Multi-infeed HVDC Systems Research Articles

Comparative Analysis on Small-Signal Stability of Multi-Infeed VSC HVDC System With Different Reactive Power Control Strategies

Voltage Source Converter based High Voltage Direct Current (VSC HVDC) technology is used more and more in modern power systems. Consequently, in the ac network, an increasing number of converters are appearing in mutual electrical proximity, thus forming Multi-Infeed (MI)-HVDC systems. For VSC technology, several aspects of MI-HVDC system operation are less well understood compared to Line Commutated Converter (LCC) HVDC systems. This paper studies an MI system which consists of two VSC HVDC schemes based on Modular Multilevel Converters (MMC) interconnected with an ac line and connected to an external grid. The aim of the study is to identify the small-signal stability properties of such a system, thereby focusing on the impact of different outer control loops for reactive power and ac voltage regulation.

Analysis and assessment standards of power stability of multi‐send HVDC systems

The asynchronously interconnected Yunnan Power Grid is a typical multi-send HVDC system. So far, research on systems with multiple high voltage direct current (HVDC) links is mainly focused on multi-infeed HVDC systems, and the assessment standards for power stability of multi-send HVDC systems have not been discussed yet. In this study, firstly the quasi-steady-state model of a two-send HVDC system is built and an analysis of power stability based on the maximum power curve is given. Secondly, the relationships between power stability and two indices (Multi-infeed Effective Short-circuit Ratio and the Multi-infeed Interaction Factor) are discussed. Lastly, the assessment standards for power stability of multi-send HVDC systems are proposed and a case study of Yunnan Power Grid is presented, which may provide useful instructions for the planning process of multi-send HVDC systems.

Impact of control system on small‐signal stability of hybrid multi‐infeed HVDC system

This paper focuses on the impact of control system on small-signal stability of the Hybrid Multi-Infeed HVDC(H-MIDC) system, in which the Line-Commutated-Converter based HVDC (LCC-HVDC) and Voltage-Sourced-Converter based HVDC (VSC-HVDC) links feed into an AC system in close mutual proximity. A comprehensive linearized small-signal model is developed for an H-MIDC system. Based on the eigenvalue analysis and participation factor analysis, the inherent oscillatory modes of the H-MIDC system are revealed. Then, the impact of the most sensitive control system parameters on the least-damped modes are investigated. The studies show that (i) the proportional gains of PLL functions of LCC and VSC stations, and AC voltage outer loop controller of VSC station are highly coupled, and presents the most sensitivity to the least damped modes; (ii) parameters of PLL in LCC station, and outer loop of AC voltage control and PLL in VSC station can highly impact the damping of the oscillatory modes and even cause instability, and (iii) smaller PLL gains in LCC and VSC stations, and smaller proportional gain of AC voltage control of VSC station are preferable and can enhance damping of dominant modes.

A Coordinated DC Power Support Strategy for Multi-Infeed HVDC Systems

A DC power support strategy utilizes the flexibility of a High-voltage direct-current (HVDC) system in power modulation to optimize the operating point or compensate the power imbalance caused by a disturbance. The major impediment to the strategy is the difficulty in maintaining DC voltage values at converter stations during the process of DC power support. To overcome the difficulty, a coordinated DC power support strategy for multi-infeed HVDC systems is proposed in this paper. Synchronous condensers are employed to provide dynamic reactive power compensation in sustaining DC voltage values at converter stations. Models are built for the optimal leading phase operation and adjusting excitation voltage reference value of synchronous condensers. Multiple HVDC links are coordinated to participate by using the DC power support factor to rank and select the links. Optimal DC power support values of the participating HVDC links are obtained with a comprehensive stability margin index that accounts for transient stability of the sending-end systems and frequency security of the receiving-end systems. An optimal load shedding model is used to ensure the frequency security of receiving-end systems. Case study results of a provincial power system in China demonstrate the effectiveness and performance of the proposed DC power support strategy.

System-Level Vulnerability Analysis for Commutation Failure Mitigation in Multi-infeed HVDC Systems

System-Level Vulnerability Analysis for Commutation Failure Mitigation in Multi-infeed HVDC Systems

A novel distance protection scheme for HVDC lines based on R-L model

HVDC transmission system is becoming increasingly desirable for its obvious technical and environmental advantages. A novel single-end distance protection scheme for HVDC lines is proposed in this paper. It is based on the linear distribution of low frequency voltage signals between the relay point and setting point. An internal fault can be detected if the low-pass filtered measure voltage is opposite in sign with the compensating voltage at the setting point, which can be calculated with the R-L differential-equation algorithm. To find a tradeoff between operating speed and accuracy, the protection is divided into K zones, each of which has different protection scopes and cut-off frequencies. A two-terminal HVDC system and a multi-infeed HVDC system are respectively built in EMTDC. Comprehensive simulation results have indicated that the proposed protection scheme has satisfactory operation performance and will be not influenced by system topologies or parameters. Besides, it is not sensitive to noises or voltage levels. Compared with many other single-end protections for HVDC lines, this novel distance protection has clear setting principles, lower sampling frequency, smaller calculation amount and faster operating speed.

