Protection of hybrid transformers in the distribution grid

Abstract

Due to the increasing integration of renewable energy sources and power electronic loads into the distribution grid, a deterioration of the grid power quality is expected. Consisting of a low frequency transformer and a fractionally rated power electronic converter, the hybrid transformer can be applied to ensure a high power quality by controlling voltage, current, active and reactive power dynamically. For the application in grids with conventional grid protection infrastructure, hybrid transformers have to withstand considerable overvoltage and -current stresses during voltage surges or grid short circuits. Since the semiconductors are less robust than low frequency transformers with respect to these stresses, the effects of possible fault scenarios and a protection concept are studied in this paper. TABLE I: Specifications of the investigated 400 kVA, 20 kV-400 V HT. Parameter Value S n 400 kVA V MV , V LV 20 kV RMS , 400 V RMS , line to line Controllability ±10% of nominal V (LV side), P and Q S conv , V ser 40 kVA, 23 V Topology 2-level voltage source inverters, back to back DC-link 70 V, 5 mF, actively balanced split DC-link

Hybrid stepless distribution transformer with four-quadrant AC/DC/AC converter at low voltage side - simulation tests

“…In the case of the distribution HT the range is 10% of the nominal voltage rating. Hence, one of the most important problems, although not covered by this article, is the proper protection of HT [18].…”

Soft start‐up scheme for the hybrid distribution transformer

“…Furthermore, [10] discusses the reactive power compensation and regulation with the help of HDT with double DC–AC converts. The author in [11] studies the effects of possible fault scenarios and a protection concept of HDT. HDT will replace the conventional distribution transformer in the future, and optimising HDT with DC–AC converters to be more reliable is a novel trend of for the next generation distribution transformer.…”

Integration of Large Photovoltaic and Wind System by Means of Smart Transformer

“…Thus, the combination of OLTCT and extra reactive power compensators or energy storage systems has been proposed to improve the voltage regulation [19,20]. A hybrid transformer (HT), based on the mixed parallel and series connection of power converter and CPT, has been proposed to achieve active and reactive power flow control and harmonic compensation [25,26]. However, the series converter current must be sized as the CPT, and, moreover, the magnetizing current impact of the CPT still remains [26].…”

“…A hybrid transformer (HT), based on the mixed parallel and series connection of power converter and CPT, has been proposed to achieve active and reactive power flow control and harmonic compensation [25,26]. However, the series converter current must be sized as the CPT, and, moreover, the magnetizing current impact of the CPT still remains [26]. STATCOM is the most common way to support grid voltage [17,18].…”