Direct Current Grids Research Articles

Modelling, analysis and virtual parallel resistor damping control of VSC‐based DC grid using master–slave control mode

This study presents a generic linearised modelling method and a virtual parallel resistor damping control for a voltage source converter-based high-voltage direct current (HVDC) system including a direct current (DC) grid. The generic modelling method is composed of DC network's equivalent node-admittance matrix and the Thevenin equivalent matrix of all converters. A simplified two-terminal HVDC system is used to identify the key factors influencing the stability of the DC grid easily. Those key factors are proved by two methods, namely, root locus and participation factors analysis. The parameters of the virtual parallel resistor damping control are designed using the simplified HVDC system. The performance of the proposed damping control is studied employing the linearised model of the DC grid. Time simulations such as instability suppressing, power-controlled station N − 1 fault, and DC system faults are carried out to validate the effectiveness of the damping control.

A Traveling-Wave-Based Fault Location Scheme for MMC-Based Multi-Terminal DC Grids

This paper presents a novel fault location scheme of DC line in modular multilevel converter (MMC)-based multi-terminal DC (MTDC) grids. Considering the low-inertia characteristics and the meshed topology, the scheme, based on traveling-wave principle, is divided into three steps, namely, faulty pole identification, faulty segment determination and fault-distance calculation. With accurate amplitude, polarities and arrival times of the first arrival current traveling waves (FACTWs) collected from time-synchronized measurements taken just at the converter stations, the proposed scheme can correctly determine the faulty pole, the faulty segment and the precise fault location. The continuous wavelet transform (CWT) is deployed to extract the required features of the input signals at the DC lines. Since the scheme merely needs the features of FACTWs, the practical difficulties of detecting subsequent traveling waves are avoided. A four-terminal MMC-based high voltage direct current (HVDC) grid was built in PSCAD/EMTDC software to evaluate the performance of the fault-location scheme. Simulation results for different cases demonstrate that the proposed fault-location scheme has high accuracy, good adaptability and reliability. Furthermore, the algorithm can be used for a MMC-MTDC grid with any number of meshes.

Hybrid AC/DC Post-Contingency Power-Flow Algorithm Considering Control Interaction of Asynchronous Area

This paper presents an approach for the calculation and estimation of control interactions between asynchronous power systems coupled via multi-terminal high-voltage direct current grids using static analysis methods. This aims at possible application within online security assessment where, as of today, static analysis methods are still a common practice. Therefore, the postcontingency power flow is evaluated using an integrated approach for the solution of AC and DC systems in the power-flow algorithm. Thereby, equations for DC voltage control and AC frequency control of converter stations, generators, loads as well as wind turbines with frequency support are embedded into the extended system Jacobian. The results show that by using the proposed approach, the post-contingency power-flow situation can be accurately determined and the frequency changes in each subsystem can be sufficiently tracked. Using an application example of an offshore power system with frequency support by offshore wind turbines, all results have been validated against dynamic simulation, while simulation models and controllers are provided. It is shown that active power balancing controls of AC and DC systems could have a high impact on line loadings and need to be respected for security analyses.

Model Predictive Control-Based AGC for Multi-Terminal HVDC-Connected AC grids

Multi-terminal high-voltage direct current (MTDC) grids are seen as the enabling technology in the development of massive scale international grids such as the European supergrid. It is expected that these grids can play a significant role in regulating ac system frequencies. To date, many proportional-integral (PI) controller-based techniques have been proposed for frequency regulation in ac MTDC-connected grids. In this paper, model predictive control (MPC) is proposed as a means of implementing automatic generation control, while minimizing dc grid power losses. The advantages of using MPC versus PI are highlighted with regard to improvements in both frequency and dc grid regulation, while explicitly considering both delays and dc voltage constraints.

Multi-Objective Optimization of VSC Stations in Multi-Terminal VSC-HVdc Grids, Based on PSO

The objective of this paper is to enhance the control performance of voltage source converter (VSC) stations within multi-terminal high voltage direct current (M-HVdc) grids under dynamic conditions. This paper presents the application of particle swarm optimization (PSO) to tune the control parameters of VSC stations. VSCs are non-linear components of the M-HVdc grids. Conventional techniques use approximated linear models to tune the control parameters which do not produce optimal results. Thus, PSO is employed to optimally tune the control parameters of VSC stations. The algorithm of the proposed objective function applies simultaneous optimization of inner and outer control layers. Dynamic simulations of a four-terminal HVdc system are developed in PSCAD/EMTDC to extol the merits of the presented optimization technique. Results show the steady state and dynamic control performances, both for classically and PSO-based tuned parameters, during load demand change from ac grids, wind power change, and eventual permanent VSC disconnection.

