The current transformer is an essential part of the power system. The basics of current transformer including construction, applications, working principles are going to be discussed in this article. Moreover, some practical aspects such as grounding and connections of current transformer and associated errors will be examined comprehensively.
Current Transformer is an 'instrumentation' transformer which steps down high values of currents to lower values.
As evident from their name, Instrumentation transformers are used to isolate the instrumentation devices from high voltages and currents to facilitate the measurement of electrical quantities.
Current Transformers are used extensively for measuring current and monitoring the operation of the power grid.The need for Current Transformer is justified by two reasons:
The construction of a Current Transformer is very similar to a normal transformer.The core of the current transformer is built up with lamination of silicon steel.
A current transformer (CT) basically has a primary coil of one or more turns of heavy cross-sectional area. In some, the bar carrying high current may act as a primary. This is connected in series with the line carrying high current.
Not only does the construction of current transformer similar to a regular transformer but the working principle is same as well.
The alternating current in the primary windings induces magnetic flux in the core which is transferred to the secondary windings and inducesalternating current there as well.
These transformers are basically step-up transformers i.e. stepping up a voltage from primary to secondary. Thus, the current reduces from primary to secondary.
Current Transformer used for metering andindicating circuits are popularly termed as Measuring CT.They have a low saturation point. In case of a fault, the core will become saturated and the secondary current will not damage the measurement devices connected to it.
Current Transformer used in conjunction with protective devices is termed as Protection CT. The purpose is to detect fault currents in the system and pass on the signal to relays. Since it operates on current values higher than its rated value, its core has a high saturation point.
This type of current transformer uses the actual cable or bus-bar of the main circuit as the primary winding, which is equivalent to a single turn. They are fully insulated from the high operating voltage.
The transformers primary winding is physically connected in series with the conductor that carries the measured current flowing in the circuit.
These do not contain a primary winding. Instead, the line that carries the current flowing in the network is threaded through a window or hole in the toroidal transformer. Some current transformers have a "split core" which allows it to be opened, installed, and closed, without disconnecting the circuit to which they are attached.
A CT is quite simple to connect in a single phase system, but for three phase system, there are 3 CTs which can be connected in two ways:
Like any other transformer, a CT also has a polarity. Polarity refers to the instantaneous direction of the primary current with respect to the secondary current and is determined by the way transformer leads are brought out of the case.
All current transformers are subtractive polarity. CT polarity is sometimes indicated with an arrow, these CTs should be installed with the arrow pointing in the direction of current flow.
It is very important to observe correct polarity when installing and connecting current transformers to power metering and protective relays.
The grounding of the current transformer is very important for the safety and correct operation of the protective relays.
As per the grounding standard of the current transformer, the current transformer secondary circuit should be connected to the station ground at only onepoint. This holds true irrespective of the number of current transformer secondary winding connected to the circuit.
Current transformer burden is defined as the load connected across its secondary. It is generally expressed in VA(volt-ampere).
In short, the connecting wires and the connected meter form the load of the current transformer. In technical terms this is called load in VA. This load influences the accuracy of the current transformer. In the design of the current transformer, internal losses and the external load of the current transformer are taken into account.
The burden is expressed in VA by multiplying the secondary current with the voltage drop across the burden (load) of the CT. A current transformer is divided into classes on the basis of accuracy which, in turn, depends on the burden of the CT.
The CT ratio is the ratio of primary current input to secondary current output at full load. For example, a CT with a ratio of 100:5 is rated for 100 primary amps at full load and will produce 5 amps of secondary current when 100 amps flow through the primary.
The current transformer has two errors – ratio error and a phase angle error.
It is mainly due to the energy component of excitation current and is given as
Where Ip is the primary current, Kt is the turn ratio and Is is the secondary current.
In an ideal current transformer, the vector angle between the primary and reversed secondary current is zero. But in an actual current transformer,there is a phase difference between the primary and the secondary current because the primary current has also supplied the component of exciting current. Thus, the difference between the two phases is termed as a phase angle error.
The ideal current transformer may be defined in which any primary condition is reproduced in the secondary circuit in the exact ratio and phase relationship. The phasor diagram for an ideal current transformer is shown in Figure 1.
In an actual transformer, the windings have resistance and reactance and the transformer also has magnetizing and loss component of current to maintain the flux (see Figure 2). Therefore, in an actual transformer the ratio of current is not equal to the turns ratio and there is also a phase difference between the primary current and the secondary currents reflected back on the primary side. Consequently, we have ratio error and phase angle error.
Kn = turns ratio = number of secondary winding turns/number of primary winding turns,
Rs, Xs = resistance and reactance respectively of the secondary winding,
Rp, Xp = resistance and reactance respectively of the primary winding,
Ep, Es = primary and secondary induced voltages respectively,
Tp, Ts = number of primary winding and secondary winding turns respectively,
Ip, Is = primary and secondary winding currents respectively,
θ = phase angle of the transformer
Φm = working flux of the transformer
δ = angle between secondary induced voltage and secondary current,
Io = exciting current,
Im = magnetizing component of exciting current
Il = loss component of exciting current,
α = angle between Io and Φm
The Current Transformer's modelling is just the same as any other transformer. The model of CT is as below:
X1 = Primary leakage reactance
R1 = Primary winding resistance
X2 = Secondary leakage reactance
Z0 = Magnetizing impedance
R2 = Secondary winding resistance
Zb = Secondary load