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We developed a new strength measure, the hybrid multi-infeed effective short-circuit ratio (HMESCR), for evaluating hybrid multi-infeed high-voltage direct current (MIDC) systems. The HMESCR defines the strength of hybrid MIDC systems with line-commutated converter (LCC) high-voltage direct current (HVDC) and voltage-source converter (VSC) HVDC systems. Unlike previous measures, the HMESCR considers the capacity of VSC HVDC systems to provide reactive power and thus, enhance the strength of AC systems connected at different buses with LCC HVDC systems. To demonstrate how the HMESCR reflects power stability (PS) and temporary overvoltage (TOV), we used MATLAB/Simulink software to perform quasi-steady-state analyses and dynamic simulations. The HMESCR can be used to determine the strength of hybrid MIDC systems. [ABSTRACT FROM AUTHOR]

In this paper, a novel distributed active power control method for interlinking converters (ICs) in an islanded hybrid ac/dc microgrid (IHMG) is proposed. The goal is to implement power sharing among all distributed generators (DGs) in an IHMG and among the ICs in a distributed manner, without the need for additional proportional-integral controllers. A mathematical formula for the active power references of the ICs is derived to achieve these objectives. Then, an estimator providing the data required for the reference calculation is proposed based on the modified dynamic consensus algorithm. In addition, a unified state-space model is derived for the entire IHMG, including the cyber-physical interactions, and a design method is presented for the parameters of the estimator. Finally, the effectiveness of the proposed method is verified by performing small-signal analysis and controller-hardware-in-the-loop verification. Compared to a previous distributed control method, the proposed method significantly improves the robustness in terms of communication delay and variation in the status of DGs while achieving the objectives. [ABSTRACT FROM AUTHOR]

A strategy was developed for reactive power control of the line-commutated converters (LCCs) of hybrid multi-terminal high-voltage direct current (MT-HVDC) systems. In the conventional method, the reactive power outputs of hybrid MT-HVDC systems are regulated discontinuously by only using switched shunt devices (SSDs). If continuous reactive power compensation is required, additional devices, including a static compensator (STATCOM), are essential, but these are costly solution. However, in the proposed method, the reactive power outputs of hybrid MT-HVDC systems can be continuously regulated without any additional compensating device by adjusting DC voltage and using SSDs. Therefore, with the proposed method, an HVDC operator can control reactive power outputs and reduce costs, simultaneously. To achieve our objectives, we first describe how to determine the DC voltage set points that regulate the reactive power outputs of multiple grid-connected LCCs, and then realize coordinated control using SSDs. The utility of the method is demonstrated by case studies simulated using PSCAD and MATLAB. [ABSTRACT FROM AUTHOR]

This article presents a current injection power flow analysis and optimal generation dispatch method for bipolar DC microgrids. In bipolar DC systems, despite a certain local voltage being balanced by the converter control, voltage unbalance can occur at buses that are electrically far from the balanced voltage. In addition, from the aspect of system operation, the optimization of operation cost and voltage unbalance has not been fully investigated yet. Herein, for steady-state analysis, the current injection Newton-Raphson power flow for a bipolar DC microgrid is developed using static models of the network, voltage balancer, sources, and loads. Afterwards, the optimization problem is formulated based on voltage sensitivity, which can be obtained from the Jacobian matrix, to reduce the operation cost while resolving the voltage unbalance. The problem of quadratic programming can be solved sequentially to find the final optimal solution. The case study validates that the power flow method is sufficiently accurate compared to the simulation in PSCAD/EMTDC and that optimal generation can be achieved while reducing the operation cost and mitigating voltage unbalance. [ABSTRACT FROM AUTHOR]

A new DC current-voltage droop control method for a hybrid high-voltage direct-current (HVDC) system featuring a voltage-sourced converter (VSC) rectifier and a line-commutated converter (LCC) inverter is presented. The proposed method utilizes DC voltage droop control to reduce the AC voltage fluctuation at the inverter side, to exploit the hybrid HVDC system for connecting offshore wind farms to weak grids without necessitating continuous reactive power compensation facilities. Additionally, an optimal droop coefficient calculation method is proposed to keep the AC voltage constant for wind power fluctuations. To evaluate the stability of the proposed method, a small-signal state-space (SS) model of the target system is derived and the root locus analysis is given. Simulation case studies are performed using an electromagnetic transient model in PSCAD and an SS model in MATLAB. The results demonstrate that a hybrid HVDC system with the proposed method successfully integrates offshore wind energy and weak grids without additional facilities because the AC voltage can be maintained at a near constant during wind fluctuations. [ABSTRACT FROM AUTHOR]