Transient Current Interruption Characteristics of a Novel Mechanical DC Circuit Breaker

With the rapid development of multiterminal direct current (MTDC) grids, high-voltage direct current circuit breakers (HV-DCCB) capable of interrupting currents within 5 ms are in urgent demand. The mechanical DCCB has always been one of the main proposals for HV-DCCBs. The current commutation is the prerequisite for mechanical DCCBs to interrupt currents successfully. Focused on the current commutation difficulty, a novel mechanical DCCB has been investigated in this paper. A simulation model of the novel mechanical DCCB in MTDC system for fault current interruption was established in PSCAD. By analyzing the simulation results, the detailed transient varying patterns of current, voltage, and energy in the novel mechanical DCCB during its interruption process are revealed. To verify the feasibility of the novel mechanical DCCB, a prototype was developed in the lab, and current interruption tests were carried out on the prototype. Experimental results have shown that a current of 5.3 kA can be interrupted by the prototype within 3 ms. In the end, failures that occurred in initial experiments are discussed and relative protection measures have been proposed. By adopting these measures, failures are successfully avoided and the normal operation of the novel mechanical DCCB are ensured.

Simulative Analysis of a Flexible, Robust and Sustainable Energy Supply through Industrial Smart-DC-Grid with Distributed Grid Management

The industrial energy supply of the production with direct current offers the chances of higher energy efficiency, better integration of generative energy and at the same time robust power supply. In addition to the technological challenges for electrical devices, an intelligent grid management, which regulates the load flow between the participants, is crucial. The distributed grid management supports a scalable, modular network with a hierarchical structure. The paper defines a direct current (DC) grid management by the different roles of every participant. Each role provides special services, needed for a stable grid operation. These services are described and a simulative analysis is carried out. The simulation shows that network stability is ensured and at the same time, the possibility of optimizing the operation management is maintained. The presented results are part of the DC-INDUSTRIE research project, funded by the German government.

Selected issues of control voltage source inverter with self-excited synchronous generator as DC grid voltage source

Synchronous self-excited generators are the most popular AC voltage sources installed in power plants of seagoing vessels. Because of fuel savings varying revolutions Diesel drives can be found on selected types of ships (platform support vessels, cable layers, tug boats etc.). Very interesting issue is use of such type of alternator working with inverter acting as rectifier in direct current grid system. In direct current type of electrical grid the problems with synchronization and reactive power balance are absent. As the control method most suitable to use is modified version of field oriented control (FOC) known from induction machines. Aforementioned method involves decoupling of currents and control voltages to flux and torque components and keeping them in the most optimal condition. Theoretical background of inverter and synchronous generator adopted FOC control method along with numerical situations and experimental results obtained in laboratory test bench of such a system were included in following article.

Modeling, control, and protection of modular multilevel converter-based multi-terminal HVDC systems: A review

Multi-terminal direct current (MTDC) grids provide the possibility of meshed interconnections between regional power systems and various renewable energy resources to boost supply reliability and economy. The modular multilevel converter (MMC) has become the basic building block for MTDC and DC grids due to its salient features, i.e., modularity and scalability. Therefore, the MMC-based MTDC systems should be pervasively embedded into the present power system to improve system performance. However, several technical challenges hamper their practical applications and deployment, including modeling, control, and protection of the MMC-MTDC grids. This paper presents a comprehensive investigation and reference in modeling, control, and protection of the MMC-MTDC grids. A general overview of state-of-the-art modeling techniques of the MMC along with their performance in simulation analysis for MTDC applications is provided. A review of control strategies of the MMC-MTDC grids which provide AC system support is presented. State-of-the art protection techniques of the MMCMTDC systems are also investigated. Finally, the associated research challenges and trends are highlighted.

Decentralized model predictive control of voltage source converters for AC frequency containment

This paper presents a novel control scheme for exchange of frequency support between asynchronous AC systems through a High Voltage Direct Current link or grid. The proposed controller bears the spirit of an emergency scheme. Using only locally available measurements, each converter can identify emergency situations that could potentially lead to unacceptable frequency values. Then appropriate control actions are taken to restrain the frequency decline and prevent it from reaching the thresholds of load shedding relays. Inspired of Model Predictive Control, the method uses simplified models of the AC and DC sides of the converter, and can incorporate various constraints. The effectiveness of the method is demonstrated on a test system consisting of two asynchronous AC areas interconnected through a five-terminal HVDC grid.

