Converter-interfaced Generators Research Articles

Inertia Estimation Through Covariance Matrix

This work presents a technique to estimate on-line the inertia of a power system based on ambient measurements. The proposed technique utilizes the covariance matrix of these measurements and solves an optimization problem that fits such measurements to the synchronous machine classical model. We show that the proposed technique is adequate to accurately estimate the actual inertia of synchronous machines and also the virtual inertia provided by the controllers of converter-interfaced generators that emulate the behavior of synchronous machines. We also show that the proposed approach is able to estimate the equivalent damping of the classical synchronous machine model. This feature is exploited to estimate the droop of grid-following converters, which has a similar effect of the swing equation equivalent damping. The technique is comprehensively tested on a modified version of the IEEE 39-bus system as well as on a dynamic 1479-bus model of the all-island Irish transmission system.

Coordination of distributed resources for frequency support provision in microgrids

To address the reduction of system inertia, transmission system operators are now requesting converter-interfaced generators (CIGs) to participate in frequency support services. Microgrids (MGs) and virtual power plants have been used as a solution for aggregation of small CIGs. It has been recently considered they may use their combined actions to provide frequency services. However, these proposals are not general enough, focusing on specific CIGs controllers and, in many cases, stability and operational limits are not considered. This paper presents an standardised methodology to design MG controllers that are in compliance with the MG transient response specifications (frequency nadir and rate-of-change-of-frequency, ROCOF). The methodology has an iterative nature and can be applied when the network conditions change. In each iteration, the control parameters are adjusted based on the sensitivity of the system eigenvalues against the frequency metrics of interest while stability and operational constraints are respected. Compared to the literature, the use of quantitative parametric sensitivity is performed for the reallocation of several eigenvalues using two sensitivity components, not only one. Moreover, it is suitable for any combination of grid-forming and grid-following devices. The proposed methodology is applied to a benchmark MG and results show that both nadir and ROCOF can be effectively modified and set as required. The methodology was validated by using a real MG comprising four 15 kW converters and one 75 kW converter.

A recursive system identification inertia estimator for traditional and converter-interfaced generators

This paper proposes a method to estimate the inertia constant of synchronous generators (SGs) and the virtual inertia provided by converter-interfaced generators (CIGs). It can be applied when the system is under normal operating conditions or after power imbalance events. The algorithm is based on a recursive system identification approach and uses the active power and bus frequency measurements proceeding from phasor measurement units. The measurements are preprocessed to improve the estimation accuracy. Subsequently, a model-independent rotor speed estimator is used to compute rotor speed variations (or internal frequency variations, in the case of CIGs). Afterwards, a recursive least-square equation error formulation is implemented to compute the SG and CIG parameters in the z-domain, which are later converted into their respective counterparts in the s-domain. The method is applied to several generation devices in the IEEE 39-bus and IEEE 118-bus test systems, including traditional SGs and CIGs providing virtual inertia. Numerical results show the accuracy and the low computational burden of the proposed method.

Forced oscillation suppression using extended virtual synchronous generator in a low-Inertia microgrid

Effects of forced oscillations (FOs) in well-damped power systems are relatively smaller than those of microgrids (MGs) in which the severity of the FOs may be intensified by converter interfaced generators (CIGs). According to the distinct system characteristics, the FOs in MGs will be challenging problems in future research topics. Without proper controls of the CIGs, the FOs may be exhibited extremely higher amplitude, resulting in the MG instability. Such influences will be exacerbated in a low-inertia MG, which can trigger critical frequency oscillation and system collapse. This paper introduces an extended virtual synchronous generator (VSG) with virtual forced components to attenuate the dynamic FO effects in the presence of a low-inertia MG. Contrastive scenarios, i.e., periodic, combined, full sine, and high-frequency FOs are conducted to validate the performances of VSG control in both stand-alone and interconnected MG environments. Numerical results verify that the extended VSG control provides promising benefits in a low-inertia MG not only for the sake of MG stability improvement but also the further FO suppression.

Power oscillation damping method suitable for network reconfigurations based on converter interfaced generation and combined use of active and reactive powers

Power system stabilisers (PSSs) are commonly used in synchronous generators for damping low-frequency oscillations. However, many of these machines have been replaced by converter-interfaced generators (CIGs), such as those used in solar power plants. This means that CIG-based power plants should also contribute to oscillation damping. Moreover, CIGs may use both active and reactive power for power oscillation damping (POD). However, their combined use has been seldom studied in the literature. Moreover, only a few articles have addressed the adaption of POD controllers when the power system changes (e.g., after a fault). In this paper, a POD controller for CIG-based power plants is proposed that is suitable for operation in power systems exposed to reconfiguration. This controller takes advantage of both active and reactive power injection to maximise the damping of the power system. This controller is based only on local measurements so communication systems are not required. Theoretical developments were validated in a laboratory using real power converters including a 75 kVA grid emulator where the two-area benchmark model is emulated and four 15 kVA CIGs operating in parallel connection. The applicability of the proposed controller was also explored for the IEEE 39-bus system.

