1. Introduction
Electrical power systems are undergoing a fundamental transition through the integration of modern power electronic-based equipment and resources while expanding and interconnecting to meet the ever-growing electrical energy demands. This, on the one hand, has increased the complexity and uncertainty of the system and, on the other hand, resulted in the elevation of the short-circuit levels above the tolerable fault current of the installed equipment [
1,
2]. Consequently, the power system components have become susceptible to failure, leading to significant costs and an increased risk of grid-wide failures. In order to ensure the continued reliability and safe operation of the power system, in terms of the robustness of components against short-circuit faults, the employment of fault current limiters (FCLs) is one of the most effective solutions [
3,
4]. Various types of FCLs have been proposed over the years, such as the reactor type [
5], solid state type [
6], superconductor type [
7], and hybrid topologies [
8]. The focus of this paper is on bridge-type DC reactor FCLs (BDCR-FCLs), which have gained popularity in the past years due to their superior capabilities.
Series BDCRs have been applied in the protection of modern power grids as efficient fault current-limiting devices [
9,
10,
11]. In principle, a DC reactor-type FCL operates based on its inductor unit’s tendency to oppose current change, thus producing fault current-limiting capabilities to the device [
10]. DC reactor-type FCLs have been employed in HVDC networks for fault current limitation as well [
11]. In another application, the capabilities of the series DC reactor-type configuration as a dual-function device for both fault current limitation and overvoltage suppression [
12]. An AC/DC reactor-type FCL was proposed in [
13] to limit the rate of rise of fault current. The study carried out in [
13] also demonstrated that the bridge-type series DC reactor FCLs do not impose any reactive power on the network.
One of the most critical challenges in modern electrical energy systems is the introduction of harmonic contents in the network arising from nonlinear loads and inverter-based resources. The IEEE standards [
14,
15] address this issue, focusing on harmonics content restriction in low and medium-voltage networks. On the consumer side, loads such as high-efficiency lights, computers, light dimmers, battery chargers, adjustable speed drives, heat pumps, electric vehicle battery chargers, etc., have caused serious power quality issues, increasing the total harmonic distortion (THD) in the network to concerning levels [
16]. Conventionally, to limit the emitted harmonic content of modern power-electronic-based loads, passive filters formed as interconnected circuits of inductors and capacitors are commonly employed [
17]. Along with filters, DC-side ripples can also be alleviated by adding a branch with capacitive and/or resistive elements in the terminals of the rectifier bridge [
18]. The addition of a series line inductance on the rectifier bridge was shown to be effective for THD reduction of the full bridge rectifier [
19].
There are various types of devices aimed at improving power quality factors [
20], utilizing passive [
21] or active [
22] mechanisms. Typically, these devices are designed to address specific power quality issues rather than functioning as multifunctional equipment [
23]. In high-voltage grids, passive power factor (PF) correctors and filters play crucial roles and have been extensively discussed [
23,
24,
25]. A comprehensive review of passive filter structures for grid-connected voltage-source converters and their ability to reduce harmonic content is provided in [
20]. Another important aspect of power quality indicators is their PF correction capability. For nonlinear consumers, a common solution for enhancing PF involves the utilization of series AC reactors, which consist of passive elements (L or C) placed in the AC side of the system [
3,
26].
In this paper, supplementary functionalities of BDCR-FCLs are investigated. In this regard, the effect of series BDCR-FCLs on THD reduction, PF correction, peak current decrement of nonlinear loads, and soft load variation in the network are studied alongside their fault current-limiting capabilities. It should be noted that although this paper tries to lay out the supplementary functionalities of the BDCR-FCLs besides their main role in limiting fault current, its aim is not solely to propose this device for power quality enhancement. The analyses have been carried out on a test system consisting of an AC microgrid connected to a simple DC microgrid through simulations conducted in PSCAD-EMTDC software and experimental verification using a laboratory test circuit. The rest of this paper is organized as follows.
Section 2 outlines the various modes of the distribution network operation with BDCR-FCLs.
Section 3 provides an analytical study of the test system.
Section 4 presents the simulation and prototype experimental test results. Discussion is made in
Section 5, and the study is concluded in
Section 6.
