HVDC Link Research Articles

Improvements in Existing System Integrity Protection Schemes Under Stressed Conditions by Synchrophasor Technology—Case Studies

In this modern era, the power system is stigmatized by a conglomeration of ultra-high voltage AC and DC, multi-terminal DC, and extra-high voltage AC transmission networks. It consists of a large distribution network beyond the country along with traditional generation having Ultra Mega Power Plants (UMPP) and growing ingress of Renewable Energy Sources (RES). The System Integrity and Protection Schemes (SIPS) play a crucial role in preserving a secure and reliable grid and facilitate efficient grid control during severe power system contingencies. In this paper, the operational experiences of three important SIPS in India are presented. Based on a thorough analysis, the drawbacks of these SIPS are identified and an algorithm is proposed to overcome the drawbacks using synchrophasor technology. The performance and capability of the proposed algorithm are evaluated by simulation studies on the 39-bus New England system embedded with HVDC link in MATLAB. Simulation results consummated confirm the effectiveness of the scheme in preserving system integrity. Results are validated using Electrical Transient Analyzer Program (ETAP), which is a very powerful design and analysis tool and has an extremely user-friendly interface.

Load Frequency Control of Multi-interconnected Renewable Energy Plants Using Multi-Verse Optimizer

A reliable approach based on a multi-verse optimization algorithm (MVO) for designing load frequency control incorporated in multi-interconnected power system comprising wind power and photovoltaic (PV) plants is presented in this paper. It has been applied for optimizing the control parameters of the load frequency controller (LFC) of the multi-source power system (MSPS). The MSPS includes thermal, gas, and hydro power plants for energy generation. Moreover, the MSPS is integrated with renewable energy sources (RES). The MVO algorithm is applied to acquire the ideal parameters of the controller for controlling a single area and a multi-area MSPS integrated with RES. HVDC link is utilized in shunt with AC multi-areas interconnection tie line. The proposed scheme has achieved robust performance against the disturbance in loading conditions, variation of system parameters, and size of step load perturbation (SLP). Meanwhile, the simulation outcomes showed a good dynamic performance of the proposed controller.

XLPE Cable for HVDC Link

XLPE Cable for HVDC Link

Operational experience and performance evaluation of some of the system integrity and protection schemes

An Indian power system is characterized by a conglomeration of ultra-high voltage AC and DC, multi-terminal DC, and extra-high voltage AC transmission networks. It consists of a large distribution network across the country along with conventional generation having Ultra-Mega Power Plants (UMPP) and increasing penetration of Renewable Energy Sources (RES). The System Integrity and Protection Schemes (SIPS) play an important role in maintaining a secure and reliable grid and assists in effective grid management during severe power system contingencies. This paper presents the operational experience of some SIPS implemented in India along with their performance evaluations. The operational experience of three important SIPS, one in the Northern Regional (NR) grid and another two in the Western Regional (WR) grid of India are presented. The drawbacks in existing SIPS are pointed out and an algorithm is proposed to overcome these drawbacks. The performance and effectiveness of the proposed algorithm are evaluated by simulation studies on the 39-bus New England system embedded with an HVDC link in MATLAB/Simulink. Simulation results consummated confirm the authenticity of the scheme in preserving system integrity. Results are validated using Electrical Transient Analyzer Program (ETAP), which is a very powerful design and analysis tool and has an extremely user-friendly interface.

Renewable Integration in Hybrid AC/DC Systems Using a Multi-Port Autonomous Reconfigurable Solar Power Plant (MARS)

As the penetration of utility-scale solar photovoltaic (PV) power plants increases, the inertia in the system is reduced and there will be increased primary frequency response requirements. To increase inertia and improve the primary frequency response, grid-forming inverters connecting PV to grid and energy storage systems (ESSs) may play an important role. Moreover, high-voltage direct current (HVdc) links can also be an enabler to transfer remote PV power generation and to improve grid stability. That is, with increased penetration of PV, discrete development of PV and ESS connecting to transmission ac grid and HVdc links is one of the solutions for stable operation of the grid. In this paper, an integrated concept for integration of PV and ESS to transmission ac grid and HVdc links is proposed that is named as multi-port autonomous reconfigurable solar power plant (MARS). The integrated development incorporates advanced control methods to provide inertial and primary frequency response, reactive power support, and transient stability to manage PV and ESS resources. In this paper, high-fidelity switched system model of the integrated system and grids are developed and detailed simulation results are provided to showcase the stable operation of the integrated system and provision of grid support functions.

