Voltage Dependent Current Order Limiter Research Articles

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.

Coordinated Optimization for Multi-Infeed LCC-HVDCs Transient Control Considering Short-Term Voltage Stability of Receiving-End Grid

With local synchronous generators in receiving-end grids being replaced by multi-infeed line-commutated converter-based (LCC) high-voltage direct currents (HVDCs), the dynamic voltage supportability of the power grid decreases. Moreover, the reactive power consumption of LCC-HVDCs during the recovery process brings risks to voltage stability. Hence, the transient control of LCC-HVDCs should be optimized coordinately to support short-term voltage stability under credible contingencies. In this paper, the dynamic reactive power characteristics between AC/DC systems are derived mathematically. Based on the coupling mechanism among the voltage-dependent current order limiter (VDCOL), AC dynamic voltage, and HVDC power recovery, an “autonomous-synergic” optimization strategy incorporating short-term voltage stability constraints is proposed for multi-infeed LCC-HVDCs. To handle the problem of large-scale differential-algebraic equations (DAEs), the optimization is iteratively solved based on time-domain simulation (TDS) and trajectory sensitivities. The stability is quantified by an indicator and the trajectory sensitivity analysis is utilized to relax stability constraints. The effectiveness and feasibility of the proposed methods are verified with a benchmark system and a real system in China. The short-term voltage stability under all credible contingencies can be guaranteed, while the total power recovery of LCC-HVDCs is improved.

A Dynamic Nonlinear VDCOL Control Strategy Based on the Taylor Expansion of DC Voltages for Suppressing the Subsequent Commutation Failure in HVDC Transmission

Subsequent commutation failure in high-voltage DC transmission systems seriously emphasizes the safe and stable operation of power systems. Via analyzing the mechanism of commutation failure and the principle of voltage-dependent current order limiter (VDCOL), this paper proposes a dynamic nonlinear VDCOL control strategy based on the Taylor expansion of DC voltage for suppressing subsequent commutation failure. To solve the problem of fluctuating DC current command value caused by a large drop in DC voltage, this paper constructs a nonlinear VDCOL control that can be dynamically adjusted according to the AC bus voltage level, and Taylor expansion of DC voltage is used to obtain its first-order and second-order differential components. Different scales of differential elements are chosen to predict the DC voltage compensation value while balancing sensitivity and accuracy. The compensated DC voltage, used as the starting voltage of VDCOL, is input to the VDCOL control constructed in this paper to suppress the subsequent commutation failure of the transmission system by reducing the fluctuation of the current command value. Finally, the standard test model of HVDC is established based on the actual parameters, and the simulation results show that the Method proposed in this paper has an effective suppressing effect in the case of single-phase or three-phase faults of different severity and is conducive to the restoration of the system power transmission.

Analysis of Secondary Controller on MTDC Link with Solar PV Integration for Inter-Area Power Oscillation Damping

Integration of renewable energy sources is important in limiting the continuous environmental degradation and emissions caused by energy generation from fossil fuels and thus becoming a better alternative for a large-scale power mix. However, an adequate analysis of the interaction with the alternating current (AC) network during network disturbance, especially during inter-area power (IAP) oscillations is needed. Insufficient damping of oscillations can significantly impact the reliability and effective operation of a whole power system. Therefore, this paper focuses on the stability of the modified Kundur two-area four-machine (MKTAFM) system. A robust secondary controller is proposed and implemented on a line commutated converter (LCC)-based multi-terminal high voltage direct current (MTDC) system. The solution consists of a local generator controller and the LCC MTDC (LMTDC) system, voltage-dependent current order limiter, and extinction angle controller. The proposed robust controller is designed for the LMTDC systems to further dampen the inter-area power oscillations. Three operational scenarios were implemented in this study, which are the local generator controller and double circuits AC line, local generator controller with LMTDC controllers, and local generator controller with LMTDC controllers and secondary controller. The simulation result carried out on PSCAD/EMTDC recorded better damping of the inter-area power oscillation with LMTDC. A considerable improvement of 100% damping of the IAP oscillations was observed when a secondary controller was implemented on the LMTDC.

