Deepak Gunasekaran

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.

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.

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.

Optimization of Fundamental Frequency Modulation for Cascaded Multilevel Inverter Based Transformer-less UPFC

This paper proposes an optimized fundamental frequency modulation (FFM) for cascaded multilevel inverter (CMI) based transformer-less unified power flow controller (UPFC). Compared to traditional FFM techniques, the proposed method features superior power sharing between H-bridge modules, thereby leading to better equalization of dc capacitor currents and voltage ripples. It also retains all the advantages of the traditional FFM such as low total harmonic distortion (THD) of output voltage and high efficiency. The proposed method is developed for a 2 MVA transformer-less UPFC system, nevertheless, it also can be applied to other applications such as CMI based static synchronous compensators (STATCOM), static synchronous series compensator (SSSC) and motor drives.

Development of 2-kW interleaved DC-capacitor-less single-phase inverter system

This paper presents the development and experimental performance of a 2-kW interleaved DC-capacitorless single-phase inverter. By utilizing an AC capacitor to absorb the ripple power pulsating at twice the line frequency, the proposed interleaved DC-capacitor-less single-phase inverter significantly reduced the total capacitor size. The adoption of SiC MOSFET with 216-kHz switching frequency and the interleaved structure further reduce the filtering components size. A comparison of existing topologies for inverter application and the design considerations of interleaved PWM scheme and integrated coupled inductors are described in details. Experimental results are provided to verify the performance and feasibility of the proposed inverter system.

Multi-level capacitor clamped DC-DC multiplier/divider with variable and fractional voltage gain - an (n/m)X DC-DC converter

Inductor-less, high gain DC-DC converter with high efficiency and high power density is a much desired circuit in Electric Vehicle (EV) powertrain and Solar photovoltaic (SPV) power converters. Modular Multi-level capacitor clamped DC-DC converter circuits (MLCCC) provide a viable solution for this case. But, their application is limited owing to their limitations in terms of fixed output voltage gains and lack of fractional output gains. The aim of this paper is to introduce an (n/m)X converter capable of both dynamically variable voltage gains and fractional voltage gains. Detailed description of the operating method along with the various stages of operation and power dissipation calculation for the proposed converter forms the main theme of the paper. Experimental results for a 1-kW prototype are presented to validate the proposed theory.

Reactive compensation of overhead AC transmission lines using underground power cables

Overhead (OH) transmission lines when loaded always consume reactive power. An underground (UG) power cable on the other hand is in itself a huge capacitor and therefore a source of reactive power. This paper highlights the use of UG power cables in conjunction with overhead lines, where the reactive power requirement of overhead line is compensated by UG power cables. To realize this compensation, an overhead line is divided into n equal sections and UG power cables are inserted in between each section. The resulting compensation leads to three advantages: 1) Unity power factor at both sending and receiving ends; 2) Increase in overall power transfer; 3) Improvement in transient stability. In addition, it partially solves the land acquisition “not in my backyard” problems associated with construction of new or already existing lines. Key equations to determine the exact cable length are formulated and validated through simulation.