High Voltage Direct Current Connection Research Articles

Novel Gramian-Based Structure-Preserving Model Order Reduction for Power Systems With High Penetration of Power Converters

Renewable energy and high voltage direct current connections increase the number of power electronic devices in the grid, changing how power systems operate and introducing new dynamic behavior. For this reason, it is fundamental to understand what modeling details are needed to represent the system in this new context. This paper proposes a novel model reduction approach using frequency-limited controllability and observability Gramians that preserves the link between the physical structure of the system and the state variables. This method allows understanding and analytically determining how vital each state is to the input-output behavior in different frequency ranges. The primary contribution of this paper is an algorithm that can provide intuitive results to help power system experts understand the simulation tools and minimum modeling details needed to perform different stability studies. Two case studies supported the applicability of the developed method, and a tool for MATLAB/Simulink was developed based on the findings, allowing visualization and immediate identification of the model components that are most relevant for the system dynamics under study. This tool can analyze dynamic behaviors where the cause is poorly understood and provide clarity around modeling detail requirements in converter-rich power systems.

Analysis of Onshore Synthetic Inertia and Primary Control Reserve Contributions of Alternating Current-Side Meshed Offshore Grids with Voltage-Source Converter and Diode Rectifier Unit High-Voltage Direct Current Connections

The increasing use of renewable energy sources in place of conventional generation units is leading to a reduction in onshore inertia and to the development of offshore wind park grids connected by multiple high-voltage direct current (HVDC) connections to the onshore alternating current (AC) grid. For AC-side meshed offshore grids with voltage-source converter (VSC) and diode rectifier unit (DRU) HVDC connections towards onshore grids, this study focuses on the energetic feasibility of synthetic inertia (SI) and primary control reserve (PCR) contributions triggered locally at the onshore converters of both connection types. To this end, the obstacles preventing contributions for VSC HVDC connections and the mechanisms allowing contributions for DRU HVDC connections are identified first. Based on these findings, the article proposes an enhancement of the offshore HVDC converter controls that is continuously active and allows locally triggered onshore contributions at all onshore HVDC converters of both connection types without using communication and requiring only minimal system knowledge. Additional simulations confirm that, although the enhancement is continuously active, the operational performance of the offshore HVDC converter controls for normal offshore grid operation and its robustness against offshore AC-side faults are not affected.

Frequency Control of a Realistic Dish Stirling Solar Thermal System and Accurate HVDC Models Using a Cascaded FOPI-IDDN-Based Crow Search Algorithm

Unbalancing the real power in power system leads to fluctuation in system frequency which can cause the several negative effects on the performance and reliability of the interconnected power system. Therefore, to deal with this, the load frequency control (LFC) of a three-area asymmetric thermal power system integrated with a solar thermal power plant (STPP), a realistic dish-stirling solar thermal system (DSTS), and an accurate high voltage direct current (HVDC) link are presented in this work. For the suggested system, a novel cascade controller called fractional-order proportional-integral and integral-double-derivative with filter (FOPI-IDDN) is designed. By minimising a newly proposed performance index called the HPA-ISE and adjusting the controller and other system model parameters using a metaheuristic method called the crow search algorithm (CS). When comparing the system dynamics, it was found that the suggested FOPI-IDDN controller outperformed the FOPI, PIDN, and FOPIDN controllers. The findings of this study show that HPA-ISE shows approximately 30% and 60% improvements in settling time (ST) and peak overshoots (POS) for frequency response, and 32% and 18% improvements for the tie power responses in terms of ST and POS over ISE criteria. Also, studies on different area capacity ratios have shown that a system connected to a greater capacity ratio operates better. The realistic DSTS system with fixed and recurring insolation in area 1 and area 2 outperforms the others, according to experiments using different DSTS insolation. Also, it is discovered that the parallel AC-AHVDC link study is superior to the AC and HVDC connection research. Moreover, it seems from the sensitivity study that the CS-optimized FOPI-IDDN controller improvements obtained under normal settings are consistent across a wide range of changes.

Life Cycle Climate Change Impact of a Cost-Optimal HVDC Connection to Import Solar Energy from Australia to Singapore

This paper aims to evaluate the life cycle greenhouse gas (GHG) emissions of importing electrical power into Singapore, generated from a large-scale solar photovoltaic (PV) power plant in Australia, through a long-distance subsea high-voltage direct current (HVDC) cable. A cost optimization model was developed to estimate the capacities of the system components. A comprehensive life cycle assessment model was built to estimate emissions of manufacturing and use of these components. Our evaluation shows that, for covering one fifth of Singapore’s electrical energy needs, a system with an installed capacity of 13GWPV, 17 GWh battery storage and 3.2GW subsea cable is required. The life cycle GHG emissions of such a system are estimated to be 110gCO2eq/kWh, with the majority coming from the manufacturing of solar PV panels. Cable manufacturing does not contribute largely toward GHG emissions. By varying full-load hours and cable lengths, it was assessed that sites closer to Singapore might provide the same energy at same/lower carbon footprint and reduced cost, despite the lower insolation as compared to Australia. However, these sites could cause greater emissions from land use changes than the deserts of Australia, offsetting the advantages of a shorter HVDC cable.