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.

Interruption method for commutation failure caused cascading reaction of HVDC with wind farm integration under grid fault

Line-commuted converter based high voltage direct current (LCC-HVDC) is a typical mode for wind power transmission. The fault of the receiving-end grid of LCC-HVDC can easily lead to the commutation failure at the inverter station. Improper controls during the recovery process of first commutation failure may cause the subsequent commutation failure, and lead to the high voltage ride through of wind generators at the sending-end grid. The regional and inter-regional cascading reactions are produced by commutation failure. Existing researches severally focus on control of suppressing subsequent commutation failure or overvoltage. The demand for commutation failure recovery of the inverter and overvoltage suppression at the sending-end grid cannot be satisfied simultaneously, and may even aggravate the cascading process. The stability of the power system will be threatened. Therefore, the new idea of feasible power region and adaptive current control of inverter station is proposed to interrupt commutation failure caused cascading reaction. The feasible ranges of DC current to avoid subsequent commutation failure and overvoltage are derived. The feasible power region is modeled by the active power and exchanged reactive power of inverter station. The principle of interrupting commutation failure caused cascading reaction based on adaptive current control of inverter station is proposed. Then, the command value calculation and implementation method of adaptive current control are further proposed. Finally, the performance of the proposed control method and voltage-dependent current order limiter (VDCOL) control is compared based on the CIGRE LCC-HVDC standard system, and the effectiveness of theoretical research is verified by the simulation results.

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.

Coordinated Control Strategy for Cascading Commutation Failure Mitigation Considering Reactive Power Interaction

Cascading commutation failure (CF) caused by the reactive power interaction between HVDC is one of the new forms of cascading failures in multi-infeed HVDC systems. However, the concept and mechanism of cascading CF have not been explained clearly, and few studies have been devoted to control strategy for cascading CF mitigation considering reactive power interaction. In this paper, the cascading CF is defined and novel coordinated control strategy for cascading CF mitigation is proposed. Firstly, the mechanism of cascading CF in multi-infeed HVDC systems is analyzed. Then influence degree of HVDC on the power system stability is evaluated considering both active power impact and reactive power interaction, based on which novel coordinated commutation failure prevention as well as voltage dependent dc current order limiter is developed to mitigate cascading CF. Simulation results based on CIGRE HVDC benchmark model in PSCAD/EMTDC verify the effectiveness and advancement of proposed strategy on cascading CF mitigation.

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

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

Overvoltage Suppression Under Commutation Failure Based on Improved Voltage-Dependent Current Order Limiter Control Strategy

Commutation failure (CF) in the line-commutated converter based high-voltage direct-current system will result in a large amount of reactive power surplus on the rectifier ac system. It will cause the transient overvoltage and lead to the massive disconnection of the renewable sources. In this article, the mechanism of the transient overvoltage caused by CF is analyzed and the suppression strategy based on improved voltage-dependent current order limiter (VDCOL) control is proposed. First, during the process of CF, the dynamic behavior of reactive power consumption of the rectifier is analyzed. It is proposed that when the inverter returns to normal commutation, the sudden increase of the inverter dc voltage will cause the decreasing of the dc current, which will result in the large decreasing of reactive power consumption of the rectifier. Then, the dynamic change process of the rectifier ac voltage is deduced. Second, the peak value of transient overvoltage as affected by the occurrence and removal time of the short-circuit fault is deduced and analyzed. Then, by improving the operating characteristic of the VDCOL control, the control strategy of overvoltage suppression is proposed. Based on the PSCAD platform's CIGRE-HVDC benchmark model, the effectiveness of the proposed strategy is verified by the simulation results.

