Current transformers (CTs) are the most widely used measurement instrumentation in power grids. Nevertheless, CT saturation gives rise to large measurement errors, which may have bad repercussions for the power system. In order to deal with the problem of CT saturation, a novel dual-core current transformer based on tunnel magnetoresistance sensor (DTCT) is proposed. Theoretical analyses, finite element method (FEM) simulations and accuracy tests are carried out to validate the practicability and effectiveness of the proposed DTCT. The simulation and experimental results manifest that the steady-state and transient characteristics of the DTCT are outstanding.

According to different application scenarios, CTs can be divided into measuring CTs and protective CTs.1 The measuring CT generally uses a closed toroidal core for high accuracy, while the protective CT utilizes a huge core probably with one or more air-gaps to avoid saturation.2 However, conventional CTs, whether for measurement or protection, are at risk of saturation.

Several software and hardware methods have been proposed in the current literature. These include:

  1. Software compensation methods. Various kinds of saturation compensation algorithms, such as artificial neural networks (ANNs), are presented in Refs. 3–5. However, existing algorithms are obliged to require a tremendous amount of training data or depend on current waveforms and hardware devices for saturation detection.

  2. Hardware demagnetization methods. Such technologies propose adding extra hardware devices in the secondary side, such as adjustable switched resistors,6 transformers,7 and negative voltage sources,8 to balance the magnetic flux density generated by the primary current. However, these devices generally require microprocessors and control algorithms.

Aside from the two methods summarized above, a variety of magnetic sensors based on different physical effects, such as the hall effect, the anisotropic magnetoresistance (AMR) effect, the giant magnetoresistance (GMR) effect and the tunneling magnetoresistance (TMR) effect, are proposed and improved for current measurement.9 As the fourth-generation magnetic sensing technology, TMR sensors combine high sensitivity with a wide dynamic range, stable temperature characteristics and low power consumption. Consequently, TMR sensors are extensively applied in various fields including open-loop (OL) or closed-loop (CL) sensors using magnetic cores, overhead transmission line parameter reconstruction and coreless current probes, etc.

Aiming to deal with the problem of CT saturation, a novel dual-core current transformer based on TMR sensor is presented in this paper. This paper investigates the basic measuring characteristics of the DTCT by means of theoretical analyses, 3D FEM simulations and accuracy tests. The results indicate that the DTCT possesses more superior steady-state and transient characteristics compared with the conventional CT.

The proposed DTCT is composed as illustrated in Fig. 1(a). The DTCT comprises of a mixed dual-core, a secondary winding evenly twined around the dual-core, a TMR sensor and a signal processing circuit. The dual-core consist of a closed core, referred to as the lower core, and a round core with a few air gaps, known as the upper core. The TMR sensor is laid inside an air gap of the upper core. According to the analysis presented below, the output of the TMR sensor is employed to restore the primary current when the core enters saturation. The signal processing circuit is utilized to process the output of the TMR sensor. As depicted in Fig. 1(b), the signal processing circuit consists of an AC-DC converter, a DC-DC converter, a temperature compensation module, a zero-calibration module, an absolute value circuit, a comparator, a power amplifier and a controlled switch. The signal processing circuit is employed to convert the voltage signal of the TMR sensor into an appropriate current signal.

FIG. 1.

Structure of the DTCT. (a) Schematic diagrams of the DTCT. (b) Flow chart of the signal processing circuit.

FIG. 1.

Structure of the DTCT. (a) Schematic diagrams of the DTCT. (b) Flow chart of the signal processing circuit.

Close modal
Ampere’s law for the lower core is given by
lHdl=Hlll=ipNsis=Nsie
(1)
Ampere’s law for the upper core is given by
lHdl=Hgnlg+Hulu=ipNsis.
(2)
where, ip is the primary current, is is the secondary current, ie is the excitation current, Ns is the turns of the secondary winding, Hl is the magnetic field intensity in the lower core, ll is the mean length of the lower core, Hu is the magnetic field intensity inside the upper core, lu is the mean length of the upper core, Hg is the magnetic field intensity in the air gap, lg is the length of each air gap, and n is the number of air gaps.
Based on (2), the primary current ip can be obtained by (3) when the magnetic field intensity Hu inside the upper core is neglected.
ip=Nsis+nlgHg
(3)

According to (3), the primary current can be rebuilt by adding the magnetic field intensity inside the air gap, which can be measured by the TMR sensor, in proportion to the secondary current.