Review of Wide-Area Controllers for Supporting Power System Stability

Countries around the world are transitioning from conventional power systems dominated by synchronous generators towards low-carbon resources, characterized by high levels of converter-interfaced generators (CIGs). With this transformation, hundreds or even thousands of fast power electronic devices will be added to the grid as CIG penetration increases, thus making the system dynamic response progressively faster and more complex. This future scenario poses significant challenges in power system stability and control. To sustain power system stability in future power systems and achieve a seamless transition, we need to depart from current control practices based on decentralized and stand-alone local control actions and begin to explore new methods. A promising technology to overcome control complexities and underlying stability issues in future low-inertia power systems are wide area control (WAC). Within this technology, the control is no longer based on a purely localized tasks but rather a set of coordinated actions across wide areas in which interplant communication plays a key role. This paper presents the state of the art of existing research efforts in the field of WAC strategies for maintaining stability in large-scale low-inertia power systems of the future. We present different WAC solutions that have been put forward hitherto, we put these control strategies into context, classify them, and finally relate them to each other. We also raise open questions and challenges that need to be addressed in order to ensure a secure transition towards power systems dominated by CIGs.

Operation strategy of battery energy storage systems for stability improvement of the Korean power system

In accordance with the energy transition roadmap of the Korean government, the Korean power system needs to significantly increase the use of renewable energy sources (RES) in the future. This may cause an amount of increase in converter-interfaced generators (CIG) for integrating RES to the utility grid substituting the conventional synchronous generators, and one can easily expect that a critical decrease in the system inertia will occur in the future power system, leading to some stability issues related to that. Besides, considering the short-term state of the Korean power system, another stability issue may arise due to the delayed reinforcement of the shared network connecting large-scaled generation plants. Several countermeasures such as generator tripping and generation curtailment are proposed to overcome stability issues. A combined action with generator tripping and generation curtailment is applied to stabilize the system after the critical event, however generation curtailment might reduce the economical operation of the system. Using their fast response characteristic, battery energy storage systems (BESS) are regarded as a countermeasure to relieve the curtailment. After adequate transmission network reinforcement for the large power plants is made, their long-term application needs to be examined, which is to maintain the frequency stability for the system in low-inertia situations. This study aims to design the BESS control strategy for improving stability as well as to determine key control parameters of the BESS. The proposed control strategy of BESS for stability enhancement is proven to be effective in overcoming stability issues in the short-term and long-term operation of the Korean power system.

Enabling Technologies for Enhancing Power System Stability in the Presence of Converter-Interfaced Generators

The growing attention to environmental issues is leading to an increasing integration of renewable energy sources into electrical grids. This integration process could contribute to power system decarbonization, supporting the diversification of primary energy sources and enhancing the security of energy supply, which is threatened by the uncertain costs of conventional energy sources. Despite these environmental and economical benefits, many technological and regulatory problems should be fixed in order to significantly increase the level of penetration of renewable power generators, which are connected to power transmission and distribution systems via power electronic interfaces. Indeed, these converter-interfaced generators (CIGs) perturb grid operation, especially those fueled by non-programmable energy sources (e.g., wind and solar generators), affecting the system stability and making power systems more vulnerable to dynamic perturbations. To face these issues, the conventional operating procedures based on pre-defined system conditions, which are currently adopted in power system operation tools, should be enhanced in order to allow the “online” solution of complex decision-making problems, providing power system operators with the necessary measures and alerts to promptly adjust the system. A comprehensive analysis of the most promising research directions and the main enabling technologies for addressing this complex issue is presented in this paper.