A New Design of Wide-Area MIMO Coupling Robust HVDC Supplementary Controller

A new multi-input multi-output (MIMO) robust coordination approach for the controller design of multi-infeed high voltage direct current (HVDC) system is proposed. This design will enhance the damping of certain low frequency oscillation mode. The dominant mode ratio (DMR) is applied to choose the appropriate feedback signals, while time delay of wide-area signals is considered. The Total Least Squares-Estimation on Signal Parameters via Rotational Technique (TLS-ESPRIT) algorithm serves in identifying the MIMO system model. To reach the best control effect and disturbance suppression level, the mixed H2/H∞ robust theory with regional poles placement is introduced to design the controller. The new MIMO design approach avoids system decoupling in complex situation, and it uses the interactions of different control loops to enhance the system's stability when most MIMO HVDC controllers are designed by decoupling strategy. Simulation results of a multi-infeed HVDC system validate the control effect and robustness of the proposed method.

Brief analysis of Jiangsu grid security and stability based on multi-infeed DC index in power system

The impact of Multi-infeed HVDC has gradually increased to security and stability operating in Jiangsu power grid. In this paper, an appraisal method of Multi-infeed HVDC power grid security and stability is raised with Multi-Infeed Effective Short Circuit Ratio, Multi-Infeed Interaction Factor and Commutation Failure Immunity Index. These indices are adopted in security and stability simulating calculation of Jiangsu Multi-infeed HVDC system. The simulation results indicate that Jiangsu power grid is operating with a strong DC system. It has high level of power grid security and stability, and meet the safety running requirements. Jinpin-Suzhou DC system is located in the receiving end with huge capacity, which is easily leading to commutation failure of the transmission line. In order to resolve this problem, dynamic reactive power compensation can be applied in power grid near Jinpin-Suzhou DC system. Simulation result shows this method is feasible to commutation failure.

An Improved Measure of AC System Strength for Performance Analysis of Multi-Infeed HVdc Systems Including VSC and LCC Converters

A small signal model of a multi-infeed high-voltage direct current (HVdc) transmission system containing a line commutated converter (LCC) and a voltage source converter (VSC) is developed. This model represents the LCC and VSC converters as operational impedances as seen from the converter ac busbar. This permits the converters to be included in the effective short-circuit ratio (ESCR) calculations. The resulting ESCR is referred to in this paper as the “Impedance based Effective Short Circuit Ratio” (IESCR). It is shown that the maximum power transfer limit (referred to as the maximum available power or MAP) of the converters is better predicted by this index compared to the conventional ESCR which ignores the operational impedances of the converters. The question also arises as to how close to this theoretical maximum power transfer limit can the HVdc system operate. Using the small-signal model, it is shown that with commonly used control strategies, the predicted MAP can only be achieved by reducing the controller gains. The results are validated using detailed electromagnetic transients simulation of the multi-infeed VSC-LCC system.

Fast Evaluation of Commutation Failure Risk in Multi-Infeed HVDC Systems

This paper presents a fast and reliable method to evaluate risk of single or multiple commutation failures for multi-infeed HVDC systems following faults occurring at the receiving-end ac system. First, a fast calculation method of multi-infeed interaction factor, describing the interactivity and mutual impact between two inverter AC buses on is proposed. It is then refined to being the AC–DC system voltage interaction factor (ADVIF) to represent the relationship in bus voltages between inverter and AC buses at the re-ceiving-end systems. Then, a critical AC–DC system voltage interaction factor (CADVIF) based on the minimum extinction angle criteria is proposed. By calculating and comparing ADVIFs and CADVIFs, which can be obtained quickly, it is possible to identify AC buses that, if a fault occurs, would cause commutation failures at one or more HVDC systems. The validity and accuracy of the proposed approach are demonstrated by comparing with simulations results using a two-infeed HVDC test system and an actual large power grid.

Impact of renewable energy sources on steady-state stability of weak AC/DC systems

In recent years, the penetration of renewable resources into AC power systems has increased tremendously, creating a significantly impact on the latter's operations and stability. In this respect, it is also important to gain a basic analytical understanding of such impact on the steady-state stability of power systems with electrically weak AC/DC interconnections, but such works are not very evident in the literature. Therefore, a classical analytic model of the single and multi-infeed HVDC system which now incorporates renewable resources is proposed. Then the well-established concept of voltage sensitivity of the AC/DC interconnection is applied to analyze the impact of the renewable resources on the steady-state stability of these composite system models, as well as on the influence of system conditions and parameters. This impact is also compared with that arising from other types of shunt devices alternatively connected at the same AC/DC interconnection, therefore their relative beneficial or negative impacts will also be benchmarked.

