Kyu-Hoon Park

The black-box arc model is a tool that facilitates efficient research based on empirical studies in breaker design and analysis. However, its application has primarily focused on the design of ACCBs, with only a few instances of its implementation in the design of low-voltage (LV) DC circuit breakers (DCCBs) using mechanical methods. In DCCBs with non-zero crossing characteristics, several factors need to be considered in the design process. Therefore, it is crucial to have a reliable and accurate DC arc model to effectively apply black-box arc model studies to actual DCCB designs. Especially, DC Current limiting circuit breaker (CLCB), which has fast operation characteristics through its own over current relays (OCR), requires reliable and accurate modeling because minor factors in the topology can contribute significantly to the interrupting performance. This paper presents a study on the characterization and modeling approach for the DC CLCB, which exhibits somewhat different arc voltage characteristics compared to the general Air Circuit Breaker (ACB). The black-box arc model incorporates existing schwarz and kema arc models, and the Levenberg-Marquardt Algorithm (LMA) is employed to optimize the parameters of each model. An efficient parameter optimization method for the CLCB model is proposed, and the characteristics of factors that should be considered in the design are identified. Consequently, the applicability and reliability of the model are verified through a comparative study with the short-circuit test results of the actual DC CLCB.

Medium voltage direct current (MVDC) transmission system are growing due to their assistive quality in conventional grid and compatibility with renewable power network. MVDC distribution links with “Mixed” overhead (OH) & underground (UG) sections could be devised based on urban planning. UG Cables or substations are indirectly exposed to lightning strikes due to adjacent tower sections. In case of MVDC converter or cable, present researchers do not specify lightning voltage impulse level for related system voltage. Therefore, preluding electromagnetic (EM) transient investigation are required to determine the maximum lightning overvoltages for system peripherals i.e. cable & Modular Multilevel Converter (MMC) substation. This research focuses on analyzing lightning performance of OH transmission towers-cable junction & tower-substation link in case of a shielding failure (SF) and back flashover (BF) for a ±35kV short-length mixed MMC-MVDC transmission scheme. This article provides broad-band modeling method for MMC substation for lightning investigation. In addition, based on a detailed time-domain parametric evaluation in PSCAD/EMTDC program, lightning impulse voltage across the transmission line’s pole insulator and embedded cable section are estimated along with numerical validation relying on travelling wave theory. Effect of project parameters such as tower grounding resistance, riser section surge impedance (which connects cable & OH line) and cable length on lightning overvoltage impacting the cable and connected tower section has been demonstrated.

The calculation of the continuous allowable current of an underground cable is determined by various characteristics. To calculate the allowable current in cables with alternating magnetic fields such as AC, special phenomena such as the proximity effect and skin effect must be applied. However, there are no standards or research related to the calculation of the continuous allowable current of a DC power cable that does not have an alternating magnetic field. In this paper, a quantitative DC cable continuous allowable current calculation formula of direct burial laying was derived by applying the existing AC cable continuous allowable current calculation method to the DC system. We developed a calculation tool that can calculate the continuous allowable current of DC cables using the derived formula. Assuming the cable conditions (cable specification, laying conditions, soil characteristics, arrangement, and number of strands, etc.), a continuous allowable current simulation of DC cables was performed. In addition, the level of contribution to the continuous allowable current value was analyzed by classifying the parameter categories into major and minor factors in the order of influence on the allowable current among the determined calculated parameters. As a result, the effectiveness of the DC cable continuous allowable current calculation tool derived by performing the allowable current calculation simulation was evaluated, and the allowable current calculation method of the HVDC cable was established.

Bipolar lightning strokes are associated with multiple polarity electrical discharge with no current intervals in between, making their behavior quite peculiar. This work presents a fast front analysis of a mixed high voltage direct current (HVDC) transmission link, evaluating the factors that influence the line transients due to shielding failures and backflashovers (BFOs), considering both overvoltage and repeated polarity reversal at the cable sending terminal. The research process includes a detailed modeling of a bipolar lightning stroke, frequency-dependent HVDC overhead, and underground transmission line sections. Noticeable findings include the occurrence of only a positive polarity insulator BFO for the adjacent and subsequent tower, despite the dual polarity of the lightning stroke with relatively small values for the lightning parameters. The influence of traveling waves on the insulator flashover performance of the line with varying parameters (such as the riser section length, the tower grounding impedance, and the location of the lightning stroke) is recorded and explained.

Many geographical constraints and aesthetic concerns necessitate the partial use of cable sections in the High Voltage DC (HVDC) transmission line, resulting in a mixed transmission line. The overhead sections of mixed lines are exposed to lightning strikes. The lightning strikes can not only result in flashover of overhead line (OHL) insulators but can enter the cable and permanently damage its insulation if adequate insulation coordination measures are not taken. In this work, we have analyzed the factors that affect the level of overvoltage inside the cable by simulating a fast front model in PSCAD. It has been determined that surge arresters must be provided at cable terminals when the length of cable sections is less than 16 km to limit the core-ground overvoltage within the lightning impulse protective level (LIPL). The level of sheath-ground overvoltage is independent of the length of cable; however, it can be limited within LIPL by lowering the sheath grounding impedance to 1.2 Ω. Insulation coordination measures do not impact the likelihood of OHL insulators’ flashover. The flashover performance of OHL can be improved by lowering the footing impedance of the second tower closest to the cable terminals, which is otherwise most likely to flashover.

