Smart Transformers as Active Interfaces Enabling the Provision of Power-Frequency Regulation Services from Distributed Resources in Hybrid AC/DC Grids

Abstract

Smart Transformers (STs) are being envisioned as a key element for the controllability of distribution networks in a future context of Renewable Energy Source (RES), Energy Storage System (ESS) and Electric Vehicle (EV) massification. Additionally, STs enable the deployment of hybrid AC/DC networks, which offer important advantages in this context. In addition to offering further degrees of controllability, hybrid AC/DC networks are more suited to integrate DC resources such as DC loads, PV generation, ESS and EV chargers. The purpose of the work developed in this paper is to address the feasibility of exploiting STs to actively coordinate a fleet of resources existing in a hybrid AC/DC network supplied by the ST aiming to provide active power-frequency regulation services to the upstream AC grid. The feasibility of the ST to coordinate the resources available in the hybrid distribution AC/DC network in order to provide active power-frequency regulation services is demonstrated in this paper through computational simulation. It is demonstrated that the aforementioned goal can be achieved using droop-based controllers that can modulate controlled variables in the ST.

Fault-Ride-Through Approach for Grid-Tied Smart Transformers without Local Energy Storage

“…The model of ST's MV inverter is illustrated in Figure 3. The power stage of the MV inverter (AC/DC Inverter) and the coupling LC filter are modeled following the assumptions adopted in [27] for the grid-tied ST, where the DAM technique is also adopted. No neutral connection was considered for the MV inverter.…”

“…The proposed FRT strategy may be less effective if most of the load/generation is located in a MV DC sub-network. • It cannot exploit power-frequency sensitive DER located in the LV AC sub-network constituting the hybrid AC/DC distribution grid [27]. This feature would enable the regulation of the net active power in the LV AC sub-network with minimal impact on non-controllable resources.…”

Smart transformer/large flexible transformer

“…Based on the Thevenin model of ST Fig. 12(a) and the Ohm' Law, the ST output voltage is polluted by the distorted load currents in case the output impedance of ST is not zero, as shown in (9). Apart from the voltage harmonic issue, the dynamic performance of grid voltage during load step change is also degraded.…”

“…Based on the included power conversion stages, the ST can also provide a full decoupling of the medium voltage (MV) and low voltage (LV) side grids frequencies. Such frequency decoupling can have an important role regarding the possibility of exploiting power-frequency droop-based controllers in LVconnected generators or specific loads (as for example electric vehicle (EV)-chargers) in order to locally promote some form of generation-load balancing [9]. Moreover, some coordination strategies between the LV and MV side frequency can be envision alongside a ST in order to allow LV-connected resources to actively respond to frequency events occurring in the upstream grid [9].…”

Exploiting OLTC and BESS Operation Coordinated with Active Network Management in LV Networks

“…Such strategies of active participation of consumers in the grid's operation have gained the interest of utilities for the past few decades by engaging, mainly, industrial consumers through demand side management schemes [5]. Several research works have discussed recently about the Smart Transformers (STs) envisioned as a key element for the controllability of distribution networks in a future context of DER massification [6]. For the smart grid development, more novel advanced control schemes have to be implemented towards the active involvement of DER.…”