ترانسفورماتور Sen با کوپلاژ چند سیم پیچی
Abstract
1- Introduction
2- An analytical electromagnetic model of the “Sen” transformer
3- Simulation case studies
4- Conclusions
References
Abstract
In flexible AC transmission systems, the “Sen” transformer (ST) is an economical and effective method for voltage regulation and power flow control, which can achieve independent control of active and reactive power. The ST is a three-phase multi-winding transformer based on the on-load tap-changing technology. In order to reveal the internal characteristics of the ST, this paper proposes an analytical electromagnetic model considering the multi-winding coupling in the ST with the three-phase three-limb structure. Firstly, according to the electromagnetic coupling relationship in the ST, a magnetic equivalent circuit model consisting of self-inductance coefficients and mutual inductance is derived via the unified magnetic equivalent circuit (UMEC). Secondly, an electric circuit model for the internal voltage and current of the ST is established based on the electrical connection between the ST and the external system. Finally, an analytical electromagnetic model considering multi-winding coupling for the ST is obtained by combining the magnetic equivalent circuit model and the electric circuit model. Two case studies have been carried out on a three-phase three-limb model of the ST with the help of MATLAB. The effectiveness of the proposed model is verified by comparing the obtained analytic results with the existing results from time-domain simulation about the series compensating voltage of the ST. In addition, the voltages and currents of windings in the ST in the conditions of different transformer core structures and ferromagnetic materials are investigated for evaluating the influence of magnetic coupling on the results.
Introduction
With the large-scale integration of renewable energy sources, it has been becoming more and more difficult for the voltage regulation and the optimal power flow control in complex power networks with UHV (Ultra High Voltage) transmission lines as the backbones[1,2]. The unified power-flow controller (UPFC) is an important way to solve the problems of voltage regulation and power flow control [3,4]. However, the high installation and operation costs prevent the UPFC from widespread applications. The “Sen” transformer (ST), as proposed in [5], has the ability of controlling active and reactive power flow independently. Compared with UPFC, the ST is lower in cost, higher in reliability and efficiency [6–8], although reducing the accuracy and dynamic performance [9]. The ST is a typical electromagnetic unified power flow controller, and is usually based on a three-phase multiwinding transformer. By changing the position of secondary winding taps, the ST can generate different voltages, achieving the adjustment of power flow. The adjustment performances between the “Sen” transformer and phase angle regulator (PAR) were compared in [10,11]. The results showed that the overall reactive power of the ST was lower than that of the PAR for enhancing the required amount of active power flow. Therefore, the “Sen” transformer is expected to be a device that will have widespread applications in flexible AC transmission systems (FACTS). As a matter of fact, the ST is a technology based on transformers and tap-changers. From the perspective of the development of electrical topologies and structural variants, many researchers have optimized the topology of the ST for the purpose of improving its power flow control performance. For example, some novel topologies of the ST were proposed in [12–15] for the sake of achieving continuous power flow control of existing transmission lines and improving the utilization rate of existing transmission lines. These topologies include the hybrid electromagnetic unified power flow controller (HEUPFC), the improved hybrid unified power flow controller (IHUPFC) and the power transistor-assisted “Sen” transformer (TAST) which consists of a high-capacity ST and a small capacity UPFC.