Why Should the CT Secondary Never be Opened?
Purpose:
It is dangerous to open-circuit the CT secondary circuit when primary circuit is energised as it may lead to fire hazard and even explosion of CT.
Introduction:
The Current Transformer ( C.T. ), is a type of “instrument transformer” that is designed to produce an alternating current in its secondary winding which is proportional to the current being measured in its primary.
Current Transformers can reduce or “step-down” current levels from thousands of amperes down to a standard output of a known ratio to either 5 Amps or 1 Amp for normal operation. The Ammeter is scaled to read the correct primary value.
Operating principle of Current Transformer:
When current flows through primary, as per Faraday's law of Electromagnetic induction, it induces primary flux ? 1 into the core which results in secondary current I2. This current I2 also induces secondary flux ? 2 but in opposite direction. Hence it opposes the primary flux ? 1. Therefore, it is called as counter balancing flux. This resultant flux ? = ? 1 - ? 2 is the flux that flows through the core. Its value is relatively small compared to ? 1. This flux maintains linear relationship between secondary and primary current depending on the core material when secondary connected with Load.
Now consider a condition when CT secondary is opened when primary is energised:
When secondary circuit is an open circuit, as per Lenz's law a very high back Emf voltage is generated. When secondary circuit is open, secondary current I2=0 and hence counter balancing flux ? 2 is not generated. Accordingly, Net flux = ? = ? 1 which is very large and this produces a very high back emf as per Lenz's Law of electromagnetic induction.
This voltage (back emf) is extremely dangerous. It can result in deadly shock to the person who opened the secondary. This extreme high voltage can exceed insulation levels of CT and result in fire hazard and even explosion of CT. CT secondary is terminated to the Meter. If there is a loose connection or open circuit at meter end, such hazard might happen.
Effect of Open Circuit:
? Due to absence of secondary current no counter magnetic field is produced and flux in core of CT rises to saturation level which can change the characteristic of CT and damage it permanently.
? Since number of turns in secondary is very high (e.g.1:1000), a very high voltage may be induced in secondary that can harm the persons working on the instruments panel.
Care to be taken while CT open circuit fault:
? Generally, CT S2 terminal are very often grounded (in star connection), if you are touching Termination Frames or Relay Racks you may become low resistance path for current if you are in contact during an open-circuit incident.
To avoid open circuit fault:
? When CT secondary is terminated at Meter, it must be ensured that the connections for secondary circuit are secure and reliable in order to avoid fire and shock hazard.
? Ensure proper cable termination practices and periodic maintenance.
Owner, Hi-Tech Marketing
5 个月Well said!