Rectifier Station Research Articles

MILP model for short-term peak shaving of multi-grids using cascade hydropower considering discrete HVDC constraints

The thriving of high-voltage direct current transmission for hydropower tackles supply-demand imbalances but complicates grids safety, especially concerning the related challenges of varying numbers of hydroelectric units operating, including the effects of discrete rectifier station altering current and vibration zones with minimum numbers of hydroelectric units operating. Here, a mixed integer linear programming model considering high-voltage direct current safety constraints for daily peak-shaving of cascade hydropower plants serving multi-grids is proposed to address these challenges. First, the model integrates the altering current constraint across multiple intervals to ensure stable operation following the passive transition of the direct current terminal into islanded system state. Second, a power distribution strategy considering load differences is adopted to compensate for shortcomings of traditional peak shaving treatments for multiple grids to achieve effective results. Finally, a method for linearizing the segmented vibration zones of hydropower station with minimum numbers of units operating is adopted during model solving to ensure that the model meets safe specifications. The case study indicates that besides guaranteeing the proposed security constraints, the model reduces the average distance by 30.4 % and 30.3 % during the dry and flood seasons, respectively, with better calculation time and variance of the residual load.

Revealing the mechanism of subsynchronous torsional interaction caused by LCC-HVDC from the perspective of vector synchronization

Line-commutated converter-based high voltage direct current transmission (LCC-HVDC) has been widely applied worldwide due to its great advantages in realizing long-distance and large-capacity power transmission. However, it may also lead to instability problems, including subsynchronous torsional interaction (SSTI). This destructive phenomenon greatly threatens the safe and stable operation of power systems and has been widely concerned since the 1970 s. Existing literature has found that SSTI is caused by DC current control of rectifier station. However, the physical mechanism of this phenomenon has not been clarified clearly, which hampers further understanding of how negative damping is generated and whether it is evitable. To fill this gap, the physical process of SSTI caused by LCC-HVDC is clarified through the perspective of vector synchronization. Based on this, the negative damping mechanism is revealed. The influence of control on damping is also studied with the contribution of different control loops quantified. All results are verified through time-domain simulations and damping torque analysis.

An Improved Suppression Method of AC Transient Overvoltage for Line Commuted Converter Based High Voltage Direct Current Considering AC-DC System Coupling

Commutation failures in line commuted converter-based high voltage direct current (LCC-HVDC) transmission systems leads to an increase in the converter bus voltage of the rectifier station, thus resulting in AC transient overvoltage in the sending-end grid. The transient overvoltage could lead to the disconnection of renewable energy generation and threaten the stable operation of the sending-end grid. However, the influences of the coupling between AC and DC systems caused by the interaction between the active and reactive power of the sending-end grid, the AC bus voltage of the rectifier station, and the DC current are ignored. The AC transient overvoltage cannot be accurately suppressed. Therefore, in this study, the transient voltage characteristics of the rectifier station under a commutation failure of the inverter station are analyzed. The influence of LCC-HVDC control on the AC bus voltage of a rectifier station through the active and reactive power of the rectifier station is analyzed. A dynamic model of the AC bus voltage of a rectifier station under an AC-DC system coupling is established. The calculation method of the command value of the DC current of the rectifier station is proposed by a predictive control model, and an improved suppression method for AC transient overvoltage is proposed. The case studies show that the accuracy and effectiveness of the suppression of AC transient overvoltage are improved by considering the coupling between AC and DC systems.

Research on Fault Identification of Hybrid Multi-Feed High-Voltage Direct Current System Based on Line Commutated Converter and Voltage Source Converter

With the rapid development of voltage source converter (VSC) and line commutated converter (LCC) technology and the relative concentration of power and load, the inverter station of the flexible DC system is fed into the same AC bus with the conventional DC rectifier station, and the high-voltage direct current (HVDC) parallel hybrid feed system is formed in structure. As the electrical distance between the converter stations is very close, when a fault occurs in the near area, the current on the AC wiring on the VSC side will fluctuate greatly, resulting in the misoperation of the AC wiring protection. For this reason, this paper proposes a fault identification method based on VSC/LCC hybrid multi-fed HVDC system, which discriminates the fault and outputs the protection signal according to the protection criterion, and logically judges the combination of the output protection signal to identify the fault type. The simulation results show that the method can identify all kinds of faults of hybrid multi-feed DC system and solve the problem of protection misoperation of the hybrid multi-feed DC system.