For evaluating the performance of the proposed DTCT and validating the aforementioned theoretical analysis, simulation results of two cases are presented in this section, separately. The geometric and electromagnetic parameters are specified in Table I.

TABLE I.

Electromagnetic and geometric parameters of the proposed DTCT.

ParameterValue
Rated primary current 1000 A 
Rated secondary current 5 A 
Rated burden 30 VA 
Power factor 0.8 inductive 
Cross-sectional area of the core 1200 mm2 
Mean length of the core 565 mm 
Length of each air gap 4 mm 
Number of air-gaps 45 
Saturation magnetic flux density 1.1 T 
ParameterValue
Rated primary current 1000 A 
Rated secondary current 5 A 
Rated burden 30 VA 
Power factor 0.8 inductive 
Cross-sectional area of the core 1200 mm2 
Mean length of the core 565 mm 
Length of each air gap 4 mm 
Number of air-gaps 45 
Saturation magnetic flux density 1.1 T 

This case researches the steady-state performance of the DTCT at the sinusoidal primary current. Figure 2(a) depicts the composite errors of the DTCT at different primary currents and power factors. It can be seen that both the secondary current and the output of the DTCT can accurately restore the waveform of the primary current when the DTCT is under normal steady-state conditions. Especially, the measurement accuracy of the DTCT output is higher than that of the secondary current.

FIG. 2.

Simulation results. (a) Simulation results of case 1. (b) Simulation results of case 2.

FIG. 2.

Simulation results. (a) Simulation results of case 1. (b) Simulation results of case 2.

Close modal

Assuming that the rms value of the rated primary short-circuit current is 5 kA, this case focuses on evaluating the performance of the DTCT at several transient short-circuit currents with different primary time constants. Figure 2(b) depicts the simulation results of three primary time constants. The secondary current and the output of the DTCT are transformed to the primary side. Similar to Case 1, when the core is unsaturated, the secondary current is accurate enough to restore the waveform of the primary current and the magnetic field intensity inside the air gap is low. Once the lower core enters saturation, the secondary current decreases, while the magnetic field intensity inside the air gap surges rapidly. As can be seen from Fig. 2(b), the distortion of the secondary current becomes more severe as the primary time constant increases. Nonetheless, the output of the DTCT is capable of reconstructing the waveform of the primary current, with peak instantaneous errors of 1.38%, 1.32%, and 1.42% respectively.

In order to further validate the practical feasibility and effectivity of the proposed device, several tests are carried out. The type of the TMR sensor used is TMR2651, which is manufactured by Multi-Dimension Technology Company Limited. This kind of sensor boasts high sensitivity, broad measurement range, and low power consumption. According to previous researches, it is found that the noise and the off-set voltage contained in the impact and the electronic circuits used have a negative impact on the measurement accuracy of the proposed device. Hence, the signal processing circuit set a comparator, of which the function is shown as a flowchart in Fig. 1(b), to overcome this problem. Simulation results in Sec. IV show that it is feasible to use the instantaneous value of the TMR sensor output (which is proportional to the magnetic field intensity in the air gap) to judge whether the lower core saturates or not. If the output of the TMR sensor is less than the designed value, it is considered that the core is not saturated. Therefore, the controllable switch is turned off for fear of the dreadful effect of the TMR sensor and the electronic circuits on the measurement accuracy. At this moment in time, the output of the DTCT only contains the secondary current. On the contrary, the controllable switch is closed so as to make the output of the TMR sensor correct the warped secondary current.

