AC System Fault Research Articles

Sensitivity analysis of commutation failure in multi-infeed HVDC systems: Exploring the impact of ac system representations and fault types

This paper investigates commutation failure (CF) in line commutated HVdc converters within multi-infeed HVDC systems and thereby provides valuable insights for the operation and design of HVdc systems. CF susceptibility across various operational scenarios, fault types, and ac system representations at different frequencies is explored. An Electromagnetic Transients (EMT) model is constructed and parameter changes are applied using Monte-Carlo Simulation. The process is fully automated with the use of a controlling program written in Python, which varies parameter values and facilitates result retrieval and post-simulation analysis. A typical analysis results in several hundred thousands of simulation runs, requiring the procedure to be implemented on a parallel computing platform. The results show that two-phase faults are the most critical CF triggers, an aspect often overlooked in previous literature focused on three-phase or single-phase-to-ground faults. Furthermore, the frequency dependent characteristics of the ac network can result in very different CF susceptibilities.

Control Strategy of Suppressing Short-circuit Current VSCs Feeding into the AC Grid in a Hybrid Cascaded UHVDC System

Hybrid cascaded ultra-high voltage dc is a new dc technique with a line commutated converter cascaded with one or several voltage source converters. It is an effective solution for large capability transmission as well as improving the stability of the ac grid with multiple dc transmission systems feeding in. Baihetan-jiangsu dc project in China is the first cascaded ultra-high voltage dc project in the world. For a receiving end with a strong ac system, the short circuit current is always large and close to the maximum ac breaker breaking capability, which is a threat to the ac system fault clearance. In this paper, the issue of excessive short circuit current in a hybrid cascaded ultra-high voltage dc system is discussed and revealed. The process of the short circuit current developing is divided into 2 stages. Characteristics and the calculation method of the short circuit are proposed, and based on this, a control method is proposed, which can suppress the short circuit current during the fault clearance effectively. The parameter setting method is proposed. Finally, based on the scenario of the Baihetan-jiangsu dc project, the strategy and the parameters setting method are validated under PSCAD/EMTDC.

Coordinated Control of an Islanded Microintegrated Energy System with an Electrolyzer and Micro-Gas Turbine

Microintegrated energy systems (MIESs) can be disconnected from power distribution systems during power system faults. This paper develops a control scheme for an islanded MIES. The VSC inverter controls the AC bus voltage and frequency using a modified AC voltage regulator and a modified frequency regulator. The control structures of the power-to-gas and PMSG-based microturbine generator (MTG) systems are improved. Renewable generation always runs at the maximum power point. The surplus renewable energy in the MIES can be converted into natural gas using power-to-gas, and the MIES can make full use of renewable energy. The proposed coordinated control scheme of the electrolyzer and the supercapacitor can achieve a power balance of the islanded MIES and reduce the DC-link voltage fluctuation. A micro-gas turbine can provide electric energy to the load and enhance distribution system resilience. A coordinated control scheme of the MTG and the supercapacitor is developed to improve MIES operation. A feature of this paper is the research on fault ride-through of the islanded MIES. A fault ride-through strategy is proposed, where the AC voltage of the VSC inverter is reduced to limit the short-circuit current during AC system faults. Islanded MIES simulations are conducted in a MATLAB/Simulink environment to test the control scheme. The simulation results verify the effectiveness of the control scheme during normal operation and failure.

Improved comprehensive energy-based control for MMC-HVDC system

The interconnection of two non-synchronous AC grids is one of the promising applications in the modular multilevel converter (MMC) based high-voltage direct-current transmission system (MMC-HVDC). Due to the complicated internal dynamics of the MMC, energy-based control is becoming an emerging technique to guarantee the robust stability of the MMC. However, conventional energy-based control cannot be applied simultaneously to the power-controlled MMC (PCMMC) and voltage-controlled MMC (VCMMC) interconnected by short or medium DC transmission lines. To address this problem, this paper proposes an improved energy-based control for the MMC-HVDC system aiming to achieve full control of the inner dynamics of MMCs regardless of the DC line length. A completed decoupled mathematical modeling with comprehensive state variables of MMCs is established. Then, a unified control framework for the PCMMC and the VCMMC considering AC grid conditions is developed. A distinct advantage of the scheme is that direct and fast regulation of the DC-side voltage can be achieved to improve the voltage behavior. The other is that the dynamic performance of the submodule capacitor voltages of VCMMC and PCMMC can be conspicuously improved under normal, AC system fault and arm parameter asymmetry conditions. Comparative simulations and hardware-in-loop (HIL) experimental results validate the effectiveness of the proposed methodology.

