Choo Fook Hoong

Development of a mathematical model for solar module in photovoltaic systems

A mathematical model of a solar module is presented. This model takes into account solar model temperature and solar radiation. The experimental data of a solar module under natural environment condition (NEC) have been obtained to determine the model parameters. The experimental results are compared with those calculated by using a mathematical model. It shows that the mathematical model accurately simulates the current-voltage characteristics of the solar module under the NEC and therefore is suitable for photovoltaic system design and performance analysis.

Multi-level control of grid-tied DC microgrids

Increase of DC-compatible loads, popularity of distributed renewable energy sources (RESs) and development of power electronic converters are boosting the applications of DC microgrids (MGs). Multi-level control is proposed to realize the coordination among RESs, battery energy storages (BESs), DC loads and utility grid in distributed manner. System elements are prioritized to schedule different operation modes with bus voltage variation. Bus voltage band is divided into a few sub-regions so that operation could be realized in distributed manner. Droop control is implemented to realize power sharing among system elements in monitoring bus (MB) mode based on their capacities. System and individual converter response regarding variation of load power, RESs generation and operation modes transition between grid-tied and islanded are investigated. Lab-scale DC MG is developed to verify the feasibility and effectiveness of proposed control method.

Energy management system for control of hybrid AC/DC microgrids

Hybrid AC/DC microgrid is getting popularized due to the higher penetration of DC-compatible loads, energy sources and storages. To maintain system power balance in both AC and DC sub-grids, the bidirectional interlinking converter (BIC) is used. Distributed control of hybrid AC/DC microgrid is normally implemented to enhance system reliability. DC bus voltage and AC frequency are used for indication of system active power balance. BIC power flow is determined based on local information to equalize the normalized deviations of DC voltage and AC frequency. However, deviations of AC frequency and DC bus voltage degrade system power quality, which deteriorate the lifetime operation of system units. Multi-level energy management system (EMS) is thus proposed in this paper to enhance control accuracy while retaining system reliability. Distributed control of hybrid AC/DC microgrid is scheduled as the primary control. Restorations of AC frequency and DC bus voltage are implemented in secondary control. In tertiary control, the power references of system units are generated based on the comparison of marginal costs. Power sharing compensation is applied to minimize power tracking errors. MATLAB/Simulink model for hybrid AC/DC microgrid is developed for the verification of the proposed multi-level EMS.

Hierarchical control of active hybrid energy storage system (HESS) in DC microgrids

Energy storages can be characterized based on energy and power densities, ramp rate, etc. Hybridization takes advantages of all energy storages to enhance system performance. Hierarchical control which is comprised of both centralized and distributed control is proposed for HESS operation in this paper. In normal state, system operates with energy management system. The central controller is used to coordinate the power sharing among various energy storages based on their characteristics and operating statuses. In case of communication failure, distributed control which operates with local information is to be activated. The bus voltage is used as indicator for system power balance to realize distributed control of system units. MATLAB/Simulink is used for the verification of the proposed methods.

Impact of the relative humidity on the LNG cold energy based inlet air cooled microturbine systems

Vaporization of liquefied natural gas (LNG) can be used as heat sinks in the external thermal cycles, and these processes are known as LNG cold utilization systems. Inlet air cooling application is one of the LNG cold utilization methods in the power generation sector, and it provides lower compressor inlet temperatures to increase the thermal efficiency and the net generated work. This study performs the simulations of energetic, environmental and economic approaches for the 30 kW and 65 kW microturbine systems for various relative humidity values under different ambient air temperatures. It is found that the relative humidity did not have a crucial impact on the net generate work rate and thermal efficiency although it causes a slight decrement from 60% RH to 90% RH. The environmental analyses show that the emission rate increases by the rising of relative humidity. Moreover, the environmental payback period increases by the rising of the relative humidity value. Lastly, the payback periods are investigated and it is seen that the relative humidity and the ambient air temperature do not have any crucial impact on the payback periods. The payback periods are found as 4.34 and 3.27 years for the 30 kW and 65 kW models, respectively.

