Low Inertia Power Systems Research Articles

Impact of wind capacity share, allocation of inertia and grid configuration on transient RoCoF: The case of the Croatian power system

Increased wind energy penetration influences the power system dynamic response to transient disturbances. Replacement of conventional production units with converter-connected wind turbines reduces natural power system inertia contained in rotational masses of synchronously connected turbine-generator units, therefore creating low-inertia power systems. Such a transition has an adverse effect on system resilience to disturbances and on the capability to maintain stable operation. This research examines the impact of high regional wind power production on system transient stability in the case of island operation of the Croatian power system. The system is divided into four geographical areas modeled as four centers of inertia with aggregated parameters. The study investigated initial transient RoCoF values in different areas for current and future wind capacity share scenarios, loading data, and primary frequency regulation settings. The modeling and scenario analysis have been performed on a detailed phasor power system model in the MATLAB/Simulink environment.

Power System Resiliency Enhancement with Ternary Pumped – Storage Hydropower

This paper investigates, how Ternary Pumped Storage Hydropower (T-PSH) can help enhance power system resiliency by contributing primary frequency regulation in both pumping and generating modes. As renewable penetration increases, power system inertia decreases. Simultaneously, the frequency of storms and earthquakes have increased. As such power system resiliency is a key issue in low inertia power systems. To cater to this issue, the authors investigate the ability of T-PSH to provide primary frequency support in pumping and generating mode. The governor dynamics of the IEEE 9-bus system and T-PSH have been modeled and integrated. When the system is subjected to a step increase or decrease in load, results display that not only can the T-PSH provide pump mode regulation using the hydraulic short-circuit, but it can also transit smoothly between pumping and generating mode within a few seconds using the clutch. By changing its mode, the T-PSH unit can provide a regulation capability equal to twice that of the unit rating.

FAPI Controller for Frequency Support in Low-Inertia Power Systems

This paper presents different forms of Fast Active Power Injection (FAPI) control schemes for the analysis and development of different mitigation measures to address the frequency stability problem due to the growth of the penetration level of the Power Electronic Interfaced Generation (PEIG) in sustainable interconnected energy systems. Among the studied FAPI control schemes, two different approaches in the form of a derivative-based control and a virtual synchronous power (VSP) based control for wind turbine applications are also proposed. All schemes are attached to the PEIG represented by a generic model of wind turbines type 4. The derivative-based FAPI control is applied as an extension of the droop based control scheme, which is dependent on the measurement of the network frequency. By contrast, the proposed VSP-based FAPI is fed by the measurement of the active power deviation. Additionally, unlike existing approaches for virtual synchronous machines, which are characterized by high-order transfer functions, the proposed VSP-based FAPI is defined by a second-order transfer function, which can contribute to fast mitigation of the system primary frequency deviations during containment period. The Great Britain (GB) test system, for the Gone-Green planning scenario for the year 2030 (GG2030), is used to evaluate the effects of the proposed FAPI controllers on the dynamics of the system frequency within the frequency containment period. Thanks to proposed FAPI controllers, it is possible to reach up to 70% for the share of wind power generation without violating the threshold limits for frequency stability. For verification purposes, a full-scale wind turbine facilitated with each FAPI controller is tested in EMT real-time simulation environment.

A Performance Comparison of PLL Implementations in Low-Inertia Power Systems Using an Observer-Based Framework

Phase-locked loop (PLL) implementations are critical components for the control and operation of grid-connected converters. Hence, they have to exhibit a highly reliable behavior under a wide range of operating conditions. Available implementations and performance analyses mainly focus on the impact of unbalances and harmonics. However, in converter-dominated low-inertia power systems an additional important type of perturbation will arise from fast variations in the grid frequency. Motivated by this, we show that the structure of several popular PLL implementations is closely related to that of high-gain observers and,byusing this framework, provide a tuning criterion for the PLL gains that mitigates the impact of the rate of change of the frequency (RoCoF) on the estimation performance. This criterion is then used to conduct a numerical comparison of four popular PLL implementations under three distorted conditions: unbalances, harmonics and frequency variations.

Active Damping of Power Oscillations Following Frequency Changes in Low Inertia Power Systems

The absolute requirement to increase the amount of energy generation from renewable sources e.g. predominantly asynchronously connected wind turbines and photovoltaic installations, may in practice during transient events (where frequency changes are examined) excite oscillatory response of the power output of large grid connected synchronous-generators. The response of such generators must be controlled either by varying the applied torque of a turbine or by altering the electromagnetic torque in the airgap. Choosing the latter, the adequacy of a voltage regulator, particularly that of the embedded Power System Stabilizer (PSS) circuit, is investigated using the IEEE PSS1A model for the automatic voltage regulator of a synchronous generator driven by a gas turbine. The response is obtained via closed form analytic solutions for both small (linear) and large (nonlinear) scale transient events in the energy grid system. In tandem with the analytical study, the behavior simulated with a computer model from MatLab-SimPowerSystems is reviewed.

