HVDC Link Research Articles

Performance Analysis of Diverse Energy Storage on Combined ALFC and AVR Control of Multiarea Multiunit System with AC/HVDC interconnection

This paper discusses the significance of various energy storage devices like redox flow battery (RFB), capacitive energy storage (CES), superconducting magnetic energy storage (SMES), and flywheel energy storage system (FESS) impact on the combined control of automatic load frequency control (ALFC) and automatic voltage regulator (AVR) of three areas interconnected power systems having thermal, hydro, gas, and geothermal plants in presence of HVDC link. A novel PD with filter coefficient (PDN) controller cascade with fractional-order PID with filter coefficient (FOPIDN), CPDN-FOPIDN is proposed as a secondary controller for combined ALFC-AVR loop. An artificial flora algorithm is utilized to obtain the various controller parameters under numerous system conditions. The proposed CPDN-FOPIDN controller provides less settling time, overshoots, undershoots, and reduces oscillation compare to the FOPIDN controller. The study also reveals that the RFB provides better system performance than SMES, CES, and FESS. Integration of HVDC link improves the system dynamic response. Furthermore, the sturdiness of the proposed CPDN-FOPIDN controller is tested against the changes in system loading conditions via sensitivity analysis.

Voltage Profile Enhancing Using HVDC for 132KV Power System: Kurdistan Case Study

Nowadays power systems are huge networks that consist of electrical energy sources, static and lumped load components, connected over long distances by A.C. transmission lines. Voltage improvement is an important aspect of the power system. If the issue is not dealt with properly, may lead to voltage collapse. In this paper, HVDC links/bipolar connections were inserted in a power system in order to improve the voltage profile. The load flow was simulated by Electrical Transient Analyzer Program (ETAP.16) program in which Newton- Raphson method is used. The load flow simulation studies show a significant enhancement of the power system performance after applying HVDC links on Kurdistan power systems. The bus voltages are significantly increased after connecting High Voltage Direct Current.

Frequency Coordinated Control for the Asynchronous Interconnected Power System With Multiple HVDC Links

The isolation of the HVDC transmission system challenges the frequency stability of asynchronous power grid. Considering the high-capacity power regulation capability of multiple HVDC links, a novel frequency coordinated control method is proposed for the asynchronous interconnected power system. This method firstly establishes a detailed frequency response analysis model by using widearea measurement data. Then, for the lowest frequency control after disturbance, based on the frequency response analysis model, the functional relationship between the lowest frequency and DC power support is deduced. Finally, considering the influence of multiple HVDC power support on node voltage, power flow of line and frequency variation of the HVDC opposite-end grids, an optimal model of frequency coordination control with minimal influence is established. By solving the model, an multiple HVDC coordinated power support scheme can be obtained to achieve the lowest frequency control target after disturbance. A typical IEEE system is improved to an asynchronous interconnected power grid for case simulation. The accuracy and effectiveness of the proposed frequency coordination control are verified by comparison with traditional method.

Advanced control to damp power oscillations with VSC‐HVDC links inserted in meshed AC grids

In this article, we consider the particular situation of a HVDC link inserted in a meshed AC grid, which has interarea modes at upper frequencies than the usual ones. In this case, the standard IEEE (lead-lag plus gain) controllers may not be efficient. As a matter of fact, in this grid situation, the control is challenged by unstable zeros and system uncertainties, systematically put into evidence. Two original robust strategies based on matching a reference model using LMI optimization approach are designed to provide coordinated active and reactive power modulation for the HVDC link. They are based on a reduced order control model of the HVDC and the neighboring AC zone, which integrates the dynamics of interest. Besides increased damping and diminished impact of the unstable zeros, this new control achieves robustness against grid variations. Investigations with both linearized and nonlinear model of the system are carried out to settle and validate the approach. The efficiency and robustness of the proposed controllers are tested and compared with the ones of several standard controllers on a realistic benchmark of 19 generators interconnected by a meshed AC grid.

