Line Commutated Converter HVDC Research Articles

Study on valve-side winding fault characteristics of converter transformer in LCC-HVDC system

This paper studies the fault analysis method and fault characteristics of valve-side winding fault of converter transformer in line-commutated-converter high-voltage direct-current (LCC-HVDC) system. The time periods are divided according to the valve conduction state within one power frequency cycle, and these time periods are classified based on the common features. For each type of time period, the flow path of the fault current is analyzed, the circuit equation is established, and the current on both sides of the transformer is solved. By combining the various time periods together, the expressions of the currents on both sides of converter transformer in one power frequency cycle after fault are obtained. The fault characteristics of the valve-side winding fault are studied based on the expressions, and the differences between them and ordinary transformer are pointed out. The research results show that when a single-phase grounding fault occurs in the valve-side winding of the low-voltage (LV) bridge converter transformer in LCC-HVDC system, both the grid-side current and the valve-side measured current contain a lot of harmonics, which will affect the operating characteristics of differential protection. Finally, a simulation model is built using PSCAD to verify the fault characteristics obtained from analysis. These works provide a theoretical basis for analyzing the action characteristics of converter transformer protection when the valve-side winding fault occurs and studying the new protection principle.© 2017 Elsevier Inc. All rights reserved.

The Strategy of Continuous Commutation Failure Suppression by Combining Turn-off Angle Compensation and Dynamic Nonlinear VDCOL

In recent years, with the continuous growth in China’s economy, the continuous advancement of urbanization and industrialization, the contradiction between rapid economic development and the continuous reduction in traditional fossil energy reserves such as coal, oil, and natural gas, the continuous aggravation of environmental pollution has become increasingly prominent. In this era, clean energy power generation technologies such as hydropower, wind power, and solar power generation, which have the advantages of renewability, environmental protection, and economy, have developed rapidly. However, wind and photovoltaic power plants are often located in remote areas, which means significant losses in the transmission process. High-voltage direct current (HVDC) transmission technology becomes the best choice to solve this problem. The HVDC transmission system based on a grid commutator is widely used in China’s AC-DC hybrid power grid. When an AC fault occurs on the inverter side, the line-commutated converter high-voltage direct current (LCC-HVDC) system is more prone to continuous commutation failure, which brings serious harm to system operation. To better suppress the problem of continuous commutation failure on the contravariant side, this paper analyzes the mechanism of continuous commutation failure from multiple angles. The DC current command sensitivity of a voltage-dependent current order limiter (VDCOL) in the LCC-HVDC system is low, which will lead to different degrees of continuous commutation failure. In addition, the rapid rise in DC current and the drop in commutation voltage during the fault will cause the turn-off angle to drop, and the probability of continuous commutation failure of the system will increase significantly. Based on the above theoretical analysis, a new control strategy combining the dynamic compensation of the turn-off angle of a virtual inductor and the suppression of continuous commutation failure by dynamic nonlinear VDCOL is proposed. A dynamic nonlinear VDCOL control strategy is proposed for the low sensitivity of current command adjustment under conventional VDCOL control. Secondly, two concepts of virtual inductance and DC current change rate are introduced, and a control strategy based on virtual inductance is proposed to comprehensively ensure that the switching angle has sufficient commutation margin during fault recovery. Finally, based on the CIGRE standard test model in PSCAD/EMTDC, the accuracy of the correlation mechanism analysis and the effectiveness of the suppression method are verified.

Mitigating commutation failures in HVDC systems with SFCL deployment

Commutation failures represent a prevalent issue encountered in line-commutated-converter high voltage direct current (LCC-HVDC) systems. As the widespread deployment of HVDC systems continues, the risk associated with commutation failures increases, posing a growing threat to power grids due to their potential to trigger severe consequences, including cascading failures and widespread blackouts. This research paper aims to address the significant issue of commutation failure within direct current (DC) systems through advocating for the use of resistive-type Superconducting Fault Current Limiters (R-SFCLs). To substantiate the efficacy of this proposed strategy, an array of simulations are executed using the PSCAD/EMTDC software. This comprehensive study investigates the performance characteristics of R-SFCLs configured with varying resistance values, scrutinizing their response under diverse fault resistance scenarios and distinct fault initiation times within the LCC-HVDC system. The outcomes of these simulations are that SFCLs confer significant advantages for mitigating commutation failures, surpassing traditional mitigation methods in terms of effectiveness. Consequently, SFCLs emerge as an optimal solution to prevent commutation failures in the HVDC systems.

