Capacitor Commutated Converter Research Articles

A Time-Domain Piecewise Analytical AC Harmonic Current Calculation Method Suitable for Capacitor Commutated Converter

The Capacitor Commutated Converter (CCC) is constructed as a conventional Line Commutated Converter (LCC) modified with a series capacitor between the converter transformer and thyristor valves in each phase. This paper proposes a time-domain piecewise analytical AC harmonic current calculation method for the CCC system. Compared to the existing calculation methods, the proposed method can calculate both characteristic and non-characteristic harmonics simultaneously when various asymmetric factors are considered. Firstly, the overlap angles are calculated considering the asymmetric factors after partitioning the piecewise intervals in a fundamental period. Secondly, the AC harmonic currents are calculated considering the DC side harmonic currents based on the Thevenin equivalent model of the DC circuit. Then, the effectiveness of the proposed calculation method is validated by simulation results in PSCAD/EMTDC. The comparison result shows that the proposed method is high-efficiency and high-accuracy for the AC harmonic current calculation of the CCC system and can meet the requirement of CCC filter design in practical projects. Finally, the influence of the asymmetric factors on the overlap angle calculation and the influence of the commutation capacitor variation on the commutation parameters are studied.

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.

Study on fault characteristics of capacitor commutated converter

The traditional line commutated converter (LCC) is prone to commutation failure when connecting to weak AC systems. Moreover, a large number of the reactive power compensation devices are needed for the LCC. A capacitor commutated converter (CCC) can overcome the above drawbacks to a certain extent. In order to explore the fault characteristics of the CCC, this paper firstly gives the mechanism of the CCC. Then simulation models of the LCC and the CCC are established in PSCAD. Finally, waveforms are presented to explore the fault ride-through characteristics of the CCC under different fault types. The transient characteristics of the CCC are compared with those of the LCC to provide theoretical support for the application of the CCC.

Calculation of Main Circuit Steady-State Parameters for Capacitor Commutated Converter System

The calculation of the main circuit parameters is the basic part of the engineering design for high voltage direct current (HVDC) transmission systems. Compared to the conventional line commutated converter (LCC), the application of the capacitor commutated converter (CCC) can reduce the probability of commutation failures and the shunt capacitor reactive compensation. This paper proposes a calculation method of main circuit parameters for the CCC-based HVDC system. Firstly, the topology of a CCC-HVDC transmission system is described. Secondly, based on the steady-state mathematical model of the CCC, the paper proposes the calculation method of the commutation capacitor to satisfy the system requirements, and the calculation formulas of the main circuit parameters are also given. Then the calculation procedure of the main circuit steady-state parameters is described in detail considering system parameters, control modes, calculation tolerances and operating conditions. Finally, a two-terminal ±500 kV/3000 MW LCC-CCC HVDC transmission system is presented to verify the validity of the main circuit parameter calculation method. The proposed method has great significance for the AC filter design in practical engineering application.

Economy Analysis of Flexible LCC-HVDC Systems with Controllable Capacitors

Commutation failure (CF) is a frequent dynamic event at inverter of LCC-HVDC systems caused by AC side faults which can lead to inverter blocking, interruption of active power transfer, and even system blackout. To eliminate CFs and improve system performance, new Flexible LCC-HVDC topologies have been proposed in previous research but with limited analysis on its economic performance. Therefore, to further validate the applicability of Flexible LCC-HVDC topologies, this paper utilizes Life-Cycle Cost Analysis model to analyze the life-cycle cost of inverter stations for conventional LCC-HVDC, Capacitor Commutated Converter based HVDC (CCC-HVDC) topology and Flexible LCC-HVDC topologies including Controllable Capacitor based Flexible LCC-HVDC, AC Filterless Controllable Capacitor based Flexible LCC-HVDC and improved Flexible LCC-HVDC. Through a case study based on a 500 kV, 1000 MW LCC-HVDC scheme, comparison results show that the AC Filterless Controllable Capacitor based Flexible LCC-HVDC topology and the improved Flexible LCC-HVDC topology have lower cost than the conventional LCC-HVDC and CCC-HVDC topologies, which proves that the elimination of CFs can be achieved with reduced cost.

