A Study of the SPWM High-Frequency Harmonic Circulating Currents in Modular Inverters

Sheng Xu,Zhendong Ji 전력전자학회 2016 JOURNAL OF POWER ELECTRONICS Vol.16 No.6

Due to detection and control errors, some high-frequency harmonics with voltage-source characteristics cause circulating currents in modular inverters. Moreover, the circulating currents are usually affected by the output filters (OF) of each module due to their filter and resonance properties. The interaction among the circulating currents in the modules increase the power loss and reduce system stability and control precision. Therefore, this paper reports the results of a study on the SPWM high-frequency harmonics circulating currents for a double-module VSI. In the paper, an analysis of the circulating-current circuits is briefly described. Next, a mathematic model of the single-module output voltage based on the carrier frequency of SPWM is built. On this basis, through mathematic modeling of high-frequency harmonic circulating currents, the formation mechanism and distribution characteristics of circular currents and their influences are studied in detail. Finally, the influences of the OF on the circulating currents are studied by mainly taking an LC-type filter as an example. A theoretical analysis and experimental results demonstrate some important characteristics. First, the carrier phase shifting of the SPWM for each module is the major cause of the SPWM harmonic circulating currents, and the circulating currents are in an odd distribution around n-times the carrier frequency nωs, where n = 1, 2, 3, …. Second, the harmonic circular currents do not flow into the parallel system. Third, the OF can effectively suppress the non-circulating part of the high-frequency harmonic currents but is ineffective for the circulation part, and actually reduces system stability.

A Study of the SPWM High-Frequency Harmonic Circulating Currents in Modular Inverters

Xu, Sheng,Ji, Zhendong The Korean Institute of Power Electronics 2016 JOURNAL OF POWER ELECTRONICS Vol.16 No.6

Due to detection and control errors, some high-frequency harmonics with voltage-source characteristics cause circulating currents in modular inverters. Moreover, the circulating currents are usually affected by the output filters (OF) of each module due to their filter and resonance properties. The interaction among the circulating currents in the modules increase the power loss and reduce system stability and control precision. Therefore, this paper reports the results of a study on the SPWM high-frequency harmonics circulating currents for a double-module VSI. In the paper, an analysis of the circulating-current circuits is briefly described. Next, a mathematic model of the single-module output voltage based on the carrier frequency of SPWM is built. On this basis, through mathematic modeling of high-frequency harmonic circulating currents, the formation mechanism and distribution characteristics of circular currents and their influences are studied in detail. Finally, the influences of the OF on the circulating currents are studied by mainly taking an LC-type filter as an example. A theoretical analysis and experimental results demonstrate some important characteristics. First, the carrier phase shifting of the SPWM for each module is the major cause of the SPWM harmonic circulating currents, and the circulating currents are in an odd distribution around n-times the carrier frequency $n{\omega}_s$, where n = 1, 2, 3, ${\ldots}$. Second, the harmonic circular currents do not flow into the parallel system. Third, the OF can effectively suppress the non-circulating part of the high-frequency harmonic currents but is ineffective for the circulation part, and actually reduces system stability.

Sheath Circulating Current Analysis of a Crossbonded Power Cable Systems

Chae-Kyun Jung,Jong-Beom Lee,Ji-Won Kang 대한전기학회 2007 Journal of Electrical Engineering & Technology Vol.2 No.3

The sheath in underground power cables serves as a layer to prevent moisture ingress into the insulation layer and provide a path for earth return current. Nowadays, owing to the maturity of manufacturing technologies, there are normally no problems for the quality of the sheath itself. However, after the cable is laid in the cable tunnel and is operating as part of the transmission network, due to network construction and some unexpected factors, some problems may be caused to the sheath. One of them is the high sheath circulating current, In a power cable system, the uniform configuration of the cables between sections is sometimes difficult to achieve because of the geometrical limitation. This will cause the increase of sheath circulating current, which results in the increase of sheath loss and the decrease of permissible current. This paper will study the various characteristics and effects of sheath circulating current, and then will prove why the sheath current rises on the underground power cable system. A newly designed device known as the Power Cable Current Analyser, as well as ATP simulation and calculation equation are used for this analysis.