A novel method of restoration path optimization for the alternating current–direct current bulk power grid after a major blackout

AbstractThe restoration control of the modern alternating current–direct current (AC–DC) hybrid power grid after a major blackout is difficult and complex. Taking into account the interaction between the line‐commutated converter high‐voltage direct current (LCC‐HVDC) and the AC power grid, this paper proposes a novel optimization method of restoration path to reconfigure the skeleton network for the blackout power grid. Based on the system strength, the supporting capability of the AC power grid for the LCC‐HVDC is first analysed from the aspects of start‐up and operation of LCC‐HVDCs. Subsequently, the quantitative relationship between the restoration path and the restoration characteristic of LCC‐HVDC is derived in detail based on the system strength indices of the short‐circuit capacity and the frequency regulation capability. Then, an optimization model of restoration path considering non‐tree paths is formulated and a feasible optimization algorithm is proposed to achieve the optimal path restoration scheme. A modified IEEE 39‐bus system and a partial power grid of Southwest China are simulated to show that the proposed method is suitable for the restoration of AC–DC power grids and can improve restoration efficiency. This research can be an important guidance for operators to rapidly restore the AC–DC power grid.

Impact Mechanisms of Commutation Failure Caused by a Sending-End AC Fault and Its Recovery Speed on Transient Stability

A sending-end AC fault may lead to commutation failure (CF) in a line-commutated converter high-voltage direct current (LCC-HVDC) system. In this paper, a theoretical analysis of the impact mechanisms of a CF and its recovery speed on the transient stability of a sending-end power system (TSSPS) is performed. Firstly, the models of the sending-end power system and DC power of CF are established; the ramp function is utilized to characterize the DC power recovery process. Secondly, the swing direction of the relative rotor angle caused by a sending-end AC fault is discussed, and the DC power flow method is employed to theoretically analyze the impacts of CF and its recovery speed on TSSPS. Next, the mathematic relations between parameters of the voltage-dependent current order limiter (VDCOL) and DC power recovery speed are further derived. It is concluded that the impacts of CF and its recovery speed on transient stability are related to the swing direction caused by a sending-end AC fault, the inertia of generators, and the location of the rectifier station. Finally, the theoretical analysis is validated by Kundur’s two-area system and IEEE 68-bus-based AC/DC asynchronous interconnection test power systems, respectively.

Modelling and analysis of LCC‐HVDC converter station for harmonic coupling of AC/DC power grid during geomagnetic storm

AbstractIn this paper, the problem of harmonic coupling between the converter transformer and the converter under the action of geomagnetically induced current (GIC) is proposed to be investigated. Firstly, a model of a 12‐pulse line commutated converter high voltage direct current (LCC‐HVDC) converter station based on harmonic state space (HSS) theory is established. Secondly, the harmonic components of the interaction between the AC‐side harmonic current and the DC‐side harmonic voltage are qualitatively analyzed by using the harmonic modulation theory. Through the simulation model and harmonic state space (HSS) calculation model, the harmonic coupling characteristics of power grid are analyzed. Finally, the harmonic current and voltage amplitude of AC / DC power grid under different GIC are calculated, and the characteristics and rules of AC / DC harmonic propagation are revealed. Among them, the amplitude of the 2nd harmonic current on the AC side and the 3rd harmonic current on the DC side increases linearly with the GIC. In addition, the 3rd and 9th harmonics on the AC side are also greatly affected by the magnetic bias of the converter.

