Renewable power systems dynamic security using a new coordination of frequency control strategy based on virtual synchronous generator and digital frequency protection

https://doi.org/10.1016/j.ijepes.2019.02.007Get rights and content

Highlights

  • This paper proposes a new frequency control strategy based on Virtual Synchronous Generator (VSG), which emulates the characteristics of a real synchronous generator and are as follows; the inertia and damping properties through the concept of virtual rotor as well as frequency control loops (i.e., primary and secondary frequency control) through the virtual primary and secondary control to compensate the reduction in system inertia that results from adding more RESs (e.g., non-inertia sources), thus stabilizing the system frequency during high penetration of RESs.

  • The proposed virtual inertia control system based on VSG is coordinated with digital over/under frequency protection for improvement the frequency stability and preservation the dynamic security of renewable power systems because of the high integration level of the RESs.

  • The virtual primary controller (i.e., proportional controller) and virtual secondary controller (i.e., integral controller) is combined to obtain on a PI controller, which is optimally designed by using Particle Swarm Optimization (PSO) for finding the optimal tuning of the virtual controller parameters, thus enhancing the power system stability and resilience.

  • The uncertainties of RESs and load are taken into consideration in the virtual controller design procedure. Thus, the proposed coordination scheme will ensure an evasion of system instability and collapsed.

  • The effectiveness of the proposed coordination scheme is tested and verified through small and large scales of power systems, Microgrid and real hybrid power system in Egypt, respectively. The simulations results proved that renewable power systems with the proposed coordinated scheme will provide better stability and performance for today’s power system, and for those of the future, which are expected to integrate more and more renewable energy; thus, the proposed coordination scheme will ensure an avoidance of power system instability and system collapse.

Abstract

The renewable power systems have become more susceptible to the system insecure than traditional power systems due to reducing of the total inertia and damping properties that result from replacing the conventional generators with Renewable Energy Sources (RESs) as well as decoupling of the RESs from the AC grid using power converters. Therefore, maintaining the dynamic security of renewable power systems is the key challenge for integrating more RESs. This paper addresses a new strategy of frequency control including virtual inertia control based on Virtual Synchronous Generator (VSG), which emulates the behavior of conventional synchronous generators in large power systems, thus adding some inertia to the system control loop virtually and accordingly stabilizing the system frequency during high penetration of RESs. Moreover, the proposed virtual inertia control system-based VSG is coordinated with digital frequency protection for improvement the frequency stability and preservation the power system dynamic security because of the high integration level of the RESs. The effectiveness of the proposed coordination scheme is tested and verified through small and large scales of power systems, Microgrid (µG) and real hybrid power system in Egypt, respectively. System modelling and simulation results are carried out using Matlab/Simulink® software. The simulation results validated that the proposed coordination scheme can effectively regulate the system frequency and ensure robust performance to maintain the dynamic system security with high share of RESs for different contingencies.

Introduction

In traditional power systems, the synchronous generators (SGs) are responsible for providing inertia to the grid through the stored kinetic energy in their rotating mass. Also, they are providing damping property for the system that results from the mechanical friction and electrical losses [1]. The intrinsic kinetic energy and damping property of the SGs play an important role in the power system stability. On the other hand, in renewable power systems, the renewable energy sources (RESs) may replace many SGs, thus the inertia of renewable power systems decreases due to lack of any rotating mass, which is the main source of inertia [2]. Therefore, with increasing the penetration level of RESs into the power systems, the influence of low system inertia and damping effect on the dynamic system performance and stability increases. Moreover, the intermittent nature of the RESs causes many control problems such as frequency/voltage instability problem, which may be limiting their high penetration [3]. As a result, renewable power systems become more susceptible to the disturbances than traditional power systems, thus are facing some of the disturbances that threaten their dynamic security such as; large frequency/voltage fluctuations, sudden load shedding, forced islanding incidents, and short-circuit faults with long clearing times [1].

