Line-commutated Converter-based High-voltage Direct-current System Research Articles

Impact of Time Delay on the Control Link in Small Signal Dynamics of LCC-HVDC System

The time delay in the control link will affect the small signal stability of the line commutated converter based high voltage direct current (LCC-HVDC) system. This paper establishes the small signal model for the LCC-HVDC system considering the time delay in the control link. Then, based on the developed models, the movement feature of the eigenvalue loci with varying time delay under different AC system strength is investigated, the impact of the time delay on the system stability margin is quantitatively evaluated, and the frequency band affected by the time delay is identified. Subsequently, influence mechanisms of time delay on system stability are revealed from the perspective of the system damping characteristic by decomposing different damping paths. Moreover, impacts of time delay on DC current/voltage controller's stability margin are investigated. Theoretical analysis and simulation results indicate that, the increase of time delay at the rectifier-side will continuously deteriorate the system stability, as well as the system dynamic performances, while in most cases the increase of the time delay at the inverter-side imposes a non-monotonic effect on the system stability, which is weakened first and then slightly strengthened. Therefore, in certain scenarios, the larger time delay at the inverter-side may play a positive role in the system stability to some extent. The results can provide certain technical reference for the identification of key influencing factors causing the system oscillation and reasonable selection of control parameters in practical LCC-HVDC projects.

Novel Controllable-Line-Commutated Converter for Eliminating Commutation Failures of LCC-HVDC System

To entirely eliminate commutation failure (CF) in conventional line-commutated converter based high-voltage direct current (LCC-HVDC) transmission system, a novel controllable line-commutated converter (CLCC) based on the combination of partially and fully controllable semiconductor devices is presented in this paper. The operating principle of CLCC and its control strategy are proposed. In addition, the voltage and current stress are analyzed. To verify correction and application of presented CLCC solution to the project, a CLCC-HVDC system model, adopting LCC at the rectifier side and CLCC at the inverter side, is developed in PSCAD/EMTDC software. Based on the model, the transient responses of the CLCC-HVDC system under various AC faults are simulated and compared with the pure LCC-HVDC. Simulation results show that the presented CLCC topology and its control strategy can not only eliminate CFs in various types of AC faults, but also improve the transient performance of the system after faults, while inheriting the attributes of traditional LCC such as large capacity, low cost and almost the same system control strategy. This is quite positive and meaningful for application of the CLCC-HVDC to multi-DC infeed system particularly.

A novel partitioning method for the power grid restoration considering the support of multiple LCC-HVDC systems

The restoration of the modern alternating current-direct current (AC–DC) hybrid power grid after a blackout is difficult and complex. By using the partitioning restoration strategy and the support of high voltage direct current (HVDC) system, the restoration speed of the blackout power grid can be greatly accelerated. Aiming at this problem, this paper proposes a partitioning method for the power grid restoration with multiple line commutated converter-based HVDC (LCC-HVDC) systems. Firstly, by introducing the system strength indices to quantify the interaction between AC and LCC-HVDC systems in the restoration process, a partitioning model considering multiple LCC-HVDCs is proposed. Wherein, the transmission power is optimized to make full use of LCC-HVDCs’ support capability. Then, by combining the system strength characteristic and community structure characteristic of the power grid, a partitioning method based on an improved Girvan Newman (GN) algorithm is proposed. Finally, the results are studied based on a modified IEEE 39-bus system and a realistic partial power grid of southwest China. Compared with two typical partitioning methods for the AC power grid restoration, the results show that the proposed method can effectively use the DC support power and speed up the restoration progress.

Commutation failure suppression method considering chain reaction in multi-infeed LCC-HVDC systems

In multi-infeed HVDC (MIDC) systems, in which HVDC means high-voltage direct current, the complex interactions between AC/DC systems and different line-commutated converter-based high-voltage direct current (LCC-HVDC) produces a special commutation failure (CF) chain reaction phenomenon. The existing methods are mostly aimed at the transient process of the single LCC-HVDC, which reduces the CF suppression effect but may become the driving force of the CF chain reaction. In view of this situation, this study proposes a new CF suppression method based on the CF chain reaction security region. First, the CF chain reaction process of multiple LCC-HVDC systems is analyzed, and the boundary of the CF chain reaction is characterized. Second, by establishing the CF chain reaction security region, the control method of the coordinating DC current and firing advance angle to suppress the CF chain reaction is proposed. Simulation results show that the proposed method avoids the subsequent CF of the LCC-HVDC while maximizing the suppression effect of the successive CF in the adjacent LCC-HVDC, thereby blocking the cascading reaction of CF that usually caused large-scale security problems.

