Active damping of oscillations in LC-filter for line connected, - - PowerPoint PPT Presentation

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Active damping of oscillations in LC-filter for line connected, current controlled, PWM voltage source converters

Authors: Olve Mo, Magnar Hernes, Kjell Ljøkelsøy SINTEF ENERGY RESEARCH, Norway Paper presented at EPE 2003, Toulouse

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The challenge

A low pass LC-filter is a commonly used method to reduce the switching ripple currents and voltages at the terminals

• f line connected PWM converters

The LC-filter introduces risk of oscillations in converter terminal voltage due to resonance between LC-filter capacitor and AC-grid reactance Oscillations can be initiated by nearby load changes or by periodic disturbances in the grid (e.g. thyristor rectifiers). The challenge is to modify the control system in such a way that oscillations are damped without affecting the basic control tasks of the converter

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Applicability

Filter oscillations is most likely to be a problem when the natural frequency of the oscillation is low (poorly damped) The worst cases will typical be those were the converter rating becomes comparable to the upstream transformer (or generator) rating. A typical example is emergency operation of isolated grids were converter ratings may become comparable to the emergency generator ratings. Other examples are ship and offshore power systems and distributed production units connected to a weak grid.

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The power circuit

Current controlled, line connected, PWM voltage source converter (For instance operated as active rectifier / inverter, STATCOM or active filter) The focused oscillation are illustrated by red arrows in the figure

AC-grid Optionally connected to DC-source / DC-load

+ + + +

UDC ib ic ia ua uc ub

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Control system

Closed loop control of filter inductor current and DC-link voltage, PWM Measures DC-link voltage, minimum two filter inductor currents and two line voltages. Synchronisation by phase locked loop Active damping is an ”add-on feature” (red in the figure)

Hreg,DC(s) + - udc,ref* udc,measure iactive

Inverse park transf.

ireactive,ref* ia/b/c,ref + + Hreg,cur(s)

PWM Gate pulses

+ _ ia/b/c,measure

Phase locked loop

ua/b/c,measure θ

Active damping

ia/b/c,damp ia/b/c,ref,mod iactive,ref*

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The active damping method

The oscillating component of the capacitor voltage is extracted (the non-fundamental component) The error in the capacitor voltage is found by subtracting the oscillating component from its reference (=0) The error is amplified and added to the reference signal for the current controller.

Hreg,damp(s) + ua/b/c,oscillation,ref* (= 0) ia/b/c,damp

• +

Park transf.

ua/b/c,measure

• θ (from phase locked loop)

ucos+ usin+

Low pass filter Inverse Park transf.

ua/b/c,fundamental,+ ucos+,filtered usin+,filtered ua/b/c,oscillation

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Demonstration by simulation

Application: PWM converter connected to the AC-grid at a point with low short circuit capacity compared to converter kVA rating Demonstration of damping of oscillations after a step reversal of the reference for reactive current

Ideal AC Network PWM converter LC-filter 1 : 1 VDC = 450 V Sconv = 20 kVA fsw = 5470 Hz

• 40 dB at fsw

L=0.82mH C=100µ F RC=1.0mΩ RL=6.4mΩ Stransf = 30 kVA xtransf = 10 % L=0.6mH, R=100mΩ fgrid = 50 Hz vgrid = 230 Vrms (l-l) Imains Imeasure

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Active damping disabled Phase a filter capacitor voltage (black) Phase a current into AC-grid (red) Active damping enabled

Time (sec)

0.13 0.142 0.154 0.166 0.178 0.19

• 0.25
• 0.15

+0.05 +0.15 +0.25 ua ia,mains

Time (sec)

0.13 0.142 0.154 0.166 0.178 0.19

• 0.25
• 0.15

+0.05 +0.15 +0.25 ua ia,mains

Time (sec)

0.14 0.144 0.148 0.152 0.156 0.16

• 0.25
• 0.15
• 0.05

+0.05 +0.15 +0.25 ua ub uc

Time (sec)

0.14 0.144 0.148 0.152 0.156 0.16

• 0.25
• 0.15
• 0.05

+0.05 +0.15 +0.25 ua ub uc

• 0.05
• 0.05

Filter capacitor voltages (phase a, b, c)

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Laboratory demonstration

Converter connected to a weak bus Three-phase diode rectifier connected to the same bus Measurements demonstrates that the active damping suppresses LC-filter filter oscillations trigged by the harmonics of the diode rectifier

PWM converter LC-filter AC Network 3-phase diode rectifier Rload,serial= 28.5Ω Lload,serial=1.2mH (no commutation reactance) imains itransf idiode Bus A 230:400 VDC = 400 V fsw = 5470 Hz CDC=3300µ F (no load or source connected to DC-link) L=0.82mH C=100µ F RC=1.0mΩ RL=6.4mΩ fgrid = 50 Hz vgrid = 400 Vrms (l-l) Transformer (ref. 230V) Ltransf = 650µH Rtransf = 130mΩ at 50Hz, 300mΩ at 600Hz imeasure

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Active damping enabled Active damping disabled Phase voltage at bus A (black) Transformer current (red)

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Conclusions

Introduction of active damping is a possible measure for reduction of oscillations due to low pass LC-filters of line connected current controlled PWM converters Simulations and measurements show that the method works as intended The voltage quality at the point of connection can be maintained in cases were filter oscillations is to be expected when the low pass LC-filter is introduced