Showing papers by "Luiz A. C. Lopes published in 2008"

Islanding Detection Assessment of Multi-Inverter Systems With Active Frequency Drifting Methods

Abstract: Islanding is one important concern for grid connected distributed resources due to personnel and equipment safety. Inverter-resident active islanding detection methods (IDMs) inject disturbances into the supply system and detect islanding based on the responses of locally measured parameters. Frequency drifting IDMs are very effective in systems with a single inverter but it is believed that their effectiveness might be reduced in multi-inverter systems due to mutual interferences. This paper discusses the effect of inverters with passive IDM on the islanding detection capabilities of inverters with active frequency drifting IDMs and also interferences between the latter. Non-detection zones (NDZs) of multi-inverter systems in a load parameter space are used as analytical tool. Simulation and experimental results are provided to validate the NDZs of the multi-inverter systems with active frequency drifting IDMs.

Voltage Regulation in Radial Distribution Feeders with High Penetration of Photovoltaic

Abstract: Overvoltages are one of the main reasons for limiting the amount of active power that can be exported by a microgrid and injected into a low voltage (LV) distribution system The well-known trade offs used in medium voltage (MV) feeders need to be revisited considering the fact that the impedance of LV feeders is mostly resistive with large R/XL ratios This digests investigates the impact of active power and reactive power variation on the voltage and losses of a radial LV distribution feeder with uniformly distributed loads and non-dispatchable (active power) sources The feeder characteristics as well as the net active power of the buses are considered in the analysis This shall give indications on how to decide between PV units with overrated inverters, for additional capacity of reactive power control, or energy storage devices, so as to minimize overvoltages during peak power production

Harmonics reduction in low switching frequency space vector modulated current source converters

Abstract: High power current source converters (CSC) are usually implemented with GTOs which present a maximum switching frequency of a few kHz. Space vector modulation (SVM) offers a very elegant way of generating CSC gating signals on-line with increased gain and reduced switching frequency. However, for very low frequency, as required by GTOs, SVM results in non-characteristics low order (5th and 7th ) harmonics. The reduction of the magnitude of these harmonics has been sought mostly through new sequences of space vectors (states), which present better performance for different ranges of modulation index and power factor. Moderate improvement can be obtained by calculating the states on times at once in the middle of a cycle. This paper shows that larger reductions can be achieved by calculating the states on times at different moments, as the states are used. The effectiveness of the proposed technique for different states sequences is demonstrated by means of simulations.

Soft-switching of a hybrid bi-directional Current Source Converter

Abstract: A three-phase Hybrid Current Source Converter (HCSC) that employs 3 SCRs and 4 switches (IGBTs) was proposed as an economical alternative to the conventional 6-switches Current Source Converter (CSC). It presents bi-directional power flow capability, does not generate low order harmonics and can operate with variable power factor. However, the force-commutated switches need to operate with increased switching frequency and remain on for minimum periods of time for safe commutation of the SCRs what leads to a reduced maximum modulation index (mamax). This paper describes a space vector modulation (SVM) technique that allows one natural (soft-switching) commutation of SCRs and IGBTs per SVM cycle. Besides, the reduced stress on the semiconductors, this allows the use of SVM sequences with three segments and an increase on the maximum modulation index (mamax) of the 3-SCR and 4-IGBT HCSC.

A hybrid bi-directional current source converter with two force-commutated switches and controlled with space vector modulation

Abstract: SCR based CSC are rugged and affordable but they create low order harmonics and do not allow active and reactive power to be controlled independently. These drawbacks can be overcome with topologies based on more costly force-commutated switches. This paper proposes a hybrid three-phase CSC that employs a three-phase SCR bridge and two switches, presenting features comparable to the fully controllable six switches CSC. Comparisons with the fully controllable CSC are provided for a fair assessment of the potential of the proposed CSC. The theoretical analysis is verified by means of simulations.