Analysis of Fault and Protection Strategy of a Converter Station in MMC-HVDC System

With the development of power energy technology, flexible high voltage direct current (HVDC) systems with high control degree of freedom flexibility, power supply to passive systems, small footprint, and other advantages stand out in the field of long-distance large-capacity transmission engineering. HVDC transmission technology based on a modular multilevel converter has been widely used in power grids due to its advantages such as large transmission capacity, less harmonic content, low switching loss, and wide application field. In the modular multilevel converter (MMC)-based HVDC system, the protection strategy of converter station internal faults is directly related to the reliability and security of the power transmission system. Starting from the MMC topological structure, this paper establishes the MMC mathematical model in a synchronous rotation coordinate system by combining the working state of sub-modules and the relationship between each variable of the upper and lower bridge arms of each phase of the MMC. It provides a theoretical basis for the design of the MMC-HVDC control system. The causes of the AC system faults and the internal faults of the converter station in the MMC-HVDC system are analyzed, and the sub-module faults and bridge arm reactor faults in the converter station are studied. The sub-module redundancy protection and bridge arm overcurrent protection strategies are designed for the faults, and the correctness of the scheme is verified by Matlab/Simulink.

Multiple commutation failure suppression method of LCC-HVDC transmission system based on fault timing sequence characteristics

Aiming at the problem of commutation failure and the poor transient response of line commutated converter-based high-voltage direct-current (LCC-HVDC) transmission systems, this paper starts from the harmonic components of an AC system fault on the inverter side. The quantitative relationship between the harmonic wave and the trigger angle of the inverter side is derived in detail, and the effect of the trigger on the commutation of the LCC-HVDC transmission system is qualitatively analyzed with a phase-locked trigger. On this basis, an improvement scheme for an anti-harmonic control system is proposed. First, a virtual separated phase-locked control method is used to determine the fault occurrence time. Additionally, the characteristics of the trigger angle of the rectifier side and extinction angle of the inverter side after the fault are used to determine the fault position and generate the corresponding fault timing sequence. Then, the generated fault timing sequence is used to control the notch filter and modify the input voltage of the voltage-dependent current order limiter to improve the robustness of the control system in terms of the harmonics. Finally, based on CIGRE benchmark LCC-HVDC transmission system model, the advantages of the improved method are verified. The results can provide a reference for actual projects.

Virtual synchronous generator based pole control in high-voltage DC transmission systems

The use of renewable energy to realize decarbonization is rapidly increasing, and considerable research is being performed to achieve highly efficient high-voltage direct current (HVDC) transmission for offshore wind power generation over long distances. However, in the case of onshore AC power transmission and distribution systems, with the mass introduction of distributed energy resources (DERs), voltage and frequency fluctuations often occur. It is thus necessary to implement the control function of smart inverters, which can facilitate the stabilization of the AC system, in the grid-connected inverters of DERs. Virtual synchronous generator (VSG) control is a smart inverter control strategy that can contribute to the stability of the AC system. In this study, conventional power control and VSG control were applied to the control strategy of modular multi-level converter-based poles of the HVDC transmission system, and the AC system fault tolerances of the entire system were examined.

