Protective Current Transformers Research Articles

Effect of Primary Cable Position on Accuracy in Non-Toroidal-Shaped Pass-Through Current Transformer.

Non-toroidal-shaped primary pass-through protection current transformers (CTs) are used to measure high currents. Their design provides them with a big airgap that allow the passing of several cables per phase though them, which is the main advantage versus toroidal types, as the number of CTs required to measure the whole phase current is drastically reduced. The cables passed through the transformer window can be in several positions. As the isolines of the magnetic field generated by the primary currents are centered in the cables, if these cables are not centered in the transformer window, then the magnetic field will be non-uniform along the transformer core. Consequently, local saturations can appear if the cables are not properly disposed, causing the malfunction of the CT. In this paper, the performance of a non-toroidal-shaped protection CT is studied. This research is focused on the influence of the cable position on possible partial saturations of the CT when it is operating near to its accuracy limit. Depending on the cable position, the ratio of the primary and secondary currents can depart from the assigned ratio. The validation of this phenomenon was carried out via finite element analysis (FEA), showing that partial transformer core saturations appear in areas of the magnetic core close to the cable. By applying FEA, the admissible accuracy region for cable positioning inside the CT is also delimited. Finally, the simulation results are ratified with experimental tests performed in non-toroidal protection CTs, varying the primary cables' positions, which are subjected to currents up to 5 kA, achieving satisfactory results. From this analysis, installation recommendations are given.

Remanent flux detection method for a protection current transformer based on fluxgate theory

Protection current transformer (CT) saturation caused by the remanent flux will affect the performance of protective relays. The saturation can be suppressed by demagnetizing the iron core. Detecting remanent flux can make demagnetization more effective. This paper proposes a remanent flux detection method based on the fluxgate theory, which can effectively measure the magnitude and polarity of the remanent flux using the second harmonic of the current generated by a single-phase electronic inverter. The proposed method can detect the remanent flux without changing it, no matter whether the CT is connected to the power system or not, which means that this method can measure the remanent flux when the CT is under normal operating conditions. The effectiveness of this method is verified in EMTP-RV software, and the simulation results show that this method can detect the remanent flux with high accuracy.

Fast and in situ remanent flux detection method for a protection current transformer based on the fluxgate theory

This paper proposes a fast and in situ remanent flux detection method for a protection current transformer (CT) based on the fluxgate theory. A high frequency voltage source is connected to the secondary side of the CT, and saturates the core slightly in the positive or negative direction. The polarity of the remanent flux is decided by the sign of integral of the voltage. The remanent flux is calculated with the linear relationship between the remanent flux and the second harmonic of the high frequency component of the voltage. The linear relationship can be obtained in advance considering the fluxgate theory. To verify the performance of the proposed remanent flux detection method, different levels of remanent flux, with and without primary current, are considered with Electromagnetic Transient Program - Restructured Version simulations. The results indicate that the proposed method can quickly and effectively detect the remanent flux of a protection CT in situ, which means that the CT is not required to be removed from the substation.

A novel non-invasion magnetic sensor array based measurement method of large current

The development of smart grid calls for pervasive deployment of current measurement in the power transmission and distribution system, serving for monitoring, control and protection purpose. However, traditional methods fail to measure current with bandwidth from direct current to kHz and are inconvenient to install or dismount. In this paper, a novel current measurement method with a clamp-like structure is proposed to alleviate the magnetic interference from random sources. This method is based on a special magnetic sensor array which consists of three pairs of sensors. In each pair, the sensitive directions of the sensors are opposite. Besides, positioning calibration of the conductor under measurement is considered in order to reduce the error contributed by the displacement uncertainty of the conductor. The proposed design is tested by numerical simulation and finite element analysis (FEA). For peak current up to 500 A, the maximum error is 0.6532%. A prototype is then implemented and tested in laboratory experiments to demonstrate the performance of the proposed approach for measurement and protective purpose. From the results, the maximum error is 1.73%, which is compatible with standard IEC 60044-8:2002 accuracy classes for protective electronic current transformer.

