How To Connect Current Transformers?

How To Connect Current Transformers?

The primary of a current transformer typically has only one turn. This is not really a turn or wrap around the core but just a conductor or bus going through the “window.” The primary never has more than a very few turns, while the secondary may have a great many turns, depending upon how much the current must be stepped down. In most cases, the primary of a current transformer is a single wire or busbar,and the secondary is wound on a laminated magnetic core, placed around the conductor in which the current needs to be measured, as illustrated in figure 1.

If primary current exists and the secondary circuit of a CT is closed, the winding builds and maintains a counter or back EMF to the primary magnetizing force.

Should the secondary be opened with current in the primary, the counter EMF is removed; and the primary magnetizing force builds up an extremely high voltage in the secondary, which is dangerous to personnel and can destroy the current transformer.

How To Connect Current Transformers?

Figure 1 – Current Transformer

CAUTION:
For this reason, the secondary of a current transformer should always be shorted before removing a relay from its case or removing any other device that the CT operates. This protects the CT from overvoltage.

Current transformers are used with ammeters, wattmeters, powerfactor meters, watt-hour meters, compensators, protective and regulating relays, and trip coils of circuit breakers. One CT can be used to operate several instruments, provided the combined loads of the instruments do not exceed that for which the CT is rated.

Secondary windings are usually rated at 5 amperes. A variety of current transformers are shown in figure 2. Many times, CTs have several taps on the secondary winding to adjust the range of current possible to measure on the primary.

Figure 2 – Photograph of Current Transformers

Figure 2 – Photograph of Current Transformers



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Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears. Professional in AutoCAD programming and web-design. Present on Google+.



8 Comments


  1. david
    Apr 03, 2014

    excellent


  2. Tony
    Mar 26, 2014

    Does a CT work properly on a MIMS cable? My thoughts are that the copper sheath would act like a faraday cage…. Has anyone had any experience with this?


  3. riaz62
    Oct 20, 2012

    excellent knowledge

  4. [...] and electrical characteristics of current transformer installed in the installation. Ensure the CT is connected to system properly in all respect (primary and secondary). 2. Test Equipment RequiredRequired equipment for [...]

  5. [...] Figure 1 – Generator stator overcurrent relayIf current transformers are not connected in the neutral ends of wye-connected generator windings, or if only the outgoing [...]


  6. sotirios
    Oct 16, 2011

    Good Morning
    I’m thinking to connect a Kwh meter in order to calculate the energy consumption but it is very difficult to connect the current transformer to the bars directly as most probably needed to be used split type as it is very difficult to dismantle the bars so I’m thinking to connect at three phases of mentioned generator smaller split type current transformers but at the cables connected with the 3 bars for transfering the power of the generator to the consumers.
    My problem is that each phase bar splitted in three cable so i have nine cables to the output to the consumers and I don’t know if each one needed Current transformer.
    In that case needed to be installed 9 transformers but I’m not sure for that.
    Your assistance will be very usefull for my problem

  7. [...] to a load or to a short circuit located at X. The conditions will be as in Fig. 15. If the two current transformers have the same ratio, and are properly connected, their secondary currents will merely circulate [...]

  8. [...] This post was mentioned on Twitter by Putu Marsya Sabrina and Electric Engineering, Electric Engineering. Electric Engineering said: How To Connect Current Transformers? http://goo.gl/fb/2akVt [...]

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