Overvoltage Mechanism and Suppression Method for LCC-HVDC Rectifier Station Caused by Sending End AC Faults

This letter analyzes the mechanism of the AC bus overvoltage at the LCC-HVDC rectifier station caused by the sending-end AC faults. Firstly, the influence of commutation failure on relevant electrical quantities is analyzed. After the commutation failure disappears, the inverter switches to constant current control. Under the joint action of the constant current control of the rectifier, the DC current and the trigger angle of the rectifier decrease rapidly, which reduces the reactive power absorbed by the rectifier. With the recovery of HVDC transmission capacity, the sending end AC bus voltage sharply rises, which increases the reactive power output of the filter. Reactive power redundancy occurs in the sending end AC bus leading to overvoltage. The suppression method proposed in this letter can effectively increase the reactive power absorbed by the LCC-HVDC rectifier during fault recovery. Besides, the method can also significantly enhance the minimum value of the inverter extinction angle and avoid possible commutation failure. The effectiveness and applicability of the strategy are confirmed through Electromagnetic Transient simulation.

Control Method for Inhibiting Chain Reaction of Commutation Failure

Due to the coupling effect between the DC/AC system as well as between different LCC-HVDC, the first commutation failure can cause a series of chain reactions, including successive commutation failure, overvoltage of the rectifier station, and subsequent commutation failure. The existing control methods for suppressing overvoltage or commutation failure cannot simultaneously meet the needs of the sending and receiving end grid of MIDC. It may expand the impact and scope of a single fault. Therefore, a commutation failure chain reaction suppression method is proposed based on the characterization of chain reaction boundary conditions. The effectiveness is verified by using the CIGRE standard test model.

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.

Control Strategy for Improving the Voltage Regulation Ability of Low-Carbon Energy Systems with High Proportion of Renewable Energy Integration

In low-carbon energy systems, due to the high proportion of renewable energy access, the voltage regulation capacity of the system will decrease. Therefore, in the event of voltage violation, it is easy to cause large-scale renewable energy off-grid and power outages. In order to improve the voltage regulation ability of low-carbon energy system, this paper proposes a two-stage overvoltage suppression strategy for sending-end power grid. Firstly, the principle of overvoltage phenomenon in the sending end power grid of low-carbon energy system with high proportion of renewable energy access is studied, and an overvoltage control strategy composed of two stages of centralized control of rectifier station and flexible resource control of distributed power grid is proposed. Then, the PSO algorithm and consensus algorithm are used to solve the established control model. Finally, a simulation system is established based on actual operating power grid data to verify the proposed control strategy through simulation. The results indicate that the control strategy proposed in this paper can effectively suppress transient overvoltage of AC buses and improve the operational stability of the high proportion of renewable energy sending-end power grid under various operating conditions. In addition, during the daytime overvoltage regulation process, the potential of flexible regulation equipment can be fully utilized. Shortening the duration of voltage exceeding the limit and reducing the peak voltage exceeding the limit can help reduce the renewable energy waste rate of the power grid.

Strategies for improving ride-through capability of three-terminal HVDC system under AC side fault

This paper considers a three-terminal hybrid DC transmission system in which the rectifier station adopts line-commutated converter (LCC) while the inverter station adopts parallel two-terminal modular multilevel converter (MMC). The focus of this paper is on the improvement of ride-through capability of LCC AC system in the presence of a fault. A strategy for improving the fault ride-through capability based on local information is proposed. First, the mathematical models of different types of converters in a hybrid DC system are established, and the influence of AC faults at the sending end on the DC transmission system is discussed. Second, in the presence of a slight fault on the AC side of LCC, the current margin control is proposed for the load center side MMC2, where the MMC2 maintains the DC port current as the minimum allowable value. Third, if a serious fault on the AC side of LCC causes a significant AC voltage drop, it is proposed that the non-load center terminal MMC1 adopts voltage margin control. In addition, the MMC1 takes over the DC system voltage control so as to maintain MMC2 to absorb minimum power from the DC system, and prevent the load center connected by the MMC2 from removing too many important loads or causing a frequency collapse. Last, a large number of simulation results verify the effectiveness of the proposed methods.

