Neutral current and Active harmonic filter

Active Harmonic Filter (AHF) and Neutral Current

Active Harmonic Filter (AHF) is used to improve power quality, by harmonics filtering, reactive power compensation and 3 phase balancing in power systems. Harmonics are voltage or current waveforms that are integer multiples of the fundamental frequency. They can cause various power quality issues, including increased neutral current in three-phase electrical power systems. AHF can significantly impact neutral current, especially in systems with non-linear loads or single phase loads.

Neutral Current in Three-Phase Systems

In a balanced three-phase system without harmonics (linear load), the sum of the currents in the three phases cancels out, resulting in no neutral current. However, several factors can cause neutral current to flow, including:

Unbalanced Loads: Unequal distribution of loads across phases.
Harmonics: Particularly, third harmonics (and multiples of three order harmonics) are in phase in all three phases and add up in the neutral conductor.

How Active Harmonic Filters Work

Active Harmonic Filter works by continuously monitoring the current waveform and injecting compensating currents to cancel out the harmonics. The process involves:

Detection: Active filter measure the harmonic content of the load current using current sensors.
Control: The control unit processes the measured current and determines the harmonic order that need to be compensated.
Injection: The AHF generates and injects currents that are 180 degrees out of phase with the detected harmonics, effectively canceling them out.

Impact of AHFs on Neutral Current

Reduction of Third Harmonics:

Third harmonics (150 Hz in a 50 Hz system) are significant contributors to neutral current in three-phase electrical distribution systems. These harmonics are in phase in all three phases and thus add directly in the neutral conductor.
AHFs can effectively reduce third harmonics by injecting compensating currents, thereby significantly reducing the neutral current.

Balancing Loads:

By mitigating harmonics, AHFs help in creating a more balanced load profile, which reduces the neutral current caused by load imbalance.

Benefits of Using AHFs

Improved Power Quality:

Reduction of harmonics leads to a cleaner sinusoidal waveform, improving overall power quality.

Reduced Neutral Current:

By eliminating or reducing third harmonics and other zero-sequence harmonics, AHFs reduce the current flowing through the neutral conductor, enhancing system efficiency and safety.

Enhanced System Reliability:

Lower neutral current reduces the risk of overheating and potential failure of the neutral conductor, improving system reliability.

Compliance with Standards:

AHFs help meet power quality standards and regulations by maintaining harmonic levels within acceptable limits.

Example Calculation: Neutral Current with AHFs

Without AHF:

Assume a system with a significant presence of third harmonics. If the phase currents (including third harmonics) are
IR=30 A,IS=30 A,IT=30 A:
IN2=IR2+IS2+IT2−IR*IS−IS*IT−IT*IR
IN=0 A
IN(3)=3×30A=90A

With AHF:

The AHF cancels out the third harmonics by injecting 30 A of third harmonic current in each phase:
IN(3)=0 A

Thus, the total neutral current is reduced to near zero, assuming perfect compensation.

Conclusion

Active Harmonic Filters (AHF) play a crucial role in reducing neutral current in three-phase systems by harmonic correction or significantly reducing total harmonics distortion, particularly the third harmonics in nonlinear load.

Meanwhile,Same as Static Var Generator, Active filter is the best power factor correction product with fast response, can By improving power quality and ensuring balanced loads, AHFs enhance the efficiency, safety, and reliability of electrical systems.