Zhirong Zeng

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.

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 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.