Zhenxiong Wang

A Digital Hysteresis Current Controller for Three-Level Neural-Point-Clamped Inverter With Mixed-Levels and Prediction-Based Sampling

For a grid-tied three-level neural-point-clamped (NPC) inverter, fully digital hysteresis controller with simple variable hysteresis band is insufficient to stabilize switching frequency (fs). The challenge mainly comes from the restriction of analogue-to-digital converter (ADC) sampling frequency, which leads to inaccurate current-boundary-touching capture. As a result, fs jitter appears. This phenomena becomes much more serious around grid voltage zero-crossing points due to zero-approaching hysteresis band, where obvious fs variation happens. To stabilize fs in a fully digital manner, two techniques are proposed in this paper: the mixed-level scheme and the prediction-based sampling method. The former one is used to cope with the obvious fs variation around grid voltage zero-crossing points. It switches operating status from three-level state into two-level state around grid voltage zero-crossing points to enlarge the hysteresis band for digital implementation. The latter one is used to resolve fs jitter. It makes the sampling and switching to happen at the moment of current boundary-touching to achieve the most effective boundary-comparing with the lowest ADC sampling frequency. With these two techniques, fs of the grid-tied three-level NPC inverter could be well stabilized with a fully digital hysteresis controller, and therefore, high-quality grid-side could be achieved.

A novel multi-level matrix converter

A new multi-level matrix converter is presented. The operation, control strategy are discussed. Simulations are intensively carried out, and comparisons are made between the multi-level matrix converter and the normal matrix converter. Experiments were carried out.

Implementation of virtual synchronous generator with an Improved Hardware Structure in PV-based microgrids

Frequency stability problem in microgrid attracts much attention with increasing penetration of the intermittent and fluctuating renewable energy. Conventional microgrid control strategy in steady state can balance power difference between sources and loads by storages or robust power grids, and hence grid frequency is maintained stable. However, due to lack of inertia in power electronic converters, characteristics of dynamic response in microgrid is neglected and thus causing problems in power system stability. Therefore, the concept of virtual synchronous generator (VSG) is proposed to solve the problem. In most available schemes of VSG, renewable energy and energy storages are combined and controlled through a dc-dc converter, which are connected to a dc-bus of inverter. The scheme is easy for imitation of synchronous generator (SG) due to their similar hardware structure, but increases complexity of hardware implementation with extra design of combined photovoltaic-storage system and lacks flexibility and reliability. In this paper, a new structure and corresponding control strategy are proposed for implementation of VSG in photovoltaic generation, where photovoltaic arrays and storages are connected to microgrid ac-bus through separate inverters. With the proposed scheme, photovoltaic inverter operates in voltage control mode and the integral system behaves like a SG, which enhances performance of VSG in flexibility and reliability. The proposed strategy is verified in simulation.

Dynamic performance analysis of paralleled virtual synchronous generators under grid-connected and islanded mode

With increasing penetration of the intermittent and fluctuating renewable energy resources, control strategy of inverters in microgrid is important. Therefore, due to lack of inertia in power electronic converters, virtual synchronous generator control is proposed to introduce backup inertia and improve dynamic performance of frequency. Researches recently have proposed different methods on control of single virtual synchronous generator and the performance is investigated. However, characteristics of paralleled virtual synchronous generators in microgrids are not sufficiently studied. In this paper, a simplified transfer function model of paralleled virtual synchronous generators is constructed and verified in simulation, which is used to study microgrids with multiple virtual synchronous generators in both grid-connected mode and islanded mode. Frequency, phase and power response of ac bus and virtual synchronous generators are investigated. Influences of inertia and grid-side inductance are analyzed in detail, which shows dynamic characteristics of paralleled virtual synchronous generators and provides some practical advices on VSG design in microgrids.