Coordination secondary control for autonomous hybrid AC/DC microgrids with global power sharing operation

This paper presents a coordination secondary control strategy for autonomous hybrid AC/DC microgrids with global power sharing operation. In the previous work, primary control with droop control method was applied to distributed generators to ensure local power sharing without communication links in either ac or dc microgrid. An extension of local power sharing for individual microgrid is the global power sharing for hybrid microgrids. With global power sharing control, all distributed generators throughout the hybrid system are capable of sharing the entire ac and dc loads without any information exchange. The coexistence of local power sharing and global power sharing in hybrid system is thereafter defined as generalized primary control, which is usually employed for fully decentralized power management throughout the whole hybrid system. To eliminate the inherent voltage/frequency deviations caused by the generalized primary control, the secondary control is usually applied to each distribution generator for voltage/frequency restoration. This however will degrade the performance of global power sharing operation because of its characteristics of voltage/frequency deviation dependence. To achieve voltage/frequency restoration while maintaining the global power sharing operation, a coordination secondary control was proposed and the effectiveness of the proposed method has been verified by the simulation results.

Hybridization of energy storages with different ramp rates in DC microgrids

Hybridization of Energy Storages (ESs) with different characteristics is an effective and economic solution to enhance system performance. Hybrid Energy Storage System (HESS) in centralized coordination is normally implemented. To maintain system operation in case of communication failure, a novel algorithm is proposed for HESS distributed control in this paper. DC bus voltage is regarded as the global information carrier for indication of system power balance. All ESs are configured as slack terminals with droop control for obtainment of the reference bus voltages. Localized Low Pass Filters (LPFs) are added to ESs with low ramp rates to realize system net power decomposition and ESs power scheduling in distributed manner. To overcome the disadvantages of distributed control including voltage deviation and power sharing errors, multi-level Energy Management System (EMS) is applied. HESS distributed control is scheduled as the primary control. Bus voltage restoration and power sharing compensation are implemented in secondary control. Autonomous SoC recovery for ESs with high ramp rates is applied in tertiary control. Load shedding and generation curtailment are used to limit system net power range. MATLAB Simulink model is developed for the verification of the proposed method.

Elimination of DC and harmonic current injection due to grid voltage measurement errors in three-phase grid-connected inverter

This study proposes an enhanced current control strategy to eliminate DC and harmonic current at the output current of three-phase grid-connected inverters caused by DC offset and scaling error in grid voltage measurement. The proposed current controller designed in the stationary (α-β) reference frame is developed with a proportional integral (PI) plus three resonant controllers where the resonant controller tuned at the fundamental grid frequency (ωs) is used to regulate the fundamental component; whereas the PI and resonant controllers tuned at 5ωs and 7ωs are employed to eliminate the DC and harmonic components in the grid current. The proposed control scheme is developed without the need of phase-locked loop, DC or harmonic detection scheme, and extra hardware circuit so that it can be integrated into the existing grid-connected inverter systems without extra cost. The effectiveness of the proposed solution is verified by experimental results.

An integral-droop based dynamic power sharing control for hybrid energy storage system in DC microgrid

Power sharing performance is a critical issue of hybrid energy storage system (HESS) in autonomous DC microgrid (MG). In this paper, a novel integral droop (ID) is proposed to mimic dynamic characteristics of the capacitor by using energy storages (ESs) with quick response. The main advantage of the proposed controller is that dynamic power sharing among ESs is automatically realized in the decentralized level. For a given HESS, ESs with the proposed ID enables to compensate fast power change while ESs with conventional voltage-power (V-P) droop provide low frequency components of power demand. With coordinated control between ID and V-P droop, high/low pass filters (LPF/HPF) are intrinsically formulated in HESS in order to obtain reasonable dynamic power allocations among ESs. Matlab/Simulink model of HESS is established for the verification of ID controller, in which the impacts of different ID coefficients on transient performances of the system are analyzed in detail. Finally, the effectiveness of proposed ID is experimentally validated on a HIL (hardware in loop) HESS platform.

A control strategy to compensate for current and voltage measurement errors in three-phase PWM rectifiers

This study proposes a compensation strategy to deal with both current and voltage measurement errors in the three-phase PWM rectifier system. The DC offset and scaling errors in the voltage and current measurements cause the injection of undesired DC and unbalanced currents into the three-phase input current and subsequently lead to voltage ripple at the DC output voltage. This paper proposes a compensation scheme for current measurement error where the DC offset and scaling errors in the current measurement are estimated from the characteristic of the DC output voltage ripple combining with simple band-pass and low-pass filters. Meanwhile, an advanced current controller designed with a proportional integral plus two resonant controllers tuned at the fundamental grid frequency (ωs) and 2ωs in the synchronous (d-q) reference frame is suggested to reject the impact of the DC offset and scaling errors in the voltage measurement. The proposed compensation method is developed without the need of extra hardware circuit, sensor, or precise information of system parameters so that it can be considered as more cost effective and robust solution. The effectiveness of the proposed solution is verified by experimental results.