Introduction to Multilevel Converters for Step-Up-Transformer-Less Direct Integration of Renewable Generation Units with Medium Voltage Smart Microgrids

Abstract For grid integration of renewable generation units, a power frequency transformer operated at 50 or 60 Hz is generally used to step-up the low output voltage (usually B690 V) to the medium voltage level (typically 6–36 kV). Because of the heavy weight and large size of the power frequency step-up transformer, the grid integration system can be expensive and complex for installation and maintenance. To achieve a compact and lightweight direct grid connection, a medium voltage multilevel converter may be an alternative approach. Different multilevel converter topologies with switching and control issues are analyzed for their medium voltage applications. The practical implementation issues are also reported in this chapter. It is expected that the medium voltage multilevel converter would have great potential for future renewable generation and smart microgrid applications.

Keywords Medium and large scale · Renewable power generation · Grid integration · Multilevel converters · Step-up-transformer-less

Introduction

The rapid increase in global energy consumption and the impact of greenhouse gas emissions have accelerated the renewable energy technology into a competitive area. Hence during the last decades, renewable energy resources have become an important part of the worldwide concern with clean power generation. Wind and solar energy has continued the worldwide success story as the wind and solar power development is experiencing dramatic growth. By 2012, there are over 282 GW of wind power generation and over 102 GW photovoltaic (PV) generation installed worldwide and renewable power plants of more than 10 MW in capacity have now become a reality. These multi-megawatt renewable power systems cover large areas of land, and thus they are usually installed in offshore and remote areas, far from cities. For example, the land area covered by a 3.6 MW turbine can be almost 0.37 km2, such that 54 turbines would cover about 20 km2 of land area [1] and the 20 MW PV power plant would cover a land area of about 500,000 m2.

Renewable energy source has very variable daily and seasonal patterns and consumer power demand requirements also have very different characteristics. Therefore, it is difficult to operate a stand-alone power system supplied from only one type of renewable energy resource unless there are appropriate energy storage facilities. If enough energy storage capacity is not available especially in medium- scale (0.1–5 MW) and large-scale ([5 MW) systems a grid connected renewable power generation may be the only practical solution. During the last few years, scientists and researchers have been introducing smart distribution grid called microgrid with distributed and renewable generation units, energy storages, and controllable loads. A medium voltage network feeder (e.g., 6–36 kV) is generally used to interconnect the renewable generation units.

Different power electronic converters have been developed using conventional topologies to fulfill the requirements of renewable generations. However, it is hard to connect the traditional converters to the grids directly, as the distortion in generated output voltages is high and a single switch cannot stand at grid voltage level. In this regard conventional systems having power frequency (i.e., 50 or 60 Hz) step-up transformer, filter, and booster not only increase the size, weight, and loss but also increase the cost and complexity of the system operation. For example, the weight and volume of a 0.69/33 kV, 2.6 MVA transformer are typically in the range of 6–8 tons and 5–9 m3, respectively [1]. A liquid-filled 2 MVA step-up transformer uses about 900 kg of liquid as the coolant and insulator, which requires regular monitoring and replacement. These penalties are critical in offshore and remote area applications, where the costs of installation and regular maintenance are extremely high. Today, the industrial trend is to move away from these heavy and large size passive components to power electronic systems that use more and more semiconductor elements controlled by powerful processor so that smart operation is ensured.

In comparison with conventional two-level converters, multilevel converters present lower switching losses, lower voltage stress on switching devices, and better harmonic performance. These remarkable features enable the connection of renewable energy systems directly to the grid without using large, heavy, and costly power transformers and also minimize the input and output filter require- ments. Although several multilevel converter topologies have been used in low voltage applications, most of the topologies are not suitable in medium voltage applications. Because of some special features, the number of components scales linearly with the number of levels, and individual modules are identical and completely modular in constriction and hence enable high-level number attain- ability; the modular multilevel cascaded (MMC) converter topology can be considered as a possible candidate for medium voltage applications [2].

The high number of levels means that medium voltage attainability is possible to connect the renewable generation units to the medium voltage grid directly and also to improve the output power quality. The component number and control complexity increase linearly with the increase in number of levels. Therefore, the optimal selection of number of converter levels is important for the best perfor- mance/cost ratio of the medium voltage converter systems. For example, the 19- level and 43-level converters are found optimal for an 11 kV and 33 kV systems, respectively [3]. Moreover, the multilevel converter requires a number of switching and control PWM signals, which cannot be generated by the available digital signal processor (DSP) because the available DSP at present only can provide about six pairs of PWM channels. In this instance, the field programmable gate array (FPGA) is the natural choice for medium voltage multilevel converters.

However, the MMC converter requires multiple isolated and balanced dc sources. In 2011, a high-frequency link operated at a few kHz to MHz was pro- posed to generate multiple isolated and balanced dc sources for MMC converter from a single source [3]. In 2013, a high-frequency link was developed and a comprehensive electromagnetic analysis was reported to verify the feasibility of the new technology [4]. Compared with the power frequency transformers, the high-frequency link has much smaller and lighter magnetic cores and windings, and thus much lower costs.

The chapter is organized as follows: The available multilevel converter topologies are presented in Sect. 2. Section 3 describes the procedure to select the multilevel converter topologies for medium voltage applications. The optimal selection of number of levels taking into account the specified system performance, control complexity, cost, and market availability of the power semiconductors for 33 kV converters are summarized in Sect. 4. The switching and control scheme for multilevel converter is presented in Sect. 5. Section 6 describes the experimental validation of the new concept by a scaled down prototype of 1 kV high-frequency link MMC converter. Finally, the chapter is concluded by brief remarks in Sect. 7.

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