A fast protection scheme for VSC based multi-terminal DC grid

This paper proposes a novel protection scheme for voltage source converter based multi-terminal direct current (VSC-MTDC) grid, in which DC line pilot protection applies polarity comparison of initial current travelling wave and DC busbar protection is based on sampled value current differential theory. This DC line protection utilizes modulus maximum of wavelet transform to get the polarity of initial current travelling wave on two ends of DC lines to distinguish internal faults from external faults. Current differential protection based on sampling values is adopted as DC busbar protection, which has the characteristics of fast operation speed and less computation compared to conventional current differential protection. A two-level voltage source converter (VSC) based four-terminal DC grid is built on the PSCAD platform and each VSC is equipped with resistive superconducting fault current limiters (R-SFCLs) to limit the short circuit current. Thus, DC circuit breakers can be used to break the DC fault current. The simulation results verify the validity and feasibility of the proposed VSC-MTDC grid protection scheme.

Grid code compliant controllers for Multi-terminal HVDC grids aimed to integrate wind power: assessing their impact on the operational security of a real-world system

Exploiting multi-terminal High Voltage Direct Current (MTDC) grids for offshore wind park integration into continental bulk AC system raises several technical challenges, in particular the verification of the compliance of MTDC grids and offshore wind parks to the performance requirements (in terms of frequency, voltage support and fault ride through) which are being established by the grid codes, and the availability of general and customizable models to simulate their response. This work is aimed to provide a set of “open access” general models, able to fulfil the grid code requirements if suitably tuned, enabling the TSOs to perform their analyses without relying exclusively on vendor-specific models. In order to verify the realistic response of these models, an example of MTDC grid is simulated, connecting it to plausible operating scenarios of a real world AC system (the West Denmark grid) in target year 2020: some indications to properly tune control parameters are provided together with the results of several simulations aimed to check the fulfilment of grid code requirements in case of typical contingencies.

Series Interline DC/DC Current Flow Controller for Meshed HVDC Grids

This paper proposes a DC/DC current flow controller (CFC) for meshed high voltage direct current (HVDC) grids. The CFC has the advantage of a simplified structure that allows us to use the minimum number of switches when unidirectional current flows through the DC lines are expected. An extended CFC topology that is able to operate with all possible current flows is also presented. Then, the operating principle of the CFC is discussed and its structure is compared with a dual H-bridge analyzing advantages and disadvantages. This paper also details the applied modulation strategy and the control methodology. Finally, a five-terminal meshed HVDC grid is used to validate the CFC by means of simulations.

Full-bridge MMC DC fault ride-through and STATCOM operation in multi-terminal HVDC grids

Abstract This paper investigates a control structure to enhance the DC fault ride-through capability of a full-bridge modular multilevel converter (MMC) station, while ensuring a stable controlled operation as a STATCOM during DC faults without the need for fault isolation. Taking advantage of the switching states of a full-bridge submodule, a DC current controller is proposed, which provides the DC voltage reference for the modulation when a DC fault is detected. By changing the outer controllers strategy from DC voltage or active power control to converter energy control during a fault, the decoupling of the converter operation from the DC side dynamics is realized. In this paper, the focus is on the control methodology at all times of operation and the evaluation of the STATCOM control during a fault. To this end, extensive simulations were performed on a three-terminal high voltage direct current (HVDC) grid in radial configuration and a pole-to-pole DC fault case was investigated. The results showed that the AC voltage and current were controlled within limits at all times, while the full-bridge MMC was able to provide reactive power support to the AC grid. Moreover, using the proposed control methodology, the transients at the operation transition points between STATCOM and inverter/rectifier operation were minimized and the stations were able to safely ride through the fault.

Probabilistic Power Flow for AC/VSC-MTDC Hybrid Grids Considering Rank Correlation Among Diverse Uncertainty Sources

A new, unscented transformation (UT) based probabilistic power flow method for Alternate Current/Voltage Source Control-Multiple Terminal Direct Current hybrid grids is presented herein. The method is able to accurately tackle various random variables, including renewable energy sources with uncertainties such as wind speeds and solar radiations, which are rank correlated and are likely to follow different types of probability distributions. The concept of Gaussian copula is adopted to transform the rank correlated random variables into a group of standard Gaussian distributions with Pearson correlation coefficients, so that the UT method can be later applied to select the critical sample points from Gaussian distributions in a proper and uniform way. The effectiveness of the proposed method is validated using a set of test results on the modified IEEE 39-bus system and IEEE 300-bus system.