Fault ride-through control for the sending-end AC system based on reactive power auxiliary in LCC-MMC hybrid HVDC system

The hybrid HVDC system based on line commutated converter and modular multilevel converter is a feasible method to address the common commutation failure at the receiving end of conventional HVDC systems. At present, project Gezhouba–Shanghai in China is under the demonstration scheme of the conventional receiving end upgrading, which introduces the difficulty in sending-end AC system failure riding through. For this purpose, an electromagnetic simulation model is established in the PSCAD/EMTDC simulation platform firstly, and then a transient analytical expression of DC current is deduced through the system equivalent circuit and verified through the simulation model. The expression deeply reveals the variation pattern of DC current and lays the theoretical foundation for fault ride-through control. Base on this expression, the decreasing level of MMC DC voltage after fault is deduced, however, it is difficult to realize the decreasing of MMC voltage due to the modulation index limitation of DC voltage. In order to solve this problem, this paper uses MMC to absorb reactive power from AC power grid to broaden the range of DC voltage drop. To solve this problem, the reactive power MMC absorbing from the AC power grid is used in this paper to broaden the decreasing range of DC voltage. On this basis, due to the conflict between the rapidity and stability of fault ride-through control, a current ride-through control based on reactive power auxiliary is proposed. This control strategy can dynamically regulate the reactive power transmission between MMC and AC grid according to fault states, assist to reduce the MMC DC voltage, and enable the constant current control so as to realize the sending-end AC system fault ride-through. Finally, the effectiveness of this control is verified with various simulation results. The sending-end AC system fault ride-through control method proposed in this paper has profound project application significance on upgrade transformation for the conventional receiving end.

Transient Performance Analysis of Reactive Power Compensators at LCC-HVDC Station Feeding Weak AC System

This paper focuses on analyzing the dynamic performance of various reactive power compensators at weak LCC-HVDC terminals. Four scenarios based on combination of active and passive reactive power compensation schemes are presented. The comparative performance analysis of various compensators in different types of AC and DC system faults are evaluated using electromagnetic transient simulation program PSCAD/EMTDC. The analysis shows that fixed capacitor has poor performance and results in reducing effective short circuit ratio of system, while STATCOM on the other hand, increases AC system strength and has significant performance in areas like immunity to commutation failure, suppression of temporary overvoltage, AC bus voltage regulation and fault recovery time.

Analysis of the Influence of Commutation Failure on the Converter Valve without Back-Inspection EHV Power Transmission

In the DC transmission system, the inverter side converter valve often has a commutation failure. The commutation failure will cause the valve control to misjudge the converter valve without back-check, which may lead to DC blocking. This paper studies the optimization of the valve control system with measures to improve the DC control system software when the AC system fault AC voltage is higher than the threshold value. It is verified through practical application that the measures are implemented well and can effectively reduce the risk of DC blocking. The measures can provide technical reference for the operation and maintenance of converter station equipment.

A novel AC line distance protection scheme for AC/DC hybrid system based on fault component energy equation

In view of the mal-operation of AC line distance protection due to DC commutation failure caused by the inverter-side AC system fault, a new AC line distance protection principle for AC/DC hybrid system based on fault component energy equation is proposed. First, by comparing the pre-fault and post-fault conducting states of converter valves, the expressions of fault components of converter bus voltage considering commutation failure are derived. Based on this, the fault supplementary network of DC system with multiple fault supplementary sources is built. Then, combining the fault supplementary network of AC system, the fault component energy equation containing the fault distance and fault resistance is established. And then, distinguish the fault direction according to different characteristics of fault component energy flowing to the converter bus in the case of the DC system fault and the AC line fault. Thus, the linear least squares method is used to solve the equations with data at multi-time sections, so that the fault parameters are calculated and distance protection criterion is constructed. Finally, the simulation results based on the RT-LAB platform verify that, this method can identify different types of internal and external faults on AC line fast and sensitively, unaffected by the commutation failure and fault resistance.

Research on Dynamic Reactive Power Compensation Scheme for Inhibiting Subsequent Commutation Failure of MIDC

Commutation failure at the inverter side of an MIDC (multi-infeed HVDC) is usually caused by AC system faults. Suppose the converter bus voltage cannot recover to the normal operation level in time: in that case, the commutation failure will then develop into more severe subsequent commutation failures or even DC blocking, which will severely threaten the security and stability of the system. Dynamic reactive power compensation equipment (DRPCE) can offer voltage support during accident recovery, stabilize voltage fluctuation and inhibit any subsequent commutation failure risk. This paper proposes the optimal DRPCE configuration scheme for maximizing both inhibitory effect and economic performance. The simulation results on MATLAB-BPA prove the scheme’s correctness and rationality, which can effectively inhibit the risk of subsequent commutation failure and obtain economic benefits.