Fast and accurate evaluation method of commutation failure in line‐commutated converter‐based high‐voltage direct current systems considering probability characteristics

Line-commutated converter-based high-voltage direct current (LCC-HVDC) systems suffer from commutation failure (CF) when receiving-end AC grid faults occur, which is a major drawback and threatens the security and stability of power systems. This paper presents a fast and accurate evaluation method of CF for the LCC-HVDC systems, considering the CF probability characteristics. Firstly, based on the analysis of the CF probability characteristics from the view of short-circuit impedance, the CF tolerance index (CFTI) and CF vulnerability index (CFVI) are proposed to quantitatively evaluate the LCC-HVDC systems' risk of the possible- and inevitable-occurrence CF, respectively. Secondly, four factors, that is, commutation voltage, fault occurrence time, DC current and firing angle command, are considered to calculate CF evaluation indices (CFEIs) and identify the CF risk area boundaries based on the theoretical analysis. Moreover, the relationships among short-circuit impedance, fault position, and extinction angle are established based on the AC/DC decoupling method, which helps realize the decoupling of multi-factors affecting CF during fault. Compared to earlier methods, this evaluation method combines the advantages of the accuracy of the electromagnetic transient simulation method and the rapidity of theoretical derivation. Finally, the validity and accuracy of the evaluation method are verified in the CIGRE HVDC benchmark model and IEEE 39-bus system.

A Calculation Method of DC Current Reference Value for Suppressing Commutation Failure in LCC-HVDC System

Line commutated converter based high voltage direct current (LCC-HVDC) transmission system is prone to suffer commutation failure, which can lead to the increase of DC current and the interruption of DC transmission power. Furthermore, the increase of DC current might lead to subsequent commutation failure. Based on the reactive power balance of the inverter AC bus, this paper studies the functional relationship between DC current and commutation voltage. The DC current reference value is obtained by using the DC current acquired from the above functional relationship, and combining with the DC current order given by voltage dependent current order limiter (VDCOL). This calculation method can adjust the DC current reference value in time according to the commutation voltage, thus limiting the increase of DC current. In order to verify the commutation failure suppression ability of the proposed method, several simulations are conducted based on CIGRE benchmark HVDC model in PSCAD/EMTDC. Simulation results show that the proposed method can not only suppress commutation failure successfully, but also improve the fault recovery performance of the LCC-HVDC system.

Prediction method of successive commutation failure for multi-infeed high voltage direct current systems under fault of weak receiving-end grid

The complex interactions between AC/DC systems and different line-commutated converter-based high-voltage direct current (LCC-HVDC) expand the effect of commutation failure (CF). The rapid prediction of CF is crucial in ensuring the stable operation of such systems. The simultaneous CF caused by grid fault has received widespread attention in recent years. However, the coupling between the LCC-HVDC with CF and the adjacent normal LCC-HVDC is not considered. When the state of the main circuit suddenly changes and the control system subsequently starts, the LCC-HVDC with CF inevitably produces an effect on the adjacent LCC-HVDC through the coupling of grid voltage. This study describes a new phenomenon, where the former produces a successive CF in the latter under the fault of weak receiving-end grid. The dynamic reactive power characteristics of inverter station and the induced mechanism of successive CF are analyzed. And the dynamic reactive power model of LCC-HVDC is established. A fast prediction method for successive CF is then proposed by calculating the extinction angle of LCC-HVDC with consideration of the effect of adjacent LCC-HVDC with CF. The proposed method can improve the accuracy of CF prediction in multi-infeed LCC-HVDC systems through theoretical derivation and simulation verification.

Power Stability Analysis and Evaluation Criteria of Dual-Infeed HVDC with LCC-HVDC and VSC-HVDC

This paper analyzes the power stability of the hybrid dual-infeed high-voltage direct-current (HVDC) system containing a line commutated converter-based HVDC (LCC-HVDC) and a voltage source converter-based HVDC (VSC-HVDC). First, the concept and the calculation method of power stability for the hybrid dual-infeed HVDC system are introduced. Second, the influence of VSC-HVDC on the power stability of the system is investigated. Third, the relationship between the power stability and the effective short circuit ratio (ESCR) is discussed under different system parameters. Then, the value range of the critical effective short circuit ratio is determined. Finally, the evaluation criteria of power stability are proposed. The results show that the evaluation criteria of the single-infeed LCC-HVDC system can still be used, if the VSC-HVDC is in constant AC voltage control mode; if the VSC-HVDC is in constant reactive power control mode, the hybrid dual-infeed HVDC system cannot operate stably when the ESCR is less than 2.0 and can operate stably with high power stability margin when the ESCR is greater than 3.0. The ESCR index can still be used to measure the power stability of the hybrid dual-infeed HVDC system.

