Grid-Forming Inverter with Simplified Virtual Synchronous Compensator Providing Grid Services and Grid Support

This paper proposes a Simplified Virtual Synchronous Compensator (S-VSC) model with grid-forming capabilities for microgrid applications. Previous works have shown how the S-VSC can provide grid services (i.e., virtual inertia, current harmonic compensation and reactive support during faults) in grid-feeding configuration. In this paper, the S-VSC model is extended to a grid-forming converter to demonstrate its capability to work in island as well, thus representing a promising solution for the control of a microgrid. The control algorithm is validated on a 15 kVA inverter connected to a scaled microgrid

Dead-Time Effect on Two-Level Voltage Source Virtual Synchronous Machines

The Virtual Synchronous Machine (VSM) concept represents a valid solution to integrate renewable energy sources into the grid to provide straightforwardly grid services (e.g., inertial behavior, harmonic sink), grid support during faults and island operation. Under non–ideal (symmetric and sinusoidal) operating conditions, VSMs can behave as harmonic and un- balance sinks, improving the voltage quality at the point of connection to the grid. However, the inverter dead–time alters the harmonic and unbalance sink capability of voltage source VSMs. To demonstrate the negative influence of the dead– time effect, this paper uses a simplified method to predict the ideal behavior of voltage source VSMs under non–ideal grid voltage conditions. The paper demonstrates through experiments that: (1) the inverter dead–time effect limits the harmonic and unbalance sink capability of voltage source VSMs under non– ideal grid voltage conditions and (2) a dead–time compensation is needed to make the voltage source VSMs behave according to the theoretical analysis. Two experimental tests under a 5% grid voltage unbalance and a 10% grid voltage fifth harmonic distortion validate the negative influence of the dead–time and the beneficial effect of its compensation

Simple Tuning Method of Virtual Synchronous Generators Reactive Control

The integration of renewable energy sources requires new control strategies to make static converters able to provide ancillary grid services, such as virtual inertia and grid support during faults. To address this issue, the idea of making inverters behave as synchronous machines is well known in the literature as the concept of Virtual Synchronous Generator. Thanks to this solution, inverters can provide both inertia and reactive grid support as traditional synchronous machines. However, the tuning of the excitation control of Virtual Synchronous Generator for proper reactive power management has not been properly analyzed in the literature. Therefore, the goal of this paper is to provide a simple tuning criterion for the VSM excitation control with improved dynamic behavior using a feed-forward term. This way, the VSM is able to provide the desired reactive support during faults and quickly track the desired reactive power setpoints. Both a theoretical analysis and experimental tests are provided for a 15 kVA system

A Lead-Lag Filter for Virtual Synchronous Machines with Improved Electromechanical Damping

Traditional power systems based on synchronous generators often feature low frequency electromechanical oscillations. However, the integration of renewable energy sources through power converters can help tackling this issue. In fact, thanks to the concept of Virtual Synchronous Machine (VSM), it is possible to make the inverters behave as real synchronous machines (SMs). This way, the inverters can be integrated into the grid as traditional SMs and can even outperform them when it comes to damping low frequency oscillations in the power system. In order to do that, proper damping algorithms must be adopted in the VSM model. Therefore, this paper presents a simple and straighforward damping method for VSMs based on a single lead-lag filter acting on the VSM active power feedback. The proposed method and its integration in the VSM model are described. Then, the proposed solution has been experimentally compared to conventional methods, along with comparison metrics, to highlight its benefits

General Method to Foresee the Behavior of Virtual Synchronous Machines working with Distorted and Unbalanced Voltage Conditions