Effects of Cyber Attacks on AC and High-Voltage DC Interconnected Power Systems with Emulated Inertia

The high penetration of renewable energy resources and power electronic-based components has led to a low-inertia power grid which would bring challenges to system operations. The new model of load frequency control (LFC) must be able to handle the modern scenario where controlled areas are interconnected by parallel AC/HVDC links and storage devices are added to provide virtual inertia. Notably, vulnerabilities within the communication channels for wide-area data exchange in LFC loops may make them exposed to various cyber attacks, while it still remains largely unexplored how the new LFC in the AC/HVDC interconnected system with emulated inertia would be affected under malicious intrusions. Thus, in this article, we are motivated to explore possible effects of the major types of data availability and integrity attacks—Denial of Service (DoS) and false data injection (FDI) attacks—on such a new LFC system. By using a system-theoretic approach, we explore the optimal strategies that attackers can exploit to launch DoS or FDI attacks to corrupt the system stability. Besides, a comparison study is performed to learn the impact of these two types of attacks on LFC models of power systems with or without HVDC link and emulated inertia. The simulation results on the the exemplary two-area system illustrate that both DoS and FDI attacks can cause large frequency deviations or even make the system unstable; moreover, the LFC system with AC/HVDC interconnections and emulated inertia could be more vulnerable to these two types of attacks in many adversarial scenarios.

Active Power Control of PV-Battery Connected MMC-HVDC System for FRT Support

Modular multilevel converter (MMC)-based VSC system has become attractive around the world for renewable energy integration. Instead of a dynamic braking resistor, this work proposes an active power reduction technique for PV systems to support the fault ride through (FRT) of the MMC-HVDC system. In addition, it develops a battery control strategy to improve transient performance during solar radiation and temperature change due to partial shading of the PV panels. Besides, a control technique for the battery to regulate the surplus energy in the HVDC transmission network is developed. Furthermore, the proposed control scheme optimally integrates solar energy using the modified incremental conductance method. A feedforward controller was employed to create a standalone AC grid. The complete system has been implemented in real-time digital simulation (RTDS). The results confirm the efficacy of active power reduction technique to protect the HVDC link voltage and battery control strategy for the improvement of transient performance during the irradiance and temperature changes. Besides, it improves the low voltage ride-through capability during balanced and unbalanced disturbances at the point of common coupling.

Principles of Operation of Grids of DC and AC Subgrids Interconnected by Power Converters

The concept of segmented AC power systems interconnected by means of HVDC is becoming more and more significant for many cases worldwide. Such systems already exist and will become more complex with the increasing emergence of HVDC links and grids. The concept of a grid composed of multiple subgrids motivates the need to define the governing roles of Interconnecting Power Converters (IPC) which connect the different subgrids. Such IPC can have various operation modes which are presented and analyzed in this paper. In addition, the principles of operation needed for each individual subgrid and for the overall system are introduced and discussed. The resulting fundamental equations are outlined and combined in a power flow formulation of the overall system. A simple case study is presented to illustrate the power flow formulation in a grid of multiple subgrids, also introducing example dynamic simulations of transitions between different operating points.

Protection of Single-Phase Fault at the Transformer Valve Side of FB-MMC-Based Bipolar HVdc Systems

Although the probability of occurrence of ac grounding faults at the valve side of the interface transformer of a high-voltage dc (HVdc) link is low, they may cause high risks to the converter when compared to grid-side ac faults. This article analyzes the characteristics of valve-side ac single-phase-to-ground faults in full-bridge modular multilevel converters (FB-MMCs)-based bipolar HVdc systems. Overcurrents in the converter arms are analyzed and it is shown that overvoltages in FB submodules occur without an appropriate protection in place. Two strategies are investigated to protect the FB-MMC during the fault and corresponding controllers are designed. The effectiveness of the presented strategies for the prevention of overcurrents and overvoltages, upon nonpermanent and permanent faults, and system postfault restoration is investigated. For completeness, the strategies are also verified by conducting simulations in PSCAD/EMTDC.

Spatio-Temporal Decomposition and Coordination for Distributed Load Restoration in AC/DC Hybrid System

A new spatio-temporal decomposition and coordination scheme is proposed for load restoration in an AC/DC hybrid transmission system. First, the framework of spatio-temporal decomposition is built for load restoration, the decoupled variables and spatial and temporal coordinators are defined, as well as two interactive blocks. Furthermore, the spatio-temporal decomposed models of subsystems and HVDC links are constructed considering security constraints and HVDC operation characteristics. Finally, a new hierarchical overlapping coordination based analytical target cascading (HOC_ATC) algorithm is developed to coordinate the decomposed models by iterative calculations between subsystems and coordinators, as well as the two interactive blocks. The proposed distributed scheme maintains independent operations of subsystems, and handles the multi-step restoration process by iteratively calculating single-step models. The developed HOC_ATC algorithm provides tractable and efficient computation for the HVDC model incorporated load restoration optimization. The effectiveness of the proposed method is validated using two IEEE test systems, showing improved computational efficiency and better convergence ability.

