Modeling techniques in DIgSILENT PowerFactory including Harmonic Source Modeling

In DIgSILENT PowerFactory, harmonic source modeling is essential for performing harmonic analysis, which evaluates the distortion in voltage and current waveforms caused by nonlinear loads and power electronic devices. Harmonic analysis helps in assessing power quality, identifying resonance conditions, and ensuring compliance with standards such as IEEE 519 and IEC 61000.

To model harmonic sources in PowerFactory, different types of harmonic sources and models are used to represent both current- and voltage-type distortions in a power system. The significance of each type of modeling varies depending on the nature of the nonlinear loads, power system characteristics, and the specific analysis requirements.

Modeling techniques in PowerFactory

1. Current Harmonic Source

2. Voltage Harmonic Source

3. Frequency-Dependent Models

4. Equivalent Impedance Modeling

5. Detailed Device-Based Modeling

1. Current Harmonic Source Modeling

Current harmonic sources are typically used to represent nonlinear loads that draw a distorted current from the power system while being supplied with a relatively sinusoidal voltage. These models are most commonly used to represent loads such as variable frequency drives (VFDs), fluorescent lighting, and other power electronic converters.

Implementation:

In PowerFactory, a current harmonic source can be modeled as a current injection at a bus. It is defined by specifying the harmonic spectrum (magnitude and phase) for each harmonic order of interest. This model assumes that the voltage at the point of connection is sinusoidal, and the harmonics are injected into the network based on the specified current distortion.

Significance:

- Accuracy: Suitable for modeling loads that predominantly act as current sources, like VFDs or rectifiers.

- Simplicity: Easier to implement when harmonic currents are known.

- Frequency Domain Analysis: The harmonic current injection can be directly used for frequency domain harmonic studies, where the current harmonic spectrum is well defined.

2. Voltage Harmonic Source Modeling

Voltage harmonic sources are used to model devices that produce harmonic voltages. These models are typically applied to power electronic converters that distort the voltage waveform. For example, an uninterruptible power supply (UPS) or an inverter might inject voltage harmonics into the system.

Implementation:

A voltage harmonic source is modeled by specifying the harmonic voltage spectrum at a particular bus. The model defines the harmonic voltages for each harmonic order and applies them to the system's voltage waveform.

Significance:

- Source Representation: Voltage harmonic sources are ideal for representing equipment like inverters or active harmonic filters that distort the voltage waveform.

- Resonance Studies: This model is useful for studying the impact of harmonics on resonance conditions in the network, particularly when harmonic voltages propagate through the system.

- Grid-Connected Inverters: In grid-connected systems, voltage harmonic sources are often used for devices that control or generate voltage waveforms (e.g., PV inverters).

3. Frequency-Dependent Models

This type of modeling is used to represent the changing characteristics of network components (such as lines, transformers, and cables) at different frequencies. This is critical in harmonic analysis because the impedance of these components varies with frequency, influencing harmonic propagation and potential resonance.

Implementation:

In PowerFactory, frequency-dependent models can be applied to various system components, including transmission lines, transformers, and loads. For example:

- Transmission Lines: Modeled using frequency-dependent impedance, considering the skin effect and frequency response of conductors.

- Transformers: Frequency-dependent transformer models include core saturation effects, eddy current losses, and winding impedance variations at different harmonic frequencies.

Significance:

- Resonance Analysis: Frequency-dependent models are essential for accurate resonance detection in harmonic studies, especially when series or parallel resonance occurs at specific harmonic frequencies.

- Accuracy: By accounting for the impedance variation with frequency, these models provide more realistic results for harmonic propagation, particularly in systems with high harmonic content.

4. Equivalent Impedance Modeling

In many harmonic studies, rather than modeling individual harmonic sources or devices in detail, an equivalent impedance or Thevenin equivalent can be used to represent the harmonic response of an entire network or part of a system.

Implementation:

An equivalent impedance is calculated based on the network configuration and the impedance seen at the point of interest. This approach is commonly used to model the grid’s harmonic impedance seen by nonlinear loads.

Significance:

- Simplification: Equivalent impedance models are often used to simplify large systems where it is impractical to model every harmonic source in detail.

- Grid Representation: When studying the harmonics injected into the grid, this model can represent the system’s ability to absorb or amplify certain harmonic frequencies.

- Resonance Detection: This model is helpful in detecting potential resonance conditions caused by interactions between the network's equivalent impedance and harmonic sources.

5. Detailed Device-Based Modeling

For more accurate harmonic studies, detailed models of specific devices (such as converters, inverters, or industrial equipment) are often required. These models include the internal structure and behavior of the devices, capturing their actual harmonic generation.

Implementation:

Detailed device models are implemented in PowerFactory by specifying the internal circuit of the device, including switches, power electronic components, and control systems. For example:

- Inverters: Modeled with detailed switching functions to simulate the harmonic content generated during operation.

- Converters: Modeled with actual diode or thyristor-based rectification circuits to capture the harmonic spectrum produced during rectification.

Significance:

- Precision: Provides highly accurate results by modeling the actual harmonic generation mechanisms of devices.

- System-Level Impact: Ideal for detailed harmonic analysis when studying the impact of specific devices on the overall system’s harmonic distortion.

- Control System Interaction: Detailed models can capture interactions between power electronic devices’ control systems and the grid, which can influence harmonic generation and mitigation.

Comparison of Modeling Techniques

Conclusion

Each harmonic source modeling approach in PowerFactory serves a distinct purpose, depending on the type of load or equipment being modeled and the depth of analysis required.

- Current harmonic sources are used for nonlinear loads drawing harmonic currents.

- Voltage harmonic sources are applied to devices that distort voltage.

- Frequency-dependent models are crucial for understanding the varying impedance characteristics of network elements.

- Equivalent impedance models provide a simplified view of the grid’s response to harmonics.

- Detailed device-based models offer the highest accuracy for simulating harmonic generation in power electronic devices.

By selecting the appropriate model based on the application, engineers can accurately perform harmonic studies, identify potential issues, and implement effective harmonic mitigation techniques.