Research on methods of measuring extinction angle and measures to suppress repetitive commutation failures through equivalent DC input resistance

Accurate, simple identification and suppression of repetitive commutation failures are of great significance to the safe and stable operation of the line-commutated converter-based high-voltage direct current (LCC-HVDC). Through the analysis of mathematical models of inverter side converter and DC transmission system. This paper proposes a variable that has an important influence on commutation failure: equivalent DC input resistance. Therefore, a measurement method is proposed, which takes into account the phase deviation of the fundamental commutation line voltage and uses equivalent DC input resistance as the input of the established mathematical model to measure the extinction angle. Meanwhile, according to the characteristics of equivalent DC input resistance, a method based on constant equivalent DC input resistance (EDCIR) to suppress repetitive commutation failures is proposed. Finally, several simulations are conducted on the CIGRE HVDC benchmark and GuiguangII model to validate the effectiveness of the proposed measurement method(TPMM) in the accuracy and suppression of commutation failure. The simulation results show that the proposed measurement method has the same accuracy as the measured extinction angle method and is higher than that of the minimum voltage drop method commonly used in practical engineering in judging commutation failure. At the same time, the proposed method to suppress can effectively reduce the number of commutation failures under single-phase and three-phase ground faults comparing with voltage-dependent current order limiter (VDCOL) and commutation failure prevention and control (CFPREV).

Rapid Recovery Control Method Based on Improved VDCOLs for Hybrid Multi-Infeed DC Transmission System After AC Failure

Continuous commutation failure is very likely to occur in the hybrid Multi-infeed high-voltage direct current (HMIDC) after AC failure. In order to improve the recovery quality after HMIDC failure, an AC-DC voltage-dependent current order limiter (VDCOL) based on system strength index is proposed in this article. Firstly, the control mode transition process and system recovery process after DC failure are analyzed based on the hybrid multi-infeed DC transmission port model. Then, considering the impact of AC voltage and DC voltage input signals of VDCOL on AC voltage recovery and DC power recovery, respectively, the interaction factor and strength index of the hybrid multi-infeed system are constructed. Moreover, the weight coefficient of AC and DC voltage is calculated according to the strength of the multi-infeed system. Finally, a three-infeed hybrid DC transmission simulation model is built in the MATLAB/Simulink digital simulation platform. The simulation results demonstrate that the rapid recovery strategy proposed in this article can effectively suppress continuous commutation failure and improve the recovery speed of AC voltage and DC power.

Line Commutated Converter Response during Total and Partial De-Blocking of a Bipolar MTDC System

This paper focuses on the fault blocking analysis and operational issues associated with MTDC systems incorporated in an AC network. The dynamic modelling of a line-commutated converter based bipolar multi-terminal direct current (LCC MTDC) system are shown, and the dynamic response of the converter during a DC converter fault is discussed. The converter controller design for both rectifiers and the inverters system was modelled for a realistic active power and extinction angle (γ) control with consideration to the VI characteristics of all the converter stations. An overall power controller was modelled for both converter pole. Two operational scenarios of converter fault were simulated using PSCAD EMTDC. The converter firing angle and extinction angle, as well as the voltage-dependent current order limiter, was monitored and plotted on a graph. Results show that the MTDC link became unstable during the full deblocking stage with a continuous occurrence of commutation failure. Furthermore, the results presented in this paper show that during partial converter de-blocking showed a favourable performance, as the power system remains stable and commutation failure of the MTDC system is prevented.

A new AC fault ride-through strategy for HVDC link with serial connected LCC-VSC hybrid inverter

HVDC link with line commutated converter (LCC) as a rectifier at the power sending end, and with a serial connected LCC and voltage source converter (VSC) hybrid inverter (SLVHI) at the power receiving end, is adopted for future Baihetan HVDC engineering project in China. To realize the AC fault ride-through (FRT) of SLVHI, a new strategy based on DC chopper (DCC) is proposed in this paper. Firstly, the mathematical model is built to investigate the VSC DC overvoltage mechanism after SLVHI commutation failure (CF) and related factors. Secondly, a modified DCC topology of SLVHI is designed to adjust unbalanced power dissipation based on voltage drop depth. Thirdly, an enhanced voltage-dependent current order limiter (VDCOL) is proposed to deal with the unbalanced power after DCC switching off. With the cooperation between modified DCC and enhanced VDCOL, the proposed FRT strategy can realize CF mitigation and VSC DC overvoltage suppression simultaneously. Finally, the inverter side AC FRT performances of the HVDC link with SLVHI are studied by PSCAD/EMTDC. The simulation results validate the proposed strategy's effectiveness and superiority, with better CF mitigation and VSC DC overvoltage suppression abilities than other existing FRT strategies under different fault scenarios.

