TECHNICAL DETAILS

Following points need to be considered for CTs for electronic control circuits.

  • CT converts primary current in to small value secondery current.
  • Generally secondory current signal is converted in to voltage signal by terminating by suitable resistance.Voltage proportional to primary current is developed across secondory terminals.
    Generally 5V &10V are selected as maximum value at rated current primary.
  • For example 30A/30mA CT,165 ohm,5V CT is having ratio1,000,core to get 5V at 50Hz. Current transformers work on magnetic field established by current passing through primary conductor.
  • For primary currents of values below 5Amps,the magnetic field is week & multiple primary turns
    give positive linkage of magnetic field.

General Information about CTs

Types of CTs

Current transformers are generaly used to convert primary current in to

  1. 5A or 1A as per IS 2705
  2. OR 1mA,5mA,10mA,30mA,100mA in case of electronic control systems

CTs as per IS 2705

  1. Metering class CT

    Class .5,1,5.These are used in metering,measurement & control in control panels. These CTs are designed to saturate beyond 120% of rated current to protect meters. It is advisable to select CT ratio that actual current is between 75 to 100%

  2. Protection class CT

    5P10,10P10 etc.These CTs are used where pick current of 10 to 20 times is to be monitered by protection relay.The accuracy is less importantant.

  3. Core balance current transformer,Zero sequence CT,earth leakage CT:-These CTs are used to detect unbalance due to leakage.All three phase wires are passed through CT,the secondery current is supposed to be zero in balanced condition.The CT ratio depends on leakage % of rated current to be detected.In electronic control systems,the ratio is 25A/5mA.The CT is round or rectangular to suit bus bar arrangement.There is no standardization about size & shape.
  4. Multiratio ratio CT

    Class .5,1,5.These CTs are used when wide range of primary currents are to be measured by single CT.ie say 20A to 200A.Taps are provided on secodery winding to select ratio to get good accuracy.

  5. Differential CT

    Class SP.These are used in supply lines to detect losses line losses.

  6. Summation CT

    These are used to get magnetic summation of currents of 2 or more feeders.There are primary windings corresponding to each feeder & one secondey winding.The ratios are 5+5/5A,5+5+5/5A etc.

CTs for electronic control applications

In electronic control citcuits the current required is in milliampers & requires low voltages. Some of the systems use secondery current directly but most of commen systems use voltage generated across secondery resistor. Earlier systems use voltages 5V,10V as rated voltage but new control systems require 1V or millivolts across load resistor. CT becomes smaller with smaller voltage & smaller seconery current combination.
In case of small currents the magnetic field is week.It is advisable to put primary turns up to 5Amps.
These CTs are generally used in PCB up to 100A primary current.Higher primary currents require higher innner diameter & are heavier.Normaly these are base mounted.The secondery currents being small the CT needs protection such as encloser or epoxy potting/moulding. Accuracy of the CT depends on selection of correct inner diameter,proper secodery load & proper ratio selection.The CT is accurate from 10% to 120%.

In order to get proper CT following informationis required

  • Primary curent,rated & surge if any, with duration of overload.
  • Secondery current at rated primary current
  • Secodery load resistor & voltage across load
  • Inner diameter or inner slot dimensions in case of bus bar.
  • Mounting required ie base/PCB/wire
  • Termination required by wire,screw or PCB pins
  • Whether resin potting/moulding required
  • Dimensional restrictions if any
  • Special moulding or colour required