A Comparison of the Dynamic Performance of Conventional and Ternary Pumped Storage Hydro

With decreasing costs of renewable energy harvesting devices, penetration of solar panels and wind turbines have increased manifold. Under such high levels of penetration, coping with increased intermittency and unpredictability and maintaining power system resiliency under reduced inertia conditions has become a critical issue. Pumped storage hydro (PSH) is the most matured and economic form of storage that can serve the purpose of capacity for over 4 to 8 h. However, to increase network inertia and add required flexibility to low inertia power systems, significant paradigm shifting modifications have been engineered to result in the development of Ternary PSH (TPSH). In this paper a test system to consider governor interaction is constructed. The dynamic models of conventional PSH (CPSH) and TPSH are constructed and integrated to the test system to examine the effect of CPSH and TPSH in the hydraulic short circuit (TPSH-HSC). The ability and the effect of mode change (from pump to turbine) without the loss synchronism of TPSH has also been examined. Results display the superior capability and effect of TPSH with its HSC capability to contribute to frequency regulation during pumping mode and the effect of rapid mode change, as compared to its primitive alternative, CPSH.

RoCoF-Constrained Scheduling Incorporating Non-Synchronous Residential Demand Response

Demand response has the potential to provide a multitude of operating reserve categories. However, if integrated at scale, the non-synchronous nature of most demand response sources must be recognized within the operational rules of low inertia power systems. Here, a candidate non-synchronous reserve resource is considered within the scheduling and frequency stability time frames. It is shown that a detrimental impact on the post-contingency rate of change of frequency (RoCoF) can arise-potentially requiring the development of system operator policies to ensure that security standards are not compromised-as the volume of reserve available from non-synchronous resources increases. A commitment-and-dispatch RoCoF constraint within scheduling procedures is proposed, which is independently verified using time domain simulations.

Mitigation of frequency stability issues in low inertia power systems using synchronous compensators and battery energy storage systems

The inertia of power systems is a key aspect of frequency dynamics and stability. The increasing penetration of non-synchronous generation reduces the available inertia and makes it fluctuating during the day depending on the online units. This causes problems for grid operators, particularly in relatively small power systems. The present work examines the impact of decreasing inertia using an aggregate model based on the swing equation, considering future lower inertia scenarios and the implementation of the current protection schemes. The frequency stability of the system is assessed by considering the reference incident, i.e. the loss of the largest operating unit in under- and over-frequency cases. New solutions to balance the grid are addressed and compared, considering the technical impacts of synchronous compensators and battery energy storage systems (BESSs) operated with a new equivalent saturation logic. The model is tested and validated using the real data of a small insular power system (Sardinia Island, Italy), outlining the importance of the high-voltage direct current and of the BESS control strategies to guarantee the frequency stability of the power system.

Effect of inertia heterogeneity on frequency dynamics of low‐inertia power systems

Based on fundamental dynamic simulation principles and illustrative case studies, this study aims to identify key changes in the dynamic behaviour of the system due to the high penetration of renewable energy sources (RES) affecting frequency dynamics and provide guidelines for performing frequency stability analysis in low-inertia power systems with a large penetration of RES. First, a critical comparison of the frequency responses obtained using a system equivalent model and a detailed dynamic model following an active power disturbance are presented. It is then demonstrated, using detailed dynamic models of two multi-area systems, such that when the proportion of RES prevails over the share of synchronous generator (SG), the dynamic coupling among areas (even separated by short-medium length lines) reduces and the inertia of the system should be considered as heterogeneous instead of a global parameter. Following this, the study discusses how frequency dynamics may be strongly and complexly affected by the physical characteristics of converter-connected generation leading to increased frequency and voltage interactions when the proportion of converter connected RES becomes greater than SG. The resulting changes in the system's dynamic behaviour due to frequency and voltage interactions can increase the actual active power imbalance size following a credible contingency so that careful considerations to protect the system against an excessively high rate of change of frequency should be made. In order to evaluate the extent of this change in system dynamic behaviour, an appropriate dynamic model of the network should be used with adequate multi-area representation of RES and loads.

Measuring effective area inertia to determine fast-acting frequency response requirements

Renewable generation and power electronic convertors increasingly displace the inertia conventionally provided by synchronous generation in the transition to a low carbon electricity network. Changes in generation profiles influence regions of the grid disproportionately, and areas of low inertia are susceptible to frequency diverging from neighboring areas, creating new risks of Rate Of Change Of Frequency (ROCOF)-based generation trips, underfrequency load shedding and islanding. A novel method of measuring area inertia is presented, with results of simulation and trials on the SP Energy Networks (Scottishpower) transmission system in south Scotland. The fundamentally new understanding of frequency and inertial effects is essential for designing mitigation measures for frequency and network stability, and enables new, flexible services for the transition to low inertia power systems.

