Article

DC-Link Voltage Control Aided for the Inertial Support During Severe Faults in Weak Grids

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Keeping the synchronization of a grid feeding converter (GFC) with a weak grid during deep voltage sags has been introduced as a serious challenge in converter-interfaced renewable energy sources dominated weak grids. To deal with this challenge, a simple yet effective solution based on the virtual inertia concept is proposed in this paper. This method is realized by adding a correction term to the DC-link voltage controller, which adjusts the active and reactive current set points and enables the converter to remain synchronized to the grid during severe faults. Closed-loop dynamics of the system in the presence of the parametric uncertainty of the grid-side impedance has been studied, in both normal and fault conditions with different voltage drops. Along these, system performance has been investigated, and in comparison with previous methods, it is revealed that the direct inertial support gain may possibly cause instability and do not propose a stable synchronization process to the GFC under deep faults. The performance of the proposed method has been verified by real-time laboratory results for different resistive/inductive weak grids with various levels of voltage sags. Real-time verification demonstrates the effectiveness of the proposed control in stabilizing GFCs for inertia emulation and its role in a better synchrony process.
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... It means that the location of renewable energy source has to be moved to distant places far from the transmission system and/or load centers, where extracting the maximum energy from renewable energy sources is possible and more cost-efficient. As a result, the need for long transmission lines, causes appearance of high impedance between the grid and renewable energy sources, which is called weak grid [6][7][8]. ...
... The technology of wind turbines are gradually become matured in emulating the characteristics of synchronous generators. This performance improvement makes wind turbines a proper replacement for synchronous generators in response to fast-growing global warming concerns [1,4,6]. Although, the technology performance of power electronic converters is promising at converter and wind turbine level, there are still some unaddressed shortcomings at plant level, that need more research investigation [7,9]. ...
... For those power plant controller brands, in which the wind turbine voltages are not involved in the control strategy, either the Low Voltage Ride Through (LVRT) or High Voltage Ride Through (HVRT) protection scheme might be triggered leading to trip of some or all wind turbines. Under weak grid condition the situation might be worse as more wind turbines may cross the voltage limits and can be considered as worst case scenario [6,7]. ...
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... The protection of an MG is a complex and challenging task, because of different factors such as the MG operation modes and its bidirectional power flow, varying fault current contribution, limited and low level of fault current in power electronics-based power converter DGUs, high impedance faults (HIF), variable configuration of MG due to plug and play characteristics of DGUs [11,12], and low-voltage ride through (LVRT) requirements during a fault to prevent from system collapse [13]. In addition, the capacity and location of DGUs are important to define the setting of directional over current relays (DOCR) [14], which may influence the setting of protection devices (PD), such as pickup current (Ip), and time multiplier setting (TMS). ...
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Citation: Hussain, N.; Khayat, Y.; Golestan, S.; Nasir, M.; Vasquez, J.C.; Guerrero, J.M.; Kauhaniemi, K. AC Microgrids Protection: A Digital Coordinated Adaptive Scheme. Appl. Sci. 2021, 11, 7066. https://doi.
Chapter
Due to the environmental reasons and growing energy needs, the share of electrical energy generated by renewable energy sources (RESs) and distributed generators (DGs), such as wind and solar power, in the modern power systems is increasing. Most of RESs/DGs are interfaced to the alternative grids via the grid connected converters (GCCs). In this chapter, the structure and essentials of GCC as the significant block of future renewable integrated power grids are discussed. Basic control loops of two main topologies, that is, grid-following and grid-forming GCCs are introduced. Then dynamic characteristics emulation as an important capability of grid-forming GCCs is emphasized. The necessity of developing required standards and grid codes is explained, and finally, the chapter is summarized.
