CPSS TPEA Vol.5 No.3(September 30, 2020)
Ken King Man Siu and Carl Ngai Man Ho
The paper presents the state-of-the-art technique Active Virtual Ground (AVG) in the design of single-phase grid-connected converters which fully covers the entire applications on AC microgrids. Based on the three main components of the microgrid, the presented topologies belonging to the AVG series are divided into three different groups, which are associated with power consumers, renewable energy, and energy storage systems.The whole series of the single-phase converters are also based on the latest AVG technique. Therefore, in each application, a corresponding single-stage AVG converter can be found, which has high efficiency, low leakage current, and continuous grid current. The topology advantages and the working principle of the AVG converter family are demonstrated through the circuit evaluation and are experimentally verified in a set of 650 to 800 W prototypes which shows a good agreement on between both of the experimental results and theoretical finding.
Md Shirajum Munir, Yun Wei Li, and Hao Tian
As renewable energy based distributed generation (DG) units are being increasingly connected throughout today’s distribution system, they can be used to mitigate harmonics caused by the wide adoption of nonlinear residential loads. To make the best use of all these DGs’ ratings, it is important to develop a method to coordinate DGs’ participation efforts in harmonic compensation according to their ratings and locations. Due to the low droop slope for the harmonic controller in DGs, traditional harmonic droop control methods can lead to significant harmonic sharing errors. Also, very limited work has been carried out in literature so far to identify the DGs’ compensation priorities according to their locations and power rating. To address this issue, a novel priority-driven, droop-based, selective harmonic compensation scheme is developed in this work. The proposed control scheme improves the harmonic sharing accuracy. The compensation priority design and the way to integrate with droop control is studied. To ensure stability, a virtual impedance model-based stability analysis is also discussed. Analysis, comparisons, and simulation results are used to verify the improvement of compensation performance.
Mingshen Li, Baoze Wei, Jose Matas, Josep Maria Guerrero, and Juan Carlos Vasquez
A simple high-performance decentralized controller based on Hopf oscillator is proposed for three-phase parallel voltage source inverter (VSI) in islanded Microgrid. In αβ frame, the oscillators equations corresponding output current and com-mon bus voltage as feedbacks are designed according to coupled oscillator synchronization properties. The enough common bus information is considered to realize external synchronization, and the current feedback is to achieve internal synchronization between VSIs. Then, the controller employs Hopf evolution dynamics to integrate their both. Therefore, a larger phase error can be eliminated when additional inverter connects, and the pre-synchronization item is proposed to be close to synchronize with the operational inverters. In addition, an integrated small-signal states pace based on averaged model for two parallel VSIs is developed, and the root locus shows the large stability margin and low sensitivity of parameters. Simulation and experiment results verifified the effectiveness of the proposed method in aspects of the fast dynamics response and precise current sharing performance.
Yuxin Liang, Hui Zhang, Mingqiao Du, and Kai Sun
Aiming at the low inertia DC micro-grid poor bus voltage quality and the energy storage SOC balanced problem, considering the urgent demand of high up/down ratio, electrical isolation and high-efficiency converter for distributed micro-source. An improved virtual capacitor (IVC) parallel coordination control strategy based on multi-port isolated DC-DC converter is proposed. First, MPIC is used to replace the traditional Buck/Boost circuit to achieve the electrical isolation from the micro sources of the energy storage system. Secondly, by analogy of IVC control, design the control frame of single voltage outside circle and multiple currents inside the ring, IVC control suitable for Four-port isolated DC-DC converter(FPIC) is obtained. Then, the parallel and coordinated control strategy under the control of IVC for multiple energy storage interface converters is established.
Alexandre Brandao Nassif
Reliable protection and grounding schemes have been well established for power systems. With the advent and proliferation of microgrids, however, these subjects need to be revisited as traditional philosophies are no longer sufficient to cope with reduced short circuit levels of distributed energy resources (DERs). A DER dominated microgrid will experience a limitation in the functionality of traditional overcurrent elements. This can degrade protection coordination and selectivity and requires a philosophy that is nonstandard in distribution systems. Furthermore, with most DERs operating as constant current sources and not naturally supplying ground current, performance grounding also becomes a fundamental problem of microgrids. Additional ground sources are required and must be appropriately sized for the needs of the microgrid. This paper proposes a practical protection and grounding scheme for an isolated microgrid that is being retrofitted with a large solar facility and a battery energy storage system (BESS). Much of the theory was developed tailored for this system and serves to reinforce the need for new philosophies that consider practical aspects of real systems.
