Giovanni De Carne

The Smart Transformer: Impact on the Electric Grid and Technology Challenges

The increasing proliferation of renewable energy resources and new sizeable loads like electric vehicle (EV) charging stations has posed many technical and operational challenges to distribution grids [1]. Encouraged by attractive tax incentives and promotion policies, local grid end consumers are becoming not only consumers of electricity but, in many cases, also producers. The actual electric distribution system limits the use of renewable energy resources, offers poor EV infrastructure, and is based on a unidirectional information flow from sources to control centers.

Integrated voltage control and line congestion management in Active Distribution Networks by means of smart transformers

Within the context of Active Distribution Networks (ADNs), smart transformers represent very powerful devices able to provide fast and efficient control actions with respect to different ADNs ancillary services. This paper discusses the benefits, in terms of ADNs voltage and line flows controls, achieved by interfacing distributed generators with the power grid by means of a smart transformer. Among several benefits, these devices allow for a phase-per-phase control of the generators active and reactive power injections. This peculiarity enables to deploy new control schemes that are analyzed and discussed in the paper with reference to a case study based on a modified IEEE 34 node test distribution feeder.

Voltage and current balancing in Low and Medium Voltage grid by means of Smart Transformer

The Smart Transformer (ST), which is a power electronics-based transformer, represents an enabling technology for providing new services to the Low Voltage (LV) and Medium Voltage (MV) grid. The 3-stage configuration, with a high frequency transformer and 2 DC links, allows the electrical separation of the LV and MV grids. This leads to the independence of the two grids: any disturbance downstream and upstream can be compensated and mitigated by the ST action, which is the case of the unbalanced load condition of the LV grids. The unbalanced currents demanded by the loads could create unbalanced voltages at the LV side and also unbalanced currents at the MV side of a traditional transformer. This paper presents the improvements achieved by ST implementation in terms of voltage and current balancing: with a proper control of the ST, it provides balanced voltage in the LV grid, demanding a balanced current from the MV grid. This feature increases the power quality in both grids and provides an improved service to the customers.

Frequency-Based Overload Control of Smart Transformers

A Smart Transformers (ST) is an automated transformer based on the latest power electronics and communication technologies. It aims not only at replacing the traditional transformer, but at providing also ancillary services to the grid, thanks to the greater flexibility offered by power electronics. However, in the case of grid overload caused by high load demand or high production from renewable energy sources, the power electronics have no extra capability. The power semiconductors can be overloaded only for few microseconds, in contrast with the grid components requirement of bearing currents higher than the rated values for several seconds. Thus the ST needs new procedures for dealing with the transients/conditions of the high current requests by the load. This paper presents the Frequency-Based Overload Control (FBOC), an innovative procedure for the overload management that acts in coordination with the droop controller of Distributed Generation (DG) systems, enabling the limitation of the transformer current.

Load Control Using Sensitivity Identification by Means of Smart Transformer

The higher variability introduced by distributed generation leads to fast changes in the aggregate load composition, and thus in the power response during voltage variations. The smart transformer, a power electronics-based distribution transformer with advanced control functionalities, can exploit the load dependence on voltage for providing services to the distribution and transmission grids. In this paper, two possible applications are proposed: 1) the smart transformer overload control by means of voltage control action and 2) the soft load reduction method, that reduces load consumption avoiding the load disconnection. These services depend on the correct identification of load dependence on voltage, which the smart transformer evaluates in real time based on load measurements. The effect of the distributed generation on net load sensitivity has been derived and demonstrated with the control hardware in loop evaluation by means of a real time digital simulator.

On-Line Load Sensitivity Identification in LV Distribution Grids

This letter proposes a new on-line load sensitivity identification by means of power electronics-based devices. Applying a voltage and frequency perturbation and measuring the consumed power of the loads, the proposed method computes in real time the voltage and frequency dependency of the load active and reactive power. In this work a Smart Transformer application has been proposed, but the method is general for any power electronics converter able to influence dynamically the voltage and frequency in the grid.

Coordinated frequency and Voltage Overload Control of Smart Transformers

A Smart Transformer (ST) is a power electronics-based transformer that aims not only to substitute the traditional transformer but to upgrade also the LV and MV grid. In order to limit the costs, the ST must be carefully designed, constraining the current carried by the ST. In this paper a Combined Frequency and Voltage Controller is proposed, in order to manage a possible overload without derating the ST. Aiming to reduce the current, this control enhances the ST security against the overload situation interacting with the local Distributed Generation (DG) and the local loads.

Multi-frequency power transfer in a smart transformer based distribution grid

The smart transformer, a solid-state transformer with control and communication functionalities should provide services to the grid. This paper proposes to use a different frequency respect to the fundamental frequency, to provide such services to the distribution grid. The approach has the main benefit to transfer energy from point to point of the grid, exploiting the lower impedance path that multi-frequency converters offer. This paper describes the control strategy of the multi-frequency converters, and verifies their impact on distribution grid.

Frequency adaptive control of a smart transformer-fed distribution grid

An advanced service provided by the Smart Transformer (ST) is the decoupling between the Medium Voltage (MV) and Low Voltage (LV) grids. In the LV side, the ST can modify the waveforms frequency in order to interact with the droop controllers of the local generators to control the power demand among the sources without affecting the MV grid. However, most of the existing controllers for power converters cannot guarantee good performances under variable frequency conditions. To address this issue, a frequency adaptive control scheme based on the Fractional-Order Repetitive Control (FORC) as well as Frequency Locked Loop (FLL) is proposed in this paper. This proposed scheme provides fast online parameter tuning capability in order to be highly adaptive to variable frequencies, and it can be implemented either in a ST converter or in distributed generators. In this work simulation and experimental results are provided to demonstrate the effectiveness and advantages of the proposed scheme.

Reverse Power Flow Control in a ST-Fed Distribution Grid

The massive integration of distributed generation in the grid poses new challenges to the system operators, like the reverse power flow from the low voltage (LV) to medium voltage (MV) grid. In the case of high DG power production and low load absorption, the voltage rises in the line reaching the upper voltage limit. At this regard the smart transformer (ST) offers a new possibility to limit the reverse power flow in the MV grids. The ST can adapt the voltage waveform modifying the frequency in order to interact with the local distributed generation, that are normally equipped with droop characteristic. However, when a fast change in the frequency is applied to avoid reverse power flow to MV grid, stability problems, so far not investigated, arise. In this paper, the stability analysis has been performed analytically and validated by means of control-hardware-in-loop in a real time digital simulator and with experimental results in laboratory.