An improved reactive power control strategy for LCC-HVDC to mitigate sending end transient voltage disturbance caused by commutation failures

The sending end transient voltage disturbance (TVD) caused by commutation failure of line commutated converter-based high voltage direct current (LCC-HVDC) systems, which is characterized by “first reduce then rise”, has received increasing attention due to its threat to the stable operation of renewable energy integrated modern power systems. To mitigate the TVD, this paper devises an improved reactive power control strategy that takes into account the signal delay between converter stations. Firstly, the mechanism of the TVD is investigated. It is found that the voltage dependent current order limiter (VDCOL) and signal delay between the rectifier and the inverter stations are two vital factors that impact the TVD, while the VDCOL cannot well cope with commutation failure suppression and TVD mitigation at the same time. On this basis, an improved closed-loop reactive power control (CLRPC) strategy is proposed for the LCC-HVDC to mitigate the TVD. The CLRPC can regulate the reactive power demand of the rectifier station by adjusting the DC current order, which replaces the conventional VDCOL during the disturbance. Furtherly, a delay compensation unit is augmented in the proposed scheme to suppress the adverse effect of signal delay. Simulation results conducted in PSCAD/EMTDC verify that the CLRPC can not only alleviate the TVD effectively but also suppress the occurrence of successive commutation failures, thus, enhancing the overall performance of the sending end power system.

Analysis of Hybrid Modular Multilevel Rectifier Operated at Nonunity Power Factor for HVDC Applications

Hybrid modular multilevel rectifier (HMMR) is a type of unidirectional modular voltage source converter (VSC) that has been proposed for the high-voltage (HV) dc transmission applications. Compared to the modular multilevel converter (MMC), up to 50% of the submodules (SMs) and capacitors can be replaced by the simple HV line-frequency diode stacks at unity power factor (PF). The nonunity PF operation is also favored for the rectifier station to support the grid. Therefore, this article proposes two new current commutation methods for HMMR to reduce the SM number at nonunity PF. In addition, an average model is given to reveal the inherent power flow relationship of HMMR. It is found that part of the active power is delivered from ac to dc side through the diode directly, which reduces the energy storage requirement. The performance of HMMR is also evaluated and compared with MMC. Both methods can successfully decrease the 30% low voltage devices. Method 1 can reduce 32% capacitor and method 2 achieves 52% reduction. The feasibility of the proposed converter is verified by the simulation results from a 200-kV 200 MVA HVdc model. Finally, a scale-down 400-V HMMR prototype is developed to validate the effectiveness of the proposed control scheme.

An AC Fault Ride Through Method for MMC-HVDC System in Offshore Applications Including DC Current-Limiting Inductors

The current-limiting inductors (CLIs) are commonly used to suppress DC fault current to safely trip DC circuit breakers in MMC-HVDC systems, but it significantly jeopardizes DC dynamics and causes instantaneous power mismatch between AC and DC sides of islanded rectifier station (RS), exposing MMC internal states to large disturbance. This paper firstly provides a detailed analysis of the influence of CLIs on AC fault, establishes RS mathematical representation including AC side dynamic, and reveals the mechanism of DC voltage oscillation, modulation saturation and PCC waveform distortion. The analysis shows that the AC and DC fault characteristics are decoupled under unsaturated AC modulation, but coupled under excessively low submodule capacitor voltage, where AC modulation is saturated and PCC waveforms are distorted. To solve this issue, a novel RS AC fault ride through method for MMC-HVDC system with CLIs is proposed to stabilize RS DC voltage, desaturate AC modulation and decouple fault characteristics by regulating the zero-sequence modulation of circulating current suppression. In addition, a current reference disturbance and an AC energy dissipation device coordinated control are designed, which adaptively recovers the submodule capacitor voltage. Finally, the feasibility of the proposed method is verified by simulation results.

S-Transform Based Traveling Wave Directional Pilot Protection for Hybrid LCC-MMC-HVDC Transmission Line

In this paper, the traveling wave protection issue of a hybrid high-voltage direct-current transmission line based on the line-commutated converter and modular multilevel converter is investigated. Generally, traveling wave protection based on voltage variation criterion, voltage variation rate criterion and current variation rate criterion is applied on hybrid high-voltage direct-current transmission lines as primary protection. There are two issues that should be addressed: (i) it has no fault direction identification capability which may cause wrong operation regarding external faults; and (ii) it does not consider the difference between line-commutated converter based rectifier station topology and modular multilevel converter based inverter station topology. Therefore, a novel traveling wave directional pilot protection principle for the hybrid high-voltage direct-current transmission line is proposed based on the S-transform. Firstly, the data processing capability of S-transform is described. Secondly, the typical traveling wave propagation process on a hybrid high-voltage direct-current transmission line is studied. Thirdly, a novel traveling wave fault direction identification principle is proposed. Eventually, based on PSCAD/EMTDC, a typical ±400 kV hybrid high-voltage direct-current transmission system is used for a case study to verify its robustness against fault location, fault resistance and fault type.

