• Progress in Superconductivity and Cryogenics
• /
• v.7 no.3
• /
• pp.39-42
• /
• 2005

In this paper, the thermal characteristics of multilayer insulation (MLI) were experimentally investigated by using boil-off calorimetry method for seeking optimum standards of thermal insulation conditions. It is necessary to design the thermal insulating efficiency for applying to cryogenic instruments such as HTS power cable system. It is well known that the thermal characteristics and heat transfer of MLI are greatly affected by various MLI structures such as the number of layers and layer density, etc. However, it is difficult to know the thermal characteristics of MLI correctly. The heat leak by MLI between room temperature and liquid nitrogen temperature was measured at various conditions using a cylindrical cryostat. The cryostat consists of two guard vessels located at both end sides and a test vessel between them. The guard vessels are also filled with liquid nitrogen to prevent radiation heat leak through the both end side of the cylindrical test vessel to measure the heat leak only through MLI.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
• /
• v.19 no.4
• /
• pp.333-338
• /
• 2007

The object of this study is to find the optimal insulation design conditions of a HTS power cable cryostat. The optimum condition of a cable cryostat is obtained by varying types of MLIs, cable core weights, spacer diameters, winding pitches and MLI layer numbers. As the weight of cable core is increased, conduction heat transfer from surroundings to cable cryostat is increased. But as the spacer pitch is increased from 120 mm to 200 mm, the heat leak of cable cryostat remains almost constant. The optimal number of MLI layers is suggested. Double ply MLI is more effective than triple ply MLI and the insulation effect is best when the number of MLI layers is 36.

• Journal of Power Electronics
• /
• v.17 no.1
• /
• pp.222-231
• /
• 2017

In this paper, a new architecture for a cost-effective power conditioning systems (PCS) using a single-sourced asymmetric cascaded H-bridge multilevel inverter (MLI) for photovoltaic (PV) applications is proposed. The asymmetric MLI topology has a reduced number of parts compared to the symmetrical type for the same number of voltage level. However, the modulation index threshold related to the drop in the number of levels of the inverter output is higher than that of the symmetrical MLI. This problem results in a modulation index limitation which is relatively higher than that of the symmetrical MLI. Hence, an extra voltage pre-regulator becomes a necessary component in the PCS under a wide operating bias variation. In addition to pre-stage voltage regulation for the constant MLI dc-links, another auxiliary pre-regulator should provide isolation and voltage balance among the multiple H-bridge cells in the asymmetrical MLI as well as the symmetrical ones. The proposed PCS uses a single-ended DC-DC converter topology with a coupled inductor and charge-pump circuit to satisfy all of the aforementioned requirements. Since the proposed integrated-type voltage pre-regulator circuit uses only a single MOSFET switch and a single magnetic component, the size and cost of the PCS is an optimal trade-off. In addition, the voltage balance between the separate H-bridge cells is automatically maintained by the number of turns in the coupled inductor transformer regardless of the duty cycle, which eliminates the need for an extra voltage regulator for the auxiliary H-bridge in MLIs. The voltage balance is also maintained under the discontinuous conduction mode (DCM). Thus, the PCS is also operational during light load conditions. The proposed architecture can apply the module-integrated converter (MIC) concept to perform distributed MPPT. The proposed architecture is analyzed and verified for a 7-level asymmetric MLI, using simulation results and a hardware implementation.

• Transactions of the Korean hydrogen and new energy society
• /
• v.8 no.2
• /
• pp.57-65
• /
• 1997

Thermal analysis has been performed to design a high-performance $LH_2$ vessel with about 1% per day evaporation loss. Analysis includes the combined insulations of MLI(Multi-Layer Insulation) and VCS (Vapor-Cooled Shield) under high vacuum. Combined insulation of MLI and VCS shows the existence of optimal location of VCS to minimize evaporation loss. Comparison of parallel-type DVCS (Double Vapor-Cooled Shield) and serial-type DVCS is also made to show the effectiveness of the system. The results indicate that the serial-type DVCS vessel is better than the parallel-type DVCS vessel with respect to overall evaporation loss. The combined insulation of SVCS (Single Vapor-Cooled Shield) with a partial MLI can give a similar performance characteristics compared to that with MLI and DVCS.

• Proceedings of the KSME Conference
• /
• 2003.11a
• /
• pp.290-295
• /
• 2003

Among the thermal analysis methods for Multi-Layer Insulation(MLI), effective emittance, diffusion MLI node and arithmetic MLI node methods are compared. The methods have been applied to the aluminum panel under the low earth orbit environment. TRASYS program is used for geometrical math modeling and SINDA program for thermal math modeling and temperature calculation. Test cases are selected according to MLI area on the panel. Temperature results are calculated and compared under the ratio of absorptivity and emissivity.

• Proceedings of the Korea Institute of Applied Superconductivity and Cryogenics Conference
• /
• 2003.10a
• /
• pp.243-245
• /
• 2003

The method to fabricate the cryostat of superconducting cable is extrusion type which is used Aluminum ingot under high temperature such as 45$0^{\circ}C$ and the cryostat is formed above cable core and MLI layer. In this case, it is expected to occur thermal injury in cable core and MLI layer, so it is necessary to study to prevent thermal injury. So in this paper, using simulation on radiation and convection which are accompany with fabricating cryostat, it is suggested to reduce the thermal injury. By measuring simulation temperature and real temperature, it is possible to check the temperature on cable core and MLI layer and using these temperature, it is possible to predict whether thermal injury is occurred or not on cable core and MLI layer.

