Leveraging storage flexibilities in power systems thanks to synthetic kinetic energy

Abstract

Genset is usually used as the electricity generator and known for its stability thanks to its rotating inertia. With the population growth, the electricity demand is increased. Due to the limited amount of fossil energy resources and the climate-friendly issues, the renewable energy integration becomes higher. However, the intermittence of these resources can destabilize the grid. To integrate these renewable energies by behaving as genset with its rotating inertia, the Virtual Synchronous Machine concept is implemented in a microgrid.

Keywords

Virtual synchronous machine microgrid storage

1. Introduction

According to the World Energy Resources 2013 [1], the population growth is always one of the key drivers of energy demand, along with economic and social development. The world population is estimated to be around 8.1 billion in 2020. The latest estimation of the World Bank indicates that there is about 1.2 billion people without access to commercial energy.

Several studies have shown that there is a strong relationship between access to energy and the social-economic development. However, the fossil energy resources become more and more expensive and the reserves become limited. On the other hand, in some regions there are unlimited renewable energies such as sun, wind, hydro or tidal. The generated electricity can then be used for powering hospitals, schools, refrigerating system for conserving captured fishes, or for producing potable water. The main question is how to exploit these renewable energies by using less or without fossil energies.

The concept of microgrid becomes a trend, since people want to generate electricity locally and to reduce the distribution costs and losses. Moreover, thousands of small islands or communities are not electrified yet or not well deserved by the utilities.

This paper presents the integration of renewable energy in microgrid by using the principle of synthetic kinetic energy, usually called Virtual Synchronous Machine (VSM), for a solar inverter and a storage element.

2. Virtual synchronous machine

Genset is well known as a grid forming or for a secured and stable power source. Nowadays its fuel becomes more and more expensive. Moreover, it is not ecological-friendly due to its CO₂ emissions. However, genset, as a synchronous machine, is stable and reliable thanks to its inertia. For these reasons, the integration of renewable energies by using the principle of genset is studied. It means that the renewable energy sources would have synthetic kinetic energy reserve, as a genset, during the fluctuation of generated power and/or consumption power.

A commercial solar inverter is taken as a case study. Indeed, as can be found in the literature [2], the limit of stability for the renewable energies is 30% of the integration ratio because of the intermittent problem. To encounter this problem, a storage element with supercapacitors is added. It is equivalent to an inertia in a synchronous machine. Moreover, by changing the control of the solar inverter, the whole system will react as a Virtual Synchronous Machine (VSM, or also called as a Virtual Generator or a Virtual Synchronous Generator (VSG)).

2.1. Concept of gensets

The principle of gensets and can be seen in Fig. 1. The gensets are driven by droop controllers, that will have a stabilizing effect to the microgrid by providing an appropriate sharing and inertia to the system.

Fig. 1.

Concept of gensets.

Indeed, as a machine, when the active power demand increases, the generator will naturally decrease the frequency and then the controller will increase the diesel fuel debit so that the generator will have more power. Finally, the frequency will be re-established on the nominal value and the genset furnishes the new value of power demand. In the case of reactive power demand, the droop control will work on the voltage value.

It can also work when there are several gensets, connected to the same grid. In this case, the gensets are synchronized to allow the aggregation of the kinetic energy providing the inertia to the system.

This method will then be applied to the renewable energy generators.

2.2. Control structure of VSM

The renewable energy resources are connected to the grid thanks to the Voltage Source Converters (VSCs). With the classic droop control, when there are several sources, these converters can highly destabilize the system because of the lack of inertia.

For the VSM [3–6], all Distributed Energy Resources (DERs) are equipped by a modified droop control. Indeed, to behave as a genset, the DER controller emulates a rotating inertia and uses power-balance synchronization mechanism of this virtual inertia, damping oscillation and reactive power control.

Fig. 2.

Control structure for a VSM [4].

Fig. 3.

VSM in microgrid with solar converter.

The control structure of a VSM can be seen in Fig. 2 [4]. The synchronous machine modeling [7] can be seen in this architecture.

Fig. 4.

Comparison between classic grid-tie inverter and VSM.

Fig. 5.

VSM with 100% solar penetration.

Fig. 6.

Microgrid Lab demonstrator architecture.

With this new control structure, the VSM behaves as a current source. However, thanks to the voltage feedback, it is functionally a voltage source.

All parts of a genset are modelled in the VSM controller. The Automatic Voltage Regulator (AVR – dark-blue box), the governor (power calculation - orange box), the inertia and active damping part are modelled. To facilitate the calculation, the Park transformation is used, so that the voltage and current control can be made on continuous values. Finally, the inversed Park transformation is applied before calculating the duty cycle for each phase. The Pulse Width Modulation (PWM) part will transform these duty cycles to switching pulses for three-phase converter.

VSM can then be used for both grid-connected and islanded mode and is capable to change smoothly from one to the other. Note that to represent the energy storage for the emulated inertia, the storage elements such as super capacitors are necessary. With this control, the storage capacity is leveraged and more flexible. Therefore, the system is more stable in case of supply or load fluctuations thanks to the integrated power management.

3. Applications of virtual synchronous machine (VSM)

The VSM concept is then applied to our solar converters. The VSM concept is shown in Fig. 3. Some simulations are carried out for a use case of an island in an off-grid situation with 7 MW of PV installation, 7.5 MVA of Gensets and 7 MW.min power storage elements. The simulation results for both the classic PV-converter and the VSM are presented in Fig. 4. Thanks to VSM, the fuel saving is higher. Moreover, by adding more PV and energy storage, the solar energy penetration can reach higher level, even up to 100% (see Fig. 5).

This concept is also applied on a demonstrator of the Microgrid Lab [5]. The 100 KVA architecture is presented in Fig. 6. A power management is implemented to make a good power sharing. A supercapacitor bench is used as the storage element. Each DER has its proper controller. The PV energy is emulated from real PV panels. The VSM concept is applied for all PV converters (20 KVA each). An experimental test of 70 KW load impact on 3 VSMs in parallel with a genset is shown in Fig. 7. The legend and scale for Fig. 7 curves are presented in Table 1.

Fig. 7.

Oscilloscope screenshot for experimental test of 70 KW load impact on 3 VSMs in parallel with a genset.

Table 1

Legend and scale for Fig. 7 curves

Time base	100 ms/div
Upper Screen	- Phase-to-phase voltages (C5, C6, C7): 350 V/div
	- Phase current of genset (C3): 50 A/div
	- Phase current of each VSM (C1, C2, C4): 50 A/div
Lower Screen	Frequency (F1): 500 mHz/div

During the impact, the frequency drops in the acceptance range and then increases by the time and reaches the nominal value thanks to the VSM controllers. Note that the system is stable.

4. Conclusions

This paper has shown the implementation of synchronous machine modeling in converter controller for renewable energy in a microgrid, so that it behaves like a genset (called VSM or VSG). By associating the concept of VSM and the appropriate power management, the installation with renewable energy resources can have some benefits such as natural paralleling between generators, and compatibility for all renewable energy resources. Moreover, the penetration ratio of renewable energies in a microgrid can overcome the 30% traditional limit and even it can be up to 100%. The storage capacity is leveraged and more flexible, so that the system is more stable during supply-demand fluctuations. Note that this principle is compatible for grid connected or islanded modes.

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