Studies of commutation failures in hybrid LCC/MMC HVDC systems

A hybrid of line commutated converters(LCCs)and modular multi-level converters(MMCs)can provide the advantages of both the technologies.However,the commutation failure still exists if the LCC operates as an inverter in a hybrid LCC/MMC system.In this paper,the system behavior during a commutation failure is investigated.Both halfbridge and full-bridge MMCs are considered.Control strategies are examined through simulations conducted in PSCAD/EMTDC.Additionally,commutation failure protection strategies for multi-terminal hybrid LCC/MMC systems with AC and DC circuit breakers are studied.This paper can contribute to the protection design of future hybrid LCC/MMC systems against commutation failures.

Calculation Method for Commutation Failure Fault Level in LCC-HVDC System Under Single-line-to-ground Faults Considering DC Current Variation

Earlier studies have reported some calculation methods for commutation failure fault level(CFFL) in line-commutated-converter based high-voltage direct current(LCCHVDC) system under single-line-to-ground(SLG) faults. The accuracy of earlier methods is limited because they only consider the commutating voltage drop and phase shift, while neglecting the DC current variation. Hence, this paper proposes a CFFL calculation method under SLG faults considering DC current variation, for better planning and designing of LCC-HVDC systems. First, the fault commutating voltage magnitude and phase shift are calculated. Then, the fault DC voltage during different commutation processes is deduced. Based on the commutating voltage magnitude and phase shift, and DC voltage during different commutation processes under SLG faults, the characteristics of CFFL with different fault time are demonstrated and analyzed. Next, the transient time-domain response of the DC current after the fault is obtained based on the DC transmission line model. Discrete commutation processes are constructed based on the commutation voltage-time area rule to solve the extinction angle under different fault levels and fault time. Finally, the CFFL is calculated considering the fault time, commutating voltage drop, phase shift, and DC current variation. The accuracy of the proposed method compared with the traditional method is validated based on the CIGRE benchmark model in PSCAD/EMTDC.

An improved prediction method of subsequent commutation failure of an LCC-HVDC considering sequential control response

Subsequent commutation failure(SCF)can be easily generated during the first commutation failure(CF)recovery process in a line-commutated converter-based high voltage direct-current system.SCF poses a significant threat to the safe and stable operation of power systems,and accurate prediction of CF is thus important.However,SCF is affected by the operating characteristics of the main circuit and the coupling effects of sequential control response in the inverter station.These are difficult to predict accurately.In this paper,a new SCF prediction method considering the control response is proposed based on the physical principle of SCF.The time sequence and switching conditions of the controllers at different stages of the first CF recovery process are described,and the corresponding equations of commutation voltage affected by different controllers are derived.The calculation method of the SCF threshold voltage is proposed,and the prediction method is established.Simulations show that the proposed method can predict SCF accurately and provide useful tools to suppress SCF.

A Continuous Fault Ride-through Scheme for DFIGs Under Commutation Failures in LCC-HVDC Transmission Systems

Experimental and theoretical studies have confirmed that,relative to a one-shot voltage fault,a doubly-fed induction generator(DFIG)will suffer a greater transient impact during continuous voltage faults.This paper presents the design and application of an effective scheme for DFIGs when a commutation failure(CF)occurs in a line-commutated converter based high-voltage direct current(LCC-HVDC)transmission system.First,transient demagnetization control without filters is proposed to offset the electromotive force(EMF)induced by the natural flux and other low-frequency flux components.Then,a rotor-side integrated impedance circuit is designed to limit the rotor overcurrent to ensure that the rotor-side converter(RSC)is controllable.Furthermore,coordinated control of the demagnetization and segmented reactive currents is implemented in the RSC.Comparative studies have shown that the proposed scheme can limit rotor fault currents and effectively improve the continuous fault ride-through capability of DFIGs.