Evaluating Strength of Hybrid Multi-Infeed HVDC Systems for Planning Studies Using Hybrid Multi-Infeed Interactive Effective Short-Circuit Ratio

The strength is an important concept in planning studies for voltage stability analysis of hybrid multi-infeed HVDC (HMIDC) systems, which include both VSC- and LCC- HVDC links. However, the accuracy of earlier strength indices is very limited because they are developed by the empirical reasoning rather than rigorous theoretical derivation. In this paper, a virtual single-infeed model of HMIDC systems is firstly established utilizing the reduced-order equivalent nodal assessment of the inverter bus current-voltage equations. While incorporating the inter-inverter interactions, the virtual model is a strict dual-bus equivalent of the original complicated multi-infeed model. This enables easier and simpler analysis to be implemented. Secondly, the virtual model is employed to propose the hybrid multi-infeed interactive effective short-circuit ratio (HMIESCR) strength index for HMIDC systems by the rigorous theoretical derivation. Compared to earlier indices, the HMIESCR can depict the voltage stability much more accurately. This is because the critical HMIESCR (CHMIESCR) indicative of the stability limit is explicitly and solely evaluated to 1.5 according to the PV sensitivity based stability criterion, while the critical values of earlier indices are ambiguous. Finally, case studies by extensive numerical simulations on the classical hybrid four-infeed HVDC system validate the HMIESCR along with good robustness.

Modular Multilevel DC–DC Power Converter Topology With Intermediate Medium Frequency AC Stage for HVDC Tapping

This article proposes an isolated dc–dc modular multilevel converter (MMC) topology intended for tapping power from the high-voltage dc (HVdc) link. The proposed converter operates with two stages of power conversion, i.e., HVdc to ac and ac to medium-voltage dc, using an intermediate medium-frequency transformer. The proposed topology reduces the number of arms in MMC to two, along with a series LC passive filter tuned at a medium frequency at the primary side of the transformer. A high-voltage transformation ratio is possible in the proposed converter. Operation at the medium-frequency ac ensures the reduction in the size of the transformer and other passive elements. A sinusoidal level-shifted pulsewidth modulation technique is implemented for modulating the arm voltages of the converter, which results in a voltage close to the sine wave with small dv/dt being impressed across the transformer primary. The design of the passive filters is also included in the article. The operation of the converter is validated by the simulation of a 400 kV, 10 MW system, and a downscaled experimental prototype of 1 kW, 400 V system.

Frequency and voltage stabilisation in combined load frequency control and automatic voltage regulation of multiarea system with hybrid generation utilities by AC/DC links

ABSTRACT This article investigates the combined analysis of load frequency control (LFC) and Automatic voltage regulation (AVR) for two-area hybrid system. Classical PID controller is habituated as secondary controller. A novel differential evolution artificial electric field algorithm (DE-AEFA) is proposed in this paper to tune controller parameters. Initially, DE-AEFA algorithm is applied to test system 1 comprising two-area non-reheat thermal turbines. Later on, the study is extended to combined model named test system 2 to investigate combined LFC and AVR problem. Further, the system is extended to the incorporation of HVDC link with the existing AC tie-line in parallel. System dynamic performance is predominantly enhanced by AC/DC rather than only AC tie-line. The proposed scheme robustness is demonstrated by conducting sensitivity analysis.

A General Steady-State Voltage Stability Analysis for Hybrid Multi-Infeed HVDC Systems

As large-scale offshore wind farms are integrated into the power grid, a hybrid multi-infeed HVDC (HMIDC) system including both LCC-HVDC and VSC-HVDC links is facing a critical steady-state voltage stability problem which mainly stems from a weak AC system under high-level DC power injections. To overcome this challenge, this paper proposes a general steady-state voltage stability analysis to accurately measure the critical voltage stability point of HMIDC systems. In this paper, a typical HMIDC system is firstly modeled as a general dual-infeed system composed of an LCC-HVDC, a VSC-HVDC, and a virtual equivalent voltage source behind a virtual equivalent impedance at the AC side. Then a hybrid power sensitive factor (HPSF) is established to indicate the critical voltage stability point with the consideration of dynamic power-voltage characteristics of HVDC links, as well as operational changes in AC system (e.g. dynamics of excitation voltage control in generator and network topology change). Finally, a general voltage stability index, called hybrid generalized short circuit ratio (HGSCR), is further derived to effectively explain the critical voltage stability point of HMIDC systems. Moreover, multiple comparative studies with existing voltage stability indices are conducted in this paper to verify the effectiveness of the proposed general steady-state voltage stability analysis by using both PSCAD/EMTDC™ power flow and time-domain simulations.

