A 1270 Hz resonance occurred between ±350 kV/ 1000 MW Luxi back-to-back voltage source converter based high-voltage dc transmission (VSC-HVDC) converter and the 525 kV ac grid after disconnection of ...several ac transmission lines. To understand the resonance and find a solution, the impedance-based stability analysis model considering different equipment is first established. Then, the resonance is analyzed and repeated in the simulation based on the established model. The system stability can be judged by the ratio of grid impedance to the equivalent impedance of all parallel-connected equipment with the converter. To evaluate the occurrence and risk of resonance, the frequency range where the impedance has a negative-real-part has been searched and studied. In order to narrow the negative-real-part region to avoid potential resonance, solutions such as control strategy improvement and passive or active impedance adapter may be applicable and are discussed. For a complex system containing various equipment, the equipment can be divided into several subsectors to avoid modeling all possible combinations of equipment, which can be exhausting. And analysis has shown sufficient but not necessary condition to stabilize the system is to avoid the negative-real-part region in each sector.
A high-voltage direct current system using modular multilevel converter (MMC-HVDC) is a potential candidate for grid integration of renewable energy over long distances. The dc-link fault is an issue ...MMC-HVDC must deal with, especially for the nonpermanent faults when using overhead lines. This paper proposed a protection scheme to implement fast fault clearance and automatic recovery for nonpermanent faults on dc lines. By employing double thyristor switches, the freewheeling effect of diodes is eliminated and the dc-link fault current is allowed to freely decay to zero. Then, the dc arc can be naturally extinguished and the insulation on the short-circuit point can be restored. The thyristor switches convert the dc-link fault into an ac short circuit of the ac grid through MMC arms. The ac short-circuit current can be cleared simply by turning off all thyristor switches. Since circuit breakers are not tripped during fault clearance, MMC can immediately and automatically rebuild the dc-link voltage and restart power transmission. Simulation results using PSCAD/EMTDC have verified the validity of the proposed protection scheme.
Modular multilevel converters (MMC) are considered a top converter alternative for voltage-source converter (VSC) high-voltage, direct current (HVDC) applications. Main circuit design and converter ...performance evaluation are always important issues to consider before installing a VSC-HVDC system. Investigation into a steady-state analysis method for an MMC-based VSC-HVDC system is necessary. This paper finds a circular interaction among the electrical quantities in an MMC. Through this circular interaction, a key equation can be established to solve the unknown circulating current. A new steady-state model is developed to simply and accurately describe the explicit analytical expressions for various voltage and current quantities in an MMC. The accuracy of the expressions is improved by the consideration of the circulating current when deriving all the analytical expressions. The model's simplicity is demonstrated by having only one key equation to solve. Based on the analytical expressions for the arm voltages, the equivalent circuits for MMC are proposed to improve the current understanding of the operation of MMC. The feasibility and accuracy of the proposed method are verified by comparing its results with the simulation and experimental results.
Modular multilevel converters (MMCs) have been widely used in voltage source converter HVdc, and the capacity of MMCs has reached 1000 MW. The accurate calculation and theoretical analysis of the ...loss has attracted more and more attention, especially in high-power applications. This paper presents a simplified analytical calculation model of average power loss for MMC based on the root mean square (RMS) and absolute values of the arm current. In this model, the analytical expressions of the conduction loss and the switching loss are derived in terms of the whole arm of MMC rather than a single submodule. It avoids the effect of the complexity of its switching pattern on the calculation accuracy of the model. The explicit expressions of the presented model show the relationship between the loss, the voltage balancing methods, and the main variables of MMC directly in comparison with the conventional loss evaluation methods. PSCAD/EMTDC simulation and experimental results of the ±350 kV/1000 MW MMC in the Luxi Back-to-Back DC Asynchronous Networking Project verify the correctness and the accuracy of the proposed simplified calculation model.
This study adopts the method of third-order harmonic voltage injection (THVI) to move the peak value of a reference voltage toward the high capacitor voltage and increase the utilization rate of the ...capacitor voltage. This method can lower the cost or enlarge the capacity of a modular multilevel converter (MMC) when applied in high-voltage direct-current transmission. In this paper, the principle of THVI is introduced. Two calculation methods for modulation waves are studied, and the mathematical expressions of the capacitor voltage under all operating conditions are deduced. Thus, the optimal magnitude and phase of the third-order harmonic voltage are determined under all operating conditions. The mathematical expressions of the optimal magnitude and phase of the third-order harmonic voltage are obtained, and their online calculation burden is reduced. Simulation and experimental results show that the proposed method of THVI is feasible and effective.
