Reactive Current Reshaping With Series Resonance Damping for Three-Phase Buck-Type Dynamic Capacitor

Abstract

By adding Buck-type AC/AC converter to conventional power capacitor, dynamic capacitor (D-CAP) can be formed to compensate variable rather than fixed reactive power. However, some nonlinear distortion factors, namely grid harmonic voltage, voltage drop of switches and dead-time, would cause harmonics in its compensation current, meanwhile, its capacitance may interact with the grid inductance to cause series resonance. Both nonlinear distortion factors and series resonance would distort the waveform of its compensation current and make the grid current failed to meet the power quality standard IEEE Std.519. This paper focuses on the output current distortion of Buck-type D-CAP in reactive power compensation. For harmonic distortion, through establishing the equivalent circuit of Buck-type D-CAP, the generation of low-order harmonic current is investigated under three nonlinear factors. A current reshaping method to simultaneously diminish the harmonics caused by the three nonlinear distortion factors is thus adopted, which is a feedback control of output current harmonics. Then for series resonance, based on the block diagram of Buck-type D-CAP within the grid, transfer function from grid voltage to the output current is deduced and the corresponding Bode diagram is depicted. An active damping method is therefore suggested to shrink the low-order series resonance peak, by detecting power capacitor's voltage for feedback control. Finally, a combined control, reactive current reshaping with series resonance damping is proposed in this paper for three-phase Buck-type D-CAP. A wide variety of experimental results from a 33kVar/220V laboratory prototype are provided to demonstrate the validity of the combined control.INDEX TERMS Buck-type dynamic capacitor, reactive power compensation, waveform distortion, current reshaping, series resonance damping.

Series and parallel resonance active damping of three‐phase buck‐type dynamic capacitor for reactive compensation and harmonic suppression

“…Buck-type D-CAP is usually considered as a variable capacitor when the duty ratio modulation signal only contains the DC term D 0 to compensate reactive power, which may interact with the grid inductor, resulting in distorted compensation current, deteriorated compensation performance, or even instability [22]. According to the Kirchhoff's voltage law (KVL) and Kirchhoff's current law (KCL) equations of an equivalent circuit of D-CAP in Fig.…”

“…According to the Kirchhoff's voltage law (KVL) and Kirchhoff's current law (KCL) equations of an equivalent circuit of D-CAP in Fig. 1d, a block diagram of D-CAP can be obtained while transfer function G 1 (s) from PCC voltage v S to compensation current i C can be expressed by (5) [22]:…”

“…Series resonance active damping can be carried out through constructing a virtual harmonic resistor D 0 R d in parallel with the D-CAP power capacitor and taking the D-CAP power capacitor voltage for feedback control, corresponding to the red part of Fig. 2a [22]. Transfer function G 2 (s) from the grid voltage v S to compensation current i C is revised as (10): (see (10)) .…”

“…Series resonance damping is usually required only at the lower series resonance frequency [22]. However, with R d decreasing in Fig.…”

“…Harmonic distortion factor (HDF) of kth harmonic PCC voltage HDF vk is then obtained through V Tk divided by V T1 . Finally, the wide-band self-adjusted damping gain G Vk can be obtained by PI controllers, which is expressed as (22):…”