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

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

Secondary Frequency Support to Weak Grids Through Coordinating Control of Hybrid HVDC System

A recently proposed Hybrid-High Voltage DC (HVDC) system comprising a Capacitor Commutated Converter connected in series with a 2-stage Voltage Source Converter called `Vernier' on the DC side and in parallel on the AC side is considered to transfer power from onshore wind farms (WFs) to the load centers. The flexible DC voltage polarity of the Vernier gives rise to circulating power. Although the circulating power leads to unwanted losses, it can be leveraged as an additional degree of freedom. This extra degree of freedom is used to provide secondary frequency control in the inverter-side grid by extracting power from the rectifier without any communication of signals while maintaining constant margin angle and firing angle at the respective ends. Moreover, to improve the rectifier-side grid frequency, the WFs provide primary frequency support while operating under deloaded condition. Analytical insight into this complex system is developed to damp frequency oscillations and increase utilization of Vernier capacity.

A Coordinating Control for Hybrid HVdc Systems in Weak Grid

A coordinating control strategy is proposed for a hybrid high voltage dc (HVdc) system comprising a capacitor commutated converter connected in series with a two-stage voltage source converter called “Vernier.” This approach ensures that 1) the tap changers and switched capacitor banks are eliminated by Vernier controls that maintain constant firing angle and margin angle at the rectifier and inverter, respectively, and provide volt-VAr control at the commutation bus, and 2) improves the power flow recovery and risk of commutation failure following sever faults. To that end, a detailed switched model of hybrid-HVdc is built in EMTDC/PSCAD platform. For the frequency-domain analysis, a new nonlinear state-space averaged model is also developed and benchmarked against the detailed model. The steady-state operating characteristics of the proposed hybrid HVdc system are established. Finally, the effectiveness of the proposed approach is demonstrated for weak ac systems interfacing the rectifier and inverter sides. Improvement in commutation failure and power flow recovery following severe disturbances are also shown through case studies.

Analytical modelling of point‐to‐point CCC‐HVDC revisited

Weak AC grids are incapable of providing the reactive power needed by the line-commutated converter (LCC) converters to maintain an acceptable system voltage, which leads to several operational challenges. Although voltage source converter-high-voltage DC (HVDC) can solve these issues, its power rating has not matched that of the LCC technology as yet. The available alternative is the capacitor-commutated converter (CCC) technology, which is currently in operation using back-to-back configuration. In order to explore the capability of this technology for long-distance transmission, this study proposes a comprehensive non-linear state-space averaged phasor model of CCC-HVDC connected to weak grids on both sides of HVDC. To that end, the analytical model of CCC is revisited and the need of including the capacitor dynamics and the contribution of unbalanced capacitor voltages to get a closer match with the detailed switched model is investigated. The proposed model is used to present the root-cause analysis of the dynamic performance and quantify the interaction of various states in the system with different modes, which is not possible through a detailed switched model.

HVDC multi-infeed analysis of the Brazilian transmission system and possible mitigation methods

The Brazilian transmission system is facing challenging problems with the distance between its generation areas and consumer centers that will be partly solved by the use of HVDC point-to-point systems. In the near future, the southeast subsystem will have a large amount of power injected through HVDC Systems in multiple points with relatively close electrical proximity. Therefore, the effects of a multi-infeed system are expected to influence the performance and operation of the network. Extensive studies and simulations will play an important role in determining the extension of the interactions among HVDC converters and determine if such interactions cause multiple commutation failures, thus disturbing the dynamic stability of the system. The use of CCC (Capacitor Commutated Converter) HVDC systems will also be assessed and is expected to diminish the need for a strong AC network (high short circuit level) and, therefore, mitigate multi-infeed interactions. The southeast subsystem of the Brazilian Power System currently has four LCC inverters, two of them belong to the Madeira power plant and the other two are from the Itaipu power plant. By the year 2024, four other HVDC systems will be arriving in the same region. This paper discusses the HVDC multi-infeed phenomena regarding the relevance of using synchronous machine models to represent important power plants and the application of mitigation methods regarding the 2020 network model, where six HVDC links will be present.