Different from voltage-source converter (VSC), reactive power absorption of line-commutated converters (LCCs) changes depending on DC voltage level. Therefore, the amount of AC voltage fluctuations at inverter side are different according to I – V droop constants in hybrid multi-terminal high-voltage direct current (MT-HVDC) system. This paper proposes a method for calculating the I – V droop constants in a hybrid MT-HVDC system to suppress these AC voltage fluctuations, based on the sensitivity of AC and DC networks. Simulations performed using PSCAD demonstrate the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

In this paper, a new control method for a line-commutated converter-based (LCC) high-voltage direct current (HVDC) system to support the frequency and AC voltage was proposed. For coordinated control of the inverter and rectifier, droop-based controllers are adopted for the outer loop of conventional controllers, and DC voltage is used to transfer the frequency information so that communication is not required. A method to determine the droop constants of the proposed controller is presented to emulate the conventional $f$ / $P$ droop characteristic and suppress the AC voltage fluctuations. Furthermore, a small-signal state-space model of the LCC HVDC system with the proposed scheme is derived, and root locus analysis is carried out to evaluate its stability. The effects of the proposed method are verified based on case studies, using the well-known CIGRE BENCHMARK model. The advantages of the proposed method can be summarized as follows: 1) system reliability is enhanced because there is no risk of communication failure during frequency information transfer and 2) voltage stability at the inverter side is enhanced as AC voltage fluctuation during frequency support is minimized. [ABSTRACT FROM AUTHOR]

We propose a new local controller to determine the active power reference for an interlinking converter (IC) of an islanded hybrid ac/dc microgrid (IHMG). The main objective of the proposed method is to improve the accuracy of global power sharing (GPS) between distributed generators throughout an IHMG to enhance the reliability and stability. Based on our analysis of the required active power of ICs for GPS, a local IC controller is proposed to enable parallel IC operation and improve GPS. The effects of the proposed controller on the system stability and GPS accuracy are also analyzed using a small-signal model. Furthermore, we provide guidelines on how to determine an appropriate control parameter for the proposed controller based on the desired GPS accuracy and the stability margin. Finally, the proposed method is verified by Simulink/MATLAB and hardware-in-the-loop experimental platform. [ABSTRACT FROM AUTHOR]

We present a new reactive power control method for hybrid high-voltage direct-current (HVDC) systems that features a line-commutated converter-based (LCC) rectifier and a voltage-sourced converter-based (VSC) inverter. Our principal objective is to regulate the reactive power of the LCC rectifier side via co-operative control of the DC voltage and switched shunt devices (SSDs) such as capacitors and reactors. To achieve these objectives, we used three operative modes: reactive power, voltage, and power factor control. For each mode, we show how to determine the DC reference voltage. Also, to maintain the voltage within operational bounds and enhance the range over which reactive power is controllable, our method features the cooperation of SSDs. Furthermore, we show how to determine the required SSD capacities. MATLAB and PSCAD simulations confirmed that our method improves reactive power controllability at the LCC rectifier side of a hybrid HVDC system. [ABSTRACT FROM AUTHOR]

This paper presents a novel supervisory control method for an islanded hybrid ac/dc microgrid (HMG) with a high renewable energy source (RES) penetration. Droop control can respond to inevitable uncertainties in the load and RES. However, this leads to frequency and voltage deviations, and the generator output power may deviate from the optimal point. To restore the frequency and voltage of both ac and dc grids while reducing generation cost at short time intervals, a model predictive control based optimization problem is formulated to update the set-points of the available resources including the interlinking converter and distributed generators. With the sensitivities, which help to linearly express the frequency and voltage for resource output power, the problem can be approximated to a quadratic programming model. In addition, to consider the effect of the reactive power on the frequency and improve the accuracy, the second-order frequency sensitivity is calculated and applied selectively. The proposed method was tested on a modified IEEE 33-bus system connected to a sample dc grid. The simulation results validate its performance for reliable and economic operation in an islanded HMG. [ABSTRACT FROM AUTHOR]

This paper presents a new generation adjustment method for achieving economical operation of power systems. Uncertainty in the loads and renewable energy sources is inevitable in power systems. The participation factor, which can be calculated using the incremental cost, is generally used to manage such unexpected variations. The incremental cost considers only the generators; however, owing to line loss, the location of the variation may also affect the operation performance. Herein, we introduce a bus-dependent participation factor (BDPF) to precisely reflect the line loss. The BDPF is calculated based on the generation-load incremental cost (GLIC), which is the cost variation for a specific generator and load. The GLIC can fairly reflect line loss changes; in addition, the second-order loss sensitivity is used to improve the accuracy of the BDPF. Using the BDPF, power generation can be instantly adjusted under unexpected variations, after which re-adjustment can be performed, if operation constraint violations exist. The proposed method is tested on the IEEE 14-bus and 118-bus systems, and the simulation results validate its effectiveness. [ABSTRACT FROM AUTHOR]

