Gum Tae Son

Design and Control of a Modular Multilevel HVDC Converter With Redundant Power Modules for Noninterruptible Energy Transfer

This paper presents design and control methods for fault-tolerant operations with redundant converter modules, one of the most prominent features in modular multilevel converter (MMC) topology. In fully implementing MMC functionalities, a nearest level control is applied as a low-switching modulation method. A dual sorting algorithm is newly proposed for effectively reducing the switching commutations of each power module as well as for voltage balancing control. Built upon these primary MMC topological and control features, its redundant operation is comprehensively investigated for fail-safe energy transfer. In particular, a novel spare process is proposed to handle an emergency situation when the number of faulty power modules exceeds the module redundancy. Since topological redundancy may cause the switching commutations of power modules in an arm to be unevenly distributed, a practical and effective mitigation measure is incorporated to keep the energy balance while avoiding the undesired switching stresses. Rigorous simulation studies for MMC and its application for high-voltage direct current are performed to demonstrate the validity and effectiveness of the proposed spare process under normal and emergency conditions.

A Study on the Direct Stability Analysis of Multi-Machine Power System With Resistive SFCL

This paper describes a method for analyzing the direct stability of a multi-machine system that consists of three machines and one resistive superconductive fault current limiter (SFCL). The resistive SFCL, which affects the transient stability of a multi-machine power system, must be taken into account. In order to evaluate the transient stability in the power system, a direct stability analysis that has rapid prediction stability is used. Although the equal-area criterion method is useful in determining the stability as a transient stability evaluation method, the method is only applicable to a one-machine system connected to an infinite bus or to a two-machine system. Therefore, the transient energy function (TEF) should be constructed. Using the TEF, the effect of the SFCL in the multi-machine system can be identified by increasing the critical clearing time.

Effect of a SFCL on Commutation Failure in a HVDC System

This paper describes an analysis of the effects of a superconducting fault current limiter (SFCL) on commutation failure in a high-voltage direct current (HVDC) system. Most commutation failures are caused by voltage disturbances at the inverter side and cannot be avoided in HVDC systems using thyristors. When ac voltage is recovered to a normal condition, the commutation failure can be removed. However, persistence of the commutation failure can lead to suspension of the converter station. The commutation process is affected by the margin of the extinction angle, commutating reactance, and fault current level. The SFCL can limit the fault current on the ac side of the converter and thus quickly restore the HVDC system to normal status. A detailed simulation based on modeling of an actual system is carried out to verify that the SFCL can reduce commutation failure in a HVDC system.

Improved PD-PWM for minimizing harmonics of multilevel converter using gradient optimization

This paper proposes the improved phase-disposition PWM (PD-PWM) method, which optimizes the voltage harmonic performance of multilevel converter. The proposed PD-PWM method adjusts the position and magnitude of the carriers to find out the optimal values, which minimize the THD of output voltages. A gradient optimization method is applied as a form of an optimization-enabled transient simulation. A neutral-point clamped converter (NPC) demonstrates that the proposed method improves the THD of output voltages using PSCAD/EMTDC and MATLAB.

Power Loss Modeling of Individual IGBT and Advanced Voltage Balancing Scheme for MMC in VSC-HVDC System

This paper presents the new power dissipation model of individual switching device in a high-level modular multilevel converter (MMC), which can be mostly used in voltage sourced converter (VSC) based high-voltage direct current (HVDC) system and flexible AC transmission system (FACTS). Also, the voltage balancing method based on sorting algorithm is newly proposed to advance the MMC functionalities by effectively adjusting switching variations of the sub-module (SM). The proposed power dissipation model does not fully calculate the average power dissipation for numerous switching devices in an arm module. Instead, it estimates the power dissipation of every switching element based on the inherent operational principle of SM in MMC. In other words, the power dissipation is computed in every single switching event by using the polynomial curve fitting model with minimum computational efforts and high accuracy, which are required to manage the large number of SMs. After estimating the value of power dissipation, the thermal condition of every switching element is considered in the case of external disturbance. Then, the arm modeling for high- level MMC and its control scheme is implemented with the electromagnetic transient simulation program. Finally, the case study for applying to the MMC based HVDC system is carried out to select the appropriate insulated-gate bipolar transistor (IGBT) module in a steady-state, as well as to estimate the proper thermal condition of every switching element in a transient state.

Planning of HVDC System Applied to Korea Electric Power Grid

This paper proposes pre-analysis on planning of high-voltage direct current (HVDC) transmission system applied to Korea electric power grid. HVDC transmission system for interface lines has been considered as alternative solution for high-voltage AC transmission line in South Korea since constructing new high-voltage AC transmission lines is challenging due to political, environmental and social acceptance problems. However, the installation of HVDC transmission system as interface line in AC grid must be examined carefully. Thus, this paper suggests three scenarios to examine the influences of the installation of HVDC transmission system in AC grid. The power flow and contingency analyses are carried out for the proposed scenarios. Power reserves in metro area are also evaluated. And then the transient stability analysis focusing on special protection scheme (SPS) operations is analyzed when critical lines, which are HVDC lines or high voltage AC lines, are tripped. The latest generic model of HVDC system is considered for evaluating the impacts of the SPS operations for introducing HVDC system in the AC grid. The analyses of proposed scenarios are evaluated by electromechanical simulation.

Trade-Off Strategies in Designing Capacitor Voltage Balancing Schemes for Modular Multilevel Converter HVDC

This paper focuses on the engineering trade-offs in designing capacitor voltage balancing schemes for modular multilevel converters (MMC) HVDC: regulation performance and switching loss. MMC is driven by the on/off switch operation of numerous submodules and the key design concern is balancing submodule capacitor voltages minimizing switching transition among submodules because it represents the voltage regulation performance and system loss. This paper first introduces the state-ofthe-art MMC-HVDC submodule capacitor voltage balancing methods reported in the literatures and discusses the trade-offs in designing these methods for HVDC application. This paper further proposes a submodule capacitor balancing scheme exploiting a control signal to flexibly interchange between the on-state and the off-state submodules. The proposed scheme enables desired performance-based voltage regulation and avoids unnecessary switching transitions among submodules, consequently reducing the switching loss. The flexibility and controllability particularly fit in high-level MMC HVDC applications where the aforementioned design trade-offs become more crucial. Simulation studies for MMC HVDC are performed to demonstrate the validity and effectiveness of the proposed capacitor voltage balancing algorithm.

Non-interruptible energy transfer algorithm applied to multi-terminal VSC-HVDC with modular multilevel converter

This paper presents a design method to provide failure-tolerant operation of converter in the multi-terminal voltage-source-converter high-voltage direct current system (VSC-HVDC). It might be connected to an offshore wind farm with modular multilevel converter (MMC) topology. To fully implement the multi-terminal VSC-HVDC, three controllers, which are the system operator controller, supervisory controller, and MMC controller, are designed. For the sustainable energy transfer in case that the DC bus voltage control of a converter is failed, the communication-based power balancing algorithm is then proposed. That is, the voltage droop control and the constant flux control in offshore wind farm converter are used for a back-up operation to regulate a constant DC bus voltage. In particular, the wind power reduction via the constant flux control is comprehensively analyzed. To evaluate the performances of proposed algorithm, several case studies are carried out by time-domain simulation based on the power systems computer aided design/electromagnetic transients including DC (PSCAD /EMTDC®) software. The results show that the offshore wind farm consistently supplies the active power to the multi-terminal VSC-HVDC even when the different terminal in a severe fault condition is to be blocked according to its current regulation.