Power stability analysis of UHVDC systems hierarchically connected to AC systems

With the development of ultra-high-voltage direct current (UHVDC) transmission systems, the receiving AC systems face the problems of power flow transferring, integrating capability and voltage stability. A novel UHVDC hierarchical connection (HC) mode has been proposed and adopted in several real projects in China. The more complex electric coupling relationship of the entire systems made the power interaction characteristics different from those of previous multi-infeed HVDC systems. Thus, it is important to analyze the power stability of such systems in a systematic manner to avoid adverse interactions. In this study, the equivalent model of HC-UHVDC systems is established and several evaluation indices concerning power stability are discussed and improved. Furthermore, the power integration capability and reactive power interaction are evaluated based on real data. The results of this research can be used to provide technical support to real HC-UHVDC projects.

Simulative Study into the Development of a Hybrid HVDC System Through a Comparative Research with HVAC: a Futuristic Approach

High Voltage Direct Current Transmission (HVDC) is considered a better solution for bulk long distance transmissions. The increased use of HVDC is a result of its advantages over the HVAC systems and especially of its fault stability nature. A better solution is proposed by using a Voltage Source Controlled–HVDC as one of the infeed for the Multi-Infeed HVDC (MIDC or MI-HVDC) systems. The main advantage with the VSC converter is its flexible power control which enhances the stability of the MIDC systems. In this paper, the behavior of an HVDC system is compared with that of an HVAC during faults. A Hybrid HVDC system that includes a LCC as a rectifier unit and a VSC converter as the inverter is being proposed. It is considered suitable for MIDC systems and particularly for supplying a weak AC system. The performance of the system during steady state and transient conditions for all the proposed topologies including HVDC, HVAC and Hybrid HVDC are studied in MATLAB/SIMULINK. All of the proposed control strategies are evaluated via a series of simulation case studies.

Rapid generation of control parameters of Multi-Infeed system through online simulation

Simulated Self-Generated - Particle Swarm optimization (SSG-PSO) toolbox that automatically generates PI control parameters very quickly in PSCAD is designed. This toolbox operates by utilizing transient simulation to evaluate objective function and converges the fitness values of objective function through PSO algorithm during run time simulation of Multi-infeed HVDC systems. Integral Square Error-Objective Function (ISE-OF) is used to accomplish the task. To make the toolbox faster, ranges are set for PSO generated value that limit the time of data acquisition for the objective function by only considering transition time of a system. This toolbox has a capability to optimize multiple controllers at same time. The PI values are generated faster and the results are showing markedly improved performance of a system during startup and under fault condition. The experimental results are presented in this paper.

Stability Analysis of a Hybrid Multi-Infeed HVdc System Connected Between Two Offshore Wind Farms and Two Power Grids

This paper presents the stability simulation results of two equivalent aggregated offshore wind farms (OWFs) based on doubly-fed induction generator fed to two power grids through a hybrid multi-infeed high voltage dc (HVdc) system. The hybrid multi-infeed HVdc system consists of a line-commutated converter (LCC)-based HVdc link and a voltage-source converter-based HVdc link. A static synchronous compensator (STATCOM) is connected at the rectifier station of the LCC-based HVdc link. A unified approach using a pole-assignment approach based on a modal control theory is employed to design the damping controllers for the two HVdc links, respectively. Steady-state results under different wind speeds of the two OWFs are carried out. Comparative dynamic responses of the studied system with and without the designed damping controllers for the two HVdc links under wind-speed changes are also achieved. Comparative transient simulations of the studied system with and without the STATCOM subject to a three-phase fault are also performed.

A Study on an Operation Strategy of Dual-Infeed HVDC System

This paper deals with the operation strategy of reactive power in a multi-infeed HVDC (MIHVDC) system, in which several converters are connected to the same or nearby separate AC buses. The potential problems concerning a MIHVDC system when feeding a weak AC network are as follows: the need for coordination of the recovery control, the possibility of voltage instability or low quality of the area connected to the MIHVDC system, and the risk of mutual commutation failures. These problems in MIHVDC systems are similar to those in single-infeed HVDC (SIHVDC) systems, but the differences with the phenomenon of the SIHVDC system are the interactions between converters. The main reason for the potential problems of HVDC systems (MIHVDC or SIHVDC) is voltage variation; therefore, to mitigate the voltage variations, the performances of the HVDC system should be enhanced. Consequently, to mitigate the potential problems of MIHVDC systems, several solutions are suggested in this study, including installing STATCOM and installing a line arrester on the tower. The study results will be applied to a multi-infeed HVDC system in Korea.

