Voltage-Based Control of a Smart Transformer in a Microgrid

Abstract

Abstract-For the islanded operation of a microgrid, several control strategies have been developed. For example, voltage-based droop control can be implemented for the active power control of the generators and the control of the active loads. One of the main advantages of a microgrid is that it can be implemented as a controllable entity within the electrical network. This requires the ability of the utility grid to control or influence the power exchange with the microgrid by communicating with only one unit. However, little research has been conducted on controlling the power transfer through the point of common coupling. This paper addresses this issue by introducing the concept of a smart transformer (ST) at the point of common coupling. This unit controls the active power exchange between a microgrid and the utility grid dependent on the state of both networks and other information communicated to the ST. To control the active power, the ST uses its taps that change the microgrid-side voltage at the PCC. This voltage-based control of the ST is compatible with the voltage-based droop control of the units in the microgrid that is used in this paper. Hence, the microgrid units can automatically respond to changes of ST set points and vice versa. Several simulation cases are included in this paper to demonstrate the feasibility of the ST concept.

Development of a smart transformer to control the power exchange of a microgrid

“…In order to reduce the required communication data for controlling the power exchange, the ST concept has been presented in [4]. Instead of exchanging new set points with all microgrid elements, only the ST needs to acquire the set point.…”

“…The transformer is smart in the sense that its control strategy is able to control the power exchange to a set value by controlling its microgrid-side voltage. [4] In medium-voltage networks, on-load tap changing transformers are sometimes already in place, hence, controlling these as STs requires only little modifications. In the lower voltage networks, most PCC transformers are manual tap changing transformers, from which the voltage can only be controlled offline and not automatically.…”

“…3, has originally been presented for ensuring a stable operation of lowvoltage islanded microgrids [5]. For the active power control of [4] the DG units, this VBD controller consists of a combination of a V g /V dc droop controller and a P/V g droop controller, with V dc the dc-link voltage, where the power of the dc link is provided by the available renewable energy, and V g the terminal voltage of the DG unit. The former enables power balancing of the DG units ac and dc side and an effective usage of the allowed tolerance on the variations of terminal voltage from its nominal value for grid control.…”

“…However, this requires a large investment in additional communications, may restrict the systems reliability and can cause delays. Therefore, in [4], another possibility is proposed. Here, the utility network communicates only with the PCC of the microgrid, where the transformer located at the PCC, which is called the smart transformer, alters its microgridside voltage accordingly.…”

A Coordinated Control for Photovoltaic Generators and Energy Storages in Low-Voltage AC/DC Hybrid Microgrids under Islanded Mode

“…However, the operating principle of an IC is not practical to coordinate both of the subgrids. References [30][31][32] propose a control strategy of V g /V DC droop and P/V DC anti-droop control strategy for PV panels. It avoids frequent energy fluctuation and improves the quality of output voltage, but does not take into account the coordination control of multiple DG units.…”