Luca Dalla Santa

Experimental Validation for Impedance-Based Small-Signal Stability Analysis of Single-Phase Interconnected Power Systems With Grid-Feeding Inverters

The interactions of more and more power electronics-interfaced power systems can worsen the power quality and the system stability in distribution networks. System stability can be addressed by analyzing the source and load impedances at the interaction section, and well-established approaches exist for dc and three-phase ac networks. Some papers also focused on single-phase ac systems, whose study is generally more difficult due to their time-varying characteristics. Similar to the approaches based on dq rotating frame impedances, this paper focuses on single-phase systems using the dynamic phasor approach to determine the 2-D source and load impedances. The system stability is then evaluated using the generalized Nyquist stability criterion. As an example of application, a single-phase ac system that includes a current-controlled inverter synchronized to the grid through a phase-locked loop is proposed. Experimental validations are provided herein to verify the feasibility of this approach and the stability criteria based on the 2-D source and load impedances in the single-phase systems.

Impact of non-simultaneous P/f and Q/V grid code requirements on PV inverters on unintentional islanding operation in distribution network

The analysis of unintentional islanding in distribution grids with a large penetration of renewable energy sources is of increasing interest due to the recently introduced requirements of P/f and Q/V supporting capabilities of Distributed Energy Resources (DERs) and the wider frequency and voltage ranges, DERs should be able to operate with, before protection tripping. DER anti-islanding protections, if present, may fail to detect this grid transition and so uncontrolled islanding operation may appear. Since this operating mode may be dangerous for the electric network operation and may cause damages for the grid equipment, in this paper the increase of its risk due to P/f and Q/V droop characteristics of generators is addressed. Analytic results are initially derived for the steady-state unintentional islanding operation and then they are validated on a laboratory-scale grid setup. The proposed investigation will show that the risk of permanent unintentional islanding increases introducing P/f or Q/V droop regulation and that such risk is maximum including both the P/f and Q/V droop regulations, rather than only one of them.

Analysis of interface protection system with Distributed Energy Resources compliant with the most recent grid codes: The unintentional islanding case

The ever growing penetration of Distributed Energy Resources (DERs), involving both conventional and renewable technologies, is changing the power system face. International Standards are being revised for including new requirements for DERs such as wider voltage amplitude and frequency protection thresholds and new local P=f and Q=V power regulation. This scenario has led to an increasing concern about the problem of uncontrolled islanding on distribution networks. The purpose of this paper is to describe a hardware in the loop (HIL) test-bed, where the effects of the DER inverter control actions and of the physical interface protection systems (IPSs) on the unintentional islanding can be addressed. The analysis has shown that the frequency measurement under transient conditions is very important, influencing the P=f DER control action and the IPS tripping. Thus, the preliminary results on the HIL test-bed suggest the need of standardization for the frequency measurement. This solution would be of fundamental importance to predict system behavior of distribution network having large DER diffusion and its protection system during unintentional islanding and other transient conditions.

Implementation of an active RLC load for unintentional islanding test

Inverters connected to electrical grids are required to properly detect islanding conditions, namely, the condition of operating isolated from the main grid. To verify the effectiveness of grid-tied converters in dealing with unintentional islanding conditions, test procedures based on resonant parallel RLC loads are recommended by standards. In this paper, we show the feasibility of using power electronic converters to implement active loads used to perform unintentional islanding tests. With this aim, the paper reports the implementation of an active RLC load by using a dc/ac converter. The equivalence of the emulated RLC load with respect to the desired one is shown by means of simulation and experimental results. An inverter having anti-islanding capabilities is considered in order to evaluate the suitability of the active load for unintentional islanding tests.

Validation of impedance-based small-signal stability analysis for single-phase grid-feeding inverters with PLL

The interactions of more and more power electronics-interfaced power systems can worsen the power quality and the system stability in distribution networks. System stability can be addressed analyzing the source and load impedances at the interaction section and well-established approaches exist for DC and three-phase AC networks. Some papers focused also on single-phase AC systems, whose study is generally more difficult due to their nonlinear and time-varying characteristics. One viable solution to address this issue is the use of equivalent dq rotating reference frame even for single phase system so as to identify a steady-state operating condition. This paper uses this approach and proposes the small-signal analysis of a single-phase AC system that includes a current-controlled inverter synchronized to the grid via a Phase-Locked Loop (PLL). Experimental validations are provided herein to asses the feasibility of this approach and to verify the stability criteria based on the source and load impedances.