Influence of electromagnetic forces in the gaps of a protective CT

Two of the main problems studied in protective current transformers (CTs) are the saturation and residual flux in the ferromagnetic core. To minimise the effects of high saturation levels and residual flux, a gapped core CT can be employed. Although solving both issues, research reports that the introduction of gaps in the ferromagnetic core can develop into a mechanical fragility when compared with a closed core. However, the fragility is not proven in the research. To investigate the mechanical fragility of a gapped core CT, this research aims to verify the influence of the electromagnetic forces in the gap borders of a protective CT under a short-circuit condition. The influence was verified in three different gap lengths: 8, 16 and 24 mm. On the basis of the results, the electromagnetic forces in the gap borders of a protective CT did not influence the CT response.

Modeling and experimental validation of a dynamic regional saturation J-A model for protective current transformer

The analysis and accuracy of modeling the protective current transformer are yet not suitable for a wide range of current levels, especially under weak transient saturation. It is necessary to develop a mathematical tool allowing to create field models of current transformers allow modeling not only steady modes, but also transients ones. With the support of the large current transient test conducted on the real protective current transformers, multiple test levels from 6 to 48 kA transients were obtained. This paper devoted to the study of modeling protective current transformers more accurately proposes an improved dynamic regional saturation J-A model. It emphasizes the excess loss of the core under transient conditions, and designs a numerical solution for the proposed dynamic regional saturation J-A current transformer model without iteration, so that it can simulate the secondary output of the current transformer online. By numerous data which are actually saturated in the transient flow test, the proposed dynamic partitioned saturation J-A model, the static J-A model and the dynamic J-A model are compared and verified, respectively. Extensive results prove the effectiveness of the proposed method, which is capable to modeling protective current transformer transient of the actual situation. And the average error under a wide range test is relatively stable.

On the Requirements of Current Transformers in Differential Protection of Transformers at Electric Power Stations

The problem of picking protective current transformers (CT) in order to ensure proper functioning of differential protection in transient regimes is examined. It is proposed that the accuracy limit factor K lim referenced to the nominal current of a power transformer be used as the main parameter in choosing CT. Amethod is presented for calculating K lim as a function of the time to saturation t s, the decay time constant of the aperiodic component of short-circuit (SC) current, and the residual induction and current of external SC. It is shown that increasing K lim results in smaller CT errors in the time interval of the saturated state of a magnetic circuit. It is pointed out that the effect of screening must be taken into account in finding K lim for CT which do not have a proper primary winding and are arranged in phase with screened conductors.

Residual Flux Mitigation of Protective Current Transformers Used in an Autoreclosing Scheme

To avoid saturation of a protective current transformer (CT) operating under an autoreclosing scheme, the CT size should be chosen much higher than a similar CT operating under a single-step fault clearing scheme. The main reason for this significant difference is that the residual flux (caused by the first stage of the fault) cannot be noticeably reduced during the deadtime interval of the reclosing process and, therefore, the occurrence of a subsequent fault can extremely saturate the CT. In this paper, a low-cost, low-power electronic device is developed and introduced to demagnetize the CT under the reclosing deadtime interval. It will be shown that by using this device, the required size of the CT is reduced by about 40%. To develop this device, an existing flux-based demagnetization technique is improved to operate faster and more efficient. The performance of this technique is independent of the CT characteristic parameters and provides promising results despite its simplicity. The proposed device can be employed to de-rate the new CTs to be installed in the system and/or to compensate the existing inservice CTs. Comprehensive computer simulations and laboratory experiments are employed to validate the reliability and effectiveness of the proposed compensation method.

