Lukasz Hubert Kocewiak

Future work on harmonics - some expert opinions Part II - supraharmonics, standards and measurements

A workshop on power system harmonics was organized in Stockholm in January 2014. On the agenda was among others a discussion on what are the main issues on harmonics at the moment and in the near future. Some of the issues discussed at that workshop are presented in this paper and its companion paper. In this paper the following issues will be addressed: the appearance of emission at higher frequencies (supraharmonics); the need for new and improved standards; measurement issues and data analysis.

Couplings in Phase Domain Impedance Modeling of Grid-Connected Converters

The output impedance of a power converter plays an important role in the stability assessment of the converter. The impedance can be expressed in different frames such as the stationary frame (phase domain) or in the synchronous frame (dq domain). To treat the three-phase system like a single-phase system, the system can be divided into positive and negative sequences in the phase domain. This paper demonstrates that there exist couplings between the positive and negative sequences, even in a balanced system due to the PLL, which is important for synchronization. Further it will be shown that even though these couplings are very small in magnitude, they are important in the stability of the converter.

Future work on harmonics - some expert opinions Part I - wind and solar power

A workshop on power system harmonics was organized in Stockholm in January 2014. On the agenda was among others a discussion on what are the main issues on harmonics at the moment and in the near future. The results of this discussion are summarized in this paper and some of the issues are discussed in more detail in this paper and in its companion paper. This paper discusses emission from wind and solar power as well as advantages and disadvantages of active and passive filters.

Harmonic resonances in Wind Power Plants: Modeling, analysis and active mitigation methods

This work reviews the state-of-the-art in the field of harmonic resonance problems in Wind Power Plants (WPPs). Firstly, a generic WPP is modeled according to the equivalent circuits of its passive and active components. Main focus is put on modeling active components, i.e. the ones based on power converters. Subsequently, pros and cons of frequency and time domain analysis methods are outlined. The next sections are devoted to mitigation methods implemented in the power electronics converters. From the wind turbine perspective, different techniques to enhance the robustness of the controller are analyzed. Subsequently, the suitability for active damping of harmonics using STATCOM devices is assessed, with focus both on control techniques and power converter technologies.

Harmonic modelling, propagation and mitigation for large wind power plants connected via long HVAC cables: Review and outlook of current research

This paper presents a state-of-the-art review on grid connection of large offshore wind power plants (OWPPs) using extra-long high voltage AC (HVAC) cables. The paper describes research by DONG Energy Wind Power in close collaboration with Aalborg University addressing related challenges through an industrial PhD project. The overall goal is to gain a better understanding of extra-long HVAC cable connected OWPPs, in order to ensure reliability and availability of OWPPs. This will reduce the cost of energy, as the risk of costly delays and modifications after the project has been commissioned can be reduced and operational availability and the reliability can be improved. Challenges hereto include lower resonance frequencies due to the long cables, frequency dependent modelling of long submarine cables, passive filtering, and PLL dynamics in the control loops (from a harmonic stability point of view).

The impact of harmonics calculation methods on power quality assessment in wind farms

Different methods of calculating harmonics in measurements obtained from offshore wind farms are shown in this paper. Appropriate data processing methods are suggested for harmonics with different origin and nature. Enhancements of discrete Fourier transform application in order to reduce measurement data processing errors are proposed and compared with classical methods. Comparison of signal processing methods for harmonic studies is presented and application dependent on harmonics origin and nature recommended. Certain aspects related to magnitude and phase calculation in stationary measurement data are analysed and described. Qualitative indices of measurement data harmonic analysis in order to assess the calculation accuracy are suggested and used.

Harmonic mitigation in wind power plants: Active filter solutions

Passive solutions are the most used harmonic mitigation methods in wind power plants, but the use of active filters is also a suitable and attractive approach. However, the best location for the connection of these active filters needs to be studied and evaluated. In this paper, a study of the use of active harmonic filters placed at different locations inside a wind power plant is performed. The results for current and voltage harmonic distortion are compared and discussed to determine the most suitable location for placing active harmonic filters within a wind power plant.

Challenges with harmonic compensation at a remote bus in offshore wind power plant

This paper investigates the challenges associated with remote harmonic compensation in offshore wind power plants through long cables and transformers. The interaction between the grid network and the wind power plant network can lead to the amplification of certain harmonics and potentially resonant conditions. Hence, the plant developer is required to maintain the harmonic distortion at the point of common coupling within the planning level limits using harmonic compensation, which is usually done by static filters. In this paper an active damping compensation strategy with a STATCOM using emulation of using emulation of resistance at the harmonic frequencies of concern is analyzed. Finally the results are demonstrated using time domain simulations in PSCAD.

Stability Boundaries for Offshore Wind Park Distributed Voltage Control

In order to identify mechanisms causing slow reactive power oscillations observed in an existing offshore wind power plant (WPP), and be able to avoid similar events in the future, voltage control is studied in this brief for a plant with a static synchronous compensator, type-4 wind turbines, and a park pilot control. Using data from the actual WPP, all stabilizing subsystem voltage proportional–integral controller parameters are first characterized based on their Hurwitz signature. Inner loop current control is then designed using internal mode control principles, and guidelines for feedforward filter design are given to obtain required disturbance rejection properties. This brief contributes by providing analytical relations between power plant control, droop, sampling time, electrical parameters, and voltage control characteristics, and by assessing frequencies and damping of reactive power modes over a realistic envelope of electrical impedances and control parameters.

Small-signal model of a decoupled double synchronous reference frame current controller

The converter behavior in harmonic studies or harmonic stability analysis can be evaluated by its output impedance. In order to overcome the nonlinearities introduced by the PLL, linearized models around the operating point are typically used. However, in the case of an imbalance in the system the dq signals are not dc anymore, and therefore, linearization cannot be done. In this paper a Decoupled Double Synchronous Frame PLL is used to eliminate the oscillations in the dq frame signals. The small signal model of this PLL including an unbalanced current controller is presented in this paper.