Novel three-phase topology for cascaded multilevel medium-voltage conversion systems in large-scale PV plants

Abstract

possible to place it in standard shipping containers. If this power-rating limit is exceeded, the transformer is subject to significant additional costs due to transportation. Consequently, a saturation of the large-scale PV plants' cost decrease is expected in the near future.In order to continue reducing the cost of large-scale PV plants, other alternatives have to be investigated. Mediumvoltage cascaded H-bridge conversion systems, MV CHB, and medium voltage modular multilevel conversion systems, MV MMC, [2]-[4] have appeared as potential solutions. They consist of conversion stages whose output is connected in series, generating in medium voltage (MV). The conventional step-up low-frequency transformer is then substituted by a high-frequency transformer in each conversion stage. LF transformers reduce the volume and therefore the cost of the transformer stage [5]. Moreover, the series connection of converters in these conversion systems enables the reduction of the power rating of each individual conversion stage while maintaining the power rating of the conversion structure. Consequently, a reduction of the number of PV modules connected to each conversion stage is achieved. This reduces the dc-wiring length, as the conversion stage is located closer to the PV modules, reducing the dc-wiring cost. In addition, as voltage is stepped up at the output of the conversion stage, current is stepped down, allowing a lower cross section in the ac-wiring than in the dc-wiring. Again, this also reduces the wiring cost. Furthermore, the series connection of conversion stages makes it possible to obtain a multilevel output voltage, reducing the grid current harmonics and therefore the cost of the required output line-filter. Fig. 1. Traditional large-scale PV plant configuration

Medium-Voltage Cascaded Sequential Topology for Large-Scale PV Plants

“…When the converter works with reactive power, output voltages and grid currents of different signs are found in several switching periods. The turn-on and turn-off transitions for one of these cases is explained in detail in [30]. The rest of the cases can be deduced in the same way.…”

“…In addition, each ac/ac converter includes a circuit that protects the conversion unit in the event of a semiconductor failure. This circuit, whose design is explained in [30], is composed of two diodes, D1 and D2, and one capacitor, Cp.…”

“…Hence, the grid currents are quickly extinguished The capacitor of the protection circuit of the phase in which the failure happens is therefore the one that charges the most. Its design can be found in [30].…”

Active control for medium-frequency transformers flux-balancing in a novel three-phase topology for cascaded conversion structures

“…3. The topology and its modulation technique are detailed in [6]. The three phases are activated sequentially and one at a time, which means that in a switching period, tSW, first the phase R is fed with power, then phase S and finally phase T. When a phase is activated, the corresponding ac/ac converter connects the phase's output to the transformer (HB1=ON and HB2=OFF) and the H-bridge applies the required voltage.…”

“…The main purpose of this paper is to study the MF transformers's flux evolution in a three-phase topology for cascaded conversion structures, and to propose an active control technique to prevent its saturation. The basic operation of the topology is explained in Section II (it is described in detail in [6]). In Section III, the technique to measure the magnetizing current and the control loop to prevent the saturation of the MF transformers are presented and explained.…”