A Simple Method for Compensating Harmonic Distortion in Current Transformers: Experimental Validation

Abstract

Conventional current transformers (CTs) suffer from nonlinearities due to their ferromagnetic cores. On one hand, it is well-known that severe core saturation may occur because of large overcurrents or unidirectional transient components: this may substantially impact the operation of relays. On the other hand, weaker nonlinear effects are also present during regular working conditions. In particular, the spectral content of typical current waveforms is characterized by a strong fundamental term responsible for harmonic distortion affecting the frequency components at the secondary side. In turn, this has a significant impact on the accuracy that can be reached as long as current harmonics must be monitored. The target of this work is implementing a simple signal processing technique that allows compensating for this effect. The method, characterized by extremely low computational complexity, is first introduced and validated using numerical simulations. After this, it was tested experimentally to improve the harmonic measurement capability of inductive CTs. The achieved results highlight a noticeable reduction of errors at low-order harmonics over a wide range of primary current amplitudes. It is worth noting that the black-box approach makes the technique suitable also for compensating nonlinearities introduced by current transducers based on different operating principles. Thanks to this peculiarity and to the low computational complexity, the proposed method is suitable to be employed in power quality analyzers and merging units. In this way, high-accuracy monitoring of current harmonics in power systems can be achieved, opening the way to advanced power quality management and to the location of disturbing users.

Refined Modeling and Compensation of Current Transformers Behavior for Line Parameters Estimation Based on Synchronized Measurements

“…When considering CTs, the situation is rather different, since the primary current may vary over a broad range. For this reason, as well as for the presence of weak nonlinear effects, assuming that CTs introduce constant ratio and phase errors does not represent, in general, a good approximation [21], [22]. As a matter of fact, considering a given accuracy class for measuring CTs, the relevant standards [23] and [24] allow significantly larger ratio and phase errors in the lowest part of the measurement range.…”

“…Experimental tests have been carried out on CTs from different manufacturers in order to assess their metrological performances. Obtained data have been firstly used for representing CT errors in numerical simulations, following the same approach adopted in [21], [22]. Moreover, these data have been also exploited to define simplified parametric models aimed at capturing the basic features of their behavior, in particular the relationship between error values and current magnitude.…”

The Influence of the 3rd Harmonic of the Distorted Primary Current on the Self-Generation of the Inductive Current Transformers

“…Therefore, its transformation accuracy may be determined only for a certain range and type of its load as well as the RMS value of the distorted primary current. The articles [24]- [27] present the methods of compensating the low order higher harmonics selfgenerated by the inductive CTs. These solutions do not consider the change in the value of the magnetic flux density in the magnetic core cause by the distortion of the transformed primary current including the influence of the 3 rd higher harmonic.…”

The Method to Determine the Turns Ratio Correction of the Inductive Current Transformer

“…The accuracy of the inductive CT depends mainly on the magnetic material that the magnetic core is made of. Therefore, compensation of their values of current error and phase displacement may be used to ensure higher accuracy at a reduced cost [4][5][6]. One proposed method is to use an active secondary circuit to increase the secondary current with the calculated value resulting from the instantaneous values of the magnetic core excitation current [4].…”

“…Another approach is to use an active secondary circuit to implement the computational algorithms from an artificial neural network to reconstruct the waveform of the distorted secondary current [5]. The compensation of the CT transducer may also be realized by appropriate analysis and processing of voltage from the secondary winding [6]. Saturation of the magnetic core of inductive CTs, which has a negative effect on their accuracy, may also be compensated [7][8][9][10][11][12].…”