Advanced Impedance-Based Control Design for Decoupling Multi-Vendor Converter HVDC Grids

A progressive interconnection of existing HVDC links to form grids and the development of completely new HVDC grids from different vendors are expected shortly. One of the current challenges of such endeavour is unintended interactions due to independently designed controllers. This article proposes a design methodology for decentralized controllers to mitigate such interactions in multi-vendor voltage source converter (VSC)-HVDC grids. The approach presented relies on the unique stand-alone input-output impedance transfer function of each VSC, and the global impedance transfer function as seen from each terminal after interconnection with other VSCs. Subsequently, network-level controllers are designed by attempting to match the global responses at selected locations based on a novel interaction analysis, to the unique transfer function model of the vendors at the corresponding location. This approach reduces the entire problem to an impedance matching problem. We demonstrate the efficacy and flexibility of both the methodology and the designed controllers in mitigating interactions due to the independent design of VSC controllers through nonlinear simulations on a four-terminal droop controlled HVDC grid.

Small‐signal stability and fault performance of mixed grid forming and grid following offshore wind power plants connected to a HVDC‐diode rectifier

This study aims at validating the simultaneous operation of grid following and grid forming wind power plants when connected to a common diode rectifier-based HVDC link. The controllers for both grid forming and grid following wind turbines include fault-ride-through capability with soft restoration of the off-shore AC-grid. Current and voltage controllers are developed in the stationary α β reference frame. The presented control and soft restoration strategies are based on local measurements only and do not require communication between wind turbine generators. The small-signal stability analysis of the mixed system in multiple d–q axis using detailed string models is carried out in order to show the sensitivity to grid following phase-locked-loop gains and to grid forming droop gains. Interoperability between grid forming and grid following wind turbines during transients is shown by means of detailed simulation during symmetric faults in different locations of the off-shore grid.

Model predictive control based coordinated control of multi-terminal HVDC for enhanced frequency oscillation damping

This paper presents a coordinated power control strategy based on Model Predictive Control (MPC) for Multi-Terminal HVDC (MTDC) to enhance the system frequency oscillation damping. Due to the available transmission capacity margin of HVDC links, MTDC buses have power regulation flexibility. With the proposed control strategy, the MTDC buses dispersed at different locations are efficiently coordinated to optimize the power flow of the grid to damp the frequency oscillations fast and timely. Besides, by the sensitivity analysis, the impact of the power injections at MTDC buses on the system frequency is quantified. By using the real-time sampling data of the Wide Area Measurement System (WAMS), the linearized power system model can be estimated on-line and used as the prediction model of the MPC. The simulation results show that the proposed controller can efficiently improve the system damping performance during frequency events.

AGC of Interconnected Thermal Power System with Any Colony Optimization Technique Optimized PID Controller and HVDC link

AGC of Interconnected Thermal Power System with Any Colony Optimization Technique Optimized PID Controller and HVDC link

Efficient method for the optimal economic operation problem in point-to-point VSC-HVDC connected AC grids based on Lagrange multipliers

This paper describes a method based on Lagrange multipliers for efficiently solving the economic dispatch in power systems including point-to-point VSC-HVDC links. The proposed formulation bases on linear models of the AC systems and HVDC links where power losses are properly considered through incremental transmission loss factors.This timely formulation preserves suitable outcome accuracy while showing a greater computational efficiency than what is achieved with classical, nonlinear OPF methods. Its main features are demonstrated using two compelling test cases: a system accommodating one point-to-point VSC-HVDC link and another comprising multi-infeed HVDC connected AC power grids. Results of this approach are compared with those calculated by the nonlinear interior point method. It is confirmed that both fundamentally different approaches are in good agreement since errors smaller than 0.5% were obtained for generation costs, whereas the computational time was reduced by more than 60% with the introduced method.To show the method applicability in realistic power systems, the 10-generator New England Test system incorporating a VSC-HVDC link is studied. The DC link effect on system operation is examined for a 24-hour period dispatch thus demonstrating its usefulness, unrivalled modelling versatility and numerical efficiency with respect to existing approaches.

Transient stability of power systems with embedded VSC‐HVDC links: stability margins analysis and control

This study investigates the impact of embedded voltage source converter-based high voltage direct current (VSC-HVDC) links on AC grids transient stability. Firstly, using transient energy functions, it is demonstrated that VSC-HVDC links controlled to track constant power references, do not inherently improve transient stability of the surrounding AC grid as an AC line naturally does. Then, a control law using the feedback linearisation technique on a simple but representative power system is derived. The control law highlights and combines the three main actions the VSC-HVDC link can offer to enhance rotor angle stability: fast power reallocation, injection of synchronising power and injection of damping power. The control law is implemented and validated in EMT simulation. It is then shown that an HVDC link can assure the synchronisation of different AC areas even if no AC transmission lines interconnect them. Through another case study, it is shown how the HVDC link can help to share dynamic frequency reserves to not jeopardise the stability of the system. The last example investigates the effect of a DC fault on AC transient stability and how the control can help improving the system response.