Fast, <italic>In Situ</italic> Demagnetization Method for Protection Current Transformers

This paper proposes a fast, in situ demagnetization method for protection current transformers (CTs). Simulations show that the method can automatically demagnetize a CT within 300 ms in situ , i.e., while the CT is deenergized but still connected to the substation. Therefore, unlike existing methods that require the CT to be disconnected from the substation, this new method can be used to demagnetize a CT during a short interruption, e.g., the time it takes an autorecloser to operate, rather than requiring an extended outage of the substation. This will dramatically increase the number of opportunities for the demagnetization of protection CTs, which will alleviate the threat of remanent flux in the CT from causing saturation and compromising the performance of power system protection. Furthermore, by demagnetizing the CT in situ , the method avoids the risk of the improper reconnection of the CT, creating a source of hidden failure. The new method uses the existing dc voltage source in the substation to charge the CT and a parallel-connected inductor. An automatic control scheme is presented that manages the charge/discharge of the inductor, and thereby the demagnetization, by controlling three switches that reconfigure an $RLC$ circuit. The additional hardware required by the method is the three switches, the switch controller, and the $RLC$ circuit. The new method is tested for different hysteresis characteristics, levels of remanent flux, and errors in the assumed element parameters and measured hysteresis characteristics using EMTP-RV simulations. The results indicate that the proposed method can quickly and effectively demagnetize a protection CT in situ .

Immunity of current-to-voltage transducer, with coils manufactured by means of multilayered PCB technology, to external magnetic fields

Purpose – The purpose of this paper is to discuss the operation of new generation electromagnetic current-to-voltage transducer. The aim of research was analysis of behaviour of considered current-to-voltage transducers during operation. The main problem was to estimate whether the external fields are able to change the value of the secondary voltage and that the replacement of the casing material by a conductive or ferromagnetic material will increase the immunity of the transducer to external magnetic fields. The immunity of current-to-voltage transducers to the external fields is very important because it influences the proper functioning of the protection system. Design/methodology/approach – The use of analytical methods to assess the influence of external fields was impossible due to the complexity of the geometry. The 3D computations were necessary because of different cross sections of circuit boards at different heights. Therefore the numerical 3D field-and-circuit method based on finite element method was applied. The wide range of dimensions in computation system, ranging from 0.15 mm (print paths) to 0.22 m, made it necessary to use the mesh of millions of elements. The division of this type of system into elements requires a diverse and extremely dense mesh in the area of printed circuits board (PCBs). Findings – The 3D analysis of magnetic field distribution was performed for different external field effect upon a current-to-voltage transducer. The magnetic field distributions and the induced secondary voltage for several different cases were presented. As a conclusion it can be said that in this particular case the magnetic shield is most effective. The influence of external magnetic fields caused by currents passing through the other neighbouring phase bars near are insignificant for the transducer with non-magnetic core. Practical implications – Commonly used in measuring and protection systems of the transmission lines are induction instrument transformers. The instrument transformers are very precise devices and their errors are counted in tenths of a per cent, and phase displacement of signals in minutes. Especially in HV systems they are very big and their cores are heavy. Replacement of instrument transformers by the current to voltage transducers cooperating with electronic measuring systems will reduce the size and cost of devices. Originality/value – The requirements set for protective current transformers concern the transformation of currents, with high accuracy, especially at transient states. Therefore magnetic characteristics of their cores should be linear. It causes that cores are large and have some air gaps. Current-to-voltage transducers based on Rogowski coil are particularly suitable for the replacement of the protective current transformers because of their linearity. The traditional technologies used for making Rogowski coil consisted in winding a wire on a non-magnetic carcass. The development of technology has enabled the use of new technologies PCB high density interconnect in the production of Rogowski coil.

Current transformer model with hysteresis for improving the protection response in electrical transmission systems

In this paper, a generic enhanced protection current transformer (CT) model with saturation effects and transient behavior is presented. The model is used for the purpose of analysis and design of power system protection algorithms. Three major classes of protection CT have been modeled which all take into account the nonlinear inductance with remanence effects. The transient short-circuit currents in power systems are simulated under CT saturation condition. The response of a common power system protection algorithm with respect to robustness to nominal parameter variations and sensitivity against maloperation is demonstrated by simulation studies.