Optimal DC-Segmentation for Multi-Infeed HVDC Systems Based on Stability Performance

This paper analyzes the optimal problems of dc-segmentation for multi-infeed HVDC systems based on stability performance and studies the selecting method for the optimal dc-segmentation scheme. First, three major stability problems of multi-infeed HVDC systems are pointed out, which implies the necessity of the optimization for dc-segmentation. Second, three typical evaluation indices which correspond to the above three stability problems are introduced: multi-infeed interaction factor, multi-infeed effective short-circuit ratio, and frequency bias factor. To improve evaluation effects of the typical indices, the subindices are presented. Third, in order to solve this multi-objective decision-making problem, a linear weighted sum model is built, and a comprehensive weight method is adopted to calculate the weight coefficients. Based on the stability problems, a selecting method is established to choose the optimal dc-segmentation scheme. Finally, a four-machine multi-infeed HVDC system and a practical multi-infeed HVDC system in China are taken as examples, respectively. Candidate dc-segmentation schemes of each system are evaluated by the proposed method, and the optimal one is selected. Detailed stability studies of the candidate schemes are executed. The result indicates that the stability of the optimal scheme is better than the stabilities of others, which verifies the validity of the proposed method.

Commutation failure and voltage sensitivity analysis in a hybrid multi-infeed HVDC system containing modular multilevel converter

Summary A typical hybrid dual-infeed high-voltage direct current (HVDC) system includes two types of links—line-commutated converter (LCC) HVDC and voltage-source converter (VSC) HVDC in proximity. When faults occur in the LCC-HVDC side, the VSC-HVDC link sufficiently can compensate power to the LCC-HVDC system; thus, voltage fluctuation is suppressed. In recent years, the modular multilevel converter (MMC) has been a promising type of VSC. This paper presents a novel hybrid dual-infeed HVDC system consisting of an LCC-HVDC link and an MMC-HVDC link, and the two links are interconnected through a tie line between buses in the inverter side. The positive contribution of the MMC-HVDC link to the LCC-HVDC link in the area of commutation failure immunity index, effective short circuit ratio, voltage sensitivity factor, temporary overvoltage, and DC line fault recovery are investigated. Investigations show that the stability of system has been enhanced significantly with MMC, and the contributors are physical structure change and the control strategy of the MMC-HVDC system. The modelling and simulation of the steady state and transient performance of hybrid HVDC system are conducted in PSCAD/EMTDC. Finally, the simulation results verify the enhancement in sensitivity and stability of the dual-infeed HVDC system. Copyright © 2016 John Wiley & Sons, Ltd.

Efficient approach to quantify commutation failure immunity levels in multi‐infeed HVDC systems

Commutation failure immunity index (CFII) is a useful indicator to quantify the immunity level to commutation failures in line commutated converter-based multi-infeed HVDC systems. Instead of using simulation tools which inevitably lead to heavy workload and long computation time, the analytical expressions of ‘local CFII’ (LCFII) and ‘concurrent CFII’ (CCFII) are derived based on the minimum extinction angle criteria. Using these expressions, the impact of the system parameters on commutation failure immunity levels in the inverter side can be determined quickly and effectively. Moreover, the derived analytical expressions will make the parametric sensitivity analysis more comprehensive and complete compared to the traditional simulation-based methods. Furthermore, in order to give an insight into how inter-inverter interactions influence the concurrent commutation failure behaviours, an improved index ‘weak coupling multi-infeed interaction factor’ (WCMIIF) is proposed. The validity and accuracy of the proposed index and analytical expressions are verified by the simulated results based on a dual-infeed HVDC system in PSCAD/EMTDC, which indicate that LCFII has an approximately linear correlation with the local effective short-circuit ratio (ESCR) of the faulted ac system, and that with the WCMIIF unchanged, CCFII has an approximately linear correlation with the local ESCR and an inverse correlation with the multi-infeed interaction factor.

An Equivalent Single-Infeed Model of Multi-Infeed HVDC Systems for Voltage and Power Stability Analysis

In this paper, an equivalent single-infeed model of a multi-infeed HVDC system is derived to lend theoretical basis for defining an index called equivalent effective short-circuit ratio (EESCR), which can resolve the ambiguity when applying the effective short-circuit ratio (ESCR) concept for the single-infeed to multi-infeed HVDC systems. Moreover, a Critical EESCR is derived analytically from the equivalent model as stability boundary of the multi-infeed HVDC system. The EESCR is also compared with other indices proposed recently to clarify their empirical or heuristic assumptions. The CEESCR stability boundary for a practical multi-infeed configuration is derived and verified by time-domain simulations to demonstrate the validity of the theoretical developments.