A Predictive Control Method for LCC HVDC Participated Black Start With Commutation Failure Prevention

Line-commutated converter (LCC) based HVDC systems can significantly speed up the restoration of infeed power systems in blackout states. But they are fragile to commutation failure (CF) under the adverse black-start conditions. To improve the adaptability of LCC HVDC systems in such situations, a predictive control method is proposed in this paper. It includes the linear discrete time predictive model, receding horizon regulator, state estimator, and CF prevention (CFPREV) module. The model is based on the quasi-dynamic and transient-state analysis of load disturbances, where the black-start state is forecasted according to voltage sags caused by inrush currents. The regulator operates in real-time to optimize reference inputs for the HVDC controller. In the estimator, process error and measurement noise are modeled and the optimal system state is estimated with a Kalman filter. Moreover, the dc current setting for CFPREV is calculated using the predicted and estimated state variables. The CFPREV module and regulator work in coordination for secure and fast dc power recovery under abnormal conditions. Small-signal stability analysis and EMT simulations are implemented on the CIGRE benchmark model based black start test grid. The results show that the proposed method i) improves dc power response to black start load startups and ii) presents better recovery performance after the fault.

Analysis of VSC-HVDC support capability for power grids with large-scale renewable energy and multi-infeed HVDC links

Henan Power Grid is located in the north of Central China Power Grid. During its energy transition, thermal power units will be decommissioned on a large scale. Therefore, the scale of the new energy grid connection and regional external power will be further greatly increased to match the load growth demand. Under this background, Henan Power Grid plans to introduce the third HVDC (High Voltage Direct Current). Based on the actual data and the current operation status of Henan Power Grid, this paper proposes an analysis strategy about VSC-HVDC (Voltage Source Converter HVDC) to improve the support capability of the receiving power grid with multi-infeed HVDC. In recent years, VSC-HVDC technology has emerged with the strengths of flexible power regulation and strong voltage support. This paper firstly analyzes the VSC-HVDC power transmission characteristics and clarifies the mechanism of the VSC-HVDC power regulation. Then, based on the actual planning of Henan power grid, the influence of two feed-in schemes (LCC-HVDC (Line Commutated Converter HVDC) and VSC-HVDC) on the voltage stability of power grid and the number of subsequent commutation failures of other conventional DC are compared and analyzed under the three-phase short-circuit fault of AC lines near HVDC terminal location by using PSD-BPA simulation software. The results show that the VSC-HVDC scheme can effectively alleviate the voltage fluctuation (duration within 1 s and fluctuation number within 2 times) after the fault and restrain the number of subsequent commutation failures of conventional DC(with 2 times and even none).

A Novel Commutation Failure Mitigation Method Based on Transformer-Side Magnetic Coupling Injection

To mitigate the commutation failure (CF), the topology of the line-commutated converter high-voltage direct current (LCC-HVDC) needs to be modified. However, the power loss and capital cost of the modification schemes proposed in the existing literature are relatively high. A novel commutation failure mitigation method based on transformer-side magnetic coupling injection (TMCI) module is proposed in this paper. The topology and working principle of the TMCI module are presented. Considering the CF prediction and turn-off time of the thyristor, the control strategy of the TMCI module is designed. For the TMCI module, the voltage of the capacitor and voltage-current stress of thyristors are analysed, and the method to select parameters is designed. Then, a test system with the TMCI module is developed based on the CIGRE HVDC benchmark model. The simulation results show that the TMCI module can effectively mitigate CFs under AC fault conditions. Furthermore, compared with the enhanced capacitor commutated converter (ECCC) and the evolutional line-commutated converter (ELCC) proposed in the existing literature, the TMCI module can not only offer the best CF immunity with the least capital cost, but also has lower steady-state active power losses and less impact on the system reliability.

LCC-HVDC's Systematical Impact on Voltage Stability: Theoretical Analysis and a Practical Case Study

Line-commutated-converter HVDC (LCC-HVDC) is generally believed to impair voltage stability. While in a China's practical power grid, increase of HVDC power unexpectedly enhances the stability. The phenomenon reveals that existing studies overlook HVDC's systematical impact on the receiving AC system. This paper empirically elaborates how the phenomenon arises, theoretically clarifies the mechanism, and demonstrates its effect on practical system operating. In this paper, systematical features of power grid are visually depicted by two vectors, i.e., network structural vector and power-flow conditional vector. A unified burden rate is proposed to evaluate voltage stability of any AC bus. Impact of HVDC is explicitly expressed by two positive-definite terms and one negative-definite term. The latter of them is overlooked by most existing literatures, and enhances voltage stability by reducing the burden rate. The proposed methodology is validated by simulations with a practical power system in Asia with tens of HVDC infeed and tens of thousands of AC nodes.