Figure 3(a) depicts the ratio and phase angle errors of the DTCT prototype at the rated frequency. According to IEC 61869-2, the DTCT can meet class 0.2 S. In comparison, the DTCT without the comparator and the controllable switch only satisfies class 0.5. Based on the comparison, it is apparent that the DTCT retains the high measurement accuracy of the conventional measuring CT since the signal processing circuit can mainly reduce the unfavourable affect of the TMR sensor while the controllable switch remains disconnected.

FIG. 3.

Experimental results. (a) Results of steady-state performance experiment. (b) Results of transient performance experiment.

FIG. 3.

Experimental results. (a) Results of steady-state performance experiment. (b) Results of transient performance experiment.

Close modal

Limited by laboratory conditions, this paper preliminarily researches the validity of the transient performance of the DTCT. Figure 3(b) shows the trial results of the DTCT under different short-circuit conditions. As shown in the graphic above, the output of the comparator alternates between low and high levels and the duration at the high level lengthens as the time constant raises, indicating that the degree of the core saturation will get more serious during a working period with the increase of the time constant. The DTCT output correctly restores the waveform of the transient short-circuit current, with peak instantaneous errors of 1.67%, 2.22%, and 1.99% respectively, which do not exceed the error limits given in IEC 61869-2.

Except for conventional CTs, common current measurement devices also include open-loop and closed-loop current sensors consisting of magnetic cores and magnetic field sensors. The performance parameters of several current measurement devices are listed in Table II. Based on the above experimental results, the ratio and phase angle errors of the DTCT satisfy the requirements of class 0.2 S, which surpasses the precision of the facilities listed in Table II besides the measuring CT. Not only that, the transient response characteristics and the measuring range of the DTCT are better than those of conventional CTs and both current sensors. As a consequence, the proposed DTCT is more suitable for power grids.

TABLE II.

Performance parameters of common current measurement devices.

Performance 0.2 S 5P and TPY OL CL 
Ratio error at rated current <0.2% <1.0% <0.5% <0.3% 
Standard accuracy limit factor a 5 to 30 
Transient performance a <10%b c c 
Performance 0.2 S 5P and TPY OL CL 
Ratio error at rated current <0.2% <1.0% <0.5% <0.3% 
Standard accuracy limit factor a 5 to 30 
Transient performance a <10%b c c 
a

The core of the measuring CT may become saturated under short-circuit conditions.

b

The core of the class P protective CT may be saturated under transient short-circuit conditions.

c

Open-loop and closed-loop current sensors can measure transient short-circuit currents, although there are no relevant data.

However, there are some limitations and disadvantages of the proposed DTCT. On the one hand, the upper core of the presented DTCT contains a number of air gaps, which will increase shaping difficulties and costs, since the measuring range of the magnetic field sensor is limited. The measurement accuracy will sharply deteriorate if the magnetic field intensity in the air gap exceeds the measuring range of the sensor used. On the other hand, the magnetic field sensor and the signal processing circuit require power supply and make actual costs higher.

In this paper, a novel dual-core current transformer based on TMR sensor is proposed to address the problem of CT saturation. Theoretical analyses, three-dimensional FEM simulations and laboratory tests are conducted to verify the practical feasibility and superiority of the proposed apparatus. The results indicate that the novel current transformer designed in this paper not only meets the accuracy class for measuring current transformers, but also satisfies the error limits for protective current transformers without significantly increasing the volume. Furthermore, the presented DTCT topology is simple and can be easily integrated into the power system, eliminating the requirement for algorithms and microprocessors.

This paper is funded by the science and technology project of state grid corporation of China. The project’s title is “Re- search on a wideband and wide-range AC/DC high voltage current measurement device based on magnetic-valve structure” and the project number is “5500-202317106A-1-1-ZN.”

The authors have no conflicts to disclose.

Zhexuan Zhang: Formal analysis (equal); Methodology (equal); Software (equal); Writing – original draft (equal). Cuihua Tian: Writing – review & editing (equal). Junhua. Guo: Conceptualization (equal). Yu Wang: Supervision (equal). Zhaojie Liu: Funding acquisition (equal). Yingying Zhao: Supervision (equal). Jian Lu: Funding acquisition (equal). Weihua Ye: Funding acquisition (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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