Fusheng Wang

Modeling and Elimination of Zero-Sequence Circulating Currents in Parallel Three-Level T-Type Grid-Connected Inverters

Unique pitfalls in parallel three-level T-type inverters (3LT2 Is) are potential zero-sequence circulating currents (ZSCCs) which are more complex than parallel two-level inverters and can cause current discrepancy, current waveform distortion, power losses, etc. In this paper, the ZSCC paths in the parallel 3LT2Is are first presented, and an equivalent model of the ZSCCs is developed. It is seen from this model that the ZSCCs consist of conduction, switching, and hybrid components. Based on the aforementioned analysis, an original sharing neutral bus structure is proposed to eliminate the conduction ZSCCs. With regard to the switching ZSCCs composed of high-frequency and low-frequency harmonics, modified LCL filters are proposed to eliminate the former, and zero-sequence control loops are put forward to suppress the latter. Furthermore, the proposed schemes are also proven to be effective to elimination of the hybrid ZSCCs. Experimental results validate the developed models and the proposed ZSCC elimination schemes.

Analysis and Control of Neutral-Point Voltage for Transformerless Three-Level PV Inverter in LVRT Operation

The modulation strategy LMZVM (large, middle, and zero vector modulation) can be widely adopted in the three-level 7 photovoltaic (PV) inverter, because it reduces the ground current and produces little neutral-point (NP) voltage in normal operation. However, in low-voltage ride-through (LVRT) operation, the NP voltage fluctuation becomes large and has not been researched nowadays. In this paper, according to this problem, four critical weighing factors about the NP voltage based on symmetrical component method are proposed. We can see that the maximum peak-to-peak value among all dip types happens when the single-phase grid voltage dips to 0. Then, an original balancing strategy, which replaces the zero vector with two opposite P-type or N-type small vectors of equal duty cycle, is put forward. Experimental results are provided to validate that the proposed strategy can reduce the NP voltage fluctuation in LVRT operation.

Optimized design of filter for transformerless three-level photovoltaic grid-connected inverter

Linking the capacitor neutral point of LCL filter to the DC midpoint for transformerless three-level photovoltaic inverter has an advantage of leakage current reduction. But it also brings large current virtual value and low-frequency resonance of bridge-side current, which leads to the reduction of the system efficiency and stability. Aiming at these problems, the common and differential mode model of three-level grid-connected inverter was constructed. Based on the equivalent model, the mechanism of the two problems was analyzed and a optimized design scheme was proposed. In the scheme, the capacitor of LCL filter was divided to two parts and the neutral point of the smaller one was linked to the DC midpoint. Then, the design principle of the improved LCL filter parameter was proposed based on bridge-side current ripple and common mode voltage source spectrum analysis. The problems of the system efficiency and stability reduction were solved when this proposed scheme was adopted. Feasibility and effectiveness of the optimized scheme are validated by experimental results.

Zero-Sequence Circulating Current Reduction for Three-Phase Three-Level Modular Photovoltaic Grid-Connected System

An equivalent model is established for zero-sequence circulating current (ZSCC) in three-phase three-level modular photovoltaic grid-connected system, and ZSCC is classified by frequency into high-frequency and low-frequency components. A parallel solution based on improved LCL filter is proposed to reduce high frequency component of ZSCC, and the low-frequency component of ZSCC can be reduced by using the ZSCC controller. The ZSCC model analysis and reduction methods are verified by experimental results based on a 10 kW three-phase three-level modular photovoltaic grid-connected system.

A novel design method of LCL filter for a grid-interconnected three-level voltage source inverter

In this paper, Firstly, maximum of ripple current is given through analysis of the current transient process. The principle of LCL filter is analyzed, and it is proved that the inductance of LCL filter is equal to one third of that of L filter in the situation of the same effect. According to power condition and ripple rejection, inductance of L filter is determined, so as to inductance of LCL filter. While satisfying the filter effect, converter current harmonic is reduced as low as possible to determine inductance ratio and capacitance. The simulation and experiment testify this design scheme is feasible.