Improved Identification Method and Fault Current Limiting Strategy for Commutation Failure in LCC-HVDC

Line-commutated converter based high-voltage direct-current(LCC-HVDC)transmission systems are prone to subsequent commutation failure(SCF),which consequently leads to the forced blocking of HVDC links,affecting the operation of the power system.An accurate commutation failure(CF)identification is fairly vital to the prevention of SCF.However,the existing CF identification methods cause CF misjudge or detection lag,which can limit the effect of SCF mitigation strategy.In addition,earlier approaches to suppress SCF do not clarify the key factor that determines the evolution of extinction angle during system recovery and neglect the influence.Hence,this paper firstly analyzes the normal commutation process and CF feature based on the evolution topology of converter valve conduction in detail.Secondly,the energy in the leakage inductance of converter transformer is presented to characterize the commutation state of the valves.Then a CF identification method is proposed utilizing the leakage inductance energy.Thirdly,taking the key variable which is crucial to the tendency of extinction angle during the recovery process into account,a fault current limiting strategy for SCF mitigation is put forward.Compared with the original methods,the proposed methods have a better performance in CF identification and mitigation in terms of detection accuracy and mitigation effect.Finally,case study on PSCAD/EMTDC validates the proposed methods.

Commutation Failure Mitigation Method Based on Imaginary Commutation Process

The commutation failure(CF) mitigation effectiveness is normally restricted by the delay of extinction angle(EA)measurement or the errors of existing prediction methods for EA or firing angle(FA). For this purpose, this paper proposes a CF mitigation method based on the imaginary commutation process. For each sample point, an imaginary commutation process is constructed to simulate the actual commutation process.Then, the imaginary EA is calculated by comparing the imaginary supply voltage-time area and the imaginary demand voltage-time area, which can update the imaginary EA earlier than the measured EA. In addition, the proposed method considers the impacts of commutation voltage variation, DC current variation, and phase angle shift of commutation voltage on the commutation process, which can ensure a more accurate EA calculation. Moreover, the DC current prediction is proposed to improve the CF mitigation performance under the single-phase AC faults. Finally, the simulation results based on CIGRE model prove that the proposed method has a good performance in CF mitigation.

Analysis on Local and Concurrent Commutation Failure of Multi-infeed HVDC Considering Inter-converter Interaction

This paper provides a comprehensive analysis of local and concurrent commutation failure(CF)of multi-infeed high-voltage direct current(HVDC)system considering multi-infeed interaction factor(MIIF).The literature indicates that the local CF is not influenced by MIIF,whereas this paper concludes that both the local CF and concurrent CF are influenced by MIIF.The ability of remote converter to work under reduced reactive power enables its feature to support local converter via inter-connection link.The MIIF measures the strength of electrical connectivity between converters.Higher MIIF gives a clearer path to remote converter to support local converter,but at the same time,it provides an easy path to local converter to disturb remote converter under local fault.The presence of nearby converter increases the local commutation failure immunity index(CFII)while reducing concurrent CFII.Higher MIIF causes reactive power support to flow from remote converter to local converter,which reduces the chances of CF.A mathematical approximation to calculate the increase in local CFII for multi-infeed HVDC configurations is also proposed.A power flow approach is used to model the relation between MIIF and reactive power support from remote end.The local and concurrent CFIIs are found to be inverse to each other over MIIF;therefore,it is recommended that there is an optimal value of MIIF for all converters in close electric proximity to maintain CFII at a certain level.The numerical results of established model are compared with PSCAD/EMTDC simulations.The simulation results show the details of the influence of MIIF on local CF and concurrent CF of multi-infeed HVDC,which validates the analysis presented.

A Commutation Failure Prediction and Mitigation Method

The mitigation of commutation failure(CF)depends on the accuracy of CF prediction.In terms of the large error of the existing extinction angle(EA)calculation during the fault transient period,a method for CF prediction and mitigation is proposed.Variations in both DC current and overlap angle(OA)are considered in the proposed method to predict the EA rapidly.In addition,variations in critical EA and the effect of firing angle(FA)on both DC current and OA are considered in the proposed method to obtain the accurate FA order for the control system.The proposed method can achieve good performance in terms of CF mitigation and reduce reactive consumption at the inverter side when a fault occurs.Simulation results based on the PSCAD/EMTDC show that the proposed method predicts CF rapidly and exhibits good performance in terms of CF mitigation.

