Leong Hai Koh

Operational Adequacy Studies of a PV-Based and Energy Storage Stand-Alone Microgrid

This paper presents a probabilistic approach in the modeling of stand-alone microgrids to predict their operational adequacy performance considering uncertainty of energy storage system (ESS), photovoltaic system (PVS) and conventional generator (CG). Instead of using the daily or hourly time step, operating period a minutely time step is considered to incorporate the effect of fast ramp up/down of system components on microgrid operating adequacy through expected energy not supplied (EENS) and expected energy not used (EENU), due to load and resource variations. A time varying state of charge (SOC) model is proposed to determine power output of an ESS in reliability modeling. The reliability of a PVS is modeled in detail based on the total cross-tied configuration (TCTC) of photovoltaic (PV) cells and arrays. The proposed technique and indices will be useful for system planners to select the type and size of microgrid systems that contain alternative energy sources and storage.

Renewable energy integration into smart grids: Problems and solutions — Singapore experience

Singapore being a city state with 712.4km2, 5.183million people, S

Comparison of Superconducting Fault Current Limiter and Dynamic Voltage Restorer for LVRT Improvement of High Penetration Microgrid

59,813 Gross Domestic Product (GDP) per capital, population density of 7,126 per square kilometer [1] and limited natural resources, identified her GHG emission as carbon dioxide (CO2) mainly from combustion of fossil fuels and natural gas to generate energy meeting development and human needs. By 2006 highly efficient combined cycle technology was deployed to generate 78% primary energy by burning natural gas [2]. One of Singapore key strategies to further mitigate GHG emissions is to increase the energy efficiency of various sectors and/or introducing renewable energy sources. Singapore Industry, Buildings and Households sector consumes 54%, 16% and 9%, respectively, of generated secondary energy [2]. Three areas of pilot test beddings are presented to enhance Singapore energy efficiency. First, Energy Market Authority (EMA) launched a pilot project Intelligent Energy System (IES) in 2010 to test and evaluate new applications and technologies around a smart grid, thereby enhancing Singapores power system efficiency and resilience, reducing wastage, saving peak loads and deferring capital investments to meet consumer demand in the future. Second, EMA will show case in 2012 how clean and renewable energy can be deployed at the system level in an environmentally, socially and economically sustainable manner for an off-grid community at Pulau Ubin, an island located at North East of Singapore. Third, Agency for Science, Technology and Research (A*STAR), and four industry partners developed a test facility Experimental Power Grid Centre (EPGC) [4], housing a 1MW power grid at Jurong Island in South West Singapore.

Compensation of DC Offset and Scaling Errors in Voltage and Current Measurements of Three-Phase AC/DC Converters

For a high penetration microgrid, improving its low-voltage ride-through (LVRT) capability under some minor or temporary faults can contribute to reinforcing power support and reducing network instability. In this paper, the comparison of a superconducting fault current limiter (SFCL) and a dynamic voltage restorer (DVR) for LVRT capability enhancement of a 10-kV microgrid is conducted. Concerning the microgrid which includes distributed photovoltaic generation, energy storage and loads, the effects of the SFCL and the DVR are compared in detail, and related theoretical analysis, simulation study, and economical evaluation are carried out. From the demonstrated results, the suggested two devices can both assist the microgrid to achieve the LVRT operation, but the economics of the DVR are weaker than the SFCL. Moreover, compared to the DVR, the SFCL enables the energy storage unit to offer better control effects in power stabilization. From technical and economic perspectives, the SFCL is a more preferable choice than the DVR.

A 13.8-MHz RC oscillator with self-calibration for ±0.4% temperature stability from −55 to 125°C

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 currents and subsequently lead to voltage ripple at the dc output voltage of the ac/dc converter. 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 voltage ripple characteristics of the dc output voltage ripple combining with simple band-pass and low-pass filters. Meanwhile, the dc offset and scaling errors in the voltage measurement can be rejected by using an advanced current controller developed with a proportional resonant plus a repetitive controller designed in the synchronous (d–q) reference frame. The proposed control algorithm is able to effectively reject the impact of both voltage and current measurement error so that the three-phase input currents are regulated to be balanced and sinusoidal with ultralow dc current component. Meanwhile, the dc output voltage is also well regulated to be pure dc with a negligibly small voltage ripple. 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.