Commutation failure prediction method based on characteristic of accumulated energy in inverter

Concerning the difficulty in predicting commutation failure in HVDC transmission system, a commutation failure prediction method based on the energy accumulation characteristic of 12-pulse inverter is proposed. First, the operation characteristic of valve-side current in the commutation process is analyzed. And then, according to the characteristic of cross commutation between Y bridge and D bridge of 12-pulse inverter, the model of accumulated energy difference between commutation inductances is built. On this basis, considering the instantaneous values of real-time firing angle and valve-side current in the DC system, the commutation failure prediction criterion is constructed, which combined with the variation rate of valve-side current can predict commutation failure. Simulation tests in RTDS verify that, the proposed method can effectively predict commutation failure in HVDC system without phasor calculation. Besides, it is strongly immune to disturbance and fault resistance and unaffected by the type of AC system fault.

A novel evaluation method for the risk of simultaneous commutation failure in multi-infeed HVDC-systems that considers DC current rise

This paper proposes a novel evaluation method for the risk of simultaneous commutation-failures (CFs) in multi-infeed high-voltage direct-current (HVDC) systems, which takes into account the DC current rise caused by the inverter AC system fault. By calculating a multi-infeed interaction factor (MIIF), and using an equivalent model for multi-infeed HVDC systems. The effect of DC-current on commutation failure is analyzed and the factors that affect the simultaneous commutation-failure are determined. Based on the equivalent equation for both the DC line and DC control, a time-domain expression for the DC-current caused by the inverter AC system fault is calculated. Based on this, this paper describes a novel CF evaluation method that takes into account DC current rise by solving for the maximum of the DC current. Several useful parameters like the MIIF, the critical multi-infeed commutation failure factor (CMICFF), and the multi-infeed commutation failure factor (MICFF) are defined, and a method for the risk of simultaneous commutation-failure in multi-infeed HVDC systems by comparing CMICFF and MICFF is proposed. The accuracy of the method proposed in this paper is validated by a comparison with a simulation using the software PSCAD/EMTDC.

A novel directional relay for AC lines close to the HVDC installation

Compared with traditional distance protection based on fundamental components, the R-L differential-equation algorithm in time domain, which is free of eliminating aperiodic component and insensitive to grid frequency, is more suitable for the AC lines close to high voltage direct current (HVDC) installation. However, there is a problem of voltage dead zone which occurs when the R-L differential equation algorithm is directly applied on the AC lines. According to the difference between fault currents’ characteristics provided by direct current (DC) system and receiving-end AC system, then forward and reverse faults on the outlets of AC lines can be identified based on the correlation of the measured voltage drop and calculated voltage drop. Therefore, a novel directional relay based on the correlation coefficient of voltage waveform (CCVW) and low voltage component is proposed. Finally, an extensive performance evaluation in PSCAD/EMTDC and MATLAB simulation verifies the effectiveness of the directional relay. Influencing factors and comparative analysis show that the directional relay has a high sensitivity, rapidity, and reliability.

Extinction angle predictive control strategy for commutation failure mitigation in HVDC systems considering voltage distortion

In this study, an extinction angle predictive (EAP) control strategy is proposed to mitigate commutation failure (CF) in high voltage direct current (HVDC) systems. The proposed strategy works effectively during both AC system faults and commutation voltage distortions. At first, an index is put forward to evaluate the prospective commutation area drop and quantify the upcoming commutation stress considering the commutation mechanism. Then, the extinction angle is predicted based on the detected commutation area drop incident. Further, the predicted extinction angle is introduced into the HVDC inverter control to design the EAP control strategy, ensuring that the CF can be prevented due to a sufficient commutation margin. The advantage of the proposed strategy is that it evaluates the upcoming commutation stress based on the transient commutation voltage waveform. Thus, a better sensitivity for commutation voltage distortion can be ensured. The proposed strategy is simulated in the PSCAD/EMTDC platform and the results are compared with that of the conventional schemes, verifying the performance of the proposed strategy in mitigating the CF during AC faults and voltage distortion.