Effects of coordinated operation management of HVDC links and PSTs on the redispatch in a Central European case study

In this paper, the effects of coordinated operation management of embedded HVDC links and phase shifting transformers on the optimal redispatch costs and volumes are examined. The underlying optimization problem, including these power flow controlling devices, is formulated for linear optimization using DC load flow equations. The evaluation is based on the results of a year-round simulation of the expected energy market and network situation of the ENTSO-E region of the year 2025. The results highlight the need for coordinated operation management of PFCs since uncoordinated operation management increases the accruing redispatch costs.

Methodology for interaction identification in modular multi-level converter-based HVDC systems

This paper suggests a methodology for the identification, classification, and evaluation of various types of interactions that may occur in an HVDC link based on modular multi-level converters (MMC). The methodology incorporates the most suitable analytic tools for the frequency-domain study of each interaction category. To do so, a detailed nonlinear model of an MMC-based HVDC link that consists of master and slave MMCs, AC grids, and the DC transmission system is derived. Then, it is linearized to obtain a multi-input multi-output (MIMO) linear model that represents the dynamics of the complete MMC-based HVDC link. Based on the control loops of interest, interactions are classified as (1) state variable interactions, (2) disturbance interactions, (3) control loop interactions, and (4) overall system interactions. Then, through the application examples, the mentioned four categories of interactions are studied in frequency domain via the relevant analytic tools. The results obtained from the frequency-domain analysis are validated by time-domain simulation.

The impact of spatial variation of inertial response and flexible inter-area allocation of fast frequency response on power system scheduling

The impact of the spatial variation of inertial response and fast frequency services has not been fully investigated in power system scheduling models incorporating constraints on network security. This paper demonstrates the importance of acknowledging the variety of local frequency dynamics in the aftermath of a generation loss. It proposes a novel scheduling methodology that enables the system security in all areas of the system thus preventing potential tripping of distributed generation triggered by locally-measured high rates of change of frequency. The proposed methodology highlights the positive contribution to local frequency dynamics from an HVDC link, whose capacity is optimally dispatched to allocate power flow and fast-frequency services. The novel power system scheduling model is applied to analyze a typical 2030 GB low-carbon scenario. Results show that there are changes in the commitment decisions compared to the solution obtained with a one-area formulation when local security is taken into account. These changes translate into additional operational cost.

Harris Hawks-Based Optimization Algorithm for Automatic LFC of the Interconnected Power System Using PD-PI Cascade Control

This paper presents a new optimization tool-based population parameter called Harris hawks optimizer (HHO) and its application study to fine-tune the gains of well-designed proportional-derivative proportional-integral (PD-PI) cascade control to suppress the load frequency control (LFC) issues. The HHO based PID and PD-PI controllers are broadly implemented at two models with many circumstances for each model to ensure the effectiveness and the robustness of the proposed scheme at a high load disturbance, nonlinearity, and some critical parameters associated with the interconnected power system. First, a two-area non-reheat power plant is implemented, and the gains of PID and PD-PI controllers are adjusted using the proposed algorithm. In order to introduce extra realistic challenges, the governor-dead band is also modeled to ensure the robustness of the HHO/PD-PI in handling nonlinearity properties. Furthermore, to guarantee the suitability of the proposed HHO/PD-PI, a model with a mixture of power plants is carried out with and without the HVDC link, which is raised for the practical problems of LFC. Simulation results proved that; the proposed techniques HHO/PID and HHO/PD-PI provide superior performance compared to other reported strategies such as DE/PID, TLBO/PID, IGWO/PID, multi-objective/PID, and TLBO/2DOF-PID controllers. Finally, the dynamic investigation has also been completed using the random load pattern in system model-2, which shows the superior performance of HHO/PID and HHO/PD-PI schemes.

Methodology for the improvements in synchrophasor based System Integrity Protection Schemes under stressed conditions

Nowadays, the power system is characterized by a conglomeration of ultra and extra-high voltage AC and DC transmission corridors, multi-terminal DC (MTDC), and a new generation having Ultra Mega Power Plants (UMPP). It consists of a large power transmission and distribution network across the country along with old conventional generating units and insertion of Renewable Energy Sources (RES). The System Integrity and Protection Schemes (SIPS) play a crucial role in maintaining a secure and reliable grid and assist in effective grid management during severe power system contingencies. In this article, the operational experiences of three prime important SIPS in India are presented. The demerits in existing SIPS are pointed out and a methodology is proposed to overcome them. The performance and effectiveness of the proposed methodology are evaluated by simulation studies on the 39-bus New England system nested with the HVDC link in MATLAB. Simulation results confirmed the authenticity of a proposed methodology 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.

