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In a typical HT sensing and low-compensation scheme:
HT sensing: The system samples voltage and current on the high-voltage side (e.g., 10kV) using CTs and PTs.
Low-side compensation: The actual compensation—either reactive power support (SVG/capacitors) or harmonic filtering (AHF)—is implemented on the low-voltage side (e.g., 400V).
This architecture is especially common in transformer-based power distribution systems, where it's convenient to monitor overall load behavior at the transformer input, while placing equipment in safer, more accessible low-voltage environments.
Centralized Monitoring: Sampling at the high-voltage side provides a more global view of the load connected to the transformer, especially useful when multiple loads are downstream.
Easier Installation & Maintenance: Installing compensation equipment on the low-voltage side is typically easier, safer, and more cost-effective.
Effective for Power Factor Correction: When used for reactive power compensation, this approach can efficiently improve the power factor at the transformer level.
Despite its convenience, this scheme poses several technical issues, especially when used for active harmonic filtering:
Because the measurement and the action point are not co-located, the control system experiences inherent delay. Data must be sampled, processed, and translated into control actions across the transformer boundary. In fast-changing load environments (e.g., welding machines or elevators), the compensation can be too slow to be effective.
Transformers inherently attenuate high-frequency signals due to their impedance characteristics. Harmonic currents—especially 5th, 7th, and higher orders—are significantly reduced or distorted when passing through a transformer.
As a result, the controller sees a harmonic profile on the HT side that does not reflect the actual low-side harmonic conditions. When the AHF attempts to compensate based on this distorted data, the filtering effect becomes ineffective or even counterproductive.
Conclusion: Harmonic filtering should not be implemented using HT sensing. For effective harmonic mitigation, sensing and compensation must be done on the same voltage level—preferably near the harmonic source.
Transformer impedance also causes phase shifts between sampled voltage and current. These shifts result in inaccurate computation of power factor and reactive power components, leading to suboptimal or incorrect compensation.
In sensitive systems, especially those relying on accurate P/Q decomposition or real-time control, this phase error can lead to control instability or overcompensation.
In systems where HT sensing relies on digital communication, delays or packet loss can occur. This affects the timing and reliability of control signals, especially in multi-device or parallel compensation systems.
While not ideal for harmonic filtering, HT sensing with low-side compensation works well for reactive power compensation in stable systems. SVGs or capacitor banks using this approach can effectively maintain transformer-level power factor, reduce grid penalties, and relieve upstream reactive burden.
Use local sensing for harmonic filtering—ideally close to the load generating harmonics.
When using HT sensing for reactive compensation, factor in transformer impedance and perform parameter tuning accordingly.
Consider systems that allow remote sensing with algorithmic correction to compensate for phase and impedance discrepancies.
For multi-device systems, ensure high-quality, synchronized communication to avoid control mismatches.
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