The job is done by the current transformer(CT) and voltage transformer is to measurements and relaying decisions are derived from current and voltage signals. Relaying hardware works mostly with a smaller range of current i.e. in amperes and not in kA and voltage in volts and not in kV, relay feed signals to feeder or transmission line currents, and bus voltages scaled to lower levels and then fed to the relays. CTs and VTs are also electrically isolated from the relaying system from the actual power apparatus in the substation or in our transmission system. The electrical isolation from the primary voltage also provides the safety of both human personnel who are working there and also the equipment.
- CT and VT function like ‘ears’ and the ‘eyes’ of the protection system in our electrical network. CT and VT listen and observe all happening in the external world. The relay itself is like the brain who gives signal what and when that processes these signals and issues decision commands implemented by circuit breakers, alarms, etc.
- CT and VTs work like sensors for the relay.
Equivalent Circuit Of Current Transformer(CT)
The equivalent circuit of a current transformer (CT) is the same as a regular transformer. The fundamental difference between the regular transformer and CT is that regular power transformers are excited by a voltage source and a current transformer has current source excitation. The primary winding of the current transformer is connected in series with the transmission line. The load on the secondary side of the current transformer is the relaying burden and the lead wire resistance.
For a better understanding of CT modeling, let assume that the CT primary is connected to a current source. Therefore, the CT equivalent circuit will look as shown in the above figure. The remaining steps in modeling are as follows:
- We can neglect the series impedance which is connecter in series with the current source and also the primary winding resistance and leakage reactance in CT modeling.
- For a better analysis of the CT circuit, we can shift the magnetizing impedance from the primary side to the secondary side of the ideal transformer as we do regular transformer.
Working Of Current Transformer(CT)
The CT equivalent circuit is as shown in the above figure. We can not neglect the secondary winding resistance and leakage reactance as it will affect the performance of CT. The total impedance on the secondary side of CT is the sum of relay burden, lead wire resistance, and leakage impedance offered by the secondary winding of CT. Therefore, the voltage developed in the secondary winding of the current transformer directly depends upon these parameters.
The secondary voltage developed by the CT has to be observed because as per the transformer emf equation, the flux level in the core depends upon secondary voltage. The transformer emf equation is given by,
where \phi_mis the peak sinusoidal flux developed in the core. If Bm is above the knee point, it is more or less understand that the CT will saturate. During saturation, CT secondary winding cannot show the accurately primary current, and hence, the performance of the CT effects.
The use of numerical relays is due to small burden vis-a-vis solid-state and electromechanical relays, that’s should improve the CT performance. CT operates in a closed condition. If the CT is open-circuited then all the current Ip/N, would flow through Xm. This will lead to the development of the dangerously high levels of voltage in the secondary winding which can even burn out the CT winding and causes a fault.
Equivalent circuit of saturated CT
The major problems seen in the protection systems are the saturation of CT on large ac currents and dc offset current present during the transient period. When the CT is saturated, the primary current source cannot be accurate as reflected on the secondary side. In other words, we can open circuit the current source in the above figure. Also, the magnetizing impedance decreases during this saturation. Then the transformer behaves like an air core device, with very little that can be neglected coupling between the primary and secondary winding. The high reluctance offered by the air path implies that the magnetizing impedance (inductance) falls down. The corresponding equivalent circuit is shown in the figure.
Classification of Current Transformers
The CTs can be classified into following types
A measurement current transformer has a much lower VA capacity than a protection grade CT. A measurement CT is accurate over its complete range e.g. from 5% to 125% of normal current. We can say that its magnetizing impedance at low current levels and hence low flux levels should be very high. Due to the non-linear nature of the B-H curve, magnetizing impedance is not constant throughout but varies over the CT’s operating range. It is not expected to give a linear response i.e. secondary current a scaled replica of the primary current is seen during large fault currents.
For a protection CT, the linear response is seen up to 20 times the rated current. Its performance is accurate in the range of normal currents and up to fault currents. Specifically, for protection grade, CT’s magnetizing impedance has a large value in the range of the currents of the order of fault currents.
When a CT is used for both the purposes i.e it has to be of required accuracy class to satisfy both accuracy conditions of measurement CTs and protection CTs. However, it has to be accurate for both very small and very large values of current. Mostly CT secondary rated current is standardized to 1A or 5A.