Evaluation of commutation failure risk in single-or multi-infeed LCC-HVDC systems based on equivalent-fault method

In single-or multi-infeed line-commutated converter-based high-voltage direct current(LCC-HVDC) systems, commutation failure(CF) induced by alternating current(AC) faults may lead to serious consequences. Considering the randomness of fault occurrences, an accurate evaluation of the CF risk(CFR) from the system point of view becomes necessary in power system planning and operation. This paper first provides a definition of the CF severity(CFS) index corresponding to an AC fault. Then,on the basis of electromagnetic transient(EMT) simulation, an approach to calculate the CFS index considering the randomness of fault-occurrence time is presented. A novel equivalent-fault method is further put forward to make the EMT simulation scalable to calculate the CFS index in terms of a fault occurring in a large-scale receiving-end grid. Thereafter, the CFR index is introduced, which is defined as the sum of the products of the CFS index of each AC fault and the corresponding fault rate.Finally, the proposed method is verified on the modified IEEE 9-bus and modified IEEE 39-bus systems using PSCAD/EMTDC.

Analysis of the Sending-Side System Instability Caused by Multiple HVDC Commutation Failure

As high-voltage direct current(HVDC)lines with large capacity are being commissioned with higher frequency,the characteristics of“strong”DC and“weak”AC transmission in the power grid are topics of interest.In particular,the coupling and interaction between the sending-side and receivingside AC systems interconnected by large-scale DC links is gaining importance.In this paper,the impact of the multiple HVDC commutation failure on the stability of the sending system under different power flow directions is analyzed based on the threearea AC/DC equivalent model.The main influencing factors and the counter-measures are discussed,and the single HVDC line blocking is taken as a comparison.Finally,the results are verified using the North China-Central China-East China power grid case system.The study provides a basis and reference to ensure security and stability of the ultra-high-voltage(UHV)AC/DC hybrid power grid.

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.

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 connections.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.All of which can have an 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 the 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 stations.Simulations conducted on the CIGRE benchmark model in PSCAD/EMTDC validate the performance of the supplementary control,which effectively mitigates successive commutation failures.

Improved Coordinated Control Approach for Evolved CCC-HVDC System to Enhance Mitigation Effect of Commutation Failure

The evolved capacitor commutated converter(ECCC),embedded with anti-parallel thyristors based dual-directional full-bridge modules(APT-DFBMs),can effectively reduce commutation failure(CF)risks of line-commutated converter-based high voltage direct current(HVDC)and improve the dynamic responses of capacitor-commutated converterbased HVDC.This paper proposes an improved coordinated control strategy for ECCC with the following improvements:(1)under normal operation state,series-connected capacitors can accelerate the commutation process,thereby reducing the overlap angle and increasing the successful commutation margin;(2)under AC fault conditions,the ability of ECCC to mitigate the CF issue no longer relies on the fast fault detection,since the capacitors inside the APT-DFBMs can consistently contribute to the commutation process and further reduce the CF probability;(3)the inserted capacitors can output certain amount of reactive power,increase the power factor,and reduce the required reactive power compensation capacity.Firstly,the proposed coordinated control approach is presented in detail,and the extra commutation voltage to mitigate the CFs provided by the proposed control approach and an existing approach is compared.Secondly,the mechanism of the improved control approach to accelerate commutation process and improve the power factor is analyzed theoretically.Finally,the detailed electromagnetic transient(EMT)simulation in PSCAD/EMTDC is conducted to validate the effectiveness of the proposed coordinated control.The results show that the proposed approach can present a further substantial improvement for ECCC,especially enhancing the CF mitigation effect.

