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• New MILP-based UC model for virtual power lines (VPL) comprising paired BESS units. • Operating policy of VPL introduced, in the context of storage as transmission only asset (SATOA). • Effective line loading levels in the range of 120–130 % attainable under normal operation, via VPL action. This paper presents a mixed-integer linear programming (MILP) unit commitment (UC) model for the envisaged virtual power line (VPL) concept, which relies on a coordinated power flow control between two battery energy storage stations (BESS) that are strategically positioned at the supply and demand side of critical transmission line corridors which could exhibit congestion in case of large scale deployment of renewable energy source (RES) stations. The proposed VPL model includes MILP constraints governing the charge/discharge mode of each BESS unit in relation to power flow constraints as well as the availability of RES power. The VPL functionality is further augmented by allocating fast-acting reserves that can be released upon detection of critical line outages, in order to provide virtual power flows through the paired BESS units. As a proof of concept, a 6-bus and the IEEE RTS-79 test system are tested under bulk RES generation. The MILP-UC model is built in GAMS software and detailed UC schedules are assessed in order to demonstrate the potential benefits that VPLs can offer as storage as a transmission-only assets (SATOAs). [ABSTRACT FROM AUTHOR]

The main scope of this paper is to propose a suitable frequency control scheme for high-voltage dc-links (HVDC) based on voltage source converters (VSCs), operating in island systems with on-site conventional generation and external ac interconnectors. The proposed droop-type and inertia emulator is built upon the power synchronization control (PSC) concept, where grid synchronization is achieved without the need of a dedicated synchronization unit. The dynamics of the proposed PSC-based scheme are assessed both in frequency and time domain, utilizing a realistic study-case system, which corresponds to the current planning for the interconnection of Crete to the Greek mainland system, involving ac and dc interconnectors operating in parallel with conventional local thermal units. To demonstrate the benefits offered by the proposed controller in the context of frequency response, a detailed average value model is developed for the VSC-HVDC link in MATLAB/Simulink, where severe contingencies are simulated, such as the sudden loss of the external ac interconnector or local generation, leading from mainland grid-connected to islanded operation. [ABSTRACT FROM PUBLISHER]

In this paper, the generation scheduling problem for autonomous island systems with significant penetration of non-dispatchable Renewable Energy Source (RES) generation is investigated, proposing a unit commitment-economic dispatch (UC-ED) model which is fully compatible with the provisions of the regulatory framework currently applicable to the Greek non-interconnected island (NII) systems. Two UC-ED model variants are presented and comparatively assessed, in order to illustrate the impact of various system parameters (technical characteristics of the thermal units, reserve requirements, etc.) on the expected operating profile of the NII system. A detailed UC-ED model is first proposed, which takes into account the reserve requirements at different time scales (primary, secondary and tertiary reserves) and respective capabilities of generating units. An alternative, simplified UC-ED model is also presented, formulated on the basis of aggregate spinning reserves, to cater for uncertainties in the actual reserve capabilities of existing thermal units. The developed UC-ED models are applied to an existing NII system to evaluate their performance and results obtained on an annual basis. [ABSTRACT FROM AUTHOR]

This paper presents an alternative control scheme for offshore wind turbines (WTs) connected to the mainland grid through high voltage dc (HVDC) links based on voltage source converters (VSCs). The proposed WT controller inherently provides rapid droop-type frequency response, without the need to implement an additional WT frequency controller. A coordinated control approach is proposed between the WT and HVDC controllers in order to achieve fault ride-through (FRT) and frequency response capabilities, relying solely on offshore frequency modulation. To demonstrate the FRT capability of the VSC-HVDC link, a detailed EMT-type model is first developed in Matlab/Simulink, where onshore grid faults are tested. Then, the offshore wind farm is connected to a study case system in order to assess the contribution of the VSC-HVDC link to frequency regulation, by exploiting the proposed WT controller along with the communication capability between the two HVDC VSCs. [ABSTRACT FROM PUBLISHER]