Series-DC connection of Offshore wind generating units - modeling, control and galvanic isolation.

This work proposes an air-core DC-DC solid state transformer to connect the wind turbines of an offshore wind farm in series with a DC transmission link. This emerging layout does not use a power transformer to increase the voltage from the offshore collector network to the transmission level, nor does it require a rectifier station to convert the electrical energy to DC. The diode-clamped rectifiers, connected to each permanent-magnet generator, are controlled to drain sinusoidal currents from the generators in order to extract the maximum power from the wind turbines. The air-core transformer, designed by means of a finite element software, ensures the galvanic isolation between the wind generator and the HVDC link. From the generator side, a single-phase full-bridge three-level inverter feeds the primary coil of the transformer, whereas the secondary coil is wound using by means of an XLPE-insulated cable. This paper also presents a strategy for designing the controllers that regulate the output voltage of each wind conversion unit. Results from digital simulations, obtained with PSCAD/EMTDC, are presented to validate the technical feasibility of the proposed DC-DC solid-state transformer scheme, as well as to demonstrate its performance under different operational conditions.

Comparative study of SBOAs on the tuning procedure of the designed SMPI controller for MIMO VSP/HVDC interconnected model

This paper presents a new application of advanced SMPI controller for a newly introduced interconnected dynamic system with VSP based HVDC links for frequency control problem. This work presents an outgrowth of analysis about the Swarm – Based Optimization Algorithms (SBOAs) in the tuning process of Multivariable Proportional – Integral (MPI) controller, which is called swarm – based MPI (SMPI). PSO, GOA and GWO algorithms are used for tuning process of the designed SMPI. The VSP based HVDC model is added for mitigation of system frequency dynamics with emulating virtual inertia. The proposed SMPI controller are designed for enhancing the dynamic performance of this system’s states during contingencies and they are compared with the conventional designed MPI controller. Deviation characteristics of the step function in MIMO transfer function of the VSP based HVDC model is considered as the common performance index in the proposed algorithms. On the other hand, the role of the proposed SMPI controller is to stabilize such interconnected system while minimizing the associated cost function. Matlab simulations next to the performed Nyquist’s stability anslysis demonstrate how the tuned SMPI can remarkably improve the frequency deviations and the damping of the inter-area oscillations excited during a fault. This enhancement is more obvious especially when SMPI controller for a VSP based virtual inertia emulation is tuned using GWO method.

Controller parameters tuning of water cycle algorithm and its application to load frequency control of multi-area power systems using TD-TI cascade control

In this study, a nature-inspired optimization tool called the water cycle algorithm (WCA) and its practical application to efficiently design a cascade controller for two area interconnected power system model is put forward to address the issues in load frequency control (LFC). The cascade control (CC) structure is one of the most effective controllers for improving the performance of a control scheme in power applications, particularly when disturbances occur. In this paper, a well-systematized combination of the WCA and a well-designed cascade tilt-derivative tilt-integral (TD-TI) cascade controller is introduced, and an error performance function, for example, the integral time absolute error, is taken. The proposed WCA-based effective TD-TI, TID, and PID strategy are executed as one broadly two area multi-source model with/without HVDC link under many scenarios to verify the effectiveness of the proposed system under a high-load perturbation and some critical parameters associated with the interconnected power system. Simulation results reveal that the proposed technique provides superior performance to that of newly published schemes such as many objective/PID, TLBO/PID, IPA/TID and IGWO/Fuzzy-PID controllers. The dynamic investigation is also completed with consideration of the random load pattern, which sufficiently reveals the superior performance of the WCA/TD-TI scheme.

