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The dynamic characteristics of converter-dominat‐d systems are governed by the controls of power converters and the interactions among the converter systems and conventional alternators. Frequency oscillations can appear during dynamic operating conditions caused by the phase-locked loop dynamics and the interactions among the converter control systems. The oscillations may be poorly damped that can result in reduced power generation, longer settling time or disconnections of sensitive components. It is foreseeable that damping services will be critical for grid stabilization in the future with high penetration of renewable generation. In this work, synchronous condensers are evaluated and applied for providing damping services to the grid in post-event conditions. An innovative supplementary controller for the automatic voltage regulator of synchronous condensers is proposed to improve frequency stabilization in a converter dominated system after disturbances. Utilizing local and remote measurements, synchronous condensers are able to modulate the reactive power output hence the terminal bus voltage, which further impacts on the power flow in the system, therefore, it provides damping to the frequency oscillations. The control is implemented on an industrial level hardware platform and the performance is verified by hardware-inthe-loop simulation.

Modern power systems are undergoing the transition from synchronous-generator-based ones towards converter-based ones. This transition tends to reduce the system strength and impair the system voltage and frequency stability. Therefore, synchronous condensers (SCs) are drawing an increasing attention world-widely in recent years, as SCs are able to enhance the system strength, provide dynamic voltage regulation, and contribute inertia to the system. However, less attention has been given to the protection systems for SCs in the literature, especially when a synchronous condenser is equipped at the point of common coupling (PCC) of a HVDC station. Firstly, an SC needs to be protected against over-speed in the rotor under the island scenario of the HVDC station and the SC. Secondly, the protection should not trip the SC undesirably when the SC is supposed to remain connected (within its physical limit) and support the gird operation subject to grid-side disturbances. This paper investigates the possible undesired trip of an SC by reverser power and over-frequency protection under a certain grid configuration through hardware-in-the-loop (HiL) tests. The tests have identified that, when there are two transmission lines emanating from the substation and the VSCHVDC station is operating under inverter mode, the trip of the line that exports active power from the substation may lead to an undesired trip of the SC due to the power swing. Losing the support from the SC during such power swing conditions can result in the instability of the grid. Therefore, it is of great importance to have suitable settings for the reverse power and over-frequency protection for the SC. Besides the physical limits of the SC, the following conditions should also be considered for the case study: (1) How much active power the HVDC is importing prior to the line trip; (2) How much active power the other transmission line is importing to the substation prior to the line trip; (3) How fast the H

Nowadays, installed capacity of renewable energy sources is increasing at a high rate and even higher increase is expected in the future. In order to accommodate renewable energy sources, integration of gas, electricity and district heating network is a promising solution. This paper provides a coordinated operation and analysis of electricity, district heating and natural gas system with integrated wind farm. The interactions among different energy sectors will provide more flexibility required by the future renewable energy system. A nonlinear optimization problem is presented with focus on decreasing the operational cost and improving efficiency of integrated system, as well as meeting the demands. Optimal operation is performed for a test case system including constraints of individual systems and linkages between each of the systems. The test system includes electrical, district heating and natural gas subsystem with thermal and gas storages, combined heat and power and power to gas units and wind turbine. Simulation results show that integration of electricity, heating and natural gas system decreases the operational cost and provides higher flexibility to the system. Moreover, wind curtailment is reduced with integration of P2G.

Automatic voltage regulator (AVR) plays an important role in volt/var control of synchronous condenser (SC) in power systems. Test AVR performance in steady-state and dynamic conditions in real grid is expensive, low efficiency, and hard to achieve. To address this issue, we implement hardware-in-the-loop (HiL) test for the AVR of SC to test the steady-state and dynamic performances of AVR in different operating conditions. Startup procedure of the system and voltage set point changes are studied to evaluate the AVR hardware response. Overexcitation, underexcitation, and AVR set point loss are tested to compare the performance of SC with the AVR hardware and that of simulation. The comparative results demonstrate how AVR will work in a real system. The results show HiL test is an effective approach for testing devices before deployment and is able to parameterize the controller with lower cost, higher efficiency, and more flexibility

