Integration of Renewable Energy Sources into Smart Grids Using Converters Based on Solid- State Transformers

Abstract

The problem of integrating renewable energy sources (RES) into regional networks is considered. The topologies of renewable energy converters are considered, their shortcomings are identified. One of the most important aspects of the introduction of alternative energy is the issue of effective integration of DC power from alternative sources into centralized regional AC networks. Integration occurs through the use of semiconductor network inverters. Typical inverting circuits are divided into two types - string circuits, in which batteries are connected in parallel to a common line by an inverter, and circuits with microinverters, in which conversion to alternating current occurs directly on each batteryThe advantage of string circuits is ease of maintenance, since the circuit has only one inverter per line, however, this reduces the reliability of the circuit. A large number of micro-inverters increases the cost of maintaining the installation, but if one of them fails, the circuit will continue to transfer electricity to the network. A distinctive feature of both types of circuits is the presence of an output power step-up transformer, which is the main element of integration. Due to the use of such transformers, the problem of accurate voltage regulation and efficient power distribution arises, since the amplitude value of the sinusoid of the primary winding depends on the voltage of the battery charge and is rarely regulated. The scheme of RES integration into smart power grids with the possibility of power and voltage regulation is considered. Such a topology of the converter can be used in both of the above integration schemes - both as a powerful string converter and as a micro converter.A converter for RES integration has been designed. The developed device consists of a non-isolated DC boost converter (BCC), a double active bridge and an inverter. The calculations of the components were made, a simulation model of the device was compiled. The principle of operation of the device is to gradually increase the DC voltage to the peak value of the mains voltage of 220 V (up to 310.2 V) with the possibility of two-level regulation and further inversion. Smoothing capacitors are provided in the DC links. Two-level regulation is performed by changing the duty cycle of the pulse-width modulation (PWM) signal of a non-isolated converter or by shifting the PWM phase of a dual active bridge. Output voltage regulation is possible both when the load increases and when the input voltage from the batteries changes.Modeling was carried out, the operation of the converter was investigated with a change in load and with a change in the input voltage from RES batteries. The simulation results showed a high efficiency of converting the DC voltage of RES batteries into the AC voltage of the regional network. The regulator effectively kept the output voltage within acceptable limits as the input voltage and load varied. The disadvantages of existing devices for integrating RES into regional smart power grids are their low control flexibility when the input voltage changes. One possible solution to this problem is the use of intelligent DC conversion systems, such as a solid state transformer based on a double active bridge and a non-isolated DC boost converter.