission for near by substations.                    The heart of a substation is nothing but our transformer, depending upon the purpose transformer can be step up or step down!! Now we are considering a load end substation as an example. Here our hero is step down transformer 😎!! A substation h
Read more

Differential Protection Scheme (Transformer)

-
Image
Hey friends! you are familiar with the power system equipment which we discussed earlier.! Now it's time to protect it to total safety. As soldiers protect our country from enemies likewise our system should be protected from fault. The line of control is decided by fixing the current transformers on the right place. So our boundaries is decided by their location. Here in a substation, we have many equipments such as transmission lines, bus-bar, transformers, etc in the system which need atmost care. Starting from the bulkiest, transformers are having different level of protections such as phase to phase and phase to ground. In other words differential protection and earth-fault protection. Over-current protection is also implemented in it. Let's talk about the differential type of protection for transformers. It's quite simple to explain the theory but some complicated while coming into the calculation. The term itself gives a brief idea about the
Read more

Capacitor banks and reactors

-
Image
Capacitor banks and Reactors We have already discussed the significance of reactive power in a power system, As we know the availability of reactive power depends on the excitation of the alternators.This reactive power can be provided by the generators themselves, through the adjustment of generator excitation, but it is often cheaper to provide it through capacitors, hence capacitors are often placed near inductive loads to inject reactive power.the so called capacitors are mainly of two, static condensers and synchronous condensers. The synchronous condensers have a variable value of VAR which can be varied by the excitation of the synchronous motor and the static capacitors have a fixed value of VAR.                                       The capacitor bank simply means that a group of  individual capacitors connected in  parallel and series accordingly to the design of the capacitor bank and together have a value in KVAR or MVAR. Capacitor Connection  A reactor is connected i
Read more