Unintentional series and/or parallel resonances, due to the tuned passive filter and the line inductance, may result in severe harmonic distortion in the industrial power system. This paper presents a hybrid active filter to suppress harmonic resonance and reduce harmonic distortion as well. The proposed hybrid filter is operated as variable harmonic conductance according to the voltage total harmonic distortion, so harmonic distortion can be reduced to an acceptable level in response to load change or parameter variation of power system. Since the hybrid filter is composed of a seventh-tuned passive filter and an active filter in series connection, both dc voltage and Kva rating of the active filter are dramatically decreased compared with the pure shunt active filter. In real application, this feature is very attractive since the active power filter with fully power electronics is very expensive. A reasonable trade-off between filtering performances and cost is to use the hybrid active filter. Design consideration are presented and experimental results are provided to validate effectiveness of the proposed method. Furthermore, this paper discusses filtering performances on line impedance, line resistance, voltage unbalance and capacitive filters.
1. Hybrid active filter
2. Harmonic resonance
3. Industrial power system
Fig. 1. The proposed hybrid active filter unit (HAFU) in the industrial power
system and its associated control.
EXPECTED SIMULATION RESULTS:
Fig. 2. Line voltage e, source current is, load current iL, and filter current
i in case of NL1 initiated. X axis: 5ms/div.
Fig. 3. Line voltage e, source current is, load current iL, and filter current i in case of NL2 initiated. X axis: 5ms/div.
(a) Terminal voltage
(b) Source current.
(c) Filter current.
(d) Load current.
Fig. 4 The HAFU is off for single-phase nonlinear load.
(a) Terminal voltage.
(b) Source current
(c) Filter current.
(d) Load current.
Fig. 5. The HAFU is on for single-phase nonlinear load.
This paper presents a hybrid active filter to suppress harmonic resonances in industrial power systems. The proposed hybrid filter is composed of a seventh harmonic-tuned passive filter and an active filter in series connection at the secondary side of the distribution transformer. With the active filter part operating as variable harmonic conductance, the filtering performances of the passive filter can be significantly improved. Accordingly, the harmonic resonances can be avoided and the harmonic distortion can be maintained inside an acceptable level in case of load changes and variations of line impedance of the power system. Experimental results verify the effectiveness of the proposed method. Extended discussions are summarized as follows:
• Large line inductance and large nonlinear load may result in severe voltage distortion. The conductance is increased to maintain distortion to an acceptable level.
• Line resistance may help reduce voltage distortion. The conductance is decreased accordingly.
• For low line impedance, THD_ should be reduced to enhance filtering performances. In this situation, measuring voltage distortion becomes a challenging issue.
• High-frequency resonances resulting from capacitive filters is possible to be suppressed by the proposed method.
• In case of unbalanced voltage, a band-rejected filter is needed to filter out second-order harmonics if the SRF is realized to extract voltage harmonics.
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