Frequency-Coupled Impedance Modeling and Resonance Analysis of DFIG-Based Offshore Wind Farm With HVDC Connection

Motivated by recent years' resonance incidents in wind-integrated power systems, this paper investigates the resonance-induced harmonic distortion and stability issues in doubly fed induction generator (DFIG)-based offshore wind farm (OWF) with high-voltage direct current (HVDC) grid connection. To accurately capture the dynamic characteristics of DFIG-based wind farm, a comprehensive impedance modelling considering the detailed PI control loop and DC dynamics of wind turbine as well as the cable connections of the medium-voltage (MV) collector system is conducted. Through stepwise simulation verifications, aggregated modelling of MV collector system is proved to be suitable for wideband resonance analysis. On this basis, the resonance analysis regarding grid topology change and controller parameter variation is conducted and the impact of frequency coupling on subsynchronous resonance (SSR), middle- and high-frequency resonances is analyzed using the aggregated models derived from a practical HVDC connected DFIG-based OWF. The strength or degree of the frequency coupling between the sequence impedances of wind farm, which is induced by the asymmetrical converter control of wind turbines, is found to be dependent on the impedances of all the components of the system. Moreover, case studies are conducted to demonstrate the importance of including the frequency coupling in SSR stability assessment. Simulations in MATLAB / Simulink validate the modelling and resonance analysis.

Static and dynamic evaluation of different architectures for an actual HVDC link project

The employment of High-Voltage Direct Current (HVDC) transmission in modern power systems is mainly driven by requirements of increasing power transfer over high distances in a cost-efficient and controllable way. HVDC control can ensure fast and accurate power flow control to improve the whole system performance as well as increased flexibility to manage power flows from remote renewable generation to load centers, especially in a zonal market configuration. However, in the network planning phase the implications of a new HVDC connection, both on the steady-state and on the dynamic operation of a power system, must be carefully evaluated with particular reference to possible outage events, depending on the operating conditions and on the exploited control schemes. In this paper, a real HVDC project within the Italian transmission network is considered in both the Voltage Source Converter (VSC) and Line Commutated Converter (LCC) technologies. The study implies a methodology to analyze the impact on steady-state and dynamic performances in the presence of faults, with a focus on grid security and market efficiency aspects. The behavior of the interconnected Italian power system is simulated in a development planning viewpoint by means of software tools exploited by the System Operator.

Performance evaluation of distance protection of transmission lines connected with VSC-HVDC system using closed-loop test in RTDS

Voltage Source Converter (VSC) based High Voltage Direct Current (HVDC) can control voltage and power flow on ac transmission lines with the priority of reactive current support during the fault as suggested in the grid codes for HVDC connections. Furthermore, the fault current contribution from the converter is limited in magnitude and modified in the waveform in order to protect the power electronic devices during the fault, which on the other hand affects the apparent impedance seen by the distance relay. This paper addresses the challenges faced by the distance protection of transmission lines connected with VSC-HVDC system by performing a closed-loop test of a commercial distance relay using Real Time Digital Simulator (RTDS). The power system with VSC is modeled in detail under a more realistic scenario in RTDS so that the performance of the distance protection can be tested under near to actual field conditions. The responses of the commercial distance relay for balanced and unbalanced faults are analyzed. The oscillograph data of the voltage, current and protection bits recorded by the relay are extracted and compared with the simulation results.

Corrective Control to Handle Forecast Uncertainty: A Chance Constrained Optimal Power Flow

Higher shares of electricity generation from renewable energy sources and market liberalization is increasing uncertainty in power systems operation. At the same time, operation is becoming more flexible with improved control systems and new technology such as phase shifting transformers (PSTs) and high voltage direct current connections (HVDC). Previous studies have shown that the use of corrective control in response to outages contributes to a reduction in operating cost, while maintaining N-1 security. In this work, we propose a method to extend the use of corrective control of PSTs and HVDCs to react to uncertainty. We characterize the uncertainty as continuous random variables, and define the corrective control actions through affine control policies. This allows us to efficiently model control reactions to a large number of uncertainty sources. The control policies are then included in a chance constrained optimal power flow formulation, which guarantees that the system constraints are enforced with a desired probability. By applying an analytical reformulation of the chance constraints, we obtain a second-order cone problem for which we develop an efficient solution algorithm. In a case study for the IEEE 118 bus system, we show that corrective control for uncertainty leads to a decrease in operational cost, while maintaining system security. Further, we demonstrate the scalability of the method by solving the problem for the IEEE 300 bus and the Polish system test cases.

Using WAMS for HVDC Control

Abstract: The paper presents how a combination of two progressive technologies - high-voltage direct current (HVDC) and wide area monitoring system (WAMS), can contribute towards secure power system operation and enable flexible power flows control. A new control algorithm based on WAMS for HVDC connection has been proposed and tested on a dynamic model of the future Central East European region.

Control of Offshore DFIG-Based Wind Farm Grid With Line-Commutated HVDC Connection

The paper considers a control solution for integration of large offshore doubly fed induction generator based wind farms with a common collection bus, controlled by a static compensator, into the main onshore grid, using line-commutated high-voltage direct current connection. The paper's main focus is a mathematically grounded study of the power system interactions. That study produced an appropriate plant model for formal control design. A design procedure is described and the controlled system is validated using power systems computer-aided design/electromagnetic transient program simulations, which confirm the high performance of the proposed control strategy in both normal operation and fault conditions