The rapid increase of converter-based generation in the grid causes significant changes in the system dynamic characteristics. These changes require verification and testing of the reliability, optimization, and functionality of power system elements and controllers, which become more important for guaranteeing a secure operation. This paper first introduces a hardware-in-the-loop (HiL) setup of an automatic voltage regulator (AVR) control system and then proposes a softwarein-the-loop (SiL) setup for the parameterization of an IEEE standard AVR/excitation system model and a power oscillation damping (POD) controller of synchronous condenser. The AVR hardware is interfaced with a simulated grid in real-time digital simulator (RTDS) using EMT simulation to evaluate its control functions. Parameter optimization for the POD and the AVR simulation model by SiL simulation is implemented in a closed loop between RTDS and Matlab/Simulink interfaced through OPC to satisfy the optimization objectives. By tests executed on the future Western Danish power system, parameter optimization of the POD and IEEE standard AVR model, and the function of AVR hardware are successfully implemented and verified.

The short circuit response of voltage source converters (VSCs) could be significantly different from that of synchronous condensers (SCs) under unbalanced faults. This paper investigates the combined effect of VSC-based sources and synchronous condensers under grid unbalanced faults considering two groups of VSC fault-ride-through (FRT) control strategies. It aims to provide an evaluation of the different control strategies and to explore the impact of equipping a synchronous condenser at the point of common coupling (PCC). By examining the combined short circuit currents, PCC voltages, DC-side voltages and system frequency responses, this study points out the merits and demerits of the examined control strategies; illustrates the benefits of synchronous condensers on mitigating the disadvantages of certain control strategies; and provides suggestions on selecting the control strategies for the VSC when incorporating a synchronous condenser at the PCC.

The parameter optimisation of designed controllers for power systems is always a big concern and needs a lot of effort of researchers especially when the electricity grid becomes larger and more complex. This study proposes a control parameterisation using genetic algorithms (GAs) for power oscillation damping (POD) incorporating synchronous condensers (SCs) via software-in-the-loop simulation to enhance the damping and frequency stability for low-inertia systems. A closed-loop interfaced setup among real-time digital simulator, MATLAB, and OLE for process communication running in real time is analysed and implemented to optimise the POD parameters of a SC. Furthermore, a Prony technique based on the system measurement is applied to find out the frequency and damping ratio of the dominant oscillation mode. The POD optimal parameters are determined by the GA objective function that maximises the damping ratio of the dominant oscillation mode. The effectiveness of the proposed method in damping power oscillations and frequency stability improvement is verified through simulation results of the future western Danish power system. Simulation results demonstrate that the proposed approach offers good performance for parameter optimisation of the POD.

The parameter optimization of designed controllers for power systems is always a big concern and needs a lot of effort of researchers especially when the electricity grid becomes larger and more complex. The paper proposes a control parameterization using genetic algorithms (GA) for power oscillation damping (POD) incorporating in synchronous condensers via softwarein-the-loop simulation to enhance the damping and frequency stability for low inertia systems. A closed-loop interfaced setup among real-time digital simulator (RTDS), Matlab, and OLE for Process Communication (OPC) running in real time is analyzed and implemented to optimize the POD parameters of a synchronous condenser. Furthermore, a Prony technique based on the system measurement is applied to find out the frequency and damping ratio of the dominant oscillation mode. The POD optimal parameters are determined by the GA objective function that maximizes the damping ratio of the dominant oscillation mode. The effectiveness of the proposed method in damping power oscillations and frequency stability improvement is verified through simulation results of the future Western Danish power system. Simulation results demonstrate that the proposed approach offers a good performance for parameter optimization of the POD.

Decentralization of generation and increasing utilization of information communication systems bring challenges to present power system modeling approaches. This work applies functional modeling for monitoring and modeling of distributed energy resources, with wind turbine generator (WTG) application as a case study. First, we established a functional model of a generic WTG through the multilevel flow modeling approach. The model acts as basis of a state estimator (SE) for monitoring the WTG. Afterwards, the application of the SE is extended for wind power plant monitoring and control. The case study results show that the SE can efficiently limit the impact of information errors from different data integrity attacks during active power curtailment.