An Optimized Third Harmonic Compensation Strategy for Single-Phase Cascaded H-Bridge Photovoltaic Inverter

Due to the unequal solar radiations or dust accumulation of photovoltaic modules in a single-phase cascaded H-bridge photovoltaic inverter, the unbalanced output power among photovoltaic modules will make the H-bridges with higher power overmodulation, resulting in deteriorated grid current. Concerning this issue, this paper proposes an optimized third harmonic compensation strategy that injects moderate amount of third harmonic into the overmodulation H-bridges to keep the amplitudes of their modulation waveforms just being unity, then compensates the same amount of opposite harmonic sequence and properly distributes it to rest of the H-bridges. The proposed method can ensure that all the power units will be free from overmodulation under some heavy power imbalance conditions and will not increase the third harmonic component of the grid current. The validity and effectiveness of the proposed method is verified by simulation and experimental results.

Filter design for three-level grid-connected inverter with low switching frequency

Aiming at the problems of poor dynamic control performance and difficult filter design for three-level grid-connected inverter with low switching frequency, a three-level discontinuous PWM strategy and its carrier-based implementation method are introduced. Considering the strategys output harmonic characteristics, an LLCL filter is adopted. Then, aiming at the resonance problem of LLCL filter, two simple passive damping schemes are comparatively analyzed from the aspects of damping effect, filtering performance and damping losses. The conclusion shows that the series resistor structure has better overall performance. Afterwards, the design principle of the filter parameter is proposed based on the bridge-side current ripple analysis and dominant harmonic component attenuation. Correctness and feasibility of the conclusions are validated by experimental results.

A Research on Power Line Communication Based on Parallel Resonant Coupling Technology in PV Module Monitoring

Intelligent photovoltaic (PV) module monitoring can automatically locate the position of panels and monitor the status of PV modules, which is of great significance for PV plants operation and maintenance. As a new communication method in the field of PV monitoring, the monitoring of PV modules based on power line communication is the future trend of PV power plants monitoring due to its low maintenance cost and strong anti-interference ability. This paper presents an intelligent PV module monitoring scheme based on the parallel resonant coupling unit, which uses the dc bus as the communication channel and modula-tes the monitoring data into a high-frequency form to carry out the carrier communication. By using the parallel resonant coupling, the high-frequency carrier wave is injected into the dc bus, which improves the system efficiency and anti-interference ability. In addition, an adaptive level decision circuit is proposed to improve the flexibility and scalability of the communication system by adaptively changing the reference level of the decision circuit. Experimental results achieved by a developed power line transceiver demonstrate both feasibility and validity of the proposed scheme.

A multiterminal Energy Interconnection System Based on MMC and its unbalanced power control

A multiterminal Energy Interconnection System Based on MMC was developed, which connected active sub-modules, DC grids and AC grids. However, the power imbalance of the active sub-modules exists in this system, for example, inconsistent maximum power points of the PV panels will lead to system power loss. In order to solve the problem, this paper first presents a power control scheme between the three-terminals, then proposed an unbalanced power control strategy, which focused on the power unbalance between upper and lower arms on one phase through injecting the fundamental frequency of the circulating current into the arms. Finally, the simulation results of a three-phase 9-level MMC validated the effectiveness and feasibility of the developed idea.

Grid fault ride through of a medium-voltage three-level full power wind power converter

According to the fault ride through (FRT) demand to wind generation systems of grid codes, a control scheme of grid FRT under the whole grid change was proposed for a medium-voltage three-level full power wind power converter. This method coordinately controls the generator-side converter (MSC), the grid-side converter (GSC) and the chopper to: 1) reduce the unloading burden of the chopper by limiting the MSC output power during the low voltage ride through (LVRT), 2) insure the controllability of the GSC by adopting the self-adaptive control strategy of dc-link voltage and the carrier-based over-modulation strategy during the high voltage ride through (HVRT), 3) ensure the neutral-point potential (NPP) into the given threshold value by adopting the NP balancing control method based on zero-level elimination during the grid fault. The simulation results validate that the proposed method can fully use the controllability of MSC and GSC to ensure the smooth switching and safe operation, while decrease the unloading burden of the chopper. Thus, the FRT ability of full-scale three-level wind generation converters can be effectively improved.