Analysis of the Operating Margin Evaluation of Multi-Infeed LCC-HVDC Systems Based on the Equivalent Impedance

In China, a structure in which multiple line-commutated converter based high-voltage direct current (LCC-HVDC) systems feed into the same AC system has been formed. For the multiple LCC-HVDC infeed system, the electrical strength and real operating condition evaluation of the receiving AC system are of great significance for system operation. The multi-infeed short circuit ratio (MISCR) is generally utilized to assess the strength based on the rated parameters; hence, it cannot analyze the real operating condition under the non-rated status. In this paper, an LCC-HVDC system is equivalent to a grounded impedance without changing the external characteristics. The multi-infeed operating short circuit ratio (MIOSCR) is then deduced from the equivalent impedance and equivalent nodal analysis. Relationships between the MIOSCR and electrical strength indices of AC and AC/DC systems are analyzed, and the MISCR is deduced as a special case of the MIOSCR under the rated status. The multi-infeed operating margin coefficient (MIOMC) is finally proposed, which can be utilized to evaluate the operating margin of the LCC-HVDC system and provide guidance for designing operating strategies. The proposed MIOSCR and MIOMC overcome the previous shortcomings of inaccuracy about operating status assessment by the MISCR. The results of the case study demonstrate the effectiveness of the MIOSCR and the MIOMC for assessing the operating condition and margin of converter buses.

Influence of Inverter Controller Parameters on the Small-signal Stability of LCC-HVDC System Based on a Practical Project

Unreasonable control parameters of the Line Commutated Converter based High Voltage Direct Current (LCC-HVDC) system may induce small-signal instability. This paper studies the impact of inverter controller parameters on steady response of the LCC-HVDC system under weak AC grid condition. Firstly, according to a practical project, the small-signal model of LCC-HVDC system is established based on the switching function. Then, the eigen-analysis method is adopted to study the impact of the controller parameters on the inverter side on the oscillation mode and damping characteristics of the LCC system, and the correctness of results is verified by PSCAD/EMTDC simulation. The conclusion shows that reasonable controller parameters can improve the stability margin of the system.

Optimal DC Voltage and Current Control of an LCC HVDC System to Improve Real-Time Frequency Regulation in Rectifier- and Inverter-Side Grids

High-voltage direct-current (HVDC) systems for constant or intermittent power delivery have recently been developed further to support grid frequency regulation (GFR). This paper proposes a new control strategy for a line-commutated converter-based (LCC) HVDC system, wherein the dc-link voltage and current are optimally regulated to improve real-time GFR in both rectifier- and inverter-side grids. A dynamic model of an LCC HVDC system is developed using the dc voltage and current as input variables, and is integrated with feedback loops for inertia emulation and droop control. A linear quadratic Gaussian (LQG) controller is also designed for optimal secondary frequency control, while mitigating conflict between the droop controllers of HVDC converters. An eigenvalue analysis is then conducted, focusing on the effects of model parameters and controller gains on the proposed strategy. Simulation case studies are performed using the models of a real HVDC system and a CIGRE benchmark system. The case study results confirm that the proposed strategy enables the HVDC system to improve GFR, in coordination with generators in both-side grids, by exploiting the fast dynamics of HVDC converters. The proposed strategy is also effective under various conditions for the LQG parameters, inertia emulation, and droop control.

A Novel Phase-Locked Loop for Mitigating the Subsequent Commutation Failures of LCC-HVDC Systems

The AC fault ride-through and support capability of line commutated converter based high voltage direct current (LCC-HVDC) have significant influence on the safe operation of power system. The ability of phase locked loop (PLL) to synchronize with the voltage phase under AC faults would affect the recovery performance of LCC-HVDC systems. This paper analyzes the influence of the conventional PLL on the subsequent commutation failure (SCF) of LCC-HVDC systems and points out that the conventional PLL has some problems such as slow dynamic response, significant frequency fluctuation, and coupling of phase and frequency detection under fault conditions. A novel PLL is proposed to address the problems. Cascade delayed signal cancellation operators are combined with a mathematical filter, forming a hybrid pre-filter to extract fundamental positive sequence voltage for the PLL. To reduce the fluctuation of PLL's frequency during AC faults, the estimated frequency is fixed when the fault is detected by a fault detector. As a consequence, the decoupling between phase detection and frequency detection is realized, and the proposed PLL's dynamic response performance is enhanced. Simulation results show that the proposed PLL can provide accurate phase reference for the DC control system and reduce the probability of SCFs of LCC-HVDC system.

