The smart grid is regarded as the next generation of power system, and substation intelligentization is a key technology in smart grids [
1,
2]. Regarding the commonly accepted current measurement methods in substation automation, traditional current transformers (CTs) are placed in a gas-insulated switchgear (GIS) to measure the currents, and their outputs are delivered through copper wires to intelligent electronic devices (IEDs) [
3]. Practically speaking, traditional CTs encounter several technically unsolved problems (e.g., DC and AC saturation, remanent magnetization). These problems are all caused by the effect of hysteresis of CT iron cores [
4,
5]. Traditional CTs also have some physical drawbacks such as large volume and heavy weight, which limits the development and application of power protective systems. In order to solve the saturation problem and speed up substation intelligentization, electronic current transformers (ECTs), a method utilizing new semiconductor materials to design novel sensing technologies, have become one of the main research directions in measurement technologies [
6,
7,
8,
9]. Nowadays, some ECT technologies are commercialized, and related standards such as International Electrotechnical Commission (IEC) 61850 have been issued for standardize the data processing, data acquisition, data transmission, and application framework in intelligent substations [
10,
11].
Table 1 presents a comparison between traditional CTs and ECTs. ECTs are classified into two types: active ECTs and passive ECTs. In the active ECTs, an external source is required to drive the sensing component. The typical techniques for active ECTs are Rogowski coil CT [
12], low power CT (LPCT) [
13], and magnetic-field-sensor-based CT [
14]. For passive ECTs, the sensing components are unnecessary to have a driving source, and the representative technique is Faraday effect CT or optical CT (OCT) [
15]. OCTs can maintain a good linearity in a wide current range, but their sensing structure is complicated and costly. Rogowski coil CTs cannot measure DC currents and are affordable but still not cheap enough. The sensing principle of LPCTs is the same as the traditional CTs, but the iron cores of LPCTs are manufactured using microcrystal alloy. Thus, LPCTs can achieve measurement accuracy with a smaller product volume due to the high permeability of microcrystal alloy. However, LPCTs are still heavy, and cannot measure DC currents. Regarding magnetic-field-sensor-based CTs, the commonly accepted sensing principles are anisotropic magneto resistance (AMR), giant magneto resistance (GMR), and the Hall effect [
16]. Based on the Lorentz force, Hall sensors have a fast response time, a wide magnetic field measurement range, and good linearity. Both GMR and AMR originate from the magneto resistance effect, and their resistance values are sensitive to the varying measured magnetic fields. In the actual design, the configuration, which employs a Wheatstone bridge with AMR or GMR sensors, is mainly used in current measurement [
17]. AMR and GMR sensors have a quick response time and are able to measure a wide current range. However, the linearity of AMR sensors is better than that of GMR sensors. The above three magnetic field sensors are all suitable for the current measurement. Nowadays, the integrated circuit (IC) technology-based Hall sensors are well developed. In this case, several compensation circuits can be integrated into Hall sensors to enhance the sensitivity and guarantee the steady performance [
18]. Also, plenty of practical applications of the Hall sensors demonstrate their economic efficiency and that of related products.
Most sensing components within ECTs are semiconductor materials, meaning the reliability of ECTs would be worse than that of traditional CTs. Our study focuses on proposing a new redundancy mechanism for the coreless Hall-effect current transformer (HCT) [
19] to enhance its reliability. The HCT is a sort of ECT, which places four Hall sensors symmetrically around the measured cable. Based on this sensor placement, the HCT can eliminate the ambient magnetic-field interference. Since there are no iron cores involved, the HCT would not encounter saturation when measuring a large current. In fact, the Hall sensor is also an electronic component, so it is not as reliable as the traditional CT. If one of the four Hall sensors fails, the HCT would not function well, which marks a weakness of HCTs as insufficient persistence. To enable the HCT have sufficient reliability for practical usage, we propose a new redundancy mechanism method. Multiple sensor modules are utilized in this method, and each sensor module consists of multiple Hall sensors. Besides, the algorithm designed for the proposed redundancy mechanism comprises three function units: (1) sensor module condition detection; (2) current measurement; and (3) sensor module failure alarm. First, the sensor module condition detection function unit performs real-time sensor module condition detection. Then, the proper sensor modules are recognized. Finally, the outputs of all proper sensor modules are allowed to be sent to the current measurement function unit to calculate the measured current. The current measurement function unit can average the outputs of all proper sensor modules, and then the average Hall voltage is transformed into an actual current waveform. The current measurement function unit could also further improve the accuracy of the measurement, since the multiple sensor modules can compensate for the measurement error. Moreover, the sensor module failure alarm function unit receives a real-time report from all sensor module conditions. If one sensor module fails, the first phase of alarm will be visualized in the monitoring system, and the failed sensor module is also indicated. If two sensor modules fail, the second phase of alarm will be visualized in the monitoring system; meanwhile, the maintenance engineer is immediately informed to arrange a HCT replacement.
In a word, the redundancy mechanism monitoring system could achieve condition-based maintenance for the HCT, and this maintenance strategy can improve the entire measurement system’s reliability and decrease the maintenance costs.
Note that the proposed HCT is mainly placed in the GIS, so the installation environment is steady without dramatic temperature change. Hence, the study is meant to develop a new redundancy mechanism for the HCT, and its measurement performance is our main concern. Additionally, in contrast to other similar industrial applications, the proposed HCT with a redundancy mechanism combines a temperature compensation mechanism, which will enable the system to perform well in a harsh environment. The system could remain stable even if the installation environment has dramatic temperature change.
The remainder of this paper is organized as follows.
Section 2 assesses the amount of sensor modules and analyzes the sensing structure. In
Section 3, a series of simulations are carried out to estimate the measurement accuracy of the HCT with a redundancy mechanism in a three-phase system.
Section 4 describes the design and the implementation of proposed redundancy mechanism. In
Section 5, lab experiments using a three-phase current measurement system are presented. Finally, this paper is concluded in
Section 6.