Grid-forming control for power converters based on matching of synchronous machines

We consider the problem of grid-forming control of power converters in low-inertia power systems. Starting from an average-switch three-phase power converter model, we draw parallels to a synchronous machine (SM) model and propose a novel converter control strategy which dwells upon the main characteristic of a SM: the presence of an internal rotating magnetic field. In particular, we augment the converter system with a virtual oscillator whose frequency is driven by the DC-side voltage measurement and which sets the converter pulse-width-modulation signal, thereby achieving exact matching between the converter in closed-loop and the SM dynamics. We then provide a sufficient condition asserting existence, uniqueness, and global asymptotic stability of a shifted equilibrium, all in a rotating coordinate frame attached to the virtual oscillator angle. By actuating the DC-side input of the converter we are able to enforce this condition and provide additional inertia and damping. In this framework, we illustrate strict incremental passivity, droop, and power-sharing properties which are compatible with conventional power system operation requirements. We subsequently adopt disturbance-decoupling and droop techniques to design additional control loops that regulate the DC-side voltage, as well as AC-side frequency and amplitude, while in the end evaluating them with numerical experiments.

Modeling, Analysis, and Experimental Validation of Clock Drift Effects in Low-Inertia Power Systems

Clock drift in digital controllers is of great relevance in many applications. Since almost all real clocks exhibit drifts, this applies in particular to networks composed of several individual units, each of which being operated with its individual clock. In the present paper, we demonstrate via extensive experiments on a microgrid in the megawatt range that clock drifts may impair frequency synchronization in low-inertia power systems. The experiments also show that—in the absence of a common clock—the standard model of an inverter as an ideal voltage source does not capture this phenomenon. As a consequence, we derive a suitably modified model of an inverter-interfaced unit that incorporates the phenomenon of clock drifts. By using the derived model, we investigate the effects of clock drifts on the performance of droop-controlled grid-forming inverters with regard to frequency synchronization and active power sharing. The modeling and analysis is validated via extensive experiments on a microgrid in the megawatt range.

On the steady-state behavior of low-inertia power systems

Whereas conventional power systems heavily rely on bulk generation by synchronous machines, future power systems will be comprised of distributed generation based on renewable sources interfaced by power electronics. A direct consequence of retiring synchronous generators is the loss of rotational inertia, which thus far was the dominant time constant in a power system, as well as the loss of the generator controls, which are the main source of actuation of the power grid. Prompted by these paradigm shifts, we study the dynamic behavior of a nonlinear and first-principle low-inertia power system model including detailed power converter models and their interactions with the power grid. In this paper, we focus particularly on the admissible steady-state behavior of such a low-inertia power grid and derive necessary and sufficient control specifications for power converters.

Graph Spectra Based Controlled Islanding for Low Inertia Power Systems

The inertia of modern power systems is decreasing and becoming more variable as more inverter-connected renewable energy sources and loads are integrated. This leads to a low and time-varied inertia power system that is more sensitive to disturbances and may not be robust enough to survive large disturbances. How to protect such a low and time-varied inertia system from blackouts is in question. This paper tries to answer this question using a graph spectra-based controlled islanding method. Eigenvector sensitivity, with respect to inertia, is used to identify the impact of reduced inertia on the spectral properties of power system graphs and, thus, coherent generator grouping. Constrained spectral clustering is then used to find the islanding boundary with minimal power-flow disruption to island low inertia systems. Simulation results, obtained using the IEEE 9-bus and 118-bus test systems, validate the effectiveness of the proposed algorithm in the case of low inertia systems.

Analytical Concepts for Reactive Power Based Primary Frequency Control in Power Systems

This paper introduces a novel idea for adding virtual inertia in power systems by controlling the frequency swing dynamics through voltage channel. The purpose is to develop a frequency-based supplementary VAR modulation to assist governor action during power imbalance events. In this aim, we propose a two-band power system stabilizer, tuned for very low-frequency common swing mode, to adaptively adjust reference voltage of a Synchronous Condenser (SC) and modulate its reactive power on a second-by-second basis. The feasibility of this idea is supported by theoretical and simulation evidences. From theoretical side, we formulate a computational procedure to measure the degree of control impact, accompanied by sensitivity analyses around varying operating points. On the simulation side, we perform extensive studies on four well-known IEEE multi-machine test systems. The results show that VAR modulation by SC has a considerable impact on the minimum post-contingency frequency (frequency nadir), even more than so-called load modulation methods in some cases, which in fact has not been given enough attention in the past. In addition, we show that the proposed method can aid governors to improve primary frequency response particularly in low inertia power systems by reducing post-event frequency settling time and bias.

Fuzzy-Logic Based Frequency Controller for Wind Farms Augmented With Energy Storage Systems

Displacement of conventional synchronous generators by non-inertial units such as wind or solar generators will result in reduced-system inertia affecting under-frequency response. Frequency control is important to avoid equipment damage, load shedding, and possible blackouts. Wind generators along with energy storage systems can be used to improve the frequency response of low-inertia power system. This paper proposes a fuzzy-logic based frequency controller (FFC) for wind farms augmented with energy storage systems (wind-storage system) to improve the primary frequency response in future low-inertia hybrid power system. The proposed controller provides bidirectional real power injection using system frequency deviations and rate of change of frequency (RoCoF). Moreover, FFC ensures optimal use of energy from wind farms and storage units by eliminating the inflexible de-loading of wind energy and minimizing the required storage capacity. The efficacy of the proposed FFC is verified on the low-inertia hybrid power system.