Reinaldo Tonkoski

Coordinated Active Power Curtailment of Grid Connected PV Inverters for Overvoltage Prevention

Overvoltages in low voltage (LV) feeders with high penetration of photovoltaics (PV) are usually prevented by limiting the feeders PV capacity to very conservative values, even if the critical periods rarely occur. This paper discusses the use of droop-based active power curtailment techniques for overvoltage prevention in radial LV feeders as a means for increasing the installed PV capacity and energy yield. Two schemes are proposed and tested in a typical 240-V/75-kVA Canadian suburban distribution feeder with 12 houses with roof-top PV systems. In the first scheme, all PV inverters have the same droop coefficients. In the second, the droop coefficients are different so as to share the total active power curtailed among all PV inverters/houses. Simulation results demonstrate the effectiveness of the proposed schemes and that the option of sharing the power curtailment among all customers comes at the cost of an overall higher amount of power curtailed.

Impact of High PV Penetration on Voltage Profiles in Residential Neighborhoods

The objective of this paper is to provide an assessment on voltage profiles in residential neighborhoods in the presence of photovoltaic (PV) systems. The network was modeled in PSCAD using common feeder characteristics that Canadian system planners use in suburban residential regions. A simulation study was performed to investigate potential voltage rise issues in the network up to 11.25% total PV penetration in the feeder and LV transformer capacity penetration up to 75%. Results indicate that the PV penetration level should not adversely impact the voltage on the grid when the distributed PV resources do not exceed 2.5 kW per household on average on a typical distribution grid. Moreover, the role of feeder impedance, feeder length, and the transformer short circuit resistance in the determination of the voltage rise is quantified.

Voltage Regulation in Radial Distribution Feeders with High Penetration of Photovoltaic

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.

Droop-based active power curtailment for overvoltage prevention in grid connected PV inverters

Photovoltaics (PV) is considered a non-controllable power source which can create overvoltages in distribution feeders during periods of high generation and low load. This is usually prevented by limiting the penetration level of PV to very conservative values, even if the critical periods rarely occur. This paper discusses the use of droop based active power curtailment (APC) techniques for overvoltage prevention in radial low voltage (LV) feeders as a means for increasing the installed PV capacity and energy yield. Two schemes are considered and tested in a typical 240V/75kVA Canadian suburban distribution feeder with 12 houses equipped with roof-top PV systems. In the first scheme, all PV inverters have the same droop coefficients. In the second, the droop coefficients are proposed to be different so as to share the total active power curtailed, and consequent loss of revenue, among all PV inverters/houses. Simulation results demonstrate the effectiveness of the droop based APC schemes and that the option of sharing the power curtailment among all customers comes at the cost of an overall higher amount of power curtailed.

Active power curtailment of PV inverters in diesel hybrid mini-grids

Renewable energy sources such as wind and photovoltaics (PV) have great potential for reducing fuel consumption in diesel-based mini-grids in remote communities. However, the fluctuating and intermittent nature of these sources can give rise to a number of problems in mini-grids with high penetration of non-dispatchable renewables with reduced or non-existent storage units. This paper proposes the use of a frequency x power droop control strategy for curtailing the output power of PV inverters during periods of excess of power in a diesel-dominated mini-grid. This approach results in reductions of frequency variation and also reduces the operation of the diesel genset with reduced loading. The theoretical analysis is verified by means of digital simulations with PSCAD.

Active Shunt Filter for Harmonic Mitigation in Wind Turbines Generators

One of the preferred technologies in variable speed wind turbines (VSWT) is formed by a wind rotor, permanent magnet synchronous generator (PMSG), three-phase bridge rectifier (BR) with a bulky capacitor, and power electronics converter for grid interface. High-intensity, low-frequency harmonic currents flows into the PMSG as electric load has a non-linear characteristic. This paper presents an analysis and simulation of an active shunt filter (ASF) for harmonic mitigation in wind turbines generators. Currents are represented in d-q synchronous reference frame (SRF) and PWM carrier strategy is used to control the active filter. A new scheme to synchronize d-q currents using mechanical rotor angular speed is adopted. A dynamic wind turbine model and the active filter are implemented on software PSIM©. Representative waveforms and spectral analysis is presented. Simulations show that the proposed active filtering configuration can mitigate harmonic content into the PMSG.