Experimental verification of impedance-based small-signal stability analysis for single-phase interconnected power systems

The interactions of more and more power electronics-interfaced power systems can worsen the power quality and the system stability in distribution networks. System stability can be addressed analyzing the source and load impedances at the interaction section and well-established approaches exist for DC and three-phase AC networks. Some papers focus also on single-phase AC systems, whose study is generally more difficult due to their nonlinear and time-varying characteristics. One viable solution to address this issue is the use of equivalent dq rotating reference frame even for single-phase system so as to identify a steady-state operating condition. This paper describes an approach to identify a steady-state operation in single-phase AC models to enable an impedance-based small-signal analysis to infer the stability of the system. Experimental validations are provided herein to verify the stability criteria based on the source and load impedances.

Impedance synthesis by inverter control for active loads in anti-islanding testbenches

This paper discusses the emulation of an active RLC load by means of an inverter. To this purpose, a suitable current control strategy is analyzed and developed. The limitations of this approach are explored, highlighting the requirements of the adopted regulator in order to avoid instability issues. The proposed analysis and model of the controlled system are verified by means of simulations and experimental tests on a 15kVA prototype. The model is able to predict the presence of the resonant peaks closed to the current control bandwidth, thus giving useful guidelines to minimize such undesired effects. Finally, as a test case, the emulation of a resonant RLC load to be used in anti-islanding tests is considered.

ΔP — ΔQ area assessment of temporary unintentional islanding with P/f and Q/V droop controlled PV generators in distribution networks

Distributed Generation in Low Voltage grids has raised the concern of unintentional islanding and its probability has increased due to the newly introduced standards for generators, which in particular impose wider frequency and voltage ranges and active and reactive power support capabilities using of P/f and Q/V droop characteristics. Anti-islanding protections that each inverter is equipped with, may fail to detect the grid transition and so uncontrolled islanding operation may appear. This operation may be dangerous especially in presence of automatic reclosing procedure, because of possible out of phase reconnections. In this paper, the temporary unintentional islanding operation (e.g. below 600 ms) is studied considering the effects of P/f and Q/V droop characteristics of generators and their response times. A potential increase of such phenomenon will be shown with simulations and experimental results.

Effects of P/f and Q/V regulations on anti-islanding detection methods in distribution networks

The diffusion of distributed energy resources (DERs) is continuously increasing and changing the structure of medium and low voltage (MV and LV) distribution networks. In this scenario the concern and the probability of unintentional islanding operations have increased due to the recently updated international standard for generator connection, which in particular require wider voltage and frequency protection thresholds and local P/f and Q/V to support the droop capabilities. Active ant-islanding detection methods have been widely used for grid-connected inverter-based DERs, such as frequency shift based protection systems. This work focuses on the performance of this active anti-islanding detection method combined with the DERs connection requirements stated by the most recent grid codes. Moreover, it describes a hardware in the loop (HIL) test-bed, where the effect of the active anti-islanding method and the droop curves on a physical interface protection system (IPS) have been tested. The obtained results emphasize the importance of having a coordination between the islanding protection system and the standard requirements for new certification procedure and connection rules for upcoming guidelines and standards.

Phase-locked loop effect on non-detection zone of unintentional islanding

The number of small power distributed generators connected to the low voltage distribution networks has increased the possibility of unintentional islanding operations. In this paper, the influence of the Synchronous Reference Frame - Phase Locked Loop (SRF-PLL) for single-phase and three-phase inverter, combined with the wider frequency and voltage protection ranges stated by the Standards, has been considered as causes of increasing possibility of unintentional islanding events. The paper is focused on the enlargement of the protection system Non Detection Zone, due to the single phase PLL on a current- controlled Inverter, as typically used in PhotoVoltaic (PV) or Renewable Energy Source (RES) applications.