The current and voltage in High Voltage DC (HVDC) line is not pure DC but contain superimposed ripple components. The current ripple in core of HVDC cable magnetically induces a voltage in the sheath, whereas the voltage ripple causes the flow of charging current from core to sheath. The knowledge of sheath voltage is necessary to ensure compliance with the specification of utility companies. In this work, we have reported that the models available in commercial Electromagnetic Transient (EMT) simulation software erroneously introduce a DC bias in steady-state sheath voltage and sheath current. We have also demonstrated that by removing the DC bias accurate steady-state evaluation of sheath voltage and sheath current is possible. Additionally, we have analyzed the sheath voltage and currents in HVDC cable considering different cable lengths and sheath grounding schemes. It has been found that grounding the sheath at the terminal of HVDC cable can limit the sheath voltage to acceptable levels without causing substantial joule loss in the sheath.

The half bridge (HB) modular multilevel converter (MMC) technology is considered a breakthrough to mitigate the shortcomings of the conventional voltage source converter (VSC) in high-voltage direct-current (HVDC) grid application. However, interruption of the DC fault is still a challenge due to fast di/dt and extremely high levels of DC fault current. The fault interruption using a DC circuit breaker (DCCB) causes enormous energy dissipation and voltage stress across the DCCB. Therefore, the use of a fault current limiter is essential, and the superconducting fault current limiter (SFCL) is the most promising choice. Past literature has focused on the operating characteristics of DCCB or limiting characteristics of the SFCL. However, there is little understanding about the fault interruption and system recovery characteristics considering both DCCB and SFCL. In this paper, we have presented a comparative study on fault interruption and system recovery characteristics considering three types of fault limiting devices in combination with circuit breaker. The transient analyses of AC and DC system have been performed, to suggest the most preferable protection scheme. It has been concluded that, amongst the three fault limiting devices, the Hybrid SFCL in combination with circuit breaker, delivers the most desirable performance in terms of interruption time, recovery time, energy dissipation and voltage transients.

The eventual goal of high-voltage direct-voltage (HVDC) systems is to implement HVDC grids. The modular multilevel converter (MMC) has been identified as the best candidate for the realization of an HVDC grid by eliminating the shortcomings of conventional voltage source converter (VSC) technology. The related research has focused on efficient control schemes, new MMC topologies, and operational characteristics of an MMC in a DC grid, but there is little understanding about the fault handling capability of two mainstream MMC topologies, i.e., half bridge (HB) and full bridge (FB) MMCs in combination with an adequate protection device. Contrary to the existing research where the fault location is usually fixed (center of the line), this paper considered a variable fault location on the DC line, so as to compare the fault interruption time and maximum fault current magnitude. From the point of view of fault interruption, AC and DC side transient analyses were performed for both MMC topologies to suggest the appropriate topology. The simulation result confirmed that the fault handling performance of an HB-MMC with a DC circuit breaker is superior due to the smaller fault current magnitude, faster interruption time, lower overvoltage magnitude, and lesser stresses on the insulation of the DC grid.

To meet the growing power demand of the future, utilities and researchers have considered the addition of dc circuits in existing transmission corridors. The previous research has focused on analyzing the field environment, steady state induction and the negative effects of induction in the hybrid transmission lines. The overhead ground wires (OHGWs) have been used only for improving the lightning performance of overhead lines (OHL). In this paper, the spatial position of OHGWs have been studied with the objective of minimizing the steady state induction between power conductors as well as poorly grounded metallic objects like railroads and fences present near the ac-dc OHL. Candidate space for placement of OHGWs having shielding failure rate (SFR) of less than 4 is identified using the conventional electro-geometric model (EGM). The steady state induction in the power conductors and nearby metallic objects is studied as a function of position of OHGWs using electro-magnetic transient (EMT) simulation. The results indicate a strong correlation between the height of OHGWs and magnitude of induced voltage in power conductors, and metallic objects underneath the transmission line. The conscious effort in placement of OHGWs can reduce the magnitude of induced voltages while ensuring adequate lightning performance.

The eventual goal of high-voltage direct-voltage (HVDC) systems is to implement HVDC grids. One of the key obstacles in realization of the HVDC grid is the absence of a dc circuit breaker (DCCB). Several prototypes of DCCBs for the HVDC grid have been proposed, but no commercial solution has been developed yet due to insufficient dc fault current breaking capabilities and an unreliable protection-coordination scheme. In this respect, application of the suitable fault current limiter is necessary, and the use of a superconducting fault current limiter (SFCL), which has already been developed for the ac grid, is a viable solution, but there is no fundamental research about performance evaluation of SFCLs for the dc grid. Therefore, we aim to verify the applicability of the conventional SFCL in HVDC grids and perform a comparative analysis of the performance of several types of SFCLs. To analyze the performance of the conventional SFCL in the HVDC grid, three well-known representative types of SFCLs are modeled in MATLAB/Simulink, and their current limiting and recovery characteristics and energy dissipation are examined in voltage-source converter HVDC system application. From the simulation results, we deduced the strengths and the weakness of different types of SFCLs. Consequently, the likely SFCL candidate for commercialization in dc grid applications is suggested.