Fault detection and diagnosis of monopolar configured VSC based high voltage direct current transmission line

The development of HVDC transmission technology using an MMC has been promoted, by overcoming the drawbacks of traditional VSC technology. The extension of flexible transmission to overhead lines, especially the use of HVDC transmission based on an MMC, raises the issue of DC fault. So identification of fault, clearing the dc fault, and design of fast-acting protection system operating against fault becomes significant. This article provides insights on the monopolar structured MMC and operational characteristics, fault analysis, and a fault protection scheme. DC line faults on HVDC lines using MMC-VSC are major issues; isolation of complete system is not a viable option. It is observed that Pole to ground fault is the most common fault, which leads to generous overcurrent in the AC grid and results in converter valves getting damaged. This article analyzes the response of MMC-HVDC under different DC and AC faults conditions for five-level MMC HVDC systems, to better understand systems under fault. Faults also have an impact on the converter stations' performance. The voltages fluctuate in faulty situations. In comparison to the inverter station, the rectifier station has the most impact. Simulation is performed out in PSCAD software. The correctness and effectiveness of DC and AC fault analysis helps to check the capability of locating fault occurring on HVDC transmission lines quickly and accurately

Low-Frequency Oscillation Analysis of VSM-Based VSC-HVDC Systems Based on the Five-Dimensional Impedance Stability Criterion

Virtual synchronous machine (VSM) based high voltage dc systems enhance the inertia of the power system. However, the dynamic interactions between the VSM-based rectifier station, the inverter station, and the grid could induce the system oscillation, which has been investigated in this article. At first, the hybrid ac–dc impedance models of the VSMs considering the coupling between the ac and dc dynamics are established. Then, the relationships between the dc impedance, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dq</i> -frame impedance, and the hybrid ac–dc impedance are presented. It is found that the dc impedance of the VSM-based inverter station and the d-d channel impedance of the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dq</i> -frame impedance of the rectifier station behave as negative resistors in the low-frequency range. Moreover, a five-dimensional impedance stability criterion based on the hybrid ac–dc impedance and generalized inverse Nyquist criterion is proposed to assess the system stability. The analysis results show that the low-frequency oscillation occurs when the grid short-circuit ratio of the rectifier station is small. Finally, the simulation and experimental results verify the impedance models and the stability criterion.

An improved control method of fault ride-through for DPMWT-based wind farm considering coupling effect of HVDC transmission system

With the continuous increase of wind power penetration, a large number of wind farm have been constructed. The large-scale wind farm is usually transmitted through high voltage direct current(HVDC) transmission system. The reactive power output on the grid-side converter is utilized to support grid voltage, which is a common method of fault ride-through for direct-drive permanent magnet synchronous generator-based wind turbine (DPMWT). The power control capability of DPMWT-based wind farm(DPMWF) is affected by safety constraints and operating characteristics of the external system. However, the control strategy of DPMWF cannot yet take into account the coupling effect of HVDC and may be difficult to meet the voltage recovery requirements of the grid during the grid fault. Even the DPMWT may oscillate due to the over-threshold of the control setting value. Therefore, a power coupling model between DPMWF and rectifier station of HVDC transmission system is established, and an evaluation method for the power controllability of DPMWF under fault considering the coupling effect of HVDC transmission system is proposed. A calculation method of maximum reference value of reactive power control of DPMWF based on model predictive control is proposed taking into account the converter constraints of DPMWF and the influence of the external system. An improved control method of fault ride-through is further proposed, which takes into account the coupling effect of HVDC transmission system. The simulation shows that the proposed method can improve the voltage recovery characteristics under the grid fault, effectively guarantee the fault ride-through of DPMWF.

Integrated maintenance planning approach to optimize budget allocation for subway operating systems

Millions of citizens worldwide heavily rely on subways as their main public transportation approach. However, public transit infrastructures are generally facing extensive deterioration that requires extensive funds by governments to retain back the desired performance levels. The real case is that most governments are heavily facing a budget scarcity dilemma, and cannot withstand the acquired maintenance costs to maintain a proper operation for subway network. This research aims at introducing a new risk-based multi-objective optimization framework for optimizing performance and budget allocation among subway systems in fulfillment to maintenance planning development in this domain. This would extensively ease the decision-making process for subway asset managers by providing the optimal allocated budget for each subway system in an automated way. The framework targets the vital subway operating systems, namely: railway tracks, overhead catenary, lighting & power supply, centralized control, communication, signaling, rolling stock, and ticketing. The research framework develops four main modules; Reliability Centered Maintenance Database, Deterioration Modelling, Multi-objective Optimization, and Decision-Making. Reliability Centered Maintenance Database where subway operating systems are decomposed to components levels showing their functions, operational failures, failure causes, failure effects, failure consequences, responsive maintenance actions, and the hourly maintenance costs. Deterioration Modeling using deterministic and artificial intelligence models to estimate future conditions of subway components. Multi-objective optimization, and decision-making are developed to decide the optimal maintenance frequencies that derive lowest maintenance costs and lowest risk of failure for each functional failure. An actual case-study is worked out to verify the proposed framework. The results reveal the greatest share of the maintenance budget was allocated to lighting & power supply rectifier station with a 54% of the total maintenance budget, followed by rolling stock system with 32%, railway tracks system with 7%, signaling system with 3%, and approximately 1% for each of the remaining systems.