Supplementary Power Control of an HVDC System and Its Impact on Electromechanical Dynamics

This paper presents a comprehensive analysis of the impact that supplementary power control of an HVDC link has on the electromechanical dynamics of power systems. The presented work addresses an interesting phenomenon that may occur when an HVDC power controller is installed to support frequency stability. In specific cases, a high gain HVDC frequency controller could deteriorate system damping. The given analytical study is the first of its kind that addresses this issue by including both: (i) the important higher-order generator dynamics that affect small signal stability simultaneously with an HVDC control as well as (ii) the available local angle/frequency input signals of the controller. The methodological approach here analytically formulates the impact an HVDC control has on the single generator dynamics. Furthermore, the relationship between the damping/synchronizing coefficients and the HVDC gain is explicitly derived when a frequency proportional HVDC controller is installed. The derived expressions confirm that, indeed, there is an optimal HVDC gain with respect to the damping coefficient and a typically positive impact of the HVDC controller on the synchronizing component. Finally, the developed theoretical foundation is demonstrated by the tools of linear and nonlinear analysis in a one-machine system case study.

Probing signal design for enhanced damping estimation in power networks

Electromechanical oscillations are inherent to power networks. Although it is possible to partially damp these oscillations, it is impossible to eliminate them completely. This oscillatory behavior can lead to major breakdowns in power networks, especially when the damping is relatively low. It is therefore important to provide accurate estimations of the network’s damping values. These estimation can be obtained via system identification for which a probing signal needs to be designed. This paper presents a framework for designing a specific probing signal that is able to provide accurate damping estimations with a user-defined variance. A power spectrum of the probing signal is determined by solving an optimization problem with Linear Matrix Inequality constraints. The objective function is defined as a weighted sum of the probing signal’s power and a level of disturbance caused by probing the network. A desired level of the damping estimation’s variance is set as a constraint. The time-domain realization of the obtained power spectrum is described by a multisine, which is the actual probing signal applied to the network. The employed framework is demonstrated through nonlinear simulations using the Kundur with an embedded HVDC link and NORDIC 44 networks.

A maximum available power algorithm for multi-infeed HVDC system based on equivalent impedance

High-voltage direct current transmission is widely used due to its asynchronous interconnection, fast and controllable transmission power adjustment and economy. The maximum available power calculation of an HVDC system is important in voltage stability analysis and control strategy design. The maximum available power is generally acquired based on repetitive power flow calculations for the system model and results in a low computational efficiency. In this paper, the equivalent impedance model of an HVDC system is deduced based on the external characteristic equivalence, and the model of an AC/DC system is converted to an AC system. For a single-infeed HVDC system, the boundary condition of the maximum available power is deduced based on the proposed impedance model. For a multi-infeed HVDC system, an equivalent model is acquired based on the impedance model, and the maximum available power algorithm is proposed. Finally, the effects of the network parameters and the operation status of neighboring HVDC links on the equivalent impedance are analyzed. The proposed method simplifies the mathematical model; hence, the computational efficiency is enhanced. The case study results demonstrate the effectiveness of the proposed method.

Comprehensive performance analysis of flexible asynchronous AC link under various unbalanced grid voltage conditions

A flexible asynchronous AC link (FASAL) system is a technology that interconnects two power system networks operating at the same or different frequencies. The FASAL system is a newly developed technology; therefore, it requires a thorough investigation under different unbalanced grid voltage conditions for better behavioral understanding and appropriate future controller design. In this paper, the FASAL system is analyzed under various symmetrical and asymmetrical unbalanced grid voltage conditions. Moreover, the performance of the back-to-back voltage source converter (VSC) based HVDC link is also analyzed to establish its comparison with the FASAL system under the same conditions. The FASAL and VSC-HVDC systems are modeled in PSCAD/EMTDC software environment. The simulation results illustrate that the FASAL system performance is better under transient as well as steady-state conditions. Furthermore, the reduction in power at the receiving end side is also lesser in the FASAL system. For example, a 10% sag contributes only a 15% dip in power in the FASAL system, whereas VSC-HVDC reports a 30% power dip for the same sag condition.

