Analysis of Smart Transformer features for electric distribution

The distribution grid is undergoing deep changes created by the integration of new generation resources, such as renewables, and new loads, like electric vehicles. These new actors impact on the distribution grid management, introducing 1) higher variability of the grid power demand and subsequent power unbalance, 2) reverse power flow with increased overvoltage conditions in case of high power production and low power consumption, cables and transformer overload in case of low power production and high power consumption, and 3) decreased system inertia, due to the power electronics-connection of the resources. The Smart Transformer (ST) enables the management of the distribution grid, absolving three main tasks: 1) adapting the voltage level from medium to low voltage grids; 2) managing the distribution grid during the aforementioned issues; and 3) offering higher controllability of distribution and transmission grid. This work describes in details the ST controllers and their tuning, taking into account the services to be provided. The ST enables the direct control of the voltage waveform in the ST-fed grid, varying the voltage amplitude and frequency. This allows to interact with the voltage-sensitive loads power consumption and droop controlled-generators in order to shape the power consumption of the ST-fed grid. Applying this control the ST can offer services to the grid, like limiting the reverse power flow in the medium voltage grid, or managing its overload conditions. The accuracy of these services can be increased if the identification of the grid power sensitivity to voltage and frequency is carried out. The ST, applying a controlled voltage amplitude and frequency variation, performs the on-line load sensitivity identification and evaluates in real time the grid sensitivity. This identification enables the offer of new ancillary services to the distribution and transmission grids

Virtual Synchronous Machine Control for Asynchronous Grid Connections

The reduced amount of large synchronous generators results in the need for fast, flexible, and intelligent power distribution devices to enhance the inertia in the modern power system. This paper proposes a new approach to control an asynchronous low-voltage grid connection, employing a virtual synchronous machine with frequency-based power control. The grid-forming converter, receiving the primary side frequency measurement, varies the fed grid frequency on the secondary side artificially, to interact with frequency-dependent resources. This enables the adjustment of the consumed or generated power in the fed grid without the need for additional communication infrastructure, and thus supports the frequency control of the mains. The performance of the proposed frequency control has been validated through simulation and using a newly developed double Power Hardware-in-the-Loop experimental test setup

Adaptive droop control strategy for Flywheel Energy Storage Systems: A Power Hardware-in-the-Loop validation

Low-inertia power systems can suffer from high rates of change of frequency during imbalances between the generation and the demand. Fast-reacting storage systems such as a Flywheel Energy Storage System (FESS) can help maintain the frequency by quickly reacting to frequency disturbances, with no concern over its lifetime. While a modern high-speed FESS has a significantly higher energy density than the conventional low-speed ones, the capacity of this storage technology is still limited. Therefore, this paper proposes a new adaptive droop controller for a FESS, considering the practical advantages and also limitations of this storage technology. The proposed controller increases the contribution of the FESS for frequency support during the first instances of a disturbance, while it reduces its output when the frequency is recovering. To verify the advantages of the proposed control strategy, the controller is implemented on a real 60 kW high-speed FESS using the concept of rapid control prototyping. Next, the performance of the FESS with the new controller is tested using Power Hardware-in-the-Loop simulations in a low-voltage microgrid. The PHIL simulation results show that the proposed adaptive controller improves the performance of the FESS in terms of limiting the frequency deviations, while preserving more energy in the FESS

Impedance-based Stability Analysis of a Power Hardware-in-the-Loop for Grid-Following Inverter Testing

Power Hardware-in-the-Loop (P-HIL) provides a reliable evaluation of real hardware interactions under realistic grid conditions in the Laboratory environment. A P-HIL setup comprises three main sectors: real-time simulator, Hardware-under-test (HuT), and interfaces. The limitations of interfaces and the delays between the sectors can result in stability issues. Therefore, a precise stability analysis is necessary before conducting laboratory experiments. This paper proposes the impedance-based approach to asses the stability constraints for a P-HIL using a grid-following inverter as HuT. The stability criterion is determined based on the impedances seen by the grid and the inverter at the PCC. The impact of interface dynamics, delays, and controller bandwidth is carefully regarded. All P-HIL components are implemented in Simulink first, then the actual setup with RTDS and linear amplifier has been configured to provide a more realistic reference for impedance verifications. The calculated impedances are verified with both simulations and experiments through frequency response. The comparison between the time domain response and the Nyquist criterion confirms the validity of the given stability criterion