Analysis and Improvement of the Multiple Controller Interaction in LCC-HVDC for Mitigating Repetitive Commutation Failure

In a line-commutated converter based high-voltage direct-current (LCC-HVDC) system, the inverter control system usually includes constant extinction angle (CEA) controller, constant current (CC) controller, current error controller (CEC), and voltage-dependent current order limiter (VDCOL). However, the interaction influence of multiple controllers may cause repetitive commutation failure (CF). For this purpose, this paper deeply investigates the mechanism and the influencing factors of repetitive CF caused by controller interaction. It is found that the lower limit value of CEA controller can affect the control switching process and thus the probability of repetitive CF. Besides, the minimum gamma measurement (MGM) unit prevents the system from acquiring the real extinction angle in time, which results in the DC current fluctuation during the recovery process. Moreover, the probability of control-induced repetitive CF is much higher when the HVDC inverter is connected to a weak AC system or the AC fault is not serious but not cleared in time. Based on the above analyses, an improved control method is proposed for recovery performance improvement, which is mainly realized by modifying the lower and upper limits of the CEA controller during recovery. Finally, the simulation and experimental results verify the theoretical analyses and the proposed control method.

A New Quantitative Analysis Method for Overvoltage in Sending End Electric Power System With UHVDC

With the development of renewable electric power generation and ultra-high voltage direct current (UHVDC) transmission, the transient overvoltage in the sending end electric power system with UHVDC occurs, but it is lack of quantitative transient voltage assessment index (TVAI) to assess the overvoltage. To deal with the above problems, this paper proposes a new quantitative transient voltage assessment index considering overvoltage and voltage dip in the sending end electric power system with UHVDC, and applies it to analyze different influencing factors of overvoltage and determine the dynamic reactive power compensation capacity. In detail, first, according to the principle of the transient voltage assessment index considering voltage dip and the overvoltage assessment index for the distribution network, a new transient voltage assessment index considering transient overvoltage in the sending end electric power system with UHVDC is proposed with the approximated step function. Then, based on the requirements of the system with UHVDC, the parameters of the proposed transient voltage assessment index are determined. Furthermore, with the absolute and relative sensitivity, the impact of the parameters of voltage dependent current order limiter and current control amplifier on the transient overvoltage is analyzed and compared quantitatively. Finally, the impact of reactive power compensation equipment, such as synchronous condenser, static var compensator, and static var generator, on the transient overvoltage of the system with UHVDC is compared quantitatively, and the reactive compensation capacity needed to maintain transient voltage security and stability is estimated. The two applications show the effectiveness and advantages of the proposed index in the sending end electric power system with UHVDC.

Real-Time Calculation Method of DC Voltage and Current to Eliminate the Influence of Signal Delay

In this study, the influences of the signal delay duration between rectifier and inverter in ultra-high voltage direct current (U-HVDC) transmission systems have been analyzed. The simulation results with different signal delay time under the same fault condition show that the signal delay time has a significant influence on the control system response during the fault and recovery periods. In the worst condition, it may lead to a commutation failure in the inverter. To eliminate the influence of the signal delay time on the DC control system, a fast calculation method for computing the voltage and current has been proposed by establishing the equivalent capacitance and inductance of the DC line. A voltage dependent current order limiter is added in the rectifier control system, which can calculate the DC current order value. Based on this concept, this paper proposes an improved control strategy to eliminate the influence of signal delay. The analysis and simulation results show that the calculation method can effectively improve the response speed and shorten the fault recovery time of the HVDC systems.

