Shuhuai Shi

Analysis and Optimization of Flexible MCPT Strategy in Submodule PV Application

Distributed maximum power point tracking (DMPPT) can effectively increase the power generation ability of photovoltaic (PV) systems in mismatch conditions. To further recover mismatch power loss inside PV panels, submodule realization is developed as an improved penetration of DMPPT concept. This paper proposes an optimized submodule level DMPPT system with a flexible time-sharing maximum current point tracking algorithm which detects the output current of series-connected dc-dc converters. Comparing with current PV optimizer and other submodule DMPPT solutions, the proposal can always guarantee that each PV submodule outputs individual true maximum power regardless of irradiance cases while eliminating the need for several MPPT units and current sensors in the system. Enormous potential of single control IC can be fully developed with fewer components (voltage sensors, current sensors, and relevant AD/DA units) for further integration in PV application.

Optimization of time-sharing output Maximum Current Tracking (MCT) strategy in subpanel level Distributed Maximum Power Point Tracking (DMPPT) application

Distributed Maximum Power Point Tracking (DMPPT) has been revealed to be a technique increasing the efficiency and reliability of Photovoltaic (PV) systems in mismatch conditions. To further recover mismatch power loss inside PV panels, subpanel realization of DMPPT solution is developed as an improved penetration of DMPPT concept. This paper proposes a double loop control based subpanel level DMPPT system with a self-adjusted time-sharing Maximum Current Tracking (MCT) algorithm which detects the output current of series connected DC-DC converters. The proposal can always guarantee that all PV units output its individual maximum power regardless of irradiance cases with a greatly reduction of components such as control IC and sensors. Comparing with current commercial panel level solutions, common subpanel level mismatch power loss inside a PV panel can be totally recovered with single current sensor and control IC. The proposal of this paper can be utilized for further integration of PV optimizer.

An improved simulated annealing maximum power point tracking technique for PV array under partial shading conditions

This paper presents an improved Simulated Annealing (SA) based Maximum Power Point Tracking (MPPT) technique designed for photovoltaic (PV) systems under partial shading conditions (PSC). Compared with existing SA MPPT algorithms, the proposed method performs faster convergence speed, higher tracking accuracy and continuously tracking of the global Maximum Power Point (GMPP) under climate changes. The new SA MPPT principle is fully explained, discussed and verified by simulation.

A 320kV hybrid HVDC circuit breaker based on thyristors forced current zero technique

This paper presents a hybrid HVDC circuit breaker topology with bidirectional interruption based on thyristors forced current zero technique for voltage sourced converter-high voltage dc (VSC-HVDC) system. In normal operation, the mechanical switch and small amount of IGBTs handles the low conduction loss. In case of interruption, the commutated current is forced to zero by injecting a current inversely in order to turn off thyristors. Moreover, snubber circuits are employed for voltage balance among semiconductors. And a conceptual breaker is applied to a high voltage application of 320kV system whose short current is 15kA. The principal operation of the beaker has been simulated and tested to demonstrate the fast dc fault clearing. Compared with other solutions, this proposal can interrupt short current with relatively low cost and increase the fault current interrupting capability, so it is more suitable for practical HVDC grid.

High power DC/AC/DC converter based on different modulation strategies

The recent researches on high voltage and power DC/DC converter mainly focus on topology. The newest topology based on modular multilevel converter(MMC) is composed of a transformer coupling two three-phase MMC, so that it can be easily extended according to the required voltage rating. However, the present topology almost adopt the same modulation strategy at two sides of converter. Because of the different applications of the high power DC/DC converter, the voltages on two sides will be different leading to the number of submodules (SM) different. For the different structure mentioned above, this paper presents the method which takes different modulation strategies at two sides of converter. It respectively selects the optimal modulation strategy according to the SM number at two sides, which could reduce switching losses and harmonic content. Finally, a simulation model of 5kV/500V DC/AC/DC converter is established in MATLAB/Simulink, which verifies the feasibility of the method.

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 new macro-model of thyristor for HVDC converter valve

The converter valve is the critical link of AC and DC system and the most commonly used switching devices for converter valve are thyristors. The performance of thyristors could significantly affect the performance of converter valve while the transient characteristics and second order effects of thyristor are critical to determine the reliability of converter valve in HVDC (High Voltage Direct Current) system. Thyristor is a relatively early kind of power electronic device and various equivalent models have been developed. However, faults of actual HVDC system could be caused by different types of thyristor failures, therefore a model which could simulate comprehensively the characteristics and failure causes of thyristor is required in the research of reliability for HVDC system. A new macro-model which has considered the main characteristics of thyristor is developed with explicit structure, also the forward and reverse recovery transients can be simulated by this model. The parameters of the model could be easily calculated by the data sheet specifications. The proposed model is verified in the simulation.

An improved control method applied to the DC/DC converter based on MMC

Recent research on high voltage and power DC/DC converter mainly focus on the face-to-face topology based on modular multilevel converters (MMC). The topology consists of two MMC which are coupled by a transformer. It is easy to extend the module number according to the voltage level. However, the present modulation method in this topology cannot regulate the voltage transformation ratio on two dc sides. The ratio of the dc voltage mainly depends on the transformer ratio. This paper proposes a modulation method which can adjust the dc voltage ratio directly. Finally, a simulation model is established to verify the feasibility of the method.

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.