The excitation current of a current transformer is the current required to establish and maintain the magnetic flux in the CT core. It is a small but critically important component of the CT's operation, as it directly affects the accuracy of the current transformation. According to the International Electrotechnical Commission (IEC), the exciting current is defined as "the r.m.s. value of the current taken by the secondary winding of a current transformer, when a sinusoidal voltage of rated frequency is applied to the secondary terminals, the primary and any other windings being open-circuited".
In the CT equivalent circuit, the excitation current flows through the magnetizing branch, which represents the core's magnetic properties. The primary current is assumed to be transformed perfectly to the secondary side, but a portion of this current is diverted through the excitation branch. The remainder is the true secondary current that flows through the external burden. The excitation current is a function of the secondary excitation voltage (Es) and the secondary excitation impedance (Ze), and it represents the ampere-turns required to magnetize the core.
The excitation current can be decomposed into two components: the magnetizing current and the loss component. The magnetizing current (Im) is the component that produces the magnetic flux in the core. The loss component (Ia) represents the active power losses in the core due to eddy currents and hysteresis. The total excitation current is the phasor sum of these two components: Ie = Im + Ia. The magnetizing current is typically much larger than the loss component, particularly in high-quality core materials with low losses.
The magnitude of the excitation current depends on several factors. The core material properties-including permeability, saturation flux density, and losses-play a primary role. Higher permeability materials require less magnetizing current to produce a given flux. The core dimensions and the number of secondary turns also affect the excitation current: a larger core and more turns generally result in lower excitation current for a given voltage. The excitation voltage, which is determined by the burden and the secondary current, is another critical factor.
The excitation current is the primary source of error in a current transformer. Because the excitation current subtracts from the current that would otherwise be available for the secondary circuit, it directly contributes to the ratio error of the CT. The relationship between excitation current and ratio error can be understood by considering that the primary ampere-turns must equal the sum of the secondary ampere-turns and the excitation ampere-turns. When the excitation current increases-due to higher burden, higher secondary voltage, or core saturation-the secondary current decreases, and the ratio error increases.
The excitation characteristic of a CT is typically presented as a curve relating the applied secondary voltage to the excitation current. This curve has two distinct regions: a linear region at low voltages where the core is not saturated, and a saturated region at high voltages where the excitation current increases rapidly with small increases in voltage. The transition between these regions is called the knee point. Below the knee point, the excitation current is small and the CT accuracy is good. Above the knee point, the excitation current increases dramatically and the CT accuracy deteriorates rapidly.
The excitation current determines the maximum accuracy that can be achieved with a given CT. Because the excitation current is an inherent property of the magnetic core, it sets a fundamental limit on the CT's performance. Even under ideal conditions, the excitation current introduces a small but non-zero ratio error and phase error. The CT designer's goal is to minimize the excitation current through careful selection of core material, core dimensions, and winding configuration.
In practical applications, the excitation current is measured during CT testing to verify the CT's performance. The excitation test involves applying a variable voltage to the secondary winding with the primary open-circuited and measuring the resulting excitation current. The excitation curve obtained from this test is used to determine the knee point voltage and to assess the CT's suitability for protection applications. A CT with a lower excitation current for a given voltage is generally considered to be of higher quality.
The excitation current also has important safety implications. If the secondary circuit of a CT is inadvertently open-circuited while primary current is flowing, the excitation current will produce a very high voltage across the secondary terminals. This can endanger personnel and damage equipment. The secondary circuit of a current transformer should never be left open-circuited while primary current continues to flow, as the CT behaves as a highly saturated choke, producing a peaked and relatively high voltage at the secondary output.
In summary, the excitation current is a fundamental characteristic of current transformers that directly determines their accuracy and performance. It represents the current required to magnetize the core and is the primary source of ratio and phase errors. Understanding the excitation current-and its relationship to core material, burden, and operating voltage-is essential for CT selection, testing, and application in electrical power systems.