Continuous Commutation Failure Suppression Method Based on Self-adaptive Auto-disturbance Rejection Proportional-integral Controller for HVDC Transmission System

Once an asymmetrical fault occurs on the AC side of the receiving-end of a high-voltage direct current(HVDC)transmission system,the current reference will be affected by the control regulation on the DC inverter side and the commutation voltage asymmetry.In this case,the advance firing angle will fluctuate periodically,causing security threats to the system.If the fault cannot be cleared in time,the effect may be even more serious.However,the traditional proportional-integral(PI)controller cannot effectively suppress the periodic components in the input error signal,which is an important cause of continuous commutation failure.Thus,the system requires more time to recover from the fault.Motivated by this,a selfadaptive auto-disturbance rejection PI controller is proposed in this study.The controller has the advantages of fast response speed and strong anti-interference ability of the auto-disturbance rejection controller.On one hand,it can automatically adjust PI,and the parameters can maintain the system’s adaptive ability.On the other hand,the discretization process satisfies the computer simulation requirements.By applying the proposed controller to a system under constant current control and extinction angle control,the dynamic response speed can be improved and the robust performance of the system can be ensured when dealing with a wide range of perturbations.Finally,simulation results show that the proposed algorithm can effectively suppress the continuous commutation failure of DC transmission systems.

Transient AC Overvoltage Suppression Orientated Reactive Power Control of the Wind Turbine in the LCC-HVDC Sending Grid

High-voltage direct current(HVDC) transmission is a crucial way to solve the reverse distribution of clean energy and loads. The line commutated converter-based HVDC(LCCHVDC) has become a vital structure for HVDC due to its high technological maturity and economic advantages. During the DC fault of LCC-HVDC, such as commutation failure, the reactive power regulation of the AC grid always lags the DC control process, causing overvoltage in the AC sending grid, which brings off-grid risk to the wind power generation based on power electronic devices. Nevertheless, considering that wind turbine generators have fast and flexible reactive power control capability, optimizing the reactive power control of wind turbines to participate in the transient overvoltage suppression of the sending grid not only improves the operational safety at the equipment level but also enhances the voltage stability of the system. This paper firstly analyses the impact of wind turbine's reactive power on AC transient overvoltage. Then, it proposes an improved voltage-reactive power control strategy, which contains a reactive power control delay compensation and a power command optimization based on the voltage time series prediction. The delay compensation is used to reduce the contribution of the untimely reactive power of wind turbines on transient overvoltage, and the power command optimization enables wind turbines to have the ability to regulate transient overvoltage, leading to the variation of AC voltage, thus suppressing the transient overvoltage. Finally, the effectiveness and feasibility of the proposed method are verified in a ±800kV/5000MW LCC-HVDC sending grid model based on MATLAB/Simulink.

Research on the Voltage Supporting Capability of Multi-VSC-HVDC Subsystems Operation Strategy to Receiving-end LCC-HVDC Network in Weak AC Grid

For the hybrid multi-infeed HVDC system in which the receiving-end grid is a strong AC grid including LCC-HVDC subsystems and multiple VSC-HVDC subsystems,it has higher voltage support capability.However,for weak AC grid,the voltage support capability of the multi-VSC-HVDC subsystems to the LCC-HVDC subsystem(voltage support capability-mVSCs-LCC)can resist the risk of commutation failure.Based on this consideration,this paper proposes an evaluation index called Dynamic Voltage Support Strength Factor(DVSF)for the hybrid multi-infeed system,and uses this index to qualitatively judge the voltage support capability-mVSCs-LCC in weak AC grid.In addition,the proposed evaluation index can also indirectly judge the ability of the LCC-HVDC subsystem to suppress commutation failure.Firstly,the mathematical model of the power flow of the LCC and VSC networks in the steady-state is analyzed,and the concept of DVSF applied to hybrid multi-infeed system is proposed.Furthermore,the DVSF index is also used to qualitatively judge the voltage support capability-mVSCs-LCC.Secondly,the influence of multiple VSC-HVDC subsystems with different operation strategies on the DVSF is analyzed with reference to the concept of DVSF.Finally,the indicators proposed in this paper are compared with other evaluation indicators through MATLAB simulation software to verify its effectiveness.More importantly,the effects of multi-VSC-HVDC subsystems using different coordinated control strategies on the voltage support capability of the receiving-end LCC-HVDC subsystem are also verified.