Improving transient stability of photovoltaic-hydro microgrids using virtual synchronous machines

Transient stability of photovoltaic-hydro microgrid systems is poor due to lack of inertia and the intermittent nature of photovoltaic systems. The stability of such systems can be improved by using virtual synchronous machines which add inertia into the system to allow for higher PV penetration without losing stability. A PV-hydro system was simulated in MATLAB/Simulink to analyze the transient stability problems due to PV fluctuations and/or load or generation changes. Virtual synchronous machine was added which reduced the frequency deviations and the high rate of change of frequency. The results indicate that the transient stability of photovoltaic-hydro microgrid systems can be improved by using virtual synchronous machines using a minimum amount of energy storage. However, the power requirements of the virtual synchronous machines converter was found to be high compared to the overall system size. Furthermore, high PV penetration levels were achieved by adding virtual synchronous machine to the system.

A single-switch three-phase boost rectifier to reduce the generator losses in wind energy conversion systems

Low and medium power wind energy conversion systems (WECS) are expected to be simple, low cost and reliable. They are usually implemented with fixed pitch wind turbines. In such a case, operation with variable speed for increased capture of wind energy requires a power electronics converter capable of adjusting the shaft speed. This is done by controlling the active power drawn from the generator. Permanent magnet synchronous generators (PMSG) are a good option for low power variable speed WECS since they do not require external excitation nor reactive power to operate. Typically, an ac-dc converter composed of a diode rectifier and a capacitive filter, connected to a voltage source inverter is used as an interface to a grid or to a load. The highly distorted input currents of the diode rectifier with capacitive filters produce additional power losses in the generator that can reduce its lifetime. This paper discusses the use of a single-switch three-phase boost rectifier in the front end of the power electronic interface. It provides means for regulating the shaft speed by duty cycle variation while reducing the generator losses when compared to the standard diode and capacitive filter scheme. Simulation and experimental results show the effectiveness of the proposed scheme for reducing the generator copper and core losses and also the internal temperature.

Harmonic Mitigation in Wind Turbine Energy Conversion Systems

Permanent magnet synchronous generators (PMSG applied to wind energy conversion system (WECS) using variable speed operation is being used more frequently i wind turbine application. Variable speed systems have several advantages over the traditional method of operating win turbines, such as the reduction of mechanical stress and a increase in energy capture. To allow the variable speed operation of the PMSG WECS a conventional three-phase bridge rectifier (BR) with a bulky capacitor associated with voltage source current controlled inverter (VS-CCI) is used This simple scheme introduces a high intensity low frequency current harmonic content into the PMSG and consequently increases the total loses in it. Subsequently, decreases the power capability of the system. This paper presents comparative simulation study between three different approaches applied to harmonic mitigation in PMSG WECS The studied techniques are: a) harmonic trap filters (HTF), b single-switch three-phase boost rectifier (PFC) and c) a three phase boost type PWM rectifier (PWMREC).

Integrated System for Intelligent Street Lighting

This reports the study and hardware implementation of a dimmable electronic ballast for high pressure sodium lamps, and a microprocessor-based system for control and energy measurement for this ballast, which uses a power line communications system to send and receive status and commands from another ballasts plugged in the same mains subcircuit. These ballasts are connected in the topology of a logic network, one of them being defined as the master of the subcircuit, and the others as slaves. The master unit distinguishes from the slaves by the additional communications system, which works through a cell phone, and enables the wireless connection to a PC-based central supervisory system. This way, any locale or town becomes able to control its entire main lighting system, in addition to the obtainment of more accurate data about energy billing, which together with the ability to control luminosity and the better power factor, will result in financial and energetic economy