This paper identifies weakness of existing method for voltage stability assessment and proposes new approach for determining point of maximum deliverable power to a given load that accounts for the variations in the Thevenin voltage magnitude. The approach uses Thevenin equivalents seen from nodes of constant voltage magnitude and load nodes in order to determine a distance to instability. A simple five-bus system together with IEEE 14 bus system were used in order to perform dynamic simulation in PSS/E. The simulation data were used to create synthetic Phasor Measurement Unit (PMU) snapshots, which served as input to the proposed approach. The new approach is demonstrated on the two test systems, where improved accuracy in determining the point of maximum deliverable power is demonstrated. The results show that the point of maximum deliverable power to the load occurs well before the Thevenin impedance matching criteria.

With converter-based renewable energy sources increasingly integrated into power systems and conventional power plants gradually phased out, future power systems will experience reduced short circuit strength. The deployment of synchronous condensers can serve as a potential solution. This paper presents an optimal synchronous condenser allocation method for improving system short circuit ratio at converter point of common coupling using a modified short circuit analysis approach. The total cost of installing new synchronous condensers is minimized while the system short circuit ratios are maintained above a certain level. The presented method is implemented on the simplified western Danish power system for a future scenario.

Inertia reduction due to high-level penetration of converter interfaced components may result in frequency stability issues. The paper proposes and analyzes different strategies using synchronous condenser (SC), synthetic inertia (SI) of wind power plant, and their combination to enhance the frequency stability of low inertia systems under various scenarios and wind conditions. Furthermore, one of the SC models includes hardware of automatic voltage regulator (AVR) for better representation of the reality is implemented. The simplified Western Danish power system simulated in real time digital simulator (RTDS) is used as a test system of low inertia to demonstrate the effectiveness of the strategies. The comparative results show that the combination of SC with AVR hardware-in-the-loop test and SI offers a better improvement not only on frequency stability (rate of change of frequency and frequency deviation) but also on the system synchronism under various operating conditions.

Voltage source converters (VSCs) are able to provide both positive- and negative-sequence short circuit currents under unbalanced faults. Their short circuit responses can be significantly different from those of conventional synchronous generators. This paper developed a static fault analysis method by considering dual-sequence current control of VSCs under unbalanced faults where VSCs are treated as voltage-dependent current sources in both positive- and negative-sequence networks. Since the control strategy of VSCs varies, flexible parameters are included in the model to reflect their diverse short circuit behaviours. The proposed method is verified through a modified IEEE 9-bus system and a simplified western Danish power system with real time simulations. This analytical method can be used to help understand and evaluate the impact of dual-sequence current control of VSCs on future converter-dominated power systems.

The short circuit response of a voltage source converter (VSC) under grid unbalanced faults mainly depends on the design of its control system. Due to the limited semiconductor overload capability, the short circuit current contributed by a VSC should be restricted within the limit for each phase. This might bring up challenges to the protection system of a converter-dominated power system. This paper derives a generic converter peak current limitation method for three different VSC control strategies. The impact of the control strategies and the combined impact of a VSC with a synchronous condenser on distance protection are evaluated using a commercial relay through hardware-in-the-loop (HIL) tests. Based on the test results, we propose to avoid using constant reactive power control strategy. It poses an adverse impact on the reliability and speed of distance protection regardless of the presence of SC at the point of common coupling (PCC), while constant active power and balanced current control strategies favor the performances of distance protection.

This paper investigates the performance of Phasor Measurement Units (PMUs) under interference conditions which can appear in a power system and are not tested by the C37.118.1 standard. Three PMUs from different vendors configured for the M-class requirements were used to test three possible interference condition scenarios. In the first scenario, noise is added to the PMU input signal. The test runs a sweep of Signalto-Noise Ratios (SNR) and the accuracy versus the noise level is obtained. The second scenario injects multiple harmonics with the input to test the influence on accuracy. The last scenario focuses on instrument transformer saturation which leads to a modified waveform injected in the PMU. This test goes through different levels of Current Transformer (CT) saturation and analyzes the effect of saturation on the accuracy of PMUs. The test results show PMU measurements will be degraded when the input signal is distorted by high noise or a saturated current waveform, but is not particularly affected by multiple harmonics. This information can be used when selecting a PMU to ensure it will provide a reliable measurement for the intended use. It can also be used for developing more robust PMUs and applications resistant to degraded measurements.

