Shuitao Yang

Impedance-Source Networks for Electric Power Conversion Part II: Review of Control and Modulation Techniques

Impedance-source networks cover the entire spectrum of electric power conversion applications (dc-dc, dc-ac, ac-dc, ac-ac) controlled and modulated by different modulation strategies to generate the desired dc or ac voltage and current at the output. A comprehensive review of various impedance-source-network-based power converters has been covered in a previous paper and main topologies were discussed from an application point of view. Now Part II provides a comprehensive review of the most popular control and modulation strategies for impedance-source network-based power converters/inverters. These methods are compared in terms of theoretical complexity and performance, when applied to the respective switching topologies. Further, this paper provides as a guide and quick reference for researchers and practicing engineers in deciding which control and modulation method to consider for an application in a given topology at a certain power level, switching frequency and demanded dynamic response.

Transformer-Less Unified Power-Flow Controller Using the Cascade Multilevel Inverter

The conventional unified power-flow controller (UPFC) that consists of two back-to-back inverters requires bulky and often complicated zigzag transformers for isolation and reaching high voltage. This paper proposes a completely transformer-less UPFC based on an innovative configuration of two cascade multilevel inverters (CMIs). The unique configuration and control of the two CMIs as a power-flow controller make it possible to independently control active and reactive power flows over a line. The new UPFC offers several advantages over the traditional technology, such as transformer-less, light weight, high efficiency, high reliability, low cost, and fast dynamic response. The transformer-less UPFC is thereby very suited for fast and distributed power flow control, such as wind and solar power transmission. Experimental results based on 13.8-kV/2-MV·A transformer-less UPFC prototype are shown to validate the theoretical analysis and operating principle.

Modulation and Control of Transformerless UPFC

In this paper, a modulation and control method for the new transformerless unified power flow controller (UPFC) is presented. As is well known, the conventional UPFC that consists of two back-to-back inverters requires bulky and often complicated zigzag transformers for isolation and reaching high power rating with desired voltage waveforms. To overcome this problem, a completely transformerless UPFC based on an innovative configuration of two cascade multilevel inverters has been proposed. The new UPFC offers several advantages over the traditional technology, such as transformerless, light weight, high efficiency, low cost and fast dynamic response. This paper focuses on the modulation and control for this new transformerless UPFC, including optimized fundamental frequency modulation for low total harmonic distortion and high efficiency, independent active and reactive power control over the transmission line, dc-link voltage balance control, etc. The new UPFC with proposed control method is verified by experiments based on 4160-V test setup. Both the steady-state and dynamic-response results will be shown in this paper.

Wireless power transfer via harmonic current for electric vehicles application

A method that only using harmonic current to transfer wireless power for electric vehicles (EV) is proposed in this paper. The frequency limit of most high voltage high current IGBT is around 20 kHz which also limits the system frequency of the transformer. Due to the gain characteristic of the resonant tank is actually a band-pass filter, using one specific harmonic current to transfer power becomes available. By doing so, the system frequency can be raised up several times while the switching frequency remain the same. Higher system frequency leads to a more compact system. A 1 kW prototype with 20 cm air gap using 3rd harmonic power is built to verify the method.

Transformer-less unified power flow controller using the cascade multilevel inverter

The conventional unified power flow controller (UPFC) that consists of two back-to-back inverters requires bulky and often complicated zigzag transformers for isolation and reaching high voltage. This paper proposes a completely transformer-less UPFC based on an innovative configuration of two cascade multilevel inverters (CMIs). The unique configuration and control of the two CMIs as a power flow controller make it possible to independently control active and reactive power flows over a line. The new UPFC offers several advantages over the traditional technology, such as transformer-less, light weight, high efficiency, high reliability, low cost, and fast dynamic response. The transformer-less UPFC is thereby very suited for fast and distributed power flow control, such as wind and solar power transmission. A simulation model is built to demonstrate the operating principle of the proposed transformer-less UPFC.