To make photovoltaic and wind power plants able to provide grid services (i.e., inertial behavior, grid support and harmonic compensation), several control algorithms have been proposed in the literature during the last years. The most promising ones make the power electronic converters behave as conventional alternators, using the concept of Virtual Synchronous Machine (VSM). Several VSM models are available in the literature, some of which can improve the voltage quality at the point of connection with the grid behaving as harmonic and unbalance sinks under non–ideal grid voltage conditions. However, the literature lacks a general method to foresee the behavior of a generic VSM configuration in such conditions along with a well–established definition of the needed features to make VSMs able to work as harmonic or unbalance sinks. Therefore, this paper proposes a simple and general method to foresee the behavior of different VSM configurations under non–ideal grid voltage conditions before any experimental verification. The proposed method accurately foresees the VSMs behavior, as experimentally demonstrated on five VSM models available in the literature, working with fifth harmonic and inverse sequence voltage distortions. Moreover, the method identifies which VSM configuration can feature a beneficial harmonic and unbalance compensation

Short-Time Transient Thermal Model Identification of Multiple Three-Phase Machines

Short-time thermal transient identification method was successfully adopted to evaluate the slot thermal parameters of induction motors for industry applications. In this work, the modeling approach and the identification methodology are extended to the more sophisticated case of multiple three-phase machines. The generalized model takes into consideration the mutual heat exchange between the windings as well as the possible causes of temperature mismatch. A complete procedure to evaluate the parameters of the modified model is provided, supported by experimental validation on a 7.5 kW machine with two three-phase winding in contact at slot level. The method covers any type of multiple three-phase machines, whatever the thermal promiscuity of the winding sets: from deep coupling as the ones presented, to the case where only the end-turns are in contact, to the completely decoupled case. The proposed technique can be useful for the machine design and for real-time temperature monitoring during operation

A Detailed Analysis of the Electromagnetic Phenomena Observed During the Flux-Decay Test

The paper analyses in detail the physical phenomena involved during the flux decay test used for the rotor time constant determination. The analysis has been performed on a 15 kW induction motor and the back e.m.f transient has been critically analysed during its evolution, finding a link between its time-by-time evolution and the physical phenomena that happen in both the stator and the rotor. In particular, the effects due to the lamination saturation, the stator and rotor leakage inductances and the stator iron losses have been associated to the transient evolution of the back e.m.f.

State-Space Modeling Techniques of Emerging Grid-Connected Converters

In modern power electronics-based power systems, accurate modeling is necessary in order to analyze stability and the interaction between the different elements, which are connected to it. State space modeling seems a valid approach to study the modes of a certain system and their correlation with its states. Unfortunately, this approach may require complicated calculations and it is difficult to model advanced or emerging control techniques for grid-tied converters, such as cascaded controllers (e.g., voltage and current) and virtual synchronous generators (VSGs). Moreover, this approach does not allow an easy reconfiguration of the modeled system by adding, removing of modifying certain elements. To solve such problems, this paper presents a step-by-step approach to the converter modeling based on the Component Connection Method (CCM). The CCM is explained in detail and a practical example is given, by modeling one exemplary VSG model available in the literature. The obtained model is finally validated experimentally to demonstrate the practical accuracy of such approach. View Full-Tex

A Test Procedure to Evaluate Magnets Thermal Time Constant of Permanent Magnet Machines

Thanks to their high torque density, permanent magnet synchronous motors (PMSMs) currently represent the most competitive solution in the electrification processes involving transports and energy production. However, it is known how the torque production of PMSMs is strictly related to the temperature of the permanent magnets (PMs) since the latter affects control performance and efficiency. This issue thus makes necessary the thermal analysis of the machine under consideration. In this scenario, the determination of the PMs thermal time constant covers a pivotal role in implementing an accurate thermal model of PMSMs. Therefore, this paper aims at proposing an experimental test procedure to evaluate the PMs thermal time constant of PMSMs. The proposed procedure can be applied to any PMSM type without being affected by factors such as rotor lamination, shaft, and PM distribution. In this way, accurate and reliable results are obtained. The experimental validation has been carried out on four PMSMs, with different rotor structures, sizes, power, and voltage/current levels. Experimental results demonstrate the validity of the proposed method