Research on Error Characteristic of the Protective Current Transformer in High Remanence Environment

Electromagnetic Current Transformer (CT) is one of the main equipment in power system, and the remanence in the measuring or protective windings of CT will influence the stability of the power system. Measuring CT is mainly used for the electric energy measurement in normal working condition, focusing on the steady operation state; while protective CT is usually used in fault or short-circuit condition, and the transient transfer characteristic is of significant important. Considering high remanence operation environment in the power system, the numeric analytical model of CT, which is working in the saturation condition, is depicted in this paper. With analyzing the amount of exciting current caused by remanence and deducing the transfer characteristic of CT, the influence of the remanence on the relay protection and the corresponding precautions are proposed, providing accurate and reliable theoretical basis for the protective CT’s correct action and safety operation after system failure. Finally, the electromagnetic transient model of protective CT (TPY) is built in PSCAD, the simulation result of which shows the validity and practicability of the proposed model.

A Method for Correcting Distorted Current Waveforms at Air-Gapped Current Transformers

A new technique for the correction of distorted current waveforms in the secondary windings of protective current transformers (CTs) is presented. The technique is based on the integration of the induced voltage in a tertiary winding. The exciting current is determined using a function that describes the magnetization process in the core, and by adding it to the distorted secondary current. In this way, a more faithful reproduction of the primary current wave is obtained. The initial magnetic flux in the core is determined by analogical integration of the voltage across the tertiary winding, from the pre-fault state up to the instant in which the current reaches 150% of the load current. Due to the large air gap length, the remanent flux in the core is ignored.

A high-current calibration system based on indirect comparison of current transformer and Rogowski coil

The calibration of the protective current transformer (CT) is of particular importance, since its accuracy at high currents is crucial to the correct operation of the subsequent relay protection devices. Conventional calibration methods have been using an electromagnetic CT which contains an iron core as the standard CT. The iron core is big and difficult to manufacture for high-current measurement, and the serious residual magnetism of the iron core at high currents can lead to excessive measurement errors. This paper proposes a calibration system based on indirect comparison of CT and Rogowski coil, i.e. using an iron-core CT to correct the error of the Rogowski coil at low currents, which may be caused by the position of the current-carrying conductor and so on, and then using the calibrated Rogowski coil as the standard transformer at high currents for its good linearity and wide dynamic range, and there is no magnetic saturation. Since the output of the Rogowski coil needs to be integrated, an improved digital integrator based on direct current (dc) negative feedback is adopted, which can effectively eliminate the influences of temperature drift, time drift and dc offset caused by the analogue circuit. The measurement errors of each part of the calibration system have also been discussed, and the test results show that the accuracy of the system can reach up to the 0.05S Class and the uncertainties are 0.038% for ratio and 0.68′ for phase in the range 500 A to 50 kA.

Using a Current Loop and Homogeneous Primary Winding for Calibrating a Current Transformer

This paper presents two methods for determining the ratio error and the phase displacement of a protective current transformer (PCT) with a transformation ratio of 10 kA/1 A and a measuring current transformer (MCT) with a transformation ratio of 20 kA/5 A. The primary conductor passed by a current (10 or 20 kA) placed in the center of the toroid is replaced by a current loop with ten turns placed in the center of the toroid. In the second case, the conductor is replaced by ten turns uniformly distributed along the circumference of the toroid. The primary current (2 or 1 kA) is measured in both cases using a toroidal current comparator, and the secondary current difference is evaluated by means of a current transformer (CT) error measurement system. The errors were also measured using a CT analyzer.