Quantifying frequency containment reserve using cross-entropy frequency-constrained contingency-state-analysis model

With the increasing penetration of converter-interfaced generators, the frequency containment reserve (FCR) from conventional generators keeps going down, leading to a potential risk of frequency instability under contingencies. Consequently, Converter-interfaced generators are required to provide FCR and participate in the corrective rescheduling. Nevertheless, how to assess the FCR and quantify the adequacy of FCR under contingencies is a big challenge in modern new power system. To address this challenge, a cross-entropy-based frequency-constrained contingency-state-analysis (FC-CSA) model is proposed in this paper. Notably, both frequency control (FC) of units (i.e., conventional synchronous generators and converter-interfaced generators), and under frequency load shedding (UFLS) are incorporated in the primary frequency response. Then a unified system frequency response (SFR) function representing frequency dynamic is derived. This SFR function is extracted and reformulated as a group of mixed-integer linear constraints and participates in the traditional CSA model. Moreover, a set of frequency dynamic indexes, i.e., Expectation of UFLS risk, Expectation of FCR from conventional and converter-interfaced generators, is extended to depict the FCR that the power system requires. These indexes are calculated by the FC-CSA in a cross-entropy-based monte carlo simulation (CE-MCs). Case studies on a modified IEEE 6-bus test system and IEEE 118-bus test system are carried out to demonstrate the effectiveness of the proposed FC-CSA model.

Estimation of Inertia for Synchronous and Non-Synchronous Generators Based on Ambient Measurements

The inertia information of both synchronous generators (SGs)/equivalent SGs and converter interfaced generators (CIGs) are crucial to maintain the frequency stability of modern power grids. This paper proposes a novel data-driven method using ambient measurements, which seems to be the first method that can estimate the inertia constants of SGs/equivalent SGs and the virtual inertia constants of CIGs simultaneously without disturbing the system’s normal operating condition. Numerical studies in the IEEE 68-bus system demonstrate that the developed estimation method can estimate the inertia constants of different generation units accurately and is robust to measurement noises, missing measurements and the variation of operating conditions.

Oscillatory Stability Region Analysis of Black-Box CIGs

Most of the existing stability analysis methods are capable of determining the oscillatory stability of black-box converter interfaced generators (CIGs) under given operating conditions only but can hardly identify the critical operating conditions. In this letter, an oscillatory stability region (OSR) is defined with the short-circuit ratio and output power as variables, and a method to obtain the OSR for the black-box CIG is proposed. First, the CIG is modeled as a variable-operating-point impedance model (VOPIM) through perturbation tests. Then, the critical operating points, for which the CIG system has a zero-damping oscillation mode, are determined by VOPIM-based oscillatory stability analysis so as to obtain the OSR. Finally, the accuracy of the OSR is verified through control hardware-in-the-loop tests. The OSR is of significant meaning for the control strategy and operation plan optimization of the CIGs.

An optimization framework for voltage stability support from active distribution networks

In recent years the number of controllable converter-interfaced generators connected to the distribution grids is rising fast. This gives rise to the concept of active distribution network (ADN), in which a local controller can maintain voltage regulation constraints, while controlling the active and reactive power exchanged with the transmission grid. This paper demonstrates how ADNs can be used in an optimal manner in order to support long-term voltage stability in a power system. The framework is based on a two-stage optimization including a central controller and a number of ADNs. The proposed method is applied on a variant of the IEEE Nordic Test System which includes a simple configuration of distribution feeders.

Management of Voltage Flexibility from Inverter-Based Distributed Generation Using Multi-Agent Reinforcement Learning

The increase in the use of converter-interfaced generators (CIGs) in today’s electrical grids will require these generators both to supply power and participate in voltage control and provision of grid stability. At the same time, new possibilities of secondary QU droop control in power grids with a large proportion of CIGs (PV panels, wind generators, micro-turbines, fuel cells, and others) open new ways for DSO to increase energy flexibility and maximize hosting capacity. This study extends the existing secondary QU droop control models to enhance the efficiency of CIG integration into electrical networks. The paper presents an approach to decentralized control of secondary voltage through converters based on a multi-agent reinforcement learning (MARL) algorithm. A procedure is also proposed for analyzing hosting capacity and voltage flexibility in a power grid in terms of secondary voltage control. The effectiveness of the proposed static MARL control is demonstrated by the example of a modified IEEE 34-bus test feeder containing CIGs. Experiments have shown that the decentralized approach at issue is effective in stabilizing nodal voltage and preventing overcurrent in lines under various heavy load conditions often caused by active power injections from CIGs themselves and power exchange processes within the TSO/DSO market interaction.

Impact of converter interfaced generators on power system long-term voltage stability monitoring and control

This paper investigates the impact of power-electronic Converter Interfaced Generators (CIG) connected at the distribution network on bulk power system long-term voltage stability. In particular, the CIG effect on instability onset, on-line stability monitoring and protection against voltage collapse is examined using an IEEE Test System properly modified to include significant CIG penetration. Several simulations are executed to evaluate the CIG impact on instability onset and determine the performance of a voltage instability monitoring method, as well as of three protection scheme variants. Different CIG connection topologies, as well as reactive control modes are examined. Conclusions drawn show significant impact of CIGs and demonstrate the need for specific countermeasures design that account for CIG interaction with traditional protection measures.

