The electric power system is currently undergoing a period of unprecedented changes. Environmental and sustainability concerns lead to replacement of a significant share of conventional fossil fuel-based power plants with renewable energy resources. This transition involves the major challenge of substituting synchronous machines and their well-known dynamics and controllers with power electronics-interfaced generation whose regulation and interaction with the rest of the system is yet to be fully understood. In this article, we review the challenges of such low-inertia power systems, and survey the solutions that have been put forward thus far. We strive to concisely summarize the laid-out scientific foundations as well as the practical experiences of industrial and academic demonstration projects. We touch upon the topics of power system stability, modeling, and control, and we particularly focus on the role of frequency, inertia, as well as control of power converters and from the demand-side.

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Foundations and Challenges of Low-Inertia Systems (Invited Paper)

Preservation of the environment has become the main motivation to integrate more renewable energy sources (RESs) in electrical networks. However, several technical issues are prevalent at high level RES penetration. The most important technical issue is the difficulty in achieving the frequency stability of these new systems, as they contain less generation units that provide reserve power. Moreover, new power systems have small inertia constant due to the decoupling of the RESs from the AC grid using power converters. Therefore, the RESs in normal operation cannot participate with other conventional generation sources in frequency regulation. This paper reviews several inertia and frequency control techniques proposed for variable speed wind turbines and solar PV generators. Generally, the inertia and frequency regulation techniques were divided into two main groups. The first group includes the deloading technique, which allow the RESs to keep a certain amount of reserve power, while the second group includes inertia emulation, fast power reserve, and droop techniques, which is used to release the RESs reserve power at under frequency events.

Inertia response and frequency control techniques for renewable energy sources: A review

A Virtual synchronous machine implementation for distributed control of power transformers in SmartGrids

There is concern that the levels of inertia in power systems may decrease in the future, due to increased levels of energy being provided from renewable sources, which typically have little or no inertia. Voltage source converters (VSC) used in high voltage direct current (HVDC) transmission applications are often deliberately controlled in order to de-couple transients to prevent propagation of instability between interconnected systems. However, this can deny much needed support during transients that would otherwise be available from system inertia provided by rotating plant.

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Inertia Emulation Control Strategy for VSC-HVDC Transmission Systems

Since the publication of the original paper on power system stability definitions in 2004, the dynamic behavior of power systems has gradually changed due to the increasing penetration of converter interfaced generation technologies, loads, and transmission devices. In recognition of this change, a Task Force was established in 2016 to re-examine and extend, where appropriate, the classic definitions and classifications of the basic stability terms to incorporate the effects of fast-response power electronic devices. This paper based on an IEEE PES report summarizes the major results of the work of the Task Force and presents extended definitions and classification of power system stability.

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Definition and Classification of Power System Stability – Revisited & Extended

The drive towards renewable energy sources (RES) is dramatically changing the dynamic electrical characteristics of Generators, which are traditionally the dominant dynamic component in Power Systems. Non-synchronous generation (NSG) approaching or even exceeding control area demand is now happening on a rapidly expanding scale. The study identifies from National Grid's perspective as Great Britain transmission system operator, the key challenges during periods of operation with high proportion of RES relative to demand. It raises some major questions. What determines system strength in an HVAC system largely without synchronous generators and what is adequate for stability? What levels of %NSG can be expected in the future energy scenarios? What are the financial consequences of constraining RES production if the technical capabilities create an upper limit of %NSG which the system can be operated at? The study finally proposes closer collaboration across the industry to find the optimal missing solutions.

https://ietresearch.onlinelibrary.wiley.com/doi/pdf/10.1049/iet-rpg.2014.0199

System strength considerations in a converter dominated power system

A generic inertia emulation controller (INEC) scheme for multi-terminal voltage-source-converter (VSC)-based high voltage direct current (HVDC) systems is proposed in this study. The INEC can be incorporated in any grid-side VSC station, allowing the multi-terminal HVDC (MTDC) terminal to contribute an inertial response to connected AC systems during system disturbances, in a fashion similar to synchronous generators. The DC-link capacitors within the MTDC are utilised by the INEC scheme to exchange stored energy with the AC system by varying the overall DC voltage level of the MTDC network within a safe and pre-defined range. A theoretical treatment of the INEC algorithm and its implementation and integration within a conventional VSC control system are presented, and the impact on the total DC capacitance required within the MTDC network to ensure that DC voltages vary within an acceptable range is discussed. The proposed INEC scheme is validated using a MATLAB/SIMULINK model under various changes in demand and AC network faults. The model incorporates a multi-machine AC power system connected to a MTDC transmission system with multiple converter-interfaced nodes. The effectiveness of the INEC in damping post-fault oscillations and in enhancing AC grid frequency stability is also investigated.

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Generic inertia emulation controller for multi-terminal voltage-source-converter high voltage direct current systems

To address the system operability challenges due to the continuous reduction of system inertia by increasing penetration of renewable power generation, this paper proposes a new synthetic inertia control (SIC) strategy for doubly fed induction generator (DFIG)-based wind turbine generators. The proposed SIC scheme enables individual DFIG generators to contribute an effective inertial response which helps to stabilise the rate of change of frequency and minimize the maximum frequency deviations in particular under severe disturbance conditions. To achieve the proposed SIC, lithium-ion supercapacitors are required in the dc link of the DFIG power electronic conversion system to increase extra energy storage capability. The DFIG's dc voltage operates at various levels following the derived SIC algorithms for energy exchange with the connected ac grid, and the conventional control system for DFIG is modified to implement the algorithms. The functions and system benefits of SIC scheme are verified using a DFIG model connected to a power system and compared with the simulations of conventional DFIG control scheme. Practical issues such as size, cost, weight, connection configuration, and charging/discharging rate of lithium-ion capacitors for the SIC are discussed and relevant recommendations are presented.

Synthetic Inertia Control Strategy for Doubly Fed Induction Generator Wind Turbine Generators Using Lithium-Ion Supercapacitors

Due to the potential future energy crisis and ever increasing population, wind power, as a renewable energy source producing no emissions and with a sufficient wind resource in many parts of the world, is attracting increasing interest and growing rapidly. Offshore wind strength is relatively much stronger than onshore levels and many large-scale wind farms (greater than 1GW) are planned to be constructed in offshore location and must transmit power over long distances. Voltage-source converter based High Voltage Direct Current (VSC HVDC) transmission system, which enables fast active and reactive power control and has relatively lower losses than conventional AC transmission, is a potential solution for offshore power transmission. Offshore wind farms usually have widely dispersed locations in a strong wind area. Furthermore, VSCs have a limited transmission capacity due to limitations on IGBT and capacitors ratings. For these reasons, a multi-terminal HVDC (MTDC) transmission system, which can extract and deliver power from and to several terminals and provide power to more than one terminal, is an attractive method for offshore wind power transmission. In addition, MTDC has been proposed in other fields such as urban sub-transmission and offshore oil/gas. A detailed description of a MTDC control scheme is presented and its operation demonstrated. The paper concludes with an overview of future research in this field.

Jiebei Zhu

Papers

Inertia Emulation Control Strategy for VSC-HVDC Transmission Systems

System strength considerations in a converter dominated power system

Generic inertia emulation controller for multi-terminal voltage-source-converter high voltage direct current systems

Synthetic Inertia Control Strategy for Doubly Fed Induction Generator Wind Turbine Generators Using Lithium-Ion Supercapacitors

Future multi-terminal HVDC transmission systems using Voltage source converters