Friedemann Moller

Experimental-Based Evaluation of PV Inverter Harmonic and Interharmonic Distortion Due to Different Operating Conditions

This paper presents the results of comprehensive testing and subsequent detailed analysis of the obtained test results, evaluating harmonic and interharmonic performances of photovoltaic inverters (PVInvs) for a range of different operating conditions. The presented results indicate significant power-dependent changes in harmonic and interharmonic emissions of tested PVInvs for different supply voltage conditions (presence of voltage waveform distortions and various source impedance values). To correctly quantify and describe these changes in PVInv performance, this paper discusses and applies measurement procedures and metrics for evaluating harmonic and interharmonic emission recommended in existing standards, as well as some additional metrics and indicators. For some operating conditions, tested PVInvs significantly increase both harmonic and interharmonic emissions, and this paper also discusses the impact of PVInv control (e.g., maximum power point tracking control) as a possible origin of the interharmonic distortion.

Stochastic Assessment of Voltage Unbalance Due to Single-Phase-Connected Solar Power

A stochastic method is presented in this paper to estimate the future voltage unbalance in a low-voltage distribution network with high-penetration of single-phase photovoltaic inverters (PVIs). The location and phase allocation of the PVIs are considered as input parameters for the stochastic simulation. The method has been applied to three different low-voltage networks: two in Sweden and one in Germany. In the Swedish networks, for 6-kW single-phase PVIs, it is likely that the contribution from single-phase photovoltaic inverters to the voltage unbalance exceeds 1%. The 2% value is unlikely to be exceeded. In the German network, for 4.6-kW single-phase PVIs, the voltage unbalance is between 1.35% and 2.62%. The risk of high-voltage unbalance can be reduced by a combination of controlled distribution over the phases and a reduction of the maximum size for a single-phase PVI.

Characterisation of harmonic interactions between electric vehicle battery chargers and PV inverters

This paper presents and discusses some initial results of the measurement and assessment of the harmonic characteristics of electric vehicle (EV) chargers and photovoltaic (PV) inverters, which are tested individually and when connected in different combinations to the same phase of a programmable three-phase LV supply in the laboratory. The range of possible harmonic interactions is assessed for three source impedance values and three characteristic supply voltage waveforms. The presented results can help in understanding how the exchange of energy between the grid, power electronic loads (EV chargers) and inverter-interfaced distributed generation (PV inverters) could be assessed in terms of the anticipated “smart grid” transformation of electricity networks.

Component-based modelling of EV battery chargers

This paper reviews typical circuit topologies and control algorithms for on-board unidirectional single-phase electric vehicle battery chargers (EVBCs) and provides simple yet reasonably accurate component-based models for their representation for power system analysis. The accuracy of the developed models is validated against measurements of actual EVBCs, for both ideal voltage supply conditions and typically distorted voltage supply waveforms. The paper also provides exponential and polynomial EVBC model interpretations, as well as some discussion of the further use of the presented models for the analysis of existing networks and future smart grids with high penetration levels of EVBCs.

Impact of operating conditions on harmonic and interharmonic emission of PV inverters

This paper presents the results of testing and analysis investigating impact of operating conditions on harmonic and interharmonic emission of PV inverters. Tests include typical supply voltage distortion and source impedances. Measurement procedures and metrics recommended by standards for evaluating harmonic and interharmonic distortion are discussed. Finally, it is investigated how the Maximum Power Point Tracking control might impact interharmonic distortion.

On the impact of operating modes and power supply conditions on the efficiency of power electronic devices

A wide range of modern low voltage power electronic (PE) devices feature sophisticated controls, which aim to improve device performance and ensure better regulation of grid-side ac currents. The implementation of these controls is expected to result in increased device efficiency, improved controllability, higher operating power factors and reduced harmonic emissions. The interactions of individual PE devices and their controls with the supplying network are analyzed in this paper in terms of the total harmonic distortion and emission of main low-order harmonics, which are then used to assess exchanges of powers at fundamental system frequency and nonfundamental (i.e. harmonic) frequencies between the individual PE device and the grid. Based on this analysis, the paper correlates the obtained results for harmonic performance and efficiencies over the entire range of operating powers of the considered PE devices, calculated in accordance with the standard definitions and using some alternative interpretations.

Stochastic assessment of voltage unbalance due to single-phase-connected solar power

A stochastic method is presented in this paper to estimate the future voltage unbalance in a low-voltage distribution network with high-penetration of single-phase photovoltaic inverters (PVIs). Location and phase allocation of the PVIs are considered as input parameters for the stochastic simulation. The method has been applied to three different low-voltage networks: two in Sweden and one in Germany. In the Swedish networks, for 6-kW single-phase PVIs, it is likely that the contribution from single-phase photovoltaic inverters to the voltage unbalance exceeds 1%. The 2% value is unlikely to be exceeded. In the German network, for 4.6-kW single-phase PVIs the voltage unbalance is between 1.35% and 2.62%. The risk of high voltage unbalance can be reduced by a combination of controlled distribution over the phases and reduction of the maximum size for a single-phase PVI.

Harmonic emission of PV inverters under different voltage supply conditions and operating powers

This paper presents the results of experimental evaluation of current harmonic emission characteristics of three photovoltaic inverters (PVIs) under different voltage supply conditions and for full range of their operating powers. In tests, various voltage supply conditions are emulated with both ideally sinusoidal and typically distorted voltage waveforms, as well as with different source impedance values, while power outputs of PVIs are adjusted in the range from their nominal/rated powers down to the lowest possible powers at which they were able to maintain a stable operation. Harmonic emission of tested PVIs is quantified in terms of their total harmonic content, total harmonic/demand distortion and individual current harmonics, which are compared with the corresponding emission limits in existing standards and recommendations. Particular attention is given to the 2nd-7th low-order harmonics.

Probabilistic household load model for unbalance studies based on measurements

The number of single phase connected devices (electric vehicle charger, photovoltaic inverters, etc.) with high electrical power and long periods of operation increases continuously and may lead to a significant increase in voltage unbalance. In order to perform a realistic simulation of voltage unbalance in public low voltage networks, the contribution of residential loads to unbalance has to be included. This paper presents a probabilistic model of household loads for unbalance studies. The model is based on a set of power measurements of households of different size in different networks. Based on these data an unbalanced load flow model for household loads is developed and validated. Both, simulation and measurement results are discussed and simplified equations to estimate unbalance and maximum power of a group of households are provided.