Improving Dynamic Performance of Low-Inertia Systems Through Eigensensitivity Optimization

An increasing penetration of renewable generation has led to reduced levels of rotational inertia and damping in the power network. The consequences are higher vulnerability to disturbances and deterioration of the dynamic response of the system. To overcome these challenges, novel converter control schemes that provide virtual inertia and damping have been introduced, which raises the question of optimal distribution of such devices throughout the network. This paper presents a comprehensive framework for performance-based allocation of virtual inertia and damping to the converter-interfaced generators in a detailed low-inertia system. This is achieved through an iterative, eigensensitivity-based optimization algorithm that determines the optimal controller gains while simultaneously preserving small-signal stability and ensuring that the damping ratio and frequency response after disturbances are kept within acceptable limits. Two conceptually different problem formulations are presented and validated on a modified version of the well known Kundur's two-area system as well as a larger 59-bus South-East Australian network.

Phasor-domain Dynamic Model of Asymmetric Current Injection Controller for Converter-interfaced Generator

This paper presents a controller model of asymmetric current injection for converter-interfaced generators suitable for root-mean-square (RMS) phasor-domain, fundamental-frequency, three-phase, and dynamic simulation tools. The effectiveness of the proposed controller is assessed with simulations in test systems with high percentage of converter-interfaced generation. The simulations focus on the operation of protection relays that use negative-sequence quantities in their directional elements. This paper also presents and compares two strategies to limit reactive negative-sequence currents, and active and reactive positive-sequence currents. A tutorial test system and a regional system part of the actual Brazilian Interconnected Power System are used to assess the correctness of the proposed controller in three-phase fundamental-frequency RMS dynamic simulations.

On-Line Inertia Estimation for Synchronous and Non-Synchronous Devices

This paper proposes an on-line estimation method able to track the inertia of synchronous machines as well as the equivalent, possibly time-varying inertia from the converter-interfaced generators. For power electronics devices, the droop gain of the Fast Frequency Response (FFR) is also determined as a byproduct of the inertia estimation. The proposed method is shown to be robust against noise and to track accurately the inertia of synchronous generators, virtual synchronous generators with constant and adaptive inertia, and wind power plants with inclusion of energy storage-based frequency control.

MPC-Based Fast Frequency Control of Voltage Source Converters in Low-Inertia Power Systems

A rapid deployment of renewable generation has led to significant reduction in the rotational system inertia and damping, thus making frequency control in power systems more challenging. This paper proposes a novel control scheme based on Model Predictive Control (MPC) for converter-interfaced generators operating in a <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">grid-forming</i> mode, with the goal of exploiting their fast response capabilities to provide fast frequency control service to the system. The controller manipulates converter power injections to limit the frequency nadir and rate-of-change-of-frequency after a disturbance. Both <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">centralized</i> and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">decentralized</i> MPC approaches are considered and compared in terms of performance and practical implementation. Special attention is given to the decentralized controller by generating an explicit MPC solution to enhance computational efficiency and reduce hardware requirements. Simulation results obtained from a detailed dynamic model of the IEEE 39-bus system demonstrate the effectiveness of the proposed control schemes.

Optimal Controller Design for Stabilizing a Power System with Multiple Converter-interfaced Generators

Abstract This paper performs optimization-based controller design to avoid control interaction among multiple converter-interfaced generators (CIGs). To assess the control interaction between CIGs, an impedance-based stability analysis. Unexpected instability resulting from the CIG disconnection is investigated in a test case where three CIGs are connected to a microgrid. Parameter sensitivity is performed to determine which control parameter has more influence on the stability, and particle swarm optimization is utilized to adjust the control parameters to enhance the stability with computational efficiency. Simulation results demonstrated that the proposed tuning procedure helps ensure the stability of the test case even if a CIG is disconnected from the grid.

Communication Enabled—Fast Acting Imbalance Reserve (CE-FAIR)

This letter presents a new frequency control strategy that takes advantage of communications and fast responding resources such as photovoltaic generation, energy storage, wind generation, and demand response, termed collectively as converter interfaced generators (CIGs). The proposed approach uses an active monitoring of power imbalances to rapidly redispatch CIGs. This approach differs from previously proposed frequency control schemes in that it employs feed-forward control based on a measured power imbalance rather than relying on a frequency measurement. Time-domain simulations of the full Western Electricity Coordinating Council system are conducted to demonstrate the effectiveness of the proposed method, showing improved performance.