Comparison of Two Control Strategies for VSC-MTDC with Wind Farm

: Two VSC-MTDC control strategies with different combinations of controllers are proposed to eliminate transient fluctuations in the DC voltage stability, resulting from a power imbalance in a VSC-MTDC connected to wind farms. First, an analysis is performed of a topological model of a VSC converter station and a VSC-MTDC, as well as of a mathematical model of a wind turbine. Then, the principles and characteristics of DC voltage slope control, constant active power control, and inner loop current control used in the VSC-MTDC are introduced. Finally, the PSCAD/EMTDC platform is used to establish an electromagnetic transient model of a wind farm connected to a parallel three-terminal VSC-HVDC. An analysis is performed for three cases of single- phase grounding faults on the rectifier and inverter sides of a converter station and of the withdrawal of the converter station on the rectifier side. Next, the fault response characteristics of VSCMTDC are compared and analyzed. The simulation results verify the effectiveness of the two control strategies, both of which enable the system to maintain DC voltage stability and active power balance in the event of a fault. Background: The use of a VSC-MTDC to connect wind power to the grid has attracted considerable attention in recent years. A suitable VSC-MTDC control method can enable the stable operation of a power grid. Objective: The study aims to eliminate transient fluctuations in the DC voltage stability resulting from a power imbalance in a VSC-MTDC connected to a wind farm. Methods: First, the topological structure and a model of a three-terminal VSC-HVDC system connected to wind farms are studied. Second, an analysis is performed of the outer loop DC voltage slope control, constant active power control and inner loop current control of the converter station of a VSC-MTDC. Two different control strategies are proposed for the parallel three-terminal VSCHVDC system: the first is DC voltage slope control for the rectifier station and constant active power control for the inverter station, and the second is DC voltage slope control for the inverter station and constant active power for the rectifier station. Finally, a parallel three-terminal VSC-HVDC model is built based on the PSCAD/EMTDC platform and used to verify the accuracy and effectiveness of the proposed control strategy. Results: The results of simulation analysis of the faults on the rectifier and inverter sides of the system show that both strategies can restore the system to the stable operation. The effectiveness of the proposed control strategy is thus verified. Conclusion: The control strategy proposed in this paper provides a technical reference for designing a VSC-MTDC system for wind farms.

Series-DC connection of Offshore wind generating units - modeling, control and galvanic isolation.

This work proposes an air-core DC-DC solid state transformer to connect the wind turbines of an offshore wind farm in series with a DC transmission link. This emerging layout does not use a power transformer to increase the voltage from the offshore collector network to the transmission level, nor does it require a rectifier station to convert the electrical energy to DC. The diode-clamped rectifiers, connected to each permanent-magnet generator, are controlled to drain sinusoidal currents from the generators in order to extract the maximum power from the wind turbines. The air-core transformer, designed by means of a finite element software, ensures the galvanic isolation between the wind generator and the HVDC link. From the generator side, a single-phase full-bridge three-level inverter feeds the primary coil of the transformer, whereas the secondary coil is wound using by means of an XLPE-insulated cable. This paper also presents a strategy for designing the controllers that regulate the output voltage of each wind conversion unit. Results from digital simulations, obtained with PSCAD/EMTDC, are presented to validate the technical feasibility of the proposed DC-DC solid-state transformer scheme, as well as to demonstrate its performance under different operational conditions.

Dynamic Phasor Modelling of LCC-HVDC System Based on a Practical Project

This paper presents a detailed dynamic phasor modeling process of a line-commutated converter-based HVDC (LCC-HVDC) system. Firstly, the dynamic phasor models of the single-ended LCC rectifier station, inverter station and the DC line are established, respectively. Secondly, LCC-HVDC is an AC-DC-AC system. The interfaces are explained to connect the converter stations with the DC line. Through block modeling, it is helpful to simplify the process and verify the accuracy of each block. Finally, based on a practical project, the model is compared with the electromagnetic-transient (EMT) simulation results in PSCAD/EMTDC to verify the accuracy of the dynamic phasor model.