Commutation Failure Elimination of LCC HVDC Systems Using Thyristor-Based Controllable Capacitors

The adverse impacts of commutation failure (CF) of a line-commutated converter (LCC)-based high-voltage direct current (HVdc) system on the connected ac system are becoming more serious for high-power ratings, for example, the development of ultra-HVdc systems. Aiming to solve the problem of CF particularly for higher power/current LCC HVdc systems, this paper proposes a new method, which utilizes a thyristor-based controllable capacitor (TBCC), to eliminate CFs. The topology of the proposed TBCC LCC HVdc and its operating principles are presented. Then, mathematical analysis is carried out for the selection of component parameters. To validate the performance of the proposed method, modified LCC-HVdc and capacitor-commutated converter (CCC)-based HVdc systems based on the modified CIGRE HVdc system are modeled in a real-time digital simulator. Simulation studies for zero impedance single-phase and three-phase faults are carried out, and comparisons are made with both LCC-HVdc and CCC-HVdc systems. Furthermore, voltage and current stress of the TBCC are investigated and power-loss calculations are presented. The results show that the proposed method is able to achieve CF elimination under the most serious faults while the increase of power losses due to the TBCC is small.

An Evolved Capacitor-Commutated Converter Embedded With Antiparallel Thyristors Based Dual-Directional Full-Bridge Module

This paper proposes an evolved capacitor commutated converter (ECCC), embedded with antiparallel thyristors based dual-directional full-bridge module (APT-DFBM), to mitigate the commutation failure of line-commutated converter-based-high-voltage direct-current (LCC-HVdc) transmission and enhance the control flexibility and dynamic response of capacitor-commutated converter-based HVdc (CCC-HVdc). The operating modes and parameter selection approach of the APT-DFBMs are provided, and the coordinated control approach between the APT-DFBMs and converter valves is presented. Then, an ECCC-HVdc system, adopting an LCC station as the rectifier and an ECCC station as the inverter, is developed in PSCAD/EMTDC. The transient responses including commutation failure probability, voltage and current stress, and fault recovery performance are compared under ac fault conditions among the proposed ECCC-HVdc, LCC-HVdc, and CCC-HVdc. The results show that the proposed ECCC-HVdc can effectively reduce the commutation failure risks of LCC-HVdc, and enhance the control flexibility and improve the dynamic responses of CCC-HVdc.

Passive AC network supplying the integration of CCC-HVDC and VSC-HVDC systems

The integration of a capacitor-commutated converter (CCC) high-voltage direct current (HVDC) (CCC- HVDC) and voltage source converter (VSC) HVDC (VSC-HVDC) is proposed in this paper to supply entirely passive AC networks. The key point of this integration is the at characteristic of the DC voltage of the CCC-HVDC, which provides the condition for the VSC to connect to the CCC DC link via a current regulator. The advantages of the proposed combined infeeding system are the requirement of only one DC line, and better dynamic responses. The structure of the proposed infeeding system, as well as its control system, is studied. Simulation results are provided to validate the eectiveness of the proposed system and its control strategy. Two other schemes for infeeding AC passive networks are studied to demonstrate the advantages of the proposed system.

Integration of CCC-HVDC and VSC-HVDC Systems to Supply an Island Network

Acombination of Capacitor Commutated Converter (CCC) HVDC and voltage source converter (VSC) HVDC is proposed to supply an island system without any local generation. The key point of this integration is the flat characteristic of dc voltage of CCC-HVDC, which provides the condition for VSC to connect to CCC dc link via a current regulator. The advantages of proposed combined in feeding system are requiring only one dc line and having better dynamic responses. The structure of the proposed in feeding system as well as its control system is shown in this study. The simulation results are presented to confirm the effectiveness of the proposed system. Two other schemes for in feeding the passive island systems are studied to demonstrate the advantages of the proposed system.