A Single-phase Harmonics Extraction Algorithm Based on the Principle of Trigonometric Orthogonal Functions

For a single-phase active power filter (APF), designing a more efficient algorithm to guarantee accurate and fast harmonics extraction with a lower computing cost is still a meaningful topic. The common idea still employs a IRPT-based Park transform, which was originally designed for 3-phase applications. Therefore, an additional virtual signal generation (VSG) link is necessary when it is used in the single-phase condition. This method, with virtual signal generation and transform, is obviously not the most efficient one. Regarding this problem, this paper proposes a novel harmonics extraction algorithm to further improve efficiency. The new algorithm is based on the principle of trigonometric orthogonal functions (TOF), and its mathematical principle and physical meaning are introduced in detail. Its implementation and superiority in terms of computation efficiency are analyzed by comparing it with conventional methods. Finally, its effectiveness is well validated through detailed simulations and laboratory experiments.

An online control method of APLC for network-wide harmonic suppression considering the coupling characteristics of nonlinear loads

Conventional researches about network harmonic suppression generally assumed the network was linear and individual at various frequencies, which ignored the coupling relations of nonlinear loads. This paper studies coupling characteristics by matrix model. Through simplifying matrix model, a novel online control method of active power line conditioners (APLC) was proposed.

A self-adaptive power balance control strategy for PV inverters in islanded microgrids

For photovoltaic inverters in islanded microgrids, droop control is a preferable strategy due to its attractive ability in bus voltage regulation and load sharing without additional communication system. However, in islanded microgrids without redundant storages, further realization of power balance between droop controlled PV sources and dynamic loads, which is beneficial to enhance PV sources penetration and system reliability, remains a serious problem. This study focuses on this issue and improves conventional droop control method with self-adaptive power balance strategy. The proposed droop method can promise stable operation when total load consumption is under maximum capacity of all PV sources, which means PV arrays quit maximum power point and shift in accordance with load demands to avoid large storage component at dc-side of inverters while keeping voltage regulation ability from droop control.

A novel control strategy of photovoltaic-battery system for restraining the photovoltaic power fluctuations and suppressing the low frequency oscillations of power system

Due to the dispersion and randomness of photovoltaic (PV) power generation, the photovoltaic output power is fluctuant. As the increasing penetration of photovoltaic system, it plays an important role for the stability of power system. In this paper, photovoltaic is combined with the battery to form a photovoltaic-battery system. And a novel control strategy is proposed to restrain the photovoltaic power fluctuations and suppress the low frequency oscillations of power system. The energy storage device of the photovoltaic-battery system is composed of a battery and a BUCK/BOOST converter. The PV array and energy storage device connect to the DC side of the grid-connected inverter in parallel. The control strategy of the BUCK/BOOST converter is composed of voltage loop and current loop, which can make the PV array work at the maximum power point and decide to charge or discharge the battery. And the control strategy of the grid-connected inverter is composed of speed loop, power loop and current loop, which can make the grid-connected inverter output active power and reactive power according to power references. Simulations are carried out with MATLAB/SIMULINK. And the practicability of the proposed control strategy is verified in simulations.

Online control of SVG for network unbalance suppression

Conventional control strategies of static var generators (SVG) mostly focus on the compensation for concentrated unbalanced loads or unbalanced voltage at the PCC. With the size of networks increases, many distributed unbalanced loads appear, the total cost applying these unbalance control methods may increase significantly. Therefore, the SVG control strategy for network unbalance suppression has become an essential problem. To solve the problem, this paper proposes an unbalance control system which contains an SVG, power quality measuring equipments and a central controller. Firstly, the network parameters are acquired by inserting a small disturbance current with the SVG and collecting information from the measuring equipments. Secondly, these parameters will be employed by the central controller to calculate the optimal current reference online, during which two objectives are considered, i.e. to minimize the summation of square of negative sequence voltage at each node and to keep unbalance factor of voltage at each node within limit. Finally, the current reference will be transmitted to the SVG. With the proposed strategy, single SVG can compensate for the power system unbalance online intelligently without predetermined system information. A 7-node network in PSCAD is employed to validate the effectiveness of these analyses.