As an increasing amount of converter-based generation on power electronics is connected to power systems, transmission system operators (TSOs) are revising the grid connection requirements to streamline the connectivity of the devices to maintain security of supply. Converter-based generation can behave significantly different from the traditional alternators under grid faults. In order to evaluate the potential impact of future converter-based power systems on protective relays, it is necessary to consider diverse current control strategies of voltage source converters (VSC) under unbalanced faults as the performance of converters primarily depends on their control objectives. In this paper, current control strategies of VSC under unbalanced faults for short circuit power provision are reviewed in two groups, namely power-characteristic-oriented and voltage-support-oriented control strategy respectively. As the fault current provided by converters should be restricted within secure operation limits considering semiconductor capabilities, converter current limit issue is also discussed.

The small-signal stability analysis of power system electromechanical oscillations is a well-established field in control and stability assessment of power systems. The impact of large wind farms on small-signal stability of power systems has been a topic of high interest in recent years. This study presents a phasor model of full scale converter wind turbines (WTs) implemented in MATLAB/SIMULINK for small-signal stability studies. The phasor method is typically used for dynamic studies of power systems consisting of large electric machines. It can also be applied to any linear system. This represents an advantage in small-signal stability studies, which are based on modal analysis of the linearised model and are usually complemented with dynamic simulations. The proposed model can represent a single WT or an aggregated wind power plant. The implemented model for small-signal stability analysis was tested in the Kundur's two area system. The results show that the proposed WT model is accurately linearised and its impact on power system oscillation is similar to that of previous research findings.

The transition towards a cyber-physical energy system (CPES) entails an increased dependency on valid data. Simultaneously, an increasing implementation of renewable generation leads to possible control actions at individual distributed energy resources (DERs). A state estimation covering the whole system, including individual DER, is time consuming and numerically challenging. This paper presents the approach and results of implementing a simplified state estimator onto an embedded system for improving DER monitoring. The implemented state estimator is based on numerically robust orthogonal factorization and used on a set of state equations of a generic wind turbine generator (WTG). The simplified state estimator is tested by simulating a generic WTG model and evaluated based on its execution time and estimation accuracy. Results show its fast execution time, its accuracy in handling normal measurement error and its ability to provide reliable data in the case of gross errors in the set of measurements.

Increasing penetration of renewable energy resources (RES) and electrification of services by implementing distributed energy resources (DER) has caused a paradigm shift in the operation of the power system. The controllability of the power system is predicted to be shifted from the generation side to the consumption side. This transition entails that the future power system evolves into a complex cyber-physical energy system (CPES) with strong interactions between the power, communication and neighboring energy systems. Current power system security assessment methods are based on centralized computation and N-1 contingencies, while these risks should still be considered in the future CPES, additional factors are affecting the system security. This paper serves as a review of the challenges entailed by transforming the power system into a CPES from a security assessment perspective. It gives an indication of theoretical solutions to CPES challenges and proposes a new framework for security assessment in CPES.

High-level integration of renewable energy sources in power system leads to the displacement of conventional generators and consequently challenges in power system frequency stability are introduced. To mitigate the negative impact of significant wind power penetration in the grid on the frequency stability, this paper proposes supplementary control methods to implement synthetic inertia for doubly-fed induction generator (DFIG) based wind energy system during frequency excursions. Different control strategies and activation schemes are analyzed and implemented on the Western Danish renewable-based system using-real time digital simulator (RTDS) to propose the best one for the synthetic inertia controller. From the comparative simulation results, it can be concluded that the method using a combination of both the frequency deviation and derivative as input signals, and the under-frequency trigger provides the best dynamic response in term of the system frequency stability improvement.