A Coordinated Control Strategy for LCC HVDC Systems for Frequency Support with Suppression of AC Voltage Fluctuations

In this paper, a new control method for a line-commutated converter-based (LCC) high-voltage direct current (HVDC) system to support the frequency and AC voltage was proposed. For coordinated control of the inverter and rectifier, droop-based controllers are adopted for the outer loop of conventional controllers, and DC voltage is used to transfer the frequency information so that communication is not required. A method to determine the droop constants of the proposed controller is presented to emulate the conventional $f$ / $P$ droop characteristic and suppress the AC voltage fluctuations. Furthermore, a small-signal state-space model of the LCC HVDC system with the proposed scheme is derived, and root locus analysis is carried out to evaluate its stability. The effects of the proposed method are verified based on case studies, using the well-known CIGRE BENCHMARK model. The advantages of the proposed method can be summarized as follows: 1) system reliability is enhanced because there is no risk of communication failure during frequency information transfer and 2) voltage stability at the inverter side is enhanced as AC voltage fluctuation during frequency support is minimized.

A supplementary control for mitigation of successive commutation failures considering the influence of PLL dynamics in LCC-HVDC systems

Line commutated converter based high voltage direct current (LCC-HVDC) links are widely employed for long distance bulk power transmission and asynchronous alternating current (AC) grid connection. However, LCC-HVDC systems often suffer from commutation failures when AC voltage is distorted, oscillating or reduced by AC faults, which leads to overheating of converter valves and interruptions in transmitted power. Seriously, it would have adverse impact on the safety and stability of the entire power system. This paper proposes a supplementary control for mitigation of successive commutation failures on the basis of analyzing the influence of phase-locked loop (PLL) dynamics on the commutation process. By analyzing the impact of PLL dynamics on the actual leading angle, it is found that changes in the AC voltage phase remarkably influence commutation. Accordingly, the error between AC voltage phase and PLL's output angle is added to the output of the extinction angle or DC voltage control to mitigate the successive commutation failures of LCC-HVDC. Simulations conducted on the CIGRE benchmark model in PSCAD/EMTDC validate the performance of the supplementary control, which effectively mitigates successive commutation failures.

Enhancing the HVRT capability of DFIG-based wind farms using cooperative rotor-side SMES considering blocking fault of LCC-HVDC system

Line-commutated converter based high voltage direct current (LCC-HVDC) has been applied for transferring bulk power from wind farms to load centers through long distances in many countries. When the blocking fault of the LCC-HVDC system occurs, the surplus reactive power accumulated at the sending end will lead to an overvoltage, thus causing the disconnection of the wind turbines. To maintain the reliable connection of wind turbines and ensure the stability of the power system, this paper introduces a superconducting magnetic energy storage (SMES) unit to connect in parallel with the rotor side of the doubly fed induction generator (DFIG). By controlling the energy side converter and rotor side converter to inject demagnetizing current and reactive current into the rotor, the proposed scheme can effectively stabilize key parameters of the wind turbine and provide desirable reactive power support, showing a favorable high voltage ride through (HVRT) performance. Several cases based on PSCAD/EMTDC & MATLAB/Simulink co-simulation together with economic analysis are conducted to demonstrate the feasibility and superiority of the proposed scheme on enhancing HVRT capability of DFIG-based wind farms.

A DC Chopper Topology to Mitigate Commutation Failure of Line Commutated Converter Based High Voltage Direct Current Transmission

To reduce the probability of commutation failure (CF) of a line commutated converter based high-voltage direct current (LCC-HVDC) transmission, a DC chopper topology composed of power consumption sub-modules based on thyristor full-bridge module (TFB-PCSM) is proposed. Firstly, the mechanism of the proposed topology to mitigate CF is analyzed, and the working modes of TFB-PCSM in different operation states are introduced. Secondly, the coordinated control strategy between the proposed DC chopper and LCC-HVDC is designed, and the voltage-current stresses of the TFB-PCSMs are investigated. Finally, the ability to mitigate the CF issues and the fault recovery performance of LCC-HVDC system are studied in PSCAD/EMTDC. The results show that the probability of CF of LCC-HVDC is significantly reduced, and the performances of fault recovery are effectively improved by the proposed DC chopper.