변압기 병렬운전시 순환전류 추정

姜龍澈(Yong-Cheol Kang),李美善(Mi-Sun Lee),李炳銀(Byung-Eun Lee),崔在宣(Jae-Sun Choi),張成一(Sung-Il Jang),金容均(Yong-Gyun Kim),柳永植(Young-Sik Lyu) 대한전기학회 2008 전기학회논문지 Vol.57 No.12

This paper proposes an algorithm to estimate the circulating currents in the transformers in parallel in an ultra high voltage system. For the Y-Y-Δ transformers operated in parallel, there exist two kinds of the circulating currents i.e. one is between the tanks and the other between the banks of the delta side. As the former is 90 deg out of phase of the load current, it is estimated by decomposing the line current into the component 90 deg out of phase of the load current in the frequency domain. The latter is estimated in the time domain from applying the Kirchhoff's voltage law on the delta winding which gives a first-order differential equation in terms of the delta winding currents. To estimate the circulating currents between the tanks, the performance of the proposed algorithm is investigated when the impedances of the two transformer tanks are different or the taps of the on-load tap changer of the transformers are mismatched temporarily. To estimate the circulating currents between the banks, the performance of the proposed algorithm is also examined under magnetic inrush and over-excitation. Test results indicate that the algorithm can estimate the two kinds of the circulating currents successfully.

Circulating Current Reduction of Parallel Connected Four-Pole Inverters

Meng-Jiang Tsai,Po-Tai Cheng 전력전자학회 2015 ICPE(ISPE)논문집 Vol.2015 No.6

Connecting the four-pole inverters (FPIs) in parallel has many benefits, like lower cost, high flexibility and excellent reliability, and becomes a favorite option in industry. However, such side effects of parallel connection as the circulating current will lead to severe distortion and extra loss. The circulating current is mainly caused by the difference of common mode voltage (CMV) and closely associated with switching states. The zero states contain extremes of CMV levels, which aims to significantly affect the circulating current. This paper presents a three-dimensional pulse width modulation (3-D PWM) approach adapted to the parallel connected FPIs. The 3-D modulation scheme substituting the zero states with active states is socalled three-dimensional active zero state PWM (3-D AZSPWM) and based classical 3-D space vector PWM (3-D SVPWM), possessing low current distortion and high dc-link utilization characteristic. By synthesizing output references without zero states, both CMV levels and circulating current can be suppressed in effect. Based on a two-module parallel connected system, the simplified model is made to discuss the phenomenon of circulating current. Besides, the simulation and test results are presented to validate the performance of 3-D PWM in balanced and unbalanced condition respectively.

$Y-{\Delta}$ 변압기 보호용 수정 전류차동 계전기

김은숙,강용철,Jin, En-Shu,Kang, Yong-Cheol 대한전기학회 2006 전기학회논문지A Vol.55 No.3

This paper proposes a modified current differential relay for $Y-{\Delta}$ transformer protection. The relay uses the same restraining current as a conventional relay, but the differential current is modified to compensate for the effects of the exciting current. A method to estimate the circulating component of the delta winding current is proposed. To cope with the remanent flux, before saturation, the core-loss current is calculated and used to modify the measured differential current. When the core then enters saturation, the initial value of the flux is obtained by inserting the modified differential current at the start of saturation into the magnetization cure. Thereafter, the core flux is then derived and used in conjunction with the magnetization curve to calculate the magnetizing current. A modified differential current is then derived that compensates for the core-loss and magnetizing currents. The performance of the proposed differential relay was compared against a conventional differential relay. Test results indicate that the modified relay remained stable during severe magnetic inrush and over-excitation, because the exciting current was successfully compensated. This paper concludes by implementing the relay on a hardware platform based on a digital signal processor. The relay does not require additional restraining signal and thus cause time delay of the relay.

Y-ㅿ 변압기 보호용 수정 전류차동 계전기

姜龍澈(Yong-Cheol Kang),金恩淑(En-Shu Jin) 대한전기학회 2006 전기학회논문지A Vol.55 No.3

This paper proposes a modified current differential relay for Y-ㅿ, transformer protection. The relay uses the same restraining current as a conventional relay, but the differential current is modified to compensate for the effects of the exciting current. A method to estimate the circulating component of the delta winding current is proposed. To cope with the remanent flux, before saturation, the core-loss current is calculated and used to modify the measured differential current. When the core then enters saturation, the initial value of the flux is obtained by inserting the modified differential current at the start of saturation into the magnetization cure. Thereafter, the core flux is then derived and used in conjunction with the magnetization curve to calculate the magnetizing current. A modified differential current is then derived that compensates for the core-loss and magnetizing currents. The performance of the proposed differential relay was compared against a conventional differential relay. Test results indicate that the modified relay remained stable during severe magnetic inrush and over-excitation, because the exciting current was successfully compensated. This paper concludes by implementing the relay on a hardware platform based on a digital signal processor. The relay does not require additional restraining signal and thus cause time delay of the relay.