Variable Extinction Angle Control Strategy Based on Virtual Resistance to Mitigate Commutation Failures in HVDC System

To mitigate commutation failures (CFs) and minimize the instability of HVDC systems due to AC faults or control mode ambiguity, a constant extinction voltage-time area based control strategy with virtual resistance is proposed. In this strategy, using sine-cosine components detector and considering zero-crossing phase shift of commutation voltage, the extinction angle setting value can be adjusted dynamically. Meanwhile, to reflect the characteristics of DC current during the fault and recovery process, a virtual resistance is introduced into the control system and DC voltage considering the voltage drop of the virtual resistance is taken as the input of voltage-dependent current order limiter (VDCOL). Through theoretical analysis, the proposed strategy not only reduces the firing angle dynamically, but also reduces DC current by lowering the current order on the rectifier side immediately when the AC voltage disturbance is detected, thereby further reducing the occurrence of CFs. Therefore, the control strategy can effectively suppress successive and intermittent CFs. The effectiveness of the proposed control strategy is verified by simulation of the single HVDC and multi-infeed HVDC model based on the CIGRE HVDC benchmark system, in which AC-DC current criterion of identifying CF and suppression ratio index are adopted. The simulation results show that the proposed control strategy can effectively mitigate CFs under single-phase and three-phase faults to a certain extent. Comparing with the existing control strategies based on controller modification, i.e. commutation failure prevention, DC current predictive control, smooth logic switching control and DC current limitation control strategy based on virtual resistance, the proposed control strategy is superior in mitigation effects. The average suppression rates of these strategies are 1.66%, 3.21%, 6.11%, 3.35%, and 8.33% under single-phase fault, respectively; with the rates of 1.4%, 2.72%, 4.97%, 0%, and 6.8% under three-phase fault, respectively.

A New Recovery Strategy of HVDC System During AC Faults

To overcome the disadvantage of voltage dependent current order limiter of HVDC control system, which cannot achieve decoupling control of reactive power and active power, a novel ac fault recovery strategy is proposed in this study. By virtue of such recovery strategy, the exchange of reactive power between the ac system and the converter station can be controlled quantitatively to find the correspondence between dc order and ac bus voltage. In order to determine the expected reactive power consumption of converters, the instantaneous reactive power theory is used to calculate the reactive power compensated by ac filters. According to the quasi steady-state calculation equations of the HVDC system and the operational range of converter firing angle, the constraint relation between the maximum permissible operating value of dc and ac bus voltage under different ac fault types is obtained. Then, by comparing the dc order and the maximum value of dc, the smallest of them is taken as the final dc order so as to improve the recovery performance of the ac–dc system in real time. Finally, the effectiveness and robustness of the proposed control strategy in suppressing subsequent commutation failures and improving the system recovery performance are verified in the CIGRE benchmark model for HVDC controls.

Master self‐tuning VDCOL function for hybrid multi‐terminal HVDC connecting renewable resources to a large power system

To take advantage of line commutated converter (LCC) and voltage source converter high-voltage, direct current (HVDC) technologies a hybrid multi-terminal HVDC (MTDC) is designed to exploit the power from renewable resources. A previous study showed that the voltage dependent current order limiter (VDCOL), an emergency function to reduce the current order of an HVDC converter station when voltage is depressed, should be installed in all of the LCC stations of the hybrid MTDC. This study presents a design for a master self-tuning VDCOL function implemented in a hybrid multi-terminal HVDC system. This controller aims to generate an adaptive current reference for each LCC station of the VDCOL in a low-voltage condition. The parameter of this adaptive VDCOL can be chosen according to the summary table of practical experiences; therefore, no tuning technique is required. The developed HVDC control algorithm is implemented on the simplified South East (SE) Australian 14 generator system using DIgSILENT PowerFactory. The simulation results demonstrate the effectiveness of the proposed control function in reducing the complexity of the control system and operation along with improving system transient stability.