Study on fault mechanism of LCC HVDC large disturbances under cloud-to-ground lightning with long continuous current

The safe and stable operation of the line commutated converter high voltage direct current (LCCHVDC) system is of great significance to the reliability of electricity supply. In order to clearly reveal the mechanism of LCCHVDC large disturbance fault under the Cloud-to-Ground (C-G) lightning with long continuous current, the HVDC-lightning simulation model considering long continuous current, actual control and protection systems is proposed. The effect of long continuous current on traveling wave protection is analyzed. Transient processes including commutation failure (CF), fault restart and blocking of the DC system under the lighting with long continuous current are simulated. The reasons for CF of LCCHVDC system caused by long continuous current are analyzed, and the influence of long continuous current amplitude and duration on fault restart process is studied. Results show that the long continuous current has little influence on the response characteristics of travelling wave protection, but during the regulation process of control system under lightning disturbance, the DC current control regulation quantity will increase due to the presence of long continuous current. If the amplitude of long continuous current exceeds 80 A, the CF will occur in the DC system. If the duration of long continuous current exceeds the deionization time, the line will fail to restart, and the line can be successfully restarted only if the long continuous current is over before the voltage begins to rise.

Implementation of a Multiterminal Line Commutated Converter HVDC Scheme with Auxiliary Controller on South Africa’s 765 kV Corridor

The deployment of a 765-kV transmission line on Eskom’s South African Grid marks the beginning of a new era in power industries. The integration of renewable energies by independent power producers (IPPs) leads to an infrastructural change in the stability performance of the entire grid. These developments are expected to bring about a multiterminal direct current (MTDC) system for practical implementation on this grid. Therefore, this study focuses on the dynamic response of the South African transmission grid during a system disturbance. In the carrying out of this study, the South African grid was modeled on PSCAD, and its performance was evaluated. The impact of the MTDC link on the grid’s interarea oscillation was also investigated. An additional current order controller for the MTDC link was developed, and its impact on the MTDC power transfer was analyzed. The results show a better system performance and reduced interarea power swings with the inclusion of the MTDC link.

Design of hybrid series converter valve considering device switching characteristics

The line commutated converter high voltage direct current (LCC-HVDC) has a weak ability to resist commutation failure (CF). In order to solve this problem, we studied a hybrid series converter valve (HSCV) composed of thyristors and the fully controllable switches in series. The working states of HSCV were designed, besides, the operating principle of HSCV was analyzed. Considering the characteristics of the device, HSCV's turn-off time control was provided. The voltage and current stress in the turn-off process of HSCV was analyzed, dynamic voltage equalization parameters of HSCV were designed also. The simulation results of SABER and PSCAD/EMTDC showed that HSCV increases the time for the thyristor to recover its forward blocking capability, thereby preventing the valve from re-conducting and switching phases. That is why HSCV-HVDC has stronger commutation failure suppression than LCC-HVDC and ELCC-HVDC. In addition, the parameter design principle proposed in this paper can reasonably design the dynamic voltage balancing branch, effectively solve the overvoltage problem of the turn-off tube, and meet the device parameter requirements.

Modeling of Efficient Control Strategies for LCC-HVDC Systems: A Case Study of Matiari-Lahore HVDC Power Transmission Line.

