Trends in active power line conditioners
Summary (2 min read)
I. INTRODUCTION
- APID variation of reactive power generated by arc fur-R naces, and harmonics produced by diode or thyristor rectifiers have been serious as they have caused flicker or harmonic interference in industrial applications and transmission/distribution systems.
- A shunt passive filter exhibits lower impedance at a tuned harmonic frequency than the source impedance to reduce the harmonic currents Rowing into the source.
- The source impedance, which is not accurately known and varies with the system configuration, strongly influences filtering characteristics of the shunt passive filter.
- Active power line conditioners, which are classified into shunt and series ones, have been studied to compensate for reactive power, negative-sequence, harmonics, andlor flicker in industrial power systems since their basic compensation principles were proposed in the 1970's [1]- [3] .
- There was almost no advance in active power line conditioners beyond a laboratory testing stage, because circuit technology was too poor to practically implement the compensation principle in the 1970's.
A. Compensation Principle
- Fig. 1 shows the basic principle of a shunt active conditioner, which is controlled in a closed loop manner to actively shape the source current i s into the sinusoid.
- Fig. 2 shows each waveform in such a case that the load is assumed to be a three-phase diode rectifier with an inductive load.
- The shunt active conditioner injects the compensating current Z c into the source to cancel the harmonics contained in the load current ZL.
B . Power Circuits
- These are the same in the power circuits as a current source PWM inverter and a voltage source PWM inverter for ac motor drives.
- The reason is that the shunt active conditioner can be controlled so as to supply the losses in the PWM inverter from the ac source.
- The author prefers the voltage source PWM inverter to the current source PWM inverter because the voltage source PWM inverter is higher in efficiency and lower in initial cost than the current source PWM inverter.
- In fact, almost all shunt active conditioners, which have been put into practical use in Japan, have adopted the voltage source PWM inverter as the power circuit.
- Table I1 shows the comparison of losses of 100 kVA shunt active conditioners using GTO thyristors, BJT's, or IGBT's, which are installed to the threephase 400-V power system [ 131.
C . Control Strategies and Filtering Characteristics
- L where dc components low-frequency components high-frequency components.
- They become strict after designing the cut-off frequency of high-pass filters which are used for the extraction of p ~, $L, PL, GL, GL, and GL from p~ and q L .
- In other words, the shunt active conditioner <an compensate for not only harmonics but also reactive power and negative-sequence component in accordance with its compensating objectives.
- In Fig. 8 , the load is a single-phase thyristor rectifier whose firing angle is constant to verify compensation characteristics for harmonic and negative-sequence currents.
- Before the start of the shunt active conditioner, the source current in the u-phase, isu is identical to the load current in the u-phase, i~,, and no source current flows in the w-phase.
nents.
- Compensation for harmonic currents and negative-sequence compo- Table IV shows the comparison between the two combined systems.
- The function of the series active conditioner is not to directly compensate for the harmonics of the rectifier, but to improve the filtering characteristics of the shunt passive filter used alone.
- Before the series active conditioner was started, a parallel resonance at the fourth-harmonic frequency occurred in the system.
- A small amount of the fourth-harmonic current included in the load current was largely amplified.
- The function of shunt active power line conditioners will be expanded from flicker compensation or voltage regulation into the improvement of stability in power systems as the capacity of shunt active power line conditioners becomes larger.
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