To overcome such problem, one of the advanced arrangements is to emulate the behavior of conventional SG in power systems to enhance the system inertia, stability, and flexibility. It is known as a virtual synchronous generator (VSG) that mimics the activity of the prime movers, thus adding some inertia to the system control loop virtually and accordingly stabilizing the system frequency [4], [5]. Where it can be established by using energy storage system (ESS) together with a power electronic inverter and a proper control mechanism [2]. Therefore, the concept of VSG can provide a basis for maintaining a large share of RESs in today’s renewable power systems without sacrificing system stability and resiliency. Some of research and studies on VSG applications for renewable power systems have been conducted in [6], [7], [8], [9], [10], [11], [12], [13]. The references [6], [7], [8] focused on the modelling of the VSG for the power electronic inverter-based RESs. The frequency stability analysis for µGs in presence of VSG have been conducted in [9], [10], [11]. While the influence of the VSG model on the grid stability has been studied in [12], [13]. On the other side, various control techniques have been implemented based on the virtual inertia control, which is a particular case of VSG execution such as; conventional control based on the derivative controller [14], fuzzy logic controller [15], [16], robust controller based on H_infinite technique [17], and Model Predictive Control (MPC) [18] for µG frequency stabilization considering high share of the RESs. Based on the aforementioned researches, the dynamic security of renewable power systems is not achieved yet as they didn’t take into consideration the protection devices role besides the frequency control loops; primary frequency control (i.e., governor action), secondary frequency control (i.e., Load Frequency Control (LFC)), and supplementary frequency control (i.e., virtual inertia control-based VSG). Moreover, the power system protection issue is considered one of the most important issues that help to achieve the dynamic security of renewable power systems during high penetration of RESs. Where, the dynamic security of renewable power systems deals with disturbances that result from low system inertia such as; large frequency and voltage fluctuations, loss of generation source, forced load shedding, and short circuit faults [19]. Hence, there are two sides for the preservation of the dynamic security of renewable power systems (i.e., system synchronism), which are frequency stability and protection of the electrical systems as addressed in this research.

Today's power systems are experiencing structural changes induced by the integration of more RESs, thus making it more complex than traditional power systems. Therefore, renewable power systems designers are endeavoring to use digital devices to cope with this complexity, thus improving cost and usability. Also, technological advancements in protective devices have constituted the basis for the emergence of digital protection relays, which enable alternative approaches to standard protection schemes [20]. The digital protection devices have many features compared to traditional schemes such as; more accurate, less sensitive to temperature, signal storage capability, and have high-speed communication [21]. Therefore, the digital protection devices are utilized in many applications of power systems operation (i.e., generation, transmission, and distributions systems). There are some endeavors have addressed the power systems protection issue from the perspective of short circuit fault as in [22], [23], [24]. While the power systems protection issue from the perspective of the frequency protection has been addressed in [25], [26], [27], [28]. However, these frequency protection strategies could not to damp the frequency deviation and returns it to the allowable limit because of energizing the frequency relay once the system frequency becomes beyond the acceptable limit. Such an issue can be overwhelmed by proposing effective coordination of frequency stability and protection as reported in this study.

Based on the above analysis, the research contribution of this work can be summarized in the following aspects:

  • i.

    Propose a new frequency control strategy based on VSG, which emulates the characteristics of a real SG; the inertia and damping properties through the concept of virtual rotor, and the frequency control loops (i.e., primary and secondary frequency controls) through the virtual primary and secondary controls, to compensate the reduction in system inertia that results from adding more RESs (e.g., non-inertia sources), thus stabilizing the system frequency during high penetration of RESs.

  • ii.

    The proposed virtual inertia control system based on VSG is coordinated with digital frequency protection for improvement the frequency stability and preservation the dynamic security of renewable power systems because of the high share of the RESs.

  • iii.

