Why is there a 50Hz component in the load current dq transformation?

In my modelling for UPFC I found that the dq component of the load current has a 50Hz component, but how can I explain this phenomenon, as far as I know, negative sequences can have harmonics of 100Hz.

Answer 1

[Sanchari]

Hello Xin,

In power systems, particularly when dealing with devices like the Unified Power Flow Controller (UPFC), the presence of a 50 Hz component in the (dq) components of the load current can often be attributed to system imbalances or non-ideal conditions. By investigating the system's balance, harmonic content, and control strategies, one can better understand and mitigate this phenomenon.

Probable explanation of the 50 Hz Component

1. Synchronization with System Frequency: The (dq) transformation is synchronized with the system's fundamental frequency (50 Hz). If the load current is purely sinusoidal and balanced, the (dq) transformation will yield a constant (DC) value in the (dq) frame. However, any unbalance or distortion in the system can introduce oscillatory components.
2. Imbalance and Non-Ideal Conditions: In practical systems, loads may be unbalanced or there may be harmonics present. These conditions can cause oscillations at the fundamental frequency (50 Hz) in the (dq) components. This is because the transformation assumes a perfectly balanced system, and any deviation from this can result in a non-zero AC component.
3. Presence of Harmonics: While negative sequence components can indeed have harmonics at twice the fundamental frequency (100 Hz), the presence of a 50 Hz component suggests that there is some aspect of the load current or system that is not perfectly balanced or is introducing a fundamental frequency component into the (dq) frame.
4. Measurement and Filtering Artifacts: Sometimes, measurement errors or inadequate filtering can introduce unwanted components into the transformed signals. Ensure that the measurements are accurate and that any filters used in the transformation process are properly designed.

Investigating the Source

1. Check for Load Imbalance: Verify if the load is balanced. An unbalanced load can introduce a fundamental frequency component in the (dq) transformation.
2. Inspect Harmonics: Analyze the harmonic content of the load current using tools like Fast Fourier Transform (FFT) to check for unexpected harmonics or interharmonics.
3. Examine Control Strategies: If the UPFC or other control devices are involved, ensure that their control strategies are not inadvertently introducing these components.
4. Simulation and Analysis: Use simulation tools to model the system and observe the behavior under different conditions. This can help identify if the 50 Hz component is due to specific system characteristics.

Please consider refering the following links for further knowledge on:

1. UPFC Phasor Model (MathWorks Documentation): https://in.mathworks.com/help/sps/ug/upfc-phasor-model.html?searchHighlight=upfc&s_tid=srchtitle_support_results_4_upfc
2. UPFC Implementation (File Exchange): https://in.mathworks.com/matlabcentral/fileexchange/133307-upfc-unified-power-flow-controller-implementation
3. UPFC model (MathWorks Documentation): https://in.mathworks.com/help/sps/ug/upfc-detailed-model.html?searchHighlight=upfc&s_tid=srchtitle_support_results_5_upfc

Hope this helps!