Enhanced Current-Type P-HIL Interface Algorithm for Smart Transformers Testing

The energy systems are evolving towards the wide integration of power electronics-based technologies, such as electric vehicles. A promising solution to increase the grid controllability is represented by grid-forming converters, such as smart transformers (STs). Being a new technology, the ST experimental testing is a fundamental step before commercialization. Instead of performing time consuming and not flexible on-field tests, the Power Hardware In the Loop (P-HIL) offers a flexible testing environment for experimentally validating new technologies. The real-time simulation of the electrical grid offers the possibility to vary quickly the testing environment, while the power amplification stage offers the validation of the real hardware. Despite the clear testing advantages, the P-HIL stability and testing accuracy is still a matter of study. This paper introduces a new P-HIL interface approach for ST application, that can guarantee high testing accuracy in a large frequency spectrum. The proposed approach combines the tracking capability of the existing controlled Current-Type P-HIL interface algorithm, with the well-known Partial Circuit Duplication approach. The accuracy and stability analysis has been performed analytically and validated by means of extensive experimental P-HIL testing

Current-type Power Hardware in the Loop (PHIL) evaluation for smart transformer application

The development of power electronics and power systems due to the massive integration of renewable energy sources is challenging the distribution grids. Among several concepts, the Smart Transformer (ST), a solid-state transformer with advanced control and communication capabilities, has been investigated by several researchers. A great challenge of this kind of system is the possibility to test the effectiveness of the physical system under a broad spectrum of operating conditions. For this reason, the Power Hardware in the Loop (PHIL) concept can be adopted to emulate the behavior of a distribution grid connected to the ST. In this case, because the low-voltage stage of the ST is voltage controlled, the test setup must be current-controlled. In this paper, the current-controlled PHIL setup is analyzed. The theorethical analysis is carried out and preliminary results obtained with the PHIL facilities are presented, highlighting how the current-controlled PHIL can be an effective means to study the ST

Distributed FACTS for Power System Transient Stability Control

The high penetration of renewable energy sources, combined with a limited possibility to expand the transmission infrastructure, stretches the system stability in the case of faults. For this reason, operators are calling for additional control flexibility in the grid. In this paper, we propose the deployment of switchable reactors and capacitors distributed on the grid as a control resource for securing operations during severe contingencies and avoiding potential blackouts. According to the operating principles, the line reactance varies by switching on or off a certain number of distributed series reactors and capacitors and, therefore, the stabilizing control rule is based on a stepwise time-discrete control action. A control strategy, based on dynamic optimization, is proposed and tested on a realistic-sized transmission system

Primary Frequency Regulation Using HVDC Terminals Controlling Voltage Dependent Loads

HVDC can provide frequency regulation during disturbances (e.g., faults) by controlling the power flow between two remote AC areas. While this action reduces the power deviation in the area affected by the disturbance, it causes a power imbalance in the other healthy AC area, leading to a frequency variation and endangering the system stability. In this work, a HVDC primary frequency regulation controlling voltage-dependent loads (PFRVDL) is proposed, where the HVDC terminal in the healthy area influences the grid voltage amplitude to shape (decreasing or increasing) the load consumption in order to cope with the power variation required by the fault-affected area. The PFR-VDL extracts the needed energy for the frequency support, not from the generators (with following frequency deviation) but from the voltage-dependent loads in the healthy area. This work analyzes the PFR-VDL performance, generalizing it with two possible HVDC connection cases: Asynchronous connection with single HVDC line, and embedded HVDC forming a parallel, hybrid connection with HVAC. The PFR-VDL application benefits and limitations are evaluated analytically and verified by means of PSCAD EMTDC simulations, and finally validated with a large interconnected IEEE 39 bus system