Luiz A. C. Lopes

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.

Performance assessment of active frequency drifting islanding detection methods

Islanding detection is a mandatory feature for grid-connected inverters. The effectiveness of passive islanding detection methods (IDMs) is usually demonstrated by means of nondetection zones (NDZs) represented in a power mismatch space (/spl Delta/P versus /spl Delta/Q). Active frequency drifting IDMs have been shown to provide improved performance but their theoretical NDZ cannot be described in the /spl Delta/P versus /spl Delta/Q space for a general RLC load. This paper shows that a load parameter space based on the values of the quality factor and resonant frequency of the local load (Q/sub f/ versus f/sub 0/) can be used in these cases. It employs a single curve to represent the NDZ of frequency drifting IDMs for any RLC loads. Equations that represent NDZs of three common active IDMs in the Q/sub f/ versus f/sub 0/ load parameter space are derived and it is shown that the slip mode frequency shift and the Sandia frequency shift IDMs can be designed to guarantee islanding detection for equivalent RLC loads with a quality factor smaller than a design value. The accuracy of the NDZs is verified with simulation and experimental results.

An improved perturbation and observation maximum power point tracking algorithm for PV arrays

This paper discusses a new implementation of a perturbation and observation (P&O) maximum power point tracking (MPPT) algorithm that can mitigate/reduce the main drawbacks commonly related to the P&O method. This is achieved with peak current control, small perturbation values and sampling of instantaneous values, instead of averaged, to speed up the system response and reduce the oscillations around the maximum power point (MPP). The number of samples per switching cycle, type (synchronized/ unsynchronized) and ideal instant for sampling (maximum or minimum current) are investigated in order to obtain fast calculation of the direction of the next perturbation. Experimental results with the proposed control scheme implemented in a DSP are presented.

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

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.

Vector Control of Single-Phase Voltage-Source Converters Based on Fictive-Axis Emulation

This paper presents an alternative way for the current regulation of single-phase voltage-source dc-ac converters in direct-quadrature (dq) synchronous reference frames. In a dq reference frame, ac (time varying) quantities appear as dc (time invariant) ones, allowing the controller to be designed the same as dc-dc converters, presenting infinite control gain at the steady-state operating point to achieve zero steady-state error. The common approach is to create a set of imaginary quantities orthogonal to those of the real single-phase system so as to obtain dc quantities by means of a stationary-frame to rotating-frame transformation. The orthogonal imaginary quantities in common approaches are obtained by phase shifting the real components by a quarter of the fundamental period. The introduction of such delay in the system deteriorates the dynamic response, which becomes slower and oscillatory. In the proposed approach of this paper, the orthogonal quantities are generated by an imaginary system called fictive axis, which runs concurrently with the real one. The proposed approach, which is referred to as fictive-axis emulation, effectively improves the poor dynamics of the conventional approaches while not adding excessive complexity to the controller structure.

Wind-driven self-excited induction generator with voltage and frequency regulated by a reduced-rating voltage source inverter

This paper discusses the regulation of the voltage and frequency of a stand-alone fixed-pitch wind energy conversion system (WECS) based on a self-excited squirrel-cage induction machine. A shunt connected voltage source inverter (VSI) and a controllable dump load are used for regulation purposes. A battery bank is included in the dc side of the VSI so that it can absorb and inject active power thus increasing the efficiency and availability of the system. A control scheme for the VSI with independent control of active and reactive power allows the state of charge of the batteries to be kept in a safe range while maximizing the voltage regulating capabilities of the VSI. The characteristics of the wind turbine, self-excited generator, and the ratings of the VSI are considered in order to determine the load range for which voltage and frequency can be regulated for a given wind speed range. The feasibility of the proposed system is verified by simulations.

An Intelligent Maximum Power Point Tracker Using Peak Current Control

The perturbation and observation (P&O) maximum power point tracking (MPPT) algorithms are commonly used in photovoltaic (PV) systems due to their easy implementation and ability to track the maximum power point (MPP) of the solar array under widely varying atmospheric conditions viz. solar irradiation, panel temperature etc. P&O algorithm based on peak current control and the use of instantaneous sampled values to calculate the next perturbation direction have the potential for faster transients and smaller oscillations around the MPP. The use of fixed variation of the reference current results in a compromise sub-optimum solution. This paper discusses a fuzzy logic based P&O MPPT with peak current control with variable variation of the reference current for improved transient as well as steady-state performance. Simulation results show a 15 % gain in the transient response and decrease of the power loss in the steady state. Besides, both the P&O scheme with fixed variation for the reference current and the intelligent MPPT algorithm were able to identify the global MPP in a partially shaded PV module, however the performance of the intelligent MPPT algorithm was better

Self-Tuning Virtual Synchronous Machine: A Control Strategy for Energy Storage Systems to Support Dynamic Frequency Control

This paper investigates the use of a virtual synchronous machine (VSM) to support dynamic frequency control in a diesel-hybrid autonomous power system. The proposed VSM entails controlling the grid-interface converter of an energy storage system (ESS) to emulate the inertial response and the damping power of a synchronous generator. In addition, self-tuning algorithms are used to continuously search for optimal parameters during the operation of the VSM in order to minimize the amplitude and rate of change of the frequency variations and the power flow through the ESS. The performances of the proposed self-tuning (ST)-VSM and the constant parameters (CP)-VSM were evaluated by comparing their inertial responses and their damping powers for different scenarios of load variations. For the simulated cases, the ST-VSM achieved a similar performance to that of the CP-VSM, while reducing the power flow through the ESS in up to 58%. Moreover, in all the simulated scenarios, the ST-VSM was found to be more efficient than the CP-VSM in attenuating frequency variations, i.e., it used less energy per Hertz reduced.

Virtual synchronous generator control in autonomous wind-diesel power systems

This paper addresses the problem of frequency stability in an autonomous wind-diesel power system with energy storage. At high wind penetration levels and due to the lack of controlled rotating machines, wind fluctuation might cause unacceptable frequency excursions in the system and they need to be mitigated. A virtual synchronous machine control is designed to emulate a virtual inertia via power injection from/to the energy storage system. The impact of this control strategy on the system is analyzed. Based on the case study, the major conclusion is that virtual inertia control reduces the maximum rotor speed deviation, but in exchange for increasing the inertia, the system becomes slower and more oscillatory. Possible solutions for the latter are pointed out, but further work is required. Parameters for sizing the energy storage are proposed.

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.