Development of Dual Converters with DC Circuit breaker for DC Distribution Systems

Abstract

Solar energy is rapidly gaining popularity due to its sustainability, clean, and wide availability. Adopting solar energy on a global scale can pave the way for a greener and more sustainable world, aligning with the goals set by the Conference of the Parties (COP). It offers a short payback period and can be used locally or fed to the grid. Solar power is in DC form and can be stored in batteries using DC-DC converters. Battery storage helps address the intermittency of solar generation, providing steady output. Standalone solar PV systems with PV, batteries, and loads are widely used in remote areas without grid access. Unpredictable load shedding further drives the adoption of standalone solar PV systems. These systems reduce reliance on diesel generators, cutting emissions. It is important to ensure (a) the development of power electronics converters that can play a dual role of DC-DC and DC-AC, (b) an adaptive solar MPPT algorithm for dynamic climatic conditions, (c) protection of the DC power system using DC circuit breakers.

Hybrid AC/DC microgrids are gaining more importance due to the apparent advantages and penetration in distributed generation and storage systems. A dual converter can serve the purpose of DC-DC conversion and DC-AC inversion. The present investigation focuses on developing dual converters with in-built short circuit protection at the load side. To obtain an AC voltage with an amplitude greater than the DC input voltage, a DC-DC converter and an inverter are conventionally required; however, the proposed dual converter can reduce one stage in the conversion process.

The first dual converter is based on the current-fed converter which is modified to achieve both positive and negative gains. By modulating the duty ratio, the converter may be made to function as an inverter. The converter circuit uses a gate driver circuit based on a gate driver IC, capable of driving devices with different gate-to-source voltages.

Experimental validation for the proposed gate driver is performed on various devices. The converter is analyzed for non-idealities and small signal behavior, leading to the design of a closed-loop PI controller. The converter is powered by a solar PV source and integrated with a standalone DC distribution system. MPPT is employed to optimize power extraction from the solar PV panels, and the PI controller determines the appropriate duty cycle for the converter switches.

The versatility of the dual converter lies in its ability to either reduce or amplify the output voltage, enabling buck or boost operations. Another dual converter is designed, which serves as a buck-boost converter with an extended gain range. The proposed converter provides enhanced functionality for DC-DC operation by incorporating two MOSFETs and two diodes while maintaining the exact count of circuit components as the conventional topology (buckboost converter). The analysis encompasses various aspects such as switch implementation, gain formulation under ideal and non-ideal conditions, design considerations, and closed-loop control using a PI controller. The standalone DC grid integration and battery backup are realized using the proposed converter. An acceleration factor is introduced in the existing Perturb and Observe (P&O) algorithm to make it faster by recording the power values at two previous time instances. After reaching the MPP, the role of the acceleration factor ceases, and the conventional P&O algorithm continues. The proposed adaptable step size in the MPPT algorithm reduces the time to reach the maximum power point for a step change in variable solar irradiation.

Replacing the semiconductor switches with four-quadrant switches, the above-developed converter can be utilized for AC load applications. The converter exhibits positive gain for duty ratios ranging from 0 to 0.5 and negative gain for duty ratios ranging from 0.5 to 1, allowing it to function as both a DC-DC converter and an inverse gain DC-DC converter. When both duty ratio ranges are combined, the converter operates as a DC-AC converter.

Another dual converter, based on three winding coupled inductor is developed, which can provide a full range of AC voltage. The converter generates highly complex waveforms (DC and AC with multiple harmonics) useful for evaluating surge protection devices in a DC distribution systems.

Further, a suitably designed DC protection scheme is required to safeguard the source and load in a DC distribution system. The DC fault current interruption differs from the AC fault because of the zero-crossing absence. A bidirectional hybrid DCCB based on coupled inductor is developed, capable of interrupting a short circuit current in a few tens of microseconds.

The topology uses the same switches to charge the commutation capacitor and fault current interruption. It is a bidirectional DCCB, which can interrupt fault current even if the voltage polarity of the source is reversed.

The study encompasses DC-DC and DC-AC dual converters with suitable protection against overcurrent and overvoltage conditions. Important results obtained and the analyses conducted during the investigations are presented.