With the recent development in power electronic devices, HVDC (High Voltage Direct Current) systems have been recognized as the most prominent solution to transmit electric power economically. Today, several HVDC projects have been implemented physically. The conventional HVDC systems use grid commutation converters, and its commutation relies on an AC system for the provision of voltage. Due to this reason, there are possibilities of commutation failure during fault. Furthermore, once the DC (Direct Current) system power is interrupted momentarily, the reversal of work power is likely to cause transient over-voltage, which will endanger the safety of power grid operation. Hence, it is necessary to study the commutation failure and transient over-voltage issues. To tackle the above issues, in this paper, the dynamic and transient characteristics of Pakistan’s first HVDC project, i.e., the Matiari–Lahore ±660 kV transmission line has been analyzed in an electromagnetic transient model of PSCAD/EMTDC. Based on the characteristics of the DC and the off-angle after the failure, a new control strategy has been proposed. The HVDC system along with its proposed control strategy has been tested under various operating conditions. The proposed controller increases the speed of fault detection, reduces the drop of AC voltage and DC and suppresses the commutation failure probability of LCC-HVDC (line commutated converter- high voltage direct current).

Mechanism Analysis of a Subsequent Commutation Failure and a DC Power Recovery Speed Control Strategy

A subsequent commutation failure (SCF) of the Line-Commutated Converter–High-Voltage Direct Current (LCC-HVDC) may occur during the recovery after the clearance of an AC fault, seriously threatening the safe and stable operation of the LCC-HVDC and the entire post-fault AC/DC hybrid power system. In this study, the mechanism of an SCF as affected by the transient stability was analyzed, and a DC power recovery speed control strategy is proposed as an additional form of control to prevent the occurrence of SCFs. First, the sending-end and receiving-end power systems were modeled as synchronous generators instead of ideal voltage sources, and the mechanism of an SCF as affected by the transient stability was analyzed and verified. Second, the ramp function was adopted to describe the recovery characteristic of DC power, and a model of its recovery was established. Then, the recovery speed control strategy is presented based on the mechanism analysis of SCFs, which can not only be used to avoid the occurrence of SCFs but also increase the transient stability margin of the sending-end power system. Finally, the effectiveness and robustness of the proposed control strategy were validated by using a hybrid electromechanical–electromagnetic model and the full electromagnetic model of the IEEE 39-bus asynchronous interconnection test power system. With the implementation of the proposed control strategy, the safe and stable operation of the LCC-HVDC and AC/DC hybrid power system can be guaranteed. The adaptive DC power recovery speed control strategy will be further investigated in future research work.

Interaction Mechanism and Coordinated Control of Commutation Failure Prevention in Multi-Infeed Ultra-HVDC System

Commutation failure (CF) is an inherent drawback of line-commutated converter high-voltage DC (LCC-HVDC) system. And CF prevention (CFPREV) control is widely applied in UHVDC lines to mitigate the subsequent CF. However, the formation of a multi-infeed UHVDC system induces power exchange and voltage interaction between AC buses during the transient process, making the behaviors and mitigation of CFs more complex. Consequently, exploring the impact of CFPREV control on adjacent stations and optimizing the CFPREV control to adapt to the interaction characteristics in a multi-infeed system is necessary. Considering the interaction characteristics, the impact of CFPREV control on the commutation process of inverters in a multi-infeed system is analyzed through formula derivation and simulation verification. It is found that although CFPREV control can effectively mitigate the CFs of the inverter near the fault, it will further increase the risk of concurrent CF (CCF) of remote inverters due to the interaction on the AC side. To solve this problem, a coordinated control scheme of CFPREV controls in a multi-infeed UHVDC system is proposed. The output of the CFPREV control of the inverter near the fault can be adjusted adaptively according to the commutation margins of remote inverters. And the probability of CCF caused by CFPREV control is consequently decreased. Case studies are conducted based on Henan provincial multi-infeed UHVDC system in China to verify the analysis result and the effectiveness of the coordinated control scheme of CFPREV controls.