해운대 해역의 파랑 및 흐름 분석

임학수(Hak-Soo Lim) 한국연안방재학회 2016 한국연안방재학회지 Vol.3 No.2

To prevent the beach erosion and to support the beach restoration project in Haeundae, the Korean government initiated R&D project, the development of coastal erosion control technology since 2013. As a part of this project, waves and currents have been observed by AWACs in the nearshore and offshore of the Haeundae beach continuously for more than two years, to understand the processes of coastal waters and sediment transport in Haeundae. The variations of residual current and wave were estimated analysing two-year-long AWAC data measured at the mean water depth of 22 m and other short term data obtained near the surf zone of the Haeundae beach, respectively, during summer and winter. IOS T_TIDE and the moving average method for the current data over a fortnightly period (MSf) were applied for estimating the speed and direction of wave-induced current and residual current. A numerical simulation using a wave and current coupled model (ROMS-SWAN) considering wave-tide interaction in Haeundae coastal waters was also conducted for determining the wave-induced current during summer and winter seasonal swell waves (Hs : 2.5 m, Tp: 12 s), for better understanding of the coastal process near the beach. By comparing the measured data and simulated result, we found that cross-shore current during summer season is mainly caused by the eddy produced by the wave-induced current near the beach, which in turn, is generated by the strong waves coming from the S and SSW directions. During other seasons, longshore wave-induced current is produced by the swell waves coming from the E and ESE directions. The wave-induced current also generates longshore current heading west toward Dong-Back Island, west end of the beach, during all the seasons and eddy current toward Mipo-Port, east end of the beach, in summer which is well matched with the estimated residual current. In this study, we especially show the circulation patterns derived from nearshore wave-induced current and offshore residual current causing beach erosion and sediment transport in Haeundae coastal waters.

Z-소스 인버터의 병렬운전 특성 개선

김윤호,이욱영,서강문,Kim, Yoon-Ho,Lee, Woog-Young,Seo, Kang-Moon 한국조명전기설비학회 2007 조명·전기설비학회논문지 Vol.21 No.3

본 논문에서는 연료전지 시스템을 위한 Z-소스 인버터로 구성된 병렬운전 연료전지 시스템의 순환전류 감쇄 방안을 검토하였다. PWM 방식으로는 Carrier phase shifted SPWM을 적용하였으며, 이 방식은 출력 전류의 고조파 성분을 줄일 수 있는 장점을 갖는다. 그러나 이 기법을 Z-소스 인버터로 구성된 병렬운전 시스템에 적용하는 경우 추가적으로 순환전류가 발생한다. Z-소스 인버터로 구성된 병렬운전 시스템이 동작 시 발생하는 순환전류를 감소시키며, 부하전류에 낮은 고조파 성분을 가질 수 있도록 커플링된 순환전류 리액터를 사용하였다. 순환전류 리액터를 상호 커플링시킴으로써 순환전류를 추가적으로 감소시킬 수 있다. 이에 대한 적합성을 시뮬레이션과 실험을 통해 입증하였다. In this paper, a circulating current reduction approach for the parallel operation of fuelcell systems with Z-source inverters is investigated. The carrier phase shifted SPWM(Sinusoidal Pulse Width Modulation) is used as a modulation method since it has an advantage in reducing output current harmonics. However, when this technique is applied to the parallel operation of Z-source inverters, it additionally produces circulating currents. A coupled circulating current reactor is used to reduce circulating current generated by the parallel operation of Z-source inverters and to reduce output current harmonics. The proposed circulating current reduction approach using coupled circulating current reactors is verified through simulation and experiment.

Analysis and Modeling of Parallel Three-Phase Boost Converters Using Three-Phase Coupled Inductor

Chang-Soon Lim,Kui-Jun Lee,Rae-Young Kim,Dong-Seok Hyun 대한전기학회 2013 Journal of Electrical Engineering & Technology Vol.8 No.5

The main issue of parallel three-phase boost converters is reduction of the low- and high frequency circulating currents. Most present technologies concentrate on low frequency circulating current because the circulating current controller cannot mitigate the high frequency circulating current. In this paper, analytical approach of three-phase coupled inductor applied to parallel system becomes an important objective to effectively reduce the low- and high frequency circulating currents. The characteristics of three-phase coupled inductor based on a structure and voltage equations are mathematically derived. The modified voltage equations are then applied to parallel three-phase boost converters to develop averaged models in stationary coordinates and rotating coordinates. Based on the averaged modeling approach, design of the circulating current controller is presented. Simulation and experimental results demonstrate the effectiveness of the analysis and modeling for the parallel threephase boost converters using three-phase coupled inductor.