• Journal of Power Electronics
• /
• v.19 no.3
• /
• pp.702-713
• /
• 2019

Multilevel Inverters (MLIs) are widely used in medium voltage applications due to their various advantages. In addition, there are numerous types of MLIs for such applications. However, the diode-less 3-level (3L) T-type Neutral Point Clamped (NPC) MLI is the most advantageous due to its low conduction losses and high potential efficiency. The power circuit of a 3L T-type NPC is derived by the conventional two level inverter by a slight modification. In order to explore the MLI performance for various Pulse Width Modulation (PWM) schemes, this paper examines the operation of a 3L (five level line to line) T-type NPC MLI for various types of Multi-Carriers Pulse Width Modulation (MCPWM) schemes. These PWM schemes are compared in terms of their voltage profile, total harmonic distortion (THD) and conduction losses. In addition, a 3L T-type NPC MLI is also compared with the conventional NPC in terms of number of switches, clamping diodes, main diodes and capacitors. Moreover, the capacitor-balancing problem is also investigated using the Neutral Point Fluctuation (NPF) method with all of the MCPWM schemes. A 1kW 3L T-type NPC MLI is simulated in MATLAB/Simulink and implemented experimentally and its performance is tested with a 1HP induction motor. The results indicate that the 3L T-type NPC MLI has better performance than conventional NPC MLIs.

• Journal of Electrical Engineering and Technology
• /
• v.12 no.4
• /
• pp.1519-1528
• /
• 2017

The paper proposes a well structured, component count waned single phase multilevel inverter (MLI) topology, which drives three different modules viz. Stipulation Based Sources Insertion (SBSI) module, Level Count Increasing (LCI) module and Inter-Linking H-Bridge (ILHB) module. The SBSI module confronts the number of basic sources needed in series/parallel to achieve required magnitude for any particular level. The LCI possesses an offsetting dc source and opuses to increase the number of levels and the ILHB module links the SBSI and LCI modules. A developed Hybrid Pulse Width Modulation (HPWM) strategy has PWM pulses for the switches of LCI module while the switches of the remaining two modules function at fundamental switching frequency. A fifteen level version of the proposed stipulation based sources insertion MLI (SBSIMLI) topology is simulated in MATLAB R2010a and a prototype of the similar specifications is constructed to validate the performance by experimental results. The comparison between the developed SBSIMLI topology and the competent topologies shows many interesting facts.

• Journal of the Korean Society of Tobacco Science
• /
• v.16 no.1
• /
• pp.43-52
• /
• 1994

To investigate the factors of the improvement of cultivation methods according to the plant type, three Burley tobacco cultivars were done. Experiments on the variability of plant type and factors representing the productivity and quality of the cultivar were subsequently carried out in relation to different fertilizer levels. Results are summarized as below. Use of more fertilizer resulted in higher plant height and longer leaf length but smaller mean leaf inclination(MLI). The horizontal leaf area distribution became greater in parts distal to the stem and the vertical leaf area distribution became greater in lower half than in upper half as the amount of fertilizer applied was increased. Br. 49 was the largest but Br. 21 and NTN 77 were similar with respect to both CGR and NAR. The yield was greater in larger MLI cultivars. These three characteristics became greater when the tobacco were heavier in the cultivars of larger MLI and increased with higher amount of fertilizer. Harvest index became greater as the amount of fertilizer decreased but not significantly differ among the cultivars. Total nitrogen content and nitrate - form nitrogen content were greater, more amount of fertilizer, and larger MLI cultivar plots. Total nitrogen content was higher in upper leaves. The filling power of tobacco leaves decreased but combustibility of leaves showed not significant trend as fertilizer application level increased.

• Journal of the Korean Society of Tobacco Science
• /
• v.16 no.1
• /
• pp.34-42
• /
• 1994

Experiments on the variability of plant type and factors representing the productivity and quality of the cultivars were subsequently carried out in relation to different plant density. Results are summarized as below. The higher was the plant density, the greater was the plant height and mean leaf inclination(MLI). As the plant density increased, the horizontal leaf area distribution became greater in proximal to the stem in NTN 77 and Br.49 but evenly in distal part in Br.21. Meanwhile, in terms of vertical leaf area distribution, it was decreased much more in middle and lower leaves than in upper in NTN 77 and Br.49 although it steadily decreased in any part of the plant in Br.21. Br.49 was the largest but Br.21 and NTN 77 were similar with respect to both CGR and NAR. The yield was greater in larger MLI cultivars(NTN 77>Br.49>Br.21). These three characteristics became greater when the tobacco were planted more densely in the field. Dry leaf weight and dry matter weight per plant were heavier in the larger MLI and increased with lower plant density. Total nitrogen content was greater in lower plant density and larger MLI cultivar. The plant density increases filling power seems to be enhanced regardless to the plant type. There was no discernible tendency on combustibility according to the plant density or plant type.