Generic Analysis Framework for Modular Multilevel Converter HVDC with Multi-Infeed Line-Commutated Converter HVDC System

This paper proposes a generic analysis framework for a grid supporting modular multilevel converter (MMC)-high voltage DC (HVDC) in a multi-infeed of line commutated converter (LCC) and MMC (MILM) system. MMC-HVDC can support the grid by compensating for the exact reactive power consumptions within the MMC-HVDC system and the varying power system conditions in the MILM system. Maximum active/reactive power capability (MPQC) curve and PQ loading curve comparison process is introduced to properly design a grid supporting MMC-HVDC. While the MPQC curve presents the maximum PQ range of the MMC-HVDC system based on the submodule capacitance value and the modulation index, the PQ loading curve presents the reactive power requirement from the power system that MMC-HVDC needs to compensate. Finally, the comparison of these two curves yields the proper value of submodule capacitance and the modulation index for sufficiently supporting the MILM system. The proposed framework is validated with detailed PSCAD/EMTDC simulation; it demonstrated that it could be applied to various power system conditions.

An adaptive DC line fault recovery strategy for LCC-HVDC system based on voltage gradient

Due to the lack of characteristic variables to determine whether the fault point persists during DC-line fault recovery in a Line-commutated-converter High Voltage Direct Current (LCC-HVDC) system, it is difficult to distinguish transient and permanent faults. Concerning this problem, an adaptive DC-line fault recovery strategy based on voltage gradient is proposed. Firstly, the variation of the topology of DC system after phase shift is analyzed, and the calculation model of voltage at the outlet of inverter station is built, thus the expression of voltage at the outlet of inverter station containing parameters of fault location, fault resistance and fault duration is obtained. On this basis, characteristics of the gradient of voltage at the outlet of the inverter station are analyzed, i.e. its value monotonically decreases and quickly attenuates to 0 in the case of permanent fault, and abruptly drops below 0 the moment fault disappears in the case of transient fault. According to the characteristics of the voltage gradient, the criterion for the identification of transient and permanent faults is constructed, thus adaptive DC-line fault recovery is realized. Finally, simulation tests in RT-LAB verify that, the proposed method can correctly identify the fault nature of permanent and transient faults on the DC line and realize adaptive fault recovery, unaffected by the fault location, fault resistance and fault duration.

Operation and Challenges of Multi-Infeed LCC–HVDC System: Commutation Failure, AC/DC Power Flow, and Voltage Stability

This paper presents a detailed analysis of commutation failure, AC/DC power flow, and voltage stability of multi-infeed high-voltage direct current (HVDC). The use of HVDC power transmission technology has become common in modern power systems. During the past two decades, HVDC technology has been extensively used for long-distance bulk power transmission to remote areas. Throughout the world, the demand for power has drastically increased in recent years due to industrialization; such situations make HVDC an economic candidate because the distance between power generation plants and load areas is significantly very long. The line-commutated converter (LCC) technology-based HVDC system is well more mature than other available conversion schemes (i.e., voltage source converters), and it is widely used in high-power projects. China had approximately 50 HVDC–LCC links in 2020, and a single LCC-based link with the highest capacity is 12 GW. The installation of several HVDC links in an existing power network has led to a situation where two or more HVDC links terminate in the electric vicinity of each other’s AC network or even in same AC busbar. Such scenarios are termed multi-infeed HVDC system. Multi-infeed HVDC systems bring various challenges related to voltage stability, local and concurrent commutation failure, and AC/DC power flow. Here, the literature available on these phenomena of LCC-based HVDC is discussed for future research. The assumptions and drawbacks of various techniques used for investigating the mentioned phenomena are also highlighted.

Transient stability margin assessment of AC/DC hybrid system with commutation failure involved

The commutation failure of Line Commutated Converter-High Voltage Direct Current (LCC-HVDC) caused by AC fault has an increasingly serious impact on the transient stability of AC/DC hybrid system. This paper presents a novel transient stability margin assessment approach based on energy function, where generators adopt 6th order electromechanical model and DC system adopts electromagnetic model. First, the network structure-preserving model of AC/DC hybrid system is established, and its energy function is deduced. Next, the energy function of post-fault system is approximately calculated by trapezoidal integral path, and energy function based transient stability criteria are put forward to determine whether the post-fault system is stable or not. Then, Bisection Method (BM) is employed to obtain Critical Clearing Time (CCT) quickly and effectively where CCT can be adopted as index of stability margin. Finally, the New England-39 bus system integrated with LCC-HVDC is used as test system to investigate the effectiveness of the proposed transient stability margin assessment approach.