Dynamic performance comparison of conventional and capacitor commutated converter (CCC) for HVDC transmission system in simulink environment

Dynamic performance comparison of conventional and capacitor commutated converter (CCC) for HVDC transmission system in simulink environment

An active-passive capacitor-commutated converter for HVDC systems with a three-phase voltage-source PWM converter

This paper introduces a new approach to the capacitor-commutated converters (CCCs) for HVDC systems. A small-rated three-phase voltage-source PWM converter is connected between a series commutation capacitor and thyristor converter through matching transformers. The PWM converter acts as auxiliary commutation-capacitor for the thyristor converter while the series passive capacitor acts as the main commutation capacitor. The capacitance, which is the sum of the small-rated active and series passive capacitors, is variable, so that stable commutation is obtained. In CCCs, commutation failure occurs when the AC bus voltage is recovered whereas the proposed combined commutation-capacitor can achieve successful commutation for both rapidly decreasing and increasing AC bus voltages. The basic principle of the proposed active–passive capacitor-commutated converter is discussed in detail. Then, constant margin angle control with a constant firing angle of the thyristor converter is proposed using a function generator block. Digital simulation demonstrates the novelty and effectiveness of the proposed active–passive capacitor-commutated converter. © 2005 Wiley Periodicals, Inc. Electr Eng Jpn, 151(1): 66–75, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20030

An Active-Passive Capacitor-Commutated Converter for HVDC Systems with a Three-Phase Voltage-Source PWM Converter

This paper introduces a new approach to the capacitor-commutated converters (CCCs) for HVDC systems. A small-rated three-phase voltage-source PWM converter is connected between a series commutation capacitor and thyristor converter through matching transformers. The PWM converter acts as auxiliary commutation-capacitor for the thyristor converter while the series passive capacitor acts as the main commutation capacitor. The capacitance, which is the sum of the small-rated active and series passive capacitors, is variable, so that stable commutation is obtained. In CCCs commutation failure occurs when the ac bus voltage is recovered whereas the proposed combined commutation-capacitor can achieve successful commutation for both rapidly decreasing and increasing ac bus voltages. The basic principle of the proposed active-passive capacitor commutated converter is discussed in detail. Then, constant margin-angle control with a constant firing angle of the thyristor converter is proposed using a function generator block. Digital simulation demonstrates the novelty and effectiveness of the proposed active-passive capacitor commutated converter.

Capacitor commutated converters for long-cable HVDC transmission

An overview of the capacitor commutated converter is given, emphasising its applications in long-cable HVDC transmission schemes. Its advantages and disadvantages in comparison with the conventional HVDC Graetz bridge converter are identified, and a critical assessment of its performance is presented using a developed steady-state formulation as well as a transient simulation based analysis.

Modeling capacitor commutated converters in power system stability studies

This paper describes a capacitor commutated converter (CCC) model suitable for power flow and transient stability studies. The CCC control structure and important operational details are described. The practical advantages of the CCC as compared to the conventional HVDC converter are discussed. Results are presented for power flow, power system stability, small signal analysis and control design studies. These results clearly demonstrate the superior performance of a CCC link, when connected to a very weak AC system.

Modeling Capacitor Comnmutated Converters in Power System Stability Studies

This paper describes a capacitor commutated converter (CCC) model suitable for power flow and transient stability studies. The CCC control structure and important operational details are described. The practical advantages of the CCC as compared to the conventional HVDC converter are discussed. Results are presented for power flow, power system stability, small signal analysis, and control design studies. These results clearly demonstrate the superior performance of a CCC link, when connected to a very weak ac system.