Soft switching resonant converter with duty-cycle control in DC micro-grid system

ABSTRACTResonant converter has been widely used for the benefits of low switching losses and high circuit efficiency. However, the wide frequency variation is the main drawback of resonant converter. This paper studies a new modular resonant converter with duty-cycle control to overcome this problem and realise the advantages of low switching losses, no reverse recovery current loss, balance input split voltages and constant frequency operation for medium voltage direct currentgrid or system network. Series full-bridge (FB) converters are used in the studied circuit in order to reduce the voltage stresses and power rating on power semiconductors. Flying capacitor is used between two FB converters to balance input split voltages. Two circuit modules are paralleled on the secondary side to lessen the current rating of rectifier diodes and the size of magnetic components. The resonant tank is operated at inductive load circuit to help power switches to be turned on at zero voltage with wide load range. The pulse-width modulation scheme is used to regulate output voltage. Experimental verifications are provided to show the performance of the proposed circuit.

Electricity system based on 100% renewable energy for India and SAARC.

The developing region of SAARC (South Asian Association for Regional Cooperation) is home to a large number of people living below the poverty line. In future, providing affordable, universally accessible, reliable, low to zero carbon electricity in this region will be the main aim. A cost optimal 100% renewable energy system is simulated for SAARC for the year 2030 on an hourly resolved basis. The region was divided into 16 sub-regions and three different scenarios were set up based on the level of high voltage direct current (HVDC) grid connections. The results obtained for a total system levelised cost of electricity (LCOE) showed a decrease from 71.6 €/MWh in a decentralized to 67.2 €/MWh for a centralized grid connected scenario. An additional scenario was simulated to show the benefits of integrating industrial gas production and seawater reverse osmosis desalination demand, and showed the system cost decreased by 5% and total electricity generation decreased by 1%. The results show that a 100% renewable energy system could be a reality in the SAARC region with the cost assumptions used in this research and it may be more cost competitive than nuclear and fossil carbon capture and storage (CCS) alternatives. One of the limitations of this study is the cost of land for installation of renewables which is not included in the LCOE calculations, but regarded as a minor contribution.

Multi-terminal DC grids: challenges and prospects

A few multi-terminal direct current (MTDC) systems are in operation around the world today. However, MTDC grids overlaying their AC counterpart might a reality in a near future. The main drivers for constructing such direct current grids are the large-scale integration of remote renewable energy resources into the existing alternative current (AC) grids, and the promotion and development of international energy markets through the so-called supergrids. This paper presents the most critical challenges and prospects for such emerging MTDC grids, along with a foreseeable technology development roadmap, with a particular focus on crucial control and operational issues that are associated with MTDC systems and grids.

A Hybrid Reliability Evaluation Method for Meshed VSC-HVDC Grids

High-voltage direct current (HVDC) grids are emerging, and their reliability has been an increasing concern for the utilities. HVDC grids are different from typical two-terminal HVDC transmission systems due to the loops in their topology, which makes it difficult to evaluate the reliability by conventional analytical methods. This paper proposes an innovative hybrid method to evaluate the reliability of meshed HVDC grids. First, steady-state models and reliability models are established for the components in HVDC grids, especially for converters and power flow controllers. In the models, virtual buses are introduced to represent the external AC connections to the HVDC grid. Then a hybrid reliability evaluation method is proposed based on an analytical approach and Monte Carlo simulation. One innovation of the paper is the application of an analytical analysis method to accelerate state evaluation in Monte Carlo simulation by skipping unnecessary optimization. The proposed models and methods are verified on two HVDC grids. Test results show that HVDC grids under most failure states (approximately 70%) tend to shed no load except on buses connected to faulted converters, and the application of the analytical method could promote evaluation efficiency significantly.

Application of new directional logic to improve DC side fault discrimination for high resistance faults in HVDC grids

This paper proposes a simple and fast way to determine the direction of a fault in a multi-terminal high voltage direct current (HVDC) grid by comparing the rate of change of voltage (ROCOV) values at either side of the di/dt limiting inductors at the line terminals. A local measurement based secure and fast protection method is implemented by supervising a basic ROCOV relay with a directional element. This directional information is also used to develop a slower communication based DC line protection scheme for detecting high resistance faults. The proposed protection scheme is applied to a multi-level modular converter based three-terminal HVDC grid and its security and sensitivity are evaluated through electromagnetic transient simulations. A methodology to set the protection thresholds considering the constraints imposed by the breaker technology and communication delays is also presented. With properly designed di/dt limiting inductors, the ability of clearing any DC transmission system fault before fault currents exceeds a given breaker capacity is demonstrated.