This paper presents an overview of the synchronization stability of converter-based resources under a wide range of grid conditions. The general grid-synchronization principles for grid-following and grid-forming modes are reviewed first. Then, the small-signal and transient stability of these two operating modes are discussed, and the design-oriented analyses are performed to illustrate the control impact. Lastly, perspectives on the prospects and challenges are shared.

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Grid-Synchronization Stability of Converter-Based Resources - An Overview

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 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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On the Inertia of Future More-Electronics Power Systems

The utilization of power electronic inverters in power grids has increased tremendously, along with advancements in renewable energy sources. The usage of power electronic inverters results in the decoupling of sources from loads, leading to a decrease in the inertia of power systems. This decrease results in a high rate of change of frequency and frequency deviations under power imbalance that substantially affect the frequency stability of the system. This study focuses on the requirements of inertia and the corresponding issues that challenge the various country grid operators during the large-scale integration of renewable energy sources. This study reviews the various control techniques and technologies that offset a decrease in inertia and discusses the inertia emulation control techniques available for inverters, wind turbines, photovoltaic systems, and microgrid. This study attempts to explore future research directions and may assist researchers in choosing an appropriate topology, depending on requirements.

Future low-inertia power systems: Requirements, issues, and solutions - A review

With an increasing capacity in the converter-based generation to the modern power system, a growing demand for such systems to be more grid-friendly has emerged. Consequently, grid-forming converters have been proposed as a promising solution as they are compatible with the conventional synchronous-machine-based power system. However, most research focuses on the grid-forming control during normal operating conditions without considering the fundamental distinction between a grid-forming converter and a synchronous machine when considering its short-circuit capability. The current limitation of grid-forming converters during fault conditions is not well described in the available literature and present solutions often aim to switch the control structure to a grid-following structure during the fault. Yet, for a future converter-based power system with no or little integration of synchronous machines, the converters need to preserve their voltage-mode characteristics and be robust toward weak-grid conditions. To address this issue, this article discusses the fundamental issue of grid-forming converter control during grid fault conditions and proposes a fault-mode controller which keeps the voltage-mode characteristics of the grid-forming structure while simultaneously limiting the converter currents to an admissible value. The proposed method is evaluated in a detailed simulation model and verified through an experimental test setup.

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Current Limiting Control With Enhanced Dynamics of Grid-Forming Converters During Fault Conditions

In this paper, different control approaches for grid-forming inverters are discussed and compared with the grid-forming properties of synchronous machines. Grid-forming inverters are able to operate AC grids with or without rotating machines. In the past, they have been successfully deployed in inverter dominated island grids or in uninterruptable power supply (UPS) systems. It is expected that with increasing shares of inverter-based electrical power generation, grid-forming inverters will also become relevant for interconnected power systems. In contrast to conventional current-controlled inverters, grid-forming inverters do not immediately follow the grid voltage. They form voltage phasors that have an inertial behavior. In consequence, they can inherently deliver momentary reserve and increase power grid resilience.

Overview on Grid-Forming Inverter Control Methods

Power systems with high penetration rates of inverter-based generation units exhibit reduced system inertia Faults like plant outages or fault-induced system splits then cause an increased rate of change of frequency and may lead to frequency instability One of the proposed schemes to provide synthetic inertia with power electronic converters is the frequency-derivative-based ( $df/dt$ ) approach Converters with this method inject an active current that is proportional to the derivative of the grid frequency to mimic the inertia of synchronous plants Specification of synthetic inertia is currently being discussed for future grid codes In this paper, we show that the frequency derivative in the $df/dt$ controller is subject to a severe bandwidth limitation Considerable low-pass filtering is indispensable to avoid instability due to the excitation of local oscillation modes in the harmonic frequency range We have found a first-order lag with a time constant around 1 s to be required for a robustly stable system with common parameters A new discrete-time linear model that accurately represents sampling effects is introduced and comprehensively described A $df/dt$ control design is proposed that leads to a robustly stable system Model and control design are validated by laboratory experiments, especially regarding mechanisms of instability The main finding of this paper—applying a $df/dt$ controller harbors the risk of merely shifting the initial danger of system-wide frequency instability to local stability problems—is important for power plant manufacturers as well as system operators

Modeling and Design of $df/dt$ -Based Inertia Control for Power Converters

Increasing shares of power electronic converters in renewable generation challenge power system stability. For very high shares the state of the art current-control approach should be replaced by grid forming control schemes. The virtual synchronous machine (VSM) approach is grid forming and fully compatible to conventional synchronous-machine-based power plants. The high fault currents of synchronous machines are also replicated and lead to excessive current ratings of the semiconductors. Current control could be applied again during faults, however the grid-forming property is lost when it is most needed. We propose a VSM with a virtual resistor that reduces fault currents while the grid forming functionality is maintained. For the most severe fault the peak current amplitude is reduced to 2.5 times the nominal current. Swing mode and high-frequency stability is investigated in detail. The results have been confirmed by experiments. The over-current requirements and thus component cost can be reduced drastically through the application of a virtual resistor. Nonetheless, an increased current rating of the hardware is required to handle severe faults. System studies should be performed to confirm the method on system level.

Virtual synchronous machine control with virtual resistor for enhanced short circuit capability

https://onlinelibrary.wiley.com/doi/pdf/10.1002/we.2457

Design and laboratory test of black‐start control mode for wind turbines

The tower is one of the major components in wind turbines with a contribution to the cost of energy of 8 to 12% [1]. In this overview the load situation of the tower will be described in terms of sources of loads, load components and fatigue contribution. Then two load reduction control schemes are described along with simulation and field test results. Pitch Balancing is described as a method to reduce aerodynamic asymmetry and the resulting fatigue loads. Active Tower Damping is reducing the tower oscillations by applying appropiate pitch angle changes. A field test was conducted on an Areva M5000 wind turbine.

http://iopscience.iop.org/article/10.1088/1742-6596/555/1/012030/pdf

Active tower damping and pitch balancing – design, simulation and field test

Motivated by emerging oscillation issues of high-voltage direct-current (HVDC) transmission system resulted from the interaction with AC grids, impedance-based stability analysis becomes an attractive approach in the de-risking studies due to the capability of black-box modelling and clear physical meaning. However, impedance-based stability assessment of HVDC is typically conducted either from the AC side or from the DC side, without fully considering both AC and DC grid dynamics, thus impairs evaluation accuracy. In this paper, the nodal admittance matrix based resonance mode analysis method is extended to include both AC and DC dynamics through formulating a hybrid AC-DC admittance matrix. To achieve this goal, HVDC interfacing-converters are modelled as three-port admittance networks with one DC port and two AC ports (in dq-frame) integrating both the AC side frequency coupling and the AC-DC dynamic coupling. In contrast to the widely adopted Nyquist-based methods, the proposed method does not requires knowing the number of right half-plane poles of system minor-loop gain, thus simplifies stability analysis, and moreover, the participation factors of both AC and DC grid nodes in critical resonance modes can be identified through eigenvalue-decomposition analysis, which is in favor of the design of mitigation measures. The effectiveness of the proposed method is validated by EMT-simulations in MATLAB/Simulink.

https://ieeexplore.ieee.org/ielx7/6287639/6514899/09780180.pdf

Daniel Duckwitz

Papers

Modeling and Design of $df/dt$ -Based Inertia Control for Power Converters

Virtual synchronous machine control with virtual resistor for enhanced short circuit capability

Design and laboratory test of black‐start control mode for wind turbines

Active tower damping and pitch balancing – design, simulation and field test

Extended Nodal Admittance Matrix Based Stability Analysis of HVDC Connected AC Grids