This paper describes a control method for distributed generation (DG) units to implement active power sharing and frequency recovery simultaneously in an islanded microgrid. Conventional active power–frequency (P –f) droop control is used for the DG controller, and the frequency deviation is recovered by the DG itself via self-frequency recovery control, without requiring secondary frequency control. Because the electrical distance (impedance) from each DG unit to a point where the load demand changes differs among DG units, the instantaneous frequency deviations may differ between DG units. These differences are fed into the integrators of the self-frequency recovery control and may result in errors in active power sharing. To solve this problem and share active power more accurately, a compensation control method is developed for active power sharing, which considers the droop coefficients of each of the DG units. Simulation results show that the proposed control method is effective. [ABSTRACT FROM AUTHOR]

This paper proposes three-phase steady-state models for a distributed generator (DG) interfaced to a main system via a three-wire current-controlled voltage-source converter. In order to represent the DG in a realistic manner, the three major factors that determine the steady-state phase outputs under unbalanced operating conditions are considered: 1) the power control strategy; 2) output filter; and 3) voltage and current sensor positions. Based on these factors, the DGs are classified into various types. According to the position of the voltage sensor, two equivalent circuit models including an equivalent three-phase current source (ETCS) are proposed. For each type of DG, the output current of the ETCS is formulated as a function of the voltage of the ETCS-connected node, the filter impedances, and the active and reactive power references. To verify the accuracy of the proposed models, the results of the power flow incorporating them are compared with those obtained from the PSCAD simulation using detailed dynamic models of the DG. [ABSTRACT FROM PUBLISHER]

In this paper, we propose a frequency and voltage control strategy for a standalone microgrid with high penetration of intermittent renewable generation systems, which might cause large frequency and voltage deviation in the system due to unpredictable output power fluctuations. To this end, a battery energy storage system (BESS) is suggested for generating the nominal system frequency instead of a synchronous generator, from a frequency control perspective. This makes the system frequency independent of the mechanical inertia of the synchronous generator. However, a BESS has a capacity limitation; a synchronous generator is used to maintain the state of charge (SOC) of the BESS at a certain value. For voltage control, we proposed that a reactive power/active power (Q/P) droop control be added to the conventional reactive power controller. By adding a Q/P droop control, renewable generation acquires a voltage-damping effect, which dramatically alleviates the voltage fluctuation induced by its own output power fluctuation. Simulation results prove the effectiveness of the proposed control strategy from both frequency and voltage control perspectives. [ABSTRACT FROM AUTHOR]

This paper is concerned with the power dispatch problem of distributed generators (DGs) for optimal operation of a microgrid. The objective is to minimize the fuel cost during the grid-connected operation, while ensuring stable operation after islanding. To achieve this goal, the economic dispatch (ED) problem and related constraints are formulated. The constraints considered in this study are: i) reserve for variation in load demand, ii) reserve for variation in the power outputs of non-dispatchable DGs, iii) flow limits between two adjacent areas, and iv) reserve for the stable islanded operation. The first three constraints, which have been employed in ED problem for conventional power systems, are modified to apply to Microgrids. We also provide a detailed formulation of the constraint for stable islanded operation in accordance with two power-sharing principles: i) fixed droop and ii) adjustable droop. The problem is solved using a modified direct search method, and the effect of the constraints on the operational cost is investigated via numerical simulations. [ABSTRACT FROM PUBLISHER]

This paper presents a new control method, in which a distributed generator (DG) actively participates in steady-state voltage control, together with an under-load tap changer (ULTC) and shunt capacitors (Sh.Cs). In the conventional DG control method, the integration of DGs into a distribution power system increases the number of switching operations of the ULTC and the Sh.Cs. To solve this problem, this paper proposes that the DG output voltage be dispatched cooperatively with the operation of the ULTC and the Sh.Cs, based on load forecasts for one day in advance. The objective of the proposed method is to decrease the number of switching device operations, as well as to reduce the power loss in the distribution lines, while maintaining the grid voltage within the allowed range. The proposed method is designed and implemented with Matlab, using two different dynamic programming algorithms for a dispatchable and a nondispatchable DG, respectively. Simulation studies demonstrate that the objective can be achieved under various grid conditions, determined by factors such as the DG output power characteristics, the location of the DG-connected bus on the feeder, and the load profile of the feeder containing the DG. [ABSTRACT FROM PUBLISHER]