4. Experimental Analysis
The experimental tests have been carried out on a scaled down prototype laboratory setup, including a BDCR-FCL, which is utilized to confirm the analytical studies. The laboratory setup is in the form of the topology in
Figure 3 (Case 3). Tests have been conducted to demonstrate the multifaceted functionalities of the BDCR-FCL to improve PF, THD, and rate of rise of current, and to smooth the load variation alongside providing fault current limitation capabilities. Setup parameters are listed in
Table 4, and the prototype configuration is shown in
Figure 9. It is to be noted that the setup is fed by an autotransformer, and the DC reactor is composed of an E-I magnetic core and 50 turns of wire. This DC reactor, under normal condition, enforces a constant current of 2 A. The signals are measured by means of a digital oscilloscope and are further imported into a computer for post-processing studies.
The results obtained from the tests conducted on the experimental setup are demonstrated in
Figure 10. The source voltage, representing the AC microgrid, is illustrated in
Figure 10a, where the visible intrinsic harmonic distortions are due to the grid line voltage. This voltage feeds the DC system, which includes the rectifier, smoothing capacitor, and DC load with and without the series BDCR-FCL.
Figure 10b,c show the source current with and without BDCR-FCL, respectively. The metrics considered to evaluate the effect of BDCR-FCL are the current peak value, the time duration of current conduction in a cycle, and the rate of rise of the current. As seen in
Figure 10b, the current peak value is 1.7 A, and the duration of current conduction is 3.5 ms, resulting in the rate of rise of current equal to 486 A/s, similar to the results depicted in
Figure 6 and
Figure 8a. As shown in
Figure 10c, the quality of the source current waveform is improved by using BDCR-FCL. In this figure, the current peak value is 0.6 A, and the current conduction duration in one half-cycle is 6 ms, thus limiting its rate of rise of current to around 100 A/s, which agrees with the results in
Figure 8a,b. In addition, as illustrated in
Figure 10d, the BDCR-FCL current matches the absolute value of the source current during the capacitor charging intervals.
Fourier analyses of the source current harmonic spectrum with and without BDCR-FCL are presented in
Figure 11a and
Figure 11b, respectively. The experimental results demonstrate that THD is improved by 18%, which verifies the harmonic restraining effect of BDCR-FCL. Moreover, according to Equation (15), the THD enhancement shown in
Figure 11 results in the improvement of distortion PF from 0.86 to 0.93, validating the effect of BDCR-FCL in improving the PF.
5. Discussion
According to the simulation analyses and laboratory experiments, it was demonstrated that BDCR-FCLs can be far more effective devices for connecting AC microgrids to DC systems because by employing BDCR-FCLs, alongside the limitation of fault current, soft load starting is achieved by decreasing the rate of rise of current, the peak current of nonlinear loads is alleviated, the THD of source current is improved, and consequently an overall improvement in power quality is attained, while no external control is involved in its operation either. In this regard, the THD was shown to be improved by 20% and 18% in the simulation and the experimental cases, respectively, and the PF was shown to be improved through enhancement of distortion PF from 0.88 to 0.947 in the simulation case and from 0.86 to 0.93 in the experimental case.
Unlike other FCL technologies, which are only present in the current flow path under the faulty condition, the BDCR-FCL is not bypassed in the non-faulty condition. Therefore, BDCR-FCLs can provide such additional functionalities as discussed by disagreeing variations even under non-faulty conditions. In this regard, the functionalities of the BDCR-FCL can be compared with the most prominent FCL technologies, i.e., the reactor type [
5] and the superconductor type [
7] FCLs, as presented in
Table 5.
It is worth mentioning that although the experimental tests were conducted for a scaled down laboratory test setup, similar outcomes, i.e., THD reduction, PF enhancement, and waveform correction capabilities, can be expected on a real scale. However, it is important to bear in mind that at the grid scale, specialized power electronic components would have to be employed, and the circuit design would require special care, complying with the insulation coordination necessities, particularly to handle the high voltage levels. The advantages of the BDCR-FCLs are listed in
Table 6.
6. Conclusions
In this paper, a comprehensive analysis of a BDCR-FCL was carried out. The investigations of this paper were performed through simulated case studies and experimental tests, validating the superior functionalities of BDCR-FCLs. This study shows that BDCR-FCLs can present additional capabilities, extending beyond its primary role as a fault current-limiting device. This device can provide waveform distortion improvement, reducing the current THD and correcting the PF in power networks. It was also proved that BDCR-FCLs enhance the current conduction duration, reduce the rate of rise of current, and thus limit the peak current of nonlinear loads and smooth the load variations. Moreover, as BDCR-FCLs are essentially series DC inductors, they do not consume reactive power during steady-state conditions. Overall, the BDCR-FCL presents promising capabilities in enhancing power quality alongside limiting fault currents. Its multifunctional characteristics make it a viable option for integration into microgrid applications, contributing to the resilience and sustainability of the power grid.