Discontinuous operation modes of current-fed Quasi-Z-Source inverter

The conventional voltage source inverter is only a buck converter and conventional current source inverter is only a boost converter. By adding a Z-Source network [1], the buck-boost capability can be achieved. The recently proposed Quasi-Z-Source inverter (qZSI) [2] is an important improvement to Z-Source inverter (ZSI), which greatly reduces the passive component stress. The current-fed qZSI can buck-boost voltage and achieve bidirectional power flow without replacing the diode with an active switch [3]. It has lower current stress on inductor compared to current-fed ZSI. However, the analysis and control methods proposed in [3] are based on the assumptions that the capacitor voltage is almost constant and equal to the input voltage. These assumptions become invalid when the capacitor is very small or the lower power factor is low in some applications that the volume is a very crucial factor. The capacitor voltage has high ripple or even becomes discontinuous. In these cases, the circuit has two new operations modes except for the normal three modes, which is called discontinuous operation modes. This paper analyzes the characteristics of the discontinuous operation modes, and derives the critical conditions for these new modes under different control strategies. Simulation and experiment results are given to verify the theoretical analysis.

Application of Transformer-Less UPFC for Interconnecting Two Synchronous AC Grids With Large Phase Difference

In this paper, the application of an innovative transformer-less unified power flow controller (UPFC) for interconnecting two synchronous ac grids with large phase difference is presented. The proposed transformer-less UPFC is based on two cascaded multilevel inverters. As is well known, the real power flow between two generators is mainly determined by their phase difference. If two grids with large phase difference are initially separate from each other, once connected, there will be huge current flowing through the transmission line and will, thus, damage the generators or other supplementary equipments. Therefore, to connect two synchronous ac grids with each other without using an extra device is impossible. For decades, researchers have been investigating different approaches to this problem but still difficult to conquer, especially for real hardware implementation. An effective solution using the transformer-less UPFC is demonstrated in this paper. The transformer-less UPFC can realize grid interconnection, independent active and reactive power control, dc-link voltage balance control, etc. Furthermore, a 1-pu equipment can compensate system with phase difference as large as 30°. Experimental results based on the 13.8-kV/ 2-MVA transformer-less UPFC prototype are shown to validate the theoretical analysis.

Application of transformer-less UPFC for interconnecting synchronous AC grids

A transformer-less UPFC based on an innovative configuration of two cascaded multilevel inverters (CMIs) has been proposed recently, which is suitable for power flow control between two interconnected synchronous AC grids. The active power as well as the reactive power can be independently controlled by the new transformer-less UPFC. In addition, the semiconductor device ratings (SDPRs) for different power flow control solutions have been investigated. It can be found that cascade multilevel inverter based transformer-less UPFC has much lower SDPR when compared to MMLC based back-to-back HVDC system, indicating significant cost saving when use transformer-less UPFC for power flow control. Experimental results based on 13.8 kV/ 2MVA transformers-less UPFC prototype are shown to validate the theoretical analysis.

Fractionally rated transformer-less unified power flow controllers for interconnecting synchronous AC grids

Recently, a novel transformer-less unified power flow controller (T-UPFC) has been proposed. The T-UPFC is a suitable candidate for performing power flow control in interconnected synchronous AC grids. By means of analytical expressions, this paper provides the overall device power rating of such a system when used to control power flow through the line to its limit. By means of these results, it is shown that the ratings of T-UPFC are only a fraction (two tenths) of that of back-to-back VSC based HVDC converter projects commercially being executed to provide power flow control in interconnected synchronous grids. The analysis and results in this paper illustrate the clear benefits of this technology.

Comparison of synchronous condenser and STATCOM for inertial response support

This paper shows a comparison between synchronous condenser (SC) and STATCOM in terms of inertia frequency response (IFR) with synchronous generator (SG). It has long been argued that, as a rotating-mass-based reactive power compensation device, SC will contribute to the total inertia of the network from its stored kinetic energy. Whereas, its counterpart, the voltage source converter (VSC) based STATCOM, will only supply reactive power and maintain voltage balance. However, the energy stored in the dc-link capacitor of the STATCOM, especially the cascaded inverter based STATCOM whose dc-link capacitance is relatively large, if properly controlled, can also contribute to the IFR. A matlab/Simulink model of SG equipped with SC and STATCOM is presented in this paper. It is demonstrated that STATCOM can provide competitive or even better IFR during disturbance condition. Both theoretical and simulated results are provided.