A novel PSO‐based technique for optimal design of protective current transformers

PurposeThe purpose of this paper is to present a new approach for the optimal design of protective current transformers (CTs). The proposed technique is applied to the design of typical CTs and results are compared with traditional CT design method and genetic algorithm optimization algorithm.Design/methodology/approachThe aim of CT design is to make current measurements more accurately, particularly for the high fault currents which arise in short circuits, and more efficient CT in terms of both size and cost. The present work formulates these objectives as an optimization problem to be solved by particle swarm optimization.FindingsSimulation results demonstrate the effectiveness of this technique in optimizing the CT design parameters. The designed CTs have smaller measurement errors compared to standard values and respond well to high fault currents. Manufacturing costs have also been reduced.Originality/valueIn addition to improved efficiency, the benefits of this method are its treatment of CT design in terms of an equivalent circuit and design parameters. The proposed algorithm also extends the linear operation area of the CT and guarantees its good response to high fault currents that may occur in power systems.

Influence of the number of core air gaps on transient state parameters of TPZ class protective current transformers

Network and protective current transformer designers at present are especially interested in the TPZ class current transformers. The behaviour of protective current transformers in a transient state is very important because it influences the proper functioning of the protection system. The basic problem in the design process is to accurately determine the instantaneous current error versus time. The application of field analysis to estimate the influence of the type of core construction, especially the distribution of air gaps, on the transformation errors of the TPZ class protective current transformers is presented. The transient state analysis, as well as the harmonic steady-state analysis, of protective current transformers was carried out using the field-and-circuit method. The 3D analyses were performed for the magnetic cores of current transformers with different numbers of air gaps. The computational results were compared with the test results. The presented application of the field-and-circuit method using professional software offers constructors the possibility of determining the properties for designing the protective current transformer during the design process before making the prototype. The investigation results presented enable one to estimate the influence of the number of magnetic core air gaps on the effective total length, which is of the highest importance for the design of linearised magnetic circuits, applied not just in current transformers.

Fault-current predetermination using time- limited CT secondary side measurements by the covariance-Prony method

Protective current transformers (CTs) are integrated into electrical power systems for assessment of the health of the system. They are intended to provide a scaled-down value of the line current. During major faults, excessive fault currents with decaying DC components could flow through the primary of the CT, causing core saturation and a consequent malfunction of the protection system. This problem is addressed in the paper, and a signal-processing scheme is proposed for fast predetermination of the peak value of the steady-state AC component, instant of occurrence of fault and the time constant of the decaying direct current. Samples of the CT secondary current are collected until the onset of saturation, and are operated upon using a covariance-Prony algorithm to estimate the aforementioned critical parameters. Simulation results in support of the proposed technique are also presented.

Design and evaluation of an algorithm for detecting current transformer saturation

When the magnetic circuit of a conventional protection current transformer (CT) enters saturation, the wave-shape of the secondary current distorts due to the increase in the excitation current. The distortion can be detected by analysing the secondary current using an algorithm that evaluates the first, second and third difference functions. The first difference contains points of inflection, which correspond to the start and end of each saturation period. The second and third differences convert the discontinuities at the points of inflection into pulses that can be used to detect saturation. The design and evaluation of an algorithm for detecting CT saturation using the third difference function is described. A low-pass filter, required for anti-aliasing and noise rejection, softens the discontinuities and reduces the magnitude of the pulses seen in the second and third difference signals. The softening effect is pronounced when the cut-off frequency of the filter is reduced. The sampling rate of 64 samples/cycle is used and the currents are passed through a first-order low-pass RC anti-aliasing filter with a cut-off frequency of from 1920 to 480 Hz. Experimental test results clearly demonstrate that the algorithm successfully detects the start and end of each saturation period. Results are obtained when a prototype detector based on a digital signal processor was used on a high current test-rig. Results indicate that the detector can correctly detect when the secondary current signal is distorted and in all the test cases the detector operated correctly.

Dynamic Model of a Current Transformer

The performance of current transformers and its errors during short-circuit conditions are very important with regard to protective devices in power networks. Core saturation of protective current transformers due to the d.c. component of fault current and the qualitative effect of core hysteresis resulting in residual flux are both well-known phenomena which can produce distorted output currents. A numerical technique for the mathematical representation of the flux density/field strength relationship, including the effects of hysteresis for a current transformer magnetic material, is presented, based on the use of exponential series. The simulation results are compared with experimental results obtained from a sample current transformer.