This paper presents a new multi-objective optimization technique for multi-terminal DC (MTDC) networks to enhance power dispatch control under stochastic operating conditions. The proposed method utilizes the DC voltage droop control concept to achieve distributed DC voltage control of the MTDC grid, while a centralized power dispatch controller monitors system operation and updates the voltage droop settings of the onshore converters in order for them to comply with scheduled power set-points dispatched by the transmission system operator, maintaining at the same time low system losses. The overall optimization problem is formulated and its effectiveness is assessed applying the Monte-Carlo simulation method. A five-terminal MTDC grid is selected as a study-case system, which corresponds to the planning for the future interconnection of Aegean Sea islands with significant wind potential to the mainland grid. [ABSTRACT FROM AUTHOR]

Photovoltaic (PV) stations are increasingly becoming subject to grid code requirements that include frequency response and active power control capability. The main goal of this paper is to propose a generic model for a two-stage grid-connected PV system with frequency response capability, suitable for power system studies. The proposed model includes a suitable control scheme, which provides both droop and inertial response, as well as the ability to operate at a scheduled active power reserve, enabling thus the provision of under-frequency response. A linearized small-signal model is developed to assess the stability of the proposed PV power control loop when the PV generator provides frequency response, whereas time-domain simulations are performed in order to quantify the benefits achieved by droop-type and inertia frequency controllers, including a discussion on the selection of their parameters. The analysis demonstrates the satisfactory performance of the proposed PV system model, which provides all functionality required by grid codes in the context of active power control. [ABSTRACT FROM AUTHOR]

Offshore wind farms connected to the mainland through High Voltage DC links based on Voltage Source Converters (VSC–HVDC) are subject to Grid Code (GC) requirements, such as frequency response, Fault Ride Through (FRT) capability, and active and reactive power control. Due to the presence of HVDC links, GC compatibility becomes a challenging task from a control and regulation perspective, since the offshore network AC voltage and frequency are decoupled from the onshore grid voltage and frequency and hence grid-side disturbances are not directly experienced by the Wind Turbines (WTs). To address this challenge of modulating the response of offshore WTs according to the onshore grid conditions, without resorting to dedicated communication schemes, which may introduce delays and reliability concerns, three alternative control strategies are presented and evaluated for the offshore HVDC VSC, which are based only on local measurements. Each strategy can cope with onshore frequency response, FRT and power control requirements at the same time. A coordinated control approach between the WT and HVDC converters is also proposed by integrating a voltage dependent current modulation strategy into the default WT current controllers. Time domain simulations are carried out in DIgSILENT PowerFactory in order to verify the effectiveness of the proposed control schemes. [ABSTRACT FROM AUTHOR]

Increasing penetration of Renewable Energy Sources (RES) to the power systems leads to the continuous evolution of grid interconnection requirements, which now include Low Voltage Ride Through (LVRT) practically for all types of generating units connected to the MV or HV networks, including photovoltaics (PV). It is essential that the Distributed Generation (DG) models used in power system studies properly represent the enhanced response characteristics of the stations. In this paper, a model is presented for a PV system with grid-code compatibility, which is suitable for power system dynamic and transient analysis studies. For this purpose, an overall control strategy is introduced to achieve LVRT capability, while elements essential for the correct representation of the dynamic performance of the system, such as the Phase Locked Loop (PLL) and the output LCL filters, are properly accounted for. Time domain simulations are provided for balanced and unbalanced voltage dips, in order to demonstrate the performance of the PV system and evaluate the accuracy of the proposed simplified model. The response of the detailed switching model is also included in the simulation results. [ABSTRACT FROM AUTHOR]

• New MILP-based UC model with embedded ac power flow constraints. • Piecewise linear ac power flow equations introduced, compatible with MILP models. • UC model perfectly applicable to small power systems containing tens of system buses. This paper presents a new network-constrained unit commitment (UC) model, which is formulated as a mixed-integer linear programming (MILP) optimization problem with embedded ac power flow (AC-PF) constraints. Piecewise linear AC-PF equations are formulated, which constrain the UC dispatch schedule within permissible transmission capacity and voltage deviation limits, solely by executing the main optimization problem. The proposed model is tested on a six-bus and the IEEE RTS-79 test systems, using GAMS software, whereas the obtained AC-PF results are validated against DIgSILENT/Powerfactory package. The UC schedules are comparatively assessed with detailed results presented in the relevant literature for the same test systems, employing different UC+AC-PF models. [ABSTRACT FROM AUTHOR]