Advanced LVRT Control Scheme for Offshore Wind Power Plant

Offshore wind farms (OWF) are being rapidly integrated to the power systems, which can be primarily attributed to saturation of the mainland onshore area, and high wind speed in offshore sites. However, to maintain stable grid operation, OWFs are required to comply with Low Voltage Ride Through (LVRT) capability. OWF during LVRT period should not only stay grid connected, but also support the grid during the disturbance. In this paper, a new communication-independent LVRT control scheme for HVDC connected OWF has been proposed. Under the proposed strategy, OWFs not only successfully ride through a fault induced voltage dip, but also comply with active power recovery ramp rate during the fault recovery period. Besides complying with LVRT requirement and ramp rate limit, the proposed strategy improves voltage dip induced frequency (VDIF) issue by increasing active power injection from the OWF during LVRT period. The proposed LVRT strategy also exploits the stored energy in HVDC link capacitors in order to reduce the impact of VDIF issue. The proposed and the conventional LVRT strategies for HVDC connected OWF have been implemented and evaluated on IEEE 39 bus benchmark system.

Research on Variable Frequency Transformer: A Smart Power Transmission Technology

The electric power systems are interconnected for the economical and reliable operation of power supply systems. An interconnection of power systems can be implemented through AC and DC links. A variable frequency transformer (VFT) has emerged as an alternative for asynchronous interconnection, like back-to-back HVDC links. Unlike back-to-back HVDC links, VFT requires reduced reactive power, offers quicker incipient transient recovery, and has coherent damping ability. Various control strategies have been proposed in the literature to achieve the controllable and reliable operation of VFT. The VFT can be scalable for high power exchange by implementing the VFT Park model. The VFT is essentially a doubly-fed induction machine (DFIM) based system that needs slip rings and brushes in the rotor circuit. Hence, brushless doubly-fed induction machine (BDFIM) based VFT systems and their various topologies are proposed in the literature. In this paper, a comprehensive review is carried out, which includes: power system networks interconnection philosophy, power flow controllers used in the interconnected networks, FACTS controllers, and HVDC link with their technical merits and limitations. A comparison of VFT with its counter technology back-to-back HVDC link is also elaborated, which suggests VFT a better option for asynchronous grid interconnection. Moreover, a thorough literature review is done concerning VFT configuration, controls, and scaling with research papers and patents. Furthermore, detailed mathematical modeling of the VFT system is also carried out along with the numerical simulations under both steady-state and various fault conditions using PSCAD/EMTDC software.

Functioning of back-to-back high voltage direct current link model in Tomsk electric power system

In this article the research results of steady-state and dynamic performance of asynchronous back-to-back HVDC link in Tomsk electric power system are presented. It is proposed to use the back-to-back HVDC link to provide an asynchronous link for parallel operation of the North and South parts of Tomsk electric power system. The steady-state stability limit and maximum allowed power flow were defined. Based on this the more effective location and power of back-to-back HVDC link were determined. Therefore, it is shown the application of back-to-back HVDC link decreases the level of short-circuit current, power fluctuations and increases the stability of load operation, especially of motor load at substation.

A new AC fault ride-through strategy for HVDC link with serial connected LCC-VSC hybrid inverter

HVDC link with line commutated converter (LCC) as a rectifier at the power sending end, and with a serial connected LCC and voltage source converter (VSC) hybrid inverter (SLVHI) at the power receiving end, is adopted for future Baihetan HVDC engineering project in China. To realize the AC fault ride-through (FRT) of SLVHI, a new strategy based on DC chopper (DCC) is proposed in this paper. Firstly, the mathematical model is built to investigate the VSC DC overvoltage mechanism after SLVHI commutation failure (CF) and related factors. Secondly, a modified DCC topology of SLVHI is designed to adjust unbalanced power dissipation based on voltage drop depth. Thirdly, an enhanced voltage-dependent current order limiter (VDCOL) is proposed to deal with the unbalanced power after DCC switching off. With the cooperation between modified DCC and enhanced VDCOL, the proposed FRT strategy can realize CF mitigation and VSC DC overvoltage suppression simultaneously. Finally, the inverter side AC FRT performances of the HVDC link with SLVHI are studied by PSCAD/EMTDC. The simulation results validate the proposed strategy's effectiveness and superiority, with better CF mitigation and VSC DC overvoltage suppression abilities than other existing FRT strategies under different fault scenarios.