Increasing the modulation index by utilizing the negative voltage states of full-bridge submodules (FBSMs) can greatly reduce capacitor usage of modular multilevel converters (MMCs), thereby ...optimizing the cost and volume. The hybrid MMC is composed of half-bridge submodules (HBSMs) and FBSMs, and the capacitor voltages of the two types of submodules (SMs) have different shapes as long as negative voltage states exist. This condition greatly complicates the analysis and design of the energy storage requirement of the hybrid MMC, which utilizes the negative voltage states of FBSMs to boost the AC voltage. A numerical calculation method for solving the capacitor voltages and designing the capacitances of FBSMs and HBSMs is proposed in order to accurately determine the minimum energy storage requirement considering the difference between the energy variations in FBSMs and HBSMs. In the numerical calculation, the energy storage and voltage of the arm are decomposed into FBSM and HBSM parts. According to the physical switching process, the output voltages of FBSM and HBSM parts are determined separately. The one-cycle waveforms of the capacitor voltages are then obtained by numerical integration of the power flows in FBSM and HBSM parts. An iterative solution procedure and the termination criterion that can ensure the accuracy of the obtained one-cycle waveforms are also proposed. Using the numerical integration and iterative solution procedure as the kernel algorithm, the proposed method can accurately analyze the capacitor voltages of the FBSMs and HBSMs and determine the minimum energy storage requirement of the hybrid MMC. Furthermore, the proposed method is applicable for various operating working conditions and various proportions of FBSMs. The simulation results verify the feasibility and accuracy of the analysis and design method.
The key technologies of ultra-high voltage hybrid LCC-VSC MTDC systems are investigated, focusing on the design of system configurations, converter topologies and the control and protection system. A ...double converter per pole of VSC connection is proposed along with the design of a 5000 MW VSC valve to develop a ± 800 kV/5000 MW large-capacity power transmission. The hybrid MMC topology capable of clearing the DC faults and the control strategy are developed to effectively improve the reliability in case of overhead line faults. The control and protection system of the LCC-VSC MTDC system is introduced to offer flexible operations under both normal and abnormal conditions, which includes voltage/current margin-based coordination, converter switch-in and switch-out, re-connection and drop-off of a third station. Simulations of an LCC-VSC MTDC system based on the LCC-VSC MTDC project are performed.
Design aspects of the hybrid HVDC system Hong Rao; Yuebin Zhou; Changyue Zou ...
CSEE Journal of Power and Energy Systems,
05/2021, Volume:
7, Issue:
3
Journal Article
Peer reviewed
Open access
Hybrid high voltage direct current transmission (Hybrid HVDC) is a new type of HVDC technology developed in recent years. It combines the characteristics of large-capacity and low cost of the line ...commutated converters (LCC) and non-commutation failure and dynamic reactive power support of the voltage sourced converters (VSCs) in one HVDC system. It has technical advantages in the fields of unidirectional power transmission and in the application of improving the stability of multi-infeed HVDCs, giving it broad application prospects. This paper defines the basic form of hybrid HVDC first and discusses the characteristics and challenges of different hybrid HVDCs. Then, it analyzes the matching characteristics between LCC and VSC in the station-station hybrid HVDC, studies the key technologies, such as VSC topologies and control strategies for clearing the DC overhead line faults, and introduces the key technologies for analyzing and suppressing the high-frequency resonances between VSC and the power grid. In this paper, using the Wudongde UHV multi-terminal HVDC as an example, the proposed key technologies are analyzed and verified.
Owing to the fast control response and distinct characteristics of the voltage source converter (VSC), various types of oscillation incidents related to the VSC-HVDC systems have been observed around ...the world. However, the causes of such oscillations are complex due to the interactions among the components of the transmission system and the various control methods of the converter station, and yet not systematically comprehended and summarised. Therefore, this study reviews several typical VSC-HVDC oscillation incidents and focuses on the root causes of the oscillation phenomenon. The analysis indicates that the oscillations in the VSC-HVDC system can occur either in subsynchronous frequency or in high-frequency range, up to 1.2 kHz. The modelling methods, including the conventional detailed model, equivalent detailed model, and average equivalent model, are also affecting the accuracy of describing the oscillations. The control methods, including adding active control scheme, passive filter at oscillation frequency, and temporary power plant disconnections, are also discussed. The advantages and drawbacks of these modelling methods, analysis methods, and control methods are summarised based on the various system conditions. In the end, the future research perspectives and key challenges are provided for further research references.
Traditional analogous π-shape transmission line models are mainly designed based on the amplitude and phase response characteristics of the distributed-parameter transmission lines at the power ...frequency. It cannot be directly used to build analogous high-voltage direct current (HVDC) transmission line models because the signals in a wide frequency band from 0 Hz to 10 kHz are used to protect HVDC transmission lines instead of power frequency components. In this paper, a novel model parameter identification method for scaled-down analogous HVDC transmission lines is proposed based on broad spectral response analysis. The broad spectral responses of distributed parameter-based and π-shape lumped parameter-based models are analyzed and compared with each other first. And then frequency-dependent per-unit parameters of HVDC transmission lines are obtained and used to form a modified π-shape HVDC transmission line model with the same zero- and positive-sequence impedance as those of distributed parameter lines. The element parameters of the π-shape circuit model are identified optimally based on the weighted sum of the results at selected several frequencies in the broadband from 0 hertz to thousands of hertz. The simulation results show that the amplitude and polarity of the dc voltage and current in our scaled-down analogous lines are close to distributed parameter-based HVDC transmission lines once a fault occurred on the lines.
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