A wide-band, frequency-dependent five-terminal model is developed that represents one HVDC transformer unit in the French-English IFA2000 HVDC interconnection. Three such interconnected 1-ph units constitute one 3-ph transformer bank needed in 12-pulse conversion. The model is obtained via admittance frequency sweep measurements on the transformer’s terminals, including common-mode measurements to capture the high-impedance coupling to earth at lower frequencies. The data set is modified to reduce the magnetizing current to a realistic level by a novel eigenvalue scaling procedure. The final data set is subjected to modeling by a stable and passive rational model while utilizing a mode-revealing transformation to retain the accuracy of the small eigenvalues that are related to the high-impedance coupling to earth. The paper describes details related to the measurements and modeling steps as well as the many intermediate accuracy validations that were done. Also is described challenges in the measurements that resulted due to interference of the measurements by a nearby 400 kV overhead line. The model is applied in a time domain simulation of the complete HVDC link in normal operation where voltage waveforms resulting from line commutations are compared against those by a classical model with added stray capacitances.

Introduction

HVDC transformers are together with thyristor valves the fundamental components in any LCC-type HVDC converter. The protection of these transformers is therefore essential for the reliable operation of the HVDC link. Overvoltages due to switching events, lightning impulses and in-station flashovers on the AC side result in steep-fronted overvoltages that stress the transformer AC side insulation parts, and voltages are also transferred to the DC-side where they stress and interfere the thyristor components [1]. Conversely, line faults and lightning strokes to a DC overhead line may result in overvoltages that impose stresses on the transformer DC-side terminals. The protection of HVDC substation against overvoltages therefore requires insulation co-ordination studies to be performed to determine the appropriate location of surge arresters [1]. Insulation co-ordination studies requires the use of models with adequate accuracy up to several hundred kHz in order to represent the actual waveshapes of the overvoltages. The representation of the HVDC transformer is a main obstacle in such studies since frequency-dependent transformer models are in general not available. Therefore, simplified models are usually employed, often consisting of a 50 Hz model in combination with lumped capacitors.

1- Introduction

2- Epitaxial structure and SME-JTE design

3- Device simulation

4- Experimental results and discussion

5- Conclusion

References

Abstract

An edge termination technique, referred to as simple multistep etched junction termination extension (SME-JTE), is presented for 4H-silicon carbide gate turn-off thyristors (4H-SiC GTO). The proposed termination technique can form over ten steps in the termination region with only requiring a few of etching steps. Numerical simulations show that a high termination efficiency over 95% with broad process window for etching depth is obtained for the SME-JTE technique. In addition, a high breakdown voltage of 7500 V has been experimentally demonstrated for the 4H-SiC GTO with 8-step JTE, and which is about 91% of the ideal breakdown voltage for the 50 μm drift layer. The high termination efficiency and simple process of SME-JTE makes it applicable for fabrication of various high-voltage power devices.

Introduction

Silicon carbide (SiC) is considered as a promising material for next-generation power devices due to its unique intrinsic properties such as high electric breakdown field strength, wide bandgap and high thermal conductivity [1, 2]. And the 4H-SiC gate turn-off thyristor (4H-SiC GTO) is of specific interest for pulsed-power applications because of its great advantages over silicon counterparts, such as high-temperature, high-voltage, and high-current applications [3, 4]. Edge termination is a critical technology for power devices. An effective edge-termination technique makes the electric field distribution more uniform at the edge of the device, and which makes the device approach the ideal breakdown voltage capability of the epitaxial layer used. And several specialized edge terminations have been developed to mitigate this effect, including floating guard rings [5], field plates [6], junction termination extensions (JTE) [7-8], and the JTE technique has become one of the preferred methods due to its simple design and high termination efficiency. The JTE technique is usually formed by ion implantation, including multiple-floating-zone JTE, space-modulated JTE and multiple-ring-modulated JTE [9-11]. However, the activation of the implanted dopants is strongly dependent on annealing temperature and time. Moreover, the ion implantation and high temperature annealing process result in crystal damage and surface roughness, which usually affects the performance of power device. Recently, the etched termination has attracted wide attentions because of its simplicity in device design and implantation-free processing. And one of the etched terminations studied in SiC BJT and PiN devices is the single-step JTE, but its BV performance shows high sensitivity to etching depth [12, 13]. Therefore, the multistep JTE formed by etching are developed for wider tolerance of etching step variation [14, 15], and the highest BV of 22kV is achieved with over ten steps JTE in 4H-SiC GTO device [16], but the complexity and cost of device fabrication are significantly increased. In order to realize multistep JTE in a cost-effective way, a method called simple multistep etched JTE (SME-JTE) is proposed in this letter. This method can effectively reduce etching process for realizing multistep JTE, and 16-step JTE only needs 5 etching process. Meanwhile, device simulations and experiments have been carried out to investigate the etching depth variation tolerance and termination efficiency of the SME-JTE technique, broad tolerance for etching depth variation and high termination efficiency have been demonstrated for the 4H-SiC GTO device with SME-JTE technique.

Abstract

The voltage source converter (VSC) excitation system is a novel excitation system based on pulse-width modulation (PWM) voltage source converter, which is proposed as improved alternatives to the conventional thyristor excitation systems. This paper aims to provide theoretical confirmation of power system stability enhancement by the VSC excitation system. The reactive current injected to generator terminals by the VSC excitation system can be controlled flexibly. Its capability of enhancing power system stability is investigated in this paper. The simplified model of VSC excitation system suitable for use in system stability studies is developed. An extended Philips–Heffron model of a single-machine infinite bus (SMIB) system with VSC excitation system is established and applied to analyze the damping torque contribution of the injected reactive current to the power system. This paper also gives a brief explanation on why the VSC excitation system can enhance the transient stability in light of equal area criterion. The results of calculations and simulations show that the injected reactive current of VSC excitation system contributes to system damping significantly and has a great effect on the transient stability. When compared with conventional thyristor excitation systems, the VSC excitation system can not only improve the small-signal performance of the power system, but also can improve the system transient stability limit.

Introduction

The generator excitation system, which provides direct current to the synchronous machine field winding, is the most important and effective means to maintain the stability of power systems. Since the 1960s, the static excitation systems based thyristor converters (thyristor excitation systems) have been extensively used, for its ability of producing almost instantaneous response and high ceiling voltages. This system has a very small inherent time constant and is easily maintainable [1]. However, the modern power systems are interconnected each other to give and take the electric power and have become much more complicated than decades ago. The presence of system instability is becoming more prominent and thyristor excitation systems with conventional PSS are not sufficient to suppress the wide range (0.1–3.0 Hz) power oscillations any more. On the other hand, the long distance power transfer with heavy load seems to be more susceptive to poor damping [2]. Studies show that the thyristor excitation system cannot provide enough damping even if PSS is equipped.