Motion Equation Modeling of LCC-HVDC Stations for Analyzing DC and AC Network Interactions

Line-commutated converter based high voltage direct current (LCC-HVDC) transmissions are widely applied in delivering large-capacity electricity in China. Multiple large-capacity LCC-HVDC transmissions strengthen dynamic coupling between AC and DC networks. Existing literature has reported some stability problems caused by dynamic interactions between AC and DC power grids, however, the mechanism behind these problems has not been totally understood and revealed. To analyze dynamic interactions between AC and DC power grids, this article presents a small-signal model to understand the external characteristics of LCC-HVDC stations based on the motion equation concept. Comparison of time-domain simulation response with the detailed switching model shows that the proposed model can hold main behaviors of concern. Dynamic interactions between AC and DC power grids are studied based on the proposed model in multi-infeed LCC-HVDC systems. Analytical results demonstrate that there exists power dynamics in receiving AC grid when sending AC grid subjects to a disturbance, and effects of converter stations’ controller parameters on power dynamics are explained by the physical relationship among state variables of the proposed model.

A Quantitative Index to Evaluate the Commutation Failure Probability of LCC-HVDC with a Synchronous Condenser

Since thyristor cannot turn off automatically, line commutated converter based high voltage direct current (LCC-HVDC) will inevitably fail to commutate and therefore auxiliary controls or voltage control devices are needed to improve the commutation failure immunity of the LCC-HVDC system. The voltage control device, a synchronous condenser (SC), can effectively suppress the commutation failure of the LCC-HVDC system. However, there is a need for a proper evaluation index that can quantitatively assess the ability of the LCC-HVDC system to resist the occurrence of commutation failures. At present, the main quantitative evaluation indicators include the commutation failure immunity index and the commutation failure probability index. Although they can reflect the resistance of the LCC-HVDC system to commutation failures to a certain extent, they are all based on specific working conditions and cannot comprehensively evaluate the impact of SCs on suppressing the commutation failure of the LCC-HVDC system under certain fault ranges. In order to more comprehensively and quantitatively evaluate the influence of SCs on the commutation failure susceptibility of the LCC-HVDC system under certain fault ranges, this paper proposes the area ratio of commutation failure probability. The accuracy of this new index was verified through the PSCAD/EMTDC. Based on the CIGRE benchmark model, the effects of different synchronous condensers on LCC-HVDC commutation failure were analyzed. The results showed that the new index could effectively and more precisely evaluate the effect of SCs on commutation failures. Moreover, the proposed index could provide a theoretical basis for the capacity allocation of SCs in practical projects and it could also be utilized for evaluating the impact of other dynamic reactive power compensators on the commutation failure probability of the LCC-HVDC system under certain fault ranges.

Comparative Study on Small-Signal Stability of LCC-HVDC System With Different Control Strategies at the Inverter Station

The constant extinction angle control and constant DC voltage control are widely used for the inverter station of Line Commutated Converter based High Voltage Direct Current (LCC-HVDC) system in practical engineering. Under different control strategies, the control system parameters would have totally different effects on the stability of the LCC-HVDC system. In this paper, the small-signal models of the LCC-HVDC system with constant extinction angle control and LCC-HVDC system with constant DC voltage control are developed, respectively. Based on the eigenvalue analysis, this paper conducts a comprehensive comparison of the effects of a phase-locked loop (PLL) and LCC controller on the small-signal stability of LCC-HVDC system with different control strategies at the inverter station under weak AC grid condition. Then, the coupling effect between the PLL and LCC controller parameters is investigated when LCC-HVDC system adopts different control strategies. This paper can provide useful guidance for the systematic design and control parameter selection of LCC-HVDC projects.

Small-signal instability and supplementary coordinated damping-control of LCC-HVDC system with STATCOM under weak AC grid conditions

Static Synchronous Compensator (STATCOM) has been utilized to improve the dynamic behaviors of Line-Commutated-Converter based High Voltage Direct Current (LCC-HVDC) system under weak AC grid conditions. However, the close coupling between STATCOM and LCC system could result in system instability especially under weak AC grid conditions, which has not been addressed in the existing literatures. This paper developed a linearized small-signal dynamic model of a LCC system with STATCOM at the inverter busbar, and validated its accuracy by comparing time-domain responses from small-signal model and simulation results from PSCAD/EMTDC. Based on the small-signal model, this paper investigated the impact of STATCOM on the small-signal stability margin of LCC-HVDC by eigen-analysis and participation factor methods. The results show that under weak AC grid conditions, the close interaction between the control system of STATCOM (especially the AC voltage controller) and PLL of LCC system, deteriorates the system stability and may cause oscillation/instability. Then, this paper proposed a novel supplementary coordinated damping-control (SCDC), for LCC-HVDC system with STATCOM under weak AC grid conditions. The results based on the eigen-analysis and electromagnetic transient (EMT) simulations indicate that the proposed SCDC approach can effectively mitigate the oscillations/instability induced by the controller interaction of STATCOM and LCC system, thus effectively improve the stability margin of the overall system.