Droop control of a bipolar dc microgrid for load sharing and voltage balancing

This paper articulates a novel oscillator design that can provide stable frequency operation over a wide temperature range using self-calibration technique. A simple ring oscillator is implemented to sense its temperature variation, and digital circuits to tune the output frequency according to the sensing outcomes has also been incorporated to the design. Simulation results show that the circuit can generate a stable frequency of 13.8 MHz. The power consumption of the whole system is only 52.8μW. The temperature coefficient is less than ±0.4% across -55 to 125°C in the worst process corner, operating at an ultra-low supply voltage of 0.6 V.

An Intelligent Microgrid Energy Management System Using Distributed Database and IEEE 1888

A bipolar dc microgrid features three voltage levels and is capable of transmitting power more efficiently than a unipolar system. This paper proposes a droop control to achieve load sharing and voltage balancing in such a microgrid system. Droop control is adopted to ensure that autonomous sharing of load power can be realized without communication. Moreover, the positive and negative pole voltages in the bipolar dc microgrid can be well regulated regardless of load changes and input voltage mismatch. Two battery energy storage systems are incorporated for balancing load and generation, and the lifetime of batteries can be improved through proper coordination control. Simulation results in the PLECS environment and experimental results obtained from an OPAL-RT system are presented to verify the effectiveness of load sharing and voltage balancing of droop control.

Optimal allocation of flux-coupling-type SFCLs for a micro-grid with wind-PV hybrid generation and battery energy storage

This paper proposes an intelligent microgrid energy management system (iEMS). It aims to develop and demonstrate an intelligent system at campus site through seamless integration of energy generation, storage and demand management facilities across the electric, thermal, and gas networks. This system will also have an enhanced information and communication technology (ICT) platform for supply and demand management as well as real-time energy market interactions. The major innovations proposed by the iEMS encompass advanced ICT tools such as forecasting, predictive modelling for harmonization of multi-energy grids and enabling real-time energy market interactions for maximizing cost savings without compromising operational effectiveness and grid stability. These innovations serve to either enhance system level energy savings or facilitate system integration by using advanced distributed database system and protocols such as IEEE 1888 for communication and control.

Operational adequacy studies of a PV-based & energy storage stand-alone microgrid

Regarding the access of a micro-grid with multiple distributed generation (DG) units into the power distribution network, superconducting fault current limiters (SFCLs) may be used to improve the micro-grids robustness against short-circuit faults. In this paper, the flux-coupling-type SFCL is selected to play the role, and the optimal allocation of the SFCLs for protection of a micro-grid with wind-PV hybrid generation and battery energy storage is performed. The SFCLs structural principle is conducted first. Further, those candidate allocation scenarios of the SFCLs used in the micro-grid are enumerated, and each of them includes installation number, mounting location and impedance parameter. Besides, a matching optimal objective function is proposed, and herein the each SFCLs current-limiting performance and the micro-grids frequency stability are considered as two key optimization criterions. Time-domain simulation is done, and the results imply that the SFCL should be located on the direct path of current flowing from the DG and the main network. Through the optimal utilization of the fluxcoupling-type SFCLs, it can be obtained that reduce of the fault current, inhibition of the frequency fluctuation and acceleration of the system restoration. Thus, the micro-grid systems transient stability is enhanced well.

Intelligent eco-building management system

Summary form only given. This paper presents a probabilistic approach in the modelling of stand-alone microgrids to predict their operational adequacy performance considering uncertainty of energy storage system (ESS), photovoltaic system (PVS) and conventional generator (CG). Instead of using the daily or hourly time step, operating period a minutely time step is considered to incorporate the effect of fast ramp up/down of system components on microgrid operating adequacy through expected energy not supplied (EENS) and expected energy not used (EENU), due to load and resource variations. A time varying state of charge (SOC) model is proposed to determine power output of an ESS in reliability modelling. The reliability of a PVS is modelled in detail based on the total cross-tied configuration (TCTC) of photovoltaic (PV) cells and arrays. The proposed technique and indices will be useful for system planners to select the type and size of microgrid systems that contain alternative energy sources and storage.