Dominik Neumayr, Dominik Bortis, and Johann Walter Kolar
In order to expedite the development of power elec-tronic systems towards higher power density and efficiency at a lower cost of implementation, Google and IEEE initiated the Google Little Box Challenge (GLBC) aiming for the worldwide smallest 2 kVA/450 VDC/230 VAC single-phase PV inverter with η > 95% CEC weighted efficiency and an air-cooled case temperature of less than 60 °C by using latest power semiconductor technology and innovative topological concepts. This paper, i.e. Part B of a discussion of The Essence of the Little Box Challenge, presents the hardware implementations and novel control concepts of two GaN-based inverter systems selected by the authors to counter the challenge: (i) Little Box 1.0 (LB 1.0), a H-bridge inverter with two interleaved bridge-legs both operated with Triangular Current Mode (TCM) modulation which features a power density of 8.18 kW/dm3 (134 W/in3) and a nominal efficiency of 96.4% and (ii) Little Box 2.0 (LB 2.0), an inverter topology with single bridge-leg DC/|AC| buck-stage operated with constant frequency PWM and a subsequent |AC|/AC H-bridge unfolder, which features a remarkable power density of 14.8 kW/dm3 (243 W/in3) and a nominal efficiency of 97.4%. Implemented using latest GaN power semiconductor technology, Zero Voltage Switching (ZVS) throughout the AC period and a variable switching frequency in the range of 200 kHz-1 MHz in order to shrink the size of filter passives, the LB 1.0 was ranked among the top 10 out of 100+ teams actively participating in the GLBC. The LB 2.0 is the result of further research and considers lessons learned from the GLBC and achieves despite moderate 140 kHz constant frequency PWM and hard-switching around the peak of the AC output current a higher power density ρ and a higher efficiency η. For both implemented prototypes experimental results are provided to confirm that all GLBC technical requirements are met. The experimental results include steady-state and step-response waveforms, EMI and ground current measurements, as well as efficiency and operating temperature measurements. The reason for the ηρ-performance improvement of LB 2.0 over LB 1.0 are then discussed in detail. Furthermore, the solutions of other GLBC finalists are described and then compared to the performance achieved with the hardware prototypes presented in this paper. This leads to findings of general importance and provides key guideline for the future development of ultra-compact power electronic converters.
Pascal Samuel Niklaus, Dominik Bortis, and Johann Walter Kolar
This work investigates three-phase electromagnetic interference (EMI) conducted emission (CE) measurements with the aim to separate the noise voltages in their common-mode (CM) and differential-mode (DM) parts. By doing so, the converter input and/or output fifilter stages can be individually optimized to improve the CM or DM attenuation depending on the origin of the CE disturbances. An overview of various ways to achieve this separation is provided and an active three-phase noise separator is presented. The main advantage of the presented active solution is the absence of magnetic components in the signal path which drastically facilitates the matching at high frequencies. The influence of asymmetries and mismatches of the component parasitics as well as in the circuit board layout are analyzed. To assess the performance of the proposed system according to widely known metrics such as CM and DM transfer functions and rejection ratios, different test methods are established and the implied limitations regarding maximum measurable performance are considered. Finally, experimental results verifying the calculated separation capabilities are provided.
Shanshan Jin, Donglai Zhang, and Zhi Fang
Space solar array simulator (SSAS) is a key power input device for spacecraft power supply testing. SSASs are also required to simulate the steady-state I-V curve with a high tangent slope of the aerospace solar panel output and to have the dynamic response performance of the fast convergence to switching working point under a spacecraft power supply test environment. This paper proposes a high-linearized and fast-convergent RmI (HLFC-RmI) look-up table control method, which is aimed at solving these problems. This study converts the conventional voltage-current UI data table into the corresponding impedance-current RmI data table without adding any auxiliary hardware circuit. The RmI data table is used as a target form for I-V outer loop control, by using the proposed equivalent impedance Rm linearization formula, thereby achieving the simulation of a high tangent slope I-V curve. In the process of looking up table control, the optimized equivalent impedance Rm_real-time linearization formula is used to approximate the high-speed convergence characteristics of the UI look-up table method under constant-voltage load switching conditions. The new I-V outer loop control method is applied to a 1 kW hardware power platform of SSAS. The SSAS can not only simulate an arbitrary I-V curve with a high tangent slope but also has the characteristics of high-speed convergence of the switching working point.