A Fault Current Limiting Approach for Commutation Failure Prevention in LCC-HVDC Transmission Systems

Commutation failure is one of the most frequent failures in line-commutated converter based high-voltage direct-current (LCC-HVdc) systems which may lead to the outage of HVdc links, and thus affect the performance of the power system. Most commutation failures are caused by voltage reduction due to ac system faults. In this paper, a commutation failure prevention module (CFPM) is developed, which inserts the optimal value of resistance proportional to the reduced extinction angle in series with the fault current path using a simple switching method. When a fault occurs at the inverter ac side, a self turn- off switch starts switching with a pre-specified frequency and controlled modulation index, which limits the fault current to the desired value and inhibits commutation failure. The salient feature of the proposed CFPM is that it is fully controllable. In addition, despite the available controller modification methods which are not effective for low-impedance faults, the proposed CFPM can prevent commutation failure regardless of the fault severity. The practical performance and feasibility of the developed CFPM is validated through laboratory testing, using the real-time Opal-RT hardware prototyping platform. The experimental results demonstrate that the proposed strategy can effectively eliminate the commutation failure or repetitive commutation failures under different fault types.

A Dynamic Equivalent Method for Electromagnetic Transient Simulation and Its Application in East China Power Grid

There is an inherent limitation when unbalanced faults in AC system are studied in large scale AC/DC power systems with electromechanical transient simulation. In order to make the electromagnetic transient simulation which is accurate in these situations possible, dynamic reduction must to be done. A method based on support strength of generators is proposed. According to the connection form of low-voltage network with 500kV skeleton buses and whether there are generators in it, three reduction schemes are used to replace the corresponding low-voltage network. The dynamic parameters of equivalent generators are obtained by a time domain aggregation method. The equivalent results of planning data of East China Power Grid in 2020 show that, the method has high accuracy in power flow analysis as well as short circuit calculation. The proposed method greatly simplifies the original system while the characteristic of dynamic response remains.

Research on the transformerless connection mode for DC power distribution system

The connection mode of the flexible DC distribution network and the AC network is not only the basis of the system design but also is one of the key technologies in the DC distribution. This paper demonstrates the feasibility of the transformerless system using modular multilevel converters (MMC) in the power distribution system. Here, in allusion to the DC power distribution network without transformer, the influence of AC system fault on DC system and the influence of DC system fault on AC system are analysed in detail separately. By establishing the simulation model with the PSCAD/EMTDC software, the transformerless system proposed is verified feasible at the appropriate voltage and the appropriate earthing way of AC system. The research proposed a solution of saving economic cost and space which is the critical issue in the conventional DC power distribution system with the transformer.

A Predictive Control Strategy for Mitigation of Commutation Failure in LCC-Based HVDC Systems

High-voltage direct-current (HVDC) systems are being widely employed in various applications because of their numerous advantages such as bulk power transmission, efficient long-distance transmission, and flexible power-flow control. However, line-commutated-converter-based HVDC systems suffer from commutation failure, which is a major drawback, leading to increased device stress and interruptions in transmitted power. This paper proposes a predictive control strategy, deploying a commutation failure prevention module to mitigate the commutation failures during ac system faults. The salient feature of the proposed strategy is that it has the ability to temporarily decrease the firing angle of thyristor valves depending on the fault intensity to ensure a sufficient commutation margin. In order to validate the performance of the proposed strategy, several simulations have been conducted on the CIGRE Benchmark HVDC model using PSCAD/EMTDC software. Additionally, practical performance and feasibility of the proposed strategy are evaluated through laboratory testing, using the real-time Opal-RT hardware prototyping platform. Simulation and experimental results demonstrate that the proposed strategy can effectively inhibit the commutation failure or repetitive commutation failures under different fault types, fault impedances, and fault initiation times.