Article
The climate emergency necessitates faster and wide-scale decarbonization of power grids and daily economic activities. As a result, it has triggered large-scale deployments of inertia-less power electronics renewable power generators with intermittent output powers as replacements for existing fossil fuel-based power plants which can dispatch their output powers and possess massive inertia. Power grid decarbonization in this manner presents significant operational and security challenges and exacerbates the risks of instability due to several factors such as low inertia, lack of spinning reserve to quickly nullify active power mismatch between demand and supply, and insufficient fault current for the correct operation of protection systems. Also, the inability to source or sink large active powers in weak ac grids that may result from decommissioning of a large number of existing generators is among the major concerns. Additionally, the transition toward power electronics dominated power systems that consist of numerous grid-following and grid-forming converters and HVdc systems is another major technical challenge that grid operators may face as many of the fundamentals that will dictate steady-state and transient behaviors in the future grid may differ, including methods for assessing grid stability. Modern power converters are versatile and can address many potential challenges that may emerge, nonetheless, their complex dynamics spread over a range of frequencies and must be understood. The highlighted challenges call for new control, protection, technologies, and solutions to be developed to enable the following: safe and reliable operation of power grids during the decarbonization period, in which coexistence of grid-forming and grid-following converters and conventional power plants must be managed carefully, and after the transition to zero-carbon energy systems; increased integration of renewable power generations at all voltage levels; holistic strategies for ac and dc fault handling in both onshore and offshore ac or dc grids; and ways for improving stability and extending active power sourcing and sinking limits in weak ac grids. This Special Issue aimed to foster and document the latest research that addresses the abovementioned emerging challenges. In response to the call for papers, the total number of manuscripts received was 63, and 42 papers were accepted for publication. The submitted manuscripts came from 19 different countries, covering all ten IEEE regions, i.e., China, Brazil, Finland, India, Hong Kong, France, Denmark, Iran, the U.K., the USA, France, Australia, Germany, South Africa, Spain, Canada, Mexico, Belgium, and Taiwan. From the accepted papers, 25 papers address issues directly relevant to this special topic. Broadly, the papers can be categorized as follows: converter topologies, faults and protection, control and stability, and review.
Chapter
The concept of virtual inertia is adopted from the moment of inertia of synchronous generator (SG) rotating masses. The conventional power systems profit from the inertial function of numerous existing SGs to solve or improve the frequency challenges, e.g., low damping and high nadir due to the load/generation disturbances. However, the modern power systems are penetrated by a large number of the inertia-less power electronic–interfaced distributed energy resources (DERs). Hence, the rotational DER portion decreases impressively and the power system frequency experiences more intensive changes due to disturbances than the conventional SG-dominated power systems. In order to solve the low-inertia challenges of the power electronic–based power systems, the concept of virtual inertia is introduced, which is realized by applying a control function on the power electronic–interfaced DERs to mimic the SG inertial dynamics and provide the inertia, virtually. The main source of the virtual inertia is the short-term stored energy in the DC link of the DER power converters, which should be injected to the AC side according to the virtual inertia control objective. In fact, the DER power converters are controlled to surmount the low-inertia challenges including high-frequency nadir and high rate of change of frequency (RoCoF), low frequency/power oscillation damping, frequency instability, and severe changes triggering protection devices mistakenly.
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Inertia plays a vital role in maintaining the frequency stability of power systems. However, the increase of power electronics-based renewable generation can dramatically reduce the inertia levels of modern power systems. This issue has already challenged the control and stability of small-scale power systems. It will soon be faced by larger power systems as the trend of large-scale renewable integration continues. In view of the urgent demand for addressing the inertia concern, this paper presents a comprehensive review of inertia enhancement methods covering both proven techniques and emerging ones and also studies the effect of inertia on frequency control. Among those proven techniques, the inertia emulation by wind turbines has successfully demonstrated its effectiveness and will receive widespread adoptions. For the emerging techniques, the virtual inertia generated by the DC-link capacitors of power converters has a great potential due to its low cost. The same concept of inertia emulation can also be applied to ultracapacitors. In addition, batteries will serve as an alternative inertia supplier, and the relevant technical challenges as well as the solutions are discussed in this paper. In future power systems where most of the generators and loads are connected via power electronics, virtual synchronous machines (VSMs) will gradually take over the responsibility of inertia support. In general, it is concluded that advances in semiconductors and control promise to make power electronics an enabling technology for inertia control in future power systems.
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