Colin Debruyne

Introducing small storage capacity at residential PV installations to prevent overvoltages

Low voltage distribution feeders are designed for unidirectional energy supply from transformer to consumer. However, the implementation of small-scale PV production units on local utilities may result in bidirectional energy flows. The simultaneous power injection at sunny moments may cause a serious voltage rise along the feeder. These overvoltages may not only damage critical loads but also switches PV inverters off causing loss of green energy at the most productive moments. This paper presents a method to limit the voltage rise by introducing small battery buffers at local production sites. A smart inverter decides whether the PV energy is injected in the grid or buffered in the batteries. The relation between battery buffer size and overvoltage reduction is presented for a typical Belgian residential distribution feeder. The influence of the buffer along the feeder is calculated by working with synthetic load profiles and solar irradiation data.

Evaluation of the Efficiency of Line-Start Permanent-Magnet Machines as a Function of the Operating Temperature

The standard squirrel-cage induction machine has nearly reached its maximum efficiency. In order to further increase the energy efficiency of electrical machines, the use of permanent magnets in combination with the robust design and the line start capability of the induction machine is extensively investigated. Many experimental designs have been suggested in literature, but recently, these line-start permanent-magnet machines (LSPMMs) have become off-the-shelf products available in a power range up to 7.5 kW. The permanent magnet flux density is a function of the operating temperature. Consequently, the temperature will affect almost every electrical quantity of the machine, including current, torque, and efficiency. In this paper, the efficiency of an off-the-shelf 4-kW three-phase LSPMM is evaluated as a function of the temperature by both finite-element modeling and by practical measurements. In order to obtain stator, rotor, and permanent magnet temperatures, lumped thermal modeling is used.

Derating factors for direct online induction machines when supplied with voltage harmonics: A critical view

Induction machines usually have a reduced efficiency when supplied with non linear currents. Most of the induction machines are not used at nominal loading, but at a 60% loading rate. In the paper it will be shown that derating factors for induction motors fed with nonlinear currents, like the NEMA MG1 or the IEC 60034-17, are correct for nominal loading. At lower loading these derating factors can underestimate the losses. The measurements shown in this paper indicate that for low loading percentages, the phase angles of the harmonics have a significant influence on the peak induction and therefore the saturation level of the induction machine. For low loading rates the constant losses, including the magnetization losses, are the dominant losses. For a correct estimation of the losses for motors working at low loading conditions, the phase of the harmonics has to be taken in account.

Preventing overvoltages in PV grids by integration of small storage capacity

The rapidly growing amount of distributed generation in low voltage distribution grids issues some undesired side effects. Simultaneous power injection may cause a serious voltage rise along the feeder. These overvoltages may not only damage critical loads but also switch off PV inverters causing loss of green energy at the most productive moments. This paper discusses the origin and different possibilities to limit this induced voltage rise. The option of introducing local energy buffering is further elaborated. The relation between buffer size and overvoltage reduction is presented for a typical Belgian residential distribution feeder. The influence of buffers along the feeder is calculated by working with synthetic load profiles and solar irradiation data.

Power injection by distributed generation and the influence of harmonic load conditions

The number of installed distributed generation (DG) in residential areas rapidly increases, specifically in the form of photovoltaics (PV), causing some undesired side effects such as voltage rise. Overvoltage can damage critical loads, but is also disadvantageous for the owner because inverters switch off in case of overvoltage, resulting in output loss.

Economic evaluation of the influence of overvoltages and the integration of small storage capacity in residential PV-installations

Implementation of decentralized production units (PV-installations) on low voltage distribution feeders may result in bidirectional energy flow. Simultaneous power injection may cause voltage rise on the feeder. Due to regulations inverters need to switch off when reaching a certain voltage causing loss of production. Because this happens mostly on bright, sunny days production loss can be significant. This paper presents an economic comparison between a situation without production loss, with production loss due to overvoltages and the introduction of storage. The production is calculated based on synthetic load profiles and solar irradiation data. The economic feasibility is calculated based on realistic prices and taking into consideration all available government funding.

Comparative study of the influence of harmonic voltage distortion on the efficiency of induction machines versus line start permanent magnet machines

Induction machines have nearly reached their maximal efficiency. In order to further increase the efficiency the use of permanent magnets in combination with the robust design of the induction machine is being extensively researched. These so-called line start permanent magnet machines have an increased efficiency in sine wave conditions in respect to standard induction machines, however the efficiency of these machines is less researched under distorted voltage conditions. This paper compares the influence of harmonic voltage distortion and the phase angle of the harmonic content on the overall motor efficiency of line start permanent magnet machines and induction machines.

Influence of harmonic currents on cable losses for different grid configurations

Nowadays, both industrial and residential consumers use power electronics with the aim of rational use of energy or the implementation of distributed generation. However, power electronics may cause harmonic currents. Since cable losses are function of the rms current, harmonic currents will increase cable losses. This contribution analyses the influence of the current harmonic spectrum on low voltage cable losses. Grid configurations are discussed with the aim to optimize cable joule losses in case of nonlinear load conditions.

Influence of Supply Voltage Distortion on the Energy Efficiency of Line-Start Permanent-Magnet Motors

Similar to standard three phase direct on line induction motors are three phase line-start permanent-magnet machines (LSPMMs) susceptible to grid voltage distortion. Although straight forward adaptation of harmonic loss mechanisms for standard squirrel-cage induction motors to LSPMMs seems appropriate, this study will indicate that harmonic loss calculations of induction machines are unsuitable to give accurate or even reasonable estimations of the influence of supply voltage distortion on the energy efficiency of LSPMMs. This research will specifically address the interaction of rotor-induced harmonic currents. Finite-element modeling is used in order to address the effect of supply voltage distortion on the segregated losses of an LSPMM. Finite-element modeling (FEM) allocates the majority of the harmonic loss to the rotor and shows shifting the relative phase angle between identical magnitudes of superimposed supply distortion of fifth and seventh order can result in a 30% reduction of rotor loss and 0.5 percentage points in overall energy efficiency. The results obtained by FEM are validated by practical measurements.

Incentives and technical considerations related to increased voltage tolerance in low voltage distribution grids

The rise of small scale decentralized production has increased the complexity of maintaining a rigid electrical grid. For a distribution network operator, knowledge of both the energy demand and supply in his portfolio has become very important. The discrepancy between production and consumption of power is buffered by the grid which can result in over/under voltages and voltage unbalance. Consequently, it is very important to balance demand and supply. For an energy supplier shortage of energy has to be purchased, which is expensive. Contradictory, an overcapacity can even result in negative energy prices. This paper suggests that, if the margins of the supply voltage variations are set more flexible, this could prove beneficial for both the distribution network operator and the energy supplier. One way to actively control the power consumption is the so-called demand side management (DSM). If the margins of the voltage variation are less strict, a higher variation in demand will be created. However, if a larger acceptable variation of supply voltage is envisioned, one has to be sure that these appliances will still work adequately. Additional to the incentives of allowing a larger spread on the voltage variation, this paper will try to explain how the influence of changing the supply voltage on household appliances can be tested and which parameters are important to interpret the results.