Flexible control strategy for HVDC transmission system adapted to intermittent new energy delivery

Intermittent new energy delivery requires increasing the flexibility of ultra-high voltage direct current(DC)power adjustment.Based on a converter steady-state model and a DC power model,the control angle constraints of a converter valve are relaxed for power regulation.In this paper,a flexible DC power control method based on a fixed tap changer position is proposed.The initial ratio of the converter transformer is optimized.The effects of the fixed-tap changer position control on the control angle,reactive power compensation,and commutation failure are analyzed.The new control method allows a DC system to operate at a large angle and increase the additional reactive power loss while improving the commutation security margin.Steady-state and electromagnetic transient simulations in the CIGRE test system verify the validity of the method proposed in this paper and the correctness of the analysis conclusions.

Overvoltage Calculation Method for Sending System Under DC Fault Disturbance Considering the Influence of Renewable Energy

Bipolar blocking and commutation failure in the renewable energy transmission system through ultra-high voltage direct current will produce significant temporary overvoltage.This paper analyzes the physical process of sending end grid overvoltage caused by two types of DC faults and establishes a DC overvoltage model considering renewable energy sources.According to characteristics of the sending end grid,the influence of the system equivalent impedance,the proportion of renewable energy output,the active and reactive power coefficients of the renewable energy sources during the low voltage ride through process on the overvoltage is analyzed respectively,and the relationship between the proportion of renewable energy output and the short-circuit ratio under the overvoltage constraint is constructed.The validity and practicality of the model and analysis method proposed in this paper are verified by combining the typical transmission system at the sending end of the actual power grid and regional power grid systems.

Transient Voltage Support Strategy of Grid-forming Medium Voltage Photovoltaic Converter in the LCC-HVDC System

The participation of photovoltaic(PV)plants in supporting the transient voltage caused by commutation failure in the line-commutated-converter-based high voltage direct current(LCC-HVDC)system is of great significance,as it can enhance the DC transmission ability.However,it is found that the gridfollowing(GFL)PV converters face the problem of mismatch between reactive power response and transient voltage characteristic when the voltage converts from low voltage to overvoltage,further aggravating the overvoltage amplitude.Thus,this article proposes a transient voltage support strategy based on the gridforming(GFM)medium voltage PV converter.The proposed strategy takes the advantage of the close equivalent electrical distance between the converter and grid,which can autonomously control the converter terminal voltage through GFM control with adaptive voltage droop coefficient.The simulation results show that the proposed strategy can ensure the output reactive power of the PV converter quickly matches the transient voltage characteristic at different stages,indicating that the proposed strategy can effectively support the transient voltage.

Quantitative Assessment for Commutation Security Based on Extinction Angle Trajectory

In order to reduce the risk of commutation failure(CF)in the AC/DC hybrid power system,the quantitative analysis on CF is required for on-line assessment and optimal control.This paper presents an accurate and reliable method to quantify the commutation security based on the trajectory due to the complexity of the high-voltage direct current(HVDC)model.Firstly,the characteristics of the extinction angle trajectory are analyzed under both commutation success and failure conditions.The commutation security margin index(CSMI)is then proposed for the HVDC systems.Moreover,a search strategy for parameter limits is put forward based on the sensitivity analysis of CSMI to accelerate the search speed with a guaranteed accuracy level.A modified IEEE 39-bus power system and an actual large-scale power system with 46 generators and 821 buses are utilized to verify the validity and robustness of the proposed index and strategy.