Harmonics
The harmonic analysis functionality is ideal for applications in Transmission, Distribution and Industrial networks for filter design, ripple control signal simulation or for the determination of network natural resonance frequencies.
For analyzing the impact of harmonics in power systems, DIgSILENT PowerFactory provides two basic harmonic analysis functions.
Harmonic Load Flow
The DIgSILENT PowerFactory harmonic load flow features the calculation of harmonic voltage and current distributions based on defined harmonic sources and grid characteristics. It allows the modeling of any user-defined harmonic voltage or current source, both in magnitude and phase including inter-harmonics. The harmonic sources can be located at any busbar in the power system and may be implemented within any network topology. Most power electronic devices carry their harmonic characteristics already and need not to be defined by the user.
- 3-phase harmonic voltage and current distribution allowing an unbalanced harmonic load flows to be carried out.
- Harmonic current sources can be associated to every load, to any SVC (TCR injection) and to any rectifier or inverter. Harmonic voltage sources can be modeled using the AC voltage source model or the PWM AC/DC converter model. The build-in rectifier models inject the spectrum of ideal 6-ulse rectifiers if no other definition has been made.
- DIgSILENT PowerFactory supports any type of characteristic harmonic, un-characteristic harmonic (even harmonics etc.) and non-integer (inter-) harmonics. Also unbalanced harmonic sources (e.g. single-phase rectifiers) are fully supported. The analysis of inter-harmonics or unbalanced harmonic sources is based on a complete abc-phase network model.
- Transformer phase shifts are completely represented why 12 pulse rectifiers can be modeled correctly using 6-pulse rectifiers and transformers with the necessary vector groups.
- DIgSILENT PowerFactory calculates all symmetrical and asymmetrical harmonic indices for currents and voltages, including harmonic current indices and harmonic losses, such as:
- HD and THD
- IT product
- Harmonic losses
- Active and reactive power at any frequency
- Total active and reactive power, displacement and power factor
- RMS quantities
- Unbalance factors
Results can be represented:
- In the single line diagram (preferably total harmonic indices)
- As histograms (Frequency domain)
- As waveform (Transformation into the time domain)
- As profile (e.g. THD versus bus bars)
- Frequency dependent representation of network elements such as lines, cables, two and three-winding transformers, machines, loads, filter banks etc. for considering skin effects.
Frequency Scan
The frequency scan performs a continuous analysis in the frequency domain. The most common application is the calculation of self- and mutual network impedances for identifying the resonance points of the network.
- All impedances are calculated simultaneously in the same run. Since DIgSILENT PowerFactory uses a variable step size algorithm, the calculation time of frequency scans is very low while the resolution around resonance points remains very high (typically 0.1 Hz).
- DIgSILENT PowerFactory also supports series voltage sources with which it is possible to identify possible series resonance points of the network (important for subsynchronous resonance studies).
- Frequency scans can either be performed with the positive-sequence network model (very fast) or the complete three-phase abc-network model.
- Calculation of self- and mutual network impedances
- Calculation of voltage amplification factors
- Impedance plots may be realized in either Bode, Nyquist or magnitude/phase forms.
- Support of flexible, user-defined libraries for voltage and current sources as well as for indices
Network Modeling
Skin effect is considered by associating frequency characteristics to line or transformer resistances and inductances. These characteristics can either be specified by setting the parameters of a polynomial expression or by entering the characteristic point by point using tables. DIgSILENT PowerFactory uses cubic splines or hermite polynoms for appropriate interpolation.
- Lines are modeled either by approximate PI sections or the fully accurate distributed parameter line model that should always be used for long lines or high frequency applications. Skin effect can be included in both line models.
-
Filters can either be specified by so-called "Layout"
parameters or by "Design" parameters. "Layout"
parameters are typically the rated reactive power, the resonance frequency
and the quality factor. Design Parameter are the actual values of R, L, and
C.
Ripple Control Signals
DIgSILENT PowerFactory provides full support for analyzing and dimensioning ripple control systems. Series and parallel coupling of ripple control systems can be modeled including all necessary filter elements.
- The level of the ripple control signal in the entire network is calculated and reported in the single line diagram, output window or the browser.
Dimensioning of Filters
DIgSILENT PowerFactory features a special, easy-to-use function for the dimensioning of all kind of filters. All relevant voltages across all components are calculated and available in the "Filter Sizing" report.