    The virtual primary controller (i.e., proportional controller) and virtual secondary controller (i.e., integral controller) is combined to obtain on a Proportional-Integral (PI) controller, which is optimally designed by using Particle Swarm Optimization (PSO) for finding the optimal tuning of the virtual controller parameters, thus enhancing the power system stability and resilience.

  • iv.

    The uncertainties of RESs and load are taken into consideration in the virtual controller design procedure. Thus, the proposed coordination scheme will ensure an evasion of system instability and collapsed.

  • v.

    The effectiveness of the proposed coordination scheme is tested and verified through small-scale renewable power systems (e.g., µG) as well as large-scale renewable power systems (e.g., real hybrid power system in Egypt).

The rest of this paper is organized as follows: Section 2 provides a brief review of the concept of VSG. The control methodology for the virtual inertia control system based on VSG is presented in Section 3. Section 4 describes the modelling of the digital frequency protection device. The configuration of the studied µG (i.e., test system 1) with the implementation of PSO algorithm for VSG design as well as the simulation results and discussions are provided in Section 5. The configuration of the real hybrid power system in Egypt (i.e., test system 2) with the implementation of PSO algorithm for VSG design as well as the simulation results and discussions are introduced in Section 6. Finally, the paper is included in Section 7.

Section snippets

Virtual synchronous generator modelling

With increasing the penetration level of RESs into renewable power systems, the RESs can bring significant impacts to the system inertia. Where the RESs exchange power to the renewable power systems through power electronic devices (i.e., inverters/converters), which are static devices. Therefore, most of RESs lack a rotating mass, which is the main source of inertia, thus the associated inertia constant is roughly zero [1]. Hence, the influence of low system inertia on the dynamic system

Virtual controller design for the VSG

From the model of VSG, the virtual primary controller and virtual secondary controller can be combined to obtain on a PI controller, which is defined as a virtual controller. The criteria for selecting virtual controller parameters are related to the stability of renewable power systems during high penetration of RESs. Therefore, this paper uses an intelligent searching method (i.e., Particle Swarm Optimization (PSO)) to find the optimal tuning of the virtual controller parameters, thus

Modelling of digital over/under frequency relay

Grid frequency stability is a balance criterion between electric power generation and load demand. Therefore, with increasing the penetration level of RESs into renewable power systems, the overall system inertia in renewable power system might be significantly reduced, thus the frequency deviation increases [17]. Therefore, maintaining the balance between the electric power generation and load demand is at the forefront of the most important issues in the operation of renewable power systems

System configuration

An islanded µG is considered as a test system to design and validate the proposed coordination scheme of frequency control including the virtual inertia control based on VSG and the digital OUFR. The test system consists of a thermal power plant with 20 MW, wind power with 8 MW, solar power with 4 MW, and domestic loads with 15 MW. The system base is 20 MW. The simplified model of the islanded µG with the proposed coordination scheme is shown in Fig. 6. Moreover, this study takes into

System configuration

The real hybrid power system in Egypt (i.e., large-scale power system) is considered as a test system to validate the effectiveness of the proposed coordinated scheme of frequency control including the virtual inertia control based on VSG and the digital OUFR. The Egyptian Power System (EPS) includes both conventional generation sources (i.e., thermal, gas, and hydraulic power plants) with inherent nonlinearities, and RESs (i.e., wind and solar energy). Where the conventional power plants can

Conclusion

With increasing the penetration level of Renewable Energy Sources (RESs) into power electronics-based renewable power systems, the overall system inertia might be significantly reduced, increasing the susceptibility of the renewable power system to the system instability. Thus, maintaining the dynamic security of renewable power systems have become the main challenge for integration more RESs. Therefore, this paper introduces a new concept of frequency control based on Virtual Synchronous

Acknowledgment

The authors are thankful for the feedbacks of anonymous reviewers, and Prof. Vladimir Terzija, Editor-in-Chief of this journal; their comments helped us a lot in improving the quality of this paper.

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